Figure 3. Catalytic enantioselective allenyl additions to trifluoromethylphenyl ketones.
a, The boron-based catalyst derived from triphenylsilyl-substituted aminophenol 1c proves to be superior for additions of allenyl units to trifluoromethylphenyl ketones (vs. 1a). b, Analysis of subtle differences in the structures of catalyst-ketone complexes for allyl versus allenyl addition reactions points to a comparatively distal positioning of the substrate molecule in the latter set of transformation. Accordingly, use of triphenylsilyl-substituted aminophenol 1c, which extends the reach of the chiral catalyst, was found to generate improved enantioselectivities. DFT calculations were carried out at M06-2X or ωB97XD/6-311++G(2df,2pd)//M06-2X/6-31G(d,p)toluene(PCM) level. (See the Supplementary Information for details.)