Homo- vs. heterometallic organoaluminum fencholates: Structures and selectivities

Homo (Al)- and heterometallic (Al, Li)-fencholates and TADDOLates (5–12) yield in methylations of benzaldehyde 1-phenylethanol with up to 90% ee. Surprisingly, the new BISFOL-based (Al, Li)-heterometallic fencholate (11) shows an strong increase and a change of the sense of enantioselectivity from 19% ee (S) to 62% ee (R) in comparison to its (Al)-homometallic fencholate (7). Despite of the presence of nucleophilic methylide groups, the O-BIFOL-based (Al, Li)-heterometallic fencholate (10) yields a stable complex with benzaldehyde, a lithium ion binds the carbonyl group.

A new heterometallic aluminum lithium-fencholate gives rise to a dramatic increase of the enantioselectivity in comparison to its homometallic aluminum-fencholate analogue, i.e. from 19% ee to 62% ee in the methylation of benzaldehyde. Benzaldehyde coordinates to a fencholate-based lithium dimethyl aluminate, this X-ray crystal structure is shown.Figure optionsDownload as PowerPoint slide