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Hydroarylations of C-C Multiple Bonds Catalyzed by Late-Metal Complexes: Mechanistic Investigations



Hydroarylations of C–C Multiple Bonds Catalyzed by Late-Metal Complexes:

Mechanistic Investigations

Chapter One

Undirected and Directed Hydroarylations Catalyzed by Late-Transition Metals: An Introductory Review. Recent investigations into homogeneous catalytic hydroarylations of olefins have focused heavily on catalyst precursors bearing a d8 electronic configuration. This chapter discusses such cases in the context of olefin hydroarylations with benzene and substituted arenes. More specifically, it discusses three types of olefin hydroarylation by d8 catalysts: those involving unfunctionalized arenes (e.g., benzene and alkyl arenes), those assisted (and directed) by ortho‐arene donor groups, and finally palladium‐catalyzed hydroarylations that involve reaction sequences of a Heck‐type coupling followed by reduction of the C=C double bond with an exogenous hydrogen source. Since hydroarylation requires both aryl C–H activation and olefin‐insertion steps, Pt has proven to be the focus in the early developments of hydroarylation catalysis. There is current interest in the development of homogeneous transition metal catalysts for selective, direct hydroarylation of α‐olefins with arenes. Given the history of research in this area, it seems that d8 precatalysts are promising candidates for future development.

Chapter Two

Olefin Hydroarylation Catalyzed by (Pyridyl-Indolate)Pt(II) Complexes: Catalytic Efficiencies and Mechanistic Aspects. A series of Pt(II) complexes of the type (PyInd)PtPh(SR2) (PyInd= 2,2′-pyridyl-indolate) were prepared, and their performance as catalysts for the hydroarylation of olefins was assessed. Evidence that the catalysis is homogeneous and Pt mediated is provided by control experiments with added hindered base (2,6-di-tert-butyl-4-methylpyridine) and Hg(0). Two potential catalytic intermediates, (tBuPyInd)PtPh(C2H4) and (tBuPyInd)Pt(CH2CH2Ph)(C2H4), were synthesized and their catalytic efficacy was explored. Additionally, decomposition and deactivation pathways, including styrene formation via β-hydride elimination and ligand reductive demetallation, were identified.

Chapter Three

Mechanistic Interrogation of Alkyne Hydroarylations Catalyzed by Highly Reduced, Single-Component Cobalt Complexes. Highly reactive catalysts for ortho-hydroarylations of alkynes have previously been reported to result from activation of CoBr2 by Grignard reagents, but the operative mechanism and identity of the active cobalt species remain unidentified. A thorough mechanistic analysis of a related system was performed using isolable reduced Co complexes. Stoichiometric treatment of Co(I) or Co(II) precursors with CyMgCl implicated catalyst initiation via a β-H elimination and deprotonation pathway. The resulting single-component Co(-I) complex is proposed as the direct precatalyst. Michaelis-Menten enzyme kinetic studies elucidated the catalytic dependence on substrate. The (N-aryl)aryl ethanimine substrate exhibited saturation-like behavior whereas alkyne demonstrated a complex dependency; rate inhibition and promotion depends on the relative alkyne to imine concentration. Activation of the aryl C–H bond occurred only in the presence of coordinated alkyne, which suggests operation of a concerted metalation-deprotonation (CMD) mechanism. Small primary isotope effects are consistent with a rate-determining C–H cleavage. Off-cycle olefin isomerization catalyzed by the same Co(-I) active species appears to be responsible for the observed Z-selectivity.

Chapter Four

Olefin Hydroarylations Catalyzed by a Single-Component Cobalt(-I) Complex. A single-component Co(-I) catalyst has been developed for olefin hydroarylations with (N-aryl)aryl imine substrates. Over 40 examples were examined under mild reaction conditions to afford the desired alkyl-arene product in good to excellent yields. Catalysis occurs in a regioselective manner to afford exclusively branched products with styrene-derived substrates or linear products for aliphatic olefins. Electrophilic functional groups were generally tolerated under the reaction conditions.

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