Hydro esterification catalysts

Very recently, a new strategy for the hydro esterification and hydroamidation of olefins was reported by Chang and coworkers [83]. They used a chelation-assisted protocol for the hydro esterification of olefins. The reaction of 2-pyridylmethyl formate with 1-hexene in the presence of a Ru3(CO)i2 catalyst gave the hydroesterification product in 98% yield as a mixture of linear and branched isomers (Eq. 54). The chain length of the methylene tether is important for a successful reaction. Thus, the reaction of 2-pyridyl formate (n=0) afforded 2-hydroxypyridine, a decarbonylation product, and the reaction of 2-pyridiylethyl formate (n=2) resulted in a low conversion (7% conversion) of the starting formate. From these results, the formation of a six-membered ruthenacycle intermediate is crucial for this chelation-assisted hydro esterification. 

Intramolecular condensation of cu-hydroxycarboxylic acids is a standard method to prepare lactones. Acid catalysts or more elaborate mediators are usually required as well as continuous removal of water. Transition-metal-catalyzed cyclocarbonylation of unsaturated alcohols is a fascinating alternative, which proceeds under neutral conditions [26]. Intramolecular hydro esterification of... 

Esterification is finally an equilibrium reaction (35 per cent methyl methacrylate), which can be continued to completion by removing one or both of the products obtained as soon as they are formed. It takes place preferably in the liquid phase, in the presence of sulfuric acid or cation exchange resins as a catalyst, with a slight excess of methanol (1.2/1 in mol), at temperatures (110 to 115°Q apd pressures (30 to 50 kPa absolute) designed to limit polymerization reactions. The addition of an inhibitor (such as hydro-quinone) is also practised. With residence time of about 1 h. once-through conversion is total and the molar yield is close to 99 per cent. 

One of the simplest and most efficient approaches to the modification of native 4-hydro)yproline is based on the esterification of the hydrojq l group with various carboxylic acids. By varying the electronic and steric properties as well as the acid counterpart s hydrophilic/hydrophobic balance, it is possible to control the corresponding characteristics of target molecules and attain the most active and stereo- or enantioselective catalysts 19-26 for asymmetric reactions (Figure 10.2). 

Finally, the a-hydroxy-y-lactones could be obtained optically pure, by esterification with acetyllactyl chloride and the resulting acetyllactyl derivatives were separated. Recently, a kinetic resolution of the a-hydro) -y-lactones has been described by Shiina, using an asymmetric acyl-transfert catalyst ((R)-BTM) in the presence of an acetylating agent. 

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