Carbonylation proposed mechanism

FIGURE 27 19 Proposed mechanism of hydrolysis of a peptide catalyzed by carboxypeptidase A The peptide is bound at the active site by an ionic bond between its C terminal ammo acid and the positively charged side chain of arginine 145 Coordination of Zn to oxygen makes the carbon of the carbonyl group more positive and increases the rate of nucleophilic attack by water... 

The second proposed mechanism involves initial ring opening of the phthalimide. Alkoxide attack on one of the imide carbonyls furnishes amide anion 26. Proton transfer affords enolate 27, which undergoes Diekmann type condensation followed by aromatization to afford the requisite isoquinoline 23. 

The presence of one carbonyl group per oligomer molecule was also ascertained. The orange colour of the resin suggested that some minor event during the photopolymerization produced chromophores in small concentrations. The presence of furoin among the products corroborated the proposed mechanism, which was shown not to involve free radical chain reactions. 

The metal complexes most commonly used in these photoredox systems are manganese and rhenium carbonyls. The proposed mechanism of the photoredox... 

At lower acidities the iV-nitrobenzamides and /V-methyl-.V-nitrobenzamides have a hydrolysis mechanism that is not acid-catalyzed for these cases plots of log kv - log h2o are linear, as for the acyhmidazoles discussed above. N-Nitroacetamide also hydrolyzes in this way.291 The proposed mechanism is given in Scheme 17, written for TV-nitroacetamide if the hydration shown is a pre-equilibrium (this is a carbonyl compound with a strong electron-withdrawing group attached, so this is likely), only one water molecule will appear in the rate expression (the difference between 3 and 2), as observed.287 Some evidence for hydroxide-catalyzed processes at the very lowest acidities was also found for some of these compounds.287... 

The reactivity of these metal hydride-metal carbonyl reactions can be correlated with the nature of the reactants in a manner consistent with the proposed mechanism nucleophilic attack by hydride on coordinated CO. Thus reactions involving the highly nucleophilic group IV hydride, Cp gZrHg, are much faster than those of group V metal hydrides. On the other hand, the relatively electrophilic neutral binary metal carbonyls all react with Cp2NbH under mild conditions (20-50° C), whereas more electron-rich complexes such as cyclopentadienylmetal carbonyls (Cp2NbH(C0), CpV(CO) ) or anionic carbonyls (V(CO)g ) show no reaction under these conditions. 

Figure 4. Proposed mechanism for the group VI metal carbonyl-catalyzed homogeneous WGSR...

FIGURE 8.21 Proposed mechanism for the BOP-Cl-mediated reaction of a carboxylate anion with a methylamino group. Formation of a mixed anhydride is followed by aminolysis that is facilitated by anchimeric assistance provided by the oxygen atom of the ring carbonyl.101 (van der Auwera Anteunis, 1987). BOP-C1 = fcw(2-oxo-3-oxazolidino)phosphinic chloride. 

Carbonylative kinetic resolution of a racemic mixture of trans-2,3-epoxybutane was also investigated by using the enantiomerically pure cobalt complex [(J ,J )-salcy]Al(thf)2 [Co(CO)4] (4) [28]. The carbonylation of the substrate at 30 °C for 4h (49% conversion) gave the corresponding cis-/3-lactone in 44% enantiomeric excess, and the relative ratio (kre ) of the rate constants for the consumption of the two enantiomers was estimated to be 3.8, whereas at 0 °C, kte = 4.1 (Scheme 6). This successful kinetic resolution reaction supports the proposed mechanism where cationic chiral Lewis acid coordinates and activates an epoxide. 

Iron-catalyzed carbonylations, 34 124-125 Fe(CO)5/amine catalysts, 34 124 homologation of methanol, 34 124 proposed mechanism for, 34 125 Iron complexes... 

I/Ru ratio critical, 34 112 proposed mechanism, 34 112 ruthenium-carbonyl complexes, 34 113 species involved, 34 110-113 -catalyzed homologation, 34 115 proposed mechanism, 34 115 compensation behavior of, 26 285, 286 complex catalyst... 

In this period, the most important reactions of the isocyanides were the formations of tetrazole derivatives from isocyanides and hydrazoic acid, a process introduced in 1910 by Oliveri-Mandala and Alagna, and then in 1921 Passerini introduced the reaction (P-3CR), which was the first 3-component reaction of the isocyanides. In the 1940s Baker,and later Dewar, proposed mechanisms of the P-3CR. The important role of the intermediate hydrogen bond between the carboxylic acid and the carbonyl compound in suitable solvents was mentioned. ... 

The outcome of reactions involving carbonyl ylides is not always as easy to predict, as the examples in Table 4.20. Depending on the basicity of the intermediates, proton migrations might occur and unexpected results can be obtained. Two examples of such peculiar conversions and the proposed mechanisms are sketched in Figure 4.16. 

Figure 11 Proposed mechanism of cyclization of dehydrated NisA by NisC. The cyclization reaction shown results in the formation of the B-ring of nisin. The possible stabilization through a /3-turn-like structure via hydrogen bonding between the amide NH of Cys and the carbonyl of Dha/Dhb is shown and may explain the high stereoselectivity observed in nonenzymatic cyclizations involving four amino acids as discussed in the text. Reprinted with permission from B. Li W. A. van der Donk, J. Biol. Chem. 2007, 282, 21169-21175.

Proposed mechanisms for polycondensations are essentially the same as those proposed in the organic chemistry of smaller molecules. Here, we will briefly consider several examples to illustrate this similarity between reaction mechanisms for small molecules and those forming polymers. For instance, the synthesis of polyamides (nylons) is envisioned as a simple Sn2 type Lewis acid-base reaction, with the Lewis base nucleophilic amine attacking the electron-poor, electrophilic carbonyl site followed by loss of a proton. 

A selective reaction takes place between H2O2 and 10-methyl-9-(p-formylphenyloxy-carbonyl)acridinium trifluoromethanesulfonate (123) in basic solution with CL emission at about 470 nm. A proposed mechanism for CL is depicted in Scheme 2. No catalyst other than an alkali is required for the process to take place. The LOD is about 5 nM, with linearity np to 60 (xM. It should be pointed out that the LOD is dictated by the technical capability of obtaining a blank of water devoid of H2O2. The method was applied for H2O2 determination in natural waters. ... 

The proposed mechanism is given in Scheme 15. Initially the dissociation of water, maybe trapped by the molecular sieve, initiates the catalytic cycle. The substrate binds to the palladium followed by intramolecular deprotonation of the alcohol. The alkoxide then reacts by /i-hydride elimination and sets the carbonyl product free. Reductive elimination of HOAc from the hydride species followed by reoxidation of the intermediate with dioxygen reforms the catalytically active species. The structure of 13 could be confirmed by a solid-state structure [90]. A similar system was used in the cyclization reaction of suitable phenols to dihydrobenzofuranes [92]. The mechanism of the aerobic alcohol oxidation with palladium catalyst systems was also studied theoretically [93-96]. 

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