Ion pair acidities and aggregation of some amide and oxazoline enolates inTHF

Equilibrium lithium ion pair acidities at 25 degrees 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), a nd 1-(diphenylmethylcarbonyl)pyrrolidine (5) (21.08). These acidity data co mpare well with those previously reported for the corresponding cesium enol ates. Lithium and cesium ion pair acidities in THF are reported for two oxa zolines: 2-(4-biphenylylmethyl)oxazoline (6) (Li, 21.53; Cs, 25.49) and (E) -(4S,5R)-2-(4-biphenylylmethyl)-4-methyl-5-phenyloxazoline (7) (Li, 20.10; Cs 24.21). Studies of their acidities over a wide range of concentrations i ndicate that all of the lithium and cesium enolates of 1-7 are essentially monomeric at concentrations ranging from 10(-8) to 10(-4) M. Traces of dime r were detected for the lithium enolates of amides 1 and 2 and for the cesi um 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 ces ium enolate probably form small amounts of dimer, but the dimerization cons tant of < 100 M-1 is below the limit of precision of our spectroscopic meth od.