Olefin formation, from pinacol

Serval reactions occurred evidenced by a complex desorption products. First, acetaldehyde (m/e 29, 15, 44) desorbed at 390 K followed by traces of ethanol at 415 K (2 % of carbon selectivity, table 2). Three other products were observed. Butadiene and butene desorbed at 540 and 673 K respectively with a combined carbon selectivity of 21.1 %. This reaction pathway follows a reductive coupling mechanism which has been observed previously on the surfaces of Ti02 single crystal and powder [19-21]. The formation of C4 olefins is a clear example of the capacity of UO2 surfaces to abstract large amounts of oxygen from surface carbonyls, via pinacolates [19], as follow... 

A titanium(Il) species formed from titanium trichloride and lithium aluminum hydride is a useful reagent for the reductive coupling of carbonyl compounds to olefins (McMurry, 1974 McMurry and Fleming, 1974). Both aliphatic and aromatic ketones can be converted to tetrasubstituted olefins in excellent yields. Reductive dimerization of retinal (CCLXXFV) affords j6-carotene (CCLXXV) in 85% yield. The course of the reaction can be accounted for by assuming pinacol formation followed by loss of titanium dioxide. 

In contrast to the preparative methods described above, a functionalized allyl-boronate can be created from a simpler allylboronate by olefin cross-metathesis [81, 82]. Here, treatment of pinacol allylboronate (2) with various olefin partners, exemplified with styrene in Equation (33), in the presence of ruthenium catalyst 58 smoothly furnishes a more elaborate 3-substituted allylboronate, the cross product 38 [81]. These reactions are noteworthy for their exceptional functional group tolerance allylboronates bearing primary halides can be directly synthesized using this method. Unfortunately, the E/Z selectivity in the formation of the 3-substituted allylboronates is variable. This metathesis approach to allylboronates was employed as the beginning of a tandem cross-metathesis/carbonyl allylation process [82] (discussed in more detail in Section 6.4.1.3). 

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