Methanesulfonyl chloride, elimination sulfene

On the basis of these findings, the reaction of acyl imines with methanesulfony 1 chloride-triethylamine is not expected to proceed via a sulfene intermediate as previously proposed [99]. Again, a carbanion intermediate accounts nicely for the experimental facts. The electrophihcity of the hetero-l,3-diene is exdemely high, therefore the carbanion, formed on reaction of triethylamme with methanesulfonyl chloride, should undergo nucleophilic attack at C-4 of the hetero-1,3-diene faster than sulfene formabon by chloride elimination. 

In certain cases in which the substrate carries an a hydrogen, there is strong evidence1728 that at least some of the reaction takes place by an elimination-addition mechanism (ElcB, similar to the one shown on p. 382), going through a sulfene intermediate,1729 e.g., the reaction between methanesulfonyl chloride and aniline. 

Methanesulfonyl chloride may be a new reagent to you. In the presence of a base (usually triethy-lamine, EtjN) it reacts with alcohols to give methanesulfonate esters, but the mechanism differs from the mechanism with TsCl. The first step is an elimination of HC1 from the sulfonyl chloride (this can t happen with TsCl, because there are no available protons) to give a sulfene. The sulfene is highly electrophilic at sulfur, and will react with any alcohol (including tertiary alcohols, which react very slowly with TsCl). Here are the two mechanisms compared, formation of toluenesuifonates (tosylates) reagents ROH + TsCl + pyridine... 

The preceding section dealt primarily with the evidence that demonstrated sulfenes to be intermediates in these processes and also, where the evidence warranted it, with the mechanism of the sulfene formation. A continual point of concern is whether or not the reaction is really proceeding by way of the sulfene or not. In this section we shall attempt to find what circumstances of substrate, base and (where different) nucleophile tend to lead to the elimination-addition (sulfene) reaction, and which to some other process, notably the direct displacement reaction. Sometimes, indeed, it takes only a rather small change in conditions to alter the mechanism completely. For example, it has been noted above that azide ion reacts with a-disulfones or sulfonyl chlorides by the direct displacement49. This is also seen in the reaction of methanesulfonyl chloride with aqueous sodium azide when the... 

Virtually all of the vast number of methanesulfonate, or mesylate , esters prepared to facilitate elimination or substitution of a hydroxyl function have been prepared from sulfene the standard procedure using methanesulfonyl chloride and triethylamine in dichloromethane has been described by Crossland and Servis138. To be sure, sulfene intermediacy is not required for alkanesulfonate ester formation the reaction, as has been noted (in Section IV.A.2), will proceed without base promotion via the direct displacement route, but it is usually sufficiently sluggish, however, that the further reaction of the ester with the alcohol competes with ester formation139. This makes the procedure a poor one for most practical purposes, although we recently encountered an instance, namely the preparation of neopentyl 2-chloroethanesulfonate87, in which the direct reaction of the alcohol and sulfonyl chloride without base was the method of choice this arose because (a) the product is stable and (b) 2-chloroethanesulfonyl chloride does not yield the corresponding sulfene with tertiary amines (Section IV.A.2). 

Methanesulfonyl chloride by contrast has a feature it shares with the acyl chlorides just above a relatively acidic proton that can be removed by base. This deprotonation, followed by loss of chloride, is the first step in the formation of a mesylate ester. It is an ElcB elimination and the product is called a sulfene. 

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