Catalytic and Photochemical Cyclopropanation of Alkenes with Methyl Diazo(trialkylsilyl)acetates: Steric Effects and Thermodynamic Stabilities of Cyclopropanes

Diazo esters 1 (N2C(SiR3)COOMe; R = Me (1a), Et (1b), i-Pr (1c)) have been decomposed in styrene, 1-hexene, and cyclohexene under Cu, Rh, or Ru catalysis as well as by photochemical means with the objective to evaluate the effectiveness and diastereoselectivity of the respective cyclopropanation reaction. With styrene and 1-hexene as substrates, the ability of the catalysts followed the order CuOTf > [Ru2(CO)4(μ-OAc)2]n ≈ Rh2(OAc)4, but even CuOTf (copper(I) triflate) did not promote cyclopropanation of 1-hexene with the bulky 1c. Cyclopropanation of cyclohexene with 1a,b succeeded only with the ruthenium catalyst. In all cases, the diastereoisomer having the silyl group anti to the vicinal methyl or phenyl substituent(s) was formed preferentially. In contrast, in the photochemical reactions of 1a,c with styrene and of 1a with 1-hexene the diastereoisomer having the silyl group syn to the vicinal substituent(s) was formed preferentially. The fluoride-induced desilylation of cyclopropanes 2a,c,e was accompanied by a loss of stereochemical integrity. The X-ray crystal structure analysis of the cyclopropane (E)-2f has been determined. The relative thermodynamic stabilities of various 2-R-1-X-cyclopropanecarboxylates (R = Me, Ph; X = Me, t-Bu, SiH3, SiMe3, Si(i-Pr)3) have been calculated by density functional theory methods. These calculations show that for X = SiMe3 and R = Ph the syn−anti energy difference is ca. 0, while for R = Me the anti isomer is more stable.