Article pubs.acs.org/Organometallics
Metal Complexes Containing Enantiopure Bis(diamidophosphite) Ligands in Asymmetric Allylic Substitution and Hydroformylation Reactions Maritza J. Bravo,† Rosa M. Ceder,*,† Arnald Grabulosa,† Guillermo Muller,† Mercè Rocamora,† J. Carles Bayón,‡ and Daniel Peral‡ †
Departament de Química Inorgànica, Universitat de Barcelona, Martí i Franquès 1-11, 08028 Barcelona, Spain Departament de Química, Universitat Autònoma de Barcelona, Bellaterra, 08193 Barcelona, Spain
‡
S Supporting Information *
ABSTRACT: Enantiopure bis(diamidophosphite) ligands with a heterocyclic terminal fragment derived from (R)- and (S)-N,N′-dimethyl-1,1′-binaphthyldiamine and bridging fragments derived from (S,S)-2,3-butanediol (a), (4R,5R)-4,5di(hydroxymethyl)-2,2-dimethyl-1,3-dioxolane (b), and (R)and (S)-1,1′-bi-2-naphthol (c) were used to prepare the palladium complexes with general formula [Pd(η3-2-CH3C3H4)(P-P)][X] (X = PF6, 1a-(S;Sal,Sal;S), 1b-(R;Ral,Ral;R), 1b-(S;Ral,Ral;S), 1c-(R;Ral;R), 1c-(R;Sal;R); X = BPh4, 2a(R;Sal,Sal;R), 2c-(R;Ral;R)), which have been fully characterized. The solid-state structure for complexes 1a-(S;Sal,Sal;S) and 1b-(R;Ral,Ral;R) has been determined by X-ray diffraction. The catalytic performance of the palladium complexes has been evaluated in asymmetric allylic alkylation and amination reactions with the benchmark substrate. The influence of the nature and absolute configuration of both the terminal and bridging fragments of the bis(diamidophosphite) ligands on the asymmetric induction is discussed. The best results in terms of enantioselectivity were obtained with 1c-(R;Ral;R), affording enantiomeric excesses up to 85% in both alkylation and amination reactions. A large match−mismatch effect between the absolute configurations of stereocenters of ligand c has been observed in the allylic amination process. Preliminary results in the rhodiumcatalyzed asymmetric hydroformylation of styrene by using bis(diamidophosphite) ligands a, b, and c disclosed in all cases low enantiomeric discrimination for the branched aldehyde. Both for the allylic alkylation and for the hydroformylation reaction, a related monodentate diamidophosphite d, derived from (R)-N,N′-dimethyl-1,1′-binaphthyldiamine and (S)-borneol, was also tested. Palladium complexes of this monodentate ligand showed fairly good enantioselectivity in allylic alkylation, but with very low rate, while the rhodium complex of d rendered better enantioselectivity (37% ee) than the bidentate ligands a−c in the hydroformylation of styrene.
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fine-tuning their donor−acceptor and steric properties through incorporation of the heteroatom into the first coordination sphere of phosphorus and a wide variation of the O- and Ncontaining building blocks as well as the N-substituents.2−6 Libraries of these chiral bidentate P−heteroatom ligands have been applied in the allylic substitution and hydroformylation among many other reactions.7 Bidentate phosphites8 and mono-9 and bisphosphoramidites10 have been successfully applied in allylic alkylation reaction. The use of diamidophosphites is mostly focused on the P-stereogenic bis(diamidophosphite) ligands with 1,3,2-diazaphospholidine rings and several diols such as 1,4:3,6 dianhydro-D-manitol,11 N-benzyltartarimide,12 N-naphthyltartarimide,13 binaphthol,14 resorcinol, and hydroquinone15 as frameworks. For the hydroformylation a great variety of chiral ligands has been
INTRODUCTION Asymmetric catalysis is one of the most cost-effective and environmentally friendly methods for the production of a large variety of enantiomerically enriched molecules. An important area of research within asymmetric catalysis involves the design of enantiopure ligands and their transition metal catalysts, which can lead to efficient and selective organic transformations. In this field, chiral bidentate C2-symmetric Pdonor ligands are successfully used because of their capability to effectively create an asymmetric environment for the active site of the catalyst.1 Phosphorus donor ligands with P−heteroatom bonds, such as phosphites (3P-O), phosphoramidites (2P-O, 1P-N), and diamidophosphites (2P-N, 1P-O) are good alternatives to chiral bidentate phosphines (3P-C), because they can be obtained in an easy way through a modular approach by reacting alcohols or amines with phosphorus halides, providing families with great structural and stereochemical diversity. They also provide ample opportunity for © XXXX American Chemical Society
Received: May 27, 2015
A
DOI: 10.1021/acs.organomet.5b00457 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics assayed, with diphosphites and diphosphonites being more widely used,16,17 although very often the best results have been achieved with hybrid diphosphorus ligands.18 Ligands containing the diamidophosphite moiety have been very scarcely investigated in hydroformylation.19,20 Bis(diamidophosphite) ligands containing terminal P-heterocyclic rings derived from C2-diamines and C2-diols as bridging fragments have been studied and applied only to a limited range of asymmetric transition metal catalyzed transformations. Interestingly, Pfaltz et al. have developed a bis(diamidophosphite) ligand to study the palladium-catalyzed kinetic resolution of allylic species.21 Trost et al. used them in the palladium-catalyzed vinylsubstituted trimethylenemethane asymmetric [3 + 2] cycloaddition.22 We recently described the synthesis of two families of C2-bis(diamidophosphite) ligands derived from N,N′substituted cyclohexyldiamine and N,N′-dimethyl-1,1′-binaphthyldiamine and several diols, which were applied in the Rhhydrogenation of different olefins.23 The best results were obtained with the diamidophosphite ligand containing heterocyclic terminal fragments derived from dimethylbinaphthyldiamine and the bridging fragment derived from dioxolane, attaining ee’s up to 99% for all the benchmark substrates tested. These results encouraged us to explore the binaphthyl-derived diamidophosphite ligands in asymmetric Pd-catalyzed allylic alkylation and amination and the Rh-catalyzed hydroformylation reactions. In this paper we describe the synthesis of the new dimethylbinaphthyldiamine-based ligands b-(S;Ral,Ral;S) and c-(R;Ral;R), diastereisomers of the previously reported ligands23 (Figure 1). We also describe the synthesis and
Scheme 1. Synthesis of Allyl Palladium Complexes
ligand d previously reported by us (Figure 1) was also assayed in these reactions. This ligand was included in the study because it showed a remarkable enantioselectivity in the palladium-catalyzed hydrovinylation of styrene.24 We report here also a preliminary study on the hydroformylation of styrene using bis(diamidophosphite)s a−c and the monodentate ligand d. It has to be noticed that Reetz et al. have described that monodentate ligands with a related skeleton shows in this reaction unusually high ee’s.20 The reactions here investigated, Pd-asymmetric allylic substitution and hydroformylation, provide the opportunity to test the efficiency of the new chiral ligands, to identify the dominant chiral fragment that determines the absolute configuration of the final product, and to study the match−mismatch effects arising from the combination of the different stereogenic elements.
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RESULTS AND DISCUSSION Synthesis and Characterization of Cationic Bis(diamidophosphite) Allylpalladium Complexes. The modular C2-symmetric bis(diamidophosphite) ligands a, b, and c were synthesized via two consecutive condensation reactions from enantiomerically pure (R)- or (S)-N,N′dimethyl-1,1′-binaphthyl-2,2′-diamine and the corresponding diols, namely, (S,S)-2,3-butanediol, (4R,5R)-4,5-di(hydroxymethyl)-2,2-dimethyl-1,3-dioxolane, and (R)- and (S)-binaphthol, in the presence of a base following our previously reported methods.23 The preparation and characterization of the new b-(S;Ral,Ral;S) and c-(R;Ral;R) ligands is reported in the Experimental Section. The cationic allylic complexes 1a−c of general formula [Pd(η3-2-CH3-C3H4)(P-P)][PF6] were obtained by reaction of the organometallic precursor [Pd(η3-2-CH3-C3H4)(μ-Cl)]2 with the stoichiometric amount of the appropriate bis(diamidophosphite) ligand, at low temperature, in the presence of NaPF6 (Scheme 1) as reported in the literature for similar compounds.24 Complexes with the general formula [Pd(η3-2CH3-C3H4)(P-P)][BPh4] (P-P = 2a-(R;Sal,Sal;R) and 2c(R;Ral;R)) were also synthesized. Complex 2a-(R;Sal,Sal;R) was prepared due to its better crystallization compared to the analogous complex with PF6− as counteranion. To study the effect of the anion on the catalytic processes, 2c-(R;Ral;R) was synthesized in order to compare the catalytic results with those obtained with 1c-(R;Ral;R). They were prepared by addition of the stoichiometric amount of NaBPh4 in MeOH to a dichloromethane solution of the hexafluorophosphate complex. The new compounds were obtained as white-yellow solids in low to moderate yields. They are stable under inert atmosphere at room temperature and soluble in common organic solvents.
Figure 1. Diamidophosphite ligands tested in asymmetric allylic substitution and hydroformylation reactions.
characterization of new allyl palladium complexes [Pd(η3-MeC3H5)(P-P)][PF6] (P-P = bis(diamidophosphite)s a−c in Scheme 1). Very few examples of the coordination chemistry of this kind of ligand have been reported in the literature.11a The new cationic palladium complexes have been used as catalytic precursors in the Pd-asymmetric allylic alkylation and amination. For comparative purposes the related monodentate B
DOI: 10.1021/acs.organomet.5b00457 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Table 1. Selected 31P{1H}a and 1Hb NMR Data for Palladium Allyl Complexes δ 1H complex
δ P
1a-(S;Sal,Sal;S)c
150.2 (d, 2JPP = 84.0) 147.4 (d, 2JPP = 84.0) [176.6 (s)]d
2a-(R;Sal,Sal;R)
139.4 (d, 2JPP = 94.9) 135.7 (d, 2JPP = 94.9) [178.3 (s)]d
1b-(R;Ral,Ral;R)
141.6 (d, 2JPP = 85.1) 139.4 (d, 2JPP = 85.1) [168.9 (s)]d
1b-(S;Ral,Ral;S)f
1c-(R;Sal;R)
1c-(R;Ral;R)f
2c-(R;Ral;R)g
31
138.7 (d, 2JPP = 85.5) 136.7 (d, 2JPP = 85.5) [163.9 (s)]d 149.9 (d, 2JPP = 65.8) 145.6 (d, 2JPP = 65.8) [174.6 (s)]d 152.3 (d, 2JPP = 66.4) 149.7 (d, 2JPP = 67.0) [173.6 (s)]d 151.1 (d, 2JPP = 67.0) 149.7 (d, 2JPP = 67.0) [173.6 (s)]d
Hsyn
N-CH3 3.28 3.20 3.09 3.04 3.15 3.00 2.98 2.92 3.34 3.20 3.13 3.04 3.34 3.17 3.14 3.03 3.30 3.25 2.49 1.79 3.49 3.26 1.71 1.56 3.46 3.24 1.71 1.56
(d, (d, (d, (d, (d, (d, (d, (d, (d, (d, (d, (d, (d, (d, (d, (d, (d, (d, (d, (d, (d, (d, (d, (d, (d, (d, (d, (d,
3
JHP 3 JHP 3 JHP 3 JHP 3 JHP 3 JHP 3 JHP 3 JHP 3 JHP 3 JHP 3 JHP 3 JHP 3 JHP 3 JHP 3 JHP 3 JHP 3 JHP 3 JHP 3 JHP 3 JHP 3 JHP 3 JHP 3 JHP 3 JHP 3 JHP 3 JHP 3 JHP 3 JHP
= = = = = = = = = = = = = = = = = = = = = = = = = = = =
14.0) 10.0) 14.0) 10.0) 14.5) 9.5) 14.5) 9.5) 15.0) 11.0) 14.5) 11.0) 14.8) 10.4) 14.8) 10.4) 9.6) 8.8) 15.2) 15.2) 13.6) 13.6) 10.8) 10.4) 13.6) 14.0) 10.4) 10.4)
Hanti
CH3 (allyl)
3.93 (bs) 3.35 (bs)
2.90 (d, 2JHP = 12.4) 2.44 (d, 2JHP = 12.8)
1.69 (s)
3.72 (bd, 2JHP = 7.0) 3.32 (bd, 2JHP = 7.5)
2.68 (d, 2JHP = 13.5) 2.49 (d, 2JHP = 13.5)
1.58 (s)
3.85e 3.42 (m)
2.98 (d, 2JHP = 14.0) 2.48 (d, 2JHP = 13.0)
1.60 (s)
3.98 (m) 3.42 (d, 2JHP = 9.0)
2.95 (d, 2JHP = 14.4) 2.35 (d, 2JHP = 13.2)
4.44 (bs) 3.55 (bs)
2.91 (d, 2JHP = 12.8) 2.81 (d, 2JHP = 13.6)
1.70 (s)
2.96 (bs) 2.30 (bs)
2.49 (bs) 2.46 (bs)
1.41 (s)
3.00 (bs) 2.28 (bs)
2.47 (bs) 2.45 (bs)
1.39 (s)
1.56 (s)
P{1H} [δ] = ppm (101.25 MHz, CDCl3, 298 K), JPP = Hz. Complexes 1a, 1b, and 1c show one heptuplet at δ = −144 ppm and 1J = 715 Hz of the PF6− anion. b1H [δ] = ppm (500 MHz, CDCl3, 298 K), JHH and JHP = Hz. c31P{1H} (121.44 MHz, CDCl3, 298 K), 1H (400 MHz, CD2Cl2, 298 K). d Free bis(diamidophosphite) ligand. eAssigned in the HSQC 13C−1H spectrum. f31P{1H} (161.9 MHz, CDCl3, 298 K), 1H (400 MHz, CDCl3, 298 K). g1H (400 MHz, CDCl3, 298 K). a31
amine fragment. Moreover, in the 13C NMR spectra, reported in the experimental part, different 2JCP coupling constants appear for the corresponding carbon atoms. All these facts suggest different orientations of the amino substituents with respect to the P−Pd bond as it has been previously reported.28 Concerning the allyl moiety, there are four signals for the terminal hydrogen atoms according to the coordination of the chiral bidentate ligand. Complexes 1a, 1b, and 1c-(R;Sal;R) showed two signals for the syn protons at lower field than the two doublets corresponding to the anti ones, as reported for similar compounds.24,26,27 For 1c-(R;Ral;R) and 2c-(R;Ral;R) unusual upfield 1H shifts were found for one Hsyn and for two methyl substituents of the binaphthyl amine fragments. Bidimensional HSQC 1H−13C and ROESY experiments were necessary to assign these signals. A ROESY experiment for 2c(R;Ral;R) showed NOE contacts between the syn protons and the methyl allyl group, allowing an unequivocal assignment. In its DFT-calculated lowest energy structure, one Hsyn appeared right below a phenyl ring of a binaphthyl diamine moiety and two methyl substituents of each binaphthyl amine fragments below different phenyl rings of the binaphthol bridging group, shifting the corresponding 1H NMR signals upfield (Figure 2). Bidimensional NOESY experiments were performed for 2a(R;Sal,Sal;R) and 1b-(R;Ral,Ral;R) complexes. For all of them NOE contacts can be observed between one Hsyn and one NMe group of one diamidophosphite terminal fragment and one
They were fully characterized in the solid state and in solution by standard methods. Suitable crystals for X-ray diffraction were obtained from solutions of CH2Cl2/hexane for complexes 1a(S;Sal,Sal;S) and 1b-(R;Ral,Ral;R). Multinuclear (31P, 1H, and 13 C) NMR study allowed us to characterize the complexes in solution. Relevant NMR data are summarized in Table 1. 31 1 P{ H} NMR spectra showed two sharp doublets showing an AB pattern, revealing two different and strongly coupled phosphorus atoms (2J = 66−95 Hz), indicating the loss of C2 symmetry of bis(diamidophosphite) ligands in the allylic palladium complex. Coupling constants are similar to those reported for related complexes with phosphite, phosphoramidite, and diamidophosphite ligands.9c,25−27 In contrast, Gavrilov11a described similar complexes with bis(diamidophosphite) ligands presenting only one 31P NMR signal. Complexes 1a−c containing different diastereoisomers of the same ligand showed different 31P chemical shifts and different coupling constants, as expected. The coordination of the free diamidophosphite ligands to the palladium-allyl fragment shifts the 31P NMR signal to higher fields probably due to the relatively low σ-donor character of the ligand based on their high JPSe values, previously reported.23 Accordingly with the nonequivalence of the two P-donor atoms, the 1H NMR spectra of complexes 1a−c showed four different doublets with two different coupling constant values corresponding to the methyl substituents of the binaphthylC
DOI: 10.1021/acs.organomet.5b00457 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
Figure 4. Molecular view of the cation corresponding to the complex 1a-(S;Sal,Sal;S) (ellipsoids drawn at the 50% probability level). Hydrogen atoms and the PF6 anion have been omitted for clarity. Selected distances (Å) and angles (deg): Pd(1)−P(1) 2.2872(9), Pd(1)−P(2) 2.2955(8), Pd(1)−C(49) 2.196(3), Pd(1)−C(50) 2.207(3), Pd(1)−C(51) 2.190(3), P(1)−N(1) 1.690(3), P(1)−N(2) 1.659(3), P(2)−N(3) 1.682(3), P(2)−N(4) 1.653(3), C(49)−C(50) 1.407(5), C(50)−C(51) 1.405(5), P(1)−N(1) 1.690(3), P(1)−N(2) 1.659(3), P(2)−N(3) 1.682 (3), P(2)−N(4) 1.653 (3); P(1)− Pd(1)−P(2) 105.60(3), ∑N(1) 351.2, ∑N(2) 354.4, ∑N(3) 353.8, ∑N(4) 359.4.
Figure 2. DFT-calculated lowest energy structure for the [Pd(η3-2CH3-C3H4)(P-P)]+ cation, (P-P) = c-(R;Ral;R).
Hanti of the other carbon with a NMe group of the other terminal fragment, showing that C2 symmetry of the bidentate ligand is broken by the allylic group. Exchange signals between Hsyn−Hanti, Hsyn−Hsyn, and Hanti−Hanti allylic protons are also observed for 1b-(R;Ral,Ral;R) and Hsyn−Hsyn for 2a-(R;Sal,Sal;R), indicating that the dynamic behavior takes place through the two well-known pseudorotation and η3−η1−η3 mechanisms. The ROESY experiment for 2c-(R;Ral;R) does not show any exchange signals between allylic protons, suggesting that the more rigid bis(diamidophosphite) ligand c-(R;Ral;R) probably prevents a fast exchange mechanism in the metal complex. For all of the complexes NOE contacts between allylic Hsyn−Hanti protons of the same carbon atom are found. Figure 3 summarizes the depicted NOE contacts and exchange signals.
Figure 5. Molecular view of the cation corresponding to the complex 1b-(R;Ral,Ral;R)-conformer A (ellipsoids drawn at the 50% probability level). Hydrogen atoms and the PF6 anion have been omitted for clarity. Selected distances (Å) and angles (deg) for both isomers: Pd(1)−P(1) 2.3143(18), Pd(1)−P(2) 2.2884(18), Pd(1)−C(52) 2.14(6), 2.25(7), Pd(1)−C(53) 2.175(13), 2.270(6), Pd(1)−C(54) 2.19(5), 2.22(8), P(1)−N(1) 1.680(6), P(1)−N(2) 1.652(6), P(2)− N(3) 1.632(6), P(2)−N(4) 1.683(6), C(52)−C(53) 1.40(4), 1.39(4), C(53)−C(54) 1.43(3), 1.42(3); P(1)−Pd(1)−P(2) 102.07(6), ∑N(1) 353.2, ∑N(2) 355.6, ∑N(3) 358.2, N(4) 347.2.
Figure 3. NOE contacts (blue) and exchange signals (red). 13
C NMR showed different chemical shifts for the terminal allylic 13C signals, whose difference is a useful tool to evaluate the asymmetry of the allyl bonding.29 In 1a and 1b complexes this difference is only around 1 ppm, but reaches 4.6 ppm in 1c(S;Ral;S) and 9 ppm in 1c-(R;RalR). Single crystals of 1a-(S;Sal,Sal;S) and 1b-(R;Ral,Ral;R), suitable for X-ray analysis, were obtained by slow diffusion of hexane into saturated dichloromethane solutions of the complexes. Their molecular structures and a selection of bond lengths and angles are shown in Figures 4 and 5. The structures consist of discrete units of the cationic complex, hexafluorophosphate anions, and dichloromethane molecules separated by typical van der Waals distances. Both complexes have a distorted square planar structure around the palladium atom. The four coordination positions are occupied by the two phosphorus atoms of the bis(diamidophosphite) ligand and the two terminal carbon atoms of the 2-methylallyl group. Bond distances and angles in the coordination sphere are in the range described for related cationic allyl palladium complexes.26,30 In the unit cell of complex 1a-(S;Sal,Sal;S) there is a single molecule. In contrast, in 1b-(R;Ral,Ral;R) there are two nonequivalent molecules, which differ in the orientation of the dioxolane group of the bridging fragment with respect to the
methyl of the allyl group (Figure 6). For 1a and 1b the three carbon atoms of the allyl group are approximately equidistant from the palladium atom. As a consequence, the carbon− carbon bond lengths of the η3-allyl group are nearly equal for 1a and 1b, which is in accordance with the similar chemical shifts observed in the 13C NMR spectra. Furthermore, for 1a the
Figure 6. Two different conformations of the bridging fragment of the bis(diamidophosphite) relative to the allyl ligand in the monocrystal of 1b-(R;Ral,Ral;R). D
DOI: 10.1021/acs.organomet.5b00457 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics relative position of the three allyl carbon atoms is interesting: the CH2 allylic carbon (C51) is situated below (−0.034 Å) and the central (C50) and the terminal (C49) allylic carbon atoms are situated above (+0.777 and +0.212 Å), showing slight rotation from the coordination plane. The same situation is observed in one molecule of the 1b complex (+0.088 Å for C52, −0.350 Å for C53, +0.415 Å for C54), but in the other one the allyl group is situated symmetrically and on one side of the coordination plane (−0.046 Å for C52, −0.727 Å for C53, −0.037 Å for C54). From the limited number of structures containing the PNNO skeleton, it is to be noted that the P−N bond distances of the bis(diamidophosphite) coordinated ligands in complexes 1a and 1b are in the range of those described for monodentate diamidophosphites both in neutral allylic palladium complexes24,27 and in borane-diamidophosphite compounds.20 The P−N bond distances suggest partial multiple-bond character when compared to the normally accepted bond lengths (P−N bond 1.77 Å and PN bond 1.57 Å).31 Two different P−N distances are observed in each terminal heterocyclic fragment of the ligand in complexes 1a and 1b. The sum of the bond angles around the nitrogen atoms reflects that their geometry tends to planarity, as occurs in other diamidophosphite28a and phosphoramidite32 allyl palladium complexes. Binaphthyl groups have torsion angles of 57.55° and 54.27° for the 1a complex and of 57.02° and 57.88° for complex 1b. The two methyl groups of each binaphthyldiamine fragment are placed at opposite sides of the plane defined by the phosphorus and the two nitrogen atoms of the heterocycle. Asymmetric Allylic Substitution Reactions. To evaluate the potential of diamidophosphite ligands a−c in the asymmetric allylic substitution, the cationic palladium complexes 1a−c, 2a, and 2c were tested as catalytic precursors using the model substrate rac-3-acetoxy-1,3-diphenyl-1-propene (racI), with sodium dimethyl malonate or benzylamine as nucleophile (Scheme 2). The reactions were performed in
Table 2. Results of the Asymmetric Allylic Substitution of rac-3-Acetoxy-1,3-diphenyl-1-propene (rac-I) Using Sodium Dimethyl Malonatea or Benzylamineb as Nucleophile entry 1 2 3 4 5 6 7
catalyst precursor 1a-(S;Sal,Sal;S) 2a-(R;Sal,Sal;R) 1b-(R;Ral,Ral;R) 1b-(S;Ral,Ral;S) 1c-(R;Ral;R) 1c-(R;Sal;R) 2c-(R;Ral;R)
NaCH(COOMe)2a eec % 47 10 41 50 85 61 83
d
(S)/38 (S) (R) (S) (R)/25 (R)d (R) (S) (R)
BnNH2b eec % 41 (R) 26 (S) 57 (S) 43 (R) 85 (S)
