Development of New Synthetic Reactions Using Organosilicon Compounds

大多数有机硅试剂具有三甲基甲硅烷基（TMS）基团或更大体积的三有机甲硅烷基基团。具有较小体积的甲硅烷基的硅试剂的合成效用仍然未被探索。我们已经发现，二甲基甲硅烷基（DMS）保护的亲核试剂比TMS保护的亲核试剂反应性更强。例如，在CaCl_2存在下，酮的DMS烯醇化物与醛在DMF中于30℃下顺利加成，高产率地得到醛醇。在相同条件下，相应的TMS烯醇化物不与醛反应。在DMS烯醇化物反应中，CaCl_2作为刘易斯碱，通过氯离子对硅的亲核进攻来活化烯醇化物。DMS烯醇化物可用于N-甲苯磺酰亚胺的曼尼希型反应和α-烯酮的迈克尔反应。类似地，α-DMS-酯比相应的基于TMS的试剂更具反应性。在金属氯化物（LiCl、MgCl_2等）存在下，一般来说，氢化硅烷不与碳亲电试剂发生自发反应，但它们的还原能力可以通过底物或它们自身的活化来获得。选择合适的活化剂可以对还原过程进行精细控制。在In（OAc_3）存在下，PhSiH_3与卤代烷反应生成脱卤代烷。这种还原将涉及由氢化铟物质介导的自由基链。PhSiH_3-In（OAc_3）体系适用于卤代烷与缺电子烯烃的分子间自由基加成反应。此外，它对α-烯酮与醛的还原Aldol反应也有价值。在这种情况下，一个合理的机制由离子1，4-还原α-烯酮与氢化铟物种和羟醛缩合反应的所得铟烯醇化物。

Most of organosilicon reagents have a trimethylsilyl (TMS) group or a more bulky triorganosilyl group. Synthetic utility of silicon reagents bearing a less bulky silyl group has remained unexplored. We have found that dimethylsilyl (DMS)-protected nucleophiles are more reactive than TMS-protected nucleophiles. For example, in the presence of CaCl_2, ketone DMS enolates added smoothly to aldehydes at 30℃ in DMF to give aldols in high yield. Under the same conditions, the corresponding TMS enolates did not react with aldehydes at all. In the reaction of DMS enolates, CaCl_2 would serve as Lewis base to activate the enolates by nucleophilic attack of the silicon by the chloride ion. DMS enolates are useful for the Mannich-type reaction of N-tosylimines and the Michael reaction of a-enones. Similarly, α-DMS-esters are more reactive than the corresponding TMS-based reagents. In the presence of metal chlorides (LiCl, MgCl_2, etc.), aldehydes and ketones underwent efficient aldol reactions of α-DMS-esters.In general, hydrosilanes do not react spontaneously with carbon electrophiles ; however, their reducing ability can be drawn by activation of the substrates or themselves. A proper choice of activator enables fine control of the reduction process. We have found that PhSiH_3 reacts with haloalkanes in the presence of In(OAc_3) to form the dehalogenated alkanes. This reduction would involve a radical chain mediated by indium hydride species. The PhSiH_3-In(OAc_3) system is applicable to intermolecular radical addition of haloalkanes to electron-deficient alkenes. Additionally, it is valuable also for reductive aldol reaction of a-enones with aldehydes. In this case, a plausible mechanism consists of ionic 1,4-reduction of a-enones with indium hydride species and aldol reaction of the resultant indium enolates.