Formyl complexes kinetics

Reaction (64) demonstrates the production of a metal formyl complex by intermolecular hydride transfer from a metal hydride which is expected to be regenerable from H2 under catalytic conditions. Further, it provides a plausible model for the interaction of [HRu(CO)4] with Ru(CO)4I2 during catalysis, and suggests a possible role for the second equivalent of [HRu(CO)4]- which the kinetics indicate to be involved in the process (see Fig. 23). Since the Ru(CO)4 fragment which would remain after hydride transfer (perhaps reversible) from [HRu(CO)4] is eventually converted to [HRu3(CO)),] [as in (64)] by reaction with further [HRu(CO)4], the second [HRu(CO)4]- ion may be involved in a kinetically significant trapping reaction. 

Unfortunately, formic acetic anhydride is not a general reagent for formyl complex synthesis (29). One reason is that formylation of a transition metal monoanion would afford a neutral formyl complex. Insofar as comparisons are valid, neutral formyl complexes tend to be kinet-ically less stable than anionic formyl complexes. In cases where neutral formyl complexes are stable (vide infra), the corresponding transition metal monoanions are unknown. Whereas formic acetic anyhydride might be of greater use for the preparation of anionic formyl complexes from transition metal dianions, only a limited number of transition metal dianions [i.e., (CO)5Cr2", (t7-C5H5)(CO)3V2 J are known (49). These appear to... 

Formyl transfers" involving neutral formyl complexes, such as shown in Eq. (22), have recently been reported (68). By precipitation of the metal carbonyl cation by-product, quite pure solutions of kinetically labile neutral formyl complexes may be obtained. 

Hiickel MO calculations have not revealed any intrinsic kinetic barrier to hydride migration to coordinated CO (93). Thus it is worthwhile to consider possibilities that might mask the occurrence of a metal hydride carbonylation reaction. For instance, metal hydrides have been observed to react rapidly with metal acyls reduction products such as aldehydes or bridging —CHRO— species form (94-96). Therefore, it is possible that a formyl complex might react with a metal hydride precursor at a rate competitive with its formation. Such a reaction could also complicate the decomposition chemistry of formyl complexes. Preliminary studies have in fact shown that metal hydrides can react with formyl complexes (35, 57), but a complete product analysis has not yet been done. 

Several trends in the kinetic stability of formyl complexes are evident in Tables I and II. First, formyl complexes of third row transition metals... 

The actinide compounds [Cp2ThH(/i-H)]2 (Cp = sterically encumbered Cp) react reversibly with CO to yield formyl complexes. The product of the reaction of Cpf(NpO)ThH (Np = neopentyl) was characterized spectroscopically, and the thermodynamics and kinetics of the insertion reaction were analyzed ... 

Kinetic Stability of Metal Formyl Complexes. Metal formyl complexes have approximately the same kinetic stability as the corresponding metal acetyl complexes. Thermal decomposition of (CH3CH2)4N [(C6-H50)3P] (CO)3FeCHO" in THF at 65°C gives a mixture of two metal hydrides in a 4 1 ratio (CO)4FeH , formed by loss of phosphite and... 

A detailed kinetic study of Reaction 2 was carried out. The rate of formation of metal hydride from metal formyl complex was followed by NMR. First-order kinetics were observed for Reaction 2 to more than two half-lives, indicating that the rate of reaction was independent of the concentration of phosphite. In related experiments we have found that the initial rate of Reaction 2 is independent of added phosphite. Only the phosphorus-containing species shown in Reaction 2 were observed by 3ip NMR. The half-life for decomposition of (CH3CH2)4Nl(ArO)3P]-(CO)3FeCHO in THF at 67.3°C was found to be 1.1 hr. Measurement of the rate of decomposition of the metal formyl complex over the temperature range 47°-79°C gave an activation energy for the process of 29.7 2 kcal/mol. (aH+ = 29.0 1.5 kcal, AS=t= = 7.9 6.1 eu at 63°C). 

Now that we have an eflScient route to metal formyl complexes, we can study the thermodynamic stability of these complexes. We have found that metal formyls are much less thermodynamically stable than the corresponding metal hydrides. Thus, metal formyl species have never been observed in the reactions of metal hydrides with CO because of the thermodynamic instability of the formyl complexes and not because of their kinetic instability. 

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... 

HPt(dmpp), has a thermodynamic and kinetic hydricity that is sufficient for the transfer of to coordinated GO in GpRe(NO)(GO)2 and Gp Re(NO)(GO)2 to give the respective Re formyl complexes (Equation (28)). The overall reaction is in principle catalytic with respect to Pt. Although the yields were low (9%), the design of this system demonstrated how access to relevant thermodynamic data may facilitate rational catalyst design. 

A hydride is one of the simplest nucleophiles, and Casey ° and Gladysz have prepared kinetically stable formyl complexes by the direct attack of hydride on a number of neutral chromium-, molybdenum-, and iron-carbonyl complexes (Equation 11.2). Although these complexes are relatively electron rich, because they possess zero-valent metal centers, the negative charge in the product can be stabilized by the remaining -ir-ac-cepting CO ligands. 

Reduction of the 4-formylbenzonitrile complex (18) also proceeds by remote attack. The subsequent aquation of the carbonyl-bound chromium(iii) complex (19) can be detected kinetically. The analogous reaction with the 3-formyl complex leads to [Cr(H20)eP+ as the only observable product, but the possibility of a labile... 

If a complex contains both carbonyl and unsaturated hydrocarbyl ligands, hydride addition may follow both of the available pathways. Not all of the factors controlling the directions taken by such reactions have been elucidated. Generally, - formyl complexes are the kinetic products, whereas, the attack of hydride on unsaturated hydrocarbyl ligands gives file thermodynamic products. 

Formylation of aromatic substrates with the dimethylformamide-car-bonyl chloride complex in chloroform68 demonstrates simple kinetics.69... 

The HCo(CO)4 complex is therefore inferred to be involved in initial hydrogen transfer to carbon monoxide. This step was initially proposed to comprise rate-determining hydrogen atom transfer from HCo(CO)4 to free CO, affording a formyl radical, HtO subsequent reaction with further HCo(CO)4 would lead to the observed products (35). However, kinetic observations (the zero-order dependence on CO partial pressure) were later made which are inconsistent with such a process (36). 

The ruthenium carbonyl complexes [Ru(CO)2(OCOCH3)] n, Ru3(CO)12, and a new one, tentatively formulated [HRu-(CO)s ] n, homogeneously catalyze the carbonylation of cyclic secondary amines under mild conditions (1 atm, 75°C) to give exclusively the N-formyl products. The acetate polymer dissolves in amines to give [Ru(CO)2(OCOCH3)(amine)]2 dimers. Kinetic studies on piperidine carbonylation catalyzed by the acetate polymer (in neat amine) and the iiydride polymer (in toluene-amine solutions) indicate that a monomeric tricarbonyl species is involved in the mechanism in each case. 

A review of formylation reactions involving methyl formate in a hydrogen fluoride-boron trifluoride medium has appeared.60 Regioselectivity and kinetic data have been reported for Gattermann-Koch formylation in superacids and provide evidence for an intra-complex reaction where the formylation electrophile HCO+ is generated by protonation of CO by the arenium ion.61 The observed selectivity results from... 

Below about 200 °C, the photo-decomposition of acetaldehyde becomes involved as a result of the increased stability of the formyl and acetyl radicals. The occurrence of new elementary steps, in addition to those already mentioned, renders the kinetics of the reaction rather complex. 

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