Synthesis of spirocyclic orthoesters by ‘anomalous’ rhodium(ii)-catalysed intramolecular C–H insertions

A tetrahydropyranyl acetal bearing a proximal phenyl diazoketone substituent underwent Rh(II)-catalysed C-H insertion via an 'anomalous' C-O bond-forming, rather than C-C bond-forming, transformation, giving spirocyclic orthoesters. Density functional theory calculations with M06 show that the formation of these anomalous products involves hydride transfer to the rhodium carbene, giving an intermediate zwitterion which undergoes C-O bond formation in preference to C-C bond formation.