An Experimental and Theoretical Study on the Interaction of N-Heterocyclic Carbene-Derived 1,3-Dipoles with Methoxycarbonylallenes: Highly Regio- and Stereoselective [3+2]-Cycloadditions Controlled by the Structures of N-Heterocycles of 1,3-Dipoles

The reactions of N-heterocyclic carbene-derived 1,3-dipoles with methoxycarbonylallenes were studied systematically by means of experimental and theoretical approach. The regioselectivity of [3+2]-cycloaddition of 1,3-dipoles toward the ester-substituted (activated) or alkyl-substituted (less activated) carbon−carbon double bond of methoxycarbonylallenes was strongly governed by the structures of N-heterocycles of 1,3-dipoles. In addition, the reaction temperature played an important part in regulating the regioselectivity of [3+2]-cycloaddition in some cases. While the reaction between benzimidazole carbene-derived 2-thiocarbamoyl benzimidazolium inner salts 5 and methoxycarbonylallenes 6 with or without heating gave predominantly adducts of C+−C−S moiety to the alkyl-substituted double bond of methoxycarbonylallenes, triazole carbene-derived triazolium salts 14 underwent mainly its [3+2]-cycloaddition of C+−C−S dipoles to the ester-substituted double bond of methoxycarbonylallenes. In the case of imidazoline carbene-derived 1,3-dipoles 10, the cycloaddition occurred between the C+−C−S fragment and the activated double bond at room temperature, while in refluxing benzene, however, the same reaction yielded cycloadducts from the addition of 10 to the less activated double bond of methoxycarbonylallenes. DFT calculation revealed asynchronous cycloaddition mechanisms for the reactions of benzimidazole and imidazoline carbene-derived 1,3-dipoles with methoxycarbonylallenes, and a concerted mechanism for the reaction of triazole carbene-derived dipoles. The different regioselectivity of the reaction originated from the combination of electronic and steric effects of the reactants and the stability of the final products.