Tosyl chloride in organic synthesis

Tosyl chloride, we also call it 4-Toluene sulfochloride or 4-Methylbenzenesulfonyl chloride. It is a white to yellow solid with Molecular Formula: C7H7ClO2S and Molecular Weight: 190.64. It is insoluble in water, stable,substances to be avoided include strong bases and strong oxidizing agents and water and moisture sensitive. 

Tosyl chloride(CAS NO: 98-59-9) is a reagent widely used in organic synthesis. Abbreviated TsCl or TosCl, it is a derivative of toluene and contains a sulfonyl chloride functional group. Being a widely available reagent, TsCl has been heavily examined from the perspective of reactivity. It is used in dehydrations to make nitriles, isocyanides, diimides. In an unusual reaction focusing on the sulfur center, zinc reduces TsCl to the sulfinate, CH3C6H4SO2Na.

In characteristic manner, Tosyl chloride converts alcohols (abbreviated ROH) into the corresponding toluenesulfonate esters, or tosyl derivatives. It can be cleaved with lithium aluminium hydride, thus, Tosyl chloride followed by reduction allows for removal of a hydroxyl group. Likewise, it is used to prepare sulfonamides from amines.

The preparation of tosyl esters and amides are conducted in the presence of a base, which absorbs hydrogen chloride. The selection of the base is often crucial to the efficiency of tosylation. Typical bases include pyridine and triethylamine. Unusual bases are also used; for example, catalytic amounts of trimethylammonium chloride in the presence of triethylamine is highly effective by virtue of the trimethylamine.

But what is the role and mechanism of action of tosyl chloride in organic synthesis?

Tosyl chloride (TsCl) is usually used as an activating group for primary alcohols. Due to its relatively large volume and the lower reactivity of secondary and tertiary alcohols, it usually doesn't come into them, being selective to primary alcohols in most of the cases. The -OTs group formed is a good leaving group for substitution reactions. So, using this reagent, it is easier to change functionalities, for example -OH for -N3, -OH for -NHR or -NR2.

Theoretically it is possible to use this group as a protective group also, recovering the -OH treating with NaOH. But I have never seen this use since there are many better protecting groups for alcohols available. 

The introduction of Tosyl chloride(CAS NO: 98-59-9) in the alcohol follows a similar mechanism than any usual ester formation between an alcohol and a acid chloride, but in this case is not a carboxilic acid, it is a sulfonic acid. For the substitution, a SN2 mechanism occurs, leaving the p-toluensulfonic acid anion.

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