We haven’t yet been very adventurous with our choice of leaving groups for eliminations: all you have seen so far are E2 from alkyl halides and E1 from protonated alcohols. This is deliberate: the vast majority of the two classes of eliminations use one of these two types of starting materials. But since the leaving group is involved in the rate-determining step of both E1 and E2, in general any good leaving group will lead to a fast elimination. You may, for example, see amines acting as leaving groups in eliminations of quaternary ammonium salts.

Both E1 and E2 are possible, and from what you have read so far you should be able to spot that there is one of each here: in the fi rst example, a stabilized cation cannot be formed (so E1 is impossible), but a strong base is used, allowing E2. In the second, a stabilized tertiary cation could be formed (so either E1 or E2 might occur), but no strong base is present, so the mechanism must be E1.

You have just seen that hydroxyl groups can be turned into good leaving groups in acid, but this is only useful for substrates that can react by E1 elimination. The hydroxyl group is never a leaving group in E2 eliminations, since they have to be done in base. A strong base would remove the proton from the OH group instead.

● OH− is never a leaving group in an E2 reaction.

For primary and secondary alcohols, the hydroxyl is best made into a leaving group for elimination reactions by sulfonylation with para-toluenesulfonyl chloride (tosyl chloride, TsCl) or methanesulfonyl chloride (mesyl chloride, MeSO2Cl or MsCl).

Toluenesulfonate esters (tosylates) can be made from alcohols (with TsCl, pyridine). We introduced tosylates in Chapter 15 because they are good electrophiles for substitution reactions with non-basic nucleophiles. With strong bases such as t-BuOK, NaOEt, or DBU they undergo very efficient elimination reactions. Here are two examples.

Methanesulfonyl esters can be eliminated using DBU, but a good way of using MsCl to convert alcohols to alkenes is to do the mesylation and elimination steps in one go, using the same base (Et3N) for both. Here are two examples making biologically important molecules. In the first, the mesylate is isolated and then eliminated with DBU to give a synthetic analogue of uracil, one of the nucleotide bases present in RNA. In the second, the mesylate is formed and eliminated in the same step using Et3N, to give a precursor to a sugar analogue.

The second example here involves (overall) the elimination of a tertiary alcohol—so why couldn’t an acid-catalysed E1 reaction have been used? The problem here, nicely solved by the use of the mesylate, is that the molecule contains an acid-sensitive acetal functional group. An acid-catalysed reaction would also have risked eliminating methanol from the other tertiary centre.

مواضيع ذات صلة

Distinguishing acid derivatives by carbon NMR is difficult

E1cB eliminations in context

Anion-stabilizing groups allow another mechanism—E1cB

The regioselectivity of E2 eliminations

E2 elimination from vinyl halides: how to make alkynes

E2 eliminations from cyclohexanes

E2 eliminations can be stereospecific

E2 eliminations have anti-periplanar transition states

E1 reactions can be stereoselective

Substrate structure may allow E1

E1 and E2 mechanisms

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