B-hydride elimination

First b-hydride elimination occurs with the formation of (c), and then migratory insertion results in the formation of (d). Elimination of the acetaldehyde and a hydrogen ion then leaves Pd (0). The Pd (0) is converted back to Pd(II) by the auxiliary Cu(II)-catalysed air oxidation. 

Kalina, D. G., Marks, T. J. Wachter, W. A. "Photochemical Synthesis of Low-Valent Organothorium Complexes. Evidence for Photoinduced B-Hydride Elimination," J. Amer. Chem. 

The products of these reactions are thermally unstable, however the nature of the organic decomposition products is Interesting. For R phenyl, the major products are benzene and biphenyl (the latter could arise via reductive elimination in a "UO2R2" intermediate). For R = CH3 and vinyl, the predominant products are the corresponding RH molecules, while for R i. 3 7> il C4H9, and -04119, the products are suggestive of B-hydride elimination processes. 

Heavy-ends Heck mechanism heteroatom-directed hydroformylation B-hydride elimination hydroaminomethylation hydrocya nation hydrolysis phosphites... 

From these data, some key information can be drawn in both cases, the couple methane/pentane as well as the couple ethane/butane have similar selectivities. This implies that each couple of products (ethane/butane and methane/pentane) is probably formed via a common intermediate, which is probably related to the hexyl surface intermediate D, which is formed as follows cyclohexane reacts first with the surface via C - H activation to produce a cyclohexyl intermediate A, which then undergoes a second C - H bond activation at the /-position to give the key 1,3-dimetallacyclopentane intermediate B. Concerted electron transfer (a 2+2 retrocychzation) leads to a non-cychc -alkenylidene metal surface complex, C, which under H2 can evolve towards a surface hexyl intermediate D. Then, the surface hexyl species D can lead to all the observed products via the following elementary steps (1) hydrogenolysis into hexane (2) /1-hydride elimination to form 1-hexene, followed by re-insertion to form various hexyl complexes (E and F) or (3) a second carbon-carbon bond cleavage, through a y-C - H bond activation to the metallacyclic intermediate G or H (Scheme 40). Under H2, intermediate G can lead either to pentane/methane or ethane/butane mixtures, while intermediate H would form ethane/butane or propane. 

Silyl(pinacol)borane (88) also adds to terminal alkenes in the presence of a coordinate unsaturated platinum complex (Scheme 1-31) [132]. The reaction selectively provides 1,2-adducts (97) for vinylarenes, but aliphatic alkenes are accompanied by some 1,1-adducts (98). The formation of two products can be rationalized by the mechanism proceeding through the insertion of alkene into the B-Pt bond giving 99 or 100. The reductive elimination of 97 occurs very smoothly, but a fast P-hydride elimination from the secondary alkyl-platinum species (100) leads to isomerization to the terminal carbon. 

A catalyst used for the u-regioselective hydroformylation of internal olefins has to combine a set of properties, which include high olefin isomerization activity, see reaction b in Scheme 1 outlined for 4-octene. Thus the olefin migratory insertion step into the rhodium hydride bond must be highly reversible, a feature which is undesired in the hydroformylation of 1-alkenes. Additionally, p-hydride elimination should be favoured over migratory insertion of carbon monoxide of the secondary alkyl rhodium, otherwise Ao-aldehydes are formed (reactions a, c). Then, the fast regioselective terminal hydroformylation of the 1-olefin present in a low equilibrium concentration only, will lead to enhanced formation of n-aldehyde (reaction d) as result of a dynamic kinetic control. 

A mechanism was proposed in which entry into the catalytic cycle is achieved via Et2AlCl-mediated cobalt hydride generation. Diene hydrometallation affords the cobalt-complexed -jr-allyl A-5, which inserts the tethered alkene to furnish intermediate B-4. Elimination of LnCoOBn provides the cyclization product. Reduction of LnCoOBn by Et2AlCl regenerates cobalt hydride to complete the catalytic cycle (Scheme 17). 

The proposed catalytic cycle of the ruthenium-catalyzed intermolecular Alder-ene reaction is shown in Scheme 21 (cycle A) and proceeds via ruthenacyclopentane 100. Support for this mechanism is derived from the observation that the intermediate can be trapped intramolecularly by an alcohol or amine nucleophile to form the corresponding five-or six-membered heterocycle (Scheme 21, cycle B and Equation (66)).74,75 Four- and seven-membered rings cannot be formed via this methodology, presumably because the competing /3-hydride elimination is faster than interception of the transition state for these substrates, 101 and 102, only the formal Alder-ene product is observed (Equations (67) and (68)). 

Enantioselective hydrogenation of 1,6-enynes using chirally modified cationic rhodium precatalysts enables enantioselective reductive cyclization to afford alky-lidene-substituted carbocycles and heterocycles [27 b, 41, 42]. Good to excellent yields and exceptional levels of asymmetric induction are observed across a structurally diverse set of substrates. For systems that embody 1,2-disubstituted alkenes, competitive /9-hydride elimination en route to products of cycloisomerization is observed. However, related enone-containing substrates cannot engage in /9-hydride elimination, and undergo reductive cyclization in good yield (Table 22.12). 

The steric effects become clear on inspecting the Newman projections of the transition states. The conformation 43 a leads to cyclopentane formation, while the conformation 43 b would give / -H elimination. As the rhodium carbenoid becomes larger, conformation 43 b is increasingly favored. Thus, as the steric bulk of the ligands on the Rh carbenoid increases on going from acetate (entry 1) to the TPA catalyst (entry 4), there is a significant increase in the proportion of /9-hydride elimination. 

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