Chemistry and Stereochemistry of the Interaction of the Water-Soluble Phosphine [HO(CH₂)₃]₃P with Cinnamaldehyde in Aqueous Media

To learn more about the bleaching action of pulps by (hydroxymethyl)phosphines, cinnamaldehyde was reacted with tris(3-hydroxypropyl)phosphine, [HO(CH₂)₃]₃P (THPP), in aqueous solution at room temperature under argon. Self-condensation of the aldehyde into two isomeric products, 2-benzyl-5-phenyl-pent-2,4-dienal and 5-phenyl-2-(phenylmethylene)-4-pentenal, is observed; this implies initial nucleophilic attack of the phosphine at the β-carbon of the α,β-unsaturated aldehyde. Reaction in D₂O gives the same products in which all but the phenyl and CHO protons are replaced by deuterons. NMR studies are consistent with carbanion formation and subsequent condensation of two phosphonium-containing aldehyde moieties to generate the products with concomitant elimination of phosphine oxide. In D₂O in the presence of HCl, THPP reversibly attacks the aldehyde-C atom to form the (α-hydroxy)phosphonium derivative [PhCHC(H)CH(OD)PR₃]Cl (where R = (CH₂)₃OD), which slowly converts into the deuterated bisphosphonium salt [R₃PCH(Ph)CD(H)CH(OD)PR₃]Cl₂ via the deuterated monophosphonium salt [R₃PCH(Ph)CD(H)CHO]Cl. The phosphonium intermediates and phosphonium products in this chemistry, although having up to three chiral carbon centers, are formed with high stereoselectivity just in enantiomeric forms. In acetone−H₂O (1:1 v/v), a cross-condensation of cinnamaldehyde with acetone to give 6-phenyl-3,5-hexadien-2-one is promoted by THPP via generation of OH^-.