High pressure and selectivity in organic reactions

Publisher Summary This chapter describes the quantitative analysis of steric effects in heteroaromatics. This chapter illustrates that, important progress in the quantitative analysis of steric effects in heteroaromatics has been made recently by linear free energy relationships (LFER), geometrical, and theoretical methods. Heterocycles have been used to propose and exemplify new concepts and new parameters in physical organic chemistry. This chapter is concerned with the importance of steric effects in rationalizing the reactivity and dynamic stereochemistry of heteroaromatics and with recent attempts to bridge the gap between them. There has been a decisive evolution in the treatment of steric effects in heteroaromatic chemistry. A better knowledge of static and dynamic stereochemistry has helped greatly in understanding not only intramolecular but also intermolecular steric effects associated with rates and equilibria. Quantum and molecular mechanics calculations will certainly be used in the future to a greater extent. Heteroaromatics have features in common with aromatic hydrocarbons. However, the variety of geometrical situations, the number of heteroatoms and their associated reactivity, and the ease of monitoring stereochemistry often associated with the presence of heteroatoms all have made, and continues to make, heterocycles exceptional probes for better understanding of reactivity, structures, and the relations between them.

The Quantitative Analysis of Steric Effects in Heteroaromatics

Comparative activation modes in organic synthesis. The specific role of high pressure

Steric effects in high pressure Knoevenagel reactions

High-pressure mechanistic delineation based on activation volumes

This paper considers the description of the pressure effect on strained transition states and proposes a kinetic formalism based on the concept of steric or strain volume of activation with a view to devising models allowing the calculation of structure effects on the position of the transition state along the reaction coordinate. The applicability of the equation is examined for several reactions (Menshutkin, solvolysis and hydrogen transfer). When steric or strain factors operate in a given reaction the pressure effect is more marked than in an unhindered analogue. Possible consequences of this phenomenon are considered, with special emphasis on mechanistic and synthetic implications at high pressure.

Gérard Jenner

Papers

The pressure effect on strained transition states. Correlation between strain and volume of activation: mechanistic and synthetic involvements