3,4 -Dihydro-1,2-dioxin, conformations

The vibrational spectrum of 1,4-dioxin was studied at the MP2 and B3-LYP levels in combination with the 6-3IG basis set [98JST265]. The DPT results tend to be more accurate than those obtained by the perturbational approach. The half-chair conformation of 4//-1,3-dioxin 164 was found to be more stable than the corresponding conformations of 3,4-dihydro-1,2-dioxin 165,3,6-dihydro-1,2-dioxin 166, and of 2,3-dihydro-1,4-dioxin 167 (Scheme 114) [98JCC1064, 00JST145]. The calculations indicate that hyperconjugative orbital interactions contribute to its stability. 

No annular tautomeric equilibrium transformations in compounds of the diox-ane series have been reported yet recently (97JCC1392), however, the optimized geometries and total energies of unsubstituted isomeric 3,4-dihydro-1,2-dioxin 22 and 3,6-dihydro-1,2-dioxin 23 were calculated using ab initio 3-21G, 6-31G, and MP2/6-31G //6-31G methods. All the methods applied revealed that the total energies for half-chair conformations of 22 and 23 are approximately the same. 

The geometries of both 3,4-dihydro- and 3,6-dihydro-l,2-dioxin have been calculated at the Hartree-Fock (HF) and MP2 ab initio levels of theory. In each case, half-chair conformers were found to be the most stable structures followed by boat conformers which represent the transition states for ring interconversion <2000JST(503)145, 1997JGC1392,... 

The microwave spectmm of 3,6-dihydro-1,2-dioxin 2 was measured in the frequency range of 10-26 GHz at dry ice temperature <1994JST(323)79> the structures on the basis of the rotational constants are half-chair conformers which readily interconvert at ambient temperature by ring puckering. This interconversion process could be frozen out spectroscopically and its free energy of activation determined (AG = 9.82 kcal moF ) by low-temperature NMR. 

Hetero-substituted cyclohexenes, e.g., 3,6-dihydro-2/7-pyran, 3,6-dihydro-2//-thiin, and 3,6-dihydro-l,2-dioxin, exist in half-chair conformations (see Section 2.2.3.2). 

In 2,3-dihydro-1,4-dioxin, -1,4-oxathiin, and -1,4-dithiin derivatives and their benzo analogues, the values of the vicinal proton coupling constant ratios and ring dihedral angles were determined and found to be consistent with half -chair conformation <820MR(18)92>. 

Whereas 1,4-dioxin is planar, 1,4-dithiin 173 and 1,4-oxathiin are significantly distorted from planarity [98JST11]. For 173 a planar nonequilibrium structure was computed to be destabilized by about 2.5 kcal/mol with respect to the boat con-former. The half-chair and boat conformations of 3,4-dihydro-l,2-dithiin, 3,6-dihydro-1,2-dithiin, 4//-l,3-dithiin, and 2,3-dihydro-1,4-dithiin 174 were investigated at the MP2/6-31G level [98JCC1064] (see the structures of the corresponding dioxines in Scheme 114). The half-chair conformers are stabilized with respect to the corresponding boat conformers, and the most stable structure was found to be the half-chair conformer of 174. 

Kondo, T, Matsumoto, M., and Tanimoto, M., Microwave and NMR studies of the structure and the conformational isomerization of 3,6-dihydro-l,2-dioxin, Tetrahedron Lett., 3819, 1978. 

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