Radical activation of Si-H bonds by organozinc and silylzinc reagents: synthesis of geminal dizinciosilanes and zinciolithiosilanes.

Organozinc compounds are very useful reagents in synthesis, and in analogy to organomagnesium and organolithium compounds are used mainly as nucleophiles. Some reports are also available for silylzinc compounds. At the same time, as Zn and Hg are both Group 12 elements, organozinc compounds are expected also to exhibit some of the characteristic reactions of organomercury compounds. For example, organomercury and silylmercury compounds undergo radical reactions both thermally and photochemically and activate Si H bonds, leading to silylmercury compounds. Such compounds are also excellent precursors for the preparation of silyl anions by transmetalation. The possibility to replace in these reactions the highly toxic organomercury compounds by the less toxic organozinc compounds is attractive. The radical chemistry of organozinc compounds, initiated by dioxygen, is a vigorously developing field of organic chemistry with many applications in synthesis. In contrast, to the best of our knowledge, there are no reports on radical reactions of silylzinc compounds. Activation of Si H bonds is a very useful reaction synthetically and therefore it is intensively studied, both in academia and in industry. There are two common strategies for Si H bond activation: by radical initiators [7] and by transition-metal catalysts. Silylzinc reagents became recently popular synthons in organic chemistry owing to their selective and mild nucleophilic nature. Therefore, a single-pot reaction leading to zinciosilanes by activation of Si H bonds has a great synthetic potential. Furthermore, in analogy to the synthesis of bismercuriosilanes, activation of dihydridosilanes by diorganozinc or disilylzinc compounds can lead, in a single-step reaction, to new geminal dizinciosilanes. These interesting dimetallic reagents are potentially very attractive reagents, in a similar fashion to geminal dizinciomethanes, their organic analogues. Herein, we present the first examples of radical activation of a Si H bond in R3SiH (R=Me3Si, Me3SiMe2Si) and Me3SiMe2SiH by R’2Zn (R’= tBu, Et) and by (tBuMe2Si)2Zn (1) (for experimental details see Supporting Information). Using this reaction we synthesized the silylenoid-type compound [Cl(tBuMe2Si)2Si]2Zn (2), which upon lithiation yields the zinc-bridged bis(silyllithium) [(thf)2Li(tBuMe2Si)2Si]2Zn (3), the first known compound with lithium and zinc bonded to the same Group 14 atom. We have also synthesized and fully characterized the novel tetrametallic bis(zinciosilyllithium)silane [(thf)3Li(tBuMe2Si)2SiZn]2Si(SiMe2tBu)2 (4). The reactions of Et2Zn, which is commercially available, and of tBu2Zn with tBuMe2SiH (5), Me3SiMe2SiH (6), and (Me3Si)3SiH (7) were first studied. The reaction of Et2Zn or of tBu2Zn with 5 was disappointing; 5 remained unchanged after 3 h at 90 8C, even in the presence of azobisisobutyronitrile (AIBN) or tBu2Hg as radical initiators. However, this situation changes upon silyl substitution of the silane. Thus, reaction of Me3SiMe2SiH (6) with tBu2Zn for 3 h at 90 8C yielded (Me3SiMe2Si)2Zn (8) in 30% yield. When a minute amount of tBu2Hg, a radical initiator, was added the yield of 8 increased to 95%. In contrast, 6 was recovered unreacted after reaction with neat Et2Zn and yielded only 10% of 8 in the presence of tBu2Hg at 90 8C for 3 h (Scheme 1).