Improved synthetic routes to methylene-bridged P-chiral diphosphine ligands via secondary phosphine–boranes

Improved methods for the preparation of methylene-bridged diphosphine ligands are described. Both enantiomers of the key intermediate tert-butylmethylphosphine–borane were prepared via resolution or by the conversion of one enantiomer into the opposite enantiomer. The precursor borane complexes of bis(tert-butylmethylphosphino)methane (t-Bu-MiniPHOS), bis((1,1,3,3-tetramethylbutyl)methylphosphino)methane (t-Oct-MiniPHOS), and (tert-butylmethylphosphino)(di-tert-butylphosphino)methane (trichickenfootphos) were prepared in good yields and enantiopure form.

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(SP,S)-tert-Butyl(methyl)(1-phenylethylcarbamoyl)phosphine–boraneC14H25BNOPDe = 97%[α]D25=-46.7 (c 1.0, CHCl3)Source of chirality: (S)-α-methylbenzylisocyanateAbsolute configuration: (Sp,S)

(S)-Methyl(1,1,3,3-tetramethylbutyl)phosphine–boraneC9H24BPEe = >99%[α]D25=+4.7 (c 1.0, CHCl3)Source of chirality: (−)-sparteineAbsolute configuration: (S)

(R,R)-Bis(boranato(methyl)(1,1,3,3-tetramethylbutyl)phosphino)methaneC19H48B2P2Ee = >99%[α]D25=-7.5 (c 1.0, CHCl3)Source of chirality: (−)-sparteineAbsolute configuration: (R,R)