Stereoselective addition of diphenylphosphine to substituted diphenylethynes: synthetic, NMR and X-ray crystallographic studies

Abstract

The base-catalysed addition of diphenylphosphine to the substituted diphenylethynes RCCR′ (R = Ph, R′ = Ph, o-tolyl, m-tolyl or 2-biphenyl; R = m-tolyl, R′ = o-tolyl or m-tolyl) yielded Ph₂PC(R)CHR′ and/or Ph₂PCH(R)CH(R′)PPh₂. Proton, ¹³C, ¹³P and two-dimensional rotating frame Overhauser enhancement ¹H NMR spectra have been used to determine the stereochemical pathways of the reactions and the stereochemistry of the products. In general the more hindered alkynes undergo monoaddition ultimately to yield phosphinoalkenes with the Ph₂P attached to the carbon bearing the least bulky substituent and cis to the olefinic proton, while for the less hindered alkynes the trans isomer is formed initially and this then reacts further to give meso/erythro-diphosphinoalkanes. Bis(o-tolyl)ethyne does not react with Ph₂PH under the same conditions. Crystal structures were determined for E- and Z-Ph₂P(Ph)CCHPh and show distortions of interbond angles consistent with the pattern of strain implied by the foregoing reactions. The sulfides of the phosphinoalkenes and the Mo(CO)₄ complexes of the diphosphinoalkanes were also prepared and their ¹H, ¹³C and ³¹P NMR spectra recorded. In several cases the pattern of ¹³CO NMR signals for the complexes was used unambiguously to determine the stereochemistry of the parent diphosphines.