Synthesis and Crystal Structure of a Palladium Metallacyclic Complex: A Key Intermediate in the Carbonylation of Azobenzene to N ‐Phenyl Urethane

Abstract

The reductive carbonylation of azoxybenzene to N‐phenyl urethane via azobenzene is promoted by the catalytic system based on palladium, bidentate nitrogen‐donor chelating ligands N‐N [N‐N = 1,10‐phenanthroline (phen) and its substituted derivatives] and the acid cocatalyst [(N‐N)H][PF6], the same catalyst as for the reductive carbonylation of nitroaromatic compounds to the corresponding carbamates. The possible key intermediate of the azobenzene → N‐phenyl urethane transformation has been synthesised and fully characterised.

Carbonylation of Nitrobenzene in Methanol with Palladium Bidentate Phosphane Complexes: An Unexpectedly Complex Network of Catalytic Reactions, Centred around a Pd–imido Intermediate

“…Note that CO insertion into the PdN bond of C4a can also occur; this would afford the palladacycle generally proposed and observed for Pd–1,10‐phenanthroline nitrobenzene reduction systems 28. 39…”

“…32, 56 During the proposed catalytic cycle the catalyst remains in the Pd II oxidation state; in the final MPC‐generating step, Pd 0 is regenerated from a palladacyclic intermediate such as that shown in Scheme 28. 39…”

“…Note that CO insertion into the Pd À N bond of C4a can also occur; this would afford the palladacycle generally proposed and observed for Pd-1,10-phenanthroline nitrobenzene reduction systems. [28,39] Alternatively, reduction of nitrobenzene must be achieved by hydrogen atoms from methanol eventually leading to Equations (5b) and (5c). This transfer-hydrogenation process most likely involves palladium hydride chemistry, without involvement of "free" H 2 , as methanol dehydrogenation to H 2 and DMC, MF or CO is endothermic by about + 21, + 48 and + 44 kcal mol À1 , respectively.…”

“…Thus far, most scientific endeavours have concentrated on the [Pd-A C H T U N G T R E N N U N G (phen) 2 ]X 2 /H + catalytic system in methanol, [24][25][26][27][28][29][30][31][32][33][34][35] leaving the Pd-phosphorus-based systems virtually unstudied. [21,31,[36][37][38] However, since it is commonly accepted that Pd 0 species function as intermediates in the catalytic cycle, [20,32,[39][40][41] it was envisaged that catalysts with bidentate P ligands could well perform differently from those with N-based ligands; their p-backbonding capability could give improved stabilization of the Pd 0 intermediates.…”

Away from phosgene: reductive carbonylation of nitroarenes and oxidative carbonylation of amines, understanding the mechanism to improve performance

“…A few years ago, Mestroni and co-workers showed that the palladium-phenanthroline system is also able to catalyse the carbonylation of azo derivatives and a metallacyclic intermediate was isolated. 99 However, it is our experience that this reaction is much slower than the carbonylation of nitroarenes and is not very effective for the remediation of an impure product.…”