Houk1 examined the Mannich reaction of the enamine formed from acetone and S-proline with N-ethylidine-N-phenylamine (see Chapter 5.3.3 in my book). Parasuk and Parasuk now extend this to the reaction of the enamine of cyclohexanone and S-proline with N-phenylmethanimine (Reaction 1).2 Geometries were optimized at B3LYP/6-31++G(d,p) and single-point energies computed with PCM (for the solvent DMSO) at both B3LYP and MP2.
First, they examined the formation of the enamine 1, which can be in the syn or anti conformation. The barrier for formation of the syn isomer is 10.2 kcal mol-1. The barrier for the formation of the anti conformer is much higher, 17.9 kcal mol-1, and this is with a single water molecule used to assist the proton migration. However, the rotational barrier between the two conformers is only 4.2 kcal mol-1. So, they conclude that the syn isomer is the only conformer directly formed by the reaction of cyclohexanone and S-proline, and then rotation can produce the anti conformer.
The located the transition state for the reaction of either syn-1 or anti-1 with phenylmethanimine. The two transition states are shown in Figure 1. The barrier for the reaction of syn-1 is 8.5 kcal mol-1, leading to the S product. The other barrier is higher, 13.0 kcal mol-1, and the R product 2R is 6.8 kcal mol-1 higher in energy than the S product 2S. Thus, the reaction to give the S product is both kinetically and thermodynamically favored. This is consistent with experiment3 which gives the S product with 99%ee. Inclusion of solvent makes the S product even more thermodynamically and kinetically favored over the R isomer.
Figure 1. B3LYP/6-311++G(d,p) optimized transition states leading to 2S and 2R.2
(1) Bahmanyar, S.; Houk, K. N., "Origins of Opposite Absolute Stereoselectivities in Proline-Catalyzed Direct Mannich and Aldol Reactions," Org. Lett. 2003, 5, 1249-1251, DOI: 10.1021/ol034198e.
(2) Parasuk, W.; Parasuk, V., "Theoretical Investigations on the Stereoselectivity of the Proline Catalyzed Mannich Reaction in DMSO," J. Org. Chem. 2008, 73, 9388-9392, DOI: 10.1021/jo801872w.
(3) Ibrahem, I.; Zou, W.; Casas, J.; Sundén, H.; Córdova, A., "Direct organocatalytic enantioselective α-aminomethylation of ketones," Tetrahedron 2006, 62, 357-364, DOI: 10.1016/j.tet.2005.08.113.