Formyl complex formation

A key step proposed in the radical chain mechanism for the formation of the formyl complex is the coordination of CO to the Rh(OEP)- monomer, to give an intermediate carbonyl complex, Rh(OEP)(CO)- which then abstracts hydride from Rh(OEP)H to give the formyl product.This mechanism was proposed without direct evidence for the CO complex, and since then, again from the research group of Wayland, various Rh(fl) porphyrin CO complexes, Rh(Por)(CO), have been observed spectroscopically along with further reaction products which include bridging carbonyl and diketonate complexes. 

Mechanistic observations on formation of hydrocarbons in Cp2Fe2(C0K with LAH. There is little doubt that the initial step in the reaction of Cp2Fe2(C0K with LAH involves formation of a formyl complex by addition of a hydride to coordinated CO. 

Reagents such as LiAlH4 and KH are not effective for the synthesis of formyl complexes. LiAlH4 does react with many metal carbonyl compounds, but it can transfer more than one and usually effects the formation of metal hydride products (50). Similar results are usually found with NaBH4(50), although some neutral formyl complexes (vide infra) can be obtained under special conditions. KH will also react with some metal carbonyls. However, rates are not very rapid, and any formyl intermediates are likely to decompose faster than they form (51). 

Two reports of H2 formation upon acidification of anionic formyls 6 (31) and 19 (38) could not be reproduced (32, 47). Thus there are no documented examples of H2 evolution upon protonation of anionic formyl complexes. It is clear, however, that rapid reactions ensue in all cases (32, 47, 66) and that good yields of neutral metal carbonyl (H loss) products are 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. 

It is significant that the only stable formyl complexes isolated to date (58-60) are coordinatively saturated, which eliminates the possible conversion to a carbonyl hydride without the prior loss of a ligand. The unfavorable thermodynamics of (5) for formyl formation are a necessary consideration in developing schemes for CO reduction by this method. 

The years 1978 and 1979 have witnessed continuing activity on the catalytic reduction of CO and models for it. Both Casey and Gladysz have established that the neutral formyl complex (C5H5)Re(CO)(NO)(CHO) which they synthesized (59b,c) is the first intermediate in the borohydride reduction of coordinated CO to methyl as reported by Graham and co-workers (55). When the neutral formyl complex is reacted with BH3 THF, the species (C5H5)Re(CO)(NO)(CH3) results. A similar reduction does not occur when H2 is used as the reductant, however (59b). While the previous report by Nesmeyanov et al. (86) of a hydroxymethyl species in the BH4 reduction process is now viewed as incorrect (59b,e), Casey has recently described (59e) unequivocal characterization of this species, and has shown how the formyl complex (C5H5)Re(CO)(NO)(CHO) can lead to its formation as shown in (23a). 

Acetylation of pyrrole is difficult because if forms a 2 1 complex with stannic chloride (29CB226). Hence, under the conditions used for the other five-membered rings (i.e., acetic anhydride in the presence of one hundredth molar equivalent of stannic chloride or iodine), no reaction occurs, and only 20% acetylation is obtained if the molar proportion of the catalyst is reduced 10-fold. The effect of complex formation also shows up in the inhibition of stannic chloride-catalyzed acetylation of fu-ran or thiophene, on addition of pyrrole (67MI4). Catalyzed acetylation of 2-cyano-, 2-formyl-, or 2-methoxycarbonylpyrrole gives mainly 4-substitution (67CJC897) indicating that the catalyst must also be coordinated with the substrate l-methyl-3-nitropyrrole acetylates only in the 5-posi-tion (57CJC21). 

The formation of a metal formyl complex by the transfer of hydrogen to a coordinated CO molecule is an attractive route for a catalytic CO iydrogena-tion. Formyl intermediates can account for C species such as methanol or formaldehyde. They can also be considered as intermediates leading to nrethane. 

Trialkylborohydride reducing agents differ from borohydride in their ability to transfer hydride directly to a carbonyl ligand without prior substitution in the coordination sphere. They are used to synthesize formyl complexes " . When formyl complexes lose CO and undergo hydride migration from the formyl ligand to the metal, a transition-metal hydride results. The process is formally similar to nucleophilic attack by [OH] on a carbonyl ligand, followed by loss of CO and formation of a transition-metal hydride. Examples of hydride syntheses via formyl complexes are ... 

Most other inorganic reactions have been carried out using ETC catalysis isomerization of octahedral complexes [39 1], disproportionation [42], metal-metal bond cleavage and formation [43, 44], CO extrusion in formyl complexes [11]. Although many studies involve electrochemical initiation, the use of a chemical oxidant is also often shown to work. It is possible to use a photoexcited state as the initiator given its enhanced redox power [45]. 

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. 

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. 

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). 

Immediately, the first step of our exercise is the most difficult and controversial one. The first reaction step in Scheme 1 is formally seen as a hydrogen addition to a CO ligand with the formation of a formyl complex. Such a formyl complex is not favoured by thermodynamics and it does not... 

To some atuhors, this situation makes it a less likely intermediate, but others state that the scarcity of known formyl complexes rather reflects the ease of their further transformation than the difficulties of their formation. 

Tris (trifluoromethyl)phosphine, (Cp3)3P, bp = 173°C, can be obtained from F3CI and white phosphorus. It is a spontaneously inflammable liquid but it is stable in boiling water. Stable formyl phosphines have been prepared [19]. (6.65a). Some unusual phosphines can be stabilised by metal complex formation [20] (6.65b). 

Of particular interest are phosphido-bridged derivatives of iron carbonyls, such as (OC)3 Fe(/i-PR2)2 Fe(CO)3. Upon reduction, these compounds give anions of the type [(OC)3 Fe(/i-PR2)2 Fe(CO)3] , in which the Fe —Fe bond is cleaved. The reduction with [BEt3H] leads to the formation of formyl complexes and the reaction with LiR furnishes acyl coordination compounds. In the reaction of... 

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