Formyl complexes alkylation

The conversion of a metal hydride into a metal formyl by hydride migration, as in (16), has been regarded for some time as a very difficult or improbable reaction. Certainly, there is evidence to suggest that for a number of metal carbonyl complexes alkyl migration to a carbonyl ligand, as follows ... 

Reactions of formyl complexes with alkylating agents can be more complex than the reductions in Eqs. (15-22). Some examples of simple hydride transfer exist. For instance, (CO)4Fe(CHO) (22) reduces octyl iodide to octane (75%) (27, 28) (C2H5)4N + 25 (Table I) reacts with heptyl iodide (overnight, room temperature, THF) to give heptane (71%) and (CO)4[(ArO)3P]Fe (37, 42) c/.s-(CO)5ReRe(CO)4(CHO) (19) converts octyl iodide to octane (68%) (47). 

Reactions of neutral formyl complexes with alkylating agents can follow different courses. Roper has observed the O-methylation reaction shown in Eq. (23) (54). Cationic methoxymethylidene complex 53 was obtained in excellent yield. 

The fact that there is such a paucity of metal formyl complexes is both interesting and significant because of the proposed intermediacy of coordinated formyl in CO reduction, and the sharply contrasting abundance of metal acyl complexes. Since many of the acyl complexes are known to form by migratory insertion of CO in an alkyl carbonyl complex (20, 20a, 22), the lack of formyl complexes from hydride carbonyls relates to the thermodynamic difference in the equilibrium (5) when Y is alkyl and when it is hydride. 

Among the types of complexes found here are formyl, acyl, alkyl, and aryl carbonyls, carbonyl cyanides," " carbonyl isocyanides and acetylides," and thiocarbonyl and seleno-carbonyl complexes." Of the greatest significance, however, are the chromium carbenes, for example, (CO)sCr=C(OR)R". This chemistry has been thoroughly reviewed " nevertheless, these compounds will be briefly discussed here. 

This mechanism is quite general for this substitution reaction in transition metal hydride-carbonyl complexes [52]. It is also known for intramolecular oxidative addition of a C-H bond [53], heterobimetallic elimination of methane [54], insertion of olefins [55], silylenes [56], and CO [57] into M-H bonds, extmsion of CO from metal-formyl complexes [11] and coenzyme B12- dependent rearrangements [58]. Likewise, the reduction of alkyl halides by metal hydrides often proceeds according to the ATC mechanism with both H-atom and halogen-atom transfer in the propagation steps [4, 53]. 

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. 

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. 

Reactions between [Cp Ir(PMe3)(Me)(OTf)] and aldehydes (RCHO) proceed with high selectivity to give the hydrocarbyl carbonyl salts [Cp Ir(PMe3)(R)(CO)]OTf (137, R = Me, Et, Pr, Ph, 1-ethylpropyl,/>-Tol, Mes, (Z)-l-phenyl-l-propen-2-yl, vinyl, Bu, 1-adamantyl). The tandem C-H bond activation/decarbonylation reaction afforded the first isolated tertiary alkyl complexes of Ir. X-ray diffraction studies were carried out on Mes, Bu, and 1-adamantyl derivatives. Hydride reduction of the /)-Tol complex provided an example of a rare transition metal formyl complex, [Cp lr(PMe3)(p-Tol)(CHO)]. ... 

Intermediate formation of formyl chloride is not necessary since the actual alkylating agent, HCO", can be produced by protonation of carbon monoxide or its complexes. However, it is difficult to obtain an equimolar mixture of anhydrous hydrogen chloride and carbon monoxide. Suitable laboratory preparations involve the reaction of chlorosulfonic acid with formic acid or the reaction of ben2oyl chloride with formic acid ... 

Transition metal complexes have been used in a number of reactions leading to the direct synthesis of pyridine derivatives from acyclic compounds and from other heterocycles. It is pertinent also to describe two methods that have been employed to prepare difficultly accessible 3-alkyl-, 3-formyl-, and 3-acylpyridines. By elaborating on reported194,195 procedures used in aromatic reactions, it is possible to convert 3-bromopyridines to products containing a 3-oxoalkyl function196 (Scheme 129). A minor problem in this simple catalytic process is caused by the formation in some cases of 2-substituted pyridines but this is minimized by using dimethyl-formamide as the solvent.196... 

In important recent work, Shriver has demonstrated that electrophiles can promote the migration of alkyl groups to coordinated CO. Lewis acid adducts of metal acyl complexes are isolated [37]. Thus it is possible that electrophilic species might also facilitate the generation of catalyst-bound formyls. 

Although the standard amidocarbonylation reaction involves an aldehyde and an amide, benzyl chloride can be used as the reactant. The amidocarbonylation of benzyl chloride was first reported by Wakamatsu eta/, in 1976 using Co2(CO)8 as catalyst precursor. This process was revisited by de Vries et al. in 1996 and iV-acetylphenylalanine 8 was obtained in 82% yield under the optimized conditions (Scheme 2)." Since the Co-catalyzed amidocarbonylation is carried out in the presence of CO and H2, formylation of benzyl chloride takes place first to form phenylacetalde-hyde in situ. In this particular case, as Scheme 2 illustrates, A-acetylenamine 10 is formed as intermediate, followed by the chelation-controlled HCo(CO)4 addition to give alkyl-Co intermediate II. Insertion of CO to the carbon-Co bond of II, forming acyl-Co complex 12, followed by hydrolysis affords 8 and regenerates active Co catalyst species. 

Another possible reason that ethylene glycol is not produced by this system could be that the hydroxymethyl complex of (51) and (52) may undergo preferential reductive elimination to methanol, (52), rather than CO insertion, (51). However, CO insertion appears to take place in the formation of methyl formate, (53), where a similar insertion-reductive elimination branch appears to be involved. Insertion of CO should be much more favorable for the hydroxymethyl complex than for the methoxy complex (67, 83). Further, ruthenium carbonyl complexes are known to hydro-formylate olefins under conditions similar to those used in these CO hydrogenation reactions (183, 184). Based on the studies of equilibrium (46) previously described, a mononuclear catalyst and ruthenium hydride alkyl intermediate analogous to the hydroxymethyl complex of (51) seem probable. In such reactions, hydroformylation is achieved by CO insertion, and olefin hydrogenation is the result of competitive reductive elimination. The results reported for these reactions show that olefin hydroformylation predominates over hydrogenation, indicating that the CO insertion process of (51) should be quite competitive with the reductive elimination reaction of (52). 

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