Sources of Error in DFT Computations of CC Bond Formation Thermochemistries: π→σ Transformations and Error Cancellation by DFT Methods

Alarming: Multiple sources of errors in DFT energetics of CC bond-forming reactions were investigated by evaluating structural transformations in Diels–Alder reactions: conversion of π into σ bonds and changes in conjugation, hyperconjugation, and branching interactions. A startling overestimation of the π to σ bond conversion is found with most methods, a central problem to all reactions involving addition of π bonds (electrocyclic processes, ene, aldol).