Normal‐to‐Abnormal NHC Rearrangement of Al III , Ga III , and In III Trialkyl Complexes: Scope, Mechanism, Reactivity Studies, and H 2 Activation

• Published in: Chemistry : a European journal 2015-12, Vol.21 (49), p.17959-17972
• Main Authors: Schnee, Gilles, Nieto Faza, Olalla, Specklin, David, Jacques, Béatrice, Karmazin, Lydia, Welter, Richard, Silva López, Carlos, Dagorne, Samuel
• Format: Article
• Language: English
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201503000

The present contribution reports experimental and theoretical mechanistic investigations on a normal‐to‐abnormal (C2‐to‐C4‐bonded) NHC rearrangement processes occurring with bulky group 13 metal NHC adducts, including the scope of such a reactivity for Al compounds. The sterically congested adducts ( n I t Bu)MMe 3 ( n I t Bu=1,3‐di‐ tert ‐butylimidazol‐2‐ylidene; M=Al, Ga, In; 1 a – c ) readily rearrange to quantitatively afford the corresponding C4‐bonded complexes ( a I t Bu)MMe 3 ( 4 a – c ), a reaction that may be promoted by THF. Thorough experimental data and DFT calculations were performed on the n NHC‐to‐ a NHC process converting the Al‐ n NHC ( 1 a ) to its a NHC analogue 4 a . A n I t Bu/ a I t Bu isomerization is proposed to account for the formation of the thermodynamic product 4 a through reaction of transient a I t Bu with THF–AlMe 3 . The reaction of benzophenone with ( n I t Bu)AlMe 3 afforded the zwitterionic species ( a I t Bu)(CPh 2 ‐O‐AlMe 3 ) ( 6 ), reflecting the unusual reactivity that such bulky adducts may display. Interestingly, the n I t Bu/Al( i Bu) 3 Lewis pair behaves like a frustrated Lewis pair (FLP) since it readily reacts with H 2 under mild conditions. This may open the way to future reactivity developments involving commonly used trialkylaluminum precursors.