Silicon−Carbon Unsaturated Compounds. 60. Reactions of Lithium Silenolates with Dienes

Abstract

The chemical properties of lithium silenolates toward dienes were studied. The reaction of lithium 2-tert-butyl-1,1-bis(trimethylsilyl)silen-2-olate (1a) with butadiene at −40 °C, followed by hydrolysis, afforded 2-tert-butyl-1,1-bis(trimethylsilyl)silacyclohex-4-en-2-ol, derived from [2 + 4] cycloaddition of 1a with butadiene, in 94% yield. Similar treatment of lithium 2-adamantyl-1,1-bis(trimethylsilyl)silen-2-olate (1b) gave the respective adduct in 87% yield. The reactions of lithium silenolates 1a,b with 2,3-dimethylbutadiene, isoprene, and 1,3-pentadiene proceeded in a fashion similar to those with butadiene to give the [2 + 4] cycloadducts in high yields. The reactions of 1a,b with 2,3-dimethylbutadiene and butadiene at room temperature gave the products originated from 1,1-bis(trimethylsilyl)silacyclohexa-1,4-diene intermediates. The crystal structure of 10-adamantyl-2,3,6,7-tetramethyl-9-(trimethylsilyl)-9-silabicyclo[4.4.0]deca-2,6-diene (6b), which was obtained from the reaction of 1b with 2,3-dimethylbutadiene at room temperature, was determined by a single-crystal X-ray diffraction study.

Silenes: Connectors between classical alkenes and nonclassical heavy alkenes

“…Finally, lithium and potassium silenolates are species that are nearly completely described by the reverse polarized resonance structure with the negative charge at Si (Scheme 13). Ohshita and co-workers found that the lithium silenolates 31 gave exclusively [4 + 2] cycloadducts with 2,3-dimethylbutadiene (Scheme 35) [140].…”

“…lithium silenolate was reacted with isoprene, only regioisomer 116 shown in Scheme 38 was detected [140].…”

“…The opposite regiochemistry in the reactions of the Brook-type silenes with isoprene was considered to originate from both steric hindrance in the transition states leading to the two products, as well as to frontier orbital interactions because the preference for the "para" product in reactions of Brook-type silenes could be explained by the large coefficients of both HOMO and LUMO on Si [138]. The lithium silenolates are even more reverse polarized than Brook's silenes, and now the regiochemistry of the [4 + 2] cycloaddition between these substrates and isoprene was completely in favor of the "para" product 116 (Scheme 38) [140].…”

Silylenes, Silenes, and Disilenes: Novel Silicon‐Based Reagents for Organic Synthesis?

“…Alkenes, alkynes and ketones/ imines undergo a [2 + 2] cycloaddition reaction whilst dienes and a,b-unsaturated carbonyl compounds normally follow a [4 + 2] pathway. (7)] [30,51] whilst the Brook-type siloxysilene 27 undergoes preferential [2 + 2] cycloaddition with the highly electron deficient diene, dimethylhexadienedioate [Eq. For example, highly reversed polarised silenes and silenolates undergo exclusive [4 + 2] cycloadditions [see Eq.…”

“…[63][64][65][66] which relates the structural features to the sum of the singlet-triplet energy gaps of the two interacting silylenes (AEDE ST ). [27,51,58] Furthermore, recent X-ray structural investigation of Ottosson revealed that silenolates are strongly influenced by the resonance structure with negative charge placed at Si. Indeed, disilenes are on the borderline between classical planar and nonclassical nonplanar structures and small structural changes, for example, rotation of the mesityl substituents in tetramesityldisilene, enforce a change in the Si=Si bond.…”