Reaction mechanisms : Part (ii) Pericyclic reactions

Abstract

This report surveys the 2009 literature on mechanisms of pericyclic reactions. Again, mechanistic studies of cycloadditions dominate, especially those of relevance to organic synthesis. Several electrocyclizations used in the synthesis of complex polycycles were also examined in detail, as were a number of sigmatropic shifts involving aza-polyenes. General theories and theoretical toolsGouverneur and co-workers provided a thorough review of pericyclic reactions involving fluoroorganic compounds, highlighting the synthetic utility of these reactions and both experimental and theoretical studies on their mechanisms with a focus on the effect of fluorine on their regio-and stereoselectivities. 1 Mandado and Ponec described the application of their multicenter bond indices to pericyclic reactions, 2 focusing in this report on electrocyclizations. The authors suggested that computed multicenter bond indices correlate with allowedness of pericyclic reactions and are likely to be applicable to other classes of pericyclic reactions. Cycloaddition reactions [4+2] ReactionsSantos and co-workers used quantum chemical calculations (MP2 and B3LYP) and NMR experiments to reexamine the mechanism of AlR 3 -promoted Diels-Alder reactions that employ dienophiles bearing the Evans chiral auxiliary (e.g., Scheme 1). 3 The authors concluded that models for the origins of stereoselectivity involving species in which Al bridges between the two carbonyl oxygens do not necessarily correspond to what actually occurs. Instead, the authors argued for the involvement of species in which Al is not chelated.