1,7-cyclization of 1-diazo-2,4-pentadiene and its heteroanalogues: DFT study

1,7-Dipolar cyclizations of 1-diazo-2,4-pentadiene 1a and its heteroanalogues 1b,c and 4c were studied using density functional theory (DFT). Although the heteroanalogue 1c has an appropriate electronic structure to allow for pseudopericyclic cyclization, natural bond order (NBO) analysis has provided evidence for the electrocyclic ring closure. Magnetic criteria (anisotropy of the induced current density [ACID], nucleus-independent chemical shifts [NICS]) confirmed the pericyclic character of the located transition states 2a,c and 5c. The activation barriers for the cyclization of 1-diazo-2,4-pentadiene la and its aza analogues 1c, 4c are 3.3, 8.2, and 12.3 kcal/mol at the B3LYP/6-31G(d) level, respectively. The higher barrier of the 1c -> 3c and 4c -> 3c reactions compared with 1a -> 3a is in line with the Hammond postulate. The out-of-plane distorted geometry of the cyclic product is an additional factor arguing against a pseudopericyclic mechanism. (c) 2006 Wiley Periodicals, Inc.