| Abstract
| - Two new xanthene-phosphole ligands are presented with a large bite angle and strong π-accepting capacity. The investigation of their reactivity toward [Pd(allyl)Cl]2 allowed the access to an unusual dimeric structure containing two Pd0 atoms and a trinuclear palladium complex containing one Pd0 and two PdI atoms. The different complexes were tested in the allylation of aniline, showing excellent catalytic activities.
- Two new xanthene-phosphole derivatives, 3 and 4, were synthesized through the nucleophilic substitutionof 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 dimethylderivative) react with [Pd(allyl)Cl]2 to afford the corresponding cationic complexes 5Cl and 6Cl. UsingAgOTf, stable triflate complexes 5OTf and 6OTf could be isolated. The chloride complexes, on theother hand, exhibit a limited stability in solution. The DPP-Xantphos derivative 5Cl eliminates allylchloride to yield the dimeric palladium(0) complex 7 of general formula [Pd(3)]2, in which each palladiumis coordinated to the two phosphorus atoms of the same ligand and to one double bond of one phospholeunit of the second ligand. Decomposition of complex 6Cl furnished a mixture of compounds featuringa 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] affordeddimer 8, which features one 18-VE Pd0 center and two Pd-Cl fragments, which are coordinated to thedienic system of the two phosphole ligands and connected through a single Pd−Pd bond. The catalyticactivity of the triflate complexes 5OTf and 6OTf was evaluated in the allylation of aniline. Whereas theDMP-Xantphos derivative 6OTf exhibited a poor catalytic activity, very good conversion yields wereobtained 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 enhancesthe 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.
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