PH OPTIMIZATION OF NUCLEOPHILIC REACTIONS IN WATER

We present a way of prescribing the pH for a reaction so as to obtain either (a) maximum yield in competition with hydrolysis or (b) selective reaction at either of two sites in such nucleophile-electrophile reactions as C-alkylation of acidic ketones and the acylation and sulfonylation of amines. First, we derive the following general equation for pH(max), the pH giving the highest yield of the product (P) of the reaction of a nucleophile (Nu) with a hydrolyzable electrophile (E) in water: pH(max) = 1/2[log (k(w)/k(OH)) + pK(w) + pK(a)] (k(w) and k(OH) refer to the water- and hydroxide-promoted hydrolyses of E, K(w) is the autoprotolysis constant of water, and K(a) is the acid dissociation constant of NuH+, the conjugate acid of Nu). pH(max) thus depends on a property of E (namely, k(w)/k(OH)) and a property of Nu (the pK(a) of NuH+), but not on the rate constant for the reaction of E with Nu or the concentration of Nu. We then deduce analogous approximate equations for maximum selectivity for reaction at either of two nucleophilic sites, specifically, equations giving pH(xmax) and pH(ymax), the pH values for the maximum yields of the respective products (P(x) and P(y)) of the reactions of E with the two nucleophiles. We find that (a) pH-yield profiles calculated from the equations concur with observed yields for reactions under pseudo-first-order conditions and (b) preparative experiments at the estimated pH values give good to excellent yields of clean products and high selectivity in both the C-alkylation and Schotten-Baumann reactions.