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Chiral metal catalysts have been widely applied to asymmetric transformations. However, the electronic structure of the catalyst and how it contributes to the activation of the substrate is seldom investigated. Here, we report an empirical approach for providing insights into the catalytic activation process in the distorted Ni(II)-catalysed asymmetric [3+2] cycloaddition of α-ketoesters. We quantitatively characterize the bonding nature of the catalyst by means of electron density distribution analysis, showing that the distortion around the Ni(II) centre makes the dz2 orbital partially ‘naked', wherein the labile acetate ligand is coordinated with electrostatic interaction. The electron-deficient dz2 orbital and the acetate act together to deprotonate the α-ketoester, generating the (Λ)-Ni(II)–enolate. The solid and solution state analyses, together with theoretical calculations, strongly link the electronic structure of the centrochiral octahedral Ni(II) complex and its catalytic activity, depicting a cooperative mechanism of enolate binding and outer sphere hydrogen-bonding activation.
Optimization in asymmetric metal-based catalysis generally requires trial-and-error approaches, whereby a metal source and a ‘privileged' chiral ligand1 are combined without understanding of the three-dimensional (3D) and electronic structure of the active catalyst. Considering that synergistic activation of two reaction components is a key mechanistic strategy to attain high rate acceleration and selectivity2,3, it is particularly important to understand the electron density distribution (EDD) of the chiral catalyst, to gain insight into how the metal and ligand(s) cooperatively activate two distinct reaction components.
Over recent decades, the use of asymmetric nickel catalysts has substantially evolved both in acid–base and redox catalysis4,5,6. Several examples of chiral nickel complexes that act as asymmetric catalysts have been sporadically characterized by X-ray structure analysis, opening up a window of opportunity to discuss the stereo-discrimination process in asymmetric nickel catalysis7,8,9,10,11,12,13,14,15,16,17. Evans's transition-state model, which was proposed on the basis of X-ray analysis of the Ni(II)–diamine–enolate complex, is a remarkable example, explaining the stereochemical course in the catalytic asymmetric Michael reaction of nitroolefins with 1,3-dicarbonyl compounds7,8. However, despite the rapid progress in this field, the electronic structure of the asymmetric nickel catalyst is seldom investigated and exploited. The difficulties in understanding nickel asymmetric catalysts arise from several features: (i) They exhibit various oxidation states (0, +1, +2, +3 and +4) and electronic configurations (typically from 16e to 20e). (ii) The steric factor and the coordination numbers of the ligand(s) drastically influence the coordination geometry and the assembly-state of nickel complexes. (iii) The structure changes dynamically depend on solvents, substrates and additives due to the coordination equilibrium18.
Here, we describe the discovery and characterization of a distorted chiral Ni(II)–diamine–acetate complex and link the electronic structure to its catalytic activity (Fig. 1). In contrast to the classical symmetric octahedral Ni(II) catalysts (Fig. 1a), the Ni(II) catalyst reported here exhibits (Λ)-chirality at the Ni(II) center19,20,21,22,23, in which the coordination about the nickel atom is a distorted octahedron (Fig. 1b). The distortion in the octahedral Ni(II) complex makes the dz2 orbital partially ‘naked' due to the labile acetate ligand, which is weakly coordinated by electrostatic interaction. This unique electronic feature, involving both Lewis acidic and Brønsted basic natures, facilitates enolization of the α-ketoester. The distorted Ni(II) complex also exhibits hydrogen bond donor ability of the coordinated amine ligand, providing an outersphere binding site for the approaching electrophile24. With the bifunctional catalytic motif on the (Λ)-Ni(II) complex, we uncover the reactivity of the Ni(II)–enolate as a formal 1,3-dipolarophile that promotes [3+2] nitrone cycloaddition. The regioselectivity for this transformation is distinct from that of classical [3+2] cycloadditions25; the vast majority of nitrone cycloadditions are predicated on the use of electron-deficient alkenes (Fig. 1c). This report provides insights into the elusive mechanistic basis for inverse-electron-demand (IED) [3+2] cycloaddition25,26,27,28,29,30,31,32,33,34 of α-ketoesters with (E)-nitrones (Fig. 1d).
Based on our Ni(II)–enolate chemistry35, we initiated this study by exploring a new Ni(II) complex, focusing on formal [3+2] cycloaddition25 of α-ketoesters 1 with nitrones 2 for the following reasons. (i) Control of the ambiphilic reactivities of α-ketoesters, which exhibit both nucleophilic and electrophilic natures, constitutes a general challenge36,37. (ii) Catalytic asymmetric [3+2] cycloaddition using transient enolate has yet to be developed, despite the long history of enolate studies25,38,39. Only a few IED cycloadditions with electron-rich alkenes (such as enol ethers and silyl ethers), not enolate, have been reported with asymmetric catalysts25,26,27,28,29,30,31,32,33,34. The [3+2] cycloaddition, which is controlled by the lowest unoccupied molecular orbital of the dipole and the highest occupied molecular orbital of the 1,3-dipolarophile, can be also categorized as ‘type III' according to Sustmann's classification40. (iii) The proposed catalytic reaction would provide stereochemically complex isoxazolidines 3 that bear three contiguous stereocenters, including a unique, stereochemically defined, hemiketal moiety.
We investigated the proposed formal [3+2] cycloaddition using cyclic nitrones 2 to construct chiral tetrahydroisoquinolines, which are one of the privileged scaffolds for drug discovery41. Among the reaction parameters we investigated, the effect of metal was most drastic (Table 1, entries 1–4). For example, Ni(OAc)2·4H2O–(R,R)-4a complex35 selectively produced anti,anti-3aa as the major product in 91% yield, 50% enantiomeric excess (e.e.) with 20/1 diastereomeric ratio (d.r.), while other metal acetate complexes displayed lower reactivity (Table 1, entries 1–4). As the counteranion, acetate gave the best results (Table 1, entries 5–7). The NiBr2–(R,R)-4a complex developed by Evans7,8 was less effective in this reaction (Table 1, entry 7). Systematic variation of the diamine ligand 4 revealed that α-branched substituents on the amine in (R,R)-4 are crucial for improving the enantioselectivity (Table 1, entries 8–11). The diamine (R,R)-4e, bearing cyclohexyl groups, was most effective, raising the e.e. value of 3aa to 83% e.e. Intriguingly, the diamine 4e has been overlooked as a chiral ligand for over a decade despite its potential utility and simplicity42. We finally found that diisopropylamine (iPr2NH: 10mol%) can further improve not only the reactivity, but also the enantioselectivity43, affording (1R,2R,10bS)-3aa in 71% yield, >50/1 d.r. and 91% e.e., with 5mol% of Ni(OAc)2·4H2O–(R,R)-4e (Table 1, entry 14). These results suggest that achiral iPr2NH may participate in tuning the chiral environments constructed by the catalyst, without interrupting enolate formation by the Ni(II) catalyst.
Removing the solvent of Ni(OAc)2·4H2O/(1R,2R)-4e=1/1 mixture in tetrahydrofuran (THF) and subsequent crystallization from CH2Cl2 at –20°C gave the rich green-colored I in 85% yield, while crystallization from n-hexane at room temperature afforded the light green-colored II in 86% yield. Both products are easily handled, air-stable complexes (Fig. 2a).
The X-ray structure analyses unambiguously revealed that I is a mononuclear Ni(II) complex: (Λ)-Ni(OAc)24e, while II is a μ-acetate-bridged trinuclear Ni(II) complex: (Λ,Λ)-[Ni3(μ-OAc)4(OAc)24e2] (Fig. 2b). In contrast to chiral nickel(II)–diamine catalysts identified so far7,8,9,10,11 (Supplementary Figs 3–5), the newly developed complexes are chiral-at-Ni(II) center19,20,21,22,23, in which the initial C2-symmetry of the ligands on I and II is desymmetrized through formation of the Ni(II) complex. Mononuclear Ni(II) complex I has a distorted octahedral architecture; the atomic distance between Ni(II) and N(2) (2.109(1)Å) is longer than that of Ni(II)–N(1) (2.091(1)Å), and the N(2)–Ni(II)–O(4) angle is 155.99(5)°, which differs markedly from 180°; we show the longer Ni(II)–N(2) bond in pseudoapical position in Fig. 2b, left side. The intermolecular H-bonding interaction of the N(1)–H with THF in the crystal is involved, indicating that N(1)–H can intermolecularly activate the Lewis basic substrate24, nitrone 2 (refs 34, 44, 45), in the present catalysis. The coordination patterns of the acetate anions in I also have a characteristic role in constructing a specific chiral environment; one of the acetates coordinates to the Ni-centre in the equatorial-equatorial mode, in which the acetate anion is symmetrically bridging [C(19)–O(1): 1.265(2)Å, C(19)–O(2): 1.266(2)Å]. In contrast, the other acetate that occupies pseudoapical and equatorial positions take asymmetric form [C(21)–O(3): 1.271(2)Å, C(21)–O(4): 1.259(2)Å]. Another feature in the distorted octahedral complex I is the distance between Ni(II) and pseudoapical O(4) [2.302(2)Å], which is significantly longer than other Ni(II)–O and Ni(II)–N distances [2.045(1)–2.140(1)Å], suggesting weak coordination. An obvious structural difference of the trinuclear Ni(II) complex II from the reported triple-bridged trinuclear Ni(II) complexes46 is the distorted units at the terminal analogous to I, in which symmetric octahedral central nickel is linked with acetate bridges (Fig. 2b, right side). The angles of the bridging acetate [O(1)–C(19)–O(2): 127.40(18)°, O(5)–C(21)–O(6): 126.47(18)°] in complex II become larger than that of the corresponding acetate [O(1)–C(19)–O(2): 120.36(15)°] in I.
The investigation for catalytic activity of complexes I and II led to identification of two key catalytic factors for attaining high enantioselectivity (Table 2). First, the mononuclear Ni(II) complex I provided better e.e. value than the trinuclear Ni(II) complex II. The mononuclear catalyst I produced the [3+2] cycloaddition adduct 3aa in 62% yield with >50/1 d.r. and 90% e.e., while the trinuclear catalyst II afforded 3aa with poor enantioselectivity regardless of the amount of catalyst (entries 2 and 3). Second, addition of a catalytic amount of achiraliPr2NH (10mol%) improved the poor enantioselectivity with trinuclear catalyst II (Table 2, entry 2: 60% e.e. vs entry 4: 90% e.e.)43.
To characterize the bonding mode around the distorted Ni(II) in I, we performed EDD analysis47 using single-crystal X-ray diffraction data (Fig. 3, Supplementary Data 1). A 3D plot of the static deformation density of I highlights its valence electron density topology (Fig. 3a). The distribution of d-orbitals along the coordination axes of the Ni(II) centre were found as electron-deficient regions. With respect to ligands, the lone pairs of oxygen [O(1), O(2) and O(3)] and nitrogen [N(1) and N(2)] atoms are directed to the electron-deficient regions at Ni(II), showing the conventional character of coordination bonds. In sharp contrast, the lone pair of O(4) is directed towards the electron-rich region at Ni(II). The 2D static model map on the O(3)–Ni–O(4) plane also demonstrates the different bonding character between the Ni–O(3) and the Ni–O(4) bonds (Fig. 3b). We can also discuss the unique bonding nature of Ni–O(4) with maps of Laplacian distribution [2ρ(r)] of total EDD along bond paths by comparing the 2ρ(r) distribution in the O(1)–Ni–O(2) plane (Fig. 3c) and the O(3)–Ni–O(4) plane (Fig. 3d)48,49,50. The valence shell charge concentration (VSCC) region at the Ni(II) centre are located between the bond paths of the Ni–O(1) and the Ni–O(2) bonds on the O(1)–Ni–O(2) plane (Fig. 3c). The VSCCs on O(1) and O(2) expand towards the charge-depletion regions around the Ni(II) centre along the bond paths. In contrast, the Ni–O(4) bond path goes through inside the VSCC region at O(4) and the VSCC region at the Ni(II) centre on the O(3)–Ni–O(4) plane (Fig. 3d). On the other hand, the Ni–O(3) bond shows similar features to the Ni–O(1) and Ni–O(2) bonds. The results described in Fig. 3 represent experimental evidence that a weaker orbital interaction between Ni(II) and O(4) is involved, while electrons are donated from lone pairs in the orbitals on other N and O atoms to the unfilled d-orbital on Ni(II) (Supplementary Figs 6–8). The density at the bond critical point (BCP) of the Ni(II)–O(4) bond [0.260(2)eÅ−3] is remarkably lower than at the other coordination bonds: Ni(II)–O(1); 0.431(2), Ni(II)–O(2); 0.480(2), Ni(II)–O(3); 0.494(2), Ni(II)–N(1); 0.559(3), and Ni(II)–N(2); 0.550(3)eÅ−3. Thus, the dissymmetric, distorted octahedral Ni(II)–diamine–acetates I possessing an elongated Ni(II)–oxygen bond has d8 18-electronic configuration with a weak electrostatic interaction with O(4) at the pseudoapical position. The density at the BCP of the N(1)–HO(5) is 0.084(10)eÅ−3, which fits reasonably with the topological properties [d(HO): 2.200Å, d(NO): 3.155(2)Å, α(N–HO): 159.20°] of the H-bonding51.
We then characterized the structure of mononuclear Ni(II) complex I in THF (Fig. 4), at the same catalyst concentration as used for the IED [3+2] cycloaddition reaction. The unique shift for carboxylates (1,598 and 1,558cm−1) in the infrared spectrum supports the idea that the two distinct acetates coordinate to the Ni-centre in a bidentate manner (Fig. 4a)52. The band observed at 3,225cm−1 also suggests that the N–H functionality on the ligand can act as a proton donor in THF53. The electronic absorption spectrum of I, which shows broad bands at 408, 753 and 1,182nm, is typical of pseudoctahedral geometry of Ni(d8) (Fig. 4b, bottom)52. The electronic circular dichroism (ECD) signal of I was observed not only in the ultraviolet–vis region, but also in the near-infrared region (Fig. 4b, top). The ECD spectrum observed in the d→d transition region demonstrated that the nickel centre in I is chiral even in solution. A feature of the observed ECD spectra is that the intensity of the near-infrared bands is significantly stronger than that of the visible absorption. Density functional theory (DFT) calculations on the noncentrosymmetric I at the level of UM06/6-311G(d,p) (SDD for Ni) were also performed (Supplementary Figs 9–14, Supplementary Tables 4 and 5)54,55. The simulated infrared and ECD spectra fitted reasonably well with the experimental data. These results suggest that (i) Ni(II)–diamine–acetate I retains pseudo-octahedral structure in the solution state, in which the two structurally distinct acetates coordinate to the Ni(II) centre in bidentate manner and (ii) chirality-at-metal in I is substitutionally and configurationally inert even in the presence of an excess of coordinative THF.
Structural analyses of I in the solid and solution states (Figs 2, ,3,3, ,4),4), as well as their catalytic activity differences (Table 2), suggested that the monomeric species should predominantly control the stereo-discrimination process (Fig. 5). We assume that the exposed nature of the d-orbital allows it to interact with α-ketoester 1a as a Lewis acid, making the ketoester susceptible to deprotonation by the acetate ligand acting as a Brønsted base, leading to the formation of (Z)-Ni(II)–enolate that contains the five-membered chelating ring (Fig. 5, step (i)). Based on the X-ray (Fig. 2b) and EDD analyses of I (Fig. 3), we propose a Ni(II)–enolate, in which the carbonyl group in the ester in 1 coordinates to Ni(II) at the pseudoapical position. A new perspective in the transient Ni(II)–enolate is its coordination pattern in the octahedral structure; the Ni(II)–enolate reported by Evans7,8 occupies the same plane with the chiral diamine. The crystallographic evidence that the N–H functionality at the equatorial position in Ni(II) complex I can contribute to activating the Lewis base (Fig. 2b) suggests that the subsequent H-bonding activation of the nitrone 2a (refs 34, 44, 45) would be a key driving force for fixing the two reaction components in close proximity (Fig. 5, step (ii)), thereby enhancing the reaction rate with high diastereo- and enantioselectivity (Fig. 5, step (iii)). In the proposed model, the centrochiral Ni(II) directly activates the α-ketoester 1a with the ligand-enabled H-bonding activation of the nitrone 2a. The model presented herein can explain the obtained absolute stereochemistry of 3aa.
The scope of the formal IED [3+2] cycloaddition using the catalytic triad of Ni(OAc)2, (R,R)-4e and iPrNH2 was examined (Fig. 6). α-Ketoesters 1 bearing various substituents on the aromatic moiety as well as longer alkyl chain substrates served as substrates, giving the corresponding [3+2] adducts 3. Substrate 1h, bearing the sterically demanding 1-adamantyl group can also participate in the catalytic reaction, affording the corresponding isoxazolidine 3ha in 74% yield, with 25/1 d.r. and 94% e.e. The terminal olefin in 1i remains intact in the reaction of 2a, giving 3ia in 77% yield, 35/1 d.r. and 88% e.e. Substituted (E)-nitrones (3ab, 3ac and 3ad) were also applicable in the developed catalytic system, and comparable reactivity and selectivity were obtained by slightly tuning the reaction conditions.
A characteristic feature in this Ni(II)-catalysis is its chemoselective recognition of α-ketoesters and (E)-nitrones. When the reaction was performed using 2a with vinyl ether 5 instead of 1a, no reaction took place (Fig. 7a); this is complementary to the reported IED [3+2] cycloadditions25,26,27,28,29,30,31,32,33,34, and supports the validity of our working model, which involves a Ni(II)–enolate as the active dipolarophile. The observation that no reaction occurred when we applied the catalytic system to the reaction of (Z)-6 and 1a (Fig. 7b) represents additional evidence that the present catalytic system selectively activates (E)-nitrones. Another issue upon which we focused is the use of E/Z-isomerizable nitrone 7 (ref. 56) in our catalytic system (Fig. 7c), because controlling the isomerization is an issue of contemporary interest in organic synthesis57 as well as in biology58. The formal [3+2] cycloaddition of E/Z isomerizable ester-conjugated nitrone 7 with 1a selectively afforded (3S,4R,5R)-anti-anti-8 as a single diastereoisomer in 90% yield with 64% e.e. The geometry selection of nitrone in the present catalytic system is complementary to that reported in (Z)-nitrone-selective reactions using chiral Cu(II) catalyst31. All these considerations indicate that the structure and geometry of both α-ketoester 1 and (E)-nitrone 2 can be discriminated in the present Ni(II)-catalyst system, facilitating the formation of stereochemically complex isoxazolidines 3 and 8.
Here, we not only showcase the first catalytic, asymmetric and direct formal [3+2] cycloaddition of transient α-ketoester enolates, but also broaden the concepts underlying the design of asymmetric catalysts. A series of spectroscopic analyses in both the solid and solution states, supported by DFT calculations, were able to relate the electronic structural features of the distorted, desymmetrized Ni(II) complex to its catalytic activity. Specifically, we demonstrated that the EDD analysis of the chiral catalyst is a powerful methodology to experimentally visualize its electron density topology, and provides a quantitative insight into how the catalyst initiates the reaction. The presence of naked d-orbital interacting with the labile acetate ligand allows this acetate ligand to act as a Brønsted base to generate the (Λ)-Ni(II)–enolate. Furthermore, the bifunctional catalytic motif that enables the merger of H-bonding activation with amine ligand and enolate formation provides a mechanistic basis to expand the potential utility of ligand-induced octahedral metal centrochirality19,20,21,22,23. The operational simplicity and the easy tunability of the present catalytic system holds a vast potential for designing chiral chemospecific spaces with high predictability.
Detailed experimental procedures, characterization of compounds, Cartesian coordinates and the computational details can be found in Supplementary Figs 1–50, Supplementary Tables 1–5 and Supplementary Methods.
To a solution of (R,R)-4e (266.6mg, 0.958mol) in EtOH was added nickel acetate tetrahydrate (237.6mg, 0.958mmol) at room temperature. The mixture was stirred for 1.5h at room temperature, and then filtered through a membrane filter (Chromatodisc 13N, KURABO). The filtrate was concentrated under reduced pressure to give the Ni(OAc)2·4H2O/(R,R)-4e=1/1 complex as a green oil (428.8mg, 0.813mmol; calculated on the basis of the formula Ni[(OAc)2(H2O)4(R,R)-4e]). Then, THF (4.07ml) was added at room temperature to prepare the stock solution of Ni(OAc)2·4H2O/(R,R)-4e=1/1 (0.2M in THF). In Fig. 6, this stock solution was used for the catalytic asymmetric formal [3+2] cycloadditions of α-ketoesters 1 and nitrones 2.
After removal of THF (6.75ml, 1.35mmol; calculated on the basis of the formula Ni[(OAc)2(H2O)4(R,R)-4e] from the stock solution [Ni(OAc)2·4H2O/(R,R)-4e=1/1 (0.2M in THF)], the residue (692.5mg) was dissolved in CH2Cl2 (~3ml) at room temperature. Slow evaporation at −25°C afforded rich green-colored crystals, which were collected by filtration to give mononuclear Ni(II)–diamine–acetates I in 86% yield (586.1mg, 1.11mmol; calculated on the basis of the formula Ni[(OAc)24e(THF)].
After removal of THF (2.07ml, 0.414mmol; calculated on the basis of the formula Ni[(OAc)2(H2O)4(R,R)-4e]) from the stock solution [Ni(OAc)2·4H2O/(R,R)-4e=1/1 (0.2M in THF)], the residue (300.1mg) was dissolved in n-hexane (~5ml) at room temperature. Slow evaporation at room temperature afforded rich green-colored crystals, which were collected by filtration to give trinuclear Ni(II)–diamine–acetates II in 85% yield (Ni-based, 182.7mg, 0.163mmol; calculated on the basis of the formula Ni3[(OAc)64e2(H2O)2]).
MS 4A (100mg, powder purchased from Nacalai Tesque) in a Schlenk flask equipped with a magnetic stirring bar was flame-dried under reduced pressure for 5min. Upon cooling to room temperature, the flask was refilled with N2, and α-ketoester 1a (23.4mg, 0.10mmol) and (E)-nitrone 2a (17.7mg, 0.12mmol) were added and dried under vacuum. The flask was backfilled with N2, THF (250μl) was added at room temperature, and the flask was cooled to –30°C. To the resulting solution was added the prepared catalyst solution (5mol%: 25μl, 0.2M in THF) and iPr2NH (10mol%: 25μl, 0.4M in THF). The reaction mixture was stirred for 24h at –30°C. Aluminium oxide 60 (~20mg, Merck) was added, and the mixture was diluted with EtOAc cooled at –30°C. The solution was passed through a pad of Aluminium oxide 60, to remove the nickel catalyst, and then eluted with EtOAc and concentrated under reduced pressure. The d.r. (>50/1) was determined from the 1H NMR spectrum of the crude sample. The residue was purified by column chromatography [CHROMATOREX NH (NH–DM1020, 100–200 mesh, Fuji Silysia Chemical) to give 3aa in 74% yield (28.2mg, 0.0740, mmol). The e.e. of (–)-(1R,2R,10bS)-3aa (91% e.e.) was determined by means of chiral HPLC analysis (CHIRALCEL OD-H, 0.46cm () × 25cm (L), n-hexane/2-propanol=95/5, 1.0mlmin−1, major; 9.8min, minor; 17.7min).
The diffraction data were collected using a RIGAKU AFC-8 diffractometer equipped with a Saturn70 CCD detector with MoKα radiation by an oscillation method at 90K. X-rays were monochromated and focused by a confocal mirror. Sixteen data sets were measured with different crystal orientations and detector positions. For all data sets, camera distance was 40mm. Bragg spots were integrated, scaled and averaged up to sinθ/λ=1.22Å−1 by the programme HKL2000 (ref. 59) Lorentz and polarization corrections were applied during the scaling processes. Analytical absorption corrections60 were applied. The initial structure of I·THF was solved by a direct method using the programs SIR2004 (ref. 61), and refined by a full matrix least-squares method on F2 using the programme SHELXL2014 (ref. 62). Refinements with a multipole expansion method using the Hansen–Coppens multipole formalism63 and topological analyses based on the resulting parameters were performed with the XD2006 package64. Crystal data of I for EDD analysis is provided in Supplementary Data 1.
The X-ray crystallographic data for compounds (1R,2R,10bS)-anti-anti-3aa, (3S,4R,5R)-anti-anti-8, mononuclear Ni complex I, trinuclear Ni complex II and multipole population parameters of I for electron density distribution analysis have been deposited at the Cambridge Crystallographic Data Centre (CCDC), with the accession codes CCDC 1482737, 1482738, 1482739, 1482740 and 1482741 (http://www.ccdc.cam.ac.uk/data_request/cif). All other data is available from the authors upon reasonable request.
How to cite this article: Sohtome, Y. et al. Naked d-orbital in a centrochiral Ni(II) complex as a catalyst for asymmetric [3+2] cycloaddition. Nat. Commun. 8, 14875 doi: 10.1038/ncomms14875 (2017).
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Supplementary figures, supplementary tables, supplementary note, supplementary methods and supplementary references.
Electron density distribution (EDD) analysis.
This work was supported in part by Scientific Research (C) from the MEXT (24550128) and Project Funding from RIKEN. RIKEN Integrated Cluster of Clusters (RICC) and RIKEN Hokusai GreatWave (GW) provided the computer resources for the DFT calculations.
The authors declare no competing financial interests.
Author contributions Y.S. and D.H. conceived and developed the concept with M.S. who supervised and guide the research. G.N. made the initial discovery of the reaction with S.L. and Y.S. G.N. and Y.S. performed chemical experiments. G.N. prepared the crystals, and D.H. performed all crystallographic studies. Y.S. performed structural characterization of Ni(II) complexes in the solution state with A.M. and M.U. Y.S. performed DFT calculations with A.M. and M.U. The manuscript was written by Y.S. with D.H. and M.S. T.T. is a supervisor of G.N. in Meiji University. All authors discussed the results.