Does a chiral alcohol really racemize when its OH group is protected with Boyer's reaction?

Abstract

Chiral reactants have been employed for assessing the real stereochemistry of the BiBr3-catalyzed synthesis of benzylic ethers, a very useful reaction for protecting alcoholic groups. The results of this investigation are in clear contrast with the conclusions of previous studies (Boyer et al., Tetrahedron 2001;57:1917-1921). Indeed, chiral GC-MS analysis of the ethereal products gives unequivocal evidence of the complete racemization of the benzylic moiety and the complete retention of configuration of the protected alcoholic substrate. Such findings make BiBr3 a powerful and stereochemically preservative catalyst for benzylation of chiral alcohols, and a potential candidate for orthogonal protecting group strategies applicable to polyhydroxy compounds.

The influence of mineral catalysts on racemization of secondary alcohols under pyrolytic temperatures

“…6 summarizes the relative yields of ethers under the influence of different solvents at 500 • C. The mechanism proposed is confirmed by the stereochemistry of the secondary products formed. Indeed, as diphenylbutanes are formed in racemic mixture they could be formed from an achiral intermediate (11), whereas the complete racemization of the -PE moiety and the complete retention of configuration of the 1-PE radical in the formation of ethers take place (see [23]). …”

Applications of bismuth(iii) compounds in organic synthesis

“…However, recently Filippi and co-workers report that contrary to the observations by Boyer and co-workers, the benzylic moiety is completely racemized and there is complete retention of configuration of the protected alcohol substrate. 358 These observations make BiBr 3 a useful catalyst for benzylation of chiral alcohols.…”