Abstract
Borane complexes of trivalent phosphorus compounds have attracted a continuously growing interest over the past four decades. Their remarkable air and configurational stability combined with a relative easiness of fully stereoretentive removal of the protecting borane moiety quickly made them reagents of choice for the elaboration of phosphine structures especially suited for the preparation of chiral phosphine ligands. Borane complexes of primary, secondary, and tertiary phosphines, phosphinites, phosphonites, and phosphites can be readily obtained by directly reacting the corresponding trivalent phosphorus compound with a convenient borane donor like BH3-THF, BH3-SMe2 etc. Similarly, straightforward access to borane complexes of free trivalent phosphorus acids has for a long time been stymied by the fact that these acids exist predominantly in the form of their tetracoordinate tautomers, which are unable to take part in the direct complexation with BH3. As a consequence, the known examples of borane complexes of trivalent phosphorus acids have been very few in number, and all have been obtained by the indirect routes. With the possible exception of borane-modified nucleotides, the properties and chemistry of borane complexes of trivalent phosphorus acids have thus remained practically unexplored. This has been especially true for phosphinous acids-boranes, for which, until very recently, only two fully identified examples could be found in the literature. Recent developments of general methods for synthesis of such compounds have made them now readily available in a great structural variety and in the nonracemic forms as well, and allowed for a systematic study of their chemistry and synthetic utility. The purpose of this account is to describe these new results and to demonstrate the utility of phosphinous acid-boranes as an emerging class of useful synthetic intermediates. Details of the studies of the reactions of secondary phosphine oxides with borane reagents that were rudimentary to the development of general synthetic routes to phosphinous acid-boranes will also be presented.
Keywords: Borane reagent, Trimethylsilyl esters, halogenating agents, crystallization, transformations, Decomplexation
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