On the Question of Cyclopropylidene Intermediates in Cyclopropene-to-Allene Rearrangements − Tetrakis(trimethylsilyl)cyclopropene, 3-Alkenyl-1,2,3-tris(trimethylsilyl)cyclopropenes, and Related Model Compounds

Abstract

Several tetrasubstituted cyclopropenes have been prepared and their pyrolyses and photolyses have been investigated. Tetrakis(trimethylsilyl)cyclopropene (10), which was obtained in 25% yield from tris(trimethylsilyl)cyclopropenylium hexachloroantimonate (9), gave tetrakis(trimethylsilyl)allene (12) as the sole product both thermally and photochemically. Kinetic studies in [D8]toluene indicated first-order behavior with Arrhenius parameters log(A/s−1) = 11.75 ± 1.20 and Ea = (37.5 ± 2.5) kcal mol−1. All three new 3-alkenyl-1,2,3-tris(trimethylsilyl)cyclopropenes (17a−c, with C1-, C2-, and C3-alkenyl groups as tethers, respectively) gave allenes upon irradiation, but thermally only two (17a, 17c) gave allenes, whilst 17b yielded a bicyclo[4.1.0]hept-3-ene derivative 22 as a result of an intramolecular ene reaction. Photolyses of two further cyclopropenes (33a,b) bearing 1,2-bis(alkenyldimethylsilyl) substituents also gave the corresponding allenes as the sole products. For none of these tethered cyclopropenes was a product found that could have originated from intramolecular trapping of a cyclopropylidene intermediate. Quantum mechanical (ab initio) calculations have been carried out on the silyl-substituted cyclopropene model compounds 3,3-dimethyl-1-silyl- (36a), 3,3-dimethyl-1,2-disilyl- (37), and tetrasilylcyclopropene (38) at the QCISD(T)/6-311G*//B3LYP/6-311G* + ZPVE level of theory, and on 3,3-dimethyl-1-(trimethylsilyl)cyclopropene (36b) at the B3LYP/6-311G*//B3LYP/6-311G* + ZPVE level. These calculations provided us with detailed energy surfaces for the potential pyrolysis pathways. Although the potential cyclopropylidene species in these rearrangements are significantly stabilized, for none of the systems was this sufficient to permit isomerization via these intermediates. 36b is calculated to rearrange via a vinylidene intermediate to give 3-methyl-1-trimethylsilyl-1-butyne (47), in agreement with experiment. Comparison of the calculations for 36a and 36b shows that H3Si− is a poor model for an Me3Si− substituent in these rearrangements. When an appropriate correction is applied, the calculations on disilyl- (37) and tetrasilylcyclopropenes (38) are consistent with the experimental findings that the trimethylsilyl-substituted cyclopropenes 48 and 10 form allenes 49 and 12, respectively, via vinylcarbene-type intermediates. These findings considerably extend our understanding of silyl group substituent effects on the various intermediates involved in cyclopropene rearrangements.