Kinetically Directed Reactivity of Magnesium Dihydropyridides with Organoisocyanates

Although
reactions between β-diketiminato magnesium species containing
monocyclic 1,4-dihydropyridide ligands and the representative ketone
benzophenone are observed to provide reduction by formal hydride transfer
to yield magnesium diphenylphenoxide, similar reactions with organic
isocyanates display only a very limited propensity toward hydride
transfer and reduction to amidate species. In these latter systems,
reaction primarily takes place with Mg–N insertion to afford
O-bound ureide complexes. Further reactions of a magnesium dihydro-isoquinolide
complex, which is constrained to hydride dearomatization at the 2-position
only, display more variable behavior. Although similar Mg–N
insertion and ureide formation is observed for reactions with isocyanates
bearing less sterically demanding N-substitution, more bulky isocyanates
provide unusual enamide C–C coupling reactivity.