Synthesis and Reactivity of Chiral, Wide‐Bite‐Angle, Hybrid Diphosphorus Ligands

Abstract

Keywords: P ligands / Synthesis design / Palladium catalysis / Rhodium catalysisEffective and modular synthetic approaches toward phosphine-phosphite ligands and phosphine-phosphonite ligands featuring a diphenyl ether backbone have been developed. The phosphine-phosphite ligands are obtained by a two-step protocol from 2-bromo-2Ј-methoxydiphenyl ether. The phosphine-phosphonite ligands are prepared in a fourstep synthetic protocol that involves a novel, unsymmetrical diphenyl ether derived phosphine-phosphorusdiamide as key building block. Structural studies on Pt II complexes with either phosphine-phosphite or phosphine-phosphonite ligands indicate strict cis coordination for these ligand systems. High-pressure NMR spectroscopy studies of Rh complexes under syngas indicate the presence of two ea isomers for Rh(H)(CO) 2 (PP). The existence of this mixture is further [a] EaSTCHEM, Scheme 1. Synthetic route to phosphine-phosphonite ligands.into the BH 3 -product 7-(BH 3 ) 2 . The 31 P NMR spectrum of 7-(BH 3 ) 2 in CD 2 Cl 2 at room temperature shows an AX spin system with broad doublets at δ = 17.8 and 88.8 ppm ( 1 J PB coupling constant of 105 Hz), assigned to the BH 3 adducts of the aminophosphine and diphenylphosphine groups, respectively.Compound 7-(BH 3 ) 2 is stable towards oxidation and hydrolysis and can be easily purified by precipitation from acetone and methanol in good yields (80 %). Crystals of 7-(BH 3 ) 2 suitable for X-ray crystallography were obtained by recrystallization by slow diffusion of hexane into a diethyl ether solution (Figure 4). The P-C bond length of the backbone to the diphenylphosphine group [1.815(4) Å] is virtually the same as the P-C bond length of the backbone to the diethylaminophosphine group [1.819(4) Å]. Both P-C bond lengths are similar to the values found for other widebite-angle ligands such as Xantphos. [16] The intramolecular distance between the two phosphorus atoms [5.803(2) Å] is larger than in Xantphos (4.080 Å), and obviously the flexible backbone has to rearrange to allow chelation of a metal atom. The P-B bonds lengths [1.910(4) and 1.918(6) Å] of both phosphorus groups are almost indistinguishable. The tetrahedral geometry is distorted around both phosphorus centres, judging from, for example, the B-P-C angles.The BH 3 protecting groups were quantitatively and smoothly removed by reaction with an equimolar amount of DABCO in toluene at 60°C overnight without affecting the P-NEt 2 bond. The 31 P NMR spectrum shows one singlet at δ = 90.9 ppm (aminophosphine) and one singlet at -18.0 ppm (phosphine). The chemical shifts of the aminophosphine and phosphine groups are in the same range as previously reported for related species. [11] Ultimately, two hybrid phosphine-phosphonite ligands were prepared by reacting (S)-BINOL derivatives with the aminophosphine moiety in refluxing toluene for one day (for 8a) or two days (8b). The longer reaction time for 8b was expected as this reaction can be hampered by steric hindrance when using too bulky diols, [19] The products we...

Recent Advances in Enantioselective Pd-Catalyzed Allylic Substitution: From Design to Applications

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“…diphosphine ligands with ether backbones [21][22][23][24][25][26][27][28][29]. Basic coordination chemistry of these potentially dinucleating ligands toward gold has already been established by several groups [30][31][32].…”

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“…DPEPhos, with a calculated natural bite angle of 102.9°, has displayed high selectivity in a variety of reactions, including rhodium-catalyzed hydroformylation, palladium-catalyzed cross-couplings, and allylic alkylation . The only chiral diphosphorus ligands based on diphenyl ether as backbone carry either chiral auxiliaries at phosphorus , ( 7 – 9 in Figure ) or possess stereogenic phosphorus atoms ( 10 ) . The former systems have been successfully applied in a number of asymmetric catalytic transformations, including conjugate addition of arylboronic acids, hydrogenation, allylic alkylation, hydrocyanation, and hydroformylation. ,− …”

“…The only chiral diphosphorus ligands based on diphenyl ether as backbone carry either chiral auxiliaries at phosphorus , ( 7 – 9 in Figure ) or possess stereogenic phosphorus atoms ( 10 ) . The former systems have been successfully applied in a number of asymmetric catalytic transformations, including conjugate addition of arylboronic acids, hydrogenation, allylic alkylation, hydrocyanation, and hydroformylation. ,− …”