Theoretical Study of Ni-Catalyzed CâN RadicalâRadical Cross

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Theoretical Study of Ni-Catalyzed C#N Radical–Radical Cross-Coupling Song Liu, Xiaotian Qi, Ruopeng Bai, and Yu Lan J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b03245 • Publication Date (Web): 19 Feb 2019 Downloaded from http://pubs.acs.org on February 19, 2019

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The Journal of Organic Chemistry

Song Liu,† Xiaotian Qi,*,§ Ruopeng Bai,† and Yu Lan*,†,‡ †

School of Chemistry and Chemical Engineering, and Chongqing Key Laboratory of Theoretical and Computational Chemistry, Chongqing University, Chongqing 400030, China ‡ §

College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China

Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA

ABSTRACT: A computational study was carried out to investigate the mechanism and the origin of chemoselectivity in nickelcatalyzed C–N radical–radical cross-coupling reaction. The global electrophilicity index ω° and global nucleophilicity index N° were used to quantitatively describe the electrophilic or nucleophilic character of the carbon radical, nitrogen radical, and Ni(II) complex. The calculated ω° and N° values indicate that introduction of nickel makes C–N cross-coupling to be a facile process. Detailed theoretical results show that the cross-coupling occurs through the combination of Ni(I) complex with a nitrogen-centered radical , a minimum energy crossing point to form the singlet Ni(II) complex and radical addition of the nucleophilic carbon-centered radical lead to C–N bond formation. Based on the theoretical results, a generalized scheme is provided to clarify the origin of the chemoselectivity in nickel-catalyzed C–N radical–radical cross-coupling.

Free radicals, because of their high reactivity, are usually used as reactive intermediates in organic reactions.1 However, the structure of the free radicals determine their stability and reactivity. With electron-withdrawing or -donating substituents, the free radicals may pronounce electrophilic or nucleophilic character.2 The global electrophilicity index ω° and global nucleophilicity index N° reported by Domingo’s group are used to evaluate the electrophilic or nucleophilic character of free radicals.3 As shown in Table 1, free radicals can be considered as strong electrophiles, when the calculated ω° is larger than 1.50 eV. While the calculated ω° is between 0.80 eV and 1.50 eV, the free radicals can be considered as moderate electrophiles. The ω° of marginal electrophiles radicals is lower than 0.80 eV. Conversely, free radicals can be divided as strong nucleophiles, whose N° is larger than 3.00 eV. The value of the calculated N° for moderate nucleophiles is between 2.00 eV and 3.00 eV. While the N° of the marginal nucleophiles is lower than 2.00 eV.3c The quantitative description of the electrophilicity or nucleophilicity of free radicals plays a critical role in understanding the reactivity as well as the selectivity in free radical reactions.4 Table 1. Classification of Free Radicals Based on the Electrophilicity and Nucleophilicity

Among the free-radical-involved reactions, radical–radical cross-coupling reactions have provided practical and efficient methods for the formation of new carbon−carbon and car-

bon−heteroatom bonds.5 In this area, extensive efforts have been devoted to the development of radical–radical crosscoupling processes.6 However, because they are highly reactive, homocoupling of free radicals is inevitable in radical–radical cross-coupling reactions.7 Usually, the cross-coupling reactions build covalent bonds between a persistent radical and a transient radical, which complies with the persistent radical effect.8 Development of methods to stabilize one of the reactive radicals could prevent two transient radicals from homocoupling. Fortunately, the transition metal can be considered to be a radical shuttle to selectively combine with the transient radicals and protect it from being quenched, thereby leading to the selective radical coupling reactions.9 Recently, a variety of radical–radical cross-coupling reactions has been realized by this strategy.10 A comprehensive mechanistic understanding is able to promote the design of transition-metal-catalyzed/mediated radical–radical cross-coupling reactions.11 Recently, a theoretical study on copper-faciliated C–N radical–radical cross-coupling was reported in our group to investigate the reaction mechanism and the origin of chemoselectivity.12 We proposed and verified a stepwise coupling model, where Cu successively stabilizes two radical species. The catalytic cycle involves the combination of nitrogen-centered radical with the Cu(I) complex, coordination of carbon radical, and the reductive elimination from Cu(III) complex finally deliver the C–N coupling product. In 2014, Lei and some of the present authors reported Niassisted oxidative radical–radical cross-coupling between a nitrogen-centered radical and a sp3 carbon-centered radical (Scheme 1).13A variety of C(sp3)–N bonds can be generated applying this radical–radical cross-coupling strategy. Density functional theory (DFT) calculations were performed to

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Scheme 1. Ni-Assisted Oxidative Radical–Radical CrossingCoupling for C(sp3)–N Bond Formation

explore the vital function of nickel in this transformation. Although the calculated results indicated that the coordination of nickel with nitrogen radical increase the stability of nitrogen radical, the detail reaction pathway of this Ni-assisted oxidative radical–radical crossing-coupling is still not clear. Here, we performed theoretical study to investigate the mechanism and the origin of chemoselectivity in the Ni-assisted oxidative radical–radical cross-coupling reaction between nitrogen radical and sp3 carbon radical. As previous mentioned, the global electrophilicity index and global nucleophilicity index play crucial roles in determining the cross-coupling selectivity in copperfacilitated C–N radical–radical coupling reaction. Therefore, global electrophilicity index and global nucleophilicity index calculations were initially performed to investigate the crosscoupling mode in this Ni-catalyzed oxidative radical–radical cross-coupling reaction.

All calculations were carried out using the Gaussian 09 series of programs.14 Geometry optimizations were performed in the gas phase using the B3-LYP15 functional. The SDD basis set was used for Ni, and the 6-31+G(d) basis set was used for other atoms. Harmonic frequency analysis was performed at the same level of theory for all of the stationary points to verify whether they are local minima or transition structures and obtain the thermochemical energy corrections. The stability of the wavefunction was tested and confirmed for singlet-, doublet-, triplet-, and quartet-state intermediates. The solvent energy corrections were considered by single point calculations of the gas-phase stationary points with a continuum solvation model (SMD).16 The single point energies were calculated at M06 functional with the 6-311+(d,p) basis set (SDD basis set for Ni) with tetrahydrofuran (THF) as the solvent, which provides highly accurate energy information.17 The energies given in this work are the M06-calculated Gibbs free energies in THF solvent. The Mulliken atomic spin densities were calculated with the same functional and basis set. Images of the 3D structures of optimized intermediates and transition states were generated using CYLview.18 Minimum energy crossing point (MECP) calculations19 were also performed using B3-LYP/6-31+G(d) (SDD basis set for Ni). The M06 functional with the 6-311+(d,p) basis set (SDD basis set for Ni) was also used to calculate the single point energies of the MECPs with THF as the solvent. The global electrophilicity index and global nucleophilicity index were calculated at the UB3LYP/6-31G(d) (SDD basis set for Ni) level using ω° = (μ°)2/2η°

(1)

N° = E °HOMO − E °HOMO (DCM) α,

α,

(2)

In eq (1), μ° refers to the global chemical potential of the free radicals. The μ° can be calculated via μ° ≈ (Eα,°HOMO +

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Eβ,°LUMO)/2. η° is the global chemical hardness of the free radicals. It can be calculated via η° ≈ (Eβ,°LUMO − Eα,°HOMO). Eα,°HOMO refers to the energy value of HOMO in the α molecular orbitals and Eβ,°LUMO correspond to the orbital energies of the LUMO for the β molecular orbitals. In addition, DCM denotes the dicyanomethyl radical.

Global electrophilicity and global nucleophilicity calculations were initially performed to investigate the cross-coupling mode in this Ni-catalyzed oxidative radical–radical crosscoupling reaction. The calculated ω° and N° values are given in Table 2. ω° = 0.89 eV for α-alkoxyl carbon radical and ω° = 4.50 eV for the N-alkoxyamide nitrogen radical, which indicate the nitrogen-centered radical is a relatively strong electrophile and the α-alkoxyl carbon radical species is only a moderate electrophile. N° = 3.80 eV for the α-alkoxyl carbon radical indicates that the α-alkoxyl carbon radical is a strong nucleophile. Table 2. Calculated ω° and N°Values

The calculated results indicate that the combination of electron-rich Ni(I) species with the electron-deficient nitrogen radical to generate the triplet Ni(II) radical complex is more facile than the electron-rich nitrogen radical. For the triplet Ni(II) radical complex, ω° = 2.47 and N° = 2.11. As a strong electrophile, the triplet Ni(II) radical complex would preferentially react with the α-alkoxyl carbon-centered radical rather than with the nitrogen radical, which would result in C–N radical–radical cross-coupling. A detailed computational study was performed to investigate the mechanism of this Ni-catalyzed oxidative radical–radical cross-coupling reaction. The calculated free energies are shown in Figure 1. Decomposition of the di-tert-butyl peroxide (DTBP) gives two tert-butoxyl radicals (ΔG = 16.9 kcal/mol, Figure 1a), which is the radical initiator in this reaction. Thus, the free energy of tert-butoxyl radical 5 is 8.5 kcal/mol. The calculated energy profiles for the reactions of the tert-butoxyl radical with tetrahydrofuran 1 and N-methoxybenzamide 2 to generate a carbon radical and a nitrogen radical, respectively, are shown in Figure 1b. The activation free energy for formation of carbon radical 6-ts is 11.4 kcal/mol, and the activation free energy for formation of nitrogen radical 8-ts is 10.5 kcal/mol. Thus, both carbon and nitrogen radicals can be generated in the reaction. The calculated results also indicate that nitrogen radical 9 is more stable than carbon radical 7. The Mulliken atomic spin density on the α-carbon atom in 7 is 0.90. The Mulliken atomic spin density of the nitrogen atom in intermediate 9 is 0.62. As shown in Figure 2, the radical addition of tert-butoxyl radical 5 to the diketonate in Ni(acac)2 can occur via transition


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Figure 1. Free energies profiles for the generation of tert-butoxyl radical 5 (a), carbon radical 7 (b), and nitrogen radical 9 (c). The Mulliken atomic spin densities on corresponding atoms are given in parentheses. All bond lengths are in angstroms.

Figure 2. Free energies profiles for the generation of active catalytic species 15-trip. The Mulliken atomic spin densities on corresponding atoms are given in parentheses. All bond lengths are in angstroms.

state 11-ts to form Ni(I) active catalyst 12. This nucleophilic addition is fast with an activation barrier of only 3.6 kcal/mol. Subsequently, the N-alkoxyamide nitrogen radical 9 coordinates to the Ni(I) intermediate 12. The triplet Ni(II) intermediate 13-trip is generated, which is exothermic by 39.2 kcal/mol. The triplet Ni(II) intermediate 13-trip is more stable than the singlet Ni(II) intermediate 13-sing by 27.9 kcal/mol (see SI). The spin density of the nickel atom in 13-trip is 1.70,

which indicates that the radical character is mainly concentrated on the Ni(II) metal atom. The spin density of the nitrogen atom is only 0.02 shows that the nitrogen atom has lost its radical character. After release of β-tert-butoxyl diketonate 14, Ni(II) β-diketonate intermediate 15-trip could be formed. The tetrahedral coordination around the nickel shows that 15-trip is a triplet Ni(II) complex. The spin density of the Ni(II) metal atom in 15-trip is 1.72, while the spin density of the nitrogen



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Figure 3. Free energy profile of the C–N radical–radical cross-coupling pathway. The Mulliken atomic spin densities on corresponding atoms are given in parentheses. All bond lengths are in angstroms.

atom in 15-trip is only 0.02, which also shows that the spin density is mainly concentrated on the nickel metal atom. In another possible pathway, coordination of carbon radical 7 to Ni(I) active catalyst 12 preferentially generates the singlet Ni(II) intermediate16-sing, which is exothermic by 13.7 kcal/mol. The singlet Ni(II) intermediate 16-sing is more stable than the triplet Ni(II) intermediate 16-trip by 2.0 kcal/mol (see SI). The free energy of the triplet Ni(II) intermediate 13trip is 25.5 kcal/mol lower than that of intermediate 16-sing. The calculated results are also consistent with the global electrophilicity index and global nucleophilicity index calculations, which indicate that the combination between electron-rich Ni(I) species and electron-deficient nitrogen-centered radical 9 is more favored over than that with carbon radical 7. The free energy profiles of the C–N bond cross-coupling pathways are shown in Figure 3. In the red pathway, the C–N bond forms by directed nucleophilic addition of carbon radical 7 to the nitrogen radical of Ni(II) triplet intermediate 15-trip. Nucleophilic addition occurs via quartet transition state 18-tsquart, which requires an energy barrier of 22.3 kcal/mol to generate the quartet intermediate 19-quart. In another possible pathway, the triplet Ni(II) intermediate 15-trip initially transforms to the singlet Ni(II) intermediate 15-sing through 20-MECP, with a structural change of Ni(II) from tetrahedral to square planar. The single point energy of the minimum energy crossing point 20-MECP is −38.9 kcal/mol, which is 4.9 kcal/mol lower than that of transition state 18-ts-quart. The C–N bond then forms through doublet nucleophilic addition transition state 21-ts with an activation

barrier of only 0.8 kcal/mol. The spin density of the nickel metal atom in Ni(I) intermediate 22 is 1.00, which indicates that the spin density is mainly concentrated on the nickel atom. Ligand exchange between intermediate 22 and Nmethoxybenzamide 2 releases cross-coupling product 3 and gives N-methoxybenzamide-coordinated Ni(I) intermediate 23. Subsequent coordination of tert-butoxyl radical 5 and hydrogen abstraction in N-methoxybenzamide gives active intermediate 15-trip to complete the catalytic cycle. The hydrogen atom transfer process bears an activation barrier of only 3.0 kcal/mol, which implies the regeneration of 15-trip is a facile step. We also calculated the free energy profiles of the N–N radical–radical homocoupling pathway to understand why homocoupling of N–N does not occur (Figure 4). Nitrogen radical 9 coordinates to the Ni atom in intermediate 15-trip to give the quartet complex 26-quart, which is exothermic by 4.9 kcal/mol. Subsequent N–N bond formation occurs via a quartet transition state 27-ts, to give the quartet Ni(I) intermediate 28-quart. The activation free energy is extremely high (40.0 kcal/mol). In an alternative pathway, the quartet Ni(I) complex 26-quart transforms to the doublet Ni(III) intermediate 30 through MECP 29-MECP. The formation of N–N bond would occur via transition state 31-ts. The activation free energy barrier of 31-ts is 40.0 kcal/mol. The high activation energies of 27-ts and 31-ts indicate that homocoupling of N–N could not occur. The calculated results are also consistent with the global electrophilicity index and global nucleophilicity index calculations, which indicate that the 15-trip intermediate would combine


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Figure 4. Free energy profiles of the N–N radical–radical homocoupling pathway.

Figure 5. Graphical illustration of the origin of the cross-coupling chemoselectivity.

with α-alkoxyl carbon-centered radical 7 rather than nitrogen radical 9. Based on the above calculations, a graphical illustration to account for the preference for C–N radical–radical crosscoupling over N–N radical–radical homocoupling is shown in Figure 5. Initially, the electron-rich Ni(I) complex would combine with the electrophilic nitrogen radical rather than with the nucleophilic carbon radical. Subsequently, the combination of the electrophilic Ni(II) species with the nucleophilic carbon radical is more favored with the electrophilic nitrogen radical to give the cross-coupling product. The nature of the Ni complex means that the Ni(acac) 2 catalyst shows excellent reactivity to selectively promote the radical coupling between the carbon radical and the nitrogen radical.

Based on our previous report, a cross-coupling mechanism for nickel-catalyzed C–N radical–radical cross-coupling has been proposed and proven. Theoretical calculations indicate that this reaction proceeds via the combination of Ni(I) species

with nitrogen-centered radical to generate the triplet Ni(II) Nmethoxybenzamide intermediate. The singlet Ni(II) Nmethoxybenzamide intermediate then forms through a MECP. Subsequent radical addition of the carbon radical to nitrogen atom constructs the C–N bond. Based on the global electrophilicity index and global nucleophilicity index calculations, the nitrogen radical is identified as a strong electrophile, while the carbon radical is identified as a strong nucleophile, and the Ni(II) complex generated by coordination of the nitrogen radical to Ni(I) is identified as a strong electrophile. The nature of the global electrophilicity and global nucleophilicity of the nitrogen radical, carbon radical, and generated Ni(II) complex accounts for the C–N crosscoupling selectivity. We expect the mechanistic insights from this theoretical study, especially the quantitative description of nucleophilicity and electrophilicity of different radicals, will aid in the design and development of radical–radical crosscoupling reactions.



Supporting Information. The following files are available free of charge. Cartesian coordinates and energies of all reported structures and full authorship of Gaussian 09 (PDF)

*E-mail: [email protected]. *E-mail: [email protected]. The authors declare no competing financial interest.

This work was supported by the National Natural Science Foundation of China (No. 21822303 and 21772020). We are thankful for the project (No. 2018CDXZ0002; 2018CDPTCG0001/4) supported by the Fundamental Research Funds for the Central Universities (Chongqing University). We are also thankful for project supported by graduate research and innovation foundation of Chongqing, China (Grant No. CYB18043).

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