Microtubing‐Reactor‐Assisted Aliphatic C−H Functionalization with HCl as a Hydrogen‐Atom‐Transfer Catalyst Precursor in Conjunction with an Organic Photoredox Catalyst

Abstract

Chlorine radical, which is classically generated by the homolysis of Cl 2 under UV irradiation, can abstract ah ydrogen atom from an unactivated C(sp 3 )ÀHb ond. We herein demonstrate the use of HCl as an effective hydrogenatom-transfer catalyst precursor activated by an organic acridinium photoredoxc atalyst under visible-light irradiation for C À Halkylation and allylation. The key to success relied on the utilization of microtubing reactors to maintain the volatile HCl catalyst. This photomediated chlorine-based CÀHactivation protocol is effective for avariety of unactivated C(sp 3 )ÀH bond patterns,e ven with primary C(sp 3 )ÀHb onds,a si n ethane.The merit of this strategy is illustrated by rapid access to several pharmaceutical drugs from abundant unfunctionalized alkane feedstocks.Dramatic developments in photocatalysis over the past decade have enabled previously inaccessible transformations. [1] In particular,t he synergistic combination of photoredox catalysts and hydrogen-atom-transfer (HAT) catalysts has offered enormous opportunities for C(sp 3 ) À Ha ctivation. [2] In general, upon irradiation with light, the activated photocatalyst can oxidize the HATc atalyst through singleelectron transfer (SET) to form ahydrogen abstractor,either in the presence or absence of abase;the hydrogen abstractor will abstract ah ydrogen atom to deliver ac arbon radical to enable C À Hactivation (Scheme 1a). In this context, avariety of HATc atalysts based on S( thiols), [3] N( quinuclidines, sulfonamides), [4] and O( benzoates, N-hydroxides) [5] have been developed (Scheme 1b). However, the development of readily available and highly efficient catalytic systems that accommodate aw ide range of CÀHp atterns without the requirement of stoichiometric amounts of additives is still highly desirable.Chlorine radical, classically generated by the homolysis of Cl 2 under UV irradiation, represents another heteroatomcentered radical capable of abstracting hydrogen atoms from unactivated aliphatic CÀHb onds.H owever,i th as not been widely recognized as aH AT catalyst under conditions of visible-light irradiation. In this regard, Doyle and co-workers and our group recently demonstrated the generation of chlorine radical by photolysis of aNi III chloride intermediate generated by photoredox-mediated single-electron oxidation of aNi II precursor and showed that it could activate C(sp 3 )ÀH bonds for arylation [6] and hydroalkylation of alkynes, [7] respectively (Scheme 2a). [8] However,t he former reaction uses stoichiometric amounts of chloride compounds,a nd the latter suffered from limited scope with respect to the C À H substrate.B oth methods required excess amounts of CÀH substrates (! 10 equiv) for effective transformations. Scheme 1. Synergistic merging of photoredox and HATcatalysis.

Photoredox-Catalysis-Modulated, Nickel-Catalyzed Divergent Difunctionalization of Ethylene

“…[18][19][20][21][22] Our group has recently developed a ''stopflow'' micro-tubing (SFMT) reactor for efficient screening of gas-involved photomediated transformations in a convenient and safe manner. 23 As part of our ongoing interests in developing visible-light-promoted transformations using inexpensive gaseous feedstocks, [23][24][25] we herein report the light-mediated ethylene difunctionalization through the synergistic combination of photoredox and Ni catalysis. Assisted by photocatalysts with different redox potentials, Ni-catalyzed reductive coupling between aryl halides and ethylene produced 1,2-diarylethanes, 1,4-diarylbutanes, and 2,3-diarylbutanes in a highly selective manner ( Figure 1C).…”

Photoinduced site-selective alkenylation of alkanes and aldehydes with aryl alkenes

“…Although much progress has been achieved, the developed systems usually require stoichiometric peroxides and a large excess of the C-H partner (over 40 equivalents of the alkanes), and the substrate scopes are limited to simple hydrocarbons and aromatic aldehydes. In pursuing methods for olefin construction [25][26][27] and photomediated C-H bond functionalization [28][29][30][31] , we aspired to develop an auxiliary-free and oxidant-free strategy for C sp3 -H and C sp2 (O)-H alkenylation with the C-H substrate as the limiting reagent (Fig. 1c).…”

Hydrogen Atom Transfer Reactions via Photoredox Catalyzed Chlorine Atom Generation

“…[6] To the best of our knowledge, few examples exist of such a process. [7] Recently, advances in Ir III /Ni 0 dual photoredox HAT catalysis have enabled the cross-coupling of alkanes (as solvent) with haloarenes through a proposed halogen atom (Br or Cl) intermediate (Scheme 1 b). [8] Herein we report the photoredox mediated catalytic generation of chlorine atoms and their ability to undergo hydrogen atom transfer (HAT) reactions with a variety of substrates such as alkanes, alcohols, ethers, ester, amides, aldehydes, and silanes, for their applications in the redoxneutral Giese-type addition to activated alkenes (Scheme 1 c).…”