The role of transiton-state bond-order on structure-reactivity relationships: the case of pericyclic and pseudo-pericyclic reactions

Abstract

This work is dedicated to Prof. António Rocha Gonsalves, on the occasion of his 70 th birthday: to the colleague and to the scientist who contributed to the development of modern organic chemistry in Coimbra AbstractThe importance of the transition-state bond-order (n ‡ ) in the interpretation of chemical reactivity, in complement to the effect of reaction energy, for decarbonylation pericyclic and pseudopericyclic reactions of 3-cyclopentenone and related molecules is demonstrated through the intersecting-state model and quantum-mechanical electronic structure calculations. Both types of methodology reveal that n ‡ is strongly related to transition state geometry.Keywords: Pericyclic reactions; structure-reactivity relationships; intersecting-state model (ISM); transition-state bond-order General AspectsBased on the idea that the ability of molecules to react rests ultimately on their own structural properties, chemists have employed several types of mathematical equations to express the relation of molecular-structure to reaction rate-constants or energy barriers. However such approaches do not currently encompass the role of electronic factors. This neglected factor often counters the energetic effects and gives rise to 'anomalies' or separate correlations. Recently Arnaut and Formosinho presented a quantification of the concept of 'chemical reactivity', emphasizing the role of molecular and electronic factors in chemistry through the Intersecting (interacting)-state model (ISM). 1ISM is based on simple chemical concepts: chemical bonds, represented in terms of potential energy curves (e.g. force constants for reactants, f r , and products, f p ) and bond lengths (l r and l p ), reaction energy, and bond orders for chemical bonds in the transition states (transition-state bond-order, n ‡ ). The model incorporates novel insights in order to understand chemical reactivity for bond-breaking-bond-forming processes.2 First, the electronic element: the encounter of two molecules triggers unique features which neither molecule possesses alone, notably an electron