New complexes of palladium chloride with bidentate diphosphine ligands on the basis of 2,2`-bis(diorganylphosphino)diphenyl oxides were prepared by three methods, and their catalytic activity in arylation of amines was studied.Previously [1] we synthesized a series of bidentate ligands on the basis of 2,2`-diphosphino-substituted diphenyl oxide, differing from their analog with phenyl substituents on phosphorus [2] in that their phosphorus atom bore secondary and tertiary alkyl radicals or their combinations. The interest in synthesis of such compounds was provoked by the data of Hamann and Hartwig [3], who showed in their study on the cross coupling reaction of aniline with N,N-dimethyl-4-bromoaniline that replacement of the diphenylphosphino groups in the catalyst, palladium(0) 2,2`-bis(diarylphosphino)diphenyloxide complex, by di-o-tolylphosphino groups increases the conversion from 30 to 80%. Comparison of the catalytic activities of palladium complexes of our synthesized ligands with variable steric and electron-donor characteristics of the phosphorus atom may aid in assessment of the role of key stages in cross coupling.The complexes of palladium chloride with ligands (2-R 1 R 2 PC 6 H 4 ) 2 O [Ia3Ig, where R 1 = R 2 = Ph (Ia), R 1 = R 2 = s-C 6 H 11 (Ib); R 1 = R 2 = i-Pr (Ic), R 1 = R 2 = t-Bu (Id), R 1 = R 2 = C 6 F 5 (Ie); R 1 = i-Pr, R 2 = t-Bu (If); R 1 = t-Bu, R 2 = Ph (Ig); were prepared by three procedures. The first procedure (procedure a) involves reaction of palladium chloride with an individual ligand I in aqueous ethanol.
IIa!IId(1)According to this procedure, complex IIa was synthesized by stirring the reagents for 3 h at 60oC. Ligands Ib and Ic with a more nucleophilic phosphorus atoms react with palladium chloride faster, but, in view of the fact that complexes IIb and IIc are thermally unstable and partially decompose at 60oC already within 30 min, for their successful synthesis reaction (1) should be accomplished at room temperature for 12 h.The second synthetic approach to complexes II (procedure b), that involves displacement by ligand I of the more basic benzonitrile, seems more promising.I + PdCl 2 (PhCN) 2 76 IIb, IId + 2PhCN. (2) Reaction (2) features high rate with various ligands, mild conditions (THF, 20oC, several hours), high yields, and facile product isolation. For comparison, the yields of complex Id by the first and second procedures are 25 and 95%, respectively.As ligands I are difficult to isolate pure in view of their tendency for oxidation in solutions and partial loss of products, we developed the third procedure for preparing complexes II (procedure c), in which a crude ligand I plays the role of substrate. To prepare complexes II by this procedure, one cannot directly use the reaction mixture containing ligand I and palladium chloride, since tetramethylethylenediamine is scarcely inferior to the phosphine ligands in complex-forming ability and successfully competes with them for place in the coordination sphere of palladium. Therefore, for preparing crude ligands I c...
