Palladium-Catalyzed Oxidative Synthesis of Unsymmetrical Azophenols

Abstract

A straightforward palladium-catalyzed oxidative hydroxylation of azobenzenes is reported. The developed methodology tolerates various functional groups and allows the synthesis of diverse unsymmetrical azophenols under mild conditions in good to excellent yields. A complementary procedure was also investigated by in situ generation of PIFA. This study represents the first general method for the synthesis of o-hydroxyazobenzenes starting from simple azoarenes.

Synthesis and Photodynamics of Stilbenyl‐Azopyrroles: Two‐Photon Controllable Photoswitching Systems

“…Initially, we performed the reaction between 1 and styrene using Pd(AcO) 2 in presence of PPh 3 , but the expected coupling product 3 a was not obtained (Table , entry 1). This result can be attributed to the strong coordinative character of the azo group included in the substrate, which can coordinate the Pd (II) source dropping the quantity of the palladium active specie to promote the cross‐coupling reaction. Recently, we reported the catalytic applications of a [Pd/(N,N)‐pyrrole ligand], which demonstrated to be robust and efficient to promote the Mizoroki‐Heck coupling, using demandant substrates as ethylene .…”

“…It is well known that azo‐compounds strongly coordinate a variety of metals including palladium, which makes it crucial to design an appropriate palladium species that catalyzes the C−C coupling reaction efficiently, avoiding the formation of palladacycles via C−H activation . In fact, the previous absence of azo compounds with an extended conjugation where additional sections are joined by unsaturated bridges is partly explained by this intrinsic synthetic difficulty.…”

Synthesis of ortho‐Azophenols by Formal Dehydrogenative Coupling of Phenols and Hydrazines or Hydrazides

“…Heterocyclic azophenols are a particularly interesting class of azo‐switches, displaying exceptionally fast and quantitative thermal isomerization . Transition metal‐catalyzed ortho ‐oxygenation of symmetric azobenzenes has been reported (Scheme A),– but suffers from poor regio‐ and chemoselectivity in non‐symmetrical substrates, complicating the synthesis of “push–pull” azophenols. A mechanistically distinct, but undeveloped transformation that leads selectively to ortho ‐azophenols occurs by a condensation and redox isomerization cascade between ortho ‐quinones and hydrazines .…”

Synthesis of Phenolic Compounds via Palladium Catalyzed C−H Functionalization of Arenes

“…Joseph et al. demonstrated an efficient Pd‐catalyzed direct C−H hydroxylation reaction that enabled synthesis of a wide variety of azophenols in good to excellent yields under mild conditions (Figure ) . The reaction exhibited very good functional group tolerance.…”