Orthometalation of Primary Amines. 4.¹ Orthopalladation of Primary Benzylamines and (2-Phenylethyl)amine

Abstract

By the refluxing of an acetonitrile solution of [Pd(OAc)2]3 and primary amines 4-XC6H4CH2NH2 (F, Cl, NO2, OMe), 3,5-X2C6H3CH2NH2 (X = OMe), or PhCH2CH2NH2 (Pd:amine = 1:1) and subsequent addition of excess NaBr, the corresponding orthometalated complexes [Pd{C6H3(CH2NH2)-2,X-5}(μ-Br)]2, [Pd{C6H3(CH2NH2)-2,(OMe)2-4,6}(μ-Br)]2, or [Pd{C6H3(CH2NH2)-2}(μ-Br)]2 are obtained. Alternatively, the hydrochloride of 4-XC6H4CH2NH2 (X = F, NO2) can also be used to prepare the corresponding [Pd{C6H3(CH2NH2)-2,X-5}(μ-Cl)]2 complexes. These results show that primary benzylamines can be orthometalated even if the substituents are electron-withdrawing groups and that 2-(phenyl)ethylamine can be orthometalated in spite of the six-membered ring that it forms. These reactions occur via intermediate complexes [Pd(OAc)2L2], which react with [Pd(OAc)2]3 to give the dimeric species [Pd(OAc)(μ-OAc)L]2 (L = amine), from which in turn the orthometalated complexes are formed. Each of these steps has been studied, and both types of intermediates have been isolated for all the amines. PPh3 reacts with the orthometalated complexes to give the corresponding products of the bridge splitting. The crystal structures of [Pd(OAc)(μ-OAc)L]2 (L = 4-O2NC6H4CH2NH2) and [Pd{C6H4(CH2CH2NH2)-2}Br(PPh3)] have been determined by X-ray diffraction.

Synthesis and characterisation of Pd(II) complexes with a derivative of aminoazobenzene

“…Further increase in concentration promotes the formation of C. Hence, the formation of C is (at least) a bimolecular process and most probably proceeds via: (i) immediate solvolysis of A producing the monomer M; (ii) subsequent dimerization of M into a chloro-bridged dimer D; which (iii) undergoes cyclopalladation and produces complex C. The presence of intermediate D is supported by the non-linear concentration dependence of the formation of C as well as by the electrospray mass spectrum obtained by dissolving A in DMF. The dimeric type intermediates have been already postulated in cyclopalladation reactions with o -tolylphosphane [28] and benzylamine [29] ligands.…”

Overview of the Mechanistic Work on the Concerted Metallation–Deprotonation Pathway

“…20 Hydrogen bonding between the acetate oxygen and proton orients the base for the CH bond cleaving step and also increases the electron density of the CH bond which in turn further favors the agostic interaction. 21 The combination of these effects leads to a nearly barrier-less proton transfer 2 , in combination with an acetate base reacted to form cyclometallated products by the same mechanism when reacted with N,Ndimethylbenzylamine. 7,22 The mechanistic work of Roberts on the mercuration of benzene and Ryabov's work on the cyclometallation of benzylamine constitute the early examples of the CMD mechanism using stoichiometric amount of metal.…”

A Theoretical Assessment of the Thermodynamic Preferences in the Cyclopalladation of Amines

“…In their original work, Cope and Friedrich tried the reaction of lithium tetrachloropalladate with N,NЈ-dimethylbenzylamine, N,NЈ-dimethyl-1-ethyl-2-phenylamine and N,NЈ-dimethyl-3-phenyl-1-propylamine in order to obtain five-, six-, and seven-membered rings, respectively, but they were successful only in the first case. [8] However, there are some recent examples of six-membered rings with primary benzyl- [10,30] and ferrocenylamines, [27e] and also with heterocycles. [31] Five-or six-membered rings can be obtained in the cyclopalladation of imines depending on the reaction conditions; [15,32] however, examples with six-membered metallacycles are less common than their five-membered counterparts.…”