Formyl species, intermediates

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... 

In CO hydrogenation, the achvity and selechvity to C1-C5 oxygenates over the bimetallic samples are higher than those of the monometallic counterparts [187-190]. Bimetallic catalysts also showed improved activity in the hydroformylation of ethylene compared to either of the monometallic catalysts [191]. The promotion for higher alcohol production is proposed to be associated with the adjacent Ru-Co sites. However, the lack of an exhaustive characterization of catalysts does not allow a clear correlation to be established between the characteristics of the active sites and the catalytic behavior. A formyl species bonded to a Ru-Co bimetallic site has been proposed to be the intermediate in the alcohol synthesis in these systems. A subsequent reaction with an alkyl-surface group would lead to the C2-oxygenate production [187]. 

Palladium and rhodium based catalysts, which yield methanol and ethanol from synthesis gas respectively, were selected for a mechanistic study. Chemical trapping showed a correlation between formyl species and the catalytic activity, indicating that these species probably are reaction intermediates. The role of the support on the activity and on the nature of the products was elucidated by chemical trapping of formyl, methoxy and formate species on palladium catalysts, and of formyl and acetate on rhodium catalysts. The rhodium catalysts were also studied by probe molecule experiments CH CHO) and by FT-IR spectroscopy (chemi-... 

The change in selectivity when using CO instead of CO is not well understood. Either the active sites differ, as mentioned above, or the chemisorption of CO is weaker, thus promoting hydrogen chemisorption and the hydrogenative power of the catalyst. The correlation between formyl species and methane suggests that the former are intermediates in the methanation of CO. ... 

This suggests that formyl species are key intermediates both to hydrocarbons and to oxygenates. The first part of the mechanism for oxygenates formation would be the same as in methanol synthesis. 

Two basic mechanisms have been proposed to interpret methanol formation in the CO + H2 reaction. When carbon monoxide adsorbed on the copper surface is hydrogenated by the stepwise addition of hydrogen atoms [Eq. (3.42)], the principal intermediate is a surface formyl species (5) ... 

Recent attention on reactions of this type with unactivated carbonyls has focused on the possibility of using an active hydride as the attacking nucleophile. The resultant formyl species is then viewed as an intermediate on the path to CO reduction. For example, Casey and Neumann (55) have shown that trialkoxyborohydride reacts with various metal carbonyls according to... 

Clearly, for all metal surfaces, the formation of formate is much more favorable, both thermodynamically and kinetically, the difference between the two activation barriers increasing from 9 to 13 to 37 kcal/mol along the series Fe/W < Ni < Ag. Qualitatively, this model prediction is in full agreement with the fact that a formate species appears to be a ubiquitous metastable intermediate in the decomposition of HCOOH on Mo (122), Ru (723), Rh (124), Ni (725), Pd (726), Pt (727), and Cu (128), but a formyl species has never been observed (102a, 129). 

According to the Sachtler-Biloen mechanism, the Fischer-Tropsch reaction is initiated through CO adsorption followed by CO dissociation. Experimental evidence for the involvement of an oxygen-free intermediate exists it was observed that predeposited C is incorporated into the product during Fischer-Tropsch synthesis when CO was included in the feed gas (3). It is important to distinguish whether during the Fischer-Tropsch s)mthesis CO dissociation is strictly monomolecular or instead involves a reaction with Hads to produce an intermediate "HCO" formyl species that in a subsequent reaction decomposes to "CH" and Oads-Another question is how the rates of CO dissociation, chain growth, and termination depend on the catalyst surface structure. Thus, it is essential to know the relative values of the rate constants for these three elementary reactions. 

The activity of supported Pt catalysts for methanol synthesis from C0-H2 is considerably enhanced when the metal is supported on oxides which exhibit themselves appreciable activity for MeOH synthesis. Furthermore it is found that the rate of methanol formation on Pt-supported catalyst is increased when Th02, Ce02 were mechanically mixed with the Pt catalyst. Such behaviour is typical for bifunctional catalysts. It has already been reported that Th02, Ce02 adsorb carbon monoxide without dissociation. Such activated CO can be hydrogenated to form a formyl species, the formyl species interacting with lattice oxygen will produce a formate intermediate. 

The first paper on methanol electrocatalysis under UHV conditions was published by Attard et al. [139] on the most active surface, Pt(110). Similar results to those on Pt(l 11) were found, that is, carbon monoxide and molecular hydrogen, but with a slightly larger methanol surface coverage of 9 = 0.10. It was the first time that methoxy species were proposed as intermediates and were different from the carbon monoxide or formyl species proposed earlier by Bagotskii et al. [140], However, traces of the formyl species were also detected on reconstructed Pt(l 10) using vibrational spectroscopy, which was able to co-adsorb this species with atomic oxygen [117]. 

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. 

With respect to mechanism of action, the most extensive kinetic and equilibrium exchange studies have been carried out on monofunctional 10-formyl-H4-folate synthetase from Cl. cylindrosporum [84]. The data support a random sequential mechanism that does not involve the formation of freely dissociable intermediates. The most likely mechanism, however, is not concerted but probably involves the formation of a formyl phosphate intermediate, since the synthetase catalyzes phosphate transfer from carbamyl phosphate but not acetyl phosphate to ADP with H 4-folate serving as an activator. Carbamyl phosphate is an inhibitor of 10-formyl-H 4-folate synthesis - an inhibition that can be eliminated only when both ATP and formate are present in accord with the concept that it spans both sites [85]. It would be of considerable interest to attempt to demonstrate positional isotope exchange employing [, y- 0]ATP for this enzyme in order to further implicate an enzyme-bound formyl phosphate species [86]. 

At short adsorption times the formation double- or triple-bonded adsorbed species (>C=0, 5C-0) are favored because most of the surface is bare. These species have a very short lifetime during which the COH species, suggested by many authors earlier as the only intermediates involved in the adsorption of MeOH on polycrystalline Pt, seems to be absent. The -COOH species also seems to be absent at short adsorption times. However, it is obvious that the two species present at short adsorption times (>C=0, C-O) block two or three adsorption sites and are rapidly transformed to the Pt-C = 0 species which occupies only one adsorption site. As a parallel process, the formation of surface formyl species takes place. 

A detailed mechanism for the electro-oxidation of methanol on platinum electrodes in acidic media is shown in Scheme 2. The important features of the reaction mechanism include the formation of reactive intermediates, such as (CHO)a 5, which form on the electrode surface and are further oxidized to either (CO)ads, which leads to the poisoning species, or the adsorbed formyl species is oxidized to (COOH)ads or directly to COj. The mechanistic pathways described... 

The irreversibility of the overall hydroformylation cycle is due to the irreversibility of the last step leading to the reductive elimination of the aldehyde from the intermediate metal-hydride-formyl species generated by oxidative addition of H2. 

The electrophile 4 adds to the aromatic ring to give a cationic intermediate 5. Loss of a proton from 5 and concomitant rearomatization completes the substitution step. Subsequent hydrolysis of the iminium species 2 yields the formylated aromatic product 3. Instead of the highly toxic hydrogen cyanide, zinc cyanide can be used. The hydrogen cyanide is then generated in situ upon reaction with the hydrogen chloride. The zinc chloride, which is thereby formed, then acts as Lewis acid catalyst. 

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