Thiabenzenes. Reassessment of their chemical and physical properties

Abstract

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 (...

A Practical Synthesis of Thiopyrylium Tetrafluoroborate from Ethyl Vinyl Sulfide

“…Lüttringhaus and Engelhard reduce thiopyran‐3,5‐dione obtaining 5,6‐dihydro‐2 H ‐thiopyran‐5‐ol, which is dehydrated and dehydrogenated by Ph 3 C + ClO 4 − (Scheme 1, B ) 4. Two groups start with 4 H ‐thiopyran which is difficult to obtain (20% in ref.,5 9% in ref 67 or Ph 3 C + ClO 4 − 8 (Scheme , C , D ).…”

Thermal rearrangements of azathiabenzenes; [1,4] shifts in cyclic sulphur–nitrogen ylides

“…However, under these conditions the materials were completely degraded and no products were isolated. Clearly, the [l, 41 shift with its consequent disruption of the benzenoid aromaticity is now much less favoured, although it is perhaps surprising that no products resulting from the alternative [1,2] shift to nitrogen were detected. This [1,2] shift is apparently a highly disfavoured process, and indeed it is rarely observed in sulphimides generally.…”

“…The thermal [1,4] migration of the sulphur substituent occurs more readily in isomeric azathiabenzenes when the fused aromatic ring is not directly involved. The sulphimide (7), m.p.…”

“…The thienothiazine (7) is less thermally stable than the isomeric ylide (3a) and rapidly decomposes in boiling toluene to give a complex a; X = C l ; b; X = SPh mixture of products. Two compounds were identified as the [1,4] shift-product (8), and the ring-expansion product (9). Similar ring-expansion products have been reported in the thermolysis of other cyclic sulphimides,6 and are best rationalised by a Stevens-type rearrangement in the tautomeric sulphonium ylide, a reaction which is well known in acyclic ~ulphimides.~…”

“…The corresponding [ 1,4] rearrangement of the benzo-fused S-phenylsulphimides (11) was so fast that the sulphimides themselves could not be isolated. The azide (10a) was prepared by condensation of 2-chloro-6-phenylthiobenzaldehyde, m.p.…”