Hydrocarbonylation, asymmetric model

A Model to Interpret the Regularities Observed in Asymmetric Hydrocarbonylation... 

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. 

Considering the experiments of hydroalkoxycarbonylation9), the number of asymmetric hydrocarbonylation results to which the model discussed in Section 4 has been applied is about 200. The wrong predictions are 15%, 83% of which concern experiments with aromatic substrates. Even if the reasons we have given to justify the success of the model may be wrong, the model is undoubtedly very useful to analyze the results of asymmetric hydrocarbonylation with different catalytic systems. 

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... 

In any case, while few results involving enantiomeric excesses smaller than 40% cannot be taken as a sound basis for speculating about the factors influencing asymmetric induction, the regularities that have become apparent in the numerous results concerning more than 90 independent experiments of hydrocarbonylation (including both hydroformylation and hydrocarbalkoxylation) in which either the substrate or the catalytic system is different, in our opinion make worthwhile an attempt to correlate the above results with a simple stereochemical model. 

Hydrocarbonylation reactions are multistep catalytic processes (14). For this reason our first problem was to determine in which step asymmetric induction takes place, since models could be very different depending on the step in which asymmetric induction is determined (e.g. in the metal alkyl complex formation or in the final reductive elimination step). 

We first established that hydrocarbonylation reactions occur with cis-stereochemistry (29, 16) and that asymmetric induction occurs before or during the formation of the metal alkyl intermediate (5, 6). This means that is either during the 7r-olefin complex formation between catalyst and substrate or during the insertion of the 7r-complexed olefin into the M-H bond. Therefore, the model should focus on the interactions between the substrate double bond and the catalytically active metal atom of the catalyst. 

Figure 2. Models for the transition states controlling asymmetric induction in the hydrocarbonylation of olefins...

---