13354
J. Am. Chem. Soc. 1998, 120, 13354-13361
Photochemical Addition of 2,2,2-Trifluoroethanol to Benzonitrile and p-, m-, and o-Methylbenzonitrile Jennifer Foster, Alexandra L. Pincock, James A. Pincock,* and Kim A. Thompson Contribution from the Department of Chemistry, Dalhousie UniVersity, Halifax, NoVa Scotia, Canada, B3H 4J3 ReceiVed July 30, 1998
Abstract: Irradiation of benzonitrile in 2,2,2-trifluoroethanol (TFE) with 254-nm light from low-pressure mercury lamps results in the formation of four addition products, the stereoisomers of 6-cyano-2-(2,2,2trifluroethoxy)bicyclo[3.1.0]hex-3-ene, 22-25. The proposed mechanism begins by formation of the 6-cyanobicyclo[3.1.0]hex-3-en-2,6-diyl biradical/zwitterion from S1 followed by both endo and exo protonation by TFE at C6. Deuterium labeling demonstrated that the resulting 6-cyanobicyclo[3.1.0]hex-3-en-2-yl cation underwent a rapid degenerate 1,4-sigmatropic rearrangement with inversion of configuration at the migrating carbon before being trapped by the nucleophilic solvent. Irradiation of p- and m-methylbenzonitrile in the same way gave six major addition products. Three of them, 32, 33, and 35 were 6-cyano-2-(2,2,2trifluoroethoxy)-4-methylbicyclo[3.1.0]hex-3-enes and the other three, 32, 33, and 35, were 6-cyano-2-(2,2,2trifluoroethoxy)-2-methylbicyclo[3.1.0]hex-3-enes. The proposed mechanism is again by TFE endo and exo protonation of the first formed biradical/zwitterion followed by trapping of the cations by the solvent. The 1,4-sigmatropic rearrangement of the cations now stops at the most stable structures, the endo- and exo-6cyano-2-methylbicyclo[3.1.0]hex-3-en-2-yl cations, and all of the products are derived from them.
Scheme 1. Mechanistic Scheme for the Photochemical Transposition Reactions of the Methylbenzonitriles 1-3
Introduction Recently, we reported that phototransposition reactions occur for p-, m- and o-methylbenzonitriles (1 (PMBN), 2 (MMBN), and 3 (OMBN)).1 Any one of three is converted, in the excited singlet state, to the other two by both 1,2- and 1,3-transpositions in a primary photochemical step. However, the reactivities are very different with relative efficiencies of conversion to the other isomers for para:meta:ortho ) 32:4:1 (i.e., PMBN is the most reactive). Moreover, deuterium labeling (circled hydrogens) demonstrated that only the cyano-substituted carbon migrates. These observations were rationalized by the mechanism shown in Scheme 1. Excitation to S1 for each creates a spectroscopic minimum; distinct fluorescence is observed for all three isomers. An activation barrier separates the excited states from the corresponding singlet bicyclo[3.1.0]hex-3-en-2,6-diyl biradicals/ zwitterions 4, 5, and 6 that are formed by the meta bonding that puts the cyano group at C6. The barrier height is dependent on which isomer is excited, being lowest for PMBN and highest for OMBN. These biradicals then equilibrate before collapsing back to one or the other of the benzonitriles. In principle, substituted benzvalenes, 7, 8, and 9, could also be formed as intermediates in these conversions, but we had no evidence for that. Moreover, the observation of both 1,2- and 1,3-transpositions as primary photochemical events seemed to exclude the possibility that benzvalenes were the only intermediates. A better understanding of the structure and reactivity of the intermediates in phototransposition reactions would be obtained if they could be trapped; on the basis of literature precedents, we have chosen 2,2,2-trifluoroethanol (TFE) as the trap. * Corresponding author: (phone) 902-494-3324; (fax) 902-494-1310; (email)
[email protected]. (1) MacLeod, P. J.; Pincock, A. L.; Pincock, J. A.; Thompson, K. A. J. Am. Chem. Soc. 1998, 120, 6443-6450.
The photochemical addition of alcohols to benzene and alkylbenzenes was first reported by Kaplan et al. in 1966.2 Thus,
10.1021/ja982712j CCC: $15.00 © 1998 American Chemical Society Published on Web 12/05/1998
Photochemical Addition of 2,2,2-Trifluoroethanol to Benzonitriles irradiation at 254 nm of benzene (0.02 M) in TFE gave two major products, 10 and 11, eq 1, in a ratio of 2:1. Similar
irradiation of 1,3,5-tri-tert-butylbenzene in methanol gave, as the only detectable product, the methyl ether, 12, eq 2.
Moreover, the positional isomerization of this substrate was supressed in favor of the addition reaction. The stereochemistry of the hydrogen (deuterium) at C6 in 12 was established by 1H NMR coupling constants to the two other cyclopropyl hydrogens. The mechanistic conclusion reached was that these products result from ground-state addition of the alcohol to the expected benzvalene primary photoproduct. Therefore, 13 was
J. Am. Chem. Soc., Vol. 120, No. 51, 1998 13355 stereoisomers of the bicyclo[3.1.0]hex-3-en-2-yl5 addition products, 15-18, R ) acetyl, but the composition varied with percent
conversion. In the earliest sample analyzed (after a 2-h irradiation), the ratio was approximately 40:41:4:16 (estimated from a plot in ref 4). Assuming rapid endo to exo ground-state conversion of the products, i.e., 16 R ) acetyl to 15 R ) acetyl, the major primary photoproduct is the endo adduct. Therefore, both the protonation and nucleophilic attack were predominantly endo, as expected for an addition to benzvalene on the basis of previously observed additions to one other bicyclobutane derivative.6 After 48 h of irradiation the product ratio was 23: 13:15:48. The photoequilibration of the products was proposed to occur by a vinylcyclopropane rearrangement as in eq 4, involving cleavage of the C1-C5 bond.
The intervention of benzvalene as an intermediate in these photoaddition reactions received even stronger support after it became available in “bounteous quantities” as a result of a nonphotochemical synthesis, developed by Katz and co-workers.7 Reaction of benzvalene in ether with D2O/D3PO4 for 30 min gave a quantitative yield of 19. the proposed precursor for 12. Similar additions of acetic acid and water (0.1 M in phosphoric acid) to benzene were observed in the same year.3 These conclusions were further supported by Berson and Hasty from the photolysis of benzene at 254 nm in D2O (0.1 N D3PO4), eq 3.4 Analysis of the 1H NMR of the isolated alcohol,
14, indicated that the deuterium at C6 was exclusively endo. The stereochemistry of the hydroxy group at C2 was exo, but any of the endo alcohol formed would have rapidly epimerized to the exo alcohol in the acidic solution. As well, irradiation of benzene in acetic acid gave all four possible regio- and (2) Kaplan, L.; Ritscher, J. S.; Wilzbach, K. E. J. Am. Chem. Soc. 1966, 88, 2881-2882. (3) Farenhorst, E.; Bickel, A. F. Tetrahedron Lett. 1966, 47, 5911-5913. (4) Berson, J. A.; Hasty, N. M. J. Am. Chem. Soc. 1971, 93, 15511552.
Finally, Kaplan and co-workers,8 using benzene-1,3,5-d3 and benzene-d6 as substrates, proved that (1) the photoaddition products 15-18 did come from protonation of benzvalene; (2) 15 and 16, the primary photoproducts, were formed by nucleophilic trapping of the cation 20; (3) both 17 and 18 were formed from 15 and 16 by benzene-sensitized secondary photolysis involving C1-C5 bond cleavage as in eq 4; and (4) 17 and 18 were equilibrated photochemically both by C1-C5 bond cleavage and C1-C6 cleavage, this latter pathway being dominant. (5) The numbering for nomenclature of these compounds can be confusing. For the unsubstituted compound, bicyclo[3.1.0]hex-2-ene, the double bond takes priority whereas, for the addition products, the alcohol functional group takes priority. (6) Wilberg, K. B.; Szeimies, G. J. Am. Chem. Soc. 1970, 92, 571. (7) Katz, T. J.; Wang, E. J.; Acton, N. J. Am. Chem. Soc. 1971, 93, 3782-3783. (8) Kaplan, L.; Rausch, D. J.; Wilzbach, K. E. J. Am. Chem. Soc. 1972, 94, 8638-8640.
13356 J. Am. Chem. Soc., Vol. 120, No. 51, 1998 Table 1.
1H
Foster et al.
and 13C NMR Chemical Shifts (δ) for Compounds 22-25 and 30-35
cmpd
H1/C1
H2/C2
H3/C3
H4/C4
H5/C5
H6/C6
CH2
22
2.34 23.8 2.47 30.2 2.30 28.1 2.13 b 2.09 29.1 2.29 32.6 2.32 24.8 2.51 33.8 2.04 29.9 2.25 28.4
5.12 85.8 4.58 84.4 4.60 84.3 5.21
5.48 130.6 5.69 129.5 5.97 131.5 5.83
6.07 134.7 6.33 137.8 6.25 135.1 6.06
2.58 31.8 2.68 30.9 2.62 29.3 2.48
1.48 13.0 0.84 16.1 2.01 16.5 1.76
3.94, 3.97 67.8 3.88 65.1 3.91 65.7 3.98
5.46 135.2 5.37 133.8 5.08 123.7 5.28 122.8 5.65 135.2 5.55 124.4
5.96 133.8 6.26 136.4
2.58 31.5 2.63 31.1 2.38 34.6 2.44 30.8 2.60 29.0 2.43 32.0
1.48 14.0 0.99 16.2 0.88 15.8 0.88 15.2 1.92 16.0 1.96 15.3
3.87 63.1 3.69, 3.64 61.1 3.89 67.6 3.87 64.8 3.68 60.5 3.84 65.2
23 24 25 30 31 32 33 34 35 a
Not detected.
b
89.1 88.1 5.00 85.5 4.52 84.7 87.5 4.54 84.3
145.6 149.3 6.26 134.6 145.6
Not enough material isolated for reliable spectra.
We now report on the photochemical reactivity and the structure of the addition products obtained from PMBN, MMBN, and OMBN in TFE. For comparison, benzonitrile (BN) itself was also examined. Results and Discussion Irradiation of Benzonitrile (21) in 2,2,2-Trifluoroethanol. Irradiation of BN (1.87 mmol) in nitrogen-purged TFE (100 mL) at 25 °C using low-pressure Hg lamps (254 nm) in a Rayonet reactor for 30 h led to 50% conversion of the starting material. GC analysis as a function of time indicated four products, later shown to be the stereisomers of 6-cyano-2-(2,2,2trifluroethoxy)bicyclo[3.1.0]hex-3-ene, 22-25, in a final ratio of 44:24:22:10 and a mass balance (GC) of 82%, eq 5. These
isomers were stable in the dark in TFE. Extrapolation to very low conversions (
