Date of Award
Doctor of Philosophy (PhD)
Gary A. Molander
Alkene functional groups are ubiquitous, and development of olefin functionalization transformations provides a unique opportunity to install Csp3 centers with great atom- and step-efficiency. Many elegant strategies have been developed toward this goal, however, the selective functionalization of unactivated olefins still remains underdeveloped. By fulfilling single electron processes under mild conditions, the emergence of photoredox/Ni dual catalysis has significantly expanded the alkyl nucleophile repertoire in C-C bond constructions. Applying this concept in C=C and C=X functionalization thus possesses great potential.
Both photoredox/Ni dual catalysis and reductive coupling transpire via single electron pathways. Although the reductive coupling requires a stoichiometric amount of reductant, photoredox/Ni dual catalysis may fulfill similar transformations in a redox-neutral form, potentially improving both atom economy and reproducibility. To take advantage of this concept, an amidation reaction using organic isocyanates and alkylsilicates was developed. Through a Csp2-Csp3 bond construction approach, various alkyl amides were synthesized with good functional group tolerance, and thanks to the mild conditions, the deleterious CO extrusion reactivity was avoided.
Heterocarbofunctionalization of olefins has great potential of rapidly building molecular complexity, however, very few mild and selective approaches have been reported. Through a photoredox proton-coupled electron transfer (PCET) pathway, reactive amidyl radicals were generated mildly to facilitate a cascade amidoarylation/nickel-catalyzed cross-coupling of unactivated olefins. This new technology grants access to an array of complex molecules containing a privileged pyrrolidinone core from alkenyl amides and aryl- and heteroaryl bromides. Notably, not only amides, but carbamates and ureas were also used. Subsequently, carbonyl-type electrophiles, such as acyl (pseudo)halides and in situ-activated carboxylic acids, were incorporated in a highly diastereoselective amidoacylation reaction. Mechanistic studies, including hydrogen-bond affinity constants, cyclization rate measurements, quenching studies, cyclic voltammetry, isomerization experiments, as well as computational studies, were central to comprehend the subtleties contributing to the integration of the two catalytic cycles and the origin of the high diastereoselectivity.
Finally, a method for Csp3-Csp3 and Csp3-X bond construction was developed by implementing photoredox/Ni dual catalysis into a Tsuji-Trost-type alkylation of allyl alcohol-derived partners. This transformation transpires with high linear and E-selectivity, avoiding the normal requirement for harsh conditions (e.g., strong base, elevated temperature). Additionally, using aryl sulfinate salts as radical precursors, allyl sulfones can also be obtained. Kinetic isotope effect experiments implicated oxidative addition of the nickel catalyst to the allylic electrophile as the turnover-limiting step, supporting previous computational studies.
In summary, photoredox/Ni dual catalysis has proven enabling toward functionalization of unsaturated systems. Through different approaches, the unsaturated systems can be implemented as electrophiles (allyl alcohols, isocyanates) or radical precursors (pendant olefins). Various connections that are pivotal for synthetic chemistry have been demonstrated, such as Csp2-Csp3, Csp3-Csp3, Csp3-S and Csp3-N bonds, delivering a series of complex structures, such as functionalized monosaccharides, (hetero)arylated pyrrolidones, allyl sulfones, as well as highly functionalized amides.
Zheng, Shuai, "Photoredox/nickel Dual Catalysis On Functionalization Of Unsaturated Systems" (2020). Publicly Accessible Penn Dissertations. 4259.