An Experimental and Theoretical Study on the Interaction of <i>N</i>-Heterocyclic Carbene-Derived 1,3-Dipoles with Methoxycarbonylallenes: Highly Regio- and Stereoselective [3+2]-Cycloadditions Controlled by the Structures of <i>N</i>-Heterocycles of 1,3-Dipoles ChengYing WangBo WangXiao-Rong ZhangJian-Hong FangDe-Cai 2009 The reactions of <i>N</i>-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 <i>N</i>-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 <b>5</b> and methoxycarbonylallenes <b>6</b> with or without heating gave predominantly adducts of C<sup>+</sup>−C−S<sup>−</sup> moiety to the alkyl-substituted double bond of methoxycarbonylallenes, triazole carbene-derived triazolium salts <b>14</b> underwent mainly its [3+2]-cycloaddition of C<sup>+</sup>−C−S<sup>−</sup> dipoles to the ester-substituted double bond of methoxycarbonylallenes. In the case of imidazoline carbene-derived 1,3-dipoles <b>10</b>, the cycloaddition occurred between the C<sup>+</sup>−C−S<sup>−</sup> fragment and the activated double bond at room temperature, while in refluxing benzene, however, the same reaction yielded cycloadducts from the addition of <b>10</b> 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.