Substituent Effects on the Reactivity of the Silicon−Carbon Double Bond. Resonance, Inductive, and Steric Effects of Substituents at Silicon on the Reactivity of Simple 1-Methylsilenes

Abstract

The reactivities of a series of substituted 1-methylsilenes RMeSidCH 2 (R ) H, methyl, ethyl, t-butyl, vinyl, ethynyl, phenyl, trimethylsilyl, and trimethylsilylmethyl) in hydrocarbon solvents have been investigated by far-UV (193-nm) laser flash photolysis techniques, using the corresponding 1-methylsilacyclobutane derivatives as silene precursors. Each of these silacyclobutanes yields ethylene and the corresponding silene, which can be trapped as the alkoxysilane RSiMe 2 OR′ cleanly upon 193-or 214-nm photolysis in solution in the presence of aliphatic alcohols. UV absorption spectra and absolute rate constants for reaction of the silenes with methanol, ethanol, and t-butyl alcohol have been determined in hexane solution at 23°C. The rate constants vary from a low of 3 × 10 7 M -1 s -1 for reaction of 1-methyl-1-trimethylsilylsilene with t-BuOH to a high of 1 × 10 10 M -1 s -1 for reaction of 1-ethynyl-1-methylsilene with MeOH. In several cases, rate constants have been determined for addition of the deuterated alcohols, and for addition of methanol over the 0-55°C range. Invariably, small primary deuterium kinetic isotope effects and negative Arrhenius activation energies are observed. These characteristics are consistent with a mechanism involving reversible formation of a silenealcohol complex which collapses to alkoxysilane by unimolecular proton transfer from oxygen to carbon. Silene reactivity increases with increasing resonance electron-donating and inductive electron-withdrawing ability of the substituents at silicon and is significantly affected by steric effects within this series of compounds. This is suggested to be due to a combination of effects on both the degree of electrophilicity at silicon (affecting the rate constants for formation and reversion of the complex) and nucleophilicity at carbon (affecting the partitioning of the complex between product and free reactants). Two 1-methyl-1-alkoxysilacyclobutanes were also investigated, but proved to be inert to 193-nm photolysis.

Silenes: Connectors between classical alkenes and nonclassical heavy alkenes

“…The attachment of larger alkyl groups at Si, as in 46 and 47 [95], had no noticeable effect Silenes with substituents that -conjugate with the Si C bond showed even more strongly redshifted values for λ max of the lowest * transition. One vinyl or ethynyl substituent at Si (54 and 55) gave bathochromic shifts of 47 and 33 nm, respectively [95].…”

“…The attachment of larger alkyl groups at Si, as in 46 and 47 [95], had no noticeable effect Silenes with substituents that -conjugate with the Si C bond showed even more strongly redshifted values for λ max of the lowest * transition. One vinyl or ethynyl substituent at Si (54 and 55) gave bathochromic shifts of 47 and 33 nm, respectively [95]. Scheme 17. From the position of the λ max of the two cyclic silabutadienes 56 and 57 (Scheme 17) it becomes clear that a C C double bond conjugates better with the Si C bond when linked to the C atom than to the Si atom [99].…”

“…A set of transient 1-substituted 1-methylsilenes RMeSi CH 2 were generated photolytically from silacyclobutanes and their reactions with methanol, ethanol, and tert-butanol were probed (Scheme 26) [95,129]. An acceptable correlation was found between the rate constant for MeOH addition to the RMeSi CH 2 silenes and a three-parameter function incorporating resonance, inductive, and steric substituent parameters.…”

“…The parent silene was characterized by an absorption maximum of the * transition at 258 nm (Table 2) [17], whereas MeHSi CH 2 (44) and Me 2 Si CH 2 (2) had λ max at 260-265 and 244-255 nm, respectively [94][95][96]. The attachment of larger alkyl groups at Si, as in 46 and 47 [95], had no noticeable effect Silenes with substituents that -conjugate with the Si C bond showed even more strongly redshifted values for λ max of the lowest * transition.…”

Steady state and time-resolved spectroscopic studies of the photochemistry of 1-arylsilacyclobutanes and the chemistry of 1-arylsilenes

“…Silacyclobutanes are well known to yield products consistent with the formation of silene-reactive intermediates upon photolysis in solution or the gas phase, or upon pyrolysis at high temperatures (1)(2)(3)(4)(5)(6)(7)(8). These compounds have proven to be tremendously useful as photochemical precursors for transient silenes, in experiments aimed at detecting the silenes directly and studying the kinetics and mechanisms of their reactions with alcohols and other "silenophiles" (9)(10)(11)(12)(13)(14)(15)(16)(17). Photochemical silene formation proceeds with useful efficiencies from silacyclobutanes that bear alkyl, vinyl, ethynyl, silyl, and phenyl substituents at silicon, and possess UV absorption maxima between 190 and 250 nm (2,4,12,13,(17)(18)(19)(20).…”