Mechanistic studies on the Cyclization of Organosilicon and Organotin compounds containing the O,C,O-Coordinating pincer-type ligand {4-t-Bu-2,6-[P(O)(OR)2]2C6H2}- (R = i-Pr, Et):: Phosphorus (POC)- versus carbon (POC)-attack

The intramolecularly coordinated triorganotin hexafluorophosphate {4-t-Bu-2,6-[P(O)(Oi-Pr)(2)](2)C6H2}SnPh2+PF6- (3a) was prepared by reaction of the [4+2]-coordinated tetraorganotin compound {4-t-Bu-2,6-[P(O)(Oi-Pr)(2)](2)C6H2}SnPh3 (2a) with Ph3C+PF6- and shown to react under intramolecular cyclization with bromide and fluoride ion, respectively, as well as with water to give the intramolecularly coordinated benzoxaphosphastannole [1(P),3(Sn)-SnPh2-OP(O)(Oi-Pr)-6-t-Bu-4-P(O)(Oi-Pr)(2)]C6H2 (4a). Analoguously, the in situ-generated intramolecularly coordinated triorganosiliconium hexafluorophosphate {4-t-Bu-2,6-[P(O)(OEt)(2)](2)C6H2}SiPh2+PF6- (6) reacts with water to give the corresponding intramolecularly coordinated benzoxaphosphasilole [1(P),3(Si)-P(O)(OEt)OSiPh2-6-t-Bu-4-P(O)(OEt)(2)]C6H2 (7). Isotope-labeling experiments with (H2O)-O-18 in combination with electrospray mass spectrometric studies reveal that in the case of the organotin compound 3a the cyclization exclusively proceeds via nucleophilic attack of water at the POC-carbon. In contrast, two pathways account for the formation of the benzoxaphosphasilole 7, that is, attack of water at the POC-carbon as well as at phosphorus. The latter pathway either is in contrast to the axial entry-axial departure principle of nucleophilic substitution at phosphorus or indicates Berry pseudorotation involving a five-membered chelate ring. The molecular structure of 3a was determined by single-crystal X-ray diffraction analysis.