Amino‐phosphanes in Rh^I‐Catalyzed Hydroformylation: Hemilabile Behavior of P,N Ligands under High CO Pressure and Catalytic Properties

Abstract

The catalytic properties of rhodium complexes containing the α‐, β‐, or γ‐amino‐phosphane ligands Ph2PCH2NEt2 (α‐P,N‐1), Ph2PCH(Ar)NHPh [α‐P,N‐2; Ar = η6(o‐C6H4Cl)Cr(CO)3], Ph2PCH2NPh2 (α‐P,N‐3), Ph2PCH2CH(Ph)NHPh (β‐P,N), Ph2PCH2(o‐C6H4–NMe2) (γ‐P,N‐1), Ph2PCH(o‐C6H4–CH2NHPh) (γ‐P,N‐2), and the α,β‐diamino‐phosphane ligand Et2NCH2P(Ph)CH2CH(Ph)NHPh (α,β‐N,P,N), in styrene hydroformylation have been examined. The results show that the activity increases when the number of backbone carbon atoms linking P and N decreases from 3 to 1. IR and 31P HPNMR studies in solution show that all P,N ligands adopt exclusively a κ1‐P coordination mode in rhodium chloride carbonyl complexes under high CO pressure. In the solid state a κ1‐P‐α‐amino‐phosphane coordination has been ascertained by X‐ray methods in trans‐[RhCl(CO)(γ‐P,N‐1)2]. In contrast, an equilibrium between the κ2‐P,N and κ1‐P‐coordination modes has been observed as a function of the CO pressure for the complex containing the β‐P,N ligand. The basicity of the dangling amino group also plays an important role on the catalytic activity and a mechanism involving the nitrogen function in the catalytic cycle is proposed.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Amino‐phosphanes in RhI‐Catalyzed Hydroformylation: New Mechanistic Insights Using D2O as Deuterium‐Labeling Agent

“…As discussed in our recent contribution, [18] hydrogenolysis could occur either by H 2 oxidative addition followed by aldehyde reductive elimination The most unexpected result of our investigation, however, is the observation of the α-D 1 and α,β-D 2 products for the [RhCl(COD)] 2 /Ph 2 PCH(Ar)NHPh catalytic system (Table 1, runs 5 and 6). A possible pathway for the D incorporation from D 2 O at the α-position could in principle involve a β-elimination reaction from the Rh-acyl complex leading to a ketene-hydride intermediate.…”

“…Neither the branched/linear regioselectivity (91/9), [18] nor the turnover frequency at the beginning of the catalytic process are affected by this solvent change. However, the catalyst starts to lose activity after ca.…”

“…Therefore, under these conditions, these ligands behave like their related α-P,N monophosphane ligands, for which the chelating mode is not observed under any operating conditions. [18] The presence of a dangling nitrogen group close to the rhodium metal center could promote the activation of dihydrogen by heterolytic cleavage (see Scheme 1). In this interaction, the Rh center acts as a Lewis acid, accepting H -, whereas the nearby amine function acts as a Lewis base, capturing the proton.…”

“…For example, when a pyridinylphosphane is only P-coordinated in a Pd II complex, the dangling amine function acts then as a "proton messenger" in alkyne methoxycarbonylation catalysis, [2] whereas a selective P,N chelation of a chiral phosphanediamine induces an effective chiral field in the rhodium complex leading to a higher selectivity in asymmetric hydrogenation of acrylic acid. [7] In our laboratory, we have developed different access ways to chiral α- [11] or β-P,N [12,13] and mixed α,β-and β,γ-P,P,N [14] ligands and their coordination properties have been explored in copper(), [15] palladium(), [13,16] and rhodium() complexes [14,17,18] Recently, we have examined the co- and PhCD(CH 2 D)CHO (α,β-D 2 ) were also produced. This result suggests a reversible deprotonation assisted by an intramolecular H-bonding interaction between the dangling ammonium function and the carbonyl moiety.…”

“…In a previous contribution, we have reported the catalytic activity and linear/branched selectivity of the above systems in dry CHCl 3 . [18] The hydroformylation experiments in the presence of D 2 O, reported in this paper, were carried out under similar conditions with the exception of the solvent, which was changed from CHCl 3 to THF in order to insure homogeneous conditions in the presence of water. In Figure 1, we compare the activity of the same Rh-P,N precatalyst under the two different solvent conditions.…”