Regio- and Stereoselective 1,3-Dipolar Cycloaddition of Cyclic

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Regio- and Stereoselective 1,3-Dipolar Cycloaddition of Cyclic Azomethine Imines to Platinum(IV)-Bound Nitriles Giving Δ2‑1,2,4Triazoline Species Andrey S. Smirnov,† Andreii S. Kritchenkov,† Nadezhda A. Bokach,*,† Maxim L. Kuznetsov,‡ Stanislav I. Selivanov,† Vladislav V. Gurzhiy,† Andreas Roodt,§ and Vadim Yu. Kukushkin*,†,∥ †

Saint Petersburg State University, Universitetskaya nab. 7/9, 199034, Saint Petersburg, Russian Federation Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisbon, Portugal § Department of Chemistry, University of the Free State, P.O. Box 339, Bloemfontein 9300, South Africa ∥ Institute of Macromolecular Compounds of Russian Academy of Sciences, V.O. Bolshoii Pr. 31, 199004 Saint Petersburg, Russian Federation ‡

S Supporting Information *

ABSTRACT: The complex trans-[PtCl4(EtCN)2] (14) reacts smoothly at 25 °C with the stable cyclic azomethine imines R 1CHNaNC(O)CH(NHC(O)C6H 4R3)CbH(C6H 4R2) (a−b) [R1/R2/R3 = p-Me/H/H (8); p-Me/p-Me/H (9); p-Me/pMeO/H (10); p-Me/p-Cl/p-Cl (11); p-MeO/p-Me/H (12); pMeO/p-Cl/m-Me (13)], and the reaction proceeds as stereoselective 1,3-dipolar cycloaddition to one of the EtCN ligands accomplishing the monocycloadducts trans-[PtCl4(EtCN){Na C(Et)NbC(O)CH(NHC(O)C6H4R3)CH(C6H4R2)NcCdHR1}])(a−d;b−c) [R1/R2/R3 = p-Me/H/H (15); p-Me/pMe/H (16); p-Me/p-MeO/H (17); p-Me/p-Cl/p-Cl (18); pMeO/p-Me/H (19); p-MeO/p-Cl/m-Me (20)]. Inspection of the obtained and literature data indicate that the cycloaddition of the azomethine imines to the C≡N bonds of HCN and of PtIV-bound EtCN has different regioselectivity leading to Δ2-1,2,3triazolines and Δ2-1,2,4-triazolines, respectively. Platinum(II) species trans-[PtCl2(EtCN){NaC(Et)NbC(O)CH(NHC(O)C6H4R3)CH(C6H4R2)NcCdHR1}](a−d;b−c) [R1/R2/R3 = p-Me/H/H (21); p-Me/p-Me/H (22); p-Me/p-MeO/H (23); p-Me/pCl/p-Cl (24); p-MeO/p-Me/H (25); p-MeO/p-Cl/m-Me (26)] were obtained by a one-pot procedure from 14 and 8−13 followed by addition of the phosphorus ylide Ph3PCHCO2Me. Δ2-1,2,4-Triazolines NaC(Et)NbC(O)CH(NHC(O)C6H4R3)CH(C6H4R2)NcCdHR1(a−d;b−c) [R1/R2/R3 = p-Me/H/H (27); p-Me/p-Me/H (28); p-Me/p-MeO/H (29); p-Me/p-Cl/ p-Cl (30); p-MeO/p-Me/H (31); p-MeO/p-Cl/m-Me (32)] were liberated from 21−26 by the treatment with bis(diphenylphosphyno)ethane (dppe). Platinum(II) complexes 21−26 were characterized by elemental analyses (C, H, N), high-resolution electrospray ionization mass spectrometry (ESI-MS), and IR and 1H and 13C{1H} NMR spectroscopies and single crystal X-ray diffraction in the solid state for 25·CH3OH, 26·(CHCl3)0.84. The structure of 26 was also determined by COSY-90 and NOESY NMR methods in solution. Quantitative evaluation of several pairs of interproton distances obtained by NMR and X-ray diffraction agrees well with each other and with those obtained by the MM+ calculation method. Platinum(IV) complexes 15−20 were characterized by 1H NMR spectroscopy. Metal-free 6,7-dihydropyrazolo[1,2-a][1,2,4]triazoles (27−32) were characterized by high-resolution ESI-MS and IR and 1H and 13C{1H} NMR spectroscopies and single crystal X-ray diffraction for 29·CDCl3. Theoretical density functional theory calculations were carried out for the investigation of the reaction mechanism, interpretation of the reactivity of Pt-bound and free nitriles toward azomethine imines and analysis of the regio- and stereoselectivity origin.

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INTRODUCTION

metal-bound nitriles is much more limited and was restricted basically to nitrones (acyclic nitrones,3 cyclic nitrones including oxazoline-N-oxides,4 imidazoline-N-oxides,5 pyrroline-N-oxides,6 and nitronates7). Application of another important

1,3-Dipolar cycloaddition of asymmetric dipoles to metal-free and ligated nitrile substrates is a powerful method for generation of a great variety of heterocyclic systems. Despite the wealth of allyl-anion type dipoles explored as reactants in alkene1 and alkyne2 cycloaddition chemistry, the range of the dipoles employed in cycloaddition with uncomplexed and © XXXX American Chemical Society

Received: September 30, 2015

A

DOI: 10.1021/acs.inorgchem.5b02246 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

dipolarophilicity. In many instances, the terminal N-coordination of RCN to platinum(II- or IV) and palladium(II) centers results in promotion of cycloadditions, which could not be realized for metal-free RCN species, and we employed this useful activation method for cycloaddition of azomethine imines.3a,e−g,7,12 As an amplification of our previous studies on metalmediated cycloadditions and in view of our permanent interest in metal-mediated reactions of ligands featuring the C≡N bond,3e−g we attempted cycloaddition of the azomethine imines (Scheme 2) to an EtCN ligand in the [PtCl4(EtCN)2] complex. We used the platinum(IV)-center as an activator of the EtCN ligand because the activation effect of a platinum(II) is not sufficient for cycloaddition of these dipoles to nitrile ligands. The scenario of this work was the following: (i) to study the reactivity of the cyclic azomethine imines toward EtCN ligands in the platinum complex; (ii) to estimate regioand stereoselectivity of this cycloaddition; (iii) to liberate Δ21,2,4-triazoline species and to characterize these new heterocycles.

category of dipoles, i.e., azomethine imines, so far has attracted only a little attention.8 In general, azomethine imines are a rather poorly explored allyl-anion type dipoles, and their cycloaddition reactions were studied only for alkene1g,9 and alkyne1g,2b,c,10 functionalities. Cycloaddition of azomethine imines to nitriles has been investigated to quite a limited extent, and a few reported reactions include cycloaddition of highly reactive azomethine imines generated in situ from diaziridines8c,d (Scheme 1, route Scheme 1. Reported Cycloadditions of Azomethine Imines to Electron-Deficient Nitriles

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RESULTS AND DISCUSSION

Cycloaddition of the Cyclic Azomethine Imines. In the current work, azomethine imines (rel-(4R,5R,Z)-4-arylmido-2arylidene-5-oxo-3-arylpyrazolidin-2-ium-1-ide) 8−13, on the one hand, and the nitrile complex trans-[PtCl4(EtCN)2] (14), on the other hand, were employed as reactants for the cycloaddition study. Rel-(4R,5R,Z) dilpoles 8−13 were obtained by condensation of the corresponding rel-(4R,5R) pyrazolidinones 1−5 with aromatic aldehydes 6 and 7 (Scheme 2) using a modified protocol reported in ref 13 and given in Experimental Section. Dipoles 8−13 were obtained as racemic mixtures with relative configuration rel-(4R,5R,Z). The Zconfiguration of these species was established by the NOESY method, whose results indicated that the distance between the C5H and C(H)N protons is 0.25 nm; this value is characteristic for the Z-species derived from rel-(4R,5R) pyrazolidinones and aromatic aldehydes.13 The reaction of any one of 8−13 with 14 in a molar ratio 1:1 was performed in CH2Cl2 at room temperature (25 °C) under Ar atmosphere for ca. 2 h. Under these conditions, we observed generation of 15−20 in good (ca. 70−90%) NMR yields (Scheme 3, route A; for plots of NMR spectra of all complexes synthesized in this work and NMR data see Supporting Information). Thus, generated complexes are not sufficiently stable in solution, and products of decomposition of platinum(IV) complexes 15−20 appear in the reaction mixture after keeping these mixtures at 25 °C for more than 2 h; among these products we identified platinum(II) complexes 21−26 and 1,3-dipoles 8−13. We isolated 15−20 (10−20%) as the solids by fast precipitation from the reaction mixture with

A) or cycloaddition of cyclic azomethine imines to nitriles8e (Scheme 1, route B). Both cycloadditions proceed exclusively to electron-deficient nitriles, and no reaction with alkyl- or arylnitriles having donor or moderate acceptor groups was observed. For this study, we addressed the stable cyclic azomethine imines (Scheme 2) as dipoles. Cycloaddition of these azomethine imines to RCN should lead to 6,7-dihydropyrazolo[1,2-a][1,2,4]triazoles species (or, for simplicity, Δ2-1,2,4triazolines), whose synthesis and chemistry are still quite unexplored. In fact, 6,7-dihydropyrazolo[1,2-a][1,2,4]triazoles represent a class of compounds that, although known, is extremely limited in the number of heterocycles synthesized. To the best of our knowledge, only two methods of their preparation were reported, and they are based on the reaction of unsubstituted or N-aryl substituted 1-amidinopyrazolidines with Ph isothiocyanates, thiocarbonyl dichloride, and formic- or acetic acid derivatives.11 The most relevant method for generation of Δ2-1,2,4-triazolines is based on cycloaddition of azomethine imines to CN groups activated by strong electronwithdrawing substituents R (Scheme 1, route B).8c−e However, we found that the azomethine imines do not react with metalfree RCN (R = Alk, Ar) due to their rather low Scheme 2. Synthesis of New Cyclic Azomethine Imines 8−13

B


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Inorganic Chemistry Scheme 3. Studied Reactions

Scheme 4. Different Regeoselectivity in Cycloaddition of Azomethine Imines to PtIV-Bound Nitriles (A) and to the Uncomplexed HCN (B)

modified protocol than that described in ref 14 (Scheme 3, route B). Moreover, we developed a one-pot procedure for synthesis of 21−26 from 14 that consists of treatment of 8−13 with 14 (molar ratio 1.5:1, 25 °C, 1−4 h, CH2Cl2) giving 15− 20 followed by addition of 1 equiv of Ph3PCHCO2Me to the reaction mixture (Scheme 3, routes A and B) furnishing 21−26 after 2 h at 25 °C. 1H NMR monitoring of the reaction mixtures of 21−26 demonstrates only one set of signals, which could correspond to a cycloaddition product (see next section for product characterization). It could be concluded that the reaction proceeds diasteroselectively. The platinum(II) species were purified by column chromatography on silica gel and isolated in 60−78% yields (see Experimental Section and Supporting Information). The theoretical calculations15 predict that cycloaddition of azomethine imines to platinum-bound nitriles is favorable from both thermodynamic and kinetic viewpoints, and a PtIV center should activate nitrile ligands toward cycloaddition of azomethine imines to a greater extent than the relevant PtII center. However, when the nitriles are activated by a platinum(IV) center, another side reaction, i.e., an oxidation of azomethine imines by the metal center could be expected.15 These predictions are in agreement with our experimental data. Indeed, 8−13 do not react with PtII-bound EtCN or with more dipolarophilic3c nitrile ligands in trans-[PtCl2(PhCN)2] or trans-[PtCl2(Me2NCN)2]. When the platinum(IV) complex is

hexane. Released 15−20 are substantially contaminated with products (1H NMR ca. 25%) of some secondary reactions, and the mother liquors contain spectrum of various platinum and organic species as verified by high-resolution electrospray ionization mass spectrometry (HRESI-MS) coupled with a thin layer chromatography (TLC) interface. The reaction between 8−13 and 14 in a molar ratio 1:2 at 25 °C proceeds similarly, affording mono-cycloaddition products 15−20, and the second cycloaddition of the azomethine imines to the remaining EtCN ligand in 15−20 does not proceed even under prolonged reaction times, e.g., five days, 25 °C, CH2Cl2. These experimental observations indicate that the formed heterocycles are rather strong donors, and they deactivate the EtCN ligand in the trans-position toward cycloaddition of the azomethine imines. Owing to the instability of platinum(IV) complexes 15−20 leading to secondary reactions, we decided to reduce these compounds into the corresponding platinum(II) species 21− 26 hoping that 21−26 would be more inert and thus allow their characterization and facilitate further liberation of the Δ2-1,2,4triazoline ligands; this speculation was indeed confirmed. The phosphorus ylide Ph3PCHCO2Me is known as an efficient reducing agent for selective and smooth reduction of (imine)PtIV into corresponding (imine)PtII species in nonaqueous media.14 We demonstrated that 15−20 could be reduced to 21−26 with Ph3PCHCO2Me by a slightly C


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Inorganic Chemistry

Scheme 5. Liberation of Δ2-1,2,4-triazolines R1/R2/R3: p-Me/H/H (27); p-Me/p-Me/H (28); p-Me/p-MeO/H (29); p-Me/pCl/ p-Cl (30); p-MeO/p-Me/H (31); p-Me/p-Cl/m-Me (32)

Figure 1. View of 25 with the atom numbering scheme. Thermal ellipsoids are drawn at the 50% probability level. The solvent was omitted for clarify.

used, cycloaddition of 8−13 proceeds selectively only in the case of trans-[PtCl4(EtCN)2], but the platinum(IV) center in trans[PtCl4(PhCN)2] or trans-[PtCl4(Me2NCN)2] activates PhCN and Me2NCN ligands so strongly that cycloaddition loses its selectivity, and even at −30 °C the reaction leads to a broad mixture of yet unidentified products. To the best of our knowledge, cycloaddition of 8−13 or other similar azomethine imines to nitriles (Scheme 4, route A) was not reported until this work. However, cycloaddition of such dipoles to the CN bond of HCN was previously described by Stanovnik and colleagues.13 It is interesting that the latter reaction has completely reverse regioselectivity and leads to 5,6-dihydropyrazolo[1,2-a][1,2,3]triazol-7(3H)-one, which then tautomerizes to 5,6-dihydropyrazolo[1,2-a][1,2,3]triazol-7(1H)-ones (Scheme 4, route B). Generation of Uncomplexed Δ2-1,2,4-Triazolines. We described above that cycloaddition of azomethine imines 8−13 to the platinum(IV)-bound EtCN followed by reduction with Ph3PCHCO2Me leads to (Δ2-1,2,4-triazoline)PtII complexes 21−26. The Δ2-1,2,4-triazolines refer to a rare class of annulated heterocycles whose properties are virtually unknown because procedures for their synthesis are poorly developed, and the reported 1,5,6,7-tetrahydropyrazolo[1,2-a][1,2,4]triazoles are very limited in number (see Introduction). In this work, we succeeded in the generation of Δ2-1,2,4triazolines via cycloaddition of the cyclic azomethine imines 8− 13 to the EtCN ligand featuring the electron-donor substituent

R. Liberation of the heterocyclic ligands from 21−26 via substitution should give uncomplexed Δ2-1,2,4-triazolines. Several methods for the liberation of the strongly bound imines and nitrogen heterocycles from their platinum complexes have been developed, and they are based on displacement with an excess of diphosphine,16 monodentate, and bidentate amines bearing two sp3-N donor centers3b,c,4 or alkali metal cyanides.3e,f,17 Platinum(II) species 21−26, which are relatively labile toward substitution, were utilized for the liberation of the Δ2-1,2,4-triazolines (27−32) by treatment with bis(diphenylphosphyno)ethane (dppe) (Scheme 5). Characterization of 15−20 and 21−26. Complexes 21− 26 were obtained as yellow solids and characterized by elemental analyses (C, H, N), high resolution ESI+-MS, IR, and 1H and 13C{1H} NMR spectroscopies (see Supporting Information for experimental data and plots of NMR spectra of all complexes synthesized in this work). Compounds 25 and 26 were characterized by X-ray diffraction, and compound 26 also in solution by COSY-90 and NOESY NMR experiments; see the next section and Supporting Information. All platinum(II) species gave satisfactory microanalyses. In the HRESI+-MS, the typical ion that was detected is [M + Na]+. The IR spectra of all complexes display ν(CN) stretching vibrations at 1660−1670 cm−1 (for 21−26). The intensive ν(CO) vibrations are in the range between 1755 and 1785 cm−1 for 21−26. D


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Inorganic Chemistry

Figure 2. View of 26 with the atom numbering scheme. Thermal ellipsoids are drawn at the 30% probability level. The solvent was omitted for clarify.

Figure 3. Structure of 26. Double-headed arrows indicate spatial interactions from the NOESY data (MM+ calculation method).

spectra of 15−20 exhibit signals from the Δ2-1,2,4-triazolines and the propiononitrile ligands. Some signals are low-field shifted relatively corresponding signals from the organic ligands in platinum(II) complexes 21−26; i.e., the CH2 protons from the ethyl of the nitrile ligand in 15−20 (2.85−3.08 ppm) are low-field shifted by ca. 0.4 ppm. Two quartets of the diastereotopic CH2 protons from the ethyl group of the heterocyclic ligands appear at 3.17−3.37 and 4.37−4.51 ppm, correspondingly; the former group of peaks is lower (by 0.2 ppm) than that in 21−26, and the latter is higher (by 0.4−0.5 ppm). The 1H NMR spectra of 15−20 also display the multiplet of C1H (6.48−6.56 ppm, s + d, JPtH 12−13 Hz), doublet of doublets of C6H (4.94−5.48 ppm), and doublet of C7H (4.40−6.67 ppm). Solid State and Solution Structures. Solid structures of 25·CH3OH and 26·(CHCl3)0.84. Complexes 25·CH3OH and 26· (CHCl 3 ) 0.84 were characterized by single-crystal X-ray diffraction (Figures 1 and 2). In both complexes, the bicyclic Δ2-1,2,4-triazoline and the nitrile ligands lie in the transposition to each other. The Pt(1)−N(2) bond lengths (1.992(3) and 1.991(5) Å in 25 and 26, correspondingly) are

1

H and 13C{1H} NMR spectra of 21−26 exhibit signals from the Δ2-1,2,4-triazolines and the propiononitrile ligands. The 1H NMR spectra display two quartets (3.42−3.53 and 3.75−3.84 ppm for 21−26) of the diastereotopic CH2 protons from the ethyl group of the heterocyclic ligands. The CH2 protons from the ethyl of the nitrile ligand (distantly located from the chiral centers) appear as one low-field shifted quartet at 2.48−2.60 ppm for 21−26. The 1H NMR spectra of 21−26 also display the singlet of C1H (5.93−6.02 ppm), doublet of doublets of C6H (5.08−5.34 ppm), and doublet of C7H (4.65−4.78 ppm). In the 13C{1H} NMR spectra of 21−26, the peaks due to the C1 (ca. 90 ppm), C6 (ca. 74 ppm), C7(ca. 60 ppm), three peaks due to C5O, NHC(O), and CN (ca. 162−167 ppm) carbons were identified. Rather unstable platinum(IV) complexes 15−20 were still isolated from the reaction mixtures as the solids. These solids are contaminated with yet unidentified products (ca. 25%), and therefore they were characterized only by the 1H NMR method. In the 1H NMR spectra of 15−20, signal integrations give evidence that the reaction of starting complex 14 and each of the azomethine imines proceeds in a 1:1 ratio. The 1H NMR E


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Inorganic Chemistry

Figure 4. View of 29 with the atom numbering scheme, the solvent was omitted for clarify. Thermal ellipsoids are drawn at the 50% probability level.

the diagonal peaks Sij/Sii on the mixing time τm.21 This dependence is exponential, and the cross-relaxation rate was defined as a derivative of the exponent at τm = 0.20c More details on the procedure can be found in Supporting Information. Table S2 displays the interproton distances for 26 obtained by NMR, rij(NMR), by X-ray diffraction, rij(X‑ray), and by MM+ calculation method, rij(MM+). Characterization of Uncomplexed Δ2-1,2,4-Triazolines. Metal-free 1,5,6,7-tetrahydropyrazolo[1,2-a][1,2,4]triazole species 27−32 were characterized by HRESI+-MS, IR, 1 H and 13C{1H} NMR spectroscopy and also by X-ray diffraction study for 29. In the ESI+-MS, the observed peaks were attributed to [M + H]+ and [M + Na]+, whereas the 13 C{1H} NMR spectra of 27−32 exhibit all signals specific for these heterocycles; the liberated species show characteristic resonances from C7 (60.0−62.7 ppm), C6 (73.0−75.2 ppm), C1 (91.0−91.4 ppm), and three low-field signals from C(O)NH, C5, and CN (155.1−156.3, 159.0−166.1, and 165.8−167.3 ppm). Solid Structure of 29·CDCl3. Solvate 29·CDCl3 was characterized by single-crystal X-ray diffraction (Figure 4). The bicyclic Δ2-1,2,4-triazoline adopt rel-(1S,6S,7S) configuration that is similar to those for coordinated Δ2-1,2,4triazolines in both 25 and 26. The CN bond distance (N2− C3 1.274(2) Å) and the N−N bond (1.461(2) Å) are comparable with those in heterocyclic ligands of 25 and 26 (see above), and in general the decoordination does not noticeably affect the geometric parameters of the Δ2-1,2,4-triazolines. Intermolecular hydrogen bonding is formed between the imine nitrogen N2 and the hydrogen of amide N(12)H group [N12··· N2 2.922(2) and 2.240(2) Å, N12−H12···N2 136.25(8)°]. Theoretical Study of the Cycloaddition. With the aim to uncover some of the mechanistic features of the processes under study and to interpret the reaction selectivity, quantumchemical calculations of cycloadditions of azomethine imine (3R,4R,Z)-4-phenylamido-2-phenylidene-5-oxo-3-phenylpyrazolidin-2-ium-1-ide (33) to free MeCN and the platinum(II)

typical for (imine)PtII species.18 The CN bond distance (N2−C3 1.281(5) and 1.285(9) Å, respectively) is in the range of typical double bonds.18 The N−N bonds in the bicyclic ligands (1.468(4) and 1.445(7) Å, correspondingly) are comparable with the typical N−N single bond (1.454(21) Å).18 In both 25 and 26, the heterocyclic ligands adopt rel(1S,6R,7R) configuration indicating the retention of the relative configuration of two C atoms, rel-(4R,5R), from the starting dipole. Solution Structure of 26. The solution structure of 26 was assessed by inspection of scalar and direct (through space) dipole−dipole interactions detected, respectively, in the COSY90 and NOESY spectra (see Supporting Information). These interactions are shown in Figure 3 by the double-headed arrows. The NH12 proton resonates at 7.09 ppm and has crosspeaks with H12 in the COSY spectrum and spatial interactions with H6, H7, H16, and H21 in the NOESY spectrum. The values of the vicinal coupling constants 3JH6−H7 = 11.4 Hz and 3JH6−H12 = 8.7 Hz favor the trans-orientation of these pairs of protons.19 In the NOESY spectrum, significantly more intensive spatial interaction between H1/H7 than that between the H6/H7 and H6/H12 pairs was detected. This observation at a qualitative level indicates the trans-orientation of the H6/H7 and H6/H12 pairs and, in addition, the spatial proximity of H1 and H7. To obtain more accurate data on the spatial solution structure of 26, several interproton distances were estimated from the values of cross-relaxation rates determined from NOESY data by calibration method.20 Interproton distances rij can be determined from the relation rij = rref (σref/σij)1/6, where σij and σref are cross-relaxation rates for the signals of protons i and j and for the reference pair of signals, respectively, in the NOESY spectrum. As a reference we have chosen the signals of nonequivalent geminal protons H9A and H9B of the ethyl group at position 3. The reference distance rref is 1.78 Å in according to standard value inter two heminal protons. Cross-relaxation rates were determined from a graph of the dependence of the integral intensities ratio of cross- and F


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Inorganic Chemistry Scheme 6. Calculated Regio- and Stereoisomeric Pathways of the Reaction of 33 with MeCN, 34, and 35

and -(IV) bis-acetonitrile complexes trans-[PtCl2(NCMe)2] (34) and trans-[PtCl4(NCMe)2] (35) (Scheme 6) have been carried out at the density functional theory (DFT) level of theory. The reaction may occur along two regioisomeric channels leading either to Δ2-1,2,4-triazoline or to Δ2-1,2,3triazoline products (pathways A and B, respectively, Scheme 6). Furthermore, two stereoisomeric pathways are possible for each regioisomeric channel (pathways Aa, Ab, Ba, and Bb). The cycloadditions along all these stereo- and regioisomeric pathways were calculated for free MeCN and platinum(IV) complex 35, but only the most plausible route (Ab) was considered for platinum(II) species 34. The calculations demonstrate that cycloadditions to MeC N and, in particular, to 34 and 35, are controlled by the HOMOdipole−LUMOnitrile type of the molecular orbital interaction belonging to the normal electron demand reactions (type I of the Sustmann’s classification).22 Indeed, the HOMOdipole−LUMOπ*(CN) gaps are 6.76 eV (MeCN), 4.82 eV (34), and 4.64 eV (35), while the HOMOπ(CN)− LUMOdipole gaps are 6.90 eV (MeCN), 6.46 eV (34), and 7.75 eV (35) (Figure 5). The lower LUMOπ*(CN) energy in 35 in comparison with MeCN (by 2.12 eV) explains, in terms of the frontier MO theory, why the platinum(IV) nitrile complexes are so significantly activated toward the azomethine imines, allowing the reaction to proceed at 25 °C, whereas no reaction with free alkylnitriles is observed even under rather drastic conditions. At the same time, the FMO theory fails to explain the large difference in the reactivity between the PtII and PtIV complexes, the LUMOπ*(CN) energy difference in 34 and 35 being only 0.18 eV. The mechanism of the reaction 33 + MeCN is concerted involving the formation of one cyclic transition state TS1a−TS 1d for each regio- and stereoisomeric pathway (Figure 6, Tables S1 and S2 in Supporting Information). Cycloaddition along pathways Aa and Ab is highly synchronous, the Sy parameter proposed as a quantitative measure of the synchronicity of concerted cycloadditions23 is 0.93−0.94 (Sy is 0 for the stepwise cycloadditions and Sy is 1 for the fully concerted cycloadditions). In contrast, the reaction along pathways Ba and Bb

Figure 5. Relative energies of the interacting HOMOs and LUMOs of reactants (the HOMO−LUMO gaps are indicated).

is clearly asynchronous, Sy being 0.67. The mechanism of the reaction 33 + 34 with the PtII complex along the Ab channel is also concerted via TS2b but significantly more asynchronous compared to the case of free MeCN (Sy = 0.70). The coordination of the nitrile to the platinum(IV) center results in a switch of the reaction mechanism to the stepwise one including the formation of acyclic zwitterionic intermediate INT1a−INT1d bearing the N(2)C(3) (pathways Aa and Ab) or C(4)C(5) (pathways Ba and Bb) bonds via TS3a−TS 3d, in the first step, and the ring closure via TS4a−TS 4d leading to final cycloaddition products 38a−38d, in the second step (Scheme 6, Figure 6, Tables S3 and S4). No transition state for the concerted mechanism of cycloaddition to the platinum(IV)bound nitrile was located during the detailed search of the potential energy surface. Such a switch of the reaction mechanism on going from MeCN and 34 to 35 may be accounted for by the steric repulsions between the bulky ligands of the azomethine imine and the {PtCl4(NCMe)} moiety of the nitrile. Indeed, the previous theoretical calculations of the reaction between 35 and the simplest G


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Inorganic Chemistry

Figure 7. Energy profiles of the reactions of 33 with MeCN, 34, and 35.

of the reaction 33 + MeCN. At the same time, cycloaddition of 33 to the platinum(II) complex 34 exhibits the activation barrier even higher than that of the metal free process, i.e., 33.0 vs 32.0 kcal/mol. Both these values are quite similar to the activation energy calculated for the reaction between the nitrone PhCH=N+(Me)O− and MeCN (34.1 kcal/mol).3f Taking into account that cycloaddition between nitrones and free acetonitrile does not occur even under harsh conditions, the reactions between 33 and MeCN or 34 are also not expected to be realized, and this is in accord with the experimental observations. Third, the platinum(IV)-mediated cycloaddition 33 + 35 demonstrates clear regioselectivity toward the Δ2-1,2,4-triazoline products and stereoselectivity toward the 5S,7S,8Sstereoisomer (pathway Ab). This pathway is significantly more favorable than others from both kinetic and thermodynamic viewpoints (Table 1). Moreover, isomer 38b is the only exoergonic product of this reaction, whereas the formation of other isomers along pathways Aa, Ba, and Bb is endoergonic, complexes 38c and 38d being completely thermodynamically forbidden. These results are in agreement with the experimental isolation of 15−20 corresponding to the rel-(1S,6S,7S)-1,2,4triazoline products. The much lower stability of Δ2-1,2,3-triazolines vs Δ2-1,2,4triazolines was previously predicted by theoretical calculations

Figure 6. Equilibrium structures of transition states and intermediate involved in the most favorable pathway Ab.

azomethine imine CH2N(Me)NH showed that in the absence of any significant steric hindrances the mechanism is concerted.15 The analysis of the calculated activation and reaction energies (in terms of Gibbs free energies in solution, ΔGs‡ and ΔGs, Table 1) indicates, first, that the rate limiting step of the reaction 33 + 35 is the second one (ring closure) along pathways Aa and Ba, the first one (intermediate formation) for pathway Bb, whereas both TS3b and TS4b have similar energies for pathway Ab (Figure 7). Second, the lowest activation barrier of the PtIV-mediated reaction 33 + 35 is noticeably lower (by 8.7 kcal/mol) than that

Table 1. Gibbs Free Energies of Transition States, Intermediates, and Products of the Reactions of 33 with MeCN, 34, and 35 in CHCl3 Solution Relative to the Reactants’ Level (in kcal/mol) reaction

pathway

TS1a−TS 1d

36a−36d

33 + MeCN

Aa Ab Ba Bb

33 + 34

Ab

33 + 35

Aa Ab Ba Bb

37.7 32.0 44.8 40.6 TS2b 33.0 TS3a−TS 3d 25.4 23.1 34.2 39.2

6.9 −1.0 22.9 16.5 37b −4.7 INT1a−INT1d 21.4 18.2 31.5 28.6 H

TS4a−TS 4d 30.7 23.3 39.1 34.8

38a−38d 8.7 −0.6 38.7 34.2 DOI: 10.1021/acs.inorgchem.5b02246 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry for the N,C-dimethyl substituted Pt-bound triazolines15 and may be explained by a strong electrostatic destabilization of the NNN fragment of the 1,2,3-triazoline cycle, in which all three N atoms have negative effective atomic charges (the NBO charges on these atoms in 38c are −0.21, − 0.18, and −0.30 e). Additionally, there is a strong steric repulsion between the electronegative pyrazolidinone oxygen atom and the chloride ligands (Figure 8). Indeed, in 38c and 38d, the two O···Cl

chemistry.24 The analysis of the equilibrium structures of 38a and 38b did not indicate any other obvious steric repulsions which could lead to the preference for one isomer over another one. Similar regio- and stereoselectivities (along pathway Ab) were predicted also for the hypothetical reaction 33 + MeC N (Table 1). The single electron transfer mechanism of cycloaddition between 33 and 35 was also investigated, but it was found unrealistic (see Supporting Information for details).

■

FINAL REMARKS The results from this work could be considered from at least five perspectives. First, we observed that the nitrile CN group bound to the platinum(IV) center exhibits dipolarophilicity, so significant that 1,3-dipolar cycloaddition of the inactive in this metal-free reaction azomethine imines proceeds efficiently under mild conditions. This activation was previously observed for other cycloadditions of both allyl- and propargyl-allenyl anion type dipoles leading to otherwise inaccessible types of heterocycles. Of course, the method for heterocycle construction via platinum-mediated activation of nitrile substrates ranks with one of the most expensive tools available for organic synthesis. However, its use pays off greatly as a last-resort method once all the other options are exhausted. In addition, the conventional recycling of Pt might strongly reduce all expenses associated with this two-step synthetic transformation. Second, we found that an EtCN ligand in trans[PtCl4(EtCN)2] undergoes facile regio- and stereoselective 1,3-dipolar cycloaddition of the cyclic azomethine imines to yield previously unknown rel-(1S,6S,7S)-6,7-dihydropyrazolo[1,2-a][1,2,4]triazoles, and the reaction proceeds rapidly at 25 °C. At the same time, the reaction of more dipolarophililc platinum(IV)-ligated NCNMe2 and PhCN does not proceed selectively at 25 °C and even at −30 °C and leads to a broad mixture of yet unidentified products. Third, the studied cycloaddition gives 6,7-dihydropyrazolo[1,2-a][1,2,4]triazoles, whereas cycloaddition of relevant azomethine imines to nitrile functionality of HCN affords other regioisomers, i.e., 6,7-dihydropyrazolo[1,2-a][1,2,3]triazoles.13 Fourth, quantum chemical calculations at the DFT level of theory have been used for the detailed investigation of the mechanism and origin of the regio- and stereoselectivity of cycloaddition between azomethine imine 33 and the platinum(IV) complex trans-[PtCl4(MeCN)2] (35) as well as of the hypothetical reactions of 33 with the platinum(II) complex trans-[PtCl2(MeCN)2] (34) and uncomplexed MeCN. The mechanism of cycloadditions to MeCN and to 34 is concerted, whereas that of cycloaddition to 35 is stepwise. The mechanism switch is accounted for by steric repulsions between the azomethine imine and the {PtCl4(MeCN)} moiety. The formation of Δ2-1,2,3-triazolines was found to be thermodynamically forbidden, and the regioselectivity toward Δ2-1,2,4triazolines is rationalized by a strong electrostatic destabilization of the NNN fragment and steric repulsions in the Δ2-1,2,3triazoline molecule. The reaction stereoselectivity is interpreted as a result of the preferred formation of the isomer with equatorial position of Ph in the pyrazolidine ring. Fifth, special attention should be drawn to the fact that heterocyclic systems featuring a Δ2-1,2,4-triazoline ring are of interest as antioxidants and glycosidase inhibitors (A),25 antibacterial agents (B),25b inhibitors of adenosine receptors (C),26 antileishmaniasis (D),27 and anti-inflammatory agents

Figure 8. Equilibrium structure of 38c (the O···Cl repulsions are indicated by the arrow).

distances are in the range of 3.13−3.28 Å, which is comparable with or lower than the sum of the van der Waals radii of the O and Cl atoms (3.27 Å). Such a repulsion results in a distortion of the pyrazolidine ring; the torsion angle N(1)N(2)C(6)C(7) is −29° and −18° in 38c and 38d, whereas it is −7° and −3° in 38a and 38b, respectively. The lower stability of stereoisomer 38a compared to 38b may be explained by the following arguments. The pyrazolidine ring of these isomers is not flat; the C(8) atom lying significantly out of the plane formed by the N(1), N(2), C(6), and C(7) atoms (Figure 9). Thus, the principal difference between 38a and 38b is on the position of the Ph substituent at the C(8) atom, i.e., axial in 38a and equatorial in 38b. The preferential formation of the cyclic isomers with a hydrocarbon substituent at the equatorial position is well-known in organic

Figure 9. Equilibrium structures of 38a and 38b (pyrazolidine rings with substituents at the C(8) atom are selected). I


Article

Inorganic Chemistry (E) (Figure 10).28 The described route for generation of novel Δ2-1,2,4-triazoline systems with their potential in biology and

harmonics, implemented in SCALE3 ABSPACK scaling algorithm. Analytical numeric absorption correction for 26·(CHCl3)0.84 was applied in the CrysAlisPro33 program using a multifaceted crystal model based on expressions derived by R. C. Clark and J. S. Reid.34 Supplementary crystallographic data for this paper have been deposited at Cambridge Crystallographic Data Centre (CCDC 1402676, 920467, and 1415308) and can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif. Computational Details. The full geometry optimization of all structures and transition states has been carried out at the DFT/HF hybrid level of theory using Becke’s three-parameter hybrid exchange functional in combination with the gradient-corrected correlation functional of Lee, Yang and Parr (B3LYP)35 using the Gaussian-0936 program package. No symmetry operations have been applied. The geometry optimization was carried out using a quasi-relativistic Stuttgart pseudopotential that describe 60 core electrons and the appropriate contracted basis set37 for the platinum atoms and the 631G(d) basis set for other atoms. As was shown previously,15,38 this approach is sufficiently accurate for the description of cycloadditions to the CN triple bond providing results close to those obtained by such methods as MP2, MP4, CCSD(T), CBS-Q, and G3B3. The Hessian matrix was calculated analytically for the optimized structures in order to prove the location of correct minima (no imaginary frequencies) or saddle points (only one imaginary frequency) and to estimate the thermodynamic parameters, the latter being calculated at 25 °C. The nature of all transition states was investigated by the analysis of vectors associated with the imaginary frequency and by the calculations of the intrinsic reaction coordinates (IRC) using the Gonzalez−Schlegel method.39 Total energies corrected for solvent effects (Es) were estimated at the single-point calculations on the basis of gas-phase geometries using the polarizable continuum model in the CPCM version40 with CHCl3 as solvent. The UAKS model was applied for the molecular cavity and dispersion, cavitation, and repulsion terms were taken into account. The entropic term in solution (Ss) was calculated according to the procedure described by Wertz41 and Cooper and Ziegler42 using eqs 1−4

Figure 10. Annelated Δ2-1,2,4-triazolines of biological importance.

possibly, in the long term, in medicine suggests further interests in developing these type of heterocyclic systems.

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EXPERIMENTAL SECTION

Materials and Instrumentation. Solvents and aromatic aldehydes 6 and 7 were obtained from commercial sources and used as received. Complex 14 was prepared as previously reported.29 Pirazolidinones 1−5 were obtained according to the previously described protocol.30 ESI mass spectra were obtained on a Bruker micrOTOF spectrometer equipped with an ESI source. The instrument was operated both in positive and negative ion mode using an m/z range of 50−3000. The capillary voltage of the ion source was set at −4500 V (ESI+-MS) and the capillary exit was set at ± (70−150) V. For ESI, species were dissolved in MeCN, whereas NaBF4 or formic acid were used as addition ionization agent. In the isotopic pattern, the most intensive peak is reported. TLC was performed on Merck 60 F254 SiO2 plates. Infrared spectra (4000−400 cm−1) were recorded on a Shimadzu FTIR-8400S instrument in KBr pellets. 1H and 13C{1H} NMR spectra were recorded on Bruker Avance II+ 400 MHz (UltraShield Magnet) and Avance II+ 500 MHz (UltraShield Plus Magnet) spectrometers at ambient temperature in CD2Cl2, CDCl3, and DMSO-d6. X-ray Diffraction Study. For single crystal X-ray diffraction experiment, crystals of 25·CH3OH, 26·(CHCl3)0.84, and 29·CDCl3 were fixed on a micro mounts and placed on Agilent Technologies SuperNova Atlas diffractometer. Crystal of 25·CH3OH and 29·CDCl3 was measured at a temperature of 100 K using microfocused monochromated CuKα radiation. Crystal of 26·(CHCl3)0.84 was measured at a room temperature of 293 K using microfocused monochromated MoKα radiation. The unit cell parameters and refinement characteristics for the crystal structures of 25·CH3OH and 26·(CHCl3)0.84 are given in Table S1. The structures had been solved by the direct methods and refined by means of the SHELXL-97 program31 incorporated in the OLEX2 program package.32 The carbon and nitrogen-bound H atoms were placed in calculated positions and were included in the refinement in the “riding” model approximation, with Uiso(H) set to 1.5Ueq(C) and C−H 0.96 Å for the CH3 groups, Uiso(H) set to 1.2Ueq(C) and C−H 0.97 Å for the CH2 groups, Uiso(H) set to 1.2Ueq(C) and C−H 0.98 Å for the tertiary CH groups, Uiso(H) set to 1.2Ueq(C) and C−H 0.93 Å for the benzene CH groups and Uiso(H) set to 1.2Ueq(N) and N−H 0.86 Å for the NH groups. Position of H atom of the OH group in the structure of 25·CH3OH was localized from difference Fourier maps and refined without any restraints. Empirical absorption correction for 25·CH3OH and 29· CDCl3 was applied in CrysAlisPro33 program complex using spherical

ΔS1 = R ln V s m,liq /Vm,gas

(1)

ΔS2 = R ln V o m/V s m,liq

(2)

α=

S o,s liq − (S o,s gas + R ln V s m,liq /Vm,gas) (S o,s gas + R ln V s m,liq /Vm,gas)

(3)

Ss = Sg + ΔSsol = Sg + [ΔS1 + α(Sg + ΔS1) + ΔS2] =Sg +[(− 11.35 cal/mol·K) − 0.19(Sg − 11.35 cal/mol·K) + 5.00 cal/mol·K]

(4)

where Sg = gas-phase entropy of solute, ΔSsol = solvation entropy, S°,sliq, S°,sgas, and Vsm,liq = standard entropies and molar volume of the solvent in liquid or gas phases (201.7 and 295.7 J/mol·K and 80.50 mL/mol, respectively, for CHCl3), Vm,gas = molar volume of the ideal gas at 25 °C (24450 mL/mol), V°m = molar volume of the solution corresponding to the standard conditions (1000 mL/mol). The enthalpies and Gibbs free energies in solution (Hs and Gs) were estimated using the expressions 5 and 6

Hs = Es − Eg + Hg

(5)

Gs = Hs − TSs

(6)

where Es, Eg, and Hg are the total energies in solution and in gas phase and gas-phase enthalpy. The Wiberg bond indices (Bi)43 and atomic charges were computed by using the natural bond orbital (NBO) partitioning scheme.44 The synchronicity of cycloadditions (Sy) was calculated using the formula:23 J


Article

Inorganic Chemistry n

Sy = 1 − (2n − 2)−1∑ i=1

|δBi − δBav | δBav

1

H NMR spectra of azomethine imines 8−13; 1H NMR spectrum of complex 14; 1H NMR and HRESI-MS spectra of complexes 15−20; 1H, 13C{1H} NMR, HRESI-MS, and IR spectra of complexes 21−26 and of heterocycles 27−32 (PDF). Crystallographic information files (CIF1, CIF2, and CIF3).

(7)

where n is the number of bonds directly involved in the reaction (n = 5 for 1,3-dipolar cycloadditions). δBi is the relative variation of a given Wiberg bond index Bi at the transition state relative to reactants (R) and products (P) and it is calculated as

δBi =

■

BiTS − BiR BiP − BiR

(8)

If the δBi value is negative it is assumed to be zero. The average value of δBi (δBav) is defined as

*(N.A.B.) Fax: +7 812 4286733; Tel: +7 812 4286890; E-mail: [email protected]. *(V.Y.K.) E-mail: [email protected].

n

δBav = n−1 ∑ δBi i=1

Notes

(9)

The authors declare no competing financial interest.

■

Synthetic Work. Synthesis of Azomethine Imines 8−13 (General Procedure). Synthesis was performed in accordance with a slightly modified literature method.13 A suspension any one of 1−5 (0.01 mol) and 6 or 7 (0.012 mol) (in the following combinations: 1−4 and 6, and 2 (or 5) and 7) in dry ethanol (30 mL) was refluxed for 5 min, whereupon trifluoracetic acid (0.05 mL) was added. The reaction mixture was then stirred for 1 h upon reflux, whereupon it was cooled to 25 °C. The precipitates of 8−13 were separated by filtration and dried in air at 25 °C. Yields 80−90%. For characterization of 8−13, see Supporting Information. Cycloaddition of Azomethine Imines 8−13 to 14 (General Procedure). Any one of azomethine imine 8−13 (0.11 mmol) was added in one portion to a suspension of [PtCl4(EtCN)4] (51 mg, 0.11 mmol) in anhydrous CH2Cl2 (2 mL) at 25 °C. The reaction mixture was stirred at 25 °C under Ar atmosphere until it became clear (ca. 2 h). Then it was diluted with hexane (4 mL); an oily precipitate formed was crystallized by stirring with a glass stick under layer of hexane. The precipitate was then separated by centrifugation, treated with anhydrous diethyl ether (3 mL), and filtered off. The precipitate was dried at 25 °C under reduced pressure for 30 min. Isolated yields of 15−20 are ca. 10−20%. On the basis of 1H NMR data, after 2 h conversion of starting 14 into cycloaddition products in CDCl3 solution is ca. 80−90%. For characterization of 15−20 see, Supporting Information. One Pot Procedure of Synthesis 21−26. A suspension of any one of azomethine imine 8−13 (0.268 mmol, 1.5 equiv) in CH2Cl2 (2 mL) was added to a suspension of trans-[PtCl4(EtCN)2] (0.179 mmol, 80 mg) in 3 mL of anhydrous CH2Cl2 under Ar at 25 °C. Reaction mixture was stirred until became clear (1 h for 22−26, and 4 h for 21) and then phosphorus ylide Ph3PCHCO2Me (60 mg, 0.179 mmol) was added at 25 °C in one portion. After stirring for 2 h the reaction mixture was evaporated in vacuo. The residue was purified by column chromatography with silica gel (eluent EtOAc:hexane 1:3−1:2 v/v). For characterization of 21−26, see Supporting Information. Liberation of the Heterocyclic Ligands. Solid dppe (94 mg, 0.238 mmol) was added in one portion to a solution of any one of 21−26 (0.119 mmol) in anhydrous chloroform (3 mL). The solution was stirred for 1 h at 25 °C, and then the precipitate formed was filtered off. The filtrate was loaded on silica gel and was purified by column chromatography (eluent EtOAc:hexane 2:1−1:1 v/v). All pure product containing fractions were combined and evaporated to dryness under reduced pressure at 25 °C giving colorless power. For experimental data for 27−32, see Supporting Information.

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AUTHOR INFORMATION

Corresponding Authors

ACKNOWLEDGMENTS The authors express their gratitude to Saint Petersburg State University for research grants (12.38.225.2014 and 12.38.781.2013) and Russian Fund for Basic Research (Grant 14-03-00080). This work was conducted within the framework of Russia−South Africa cooperative program, and support from RFBR (14-03-93959) and the South African NRF (UID 92196) is gratefully acknowledged. M.L.K. thanks Fundaçaõ para a Ciência e a Tecnologia (FCT), Portugal, for the financial support (project UID/QUI/00100/2013). Dr. P. M. Tolstoy is thanked for valuable suggestions and stimulating discussions. We thank the Computer Center (at Petrodvorets) of Saint Petersburg State University for providing their computer facilities for the theoretical calculations and also acknowledge Center for Magnetic Resonance and X-ray Diffraction Center of Saint Petersburg State University for performing of NMR and XRD studies.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02246. Experimental data for 8−13 and 15−32; Table S1: solution structure of 26; single electron transfer mechanism; Figures S1−S5; Table S2−S4 (PDF). K


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