Thiirane

Thiiranes are also known as episulfides. As a result of the greater atomic radius of the S-atom, -----1 the three atoms form an acute-angled triangle (see Fig. 3.2). 

The thermochemically determined strain enthalpy of thiirane of 83 kJ mol is less than that of oxirane. The ionization potential amounts to 9.05 eV, the dipole moment to 1.66 D. Both values are below those of oxirane. The chemical shifts in the NMR spectra are = 2.27, Sq =18.1. 

The properties of the thiiranes are primarily due to ring strain. In spite of the smaller strain enthalpy, thiirane is thermally less stable than oxirane. Even at room temperature, linear macromolecules are formed because of polymerization of ring-opened products. Substituted thiiranes are thermally more stable. The following reactions are typical for thiiranes [13]  

Ammonia, or primary or secondary amines, react with thiiranes to give / -arnino thioles  

The mechanism is the same as described on p 18 for oxiranes. However, the yields are lower, due to competing polymerization. Concentrated hydrochloric acid reacts with thiiranes to give y chlorothioles (protonation on the S-atom and ring-opening by the nucleophilic chloride ion). 

The properties of the thiiranes are primarily due to ring strain. In spite of the smaller g 

Tfhiiranes are oxidized by sodium periodate or peroxy acids to give thiirane oxides. These undergo fragmentation at higher temperature to give alkenes and sulfur monoxide  

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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). 

Photolysis of 4-aminothiazolium salts in aqueous solution may cause ring opening, probably via a thiirane. to give a-cyano-jS-aminovinyl-disulfide or a-acylacetamidine derivatives (22). 

Methylthio)cyclopentane Thiirane Cyclopentyl methyl sulfide... 

The facility with which oxiranes may be prepared and the ease with which they undergo ring opening with nucleophiles or electrophiles makes them useful synthons. Aziridines and especially thiiranes have been less widely exploited in this respect. 

Thiiranes can be made from nascent sulfur , such as is obtained by the thermolysis of diethyl tetrasulfide (64HC(l9-l)59l). 

Three-membered rings containing one sulfur atom are named as thiiranes (1) or thiirenes... 

Table 1 Bond Lengths and Angles for some Thiiranes and Thiirenes...

The configurations of chiral thiiranes and thiirane 1-oxides can be determined by H NMR experiments in chiral solvents such as (R)-(-)- -phenyl-2,2,2-trifluoroethanol. The... 

Chemical shifts ( H, C) and coupling constants for some thiiranes and thiirenes are given in Table 2. The shifts of substituted derivatives may be calculated by the use of additivity relationships found in textbooks on NMR. 

The H NMR spectrum of thiirane 1-oxide is complex (AA BB ) at 60 MHz 24 lines are cfbserved consisting of two sets of 12 centered about a midpoint. The H NMR chemical shift in thiirane 1,1-dioxide is fairly sensitive to solvent variations partly because of the high dipole moment (4.4 D) of the sulfone. The benzene-induced shift, A5 (CeDe-CCLt), is large (-1.04 p.p.m.), as expected from the presence of a sulfone group. Oxygen-17 chemical shifts for thiirane 1-oxide and thiirane 1,1-oxide are -71 and +111 p.p.m. respectively, relative to H2O. 

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