Silicon(IV) reagents

Abstract Organosilicon compounds are one of the most widely and frequently used organometallics in organic synthesis. They are easy to handle and synthesize due to their thermal and aerial stability, and exhibit a variety of reactivities derived from the electropositive nature of silicon as well as the steric and electronic effects of the silyl group.1-2 Silicon reagents play important roles as not only protecting agents and masked nucleophiles but also Lewis acids. The Lewis acidity of alkyl- and alkoxysilanes is much weaker than that of the corresponding group III metal compounds. However, when one of the four ligands on silicon is changed to a soft Lewis base such as triflate (OTf) or iodide (I), the acidity becomes strong enough for synthetic use as Lewis acid catalyst.3-8 Trimethylsilyl trifluoromethanesulfonate (TMSOTf) and iodotrimethylsilane (TMSI), bearing one Lewis-acidic co-ordination site, interact strongly to various heteroatoms, particularly oxygen, to activate the carbonheteroatom bond. Although it is also known that five co-ordinated silicates and angle strained silanes have moderate Lewis acidity, these types of Lewis acids are only utilized to intramolecularly assist the addition of silylated nucleophiles as shown in the reactions of allylsilicates9.1° and enoxysilacyclobutane.11 The synthetic reactions mediated by TMSOTf3-5 and TMSI6-8 have been extensively developed in the last two decades.