Interaction of tertiary phosphines with lignin-type, alpha,beta-unsaturated aldehydes in water

To learn more about the bleaching action of pulps by (hydroxymethyl)phosphines, lignin chromophores, such as the alpha,beta-unsaturated aromatic aldehydes, sinapaldehyde, coniferylaldehyde, and coumaraldehyde, were reacted with the tertiary phosphines R2R’P [R = R’ = Me, Et, (CH2)(3)OH, iPr, cyclo-C6H11, (CH2)(2)CN; R = Me or Et, R’ = Ph; R = Ph, R’ = Me, m-NaSO3-C6H4] in water at room temperature under argon. In all cases, initial nucleophilic attack of the phosphine occurs at the activated C=C bond to form a zwitterionic monophosphonium species. With the phosphines PR3 [R = Me, Et, (CH2)(3)OH] and with R2R’P (R = Me or Et, R’ = Ph), the zwitterion undergoes self -condensation to give a bisphosphonium zwitterion that can react with aqueous HCI to form the corresponding dichloride salts (as a mixture of RR- and S,S-enantiomers); X-ray structures are presented for the bisphosphonium chlorides synthesized from the Et3P and Me3P reactions with sinapalclehyde. With the more bulky phosphines, iPr(3)P, MePPh2, (cyclo-C6H11)(3)P, and Na[Ph2P(m-SO3-C6H4)]b only an equilibrium of the monophosphonium zwitterion with the reactant aldehyde is observed. The weakly nucleophilic [NC(CH2)(2)](3)P does not react with sinapaldehyde. An analysis of some exceptional H-1 NMR data within the prochiral phosphorus centers of the bisphosphonium chlorides is also presented.