Reaction Possibilities of Carbene Complexes

I now wish to confine my attention to carbene complexes of our type and to show with recent examples the kinds of reactions we wrere able to produce. 

We have already established that the carbene carbon is an electrophilic center and, hence, it should be very easily attacked by nucleophiles. In most reactions we believe that the first reaction step probably involves attachment of a nucleophile to the carbene carbon. In some cases, for instance with several phosphines (49) and tertiary amines (50), such addition products are isolable analytically pure under certain conditions (1 in Fig. 3). For the second step there exists the possibility that the nucleophilic agent may substitute a carbon monoxide in the complex with preservation of the carbene ligand (2 in Fig. 3). One can also very formally think of the carbene complex as an ester type of system [X=C(R )OR with X = M(CO)j instead of X = 0], because the oxygen atom as well as the metal atom in the M (CO) 6 residue are each missing 2 electrons for attainment of an inert gas configuration. So, it is not surprising that the 

OR group can be replaced by amino, thio, or seleno groups (3 in Fig. 3). In this way, the amino- (36, 51-54), thio- (51, 55), and seleno (organo)-carbene complexes (56) are accessible, but the synthesis of the two latter species requires a special experimental skill. 

We can also observe reactions that lead to a more stable arrangement of the whole system very probably via primary addition and subsequent rearrangement (4 in Fig. 3). In addition, it can be established that because of the electron withdrawal of the M(CO)5 moiety, hydrogen atoms in cv-alkyl positions to the carbene carbon develop such an acidic character that their acidity corresponds to that of nitromethane (5 in Fig. 3). Finally, by cleavage of the carbene ligand from the metal complex, pathways in synthetic organic chemistry are opened (6 in Fig. 3). 

Upon irradiation of solutions of these ylide complexes in hexane-toluene mixtures at — 15°C, a CO ligand is eliminated from the cis position and thereby the ds-tetraearbonyl[alkoxy (organo)carbene]phosphine complexes are obtained (58). The phosphine that was initially added to the carbene carbon of the starting material thus takes the place of a CO 

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