Formyl complexes decomposition

Bimolecular decomposition of a non-metal hydride organometallic complex, such as metal formyl complexes, which may involve metal hydride precursors MC( = 0)H + H20 <-> M(CO)+ + OH + H2 N/A 36... 

All anionic transition metal formyl complexes described in the literature through the end of 1980 (21-47) are compiled in Table I. Since several of these have been prepared with more than one counterion, cations are not specified in the table. Geometric isomers are not assigned unless warranted by direct spectroscopic evidence. Also, the stability data in Table 1 should be regarded as qualitative, since decomposition rates have been shown to be dependent on both purity and counterion. When half-lives are specified, they are usually based upon measured rate constants. 

TOPICS RELATING TO THE STABILITY AND DECOMPOSITION OF TRANSITION METAL FORMYL COMPLEXES... 

Discussion of the decomposition chemistry of formyl complexes has been deferred until this stage because some of the reactivity modes described in Section IV can play important roles. 

Other anionic formyl complexes decompose by more complex pathways. Unstable formyl 6 (Scheme 4) yielded approximately equimolar amounts of (CO)5Mn, (CO)5Mn(COC6H5), and (after protonation) benzyl alcohol (31, 32). The rate of decomposition was first order, accelerated by... 

Neutral formyl complexes which contain ligating CO often decompose by decarbonylation however, several exceptions exist. For instance, the osmium formyl hydride Os(H)(CO)2(PPh3)2(CHO) evolves H2(54). Although the data are preliminary, the cationic iridium formyl hydride 49 [Eq. (14)] may also decompose by H2 evolution (67). These reactions have some precedent in earlier studies by Norton (87), who obtained evidence for rapid alkane elimination from osmium acyl hydride intermediates Os(H)(CO)3(L)(COR) [L = PPh3, P(C2H5)3], Additional neutral formyls which do not give detectable metal hydride decomposition products have been noted (57, 65) however, in certain cases this can be attributed to the instability of the anticipated hydride under the reaction conditions (H2 loss or reaction with halogenated solvents). 

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. 

The decomposition reactions of metal formyl complexes have been reviewed by Gladysz and coworkers ". ... 

Ru( CHO)( CO)(dppe)][SbFg] have been used also " to identify more fully the radicals formed in the decomposition of cationic formyl complexes of ruthenium. The conclusion has been reached that a radical mechanism does not constitute major pathway for these decompositions ... 

We have developed a new synthesis of metal formyl compounds from the addition of metal trialkoxyborohydrides to metal carbonyls (10,11). The formyl proton characteristically appears at very low field, 14-16 8, in the NMR spectrum of metal formyl complexes. This low field resonance has allowed us to rapidly survey the reactions of trialkoxyborohydrides with a series of metal carbonyls. Initially, Na HB(OCH3)3 was used as the borohydride reducing agent, but we have subsequently found that K HB(0-i Pr)3" is a more rapid and eflFective hydride donor (JO). We have obtained NMR evidence for the formation of metal formyl complexes in the reactions of K HB(O-f-Pr)3" with Fe(CO)5 (14.9 8) (C6H50)3PFe(C0)4 (14.8 8, d, / = 44) (C6H5)3PFe(CO)4 (15.5 8, d, / = 24) Cr(CO)6 (15.2 8) W(CO)e 5.9 8) and Re2(CO)io (16.0 8). In some cases we have isolated the metal formyl complexes. In other cases, such a Cr(CO)6, the maximum observed conversion to (CO)5Cr-CHO was 76% after 25 min at room temperature, and the formyl complex underwent subsequent decomposition with a half-life of 40 min at room temperature. 

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). 

Hydride Transfer Reactions of Metal Formyl Complexes. We have found that metal formyl complexes can act as hydride donors to electrophiles such as ketones, alkyl halides, and metal carbonyls. EUNHrans-[ (CeHsO) 3P] (CO) 3FeCHO" reacts with 2-butanone overnight at ambient temperature to give a 95% yield of 2-butanol. The possibility that 2-butanone is reduced by (CO)4FeH formed in situ from decomposition of the metal formyl complex is excluded since the metal formyl complex reacts with 2-butanone much faster than it decomposes to (CO)4FeH and since no reaction between (CO)4FeH and 2-butanone was observed by IR spectroscopy. 

Dissociation of formaldehyde, CHgO, at comparably low temperatures is obviously determined by a complex decomposition mechanism. Conclusions on the unimolecular dissociation can only be drawn from measurements at high temperatures under shock wave conditions. In this system the primary dissociation leading to formyl radicals is followed by decomposition of CHO and subsequent reactions of H atoms with CH2O and CHO. By analysing the chain mechanism the rate constant of the unimolecular reaction was derived. ... 

Lapinte and Astruc have studied the effects of solvent on the extent to which reduction of a carbonyl ligand occurs in [Fe CO)3(ti5-Cp )][PF6] upon treatment with sodium borohydride. The complex, [FeH(CO)2(il Cp )], was shown to form in aqueous solution or in a THF/water mixture. It was suggested 3 that it is formed by decomposition of an unstable formyl complex or its borane adduct. In dichloromethane, however, the reduction leads to formation of the hydrox)rmethyl product, [Fe(CH H)(CO)2(Tl5-Cp )], while the methyl complex, lFe(CO)2Me(Ti5-Cp )], is formed in anhydrous THF. 

A theoretical investigation showed that the most favourable unimolecular decomposition path of primary fluorozonide is a concerted cleavage to carbonyl oxide and formyl fluoride. The secondary fluorozonide decomposition takes place most readily in a stepwise manner initiated by the 0-0 bond rupture.153 DFT calculations have shown that ozone-difluoroethylene reactions are initiated by the formation of van der Waals complexes and then yield primary ozonides, which rapidly open to carbonyl oxide compounds. The formation of primary ozonide has been predicted to be the rate-controlling step of the oxidation process.154... 

The decompositions of the formyl and acetyl radicals are certain to be in the fall-off region at the pressures used in the investigations of the acetaldehyde pyrolysis. However, the complexity of the mechanism impedes any conclusion to be drawn from this system. 

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. 

The kinetics of the photolysis is much more complex at lower temperatures than at around 300 °C. The role of rate-determining step, i.e. the hydrogen atom transfer reaction (20) at high temperatures, is taken over by the decomposition of the acetyl radical as the temperature decreases. At the highest temperatures, the chains are terminated almost exclusively by the recombination of the methyl radicals, while at medium and low temperatures the disproportionation step (26) as well as self combination of the formyl and acetyl radicals are dominant. The first-order wall reaction of the radicals, such as reactions (22) and (31), may also play an important role, especially at low light intensities and pressures. On account of the aforesaid, it seems almost impossible to attempt a general discussion of the kinetics of the reaction. Instead, only selected questions will be dealt with in detail. 

Chiral electrophilic cyclopropanes (63) are prepared in high enantiomeric excess starting from butadiene-iron tricarbonyl complexes (60) containing a non-complexed double bond. Reaction with diazomethane and decomposition of the resulting pyrazolines (61) in the presence of Ce" gave the corresponding chiral cyclopropanes (62). Breakdown of the dienic substituent of electrophilic cyclopropane (62) by means of ozonization resulted in the formation of formyl-substituted electrophilic cyclopropane (63) still carrying the asymmetric centre (equation 10) " . ... 

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