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

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-j to the corresponding allenes 7a-j 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 pi 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 (C) 2015 John Wiley & Sons, Ltd.