In this paper, the competing mechanisms of PPh3-catalyzed [3 +3] annulation reaction (including possible pathways a, b and c) and [3 +2] annulation reaction (including possible pathways a', b' and c') between 5-acetoxypenta-2,3-dienoate and 1C,30-bisnucleophile have been theoretically investigated using density functional theory (OFT). Among the six possible pathways, our computational results indicate that pathway c is the most energy favorable pathway at acid condition, whereas pathway b' is the most energy favorable pathway at base condition. Thus, the different acid/base condition would be the switch for the two competing reactions, which is in agreement with the experiment results. Moreover, the analysis of global reactivity indexes has been carried out to explore the role of the catalyst PPh3, which demonstrates that Lewis base catalyst PPh3 noticeably strengthens the nucleophilicity of the reactant and makes the annulation reactions easier to occur. This work should be helpful for not only understanding the role of PPh3 catalyst in the [3 +3] and [3 + 2] annulations, but also the role of the real switch (i.e., the acid/base additives) in the competing [3 +3] and [3 +2] annulations, and thus provides valuable insights on the rational design of more efficient catalysts for this kind of reactions. (C) 2015 Elsevier B.V. All rights reserved.