UVA- and Visible-Light-Mediated Generation of Carbon Radicals from Organochlorides Using Nonmetal Photocatalyst

Abstract

Carbon radicals are reactive species useful in various organic transformations. The C-X bond cleavage of organohalides by photoirradiation is a common method to generate carbon radicals in a controlled fashion. The use of organochloride substrates is still a formidable challenge due to the low reduction potential and the high dissociation energy of the C-Cl bond. In this report, we address these issues by using a nonmetal organic molecule with a relatively simple structure as a photocatalyst. In this catalyst (bis(dimethylamino)carbazole), the amino groups increase both the HOMO and LUMO energy levels, especially in the former. As a result, compared to the parent molecule, the new catalyst shows experimentally red-shifted absorption in the visible region and forms an excited state with better reducing capability. This photocatalyst was used in the reduction of unactivated aryl chlorides and alkyl chlorides in the presence of hydrogen atom donor at room temperature. The catalytic system can also be applied to the coupling of aryl chlorides with electron-rich arene and heteroarenes to affect the C-C bond-forming reactions. Our mechanistic study results support the assumption that carbon radicals are formed from the organochlorides via a single-electron-transfer step.

Implementing Hydrogen Atom Transfer (HAT) Catalysis for Rapid and Selective Reductive Photoredox Transformations in Continuous Flow

“…Turning towards mechanistic investigation, dehalogenation experiments using DCOOD displayed only a minor extent of deuterium incorporation, similar to previous observations. [10b] Furthermore, no change in quenching constant was observed by the addition of CySH in fluorescence quenching studies . Therefore, the mechanism by which this thiol additive accelerates the reaction is proposed to be in mediating the regeneration of ground state photocatalyst .…”

“…Recently, these reductions have also been achieved using organocatalysts with strongly reducing excited states . Amongst the most reducing of these are phenothiazine‐based catalysts, such as N ‐phenylphenothiazine (PTH), whose excited state reduction potential reaches –2.1 V vs. SCE (Figure b).…”

“…Amongst the most reducing of these are phenothiazine‐based catalysts, such as N ‐phenylphenothiazine (PTH), whose excited state reduction potential reaches –2.1 V vs. SCE (Figure b). [10b], Accordingly, this type of readily available catalyst has enabled photochemical transformations which were previously unachievable by the most reducing catalysts available, for example couplings of aryl chlorides with electron rich olefins . However, these transformations can be extremely slow, often requiring reaction times of several days.…”

Laser‐Induced Wurtz‐Type Syntheses with a Metal‐Free Photoredox Catalytic Source of Hydrated Electrons

“…[22][23][24] Ta ble 1c ollectsy ields at illumination end points for a number of synthetic applicationst ested with our systems. Its five times longer optical path length made better use of the laser light by absorbing all of it but influenced the product distribu- tion only marginally,a gain in accordance with the results of SI-4.2.2.…”

“…This compares very favourably with reaction durations and turnover rates reported for other photon-pooling systemsc apable of cleaving carbonÀ chlorine bonds. [22][23][24] Ta ble 1c ollectsy ields at illumination end points for a number of synthetic applicationst ested with our systems. Being sum parameters, thesey ields cannot provide as specific information as do the concentration profiles (Figure 2b), let alone the surgicall aser-flash investigations of the individual processes ( Figure 1b-d);b ut on the basis of those preceding results, the trends in the Ta ble can be rationalized by two key factors each for the substrate conversion and for the product distribution.…”