Hydrocarbonylation, asymmetric

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... 

Nozaki, K. Hydrocarbonylation of Carbon-carbon Double Bonds. In Comprehensive Asymmetric Catalysis, Jacobsen, E. N. Pfaltz, A., Yamamoto, H. Eds. Springer Heidelberg, 1999 Vol. I, pp 381-409. 

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

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. 

Table V. Influence of the Structure of the Substrate on the Prevailing Chirality and on the Maximum Optical Yield Obtained in Asymmetric Hydrocarbonylation with Rhodium or Palladium Catalysts in the Presence...

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

Asymmetric hydrocarbonylation is a promising method for synthesizing optically active oxygenated compounds from prochiral olefins. Despite the reaction conditions, which include high carbon... 

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. 

Methylstilbenes, asymmetric hydrogenation, 10, 39-40 a-Methylstyrene, hydrocarbonylation, 11, 465 Methyl-substituted phenol, arylation with lead triacetates,... 

In this review the synthetic aspects of asymmetric hydroformylation will be discussed first the experimental data relevant to attempt a rationalization of the results will then be considered. The closely related synthesis of optically active aldehydes by hydroformylation of optically active olefinic substrates in the presence of achiral catalysts7,8 and the different asymmetric hydrocarbonylation reactions, such as the synthesis of esters from olefins, carbon monoxide and alcohols in the presence of optically active catalysts9 , are beyond the scope of this review and will not be discussed here. 

Table 15. Influence of the structure of the substrate on the prevailing chirality and on the maximum optical yield obtained in asymmetric hydrocarbonylation with different metallic components of the catalyst in the presence of the same asymmetric ligand [(—)-DIOP]... 

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. 

The relatively low enantiomeric excess achieved up to now in hydroformylation does not prove that the synthetic potential of this reaction in the synthesis of optically active oxygenated products is low. In contrast to the research on asymmetric hydrogenation which has mainly been directed to the synthesis of a single class of compounds of practical importance, no comparable effort has been made to synthesize one single compound with high optical purity through hydrocarbonylation. 

Examples include acetal hydrolysis, base-catalyzed aldol condensation, olefin hydroformylation catalyzed by phosphine-substituted cobalt hydrocarbonyls, phosphate transfer in biological systems, enzymatic transamination, adiponitrile synthesis via hydrocyanation, olefin hydrogenation with Wilkinson s catalyst, and osmium tetroxide-catalyzed asymmetric dihydroxylation of olefins. 

K. Nozaki, Hydrocarbonylation of Carbon-Carbon Double Bonds, in Comprehensive Asymmetric Catalysis (Eds. E. N. Jacobsen, A, Pfaltz, H. Yamamoto), Springer, Berlin, 1999, Vol. 1, p. 381. 

The historic discovery of Rh complexes of chiral bisphosphites and phosphine-phosphites dramatically raised the enantioselectivities of asymmetric hydrocarbonylation from -50% ee to almost quantitative values in the first half of the 1990s. The successes with Rh catalysts seemed to replace the earfier used Pt catalysts which often suffered from extensive side reactions such as hydrogenation and isomerization, and low selectivity to fso-aldehydes. At this stage, asymmetric hydroformylation has reached the level of enantioselectivity of asymmetric hydrogenation, the most studied asymmetric reaction. 

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