be isolated in 61% yield by short-path distillation at 32- 36"C/0.08 torr as a colorless oil having an unpleasant odor. 4 gives with potssium iodide a distinct peroxide spot on the thin-layer chromatogram (R,= 0.70; S O z , CH2CIZ). The titrimetrically determined peroxide content amounted to 27.65"O (calc. 28.04%), corresponding to a purity of 9 8.6%.

[ I ] F. D. Greene, W. Adam. G. A. Knudsen, J . Org. Chem. 31 (1966) 2087. [2] W. Adam, C . I. Rojas, Syn/hesis 1972. 616. 131 W. Adam, Acr. Chern. Res. 12 (1979) 390. [4] W. Adam, 0. Cueto, L. N. Guedes, J. Am. Chem. Soc. 102 (1980) 2106. [ S ] k(cis-2-butene)/k(l) = 8.0; k(trans-2-butene)/k(3) =7.7; k( l ) /k(3) =3.8

[6] W. G. Dauben, L. Salem, N. J. Turro, Arc. Chrm. Res. 8 (1975) 41. (in CCI, at 10°C).

Table I . ' H dnd "C-NMR data and selected IR data of 2 and 4.

'H-NMR 1.38 (d, 3H, J=6 .2 Hz) (li values) [a] 4.95 (4, I H, J=6.2 Hz)

6.02 (s, I H) 6.48 (s, I H ) 9.67 (br. s, 2 H)

"C-NMR 18.34 (4) (is values) [a] 79.13 (d)

128.32 (t) 139.91 (s) 171.90 (s)

IR [cm ~ '1 3300 (s), 2990 (m), [bl 1700 (s), 1628 (m),

I272 (m), I178 (m)

1.83 (d, 3H, J = 6 Hz) 5.42 (ddd, H,, ' J = 6 Hz, Jh,=2.8 Hz, J,%,=3.0 Hz) 5.77 (dd, Hh, Jh,,=0.7S Hz, J, ,=2.S Hz) 6.30 (dd, H,,, J,,,=O.75 Hz, J.,,'= 3.0 Hz)

17.76 (4) 81.27 (d)

122.11 (t) 138.66 (5)

168.48 (s) 2990 (w). 1790 (s), 1672 (w), 1410 (s), 1246 (s), 1132 (s)

[a] 400 MHz, CDCI,, S values referred to TMS. [b] Film (NaCI plates). [c] Correct elemental analyses. [d] Molecular weight: experimental, 116 (osmo- metric): theoretical, 114.

A comparison of the IR and of the ' H - and I3C-NMR data of 2 and 4 (Table 1) lends support to the proposed structures. A striking feature of the 'H-NMR spectrum of 4 is the additional 4JHH-couphg between H, and H, and between H h and H,, which d o not occur in the open-chain compound 2. 4 can be stored without deterioration at -2O"C, but on heating to 70-80°C it undergoes complete polymerization.

4 proves to be exceptionally stable upon flash photoly- sis: Even on passage through a quartz tube (60-cm long, 14-mm diameter, unpacked) heated to 1000°C at 0.08 torr only 15"'o of the substance polymerizes; 85% thereof can be recovered in the cold trap. This finding can be explained in terms of the diradical state 5,16] which is proposed as inter- mediate.

n-5 0 -5

Since the relative energies of TI-5 and 0-5 are unknown, it is impossible to make any definite statement about the electronic nature of the diradical. Decarboxylation of TI-5 should lead to a n,n*-excited C02-molecule-an energeti- cally very expensive fragmentation path. The cleavage of C 0 2 from 0 - 5 would lead to a vinyl radical or, at least, require a concomitant 1,2-methyl shift to an olefinic C- atom, both of which are unfavorable for stereoelectronic reasons.

Received: August 8. 1988; supplemented: October I , 1988 [Z 1416 IE]

German version: Angew. Chem. 97 (1988) 1071

Sensitized UV/Laser Photolysis of Azoalkanes: Conformational Influences on Intersystem-Crossing and Lifetimes of Triplet Diradicals** By Waldemar Adam,* Klaus Hannemann, and R . Marshall Wilson Dedicated to Professor Rolf Huisgen on the occasion of his 65th birthday

The importance of diradicals in chemical reactions is re- flected in the large number of studies devoted to the detec- tion and determination of the lifetime of these short-lived intermediates."] Although their behavior is qualitatively well understood,['] experimental data essential for testing theoretical models are still greatly lacking. For example, the law of conservation of total angular momentum for tri- plet-singlet intersystem-crossing requires a conformational arrangement of the radical orbitals in which the radical or- bital axes are oriented orthogonally to one another and also to the axis about which the orbital angular momentum is generated (Fig. la). In contrast, a parallel arrangement (Fig. lb) of the radical orbitals proves to be unfavorable for spin-orbital coupling.['l To our knowledge no experi- mental evidence has hitherto been put forward in support of this theoretical prediction. Either the geometrical pre- requisites are not given or the diradical is so strongly per- turbed by substituents that no influences of the conforma- tion on intersystem-crossing and, thus, on the lifetime of the triplet diradical, can be recognized."'

z b) z

t a)

t

Fig. I . Optimal orthogonal (a) and unfavorable parallel (b) arrangements of the radical orbital axes for intersystem-crossing in triplet diradicals.

Using the localized diradicals 1,3-cyclopentadiyl la , 1,4- cyclohexadiyl l b , and 2,7-bicyclo[2.2. Ilheptadiyl l c , which are not perturbed by substituents, as examples, we show that the arrangement of the radical orbitals consider- ably influences the lifetimes of these transient species- they differ by more than four orders of magnitude. The di-

[*I Prof. Dr. W. Adam, Dr. K . Hannemann lnstitut fur Organische Chemie der Universitat Am Hubland, D-8700 Wiirzburg (FRG) Prof. Dr. R. M. Wilson Department of Chemistry, University of Cincinnati Cincinnati, OH 48221 (USA)

[**I This work was supported by the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie, the National Science Foundation (USA), and the North Atlantic Treaty Organization.

Anqew. Chrm. Inr. Ed. Engl. 24 (1985) No. 12 0 VCH Verlagsgesellschaft mbH. 0-6940 Weinherm. I985 0570-0833/85/1212-1071 !$ 02.50/0 107 1

radicals l a - l c were generated by photochemical benzo- phenone-sensitized Nz-cleavage from the corresponding azo compounds 2a-2c. The laser photolyses (Coherent su- pergraphite CR18 argon ion laser) were carried out in inert solvents (CCI,, CFCI,) under oxygen (10 bar) in order to trap and detect the short-lived diradicals l a - lc as perox- ides. The lifetimes could be determined from the 0,-pres- sure-dependent product distribution between trapping product (peroxides) and cyclization product (hydrocarbon) with the aid of a Stern-Volmer plot.[31

exo-[D2]-2a, yielding the isomeric peroxides 3a in a 1 : I ratio (D-NMR).["] The 1,3-cyclopentadiyl triplet l a must therefore be planar on the average during the time of the oxygen trapping reaction, thus requiring the parallel ar- rangement of the radical orbitals (Q= 0"): hence the long lifetime of la becomes understandable on the basis of the

l a l b Ic

2a 2b 2c

1,3-Cyclopentadiyl l a can readily be trapped with oxy- genI4l and a quantitative evaluation of the reaction in CCI, at 8°C yields a lifetime of .sT=9O0+4O ns,l3] whereas 1,4- cyclohexadiyl l b can merely be detected by trapping with oxygen.[51 The amounts of peroxides are too small for a re- liable determination of the oxygen-dependent product dis- tribution. It can be concluded from this finding that the triplet lifetime of l b is very short (an estimated 0.1- 1.0 ns)."] All attempts to detect even traces of peroxides by trapping with oxygen met without success in the case of 2,7-bicyclo[2.2. Ilheptadiyl l c . Even the very sensitive KI/ HOAc test (limit of detection ca. 0.01%) gave no indication of the presence of peroxide in the concentrated photoly- zate. The triplet diradical l c is apparently too short-lived to be intercepted by bimolecular reactions such as trapping with oxygen. Accordingly, the lifetime of this triplet diradi- cal must be shorter than 0.1 ns.

The sequence of the lifetimes (rT( la ) > sT( lb) > rT( Ic)) and their large differences (a factor of lo4) cannot be ex- plained in terms of steric or energy factors. This is only possible if the arrangement of the radical orbitals are taken into consideration. Consistent with this explanation is the result of the photolysis of the deuterated compound

unfavorable intersystem crossing (Fig. 1). In contrast, the arrangement of the radical orbitals in the triplet diradical l c (H=60" ) is almost optimal for rapid intersystem-cross- ing as a result of the rigid bicyclo[2.2.l]heptane skeleton (Fig. la). A twist-boat conformation is assumed for the 1,4- cyclohexadiyl lb,''] in which the radical orbital axes make an angle of 20" to each other. As expected, the lifetime of the triplet diradical l b lies between those of the two ex- treme cases la and lc , as demonstrated by the successful trapping with oxygen; unfortunately the oxygen trapping cannot be quantified in this case.

All these results indicate that the very different lifetimes (factor of lo4) of the triplet diradicals la -c can be as- cribed to conformational influences on the intersystem- crossing rates. Thus, the above theoretical predictions['] have, for the first time, been confirmed experimentally."1

Received: August 5, I985 [Z 1417 IE] German version: Angew. Chem. Y7 (1988) 1072

I l l a ) R. M. Wilson in A. Padwa (Ed.): Organic PhururhemiJtrr;, V d . 7, Mar- cel Dekker, New York 1985, p. 339: b) R. A. Caldwell, Pure Appl. Cbenr. 56 (1984) 1167: c) J . Wirz, h i d . 86 [ 1984) 1289: d) J. C. Scaiano, Acc. Chem. Re\. I5 (1982) 2 5 2 .

[2] L. Salem, C. Rowland, Angew. Chem. 84 (1972) 86: Angen. Chem. I n r . Ed. Engl. i l (1972) 92.

131 The mechanistic and experimental details of this novel method for deter- mining the lifetimes of triplet diradicals by trapping with oxygen have already been described: W. Adam, K. Hannemann, R. M. Wilson. J . Am. Chem. Sor. 107 (1985), in press.

141 R. M. Wilson. F. Geiser, J . A m . Chem. Soc. 100 (1978) 2225. [S] W. Adam. K. Hannemann, R. M. Wilson, J . A m . Chem Sor. 106 (1984)

1646. 161 K. Hannemann, Dirserrarron. Universitat Wiirzburg 1984. [7] N. J. Turro, W. K. Cherry, M. F. Mirbach (J . A m . Chem. Sac. 99 (1977)

7388) explains the very rapid intersystem-crossing in tricy- cl0[2.2. I .O'.']heptadiyl in the triplet-sensitized photocycloaddition of nor- bornadiene to quadricyclane in terms of the Salem rule [?].

1072 0 VCH Verlagsgesellsrhali mbH. D-6940 Weinheim, lYNS 0570-0833/~5/1212-1072 $ 02.80/0 Angen. Chem. Ini. Ed. Engl. 24 119851 No. 12