Hydroxyl Assisted, Photoredox/Cobalt Co‐catalyzed Semi‐Hydrogenation and Tandem Cyclization of o‐Alkynylphenols for Access to 2,3‐Dihydrobenzofurans

Abstract

Herein, a hydroxyl assisted, photoredox/cobalt co‐catalyzed semi‐hydrogenation and tandem cyclization of o‐alkynylphenols is developed towards direct assembly of 2,3‐dihydrobenzofurans. Moderate to good yields were obtained for a range of sterically and electronically diverse 2‐propynolphenols under mild conditions. Mechanistic studies demonstrated the inevitable role of the alcoholic hydroxyl group with (Z)‐alkene as the real intermediate. Finally, a key low‐valent cobalt catalyzed intramolecular hydroetherification of alkene is proposed.magnified image

Dual Cobalt and Photoredox Catalysis Enabled Redox‐Neutral Annulation of 2‐Propynolphenols

“…Meanwhile, we also conducted the experiments that using prepared (Z)‐ 8 and (E)‐ 8 as substrates under standard conditions, but none of reactions were occurred (Scheme S6). Since (Z)‐ 8 was proved to be an intermediate in our previous work (Scheme 1b), we thus deduced that this work should proceed via a highly different mechanism from our previous work [11] . Third, the light on/off experiment was conducted (Figure 1b).…”

“…We started to verify our hypothesis by using similar conditions in our previous work, [11] that is, selecting 1 a as the model substrate, Ir[dF(CF 3 )ppy] 2 (dtbbpy)PF 6 as photocatalyst, 4,4′‐di‐tert‐butyl‐2,2′‐bipyridine ( L1 ) as ligand, and i ‐Pr 2 NEt as radical sacrificial in n ‐propyl alcohol at room temperature (table 1, entry 1). Unfortunately, none of corresponding benzo[ b ]furan 2 a was detected.…”

“…Merging photoredox catalysis with transition‐metal catalysis has been identified as vital ways to achieve novel and unique transformations under mild conditions [10] . Recently, we developed an visible‐light‐promoted iridium/cobalt dual catalytic system for efficient construction 2,3‐dihydrobenzofurans from 2‐propynolphenols via tandem semi‐hydrogenation and intramolecular cyclization processes (Scheme 1b) [11] . A low‐valent cobalt catalyzed phenolic O−H activation or a further hydrogenation of (Z)‐alkene followed by phenolic oxygen substitution on cobalt center is supposed to contribute to the cyclization.…”

“…Solvent investigation revealed that EtOH is the best choice, delivering 2 a in 56% yield (entries 7-9). Increasing the amount of base and solvent further elevated the yield up to 74% (entries 10,11). The ligand screening showed that 5,5'-dimethyl-2,2'-bipyridine (L3) is the best choice (entries [12][13][14][15].…”

Recent Advances Towards the Synthesis of Dihydrobenzofurans and Dihydroisobenzofurans

“…In 2020, the research group of Qiang Xiao illustrated the hydroxyl assisted photoredox/cobalt Co-catalyzed semi-hydrogenation and tandem cyclization of o-alkynylphenols 65 for the synthesis of 2,3-DHBs 66. [101] In this context, an efficient catalytic system has been established using 2-propynolphenols 65 as a substrate in the presence of blue LED irradiation at ambient temperature (Scheme 15). It was observed that the alcoholic hydroxyl group played an essential role in the reaction, and (Z)alkene 67 was the intermediate for the conversion.…”