Sodiated Oppolzer enolates: solution structures, mechanism of alkylation, and origin of stereoselectivity

NMR spectroscopic studies reveal camphorsultam-derived sodium enolates known as Oppolzer enolates reside as monomers in neat THF and THF/HMPA solutions and as dimers in toluene when solvated by N,N,N & PRIME;,N & PRIME;-tetramethylethylenediamine (TMEDA) and N,N,N & PRIME;,N & PRIME;& PRIME;,N & PRIME;& PRIME;-pentamethyldiethylenediamine (PMDTA). Density functional theory (DFT) computations attest to the solvation numbers. Rate studies show analogy with previously studied lithiated Oppolzer enolates in which alkylation occurs through non-chelated solvent-separated ion pairs. The origins of the selectivity trace to transition structures in which the alkylating agent is guided to the exo face of the camphor owing to stereoelectronic preferences imparted by the sultam sulfonyl moiety. Marked secondary-shell solvation effects are gleaned from the rate studies.

Automated summary

NMR spectroscopic studies show that camphorsultam-derived sodium enolates known as Oppolzer enolates exist as monomers in neat THF and THF/HMPA solutions and as dimers in toluene when solvated by TMEDA and PMDTA. DFT computations confirm the solvation numbers. Rate studies indicate similarity to previously studied lithiated Oppolzer enolates, where alkylation occurs through non-chelated solvent-separated ion pairs. The selectivity originates from transition structures in which the alkylating agent is guided to the exo face of the camphor due to the stereoelectronic preferences imparted by the sultam sulfonyl moiety. Secondary-shell solvation effects are observed from the rate studies.