Asymmetric hydrocarbalkoxylation

Asymmetric hydrocarbalkoxylation of alkenes has been studied since the early 1970s, but the number of papers published on this subject is much less than that of asymmetric hydroformylation. This difference is mainly due to the fact that these reactions catalyzed by palladium complexes with chiral phosphine ligands usually require a very high pressure of carbon... 

The influence of the type of alcohol and of the carbon monoxide partial pressure on the asymmetric induction has been investigated in the asymmetric hydrocarbalkoxylation of 2-phenyl-1-propene using (-)DIOP as the chiral ligand (18). For the same substrate different chiral phosphines and diphosphines were used also (17, 24). 

Table I. Influence of the Structure of the Alcohol on the Enantiomeric Excess in the Asymmetric Hydrocarbalkoxylation of 2-Phenyl-l-propenea in the Presence of (-)DIOP...

Figure 1. Influence of carbon monoxide pressure on the ratio between the enantiomers of t-butyl-3-phenylbutanoate arising from asymmetric hydrocarbalkoxylation of 2-phenul-l-propene by (—)DIOP/PaCl2 in a benzene solution at 100 °C...

Some data are also available on the effect of the structure of the phosphine or diphosphine on the optical yield in the asymmetric hydrocarbalkoxylation of 2-phenyl-1-propene (24) (see Table III). The best optical yield was obtained using 2,2-dimethyl-4,5-bis(dibenzo-phosphol-5-ylmethyl)-l,3-dioxolane as the chiral ligand. Much lower optical yields were obtained using aminophosphines or monophosphines. 

Table IV. Palladium-Catalyzed Asymmetric Hydrocarbalkoxylation of Some Olefinic Substrates at 100°C...

Considering isomeric and stereoisomeric composition of the products obtained in the asymmetric hydrocarbalkoxylation investigated using the palladium-(-)DIOP catalytic system, some interesting regularities are apparent. This is also true of the rhodium-(-)DIOP-catalyzed hydroformylation using the same substrates (13). 

Table VI. Comparison Between Experiments and Forecast for the Prevailingly Carbonylated Unsaturated Carbon Atom and for Absolute Configuration in Asymmetric Hydrocarbalkoxylation by PdCl2/(-)DIOP on the...

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. 

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. 

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