Carbon acidity. 105. Ion pair acidities and aggregation of some amide and oxazoline enolates in THF

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), 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-methyl-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(-8) 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.