Atropisomeric [(diphosphine)Au2Cl2] Complexes and their Catalytic Activity Towards Asymmetric Cycloisomerisation of 1,6-Enynes

X-ray crystal structures of two [(diphosphine)Au2Cl2] complexes (in which diphosphine=P-Phos and xylyl-P-Phos; P-Phos=[2,2,6,6-Tetramethoxy-4,4-bis(diphenylphosphino)-3,3-bipyridine]) were determined and compared to the reported structures of similar atropisomeric gold complexes. Correlations between the AuAu distances and torsional angles for the biaryl series of ligands (MeOBIPHEP, SEGPhos, and P-Phos; BIPHEP=2,2-bis(diphenylphosphino)-1,1-biphenyl, SEGPhos=[(4,4-bi-1,3-benzodioxole)-5,5-diyl]bis[diphenylphosphine]) can be made; these measurements appear to be very dependent upon the phosphorous substituent. Conversely, the same effect was not observed for ligands based on the binaphthyl (BINAP) series. The catalytic activity of these complexes was subsequently assessed in the enantioselective cycloisomerisation of 1,6-enynes and revealed an over-riding electronic effect: more-electron-rich phosphines promote greater enantioselectivity. The possibility of silver acting as a (co-)catalyst was ruled out in these reactions.