Catalytic and photochemical cyclopropanation of alkenes with methyl diazo(trialkylsilyl) acetates: Steric effects and thermodynamic stabilities of cyclopropanes

Diazo esters I. (N-2=C(SiR3)COOMe; R = Me (1a), Et (1b), i-Pr (1c)) have be en decomposed in styrene, 1-hexene, and cyclohexene under Cu, Rh, or Ru cat alysis 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 catal ysts followed the order CuOTf > [Ru-2(CO)(4)(mu -OAc)(2)](n) approximate to Rh-2(OAc)(4), but even CuOTf (copper(I) triflate) did not promote cyclopro panation of 1-hexene with the bulky 1c. Cyclopropanation of cyclohexene wit h 1a,b succeeded only with the ruthenium catalyst. In all cases, the diaste reoisomer having the silyl group anti to the vicinal methyl or phenyl subst ituent(s) was formed preferentially. In contrast, in the photochemical reac tions of 1a,c with styrene and of la with 1-hexene the diastereoisomer havi ng the silyl group syn to the vicinal substituent(s) was formed preferentia lly. The fluoride-induced desilylation of cyclopropanes 2a,c,e was accompan ied by a loss of stereochemical integrity. The X-ray crystal structure anal ysis of the cyclopropane (E)-2f has been determined. The relative thermodyn amic 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 func tional theory methods. These calculations show that for X = SiMe3 and R = P h the syn-anti energy difference is ca. 0, while for R = Me the anti isomer is more stable.