Carbenes transition-metal-assisted reactions

Despite the fact that transition metal complexes have found wide application in the synthesis of carbo- and heterocycles, [3+3] cyclisation reactions mediated or assisted by transition metals remain almost unexplored [3, 86]. However, a few examples involving Fischer carbene complexes have been reported. In all cases, this complex is a,/J-unsaturated in order to act as a C3-synthon and it reacts with different types of substrates acting as C3-synthons as well. 

C-H insertion reactions of carbenes and nitrenes assisted by transition metals have been overviewed. Transition-metal-catalysed carbenoid insertions as a tool for C-H functionalization have been detailed. The intermolecular versions of these C-H insertions have been discussed from the point of view of their mechanisms and stereochemical outcomes. ... 

For recent reviews, see (a) J. Santamaria, Curr. Org. Chem. 2009, 13, 31-46. Fischer-type group 6 carbene complexes in the synthesis of optically active molecules, (b) K. H. Dotz, J. Stendel, Chem. Rev. 2009, 109, 3227-3274. Fischer carbene complexes in organic synthesis metal-assisted and metal-tem-plated reactions, (c) J. W. Herndon, Coord Chem. Rev. 2010, 254, 103-194. The chemistry of the carbon-transition metal double and triple bond annual survey covering the year 2008. 

Unusual type of P-based cyclophane 154 containing NH,NH-functionalized carbene heterocyclic fragment coordinated to the transition metal was obtained by metal template assisted cyclization reactions of the diphosphine hgand bearing reactive 2-fluoro substiments at phenyl groups on phosphorus atoms and carbene heterocycle in coordination sphere of 153 (Scheme 12.57). Aerial oxidation of the complexes of the cyclophane hgand resulted in the isolation of the free macrocycle as di(phosphine oxide)/imidazolidinium salt [142, 143]. 

In addition to carbenes exerting a strong trans influence (thermodynamic), which may be manifest as a trans effect (kinetic) in ligand substitution reactions, heteroatom-functionalized carbene ligands (with their associated dipolar resonance contributor, Figure 1.16) may assist in the stabilization of transition states of reduced coordination number by electron donation to the metal. 

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