Keto–enol tautomerism of some ortho-substituted a-methylthio-a-diethoxyphosphorylacetophenones

The existence of the keto–enol equilibrium in diluted solutions of ortho-substituted a-methylthio-a-diethoxyphosphorylacetophenones X-PhC(O)CH[SMe][P(O)(OEt)2] (X ¼ OMe 1, Me 2, F 3, Cl 4 and Br 5) has been detected through IR (nOH; nCO and nPyO) and NMR (1H, 13C and 31P) analyses. The enol form exists as a strong intramolecular hydrogen bonded ‘conjugated chelate’. For compounds 1 and 3 for which the ortho-OMe and F substituents are almost coplanar with respect to the CyO group, their strong þM effect gives rise to a strong pX–pPhC(O) conjugation, which hinders the enolization of the a-methine hydrogen atom (circa ,4% for the enol form). Similarly, the weaker pCH3–pPhC(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 ,22%), while for 4 and 5, the Cl and Br substituents lead to deviation from planarity in relation to the CyO group and only the strong 2I effect plays a significant role, being responsible for the presence of larger amounts of the enol form (.50%). q 2004 Elsevier B.V. All rights reserved