PATTERNS OF THERMAL REARRANGEMENTS INVOLVING 3RD ROW ELEMENTS

Abstract

Abstract The thio-Claisen rearrangement of allyl aryl sulfides was found to be catalyzed by nucleophiles exerting only a small fraction of their nucleophilic abilities. A mechanism of nucleophilic triggering of concerted sigmatropic rearrangement was proposed and successfully tested against the rival proposal of cyclization-induced rearrangement on the basis of substituent-rate and secondary D-isotope effects. On the basis of the evidence presented the thioallylic rearrangement of allyl aryl sulfides under light-free, anaerobic conditions was shown to be a 2-step cyclic process in which a zwitterionic thiacyclobutane intermediate intervenes, involving octet expansion of the central sulfur. This is transformed to its rearranged isomer via permutational isomerism. By contrast, the corresponding silaallylic rearrangement, however, follows a concerted pericyclic pathway in which a 3p orbital of silicon bridges the allylic framework with complete inversion of configuration and unattended by any racemization. The rearrangement of β-silylketones to siloxyalkenes, however, shows an entirely different pattern one quite similar to the thioallylic arrangement in which silicon experiences octet expansion through formation of an intramolecular, apical Si[dbnd]O bond in the structure of a trigonal bipyramid intermediate. The occurrence of permutational isomerism in this TBP intermediate is confirmed by the observation of retention and racemization of the configuration of an initially chiral silicon reactant.