Toshikazu Matsuoka

Ab initio and PM3 analysis of 1,3-dipolar cycloaddition reaction between pyridine N-oxides and isocyanates. Theoretical evidence of concerted and nonsynchronous mechanism with zwitterionic character

Abstract The 1,3-dipolar cycloaddition of nitrone (1a) and pyridine N -oxide (1b) with isocyanates (2) was evaluated in terms of ab initio and semiempirical PM3 calculations. In the ab initio transition structure of 1a with isocyanic acid (2a) by MP2/6-31G ∗ level, the length of the newly forming bonds of O-C and C-N are 1.740 and 2.197 A, respectively, indicative of the former bond formation being significantly advanced than that of the latter. The PM3 transition structure (TSba) of the cycloaddition between 1b with 2a showed similar result. The O-C bond (1.445 A) is ca . 76% formed at the transition state, significantly advanced than the C-N bond (2.165 A, ca . 20%). Moreover, the atoms forming the C-N bond have opposite charge, indicating a zwitterionic character of TSba.

Retro-ene type fragmentation of allylic dithiolcarbonates

Abstract The formation of 2-alkenyl alkyl sulfides from S-(2-alkenyl) S-alkyl dithiocarbonates with extrusion of COS was found to be effectively catalyzed by Lewis acids. The ab initio calculations strongly suggested that the reaction falls into category of “retro-alkylthio-ene” reaction.

Reaction of aromatic N-oxides with dipolarophiles. XVII. Cycloaddition behavior of allenes toward pyridine N-oxides and formation of azetidine-type cycloadduct

3,5-Dimethylpyridine N-oxide was allowed to react with phenylsulfonylpropadiene in CHCl 3 at room temperature to give a mixture of the 1:1 [1,5] sigmatropic rearrangement product and the 1:2 azetidine-type cycloadduct. The structure of the azetidine-type cycloadduct was determined by single crystal X-ray analysis. The reaction behavior and the regioselectivity are discussed in terms of the frontier molecular orbital considerations

Reaction of aromatic N-oxides with dipolarophiles. IXI: PM3 analysis of sequential pericyclic reactions between pyridine N-oxides and allenes

Sequential pericyclic reactions between pyridine N-oxides and allenes were evaluated in terms of PM3 calculation. Structures corresponding to the perturbating system were calculated by varying the distance of the addends. The cycloaddition occurs via π-complex. In the transition structure TS1 of primary cycloaddition, the lengths of the newly created bonds of O-C and C-C are 1.864 and 2.204 A, respectively. The bond between the O atom of pyridine N-oxide and the central carbon of allene is ca. 35% formed at TS1, slightly advanced than the other (ca. 30%). The PM3 calculation at the UHF level predicts that [1,5]-sigmatropic rearrangement of the primary cycloadduct proceeds via a transition state with a biradical character

Cycloaddition of imines with allene. formation mechanism of azetidine ring

Abstract PM3 calculations of possible pathways for azetidine-ring formation reaction of imine with allene was performed, indicating that the reaction proceeds via a nonconcerted two-step mechanism in which the allene attacks toward the lone pair of nitrogen atom of imine moiety. Isolation of two stereoisomeric 2:1 cycloadducts from the reaction of 3,3-dimelhyl-3H-indole with phenylsulfonyl propadiene strongly supports the presence of a zwitterionic reaction intermediate.

Reaction of aromatic N-oxides with dipolarophiles. Part 18. Formation mechanism and X-ray structure of the cycloadduct from sequential pericyclic reaction of pyridine N-oxides with phenylsulfonylallene

Sequential pericyclic reactions of pyridine N-oxides (1) with phenylsulfonylpropadiene (2) and 1-phenylsulfonylpropyne (3) were investigated. 3,5-Dimethylpyridine N-oxide (la) was allowed to react with 2 in CHCl3 at room temperature to give a mixture of the [1,5]-sigmatropic rearrangement product (4a) of the 1 : 1 cycloadduct and the 1 : 2 azetidine-type cycloadduct (5a). The structure of 5a was established by single crystal X-ray analysis. The reaction rate of 1a with 2 was about 50 000 times that of 1a with N-phenylmaleimide. The reaction of 1a with 3 did not give 5a but 4a as the sole product. The reactivity, regio- and peri-selectivity and formation mechanism of 4 and 5 are discussed in terms of the frontier molecular orbital consideration based on kinetic and molecular orbital calculation data.