Hydride insertion, reversibility

The ion 28 loses H2 by CID with argon to form [(PHOX)Ir(styrene)]+ (29). Compound 29 then undergoes H-D exchange with D2 gas to form the mixture of iso-topomers 29, 29-dh and 29-d2 (Scheme 13.3). When combined, these observations show that the oxidative addition of H2 to 29 is followed by alkene hydride insertion, and that both these steps occur rapidly and reversibly in the gas phase. These results thereby provide gas-phase analogues for catalytic elementary steps that are proposed to occur in solution. Support for this proposed sequence of steps was obtained from a solution-phase catalytic deuteration of styrene. Analysis showed no deuterium incorporation in the unreacted styrene at various conversions, and clean formation of dideuterio ethylbenzene as sole product. 

Probably the nickel carbonyl-catalyzed synthesis of acrylates from CO, acetylene, and hydroxylic solvent (78) involves an acetylene-hydride insertion reaction, followed by a CO insertion, and hydrolysis or acyl halide elimination. The actual catalyst in the acrylate synthesis is probably a hydride formed by the reversible addition of an acid to nickel carbonyl. 

Mechanistic smdies of Pd(MeCN)2Cl2-catalysed hydroalkylation reactions of allylic amine derivatives by n-BuZnBr in the presence of Zn(OTf)2, benzoquinone, and DMA suggested a reversible )3-hydride elimination/hydride insertion process leading to the primary Pd-alkyl intermediate, which underwent trans-metalation followed... 

Palladium Pd(II)-catalysed hydroalkylation of Al-protected allylic amines PG(R )N-CH(R )C=CH2 (PG = protecting group) by Bu"ZnBr and other alkylzinc reagents has been reported to afford anti-Markovnikov products PG(R )N-CH(R )CH2CH2-Bu". Mechanistic studies suggest that a reversible jS-hydride elimination/hydride insertion process furnishes the primary Pd-alkyl intermediate, which undergoes transmetallation followed by reductive elimination to form a new sp -sp carbon-carbon bond. ° DFT PBE/3z calculations have been employed to elucidate the solvent effect on hydroxymethoxycarbonylation of cyclohexene catalysed by (Ph3P)2Pd. ... 

An a-hydrogen elimination is the microscopic reverse of hydride insertion/imino formyl formation and affords the nickel(ii) hydride complex (c. Scheme 2). Subsequent olefin insertion and isocyanide insertion gives hydrocarbation product (f. Scheme 2). Isotopic labeling experiments by using < 4-ethylene or [Ni C(D)N(D)xylyl (triphos)](CF3S03)2 showed deuterium at both the methylene group and the methyl group of the a-ethyl carbene (f. Scheme 2), not expected in an alkene pathway. 

Another reaction in the last step is the syn elimination ofhydrogen with Pd as H—Pd—X, which takes place with alkyl Pd complexes, and the Pd hydride and an alkene are formed. The insertion of an alkene into Pd hydride and the elimination of, (3-hydrogen are reversible steps. The elimination of, 3-hydrogen generates the alkene, and both the hydrogen and the alkene coordinate to Pd, increasing the coordination number of Pd by one. Therefore, the / -elimination requires coordinative unsaturation on Pd complexes. The, 3-hydrogen eliminated should be syn to Pd. 

The main aim of this review is to survey the reactions by which the Co—C bond is made, broken, or modified,.and which may be used for preparative purposes or be involved in catalytic reactions. Sufficient evidence is now available to show that there exists a general pattern of reactions by which the Co—C bond can be made or broken and in which the transition state may correspond to Co(III) and a carbanion (R ), Co(II) and a radical (R-), Co(I) and a carbonium ion (R ), or a cobalt hydride (Co—H) and an olefin. Reactions are also known in which the organo ligand (R) may be reversibly or irreversibly modified (to R ) without cleavage of the Co—C bond, or in which insertion occurs into the Co—C bond (to give Co—X—R). These reactions can be shown schematically as follows ... 

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. 

The reversal of the insertion reaction [Eq. (10)] is not normally observed [in contrast to nickel hydride addition to olefins, Eq. (9)]. An exception is the skeletal isomerization of 1,4-dienes (88, 89). A side reaction—the allylhydrogen transfer reaction [Eq. (5)]—which results in the formation of allylnickel species such as 19 as well as alkanes should also be mentioned. This reaction accounts for the formation of small amounts of alkanes and dienes during the olefin oligomerization reactions (51). 

The metal hydride bond is stronger than a metal carbon bond and the insertion of carbon monoxide into a metal hydride is thermodynamically most often uphill. Alkene insertion into a metal hydride is thermodynamically allowed and often reversible. 

Reversibility of hydrogen charging was very good at room temperature. This is because the hexagonal lattice of the metal host, like this of LaNi, does not undergo major transformation as hydrogen is inserted interstitially. This was the first AB -type interstitial hydride in which hydrogen is stored between metal atoms. 

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