Reductive coupling reactions with alkenes

In the presence of dienes or alkynes, identical reduction of (136) yields ReCp Cl2(diene) and ReCp Cl2(RC=CR). The latter has an interesting chemistry which includes inter alia hydrolysis to form ReCp (0)(RC=CR), alkylation to give ReCp Me2(RC=CR), and a series of H+-catalyzed coupling reactions with alkenes to form ReCp Cl2(diene) and ReCp Cl(dienyl). ... 

Stahl and Sheldon have shown how oxidations can be driven by air as primary oxidant, or source of stochiometric oxidizing power. Like the catalysts in this subsection, biological oxidases are enzymes that use O2 but do not incorporate its O atoms into the substrate. For example, Pd(OAc)2-pyridine is active for alcohol oxidation, intramolecular hetero- and carbocyclization of alkenes, intermolecular O-C and C-C coupling reactions with alkenes, and oxidative C-C bond cleavage of tertiary alcohols. A pathway for alcohol oxidation is shown in Eq. 9.27. Normally a 4e process, reduction of O2 can be hard to couple with oxidation of the catalytic intermediates, processes that often proceed in 2e steps. In this case, intermediate rj -peroxo Pd(II) complexes can be formed from reaction of Pd(0) intermediates with O2, which thus acts as a 2e oxidant. 

A tremendous amount of progress has been made over the past decade in the understanding of the catalyzed reductive coupling of unactivated alkenes and alkynes. Both early and late transition metal complexes accomplish the reaction with good yields and with low catalyst loadings. Enynes and dienes can... 

This coupling procedure with the thioesters proved sensitive to the substitution pattern of both the amino acid and alkene. In contrast, coupling reactions with the M-acyl oxazolidinone derivatives such as 22 proved to be much more effective (Scheme 14) [20]. Mechanistic studies suggested that an alternative pathway was operating in these cases, where reduction of the al-... 

Enal-derived nickel-Jt-allyl complexes are efficient partners in cross-coupling reactions with alkenyl- and arylstannanes leading to products such as 39 (Scheme 23).P 1 As described for the aUylic ether derived complexes, the mechanism of the coupling process involves transmetalation followed by reductive elimination. The reaction is catal hic in nickel, unlike couplings of enals with allstoichiometric quantities of nickel. Many other variants of nickel-catalyzed conjugate additions may involve Ji-allyl complexes, but these are treated separately in a section on alkenes (Section 1.1.4.2) because the mechanism is poorly defined in most nickel-catalyzed conjugate additions. 

It is clear that McMurry reactions can be performed with low-valent titanium species in various oxidation states. Pinacol and alkene-forming reductive coupling reactions have been achieved with organometallic complexes of Ti(0), Ti(II), and Ti(III) (see Section 6.2.2.3). While no evidence was found for the presence of 11(0) on reduced Ti02 surfaces active in benzaldehyde coupling, X-ray photoelectron spectroscopy showed that the active site required for gas-solid reductive coupling is an ensemble of Ti cations in the +1, +2, and +3 oxidation states that collectively effect the four-electron reduction [258]. 

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