Transition Cornforth

If it is assumed that the Curtin-Hammett principle applies, one need only to compare the energies of the minima on the solid and dashed curves to be able to predict the structure of the major product. These curves also allow a direct comparison of Cram s, Cornforth s, Karabatsos s and Felkin s model for 1,2 asymmetric induction. Both Figures show the Felkin transition states lying close to the minima. The Corn-forth transition states (Fig. 3) are more than 4 kcal/mol higher and should contribute little to the formation of the final products assuming a Boltzmann distribution for the transition states, less than one molecule, out of a thousand, goes through them. Similarly, Fig. 4 shows the Cram and Karabatsos transition states to lie more than 2.7 kcal/mol above the Felkin transition states, which means that they account for less than 1% of the total yield. 

Figures 3, 4, 5, 6 all locate the Cram and Cornforth transition states very near to a crossing of the solid and dashed curves. It follows that a small conformational change of the substrate may reverse the direction of the preferential attack by putting the dashed transition state below the solid one. The reagent then arrives from the apparently more hindered side, thus violating the Cram s (Cornforth s) ruie.

For carbonyl addition, three transition state models have been proposed the Felkin-Anh model36, the chelate Cram model37 and the dipolar Cornforth model37 . 

Felkin-Ahn/Cornforth (anti) product is predominant with MgBr2 as a consequence of strong solvent-Lewis acid association at the expense of chelation. The same trend is observed with the a-OTBS derivative and MgBr2 owing to the diminished capacity of TBS ethers to complex with MgBr2- Probable transition states for these additions are depicted in Fig. 2. 

In the 1950s, Cram and Cornforth proposed models to rationalize results of this kind, invoking ideas about steric effects and their influence on transition-state con-... 

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