Addition reactions, pressure effects, high

Jenner investigated the kinetic pressure effect on some specific Michael and Henry reactions and found that the observed activation volumes of the Michael reaction between nitromethane and methyl vinyl ketone are largely dependent on the magnitude of the electrostriction effect, which is highest in the lanthanide-catalyzed reaction and lowest in the base-catalyzed version. In the latter case, the reverse reaction is insensitive to pressure.52 Recently, Kobayashi and co-workers reported a highly efficient Lewis-acid-catalyzed asymmetric Michael addition in water.53 A variety of unsaturated carbonyl derivatives gave selective Michael additions with a-nitrocycloalkanones in water, at room temperature without any added catalyst or in a very dilute aqueous solution of potassium carbonate (Eq. 10.24).54... 

Boron enolates bearing menthol-derived chiral ligands have been found to exhibit excellent diastereo- and enantio-control on reaction with aldehydes34 and imines.35 Highly diastereo- and enantio-selective aldol additions of geometrically defined trichlorosilyl ketone enolates (31) and (32) have been achieved by promoting the reactions with chiral Lewis bases, of which (,S., S )-(33) proved to be the most effective.36 Moderate enantiomeric excesses have been achieved by using chiral ammo alcohols as catalysts for the Baylis-Hillman condensation of aldehydes with methyl vinyl ketone the unexpected pressure effect on the reaction has been rationalized.37... 

Interest has been generated in the high-pressure method since it was demonstrated that high pressure is not only useful in effecting cyclo addition reactions, but also several kinds of ionic reactions [9-16]. The aim of the present article is to review recent examples of the use of high pressure for the synthesis of hetero cycles related to biologically interesting molecules, and to predict some further possibilities. The present review covers either representative or most recent examples. 

In these cases the size of the activation volumes obviously depends not only on the effective packing probably caused by the restriction of vibrations and rotations in the transition state, but also on the transition-state polarization enhanced by the polar groups leading to a further decrease in volume which is not observed in the less polarized cycloadducts. Blake and Jorgensen [51] have assumed similar effects to explain the acceleration of Diels-Alder reactions in water. Catalytic and solvophobic promotion of high pressure addition reactions will be discussed in Chapter 11. 

A negligible pressure effect on the diastereoselectivity was also observed for the cycloaddition of the enamine carbaldehyde (81a) carrying an electron-withdrawing group at position 3 and 61a to yield the dihydropyrans 82 and 83 (Scheme 8.21). This reaction was again studied by direct quantitative infrared spectroscopy up to 300 MPa between 45 and 95 °C in different solvents. The activation volume was found to be —(25.1 1.7) cm mol in dichloromethane and —(25.0 + 1.8) cm mol in isodurene. Thus, in this reaction solvent polarity had no influence on the pressure dependence of the rate coefficient in addition, the ratio of the two diastereo-meric products is not changed under high pressure thus the AAV value is very small (AAV < 1 cm mol ). 

The first example of a true positive high pressure effect on the enantioselectivity was found for the intramolecular hetero-Diels-Alder reaction of the l-oxa-1,3-butadiene (173) in the presence of the Narasaka catalyst (164) to give the two enantiomeric bridged cycloadducts 174 and 175 (Scheme 8.44) [80]. At atmospheric pressure the two enantiomers were formed with 4.5 % ee, whereas at 500 MPa an increase to 20.4 % ee was observed which corresponds to a AAV = —(1.7 + 0.2) cm mol . In addition, the yield was improved from 50 to 89 %. It was assumed that under high pressure complexes of different stoichiometry may be formed which are more favorable towards a facial selective addition. However, a clear interpretation of the results cannot be given at this point. 

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