Unusual Conformational Isomerization of 1,3

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Unusual Conformational Isomerization of 1,3-Oxathianes ... behavior of cyclohexane heteroanalogs [1]. ... thianes, apart from chair (C) conformers, give rise.
ISSN 1070-4280, Russian Journal of Organic Chemistry, 2010, Vol. 46, No. 10, pp. 1583–1584. © Pleiades Publishing, Ltd., 2010. Original Russian Text © V.V. Kuznetsov, 2010, published in Zhurnal Organicheskoi Khimii, 2010, Vol. 46, No. 10, pp. 1576–1577.

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Unusual Conformational Isomerization of 1,3-Oxathianes V. V. Kuznetsov Institute of Molecular and Crystal Physics, Ufa Research Center, Russian Academy of Sciences, pr. Oktyabrya 151, Ufa, 450075 Bashkortostan, Russia e-mail: [email protected] Ufa State Petroleum Technological University, pr. Kosmonavtov 1, Ufa, 450062 Bashkortostan, Russia Received April 24, 2010

DOI: 10.1134/S107042801010026X 1,3-Oxathianes having intrinsically asymmetric ring turned out to be convenient model compounds for studying effects of heteroatoms on the conformational behavior of cyclohexane heteroanalogs [1]. It is known that chair–chair conformational isomerizations of 1,3-dioxanes [2] and 1,3-dithianes [3] never occur directly, i.e., they necessarily involve an intermediate local minimum corresponding to a flexible structure. In order to estimate the scope of the above conclusion we performed computer simulation at the HF/6-31G(d) level of theory (using HyperChem software [4]) of conformational isomerization of 1,3-oxathiane (I) and its 2,2-dimethyl-, 5,5-dimethyl-, and 2,2,5,5-tetramethyl-substituted derivatives II–IV. Molecules I–IV at room temperature undergo fast (on the NMR time scale) inversion of the six-membered ring [5].

O C

S

S

III

O

S

O 3,6-T

2,5-T

The transition states correspond to half-chair conformer for unsubstituted compound I and tetramethylsubstituted derivative IV and to sofa for compounds II and III (see figure). The presence of two methyl groups on C2 appreciably reduces the calculated barrier to inversion, as compared to compound I, whereas O S I (12.3)

Me

Me O

Me

O

1,4-T

II

Me

O

S

Me

O

I

Me

S

S

S O

Cinv) may occur in one step, as was reported previously for cyclic carbonates [7] and boric acid esters [8].

R

Me

S O

Me

S

R'

Me

O

S Me II (6.6)

R

IV

C

We previously [6] showed in terms of empirical and semiempirical approximations that substituted 1,3-oxathianes, apart from chair (C) conformers, give rise to flexible 1,4-twist (1,4-T), 2,5-twist (2,5-T), and 3,6-twist (3,6-T) conformers on the potential energy surface. However, our study on conformational isomerization of compounds I–IV revealed that the transition between energy-degenerate chair invertomers (C ↔ 1583

O

Me

R'

S

R R

O Me

S

R' R'

Cinv

III (13.2) Me Me

Me S

O Me

IV (11.5)

Transition states in the conformational isomerizations of compounds I–IV. Energies of activation (kcal/mol) are given in parentheses.

KUZNETSOV

1584

analogous substitution at the C5 atom increases the inversion barrier. In the transformation of compound IV, the effect of the substituents on C2 predominates. Our results indicated that 1,3-oxathiane molecules are conformationally less rigid than 1,3-dioxanes and 1,3-dithianes. Presumably, the main reasons are different lengths of the C–O and C–S bond and electron density redistribution in the heteroatom fragment of the 1,3-oxathiane ring [9].

3. 4. 5. 6.

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7. 8.

9.

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RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 46 No. 10 2010