Head-to-Head versus Head-to-Tail Dimerizations of Transient Silenes – The Solvent-Dependent Regiospecifity of the Dimerization of 2-(2-Methoxyphenyl)-1,1-bis(trimethylsilyl)silene

Abstract

The regiospecifity of the dimerization of the transient 2‐(2‐methoxyphenyl)‐1,1‐bis(trimethylsilyl)silene (6), synthesized by base‐initiated trimethylsilanolate elimination from (2‐methoxyphenyl)[tris(trimethylsilyl)silyl]methanol (5) according to a modified Peterson mechanism, decisively depends on the donating ability of the solvent in which the silene is generated. In ether, 6 undergoes a formal [2 + 2] dimerization to afford 3,4‐bis(2‐methoxyphenyl)‐1,1,2,2‐tetrakis(trimethylsilyl)‐1,2‐disilacyclobutane (12), whereas in toluene the head‐to‐tail [2 + 2] cyclodimer, 2,4‐bis(2‐methoxyphenyl)‐1,1,3,3‐tetrakis(trimethylsilyl)‐1,3‐disilacyclobutane (13) was obtained. 1,1‐Bis(trimethylsilyl)‐2‐(2,4,6‐trimethoxyphenyl)silene (18), similarly made by a modified Peterson reaction, under the same conditions reacted with the eliminated trimethylsilanolate with readdition at the polar Si=C bond to give, after hydrolysis, 2‐(2,4,6‐trimethoxybenzyl)‐1,1,1,3,3,3‐hexamethyl‐2‐(trimethylsiloxy)trisilane (19). A possible mechanism explaining the solvent‐dependent regiospecificity of the dimerization of 6 is discussed. The structures of 12 and 13 were determined on the basis of NMR and MS data as well as X‐ray structural analyses.

From a Transient 3-Germa-1-phosphabutadiene to a 1,2-Bis(phosphaalkenyl)-1,2-digermacyclobutane

“…The head-to-head dimerization is a less common mode of dimerization . The same behavior was observed for some silenes >SiC<, , but until now only head-to-tail dimerization has been observed for stannenes >SnC< …”

“…In the dimerization of silenes involving the preliminary formation of a Si−Si bond, various types of dimers (head-to-head, disilanaphthalene, or linear) are obtained, depending on the substituents on the silene carbon …”

Addition of Organometallic Reagents to a Stable Silene and Germene

“…The bond polarity of the SiC bond is the most important factor governing the reactivity of the silene and plays a significant role in the regioselectivity of the addition of water and alcohols. For silenes of natural polarity, such as silene 1 or Me 2 SiCPh­(CH 2 SiMe 3 ) studied by Ishikawa, or even silenes of reduced polarity, such as the bis­(silyl)-substituted silenes studied by Oehme, − the regiochemistry might be expected. On the other hand, the complete regioselectivity of the addition of organometallic reagents to Brook silenes ((Me 3 Si) 2 SiC­(OSiMe 3 )­R), which have reversed polarity, is, at first, surprising.…”

“…Mes 2 Si(Me)CH 2 CH 2 t-Bu ( 9): 1 H NMR (C 6 D 6 ) δ 0.74 (s, 3 H, SiCH 3 ), 0.82 (s, 9 H, C(CH 3 ) 3 ), 1. 25 (15): IR (cm −1 ) 793 (m), 847 (m), 1285 (m), 1449 (m), 1605 (m), 2978 (s); 1 H NMR (C 6 D 6 ) δ 0.76 (s, 3 H, SiCH 3 ), 2.10 (s, 6 H, Mes p-CH 3 ), 2.29 (s, 12 H, Mes o-CH 3 ), 5.63 (dd, J = 5, 20 Hz, 1 H, SiCHCH 2 ), 5.86 (dd, J = 4, 14 Hz, 1 H, SiCHCH 2 ), 6.68 (dd, J = 5, 20 Hz, 1 H, SiCHCH 2 ), 6.70 (s, 4 H, Mes-H); 13 In the addition of MeLi to 4, a viscous, colorless oil, identified as a mixture of 12 and 16, was obtained in a ratio of 1:0.03 (10 mg, 92% yield).…”

“… The most extensive study of the addition of organometallic reagents to silenes to date was reported by Oehme. His group studied the addition of organometallic reagents to transient 1,1-bis­(trimethylsilyl)­silenes ((Me 3 Si) 2 SiCR 2 ), generated by Petersen elimination of the corresponding carbinols upon treatment with organolithium reagents. − In most cases, the excess organolithium reagent or the released lithium silanolate adds regioselectively to the silicon of the transient silene to give 1,2-addition products in moderate to good yields after hydrolysis (Scheme ), although, on occasion, migration of the silyl substituents was observed . Only a few studies of the addition of organometallic reagents to stable, naturally polarized silenes have been reported.…”