Keto-enol tautomerism of some ortho-substituted α-methylthio-α-diethoxyphosphorylacetophenones

The existence of the keto-enol equilibrium in diluted solutions of ortho-substituted alpha-methylthio-alpha-diethoxyphosphorylacetophenones X-PhC(O)CH[SMe][P(O)(OEt)(2)] (X = OMe 1, Me 2, F 3, Cl 4 and Br 5) has been detected through IR (v(OH), v(CO) and v(P=O)) and NMR (H-1, C-13 and P-31) analyses. The enol form exists as a strong intramolecular hydrogen bonded 'conjugated chelate'. For compounds I and 3 for which the ortho-OMe and F substituents are almost coplanar with respect to the C=O group, their strong +M effect gives rise to a strong pi(X)-pi(PhC(O)) Conjugation, which hinders the enolization of the alpha-methine hydrogen atom (circa similar to4% for the enol form). Similarly, the weaker pi(CH3)-pi(PhC(O)) hyperconjugative effect of the coplanar o-Me-PhC(O) moiety, in compound 2, is responsible for a slightly larger amount of the enol form (circa similar to 22%), while for 4 and 5, the Cl and Br substituents lead to deviation from planarity in relation to the C=O group and only the strong - I effect plays a significant role, being responsible for the presence of larger amounts of the enol form (> 50%). (C) 2004 Elsevier B.V. All rights reserved.