Silicon−Carbon Unsaturated Compounds. 55. Synthesis and Reactions of Lithium Silenolates, Silicon Analogs of Lithium Enolates

Abstract

Chemical behavior of acylpolysilanes, (Me3Si)3SiCOR (1a, R = Mes; 1b, R = o-Tol; 1c, R = Ad; 1d, R = t-Bu; 1g, R = Ph; 1h, R = Me), toward silyllithium reagents has been studied. Reactions of 1a−d with [tris(trimethylsilyl)silyl]lithium gave the corresponding lithium silenolates, (Me3Si)2SiCROLi (5a, R = Mes; 5b, R = o-Tol; 5c, R = Ad; 5d, R = t-Bu), in high yield by removal of a trimethylsilyl group from the acylpolysilane. However, a similar reaction of 1g gave an unstable lithium silenolate, which undergoes dimerization and then further reactions under the conditions used. Acetylpolysilane 1h did not afford the lithium silenolate but gave lithium [tris(trimethylsilyl)silyl]ethenolate. Lithium silenolate 5a was produced also by treating 1a with (dimethylphenylsilyl)lithium. The reactions of 1b,c with (dimethylphenylsilyl)lithium proceed in a different fashion from that of 1a to afford products arising from addition of the silyllithium to the carbonyl bond as major products. Lithium silenolates 5a,c,d are stable at low temperature in THF solution and can be characterized by NMR spectroscopy. Silenolates 5a−d reacted with alkyl halides to afford Si-alkylated products in high yield. Treatment of 5a,b with chlorotriethylsilane led to the formation of silenes arising from O-silylation of the lithium silenolates, almost quantitatively, while 5c,d gave the corresponding acylpolysilanes by Si-silylation in high yield. Results of theoretical studies which were carried out using the reaction of (H3Si)3SiCOCH3 with H3SiLi as a model also are described.

Silenes: Connectors between classical alkenes and nonclassical heavy alkenes

“…The 13 C shift of 268.3 ppm revealed a very deshielded central C atom, and the 29 Si shift of −64.5 ppm for the central Si atom indicated that is was very shielded. This 29 Si shift is similar to that of Kira's 4-silatriafulvene (−71.9 ppm [58]) and those of the silenolates prepared by Ohshita, Ishikawa, and Ottosson (−79 to −60 ppm [72,73]). …”

“…These species have been extensively investigated by the groups of Bravo-Zhivotovskii, Apeloig, Ishikawa, Ohshita, and Ottosson. In 1996, Ishikawa and Ohshita reported the synthesis of four lithium silenolates (Me 3 Si) 2 Si CROLi with R = Mes (32a), o-Tol (32b), 1-Ad (32c), and tBu (32d), together with detailed NMR spectroscopic studies of 32a, 32c and 32d [72]. The 29 Si NMR spectrum of 32a indicated that its central Si atom was sp 2 hybridized.…”

“…In the thermolytic formation of the transient 2-amino-2-siloxysilenes (75a-75c, Scheme 22) very little dimer formation was observed [8], and potassium silenolates revealed no tendency for dimerization [73]. Formation of dimers from lithium silenolates was suggested but not proven [72,121].…”

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

“…[22] Increased stability is also realised with silenes influenced by reversed Si=C bond polarisation, that is, an Si dÀ =C d + instead of Si d + =C dÀ polarisation, effected by p-donor substituents on the carbon atom. [28,29] In particular, Ottosson has exemplified this concept in a recent report describing an isolable silenolate 10 a although, having a very long Si À C bond (1.926 ), this is probably better described by the alternative resonance structure 10 b. [25,26] Whilst the 2-siloxysilenes of Brook show this reversed Si=C bond polarisation effect, other examples are seen with the silatriafulvene 9 [27] and silenolates.…”

Heavy Analogs of Alkenes, 1,3‐Dienes, Allenes and Alkynes: Multiply Bonded Derivatives of Si, Ge, Sn and Pb

“…138,139 Given the full negative charge at the oxygen atom of 2-silenolates A, one can expect a significant contribution from the resonance form B featuring a negative charge at the silicon center adjacent to the carbonyl group (Scheme 5.47). 138,139 Given the full negative charge at the oxygen atom of 2-silenolates A, one can expect a significant contribution from the resonance form B featuring a negative charge at the silicon center adjacent to the carbonyl group (Scheme 5.47).…”