1,3-Dipolar cycloaddition reactions of substituted 3,4-dihydroisoquinoline N-oxides with allenes

Reactions of ruthenium-allenylidene complexes tethering a cyclopropyl group

Two cyclopropyl allenylidene complexes [Ru]=C=C=C(R)(C(3)H(5)) ([Ru]=[RuCp(PPh(3))(2)], Cp=Cyclopentadienyl; R=thiophene (2a) and R=Ph (2b)) are prepared from the reactions of [Ru]Cl with the corresponding 1-cyclopropyl-2-propyn-1-ol in the presence of KPF(6). Thermal treatment, halide-anion addition, and palladium-catalyzed reactions of 2a and 2b all lead to a ring expansion of the cyclopropyl group, giving the vinylidene complexes 4a and 4b, respectively, each with a five-membered ring. This ring expansion proceeds by C-C bond formation between Cβ of the cumulative double bond and a methylene group of the cyclopropyl ring. In the reaction of 2a with pyrrole, consecutive formation of two C-C bonds, one between C-2 of pyrrole and Cγ of 2a and the other between C-3 of pyrrole and Cα, results in the formation of 6a. The reaction proceeds by addition of pyrrole and 1,3-proton shifts. The hydrogenation of 2a by NaBH(4) is carried out in different solvents. The cumulative double bonds are reduced regioselectively to give a mixture of 7a and 8a. Interestingly, use of different solvents leads to different ratios of 7a and 8a. Presence of a protic solvent like methanol in dichloromethane or chloroform solution increases the yield of 8a, thus revealing that both the rates of hydroboration and deboronation increase. The structures of two new complexes 4a and 6a have been firmly established by X-ray diffraction analysis.

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.

Organocatalysis “on water”. Regioselective [3 + 2]-cycloaddition of nitrones and allenolates

The first example of a regioselective and organocatalyzed 1,3-dipolar cycloaddition reaction between conjugated alkynoates and nitrones "on water" is described.