Thiirane, protonation

The NMR study of steroidal epoxides (discussed in section II-F) parallels that of the analogous thiiranes. It is possible to relate the location and configuration of the thiirane group with the angular methyl and thiirane proton resonances. The proton NMR relationships for the intermediate thiocyanatohydrins have been included inageneral NMR study of steroids. Electronic spectra may be used in the analysis of steroidal thiiranes. Spectroscopic measurements have shown the existence of a low intensity absorption in the 240-260 m region. The regular patterns of rotatory contributions of thiiranes which are comparable with those of ketones prompted an accumulation of ORD and CD data for steroidal thiiranes. 

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. 

Acids are poor catalysts for ring cleavage of thiirane 1,1-dioxides but are good catalysts for reactions of thiirane 1-oxides with nucleophiles. These reactions of episulfoxides are believed to proceed by protonation of the oxygen atom (but see the NMR evidence cited above for 5-protonation in fluorosulfonic acid) and will be treated in the section on nucleophilic reactions. 

The formation of alkenes from thiirane dioxides may not be stereospecifically controlled in the presence of a sufficiently strong base and sufficiently acidic protons in the three-membered ring. Under such conditions (essentially those typical for the Ramberg Backlund reaction), epimerization via a carbanion intermediate produces an equilibrium mixture of thiirane dioxides19,99 and consequently a mixture of cis- and trans-alkenes. 

A mechanism analogous in many ways to that of the acid-catalyzed ring opening reaction was advanced for the reaction of the thiirane oxide with alkyl chloromethyl ethers S . The first step is the displacement of the chloride by the sulfoxy oxygen (equation 24). In view of the above mechanistic interpretation, it is quite surprising that the parent thiirane oxide (16a) was found to be protonated on sulfur and not at oxygen in FSOjH-SbFfi at — 78 °C, according to NMR studies s . 

Attempts to generate thiocamphor (S)-methylide (44) by the addition of diazomethane to thiocamphor and subsequent N2-elimination from the [3-1-21-cycloadduct 43 led to enethiol ether 45 via a 1,4-H shift (Scheme 5.17). The formation of an unstable intermediate 43 was proposed on the basis of the proton nuclear magnetic resonance ( H NMR) spectrum of the crude mixture. The postulated intermediate 44 could not be intercepted by dipolarophiles or methanol, and did not undergo electrocyclization to give the corresponding thiirane (41). 

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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