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. 

One step of the mechanism determined for the reaction of Rh(OEP)H with CO to give the formyl complex Rh(OEP)CHO involved the coordination of CO to the chain-carrying Rh(ll) porphyrin Rh(OEP)-, although there had been no direct evidence observed for this species.Since that time. Wayland has systematically developed the chemistry of Rh( 11) porphyrins coordinated to CO, and the chemistry is now well understood. 

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) ... 

Figure 5. An isolable, crystalline neutral formyl complex...

There is substantial hydride mobility associated with homogeneous formyl complexes (particularly those which are anionic) [10,11,12,13]. Therefore, the generation of small quantities of catalyst-bound formyls (a step which based upon homogeneous precedent is likely uphill thermodynamically) might be accompanied by a similar electrophile-induced disproportionation. 

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... 

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. 

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. 

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... 

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. 

Figure 2.26 shows an alkoxide attack at co-ordinated CO giving a carboalkoxy complex, and a borohydride attack at co-ordinated CO in which the boron simultaneously acts as a Lewis acid. The BH3 complexation now stabilises the formyl complex that would otherwise be thermodynamically inaccessible. So far the latter reaction has only been of academic interest in homogeneous systems (it may be relevant to heterogeneous systems though proof is lacking). 

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). 

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. 

The formyl complex [(H20)5CrCH0]2+ is formed in aqueous solution via the reaction of aqueous Cr(II) with the dihydroxymethyl radical (52). The latter radical is derived from the reaction of formaldehyde (aqueous formaldehyde exists almost exclusively in the acetal form) with "OH radicals. The initial complex formed is that of the hydrated formyl complex, which transforms rapidly to the formyl complex ... 

Apparently the preference of the formyl ligand for the aldehyde form over the hydrated form stems mainly from the large steric requirements of the (H20)sCr moiety. Surprisingly the transient formyl complex acts as a reducing agent towards the formaldehyde present in the solution, via hydride transfer to yield CO and methanol (52) ... 

Zirconium hydride reactivity with carbon monoxide demonstrates the strong driving force toward products with a Zr-O bond. Indeed, the facility of the CO migratory insertion into Zr-C and especially Zr-H bonds may be from a carbonyl oxygen-zirconium interaction that stabilizes the transition state to the acyl and formyl complexes. 

Organometallic complexes frequently are susceptible to nucleophilic attack by an external reagent. In some instances the attack takes place on the metal center (see substitution reactions, page 686). while in others it occurs on a bound ligand. Already in this chapter we have seen many instances in which coordinated carbon monoxide undergoes nucleophilic attack. Examples include reactions with H to produce a formyl complex (Eq. 15.19). with R to form an acyl complex (Eq. 15.49). and with OH to give a hydroxycarbonyl complex (Eq. 15.21). 

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