DFT Studies on the Mechanism of the Iridium-Catalyzed Formal [4 + 1

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DFT Studies on the Mechanism of the Iridium-Catalyzed Formal [4 + 1] Cycloaddition of Biphenylene with Alkenes Hideaki Takano,† Natsuhiko Sugimura,‡ Kyalo Stephen Kanyiva,§ and Takanori Shibata*,†,∥ †

Department of Chemistry and Biochemistry, School of Advanced Science and Engineering, ‡Materials Characterization Central Laboratory, and §International Center for Science and Engineering Program (ICSEP), Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan ∥ JST, ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan S Supporting Information *

ABSTRACT: Recently, we reported an Ir-catalyzed formal [4 + 1] cycloaddition of biphenylenes with alkenes, which gave 9,9-disubstituted fluorenes in moderate to excellent yields. We proposed a reaction mechanism that involved the intermolecular insertion of alkenes, β-elimination, and intramolecular insertion based on the results of experimental mechanistic studies. Herein, we further support the proposed mechanism by density functional theory calculations and explain why [4 + 1] cycloaddition proceeds rather than conventional [4 + 2] cycloaddition.

1. INTRODUCTION Biphenylene contains a strained cyclobutadiene skeleton and is an antiaromatic compound due to its planar 12-π system. Biphenylene readily reacts with transition metal complexes to give dibenzometallacyclopentadienes along with C−C bond cleavage because the bond dissociation energy of the sp2 C (aryl)−sp2 C (aryl) bond of biphenylene (65.4 kcal/mol) is much lower than that of biphenyl (114.4 kcal/mol).1 From a synthetic point of view, the metallacyclopentadienes can be used as a C4 unit. For example, biphenylene reacted with a stoichiometric amount of Ni(0) complex to generate dibenzonickelacyclopentadiene at 0 °C. Subsequent reaction with diphenylacetylene gave a disubstituted phenanthrene as a formal [4 + 2] cycloadduct (Scheme 1).2 Although many examples of the catalytic formal [4 + 2] cycloaddition of biphenylene with unsaturated motifs such as alkynes and alkenes have been reported,3 there has been only a few examples of formal [4 + 1] cycloaddition.4 We recently reported an Ir-catalyzed formal [4 + 1] cycloaddition of biphenylenes with alkenes (Scheme 2).5 On the basis of the results of experimental mechanistic studies, we proposed a reaction mechanism that included the intermolecular insertion of alkenes, β-elimination, and intramolecular insertion (Scheme 3). Herein, we explore this reaction mechanism with the use of density functional theory (DFT) calculations and elucidate why [4 + 1] cycloaddition proceeds exclusively instead of the usual [4 + 2] cycloaddition.6

(diphenylphosphino)-1,1′-binaphthyl (BINAP) ligand were replaced with methyl groups for simplicity (Figure 1).7 Int0 shows a catalytic active species generated from [Ir(cod)Cl]2 and the diphosphine ligand along with the liberation of cycloocta-1,5-diene (thermodynamic consideration of the generation of Int0 is shown in the Supporting Information).8 The first step is the coordination of biphenylene (1) to Int0 to give a η2-coordinated intermediate Int1, which was judged by the natural bond orbital (NBO) analysis.9 Oxidative addition of the iridium center to the C−C bond of biphenylene gave Int2 via the transition state TS1 (activation energy: ΔG = 19.1 kcal/ mol). This pathway was very exothermic due to the release of the ring strain in the cyclobutadiene skeleton. Next, the coordination of styrene (2) to Int2 gave Int3, which was inserted into the Ir−C bond to afford Int4 via TS2 (activation energy: ΔG = 16.9 kcal/mol). The direction of styrene coordination and insertion was possibly controlled by steric repulsion between the phenyl group of styrene and substituent(s) on the phosphine atom. The details of these results are shown in the Supporting Information. From Int4, β-hydrogen elimination proceeded to give iridium hydride intermediate Int5 through TS3 (activation energy: ΔG = 20.6 kcal/mol). In our previous work, the hydrolyzed product of Int5 was isolated and fully characterized.5 For the next intramolecular insertion, the isomerization of the exo olefin moiety gave Int6, wherein the carbon− carbon double bond and Ir−C bond could become parallel.

2. RESULTS AND DISCUSSION We performed the DFT calculations for all of the proposed intermediates, wherein phenyl groups of the 2,2′-bis-

Received: April 5, 2017 Accepted: August 14, 2017 Published: August 30, 2017

© 2017 American Chemical Society

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DOI: 10.1021/acsomega.7b00403 ACS Omega 2017, 2, 5228−5234

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Scheme 1. Pioneering Example of Formal [4 + 2] Cycloaddition of Biphenylene2a

On the basis of these DFT calculations, we considered the preference formation of [4 + 1] cycloadduct 3 over [4 + 2] cycloadduct 4 and exo olefin 5. Judging from the slight difference in the activation energies, there are equilibriums from Int4 to Int8. [4 + 2] Cycloaddition from Int4 via TS6 is entropically unfavored because the biphenyl moiety loses the flexibility compared with Int4. The intramolecular coordination of the olefin moiety in Int5 and Int6 slows the reductive elimination for the formation of the exo olefin 5. The most stable intermediate Int7 gives [4 + 1] the cycloadduct 3. The reductive elimination for the formation of sp3 C−H bond readily proceeds. The formal [4 + 2] cycloadditions of biphenylene with alkynes have been widely reported, and phenanthrene derivatives have been obtained.3a−c,e,f To gain an insight into the different reactivities between alkenes and alkynes, we performed the calculations for the [4 + 2] cycloaddition of biphenylene (1) with diphenylacetylene (6) (Figure 5). The alkyne was coordinated to Int2 to form the intermediate Int3, which is similar to Int3 in [4 + 1] cycloaddition (Figure 1). The alkyne insertion gave the seven-membered metallacycle Int14 via TS9 (17.3 kcal/mol). Finally, the reductive elimination smoothly proceeded to give phenanthrene 7 through TS10; the activation energy of this step was 21.4 kcal/mol, which is much smaller than that of the reductive elimination from Int4 in Figure 3 (39.6 kcal/mol). This difference has already been documented to be due to the faster reductive elimination for the formation of sp2 C−C bond than that of sp3 C−C bond.10 The activation energy of reductive elimination is higher than that of oxidative addition (19.1 kcal/mol), which means that the rate-determining step was the reductive elimination in the [4 + 2] cycloaddition with alkynes.

Scheme 2. Our Study of Ir-Catalyzed Formal [4 + 1] Cycloaddition5

The intramolecular insertion of Int6 afforded Int7 via TS4 along with the formation of a fluorene skeleton (activation energy: ΔG = 19.5 kcal/mol). After isomerization of Int7 to Int8, subsequent reductive elimination through TS5 gave 9,9disubstituted fluorene 3 and regenerated Int0 (activation energy: ΔG = 22.9 kcal/mol). On the basis of these results, this reductive elimination was found to be a rate-determining step in the [4 + 1] cycloaddition reaction (Figure 2). We next performed the calculations for the reductive elimination of Int4 via TS6, which gives [4 + 2] cycloadduct 4 (Figure 3). The activation energy was 39.6 kcal/mol, which was much greater than that of the rate-determining step of [4 + 1] cycloaddition (22.9 kcal/mol) (Figure 2). Furthermore, we calculated the transition state of reductive elimination to give the exo olefin compound 5 (Figure 4). The exo olefin 5 can be generated from both Int5 and Int6, but the corresponding transition states, TS7 and TS8, showed higher relative energies than any other steps of [4 + 1] cycloaddition. Actually, when exo olefin 5 was subjected to the reaction, no reaction proceeded (Scheme 4). Therefore, the transformation of 5 into fluorene 3 via the C−H bond activation did not occur, and this reductive elimination did not proceed in the reaction of biphenylene (1) with styrene (2).

Scheme 3. Proposed Mechanism of the Formal [4 + 1] Cycloaddition

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Figure 1. Gibbs free energy profile from oxidative addition to migratory insertion at 298 K (kcal/mol). Relative energies are in parentheses.

Figure 2. Gibbs free energy profile from β-hydrogen elimination step to product at 298 K (kcal/mol). Relative energies are in parentheses.

3. CONCLUSIONS The present DFT calculations supported our previously reported reaction mechanism for the iridium-catalyzed formal [4 + 1] cycloaddition of biphenylene with alkenes: intermolecular insertion, β-hydride elimination, and intramolecular insertion. Furthermore, we determined the ratedetermining steps in [4 + 1] and [4 + 2] cycloadditions and explained why the [4 + 1] cycloaddition and reductive elimination of exo olefin compound exclusively proceeds rather than [4 + 2] cycloaddition in the reaction of alkenes. In addition, we experimentally and theoretically disclosed that the reaction of alkene required higher energy rather than that of alkyne.

To ascertain these DFT calculations, we examined the reaction of biphenylene (1) with styrene (2) and diphenylacetylene (6) (Table 1). Although the reaction with alkyne 6 proceeded at as low as 100 °C to give [4 + 2] cycloadduct 7 in a moderate yield (entry 1), the reaction with alkene 2 hardly proceeded at 100 °C, and only a trace amount of [4 + 1] cycloadduct 2 was obtained (entry 2). In contrast, biphenylene (1) was completely consumed at temperatures higher than 120 °C (entries 3−5). These results supported the finding that the C−C bond cleavage of biphenylene is not a rate-determining step in the formal [4 + 1] cycloaddition, and the activation barrier of [4 + 1] cycloaddition with styrene (2) is higher than that of [4 + 2] cycloaddition with diphenylacetylene (6). 5230


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Figure 3. Gibbs free energy profile of prospected [4 + 2] cycloaddition at 298 K (kcal/mol). Relative energies are in parentheses.

Figure 4. Gibbs free energy profile of the prospected reductive eliminations to generate exo olefin 5 at 298 K (kcal/mol). Relative energies are in parentheses.

4. COMPUTATIONAL METHODS All of the calculations were performed at the M0611 level of theory, using the Gaussian 09 package.12 The geometry optimizations was carried out with a mixed basis set of LanL2DZ13 for Ir atom and 6-31G(d) for other atoms. Vibration frequency was computed at the same level of theory to confirm whether the structures are minima (no imaginary frequencies) or transition states (only one imaginary frequency). Energies of optimized structures were calculated by the single-point calculation at the same level of theory with

Scheme 4. Reaction of exo Olefin Compound 5 under Optimal Conditions

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Figure 5. Gibbs free energy profile of [4 + 2] cycloaddition of biphenylene (1) with diphenylacetylene (5) at 298 K (kcal/mol). Relative energies are in parentheses.

(13C NMR) spectra were recorded with ECX-500 (500 MHz) spectrometer. Chemical shift values for protons are reported in parts per million (δ) relative to the internal standard TMS (0.0 ppm). 13C NMR spectra were obtained by JEOL ECX-500 (125 MHz) spectrometers and referenced to the internal solvent signals (central peak is 77.0 ppm in CDCl3). Data are presented as follows: chemical shift, multiplicity (d = doublet, m = multiplet), coupling constant in Hertz (Hz), and area integration. High-resolution mass spectra (HRMS) were measured with an electrospray ionization (ESI)-orbitrap mass spectrometer. Preparative thin-layer chromatography (PTLC) was performed with silica gel-precoated glass plates (Merck 60 GF254) prepared in our laboratory. All of the reagents were weighed and handled in air and backfilled under argon at room temperature. All of the reactions were performed under an argon atmosphere. Unless otherwise noted, organic compounds and solvents were purchased from Tokyo Kasei Co., Aldrich Inc., and other commercial suppliers and were used without further purification. 5.2. Synthesis of exo Olefin 5. To the two-neck roundbottom flask, methyl triphenylphosphonium bromide (2.0 mmol) and tetrahydrofuran (THF) (2 mL) were added. After cooling to 0 °C, n-BuLi in hexane (2.0 mmol, 1.6 M/1.25 mL) was added to the reaction mixture and this solution was stirred for 1 h at 0 °C. [1,1′-Biphenyl]-2-yl(phenyl)methanone, which was synthesized by reported method (Chem. Commun. 2014, 50, 1131), in THF (1.0 mL) was added to the reaction mixture and the solution was stirred for 2 h. This was quenched by pH 7 buffer and extracted by ethyl acetate. The organic phase was dried by sodium sulfate and dried in vacuo. The obtained crude product was purified by column chromatography and exo olefin 5 was obtained in 84% yield. 5.2.1. 2-(1-Phenylvinyl)-1,1′-biphenyl 5. Isolated by PTLC (hexane, Rf = 0.3). The title compound was obtained as white solid (216.3 mg, 84%); mp 61 °C; 1H NMR δ 7.41−7.33 (m, 4H), 7.22−7.19 (m, 2H), 7.15−7.06 (m, 8H), 5.56 (d, J = 1.4 Hz, 1H), 5.19 (d, J = 1.4 Hz, 1H); 13C NMR δ 149.5, 141.6,

Table 1. Screening of Temperature in [4 + 2] and [4 + 1] Cycloadditions

a

entry

reactant

product

temp. (°C)

time (h)

yield (%)a

1 2 3 4 5

6 2 2 2 2

7 3 3 3 3

100 100 120 135 150

24 24 24 24 16

33 trace 59 83 78

Isolated yield.

solvation effects (xylene mixture, ε = 2.387900) using the SMD model.14 The molecular geometries of the transition states were first estimated by the Reaction plus pro software package (software to optimize reaction paths along the user’s expected ones, HPC Systems Inc., http://www.hpc.co.jp/chem/react1. html (written in Japanese)), based on the nudged elastic band (NEB) method,15 and subsequently reoptimized by the Synchronous Transit-guided Quasi-Newton method with the keyword QST2 or QST3.16 Transition-state structures were confirmed to connect the corresponding reactants and products through the use of intrinsic reaction coordination (IRC) calculations.17

5. EXPERIMENTAL DETAILS 5.1. General Information. Proton nuclear magnetic resonance (1H NMR) and carbon nuclear magnetic resonance 5232


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141.3, 141.3, 140.7, 130.8, 130.2, 129.1, 127.7, 127.7, 127.5, 127.1, 127.1, 127.0, 126.4, 116.4; HRMS (ESI): m/z calcd for C20H16Na [M + Na] 279.1144, found 279.1145. 5.3. General Procedure for Formal [4 + 1] Cycloaddition in Table 1. [Ir(cod)Cl]2 (0.01 mmol), BINAP (0.02 mmol), and biphenylene (1) (0.10 mmol) were placed in a Schlenk tube, which was then evacuated and backfilled with argon (×3). Styrene (2) (0.4 mmol) and anhydrous xylene (0.5 mL, pretreated by argon bubbling for 30 s) were added to the reaction vessel. The solution was then stirred for 24 h at the indicated temperature (bath temperature). The reaction mixture was cooled to room temperature and the solvent was evaporated to dryness. The obtained crude product was purified by PTLC to give product 3. 5.4. Procedure for Formal [4 + 2] Cycloaddition in Table 1. [Ir(cod)Cl]2 (0.01 mmol), BINAP (0.02 mmol), and biphenylene (1) (0.10 mmol) were placed in a Schlenk tube, which was then evacuated and backfilled with argon (×3). Diphenylacetylene (6) (0.4 mmol) and anhydrous xylene (0.5 mL, pretreated by argon bubbling for 30 s) were added to the reaction vessel. The solution was then stirred for 24 h at 100 °C (bath temperature). The reaction mixture was cooled to room temperature and the solvent was evaporated to dryness. The obtained crude product was purified by PTLC to give product 7 (33% yield).

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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/acsomega.7b00403. NMR spectra of compound 5, thermodynamic consideration of the generation of Int0, other conformations of Int3, total electronic energy, and free energy with solvation effects (xylene mixture, ε = 2.387900) using SMD model, structures optimized at M06/LanL2DZ for Ir and M06/6-31G(d) for other atoms, Cartesian coordinates of stationary points optimized at M06/ LanL2DZ for Ir and M06/6-31G(d) for other atoms, and second-order perturbation theory analysis of Fock matrix of Int1 in NBO basis are listed (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax: +81-3-5286-8098. ORCID

Takanori Shibata: 0000-0003-4436-8264 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by JST, ACT-C, Japan, and Waseda University Grant for Special Research Projects. We would like to thank Prof. Ken Sakata (Hoshi University) for helpful discussions. All of the calculations were performed at The Materials Characterization Central Laboratory of Waseda University: an open-end laboratory program for fourth-year undergraduate and graduate students.18 We are grateful to Umicore for generous supports in [IrCl(cod)]2 supply. 5233


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(15) Jónsson, H.; Mills, G.; Jacobsen, K. W. Nudged Elastic Band Method for Finding Minimum Energy Paths of Transitions, in Classical and Quantum Dynamics in Condensed Phase Simulations, Ed. Berne, B. J.; Ciccotti, G.; Coker, D. F. 385 (World Scientific, 1998); Henkelman, G.; Jónsson, H. Improved tangent estimate in the nudged elastic band method for finding minimum energy paths and saddle points. J. Chem. Phys. 2000, 113, 9978−9985. (16) (a) Peng, C.; Schlegel, H. B. Isr. J. Chem. 1993, 33, 449−454. (b) Peng, C.; Ayala, P. Y.; Schlegel, H. B.; Frisch, M. J. J. Comput. Chem. 1996, 17, 49−56. (17) (a) Fukui, K. J. Phys. Chem. 1970, 74, 4161−4163. (b) Fukui, K. Acc. Chem. Res. 1981, 14, 363−368. (18) Izutani, C.; Fukagawa, D.; Miyasita, M.; Ito, M.; Sugimura, N.; Aoyama, R.; Gotoh, T.; Shibue, T.; Igarashi, Y.; Oshio, H. J. Chem. Educ. 2016, 93, 1667−1670.

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DFT Studies on the Mechanism of the Iridium-Catalyzed Formal [4 + 1

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