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.
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