New optically active camphor-derived cyclopalladated complexes with an asymmetric carbon bonded to the metal

Journal of Organometallic Chemistry 900 (2019) 120917

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New optically active camphor-derived cyclopalladated complexes with an asymmetric carbon bonded to the metal Purna Chandra Rao Vasireddy a, Gerard C. Dickmu a, Angel Ugrinov b, Irina P. Smoliakova a, * a b

Department of Chemistry, University of North Dakota, 151 Cornell Street, Mail Stop 9024, Grand Forks, ND, 58202-9024, USA Department of Chemistry and Biochemistry, North Dakota State University, Dept. 2735, Fargo, ND, 58108-6050, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 August 2019 Received in revised form 25 August 2019 Accepted 26 August 2019 Available online 27 August 2019

N,N-Dimethylhydrazone of D-camphor 1 was obtained as the single E isomer in 80% yield by treating the enantiopure ketone with N,N-dimethylhydrazine in the presence of an equimolar amount of p-toluenesulfonic acid in ethanol. Direct cyclopalladation of hydrazone 1 was accomplished at the C(3)H2 group using [Pd(MeCN)2Cl2] and NaOAc in MeCN at the reflux temperature. The product of the reaction, dinuclear cyclopalladated complex 2, was isolated in 89% yield as a mixture of diastereoisomers, which differ by the absolute configuration of the chiral carbon bound to the metal. Compound 2 was converted to the mononuclear complexes 3 and 4 by treating the dimer with PPh3 and Na(acac), respectively. Compounds 3 and 4 were also mixtures of two diastereomers with the Pd atom either in endo or exo position at the new chiral center in the norbornane moiety. Isomers of complex 3 were partly separated by column chromatography to obtain samples of (1S,2S,4R)-3 and (1S,2R,4R)-3 with 96% and 86% de, respectively. The structures of all new compounds were supported by 1H, 13C{1H}, and 31P{1H} spectra as well as 1D NOE experiments. X-ray crystallographic data of complexes 2, (1S,2S,4R)-3 and (1S,2R,4R)-3 supported the cyclopalladation at the C(3)H2 group of the ligand and the absolute configuration of the Pd-bound chiral carbon in both endo and exo palladacycles. © 2019 Elsevier B.V. All rights reserved.

Keywords: Cyclopalladated complex Camphor N,N-dimethylhydrazone (sp3)C
Pd bond Metalation of a secondary carbon Pd bonded to a chiral carbon

1. Introduction Direct cyclopalladation via (sp3)CeH bond activation using Pd(II) salts remains challenging despite a number of studies focused on this topic. As a rule, cyclopalladation at a primary (sp3)C is the most straightforward task [1e7a], while the formation of a tertiary (sp3)CePd bond requires special conditions [8]. Cyclopalladation of alkyl groups is likely to be successful when (sp3)CeH bond activation takes place (i) at the benzylic position [1,9], (ii) at the tert-butyl [3c,d,4] or a structurally similar [5e7a] fragment, (iii) next to a heteroatom or an electron-withdrawing group [2,3,10,11], or (iv) when a pincer complex can be produced [7b,8,12]. Palladacycles formed via secondary (sp3)CeH bond activation are relatively rare [1a,7,9e11], and all of them were formed from the ligands with the features mentioned above. Metalation at the CH2 group of nonsymmetrical ligands deserves special attention because it results in

* Corresponding author. E-mail address: [email protected] (I.P. Smoliakova). https://doi.org/10.1016/j.jorganchem.2019.120917 0022-328X/© 2019 Elsevier B.V. All rights reserved.

the formation of a stereogenic center. The presence of a chiral center near the metal is likely to increase chirality induction in asymmetric reactions catalyzed by such complexes. Also, optically active palladacycles with a chiral center bonded to the metal are excellent models for studying mechanisms, including stereochemistry, of various known transformations at a CePd bond. Such reactions, in their turn, may be used to predict stereoselectivity of Pd-catalyzed reactions occurring with the formation of palladacycles as intermediates. So far, only a limited number of chiral nonracemic cyclopalladated complexes with an asymmetric carbon bonded the metal have been reported [1a,7,9a,c,10a,c,d,11] (Chart 1). All of them were formed by direct cyclopalladation of an (sp3) C atom. There are also racemic cyclopalladated complexes with an asymmetric carbon attached to the metal. They were obtained either through CeH bond activation [9b,d,10b,13] or transmetalation [14]. Previously, the Kuchin group and we reported direct cyclopalladation of N-benzyl imine and oximes of D-camphor [6b,c] and closely related L-fenchone [6a] (Chart 2, structures VIIIa,b and IXa,b). In all the cases, metalation occurred at the CH3 group

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P.C.R. Vasireddy et al. / Journal of Organometallic Chemistry 900 (2019) 120917

Chart 1. Optically active cyclopalladated complexes with the metal bonded to a chiral center and obtained by secondary (sp3)CeH bond activation using Pd(II) salts.

Chart 2. Reported (VIIIa,b [6b,c] and IXa,b [6a]) and possible (VIIIc and X) palladacycles derived from N-benzylimine, O-methyloxime, and N,N-dimethylhydrazone of Dcamphor and L-fenchone.

attached to one of the two bridgehead quaternary carbons of the norbornane framework. In the present study, we report the regioselective cyclopalladation of D-camphor N,N-dimethylhydrazone at a CH2 group resulting in metallacycle X (Chart 3) with a new chiral center bonded to a metal. 2. Results and discussion 2.1. Cyclopalladation of N,N-dimethylhydrazone of D-camphor N,N-Dimethylhydrazone of D-camphor (1, Scheme 1) was synthesized from the ketone and N,N-dimethylhydrazine in the presence of p-toluenesulfonic acid (p-TSA) using a modified procedure reported by Chelucci et al. [15]. In our hands, the 18-h reaction using a stoichiometric amount of p-TSA afforded a higher yield of the hydrazone, 80% compared to a catalytic amount of the acid and eight days recommended in the procedure. Compound 1 was isolated as a single isomer (1H and 13C{1H} NMR data). An NOE test was carried out to determine whether the compound has either E or Z geometry. Irradiation of the protons on the NMe2 group showed a positive NOE for the signal of the endo hydrogen on C(3) suggesting that the hydrazone has the E geometry. The hydrazone moiety is a well-known directing group in

cyclopalladation [3]. Reported hydrazone-based palladacycles with an (sp3)CePd bond are five-membered and have either the (sp2)N or (sp3)N atom forming the dative bond with the metal (Chart 3). To form the dinuclear derivative XI, either [Na2PdCl4] [3d,e] or the coordination complex [Pd(HL)2Cl2] (HL ¼ pinacolone) [3c] in MeOH in the presence of NaOAc were used [3a,b]. Compound XII was obtained by reacting pinacolone with [Pd(PhCN)2Cl2] in benzene for seven days [3c]. Mononuclear complexes of type XIII were prepared by treating the corresponding hydrazones with [Pd(PPh3)2Cl2] and NaOAc in MeCN at 65e75  C for 24e48 h [3a,b]. It is noteworthy that the reported attempt to form a cyclopalladated complex by reacting N,N-dimethylhydrazone of D-camphor with [Pd(PPh3)2Cl2] was unsuccessful [3a]. Assuming the formation of a five-membered metallacycle, direct cyclopalladation of N,N-dimethylhydrazone of D-camphor (1) may result in the formation of two types of palladacycle, VIIIc and X (Chart 2), with an (sp2)NePd bond and an (sp3)NePd bond, respectively. We tested three Pd(II) sources and different conditions in reaction with hydrazone 1 (Scheme 2). The successful results are summarized in Table 1. The best yield, 89%, of the cyclopalladated complex 2 was achieved with [Pd(MeCN)2Cl2]/NaOAc in MeCN. It is noteworthy that the use of AcOH as a solvent resulted in deprotection of the carbonyl group to give D-camphor. All successful cyclopalladation reactions of 1 were regiospecific and resulted in the formation of only one type of palladacycle, X. This conclusion was made based on the 1H and 13C{1H} NMR spectra of the isolated product 2, which showed signals of the three methyl groups on the camphor moiety in addition to the nonequivalent methyl groups on the (sp3)N atom. Overall, the 1H

Scheme 1. Synthesis of D-camphor N,N-dimethylhydrazone 1.

P.C.R. Vasireddy et al. / Journal of Organometallic Chemistry 900 (2019) 120917

Chart 3. Known hydrazone-derived palladacycles with an (sp3)CePd bond.

Scheme 2. Cyclopalladation of D-camphor N,N-dimethylhydrazone 1.

and 13C{1H} NMR spectra of 2 were somewhat challenging to interpret. The spectra complexity appears to be due to the presence of stereoisomers of 2, which differ from each other by the absolute configuration of the new stereogenic carbon center as well as cis and trans forms of the dimeric isomers. Repeated attempts to get preferentially or exclusively one of the stereoisomers of complex 2 by varying the reagents and reaction conditions gave the same mixture (1H and 13C{1H} NMR data). Our effort to separate the isomers by preparative thin-layer chromatography (TLC) or recrystallization using different solvents were unsuccessful. In an attempt to form a single isomer, a solution of 2 in MeOH was refluxed for 4 h. However, instead of the isomerization, deprotection of the carbonyl group took place, and D-camphor and unidentified products were formed. Refluxing the complex over 6 h in aprotic solvents like MeCN and toluene also led to the breakdown of the complex with the deposition of Pd black. In order to simplify 1H and 13C{1H} NMR spectra, complex 2 was reacted with 2 equiv. of PPh3 in acetone to give the mononuclear adduct 3 (Scheme 3). The 1H and 13C{1H} NMR spectra of complexes 3 showed a mixture of two stereoisomers in a 1:1 isomeric ratio, one with an endo C
Pd bond [endo-3 or (1S,2S,4R)-3] and the other with an exo C
Pd bond (exo-3 or (1S,2R,4R)-3, Scheme 3) in the norbornane moiety. The diastereomers were partly separated by column chromatography on silica gel using a 1:4 mixture of ethyl acetate and hexanes. According to the 1H NMR spectra, the isomeric purity of endo-3 and exo-3 were 96 and 86% de, respectively. Similar to dimer 2, other attempts to get diastereomerically pure samples of complex 3 were unsuccessful. The solutions of endo(96% de) and exo-3 (86% de) in CDCl3 slowly epimerize at rt and became 1:1 mixtures in two weeks. After the third week, the 1H NMR spectrum contained several new signals suggesting decomposition of the isomers. Some of the new signals were identified as those belonging to triphenylphosphine oxide and dimer 2. Neither epimerization nor degradation was observed for isometrically

3

enriched samples of endo- and exo-3 kept in C6D6 solutions for five months at þ5  C. Dinuclear complex 2 was also converted to the mononuclear acetylacetonate adducts endo-4 [(1S,2S,4R)-4] and exo-4 [(1S,2R,4R)-4] in 96% yield by stirring with 3 equiv. of Na(acac) in acetone (Scheme 3). The 1H and 13C{1H} NMR spectra of complex 4 showed a mixture of two diastereomers, which differ by the absolute configuration of C(2). Attempts to isolate pure isomers using preparative TLC were unsuccessful. The best ratios of endo-4 and exo-4, 83:17 (de 66%) for the former and 19:81 (de 62%) for the latter, were obtained using column chromatography on SiO2. The 83:17 mixture of endo-4 and exo-4 was kept in C6D6 for 3 weeks at rt and then analyzed by NMR spectroscopy. The 1H NMR spectrum showed a 3:2 ratio of the endo and exo isomers suggesting a slow isomerization of the former compound to the latter at rt. Two isomers, exo-4 and endo-4, can be differentiated by a distinct 1H NMR signal of the hydrogen bonded to C(2). The singlet at d 3.99 ppm was assigned to isomer exo-4 with the endo hydrogen at C(2) since no coupling is expected between hydrogens of C(2) and C(1). The exo hydrogen of C(2) in endo-4 was coupled with both the hydrogen of C(1) and the exo hydrogen of C(6) providing a triplet at d 4.82 ppm with a coupling constant of 3.5 Hz. These data point to the fact that endo-4 has the C‒Pd bond in endo position, while the other isomer has the metal in the exo position. The identity of the endo isomer could further be confirmed by comparing the 1H NMR signals of the three methyl groups of the camphor fragment as they appear in three compounds: hydrazone 1 and complexes endo-4 and exo-4. The three methyl groups of the camphor moiety of the endo isomer are arranged in a similar pattern to those of free hydrazone 1 while those in the exo isomer are not. The interaction of the acac ligand with the (pro-S)-Me group bonded to C(7) in the exo isomer led to a downfield shift of its 1 H NMR signal. 2.2. X-ray crystallographic studies of CPC 2 and 3 The X-ray single crystal studies of complexes 2 and 3 unambiguously proved their cyclopalladated structure and the palladation at the CH2 group next to the hydrazone functionality. The molecular structures of the compounds and the numbering schemes are presented in Figs. 1 and 2. To the best of our knowledge, there are no reported crystal structures for CN-CPCs with a 2 (sp3)CePd bond; however, there are X-ray data for the related CNNpincer complex (1S,2R,4S,6S)-VI with PPh3 as the auxiliary ligand (see Chart 1). Several crystallographic studies of chloro-bridged dimeric five-membered CN-CPCs with 1 (sp3)CePd bonds have been reported, including structures 5e10, which are used for comparison in this section (Chart 4) [3c,e,6a,16,17]. Two of these studies describe the molecular structures of cyclopalladated hydrazones with an (sp3)CePd bond (6 and 7); these hydrazones were obtained from tert-butyl methyl ketone [3c]. Complex 2 crystallizes from hexane/dichloromethane in the monoclinic crystal system and the space group P21. The dimeric molecule consists of two independent halves, which are slightly different in their structural parameters. One of the two palladacycles has the S absolute configuration of the chiral carbon, C2,

Table 1 Cyclopalladation of D-camphor N,N-dimethylhydrazone 1. Entry

Pd source/base

Reaction temp. ( C)

Reaction time (h)

Solvent

Yield of 2 (%)

1 2 3 4

Pd(OAc)2 [Na2PdCl4]/NaOAc Pd(OAc)2 [Pd(MeCN)2Cl2]/NaOAc

reflux reflux 60 reflux

4 4 6 0.5

MeCN MeCN PhMe MeCN

44 72 46 89

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Scheme 3. Reactions of complex 2 with PPh3 and Na(acac).

Fig. 1. ORTEP drawing of the molecular structure of CPC 2. Thermal ellipsoids are shown at the 50% probability level.

bonded to the metal and, therefore, can be described as the endo metallacycle. The second metal-containing five-membered ring can be described as the exo palladacycle with the R configuration of the stereogenic center, C2A. The structure showed trans geometry of the cyclopalladated ligands typical for the majority of known chloro-bridged CN-CPCs with a five-membered palladacycle in the solid state. The Pd2Cl2 ring in 2 is almost planar as in many other chloro-bridged CN-CPCs with trans configuration. Four torsion angles in the Pd2Cl2 ring are between 2.17 and 2.38 . The Pd…Pd distance in the complex is 3.466 Å, which is similar to those reported for CN-CPCs with trans-geometry [18]. For comparison, the closest analog 10 has a Pd…Pd distance of 3.500 Å [6a] while the other related complex 9 possesses very rare cis ligand geometry in the solid state and displays a significant bending of the Pd2Cl2 ring that results in an unusually short Pd…Pd distance of 2.99 Å [3e]. The PdeCl bond trans to the metalated carbon in CPC 2 is longer, 2.4906 Å (here and later, the given values represent the average of two numbers obtained for each half of the dimeric molecule), than that trans to the nitrogen, 2.3374 Å (D 0.1532 Å). Similar findings were reported for trans complexes 5e10, in which the PdeCl bond length differences are 0.188, 0.169, 0.156, 0.162, 0.205, and 0.128 Å, respectively [3e,6a,16,17]. For three representative chloro-bridged dimeric CN-CPCs with (i) the (sp2)CePd bond, (ii) a fivemembered palladacycle and (iii) trans ligand geometry, the difference between two PdeCl bonds (cis and trans to the aromatic carbon) has also been observed, although that difference is smaller, 0.1053, 0.125 and 0.131 Å [19e22]. These data reflect a stronger trans influence of (i) the carbon donor atom compared to nitrogen and (ii) the (sp3)C atom compared to (sp2)C. Complex 3 crystallizes from ethyl acetate at rt in the space group P1. Two isomers, endo-3 and exo-3, were found in a crystallographic unit cell. Both structures have the NP-trans geometry typical for mononuclear CN-complexes with PPh3 as the auxiliary

ligand. The (sp3)CePd bond length in complexes 2, endo-3, and exo-3 varies noticeably. First, these lengths are shorter in the dimer regardless of the metallacycle type. Secondly, the CePd distances in the endo palladacycles of complexes 2 and endo-3 are 1.982 and 2.031 Å, while in the corresponding exo metallacycles those lengths are longer, 2.006 and 2.072 Å, respectively. The last value is higher than those reported for complexes 5e10 and VI (1.959e2.034 Å). The (sp3)NePd bonds in 2, exo-3 and endo-3 are a little bit longer than the (sp3)CePd bonds in the corresponding metallacycles that is typical for chloro-bridged dimeric CN-CPCs with the (sp2)N and (sp3)C or (sp3)N and (sp3)C donor atoms and transgeometry of cyclopalladated ligands [6b,19e21,23]. In dimer 2, both NePd distances are practically the same: 2.077 Å in the endo palladacycle and 2.078 Å in the exo analog. As in the case of the CePd bond, the PdeN distances in the mononuclear complexes endo- and exo-3 are longer than those in the dimer, 2.144 and 2.157 Å, respectively. For comparison, the (sp3)NePd bond length in the closest analog 7 is 2.063(1) Å [3c]. The C-Pd-N bite angles in both palladacycles of complex 2 are practically the same: 80.81 for the endo palladacycle and 80.80 for the exo analog. The C-Pd-N torsion angles in mononuclear exoand endo-3 slightly differ from each other: 80.42 and 81.16 , respectively. These four values fall in the range reported for compounds 5e7 and 10: 84.5(1), 80.7(7), 82.9(6), and 82.14(10)o, respectively [3c,6a,16]. In complex 8 with a silicon atom in the metallacycle, the angle reaches 86.81(9)o [17]. For comparison, the C-Pd-N angle for chloro-bridged CPCs with the (sp2)N and (sp2)C donor atoms varies from 80.3 to 81.2 [19e21]. For the corresponding complexes with (sp3)N and (sp2)C, the C-Pd-N bite angle is slightly larger, 80.6e82.8 [18,23]. The palladium atoms in complexes 2, endo-3, and exo-3 are nearly in square-planar coordination with a slight tetrahedral

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5

The endo and exo metallacycles of dimer 2 can be described as slightly twisted envelopes with C2 and Pd1A serving as the envelope flaps. The distortion of each metallacycle from planarity was estimated using the sum of absolute values of intrachelate torsion angles [25]. The sums are equal to 89.68 and 72.32 for the endo and exo palladacycles in the dimer. These values are similar to those calculated for the palladacycles in endo-3 and exo-3, 88.26 and 71.60 . A similar distortion of the CN-palladacycle was reported for the oxime camphor-derived complex 10. Interestingly, the palladacycle's distortion in camphor-derived complexes 2, endo-3, exo3, and 10 is significantly less than that reported for the aminederived dimer 5, 158 [16]. It is noteworthy that the NeN distances in the structures of the exo and endo palladacycles of complex 2 are unusually long, 1.489 and 1.495 Å. The same distances in exo-3 and endo-3 are also above average, 1.492 and 1.485 Å, respectively. For comparison, the NeN bond length in the known compound 7 is only 1.45(2) Å [3c]. It appears that there is only one reported complex, a cationic Pt(II) N,N-dimethylhydrazone derivative, with the NeN bond longer than that found in compound 2, 1.497(6) Å [26]. 3. Conclusions Enantiopure D-camphor N,N-dimethylhydrazone 1 was synthesized as the single E isomer in high yield. Direct cyclopalladation of hydrazone 1 with either [Pd(MeCN)2Cl2], Pd(OAc)2 or [Na2PdCl4] took place at a secondary carbon affording a new optically active aliphatic cyclopalladated complex 2 with an asymmetric carbon bonded to the metal. Complex 2 appeared to be a mixture of cis/ trans isomers having palladacycles with the exo and endo positions of the CePd bond. Diastereomeric mixtures of mononuclear complexes 3 and 4 with PPh3 and acetylacetonato ligands, respectively, were partially separated. It appears that epimerization of endo- and exo- complexes of 3 and 4 slowly occurs in CDCl3 and benzene at rt, while the isomers are stable in solid form at rt and in C6D6 solutions at þ5  C. X-ray single crystal structures for dimer 2 and mononuclear complexes endo-3 and exo-3 represent the first crystallographic data for a C,N-palladacycle with the metal directly connected to a chiral center. Fig. 2. ORTEP drawings of the molecular structures of two complexes, endo-3 (A) and exo-3 (B) found in the same crystallographic unit. (The structures of two compounds are shown separately for better visualization.) Thermal ellipsoids are shown at the 50% probability level.

4. Experimental

distortion. The angles between the planes {N-Pd-C} and {Cl-Pd-Cl} for the endo palladacycles in two compounds, endo-3 and 2, are only 6.00 and 6.02 . The angles between the corresponding planes determined for the exo palladacycles in two complexes, exo-3 and 2, are even smaller, 3.30 and 3.61, respectively. Such almost ideal square-planar geometry has been reported for many aliphatic palladacycles [24].

Materials. The laboratory-grade D-camphor (Alfa Aesar), [PdCl2(MeCN)2] (Strem Chemicals), N,N-dimethylhydrazine (Acros Organics), and anhydrous NaOAc (Sigma-Aldrich, 99%) were used as purchased. PPh3 (Sigma Aldrich) was recrystallized from ethanolwater. Pd(OAc)2 (Strem Chemicals) was dissolved in hot benzene, and the resulting solution was filtered. After filtration, benzene was removed using a rotavapor, and Pd(OAc)2 was dried in vacuum. Na(acac) was synthesized using a published procedure [27]. All

4.1. General methods and material

Chart 4. Examples of chloro-bridged dimeric CN-CPCs with an (sp3)CePd bond and a known molecular structure.

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solvents were purified using standard methods [28]. Deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc. and kept over molecular sieves 4 Å. General Methods and Instrumentation. Reactions were monitored using Merck TLC aluminum sheets precoated with silica gel 60 F254. Column purifications were carried out by using 100e200 mesh silica gel from Natland International Corporation. 1 H, 13C{1H}, 31P{1H}, DEPT, COSY, HMQC, and 1D NOE NMR spectra were recorded on a Bruker AVANCE 500 NMR spectrometer. Chemical shifts are reported in ppm relative to SiMe4 as an internal standard for 1H and 13C{1H} NMR spectra and P(OEt)3 as an external standard for 31P{1H} NMR spectra. Coupling constants, J, are given in Hz. IR spectra were recorded using a Thermo Scientific Nicolet iS5 FT-IR spectrometer. Optical rotations were measured using a Jasco P-2000 polarimeter. Melting points were determined using a Melt-Temp II apparatus. CHN elemental analyses were carried out by Atlantic Microlabs, Inc, Norcross, GA. 4.2. Preparation of complexes and their spectra (E)-1,1-Dimethyl-2-[(1S,4R)-bicyclo[2.2.1]heptan-2-ylidene]hydrazine (D-Camphor N,N-Dimethylhydrazone) (1). Monohydrate of p-toluenesulfonic acid (6.24 g, 32.8 mmol) and N,N-dimethylhydrazine (5.74 mL, 75.4 mmol) were added to a solution of Dcamphor (4.99 g, 32.8 mmol) in ethanol (40 mL) at rt. The reaction mixture was reflux for 18 h and then cooled to rt. The solvent was removed on a rotavapor, then water (50 mL) was added to the residue. The mixture was extracted with ethyl acetate (3  20 mL). Organic layers were combined and washed with saturated aq. NaHCO3 solution (2  25 mL) followed by water (30 mL) and saturated brine solution (30 mL). The organic layer was dried over anhydrous Na2SO4, filtered and concentrated in vacuum to yield 5.2 g (80%) of compound 1 as a colorless liquid. Rf 0.52 (1:4,
1 EtOAcehexanes); [a]22 D ¼ þ30.5 (c 25.6, acetone). IR (neat, n, cm ): 1 1665 (C]N). H NMR (d, ppm, C6D6): 0.72 (s, 3H, C(9)H3), 0.73 (s, 3H, C(10)H3), 0.99e1.06 (m, 1H, exo-C(6)H), 1.16 (s, 3H, C(8)H3), 1.39 (ddd, 2Jendo-5,exo-5 ¼ 12.5, 3Jexo-5,exo-6 ¼ 9.5, 4Jexo-5,exo-3 ¼ 4, 1H, exoC(5)H), 1.53 (td, 2Jendo-5,exo-5 ¼ 3Jendo-5,endo-6 ¼ 12.5, 3Jendo-5,exo6 ¼ 3.4, 1H, endo-C(5)H), 1.58e1.65 (m, 2H, C(4)H and endo-C(6)H), 1.99 (d, 2Jendo-3,exo-3 ¼ 18, 1H, endo-C(3)H), 2.48 (s, 6H, N(CH3)2), 2.53 (dt, 2Jendo-3,exo-3 ¼ 18, 3Jexo-3,4 ¼ 4Jexo-3,exo-5 ¼ 4, 1H, exo-C(3)H). 13 1 C{ H} NMR (d, ppm, C6D6): 12.3 (C8), 19.2 (C9), 19.9 (C10), 28.1 (C6), 33.3 (C5), 36.1 (C3), 44.7 (C4), 47.7 (N(CH3)2), 52.8 (C1 or C7), 177.0 (C(2) ¼ N). Di-m-chlorobis[(1S,2R,4R)and (1S,2S,4R)-3-(2,2dimethylhydrazono)-4,7,7-trimethylbicyclo[2.2.1]heptan-2-yl-C,N] dipalladium(II) (2). Anhydrous NaOAc (0.063 g, 0.77 mmol) and [Pd(MeCN)2Cl2] (0.20 g, 0.77 mmol) were added to a solution of hydrazone 1 (0.15 g, 0.77 mmol) in MeCN (15 mL). The mixture was refluxed for 30 min, cooled to rt, and then passed through a plug of celite. The filtrate was concentrated under reduced pressure. Icecold water (15 mL) was added to the residue, and the resulting mixture was stirred for 10 min. The solid formed was filtered and dried. Then ice-cold ethyl acetate (5 mL) was added to the crude product, and the mixture was stirred for 5 min at 0e5  C. The yellow solid was collected and dried to yield 0.23 g (89%) of compound 2. According to 13C{1H} NMR data, the product exists in solution as a mixture of at least 4 isomers. The following data are given for the mixture as a separation of these isomers was unsuccessful. Mp 209e210  C; Rf 0.4 (1:3 EtOAcehexanes); [a]21 D ¼ þ0.0795 (c 5.12, chloroform). IR (thin film in mineral oil, n, cm
1): 1667 (C]N). 1H NMR (d, ppm, C6D6, integration values are tentative): 0.62e0.71 (m, 9H), 0.93e0.95 (m, 3H), 1.00 (m, 3H), 1.04e1.41 (m, 4H), 1.41e1.60 (m, 5H), 1.72e1.86 (m, 1H), 1.95e2.23 (m, 2H), 2.22e2.40 (m, 7H), 2.93e3.06 (m, 6H), 4.42e4.29 (m, 1H), 5.07e5.23 (m, 1H). 13C{1H}

NMR (d, ppm, C6D6): 11.23, 11.28, 12.39, 19.77, 19.80, 20.27, 20.30, 20.34, 20.37, 21.12, 21.15, 21.42, 21.45, 21.53, 26.35, 26.38, 26.41, 29.00, 29.07, 30.72, 30.79, 37.01, 37.07, 47.87, 48.96, 49.29, 49.35, 50.56, 50.67, 50.75, 50.95, 51.15, 51.32, 51.38, 51.48, 51.53, 51.57, 51.60, 52.07, 52.15, 52.52, 52.64, 53.35, 53.44, 53.82, 53.89, 53.99, 54.24, 54.46, 54.81, 54.85, 195.91, 196.13, 196.85, 196.99. Anal. Calcd for C24H42Cl2N4Pd2: C 43.00, H 6.32, N 8.36. Found: C 43.18, H 6.18, N 8.38. Chloro[(1S,2S,4R)- and (1S,2R,4R)-3-(2,2-dimethylhydrazono)4,7,7-trimethylbicyclo[2.2.1]heptan-2-yl-C,N](triphenylphosphine-P) palladium(II) (endo-3 and exo-3). Triphenylphosphine (0.0401 g, 0.153 mmol) was added to a yellow suspension of complex 2 (0.0492 g, 0.0733 mmol) in acetone (5 mL). The mixture was stirred at rt for 20 min. During that time, the bright yellow reaction mixture turned a clear and pale-yellow solution. The crude product was isolated in the amount of 0.0891 g (96%) after removal of the solvent in vacuum. According to 1H NMR data, the crude product was a mixture of (1S,2S,4R) (endo-3) and (1S,2R,4R) (exo-3) diastereomers in a ratio of 1:1. The isomers were partly separated by silica gel column chromatography using a 1:4 mixture of ethyl acetate and hexanes. The yield was 0.030 g (32%, 96% de) of endo-3 (a pale yellow solid) and 0.025 g (28%, 86% de) of exo-3 (a pale yellow solid). Anal. Calcd. for C30H36ClN2PPd: C 60.31, H 6.07, N 4.69. Found: C 60.04, H 6.01, N 4.66. Data for (1S,2S,4R) isomer (endo-3): mp 190e194  C (decomp.). Rf 0.72 (3:5 EtOAcehexanes), [a]22D ¼ þ253 (c 0.450, acetone), IR (thin film in mineral oil, n, cm
1): 1660 (C]N). 1H NMR (d, ppm, C6D6):
0.10 (t, Jexo-2,exo-6 ¼ 4, 1H, CH, C1), 0.49 (s, 3H, C(9)H3), 0.68 (s, 3H, C(10)H3), 1.14 (s, 3H, C(8)H3), 1.17e1.26 (m, 2H, exo-C(5)H and exo-C(6)H), 1.33e1.43 (m, 2H, endo-C(5)H, endo-C(5)H), 2.87 (d, 4JHP ¼ 2.9, 3H, NCHA 3 ), 3.35 (d, 4 JHP ¼ 2.0, 3H, NCHB3), 4.56 (t, 3J1,2 ¼ 4Jexo-2,exo-6 ¼ 4, 1H, PdC(2)H), 6.99e7.05 (m, 9H, m,p-PPh3), 7.91e7.95 (m, 6H, o-PPh3). 13C{1H} NMR (d, ppm, C6D6): 11.2 (C8), 19.7 (C9), 20.8 (C10), 27.9 and 36.3 (C5, C6), 47.6 (C7), 48.7 (C1), 50.1 and 51.7 (N(CH3)2), 52.9 (C4), 57.8 (d, 2JCP ¼ 2.6, C2), 128.0 (d overlapped with C6D6, m-PPh), 130.0 (d, 1 JCP ¼ 2.8, p-PPh), 133.2 (d, 1JCP ¼ 49, PC), 135.1 (d, 2JCP ¼ 11, o-PPh), 194.5 (C3). 31P{1H} NMR (d, ppm, C6D6): 32.8. Data for (1S,2R,4R) isomer (exo-3): mp 172e176  C (decomp.); Rf 0.67 (3:5 EtOAcehexanes); [a]22D ¼
181 (c 0.515, acetone), IR (thin film in mineral oil, n, cm
1): 1665 cm
1 (C]N). 1H NMR (d, ppm, C6D6): 0.47 (s, 3H, C(10)H3), 0.56 (ddd, 3Jexo-5,exo-6 ¼ 13, 2Jendo-6,exo-6 ¼ 10, 3 J1,exo-6 ¼ 4, 1H, exo-C(6)H), 0.63 (t, J ¼ 4, 1H, C(1)H), 1.01 (s, 3H, C(9) H3), 1.04 (s, 3H, C(10)H3), 1.36e1.32 (m, 2H, endo-C(5)H and endoC(6)H), 1.48 (ddd, 3Jexo-5,exo-6 ¼ 13, 2Jexo-5,exo-6 ¼ 8, 3Jexo-5,endo-6 ¼ 4, 4 1H, exo-C(5)H)), 2.86 (d, 4JHP ¼ 2.8, 3H, NCHA 3 ), 3.71 (d, JHP ¼ 1.9, 3H, NCHB3), 3.73 (s, 1H, PdC(2)H), 7.05e7.00 (m, 9H, m- and p-PPh3), 7.86e7.82 (m, 6H, o-PPh3). 13C{1H} NMR (d, ppm, C6D6): 12.8 (C8), 20.1 (C9), 20.5 (C10), 30.5 (C5), 30.9 (d, JCP ¼ 3, C6), 49.47 (d, JCP ¼ 6, C1), 50.5 (d, JCP ¼ 2.5, C7), 51.1 and 52.8 (N(CH3)2), 55.6 (d, 2JCP ¼ 4, C2), 128.0 (d overlapped with C6D6, m-PPh), 130.7 (d, 3JCP ¼ 3, pPPh), 132.9 (d, 1JCP ¼ 48, PPh), 134.9(d, 2JCP ¼ 11, o-PPh), 193.6 (C3). 31 1 P{ H} NMR (d, ppm, C6D6): 33.4. [(1S,2S,4R)- and (1S,2R,4R)-3-(2,2-dimethylhydrazono)-4,7,7trimethylbicyclo[2.2.1]heptan-2-yl-C,N](acetylacetonato-O,O’)palladium(II) (endo-4 and exo-4). A freshly prepared Na(acac) (0.0657 g, 0.469 mmol) was added to a bright yellow suspension of 2 (0.1511 g, 0.2254 mmol) in acetone (15 mL). The reaction mixture was stirred at rt for 90 min. During this time, the color of the suspension turned pale yellow. The reaction mixture was concentrated under reduced pressure. Water (10 mL) was added to the residue, and the resulting mixture was stirred for 15 min. The precipitated pale yellow solid was filtered and dried to yield 0.148 g (83%) of complex 4 as a mixture of endo and exo isomers in a ratio of 1:1. An attempt to separate the diastereomers by column chromatography using a 1:99 mixture of acetone and hexanes provided two major fractions:

P.C.R. Vasireddy et al. / Journal of Organometallic Chemistry 900 (2019) 120917

one containing exo- and endo-4 in a ratio of 1:5 and the other in a ratio of 3:8. Anal. Calcd. for C17H28N2O2Pd: C 51.19, H 7.08, N 7.02. Found: C 50.89, H 6.89, N 7.03. Data for a 1:1 mixture of two isomers: mp 120e122  C, Rf 0.65 (3:5 EtOAcehexanes). IR (thin film in mineral oil, n, cm
1): 1656 (C]N), 1581 (C]O). 1H NMR (d, ppm, C6D6, the signals, which we could assign to the endo and exo isomers are marked with * and **, respectively): 0.78 (s, 6H)*,**, 0.86 (s, 3H)*, 1.06 (s, 3H)**, 1.13 (s, 3H)*, 1.30e1.25 (m, 1H)**, 1.43e1.38 (m, 1H)*, 1.59e1.51 (m, 4H)*,**, 1.62 (s, 3H)**, 1.74e1.68 (m, 1H)**, 1.83 (s, 6H)**, 1.85 (s, 3H)*, 1.86 (s, 3H)*, 2.10 (d, J ¼ 3.5 Hz, 1H)**, 2.26e2.21 (m, 2H)*, 2.37 (s, 3H)**, 2.39 (s, 3H)*, 2.97 (s, 3H)*, 3.03 (s, 3H)**, 4.17 (s, 1H)**, 4.94 (t, J ¼ 3.5 Hz, 1H)*, 5.18 (s, 1H)**, 5.20 (s, 1H)*. 13C{1H} NMR (d, ppm, C6D6): 11.36*, 12.35**, 20.04, 20.78, 21.38*, 21.50**, 26.85, 27.87, 28.02, 28.62, 28.67, 29.79, 31.26, 37.70, 47.35**, 47.46*,**, 48.37 (quat.), 48.65*, 49.66*, 50.70 (quat.), 51.55 (quat.), 51.72*, 51.95**, 52.20*, 52.79**, 53.09 (quat.), 100.02*, 100.24**, 186.36 (quat.), 186.51 (quat.), 187.68 (quat.), 187.71 (quat.), 197.66 (quat.), 199.48 (quat.) 4.3. Crystallographic studies Complex 2 (CCDC 1942702). A crystal (approximate dimensions 0.220  0.200  0.180 mm3) was placed onto the tip of a 0.1 mm diameter glass capillary and mounted on a Bruker APEX-II CCD diffractometer for a data collection at 123(2) K. A preliminary set of cell constants was calculated from reflections harvested from three sets of 20 frames. These initial sets of frames were oriented such that orthogonal wedges of reciprocal space were surveyed. This produced initial orientation matrices determined from 129 reflections. The data collection was carried out using MoKa radiation (graphite monochromator) with a frame time of 5 s and a detector distance of 6.0 cm. A randomly oriented region of reciprocal space was surveyed to the extent of one sphere and to a resolution of 0.77 Å. Four major sections of frames were collected with 0.50 steps in u at four different f settings and a detector position of
28 in 2q. The intensity data were corrected for absorption and decay (SADABS) [29]. Final cell constants were calculated from the xyz centroids of 2979 strong reflections from the actual data collection after integration (SAINT) [30]. Please refer to Table S1 for additional crystal and refinement information. The structure was solved using SHELXS-97 (Sheldrick, 2008) [31] and refined using SHELXL-2014 (Sheldrick, 2014) [32]. The space group P21 was determined based on systematic absences and intensity statistics. A direct-methods solution was calculated which provided most non-hydrogen atoms from the E-map. Full-matrix least squares/difference Fourier cycles were performed which located the remaining non-hydrogen atoms. All non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were placed in ideal positions and refined as riding atoms with relative isotropic displacement parameters. The final full matrix least squares refinement converged to R1 ¼ 2.98% and wR2 ¼ 5.96% (F2, all data). Data collection and structure solution were conducted at the X-Ray Crystallographic Laboratory, 192 Kolthoff Hall, Department of Chemistry, University of Minnesota. All calculations were performed using Pentium computers using the current SHELXTL suite of programs. Complexes endo- and exo-3 (CCDC 1942704). A crystal (approximate dimensions 0.220  0.180  0.120 mm3) was placed onto the tip of a 0.1 mm diameter glass capillary and mounted on a Bruker APEX-II CCD diffractometer for a data collection at 110(2) K. A preliminary set of cell constants was calculated from reflections harvested from four sets of 30 frames. These initial sets of frames were oriented such that orthogonal wedges of reciprocal space were surveyed. This produced initial orientation matrices determined from 259 reflections. The data collection was carried out

7

using ImS Cu radiation with a frame time of 10 s and a detector distance of 4.0 cm. A randomly oriented region of reciprocal space was surveyed to the extent of one sphere and to a resolution of 0.84 Å. Forty-three major sections of frames (39 u and 4 f scans) were collected with 2.0 steps at different 2q detector positions in order to achieve the desired completeness. The intensity data were corrected for absorption and decay (SADABS) [29]. Final cell constants were calculated from the xyz centroids of 9757 strong reflections from the actual data collection after integration (SAINT) [30]. Please refer to Table S2 for additional crystal and refinement information. The structure was solved and refined using SHELX (Sheldrick, 2014) [31] set of programs with Olex 2 software package [32]. SHELXT solution was calculated which provided most nonhydrogen atoms and full-matrix least squares/difference Fourier cycles were performed which located the remaining non-hydrogen atoms. All non-hydrogen atoms were refined with anisotropic displacement parameters. All hydrogen atoms were placed in ideal positions and refined as riding atoms with relative isotropic displacement parameters. The final full matrix least squares refinement converged to R1 ¼ 4.65% and wR2 ¼ 12.15% (F2, all data). Data collection and structure solution were conducted at the X-Ray Crystallographic Facility, Department of Chemistry and Biochemistry, NDSU, Fargo, ND. Acknowledgements The authors are grateful to the University of North Dakota for financial support. We would like to acknowledge Victor G. Young, Jr., and the X-ray Crystallographic Laboratory in the Department of Chemistry of the University of Minnesota for the X-ray crystallographic analysis of complex 2. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jorganchem.2019.120917. References [1] Representative examples of cyclopalladation at the benzylic CH3 group: a) V.V. Dunina, O.N. Gorunova, M.V. Livantsov, Y.K. Grishin, K.A. Kochetkov, A.V. Churakov, L.G. Kuz’mina, Polyhedron 30 (2011) 27e32; b) V.V. Dunina, O.N. Gorunova, V.A. Stepanova, P.A. Zykov, M.V. Livantsov, Y.K. Grishin, A.V. Churakov, L.G. Kuz’mina, Tetrahedron: Asymmetry 18 (2007) 2011e2015; c) V.V. Dunina, O.N. Gorunova, E.D. Kuznetsova, E.I. Turubanova, M.V. Livantsov, Y.K. Grishin, L.G. Kuz’mina, A.V. Churakov, Russ. Chem. Bull., Int. Ed. 55 (2006) 2193e2211; d) S. Stoccoro, B. Soro, G. Minghetti, A. Zucca, M.A. Cinellu, J. Organomet. Chem. 679 (2003) 1e9; e) M.A. Zhuravel, D.S. Glueck, L.N. Zakharov, A.L. Rheingold, Organometallics 21 (2002) 3208e3214; f) J. Vicente, M.C. Lagunas, E. Bleuel, M.C.R. de Arellano, J. Organomet. Chem. 648 (2002) 62e71; g) V.V. Dunina, O.N. Gorunova, E.B. Averina, Y.K. Grishin, L.G. Kuz’mina, J.A.K. Howard, J. Organomet. Chem. 603 (2000) 138e151; € € ssmer, C.-P. Reisinger, T.H. Riermeier, K. Ofele, h) W.A. Herrmann, C. Bro M. Beller, Chem.dEur. J. 3 (1997) 1357e1364; i) P.L. Alsters, P.F. Engel, M.P. Hogerheide, M. Copijn, A.L. Spek, G. van Koten, Organometallics 12 (1993) 1831e1844; j) J. Albert, R.M. Ceder, M. Gomez, J. Granell, J. Sales, Organometallics 11 (1992) 1536e1541; k) A.J. Deeming, I.P. Rothwell, M.B. Hursthouse, K.M. Abdul Malik, J. Chem. Soc., Dalton Trans. (1979) 1899e1911; l) G. Longoni, P. Chini, F. Canziani, P. Fantucci, Chem. Commun. (1971) 470; m) G.E. Hartwell, R.V. Lawrence, M.J. Smas, Chem. Commun. (1970) 912. [2] Cyclopalladation at the CH3 group next to a heteroatom: a) F. Cocco, A. Zucca, S. Stoccoro, M. Serratrice, A. Guerri, M.A. Cinellu, Organometallics 33 (2014) 3414e3424; b) J. Dupont, N. Beydoun, M. Pfeffer, J. Chem. Soc., Dalton Trans. (1989) 1715e1720.

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[3] Cyclopalladation at either the CH3 or CH2 group next to an electronwithdrawing moiety: a) D.J. Cardenas, A.M. Echavarren, Organometallics 14 (1995) 4427e4430; b) D.J. Cardenas, A.M. Echavarren, A. Vegas, Organometallics 13 (1994) 882e889; c) B. Galli, F. Gasparrini, B.E. Mann, L. Maresca, G. Natile, A.M. Manotti-Lanfredi, A. Tiripicchio, J. Chem. Soc., Dalton Trans. (1985) 1155e1161; d) B. Galli, F. Gasparrini, L. Maresca, G. Natile, G. Palmieri, J. Chem. Soc., Dalton Trans. (1983) 1483e1487; e) A.G. Constable, W.S. McDonald, L.C. Sawkins, B.L. Shaw, J. Chem. Soc., Dalton Trans. (1980) 1992e2000; f) A.G. Constable, W.S. McDonald, L.C. Sawkins, B.L. Shaw, J. Chem. Soc., Chem. Commun. (1978) 1061e1062. [4] Examples of cyclopalladation at the tert-butyl group: a) H.R. Thomas, R.J. Deeth, G.J. Clarkson, J.P. Rourke, Organometallics 30 (2011) 5641e5648; b) R.Y. Mawo, S. Mustakim, V.G. Young Jr., M.R. Hoffmann, I.P. Smoliakova, Organometallics 26 (2007) 1801e1810, and references cited therein; c) H.-P. Chen, Y.-H. Liu, S.-M. Peng, S.-T. Liu, Dalton Trans. (2003) 1419e1424; d) V.V. Dunina, O.N. Gorunova, E.V. Averina, Y.K. Grishin, L.G. Kuz’mina, J.A.K. Howard, J. Organomet. Chem. 603 (2000) 138e151; e) K. Hiraki, M. Nakashima, T. Uchiyama, Y. Fuchita, J. Organomet. Chem. 428 (1992) 249e258. [5] Examples of cyclopalladation at the CH3 group attached to a quaternary carbon: A. McNally, B. Haffemayer, B.S.L. Collins, M.J. Gaunt Nature 510 (2014) 129e133. [6] Cyclopalladation at the CH3 group attached to a quaternary carbon in Dcamphor and L-fenchone: a) G.C. Dickmu, I.P. Smoliakova, J. Organomet. Chem. 772e773 (2014) 42e48; b) G.C. Dickmu, L. Stahl, I.P. Smoliakova, J. Organomet. Chem. 756 (2014) 27e33; c) Y.A. Gur’eva, O.A. Zalevskaya, L.L. Frolova, I.N. Alekseev, P.A. Slepuchin, A.V. Kuchin, Russ. J. Gen. Chem. 82 (2012) 1117e1123. [7] Cyclopalladation of a camphor-based ligand at either a primary or secondary carbon: a) K.M.A. Malik, P.D. Newman, Dalton Trans. (2003) 3516e3525; b) C.E. Coomber, L. Benhamou, D.-K. Bucar, P.D. Smith, M.J. Porter, T.D. Sheppard, J. Org. Chem. 83 (2018) 2495e2503. [8] Pincer complex formation via tertiary CeH bond activation: a) Z. Ren, G. Dong, Organometallics 35 (2016) 1057e1059; b) A. Yoneda, T. Hakushi, G.R. Newkome, F.R. Fronczek, Organometallics 13 (1994) 4912e4918. [9] Examples of cyclopalladation at the benzylic CH2 group: a) J. Spencer, F. Maassarani, M. Pfeffer, A. DeCian, J. Fischer, Tetrahedron: Asymmetry 5 (1994) 321e324; b) M.T. Pereira, M. Pfeffer, M.A. Rotteveel, J. Organomet. Chem. 375 (1989) 139e145; c) V.I. Sokolov, T.A. Sorokina, L.L. Troitskaya, L.I. Solovieva, O.A. Reutov, J. Organomet. Chem. 36 (1972) 389e390; d) D.F. Gill, B.L. Shaw, J. Chem. Soc., Chem. Commun. (1972) 65e66. [10] Cyclopalladation at the CH2 group next to an electron-withdrawing group: a) X.-Y. Yang, W.S. Tay, Y. Li, S.A. Pullarkat, P.-H. Leung, Chem. Commun. 52 (2016) 4211e4214; b) H.A. Ankersmit, P.T. Witte, H. Kooijman, M.T. Lakin, A.L. Spek, K. Goubitz, K. Vrieze, G. van Koten, G. Inorg, Chem. 35 (1996) 6053e6063; c) W.S. Tay, X.-Y. Yang, Y. Li, S.A. Pullarkat, P.-H. Leung, RSC Adv. 6 (2016) 75951e75959; d) L.B. Bal azs, W.S. Tay, Y. Li, S.A. Pullarkat, P.-H. Leung, Organometallics 37 (2018) 2272e2285; e) W. Henderson, A.G. Oliver, C.E.F. Rickard, L.J. Baker, Inorg. Chim. Acta 292 (1999) 260e265;

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