Mechanistic Studies on the Catalytic Oxidative Amination of Alkenes by Rhodium(I) Complexes with Hemilabile Phosphines

Abstract

Abstract:Cationic rhodium(I) complexes having P,O-functionalized arylphosphine ligands are efficient catalysts for the regioselective antiMarkovnikov oxidative amination of styrene with piperidine. The mechanism of the catalytic reaction has been investigated by spectroscopic means under stoichiometric and catalytic conditions. In the presence of piperidine the catalyst precursor [Rh{κ 2 -P,O-Ph 2 P(CH 2 ) 3 OEt} 2 ] + (5) gives the piperidine complex [Rh{κ 1 -PPh 2 P(CH 2 ) 3 OEt} 2 (HNC 5 H 10 ) 2 ] + (8) that was transformed into the neutral amidopiperidine species [Rh{κ 1 -PPh 2 P(CH 2 ) 3 OEt} 2 (NC 5 H 10 )(HNC 5 H 10 )] (9) under thermal conditions. NMR studies carried out in the presence of styrene under catalytic conditions showed that 9 is a key species in the catalytic oxidative amination of styrene. Related cyclooctadiene-containing catalyst precursors [Rh(cod){κ 1 -PPh 2 P(CH 2 ) 3 OEt} n ] + (n = 1, 2) also gave 9 under the same conditions. The proposed catalytic cycle has been established by a series of DFT calculations including the transition states of the key steps that have been found and characterized. These studies have shown that, after the elimination of the enamine, the regeneration of catalytic active species takes place by direct transfer of the proton of a piperidine ligand to the alkyl group, resulting from the insertion of styrene into Rh-H bond, with formation of ethylbenzene. Against the expectations, the formation of a dihydride intermediate by NH oxidative addition is a highly energy-demanding process. Catalyst 5 has also been applied for the oxidative amination of substituted vinylarenes with several secondary cyclic and acyclic amines.

The Origin of Anti‐Markovnikov Regioselectivity in Alkene Hydroamination Reactions Catalyzed by [Rh(DPEphos)]+

“…The oxidative amination process for a Rh complex with a P,O‐functionalized phosphine ligand has been analyze by Polo, Oro et al . A mechanism with an amine activation pathway (deprotonation of the amine in the first step, followed by insertion of the coordinated styrene into the Rh−amido bond to yield the enamine product) has been proposed . Then it takes place for the hydrogenation of the alkene to recover the catalyst.…”

Palladium-Catalyzed Intermolecular Oxidative Amination of Alkenes with Amines, Using Molecular Oxygen as Terminal Oxidant

“…Oxidative coupling for the formation of nitrogen-containing compounds such as enamines using metal-catalyzed oxidative amination of olefins by dioxygen, known as the aza-Wacker reaction, has been the subject of considerable research [10][11][12][13][14][15][16][17][18]. Although significant developments in aza-Wacker-type reactions have been achieved, the existing aza-Wacker process generally requires non-basic nitrogen nucleophiles such as carboxamides, carbamates, and sulfonamides as amination reagents, under relatively high oxygen pressures (4-10 atm.)…”

Polymerization of phenylacetylene catalyzed by rhodium(i) complexes with N-functionalized N-heterocyclic carbene ligands

“…20 It is worth mentioning that decoordination of the hemilabile ethoxy fragment of the functionalized phosphine ligand in compound [Rh(cod){κP-(4-MeC 6 H 4 ) 2 P(CH 2 ) 3 OEt}] + in coordinating solvents has also been observed. 27 The neutral amido complex [Rh(nbd){κ 2 C,N-t-BuIm(CH 2 ) 3 Nt-Bu}] (9) was prepared by double deprotonation of [t-BuImH (CH 2 ) 3 NH-t-Bu]Br•HBr with NaH followed by reaction with [Rh (μ-Cl)(nbd)] 2 (Fig. 1, ii) and isolated as a yellow solid in moderate yield (53%) (Chart 1).…”