J. Org. Chem. 1988,53, 4748-4758
4748
Photochemistry of Azocyclopropane Paul S. Engel* and Gregory A. Bodager Department of Chemistry, Rice University, Houston, Texas 77251 Received M a r c h 21, 1988 trans-Azocyclopropane (It) has been irradiated in t h e gas phase at 254 n m and in hydrocarbon solution at several wavelengths. As with most acyclic azoalkanes, the dominant reaction is isomerization to t h e cis-azoalkane IC. In t h e gas phase or with s h o r t wavelength light, I t undergoes competitive C-N homolysis, giving cyclopropyl radicals, and ring fragmentation to ethylene. Triplet-sensitized isomerization of t h e azo linkage proceeds with unusually high efficiency (@* = 0.2), but n o experimental support could be found for the notion that cyclopropane ring cleavage leads to azo group isomerization. Since azocyclopropane behaves as a “reluctant azoalkane” that undergoes multiple photoreactions, it is n o t a particularly useful source of cyclopropyl radicals.
Seven years ago, we reported the synthesis and thermal chemistry of the simplest azocycloalkane, azocyclopropane 1.’ Unlike typical a z o a l k a n e ~1, ~does ~ ~ not lose nitrogen thermally or photochemically. Instead, UV irradiation interconverts the trans and cis isomers (It and IC) while thermolysis affords pyrazoline 2. Perusal of the literature
V
I C
It
k 6,Pb \ 2
reveals only a few compounds with a cyclopropyl ring attached to the azo linkage.4-8 We recently reinvestigated one of these (3) and found that cis-trans isomerization of the azo linkage is very much faster than any reported photorea~tion.~ The photochemistry of [ (trifluoromethyl)azo]cyclopropane (4) has also been studied,6 but as we shall see below, the reported products are severely inconsistent with those of related azoalkanes. Thus there
Table I. Products of Gas-Phase Azocyclopropane Photolysis” product yield, % product yield, % methane 0.5 hexamethylethane 4.6 ethane 2.1 isooctane 1.0 ethylene 11.0 4,4-dimethylpentene 0.91 cyclopropane 9.2 bicyclopropyl 0.22 propene 0.5 allylcyclopropane 0.02 isobutene 12.5 tert-butylcyclopropane 0.06 “Yields for 254-nm irradiation of -20 m m It and 700 m m isobutane; 70% of 1 disappeared.
study of 1. Photolysis is examined under the following conditions: (1)in the gas phase a t 254 nm with excess isobutane, (2) in hydrocarbon solution irradiating with an excimer laser and mercury lamps at several wavelengths, and (3) in solution under triplet sensitization. It is found that azocyclopropane is unusudy photostable (a “reluctant azoalkane”), but when it does react, cleavage of a cyclopropane ring bond competes with the expected C-N homolysis. Results and Discussion Electronic Spectrum of 1. Figure 1 shows that in accord with the usual behavior of azoalkanes,15-17 IC absorbs at longer wavelength and with a greater extinction coefficient than It. However the wavelength maximum for It (332 nm) is shorter than that of trans-azoisopropane (AIP, 358 nm), a reasonable model compound, while the e of It (51.5) is higher than that of AIP (14.8).lS These differences suggest mixing of the weak azoalkane n a* transition with the more strongly allowed transitions of the cyclopropyl The window in the 280-nm region of the azocyclopropane spectrum allows for selective irradiation of triplet photosensitizers while the strong absorption below 250 nm provides an opportunity to study the short wavelength photochemistry of 1. Interestingly, the cyclopropyl group lowers the energy of the short wavelength transition, in contrast to its effect on the n,a* transition. Thus the A, of gaseous It (207 nm) is longer than that of gaseous AIP (195 nm).20 Furthermore, the short wavelength band of IC is at higher energy than the corresponding band of It (IC, 204 nm; It, 213 nm, both in pentane). This difference is again in the opposite direction from the n,a* band where the cis isomer absorbs
-
3
41
is a shortage of reliable data on photoreactions of azocyclopropanes. Our interest in forcing decomposition of reluctant azoalkanes,1°in comparing laser irradiation with conventional light sources,”JZ and in the chemistry of cyclopropyl radicals13J4 prompted the present detailed (1)Engel, P. S.; Gerth, D. B. J. Am. Chem. SOC. 1981, 103, 7689. (2) Engel, P. S. Chem. Rev. 1980,80, 99.
(3) Adam, W.; De Lucchi, 0. Angew. Chem., Int. Ed. Engl. 1980,19, 762. (4) Rosenkranz, H. J.; Schmid, H. Helu. Chim. Acta 1968,51,1628. (5)Bonnekessel, J.; Ruchardt, C. Chem. Ber. 1973,106,2890. (6) Chakravorty, K.; Pearson, J. M.; Szwarc, M. J. Phys. Chem. 1969, 73,746. (7) Vilsmaier, E.; Penth, B.; Troger, W. Tetrahedron Lett. 1982,3475. (8) Engel, P. S.; Gerth, D. B. J. A m . Chem. SOC. 1983, 105, 6849. (9)Engel, P. S.; Bodager, G. A. J . Org. Chem. 1986,51,4792. (IO) Adam, W.; Mazenod, F.; Nishizawa, Y.; Engel, P. S.; Baughman, S. A.; Chae, W. K.; Horsey, D. W.; Quast, H.; Seiferling, B. J. Am. Chem. SOC.1983,105,6141. (11) Turro, N. J.; Aikawa, M.; Gould, I. R. J . A m . Chem. SOC.1982, 104,856. (12) Scaiano, J. C.; Johnston, L. J.; Gimpsey, W. G.; Weir, D. Acc. Chem. Res. 1988 21,22.
0022-3263/88/1953-4748$01.50 I O
(13) Walborsky, H.M. Tetrahedron 1981,37,1625. (14) Johnston, L. J.; Ingold, K. U. J. Am. Chem. SOC. 1986,108,2343. (15) Rau, H. Angew. Chem., Int. Ed. Engl. 1973, 12,224. (16) Chae, W. K.; Baughman, S. A.; Engel, P. S.; Bruch, M.; Ozmeral, C.; Szilagyi, S.; Timberlake, J. W. J . Am. Chem. SOC. 1981,103,4824. (17) Schmittel, M.; Ruchardt, C. J. Am. Chem. SOC. 1987,109,2750. ( H ’ F o g e l ,L.D.; Steel, C. J . Am. Chem. SOC. 1976,98,4859. (19) deMeijere, A. Angew. Chem., Int. Ed. Engl. 1979,18,809. (20)Robin, M. B.; Hart,R. R.; Kuebler, N. A. J. Am. Chem. SOC. 1967, 89, 1564.
0 1988 A m e r i c a n C h e m i c a l Societv
J. Org. Chem., Vol. 53, No. 20, 1988
Photochemistry of Azocyclopropane
4749
Scheme I. Photolysis of Gaseous Azocyclopropane in Isobutane CIS-ACP
/A
11
v 7
8
4NA.. CH
WAVELENGTH,
Figure 1. UV
nm
spectrum of trans- and cis-azocyclopropane.
10
+
at lower energy. These effects are not readily explained, particularly since the spectral assignment of the azoalkane short-wavelength band is u n ~ e r t a i n . ~ ~ ~ ~ ~ ~ ~ ~ material on the vessel walls, especially since none of the components of the photolysis mixture are reactive enough Gas-Phase Photolysis. A sample of It at approxito trap 10 ~leanly.~'The other primary photoreaction of mately 20 mm of pressure and a t 47 f 3 "C was continuIt is cleavage to cyclopropyl radicals that mainly abstract ously mixed with 700 mm of isobutane while irradiating hydrogen from the bath gas. Very few of the cyclopropyl at 254 nm through vycor glass. GC analysis of the gaseous radicals afford radical recombination products, but some mixture showed the major products to be ethylene, cyare sufficiently excited to rearrange to allyl radi~a1s.I~ clopropane, isobutane, and hexamethylethane (cf. Table Although these allyl radicals show up in products 5 and I). Unlike most gas-phase azoalkane photolyses, irradia7,they do not reach a high enough concentration to dition of It is far from quantitative. Not only does a polymerize. The tert-butyl radicals undergo their usual remeric material form on the vessel's walls, but the total yield combination and disproportionation reactions, the isoof tert-butyl products is only 37.2%, while that of cyclobutane from the latter process being undetectable in the propyl products is even lower at 11.2%. presence of the large amount of this gas present initially. Authentic samples of the minor products allylcycloApparently some tert-butyl radicals attack isobutene, afpropane (5)22and tert-butylcyclopropane (6)23were synfording tert-octyl radicals that appear as isooctane but not thesized independently while 4,4-dimethyl-l-pentene (7) as isooctene. was identified by comparison with purchased material. Whereas all of the above reactions are reasonable in light Bicyclopropyl (8) was recognized by its characteristic mass of prior knowledge about azoalkanes and radicals, they spectrum.% At least four GC peaks accounting for 5-10% contrast sharply with earlier results obtained by CPS6on of the starting material remain unidentified, but a small [ (trifluoromethyl)azo]cyclopropane(4). Their molar ratios amount of IC could be detected. Certain products exof products relative to Nz are reproduced below. CPS pected on the basis of the work of Chakravorty, Pearson, and Szwarc (CPS)6were specifically shown to be absent. Thus the NMR spectrum of material washed down from h v . 6 5 'C CF3H + F 3 C w / + the walls after the photolysis of It did not exhibit any 0 . 2 5 peaks in common with authentic unsubstituted 2vapor 4 pyrazoline (9)25or with 2.l The concentrated gaseous photolysate was then examined by GC and found not to CN,H + contain either pyrazoline. Known samples of 1,Bhexadiene 0.25 0 trace and isooctene were used to show that these compounds 0 15 were not present in detectable amounts. found very little cyclopropane, our major cyclopropyl The nature of the gas-phase products immediately product, and they did not mention ethylene, the manisuggests two primary processes of It (cf. Scheme I). festation of a new primary p r o c e ~ s .Moreover, ~ they obCleavage of the cyclopropyl ring affords ethylene in a retained a substantial quantity of a formal recombination action well-known in cyclopropanesz6but discovered only product CF3CH2CH=CHz in the gas phase but not in recently in an azocy~lopropane.~ Interestingly, the highsolution. The cyclic mechanism written to rationalize this resolution mass spectrum of It shows more C2H4than Nz. product has no analogy in azoalkane chemistry and if apAlthough we were unable to detect products from 10, this plied to It, would give allylcyclopropane 5. In accord with nitrile imine might have contributed to the polymeric the usual ideas about cage effects,28we found very little of this recombination product or bicyclopropyl8 in the gas phase. CPS reported a considerable amount of propene, (21) Robin, M. B. In The Chemistry of the Hydrazo, Azo, and Azoxy Groups; Patai, S., Ed:, Wiley: New York, 1975; Vol. 1, pp 1-22. but our study gave only 0.5% of this hydrocarbon, as ex(22) Manning, T. D. R.; Kropp, P. J. J. Am. Chem. SOC.1981,103,889. pected from the poor hydrogen abstracting ability of the We used 193-nm laser irradiation of 1,5-hexadiene to produce isolable allyl radical. Finally, we proved the absence of 2quantities of 5. (23) Kropp, P. J.; Pienta, N.; Sawyer, J. A.; Polniaszek, R. P. Tetrapyrazoline, another of CPS's reported products. It was hedron 1981,37, 3229. Because we had difficulty separating the product proposed that cyclopropyldiazenyl radical 11 rearranges from CH2C12, 6 was prepared by the photolysis of (tert-buty1azo)cyclo-
-
pcF3 +
propane. We thank Professor Kropp for his suggestions. (24) Farneth, W. E.; Thomsen, M. W. J . Am. Chem. SOC.1983, 105, 1843. (25) Curtius, T. J . Prakt. Chem. 1894, 50, 531. (26) Griffin, G. W. Angew. Chem., Int. Ed. Engl. 1971, 10, 537.
(27) Caramella, P.; Grunnager, P. In 1,3-Dipolar Cycloaddition Chemistry; Padwa, A., Ed.; Wiley: New York, 1984; Vol. 1, p 291. (28) Lyon, R. K. J. Am. Chem. SOC. 1964,86, 1907.
Engel and Bodager
4750 J . Org. Chem., Vol. 53, No. 20, 1988 Table 11. Q u a n t u m Yields of trans -Azocyclopropane Photolysis wavelength, nm
solvent
a,, 0.6 0.6 0.2 0.6 0.2
366
n-C5H,z
313
C6D6
254 254 193
n-C5HI2 CCD6 n-C6H,,
@,-No
%
IC a t pssa
