Pd-catalyzed inter- and intramolecular carbene transfer from group 6 metal-carbene complexes

The use of group 6 metal-carbene complexes in inter- and intramolecular car bene transfer reactions has been studied. Thus, pentacarbonyl[(aryl)(methox y)carbene]chromium(0) and tungsten complexes, 10, efficiently dimerize at r oom temperature in the presence of diverse Pd(0) and Pd(II)/Et3N catalysts. The effect of additives (PPh3, AsPh3, or SbPh3) on the nature and the isom eric ratio of the reaction products is negligible. The nature of the reacti on products is more catalyst-dependent for metal carbenes 12 bearing alkyl groups attached to the carbene carbon. In these cases, either carbene ligan d dimerization or beta -hydrogen elimination reactions are observed, depend ing on the catalyst. The carbene ligand dimerization reaction can be used t o prepare conjugated polyenes, including those having metal moieties;at bot h ends of the polyene system, as well as enediyne derivatives. The intramol ecular carbene ligand dimerization of chromium bis-carbene complexes 28 and 30 allows the preparation of mono- and bicyclic derivatives, with ring siz es from six to nine members. For bis-carbene derivatives the beta -hydrogen elimination reaction is inhibited, provided that both metal centers are te thered by an omicron -xylylene group. Other alkyl complexes 32 form new mon onuclear carbene complexes 37 or decompose to complex reaction mixtures. Th e results obtained in these reactions may be explained by transmetalation f rom Cr(0) to Pd(0) and the intermediacy of Pd-carbene complexes. Aminocarbe ne-chromium(0) complexes 15, need harsher reaction conditions to transfer t he carbene ligand, and this transfer occurs only in the presence of deactiv ated olefins. The corresponding insertion/hydrolysis products 48 resulted i n these cases. A catalytic cycle involving transmetalation from a chromacyc lobutane to a palladacyclobutane is proposed to explain these results.