In an Evans aldol reaction, the enolate is typically generated by addition of a boron Lewis acid (e.g. $\ce{(n-Bu)2BOTf}$) and an amine, which acts as a weak base. I'd like to know whether there's any difference between
- Triisobutylamine, $\ce{(i-Bu)3N}$
- N,N-Diisopropylethylamine (Hünig's base), $\ce{(i-Pr)2NEt}$
- Triethylamine, $\ce{Et3N}$
in the selectivity of the enolate formation and subsequent aldol reaction. I'm thinking about the different $\mathrm pK_\mathrm a$ values (1>2>3) and the different steric demand (1>2>3). Probably the base with the highest $\mathrm pK_\mathrm a$ will be kinetically fastest. Also, the cis-enolate should be favoured as it avoids a destabilising syn-pentane interaction between the terminal methyl group and the auxiliary. However, do all of the bases lead to the same ratio of cis/trans-enolate formation (and hence diastereoselectivity in the subsequent aldol reaction)?