Heterocycles Thiiranes

Diastereomeric heterocyclic thiiranes 15 are prepared as diastereomeric mixtures by reaction of phenyldiazomethane with 1,3-thiazol-5(4//)-thiones 14145. 

The problem of the synthesis of highly substituted olefins from ketones according to this principle was solved by D.H.R. Barton. The ketones are first connected to azines by hydrazine and secondly treated with hydrogen sulfide to yield 1,3,4-thiadiazolidines. In this heterocycle the substituents of the prospective olefin are too far from each other to produce problems. Mild oxidation of the hydrazine nitrogens produces d -l,3,4-thiadiazolines. The decisive step of carbon-carbon bond formation is achieved in a thermal reaction a nitrogen molecule is cleaved off and the biradical formed recombines immediately since its two reactive centers are hold together by the sulfur atom. The thiirane (episulfide) can be finally desulfurized by phosphines or phosphites, and the desired olefin is formed. With very large substituents the 1,3,4-thiadiazolidines do not form with hydrazine. In such cases, however, direct thiadiazoline formation from thiones and diazo compounds is often possible, or a thermal reaction between alkylideneazinophosphoranes and thiones may be successful (D.H.R. Barton, 1972, 1974, 1975). 

The proton affinities (gas phase) of thiirane and other three-membered heterocycles have been determined azirane (902.5), thiirane (819.2), phosphirane (815.0), oxirane (793.3 kJ moF ) (80JA5151). Increasing s character in the lone electron pairs decreases proton affinities. Data derived from NMR chemical shifts in chloroform indicate the order of decreasing basicity is azirane > oxirane > thiirane (73CR(B)(276)335). The base strengths of four-, five- and six-membered cyclic sulfides are greater than that of thiirane. 

A few special syntheses of thiirane derivatives from four-membered sulfur heterocycles are known. Methylenethiiranes are derived by thermolysis of the tosylhydrazones of thietan-3-ones (Scheme 143) (81TL4815), and 2,2,3,3-tetrakis(trifluoromethyl)thiirane is obtained by thermolysis of 2,2,4,4-tetrakis(trifluoromethyl)-l,3-dithietane 1,1-dioxide with loss of sulfur dioxide. 2,2,3,3-Tetrakis(trifluoromethyl)thiirane 1,1-dioxide is obtained in low yield by heating a mixture of 3,3-bis(trifluoromethyl)-l,2,4-oxadithietane 2,2,4,4-tetroxide and bis(trifluoromethyl)ketene. The mono-episulfide of norbornadiene is obtained from thietane derivative (65 Scheme 144) (73JOC649). 

R] Deyrup, 1. A. In Small Ring Heterocycles, Part 1, Aziridines, Azirines, Thiiranes, Thiirenes Hassner, A., ed. John Wiley Sons New York, 1983, Chapter 1, ppl-214. 

Small ring compounds represent a fair portion of strained organic systems in which the geometry of sp and that of sp carbons have been distorted from the ideal configurations. Foremost among these reactive molecules are the small ring heterocycles, such as thiirane and thiirene oxides and dioxides. ... 

The classification adopted in this review is by type of sulfur reagent it is useful to list by section the more important heterocycles that can be made thiiranes, II,C, IV,B, VIII,D thietanes and thietanones, V,B,2, VI,B,... 

The use of other phosphorus-sulfur reagents for heterocyclic synthesis appears rare. It would be interesting to investigate in more detail the reaction of compounds, such as the phosphine sulfides, with organic substrates. Triphenylphosphine sulfide is an effective sulfur transfer agent, as it converts oxiranes into thiiranes in good yield. The reaction proceeds with retention of configuration.128... 

Simple heterocyclic compounds Three-membered heterocycles containing one heteroatom Oxiranes (epoxides), thiiranes (episulfides), aziridines Three-membered heterocycles containing more than one heteroatom Oxaziridines, dioxiranes, diazirines Four-membered heterocycles containing one heteroatom Oxetanes, thietanes, azetidines... 

The electrocyclic reactions of 3-membered rings, cyclopropyl cation and cyclopropyl anion, may be treated as special cases of the general reaction. Thus the cyclopropyl cation opens to the allyl cation in a disrotatory manner (i.e., allyl cation, n = 0), and the cyclopropyl anion opens thermally to the allyl anion in a conrotatory manner (i.e., allyl anion, m = 1). Heterocyclic systems isoelectronic to cyclopropyl anion, namely oxiranes, thiiranes, and aziridines, have also been shown experimentally and theoretically to open in a conrotatory manner [300]. 

The basicities of saturated heterocycles are similar to those of analogous open chain systems, with the exception of three-membered heterocycles, in which the basicity is markedly reduced. Table 1 gives pvalues for the equilibria between free and monoprotonated heterocycles. As the ring size increases, the protonated species become more stable and the pKa values approach those of the open chain analogues. Increasing basicity (thiirane < oxirane < aziridine) prevails in gas phase proton affinities (Table 2) (80JA5151). 

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