Fig. 6: Stereochemical consideration and proposal.
Our results indicate that ligands derived from prolinols and pipecolinols give hydrosilylation products of opposite configuration (92 and 93). The distinct stereoselectivity is proposed to arise from the different catalyst pockets. In the model of prolinol-derived ligand (88 and 90), the bulky diaryl motif of the prolinol would repel the large substituent of the malonic ester to the back and block the right front area in 90. Therefore, the hydrosilylation takes place on the left ester. Meanwhile, the chair conformation of the pipecolinol would lower the barrier of the right front area in 91 significantly. While the larger substituent still points to the back, the hydrosilylation is mediated by the more exposed alkoxide on the right. RL larger substituent, RS smaller substituent.
