Insertion, protonolysis reductive elimination

The hydroamination of olefins has been shown to occur by the sequence of oxidative addition, migratory insertion, and reductive elimination in only one case. Because amines are nucleophilic, pathways are available for the additions of amines to olefins and alkynes that are unavailable for the additions of HCN, silanes, and boranes. For example, hydroaminations catalyzed by late transition metals are thought to occur in many cases by nucleophilic attack on coordinated alkenes and alkynes or by nucleophilic attack on ir-allyl, iT-benzyl, or TT-arene complexes. Hydroaminations catalyzed by lanthanide and actinide complexes occur by insertion of an olefin into a metal-amide bond. Finally, hydroamination catalyzed by dP group 4 metals have been shown to occur through imido complexes. In this case, a [2+2] cycloaddition forms the C-N bond, and protonolysis of the resulting metallacycle releases the organic product. 

Like alkynes, a variety of mechanistic motifs are available for the transition metal-mediated etherification of alkenes. These reactions are typically initiated by the attack of an oxygen nucleophile onto an 72-metalloalkene that leads to the formation of a metal species. As described in the preceding section, the G-O bond formation event can be accompanied by a wide range of termination processes, such as fl-H elimination, carbonylation, insertion into another 7r-bond, protonolysis, or reductive elimination, thus giving rise to various ether linkages. 

In addition to /3-H elimination, olefin insertion, and protonolysis, the cr-metal intermediate has also proved to be capable of undergoing a reductive elimination to bring about an alkylative alkoxylation. Under Pd catalysis, the reaction of 4-alkenols with aryl halides affords aryl-substituted THF rings instead of the aryl ethers that would be produced by a simple cross-coupling mechanism (Equation (126)).452 It has been suggested that G-O bond formation occurs in this case by yy/z-insertion of a coordinated alcohol rather than anti-attack onto a 7r-alkene complex.453... 

P-H oxidative addition followed by alkyne insertion into a Pd-P bond gives the re-gio-isomeric alkenyl hydrides 15 and 16. Protonolysis with dialkyl phosphite regenerates hydride 17 and gives alkenylphosphonate products 18 and 19. Insertion of alkene 18 into the Pd-H bond of 17 followed by reductive elimination gives the bis-products, but alkene 19 does not react, presumably for steric reasons. P-Hydride elimination from 16 was invoked to explain formation of trace product 20. 

Since the first reports of Pd(II)-catalyzed hydroselenation and hydrothiolation 1992, considerable investigations have accumulated experimental evidence for the mechanism, in particular for Type I mechanism. Each step of Type I mechanism, structures of active catalysts, the reaction of alkynes with the active catalysts, and the protonolysis of the resulting vinyl metal complexes, has been verified for Pd, Ni, Zr, Ln, and An-catalyzed hydrochalcogenations by isolation of intermediates, isotope-labeled experiments, and kinetic studies. With regard to Type II mechanism, while the initial oxidative addition of REH (E = S, Se) to a low-valent transition metal catalyst (metal = Pd and Pt) has been verified by direct (for Pt) or indirect (for Pd) experimental evidence, the following steps of alkyne insertion to chalcogenolate-hydrido complex and reductive elimination of resultant vinyl metal complexes leave room for further mechanistic investigations to obtain direct evidence. On the other hand, a hybrid mechanism of Type I and Type II has been clarified for the hydrothiolation with Rh(I) complexes. 

In tenns of understanding the mechanistic aspects involved in such additimis on vinylic substrates via organometallic catalysts, analogies have been drawn to the hydroamination reactimis [28-30], Chiral metal complex-promoted asymmetric hydroaminations have been proposed to follow two different pathways. The first involves a sequence that commences with the oxidative addition of the N-H bond onto the metal ion followed by the insertion of the olefin and subsequent reductive elimination of the chiral substrate. An alternative pathway has also been proposed which involves the nucleophilic attack by the free amine on a coordinated olefin and a final protonolysis sequence, which leads to the release of the final product. Similar studies on metal irm-induced hydrophosphinations have been reported, and the mechanisms suspected to be in play include those proposed by Glueck and coworkers which basically involves the oxidative addition of a secondary phosphine followed by an olefin insertion [31], Togni and coworkers have also observed in certain scenarios the coordination of the olefin to the catalyst metal center followed by the addition of a secondary phosphine across the C-C double bond [32]. 

Scheme 2 shows an alternative inner-sphere mechanism, first involving the initial oxidative addition of NuH to the metal followed by olefin insertion into the M-Nu bond. The resulting M-C bond is cleaved by a C-H reductive elimination or by protonolysis (Scheme 2).While this mechanism is generally preferred for more electron-rich metals such as rhodium and iridium, several studies suggest that platinum and palladium-catalyzed additions of N-H or O-H nucleophiles more likely run by the outer-sphere electrophilic activation mechanism shown in Scheme 1.

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