Deoxygenation reactions McMurry coupling

Titanium promotes the pinacol coupling and then deoxygenates the products the McMurry reaction... 

In 1977, McMurry and Kees [152] developed a titanium-induced intramolecular coupling procedure to form cycloalkenes from dicarbonyl compounds. Mechanistically, as shown in Scheme 85, the coupling reaction proceeds by an initial pinacol dimerization of the dicarbonyl 253 to 254, followed by titanium-induced deoxygenation to afford alkene 255. 

The inter- and intramolecular coupling of two carbonyl groups of aldehydes or ketones in the presence of a low-valent titanium species produces a C-C bond with two adjacent stereocenters, a 1,2-diol (a pinacol). These may be further elaborated into ketones by the pinacol rearrangement or be deoxygenated to alkenes (McMurry reaction). 

Several other methods for the synthesis of PPVs have been described in the literature. The McMurry reaction, which consists of the deoxygenative coupling of aromatic dialdehydes in the presence of titanium compounds, has been employed to obtain 2,5-dihexyl-PPVs [69], para- and meta-FFV (20) [70], Scheme 11, the latter very unlikely to be prepared by the Wessling-Zinmerman or electrochemical routes, since there is no possibility of forming the required quinodimethane intermediate. Nevertheless, a drawback of this method is the somewhat tedious workup needed for the isolation of insoluble PPVs from the elementary metal powder (Ti and/or Zn), which is also produced. 

---