Methylation of Secondary Alkyl Halides

Relative to tertiary alkyl halides, secondary derivatives react considerably slower. At room temperature and long reaction periods ( 24h) cyclohexyl chloride is almost quantitatively methylated with dimethyltitanium dichloride (prepared in situ from dimethylzinc and catalytic amounts of TiQ4)137 , but other cyclic or acyclic halides tend to undergo competing rearrangements prior to C—C bond formation 77). The same applies to 1,2-dihalides such as 1,2-dibromocyclohexane which affords 1,1-dimethylcyclohexane instead of the 1,2-dimethyl derivative137. In complete contrast, activated secondary chlorides behave much like tertiary derivatives, i.e., methylation is fast and position specific at low temperatures. Examples are shown in Equation 86137 . It should be noted that in such cases cuprate chemistry affords less than 40 % of methylation products138). 

The so-called Tebbe reagent 272 reacts with carbonyl compounds to form olefinic products 139a , a reaction which is of particular synthetic value in case of esters (Equation 87) 139b). 

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