R
O
O
O
R
O
3, 4
1, 2
1, 3 R = Me; 2, 4 R = Ph
The results of the determination of ∆G0 of the substituents at 293 K are presented in Table 1. It is easy
to see that for the formals 1 and 2 the best agreement with the data from the configuration isomerization method
[1, 2] (the smallest value of ∆) is observed with the use of AM1, CNDO, INDO, and Eq. (A). Correct values for
the free conformational energy of the 5-CH3 group were also obtained in the PM3, MNDO, and MINDO/3
methods with Eq. (B). In the case of the substituted analogs 3 and 4 the ∆G0 values closest to the data from an
independent experiment were only obtained by means of the empirical MM+ method and Eq. (A). Thus, the
accuracy of reproduction of the optimum geometry of forms Ke and Ka in terms of the employed methods of
calculation is determined to an appreciable degree by the position of the substituting group. An important factor
that was not considered is also the specific dependence of ∆G0 of the substituent (particularly Ph) on the
dielectric characteristics of the medium [11, 12]. (All the calculations were made for an isolated molecule in a
vacuum.) Nevertheless, with allowance for the above-mentioned restrictions the proposed approach opens up a
possibility for relatively simple determination of the ∆G0 values of alkyl and aryl substituents at the C(4) and C(5)
atoms of the ring in the molecules of 1,3-dioxanes by means of separate calculation methods and the
1
experimental spin–spin coupling constants from the H NMR spectra of cyclic formals. A similar solution is
possible in the case of other six-membered 1,3- and 1,3,2-heterocycles and, in particular, substituted 1,3-dioxa-
2-silacyclohexanes [13, 14] and 1,3,2-dioxaborinanes [15, 16].
EXPERIMENTAL
The parameters of the 1H NMR spectra of 5-methyl-1,3-dioxane 1 were described in [17]. The 1H NMR
spectra of the formals 2-4 were determined on a Bruker AM-300 spectrometer at 300 MHz for 10% solutions of
the investigated compounds in deuterochloroform with TMS as internal standard.
1,3-Dioxanes 2 and 3. These compounds were obtained with yields of 64 and 56% respectively by
condensation of the respective 1,2-diols with paraform in the presence of catalytic amounts of p-toluenesulfonic
acid.
Formal 4. The formal was obtained with a yield of 70% by the reaction of styrene with paraform in the
presence of sulfuric acid. The physicochemical constants of compounds 3 and 4 agreed with data in [18].
5-Phenyl-1,3-dioxane (2). Bp 110-112°C (8 mm Hg), nD20 1.5331.
REFERENCES
1.
W. J. Orville-Thomas (editor), Internal Rotation of Molecules [Russian translation], Mir, Moscow
(1977), p. 355.
2.
3.
4.
5.
M. Anteunis, D. Tavernier, and T. Borremans, Heterocycles, 4, 293 (1976).
J. E. Anderson, F. G. Riddel, and M. J. T. Robinson, Tetrahedron Lett., 2017 (1967).
A. I. Gren', V. V. Kuznetsov, and V. A. Bacherikov, Ukr. Khim. Zh., 65, No. 10, 73 (1999).
N. S. Zefirov, V. S. Blagoveshchenskii, I. V. Kazimirchik, and O. P. Yakovleva, Zh. Org. Khim., 7, 594
(1971).
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