Xanthene-Phosphole Ligands:  Synthesis, Coordination Chemistry, and Activity in the Palladium-Catalyzed Amine Allylation

Abstract

Two new xanthene-phosphole derivatives, 3 and 4, were synthesized through the nucleophilic substitution of the cyano group in 1-P-cyano-2,5-diphenylphosphole (1) and 1-P-cyano-3,4-dimethylphosphole (2) by the 4,5-dilithium salt of 9,9‘-dimethylxanthene. For this, a new synthetic procedure was developed, allowing the synthesis of the required 1-P-cyano-2,5-diphenylphosphole from the 1,2,5-triphenylphosphole. Both ligands (DPP-Xantphos 3 for the diphenyl derivative and DMP-Xantphos 4 for the dimethyl derivative) react with [Pd(allyl)Cl]2 to afford the corresponding cationic complexes 5Cl and 6Cl. Using AgOTf, stable triflate complexes 5OTf and 6OTf could be isolated. The chloride complexes, on the other hand, exhibit a limited stability in solution. The DPP-Xantphos derivative 5Cl eliminates allyl chloride to yield the dimeric palladium(0) complex 7 of general formula [Pd(3)]2, in which each palladium is coordinated to the two phosphorus atoms of the same ligand and to one double bond of one phosphole unit of the second ligand. Decomposition of complex 6Cl furnished a mixture of compounds featuring a dimeric trinuclear species of general formula [Pd3(4)2Cl2]. This complex was also synthesized directly. Reaction of the DMP-Xantphos ligand 4 with 1 equiv of [Pd(COD)Cl2] and 2 equiv of [Pd(dba)2] afforded dimer 8, which features one 18-VE Pd0 center and two Pd-Cl fragments, which are coordinated to the dienic system of the two phosphole ligands and connected through a single Pd−Pd bond. The catalytic activity of the triflate complexes 5OTf and 6OTf was evaluated in the allylation of aniline. Whereas the DMP-Xantphos derivative 6OTf exhibited a poor catalytic activity, very good conversion yields were obtained with the DPP-Xantphos complex 5OTf. On the basis of X-ray structure data and DFT calculations, it was concluded that the high catalytic activity of the DPP-Xantphos derivative complex [Pd(allyl)3][OTf] (6OTf) results from the combination of two effects: a large P−Pd−P bite angle, which enhances the reactivity of the allyl ligand, and the strong π-accepting capacity of the diphenylphosphole moiety, which allows the easy formation of a 14-VE complex.

Gold(I)-Catalyzed Intramolecular Amination of Allylic Alcohols With Alkylamines

“…1 Since this time, a number of transition metal complexes including Pd(0), 2,3 Pt(II), 4 Mo(VI), 5 Bi(III), 6 Au(I), 7 and Au(III) 7 have been shown to catalyze the intermolecular amination of allylic alcohols in the absence of Lewis acid activators. 8 Included in this family of transformations are a subset that employ simple alkylamines as nucleophiles.…”

Why Platinum Catalysts Involving Ligands with Large Bite Angle Are so Efficient in the Allylation of Amines: Design of a Highly Active Catalyst and Comprehensive Experimental and DFT Study

“…-allyl)(DPP-Xantphos)]OTf and 108.11 (7) [14,42] probably due to the steric crowding provided by the two diphenylphosphole moieties. We therefore postulated that platinum complexes of this ligand could be efficient catalysts.…”

“…NMR spectra were recorded on a Bruker AC-300 SY spectrometer operating at 300. [14] [PtA C H T U N G T R E N N U N G (cod)Cl 2 ] (cod = 1,5-cyclooctadiene) and [PtCl 2 A C H T U N G T R E N N U N G (dppe)] were prepared according to established procedures; [57,58] dppe, allyl alcohol, cinnamyl alcohol, allyltrin-butyltin, sodium borohydride, ammonium hexafluorophosphate, silver trifluoromethanesulfonate (AgOTf), silver hexafluorophosphate, and all amines were obtained from commercial suppliers and used as received. Elemental analyses were performed by the Service danalyse du CNRS, at Gif sur Yvette, France.…”

“…[14,36] This catalytic system, which relies on the use of large-bite-angle ligand DPPXantphos (Scheme 2), could achieve complete allylation of aniline at room temperature with 0.1 mol % of catalyst. However, as observed with other palladium complexes, only a very limited scope of amines (such as aryl amines and a handful of secondary amines) could be converted.…”

Scope of the allylation reaction with [RuCp(PP)]+ catalysts: changing the nucleophile or allylic alcohol

“…Given the low acidity of aliphatic alcohols, it would be interesting to see if the allylation of aliphatic alcohols proceeds with a similar catalytic system in the absence of acid. Apart from alcohols, the scope of the [RuCp(PP)] + -catalyzed reactions is expanded by investigating the reactivity of other substrates often used in allylation reactions, such as amines, [14,15] indole, [13] thiols [16,17] and activated diketones. [18] Apart from allyl alcohol as allylating agent, substituted allylic alcohols have also been used in allylation reactions.…”