Rhodium(I) Complexes with Hemilabile Phosphines: Rational Design for Efficient Oxidative Amination Catalysts

Abstract

and excellent enamine selectivity (96%).

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.)…”

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

“…14,15 In particular, complexes having arylphosphine ligands with a 3-ethoxypropyl hemilabile moiety exhibited a superior catalytic activity. 17 Table 1.…”

“…19 The simultaneous formation of 9 and [H 2 NC 5 H 10 ] + strongly supports the formulation of 9 as an amido rhodium(I) complex whose structure, supported by DFT calculations, is shown in Scheme 2. Unfortunately, we were unable to observe the amido ligand by 15 N NMR, however, it has been possible to identify the protons and carbons of the amido ligand, all of them inequivalent, through 1 H, 1 H-COSY and 1 H, 13 C-HSQC correlation experiments (Supporting information). In sharp contrast, when a solution of complex [Rh{Ph 2 P(CH 2 ) 3 OEt} 2 (HNC 5 H 10 )] + (7) in THF-d 8 was heated for 20 h the formation of 9 was never observed.…”

“…The determination of the molecular structure of complexes 5 and 6 allowed the identification of a π-π intramolecular interaction between two phenyl groups of the two phosphines. 14,15 This interaction provides a rigid scaffold in the intermediates that directs both hemilabile arms of the phosphine ligands on the same side of the molecular plane towards the metal centre facilitating the coordination of both ethoxypropyl fragments and in turn, the protection of the two accessible coordination sites necessaries for the catalysis (Figure 14). Thus, the observed …”

“…13 We have recently found that rhodium(I) complexes containing hemilabile phosphine ligands of the type Ar 2 P(CH 2 ) n Z (Z = OR, NR 2 , SR; n = 2, 3) are active catalysts for the regioselective antiMarkovnikov oxidative amination of styrene with piperidine. 14,15 Interestingly, the catalyst containing P,O-functionalized arylphosphine ligands having a 3-alkoxypropyl hemilabile moiety, Ar 2 P(CH 2 ) 3 OR (R = Me, Et, nBu), exhibited a higher catalytic activity than those containing P,N-functionalized phosphine ligands. Important features of these flexible ligands are their potential for a weak interaction with the rhodium centre through the oxygen atom forming a six-membered metallocycle ring.…”