Thiiranes deprotonation

The Ramberg-Backlund Reaction. a-Halosulfones undergo a related rearrangement known as the Ramberg-Backlund reaction.91 The carbanion formed by deprotonation gives an unstable thiirane dioxide that decomposes with elimination of sulfur dioxide. This elimination step is considered to be a concerted cycloelimination. 

In the middle of the 1950s, Knott reported the synthesis of dyesmffs based on benzothiazole derivatives. Alkylation of Al-methylbenzo-l,3-thiazole-2-thione with a-bromoacetophenone and deprotonation of the resulting thiocarbonylium salt 5 yielded, after spontaneous desulfurization of the intermediate thiirane (7), the alkylidene derivative 8 (18) (Scheme 5.1). In order to rationalize the reaction, thiocarbonyl ylide 6 was proposed as the precursor of thiirane 7. To the best of our... 

Thiocarbonyl ylides are both nucleophilic and basic compounds (40,41,86). For example, adamantanethione (5)-methylide (52) is able to deprotonate its precursor, the corresponding 2,5-dihydro-1,3,4-thiadiazole, and a 1 1 adduct is formed in a multistep reaction (40,86). Thioxonium ion (56) (Scheme 5.22) was proposed as a reactive intermediate. On the other hand, thiofenchone (S)-methylide (48) is not able to deprotonate its precursor but instead undergoes electrocyclization to give a mixture of diastereoisomeric thiiranes (41,87,88). The addition of a trace of acetic acid changes the reaction course remarkably, and instead of an electrocyclization product, the new isomer 51 was isolated (41,87) (Scheme 5.18). The formation of 51 is the result of a Wagner-Meerwein rearrangement of thioxonium ion 49. 

In the second case, the 3-(acylmethylthio)-l,2-dithiolylium cation is first deprotonated to a thiirane which finally yields an oxadithiapentalene according to equation (23). In this case, sulfur extrusion is not always complete and a disulfide (25) may also be formed. 

Ring-opening reactions of thiirane, thiirane-l-oxide, and thiirane-1,1-dioxide are to be expected, as such attacks could lead to relief of ring strain. Such attack could occur at carbon or at sulfur and the attacking species could be an electrophile, a nucleophile, or a radical. Deprotonation from a ring carbon is also to be expected, especially in the cases of the more acidic episulfoxides and episulfones. 

Table 2 Examples of deprotonation of a fused thiirane S,S-dioxide followed by alkylation...

Fused-ring thiirane A-oxides have been enantioselectively deprotonated by chiral lithium amides 78 or 79. The desymmetrized products were obtained after oxidation of the sulfoxide to remove the additional stereogenic center in good yield and up to 88% ee (Schemes 41 and 42) <1997SL919, 2000J(P1)153>. 

Quinazoline-4(3// )-thione condenses with ethyl bromocyanoacetate " and ethyl 2-chloro-3-oxobutanoate in the presence of sodium ethoxide in ethanol with sulfur extrusion to form ethyl 2-cyano-2-(quinazolin-4-ylidene)acetate (5, R = CN) and ethyl 2-(quinazolin-4-ylidene)-acetate (5, R = H), respectively. It has been suggested that the initial adduct of quinazoline-4(3//)-thione and a methylene compound is deprotonated to give the corresponding ylide which undergoes electrocyclic closure to a thiirane derivative and then desulfurization. A C - C double bond is formed between the methylene carbon atom and the electrophilic carbon C4, providing an alternative to the Wittig reaction. 

The mechanism of the Ramberg-Backlund reaction is rather straightforward. When a-halosulfone 1 is treated with a strong base, deprotonation rapidly takes place to give a-anion 3, which undergoes a backside displacement (intramolecular nucleophilic substitution, SNi) to provide thiirane dioxide 4 (also known as episulfone) as the key intermediate.10 The Swi reaction with loss of halide is the rate-limiting step. Finally, the unstable 4 releases sulfur dioxide and the ring strain to deliver alkene 2. 

A new thiophen synthesis is based on deprotonation and cyclization of the resultant carbanion (65) of a keten dimethylthioacetal. Other applications of cr-(phenylthio)- or cr-(alkylthio)-alkyl-lithiums include preparations of disparlure (the sex pheromone of the gypsy moth) and other chiral epoxides,of an anti-peptic-ulcer diterpene from Croton sublyratus, of carbene-thiometal chelates through reaction with fCr(CO)gl or fW(CO)jl, of olefins via )ff-hydroxy-sulphides, and of Michael adducts of enones. In other work, the chiral solvent l,4-dimethylamino-2,3-dimethoxybutane is employed in enantioselective addition reactions of cr-thio-carbanions with ketones and Michael acceptors. a,P -Elimination of cr-thio-carbanions such as (66) provides a useful synthesis of olefins, and a 2-(lithiomethylthio)-A -oxazoline (67) has been used to prepare a C-labelled thiiran for a microwave study of the valence tautomerism of allene episulphide. ... 

An c ff-unsaturated sulphoxide (72) is the final product isolated when cis-1,2-diphenylthiiran 5-oxide (70) is treated with butyl-lithium, followed by alkylation. The lithio-thiiran 5-oxide (71) is a presumed intermediate. A number of papers have dealt with the reactions of alkyl-lithiums or other strong bases with c S-un-saturated sulphoxides. The reactions observed include (a) or-deprotonation, affording a-sulphinylvinyl anions, which turn out to be surprisingly configuration-... 

Arguments based on the principle of least motion and conservation of orbital symmetry have been used to explain the ring-opening reactivity of the cyclic carbanions obtained on deprotonation of thiirane, thiirane S-oxide, and thiirane 5, 5 -dioxide in... 

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