Photoredox catalysis has driven a revolution in the field of organic chemistry, but direct mechanistic insights into reactions of genuine synthetic utility remain relatively scarce. Herein we report ultrafast time-resolved spectroscopic observation of a bimolecular organocatalyzed photoredox reaction, from catalyst photoexcitation through to photoinduced electron transfer (PET) and intermediate formation, using transient vibrational and electronic absorption spectroscopy with sub-picosecond time resolution. Specifically, the photochemical dynamics of initiation in organocatalyzed atom-transfer radical polymerization (O-ATRP) are elucidated for two complementary photoredox organocatalysts (N,N-diaryl-5,10-dihydrophenazines). Following photoexcitation, a dissociative bimolecular electron transfer is observed from the first excited singlet state of both photocatalysts to methyl 2-bromopropionate in dichloromethane, toluene, and dimethylformamide. The photocatalyst excited donor state, ground state, and radical cation are tracked in real time alongside the debrominated radical fragment. Our work challenges previously proposed mechanisms of initiation in O-ATRP and indicates that PET from short-lived excited singlet states can exert control of polymer molecular weight and dispersity by suppressing the steady-state concentration of the reactive debrominated radical. More broadly, we aim to demonstrate the potential of ultrafast absorption spectroscopy to observe directly transient, open-shell intermediates in mechanistic studies of photoredox catalysis.

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Diarylamines find use as metal ligands and as structural components of drug molecules, and are commonly made by metal-catalyzed C-N coupling. However, the limited tolerance to steric hindrance of these couplings restricts the synthetic availability of more substituted diarylamines. Here we report a remarkable variant of the Smiles rearrangement that employs readily accessible N-aryl anthranilamides as precursors to diarylamines. Conformational predisposition of the anthranilamide starting material brings the aryl rings into proximity and allows the rearrangement to take place despite the absence of electron-withdrawing substituents, and even with sterically encumbered doubly ortho-substituted substrates. Some of the diarylamine products are resolvable into atropisomeric enantiomers, and are the first simple diarylamines to display atropisomerism.

Chemists have many options for elucidating reaction mechanisms. Global kinetic analysis and classic transition-state probes (e.g., LFERs, Eyring) inevitably form the cornerstone of any strategy, yet their application to increasingly sophisticated synthetic methodologies often leads to a wide range of indistinguishable mechanistic proposals. Computational chemistry provides powerful tools for narrowing the field in such cases, yet wholly simulated mechanisms must be interpreted with great caution. Heavy-atom kinetic isotope effects (KIEs) offer an exquisite but underutilized method for reconciling the two approaches, anchoring the theoretician in the world of calculable observables and providing the experimentalist with atomistic insights. This Perspective provides a personal outlook on this synergy. It surveys the computation of heavy-atom KIEs and their measurement by NMR spectroscopy, discusses recent case studies, highlights the intellectual reward that lies in alignment of experiment and theory, and reflects on the changes required in chemical education in the area.

Alkylidene carbenes undergo rapid inter-and intra-molecular reactions and rearrangements, including 1,2-migrations of β-substituents to generate alkynes. Their propensity for substituent migration exerts profound influence over the broader utility of alkylidene carbene intermediates, yet prior efforts to categorize 1,2-migratory aptitude in these elusive species have been hampered by disparate modes of carbene generation, ultrashort carbene lifetimes, mechanistic ambiguities, and the need to individually prepare a series of 13 C-labelled precursors. Herein we report on the rearrangement of 13 C-alkylidene carbenes generated in situ by the homologation of carbonyl compounds with [ 13 C]-Li-TMS-diazomethane, an approach that obviates the need for isotopically labelled substrates and has expedited a systematic investigation ( 13 C{ 1 H} NMR, DLPNO-CCSD(T)) of migratory aptitudes in an unprecedented range of more than 30 alkylidene carbenes. Hammett analyses of the reactions of 26 differentially substituted benzophenones reveal several counterintuitive features of 1,2-migration in alkylidene carbenes that may prove of utility in the study and synthetic application of unsaturated carbenes more generally.