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]2C6H2}SnPh2+PF6- (3a) was prepared by reaction of the [4+2]-coordinated tetraorganotin compound {4-t-Bu-2,6-[P(O)(Oi-Pr)2]2C6H2}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)-SnPh2OP(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]2C6H2}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 H218O 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.