The syntheses of divers thiabenzene systems are described, and several chemical and physical properties of this class of compounds are evaluated. Observations are recorded which point to the similarity between thiabenzenes and acyclic sulfonium ylides: both classes of compounds are stabilized toward thermal decomposition by substituents which delocalize or stabilize electronic charge, both classes undergo thermal Stevens rearrangements, and both classes are stably pyramidal at sulfur with a barrier to pyramidal inversion >20 kcal/mol. These observations, as well as the marked upfield chemical shifts noted for protons on carbons CY to sulfur in thiabenzene systems, rule out an aromatic bonding description for thiabenzenes and can be reconciled only with the view of thiabenzenes as cyclic sulfonium ylides.In the course of our studies on pyramidal i n~e r s i o n ,~ we had occasion to develop a specially parametrized C N D 0 / 2 scheme4 which proved useful in providing reliable quantitative estimates for barriers to pyramidal inversion in systems containing elements from the first and second row of the periodic table. These calculations also offered many predictions which, it was hoped, would stimulate and direct further experimentation. c)One such prediction was that 1 -methylthiabenzene (1) should have a relatively high barrier to pyramidal inversion at sulfur (ca. 43 kcal/m01).~ As was subsequently pointed this high barrier contrasts sharply with a previously expressed opinion that 1-phenylthiabenzene (2), and other similar unhindered thiabenzenes, should be planar or have a very low bending barrier for the sulfur-phenyl b~n d .~-~ To resolve these conflicting views, we decided to resort to an experimental test. Nuclear magnetic resonance spectroscopy is a convenient technique for gaining insight into the question of pyramidality. By incorporation of groups containing enantiotopic nuclei, which become diastereotopic in the chiral molecular environment that would be associated with appropriately substituted pyramidal sulfur, one could probe the conformation of thiabenzenesg If thiabenzenes proved to be nonplanar, on the time scale of the N M R observations, then dynamic nuclear magnetic resonance (DNMR) would provide a means of determining the inversion barrier so long as the activation energies were in the range of ca. 5-25 kcal/mol. Hence, the synthesis of an appropriately substituted thiabenzene was undertaken.Since 2 was reported5 to be quite stable, and since Sarylthiabenzenes might be expected'O to be more stable than S-alkylthiabenzenes (and thus 2 more stable than l), we felt that a derivative of 2 would be ideally suited for a preliminary study along the lines described above. Accordingly, we attempted to synthesize 2-isopropyl-1 -phenylthiabenzene (3b), in which the sulfur atom serves as a potential center of chirality, by the route that was employed for the preparation of 2 (eq The reaction of 3a" with phenyllithium was examined by NMR spectroscopy. In an N M R tube under a dry nitrogen atmosphere, 3a (...
ring, but the striking similarity in the spectral characteristics of 2 and 3 suggests that it is highly probable that 3 also exist in a chair conformation (4b). It is very tempting to carry this analogy over to 1 for which no spectral change was observed but caution is suggested in light of the recently observed change from a twist-boat conformation for cyclohexane-l,4-dione to a chair for 1,4-dimethylenecyclohexane.8 On the other hand, the negative results for 1 and 2,2-dibenzyl-5,5dimethylcyclohexane-l,3-dione1 23 do not necessarily rule out a chair conformation since cyclohexanone, which exists as a chair, does not give rise to a spectral change down to -170°.9Our work certainly indicates that more information is necessary in this area of chemistry and our objective will now be a systematic study of various derivatives of cyclohexane-1,3-dione in an attempt to determine all the relevant factors influencing conformation.
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