Technical Library


In addition to the ability to moderate the reactivity of the organosilane groups both sterically and electronically, another distinct advantage of the organosilanes is the ability to take advantage of the tetravalent nature of the silicon atom to employ multiple reactivities in the same reagent. This has successfully been used in a templating fashion wherein the silyl group is attached to an alcohol substituent and then the silyl group is intramolecularly reacted with a second functional group in the molecule. The sequential silylation/hydrosilylation of suitably unsaturated alcohols can lead to oxa-silaheterocycles, which can, in turn, be converted to various organic systems via functionalization of the resulting silicon-carbon bond. The silylation-hydrosilylation-oxidation sequence shown in Scheme 1 is a good example.52 A similar sequence was conducted in an enantioselective manner.53 In the case of allyl amines intramolecular ring closure occurs to give the four-membered ring structure oxidation of which provides the b-aminoethanol derivative.54 The stereoselectivity can be impressive (Scheme 2).

Scheme 1: Synthesis of 1,3-Diketones from Homoallylic Alcohols

Scheme 2: Diastereoselective Synthesis of 1,2-Aminoalcohols

Denmark and Yang established a vinylsilane butenyl ether geometry to carry out a ring-closing-metathesis reaction that set up the alkoxysilane 6 for an intramolecular cross-coupling step to prepare 7 in a synthesis of (+)-brasilenyne (Scheme 3).55

Scheme 3: Synthesis of (+)-Brasilenyne Intermediate by Intramolecular Metathesis

In a somewhat different use of a silyl protecting group for the introduction of organic functionality aryloxydi-tertbutylsilanols were shown to direct the ortho-vinylation of the aryl group via a C-H vinylation process. This provides a combination of phenol protection, directing effect and cross-coupling as shown in Eq. 27. In addition other silanol directing chemistries are demonstrated in this work, as, for example, that shown in Eq. 28.56

In a clever application of a silicon-directed cross-coupling aryloxy-substituted o-bromoarylsilanes are intramolecularly cross-coupled to form an oxa-silabiphenyl ring system, which can be further modified including desilylation and oxidation (Eqs. 29 – 31). The conversion works well with aminofunctional system to form the aza-silabiphenyl ring system (Eq. 32).57

Simmons and Hartwig have utilized an iridium-catalyzed dehydrogenative silylation-C-H –activation sequence to prepare oxasilacyclopentanes, which can be oxidized to 1,3-diols. The C-H activation occurs at the g-position from the siloxy group (Eqs. 33 & 34).58