In Situ Infrared Spectroscopic Study of Iron-Catalyzed Hydromagnesiation of Vinyl Arenes

Iron catalysis, especially homogeneous catalysis, has been a resurging topic of organometallic chemistry research. Discussions of past and present mechanistic analyses for homogeneous iron catalysis will be discussed in Chapter 1. As an expansion of homogeneous iron catalysis, in situ infrared spectroscopy will be used to develop a full mechanistic study of iron-catalyzed hydromagnesiation of vinyl arenes. The kinetic analyses by both initial and observed rate measurements indicate complex concentration dependencies on the (PDI)iron catalyst as well as sacrificial Grignard reagent and styrene. These complexities are not limited to non-linear catalyst and Grignard initial rate and inhibition by styrene/Grignard at low concentrations which then change upon reaching concentrations of a 1:1 ratio by observed rates. The process of numeric timecourse simulation of probable mechanisms using COmplex PAthway Simulator (COPASI) led to the identification of a twelve-step mechanism that accurately reproduces experimental timecourse data over a wide variety of reaction conditions.1 This initial analysis was used as a building block for further identification of kinetic complexities when varying the electronics and sterics of the substrates and precatalysts. The development of an unexpected kinetic complexity with respect to the electronics of the styrene derivatives resulted in a strange “arrow-head” shaped Hammett correlation. DFT calculations and numeric timecourse simulation suggest a change in electronic character of a p-methoxystyrene or turnover limiting step, wherein the rate constants for the competitive 2,1- vs 1,2- insertion is significantly lower than that of transmetallation. Furthermore, predictions using the hydromagnesiation mechanism reveal the origins of the observed kinetic complexity. Identification of the limitations of some sacrificial Grignard reagents leads to the explanation of reaction complexity based upon the generation of a gaseous alkene byproduct. Following the kinetic analysis of Grignard reagents, electron-rich styrenes, and pre-catalysts bearing less sterically bulky PDI ligands led to the development of a mechanism consisting of 16 elementary steps.