Metal‐Catalyzed Hydrogen Evolution Reactions Involving Strong C−H Bonds Activation via Hydrogen Atom Transfer

Abstract

Hydrogen evolution reactions have been proved to be a powerful and green strategy in organic synthesis. Differing from hydrogen evolution reactions catalyzed by noble metal pincer complex with some limitations, the reactions facilitated by merging hydrogen atom transfer (HAT) and metal‐catalyzed hydrogen evolution (MCHE) represents a more attractive and promising alternative for direct formation of chemical bonds from available feedstocks in synthetic chemistry. Given the significance, this review summarizes the state of the art with highlighting the mechanism, synthetic applications and meanwhile enlightens the future development of this area.

Stop-Flow Microtubing Reactor-Assisted Visible Light-Induced Hydrogen-Evolution Cross Coupling of Heteroarenes with C(sp3)–H Bonds

“…However, stoichiometric oxidants are generally required, which results in a diminished atom economy of the overall process with unwanted waste generation and poor functional group compatibility. Over the past decade, a greener approach involving CDC reactions with hydrogen evolution in the absence of any external oxidants has been achieved through transition metal catalysis, , photoredox/cobalt dual catalysis, − quantum dot photocatalysis, − and electrocatalysis. ,, Although the strategy of hydrogen-evolution cross coupling of C­(sp 3 )–H bonds has gained significant momentum, such approaches have several limitations, including a reliance on transition metal catalysts or specialized electrochemical equipment. Therefore, the development of new strategies of hydrogen-evolution cross coupling with readily available C­(sp 3 )–H feedstocks in a metal-free and operationally simple manner is highly desirable.…”

“…Our investigation of the conditions indicated that a catalytic amount of mesityl-10-methylacridinium perchlorate (Mes-Acr + ClO 4 – ) and Co­(dmgh) 2 PyCl in the presence of two equivalents of diphenyl phosphate in acetonitrile at 60 °C under blue LED light (475 nm, 36 W) irradiation for 24 h gave desired coupling product 3 in 87% yield (Table , entry 1). Unexpectedly, an improved product yield (95%) was obtained using 1,2-dichloroethane (DCE) as the solvent even in the absence of cobalt catalyst, which was essential in the existing reported hydrogen evolution dehydrogenative couplings − , (entry 2). Hydrogen gas was detected as the coupled product (Figure S4).…”

“…The reaction of benzylic C(sp 3 )−H bonds was less efficient, delivering the corresponding product (10) in 30% yield when 4CzIPN was used as a photocatalyst. Alcohols participated in the present transformation effectively to afford the coupling product (11) in 60% yield, with the reaction occurring exclusively at the α position of the hydroxy group. Cyclic ethers (12 and 13) are feasible substrates to deliver α-heteroatom C−H functionalized ethereal products.…”