UC Merced

Transition states are the gate keepers of chemical reactions. The free energies associated with transition states determine everything of interest in a chemical reaction, namely the rate and product distribution. Attempts to categorize the repulsive and attractive forces that dictate structural and energetic features of transition states dominate mechanistic organic chemistry. Kinetic isotope effects (KIEs) provide a direct quantitative probe into transition structure because they report directly upon changes in bond strength. Furthermore, KIEs provide a satisfying means of comparison with computational efforts. However, KIE methods are almost exclusively applied to spot to spot reactions with only a single product. The information provided by studying these somewhat rare, well-behaved reactions, can be helpful, but more versatile methods are needed to investigate reactions that yield more than one product.

In this dissertation, product-specific KIE, a new method developed in the lab which can simultaneously measure the KIE of all pathways in a multi-product reaction, will be introduced. In principle, the product-specific KIE methodology is able to reveal the energetic landscape of all pathways by analysis of the isotopic fractionation which records the bonding changes that occur along a reaction pathway. The information provided can be illuminating to discover the branching events in chemo-, regio- and stereo-selective reactions. Secondly, the mechanisms of two transition-metal catalyzed multi-product reactions have been discussed as examples for the application of this method with both natural abundance 13C and deuterium KIE experiments. One of them is cobalt catalyzed [2+2] and Alder-ene reaction, where a common intermediate was found before it branches to two different products. In another case, a dirhodium tetraacetate catalyzed C-H insertion was found to proceed via an initial hydride transfer transition state, followed by formation of two interconverting zwitterion intermediates before the formation of two diastereomers. Product-specific KIEs have the potential to address mechanistic questions in reactions under development and provide a basic understanding of the key transition state features necessary to develop more selective catalysis.