Effect of the metal fragment in the thermal cycloaddition between alkynyl metal(0) Fischer carbene complexes and nitrones

The thermal cycloaddition between alkynyl metal(0) Fischer carbenes and nitrones has been studied computationally within the Density Functional Theory framework. It is found that the 3 + 2] cycloaddition takes place via transition structures that are more asynchronous and less aromatic than their nonorganometallic analogues. These reactions are also found to be completely regioselective in favor of the cycloadduct possessing the Fischer carbene moiety and the oxygen atom of the nitrone in a 1,3-relative disposition. These results are consistent with the role of the Fischer carbene moiety as an electron withdrawing group that enhances the electrophilic character of the alkyne group acting as a Michael acceptor as a dipolarophile. In terms of the isolobal analogy model, it can be concluded that alkynylalkoxy metal(0) carbene complexes act in this reaction as organometallic analogues of organic alkyl-propiolates with enhanced electrophilic character.