Formyl metal complexes 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). 

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... 

The (5)-tryptophan-derived oxazaborolidenes utilized in this aldol study have been previously examined by Corey as effective catalysts for enantioselective Diels-Alder cycloaddition reactions [6]. Corey has documented unique physical properties of the complex and has proposed that the electron-rich indole participates in stabilizing a donor-acceptor interaction with the metal-bound polarized aldehyde. More recently, Corey has formulated a model exemplified by 7 in which binding by the aldehyde to the metal is rigidified through the formation of a hydrogen-bond between the polarized formyl C-H and an oxyanionic ligand [7], The model illustrates the sophisticated design elements that can be incorporated into the preparation of transition-metal complexes that lead to exquisite control in aldehyde enantiofacial differentiation. 

As already indicated in the discussion on formyl formation, the transition state on a metal surface may be closely related to that found in the organometallic complexes used in homogeneous catalysis. The p orbital stabilizing interactions have also been shown to play an important role in insertion reactions occurring in organic-metallic complexes [84,85]. It explains, for instance, the higher activa-... 

Third, and not least, the mechanistic features of the Fischer-Tropsch hydrocarbon synthesis mirror a plethora of organometallic chemistry. More precisely Molecular models have been invoked that could eventually lead to more product selectivity for eq. (1). Although plausible mechanistic schemes have been considered, there is no way to define precisely the reaction path(s), simply because the catalyst surface reactions escape detection under real process conditions (see Section 3.1.1.4). Nevertheless, the mechanism(s) of reductive hydrocarbon formation from carbon monoxide have strongly driven the organometallic chemistry of species that had previously been unheard of methylene (CH2) [7-9] and formyl (CHO) [10] ligands were discovered as stable metal complexes (Structures 1-3) only in the 1970s [7, 8]. Their chemistry soon explained a number of typical Fischer-Tropsch features [11, 12]. At the same time, it became clear to the catalysis community that molecular models of surface-catalyzed reactions cannot be... 

For the Reppe carbonylation, it is proposed that the reaction involves the initial oxidative addition of a nucleophile to the transition metal complex, followed by the complexation of an unsaturated hydrocarbon to the metal and insertion into a metal-H bond. Subsequently, migration of hydrocarbon species to CO followed by a reductive elimination afford the corresponding product." Scheme 1 illustrates the formation of ester from acetylene, CO, and methanol in the presence of a catalytic amount of Pd[CO]4. In addition, a mechanism analogous to that of Hydroformylation is proposed and displayed in Scheme 2 for the Reppe formylation. 

This type of side-on bending, which has been established by X-ray crystallographic methods for the related acyl complexes (r 5-C5H5)2Zr(COMe)Me (38) and (T>5-C5H5)2Ti(COMe)Cl (39), could overcome the thermodynamic objection, previously discussed, against the formation of a normal, linearly bonded formyl by CO insertion into a metal-hydride bond. Thermochemical data obtained from alcoholysis of zirconium tetralkyl species show that the mean bond energy of Zr—O is 50 kcal/mole greater than that of Zr—C (40). 

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. 

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