Stereoselective cyclopalladation of 2,3 camphorquinone 3

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Mar 7, 2014 - 1543—1546, July, 2014. 1543. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1543—1546, July, 2014.
Russian Chemical Bulletin, International Edition, Vol. 63, No. 7, pp. 1543—1546, July, 2014

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Stereoselective cyclopalladation of 2,3camphorquinone 3diphenylmethylimine* Ya. A. Gur´eva,a O. A. Zalevskaya,b I. N. Alekseev,a L. L. Frolova,a P. A. Slepukhin,c and A. V. Kuchina aInstitute

of Chemistry, Komi Scientific Center of the Ural Branch of the Russian Academy of Sciences, 48 ul. Pervomaiskaya, 167982 Syktyvkar, Russian Federation. Fax: +7 (821 2) 21 8477. Email: gurjeva[email protected] bSyktyvkar State University, 55 Octyabr´skii prosp., 167001 Syktyvkar, Russian Federation. Fax: +7 (821 2) 43 6820 cI. Ya. Postovsky Institute of Organic Synthesis, Ural Branch of the Russian Academy of Sciences, 22 ul. S. Kovalevskoi, 620137 Ekaterinburg, Russian Federation. Fax: +7 (834 3) 369 3058 A reaction of (1R,4S)3diphenylmethylimino1,7,7trimethylbicyclo[2.2.1]heptan2one with lithium tetrachloropalladate gives a palladium(II) complex with a monodentately coordi nated ligand, whereas chiral palladacyclic compounds are formed in the case of palladium acetate. Key words: palladium complexes, cyclopalladation, palladacycles, camphor, bicyclic mono terpenoids, imines, Xray diffraction analysis.

At the present time, palladium complexes of different types, especially palladacycles, are successfully used as catalysts in asymmetric synthesis. 1,2 Despite a large amount of synthesized chiral ligands and complexes on their basis, scientists continuously search for new struc tures possessing high enantioselectivity. Naturally occur ring terpenoids isolated from renewable plant resources are the most attractive starting compounds for the devel opment of chiral ligands.

Scheme 1

Results and Discussion In the present work, the ligand was synthesized start ing from optically pure (–)camphor 1. Oxidation of bromocamphor with air oxygen in DMSO gave (1R,4S) camphorquinone 2, then its imine 3 was obtained accord ing to the procedures described earlier3,4 (Scheme 1). The condensation of 2,3camphorquinone and di phenylmethylamine proceeds regio and stereoselectively and leads to imine 3 in 62% yield. The NMR spectra of this imine exhibit one set of signals, which indicates that only one isomer is formed. The NOESY spectrum of imi ne 3 shows the interaction of benzyl proton H(11) and proton H(4) of the terpene fragment, that confirms the Econfiguration of the imine. * Dedicated to Academician of the Russian Academy of Sciences O. N. Chupakhin on the occasion of his 80th birthday.

Earlier,5—7 we have found that the result of cyclopalla dation of bornane and pinane nitrogencontaining ligands depends on the nature of palladating agent. In our case, the reaction of imine 3 with lithium tetrachloropalladate leads to complex 4 with the monodentately coordinated ligand (Scheme 2). The structure of complex 4 was con firmed by elemental analysis and NMR spectroscopy. The use of palladium acetate leads to cyclopalladation of imino ketone 3 to give two isomeric binuclear pallada cycles 5 and 6, which were separated by column chroma tography in 3 and 61% yields, respectively (see Scheme 2). This reaction has an interesting stereochemical specific feature: the orthopalladation is accompanied by isomer

Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1543—1546, July, 2014. 10665285/14/63071543 © 2014 Springer Science+Business Media, Inc.

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Scheme 2

ization of the starting ligand. In complex 5, we observed structural rearrangement of imine, whereas in compound 6, the configuration of the C=N bond was changed. A new chiral center was formed in both cases. The structure of complex 5 was established by NMR spectroscopy. The stereoselective orthopalladation can be confirmed by the following data. 1H NMR spectrum ex hibits two groups of signals for aromatic protons in the region  6.52—7.65, which correspond to the orthodisub stituted ring (four nonequivalent aromatic protons with indicative splitting (two doublets and two doublets of dou blets)) and the monosubstituted ring (multiplets with in tensities 2 H + 3 H). The shift of the double bond at the N atom was confirmed by the presence of the H(3)—H(4) correlation with the coupling of 4 Hz in the COSY spec trum. Molecular geometry, viz., the (S)configuration of the chiral center C(3) and the arrangement of substituents around the C=N double bond, corresponds to the charac teristic NOE interactions shown in Fig. 1. The (S)con figuration of the chiral center C(3) corresponds to the endosubstituent and the exoproton. If (R)configuration is

assumed, the proton should be in the endoposition, that leads to alternative diastereomer, whose NMR spectra should exhibit another NOE interaction and another split ting pattern of 1H signals. For comparison, we can con sider spectra of model borneol and isoborneol: borneol has the H(2)—H(9) NOE, whereas isoborneol has not. The structure of palladacycle 6 was established by Xray diffraction studies (Fig. 2). Diastereomerically pure com plex 6 was isolated in the crystalline state. First of all, it should be noted that the orthopalladation is accompanied

Cl(1) O(2)

N(2) C(30)

Pd(2) Pd(1)

C(13) N(1)

O(1)

Cl(2)

Fig. 1. Key NOE interactions in palladacycle 5.

Fig. 2. Molecular structure of palladacycle 6 in the thermal el lipsoids of 30% probability and the atom numbering system used in the structural experiment. Hydrogen atoms and the solvate part are omitting.

Cyclopalladation of imine camphorquinone

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by the change of configuration of the imine C=N bond: from the (E) in the free ligand to the (Z) in the complex. Besides, a separation to two enantiomers of prochiral cen ter C(11) in ligand 3 occurs (see Scheme 1), and in the orthopalladated complex 6 atoms C(13) and C(30) are fixed in the (R) and (S)configurations, respectively. The Xray diffraction studies showed that the binucle ar complex with the bridging chlorine atoms crystallizes as a 1 : 1 benzene solvate in the chiral space group of – symmetry P1. However, it should be noted that the greater part of the molecule which includes the metallochelate fragment is centrosymmetric. This causes some difficul ties in the refinement of anisotropic thermal parameters of the corresponding pairs of symmetrically arranged atoms, which were solved by using a command for their enforced alignment (the EADP command of the instruction file .ins of the SHELXTL software8). Besides, a symmetric arrangement of heavy atoms leads to a small Flack abso lute structural parameter with a large error of its determi nation (0.02(3)), which did not allow us to assign the absolute configuration of the molecule using exclusively effect of the anomalous scattering. The complex configu ration was determined based on the known configuration of the camphor fragment. This structure can be also solved – in the centrosymmetric space group P1 with satisfactory convergence parameters, which, however, leads to the ap pearance of statistical disordering of camphor fragment not observed when solved in the chiral group. Thus, a retention of enantiomeric purity of the camphor frag ment involved in the reaction was confirmed. The central ions in the complex have a distorted square surrounding characteristic of Pd atoms. For the com pound, a slight asymmetry of the bond distances involving the bridging chlorine atoms is observed, which is consis tent for the corresponding pairs of atoms: Pd(1)—Cl(1), 2.323(3) Å; Pd(1)—Cl(2), 2.480(3) Å; Pd(2)—Cl(2), 2.316(3) Å; Pd(2)—Cl(1), 2.488(3) Å. Generally, in the structure the bond distances are consistent for the corre sponding symmetric pairs of atoms, the bond distances and bond angles are typical of this class of compounds. Metallochelate cycles are nonplanar, the sp3hybridized atoms C(30) and C(13) with the pseudoaxially arranged phenyl substituents deviate especially strongly from the meansquare plane. The cycle distortion leads to the turn of the plane of the phenylene fragments relative to the plane Pd(1)Cl(1)Pd(2)Cl(2) by 19.5. The oxygen atoms of the carbonyl groups are maximally distant from the metal centers, probably, to avoid unfavorable contacts with the chlorine atoms. Molecular crystal packing is formed by nonspecific van der Waals contacts. The packing does not have important shortened contacts. The packing along the axis 0a forms pores filled with molecules of solvent (benzene). The NMR spectra of palladacycle 6 are difficult to interpret. It is possible that several isomeric forms exist in

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equilibrium in the solution. However, we succeeded in characterization of the predominant component, varying concentration of compound, solvent, and temperature in our NMR experiments. The inversion of configuration of imine is indirectly confirmed by the absence of the NOE interaction between protons H(11) and H(4). The exist ing NOE interaction H(9)—H(11) corresponds to the (S)configuration of the chiral center at position 11 and agrees with the Xray diffraction data. In conclusion, cyclopalladation of prochiral 2,3cam phorquinone 3diphenylmethylimine using palladium ac etate proceeds with high diastereoselectivity. Structural rearrangements of such ligands are known4 and are quite explainable by the fact that in the presence of a base (in this case, acetate ion), the conjugate imine can under go ketoenol rearrangement with the transfer of labile benzyl proton. Experimental IR spectra were recorded on a Shimadzu Prestige 21 IR spectrometer for neat samples or in KBr pellets. 1H and 13C NMR spectra were recorded on a Bruker AVANCEII300 spectro meter (300 (1H) and 75 MHz (13C)) in CDCl3. Residual signals of chloroform (H 7.27, C 77.00), DMSO (H 2.62, C 40.76), and benzene (H 7.36, C 128.37) were used as references. Sig nals were assigned using 13C NMR spectra recorded in the Jmodulation regime and 2D 1H—1H (COSY, NOESY) and 1H—13C (edHSQC, HMBC) correlation spectra, using the Bruk er standard pulse programs. Optical rotation was measured on a Kruss P3002RS automated polarimeter (Germany). Elemental analysis was carried out on an EA 1110 analyzer (CHNSO). Reaction progress was monitored by TLC on Sorbfil plates, using the C6H14—Et2O and benzene—acetone solvent systems. Compounds were visualized by treatment of plates with iodine vapors. Column chromatography was carried out on Alfa—Aesar silica gel (70—230). Diphenylmethylamine was purchased from Acros Organics, camphor 1 was recrystallized from ethanol: []D –40.9 (c 1.0, CHCl3). Palladium chloride and acetate were used without additional purification. (1R,4S)Camphorquinone 2, []D20 –97.8 (c 1.5, toluene) was synthesized according to the known procedure.3 (1R,4S)3Diphenylmethylimino1,7,7trimethylbicyclo[2.2.1] heptan2one (3). (–)Camphorquinone (0.88 g, 5 mmol) and diphenylmethylamine (0.97 g, 5 mmol) were dissolved in an hydrous benzene (20 mL), and a solution obtained was refluxed for 5 h. The solvent was evaporated in vacuo. The reaction prod uct was isolated from the residue by column chromatography (SiO2, eluent a mixture of hexane—diethyl ether). Compound 3 was obtained as yellow crystals. The yield was 1.01 g (62%), []D23 +52.6 (c 0.3, CHCl3). Spectral characteristics of com pound obtained agree with the literature data.4 transDichlorobis[(1R,4S)3diphenylmethylimino1,7,7tri methylbicyclo[2.2.1]heptan2oneN]palladium(II) (4). A suspen sion of palladium(II) chloride (50 mg, 0.3 mmol) and lithium chloride (30 mg, 0.6 mmol) in methanol (5 mL) was refluxed for 1 h on a water bath. A dark red solution of lithium tetrachloro palladate was added to a solution of imine 3 (0.11 g, 0.3 mmol) in methanol (2 mL), the mixture was stirred at room temperature

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for 3 h, during which it acquired dark red color. The solvent was evaporated in vacuo, the residue was dissolved in benzene and purified by chromatography (SiO2, eluent benzene—acetone, 10 : 1). The solvent was evaporated, a bright red residue contained (the TLC data: Sorbfil, benzene—acetone, 10 : 1, visualization in iodine vapors) the product with Rf 0.7. Complex compound 4 was isolated by crystallization from the system benzene—hexane as a yellow powder, soluble in chloroform, acetone, benzene, DMSO. The yield was 130 mg (52%), m.p. 157—158 C (with decomp.), []D20 +87.7 (c 0.2, CHCl3). Found (%): C, 67.8; H, 6.69; N, 3.37. C46H50Cl2N2O2Pd. Calculated (%): C, 68.1; H, 6.65; N, 3.45. IR, /cm–1: 1751 (C=O), 1641 (C=N). 1H NMR (DMSOd6), : 0.61 (s, 3 H, C(9)Me); 0.93 (s, 3 H, C(10)Me); 0.94 (s, 3 H, C(8)Me); 1.02 (m, 1 H, Hn(5)); 1.34 (m, 1 H, Hn(6)); 1.77 (m, 1 H, He(6)); 1.95 (m, 1 H, He(5)); 3.22 (d, 1 H, H(4)); 5.81 (s, 1 H, H(11)); 7.15—7.45 (m, 10 H, H arom.). 13C NMR (DMSOd ), : 9.31 (C(10)); 17.63 (C(8)); 20.65 6 (C(9)); 23.32 (C(5)); 30.07 (C(6)); 44.17 (C(1)); 49.29 (C(4)); 58.01 (C(7)); 69.69 (C(11)); 127.38, 127.51 (C(15), C(15´)); 127.55, 127.86 (C(13), C(13´)); 128.86, 128.95 (C(14), C(14´)); 144.02, 144.21 (C(12), C(12´)); 171.85 (C(3)); 206.73 (C(2)). Complexes 5 and 6. A suspension of imine 3 (0.12 g, 0.4 mmol) and palladium acetate (0.1 g, 0.4 mmol) in benzene (15 mL) was heated for 3 h at 60 C. The solvent was evaporated in vacuo. A solution of lithium chloride (0.08 g, 0.2 mmol) in methanol was added to the residue, and the mixture was stirred for 2 h at room temperature. Formation of two products was observed in the course of the reaction. The solvent was evaporated in vacuo from the dark red reaction mixture, the residue was dissolved in benzene and purified by chromatography (SiO2, eluent benz ene—acetone). After evaporation of the solvent and recrystalli zation from a mixture of benzene—hexane, complex compounds 5 and 6 were obtained in separate fractions with Rf 0.2 and 0.7 as yellow powders. Dichlorobis(2{[(1R,3S,4S)2oxo1,7,7trimethylbicyclo [2.2.1]hept3ylimino](phenyl)methyl}phenylC,N)dipalladium(II) (5). A yellow powder, soluble in chloroform, acetone, benzene, DMSO, Rf 0.2 (Sorbfil, benzene—acetone, 10 : 1, visualization in iodine vapors), the yield was 6 mg (3%). 1H NMR (CDCl3), : 0.87 (s, 3 H, C(8)Me); 0.99 (s, 3 H, C(9)Me); 1.06 (s, 3 H, C(10)Me); 1.32 (m, 1 H, H(4)); 1.45—1.55 (m, 2 H, H(5)); 1.68 (m, 1 H, Hn(6)); 1.87 (m, 1 H, He(6)); 5.23 (d, 1 H, H(3), J = 4.0 Hz); 6.52 (d, 1 H, J = 7.4 Hz), 6.84, 6.94 (both dd, 1 H each, J = 7.4 Hz, J = 7.6 Hz), 7.61 (d, 1 H, J = 7.6 Hz) (odisubstituted benzene); 7.47 (d, 2 oH, J = 7.5 Hz), 7.55—7.65 (m, 3 H) (monosubstituted benzene). 13C NMR (CDCl3),  (without quaternary C toms): 8.3 (C(10)); 18.9 (C(8)); 19.5 (C(5)); 20.7 (C(9)); 34.5 (C(6)); 48.9 (C(4)); 76.4 (C(3)); 124.5, 130.1, 130.8, 136.4 (CHCPd) (odisubstituted benzene); 126.4, 128.8, 130.0 (monosubstituted benzene). Dichlorobis(2{(S)[(1R,4S)2oxo1,7,7trimethylbi cyclo[2.2.1]hept3ylidenamino](phenyl)methyl}phenylC,N) dipalladium(II) (6). A yellow powder, soluble in chloroform, ace tone, benzene, DMSO, Rf 0.7 (Sorbfil, benzene—acetone, 10 : 1, visualization in iodine vapors), the yield was 200 mg (61%), m.p. 165—166 C (with decomp.), []D23 +175.3 (c 0.4, CHCl3). Found (%): C, 58.36; H, 5.12; N, 3.03. C46H48Cl2N2O2Pd2. Calculated (%): C, 58.48; H, 5.08; N, 2.98. IR, /cm–1: 1745 (C=O), 1655 (C=N). 1H NMR (C6D6), : 0.60 (s, 3 H, C(9)Me); 0.64 (s, 3 H, C(8)Me); 0.82 (s, 3 H, C(10)Me); 1.06 (m, 1 H, Hn(6)); 1.31 (m, 1 H, He(6)); 1.64 (m, 1 H, Hn(5)); 1.91 (m, 1 H,

He(5)); 3.80 (d, 1 H, H(4), J = 4.5 Hz); 6.92, 6.95, 7.06, 8.32 (all m, 4 H, C6H4); 6.97, 7.27, 7.65 (all m, 5 H, C6H5); 7.67 (s, 1 H, H(11)). 13C NMR (C6D6), : 8.97 (C(10)); 17.01 (C(8)); 20.41 (C(9)); 24.65 (C(5)); 29.40 (C(6)); 44.09 (C(7)); 58.19 (C(4)); 59.94 (C(1)); 78.40 (C(11)); 121.40, 125.69, 127.60, 128.82 (CH disubstituted benzene); 125.57, 128.69, 132.0 (CH mono substituted benzene); 133.70 (CAr); 141.73 (CAr); 150.78 (C—Pd); 176.09 (C(3)); 200.92 (C(2)). Xray diffraction studies of palladacycle 6 was performed on Xcalibur 3 automated single crystal diffractometer (T = 295(2) К, MoK radiation, graphite monochromator, /2 scanning in the region 2.89 <  < 33.54). A pale yellow crystal used for the analysis of the compound had the size of 0.22×0.11×0.03 mm, a correction for absorption was made analytically, using a poly hedral crystal model ( = 0.913 mm–1). The structure was solved and refined using the SHELXTL software.8 The structure was solved by direct method and refined by the fullmatrix least squares method on F2 in anisotropic approximation for all the nonhydrogen atoms. Hydrogen atoms were found from the peaks of electron density and included in refinement in isotropic ap proximation, using a riding model. According to the Xray – diffraction data, the crystal is triclinic, space group P1 , a = 8.9986(8) Å, b = 10.3109(14) Å, c = 14.9176(13) Å,  = = 94.803(12),  = 104.726(8),  = 115.035(10), V = 1183.6(2) Å3, dcalc = 1.435 g cm–3. There were collected 15056 reflections, 9571 of them were independent (Rint = 0.0271), including 5451 with I > 2(I). Completeness of the set for  < 26.00 was 99.1%. The final parameters of refinement: S = 1.015, R1 = 0.0320, wR2 = 0.0574 (for reflections with I > 2(I)), R1 = 0.0707, wR2 = = 0.0599 (on all the reflections). The maximal and minimal peaks of residual electron density were 0.981 and –0.483 e Å–3.

This work was financially supported by the Russian Foundation for Basic Research and the Government of the Komi Republic (Project No. 130398805 r_sever). References 1. J. Dupont, C. S. Consorti, J. Spencer, Chem. Rev., 2005, 105, 2527. 2. V. V. Dunina, O. N. Gorunova, P. A. Zykov, K. A. Kochet kov, Russ. Chem. Rev., 2011, 80, 53. 3. K. Hattori, T. Yoshida, K. Rikuta, T. Miyakoshi, J. Chem. Soc. Jpn. Chem. Lett., 1994, 1885. 4. G. Cainelli, D. Giacomini, A. Trere, P. P. Boyl, J. Org. Chem., 1996, 61, 5134. 5. O. A. Zalevskaya, Y. A. Gur´eva, L. L. Frolova, I. N. Alek seev, A. V. Kutchin, Nat. Sci., 2010, 2, 1194. 6. A. V. Kutchin, Ya. A. Gur´eva, L. L. Frolova, I. N. Alekseev, O. A. Zalevskaya, Russ. Chem. Bull. (Int. Ed.), 2013, 62, 745 [Izv. Akad. Nauk, Ser. Khim., 2013, 745]. 7. Ya. A. Gur´eva, O. A. Zalevskaya, L. L. Frolova, I. N. Alek seev, P. A. Slepukhin, A. V. Kutchin, Zh. Obshch. Khim., 2012, 82, 1117 [Russ. J. Gen. Chem. (Engl. Transl.), 2012, 82]. 8. G. M. Sheldrick, Acta Crystallogr., Sect. A, 2008, 64, 112.

Received March 7, 2014; in revised form May 15, 2014