Combined Theoretical and Experimental Study on High Diastereoselective Chirality Transfer Based on [2.2]Paracyclophane Derivative Chiral Reagent

We report a paracyclophane N-Me thioamide chiral reagent for the asymmetric thio-Claisen rearrangement with high diasteroselectivity. Comparisons between candidate chiral reagent N-phenyl-N-([2.2]paracyclophan-4-yl)amide, N-methyl amide, N-phenyl thioamide, and N-methyl thioamide are made both by experiment and theoretical calculations to clarify the principle behind the high diasteroselectivity. Dynamic H-1 NMR phenomenon tested by varying temperature (VT) experiments has proved that N-Ph amides have triple splitting peaks, while N-Ph thioamide would reduce the number to two, further substituting the Ph to Me made dynamic phenomenon disappear. So the side chain is thought to be the most rigid in N-Me thioamide, which accounts for a structure prerequisite favoring high efficient chirality transfer. This is confirmed by theoretical calculation: remarkable energy difference exists between the Re and Si faces of the chiral molecule. To further clarify the possible pathways for thio-Claisen rearrangement, theoretical prediction is adopted. The result implies that the cisoid pathways will dominate the process. Further experiment confirmed this: with N-Me thioamide, the asymmetrical reaction affords gamma-unsaturated thioamides in good yields and high diastereoselectivities up to 98%. After removing the thioamide auxiliaries under hydrolysis conditions, product beta,gamma-substituted chiral alcohols reached high enantiopurity of 98% ee.