Changing the regioselectivity and asynchronicity of the possible concerted [2+2] and [4+2] cycloadditions of ketene and halogenated ketenes with cyclopentadiene: a DFT survey

Two concerted Diels-Alder [4+2] and two concerted Staudinger [2+2] reactions between ketene and/or dihaloketenes with cyclopentadiene are examined at B3LYP/6-31+G* level. For the parent ketene, two cycloaddition modes including [2+2] across the CC and [4+2] across the CC bond of ketene are thermodynamically favorable (ΔE=−13.4 and −17.3 kcal/mol, respectively) while the two other including [2+2] across the CO and [4+2] across the CO bond of ketene are endoergic. The corresponding barrier heights change to a lesser amount and ranging from 20.8 to 35.1 kcal/mol. Halogenated ketenes significantly affect both the thermodynamics and kinetics of these reactions in a way that they increase the exoergicity and reduce the barrier heights result in the decrease of regioselectivity. Unsymmetrical bond formations in the structures of TSs indicate that all the reaction modes are concerted and synchronous but halogens increase the asynchronicity due to the electronic effects. Moreover, three transient halogenated cycloadducts obtained through the [4+2] cycloaddition across the CO bond are converted to some new products via a [3,3] sigmatropic rearrangement. The AIM pictures show that in the [4+2] cycloadditions four TS structures represent pericyclic reactions, while all the [2+2] reactions are classified as pseudopericyclic. FMO theory is applied to justify the findings. The reliability of B3LYP results is confirmed through comparison with the HF and MP2 methods.