Iminocarbene complexes

Fischer carbene complexes are valuable C3 building blocks for the formal [3C+2S] carbo- and heterocyclisation reactions [55]. Thus, not only the traditional a,/J-unsaturated but also aryl and iminocarbene complexes have been used to get a great variety of compounds derived from the [3C+2S] reaction with different C2 counterparts. 

Iminocarbene complexes of chromium and tungsten are useful isolable synthetic equivalents to nitrile ylides having the advantage that the range of 1,3-dipo-larophiles is not limited to electron-acceptor substrates and can be extended to electronically neutral as well as to electron-rich systems [56] (Scheme 18). 

The regioselectivity observed in these reactions can be correlated with the resonance structure shown in Fig. 2. The reaction with electron-rich or electron-poor alkynes leads to intermediates which are the expected on the basis of polarity matching. In Fig. 2 is represented the reaction with an ynone leading to a metalacycle intermediate (formal [4C+2S] cycloadduct) which produces the final products after a reductive elimination and subsequent isomerisation. Also, these reactions can proceed under photochemical conditions. Thus, Campos, Rodriguez et al. reported the cycloaddition reactions of iminocarbene complexes and alkynes [57,58], alkenes [57] and heteroatom-containing double bonds to give 2Ff-pyrrole, 1-pyrroline and triazoline derivatives, respectively [59]. 

Fig. 2 Reaction of an iminocarbene complex of chromium with an ynone...

The coupling of alkynes and iminocarbene complexes (e.g., 4, Scheme 17.2) affords pyrrole ring systems (e.g., 6) in a formal [3-1-2]-cycloaddition process [2,3]. The pyrroles obtained are analogous to those obtained from nitrile ylide cycloadditions. A mechanism was proposed involving a net metalla-[4- -2] cycloaddition to afford azametallacycle 7 followed by reductive elimination and alkene isomerization. The regiochemistry arises through a nonconcerted addition to the triple bond via... 

Several examples of imino-functionalized carbene complexes of palladium have been described. " The imidazolium salt 97 was prepared and coordinated to Pd by an Ag-transfer reaction. Tautomerization of the carbene-imine bond occurs during the transfer reaction to give the the enamine complex 98. Neutral and cationic Pd complexes of a second closely related chelating iminocarbene ligand, 99, were also prepared via an Ag-transfer route. " In this case, no tautomerization was reported. However, evidence of solvent-dependent hemilability was observed. 

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