Ion Pair Acidities and Aggregation of Some Amide and Oxazoline Enolates in THF,

Equilibrium lithium ion pair acidities at 25 °C have been determined in THF for the following acetamides: N, N -dimethyl-(4-biphenylyl)acetamide ( 1 ) (19.69), N, N -diethyl-(4-biphenylyl)acetamide ( 2 ) (20.30), N, N -dimethyl-diphenylacetamide ( 3 ) (20.76), N, N -diethyl-diphenylacetamide ( 4 ) (21.99), and 1-(diphenylmethylcarbonyl)pyrrolidine ( 5 ) (21.08). These acidity data compare well with those previously reported for the corresponding cesium enolates. Lithium and cesium ion pair acidities in THF are reported for two oxazolines: 2-(4-biphenylylmethyl)oxazoline ( 6 ) (Li, 21.53; Cs, 25.49) and ( E )-(4S,5R)-2-(4-biphenylylmethyl)-4-methoxymethyl-5-phenyloxazoline ( 7 ) (Li, 20.10; Cs 24.21). Studies of their acidities over a wide range of concentrations indicate that all of the lithium and cesium enolates of 1 − 7 are essentially monomeric at concentrations ranging from 10 - 3 to 10 - 4 M. Traces of dimer were detected for the lithium enolates of amides 1 and 2 and for the cesium enolate of oxazoline 6 . Their dimerization constants (M - 1 ) were found to be about 450, 200, and 150, respectively. All of the other lithium and cesium enolate probably form small amounts of dimer, but the dimerization constant of <100 M - 1 is below the limit of precision of our spectroscopic method.