From intramolecularly [4+1]- and [4+2]-coordinated tri- and tetraorganosilanes to hypercoordinated benzoxasilaphospholes

The synthesis of the intramolecularly coordinated triorganosilane {4-t-Bu-2 ,6-[P(O)(OEt)(2)](2)C6H2}Si(H)Ph-2 (3) is reported. The reaction of both 3 and the corresponding tetraorganosilane 4-t-Bu-2,6-[P(O)(OEt)(2)](2)C6H2SiP h3 (2) with [Ph3C](+)[PF6-] results in the in situ generation of the silico nium ion {4-t-Bu-2,6-[P(O)(OEt)(2)](2)C6H2}SiPh2+PF6- (4). The siliconium i on of 4 reacts with water under intramolecular cyclization to give the nove l hypercoordinated benzoxasilaphosphole [1(P),3(Si)-P(O)(OEt)OSiPh2-6-t-Bu- 4-P(O)(OEt)(2)]C6H2 (5). Compound 5 and the related methyl/phenyl- and dime thyl-substituted derivatives [1(P),3(Si)-P(O)(OEt)OSiRR'-6-t-Bu-4-P(O)(OEt) (2)]C6H2 (6, R = Me, R' = Ph; 7, R = R' = Me) are also prepared by reaction of {4-t-Bu-2,6-[P(O)(OEt)(2)](2)C6H2}Li (1) with the corresponding dichlor odiorganos ilanes. Compound 6 is a mixture of diastereomers from which the 1S*,3S* diastereomer 6a was separated. In solution, the latter undergoes an acid-catalyzed epimerization which was monitored by P-31 NMR spectroscopy. The mechanism proposed to account for this process is supported by ab init io MO calculations. The molecular structures of 3, 5, and 6a were determine d by single-crystal X-ray diffraction.