Hydride-formyl equilibrium

Participation of the hydride-formyl equilibrium in (16) is also plausible in light of an apparent inverse kinetic deuterium isotope effect for the catalytic process. Use of deuterium gas instead of hydrogen (cf. Expts. 6 and 4 in Table II) causes an increased rate, with kH/k = 0.73 (37). The existence of an isotope effect implies that hydrogen atom transfer occurs before or during the rate-determining step, and an inverse kinetic isotope effect may be possible in the case of a highly endothermic, product-like transition state (73). On the other hand, Bell has concluded that inverse kinetic isotope... 

While the mechanism for the formation of the methoxy complex (14) is not established, it is significant that the dihydride Zr(C5Me5)2H2 is needed for the reduction of the CO coordinated in (13). A reasonable proposal for this reaction can be formulated if it is assumed that since complex (13) is formally d°, Zr—CO backbonding will not be of major importance, and that hydride complexes of the group 4 elements possess substantial hydridic character. The first assumption may lead to a more favorable equilibrium constant for carbonyl hydride formyl interconversion as in (5), while the second suggests H" attack in this sequence presumably on a coordinated formyl. If the latter results in Zr—H addition across C=0, then reductive elimination of a C—H bond leads to the observed product. This is shown in (21). 

Another possible reason that ethylene glycol is not produced by this system could be that the hydroxymethyl complex of (51) and (52) may undergo preferential reductive elimination to methanol, (52), rather than CO insertion, (51). However, CO insertion appears to take place in the formation of methyl formate, (53), where a similar insertion-reductive elimination branch appears to be involved. Insertion of CO should be much more favorable for the hydroxymethyl complex than for the methoxy complex (67, 83). Further, ruthenium carbonyl complexes are known to hydro-formylate olefins under conditions similar to those used in these CO hydrogenation reactions (183, 184). Based on the studies of equilibrium (46) previously described, a mononuclear catalyst and ruthenium hydride alkyl intermediate analogous to the hydroxymethyl complex of (51) seem probable. In such reactions, hydroformylation is achieved by CO insertion, and olefin hydrogenation is the result of competitive reductive elimination. The results reported for these reactions show that olefin hydroformylation predominates over hydrogenation, indicating that the CO insertion process of (51) should be quite competitive with the reductive elimination reaction of (52). 

The rate of decomposition of (C2H5)4N+ 25 was found to be first order and independent of added phosphite. At 63°C, the following activation parameters were obtained A//1 = 29.0 1.5 kcal/mol A5 = 7.9 6.1 eu. These data suggest that (provided the reaction is analogous to well-established metal acyl decarbonylation mechanisms) (4, 5) loss of phosphite is followed by rapid hydride migration to the metal, as shown in Eq. (31). None of the formyl could be detected to be in equilibrium with (CO)4FeH , even in the presence of excess (ArO)3P. 

Some of the decomposition reactions in Section V,A above bear upon the thermochemistry of Eq. (2) (Section II). Since a metal hydride has never been observed to be in equilibrium with a measurable quantity of a metal formyl (in Eq. (31), 19f of 25 would have been detectable) (42), AGrsn for Eq. (2) (forward direction) is commonly >3 kcal/mol (see, however, Addendum, p. 34). In contrast to Eq. (31), the acyl-alkyl equilibrium in Eq. (34) lies far to the left (42). 

The fact that there is such a paucity of metal formyl complexes is both interesting and significant because of the proposed intermediacy of coordinated formyl in CO reduction, and the sharply contrasting abundance of metal acyl complexes. Since many of the acyl complexes are known to form by migratory insertion of CO in an alkyl carbonyl complex (20, 20a, 22), the lack of formyl complexes from hydride carbonyls relates to the thermodynamic difference in the equilibrium (5) when Y is alkyl and when it is hydride. 

For the mono-insertion step with metal carbonyl compounds, very fast reactions were noticed, which led to complete formation of the p-formyl complexes, as displayed in Scheme 3. The hydride Mo(NO)(dmpe)2H showed even such a high activity that it inserted into a Re-(CO) bond of Re CCO), twice. The second insertion step, however, again represents an equilibrium reaction, lying far on the product side. 

Equilibrium between Metal Formyl Complexes and Metal Hydrides ... 

Octaethylporphyrin rhodium II dimer, [(OEP)Rh]2r reacts with H2 and CO to produce an equilibrium distribution of hydride and formyl complexes (Equations 1-3).Thermodynamic and kinetic measurements for this system have... 

Metalloformyl complexes are the most probable first organometallic intermediates in metal complex promoted reactions of H2 and CO that produce organic oxygenates. Production of large equilibrium concentrations of T Carbon bonded formyl complexes from reactions of metal hydrides with CO (Equation 6) requires that the M-H bond... 

The formal transfer of hydride is a fundamental reaction in biological catalysis. The Re formyl complex GpRe(NO)(GO)(GHO), the hydricity of which was determined by equilibrium measurements (Equation (25)), engaged in a reversible hydride transfer with an NAD/NADH model system, BzNAD /BzNADH (Equation (31)). 

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