Cyclopropenylidene" * By Hans Peter Reisenauer, Gunther Maier*, Achim Riemann, and Reinhard W. Hoffmann*

Cyclopropenylidene 2a ++ 2b is of considerable theoreti- cal interest"]. Until now, only the diphenyl[*] and bis(dia1- kylamino) derivatives[31 had been detected indirectly in trapping reactions. We report here the generation of cyclo- propenylidene 2 by flash thermolysis and its matrix isola- tion.

1 2a 2 b 3

4 5 ti

The quadricyclane derivative 114] was heated to 523 K under conditions of preparative vacuum flash pyrolysis at 1 torr. The products, which were trapped at 77 K, under- went exothermic polymerization upon thawing; in addi- tion, benzene 3 was isolated in almost quantitative yield. If water was not completely excluded from the reaction, cy- clopropenol 4 and dicyclopropenyl ether 5 were formed[']. This indicates that in the pyrolysis of 1 a C,H2-fragment having an intact cyclopropene ring is cleaved off.

If the product of the pyrolysis is condensed with a large excess of argonL6] onto a window at 10 K immediately after it leaves the hot zone, the IR spectrum shows, apart from the bands from benzene, four additional absorptions (Ta- ble 1). Within the normal variations of calculations of this type (the calculated frequencies are ca. 10% too high), the frequencies and relative intensities are consistent with those of the four most intense IR bands predicted for 2['"]. Four other bands are too weak to be observed under our experimental conditions.

Table 1. Experimental and calculated [la] IR spectrum of cyclopropenyli- dene 2.

Band frequencies [cm- '1 Found Calc.

Rel. intensities Found Calc.

-

1279 1063 - -

888 789

3457 3418 1759 1419 1191 1071 998 983 854

-

1.0 0.16 - -

0.33 0.34

[*I Prof. Dr. G . Maier, Dr. H. P. Reisenauer lnstitut fur Organische Chemie der Universitat Heinrich-Buff-Ring 58, D-6300 Giessen (FRG)

The presence of cyclopropenylidene 2 is confirmed by the following experiment: Upon irradiation (A > 360 nm) of the matrix-isolated pyrolysis product of 1, only the four IR bands belonging to 2 disappear. Simultaneously, new absorptions at 3390,3366,412,405,261, and 248 cm-' ap- pear, whose frequencies correspond to the known values for matrix-isolated propynylidene 6"l.

The photolytic conversion of 2 into 6 can also be ob- served using ESR spectroscopy. In accord with the singlet ground-state of 2"], the ESR spectrum of the matrix-iso- lated product of pyrolysis exhibits no triplet signal. After irradiation, the known spectrum[" of the triplet carbene 6 is obtained, consistent with the results of IR studies.

2 does not react when the matrix is tempered for several hours at 35-40 K. After evaporation of the argon, a brown, polymeric residue remains. Evidence for the forma- tion of the interesting dimer of 2, 1,l-bicyclopropenyli- dene, could not be obtained.

Summary: Cyclopropenylidene 2 is stable in an argon matrix at 10 K and has, as predicted"], a singlet ground- state.

Received: April 25, 1984 [ Z 807 IE] German version: Angew. Chem. 96 (1984) 596

[I] a) T. J. Lee, A. Bunge, H. F. Schaefer 111, J . Am. Chem. Soc., in press. ~

Prof. Schaefer is thanked for calculating the IR spectrum of 2 and for a preprint; b) R. Gleiter, R. Hoffmann, J. Am. Chem. Soc. 90 (1968) 5457; c) W. J. Hehre, J . A. Pople, W. A. Lathan, L. Radom, E. Wasserman, Z. R. Wasserman, ibid. 98 (1976) 4378; d) N. C. Baird, K. F. Taylor, ibid. 100 (1978) 1333; e) H. Kollmar, ibid. 100 (1978) 2660; 0 R. Shepard, A. Ba- nerjee, J. Simons, ibid. 101 (1979) 6174; g) W. W. Schoeller, Tetrahedron Left. 21 (1980) 1509; h) P. Saxe, t i . F. Schaefer 111, J. Am. Chem. Soc. 102 (1980) 3239.

[2] W. M. Jones, M. E. Stowe, E. E. Wells, E. W. Lester, J . Am. Chem. Sor. 90 (1968) 1849, and literature cited therein.

[3] Z. Yoshida, Pure Appl. Chem. 54 (1982) 1059, and literature cited there- in.

[4] a) D. N. Butler, 1. Gupta, Can. J. Chem. 60 (1982) 415; h) A. Riemann, R. W. Hoffmann, J. Spanget-Larsen, R. Gleiter, Chem. Ber., in press.

[S] 4 and 5 polymerize above 243 K, and, hence, only NMR data (203 K, [D,]acetone) could be obtained. 4 : 'H-NMR: 6=7.91 (d, 2H, J= 1.5 Hz), 3.76 (t, 1 H, J = 1.5 Hz), 3.35 ( I H); '?C-NMR: S= 120.8 (d, J(C-H)=227 Hz), 43.9 (d, J(C-H)=200 Hz). 5 : 'H-NMR: 6=7.86 (d, 4H, J= 1.6 Hz), 3.89 (t, 2H, J=1.6 Hz); "C-NMR: 6= 128.6 (d, J(C-H)=224 Hz), 48.7 (d, J/C-H)=200 Hz).

[6] Pyrolysis oven (G. Maier, G. Mihm, H . P. Reisenauer, Chem. Ber. 115 (1982) 801) flanged directly onto the vacuum shroud of the cryostat (Dis- plex Closed Cycle Refrigerations System CSA 202, Air Products). 1 was cooled to 223 K in a flask attached directly to the pyrolysis tube lo-' torr), pyrolyzed at 820-Y20 K within 5 h, and condensed with a large excess of argon onto the C's l window (10 K, distance from end to the hot zone ca. 5 cm).

[7] C. F. Kang, dissertation, Michigan State University 1972. [S] R. A. Bernheim, R. J. Kempf, J. U. Gramas, P. S. Skell, J . Chem. Phys. 43

(1965) 196.

0.006 0.012 0.004 1.000 0.321 inactive

Direct Photochemical Cleavage of the Cyclobutane Ring in Bicyclo[4.2.0loctane on 185nm Irradiation in Solution** By Waldemar Adam* and Thomas Oppenlander

By analogy to cyclopropanes, which recently have been shown to exhibit interesting and diversified photochemis- try on 185 nm irradiation in solution"], cyclobutanes are

0.031 0.309 0.395

Prof. Dr. R. W. Hoffmann, Dr. A. Riemann Fachbereich Chemie der Universitat Hans-Meerwein-Strasse, D-3550 Marburg (FRG)

[**I Small Rings, Part 51. This work was supported by the Deutsche For- schungsgemeinschaft and the Fonds der Chemischen 1ndustrie.-Part 50: G. Maier, K. Euler, H. Irngartinger, M. Nixdorf, Chem. Ber., in press. the Deutsche Forschungsgemeinschaft.

[*] Prof. Dr. W. Adam, Dipl.-Chem. T. Oppenlander ['I Institut fur Organische Chemie der Universitat Am Hubland, D-8700 Wurzburg (FRG)

['] Doctoral fellow of the Fonds der Chemischen Industrie. [**I This work was supported by the Fonds der Chemischen Industrie and

Angew. Chem. Int. Ed. Engl. 23 (1984) No. 8 0 Verlag Chemie GmbH, 0-6940 Weinheim, 1984 0570-0833/84/0R08-064I $ 02.50/0 641

also expected to be photoactive in the far UV region since they possess appreciable absorption below 200 nm ( o * t o and Rydberg excitations)[21. Cyclobutane rings can be cleaved efficiently by photochemical electron transfer with quinones as electron acceptor^'^]. This photo-cleavage, which involves radical cation intermediates, plays a central role for the dissociation of thymine dimers in photoda- maged DNA by photo-repairing enzymes[41. Since o* to- Rydberg excitation does lead to intermediates with radical cation character[‘], it appeared important to explore the possibility of promoting direct photochemical cleavage of cyclobutanes that are devoid of chromophoric substituents by means of 185 nm irradiation in solution. That this cleav- age indeed occurs is demonstrated using bicyclo[4.2.0]oc- tane 1 as example.

1 2

ground state 1,4-~yclooctanediyl intervenes, both 1 and cy- clooctene are produced[’]. Finally, the fact that the vacuum flash pyrolysis of 1 leads mainly to cyclohexene and ethy- leneI6] is evidence against the involvement of the 1,4-cy- clooctanediyl as preferred intermediate in the 185 nm pho- tolysis of 1.

Since cyclobutane rings are efficiently cleaved in the electron-transfer photolysis of thymine dimers with qui- nones via intermediary radical cations[31, we postulate that the observed cleavage of 1 into 1,7-octadiene and cyclo- hexene in the 185 nm photolysis proceeds via (o,3s)-Ryd- berg excitation. Promotion of an electron into a 3s Ryd- berg molecular orbital generates a species 2 with radical cation character“’], which on cyclobutane ring-cleavage and electron demotion would be expected to afford the ob- served products. Orbital symmetry arguments[’’] also sug- gest that such cleavage processes should be facile. The demonstration that cyclobutane rings can be directly cleaved by 185 nm excitation provides novel opportunities for mechanistic investigations.

Received: April 9, 1984; revised: June 6, 1984 [ Z 792 IE]

German version: Angew. Chem. 94 (1984) 599

Irradiation of a 0.044 M solution of 1 in n-pentane with the full output of a mercury low-pressure lamp (Grantzel, Karlsruhe) affords 1,7-octadiene, cyclohexene, and ethy- lene. These photo-products were identified by co-injecting authentic materials onto three different capillary GC co- lumns (86 m polypropylene glycol, 50 m OV-101, and 50 m Carbowax 20 M). In addition, 1,7-octadiene and cyclohex- ene were identified by their characteristic fragmentation patterns on capillary G U M S analysis. Finally, 1,7-octa- diene was collected by preparative gas chromatography (packed glass column 1.5 m x 8 mm, 10% P,P’-oxydipro- pionitrile on Chromosorb WHP, 100/200), and its 90 MHz ’H-NMR spectrum found to be identical to that of an au- thentic sample.

Control experiments revealed that not even traces of cy- clooctene were produced in the 185 nm photolysis, and at 254nm (the major output of the mercury low-pressure lamp) 1 was completely photo-stable. Quantitative capil- lary GC analysis (50 m SE 30) gave a relative product com- position of 1,7-octadiene: cyclohexene = 70 : 30, with a mass balance of >98% during the first 40 min of 185 nm irradiation. The quantum yields determined using the (Z,E)-isomerization of cyclooctene as actin~meter[~] were Qs = 0.12 f 0.01 for substrate consumption and Qp=O.lO~O.O1 and @p=0.030+0.003 for 1,7-octadiene and cyclohexene, respectively.

In comparison, vacuum flash pyrolysis of 1 (18 torr, dis- tillation through a Quartz tube, 450°C) led to a relative product composition of 1,7-octadiene : cyclohex- ene = 11 : 89I6I. Ethylene was also formed, but the amount was difficult to quantify.

There are no analogous photoreactions of 1 at higher wavelengths because this alkyl-substituted cyclob~tane[’~ has no UV absorption above 200 nm. Mercury-sensitized irradiation of cyclooctene at 254 nm leads to 1, postulated to be formed via cyclization of the intermediate tetrame- thylene diradical 1,4-~yclooctanediyl[~~. The fact that cy- clooctene is not formed in the 185 nm photolysis of 1 pre- cludes this 1,4-diradical as reactive intermediate, at least in its ground state. For example, in the thermolysis of the azoalkane 7,8-diazabicyclo[4.2.2]dec-7-ene, in which a

[ l ] a) R. Srinivasan, J. A. Ors, T. Baum, J . Org. Chem. 46 (1981) 1950; b) W.

[2] M. B. Robin: Higher Excited States of Polyatomic Molecules, Val. 1, Aca-

[3] H . D. Roth, A. A. Lamola, J . Am. Chem. Soc. 94 (1972) 1013. 141 J. C. Sutherland, Photochern. Photohiol. 25 (1977) 435. [5] W. Adam, T. Oppenlander, Photochem. Photobiol. 39 (1984) 719. [6] J. E. Baldwin, P. W. Ford, J. Am. Chem. Soc. 91 (1969) 7192 reported cy-

clohexene as major product. [7] Cyclobutanes with cbromophores can be cleaved by direct photolysis

above 200 nm, cf. E. Schaumann, R. Ketcham, Angew. Chem. 94 (1982) 231; Angew. Chem. Int . Ed. Engl. 21 (1982) 225. For the direct photo- chemical cleavage of a functionalized cyclobutane with radiation below 200 nm cf. M. G. Steinmetz, E. J. Stark, Y.-P. Yen, R. T. Mayes, R. Srini- vasan, J . Am. Chem. Soc. 105 (1983) 7209.

[8] Y. Inoue, K. Moritsugu, S. Takamuku, H. Sakurai, J. Chem. Soc. Perkin 11 1976, 569.

191 C. J. Samuel, J. Chem. Soc. Chem. Commun. 1982, 131.

Adam, T. Oppenlander, Tetrahedron Letf. 23 (1982) 5391.

demic Press, New York 1975, p. 146.

[lo] P. J. Kropp in A. Padwa: Organic Photochemistry, Vol. 4, Marcel Dek-

[Il l N. L. Bauld, D. J. Bellville, R. Pabon, R. Cbelsky, G. Green, J. Am. ker, New York 1980, pp. 1-134.

Chem. Soc. I05 (1983) 2378.

Formation of Phenanthrenequinone from Benzil : A Novel Reaction of Graphite-Potassium Intercalates By Dou Tamarkin, Daphna Benny, and Mordecai Rabinouitz*

Reactions of the highly ordered potassium-graphite in- tercalate C,K have evoked recently much interest. Due to its structure it reacts in a different mode, as compared with non-intercalated dispersed potassium[’]. Recently, we re- ported that vic-dibromides undergo with C8K a stereospe- cific debromination reaction, and ketones undergo a bimo- lecular reduction in high yields to the respective pinacols. It has been suggested that the “layer-edge mechanism” is responsible for the specific reactivity of this reagentl2]. Ac- cording to this mechanism ketones which are linked to ad- jacent potassium atoms at the graphite layer edge, form ra- dical anions. These radical anions can then undergo a C-C bond formation reaction at those carbons bearing high spin densitylZb1.

[*] Prof. Dr. M. Rabinovitz, D. Tamarkin, D. Benny Department of Organic Chemistry, The Hebrew University of Jerusalem Jerusalem 91 904 (Israel)

642 0 Verlag Chemie GmbH, 0-6940 Weinheim, 1984 0570-0833/84/0808-0642 $ 02.50/0 Angew. Chem. lnt. Ed. Engl. 23 (1984) No. 8