Conformational Analysis of the 1,3-Oxaphosphorinanes Formed from α,β-Unsaturated Aldehydes and the (Hydroxymethyl)phosphines RP(CH2OH)2 (R=Ph or CH2OH)

Equimolar reactions of cinnamaldehyde or its 3,5-dimethoxy-4-hydroxy derivative (sinapaldehyde) with RP(CH2OH)(2) (R=Ph or CH2OH) were studied in MeOH or CD3OD at room temperature by NMR spectroscopy. In MeOH, nucleophilic attack of the phosphine at the CC bond, with concomitant loss of CH2O, affords the tertiary phosphine HOCH2P(R)CH(Ar)CH2CHO (3) that rapidly converts mainly into a 1,3-oxaphosphorinane derivative (5) formed as a mixture of four diastereomers. Conformational analysis reveals that the Ar group in these is exclusively in an equatorial position while the OH and R groups can be equatorial-oriented or axial-oriented. In CD3OD, 1,3-oxaphosphorinanes monodeuterated in the C5 position are obtained as a mixture of eight diastereomers where the dominate diastereomers have an axial D-atom. Diastereomeric ratios depend on the nature of the Ar and R groups.