Transition‐Metal‐Free C(sp²)–C(sp²) Cross‐Coupling of Diazo Quinones with Catechol Boronic Esters

Abstract

A transition‐metal‐free C(sp2)−C(sp2) bond formation reaction by the cross‐coupling of diazo quinones with catechol boronic esters was developed. With this protocol, a variety of biaryls and alkenyl phenols were obtained in good to high yields under mild conditions. The reaction tolerates various functionalities and is applicable to the derivatization of pharmaceuticals and natural products. The synthetic utility of the method was demonstrated by the short synthesis of multi‐substituted triphenylenes and three bioactive natural products, honokiol, moracin M, and stemofuran A. Mechanistic studies and density functional theory (DFT) calculations revealed that the reaction involves attack of the boronic ester by a singlet quinone carbene followed by a 1,2‐rearrangement through a stepwise mechanism.

Rhodium-catalyzed reaction of diazoquinones with allylboronates to synthesize allylphenols

“…To gain insight into the reaction mechanism, control experiments were performed. In the absence of Rh 2 (esp) 2 , no reaction was observed (Scheme 4a), which indicates that the reaction is unlikely to proceed via the intermediacy of free quinoid carbene 24 or the Hooz reaction mechanism 25 which involves a nucleophilic attack of diazo compounds to boron reagents and subsequent 1,2-migration of the substituent from boron to the diazo carbon atom and release of N 2 . Although no cyclopropanation products of diazoquinones and allylboronates were observed, probably due to their lability, we obtained cyclopropane 8 in 78% yield when diazo compound 7 was treated with 2a in the presence of a catalytic amount of Rh 2 (esp) 2 (Scheme 4b).…”

Metal-Quinoid Carbene Chemistry: From Bonding to C–H Activation Catalysis

“…The use of quinone diazides as arylating reagents also merits further investigation, and we recently discovered a metal-free C(sp 2 )-C(sp 2 ) bond-formation reaction through cross coupling of quinone diazides and alkenyl/aryl boronic esters. 19…”

Ruthenium-catalyzed reaction of diazoquinones with arylamines to synthesize diarylamines

“…Diazoquinones are an unusual class of diazo compounds that feature a planar six-membered ring structure with diazo, alkene and carbonyl groups in conjugation. 10 This unique structure renders the quinoid carbene distinctive reactivities compared with the commonly used diazo esters, 11–14 which include high electrophilicity, 12 a , d ,13 c hydrogen atom transfer reactivity 12 g ,14 b and a propensity for aromatization to form phenols. 11–14 These unusual reactivity profiles hold great promise in organic synthesis and have attracted increasing attention in transition metal catalysis in recent years.…”

“…10 This unique structure renders the quinoid carbene distinctive reactivities compared with the commonly used diazo esters, 11–14 which include high electrophilicity, 12 a , d ,13 c hydrogen atom transfer reactivity 12 g ,14 b and a propensity for aromatization to form phenols. 11–14 These unusual reactivity profiles hold great promise in organic synthesis and have attracted increasing attention in transition metal catalysis in recent years. 11–14 Compared with diazo esters, however, the synthetic potential of diazoquinones has remained underexploited.…”