Thiocarbonyls/sulfur containing compounds

In summary, it seems well established that supplementary d functions are indispensable in molecular orbital calculations of sulfur containing compounds. This is particularly true for thiocarbonyl compounds, wherein the anisotropy of the electrondensity distribution around sulfur is significantly greater34 than that around oxygen in carbonyl systems. 

In comparison with other 1,3-dipoles that have been extensively explored in organic synthesis (7), sulfur-centered 1,3-dipoles (1-4) are rather uncommon species. However, within the last two decades, remarkable progress has been made regarding both methods of generation and synthetic applications. In particular, thiocarbonyl ylides (1) were established as key intermediates useful for the preparation of sulfur-containing heterocyclic compounds. General methods for the preparation of thiocarbonyl ylides and their chemical reactivity have been reviewed (8-11). 

The following types of dipolarophiles have been used successfully to synthesize five-membered heterocycles containing three heteroatoms by [3 + 2]-cycloaddition of thiocarbonyl ylides azo compounds, nitroso compounds, sulfur dioxide, and Al-sulfiny-lamines. As was reported by Huisgen and co-workers (91), azodicarboxylates were noted to be superior dipolarophiles in reactions with thiocarbonyl ylides. Differently substituted l,3,4-thiadiazolidine-3,4-dicarboxylates of type 132 have been prepared using aromatic and aliphatic thioketone (5)-methylides (172). Bicyclic products (133) were also obtained using A-phenyl l,2,4-triazoline-3,5-dione (173,174). 

Since in many instances we shall try to compare the properties of thiocarbonyl compounds with their carbonyl analogs, we recall that, in general, the theoretical treatment of sulfur containing systems is more demanding, essentially because of the larger number of electrons in the sulfur atom. This has restricted the ab initio calculations to relatively small compounds or to the use of small basis sets. Until quite recently, high level ab initio calculations were only reported for a few, small thiocarbonyl systems. 

So far we have considered sulfur-containing carbanions obtained by metallation with appropriate bases of a to sulfur atom(s) acidic C-H bonds. For synthetic purposes this method has been by far the most used. Among the variety of other routes to such carbanions [203] we shall consider two methods which seem to be general and promising the reductive lithiation of phenyl thioethers and the thiophilie addition of organometallics to thiocarbonyl compounds (vide infra Section 4.1.2). 

Implications O- and S-containing compounds are ubiquitous in organic and supramolecular chemishy and we will see multiple examples of their involvement in stereoelectronic interactions where the fundamental and structural features described above come into play. For example, the differences between energies and hybridizations of oxygen and sulfur lone pairs readily explain the large differences in directionality of H-bonds formed by carbonyl and thiocarbonyl compounds (Figure 5.15). For sulfur, the hybridized lone pair has so much s-character that it is hardly available for H-bonding. 

The known nonclassical A,B-diheteropentalenes consist of compounds containing an annelated thiophene or selenophene ring, the only uncharged nonradical representations of which contain a tetravalent sulfur or selenium, while the charged structures represent carbonyl, azomethine, thiocarbonyl or selenocarbonyl ylides. The parent systems have not yet been synthesized, only substituted compounds being known. The properties of these substituted derivatives provide a good measure of understanding of the reactivities of the parent systems. 

One more approach to producing six-membered N-containing rings involves thiocarbonyl ylide formation through reaction of a carbenoid with the thiolactam sulfur atom. In this case, the initially formed ylides cyclize to the corresponding episulfides. The latter compounds, through consecutive... 

---