Carbonylation reactions, catalysis

Carbonyl reactions are extremely important in chemistry and biochemistry, yet they are often given short shrift in textbooks on physical organic chemistry, partly because the subject was historically developed by the study of nucleophilic substitution at saturated carbon, and partly because carbonyl reactions are often more difhcult to study. They are generally reversible under usual conditions and involve complicated multistep mechanisms and general acid/base catalysis. In thinking about carbonyl reactions, 1 find it helpful to consider the carbonyl group as a (very) stabilized carbenium ion, with an O substituent. Then one can immediately draw on everything one has learned about carbenium ion reactivity and see that the reactivity order for carbonyl compounds ... 

Another noncatalytic step proposed by King et al. (18) in iron carbonyl/base catalysis of the WGSR involves the formation of formate ion however, we recently observed that formate formation appears to have little importance in the related rhodium catalysis of hydrohydroxymethylation. We plan to perform studies of the CO + KOH and C02 + KOH reactions independent of catalysis to more fully appreciate the relationship of these reactions to solution pH and thus the catalytic activity. 

Recent studies indicate that phase transfer catalysis is useful for effecting a variety of interesting metal catalyzed reactions. Developments in the author s laboratory, in three areas, will be considered reduction, oxidation, and carbonylation reactions. 

In conclusion, phase transfer catalysis is a method of considerable potential for metal complex catalyzed reduction, oxidation and carbonylation reactions. 

Progress was made by the discovery of electrophilic catalysis by acyl cations in carbonyl reactions (91ZOR1588). This catalysis type consists in the conversion of aldehydes or ketones into highly active acyloxycarbo-cations 51 by the addition of acyl cations regardless of the origin of the latter. In contrast to related hydroxy- and alkoxycarbocations (R R —OR ... 

From the temperature variation of the equilibrium constant, thermodynamic parameters for the reaction were also obtained. The extent of formation of [Mo(CO)5l]" was found to be cation-dependent, and while equilibrium constants of 39 and 21 atm L moF were obtained for Bu4P and pyH+, none of the anionic iodide complex was observed for Na. Despite this variation, there seemed to be no correlation between the concentration of [Mo(CO)5l]" and the rate of the catalytic carbonylation reaction. It was proposed that [Mo(CO)5] and [Mo(CO)5l] are spectator species, with the catalysis being initiated by [Mo(CO)5]. Based on the in situ spectroscopic results and kinetic data, a catalytic mechanism was suggested, involving radicals formed by inner sphere electron transfer between EtI and [Mo(CO)5]. 

A chapter written in 1996 covers hydroformylation catalyzed by organometallic complexes in detail,219 whereas a review written 5 years later gives a summary of the advances on hydroformylation with respect to synthetic applications.220 A selection of papers in a special journal issue has been devoted to carbonylation reactions.221 A major area of the research has been the development of fluorous biphasic catalysis and the design of new catalysts for aqueous/organic biphasic catalysis to achieve high activity and regioselectivity of linear or branched aldehyde formation. 

The early workers in coordination chemistry were more interested in the theory of bonding and structure than in any practical usefulness which the compounds might have. In more recent times, however, applications have developed. Perhaps the most important of these is in catalysis, especially for hydrogenation and the activation of carbon-hydrogen bonds. Metal carbonyls and their derivatives have played a large part in this application, as well as in carbonylation reactions such as the recently developed process for converting methanol to acetic acid 42... 

A number of simple and inexpensive materials catalytically promote the cobalt-carbonylation (Reaction 2) in aqueous solution. These include ion-exchange resins, zeolites, or special types of activated carbon. Formation of the active catalyst in a separate reactor is thus economically feasible. The mechanism of this catalysis has not yet been elucidated and seems to differ for each promoter mentioned. After an induction period during which the cobalt fed to the reactor is partially retained by the promoter, fully active materials have absorbed cobalt carbonyl anion Co(CO)4 (ion exchange resins), Co2+ cation (zeolites), or a mixture of Co2+, cobalt carbonyl hydride, and cluster-type cobalt carbonyls (activated carbon). This can be shown by analytical studies (extraction, titration, and IR studies) of active material withdrawn from the reactor. 

Palladium has been extensively used in organic syntheses and in homogeneous catalysis (ref. 1-3), but industrial applications have remained relatively rare so far (ref. 4). The main reason lies in the de-activation of the catalyst by precipitation of metallic palladium under catalytic conditions. Such a process is actually observed in the carbonylation reactions under CO pressure. 

We have already encountered general catalysis in Section 7.1 (p. 340). Because it is so important to the understanding of carbonyl reactions, we shall consider it here in more detail. The discussion will be restricted to aqueous solutions, because these have been the most thoroughly studied. 

The mechanism of the caprolactam polymerization, i. e. the transamid-ation reaction catalysis by the system imide + salt can be interpreted by a nucleophillic attack of the amide anion on the carbonyl group of the imide which represents the strongest electrophillic reagent in the polymerizing system. 

Two approaches to asymmetric carbonylation reactions have met with some success. In the carbonylation reactions of a-methylbenzyl bromide under phase transfer catalysis, the... 

Two other Ni(CO)4 substitutes, Ni(CO)3PPh3 and Ni(COD)2/dppe, prove to be appropriate for the catalysis of tandem metallo-ene/carbonylation reactions of allylic iodides (Scheme 7)399. This process features initial oxidative addition to the alkyl iodide, followed by a metallo-ene reaction with an appropriately substituted double or triple bond, affording an alkyl or vinyl nickel species. This organonickel species may then either alkoxycar-bonylate or carbonylate and undergo a second cyclization on the pendant alkene to give 51, which then alkoxycarbonylates. The choice of nickel catalyst and use of diene versus enyne influences whether mono- or biscyclization predominates (equations 200 and 201). 

Most of the books listed under Sections 1.3 and 2.1-2.3.4 contain information on carbonylation reactions and should be consulted. Especially useful are the book by Parshall and Ittel and Section 2.1.2 of Vol. 1 of Applied Homogeneous Catalysis with Organometallic Compounds, ed. by B. Comils and W. A. Herrmann, VCH, Weinheim, New York, 1996. 

P. M. Maitlis, and A. Haynes, Carbonylation Reactions of Alcohols and Esters, in Metal-catalysis in Industrial Organic Processes (Eds. G. P. Chiusoli, and P. M. Maitlis, RSC Publishing, Cambridge, 2006, Chap. 4.2). 

In the course of acyloxycarbocation investigations112 it has been noted that the reactions of both aldehydes and ketones follow an unusual course or are strongly accelerated if either acylium ions RCO+ are present in the reaction mixtures or conditions to generate them in situ arise. These observations are explained by a transformation of carbonyl compounds into the highly reactive acyloxycarbocations 163 which easily react with weak nucleophiles such as vinyl ethers, vinyl esters, etc. Hence the electrophilic catalysis by acyl cations in carbonyl reactions takes place regardless of the origin of the latter. This catalysis was used in the reaction of ketones with nitriles. 

A long-standing success in transition metal catalysis is the carbonylation reaction [66], in particular the synthesis of acetic acid [67]. Formally this is the insertion of CO into another bond, in particular into a carbon-halogen bond. After the oxidative addition to the transition metal (the breaking of the carbon-halogen bond), a reaction with a CO ligand takes place. This reaction is often called an insertion. Mechanistic studies have, however, shown that the actual reaction... 

The growing demand for efficient chemical transformations and catalysts has inspired a few research groups in recent years to develop rare earth metal catalysts for organic synthesis [1, 2]. Triflates of rare earth metals are strong Lewis acids, which are stable in aqueous solution. Rare earth metal alkoxides on the other hand are of interest as Lewis bases, e.g. in the catalysis of carbonyl reactions, because of the low ionization potentials (5.4-6.4 eV) and electronegativities (1.1-1.3) of the 17 rare earth elements. Rare earth metal-alkali metal complexes in contrast show both Brpnsted-basic and Lewis-acidic properties. Impressive applications of such catalysts are presented and discussed here. 

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. ... 

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