Mechanism, metal hydride

Another difference between the two mechanisms is that the former involves 1,2 and the latter 1,3 shifts. The isomerization of 1-butene by rhodium(I) is an example of a reaction that takes place by the metal hydride mechanism, while an example of the TT-allyl complex mechanism is found in the Fe3(CO)i2 catalyzed isomerization of 3-ethyl-l-pentene. " A palladium acetate or palladium complex catalyst was used to convert alkynones RCOCSCCH2CH2R to 2,4-alkadien-l-ones RCOCH= CHCH = CHCHR. ... 

The conjugated diene (including the trans-trans, trans-cis, and cis-cis isomers) can further add ethylene to form Cg olefins or even higher olefins (/). The mechanism of isomerization is proposed to be analogous to butene isomerization reactions (4, 8), i.e., 1-butene to 2-butene, which involves hydrogen shifts via the metal hydride mechanism. A plot of the rate of formation of 2,4-hexadiene vs. butadiene conversion is shown in Fig. 2. 

The metal hydride mechanism was first described for the cobalt-carbonyl-catalyzed ester formation by analogy with hydroformylation.152 It was later adapted to carboxylation processes catalyzed by palladium136 153 154 and platinum complexes.137 As in the hydroformylation mechanism, the olefin inserts itself into the... 

The metal hydride mechanism has been written particularly for hydrocarboxylation reactions with a palladium catalyst.67,68 In the reactions of propene in the presence of (PhaP dCk, the acyl complex (18) was isolated from the reaction mixture, and also shown to be a catalyst for the reaction. 

Both mechanisms are predicted to show syn addition of hydride and caiboxylate to the alkene. In the metal hydride mechanism (equation 36) alkene insertion is syn and CO insertion proceeds with retention of configuration at carbon. In the metal carboxylate mechanisms (equation 37) alkene insertion is syn and cleavage of the metal-carbon bond can take place with retention at carbon. The palladium-catalyzed hydroesterification reaction produces the erythro ester from (Z)-3-methyl-2-pentene (equation 38) and the threo ester from ( )-3-methyl-2-pentene (equation 39).w... 

At low temperatures the intermediate /3-allylrhodium(III) hydride PF3 complex was detected by 1H and 19 F NMR spectroscopy and provided supportive evidence for the isomerisation of alkenes via an allyl-metal-hydride mechanism (285, 293). 

The fundamental differences between these two mechanisms are that 1) the jr-allyl metal hydride mechanism involves a 1,3-hydrogen shift while the metal hydride addition-elimination mechanism involves a 1,2-hydrogen shift and 2) the hydrogen shift in the Jt-allylhydride mechanism proceeds in an intramolecular fashion while that in the metalhydride addition-elimination mechanism proceeds in an intermolecular fashion. 

A crossover experiment using 3 and 4 under standard conditions demonstrated the intramolecularity of this hydrogen shift. Intramolecularity of the isomerization and the 1,3-hydrogen shift strongly indicates that the reaction proceeds via the Jt-allyl metal hydride mechanism as depicted in Scheme 12.1. 

Isomerization of allylic and homoallylic alcohols is also catalyzed by the zwitterion-ic Rh(I) complex (sulphos)Rh(COD), with sulphos = [03S(C6H4)CH2C(CH2PPh2)3], in water/n-octane to give the corresponding aldehyde or ketone in high yields and chemoselectivity. A jr-allyl metal hydride mechanism was proposed on the basis of various independent experiments in both homogeneous and biphasic systems [3a]. 

The mechanism of Ni-catalyzed ethylene oligomerization involves both nickel hydride and nickel alkyl species. The mechanism is known in the literature as the metal-hydride mechanism, Cossee-Arlman mechanism, or ethylene insertion - -hydride elimination mechanism and results in a Schulz-Flory distribution of the oligomerization products. The mechanism is depicted in Figure 6.16.4. Note that two other coordination sites at the nickel are occupied by one bidentate ligand or two monodentate ligands (see Section 2.4 for details) that have been omitted in Figure 6.16.4 for clarity. 

Figure 6.16.4 Formation of linear a-olefins according to the metal hydride mechanism.

Alkenes.—Deuterium exchange of propene with MeOD homogeneously catalysed by complexes of platinum, rhodium, and nickel can be monitored by microwave spectroscopy. The results show considerable incorporation of deuterium at C-2, a result which cannot be accommodated by the 7t-allyl-metal hydride mechanism for exchange/isomerization of olefins. However, the n- or /i -allyl-metal hydride mechanism for olefin isomerization has received some useful supporting evidence. The compound (54) can be generated... 

The reduction of iminium salts can be achieved by a variety of methods. Some of the methods have been studied primarily on quaternary salts of aromatic bases, but the results can be extrapolated to simple iminium salts in most cases. The reagents available for reduction of iminium salts are sodium amalgam (52), sodium hydrosulfite (5i), potassium borohydride (54,55), sodium borohydride (56,57), lithium aluminum hydride (5 ), formic acid (59-63), H, and platinum oxide (47). The scope and mechanism of reduction of nitrogen heterocycles with complex metal hydrides has been recently reviewed (5,64), and will be presented here only briefly. 

The mechanism of the poisoning effect of nickel or palladium (and other metal) hydrides may be explained, generally, in terms of the electronic theory of catalysis on transition metals. Hydrogen when forming a hydride phase fills the empty energy levels in the nickel or palladium (or alloys) d band with its Is electron. In consequence the initially d transition metal transforms into an s-p metal and loses its great ability to chemisorb and properly activate catalytically the reactants involved. 

There is no clear reason to prefer either of these mechanisms, since stereochemical and kinetic data are lacking. Solvent effects also give no suggestion about the problem. It is possible that the carbon-carbon bond is weakened by an increasing number of phenyl substituents, resulting in more carbon-carbon bond cleavage products, as is indeed found experimentally. All these reductive reactions of thiirane dioxides with metal hydrides are accompanied by the formation of the corresponding alkenes via the usual elimination of sulfur dioxide. 

Remarkable solvent effects on the selective bond cleavage are observed in the reductive elimination of cis-stilbene episulfone by complex metal hydrides. When diethyl ether or [bis(2-methoxyethyl)]ether is used as the solvent, dibenzyl sulfone is formed along with cis-stilbene. However, no dibenzyl sulfone is produced when cis-stilbene episulfone is treated with lithium aluminum hydride in tetrahydrofuran at room temperature (equation 42). Elimination of phenylsulfonyl group by tri-n-butyltin hydride proceeds by a radical chain mechanism (equations 43 and 44). 

The racemization mechanism of sec-alcohols has been widely studied [16,17]. Metal complexes of the main groups of the periodic table react through a direct transfer of hydrogen (concerted process), such as aluminum complexes in Meerwein-Ponn-dorf-Verley-Oppenauer reaction. However, racemization catalyzed by transition metal complexes occurs via hydrogen transfer processes through metal hydrides or metal dihydrides intermediates (Figure 4.5) [18]. 

The mechanism of homogeneous hydrogenation catalyzed by RhCl(Ph3P)3 ° involves reaction of the catalyst with hydrogen to form a metal hydride (PPh3)2RhH2Cl (43), which rapidly transfers two hydrogen atoms to the alkene. 

The proposed mechanism for the metallocene complexes of Group 5 metals and lanthanides involves the coordination of alkene to the metal hydride (51), followed... 

Catecholborane and pinacolborane are especially useful in hydroborations catalyzed by transition metals.163 Wilkinson s catalyst Rh(PPh3)3Cl is among those used frequently.164 The general mechanism for catalysis is believed to be similar to that for homogeneous hydrogenation and involves oxidative addition of the borane to the metal, generating a metal hydride.165... 

A related study with a similar ruthenium catalyst led to the structural and NMR characterization of an intermediate that has the crucial Ru—C bond in place and also shares other features with the BEMAP-ruthenium diacetate mechanism.33 This mechanism, as summarized in Figure 5.4, shows the formation of a metal hydride prior to the complexation of the reactant. In contrast to the mechanism for acrylic acids shown on p. 378, the creation of the new stereocenter occurs at the stage of the addition of the second hydrogen. 

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