Silicon hydrides in organic synthesis

In the past few years we have developed an array of composite reducing systems based on combinations of tin or silicon hydrides with various transition-metal homogeneous catalysts such as palladium(O), ruthenium(I1) and molybdenum(0) complexes. These represent a useful family of reducing media that effect highly selective reductions of various organic functional groups, such as allylic heterosubstituents, or,/3-unsaturated carbonyl compounds and or-halo ketones. More recently, we have discovered that diiodosilane (DIS), which has never been used previously in organic synthesis, transforms a broad variety of useful functional-groups under mild conditions. This new reagent was found to exhibit properties and reactivities that are, in some cases, either superior or complementary to those of the well-known iodotrimethylsilane. In addition t o its ability t o cleave ethers, alcohols, ketals, acetals, or-haloketones, etc., this new reagent may also serve as a unique. reducing agent. As a multipurpose reagent, DIS allows sequential cleavage, reduction and iodination of oxygen functionalities, all of which are carried out by a single reagent. COMPOSITE REDUCING SYSTEMS Despite the bewildering variety of reducing agents available for synthetic tasks, new, and selectivity-en'hanced reductants are still being sought. Most popular of selective reducing agents are the various metal hydrides, mainly those of boron and aluminum, an abundance of which have been developed over the past four decades. However, the hydridic nature of most of these group-13 and other metal hydrides can limit their usefulness when high chemoselectivity is required. Several years ago, we initiated a research program aimed at designing a new family of reducing systems that selectively transfer a hydride ion to various electrophilic functional groups. We anticipated that one promising approach would utilize systems comprised of a t least two components, i.e. a relatively inactive source of hydride entities and a transfer agent to deliver the hydride selectively from that donor to the target functionality. Group-14 metal hydrides, especially those of silicon and tin, represent a satisfactory choice of nonreactive hydride donors, as in the absence of a catalyst they are, generally, poor reducing agents. Moreover, transition-metal complexes are attractive transfer agents because they readily insert into Si-H or Sn-H bonds and also bind specifically to various functional groups. Such multiple-component reducing systems could offer high flexibility because they involve a large number of independent variables that can be tailored to various synthetic tasks, especially when compared to single-reagent metal-hydride reductions. Thus, by appropriate modification of the hydridosilane, judicious selection of a transition-metal transfer agent, and in some cases, use of a cocatalyst, opportunities arise for creating a wide variety of reducing systems that exhibit improved chemoselectivity, a s well as regioand stereocontrol.