Mechanistic Studies on the Cyclization of Organosilicon and Organotin Compounds Containing the O,C,O-Coordinating Pincer-Type Ligand {4-<i>t</i>-Bu-2,6-[P(O)(OR)₂]₂C₆H₂}^- (R = <i>i</i>-Pr, Et):  Phosphorus (<i>P</i>OC)- versus Carbon (PO<i>C</i>)-Attack

Abstract

The intramolecularly coordinated triorganotin hexafluorophosphate {4- t -Bu-2,6-[P(O)(O i -Pr) 2 ] 2 C 6 H 2 }SnPh 2 + PF 6 - ( 3a ) was prepared by reaction of the [4+2]-coordinated tetraorganotin compound {4- t -Bu-2,6-[P(O)(O i -Pr) 2 ] 2 C 6 H 2 }SnPh 3 ( 2a ) with Ph 3 C + PF 6 - 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)-SnPh 2 OP(O)(O i -Pr)-6- t -Bu-4-P(O)(O i -Pr) 2 ]C 6 H 2 ( 4a ). Analoguously, the in situ-generated intramolecularly coordinated triorganosiliconium hexafluorophosphate {4- t -Bu-2,6-[P(O)(OEt) 2 ] 2 C 6 H 2 }SiPh 2 + PF 6 - ( 6 ) reacts with water to give the corresponding intramolecularly coordinated benzoxaphosphasilole [1(P),3(Si)-P(O)(OEt)OSiPh 2 -6- t -Bu-4-P(O)(OEt) 2 ]C 6 H 2 ( 7 ). Isotope-labeling experiments with H 2 18 O 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 PO C -carbon. In contrast, two pathways account for the formation of the benzoxaphosphasilole 7, that is, attack of water at the PO C -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.