Six-Membered, Chiral NHCs Derived from Camphor: Structure

Article pubs.acs.org/Organometallics

Six-Membered, Chiral NHCs Derived from Camphor: Structure− Reactivity Relationship in Asymmetric Oxindole Synthesis Markus J. Spallek, Dominic Riedel, Frank Rominger, A. Stephen K. Hashmi, and Oliver Trapp* Organisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany S Supporting Information *

ABSTRACT: A series of three chiral, expanded six-membered NHC−palladium(II) complexes was prepared with successively increased sterical demand, while retaining natural d-(+)-camphor as a chiral motif. The catalysts showed different reaction profiles in the asymmetric, intramolecular α-arylation of amides. The molecular structure of two N-heterocyclic and one nitrogen acyclic carbene palladium isonitrile complex was unequivocally determined by X-ray crystallographic analysis. The results reported herein account for a correlation of catalytic activity and enantiodiscrimination in relation to the degree of chiral substitution and steric congestion at the metal center. The modular and convergent synthetic route of these air- and moisturestable palladium isonitrile complexes underlines the usefulness of this approach.

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INTRODUCTION N-Heterocyclic carbenes (NHCs) have been known since the late 1960s and have become an important class of ligands after Arduengo et al. were able to isolate the first stable derivative in 1991.1 NHC ligands possess a strong σ-donor character and a weak π-acceptor capability.2 The resulting metal complexes are typically characterized by a high stability3 of the NHC−metal bond, making them highly attractive for metal complex formation. Due to their properties and reactivity, NHCs revolutionized today's organometallic chemistry and homogeneous transition-metal catalysis.4−7 Particularly, chiral ligands and their potential in asymmetric catalysis are receiving growing attention.8−13 Various approaches toward the synthesis of (chiral) NHC ligands were established.14−19 Besides the need for a short access to chiral ligand frameworks and their complexes, focus on modular synthetic pathways, preferably high yielding and convergent, to investigate the impact of ligand design on catalysis is emerging. Especially steric demand is an important factor20 for effective chirality transfer as well as for stabilization of low-coordinated metal complexes during catalysis. The latter are needed for challenging transformations, such as cross-coupling reactions of nonactivated aryl chlorides.21−23 Furthermore, besides steric encumbrance of the metal complex enhancing chirality transfer, retention of certain flexibility24 within the catalyst seems to be a beneficial factor as well. On the basis of the pioneering work of Hartwig et al.25 on the intramolecular α-arylation of amides, we wish to contribute to the growing field of ligands used for this transformation. The reaction provides efficient and direct access to chiral 3,3disubstituted oxindoles, a common structural motif present in many natural products. Currently there are various routes © 2012 American Chemical Society

toward enantiomerically enriched 3,3-disubstituted oxindoles with overall moderate success. Dorta et al.,26 Glorius et al.,27 Kündig et al.,28−30 and more recently Murakami et al.31 managed this challenging type of α-amide arylation with high enantiomeric excess and yields.32 While many of the ligands employed are based on five-membered imidazole,25,28−30 imidazolines (or oxazoles),27,33 the use of expanded NHCs and furthermore unsymmetrical substituted chiral NHC−metal complexes have rarely been reported or investigated for this type of transformation. Rather different properties are reported for six-,15,34−39 seven-,15,40−43 and most recently eightmembered44 derivatives including increased basicity (nucleophilicity),36,37,39 greater steric demand, and higher congestion around the metal center (larger N−CNHC−N angle).37 We developed a set of three chiral palladium catalysts featuring a six-membered N-heterocyclic hexahydropyrimidine core. By modification of the flanking substituents and finally by installing an additional group at the NHC backbone the steric demand and the chiral information on the catalyst were successively increased. Using these catalysts we want to quantify the impact of substitution and steric hindrance on catalyst reactivity and selectivity in the asymmetric oxindole synthesis. This reaction, with rather low enantiomeric excess, was chosen to be able to observe maximum effects of the catalyst substitution. To get reliable insights, natural d(+)-camphor (respectively bornylamine) was chosen as the chiral building block, and aryl bromides as well as aryl chlorides with different substitution patterns were employed in the asymmetric α-arylation of amides. Received: November 22, 2011 Published: January 30, 2012 1127

dx.doi.org/10.1021/om201166b | Organometallics 2012, 31, 1127−1132

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RESULTS AND DISCUSSION The synthesis of the six-membered, chiral NHC complexes was achieved utilizing a straightforward synthetic protocol, developed in the Hashmi group.45−49 The modular and quite convergent pathway allowed us the unprecedented short synthesis of three bornylamine-derived palladium catalysts with varying types of backbone and wingtip substitution (cf. Scheme 1).

Scheme 3. Palladium−Bis(isonitrile) Complex Formation

Scheme 1. Chiral, Six-Membered NHC−Pd Complexes with Enhanced Steric Demand

as their corresponding chloride salts (94% for 6a, 88% for 6b) (cf. Scheme 4). Scheme 4. Preparation of NHC-Backbone Synthons 6a and 6b

The cyclododecanone derivative of 6 was prepared in a similar manner and obtained as colorless crystals after recrystallization (79%, cf. Supporting Information). The catalysts I−III were prepared by reaction with the appropriate aminoalkyl chloride in the presence of excess triethylamine in tetrahydrofuran. Nucleophilic attack of the amine nitrogen at the isonitrile carbon atom followed by in situ intramolecular cyclization of the resulting anion gave the bornyl-derived Pd− isonitrile complexes I−III in 67% (I), 64% (II), and 41% (III) yield (cf. Scheme 5). The reactions showed full conversions to

Bornylisonitrile 1 was prepared by neat reaction of ethyl formate and 1R,2S-bornylamine in an autoclave at 200 °C for 12 h and an additional 5 h at 250 °C. This reaction furnished pure bornylformamide 2 after precipitation, washings (npentane), and recrystallization (acetone, petroleum ether) as colorless crystals in 87% yield. Due to amide resonances, two diastereomers were detected by NMR spectroscopic measurements. Dehydration with phosphorus trichloride and excess triethylamine in dichloromethane at −60 °C and then at room temperature gave bornylisonitrile 3 in 83% yield as an off white solid (cf. Scheme 2).

Scheme 5. Formation of Chiral Pd−Isonitrile Complexes I− III by Intramolecular Cyclization

Scheme 2. Synthesis of Chiral Bornylisonitrile 3

The chiral palladium(II) isonitrile precursor 4 was obtained in excellent yields (quantitative) by ligand exchange reaction of Pd(MeCN)2Cl2 and bornylisonitrile 3 in toluene for 12 h at room temperature (cf. Scheme 3). Aminoalkyl chlorides were used as synthons for the installation of the NHC backbone. Therefore, 1R,2S-bornylamine (1) was reacted with 1-bromo-3propanol in benzonitrile at elevated temperatures (95 °C) to furnish 3-hydroxypropylbornylamine (5a) and 3-phenyl-3hydroxypropylbornylamine (5b) as colorless liquids after purification. The chloride salts of 5a and 5b were prepared in very good yields using thionyl chloride. To ease purification and handling in further synthetic steps, 6a and 6b were isolated

the unsymmetrical, chiral isonitrile−NHC−palladium complexes, as evidenced by IR-stretching frequencies and NMR spectroscopic measurements of the isonitrile ligands. Formation and cyclization of the backbone were validated by fading of the carbonyl 13C-resonance of the isonitrile carbon at 168.0 ppm and of the tertiary bornyl-bridgehead CH carbon at 63.7 ppm. 1128


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A shift of the corresponding characteristic isonitrile resonance at 2233 cm−1 was observed as well. Crystals suitable for X-ray diffraction studies were obtained by vapor diffusion of diethyl ether into a saturated dichloromethane solution of either I or II. The molecular structures of palladium complexes I and II were determined by crystal structure analysis (cf. Figures 1 and 2). Two independent

Table 1. Selected Bond Lengths [Å] and Angles [deg] of Camphor-Derived Pd−Isonitrile Complex I Pd−C(1) Pd−C(5) Pd−Cl(1) Pd−Cl(2) C(5)−N(3)

2.027(1) 1.890(2) 2.364(6) 2.317(6) 1.140(2)

N(3)−C(6) Pd−C(5)−N(3) C(5)−N(3)−C(6) C(1)−Pd−C(6) N(2)−C(1)−Pd−C(5)

1.440(2) 170.0(2) 171.30(3) 89.60(9) −79.00

Table 2. Selected Bond Lengths [Å] and Angles [deg] for Camphor-Derived Pd−Isonitrile Complex II Pd−C(1) Pd−C(5) Pd−Cl(1) tPd−Cl(2) C(5)−N(3)

2.014(4) 1.917(5) 2.331(1) 2.357(0) 1.160(6)

N(3)−C(6) Pd−C(5)−N(3) C(5)−N(3)−C(6) C(1)−Pd−C(6) N(2)−C(1)−Pd−C(5)

1.436(6) 178.0(5) 178.2(6) 88.45(17) −73.54

Interestingly, an unprecedented dehydrochlorination side reaction at the backbone of the palladium complex prior to the cyclization step took place by reaction of 6b with bis(isobornylnitrile) 4. At room temperature this competitive dehydrochlorination step to form the nitrogen acyclic carbene (NAC) palladium complex 7 is favored. This clearly demonstrates the high steric demand of the system. Higher temperatures were proven to be beneficial for the cyclization, thus suppressing dehydrochlorination to a certain extent. Therefore preparation of III was conducted under reflux conditions. Although, dehydrochlorination becomes less favored, yields of the six-membered, phenyl-substituted NHC palladium complex III did not exceed 41%. Furthermore, we found that purification of the mixture by simple column chromatography yields the pure cyclization product III (cf. Scheme 6). The solid-state structure of NAC complex 7

Figure 1. Solid-state structure of the six-membered NHC−Pd− isonitrile complex I.

molecules of I were obtained due to a conformational change of the cyclododecane N-substituent. However, only the ORTEP diagram of one isomer is depicted because of the small changes observed between the two isomers. Note that the dihedral torsion angle between N(2)−C(1)−Pd−C(5) of −79.0° (11° deviation from the orthogonality) is indicative of steric congestion around the quadratic planar metal center and generally observed for NHC complexes with bulky substituents. Selected bond lengths and angles of Pd−NHC complex I are shown in Table 1. The solid-state structure of complex II, bearing one chiral bornylamine moiety at the N-atoms each, is depicted in Figure 2. The structure is in accordance with the structural features observed for complex I. Due to the higher steric demand and in particular the orientation of the exomethyl groups (C25 and C27) of the bornyl substituents, a decrease of the dihedral torsion angle N(2)−C(1)−Pd−C(5) to −73.5° is observed. However, packing effects in the crystal structure cannot be excluded. In relation to the NHC axis the isonitrile ligand points toward the sterically less crowded camphor backbone (C24, C23, and C19). A significant change of the Pd−carbene bond and Pd−isonitrile distances is not observed, and the bornylisonitrile 3 is almost linearly coordinated to the palladium center (Pd−C(5)−N(3) and C(5)−N(3)−C(6), both 178°) (cf. Figure 2, Table 2).

Scheme 6. Dehydrochlorination to NAC−Palladium Complex 7, Bearing an Unsaturated Styrenyl Backbone

Figure 2. Solid-state structure of six-membered NHC−Pd−isonitrile complex II: (a) side view and (b) top view. 1129


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converted to their (R)-oxindole derivatives in very high yields as well (entries 2−6, as determined by chiral HPLC). Benzylsubstituted substrates were obtained with an enantiomeric excess of 58% ee (bromide derivative, entry 3) and 63% ee (chloride derivative, entry 4). The corresponding naphthyl substrates showed higher enantioselectivities (68% ee and 72% ee, entries 5 and 6). The results are noteworthy, since temperature has a pronounced influence on the enantioselectivity. The related five-membered NHC showed 70% ee, however only 58% ee at 0 °C with catalyst loadings of 10 mol % catalyst.25 We attribute the higher enantioselectivity of 72% ee (entry 6) at higher temperatures to the NHC−metal angle being enlarged within a six-membered NHC, thus increasing steric congestion and chiral induction compared to the fivemembered derivative with the same chiral ligand pattern. A slight increase of 5% in enantioselectivity was observed for the chloro derivatives in each case (entries 3, 4 and 5, 6). These data suggest that the halide is present on or in close proximity to the catalytic active species during the enantiodiscriminating step. Introduction of a cyclobutyl substituent at the N-terminus of the substrate had no significant influence on enantioselectivity compared to the N-methylated substrate (entries 2 and 3). Interestingly, catalyst III bearing the same chiral bornyl ligand pattern but exhibiting a phenyl substituent at the NHC backbone in close proximity to the N-substituent (and thus to the chiral substituents) showed a different reactivity. Employing the bromo-phenyl derivative (entry 7) high conversions were observed as well, but enantioselectivity dropped almost completely (8%). Besides small amounts of oxindole product dehalogenation of the starting material was observed in 64% yield (rac), as validated by NMR spectroscopic and chiral HPLC-MS measurements. By changing the substrate to the even more sterically demanding bromo-naphthyl derivative, only the racemic dehalogenation products were observed.

revealed a distorted structure with the styrenyl unit being displaced about −67.9° out of the C(1)−N(2)−C(2)−C(3) plane. The isobornyl ligand is located in the opposite direction with a deviation of −73.6° out of the N(3)−C(1)−N(2)−C(2) plane. Note that due to rotational freedom, the chiral-bornyl Nsubstituents changed orientation in the NAC complex compared to the fused six-membered NHC Pd complexes I and II (cf. Figure 3).

Figure 3. Solid-state structure of six-membered NHC−Pd−isonitrile complex 7.

After synthesis, characterization, and structural analysis of the novel, chiral palladium−isonitrile complexes their catalytic performance in the asymmetric α-amide arylation was investigated. For a representative study overall seven different substrates were synthesized according to the literature.25,50−53 Substrates bearing benzyl and naphthyl substituents and Nmethylation were chosen to evaluate the influence of the substitution pattern on selectivity. The corresponding bromide and chloride analogues and one substrate with an Ncyclobutylated amide were prepared as well. Initially, solvent screening (THF, diglyme, dioxane, DMSO) revealed dry dimethoxyethane and sodium tert-butylate to be ideal, with catalyst loadings of 2.5 mol % being sufficient for catalysis. Unfortunately, related Pd−isonitrile complexes, which are highly active catalysts in alcoholic media for Suzuki crosscoupling of boronic acids,47 were not suitable. All reactions were performed at 50 °C to furnish complete conversions within a maximum of 18 h. With chiral catalyst I, featuring a flexible cyclododecanyl substituent at one N-terminus, the desired oxindole was obtained and isolated in quantitative yields (cf. Table 3, entry 1). Enantioselective HPLC analysis of

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CONCLUSION In summary, we have presented the synthesis of three sixmembered (hexahydropyrimidine core), camphor-derived NHC−Pd−isonitrile complexes and their application as catalysts in the asymmetric α-amide arylation. The structure of two Pd complexes and one nitrogen acyclic carbene Pd complex was determined by X-ray crystallographic analysis. Taking advantage of a convergent, modular synthetic pathway the preparation of NHC ligands bearing different structural motifs present at the N-termini was accomplished. By successive increase of the chiral information and congestion within the catalyst patternwhile retaining the chiral motif (bornylamine) at the same timewe demonstrated their influence on enantioselectivity. All catalysts showed good conversions of bromo as well as chloro substrates with catalyst loadings of 2.5 mol %. Whereas catalyst I showed no enantiodiscrimination in the oxindole synthesis, catalyst II proved to be more effective. Higher enantioselectivities compared to the related, five-membered bornylamine-derived ligand were observed at even higher temperatures. With the sterically most demanding catalyst III a reaction profile leading either to an arylation or dehalogenation product depending on the employed substrate was observed. Furthermore, the calculated buried overlap volume54,55 of catalyst II (%Vbur = 40.9) exceeds the value of 1,3-di(1-adamantyl)imidazol-2ylidene (IAd, %Vbur = 36.1)56 and represents the second highest volume reported so far for chiral N,N-heterocyclic carbene ligands (IBiox[(−)-menthyl], %Vbur = 47.7, Au

Table 3. Selected Bond Lengths [Å] and Angles [deg] for NAC Pd−Isonitrile Complex 7 Pd−C(1) Pd−C(5) Pd−Cl(1) Pd−Cl(2) C(5)−N(3) C(4)−C(5)

1.994(3) 1.937(3) 2.366(9) 2.319(3) 1.140(4)

N(3)−C(6) Pd−C(5)−N(3) C(5)−N(3)−C(6) C(1)−Pd−C(6) N(3)−C(1)−Pd−C(5) C(1)−N(2)−C(2)−C(3)

1.461(4) 174.9(3) 178.2(6) 90.20(11) −73.58 −67.85

the product showed a racemic mixture of oxindole enantiomers and proved that no chiral induction takes place within a system bearing only one chiral substituent. Therefore, catalysis was continued using NHC−Pd−isonitrile complex II, featuring two chiral bornyl substituents. With this catalyst all substrates were 1130


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Table 4. Asymmetric Oxindole Synthesis Using NHC−Pd−Isonitrile Complexes I−IIIa

entry

catalyst

X

R1

Ar

A

1 2 3 4 5 6 7d 8d

I II II II II II III III

Br Br Br Cl Br Cl Br Br

Me c-C4H8 Me Me Me Me Me Me

1-naphthyl Ph Ph Ph 1-naphthyl 1-naphthyl Ph 1-naphthyl

100 100 100 100 100 100 36

B

yield [/%]b

ee [A; %]c

64 100

quant. 95 90 98 92 95 89 91

rac 55 58 63 68 72 8

a Reaction conditions: 0.3 mmol scale, NaOtBu (0.45 mmol), catalyst (2.5 mol %) in DME (5 mL) at 50 °C, 14−18 h. bIsolated yield. cDetermined by chiral HPLC (Chiralpak IA). Product configuration: R, determined by chiral HPLC (Chiralpak IB) of known compounds. dReaction at 80 °C. Isolated yield of A and B.

complex).27 The results obtained show that higher hindrance of the metal by substituents is still beneficial for enantioselectivity, but at a certain level of congestion or restriction of flexibility a further improvement of chiral induction within a given system is limited.

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ASSOCIATED CONTENT

* Supporting Information S

Crystallographic information files (CIF) of compounds I, II, and 7, as well as characterization data of all products including HPLC data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +49 6221-54-8470. Fax +49 6221-54-4904. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Deutsche Forschungsgemeinschaft (DFG SFB 623 “Molecular Catalysts: Structure and Functional Design”) for generous financial support. M.J.S. is grateful for a doctoral fellowship (DFG GK 850 “Molecular Modeling”).

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Six-Membered, Chiral NHCs Derived from Camphor: Structure

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