Radical Activation of SiH Bonds by Organozinc and Silylzinc Reagents: Synthesis of Geminal Dizinciosilanes and Zinciolithiosilanes

Abstract

Si‐H‐Bindungen können durch Organozink‐ und Silylzinkverbindungen in Gegenwart von Spuren von Radikalstartern wie tBu2Hg oder AIBN aktiviert werden. Dabei entstehen in guten Ausbeuten Zinkiosilane (siehe Schema). Werden Dihydridosilane eingesetzt, erhält man geminale Dizinkiosilane. Auch Zinkio‐verbrückte Disilyllithiumverbindungen mit drei (1) und vier Metall‐Silicium‐Bindungen wurden synthetisiert.

Radical Germylzincation of α-Heteroatom-Substituted Alkynes

“…The presence of TEMPO also thwarted completely the germylzincation (Scheme 6, c): no conversion was observed if amounts as low as 10 mol % were added at the beginning and only 26% of E-2 was formed upon addition after 5 min of reaction. While these experiments lend clear evidence for the intervention of radicals in the mechanism, we could not at this point exclude a scenario wherein Et 2 Zn would react with R 3 GeH by a radical alkylzinc group-transfer process (reminiscent of that involved in the formation of [(Me 3 Si) 3 Si] 2 Zn from (Me 3 Si) 3 SiH and Et 2 Zn) 48 and provide a putative germanium−zinc intermediate IX (Scheme 6, d) that would add across the C−C triple bond of the ynamide through a polar mechanism. Notably, the reported formation of (Ph 3 Ge) 2 Zn by reaction between Et 2 Zn and Ph 3 GeH in THF required to exclude this possibility.…”

Thirty Years of (TMS)3SiH: A Milestone in Radical-Based Synthetic Chemistry

“…TTMSS reacts with Et 2 Zn at 60 °C to provide the bissilylzinc reagent 148 in only 30% yield (Scheme 45). 126 Addition of a small amount of AIBN or t-Bu 2 Hg (0.03 equiv) increased the yield to 90%, indicating that the reaction likely proceeds through a radical pathway. Although oxygen is known to initiate efficiently reactions with Et 2 Zn, it had to be excluded from these reaction media, the silyl radical also reacting with oxygen.…”

“…The mechanism was shown to proceed through a radical pathway as illustrated in Scheme 61. On the basis of pioneering studies by Apeloig et al, 126 it was proposed that reaction of R 2 Zn with oxygen would lead to the generation of an alkyl radical R • , which abstracts hydrogen from TTMSS to form the (TMS) 3 Si • radical precursor. The latter would then add selectively on the β-carbon of the ynamide, affording a vinyl radical I, which has two options: it may be reduced by TTMSS to form vinylsilane II or react with R 2 Zn through a homolytic substitution (S H 2) process at zinc to form zinc-chelated species III, which leads to reduction under acidic conditions, and the R • radical that sustains the chain.…”

Radical Silyl‐ and Germylzincation of Propargylic Alcohols

“…A Schlenk tube was charged with the appropriate terminal propargylic alcohol (0.25 mmol) and a suspension of [(Me 3 Si) 3 Si] 2 Zn [40] (308 mg, 0.55 mmol) in hexane (2.0 mL) was added at −30 °C, followed by Et 2 Zn (1.0 m in hexane, 0.28 mL, 0.28 mmol). The turbid mixture was stirred at this temperature for the given time.…”