Radicals are core reactive species that occur in almost every subfield of chemistry. In particular, solution phase radicals find their way into biochemistry (e.g. vitamin B12), and in polymer chemistry (e.g. radical polymerizations) just to name a few. Yet, given the proliferation of radical chemistry, there are still fundamental aspects of it that are poorly understood. This dissertation probed factors that influence the solvent cage effect. The solvent cage effect is where two radicals are held in close proximity to one another and prevented from easily escaping (to form free radicals) by a cage of solvent molecules. A convenient metric of the solvent cage effect is the radical recombination efficiency (FcP). Typically, FcP correlates with the bulk viscosity of the solution, however, this parameter only produces qualitative assessments. This dissertation outlines a method to quantitatively predict FcP using the microviscosity. This microviscosity dependence holds for non polar, aromatic, polar, and hydrogen-bonding solvents, along with solutions that contain polymers. Microviscosity is a great metric because it addresses an underlying reason for the solvent cage effect, the strength of the cage. Not only does the strength of the solvent cage around the radical pair affect FcP, but so does the identity of the radicals themselves. That is, the strength of the solvent cage is one piece to forming a total predictive model. FcP for the Cp'2Mo2(CO)6 dimer also varies with the wavelength of irradiation. Identifying the mechanism by which this wavelength dependence occurs may also provide another factor to include in an overall model of the solvent cage effect. Also, an attempt at synthesizing an asymmetric molybdenum dimer was performed. This asymmetric dimer would allow the study of solvent caged radical pairs that are different from each other. Predicting the photochemical cage pair recombination efficiency (FcP) is the major topic of this dissertation. However, there is also the collisional cage recombination efficiency (Fcʹ). This is where free radicals come together in what is called a collisional solvent cage pair. A method and values of Fcʹ are detailed later in this dissertation. This dissertation contains previously published and unpublished co-authored material.

Mechanistic studies on three different ruthenium-based catalysts have been performed. The catalysts have in common that they have been employed in hydrogen transfer reactions involving alcohols and ketones, amines and imines or both. Bäckvall’s catalyst, η5-(Ph5C5)Ru(CO)2Cl, finds its application as racemization catalyst in dynamic kinetic resolution, where racemic alcohols are converted to enantiopure acetates in high yields. The mechanism of the racemization has been investigated and both alkoxide and alkoxyacyl intermediates have been characterized by NMR spectroscopy and in situ FT-IR measurements. The presence of acyl intermediates supports a mechanism via CO assistance. Substantial support for coordination of the substrate during the racemization cycle is provided, including exchange studies with both external and internal potential ketone traps. We also detected an unexpected alkoxycarbonyl complex from 5-hydroxy-1-hexene, which has the double bond coordinated to ruthenium. Shvo’s catalyst, [Ru2(CO)4(μ-H)(C4Ph4COHOCC4Ph4)] is a powerful catalyst for transfer hydrogenation as well as for dynamic kinetic resolution. The mechanism of this catalyst is still under debate, even though a great number of studies have been published during the past decade. In the present work, the mechanism of the reaction with imines has been investigated. Exchange studies with both an external and an internal amine as potential traps have been performed and the results can be explained by a stepwise inner-sphere mechanism. However, if there is e.g. a solvent cage effect, the results can also be explained by an outer-sphere mechanism. We have found that there is no cage effect in the reduction of a ketone containing a potential internal amine trap. If the mechanism is outer-sphere, an explanation as to why the solvent cage effect is much stronger in the case of imines than ketones is needed. Noyori’s catalyst, [p-(Me2CH)C6H4Me]RuH(NH2CHPhCHPhNSO2C6H4-p-CH3), has successfully been used to produce chiral alcohols and amines via transfer hydrogenation. The present study shows that the mechanism for the reduction of imines is different from that of ketones and aldehydes. Acidic activation of the imine was found necessary and an ionic mechanism was proposed.