Carbonylation process mechanism

Before looking in detail at the mechanisms of the various reactions and processes it is worth summarizing some relevant observations about the carbonylation processes as a whole that need to be explained by these mechanisms. It should be noted that, although the overall feedstock is MeOH for an AcOH process, for practical reasons it is usual to operate with relatively high [AcOH] in the reactor and that most of the substrate is present as MeOAc through esterification (Eq. (3)). 

In this chapter, the recent advances in amidocarbonylations, cyclohydrocarbonylations, aminocarbonylations, cascade carbonylative cyclizations, carbonylative ring-expansion reactions, thiocarbonylations, and related reactions are reviewed and the scope and mechanisms of these reactions are discussed. It is clear that these carbonylation reactions play important roles in synthetic organic chemistry as well as organometallic chemistry. Some of the reactions have already been used in industrial processes and many others have high potential to become commercial processes in the future. The use of microwave irradiation and substitutes of carbon monoxide has made carbonylation processes suitable for combinatorial chemistry and laboratory syntheses without using carbon monoxide gas. The use of non-conventional reaction media such as SCCO2 and ionic liquids makes product separation and catalyst recovery/reuse easier. Thus, these processes can be operated in an environmentally friendly manner. Judging from the innovative developments in various carbonylations in the last decade, it is easy to anticipate that newer and creative advances will be made in the next decade in carbonylation reactions and processes. 

We established in Chapter 12 a hierarchy for the electrophilic reactivity of acid derivatives that should by now be very familiar to you—acyl chlorides at the top to amides at the bottom. But what about the reactivity of these same derivatives towards enolization at the a position, that is, the CH2 group between R and the carbonyl group in the various structures You might by now be able to work this out. The principle is based on the mechanisms for the two processes, mechanism of nucleophilic attack mechanism of enolate formation... 

The first methanol carbonylation process, commercialized in the 1960 s by BASF, used an iodide promoted cobalt catalyst but required very high pressures ( 700 atm) as well as high temperatures ( 250°C), and gave only ca. 90% selectivity. Few mechanistic studies have been published and little is known for certain about the mechanism. 

The reaction of an aUcene (or aUcyne), CO, and H2O to directly produce a carboxylic acid is called Reppe carbony-lation chemistry or, more recently, hydrocarboxylation (see Reppe Reaction). An excellent review of palladium-catalyzed Reppe carbonylation systems has been published recently by Kiss, and coverage of this important material will not be repeated here. This catalytic reaction has been known for quite some time, although the stoichiometric Ni(CO)4-based carbonylation of acetylene was the first commercial carbonylation process implemented (equation 13). The extreme toxicity of Ni(CO)4, however, has limited practical applications (see Nickel Organometallic Chemistry). Co, Rh, and Pd catalysts have certainly replaced Ni(CO)4 in smaller-scale laboratory reactions, though for historical reasons a number of the fim-damental mechanisms discussed in this section are based on Ni(CO)4. 

Unlike the hydrogenation catalysts, most iridium catalysts studied for hydroformylation chemistry are not particularly active and are usually much less active than their rhodium counterparts see Carbonylation Processes by Homogeneous Catalysis). However, this lower activity was useful in utihzing iridium complexes to study separate steps in the hydroformylation mechanism. Using iridium complexes, several steps important in the hydroformylation cycle such as alkyl migration to carbon monoxide were studied. Another carbonylation reaction in which iridum catalysis appears to be conunercially viable is in the carbonylation of methanol. ... 

Carbonylation Processes by Homogeneous Catalysis Hydrocyanation by Homogeneous Catalysis Mechanisms of Reaction of OrganometaUic Complexes Ohgomeriza-tion Polymerization by Homogeneous Catalysis Osmium Inorganic Coordination Chemistry. 

This final reaction step of the carbonylation mechanism is the primary distinguishing feature of each carbonylation process. A sufficient concentration of water or acetic acid in the reactor is therefore necessary to achieve high acetic acid or acetic anhydride formation rates respectively. 

Since 1979, numerous reviews have appeared on the kinetics, mechanisms, and process chemistry of the metal-catalyzed methanol carbonylation reaction [11, 14-20], especially the Monsanto rhodium-catalyzed process. In this section, the traditional process chemistry as patented by Monsanto is discussed, with emphasis on some of the significant improvements that Monsanto s licensee, Celanese Chemicals (CC) has contributed to the technology. The iridium-based methanol carbonylation process recently commercialized by BP Chemicals Ltd. (BP) will be discussed also. 

The reaction chemistry of the rhodium-catalyzed methanol carbonylation process under Monsanto conditions has been investigated extensively [6-8, 10, 12, 21, 26-29] (cf Section 2.1.2.1.1). The overall reaction kinetics are first order in both rhodium catalyst and methyl iodide promoter. The reaction is zero order in methanol and zero order in carbon monoxide partial pressure above 2 atm (eq. (6)) [27]. The kinetics agree well with the basic mechanism common to the three carbonylation reactions (see Section 2.1.2.1.1 and Tables 1 and 2). 

The proposed mechanism for this carbonylation reaction, which occurs in the absence of water, involves basic catalytic steps similiar to the rhodium-catalysed methanol carbonylation process (see Section 2.1.2.1.1). The mechanism leads to the formation of acetyl iodide, which reacts with methyl formate to produce the mixed anhydride [133]. 

NaH/NaOCH2CMe3/Co(OAc)2 system, in which [Co(CO)4] is generated in situ (Eq. 9) [68]. Although a plausible SRNl-type reaction path has been proposed to account for the unusual reactivity of the cobalt system [23], the intimate mechanism of this carbonylation process remains unknown. 

The precise mechanism employed by the DHS domain to introduce the cis double bond after the fourth condensation is not yet known. Three models to describe the events are presented here. The intermodular model has the DHS domain acting directly on the 2,3 atoms of nascent thiazoacyl chain tethered to ACP4 to produce the cis double bond (Fig. 8). The DHS domain, therefore, would have to orient itself to interact with ACP4. After the dehydration event, the chain would be passed to the KS domain of module S for the fifth condensation event and subsequent P-carbonyl processing, including the DHS-mediated dehydration, followed by ERS-mediated enoyireduction. Direct interaction of a P-carbonyl processing domain of one module with the ACP domain of the upstream module has not been described previously and would represent a truly unique process in polyketide biosynthesis. 

However, an alternative mechanism similar to that described in scheme 2, that considers the oxidative addition of aniline to the Rh° finely dispersed on the support, cannot be completely excluded. The evolution of carbamoyl intermediate to DPU should occur still via iodoformamide. The last mechanism could be also operative in the reductive carbonylation of nitrobenzene, when aniline is necessary for its conversion. In this case, the reaction could be better considered as an oxidative carbonylation process in which the nitrobenzene is playing the role of the oxidant in place of the oxygen. It has been ascertained that under these conditions the carbonylation occurs with the stoichiometry of reaction (11) [14], different from the one reported in reaction (4). 

Support for the intermediacy of the carbonyl oxide mechanism stems mainly from the observation of stoichiometric 5-amino group expulsion from the pyrimidine cofactors during PAH turnover regardless of the extent of coupling [106]. However, an attempt to demonstrate PAH-catalyzed cyclization of 25 to 26 proved unsuccessful, despite the requirements for such a process if the tetrahydropterin follows a similar reaction course [102]. Thus the case for their intermediacy is flawed. Carbonyl oxides are rather poor electrophilic reagents so that the hydroxylation of an aromatic ring probably proceeds via a radical species [115]. 

The behavior of early transition metal alkyls toward CO is somewhat different from that of late transition metal alkyls. Very little applications using early transition metal complexes for carbonylation processes have been reported in contrast to the abundant examples of applications of late transition metal complexes to carbonylation of organic substrates. However, fundamental studies on the chemistry of early transition metal alkyls toward CO insertion provide us with important information regarding the mechanisms of catalytic carbonylation processes. Thus we deal here with the chemistry of CO insertion into early transition metal alkyls and into late transition metal alkyls separately. 

Mechanism b), the oxa-di-7r-methane mechanism, is in line with CNDO calculations which show that u-orbital interaction between the carbonyl and the p-carbon is favored in the tt—n triplet state. 7) The concerted process, mechanism (c) has been pointed out to be unlikely in that a cyclopropyl ketone triplet is of higher energy than the triplet of the starting ketone. It is reasoned that the rearrangement thus cannot take place entirely in the triplet manifold and a spin inversion to yield an intermediate during the overall reaction is needed. On the other hand, this explanation may be too simple, there is no a priori reason why a reaction of triplet state to ground state should not proceed with synchronous bond breaking and formation. 

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