Aldehydes formyl complexes

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) ... 

Formyl complexes are characterized by distinct spectroscopic features. The most frequently used diagnostic probe has been H NMR. Chemical shifts in the 8 12-17 range (several ppm downfield from normal aldehyde or formamide H-NMR resonances) (69) are observed. However, since chemical shifts measured in the presence of trialkylborane by-products can show marked temperature dependence (32, 38, 47, 66), absolute values must be treated with caution. 

Most aldehydes also show two weak i c-h in the 2830-2695 cm-1 region (72). Similar absorptions have been observed in some formyl complexes (28, 29, 52 -54, 67). However, these are not intense enough to be diagnostic, and confirmation of this assignment by study of the corresponding deuteroformyls (—CDO) would seem desirable. 

Transition metal formyl complexes are capable of donating hydride to several classes of substrates, of which ketones and aldehydes are pro-... 

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]+. 

Formyl complexes show varying behavior when treated with protonating agents. Reaction of (CO)4Fe(CHO)" (22) with acid gives formaldehyde in 13-20% yield (27-29). Homologous (CO)4Fe(COR) acyls afford excellent yields of aldehydes when protonated, presumably via... 

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. 

Formyl complexes are themselves strong hydride donors, as is evident from the conversion of (n Cp)Re(COXNOXCllO) giving (14), in which hydride transfer from one formyl ligand to another, or hydrogenation of aldehydes to alcohols occurs [22. ... 

The lack of spectroscopic evidence for Lewis acid complexation anti to the formyl hydrogen in aldehyde-derived complexes does not imply that such complexes do not exist. Indeed, ab initio molecular orbital calculations suggest that the energy difference for E and Z BF3-aldehyde complexes can be as small as... 

Still another powerful method for the regeneration of carbonyl compounds from dialkylhydrazones is copper-catalyzed hydrolysis. The reagents that have been tested for this purpose are 2% aqueous cop-per(II) acetate solution at pH 4, copper(II) chloride in 0.05M phosphate buffer and 75% tetrahydrofu-ran/water, and copper(II) sulfate pentahydrate . Under the conditions of the hydrolysis, no reaction is observed in the absence of the copper(II) ion. Typical yields are 85-100%. Other functional groups like a-dicarbonyl, a-tricarbonyl, acetal and aldehydic formyl groups were not affected by this hydrolysis procedure. Nitrile formation in the case of aldehyde dimethylhydrazones was not a significant side reaction. However, reaction times ranged from 1 to 15 h. The reaction is believed to be nonoxidative in nature rather, the copper is believed to activate the C=N bond and catalyze hydrolysis. The dimethylhydrazine produced during hydrolysis also complexes irreversibly with the copper(II) ion to drive the reaction to completion. 

The tridimensional structure of carbonyl-Lewis acid complexes has been determined by X-ray studies. The crystal structure of the benzaldehyde BFs complex, shows that the BF3 group coordinates anti to the Ph group as in I iP Likewise, the crystal structure of (4-(-/-butyl)benzaldehyde)2 SnCl4 shows an anti coordination of SnCU. Importantly, a formyl hydrogen bond has been postulated by Corey as an organizing element to rationalize the outcome of a number of reactions involving aldehyde-metal complexes... 

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)]. ... 

Treatment of tricarbonylh4-1-(ethoxycarbonyl)-l//-azepine]iron(0) (30) with acetic anhydride and tetrafluoroboric acid at 0 C (Method A) yields the 3-acetyl derivative 31.226 The acetyl derivative is also formed, but in lower yield, by the action of acetic anhydride and tin(IV) chloride (Method B) on complex 30. The 3-propanoyl derivative (20% mp 95-96 C) can be prepared similarly, whereas formylation to give aldehyde 32 is successful under Vilsmeier conditions. 

The enantioselectivity of the BINOL-Ti(IV)-catalyzed reactions can be interpreted in terms of several fundamental structural principles.42 The aldehyde is coordinated to Ti through an apical position and there is also a 0-HC=0 hydrogen bond involving the formyl group. The most sterically favored approach of the alkene toward the complexed aldehyde then leads to the observed product. Figure 10.2 shows a representation of the complexed aldehyde and the TS structure for the reaction. 

Friedel-Crafts acylation reactions of aromatics are promoted by Tilv complexes.104 In some cases, a catalytic amount of the titanium compound works well (Scheme 28). In addition to acyl halides or acid anhydrides, aldehydes, ketones, and acetals can serve as electrophile equivalents for this reaction.105 The formylation of aromatic substrates in the presence of TiCl4 is known as the Rieche-Gross formylation metalated aromatics or olefins are also formylated under these conditions.106... 

Aldehyde-containing macromolecules will react spontaneously with hydrazide compounds to form hydrazone linkages. The hydrazone bond is a form of Schiff base that is more stable than the Schiff base formed from the interaction of an aldehyde and an amine. The hydrazone, however, may be reduced and further stabilized by the same reductants utilized for reductive amination purposes (Chapter 3, Section 4.8). The addition of sodium cyanoborohydride to a hydrazide-aldehyde reaction drives the equilibrium toward formation of a stable covalent complex. Mallia (1992) found that adipic acid dihydrazide derivatization of periodate-oxidized dextran (containing multiple formyl functionalities) proceeds with much greater yield when sodium cyanoborohydride is present. 

Treatment of a polyfunctional chromium-tricarbonyl-complexed hydroxy aldehyde with an excess of Et3SiFFTFA for 4.5 hours gives an 82% yield of fully reduced product with both the formyl and hydroxy groups completely and selectively reduced (Eq. 193).352... 

A tandem hydroformylation/carbonyl ene reaction can be observed in cases, in which substrates with at least two isolated oleftnic bonds are hydro-formylated at only one double bond selectively. Thus hydroformylation of limonene with PtCkCPPlH /SnCk/PPlH or PtCl2(diphosphine)/SnCl2/PPh3 gives a mixture of two diastereomeric alcohols upon carbonyl ene reaction of the intermediate aldehyde, (Scheme 36). Best results are achieved with a PtC Cdppb) complex. The mechanism of the final intramolecular cycli-zation step resembles an acid catalyzed carbonyl ene reaction [89]. 

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