Cooperative Activation of Isocyanates by Al–N-Based Active Lewis Pairs and the Generation of a C5 Chain by Simultaneous Formation of Two C–C Bonds

Martinewski K., Holtrichter-Rößmann T., Rösener C., Hepp A., Würthwein E., Uhl W.

Zusammenfassung

The active Al/N Lewis pair, (Me3C)2Al−C(SiMe3)=C(H)−N(CHMe-CH2)2CH2 (2), reacted with isocyanates to afford a fascinating variety of products. One equivalent of Ph−N=C=O yielded by the release of H−C≡C−SiMe3 an urea-type ligand which coordinated the Al atom in a chelating manner (4). Dipp-N=C=O gave a similar product, but the bulky substituent hindered the approach of the N-aryl group to Al. A situation similar to that of frustrated Lewis pairs resulted in the coordination of the alkyne to the Al and N atoms (6) by C−H bond activation. Dual insertion was observed upon treatment of 2 with two equivalents of isocyanates (8 to 11). The preferred formation of cyclic oligomers is prevented by the specific cooperative properties of the Lewis pair. A metal-free dimeric isocyanate (13) was formed by hydrolysis. Replacement of the CMe3 groups in 2 by less bulky isobutyl groups (7) afforded the insertion of two isocyanate molecules into the Al–vinyl bonds without alkyne elimination. The resulting highly functionalised compound had a chain formed by two isocyanates and the organic backbone of the Lewis pair. Me3C−N=C=O and 2 afforded a unique compound (14) in which an isocyanate ligand connects two molecules of 2 by the release of dimethylpiperidine. The combination of a C1 building block and two C2 groups gave an unsaturated branched C5 moiety by the simultaneous formation of two C−C bonds. The molecular structure showed an interaction between an Al atom and a C−C π-bond.