2-Thiirene antiaromaticity

For the antiaromatic three-membered heterocycles, experimental data are available only for thiirenes (and there is some doubt about the true antiaromaticity of thiirenes). Bond lengths have been calculated, however, for these antiaromatic 47r-systems (80PAC1623). In comparison with the corresponding saturated heterocycles, the C—X bond lengths are increased by 0.05 to 0.17 A and the C—C bond length is decreased by 0.2 A. 

The thiirene oxide system is of particular interest due to it being simultaneously both a potentially nonbenzenoid aromatic (4n + 2)n and antiaromatic 4nn Hiickel system. 

Since it is clear that the presence of an unshared pair of electrons on the sulfur of the sulfoxide group leads to no special instability in the case of the known thiirene oxides (i.e., 18a, 28a,b and the first alkyl-substituted thiirene oxide 30 recently synthesized60), the reduced antiaromatic properties of the thiirene oxides relative to that of thiirenes have been manifested experimentally. As far as the possibility of electron-attracting conjugative stabilization involving the sulfur atom in thiirene oxides is concerned, the experimental evidence accumulated so far is not decisive. Thus, the chemical shift of the vinylic carbon of... 

Unlike aromatic borirene, only thiirene of the antiaromatic three-membered ring species (100), (123), and (127) (see Table XII) could be experimentally detected by means of matrix isolation at low temperature [81JA(103)486]. The IR spectrum of thiirene is consistent with the data of ab initio calculation [80JA2507 also see 83JA( 105)396]. 

The suggestion that three-membered heterocycles (362) are involved is illustrated by the postulated generation of derivatives of IH-diazirine (315 and 345), thiiren (328), thiazirin (337), and lif-triazirine (358). These three-membered heterocycles belong to the class of 4n-antiaromatic heterocycles their possible role as reaction intermediates is of general interest. There is, however, an interesting difference between the photochemistry of sydnones (Fig. 4) and meso-ionic l,2,4-triazol-3-ones (Fig. 6). In both cases IH- azirines (315 and... 

Photolysis of anhydro-4-hydroxy-2,5-diphenyl-1,3-dithiolium hydroxide (134, R1 = R2 = Ph) in benzene solution yields tetraphenyl-1,4-dithiin (138, 19% yield), diphenylacetylene (16% yield), and sulfur. This result has been interpreted95 as involving the bicyclic photointermediate 139, which loses carbonyl sulfide to give the 4 -antiaromatic diphenyl thiiren (140), which is the precursor of diphenylacetylene and tetraphenyl- 1,4-dithiin (138). 

Dihydro-4,4,6,6-tetramethylfuro[3,4-rf]-l,2,3-thiadiazole (37) has been used to prepare some highly reactive intermediates. Photolysis of compound (37) in the presence of furan gave the adduct (38) in 11% yield a similar reaction in the presence of 2,5-dimethylfuran gave the adduct (39) in 12% yield. The authors claim that formation of adducts (38) and (39) unequivocally proves the intermediacy of the antiaromatic thiirene (40) <87TL2867>. 

Interestingly, the antiaromatic thiirene (22) has been observed on irradiation of the l,3-dithiole-2-thione (21) in an argon matrix (79NJC149). 

Semiempirical calculations have been applied to antiaromatic three-membered rings and indicate that they correspond to energy minima <1973CC688>. For example, thiirene 13 corresponds to a local energy minimum relative to the isomeric carbene 14, zwitterion 15, heterocumulene 16, and heterocyclic carbene 17. 

Jenks et al. studied the effects of conjugation and aromaticity on the sulfoxide bond by means of ab initio computation <1996JOC1275>. They calculated S-O bond dissociation energies (BDEs) and found that, in a formally aromatic system such as thiophene sulfoxide, the SO BDE is decreased by as much as 25kcalmoP relative to the BDE of DMSO. Although the BDE of the formally antiaromatic thiirene sulfoxide increased by about 15 kcal moP the authors concluded, on the basis of calculated geometries and isodesmic reactions with pure hydrocarbons, that cyclic unsaturated sulfoxides are neither significantly aromatic nor antiaromatic. 

Similar to their open chain counterparts, fused thiirenes are highly reactive substances due to their antiaromatic character <1996CHEC-II(1)174>. They are thought to be in equilibrium with diradicals and carbenes (Scheme 1). In addition to behaving as dienophiles, they can therefore undergo typical carbene and radical reactions <1987TL2867>. 

Thiirene oxides are more stable as the electrons on the sulfur are less available to make the ring antiaromatic. Stable fused thiirene oxides have been isolated and shown to be powerful dienophiles and dipolarophiles <1996CHEC-II(1)241, 1982JA4981, 1984JA2216>. Other reactions of fused thiirenes and their oxides remain relatively unexplored. 

From consideration of resonance energy, thiirenes should be antiaromatic, and their existence consequently highly unfavored. Nevertheless, their formation in chemical or photochemical reactions has been discussed <72MI 105-01). 

Due to their strain and putative electronic destabilization, such three-membered heterocycles possessing a cyclic array of 47r-electrons offer a considerable challenge to synthesis. Such molecules are expected to be both unimolecularly and bimolecularly reactive, if they exist at all as energy minima. Since no isolable tellurirenes have been reported, only some reactions of selenirenes are described in this section. Selenirene and its kindred systems, oxirene, azirine, and thiirene, are of interest because of their theoretical significance as prototypes of antiaromatic species. 

Finally, it should be noted that S-methylthiirenium salts (106) are stable below -40°C 106 (R = tert-butyl) is even stable at room temperature.145 This may be ascribed to reduced antiaromaticity of the thiirene ring. 

A-2-Unsaturated three-membered systems are unknown as stable molecules because they would have a four-electron 7t-system, and thus be antiaromatic. 1//-Azirines occur as reactive intermediates and there is evidence for the existence of 2-thiirene in a low-temperature matrix. Azirines, by contrast, are well-known stable compounds. Thiirene A.A-dioxides are also stable molecules, probably best likened to cyclopropenones. The chemistry of saturated three-membered heterocycles is, however, very extensive - epoxides (oxiranes), and to a lesser extent, aziridines are important intermediates in general synthesis. 

Krantz, A. and Laureni, J., A methodology for the preparation and characterization of three membered, potentially antiaromatic molecules. Preparation of matrix-isolated thiirene and sele-nirene, /. Am. Chem. Soc., 99, 4842,1977. 

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