Acylcarbene complexes

In contrast with non-acceptor-substituted carbene complexes, most of which are rather stable compounds, only few acceptor-substituted carbene complexes have been isolated [500,502,947,948]. In particular, acceptor-substituted carbene complexes relevant to organic synthesis (e.g. copper or rhodium acylcarbene complexes) are normally highly reactive and have remained elusive to spectroscopic characterization (for theoretical treatments, see Section 1.2). The inference that these intermediates are indeed carbene complexes is in part based on the observation that the modes of generation and the reactivity of these reactive species correlate well with those of less reactive carbene complexes. 

The intermolecular reaction of alkynes with acylcarbene complexes normally yields cyclopropenes [587,1022,1060-1062]. Because of the high reactivity of cyclopropenes, however, in some of these reactions unexpected products can result. In particular intramolecular cyclopropanations of alkynes, which would lead to highly strained bicyclic cyclopropenes, often yield rearrangement products of the latter. In many instances these products result from a transient vinylcarbene complex, which can be formed by two different mechanisms (Figure 4.3). 

The intramolecular addition of acylcarbene complexes to alkynes is a general method for the generation of electrophilic vinylcarbene complexes. These reactive intermediates can undergo inter- or intramolecular cyclopropanation reactions [1066 -1068], C-H bond insertions [1061,1068-1070], sulfonium and oxonium ylide formation [1071], carbonyl ylide formation [1067,1069,1071], carbene dimerization [1066], and other reactions characteristic of electrophilic carbene complexes. 

The generation of electrophilic carbene complexes in the presence of nitriles or other cyano-group-containing compounds can lead to the formation of nitrile ylides. With acylcarbene complexes the final products are often 1,3-oxazoles [1194], presumably formed by the mechanism sketched in Figure 4.10. 

Fig. 4.10. Possible mechanism for the formation of 1,3-oxazoles from acylcarbene complexes and nitriles.

Experimental Procedure 4.2.10. Cycloaddition of an Acylcarbene Complex to an Enol Ether Ethyl 5-Ethoxy-2-trifluoromethyl-4,5-dihydro-3-furoate [1417]... 

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