Stereoselective Synthesis
of <i>o</i>-Bromo
(or Iodo)aryl P-Chirogenic Phosphines Based on Aryne Chemistry
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Abstract
The efficient synthesis of chiral or achiral tertiary
phosphines
bearing an <i>o</i>-bromo (or iodo)aryl substituent is described.
The key step of this synthesis is based on the reaction of a secondary
phosphine borane with the 1,2-dibromo (or diiodo)arene, owing to the
formation in situ of an aryne species in the presence of <i>n</i>-butyllithium. When P-chirogenic secondary phosphine boranes were
used, the corresponding <i>o</i>-halogeno-arylphosphine
boranes were obtained without racemization in moderate to good yields
and with ee up to 99%. The stereochemistry of the reaction, with complete
retention of the configuration at the P atom, has been established
by X-ray structures of P-chirogenic <i>o</i>-halogenophenyl
phosphine borane complexes. The decomplexation of the borane was easily
achieved without racemization using DABCO to obtain the free <i>o</i>-halogeno-arylphosphines in high yields