Intermediates hydroformylation model

Recently, several reports have appeared in which PHIP has been used to probe the mechanism of, and look for intermediates in model reactions related to, catalytic hydroformylation. Two of these studies focused on the detection and identification of species formed in small quantities, while a third involved the discovery of an unexpected parahydrogen effect. ... 

Platinum complexes with chiral phosphorus ligands have been extensively used in asymmetric hydroformylation. In most cases, styrene has been used as the substrate to evaluate the efficiency of the catalyst systems. In addition, styrere was of interest as a model intermediate in the synthesis of arylpropionic acids, a family of anti-inflammatory drugs.308,309 Until 1993 the best enantio-selectivities in asymmetric hydroformylation were provided by platinum complexes, although the activities and regioselectivities were, in many cases, far from the obtained for rhodium catalysts. A report on asymmetric carbonylation was published in 1993.310 Two reviews dedicated to asymmetric hydroformylation, which appeared in 1995, include the most important studies and results on platinum-catalogued asymmetric hydroformylation.80,81 A report appeared in 1999 about hydrocarbonylation of carbon-carbon double bonds catalyzed by Ptn complexes, including a proposal for a mechanism for this process.311... 

Today, iridium compounds find so many varied applications in contemporary homogeneous catalysis it is difficult to recall that, until the late 1970s, rhodium was one of only two metals considered likely to serve as useful catalysts, at that time typically for hydrogenation or hydroformylation. Indeed, catalyst/solvent combinations such as [IrCl(PPh3)3]/MeOH, which were modeled directly on what was previously successful for rhodium, failed for iridium. Although iridium was still considered potentially to be useful, this was only for the demonstration of stoichiometric reactions related to proposed catalytic cycles. Iridium tends to form stronger metal-ligand bonds (e.g., Cp(CO)Rh-CO, 46 kcal mol-1 Cp(CO)Ir-CO, 57 kcal mol ), and consequently compounds which act as reactive intermediates for rhodium can sometimes be isolated in the case of iridium. 

In 2004 Caporali investigated the hydroformylation of 1-hexene and cyclohexene using HRh(CO)(PPh3)3 [61]. The collected data indicated that the rate-determining step in the hydroformylation cycle depends upon the structure of the olefin. With an alpha-olefin like 1-hexene, the slowest step seems to be the hydrogenolysis of the acyl rhodium complex. In the presence of cyclohexene as a model for an internal olefin, the rate-determining step is the reaction of the olefin with the rhodium hydride complex (intermediate II in Fig. 6). 

The fact that a model for the transition state controlling asymmetric induction based on steric interactions allows us to correctly predict the type of prevailing regio- and stereoisomer for about 85% of the asymmetric hydrocarbonylation experiments (including hydroformylation and hydrocarbalkoxylation) is an indication that asymmetric induction in these catalytic reactions is based mainly on steric interactions. The data obtained so far do not allow us to establish whether the more stable or the less stable 7r-olefin complex intermediate is the one that reacts preferentially. However, the regularities that we observed indicate that the kinetic features are the same, at least in most of the experiments. 

The reduced models in Table 11.1 rely on the validity of the Bodenstein approximation for all intermediates except the aldehyde in hydroformylation, but are otherwise free of assumptions. In every case, equations that are as simple or even simpler have long been derived, but only with much more restrictive assumptions, most commonly that of a single rate-controlling step and quasi-equilibrium everywhere else. Of course, such equations should be used in preference if their assumptions can be substantiated. 

Industrial practice often confronts the development engineer with networks that are considerably more complicated than that of cyclohexene hydroformylation in the example above. Additional simplifications may then be desirable or necessary in order to arrive at a model that remains manageable in the highly iterative applications called for in reactor design and optimization and possibly on-line process control. A useful and usually successful way of achieving such streamlining is to place all network nodes at end members or non-trace intermediates, ignoring the fact that some of them may be at trace-level intermediates [10]. 

The mechanism of hydroformylation by Pt/Sn systems has been investigated with the help of model complexes (Scheme 42). Only platinum SnCla complexes react with H2 to give EtCHO and close the cycle. 4-Pentenal is cyclized to cyclopentanone by cationic rhodium catalyst such as [Rh(dppe)2] in nitromethane or dichloromethane at 20 °C. The initiating step of the process is the oxidative addition of aldehyde-CH to the Rh(I) centre, a reversal of the final step in an olefin hydroformylation sequence. The mechanism was probed by deuteration studies direct evidence for the catalytic intermediates by NMR was unobtainable. The intermediates are involved in the reversible formation of side products, although selectivity to cyclopentanone can be as high as 98%. The essential features of the reaction are outlined in Scheme 43. ... 

The ease of H2 dissociation in some dihydrogen complexes makes them ideal precursors for 16-electron intermediates, useful in both stoichiometric and catalytic reactions. The loss of H2 may also be of use for reduction reactions. Compound (7) has been modeled as a hydroformylation inter-mediate and (8) has been shown to be a hydrosilation intermediate (in... 

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