Photo‐Induced Ruthenium‐Catalyzed C−H Benzylations and Allylations at Room Temperature

Abstract

The ruthenium‐catalyzed synthesis of diarylmethane compounds was realized under exceedingly mild photoredox conditions without the use of exogenous photocatalysts. The versatility and robustness of the ruthenium‐catalyzed C−H benzylation was reflected by an ample scope, including multifold C−H functionalizations, as well as transformable pyrazoles, imidates and sensitive nucleosides. Mechanistic studies were indicative of a photoactive cyclometalated ruthenium complex, which also enabled versatile C−H allylations.

Site-selective desaturation of C(sp3)–C(sp3) bonds via photoinduced ruthenium catalysis

“…6d), which is in agreement with the reported results. 9 Deuterium labeling studies involving 1a and 1a-γ-d 2 or 1a-ö-d 2 yielded KIE values of 2.4 or 1.2, respectively, suggesting that the cleavage of the γ-C(sp 3 )–H bond is likely a turnover-limiting step (Fig. 6e).…”

“…8 However, as the coordination sites of the central ruthenium( ii ) are fully blocked by three bidentate ligands, the innate transition metal catalytic activity of the Ru complexes is ignored. The exciting recent development of visible-light-induced Ru catalytic systems by Ackermann 9 and Greaney, 10 where the Ru complexes act as photocatalysts for generating radical species and simultaneously activate heteroarene substrates, enables meta -C–H alkylation and ortho C–H arylation. This dual-function Ru catalytic strategy 11 is conceptually novel, whilst chemical transformations have been known to occur at elevated temperatures (Fig.…”

Visible‐Light‐Induced, Single‐Metal‐Catalyzed, Directed C−H Functionalization: Metal‐Substrate‐Bound Complexes as Light‐Harvesting Agents

“…This ortho-CÀ H functionalization was recently studied in more detail by the Ackermann group using benzyl chlorides and allyl bromides. [48] Two key strategies were pursued to achieve monoalkylation reactions: a) by blocking the second site for CÀ H functionalization with a methyl group or b) by the introduction of steric repulsion through the use of a modified directing group. The versatility of this photochemical CÀ H alkylation was further demonstrated using water as a green reaction medium.…”

“…Highly activated benzyl bromide smoothly reacts to afford the double ortho ‐benzylated product of 2‐phenylpyridine in 90 % yield under photochemical reaction conditions—the corresponding thermal reaction has not been reported to date. This ortho ‐C−H functionalization was recently studied in more detail by the Ackermann group using benzyl chlorides and allyl bromides [48] . Two key strategies were pursued to achieve monoalkylation reactions: a) by blocking the second site for C−H functionalization with a methyl group or b) by the introduction of steric repulsion through the use of a modified directing group.…”

C–H activation by immobilized heterogeneous photocatalysts

“…1b), unlocked a dormant area of photocatalysis [13,14]. In sharp contrast, Ackermann [15][16][17] and later Greaney [18,19] elegantly disclosed C-H activation by ruthenaphotoredox catalysis under exogenous-photosensitizer-free conditions. While C-H activation has predominantly exploited noble 4d or 5d transition metals, the introduction of Earth-abundant and cost-effective 3d metal catalysts [20][21][22][23], particularly copper and cobalt, allows for more sustainable and photoinduced C-H activation.…”