C(sp ³ )–H functionalizations of light hydrocarbons using decatungstate photocatalysis in flow

Abstract

<jats:p>Direct activation of gaseous hydrocarbons remains a major challenge for the chemistry community. Because of the intrinsic inertness of these compounds, harsh reaction conditions are typically required to enable C(sp<jats:sup>3</jats:sup>)–H bond cleavage, barring potential applications in synthetic organic chemistry. Here, we report a general and mild strategy to activate C(sp<jats:sup>3</jats:sup>)–H bonds in methane, ethane, propane, and isobutane through hydrogen atom transfer using inexpensive decatungstate as photocatalyst at room temperature. The corresponding carbon-centered radicals can be effectively trapped by a variety of Michael acceptors, leading to the corresponding hydroalkylated adducts in good isolated yields and high selectivity (38 examples).</jats:p>

Photo-triggered hydrogen atom transfer from an iridium hydride complex to unactivated olefins

“…In our case, the overall HAT reaction is catalytic and occurs to olen substrates rather than carbon-centered radicals derived from CH 2 Cl 2 solvent. Contrary to previously explored polyoxometallate-based photo-HAT systems, [38][39][40][41][42][43] the iridium hydride explored herein operates under visible-light irradiation.…”

“…[34][35][36][37] Photoreactions, in which a metal complex donates or accepts an H-atom in an electronically excited state, are rare. 33,[38][39][40][41][42][43] For the conceptually related proton-coupled electron transfer (PCET) reactions, there exist numerous compounds which exhibit PCET reactivity from electronically excited states, but these systems typically rely on polar O-H or N-H groups which can form hydrogen bonds. [44][45][46][47] The HAT reactions considered herein do not proceed directly from an electronically excited state, but instead occur aer an initial reductive excited-state quenching process with a tertiary amine.…”

Optimization of a Decatungstate-Catalyzed C(sp3)–H Alkylation Using a Continuous Oscillatory Millistructured Photoreactor

“… 7 The decatungstate anion has been proven to be a robust and versatile catalyst for the functionalization of unactivated aliphatic C–H bonds upon irradiation with near-UV light ( Scheme 1 ). 7a Notable examples are alkylation, 3a , 8 arylation, 9 fluorination, 10 trifluoromethylation, 11 and oxygenation. 12 …”

Photocatalytic (Hetero)Arylation of C(sp3)–H Bonds with Carbon Nitride

“…Surprisingly, mpg-CN has rarely been used as a photocatalyst for direct C(sp 3 )−H functionalization by hydrogen atom transfer (HAT) chemistry 20,21 . The synergistic combination of homogeneous photocatalysis, HAT and transition metal catalysis has provided several methods for selective C−H bond functionalization [22][23][24] , most of which remain restricted to either the use of i) activated Michael acceptors (electron-deficient olefins) [25][26][27] or ii) stoichiometric amount of HAT agents 28 . Taking lessons from free radical halogenation chemistry of alkanes, we anticipated that the catalytically-formed halogen radicals on the mpg-CN surface might involve in a HAT event for the generation of carbon radicals, which in combination with transition metal catalysis would enable arylation of C(sp 3 )−H bonds with aryl halides (Fig.…”

“…The mpg-CN photochemical protocol was even effectively employed to polycyclic aryl bromides(27,28) in spite of their tendency for ring opening reaction in the presence of a nickel catalyst.Pleasingly, the developed conditions easily translated on a gram scale in batch-mode processing without noticeable erosion in yield (1a, 83%). Elsewhere, continuous flow chemistry is widely appreciated as an enabling technology for the scale-up of photochemical processes23 . Scalability was evaluated in two commercially-available suspension/slurry-handling continuous flow reactors (10-15 mL reactor sizes), one tubular reactor and one which used state-of-the-art oscillatory flow and pulsation technology together with hi-power LED modules (24-45 W radiant power).…”