8760
J. Am. Chem. Soc. 2001, 123, 8760-8765
Thermal and Photochemical Solvolysis of (E)- and (Z)-2-Phenyl-1-propenyl(phenyl)iodonium Tetrafluoroborate: Benzenium and Primary Vinylic Cation Intermediates Roel Gronheid,† Gerrit Lodder,*,† Masahito Ochiai,‡ Takuya Sueda,‡ and Tadashi Okuyama*,§ Contribution from the Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden UniVersity, P.O. Box 9502, 2300 RA Leiden, The Netherlands, Faculty of Pharmaceutical Sciences, UniVersity of Tokushima, Tokushima 770-8505, Japan, and Faculty of Science, Himeji Institute of Technology, Kamigori, Hyogo 678-1297, Japan ReceiVed April 2, 2001 Abstract: The thermal and photochemical solvolysis of the two stereoisomeric 2-phenyl-1-propenyl(phenyl)iodonium tetrafluoroborates has been investigated in alcoholic solvents of varying nucleophilicity. The product profiles and rates of product formation in the thermal reaction are all compatible with a mechanism involving cleavage of the vinylic C-I bond assisted by the group in the trans position (methyl or phenyl), always leading to rearranged products. Depending on the nucleophilicity of the solvent, the primarily formed cations may or may not further rearrange to more stable isomers. The less reactive Z compound also yields some unrearranged vinyl ether product in the more nucleophilic solvents via an in-plane SN2 mechanism. The mechanism of the photolysis involves direct, unassisted cleavage of the vinylic, and aromatic, C-I bond in an SN1 mechanism. This produces a primary vinyl cation, which is partially trapped prior to rearrangement in methanol. The unrearranged vinyl ethers are mainly formed with retention of configuration via a λ3-iodonium/solvent complex in an SNi mechanism. Thermal and photochemical solvolyses of iodonium salts are complementary techniques for the generation of different cation intermediates from the same substrate.
Introduction The (E)-styryliodonium salt E-1 undergoes thermal solvolysis through a vinylene benzenium ion I1 as intermediate.1 The departure of the iodonio leaving group has been suggested to proceed with anchimeric assistance of the β-phenyl group.1,2 That is, the benzenium ion is formed directly without intervention of the primary vinylic cation I2 (eq 1).
Stereoelectronic effects require that the participation of the β-phenyl group in this reaction occurs in an anti-periplanar fashion. This implies that only the E isomer of substrate 1 can undergo phenyl participation but the Z isomer (Z-1) cannot. It is therefore interesting to examine effects of the β-phenyl group * Corresponding authors. E-mail:
[email protected]; okuyama@ sci.himeji-tech.ac.jp. † Leiden University. ‡ University of Tokushima. § Himeji Institute of Technology. (1) (a) Okuyama, T.; Ochiai, M. J. Am. Chem. Soc. 1997, 119, 47854786. (b) Okuyama, T.; Ishida, Y.; Ochiai, M. Bull. Chem. Soc. Jpn. 1999, 72, 163-170. (2) Hinkle, R. J.; Thomas, D. B. J. Org. Chem. 1997, 62, 7534-7535.
in the reactions of Z-1. Unfortunately, Z-1 is too unstable to be isolated as a salt owing to its very facile anti-β-elimination.3 We therefore studied the solvolysis reactions of the E and Z isomers of the methyl-substituted derivative, 2-phenyl-1-propenyl(phenyl)iodonium tetrafluoroborate (E-2 and Z-2).
The solvolysis reactions were performed thermally as well as photochemically. Irradiation of β-phenyl-substituted vinyl halides is known to be an excellent entry to vinyl cations.4 Information about the photochemistry of vinyl iodonium compounds, which are pseudo-vinyl halides, was not available, but formation of vinyl cations by direct C-I bond cleavage was anticipated. This was based on the knowledge of the photochemical behavior of diaryliodonium salts, which do yield aryl cations5 and the leaving group effects in photochemistry of vinyl halides,6 where more heterolytic photocleavage occurs with better leaving groups. In this paper, the results of the thermal (3) Ochiai, M.; Oshima, K.; Masaki, Y. J. Chem. Soc., Chem. Commun. 1991, 869-870. (4) Lodder, G. In Dicoordinated Carbocations; Rappoport, Z., Stang, P. J., Eds.; John Wiley & Sons: Chichester, U.K., 1997; pp 377-431. (5) (a) Dektar, J. L.; Hacker, N. P. J. Org. Chem. 1990, 55, 639-647. (b) Dektar, J. L.; Hacker, N. P. J. Org. Chem. 1991, 56, 1838-1844. (c) Hacker, N. P.; Leff, D. V.; Dektar, J. L. J. Org. Chem. 1991, 56, 22802282. (d) DeVoe, R. J.; Olofson, P. M.; Sahyun, M. R. V. In AdVances in Photochemistry; Volman, D. H., Hammond, G. S., Neckers, D. C., Eds.; John Wiley & Sons: New York, 1992; Vol. 17, pp 313-355.
10.1021/ja010861n CCC: $20.00 © 2001 American Chemical Society Published on Web 08/16/2001
(E)- and (Z)-Vinyl(phenyl)iodonium Salt SolVolysis
J. Am. Chem. Soc., Vol. 123, No. 36, 2001 8761
Table 1. Product Distributions (in percent yields) in the Thermal Solvolysis of 2 at 60 °C substrate solvent time/h 3 (E/Z)b 4 (E/Z)c 5 (E/Z)d 6 7 8
E-2
Z-2
E-2
E-2
Z-2
E-2
Z-2
MeOH 1 0 29 (15/0) 0 28 30 97
MeOH 95 5.8 (5.8/0) 0 4.9 81 0 87
MeOHa 0.5 0 0 0 95 0 86
AcOH 1 0 42 (35/7) 8.1 (3.1/5.0) 35 7.3 97
AcOH 122 12.8 (5.2/7.6) 0 56 (20/36) 30 0 92
TFE 0.5 0 20 11 17 6.4 85
TFE 170 0 0 14 6.7 0 22
a In the presence of 0.05 M of sodium acetate. A similar result was obtained by using Z-2 as substrate. b Total yield of 3 and the yields of E-3 and Z-3 in parentheses. c Total yield of 4 and 4C and the yields of E-4 and Z-4 in parentheses. d Total yield of 5 and 5C and the yields of E-5 and Z-5 in parentheses.
and photochemical solvolysis reactions of the stereoisomeric iodonium salts 2 are compared and shown to yield mutually supportive information about mechanisms and the nature of the reactive intermediates.
Table 2. Rate Constants (104kobsd/s-1) and Rate Ratios for the Thermal Solvolysis of 2 in Various Solvents at 60 °C
Results Preparation. The tetrafluoroborate salts of the stereoisomeric 2-phenyl-1-propenyliodonium ions, E-2 and Z-2, were prepared stereoselectively from phenyl- and methylacetylene, respectively, by bromoboronation7 and methylation/phenylation8 followed by treatment with (diacetoxyiodo)benzene9 (eq 2). Both isomers are stereochemically >98% pure according to 1H NMR.
solvent
E-2
Z-2
E-2/Z-2
MeOH EtOH 80Ea 60Ea 40Ea 20Ea 50ETb AcOH TFE AcONa/MeOHc
17.1 16.3 15.7 15.3 16.0 16.3 45.0 19.9 80.4 226
0.82 1.10 0.8d 0.5d 0.4d 0.6d 1.5 0.42 0.02e 823
21 15 20 30 40 27 30 47 4000 0.27
a Aqueous ethanol containing a specified volume percent of ethanol. 50 vol % EtOH-TFE. c Sodium acetate (0.05 M) in methanol, reaction at 25 °C. d Approximate value, see text. eObtained from the calculated infinite absorbance.
b
Thermal Solvolysis. The thermal solvolysis of 2 was carried out in various alcohols, acetic acid, and aqueous alcoholic solvents at 60 °C. The reaction of E-2 was complete within 1 h in all solvents but that of Z-2 was much slower, taking over a week for completion in TFE. The products obtained in methanol, acetic acid, and TFE were identified on the basis of their gas chromatographic retention times and MS spectra compared with those of independently prepared authentic samples (eq 3).
(6) (a) Sˇ ket, B.; Zupan, M.; Pollak, A. Tetrahedron Lett. 1976, 783784. (b) Sˇ ket, B.; Zupan, M. J. Chem. Soc., Perkin Trans. 1 1979, 752756. (c) Kitamura, T.; Kobayashi, S.; Taniguchi, H. J. Org. Chem. 1982, 47, 2323-2328. (d) Zupancic, N.; Sˇ ket, B. J. Photochem. Photobiol. A: Chem. 1991, 60, 361-367. (e) Krijnen, E. S.; Lodder, G. Tetrahedron Lett. 1993, 34, 729-732. (f) Verbeek, J. M.; Stapper, M.; Krijnen, E. S.; Van Loon, J. D.; Lodder, G.; Steenken, S. J. Phys. Chem. 1994, 98, 95269536.
The product yields were determined using gas chromatography with tetradecane as an internal standard and are summarized in Table 1. The data show that in each of the solvents the product distribution derived from E-2 and Z-2 is quite different. From E-2 two elimination products, 1-phenylpropyne (6) and phenylpropadiene (7), are formed, as well as a phenylmigrated (4(S)) and a methyl-migrated (5(S)) substitution product, and their hydrolysis products 1-phenylpropanone (4C) and propiophenone (5C), accompanied by iodobenzene (8). The Z isomer Z-2 gave mainly methyl-migrated products, 5 and 6, also accompanied by 8. In the more nucleophilic solvents, methanol and acetic acid, in addition a small amount of unrearranged substitution product 3(S) was obtained. However, no phenyl-migrated products, 4 and 7, were found. In the presence of a base, sodium acetate, in methanol, the very rapid reaction gave 1-phenylpropyne (6) and 8 as the exclusive products, irrespective of the substrate structure. Rate constants for the solvolysis of 2 were determined spectrophotometrically in both alcoholic and aqueous alcoholic solutions using freshly distilled solvent containing 1 mM trifluoroacetic acid to maintain the reproducibility of the results.1b The (pseudo-)first-order rate constants kobsd are summarized in Table 2. For the slow reactions of Z-2 in aqueous alcoholic solutions, no good first-order kinetics were observed, and approximate rate constants were evaluated from the change in absorbance during the initial two half-lives. Photochemical Solvolysis. The photochemical solvolysis of 2 was carried out in methanol and TFE at 20 °C by irradiation (7) Hara, S.; Hyuga, S.; Aoyama, M.; Sato, M.; Suzuki, A. Tetrahedron Lett. 1990, 31, 247-250. (8) Takada, E.; Hara, S.; Suzuki, A. Tetrahedron Lett. 1993, 34, 70677070. (9) Ochiai, M.; Toyonari, M.; Nagaoka, T.; Chen, D.-W.; Kida, M. Tetrahedron Lett. 1997, 38, 6709-6712.
8762 J. Am. Chem. Soc., Vol. 123, No. 36, 2001
Gronheid et al.
Table 3. Product Distributions (in Percentages) in the Photochemical Solvolysis of 2a
Scheme 1. Mechanism of the Thermal and Photochemical Solvolysis of 2
substrate solvent E-3 Z-3 E-4 Z-4 4C E-5 Z-5 5C 6 8 9 E-10 Z-10 11
E-2a
Z-2a
E-2a
Z-2a
MeOH 34.1 13.4 6.5 0 0 8.2 8.6 0 15.3 66.3 10.8 33.7 0c 6.5
MeOH 14.8 38.5 7.5 0 0 10.8 11.5 0 16.7 71.5 5.7 0c 28.5 7.1
TFE 0 0 32.6 4.4 9.9
TFE 0 0 11.0 2.2
