Kinetic or Dynamic Control on a Bifurcating Potential Energy Surface? An Experimental and DFT Study of Gold-Catalyzed Ring Expansion and Spirocyclization of 2-Propargyl-beta-tetrahydrocarbolines

In classical transition state theory, a transition state is connected to its reactant(s) and product(s). Recently, chemists found that reaction pathways may bifurcate after a transition state, leading to two or more sets of products. The product distribution for such a reaction containing a bifurcating potential energy surface (bPES) is usually determined by the shape of the bPES and dynamic factors. However, if the bPES leads to two intermediates (other than two products), which then undergo further transformations to give different final products, what factors control the selectivity is still not fully examined. This missing link in transition state theory is founded in the present study. Aiming to develop new methods for the synthesis of azocinoindole derivatives, we found that 2-propargyl-beta-tetrahydrocarbolines can undergo ring expansion and spirocyclization under gold catalysis. DFT study revealed that the reaction starts with the intramolecular cyclization of the gold-activated 2-propargyl-beta-tetrahydrocarboline with a bPES. The cyclization intermediates can not only interconvert into each other via a [1,5]-alkenyl shift, but also undergo ring expansion (through fragmentation/protodeauration mechanism) or spirocyclization (through deprotonation/protodeauration mechanism). Detailed analysis of the complex PESs for substrates with different substituents indicated that the reaction selectivity is under dynamic control if the interconversion of the intermediates is slower than the ring expansion and spirocyclization processes. Otherwise, the chemical outcome is under typical kinetic control and determined by the relative preference of ring expansion versus spirocyclization pathways. The present study may enrich chemist's understanding of the determinants for selectivities on bPESs.