Structures of Reactive Donor/Acceptor and Donor/Donor Rhodium Carbenes in the Solid State and Their Implications for Catalysis

Owing
to its tremendous preparative importance, rhodium carbene
chemistry has been studied extensively during past decades. The invoked
intermediates have, however, so far proved too reactive for direct
inspection, and reliable experimental information has been extremely
limited. A series of X-ray structures of pertinent intermediates of
this type, together with supporting spectroscopic data, now closes
this gap and provides a detailed picture of the constitution and conformation
of such species. All complexes were prepared by decomposition of a
diazoalkane precursor with an appropriate rhodium source; they belong
to either the dirhodium­(II) tetracarboxylate carbene series that enjoys
widespread preparative use, or to the class of mononuclear half-sandwich
carbenes of Rh­(III), which show considerable potential. The experimental
data correct or refine previous computational studies but corroborate
the currently favored model for the prediction of the stereochemical
course of rhodium catalyzed cyclopropanations, which is likely also
applicable to other reactions. Emphasis is put on stereoelectronic
rather than steric arguments, with the dipole of the acceptor substituent
flanking the carbene center being the major selectivity determining
factor. Moreover, the very subtle influence exerted by the anionic
ligands on a Rh­(III) center on the chemical character of the resulting
carbenes species is documented by the structures of a homologous series
of halide complexes. Finally, the isolation of a N-bonded Rh­(II) diazoalkane
complex showcases that steric hindrance represents an inherent limitation
of the chosen methodology.