Browsing by Author "Kelley, Alexandra S."

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    Greener Photoredox-Catalyzed Phosphonations of Aryl Halides
    (2024-05) Kelley, Alexandra S.; Laulhe, Sébastien; Minto, Robert; Deng, Yongming
    Aromatic phosphonates and phosphine oxides are highly desirable synthetic targets used in pharmaceuticals, natural products, agrichemicals, catalysis, and materials science. While a variety of aromatic precursors have been used to access these motifs, aryl halides remain one of the most desirable coupling partners owing to their low cost, commercial availability, and regioselective reactivity. Traditional phosphonation often requires the use of harsh reductants in the presence of liquid ammonia, which are caustic and pose incredible environmental concerns. Milder, transition metal-catalyzed approaches have been developed, but can be limited by air sensitivity, cost, low reaction selectivity, and low functional group compatibility. Photoredox catalysis has been significantly advanced in the past decade in the pursuit of greener, more sustainable avenues to facilitate desirable reaction transformations under mild conditions. These methods most commonly use a dual catalytic strategy in which a metal is paired with an organocatalyst. While these approaches enable facile phosphonation of a variety of aromatic precursors, the metals and organocatalysts used are often expensive and toxic. Indeed, there remains unexplored chemical space for transition metal-free photoredox-catalyzed aryl C-P bond formations. Herein, we present a series of transition metal-free, photoredox-catalyzed approaches to the phosphonation of aryl halides. The approaches and mechanistic works will be discussed in the following order: First, the discovery that 10H-phenothiazine (PTZ) enables the transition metal-free phosphonation of aryl halides using trialkyl phosphites will be presented. PTZ serves as a photocatalyst capable of reducing the aryl halide to access aryl radicals, which readily couple with phosphite esters. This transformation exhibits broad functional group tolerance in good to excellent yields. Then, photoredox catalysis by PTZ enables the formation of unsymmetrical aromatic phosphine oxides using triphenylphosphine (PPh3) and aryl halides. This is the first work in which PPh3 has been used as the starting material, and the reaction proceeds via the alkaline hydrolysis of quaternary phosphonium salts. The final work exhibits novel photocatalytic activity of N-heterocyclic carbenes (NHC) to activate aryl halides, form aryl radicals, and enable phosphonation. This method displays broad functional group tolerance under mild conditions and highlights its untapped synthetic utility as a photocatalyst.
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    Synthesis of Imide and Amine Derivatives via Deoxyamination of Alcohols Using N-Haloimides and Triphenylphosphine
    (Wiley, 2021) Irving, Charles D.; Floreancig, Jack T.; Gasonoo, Makafui; Kelley, Alexandra S.; Laulhé, Sébastien; Chemistry and Chemical Biology, School of Science
    A deoxyamination methodology of activated and unactivated alcohols is presented. The reaction is mediated by phosphonium intermediates generated in situ from N-haloimides and triphenylphosphine. The protocol allows for the synthesis of phthalimide and amine derivatives in moderate to good yields at room temperature. A series of NMR experiments have provided insight into the reactive intermediates involved and the mechanism of this deoxyamination reaction.
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    Transition-Metal-Free Photoredox Phosphonation of Aryl C–N and C–X Bonds in Aqueous Solvent Mixtures
    (American Chemical Society, 2022) Pan, Lei; Kelley, Alexandra S.; Cooke, Maria Victoria; Deckert, Macy M.; Laulhé, Sébastien; Chemistry and Chemical Biology, School of Science
    Herein, we present an efficient and mild methodology for the synthesis of aromatic phosphonate esters in good to excellent yields using 10H-phenothiazine, an inexpensive commodity chemical, as a photoredox catalyst. The reaction exhibits wide functional group compatibility enabling the transformation in the presence of ketone, amide, ester, amine, and alcohol moieties. Importantly, the reaction proceeds using a green solvent mixture primarily composed of water, thus lowering the environmental footprint of this transformation compared to current methods. The transformation also proceeds under atmospheric conditions, which further differentiates it from current methods that require inert atmosphere. Mechanistic work using fluorescence quenching experiments and radical trapping approaches support the proposed mechanism.