Chiral 1,8-bis(tert-butylphenylphosphino)naphthalene oxides and sulfides: resolution and structures

Chiral racemic 1,8-bis(tert-butylphenylphosphino)naphthalene oxide (2a) was resolved into enantiomers by fractional crystallization of its diastereomeric adducts 3a with (+)-(2S,3S)-di-O-benzoyl tartaric acid (DBTA), followed by neutralization. Racemic 1,8-bis(tert-butylphenylphosphino)naphthalene (1a) was oxidized with sulfur to two isomers of 1,8-bis(tert-butylphenyl-phosphino)naphthalene monosulfide, rac-4a and rac-4a′, or 1,8-bis(tert-butylphenylphosphino)naphthalene disulfide, rac-5a. The compounds were characterized by NMR spectroscopy (1H, 31P, 13C). Crystal structures were determined by the X-ray method for (−)-3a,rac-4a, rac-4a′ and rac-5a; this allowed the determination of the absolute configuration (S,S) for the more soluble adduct (−)-3a. It was found that rac-1a undergoes partial epimerization in boiling xylene to give the meso form.