Intermediates formyl complexes

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. 

The CO reductions generally could likely proceed through formyl intermediates, probably at a multinuclear site (420) hydride migration to a coordinated CO [e.g., as in the hypothetical scheme outlined in Eq. (72)] has not yet been observed, although metal formyl complexes have been synthesized via other methods (422-425). A ir-bonded formyl also seems plausible (426), since 7r-bonded acyl groups have been demonstrated (427). A stoichiometric hydrogen reduction of CO to methanol under mild conditions via a bis(pentamethylcyclopentadienyl)zirconium complex is considered to go through a formyl intermediate (428, 429) ... 

This was the first example in which models for presumed Fischer-Tropsch intermediates have been isolated and their sequential reduction demonstrated. Neither methane nor methanol was observed from further reduction of the methyl and the hydroxymethyl complexes. The use of THF/H20 as solvent was crucial in this sytem in THF alone CpRe(C0)(N0)CH3 was the only species observed, probably because the initial formyl complex was further reduced by BH3.— When multihydridic reagents are reacted with metal carbonyl complexes, formyl species are usually not observed. The rapid hydrolysis of BH3 by aqueous THF allowed NaBH to act as a... 

We believe the initial site of attack is on a terminal CO which should be more susceptible than the more electron rich bridging CO s.— The formyl complex will not be "free" but will almost certainly have aluminum coordinated to the oxygen. Further reduction to a methyl could occur as was observed in NaBHi, reduction of CpRe(C0)2N0. We would concur with the statement that the intermediates will all have coordination of the aluminum to the oxygen during the reduction. We have demonstrated in a separate experiment that methane is formed when CH3FeCp(C0)2 is reacted with LAH. 

In contrast, spectroscopic and crystal structure analysis indicates that nucleophilic attack of hydride on 72 occurs on the face of the ligand which is coordinated to the metal (Scheme 17). No intermediate species could be detected for this latter reaction. Monitoring of the reduction of the rhenium analog 74 with sodium borohydride indicated the intermediacy of a rhenium formyl complex 75, presumably formed by attack on a coordinated carbon monoxide. Signals for 75 eventually disappear and are replaced by those of the (diene)rhenium product 76 (Scheme 18)95. 

Ojima has proposed a mechanism for the rhodium-catalyzed cyclization/silylformylation of enynes that invokes several of the same intermediates proposed for the rhodium-catalyzed cyclization/hydrosilylation of enynes (Scheme 7). Silylmetallation of the G=G bond of the enyne followed by / -migratory insertion of the pendant G=G bond into the resulting Rh-G bond could form rhodium cyclopentyl complex Illf. a-Migratory insertion of GO into the Rh-G bond of Illf followed by silane-promoted reductive elimination from the resulting rhodium formyl complex rVf could release the silylated cyclopentane carboxaldehyde with regeneration of silylrhodium hydride complex If (Scheme 7). 

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). 

As shown in Eqs. (17) and (18), the isolated formyls 19 and 24 are capable of reducing aldehydes and ketones (37, 38, 42. 47, 66). Thus there is no doubt that hydride transfer is an intrinsic chemical property of anionic formyl complexes. One reaction of a neutral formyl complex with an aldehyde has been reported addition of benzaldehyde to (i7-C5H5)Re(NO)(CO)(CHO) (38) yields the alkoxycarbonyl complex (i7-C5H5)Re(NO)(CO)(C02CH2C6Hs) (62). This transformation, which appears to require catalysis by adventitious acid, can be viewed as occurring via attack of initially formed benzyl alcohol upon the intermediate carbonyl cation [(i -C5H5)Re(NO)(CO)2]+. 

Two formyl transfer" reactions of isolated formyl complexes are shown in Eqs. (20) (37, 42) and (21) (37, 38, 42, 47, 66). Control experiments indicate that these reactions do not involve metal hydride intermediates (formed via decarbonylation). Straightforward intermolecular H transfer (rather than formyl ligand transfer) is believed to be taking place. 

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). 

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). 

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. 

Two neutral formyl complexes have been synthesized and isolated via equation (a), beginning with the cationic carbonyl compounds, CpRe(NO)(CO)L where L = CO and PPh3 . These compounds are intermediates in the BH4 reduction of coordinated... 

This was confirmed by a number of experiments using a tethered binuclear [Rh (por)] analogue with steric properties comparable to [Rh (TMP)] (Fig. 45) (142). Whereas hydride species obtained by reaction with H2 failed to react subsequently with CO over a period of weeks, the sequential reaction in the reversed order (i.e., first CO, and then H2) gave the bis(formyl) species as the only species observed. Reactions with water yielded formyl-hydride species, presumably via an intermediate formyl-carboxylic acid complex that loses CO2. 

Metal formyl complexes have been proposed as important intermediates in the metal-catalyzed reduction of CO by H2 1,2, 3, 4). While the insertion of CO into alkyl and aryl carbon-metal bonds is well known (5), the insertion of CO into a metal-hydrogen bond to give a metal formyl complex has not been observed. (The intermediacy of metal formyl compounds in the substitution reactions of metal hydrides has been considered.) To ascertain the reasons for the failure to observe metal formyl complexes in the reactions of metal hydrides with CO, we have developed a new synthesis of metal formyl complexes and have studied their properties. 

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. 

Summarizing the literature, one can cautiously conclude that so far stable formyl complexes have only been synthesized and isolated when the co ordinated centre was fully saturated. This might be an indication that with heterogeneous catalysts also such complexes can only be formed when the adsorption sites are isolated. This is the situation with, for example, ions or atoms isolated in a matrix of an inactive or much less active support or promoter, but not with bulk metals. As mentioned elsewhere, i.r. spectra obtained at slightly elevated temperatures with CO/Hj mixtures adsorbed on Cu/ZnO catalysts revealed two small bands which the authors ascribed to the frequently postulated formyl intermediates. 

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... 

A simple catalytic cycle for hydroacylation is shown in part A of Scheme 18.19. Hydroacylation occurs by oxidative addition of the formyl C-H bond to generate an acyl hydride complex. Insertion of olefin into the metal hydride then generates an alkyl acyl intermediate. These complexes undergo reductive elimination, as described in Chapter 8. Although these basic steps constitute the catalytic cycle, many other processes occur outside of this cycle in the catalytic system. Some of these steps lying off the cycle lead to poisoning of the catalyst and others are unproductive reversible processes that have been revealed by H/D exchange experiments. Part B of Scheme 18.19 shows a catalytic cycle that includes these side processes. 

An ab initio study of addition of H to [CpFe(CO)3] considered the possibility of a formyl complex however the thermodynamically stable product was predicted to be [(T -C5H6)Fe(CO)3]. Low temperature NMR studies have shown that a formyl intermediate is involved in the addition of H to the butadiene complex [(r -C4H6)Fe(CO)3] in which the ultimate product is [(ti3-MeCHCHCH2)Fe(CO)3]-.97... 

CO Reduction via Formyi intermediates In planning an attack on the problem of CO reduction, one might think that the migratory insertion of CO into an M—H bond to give a formyl complex M—CHO (by analogy to... 

Interest in the stepwise reduction of coordinated CO continues. Further work on determining whether surface methylenes could arise from formyl intermediates has been reported using 0s3(C0)i2 as a model system. On hydride reduction, 033(00)12 yields [OS3 (C0)u (GH0)] which can be converted into [OS3(C0)u(U-CH2)] by protonation. The methylene complex eliminates CHi, on heating in H2 gas, and forms [033112(00)9(113-000)] in the absence of H2. The first stable neutral formyl complex of a 3d-transltion metal has been claimed. Reduction of trans-[Mn(C0) 1,(P(0R) 3 2 ] or mer-[Mn(C0) 3(P(0R) 3 3 ] yields [Mn(CO)3(OHO)(P(0R)3 2], and the crystal structure of the complex with R = Ph was determined. Other reports on the reduction of Or-, Mo- or Fe-coordinated 00 are referenced below. ... 

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