Alkenes using titanium or chromium reagents

Alkenes can be obtained from aldehydes or ketones on reductive dimerization by treatment with a reagent prepared from titanium(III) chloride and zinc-copper couple (or L1A1H4), or with a species of active titanium metal formed by reduction of titanium(III) chloride with potassium or lithium metal. This McMurry coupling reaction is of wide application, but in intermolecular reactions generally affords a mixture of the E- and Z-alkenes (2.99). 

Mixed coupling reactions, using two different carbonyl compounds, can be effected, but they generally lead to mixtures of products and are of limited use in synthesis. Intramolecular reactions with dicarbonyl compounds, on the other hand, provide a good route to cyclic alkenes. The keto-aldehyde 80, for example, gave the cyclic diterpene kempene-2, despite the presence of a saturated ketone 

Another reagent for the methylenation of aldehydes or ketones is the Oshima-Lombardo reagent TiCl4-Zn-CH2l2 (or CH2Br2). This reagent is non-basic and can therefore be advantageous for base-sensitive substrates (2.105). 

A drawback of the Tebbe and related reagents is that they are generally suitable only for methylenation and do not permit the formation of higher alkyl analogues. However, the alkenylation of esters (or amides) has been found possible using the Oshima-Lombardo conditions in the presence of TMEDA (tetramethylethylene-diamine) (2.106). This chemistry requires the prior formation of the alkyl gem-dibromide and a more-convenient method, using a dithioacetal, has been reported (2.107).  

Oshima and K. Utimoto, Tetrahedron Lett. (1978), 2417 L. Lombardo, Tetrahedron Lett., 23 (1982), 4293 J. Hibino, T. Okazoe, K. Takai and H. Nozaki, Tetrahedron Lett., 26 (1985), 5579. 

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