Stereocontrol in Preparation of Cyclopalladated Alkylaromatic Oximes

Aug 18, 2017 - The stereochemistry of 2′-methylbutyrophenone oxime, the rates of ortho-palladation of its E- and Z-isomers, and catalytic activity o...
2 downloads 16 Views 1MB Size
Download PDF
Article pubs.acs.org/Organometallics

Stereocontrol in Preparation of Cyclopalladated Alkylaromatic Oximes and Evaluation of Their Stereoselective Esterase-Type Catalytic Activity Sergey Z. Vatsadze,*,† Aleksei V. Medved’ko,† Sergey A. Kurzeev,† Oleg I. Pokrovskiy,‡ Olga O. Parenago,†,‡ Mikhail O. Kostenko,†,‡ Ivan V. Ananyev,§ Konstantin A. Lyssenko,§ Dmitri A. Lemenovsky,† Gregory M. Kazankov,† and Valery V. Lunin† †

Department of Chemistry, Lomonosov Moscow State University, Moscow 119991, Russia N.S. Kurnakov Institute of General and Inorganic Chemistry and §A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Moscow 119991, Russia

‡

S Supporting Information *

ABSTRACT: The stereochemistry of 2′-methylbutyrophenone oxime, the rates of ortho-palladation of its E- and Zisomers, and catalytic activity of the respective Pd complexes were studied. The full stereoisomeric composition of oximes was established for the first time by means of supercritical fluid chromatography on chiral polysaccharide column. It was shown that enantiomeric excesses of both E/Z-isomers of (S)-2′-methylbutyrophenone oxime (1S) and (R)-2′-methylbutyrophenone oxime (1R) were equal to 92 ± 2. The cyclopalladation study revealed that while E-isomer is orthopalladated very quickly its Z-counterpart does not enter this reaction. However, upon coordination to Pd(II), Z-oxime slowly isomerizes into E-form with fast subsequent cyclopalladation, so it was possible to perform ortho-palladation of E-oxime in kinetic resolution mode with removal of unreacted Zoxime. Comparatively rare cis-structure of cyclopalladated oxime dimer was proved by means of single-crystal X-ray study. For the first time, it was shown that ortho-palladated chiral oximes behave as enantioselective catalyst in the hydrolysis of chiral esters.

■

INTRODUCTION The key trends in the field of modern organic synthesis include catalytic cross-coupling and oxidative cross-coupling reactions, metathesis processes, design of new homogeneous and heterogeneous catalytic systems, and the development of new approaches to investigation of mechanisms of catalytic reactions.1 The study of new enantioselective metallocatalysts that mimic principles of enzyme reactions is one of the most important purposes of biomimetics.2 These catalysts usually represent coordination complexes of transition metals such as Pd(II), Pt(II), Zn(II), and Cu(II).3−5 Among them, cyclometalated complexes seem to be extremely promising, due to their high activity and ease with which they make structurally diverse systems. Palladium compounds containing Pd−C bond and stabilized by intramolecular bond with electron-donating atom are referred to as cyclopalladated complexes or palladacycles (Figure 1).6 ortho-Palladated oximes are among promising catalysts for performing cross-coupling reactions.7,8 If a water molecule is coordinated in the trans-position with respect to aromatic carbon atom, then it easily loses a proton (pKa ca. 4), forming a strong nucleophilic center that is generally responsible for the activity of cyclopalladated complexes in nucleophilic catalysis.9,10 In addition, the high © XXXX American Chemical Society

Figure 1. General formulas of cyclopalladated oxime.

activity of cyclopalladated oximes in nucleophilic catalysis is due not only to the presence of this hydroxide fragment but also to high acidity of the hydroxyl group of oxime moiety. For example, pKa of free acetophenone oxime is about 11, but being ortho-palladated, it has a pKa value of 6−7. Thus, the oximate anion acting together with hydroxide as a nucleophile enhances the rate of hydrolysis of the substrate significantly.11 While cyclopalladation of oximes was performed for the first time more than 40 years ago,12,13 these complexes are still popular precursors for synthesis of more complicated palladium complexes.14−17 Although their catalytic properties were Received: June 4, 2017

A

DOI: 10.1021/acs.organomet.7b00410 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Scheme 1. General Routes for Preparation of Racemic (a) and Chiral (b, c) 2′-Methylbutyrophenone oximesa

a

SAMP: (S)-N-amino-2-methoxymethylpyrrolidine.

already explored in the hydrolysis processes,11 no information is found on the application of chiral oximes in such reactions. The presence of a chiral center at the α-position with respect to the benzene ring influences many of the structural features of the ortho-palladated complexes including the unique formation of trimeric forms in solid state and solution.18 In contrast, in the case of oximes one could only have a chiral center one carbon atom further (Figure 1). The creation of such asymmetric carbon atom at the α-site of oxime that is located in distal proximity to reacting Pd center (Figure 1, marked with asterisk) might slightly influence the energy of the “substrate− catalyst” transition complex. This idea deserves to be proved or disproved. If we could prepare palladium complexes of both enantiomers of such oxime, then we can try to catalyze the hydrolysis of chiral esters. If enantiomers are being hydrolyzed with different rates, then such complexes could be named as enantioselective hydrolytic catalysts. Therefore, the aim of this work was the evaluation of catalytic activity of chiral orthopalladated oximes. To achieve our aim, we fulfilled the following tasks: (i) preparation of simplest chiral alkylaromatic oxime of 2′-methylbutyrophenone, (ii) study of its stereochemistry, (iii) study of its cyclopalladation, and (iv) testing hydrolytic activity of the chiral oxime complexes in reactions with model chiral substrates.

multistep recrystallization of diastereomeric mixture leading to a large loss of oxime.26 In this work, we applied the oximation of both enantiomerically pure forms of chiral ketone. Oximation of racemic and chiral 2′-methylbutyrophenones by hydroxylamine hydrochloride in dry pyridine gave corresponding oximes in almost quantitative yields (Scheme 1), but the question of their stereochemistry had to be solved. The oximation of racemic 2′-methylbutyrophenone led to two pairs of enantiomers, hereafter called (E)-1S/(E)-1R and (Z)-1S/ (Z)-1R (Scheme 1). First, we studied the diastereochemistry of the oximes. In our procedure, oximes 1rac, 1S, and 1R were obtained as mixtures of E- and Z-isomers with ratios close to 1.2:1 (on the basis of proton NMR spectra, Figure 2b), in accordance with literature data for similar unsymmetrical oximes.27−29 Assignment of isomers’ chemical shifts was based on the NMR spectrum of pure racemic E-isomer ((E)-1rac) that spontaneously formed from racemic E/Z mixture upon storage for ca. 1 year in a glass vessel (Figure 2a).29 The diastereochemistry of racemic (E)-1rac was proved by X-ray diffraction study (Figure 3). The molecules of (E)-1rac crystallize in the centrosymmetric space group (I41/a, Z′=1), and despite the disorder attributed to the superposition of two enantiomers with occupancies 0.84(1) and 0.16, the crystal of (E)-1rac is a racemate. Note that this disorder and 1:1 ratio of enantiomers remains even if the crystal structure is solved and refined in the noncentrosymmetric space group (I41, Z′=2). In the crystal, the (E)-1rac molecules form N···H−O hydrogen bonds (N···O 2.751(2) Å) assembling molecules into tetramers. A similar supramolecular motif was reported recently for E-isomer of oxime of 2-methylpropiophenone (N···O 2.753 Å)30 Oxime Diastereo- and Enantiomeric Composition. Before proceeding to the preparation of palladium complexes, we tried to analyze the composition of oxime stereoisomers mixtures. The diastereo- and enantiomeric composition of oximes was established by supercritical fluid chromatography (SFC) using a Kromasil Amycoat column as a chiral stationary phase and a CO2−iPrOH mixture as a mobile phase. No chiral separation was achieved with cellulose-based chiral phases and methanol as a cosolvent. Enantiorecognition on amylose-based chiral stationary phases is known to be mobile-phase-dependent because amylose swells differently in different cosolvents, which leads to changes in polymer conformation and hence in chiral centers accessibility.31

■

RESULTS AND DISCUSSION 2′-Methylbutyrophenone Preparation. Friedel−Crafts reaction is the easiest way to obtain achiral alkylaromatic ketones.19,20 However, in the case of chiral ketones this method is limited by the commercial availability of the corresponding enantiopure carboxylic acids as well as by the lability of the resulting ketone toward racemization. It is known that our target molecule 2′-methylbutyrophenone easily undergoes racemization in alkaline conditions21 even during storage in glass vessels.22 At the same time, the stereochemistry of this ketone does not change in dry pyridine.19 In this work, racemic 2′-methylbutyrophenone (1rac) and chiral (S)-2′-methylbutyrophenone (1S) were obtained by acyl chloride preparation and sequential Friedel−Crafts acylation of benzene19 using racemic and chiral (S)-2-methylbutiric acid, respectively, as shown in Scheme 1a,b. Enantiopure (R)-2′-methylbutyrophenone (1R) was prepared by Enders’ asymmetric alkylation of propiophenone with iodomethane23,24 (Scheme 1c). Unlike aldoximes, no general methods to prepare stereochemically pure ketoximes exist.25 The only separation technique found in the literature includes a B

DOI: 10.1021/acs.organomet.7b00410 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

stereoisomers is shown in Figure 4. The enantiomeric excesses of both E/Z-isomers of (S)-2′-methylbutyrophenone oxime 1S

Figure 4. Separation of 2′-methylbutyrophenone oxime diastereomers 1rac. Kromasil Amycoat 100−3 150 × 4.6, CO2/iPrOH 93/7 v/v, 100 bar, 40 °C, 4 mL/min, 220 nm.

and (R)-2′-methylbutyrophenone oxime 1R were equal to 92 ± 2% (Figures S2 and S3). Z- and E-oximes have different enantiomer elution orders. For E-oxime, (S)-enantiomer elutes earlier than (R), whereas for Z-oxime, the elution order is the opposite. To assign the chromatographic peaks to the corresponding isomer, enantiomerically enriched samples with known major component were used as standards. Oxime Cyclometalation. Since the preparation of orthopalladated product in fact requires only E-isomer as a starting organic substrate, we made several attempts to increase its content in the diastereomeric mixture. We increased bulkiness of substituent replacing hydrogen atom attached to oxygen for a methyl group. However, E/Z-ratio reversed to become 1:1.7 for 2′-methyl-1-phenylbutan-1-one O-methyl oxime (Figure S1). Then, we tried to improve the stereoselectivity of oximation by performing the synthesis in the presence of α-cyclodextrin which is known as a host molecule with hydrophobic internal cavity, but the best E/Z isomeric ratio was 60/40 with low chemical yield (50%). Since no positive results in improving the diastereomeric ratio were achieved, we decided to perform the cyclopalladation of 1S and 1R using their respective E/Z mixtures (Scheme 2). Surprisingly, the product distribution showed that the presence of Pd(II) affected the ratio between nonpalladated E- and Z-

Figure 2. High-field part of 1H NMR spectra (400 MHz, CDCl3) of racemic (E)-2′-methylbutyrophenone oxime (E)-1rac (a) and E/Z mixture of racemic 2′-methylbutyrophenone oxime 1rac (b).

Scheme 2. Cyclopalladation of Isomeric Mixtures of 2′Methylbutyrophenone Oximesa,b,c

Figure 3. General view of the hydrogen bonded tetramer in the racemic crystal of (E)-1rac in representation of atoms by thermal ellipsoids (p = 50%). The disorder corresponding to the superposition of two enantiomers with occupancies 0.84 and 0.16 is shown only for the asymmetric part.

a

Compound 1 is (E)-1. Reflux time is 15 min. Isolated yield corrected on purity determined by NMR. bCompound 1 is E/Z mixture. Reflux time is 4 h. NMR-based yield corrected on purity determined by NMR. cCompound 1 is E/Z mixture. Reflux time is 6 h. NMR-based yield corrected on purity determined by NMR.

This makes the choice of a cosolvent type and concentration as the key factor of selectivity control in separations on these stationary phases. A typical SFC separation of four oxime C

DOI: 10.1021/acs.organomet.7b00410 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics isomers. The ratio was checked by NMR of crude sample with slight excess of pyridine. Indeed, Z-oxime content in E/Z mixture decreased from 45 to 16% during the reaction. This means that partial isomerization of oxime occurred during interaction of a ligand with Pd(II). We assume that the palladium ion being bound to the N atom of oxime weakens the CN double bond to allow easy oxime isomerization. To enhance the degree of oxime isomerization, the cyclopalladation of E/Z mixture was performed in refluxing 1,4dioxane−water mixture. The pure E-isomer is ortho-palladated completely after 15 min under these conditions. However, the formation of cyclopalladated complexes is also accompanied by reduction of Pd(II) to Pd(0) even in inert atmosphere.32 Oxime itself does not reduce palladium at reflux temperature, but after addition of basic sodium acetate, the Pd(0) formation is much more accelerated and occurs concurrent to ortho-palladation. NMR analysis showed that after 4 h ca. 10% of Z-oxime was isomerized, and after 6 h, ca. 25% was isomerized. At the same time, the complete isomerization is suppressed because free Zoxime could be bound by cyclopalladated product to form byproducts shown on Scheme 3.12

Figure 5. General view of one independent molecule in crystal of 2rac in representation of atoms by thermal ellipsoids (p = 50%). The disorders of the sec-butyl substituents are omitted for clarity. The hydrogen bonds to chlorine atom are shown as dotted lines.

Scheme 4. Sequential Cyclopalladation-Monomerization of Oximesa

Scheme 3. Supposed Byproducts of Cyclopalladated Dimeric Complex Reaction with Free Z-Oxime a

Starting oximes are E/Z mixtures.

procedure was found to be equal to 87 ± 1 and 79 ± 1%, respectively (Figure 6). This shows that partial isomerization

Racemic O-methyl-2′-methylbutyrophenone oxime is not involved in cyclopalladation under these conditions. Only adduct L2PdCl2 is formed as proved by X-ray study (Figure S4). The structure of racemic complex 2rac was also proved by means of X-ray (Figure 5). This complex has cisoid arrangement of cyclopalladated ligands,33−35 while for other cyclopalladated complexes, i.e., amines, the trans orientation is common.36−38 Also, the ligand molecule is highly bent with dihedral angle between Pd−C−N mean planes equal to 121°. The observed bending of Pd2Cl2 cycle in 2rac is probably due to intramolecular O−H···Cl bonds (H···Cl 2.37−2.49 Å, O−H···Cl 138−148°). For known dimeric palladacycles, the bending angle is close to 180° with very few exceptions.36,38−41 The fast cyclopalladation of E-isomers and significant difference of solubility of starting oximes 1 and cyclopalladated complexes in n-hexane allowed us to perform the reaction in kinetic resolution mode (Scheme 4). NMR study of the reaction mixture after separation of cyclopalladated oxime confirmed that only E-oxime can be cyclopalladated with some isomerization Z- to E-isomer (Figure S24). Enantiomeric excesses of compounds 3S and 3R measured by another SFC

Figure 6. Separation of enantiomers of 3S. Kromasil 3-CelluCoat 4.6 × 150 mm column with CO2/methanol/isopropylamine 70/30/1 mobile phase.

takes place during cyclopalladation. In contrast, chiral cyclopalladated oximes are stable for months in solid state and give no sign of racemization in methanol solution for at least 10 days (Figure S25), suggesting the absence of racemization during catalysis. The structure of pyridine-containing monomer 3S was proved by X-ray (Figure 7) and is typical for cyclopalladated oximes.9 In crystal, two independent molecules are distinguished by two different rotation angles of a pyridine ring. Also, short distances H(1O)...Cl(1) and H(1OA)...Cl(1A) (∼2.23 Å) prove the existence of intramolecular hydrogen bond H···Cl, which is typical for such oxime systems.14,15,17 The H-bond sustains in solution, which is clearly seen by shifting of OH D

DOI: 10.1021/acs.organomet.7b00410 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

(k2(R)/k2(S)) compared to 1.55 (k2(S)/k2(R)) for the first substrate (Scheme 5). Scheme 5. Proposed Mechanism of Ester Cleavage

Figure 7. General view of one independent molecule in crystal of 3S in representation of atoms by thermal ellipsoids (p = 50%).

chemical shift from 8.2 to 10.2 ppm in 1H NMR spectrum in chloroform-d. Catalytic Activity of Oximes. The study of catalytic activity of racemic cyclopalladated oxime 3rac in hydrolysis of esters was started using standard substrate 2,4-dinitrophenyl acetate. Because of low catalytic activity of cyclopalladated oximes, the hydrolysis was carried out in pseudo-first-order regime with variable excess of catalyst to determine k2 from kobs = k0+k2[Pd],11 where [Pd] is palladium complex concentration. The catalytic constant for model substrate and catalyst 3 is equal to 224 ± 8 M−1 s−1 (r2 = 0.988). It is 150 times higher than the respective constants for amine-based palladacycles.42 This proves the initial idea of using nucleophilic oxime-type ligands. As was mentioned in the Introduction, to evaluate the enantioselectivity of chiral palladium complexes, one should compare the reaction rates of chiral substrate using both enantiomeric catalysts. These reactions with chiral oxime palladacycles 3S and 3R were tested on 2,4-dinitrophenyl (S)-2-methylbutanoate and commercially available N-Cbz-Lvaline 4-nitrophenyl ester as substrates (Table 1). Although the rate of hydrolysis of 2,4-dinitrophenyl (S)-2methylbutanoate by palladacycles 3S and 3R is two times lower than that for 2,4-dinitrophenyl acetate, the reaction now proceeds in a stereoselective manner. The second substrate was hydrolyzed much more slowly, which was obvious because it had only one nitro substituent on its phenolic leaving group, but this decrease in reactivity allowed us to gain an increase in selectivity. Indeed, the catalytic constants ratio reached 1.98

Interestingly, 3R isomer was more active for Cbz-Val than was 3S, while the latter was more active in 2,4-dinitrophenyl (S)-2-methylbutanoate hydrolysis. We are not going to speculate on the possible catalytic mechanism,11 but it seems both reactions proceed similarly. The difference found in rate constants could be only a consequence of the difference in energy of diastereomeric “substrate−catalyst” transition complexes. This proves our second initial idea that the creation of asymmetric carbon atom in disital proximity to catalytic site would provide an effect on catalytic reaction rate. What is surprising is that even such small difference in bulkiness of methyl and ethyl groups in sec-butyl substituent exhibits such a noticeable effect on ester cleavage rate.

■

CONCLUSIONS The oxime of the 2′-methylbutyrophenone exists as a racemic mixture of E- and Z-isomers. Supercritical fluid chromatography on chiral polysaccharide columns allows separating all four oxime stereoisomers and both cyclopalladated enantiomers within reasonable time. During the ortho-palladation reaction, partial isomerization of Z-isomer to E-isomer occurred. orthoPalladated oxime adopts a very rare syn-configuration of the

Table 1. Hydrolysis Rate Constantsa substrate 2,4-dinitrophenyl(S)-2-methylbutanoate N-Cbz-L-valine 4-nitrophenyl ester a

catalyst

k2 (M−1 s−1) 119 77 5.6 11.1

3S 3R 3S 3R

± ± ± ±

4 2 0.2 0.3

k2(S)/k2(R) 1.55 0.51

Rate constants were normalized to 100% enantiomer of catalysts 3S and 3R according to kR =

kS =

k 2RXSS − k 2SXSR XRR XSS − XRS XSR

k 2S − kRXRS XSS

where kR, kS are catalytic constants for pure enantiomers, kR2 , kS2 are catalytic constants for enriched mixture of R or S enantiomer, XSS, XSR are parts of S and R enantiomers in mixture enriched of S enantiomer, and XRR, XRS are parts of R and S enantiomers in mixture enriched of R enantiomer. E

DOI: 10.1021/acs.organomet.7b00410 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

CH), 1.46−1.63 (m, 2H, CH2CH3), 1.70−2.08 (m, 5H, 2*CH2−Pyr, N−CHH), 2.63 (1H, m, N−CHCH3), 3.15−3.34 (3H, m), 3.37 (3H, s, OCH3), 3.41−3.48 (1H, m), 3.61 (1H, m), 7.32−7.34 (3H, m, C(3,4,5)H−Ph), 7.40−7.42 (2H, m, C(2,6)H−Ph). (R)-2-Methylbutyrophenone.24 A solution of 1.43 g (5.2 mmol) of (2′S)-2-methoxymethyl-1-((2R)-2-methyl-1-phenylbutyliden-amino)pyrrolidine in 21 mL of hexane was vigorously stirred with 7.4 mL of oxalic acid saturated aqueous solution for 2 h. The organic phase was separated and water phase was washed with 10 mL of hexane. The organic phases were combined and dried over anhydrous magnesium sulfate. Solvent was removed and residue was distilled in vacuum. Product was obtained as yellow oil (0.59 g, 60%). Bp 72−73 °C (2 Torr) (lit.45 40−45 °C (0.01 Torr)). [α]D − 32.7° (c 0.2 in Et2O) (lit.39,45 −27.9° (c 1.0 in Et2O)). 1H NMR (400 MHz, CDCl3, Me4Si) 0.93 (3H, t, 3J = 7.5 Hz, CH2CH3), 1.21 (3H, d, 3J = 6.9 Hz, CHCH3), 1.47−1.54 (1H, m, CH2), 1.81−1.88 (1H, m, CH2), 3.37−3.46 (1H, m, CH), 7.45−7.49 (2H, m, C(2,6)H), 7.55−7.58 (1H, m, C(4)H), 7.95−7.97 (2H, m, C(3,5)H). General Procedure of Oximation. First, 15 mmol of hydroxylamine hydrochloride was dissolved in 60 mmol of dry pyridine. Then, 4 mmol of ketone was added, and reaction mixture was left for 3 days at rt. A solution of 5 mL of concentrated hydrochloric acid in 34 mL of ice water was added. The product was extracted with 2 × 25 mL of diethyl ether. The organic phase was separated and dried over anhydrous sodium sulfate. The solvent was evaporated to dryness. 2-Methyl-1-phenylbutanone oxime (1rac). Slightly pink powder. Yield 91%. E/Z mixture 1.2:1. 1H NMR (400 MHz, CDCl3, Me4Si) 0.93−0.99 (5.66 H, m), 1.17 (2.55H, d, 3J = 6.8 Hz), 1.27 (3.00 H, d, 3 J = 7.0 Hz), 1.34−1.45 (0.90H, m), 1.51−1.69 (1.90H, m), 1.72−1.83 (1.01H, m), 2.62 (0.83H, m), 3.41 (1.00H, m), 7.30−7.46 (9.04H, m). δH (400 MHz, DMSO-d6) 0.83 (5.55H, m), 1.04 (2.46H, d, 3J = 6.9 Hz), 1.15 (3.00H, d, 3J = 7.2 Hz), 1.22−1.31 (0.88H, m), 1.42−1.55 (1.84H, m), 1.61−1.72 (1.02H, m), 2.57 (0.81H, m), 3.21−3.30 (1.00H, m), 7.21−7.43 (8.98H, m, Ph), 10.46 (0.77H, s, OH), 10.99 (0.95H, s, OH). 13C NMR (100 MHz, CDCl3, Me4Si) 11.50, 12.40, 17.06, 17.40, 26.66, 26.80, 35.12, 41.26, 127.50, 127.56, 128.07, 128.11, 128.38, 128.45, 133.72, 136.17, 162.34, 163.82. (S)-2-Methyl-1-phenylbutanone oxime (1S). Yield 92%. Slightly pink powder. E/Z mixture 1.2:1. 1H NMR (400 MHz, CDCl3, Me4Si) 0.93−0.98 (5.68H, m), 1.16 (2.56H, d, 3J = 6.8 Hz), 1.26 (3.00H, d, 3J = 7.0 Hz), 1.35−1.44 (0.86H, m), 1.50−1.67 (1.94H, m), 1.71−1.82 (1.12H, m), 2.64 (0.79H, m), 3.40 (1.00H, m), 7.29−7.46 (8.67H, m, Ph), 9.09 (0.71H, br s, OH), 9.64 (0.83H, br s, OH). Elemental composition calcd (%) for C11H15NO: C 74.54, H 8.53, N 7.90. Found (%): C 74.41, H 8.38, N 7.98. (R)-2-Methyl-1-phenylbutanone oxime (1R). Yield 89%. Slightly pink powder. E/Z mixture 1.2:1. 1H NMR (400 MHz, CDCl3, Me4Si) 0.90−0.96 (5.51H, m), 1.13 (2.47H, d, 3J = 6.8 Hz), 1.23 (3.00H, d, 3J = 7.0 Hz), 1.30−1.40 (0.88H, m), 1.48−1.65 (1.96H, m), 1.69−1.80 (1.09H, m), 2.62 (0.78H, m), 3.37 (1.00H, m), 7.27−7.43 (8.18H, m, Ph). Elemental composition calcd (%) for C11H15NO: C 74.54, H 8.38, N 7.90. Found (%): C 74.32, H 8.38, N 7.89. General Procedure of Cyclopalladation. First, 0.77 mmol of palladium chloride was dissolved in a solution of 1.76 mmol of lithium chloride in 36 mL of water with slight heating. A solution of 0.75 mmol of (E)-oxime in 130 mL of 1,4-dioxane was added followed by addition of 0.77 mmol of sodium acetate trihydrate in 1 mL of water. Reaction mixture was refluxed. Suspension was cooled to rt and filtered. Solvent was removed, 10 mL of water was added and product was extracted with 3 × 10 mL of benzene. The organic phase was dried over anhydrous sodium sulfate. The solvent was removed, and the product was precipitated with hexane from benzene solution. cis-Di-μ-chlorobis[hydroxyimino-2-methyl-1-phenylbutanoneκ2C2′,N]dipalladium(II) (2rac). Reflux time 15 min. Yield 91%. Yellow powder. 1H NMR (400 MHz, CDCl3, Me4Si) 0.93 (3H, t, 3J = 7.5 Hz, CH2CH3), 1.38 (3H, d, 3J = 7 Hz, CHCH3), 1.75 (1H, m, CH2), 1.94−2.05 (1H, m, CH2), 3.17 (1H, br s, CH), 6.95−6.99 (1H, m, C(5)H−Ph), 7.04−7.08 (1H, m, C(4)H−Ph), 7.12−7.14 (1H, m, C(3)H−Ph), 7.35 (1H, d, 3J = 7.7 Hz, C(6)H−Ph), 8.14 (1H, br s, OH). Mp 132−135 °C. Elemental composition calcd (%) for

ligand molecules in a solid state. Pyridine-containing monomers of ortho-palladated chiral oximes catalyze hydrolysis of chiral esters in enantioselective manner; this finding would allow kinetic resolution of racemic carboxylic acids in future. The exploitation of bipy-type bidentate ligands for the synthesis of dinuclear palladium catalysts is under current study at our laboratories.

■

EXPERIMENTAL SECTION

Materials and Methods. All reagents were obtained from commercial sources. Solvents were dried prior to use by conventional methods. 2,4-Dinitrophenyl (S)-2-methylbutanoate43 and (S)-Namino-2-methoxymethylpyrroli-dine44 were prepared as described elsewhere. Melting points were measured on Thermoscientific IA9100. 1H and 13C NMR spectra were processed on Bruker DRX400 spectrometer. J values are given in Hz. Optical rotation of polarized light was measured on Autopol II polarimeter in a 1 dm length cuvette. Synthetic Procedures. General Procedure for Synthesis of Ketones. First, 34 mmol of anhydrous aluminum chloride was mixed with 52 mL of benzene and cooled to 5 °C. Then, 31 mmol of chloroanhydride of (±)-2-methylbutyric acid was added dropwise for 0.5 h. The reaction mixture was stirred for 2 h in ice bath and further at rt overnight. A solution of 5 mL of concentrated hydrochloric acid in 45 mL of water was added. The organic layer was separated and washed with 100 mL of ice water and 100 mL of 1% aqueous solution of sodium hydrocarbonate. The organic layer was dried over anhydrous calcium chloride, and the solvent was evaporated and residue was distilled in vacuum. (±)-2-Methylbutyrophenone. Bp 92 °C (3 Torr) (lit.13 103 °C (8 Torr)). 1H NMR (400 MHz, CDCl3, Me4Si) 0.84 (3H, t, 3J = 7.4 Hz, CH2CH3), 1.12 (3H, d, 3J = 6.9 Hz, CHCH3), 1.37−1.47 (1H, m, CH2), 1.72−1.82 (1H, m, CH2), 3.34 (1H, CH), 7.37−7.41 (2H, m, C(3,5)H), 7.45−7.49 (1H, m, C(4)H), 7.90−7.92 (2H, m, C(2,6)H). 13 C (100 MHz, CDCl3, Me4Si) 11.42, 16.46, 26.37, 41.73, 127.90, 128.30, 132.46, 136.52, 203.9. (S)-2-Methylbutyrophenone. Bp 84−85 °C (3 Torr). (lit.45 40−45 °C (0.01 Torr)) [α]D + 36.2° (c 1.6 in Et2O). (lit.15,19 +36.6° (c 4.7 in Et2O)). 1H NMR (400 MHz, CDCl3, Me4Si) 0.93 (3H, t, 3J = 7.45 Hz, CH2CH3), 1.21 (3H, d, 3J = 6.9 Hz, CHCH3), 1.47−1.54 (1H, m, CH2), 1.81−1.88 (1H, m, CH2), 3.37−3.46 (1H, m, CH), 7.45−7.49 (2H, m, C(3,5)H), 7.55−7.58 (1H, m, C(4)H), 7.95−7.97 (2H, m, C(2,6)H). (2S)-2-Methoxymethyl-1-(1-phenylbutylidenamino)-pyrrolidine.45 1.43 g (9.6 mmol) of butyrophenone and 1.24 g (9.6 mmol) of (S)-N-amino-2-methoxymethylpyrrolidine were dissolved in 30 mL of benzene and refluxed with Dean−Stark trap for 144 h in argon atmosphere. The product was purified by gradient column chromatography on silica. Eluent: benzene to benzene/acetone 10:1. The product was obtained as a yellow oil (1.84 g, 74%). [α]D + 684° (c 1.3 in benzene) (lit.45 +640° (c 2.1 in benzene). 1H NMR (400 MHz, CDCl3, Me4Si) 0.90−0.95 (3H, m), 1.46−2.08 (6H, m), 2.42− 2.89 (3H, m), 3.27−3.63 (7H, m), 7.32−7.38 (3H, m), 7.65−7.68 (2H, m). (2′S)-2-Methoxymethyl-1-((2R)-2-methyl-1-phenylbutylidenamino)pyrrolidine.45 A round-bottomed flask with an addition funnel was filled with solution of 1 mL (7.1 mmol) of diisopropylamine in 14 mL of dry diethyl ether. The solution was cooled in an ice−salt bath, and 3.6 mL of 2 M BuLi solution in cyclohexane was added dropwise. The solution was stirred for 15 min on cooling bath. A solution of 1.59 g (6.1 mmol) of (2S)-2-methoxymethyl-1-(1-phenylbutylidenamino)pyrrolidine in 1.5 mL of dry diethyl ether was added dropwise. The suspension was stirred for 4 h on cooling bath, and the reaction mixture was cooled to −110 °C. A solution of 0.417 mL (6.7 mmol) of iodomethane in 1.5 mL of dry diethyl ether was added and stirred for 2 h. The solution was stirred at rt overnight and further poured in mixture of 30 mL of diethyl ether and 15 mL of water. Product was obtained as yellow oil (1.51 g, 91%). 1H NMR (400 MHz, CDCl3, Me4Si) 0.94 (3H, t, 3J = 7.4 Hz, CH2CH3), 1.08 (2H, d, 3J = 7 Hz, F

DOI: 10.1021/acs.organomet.7b00410 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

consisting of CO2 and cosolvent pumps, Acquity UPLC autosampler with partial loop injection, Acquity column manager for two columns of up to 15 cm length, Acquity convergence manager responsible for the conditioning of inlet and outlet CO2, and Acquity UPC2 PDA detector equipped with high-pressure flow cell. Chromatographic separations on this system were conducted under the following conditions: back pressure 105 bar, temperature 10 °C, flow rate 1.5 mL/min, injection volume 2 μL, and detection wavelength 210 nm. Separation was performed on Kromasil 3-CelluCoat 4.6 × 150 mm column with CO2/methanol/isopropylamine 70/30/1 mobile phase. Samples were made in methanol at 1 g/L concentration. X-ray Diffraction Experiments. Data were collected with a Bruker Apex2 CCD diffractometer using graphite monochromated Mo Kα radiation (λ = 0.71073 Å, ω-scans). The structures were solved by direct method and refined by the full-matrix least-squares against F2 in isotropic−anisotropic approximation. Hydrogen atoms positions with the exception of OH ones were calculated using geometric criteria. All hydrogen atoms were refined in isotropic approximation in riding model. Crystal data and structure refinement parameters for (E)-1rac, 2rac, and 3S are given in Table S1. All calculations were performed using the SHELXTL 2014 software.46 CCDC 1430829 (2rac), 1430830 (3S), and 1431462 ((E)-1rac) contain the supplementary crystallographic data. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge, CB21EZ, UK, or deposit@ ccdc.cam.ac.uk). Kinetic Experiments. Experiments were carried out in 0.01 M phosphate-buffered saline at pH 8.0. Optical density was detected at 4nitrophenolate adsorption wavelength of 405 nm and at 2,4dinitrophenolate adsorption wavelength of 360 nm. Catalyst was added as solution in acetonitrile. Final concentrations of catalyst in reaction mixture were in the range of 10−5−10−4 M. The final concentration of acetonitrile in reaction mixture was equal to 10 vol %. Pseudo-first-order rate constants kobs were approximated by nonlinear regression equation using SigmaPlot 11 software. All kinetics curves followed this equation for more than 5 half times of reaction. Second-order rate constants k2 were calculated from eqs 3 and 4.

C22H28Cl2N2O2Pd2: C 41.54, H 4.44, N 4.40. Found (%): C 41.83, H 4.63, N 4.40. X-ray quality crystals were obtained by slow evaporation of chloroform solution. General Procedure of Sequential Cyclopalladation−Monomerization. First, 0.6 mmol of palladium chloride was dissolved in a solution of 1.2 mmol of lithium chloride in 12 mL of water with slight heating. A solution of 1.1 mmol of mixture of E/Z-oximes in 23 mL of 1,4-dioxane was added, followed by addition of 0.6 mmol of sodium acetate trihydrate in 6 mL of water. The reaction mixture was refluxed for 15 min, and the suspension was cooled to rt and filtered. The solvent was removed; 10 mL of water was added and the product extracted with 15 mL of dichloromethane. The organic phase was dried over anhydrous sodium sulfate. Next, 0.6 mmol of pyridine was added, and the reaction mixture was stirred for 3 min. The solution was evaporated to dryness. The residue was dissolved in 2 mL of dichloromethane, and the palladacycle was precipitated by adding this solution to 50 mL of n-hexane under vigorous stirring. Obtained precipitate was filtered, washed with 10 mL of n-hexane, and dried. Chloro[hydroxyimino-2-methyl-1-phenylbutanone-κ 2 C 2 ′ ,N](pyridine)palladium(II) (3rac). Yield 85%. White powder. Mp 132− 135 °C. Calcd: C 48.38%, H 4.82%, N 7.05%. Found: C 48.40%, H 4.79%, N 6.93%. 1H NMR (400 MHz, CDCl3, Me4Si) 0.96 (3H, t, 3J = 7.4 Hz, CHCH3), 1.40 (3H, d, 3J = 7.1 Hz, CHCH3), 1.69−1.79 (1H, m, CH2), 1.99−2.07 (1H, m, CH2), 3.19 (1H, br s, CH), 6.23 (1H, d, 3 J = 7.5 Hz, C(6)H−Ph), 6.90 (1H, m, C(5)H−Ph), 7.06 (1H, m, C(4)H−Ph), 7.21 (1H, d, 3J = 7.2 Hz, C(3)H−Ph), 7.47−7.50 (2H, m, C(3,5)H−Py), 7.89−7.92 (1H, m, C(4)H−Py), 8.86−8.88 (2H, m, C(2,6)H−Py), 10.16 (1H, br s, OH). 13C NMR (100 MHz, CDCl3, Me4Si) 12.63, 16.21, 26.03, 34.74, 124.69, 125.60, 126.02, 128.86, 131.26, 138.41, 143.29, 152.98, 153.95, 172.02. Chloro[hydroxyimino-(S)-2-methyl-1-phenylbutanone-κ2C2′,N](pyridine)palladium(II) (3S). Yield 76%. White powder. Mp. 129−131 °C. ee 86%. [α]D + 10° (c 0.2 in CH2Cl2). Calcd: C 48.38%, H 4.82%, N 7.05%. Found: C 48.44%, H 4.54%, N 6.99%. 1H NMR (400 MHz, CDCl3, Me4Si) 0.95 (3H, t, 3J = 7.4 Hz, CH2CH3), 1.40 (3H, d, 3J = 7.1 Hz, CHCH3), 1.70−1.80 (1H, m, CH2), 1.99−2.07 (1H, m, CH2), 3.18 (1H, br s, CH), 6.23 (1H, d, 3J = 7.5 Hz, C(6)H−Ph), 6.90 (1H, m, C(5)H−Ph), 7.06 (1H, m, C(4)H−Ph), 7.21 (1H, d, 3J = 7.2 Hz, C(3)H−Ph), 7.47−7.50 (2H, m, C(3,5)H−Py), 7.89−7.92 (1H, m, C(4)H−Py), 8.86−8.88 (2H, m, C(2,6)H−Py), 10.19 (1H, s, OH). 13 C NMR (100 MHz, CDCl3, Me4Si) 12.63, 16.21, 26.00, 34.70, 124.69, 125.61, 126.02, 128.85, 131.24, 138.41, 143.26, 152.95, 153.93, 171.99. X-ray quality crystals were obtained by slow evaporation of chloroform solution. Chloro[hydroxyimino-(R)-2-methyl-1-phenylbutanone-κ2C2′,N](pyridine)palladium(II) (3R). Yield 86%. Pale yellow powder. Mp 132−135 °C. ee 79%. [α]D − 9° (c 0.2 in CH2Cl2). Calcd: C 48.38%, H 4.82%, N 7.05%. Found: C 48.26%, H 4.63%, N 7.03%. 1H NMR (400 MHz, CDCl3, Me4Si) 0.95 (3H, t, 3J = 7.4 Hz, CH2CH3), 1.40 (3H, d, 3J = 7.1 Hz, CHCH3), 1.70−1.80 (1H, m, CH2), 1.99−2.07 (1H, m, CH2), 3.18 (1H, br s, CH), 6.23 (1H, d, 3J = 7.5 Hz, C(6)H− Ph), 6.90 (1H, m, C(5)H−Ph), 7.06 (1H, m, C(4)H−Ph), 7.21 (1H, d, 3J = 7.2 Hz, C(6)H−Ph), 7.47−7.50 (2H, m, C(3,5)H−Py), 7.89− 7.92 (1H, m, C(4)H−Py), 8.86−8.88 (2H, m, C(2,6)H−Py), 10.20 (1H, s, OH). 13C NMR (100 MHz, CDCl3, Me4Si) 12.64, 16.22, 26.03, 34.77, 124.70, 125.61, 126.02, 128.86, 131.26, 138.41, 143.29, 152.98, 153.95, 172.02. Chiral Supercritical Fluid Chromatography. Enantiomeric compositions of oximes 1rac, 1S, and 1R were established by supercritical fluid chromatography (SFC) using the Investigator system (Waters Corp, Milford, MA, USA). Simultaneous separation of all four oxime stereoisomers was performed on Kromasil 3-AmyCoat 4.6 × 150 mm column with CO2/isopropanol 93/7 mobile phase. Conditions: back pressure 100 bar, temperature 40 °C, flow rate 4 mL/min, injection volume 2 μL, and detection wavelength 220 nm. Samples were made in hexane at 1 g/L concentration. Enantiomeric composition of oxime palladium complexes 3rac, 3S, and 3R were established by supercritical fluid chromatography (SFC) using Acquity UPC2 system (Waters Corp, Milford, MA, USA). The system is equipped with the following: Acquity ccBSM pump module

D(t ) = D∞ − (D∞ − D0) ekobs

t

(3)

kobs = k 0 − k 2[Pd]

(4)

where k0 is the rate constant of hydrolysis of ester without catalyst and [Pd0] is the initial concentration of palladacycle.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00410. SFC chromatograms, X-ray details, and NMR spectra (PDF) Accession Codes

CCDC 1430829−1430832 and 1431462 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Sergey Z. Vatsadze: 0000-0001-7884-8579 Ivan V. Ananyev: 0000-0001-6867-7534 G

DOI: 10.1021/acs.organomet.7b00410 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Notes

(24) Enders, D.; Hundertmark, T.; Lazny, R. Synlett 1998, 1998, 721−722. (25) Porcheddu, A.; Giacomelli, G. In The Chemistry of Hydroxylamines, Oximes and Hydroxamic Acids; Rappoport, Z., Liebman, J. F., Eds.; John Wiley & Sons, Ltd.: Chichester, U.K., 2009; p 164. (26) Grob, C. A.; Ide, J. Helv. Chim. Acta 1974, 57, 2571−2583. (27) Mizutani, Y.; Tanimoto, H.; Morimoto, T.; Nishiyama, Y.; Kakiuchi, K. Tetrahedron Lett. 2012, 53, 5903−5906. (28) Kuroda, Y.; Imaizumi, K.; Yamada, K.; Yamaoka, Y.; Takasu, K. Tetrahedron Lett. 2013, 54, 4073−4075. (29) Politzer, P.; Murray, J. S. In The Chemistry of Hydroxylamines, Oximes and Hydroxamic Acids; Rappoport, Z., Liebman, J. F., Eds.; John Wiley & Sons: Chichester, U.K., 2009; p 18. (30) Shimbayashi, T.; Okamoto, K.; Ohe, K. Organometallics 2016, 35, 2026−2031. (31) Phinney, K. W.; Stringham, R. W. In Chiral Separation Techniques; Subramanian, G., Ed.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2007; pp 135−154. (32) Nosova, V. M.; Ustynyuk, Y. A.; Bruk, L. G.; Temkin, O. N.; Kisin, A. V.; Storozhenko, P. A. Inorg. Chem. 2011, 50, 9300−9310. (33) Zanini, M.; Meneghetti, M. R.; Ebeling, G.; Livotto, P. R.; Rominger, F.; Dupont, J. Inorg. Chim. Acta 2003, 350, 527−536. (34) Hockless, D. C. R.; Gugger, P. A.; Leung, P. H.; Mayadunne, R. C.; Pabel, M.; Wild, S. B. Tetrahedron 1997, 53, 4083−4094. (35) Yap, J. S. L.; Li, B. B.; Wong, J.; Li, Y.; Pullarkat, S. A.; Leung, P.H. Dalton Trans. 2014, 43, 5777−5784. (36) Nieto, S.; Arnau, P.; Serrano, E.; Navarro, R.; Soler, T.; Cativiela, C.; Urriolabeitia, E. P. Inorg. Chem. 2009, 48, 11963−11975. (37) Li, Y.; Selvaratnam, S.; Vittal, J. J.; Leung, P.-H. Inorg. Chem. 2003, 42, 3229−3236. (38) Mentes, A.; Kemmitt, R. D. W.; Fawcett, J.; Russell, D. R. J. Mol. Struct. 2004, 693, 241−246. (39) Ding, Y.; Li, Y.; Pullarkat, S. A.; Leng Yap, S.; Leung, P.-H. Eur. J. Inorg. Chem. 2009, 2009, 267−276. (40) Ding, Y.; Chiang, M.; Pullarkat, S. A.; Li, Y.; Leung, P.-H. Organometallics 2009, 28, 4358−4370. (41) Ng, J. K.-P.; Tan, G.-K.; Vittal, J. J.; Leung, P.-H. Inorg. Chem. 2003, 42, 7674−7682. (42) Medved’ko, A. V.; Kurzeev, S. A.; Vatsadze, S. Z.; Kazankov, G. M. Moscow Univ. Chem. Bull. 2008, 63, 255−259. (43) Vatsadze, S. Z.; Medved’ko, A. V.; Kazankov, G. M.; Kurzeev, S. A. ChemSpider Synth. Pages 2013, 70510.1039/SP705 (44) Enders, D.; Fey, P.; Kipphardt, H. Org. Synth. 1987, 65, 173. (45) Enders, D.; Eichenauer, H.; Baus, U.; Schubert, H.; Kremer, K. M. A. Tetrahedron 1984, 40, 1345−1359. (46) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122.

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The synthetic, catalytic, and structural parts of this work were supported by RFBR (grant no.# 13-03-12054-ofi-m). Parts of the work concerning supercritical fluid chromatography and stereochemistry assignments were supported by RSF (grant no. 14-33-00017-p).

■

REFERENCES

(1) Ananikov, V. P.; Khemchyan, L. L.; Ivanova, Y. V.; Bukhtiyarov, V. I.; Sorokin, A. M.; Prosvirin, I. P.; Vatsadze, S. Z.; Medved’ko, A. V.; Nuriev, V. N.; Dilman, A. D.; Levin, V. V.; Koptyug, I. V.; Kovtunov, K. V.; Zhivonitko, V. V.; Likholobov, V. A.; Romanenko, A. V.; Simonov, P. A.; Nenajdenko, V. G.; Shmatova, O. I.; Muzalevskiy, V. M.; Nechaev, M. S.; Asachenko, A. F.; Morozov, O. S.; Dzhevakov, P. B.; Osipov, S. N.; Vorobyeva, D. V.; Topchiy, M. A.; Zotova, M. A.; Ponomarenko, S. A.; Borshchev, O. V.; Luponosov, Y. N.; Rempel, A. A.; Valeeva, A. A.; Stakheev, A. Y.; Turova, O. V.; Mashkovsky, I. S.; Sysolyatin, S. V.; Malykhin, V. V.; Bukhtiyarova, G. A.; Terent’ev, A. O.; Krylov, I. B. Russ. Chem. Rev. 2014, 83, 885−985. (2) Breslow, R. Chem. Soc. Rev. 1972, 1 (4), 553−580. (3) Xia, J.; Xu, Y.; Li, S.; Sun, W.; Yu, K.; Tang, W.-X. Inorg. Chem. 2001, 40, 2394−2401. (4) Li, S.-A.; Yang, D.-X.; Li, D.-F.; Huang, J.; Tang, W.-X. New J. Chem. 2002, 26, 1831−1837. (5) Hong, J.; Jiao, Y.; Yan, J.; He, W.; Guo, Z.; Zhu, L.; Zhang, J. Inorg. Chim. Acta 2010, 363 (4), 793−798. (6) Dupont, J.; Consorti, C. S.; Spencer, J. Chem. Rev. 2005, 105 (6), 2527−2571. (7) Alonso, D. A.; Nájera, C.; Pacheco, M. C. Adv. Synth. Catal. 2002, 344 (2), 172−183. (8) Alonso, D. A.; Nájera, C. Chem. Soc. Rev. 2010, 39 (8), 2891− 2902. (9) Ryabov, A. D.; Kazankov, G. M.; Yatsimirskii, A. K.; Kuz’mina, L. G.; Burtseva, O. Y.; Dvortsova, N. V.; Polyakov, V. A. Inorg. Chem. 1992, 31, 3083−3090. (10) Dunina, V. V.; Gorunova, O. N.; Zykov, P. A.; Kochetkov, K. A. Russ. Chem. Rev. 2011, 80, 51−74. (11) Yatsimirsky, A. K.; Kazankov, G. M.; Ryabov, A. D. J. Chem. Soc., Perkin Trans. 2 1992, 8, 1295−1300. (12) Onoue, H.; Minami, K.; Nakagawa, K. Bull. Chem. Soc. Jpn. 1970, 43, 3480−3485. (13) Selvakumar, K.; Vancheesan, S.; Varghese, B. Polyhedron 1997, 16, 2257−2262. (14) Vicente, J.; Chicote, M.-T.; Abellán-López, A.; Bautista, D. Dalton Trans. 2012, 41, 752−762. (15) Abellán-López, A.; Chicote, M.-T.; Bautista, D.; Vicente, J. Dalton Trans. 2014, 43, 592−598. (16) Abellán-López, A.; Chicote, M.-T.; Bautista, D.; Vicente, J. Organometallics 2012, 31, 7434−7446. (17) Abellán-López, A.; Chicote, M.-T.; Bautista, D.; Vicente, J. Organometallics 2013, 32, 7612−7624. (18) Vatsadze, S. Z.; Medved’ko, A. V.; Zyk, N. V.; Maximov, A. L.; Kurzeev, S. A.; Kazankov, G. M.; Lyssenko, K. A. Organometallics 2009, 28, 1027−1031. (19) Potapov, V. M.; Demyanovich, V. M.; Zaitsev, V. P.; Karnaukh, A. P. Moscow Univ. Chem. Bull. 1974, 15, 594−600. (20) Potapov, V. M.; Demyanovich, V. M.; Zaitsev, V. P. Zh. Org. Khim. 1978, 14, 91−97. (21) Aurelly, M.; Lamaty, G. Bull. Soc. Chim. Fr. 1980, 7−8, 389− 394. (22) Bartlett, P. D.; Stauffer, C. H. J. Am. Chem. Soc. 1935, 57, 2580− 2583. (23) Job, A.; Janeck, C. F.; Bettray, W.; Peters, R.; Enders, D. Tetrahedron 2002, 58, 2253−2329. H

DOI: 10.1021/acs.organomet.7b00410 Organometallics XXXX, XXX, XXX−XXX