Competing nature of intramolecular [4 + 2] and [3 + 2] cycloaddition reactions: a theoretical study

Comparative study of the intramolecular alkyne triple bond addition reaction to the conjugated ­C≡C—CHX moiety (X = CH 2 ,O,S,NH) revealed that two different pathways are possible in the system, namely [4 + 2] and [3 + 2] cycloaddition reactions. The energetically preferred pathway for enynes (X = CH 2 ) involves [4 + 2] cycloaddition leading to benzene derivatives, whereas heteroatom‐substituted substrates undergo [3 + 2] cycloaddition resulting in an five‐membered aromatic ring in the final product. This paper reports a detailed mechanistic study based on full potential energy surface calculations at the MP2 and B3LYP theory levels, with MP4(SDTQ) energy evaluation. The effect of solvent was included within the PCM approach.