Visible Light Photocatalysis for the Generation and Use of Reactive Azolyl and Polyfluoroaryl Intermediates

Abstract

Photocatalysis offers several mechanistically unique pathways that are not rivaled by mainstream catalysis. Primarily, the ability to convert photochemical energy into single electron oxidation and reduction events provides a new dimension for chemists to consider when choosing how to activate a molecule or approach a complex synthesis. Since most organic molecules do not absorb light in the visible region, they are impervious to direct visible light photochemistry, which provides an opportunity for photocatalysis in which a visible light absorbing compound can serve as a mediator. In this Account, we discuss the consequences of catalyst mediated, photoinduced electron transfer to several classes of reducible arenes. While the bulk of the work discussed within this Account utilizes iridium-based photocatalysts, in principle the chemistry is not limited to this class of photocatalyst, and the principles should be more general. Instead, this Account focuses largely on the consequences of single electron transfer to poly- and perfluorinated arenes and 2-halo azoles. Electron transfer converts these stable molecules into reactive intermediates whose behavior often depends entirely on the identity of the halogen that undergoes substitution. The result is both diverse chemistry and an alternative way of thinking about the chemical reactivity of these motifs. Specifically, we discuss our efforts and those of others to develop strategies for the generation of radicals or radical anions from perfluoroarenes and azoles and the behavior of these intermediates as implied by reactions in which they participate. The divergent pathway is illustrated by 2-bromoazoles, which yield azolyl radicals and can be utilized for addition to π-bonds, while use of the 2-chloroazole substrate leads to an entirely different reaction profile. Under the appropriate reaction conditions, the reactive and transient intermediates are useful coupling partners and often provide unrivaled access to new chemical space. The odd electron species can form challenging bonds with minimal prefunctionalization of the coupling partner. For instance, some of the intermediates can be utilized for C-H functionalizations to selectively make crowded amines or to synthesize biarenes substituted at every ortho position. While photocatalysis is not the only manner of accomplishing electron transfer, the catalytic generation of the reactive species in which the concentration of the transient odd electron species is kept low, provides a synthetic handle that can be used to improve reaction outcomes. This is elegantly demonstrated in a number of examples in which redox sensitive groups located on substrates survive the reaction. In addition, the underlying basic concepts associated with radical anion fragmentation are reviewed and provide the backdrop for discussion throughout the Account.

Consecutive Transformations of Tetrafluoropropenes: Hydrogermylation and Catalytic C−F Activation Steps at a Lewis Acidic Aluminum Fluoride

“…Thus the hydrogermylations have two roles,n amely to generate the C(sp 3 ) À Fb ond and to functionalize the olefin after the C À Fb ond activation step.T he transformation of olefinic C À Fb onds induced by Lewis acids,f or example, silylium ions,h ad previously been elusive and had only been described at transition-metal centers. [1,4,9,30]…”

“…[1,4,9,30] This results in an unprecedented reactivity pattern for an olefinic CÀFb ond at ACFand the functionalization of the starting material by germylation.…”

Radical‐Polar Crossover Annulation: A Platform for Accessing Polycyclic Cyclopropanes

“…The reaction also tolerated a polyfluorinated arene without evidence of products derived from radical/polar S N Ar-type processes. [17] Alteration of the tetralone structure was similarly welltolerated and allowed various groups to be installed on the cyclopropane fragment (16,17) as well as on the arene (6 b-c, 10 b) emphasizing the modular nature of this approach. Electronic changes on the aryl ring appear to have little impact on cyclopropanation reactivity, as evidenced by the lack of variability in yield between 6 a-6 c and the results of a head-to-head competition study (See Supporting Information).…”

Photoredox‐gesteuerte Mono‐ und Di‐Multifluorarylierung von C(sp3)‐H‐Bindungen mit Arylfluoriden

“…[3] Vork urzem gelang der Gruppe von Weaver eine milde C-F-Bindungsfunktionalisierung von Multifluorarenen mittels Photokatalyse,e ine neue Option fürC -C-Kupplungen mit Alkenen, Alkinen und Arenen. [4] Alternativ dazu wurden auch durch sichtbares Licht vermittelte,r adikalische Hydrodefluorierungen verçffentlicht. [4c,f] Tr otz großer Vorteile sind durch sichtbares Licht vermittelte und hoch selektive Multifluorarylierungen von nichtaktivierten C(sp 3 )-H-Bindungen mit Arylfluoriden noch immer selten.…”