Metal insertion hydroformylation

Noteworthy, the stereochemistry of the vinyl moiety in both products was found to be that resulting from a trans addition to the triple bond, in contrast to the cis addition products expected from a concerted single-metal insertion.[67] This suggests that insertion reactions leading to trans addition products are entirely feasible in dinuclear species, probably as a consequence of the concerted action of both metal centers. This observation may be relevant in the context of some catalytic processes, such as alkyne hydrosilylations, which rather frequently afford trans addition products.[68] This unusual selectivity might result from intermolecular hydride transfer steps, similar to those recognized during catalytic hydroformylations,[69] or from the formation of dinuclear active species under catalytic conditions.[70]... 

Tandem procedures under hydroformylation conditions cannot only make use of the intrinsic reactivity of the aldehyde carbonyl group and its acidic a-position but they also include conversions of the metal alkyl and metal acyl systems which are intermediates in the catalytic cycle of hydroformylation. Metal alkyls can undergo -elimination leading to olefin isomerization, or couplings, respectively, insertion of unsaturated units enlarging the carbon skeleton. Similarly, metal acyls can be trapped by addition of nucleophiles or undergo insertion of unsaturated units to form synthetically useful ketones (Scheme 1). 

The insertion of ligated CO into metal-carbon -bonds (or rather the migration of an alkyl group to a coordinated CO) is a key step in a variety of synthetic and catalytic important processes, e.g., in hydroformylation (145), the Fischer-Tropsch reaction (146) and the synthesis of acetic acid from methanol (147). 

The metal hydride mechanism was first described for the cobalt-carbonyl-catalyzed ester formation by analogy with hydroformylation.152 It was later adapted to carboxylation processes catalyzed by palladium136 153 154 and platinum complexes.137 As in the hydroformylation mechanism, the olefin inserts itself into the... 

Once metal hydride addition (alkene insertion) has taken place, for example (3) —> (4), -elimination (4) - (3) and readdition can occur (Scheme 2). Accordingly, alkene isomerization can take place in the hydroformylation process (equation 3). 

Another important reaction typically proceeding in transition metal complexes is the insertion reaction. Carbon monoxide readily undergoes this process. Therefore, the insertion reaction is extremely important in organoiron chemistry for carbonylation of alkyl groups to aldehydes, ketones (compare Scheme 1.2) or carboxylic acid derivatives. Industrially important catalytic processes based on insertion reactions are hydroformylation and alkene polymerization. 

Hydroformylation (the oxo process) involves the addition of H2 and CO to an olefin to form aldehydes (eq. 2.8), which have a number of important industrial applications. Extensive mechanistic studies have shown that this reaction involves migratory insertion of a bound alkyl group (formed by insertion of an olefin into a metal hydride) into a bound CO, followed by reductive elimination of the aldehyde. The rate-limiting step for the hydroformylation in liquids is either the reaction of olefin and HCo(CO)4 or the reaction of the acyl complex with H2 to liberate the product aldehyde. The high miscibility of CO in sc C02 is therefore not necessarily a major factor in determining the rate of the hydroformylation. Typically, for a-olefins, linear aldehydes are preferred to branched products, and considerable effort has gone into controlling the selectivity of this reaction. 

Therefore, for either antipode, the diastereomeric activated complex controlling optical yield could be either the one corresponding to the formation of the x-complex or the one corresponding to the olefin insertion into the metal-hydrogen bond. In the case of rhodium, it appears from the results of the hydroformylation of 1,2-dimethylcyclohexene and of 2-methylmethylidencyclohexane, that the second case is more probable 10). In the case of platinum, the fact that isomerization of the substrate, which is very likely to occur via metal alkyl-complex formation, proceeds at a rate similar to or even higher than the hydroformylation rate seems to indicate that the same situation can also be assumed. 

The two different ways of inserting an alkene into a metal-hydrogen bond, as shown by 5.4 and 5.5, are called anti-Markovnikov and Markovnikov addition, respectively. Insofar as hydroformylation with high selectivity to n-butyralde-hyde is concerned, it is considered to be primarily an effect of steric crowding around the metal center. The normal alkyl requires less space and therefore formed more easily than the branched one in the presence of bulky ligands. 

The catalytic cycle for the cobalt-based hydroformylation is shown in Fig. 5.7. Most cobalt salts under the reaction conditions of hydroformylation are converted into an equilibrium mixture of Co2(CO)8 and HCo(CO)4. The latter undergoes CO dissociation to give 5.20, a catalytically active 16-electron intermediate. Propylene coordination followed by olefin insertion into the metal-hydrogen bond in a Markovnikov or anti-Markovnikov fashion gives the branched or the linear metal alkyl complex 5.24 or 5.22, respectively. These... 

The most intensely studied insertion reactions are those of CO into metal—carbon bonds to form metal acyls. These reactions are fundamental to industrially important catalytic reactions such as carbonylation and hydroformylations (Sections 22-5 and 22-6). 

We then focus on the Pichler-Schulz CO insertion mechanism (39). This reaction has been much less investigated than the carbide mechanism. We recognize that in homogeneous catalysis, alkene hydroformyla-tion has been investigated extensively it appears that hydroformylation is much more difficult on metallic surfaces than in the presence of mononuclear cationic metal complexes (40). 

The study of stoichiometric CO insertions into transition metal complexes is of great importance because this reaction is the first step m the catalytic conversion of carbon dioxide. Hence, these investigations can lead to the possibility of introducing carbon dioxide into transition metal-catalyzed synthetic processes. Analogies with carbon monoxide chemistry may be drawn, for instance. from the CO insertion into metal alkyl bonds leading to such important industrial processes as hydroformylation and carbonylalion. 

A bimetallic system of Ru3(CO)i2/Co2(CO)g shows high catalytic activity for the hydroformylation of cydohexene. Synergistic effects may play an important role in the insertion of alkene into a hydrido-metal bond [43],... 

Among the steps in Scheme 1, those related to the hydrogen transfer (step 2) or the CO insertion (step 3) must be very fast since no isomerization nor hydrogenation processes are concomitant with hydroformylation. A mechanism analogous to that proposed for rhodium, that is, occurring on a single metal center, can account for all these observations (Scheme 1). 

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