Ring transformation equilibrium (bond switch) in 5-(2-aminovinyl)isothiazole system via hypervalent sulfurane. Synthesis, structure determination, and kinetic study

Akiba, Kin‐ya; Kashiwagi, Kohichi; Ohyama, Yoshihiko; Yamamoto, Yohsuke; Ohkata, Katsuo

doi:10.1021/ja00295a026

Cited by 50 publications

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“…The solvent effect by benzene and DMSO leads minor change for the barriers. The substitutions at Y (4) and Y (6) do not change the energy diagrams in any significance except for the reactant and product structures. Therefore, it is concluded that the unimolecular bond-switching reactions cannot proceed thermally.…”

Section: ¹1mentioning

confidence: 93%

“…Therefore, the hypervalent NSN bond energy itself could not been evaluated experimentally. On the other hand, such phenomenon cannot be found for the corresponding oxygen analogs, i.e., 1,2,4-oxadiazole and isoxazole systems (6) in Scheme 4. 6 The difference between 2 and 6 is only the center atom.…”

mentioning

confidence: 89%

“…On the other hand, A¤(¡) is more stable in 8 and 10 with Y (4) = Y (6) = N, in which the hydrogen-bonding interaction between N (4) and H a is considered to stabilize the A¤(¡) form. The energy barriers with respect to the stable structures were estimated to be 4.2, 8.2, and 8.2 kcal mol ¹1 in 7, 9, and 10, respectively.…”

Section: -A′(α)mentioning

confidence: 99%

“…Due to the symmetry, the relative energies of the intermediates and TSs from N (2) S (1) C (5) C (4) N (8) C (7) C (6) …”

Section: ¹1mentioning

confidence: 99%

“…6 Since the discovery of bond-switching equilibration, a mechanism assuming consecutive 1,5-hydrogen shift (pathway A) has been proposed, based on the fact that the rate of equilibration is faster in a non-polar benzene solvent than in a polar dimethyl sulfoxide (DMSO) solvent. Pathway B was also shown to take part based on the shift of deuterium [ND to CD].…”

mentioning

confidence: 99%

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Theoretical Study of Bond-Switching in 1,6-Dihydro-6a-thia-1,6-diazapentalene (10-S-3) Systems Compared with Corresponding Oxygen Analogues

Atsumi¹,

Abe²,

Akiba³

et al. 2010

Bulletin of the Chemical Society of Japan

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Theoretical study of the mechanism of bond-switching of 5-(1-aminoethylimino)-3-methyl-1,2,4-thiadiazole and 5-(2-aminovinyl)isothiazole was carried out by using simplified models of 1,6-dihydro-6a-thia-1,6-diazapentalene (10-S-3) systems and corresponding oxygen analogs. Geometries and energetics were examined along unimolecular and bimolecular reaction paths by hybrid density functional theory (DFT) calculations with triple-zeta class basis sets by taking into account solvent effects which is estimated by the polarizable continuum model. It was clarified that the unimolecular reactions cannot proceed due to the high energy barriers around 70 kcal mol ¹1. On the other hand, the bimolecular processes in neutral and acidic conditions can be accomplished for the sulfur compounds, not for the oxygen ones. The differences of the reactivities between the sulfur and oxygen compounds were found to be due to the difference of the stability of the symmetric intermediates with the hypervalent three-center four-electron bonds.Bond-switching equilibration, i.e., ring transformation equilibrium, of a 5-(1-aminoethylimino)-3-methyl-1,2,4-thiadiazole (1) system, as shown in Scheme 1, was first reported 1,2 in 1979 and firmly established by the use of a 15 NH 2 group. 3,4Furthermore, it was found that the rate was extremely accelerated by acid. An intermediate 1-B, which has 10-S-3 sulfurane (three coordinate hypervalent sulfurane bearing two equatorial lone pair electrons) consisting of a three-center fourelectron (3c-4e) bond, was invoked to realize the equilibrium between 1-A(¡) and 1-A(¢). The same type of bond-switching equilibration was reported for a 5-(2-aminovinyl)isothiazole (2) system, 5 as shown in Scheme 2, and detailed kinetic study was also carried out by using a 15 NH 2 group. 6Since the discovery of bond-switching equilibration, a mechanism assuming consecutive 1,5-hydrogen shift (pathway A) has been proposed, based on the fact that the rate of equilibration is faster in a non-polar benzene solvent than in a polar dimethyl sulfoxide (DMSO) solvent. Pathway B was also shown to take part based on the shift of deuterium [ND to CD]. Such type of compounds 2-B, i.e., Sulfurane-II, were synthesized as stable intermediates, in which two hydrogens are substituted by methyl groups (3 and 4 in Chart 1). 710Attempts to estimate the hypervalent NSN bond energy was carried out by employing the skeleton of 1 fused with two pyrimidine rings (5), as shown in Scheme 3. The rotational barrier of this process was experimentally determined to be 16.6 kcal mol ¹1 (1 kcal mol ¹1 = 4.184 kJ mol ¹1 ) by measuring coalescence of the two methyl groups.11 The theoretical study 12 gave a close number, 15.7 kcal mol ¹1 . However, this process involves not only the bond change from the hypervalent NS N bond to the normal NS single-bond but also the aromaticity change of the skeleton. Therefore, the hypervalent NSN bond energy itself could not been evaluated experimentally.

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Section: ¹1mentioning

confidence: 93%

mentioning

confidence: 89%

Section: -A′(α)mentioning

confidence: 99%

“…Due to the symmetry, the relative energies of the intermediates and TSs from N (2) S (1) C (5) C (4) N (8) C (7) C (6) …”

Section: ¹1mentioning

confidence: 99%

mentioning

confidence: 99%