Substituent effects on the ring-opening mechanism ofgem-dibromospiropentanes to related allenes: a theoretical study

Abstract

Density functional theory computations at B3LYP and X3LYP levels were performed for ring openings of substituted gem‐dibromospiropentanes (R = ―H, ―Cl, ―Br, ―CH3, ―SiH3, ―OH, ―OCH3, ―CF3, ―BF2, and ―SH) to related allenes. The conversion of spiropentanoids 5a, 5b, 5c, 5d, 5e, 5f, 5g, 5h, 5i, 5j to the corresponding allenes 7a, 7b, 7c, 7d, 7e, 7f, 7g, 7h, 7i, 7j can proceed in both concerted and stepwise mechanism except for R = ―H. Both ring‐opening mechanisms have similar activation energy barriers to open the spiropentanylidene ring and generate the structure of allene at all theoretical levels used herein. Generally the π electron‐donating group (―OH or ―SH) decreases the activation barrier for the follow‐up reaction of 1‐bromo‐1‐lithiospiropentanoid and free spiropentanylidene. Hence, both bearing electron‐donating substituents are more reactive than those with electron‐withdrawing group, and the first one to open the ring to the LiBr–allene complex does so more readily than the second. The sEDA index used to measure sigma‐electron excess/deficiency of the cyclopropylidene ring is mutually correlated for the studied systems. Copyright © 2015 John Wiley & Sons, Ltd.