Leaving groups tosylate anion

We have also seen good leaving groups other than halide ion, e.g. tosylate anion (cf. p. 88),... 

Direct nucleophilic SN2 exchanges F/leaving group (mesylate, tosylate, nosy-late, triflate, iodide or bromide) in a complex structure using [ Fj fluoride anion are more frequent in aliphatic than in aromatic series. The choice of the leaving group is strongly dependent on the substrate to label [206] and it is often neces-... 

She and co-workers took advantage of the acyl anion equivalent formed from the addition of an NHC to an aldehyde to catalyze the formation of benzopyranones via an intramolecular S 2 displacement (Scheme 50) [167], Various aromatic aldehydes provide alkylation products in moderate yields when the leaving group is either tosylate or iodide. No reaction was observed when phenyl or methyl was placed alpha to the leaving group. 

As in part (a), identify the nucleophilic anion in each part. The nucleophile replaces the p-toluene-sulfonate (tosylate) leaving group by an SN2 process. The tosylate group is abbreviated as OTs. 

The epoxide evidently could not be used as the leaving group for the cyclisation so it was opened with benzyloxide anion 48 and activated by tosylation 49. After deprotection, enolate cyclisation gave 50 from which copaene was made. There are two possible enolates from 49 the other would also give a four-membered ring if it cyclised but it is too far away. 

A better leaving group to exchange with halide anions is a methanesulfonate orp-toluene-sulfonate (tosylate). Thus, potassium salts in aqueous solutions of all the halides react with methyl octanesulfonate under mild conditions in presence of either tributylhexadecylphosphonium bromide200,201 or dicyclohexano-18-crown-6202 catalysts (equation 17) ... 

An ester of an alcohol with para-toluenesulfonic acid. Like halide ions, the tosylate anion is an excellent leaving group, (p. 477)... 

At lower temperatures, a tosylate is formed from the reaction of p-toluenesulfonyl chloride and an alcohol. The new bond is formed between the toluenesulfonyl group and the oxygen of the alcohol. At higher temperatures, the chloride anion can displace the -OTos group, which is an excellent leaving group, to form an organochloride. 

As indicated below, the weakest bases (that is, the anions derived from the strongest acids) are indeed the best leaving groups. The / -toluene-sulfonate (tosylate) leaving group is very easily displaced, as are I" and Br, but Cl" and F" are much less effective as leaving groups. 

Intramolecular displacement of aliphatic amines having a suitable leaving group also affords cyclic amines. In order to avoid further alkylation, the amino function is protected by appropriate groups. The tosyl group has frequently been utilized to this end, where cyclization proceeds through an anionic displacement reaction. 

The best nucleophiles for the SrnI mechanism can make a relatively stable radical in the initiation part, either by resonance (enolates) or by placing the radical on a heavy element (second-row or heavier nucleophiles). The best electrophiles for the SrnI mechanism are able to delocalize the odd electron in the radical anion (aromatic leaving groups, carbonyl compounds), can make a stable radical (3° alkyl halides), and have a weak R-X (Br, I) bond. Tosylates and other pseudohalides are very poor SrnI electrophiles. If light is required for substitution to occur, the mechanism is almost certainly SrnI. 

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