Manganese-Mediated C–H Bond Activation of Fluorinated Aromatics and the ortho-Fluorine Effect: Kinetic Analysis by In Situ Infrared Spectroscopic Analysis and Time-Resolved Methods

Insights into the factors controlling the site selectivity of transition metal-catalyzed C–H bond functionalization reactions are vital to their successful implementation in the synthesis of complex target molecules. The introduction of fluorine atoms into substrates has the potential to deliver this selectivity. In this study, we employ spectroscopic and computational methods to demonstrate how the “ortho-fluorine effect” influences the kinetic and thermodynamic control of C–H bond activation in manganese(I)-mediated reactions. The C–H bond activation of fluorinated N,N-dimethylbenzylamines and fluorinated 2-phenylpyridines by benzyl manganese(I) pentacarbonyl BnMn(CO)5 leads to the formation of cyclomanganated tetracarbonyl complexes (2a–b and 4a–e), which all exhibit C–H bond activation ortho-to-fluorine. Corroboration of the experimental findings with density functional theory methods confirms that a kinetically controlled irreversible σ-complex-assisted metathesis mechanism is operative in these reactions. The addition of benzoic acid results in a mechanistic switch, so that cyclomanganation proceeds through a reversible AMLA-6 mechanism (kinetically and thermodynamically controlled). These stoichiometric findings are critical to catalysis, particularly subsequent insertion of a suitable acceptor substrate into the C–Mn bond of the regioisomeric cyclomanganated tetracarbonyl complex intermediates. The employment of time-resolved infrared spectroscopic analysis allowed for correlation of the rates of terminal acetylene insertion into the C–Mn bond with the relative thermodynamic stability of the regioisomeric complexes. Thus, more stable manganacycles, imparted by an ortho-fluorine substituent, exhibit a slower rate of terminal acetylene insertion, whereas a para-fluorine atom accelerates this step. A critical factor in governing C–H bond site selectivity under catalytic conditions is the generation of the regioisomeric cyclomanganated intermediates, rather than their subsequent reactivity toward alkyne insertion.