Computational insight into Wilkinson's complex catalyzed [2 + 2 + 2] cycloaddition mechanism leading to pyridine formation

• The rate-determining step of the [2 + 2 + 2] cycloaddition catalyzed by the Wilkinson complex is the initial oxidative addition.
• Oxidative addition between two acetylenes is more favorable than between acetylene and nitrile moieties.
• Insertion of acetylene to the rhodacyclopentadiene intermediate is somewhat more favorable than insertion of hydrogen cyanide.
• Electron withdrawing groups in the nitrile favors its addition in the insertion process and improves the ratio of pyridine vs. benzene.

An interesting reaction for the synthesis of aromatic carbo- and heterocycles is the transition-metal catalyzed [2 + 2 + 2] cycloaddition reaction. In this elegant and atom-efficient process three C–C or C–X (X = N, O, S…) bonds are formed in a single synthetic process. The [2 + 2 + 2] cycloaddition is catalyzed by a variety of organometallic complexes, including more than 15 different metals. In this work, we carry out computational studies of the reaction mechanism between two acetylenes and a nitrile to form substituted pyridines catalyzed by the Wilkinson complex. Our results show that initial oxidative addition between the two acetylene molecules is the rate-determining step. Oxidative addition between two acetylenes is more favorable than between acetylene and nitrile moieties. Our results are in agreement with the experimental finding that the formation of pyridines in some cases requires the use of an excess of nitrile. Nitriles with electron withdrawing substituents favor the incorporation of nitrile in the insertion process and improve the ratio of pyridine vs. benzene.

Nitriles with electron withdrawing substituents favor the incorporation of nitrile in the insertion process and improve the ratio of pyridine vs. benzene in the [2 + 2 + 2] cycloaddition of acetylene and nitrile molecules catalyzed by the Wilkinson's catalyst. Figure optionsDownload as PowerPoint slide