‘Tcfmhrdmn Vol. 39. No. 7. pp. 115 I to 1160. 1983 al4o-mo183/01115laso3.03l0

Printed in Great Britain Pergamon Press Lld

PHOTOREARRANGEMENTOFHOMODIBENZOBARRELENES

HELFRIED HEMETSBERGER,* WOLFGANG HOLSTEIN and FRIEDRICH WERRES Lehrstuhl fiir Organische Chemie 2, Ruhr-Universitit Bochum. 4630 Bochum. Federal Republic of Germany

(Received in Grnnany 16 April 1982)

Abstract-Direct irradiation of 61 affords the rearrangement products 1L formed from the lowest triplet state and 111 from S, or a higher triplet state, 101 via a d&r-methane rearrangement under am-aro bridging followed by a cyclopropylcarbinyl-homoallyl rearrangement in a later step along the reaction co-ordinate. Deuterium labelling demonstrated that the rearrangement showed a regiospeciiity of 78 and 85% for bond breaking on the backsite to the cyclopropyl ring. Dimethylsubstituted substrates show different photochemistry as a function of the position exo- or endo- to the cyclopropane ring. Quantum yield determinations show ET > 330 W mol.’ for the lowest triplet, also that energy transfer from acetone is not totally efficient.

The generality of the d&r-methane photorearrangement was demonstrated by a thorough and methodical study by Zimmerman.’ The interactions of n-systems in the course of the photorearrangement were intensively stu- died in bicyclic systems as barrelenes: are-3 and diaro- harrelenes”” and benzobicyclo[2.2.2]octadienesS and aronorbornadienes.” In these cases the photorear- rangements proceed by a vinyl-vinyl or aryl-vinyl bond- ing from the photoreactive triplet state. In systems, in which aryl-ar I bonding is the only accessible route, as in triptycenes Y or diaronorbornadienes,’ no d&r-methane rearrangement could be observed. In these cases a car- bene is formed, which can add intramolecularly to an arylring, can be intercepted by solvent“‘.’ or show a carbene rearrangement.’ An exception to these results was demonstrated by the photorearrangement of the homodibenzobarrelene (l), which isomerized pre- dominantly by the d&r-methane route and only to a minor extent by the carbene route (Scheme l).9

Zimmerman reported a cyclopropyl-vinyl bonding in the photorearrangement of 3-(2,2-diphenylcyclopropyl)- 3-methyl-l,l-diphenyl-l-butene.‘” An analogous cyclo- propyl-aryl bonding in the photorearrangement of 1 did not take place.

Direct or sensitized irradiations of exo- and endo- dicarbomethoxy substituted homobenzobarrelenes Is and 4b deliver a rearrangement product (S), which has been formed probably by a d&r-methane rearrangement in the first step followed by a cyclopropylcarbinyl- homoallyl rearrangement in a later step along the reac- tion co-ordinate.”

To gain more information in detail on the interference of the cyclopropyl ring an investigation of the photo- chemistry of the dicarbomethoxy homodibenzobar- relenes 6 seemed lo be promising.

& /\ -

RESULTS

Synthesis of photoadducts The photoadducts were synthesized as outlined in

Scheme 3. endo- and exodc and 6d could be separated by pre-

parative HPLC and obtained pure for the photochemical experiments. endo- and exo- 6d were subsequently con- verted to the monodeutero derivatives endo- and exode by reaction with n-Bu&D.

Photoreanangement. Overall reaction course Direct or acetone sensitized irradiation of 6a,b del-

ivered as the only primary photoproducts 10 and 11. The ratio of 10 to 11 varied depending on the inadia-

tion conditions and will be discussed later. The structure of 11 could be easily deduced from the ‘H-NMR spectra, from the isotopic labeling and the double resonance experiments. With the structure of 10 some difficulties arose. Hydrogenation of 10 under atmospheric pressure led to a fast consumption of two equivalents of hydrogen. The hydrogenation of the third double bond proceeded slowlier and competed with the hydrogenation of the cyclopropyl ring. Addition of tetracyanoethene led to two Diels-Alder products, which could not be separated. ‘H-NMR- spectra of 1Oa showed a doublet of doublets at 6 = l.OSppm with the intensity of one hydrogen atom. This proton was coupled with a signal at 7.14 ppm within the aromatic region which does not couple for lob. The proton was further coupled with a proton showing a signal at 3.76ppm with a coupling constant of I9 Hz, typically for geminal hydrogen atoms. The hydrogen atoms of the carbomethoxy groups gave rise to two singlets at 3.36 and 3.72ppm demonstrating that the symmetry plane was lost in the photorear- rangement. A complex multiplet in the olefinic region

hv dir-

1

TET Vol. 39. No. 7-J

2 3

Scheme I.

I151

1152

X 0 COzMe

Scheme 2.

‘3’ m I’ \ \ R’

0 0 Iv +MeO,CCZCCO,Me -

R’

7

6 ChN2 _

9a ero -/ endo - 9b -9e

X= COpMe

hv acetone

+

R'

a : R’s R2rR’. R3= H

b : R’s R2rD ; R3rR4=H

c : R’=R’rH ; RkR’=fde

d : R’s R2rH ; R3= 8r ; R&H

e : R’mR2sH ; R%D ; R’tH

Scheme 3.

Photorearrangement of homodibcnxobsrrelems 1153

6 10 11

a: RI= R2= H b: R’rR2= D

X = CO*Me

Scheme 4.

with intensity of four hydrogen atoms at 5.46-6.OOppm was well separated from the signals of the aromatic hydrogen atoms. Further, a singlet with the intensity of one H-atom could be observed at 3.53ppm, which did not show up in the spectrum of 1Oc. The “C-NMR of lOa showed a triplet at 21.66 ppm, a singlet at 43.67ppm, a singlet and a doublet superimposed at 46.98 and a singlet at 51 .S8 ppm. All other signals appeared in the expected region (ExperimentaJ). From the spectral and chemical evidence it was first concluded that the structure of the barbaralane derivative (12) has to be assigned to the photorearrangement product.” Recent investigations of the photorearrangement of dicarbomethoxyhomoben- zobarrelene’ ’ and carboxyhomodibenzobarrelene” revealed that the structure of 12 has to be rejected and the structure according lo 10 has to be assigned to the photo-product.

The positions of the resonance signals and the coup-

X

& / X

12

ling constants in the ‘H- and “C-NMR spectra of 10 correspond closely to those found for 5.”

Rcgiospecijty studies In the photorearrangement of 6 the products 10 and 11

are formed (Scheme 4). In 10 one aromatic ring retained its electronic structure whereas the other was converted to a cyclohexadiene, as was the case in the photorear- rangement of 1 to 2 (Scheme 1). From the exoconfigura-

Me

Me

Me X X

\ Me 4 \

15

X =C02Me

1154 H. HEMEISIJERGER et al.

tion of the cyclopropane ring in 2, it was concluded that the exo-benzene ring in 1 retained the aromatic elec- tronic structure whereas the aromatic structure got lost in the endo-ring. Since the cyclopropyl ring was cleaved in the photorearrangement of 6, no direct answers on the regiospecifity of the rearrangement can be given. An investigation of the regiospecifity of the photorear- rangement requires the introduction of suitable lables in the benzene rings. To find an answer to this question the photorearrangement of endo- and exodc and df was studied. The results are outlined in Schemes 5 and 6.

Whereas acetone-sensitized irradiation of endodc yielded the rearrangement product 14 and a mixture of tle isomers of 13, irradiation of exodc gave endode and the same mixture of syn- and anti-13. No 15 could be observed in either photoreaction. This unexpected result gives an answer to the question of the regiospecifity to some extent. In the rearrangement of endodc to 14 the exo-benzene ring retained its electronic structure as was observed with 1. Surprisingly exodc did not yield either 14 or IS. This finding needs further comment (see Dis-

Since the methyl groups in endo- and exode intro- duced an excessive perturbation into the electronic structure, the photorearrangement of the deuterium- labeled exe- and endo-6e was investigated. (Scheme 6) Deuterium substitution in the aromatic moiety should only show a minor effect. Franck-Condon factors are expected to be only of minor importance, since the position of due- terium labeling and the site of bond reorganization are far apart. The deuterium distribution in the isotopomeric rearrangement products d-10 was determined by the in- tegration differences in the ‘H-NMR spectra of the well separated aromatic and olefinic regions. The intensities were measured over the entire regions and no efforts were made to assign the positions of the deuterium atoms in the aromatic and the olefinic moieties, respec- tively. The deuterium distribution determined by mass spectroscopy showed that the amount of deuterium con- tent did not change during photoreaction and product isolation. exo&, with deuterium substitution d,, = 8.5% and d, = 91.5% yielded d-10 with do = 7.2% and d, = 92.8 and endo4, with Q = 8.4%, and d, = 91.6% yielded d-10

cussion). with d,,= 6.6% and d, = 93.4%. Evaluation of the in-

X

X

A /\ - 6

I X=C02Me I

exo-d-10 18a

endo-d-10

. = deuteriumlabel

Scheme 7.

Photorearrangement of homodibenzobarrelenes 1155

tegrals in the ‘H-NMR spectra of d-10 obtained from The quantum yields of product formation were irradiation of exo-6e gave the intensity ratio Jrrom/Jo~c, = obtained with an apparatus consisting of a HBO 200 1 S47 and from irradiation of endo-6e Jarum/Jorcr = 1.052. lamp, and Bausch & Lomb high intensity monoch- Taking into account that the deuterium content was less romator. The light quanta were measured by a unit than 100% the acetone sensitized photorearrangement of described by Amrein et al.” which was calibrated by exodc led to exod-10 with a regiospecifity of 85% and ferrioxalate actinometry.” The quantum yields measured endodc to endod-10 with 78%. These values agree well were obtained by conversions of less than 5% and are and can be considered to be within experimental error. collected in Table I.

Multiplicity study and quantum yields Since the photorearrangement proceeded by direct or

sensitized irradiation, a singlet and/or triplet state could be involved. A quantitative analysis of the product dis- tribution of 10 and 11 revealed that in an acetone sen- sitized irradiation of 6s 31.8% lOa and 5.6% lla were formed. Contrary, in the direct irradiation of 6a to a conversion of 61.3%, only 7.8% 101 and 15.7% 11 plus a small amount of a third not identified product were formed. 1Oa could be shown to be sensitive to direct irradiation. Consequently, the figures obtained do not reflect the real situation, To gain information on the triplet energy needed, sensitizers of different triplet energy were used. With acetone the product ratio of 10s to lla was 4.31 and dropped to 0.453 with propio- phenone. 0.77 with p-methoxybenzophenone, 0.34 with ttiphenylene and 0.36 with 4,4’dimethylaminoben- zophenone. These results imply that 101 and lla are not formed from the same photoreactive states. Since the product ratio for the sensitizers propiophenone, ET= 301 Wmol-’ to 4,4’dimethylaminobenzophenone, ET = 259 Wmol-’ is almost constant and close to 0.5, observed in direct irradiations, it must be assumed that the pho- toreactions observed in the sensitized runs with sen- sitizer energies E.r < 301 W mol-’ are the results of direct light capture. Parallel with the change in the product ratio by changing the sensitizer energy from 330 W mol-’ using acetone to 301 W mol-’ using propiophenone the efficiency of the formation of 10s dropped, too. This implies that a sensitizer energy of ET > 330 W mol.’ is required.

The finding that ddir > drcnr (acetone) for the for- mation of lOa implies that the energy transfer is

it has to of the is higher EY

(sensitizer). A similar observation was reported in the d&-methane rearrangement of 6,7&n- zobicyclo[3.2.1]2,6-octadiene.‘” It was shown that the efficiency of the photoreaction increased with the acetone concentration.

DISCUSSION

Irradiation of 1 (X = H) yields 2 as the main product in a di-n-methane rearrangement from the singlet state. In this rare case a di-n-methane product was formed by am-aro bridging and the involvment of a diradical 16 was assumed. 16 is vinylogous to the cyclopropyldicarbinyl radicals which are generally assumed to be intermediates along the reaction co-ordinate of the di-n-methane rear- rangement. 16 may react by breaking bond (a) with concomitant rearomatization of one benzene ring to yield a diradical 170, which if a singlet electronic configuration is attained, will close to 2 (Scheme 7).

To get more definite information on the nature of the photoreactive states, quenching experiments using a piperylene as quencher were performed and a linear Stem-Volmer plot was obtained in the range of concen- trations of piperylene of 0.2 to 1.4 X 10m3 hi for 101. In contrast the formation of lla could not be quenched. These results show clearly that two different photoreac- tive states are involved. 101 will be formed most prob ably from T, of 60, whereas 11s results from S, or a higher triplet state which cannot be quenched nor sen- sitized with the usual sensitizer.

The diradicals 16 and 170 are to be considered as species on the reaction hypersurface, which do not necessarily correspond to energy minima. The involve- ment of triplet diradicals in the di-r-methane rear- rangement was made highly probably by Zimmerman.” SchaBner could show in the di-r-methane rearrangement of benzoylnaphthobarrelene for the first time in the di-r-methane photochemistry that two triplet ground state diradical intermediates intervene consecutively.” The adoption of diradical intermediates was of help to explain the regiospecifity of the di-n-methane rear- rangements. In numerous cases of di-a-methane systems it could be shown that the dominating break step was the more exothermic one.19

In the direct and sensitized irradiation of 6a the triplet state is photoreactive and yields 10s. Most likely lOa will have been formed on a pathway similar to that one used in the formation of 5 from 4a,b (Scheme 2). The diradi- cals 170 and 17b will be formed by breaking bond (a) and bond (b) in 16.

Table I. Quantum yields of formation of photoproduct from C

Product ‘dir. x 102a Dsens*x 102b

J& 6.5 t 0.2 3.25 ! 0.15

lla 4.0 f 0.1 0.7L5 ? 0.005 -

a) direct irradiations were performed in cyclohexane

at 251 nm.

b) sensitized irradiations were conducted in acetone

at 316 nm.

11S6 H. HEYETSBERGER rt al.

b&-J - fg exe-6e

/,

\

exo- d-10

/

&$&- cg& endo - 6e endo -d-IO

X= C02Me l = deutetiumtabel

Scheme 6.

In these diradicals one of the odd electrons is placed on a carbon belonging to a cyclopropyl-carbinyl system, which is known to rearrange easily to a homoallyl sys- tern... The driving force for the conversion from 17a,b+ l&,b will derive from the removal of diester strain energy and the stabilization of the radical centre by the carbomethoxy groups. In addition the cyclopropyl bond between the two carbomethoxy substituted carbon atoms is predicted to be weak on the basis of ground state electronics.*’ Closure of the cyclopropyl ring should ultimately lead to the photoproduct 10s.

To study the regiospecitity of the bond breaking step the deuteriumlabeled compounds exe- and endo& were irradiated. From ‘H-NMR analysis of the deuterium dis- tribution of the photoproducts it was found that the reaction proceeded with a high degree of regiospecifity (78% and 85%) and it has to be concluded that reaction along path (a) is 4.4 times more probable than path (b). This result compares well with the quantum yields obtained for the photorearrangement of 4a, @ = 0.099 and 4h, @ = 0.069. The endo-homobenzobarrelenes show the lower quantum yields.” The reason for favouring path (a) over (b) has to be searched in the different

radical stability of 17s and 17b. As was pointed out earlier”” the cleavage of bond a will produce a radical centre in which the p-orbital will be aligned more favourably for overlap with the Walsh r-orbital of the cyclopropyl ring than is possible with 17b.” Certainly, the stabilization of cyclopropylcarbinyl radical by orbital interaction is known to be Iow,~* but will be sufficient to account for the differences observed.

The photochemistry of endo- and exodc shows some peculiarities. Irradiation of endodc gives rise to the expected products 13 and 14 (Scheme 5). 14 will have been formed by a pathway as outlined in Scheme 7 by cleavage of bond a. Surprisingly, no 15 (which could have been formed by breaking bond b) could be obser- ved. The difference may be attributed to a difference in the biradical stability and the tendency to rearomatize the benzene- and the dimethylbenzene ring. The behaviour of exode is even more surprising since only endo& and 13 were formed. No 4 or 15 could be detected.

This result demonstrates that rearomatization of the dimethylbenzene ring does not occur, even if path a is available. Path b used to ca 18% in fie is not used either. endo- and exo-6c are constituted of the same chromo-

X = COgMe

Photorearrangement of homodibcnzobanelenes 1157

/ 15

X= C02Me

phores, an o-xylene and a durene moiety. The triplet excitation energies for o-xylene and durene are 82.323 and 80.0 kcal mall’,*’ respectively, and are close enough that either chromophore will be excited. Excitation does not delocalize over the entire molecule as was shown with triptycenes, but excitation transfer between the aromatic rings is fast.2s Consequently, the situation is identical at the beginning of the photoreaction for exo- and endodc and formation of the triplet diradicals 19 and u) by aryl-aryl bridging should be equally probable. The differences in the photo-chemical behaviour of endo- and exo&I can only be the result of differences in the tendencies to cleave bond a and b in the diradicals. Since energy differences in the rearomatization processes for the o-xylene and the durene moiety do not differ greatly, the observed regiospecific control can hardly be explained by differences of the exo-thermicity of the rearomatization step. In many instances in which only one of the a-moieties was an arene, the regiospecifity in the break step has been totally dominated by the exo- thermicity of the rearomatization.” As the consequence of the observed regiospecifity the structures of the excited diradicals 19 and 20 might be a gross over simplification. It seems reasonable to assume that a charge transfer from the durene to the o-xylene moiety, at least partially, occured in the diradical and a charge resonance structure has to be considered. The ionization potentials of o-xylene and durene are 8.56 and 8.03 eV,26 respectively, and a triplet configuration with a doubly occupied o-xylene-HOMO and a singly occupied o- xylene-LUMO and durene-HOMO has to be taken into account. This polarization introduces a negative charge on the o-xylene and a positive charge on the durene moiety. The importance of a polarization was already detected in di-r-rearrangement of norbornadienes sub- stituted with polar groups, in which the vinyl groups behaved as weak electrophiles.*‘“-’

The photorearrangement described here belongs to the large class of di-r-methane rearrangements. Despite cyclopropane interference in a later step of the reaction co-ordinate, it shows the same typical stages; the “bridge” and the “break” step. In singly connected di-a- methane systems (i.e. two r-moieties connected by one sp’ carbon) only one bridge step is possible and the regiospecifity will be determined by the break step. In a doubly connected din-methane system (i.e. two ‘II- moieties are connected by two sp’ carbons) such as

benzo-norbomadiene the bridge and the break step can determine the regiospecility’6*27”-r and it is sometimes difficult to distinguish between the two modes. The sys- tem studied in this work is unique in that way that it allows an investigation of substituent effects on the bridge and on the break step, which in turn will help to increase the understanding of the reaction in the early stages along the reaction co-ordinate.

BXPBlllMENTAL

‘H-NMR spectra were recorded with a VarianT60 and a Bruker WP80. The later was used for “C-NMR spectra. Mass spectra were obtained using a Varian MATCH-S. UV spectra were determined with a Varian Cary 17. IR spectra on a Perkin- Elmer 257 and 325.

Preparative irradiations were performed in a Rayonet Photo- chemical Reactor, RPC 100 of the Southern New England Ultraviolet Company equipped with RPR 2537 or RPR 3000 lamps, respectively.

Analytical and semipreparative separations were conducted by HPLC.. The apparatus consisted of a Waters M 6ooo pump, Waters UK6 iniector and LDC UV 111 detector at 254 nm. Peak height and area determinations were performed with LDC 308 computing integrator.

Acetone used for the irradiations was purified by refluxing over CaC12 and distillation through a Vigreux column. The pho- tolysis solutions were purged prior and during photolysis using vanadous-purified N2.m

Silica gel used for column chromatography was Merck, Kieselgel 60, 0.063-0.200 mm. Column chromatography was performed using quartz columns. 2% Fluorescence indicator F2,,, Merck were mixed with the silica before packing, thus allowing the separations to be monitored by a fluorescence lamp.

Quantum yield irradiations were performed using a microbench apparatus similar to that described by Zimmerman.29 A HBO 200 W Osram mercury high pressure lamp was used as light source. A Bausch & Lomb high intensity monochromator model 33-86-75 gave a band pass of 20nm at half peak height with 5.4 mm entrance and 3.0 mm exit slit. Samples were irradi- ated in 1 cm quartz cells in an electronic actinometer,” calibrated with ferrioxalate.” Cyclohexane, acetone and benzene were used as solvents, which were degassed I5 min prior to and during the photoreaction using deoxygenated nitrogen. After irradiations of the solutions an aliquot of a solution with a standard was added. Analysis was performed with HPLC. The photochemical con- versions were less than 5% and may be taken to be kinetic. The quantum yields were determined in at least two independent runs.

LXmcthyl 9.10 - dihydm - 9,10(3’,4’]pyratolidinoanthracene - 3’.4’ - dicarboxylatr )r

8.0 8 (0.024 mol) Dimethyl 2,3.5,6 - dibenzobicyclo-

II58 H. HEMEIMERGER CI al.

[2.2.2]octa - 2,5,7 - triene - 7,8 - dicarboxylate @a, R’ = R* = R’ = R’ = H)W were dissolved in 90 cm’ of ethereal CH,N, and left for seven days in a refrigerator at 4”. Excess CH$, was decom- posed wiih acetic acid. 8.4g (96.7%) pure 9 were collected as colorless crvstals: m.o. 14&W. Found: C. 69.46: H. 5.40. Calc for C H fi 0 . .C, ‘69.60; H, 5.W%. MS: no M’.pe.ak; UV 21 IS 2 1. (EtOti, A,., (nm)/r (Imol-‘cm-‘)) 321/175, 275/79$, 268/759, 251/1060. 1R (CCL): 2950. 1728-1758. 1430. 1255. 1217. 1068. 630cm-‘; ‘If-iWi (CCl,j: 6 (ppm) j.3 (s, CH,O; 3H),‘3.5 (s; CH,O, 3H). 4.2 (s, H-bridgehead, IH), 5.4 (s, H-bridgehead, IH), 4.2-5.0 (AB-system, CH2, 2H). 6.9-7.7 (m, H-arom, 8H).

Dimethyl 9.10 - dihydro - 9,lO _ cyclopropanoanfhractne - I I.12 - dicarboxylale 6a

8.0 g (0.022 mol) 9a were dissolved in I I acetone and irradi- ated at 300 nm for 3 h in a quartz vessel. The solution was stirred and purged with N*. The acetone was removed in vacua, the residue was dissolved in benzene and filtered through a silica layer to remove polymers. After distillation of the solvent, recrystallization from methanol afforded 6.8g (93%) Q, m.p. 149-150. Found: C, 75.5; H, 5.%. Calc for C2,H,aOl: C, 75.43: H, 5.43%. MS: hi- 334; UV (EtOH. A,,, (nm)/r,,, (Imol-’ cm-‘)) 275/1200, 268/930, 259/620, 251/610; IR (Ccl,): 2945, 1725-1740, 1458, 1435, 1322, 1310. 1290, 1247, (Ccl,): 6 (ppm) 0.6

1207, 1105, 617; ‘H-NMR (d, H-cyclopropane (endo), IH), 1.9 (d,

H-cyclopropanecxo, IH), 3.5 (s, CH,O, 3H), 4.6 (s, H-bridge- head, 2H), 6.9-7.4 (m, , H-arom, 8H).

Dimethyl 9,lO - dideutero - 9.10 - cyclopropanoanthracene - II,12 - dicarboxylate 6b

9,WDideuteroanthracene was prepared by dehydrogenation of 9,lOdideutero-9,IOdihydroanthracene which was obtained from anthracene bv reaction with Na followed bv D,O?’ Reoetition of the process ihree times gave 9,lOdideut&o~nthrace~e with a degree of deuteration of 86.5% by ‘H-NMR. 6h was synthesized as outlined for 6a and was deuterated to 70.5% on the bridgehead positions (by ‘H-NMR).

LXmerhyl 2.3 - dimefhyl - 9,lO - etheno - 9.10 - dihydroanthracene - II,12 - dicarboxylale &

8.2g (4.lO~*mol) 2,3dimethylanthracene and I5 cm’ dimethyl acetylenedicarboxylate were mixed, heated to 200” and kept 5 min at this temperature. The reaction mixture was dissolved in benzene and the solution was filtered through a silica column, IOcm x I.5 cm I$. After removal of the solvent in uacuo the product was recrystallized from methanol. Yield 9.8g (72%); m.p. 177-179”. Found: C, 75.82; H, 5.88. Calc for C12Hm0,: C. 75.84; H, 5.7%. MS: 348 M’, UV (cyclohexane, A,, (nm)/c,, (I mol-’ cm-‘)): 276/24&I, 284/2760, IR (&A,): 2940. 1725, 1640. 1490, 1475, 1440, 1340, 1275, 1225, 1155, 1120, 1070cm-‘; ‘H- NMR (CD&): 6 (ppm) 2.25 (s, CH,-, 6H), 3.80 (s, CHIO, 6H), 5.45 (s, CH-bridge-head, 2H), 6.8-7.5 (m, H-arom, 6H).

exe- and mdo-Dimethyl 2.3 - dimelhyl - 9.10 - dihydro - 9,10(3’,4’]pyratolidinoanthracene - 3’,4’ - dicarboxylate 9c

9.5 g (2.73 x IO-‘ mol) Dimethyl 2,3dimethyl-9, IO - dihydro - 9.10 - ethenoanthracene - I I ,I2 - dicarboxylate k were dissolved in ethereal CHIN* and kept 24 hr at 4” in a refrigerator. Excess CH2N2 was reacted with acetic acid and the ether removed under reduced pressure. The resulting solid was recrystallized from methanol. The mixture of exe- and endo- could not be separated by chromatography. Yield 9.3 g (87%), m.p. 155” decomp. Found: C, 70.66; H, 5.63. Calc for C H N 0 . C, 70.75; H, 5.68%. MS: 23 22 2 + 362 M+-N,; IR (Ccl,): 2950, 1770, 1740, 1475,. 1450, 1270, 1235, 1215. ll9Ocm~‘: ‘H-NMR (CDCI,): 6 (ppm) 2.15 (s, CH,), 2.25 (s, CH,), 3.45 (s, CH,O), 3.55 (s, dH,O); j.60 (s. C&O), j.65 (s, CHD, 4.30 (s, CH-bridgehead). 4.64.8 (m, CHJ, 5.3 (s, CH- bridgehead). 6.9-7.4 (m, H-arom).

exe- and mdo- Dimethyl 2,3 - dimethyl - 9,lO - dihydro - 9.10 - cyclopropano - anlhracene - I I .I 2 - dicarboxylate 6c

8.5 g (2.2 x IO-* mol) exo-lendo-k was dissolved in I I. acetone and irradiated as described for 6a. The solvent was removed by distillation in oacuo and the residue (mainly endok) crystallized

from MeOH. The crystals and the mother liquor were puriiied separately by chromatography (silica column 50 x 3.5 cm, ben- zene 5OCcm’ was used as mobile phase). Fractions were collec- ted and checked for ouritv bv HPLC. Fractions 8-16 solely contained endo-(d, 17-25 a-mixture and 26-31 only exde. 2.2i pure endodc (27%) and I .9 g.exo& (24%) and 3.05 g endo-lexo- 6e mixture (37%) were obtained. endode. m.o. (190-191.5”. Found: C. 76127: h. 6.08. Calc for C2,H2201: ?, 7a.22; H, 6.12%. MS: 362 M’, UV (cyclohexane, A,., (nm)/r,, (I mol-’ cm-‘)): 276/2870,282/3310; IR (Ccl,): 2940, 1740, 1500. 1480, 1470. 1445, 1355, 1325, 1255, 1220, 1170, 1155, ll20cm-‘; ‘H-NMR (CD&): 6 (ppm) 0.70 (d, CH-cyclopropane, IH), 2.05 (d, H-cyclopropane, IH). 2.20 (s. CH,. 6H). 3.65 (s. CH,O. 6H). 4.60 (s. H-bridgehead. 2Hj; 6.95‘6, H&on&do,‘ 2H), 6.9-7.6’ (m, &rom&o, 4Hj exo& m.p. 156-158”. Found: C, 76.09; H, 6.00. Calc for &HZOI: C, 76.22: H, 6.12%. MS 362 M+; UV (cyclohexane, AmX (nm)/c,., (I mol-’ cm-‘)): 281/2750, 276/2240; IR (CC4): the spectrum resembled closely to that of endok; an additional band appeared at 13lOcm- , ‘. ‘H-NMR (Ccl,): 6 (ppm) 0.5 (d. H-cyclopropane, IH), I.8 (d, Hcyclopropane, IH), 2.1 (s, CH>, 6H), 3.4 (s, CH,O, 6H), 4.4 (s, H-bridgehead, 2H), 7.0 (s, H-arom, 6H). The structural assignment was possible by compat ison with the spectra of 4a and 4b.”

Dimethyl 2 - bromo - 9,lO - dihydro - 9,lO - etheno - anthracene - I I ,I2 - dicarboxylate IM

10.0 g (3.98 x lo-’ mol) 2 - bromoanthracene and l6.6g (0. I I7 mol) dimethyl acetylenedicarboxylate were mixed as described for Q. 8.2g (53%) Ild were obtained, m.p. 151”. MS: 398 and 400 M+; IR (KBr): 2960, 1737, 1715, 1640, 1440, 1410, 1335, 1305, 1275, 1225, ll60,, 1120, 1060, 1033,950,905,875,830, 800,762,675 cm-’ ’ H-NMR (CDCI,): S (ppm) 3.8 (s, CH>O, 6H), 5.4 (s, H-bridgehead, 2H), 6.8-7.6 (m. H-arom, 7H).

exo-lendo-Dimethyl 2(3) - bromo - 9,lO - dihydro - 9.10 - [3,4]pyrazolidino - anlhracene - 3’,4’ - dicarboxylate pd

l g (2.51 x IO-’ mol) &l were reacted with CHzN2 in ether and the product mixture isolated as described for h. Yield l.03g (93%).

endo-lexo-Dimethyl 2 - bromo - 9,lO - dihydro - 9,lO - cyclo- propanoanthracme - I I,12 - dicarboxylate 6d

20.0 g (4.53 x IO-’ mol) exolendo-9d were dissolved in 200 cm’ acetone and irradiated as described for 61. The solvent was removed under reduced pressure and the solid crystallized from ether giving a fraction consisting mainly of endodd. The crystal- line fraction and the fraction contained in the mother liquor were chromatographed separately (silica, 100 x 2.5 cm, eluted with benzene). Exo-6d moved faster than endo&l. 0.94 g (50.5%) exo-, 0.55 g (29%) endo-(d and 0.22 g (12%) mixture of isomers were obtained. The endo-/exobd ratio was determined by reversed phase HPLC as 36:64. endoa. m.p. I59-161”. MS: 412 and 414 M’: UV Icvclohexane. A (nm)/r (I mol-’ cm-‘): 27811545. 285/l&44; IR (KBr): 2950, 2hO,‘.l725, 1460, 1425, 1350. 1330, 1265. 1222. 1205. 1172. 1110. 896. 810. 755cm.‘: ‘H-NMR (CDCI,): 6 (ppm) 0.7 (d; H-cycloprdpane; IH), 2.1 (d, Hcyclo- propane, IH), 3.6 (s, CH,O-, 6H), 4.7 (s. CH-bridgehead) 6.9-7.6 (m H-arom, 7H). uoa, m.p. 135-137”. MS: 412 and 414 M’; UV (cyclohexane, A,,,,” (nm)/t,.. (I mol-‘cm-‘)): 2781141 I, 284/1312; IR (KBr): The spectrum resembks closely that of endodd. An additional band of medium intensity appears at 1217cm-‘; ‘H-NMR (CDCIJ: 6 (ppm) 0.65 (d, Hcyclopropane. IH), 2.1 (s, CH,O-, 6H), 4.7 (s, H-bridgehead ZH), 7.1-7.6 (m. H-arom, 7H). 3.6 (s, CH,O, 6H), 4.7 (s, H-bridgehead, 2H). 7.1-7.6 (m, H-arom. 7H).

exo-(endo-) Dimethyl 2 - deulero - 9.10 - dihydro - 9.10 - cyclopropano - anthracene bt

One mol exolendo- dimethyl 2 - bromo - 9.10 - dihydro - 9,lO - cyclopropano - anthracene u were reacted with 2 mol triphenyl- tindeuteride durinn 2 hr at IW. Chloroform was added and the reaction mixture was filtered. The products were isolated by liquid chromatography (25 x 2 cm), silica, n-hexanelether = 80:20. Degree of deuteration: exodc: &= 8.546, d, =91.5%,

Photorearrangement of homodibenzobarrelenes 1159

endok: &, = 8.4, dl = 91.6%. exo- and endodc or (e show dis- tinct dPercnces in the ‘II-NMR suectra in the aromatic region. The aromatic protons of the unsudstituted benzene ring in endo- con&ttration to the cyclopropane ring give rise to a singlet, whereas the protons in exo-position show an AA’BB’ spectrum as was found for 4a and 4b.”

Accronr-sensitized irradiation of 6a 2.08 (6x IO-‘mol 6s were dissolved in 1 I. acetone and

irradiated at 300 nm in a quartz vessel in N2. Nine identical runs were combined, filtered through a layer (I cm high) of silica with benzene and the solvent removed -in oucuo. The residue was crvstallized from methanol to nive I2 R (66.6%) starting material. tie products in the mother liquor were chiomato&phed on silica, 60 X 4 cm, with be.nzene. The first fractions, 3 I. in total, contained 3.4g dimethyl 4a.10 - [1,3]propenylene - cyclo- propane[l]lIuorene - IO.12 - dicarboxylate 1L. Crystallization from methanol afforded pure 10, m.p. 117-I WC, Found: C, 75.37; H, 5.43. Calc for C2,HIsOI: C, 75.43; H, 5.43%. MS: no W-peak, 275 (M+CO,Me); UV (cyclohexane, Aaur (nm)/c,., (I mol-’ cm-‘)) 282/2760, 274/2770; ‘H-NhfR (CCL& S (ppm) I.08 (d, d, H(ll)-endo, lH, Jcro.mdo = I9 Hz, Jcndo-~(~)) = 3.6 Hz) 3.53 (s, H(9), IH), 3.63 (s, CH,O-, 3H), 3.72 (s, C&O-, 3H), 3.76 (4 H(l lkxo, IH, Jmdorxo = 19 Hz), 5.46-6.0 (m, H(1,2,3,4), 4H), 6.84-7.29 (m. H-arom and H(13). SH): Double resonance exoeri- ments: &diation at 7.14 ppm ied to’ becoupling of the signal at 1.08ppm; “C-NMR (CD&: off Aeld dec&pl;d 6 (pprnj 22.88 (1, CHJ, 43.38 (s), 46.98 (s), 46.98 (d, CH-cyclopropane), 51.58 (s, C-CO+ZH,), 51.58 (q, CHIO), 52.35 (q, C&O-), 121.46 (d), 122.82 (two d), 123.99 (d). 124.12 (d), 126.45 (d), 127.29 (d) 127.1 (d), 130.00 Is). 138.35 (s). 145.15 Id). 145.93 fs) 165.66 Is). 170.26 Is). . ,. , .

The mother liquor from 1Oa was con&rated ati ihe mixture of compounds was separated by HPLC, LiChrosorb Si 100 chemically bonded with C21, 25 cm x 4.1 mm, CHJOH/H20 = 70: 30, 5 cm3/min. The Rrst peak eluted in 2.7 min. Tlte fractions were collected and then extracted with benzene. After removal of the solvent dimethyl 9.10 - diiydro - 9.10 - [1,3]propenylene - anthracene - I I,13 - dicarboxylate (111) was obtained, which was crystallized from methanol, m.p. 128.5-W’. talc C 75.43, H 5.43; found, C 75.33, H 5.61%. MS: 334 M’; UV (cyclohexane. A,,, (nm)lf,, (1 mol-’ cm-‘)): 273.511470. 266.511540: ‘H-NMR (Ccl,): 6 (ppm) 3.48 (t, H(l3)), 3.60 (s, ‘CH,O); 3.71’(s, CHIO), 4.56 (m, H(l0)). 5.08 (s, H(9)), 6.46 (d. H(l2)); double resonance experiments: irradiation at 4.56ppm led to decoupling at S 3.48ppm, J12,1) = 3.2 Hz); irradiation at 3.48ppm led to broad singlets at 4.56 and 6.46 ppm.

Direct irradiation of b 0.5 g (I.5 x lo-’ mol) 6a were irradiated in 800 cm3 n-hexane at

254 nm in a quartz vessel. After removal of the solvent in uacuo 1L and llr were isolated as the only photoproducts.

Sensihzed irradiaGon of 6b The irradiation was performed as described for 6a. The pho-

toproducts were isolated and the ‘H-NMR spectra recorded. In the ‘H-NMR of lob the signal at 3.53ppm vanished and the integral value in the aromatic region was reduced showing the positions of the deuterium at C (9) and C (13).

Acetone sensitized irradiahon of exok I.8 g (4.96 x IO-’ mol) exode were dissolved in I I. acetone and

irradiated as described for 6e. The acetone was removed in uocuo and the residue crystallized from methanol. 5OOmg starting material were recovered. The products in the mother liquor were separated by HPLC. Two columns Lichrosorb Si 100, Cl8, 280 x 7.9 mm, methanol/H?0 = 70: 30, 8 cm’min-‘. Four peaks were collected: Peak I. retention time 5.2 min starting material; peak 2. retention time 7.8 min endo&: peak 3, retention time 9.6 min and peak 4, retention time 13.5 are to be assigned to syn- and antidimethyl 2.3 - dimethyl - 9.10 - dihydro - 9.10 - [1,3]pro- penylenanthracene I I,13 - dicarboxylate (13). Syn- and anti-13 isomerized easily in protic solvents to give a I : I mixture. Con- sequently, the elemental and spectroscopic analysis were per- formed on the diastereomeric mixture of 13. Found: C, 76.X; H,

5.43. Calc for CZjHZ201: C, 76.22; H, 6.12%. MS: 362 M’; UV (cyclohexane, A,. (nm)/r,, (I mol-‘cm-‘)): 241/2710,250/3650, 263/2480,270/2430; IR (Ccl,): 2920, 1740,1715,1645,1490.1440, 1310,1280,1240,1215,1200,1175,1120,1070,1@20cm~‘; ‘H-NMR (CD&): S (ppm) I.8 (s-broad, CHI, 6H); 3.1 (1, H(l3), 1H). 3.2 (s-broad, CH,O, 3H)-the signal splits into two signals in Ccl,- 3.3 (s, C&O-, 3H), 4.1 (s-broad, H(IO), IH), 4.65 (s, H(9), IH). 6.1 (s-broad, H(l2), IH). 6.4-7.0 (m, H-arom, 6H).

Acetone sensihed irradiation of endo& l.9g (5.25 x IO-’ mol) end&c were dissolved in I I. acetone

and irradiated as described for 6a. After removal of the solvent in uucuo the reaction mixture was crystallized from methanol and I.140 (60%) starting material recovered and irradiated ano- ther time. The product mixture in the united mother liquors were chromatographed on silica, 41 x 3.5 cm, benzene as eluent. The first fraction~obtained was found to be dimethyl 2.3 - dimethyl - 9, 10 - dihydro - 9.10 [1,3]propenylen - 11 - I3 - dicarboxylate. (syn- and anti-13) The second fraction was identiied as dimethyl 2,3 - dimethyl - 4a.10 - [1,3]prop - 2 - enylen - cyclo- propa[1]8uorene - IO.12 - dicarboxylate (14). The last fraction was endo& syn-/anti-13 (analytical data above). 1J: m.p. loo” (decomp), Calc C76.22, H 6.12; found: C 76.39, H 6.04%. MS: 362 M ; UV (cyclohexane, A,,,= (nm)/c, (I mol-’ cm-‘)): 274/3548,281/3589; IR (Ccl,): 2940, 1720, 1645, 1440. 1290, 1285, 1215, 1095 cm-‘; ‘H-NMR (CDCl& 6 (ppm) 1.1 (dd, H(I l)-endo, IH), 1.6-1.9 (two d, CHI, 6H), 3.5 (s, H(9), lH), 3.72 (d, H(ll)- exo, IH), 3.7 (s, CH,O, 3H), 3.8 (s, CHsO, 3H), 5.2 (s-broad, H(1) lH), 5.8 (s-broad, H(4), IH), 6.8-7.4 (m, H-arom t H(13), SH).

Acetone sensitized inadiotion of endo-(exo-)-& The irradiations of endolexodc were conducted as described

for (1. The rearrangement products endolexo-10c were isolated by semi-ureoarative HPLC and the deuterium distribution determind dy ‘H-NMR spectroscopy. endodc: 4 = 8.496, d, = 91.6%; lfk (isolated): 4 = 6.6%. d, =93.4%; Intensity ratio JUO./Jo*r, = 1.547. Correcting for a degree of deuteration of 93.4%, breaking of bond (a) happened with a regiospecility of 78%. exo& &= 8.51, d, = 91.5%; lfk (isolated): d,,=7.2%. d, = 92.8%. Intensity ratio J&Jolel = 1.052. Taking into account the degree of deuteration of 92.81, bond (a) was broken with a regiospecifity of 85%.

Quantum yield determinations The data are given in the following order: irradiated educt.

solvent concentration in mol I-‘, wavelength of irradiation A (nm). HPLC-column used, photoproduct+(formation). The fol- lowing HPLC-columns were used: A: LiChrosorb Si 100. IO pm, chemically bonded (PhCH2Ph)CH2CH2CH2-; Ij: LiChrosorb Si 100, IO pm, chemically bonded (Cl8h Si; Naphthalene was used as standard. Run I: 6, acetone, 7.44 x IO-‘, 3j6, A, lol-4 = 3.4 x

10~‘/3.1 x IO-‘: lla-) = 7.5 x 10-‘/7.4x IO-’ Run 2: Q: cvclo- hexane, 6.47 x’lO_‘, i5l, A, lga-4 = 6.3 x 10-‘/6.7x IO-‘, il.14 = 4.0~ IO-’ Run 3: 6c. acetone, 4.14~ IO-‘, 316 B, 14-O = I.2 x 10-‘/1.3x lo-‘.

Sensirized experiments In the sensitization experiments benzene solutions of the sen-

sitizer were prepared in which > 99% of the light was absorbed by the sensitizer. The solutions were degassed by three freeze- thaw cycles. The irradiations were performed in a Rayonet Reactor using a “Merry-go-round” turntable (RPR 3000 lamps). The quantum yields were measured relative to those found for acetone. The data are listed in the following order; adduct, sensitiier (ET &J/mole)), quantum yield (product). (a) 6a, uro- piophe&te (312). 0 (lb) = 7.26 x I&‘, #-(111) = 1.6 x IO-‘: (b) (r. pmethoxybenzophenone (287). 4 (1L) = 3.1 I X IO-‘, I$ (llr) =4.0x IO-‘. (c) 6a. triphenylene, (278), 4 (l)r)= 1.42 x IO-‘. & (lla) = 4.1 x IO-‘. (dl 6a. u.o’dimethvlaminoben- zophenone (as), 4 (lb) = 2.01 i iO_‘.. i.-(lla) = 5.i x IO-‘. (e) endo&, acetone (330). I$ (14) = 1.25 x IO-‘. (f) endode aceto- phenone (308). 4 (14) < 2.4 x lo-‘.

1160 H. HEMETSBERGER et al.

Quenching experiments Five quartz tubes were charged with 4 cm3 of a 5.34 X IO-’ h4

THF solution of 6a and 2cm3 of THF solutions of pipcrvlcne added. The resulting pipcrylene solutions were 0.009, 0.267, 0.534,0.854, 1.068 and 1.335 x IO-’ hf. The solutions were purged during 10 min with N2 and irradiated in a Rayonct-Reactor using a turntable at 254nm. The solvent was removed in DPCYO and 2cm’ THF solution containing dimethyl 9,lO - diydro - 9,10 - etheno - anthracene as internal standard were added. The amount 101 and 111 formed was determined by reversed phase HPLC on C18, CH,OH/H,O=68:32 and the ratio @Id calculated. @‘Id for 101: 1.175, 1.361, 1.551, 1.691, and 1.844 do/d llr: 1.101, 1.004, 1.029, 10.45, 1.019. Stem-Volmer plot for 1L gave a straight line with a slope of 0.634, for llr a slope of - 0.005.

REl’JlRENCE9

‘For the most recent review see H. E. Zimmerman, Rcur- rangcmcnts in Ground and &cited Slates (Edited by P. de Mayo), pp. 131-166. Academic Press, New York (1980). tiH: E. Zimmerman and G. L. Grunewald, 1. Am. Chem. Sot. 1. 183 (1%6): bH. E. Zimmerman. R. W. Binklev. R. S. Givens and M. fi. &twin, Ibid. 89,3932 (1967); ‘HI-i. khmerman, R. W. BinWey, R. S. Givens, G. L. Grunewald and M. A. Sherwin, Ibid. 91, 3316 (1969).

“J P N. Brewer and H. Heaney, Chcm. Commun. 811 (1967); . . bH. E. Zimmerman, R. S. Givens and R. M. Pagni, 1. Am. Chem. Sot. 9& 4191, 6096 (1968); ‘P. W. Rabideau, J. B. Hamilton and L. Friedman, Ibid W, 4465 (1968); ‘H. E. Zim- merman and C. 0. Bender, ibid. 91, 7516 (1%9); 92, 4366 (1970); ‘C. 0. Bender and H. D. Burgess, Ibid 51, 3486 (1973); ‘H. E. Zimmerman. D. R. Amick. Ibid. 95. 3977 119731: ‘H. E. Zimmerman, D. R. ‘Amick and H.‘Hemetsdrger, jbid.‘&,&CUi (1973); hC. 0. Bender and E. H. King-Brown, I. C. S. Chcm. Commun. 878 (1976); ‘C. 0. Bender and 1. Wilson, He/v. Chim. Acto 59, 1469 (1976); ‘hf. Demuth, W. Amrein, C. 0. Bender, S. E. Braslavsky, U. Burger, M. V. George, D. Hemmer and K. Schaffner, Tetrahedron 37, 3245 (1981).

tiE. Ciganek, 1. Am. Chum. Sot. 88, 2882 (1966); bK. E. Richards, R. W. Tillman and G. I. Wright, Austral. 1. Chcm. 28, 1289 (1975); ‘R. G. Paddick, K. E. Richards and G. J. Wright, Ibid. 29. 1005 (1976).

“H. Hart and R. K. Murray, 1. Am. Chcm. Sot. 91.2183 (1%9); bH. Hart and G. M. Love,. Ibid. %,4592 (1973).

“J. R. Edman. J Am. Chcm. Sot. 91.7103 (19691: bB. hf. Trost. I. Org. Cheml 34, 3644*(1%9); ‘M. &ight&, d: S. Hammond and H. B. Gray, 1. Am. Sot. 92, 6068 (1970); dJ. J. Tufariello and D. W. Rowe, 1. Org. Chcm. 36, 2057 (1971); ‘S. I. Christol, T. D. Ziebarth. N. J. Turro. P. Stone and P. Scribe. J. Am. Chcm. Sot. 96, 3016 (1974); ‘L. A. Paquette, D. M. Coitrell, R. A. Snow, K. B. Grifkins and J. Clardy, Ibid Y?,3275 (1975).

“T. D. Walsh, 1. Am. Chcm. Sot. 91, 515 (1969); bN. 1. Turro, M. Tobin. L. Friedman and 1. B. Hamilton, 16id. 91,516 (1969);

‘H. Iwamura. 1. C. S. Chcm. Commun. 232 (1973): ‘Chcmistrv Letters 5 (1974); ‘H. Iwamura and K. Yoshii~ra, i Am. Chk. Sot. 96. 2652 (1974): ‘H. lwamura and H. T&da. I. C. S. Chcm. ?omm&. WF(l975); ‘H. Iwamura, H. T&la, T&a: hcdron Letters 3451 (1978).

‘J. Ipaktschi, Chcm. Bcr. 105, 1989 (1972). ‘H. Hemetsberger, W. Briiuer and D. TartIer, Chcm. Err. 110. 1586 (1977).

‘(‘H. E. Zimmerman and C. J. Samuel, 1. Am. Chcm. Sot. 97,448 (1975): bn, 4025 (1975).

“H. Hemetsberger and W. Holstein, Tetrahedron 38.3309 (1982). “H Hemetsberger and W. Holstein, 1. C S. Chemical Commun

9; (1977). “H. Hemetsberger and F. Werres, unpublished work. “W. Amrein, J. Gloor and K. Schagner, Chimia 28, 185 (1974). “C. G. Hatchard and C. A. Parker, Proc. R. Sot. 235,518 (1956). 16R. C. Hahn and R. P. Johnson, /. Am. Chcm. Sot. 9, 1508

(1977). “H E. Zimmerman, R. J. Boettcher, N. E. Buehler, G. E. Keck

aid M. G. Steinmetz, 1 Am. Chcm. Sot. 98.7680 (1976). ‘*M. Demuth, W. Amrein, C. 0. Bender, S. E. Braslaviky, U.

Burner. M. V. Georne. D. Lemmer and K. SchafTner. Tctra- hcdkn 37, 3245 (198i).’

19S S Hixson, P. S. Mariano and H. E. Zimmerman, Chcm. Rct~. 74, j31 (1973).

%J. K. Kochi and P. J. Krusic. Special Publication No 24, p. 147. The Chemical Society, London (1970); bB. Maillard, D. Forrest and K. U. Ingold, 1. Am. Chcm. Sot. 98, 7024 (1976).

“H. Gilnther, Tcrrahcdron Utcrs 5173 (1970). 2tiD F McMill, D. M. Golden and S. W. Benson, Inr. 1. Chcm.

Kin. i, 359 (1971); bJ. C. Martin, 1. W. Timberlake, 1. Am. Chcm. Sec. 92,978 (1970); ‘H.-D. Beckhaus and C. Rtichardt, Tetrahedron Lctrcrs 1971 (1973).

“L. A. Blackwell, Y. Kanda and H. Sponer, 1. C/urn Phys. 32, 1465 (1960).

UH. Sponer and Y. Kanda, 1 Chcm. Phys. 0,778 (1964). ?f. S. de Goot and J. H. van der Waals, Molcc. Phys. 13, 545

(1%3). xK. Watanabe, T. Nekayama and 1. Mottl. Final Report on

Ionization Potential of Molecules by a Photo-ionization Method, Lr.S. Army Rcpr. ORD-TB2M)R-1624 (1959).

*“L. A. Paquette, D. M. Cottrell, R. A. Snow, K. B. Gifkins and J. Clardy, /. Am. Chcm. Sot. 97, 3275 (1975); “L. A. Paquette, D. M. Cottrell and R. A. Snow. 1. Am. Chcm. Sot. 99. 3723 (1977); ‘R. A. Snow, D. M. Cottiell and L. A. Paquette, i Am. Chcm. Sot. 9, 3734 (1977); dC. Santiago, K. N. Houk, R. A. Snow and L. A. Paquette, J. Am. Chcm. Sot. 98.7443 (1976); ‘C. Santiago and K. N. Houk. 1. Am. Chem. SC. W. 3380 (1976). -

=L. Meites and T. Meites, Anal. Chcm. 16,984 (1948). *‘H. E. Zimmerman. Moles. Phorochcm. 3. 281 (1971). wO. Diels and K. Aider, Ann. 496, 191 (1931). ’ “hi. lwaizumi and J. R. Bolton, 1. Magn. Rcs. 2,278 (1970).