7-Metalla-1,4-diphosphanorbornadienes: cycloaddition of monovalent group 13 NacNac complexes to a stable 1,4-diphosphinine.

This work presents the first example of a dienophilic property of the monovalent group-13 NacNac complexes, which are well-known for their carbene-like reactivity, e.g., addition to multiple bonds, insertion into single bonds, and oxidative cleavage of some multiple bonds. In reactions with imidazole-2-thione based tricyclic 1,4-diphospinine 4, the monovalent compounds NacNacM (M = Al,Ga) showed dienophilic behaviour and produced the corresponding 7-metalla-1,4-diphosphanorbornadienes (5-6), leaving the C[double bond, length as m-dash]S functionality of the imidazole-2-thione intact, while the NacNacIn complex did not show any reactivity because of unfavourable thermodynamics. DFT calculations revealed that for NacNacAl, the cycloaddition was kinetically more favoured (due to a small reaction barrier) than the oxidative cleavage of the C[double bond, length as m-dash]S functionality, despite the fact that the product of the C[double bond, length as m-dash]S bond addition has a somewhat higher stability. For NacNacGa, the cycloaddition reaction was both kinetically and thermodynamically favourable than the cleavage of C[double bond, length as m-dash]S. Furthermore, these reactions were found to be reversible in nature and diphosphinine 4 showed a clear preference towards Al than towards Ga which reflects the better inclination of Ga to stay monovalent than be in the trivalent state.