Factors Controlling the Reactivity and Chemoselectivity of Resonance

Oct 10, 2017 - N-Glutarimide amides have recently emerged as an exceptional group of compounds with unusually high reactivity in amide C–N bond acti...
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Article Cite This: J. Am. Chem. Soc. 2017, 139, 15522-15529

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Factors Controlling the Reactivity and Chemoselectivity of Resonance Destabilized Amides in Ni-Catalyzed Decarbonylative and Nondecarbonylative Suzuki-Miyaura Coupling Chong-Lei Ji and Xin Hong* Department of Chemistry, Zhejiang University, Hangzhou, 310027, China S Supporting Information *

ABSTRACT: N-Glutarimide amides have recently emerged as an exceptional group of compounds with unusually high reactivity in amide C−N bond activation. To understand the key factors that control the remarkable reactivity of these resonance destabilized amides, we explored the Ni-catalyzed decarbonylative and nondecarbonylative Suzuki-Miyaura coupling with N-glutarimide amides through density functional theory calculations. Two leading effects are responsible for the C−N cleavage activity of N-glutarimide amides, the coordinating N-substituents and the geometric twisting. The carbonyl substituent of the N-glutarimide amides provides crucial nickel−oxygen interaction, which essentially acts as a directing group to facilitate the formation of the reactive intermediate for the amide C−N bond cleavage. The geometric twisting weakens the resonance stability by removing the acyl-nitrogen conjugation, which lowers the energy penalty for the C−N bond stretch during oxidative addition. For the chemoselectivity of decarbonylation versus carbonyl retention, we found that the C−C reductive elimination for ketone formation is kinetically faster than that for biaryl formation, while ketone is thermodynamically less stable with respect to the decarbonylated biaryls. The computations also suggest that the nickel catalyst is able to promote the decarbonylation of biaryl ketones via an unexpected C− C bond activation.



INTRODUCTION Ni-catalyzed C−N bond activation of amides has recently emerged as a distinctive strategy to utilize amides in organic synthesis.1 A number of exciting transformations, contributed by Garg and co-workers,2 Szostak and co-workers,3,4 Shi and co-workers,5 Zou and co-worker,6 Rueping and co-workers,7 Maiti and co-workers,8 Stanley and co-workers,9 and Molander and co-workers,10 have been realized based on the Ni-mediated C−N cleavage of amides. Compared to the common planar amides, Szostak and co-workers discovered that N-glutarimide amides exhibit significant geometric distortions and unusually high reactivities in C−N bond activations,3,4 even in metal-free conditions.11 The high reactivity of these N-glutarimide amides are usually rationalized by the twisting geometric nature (Scheme 1).3a,12 Despite the vast development of synthetic transformations with N-glutarimide amides,3,4 the controlling factors that differentiate the C−N bond activation reactivity of Nglutarimide amides from those of other amides remain elusive. In addition to this key question of reactivity, the SuzukiMiyaura coupling with N-glutarimide amides produced the decarbonylated biaryl products,3a which is distinctive compared with the nondecarbonylative Suzuki-Miyaura couplings involving other amides (Scheme 1).13 This chemoselectivity is another general issue that exists in all transformations involving © 2017 American Chemical Society

C−N cleavage of amides. Understanding these two fundamental mechanistic questions with N-glutarimide amides is critical toward the rational design of amide C−N bond activation and functionalization. Regarding the mechanistic studies of Ni-catalyzed amide C− N bond activation, Houk and Garg elucidated the mechanism of Ni/NHC-catalyzed esterification of anilides with DFT calculations, and the three-centered oxidative addition model successfully explains the reactivities of N-substituted aryl amides.2a In addition, the group of Zhao and Zhu,14 as well as the group of Fu and Yu,15 independently studied the mechanism of the Ni/NHC-catalyzed Suzuki-Miyaura coupling with Boc-activated amides, which elucidated the roles of K3PO4 and water in facilitating the transmetalation step. To elucidate the origins of reactivity and chemoselectivity with N-glutarimide amides, here we report the first computational study on the C−N bond activation of this special group of amides, focusing on the Ni/PCy3-catalyzed Suzuki-Miyaura coupling with aryl boronates. Two essential factors were found to contribute to the exceptional reactivities of N-glutarimide amides in the Ni-mediated C−N cleavage, the carbonyl coordination, and the geometric twisting. The coordinating Received: September 5, 2017 Published: October 10, 2017 15522

DOI: 10.1021/jacs.7b09482 J. Am. Chem. Soc. 2017, 139, 15522−15529

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Journal of the American Chemical Society Scheme 1. Twisting Nature of N-Glutarimide Amides and Comparisons between the N-Glutarimide Amides and the Boc-Activated Amides in Ni-Catalyzed Suzuki-Miyaura Coupling2b,3a

reactions with acyl derivatives,2a,14,15,23 the proposed catalytic cycles of the Ni-catalyzed Suzuki-Miyaura coupling between amides and arylboronic acids are shown in Scheme 2. Starting Scheme 2. Proposed Catalytic Cycles of Ni-Catalyzed Suzuki-Miyaura Coupling between Amides and Arylboronic Acids

glutarimidyl moiety acts as a directing group which facilitates the generation of the reactive intermediate for C−N bond cleavage. In addition, the geometric twisting alleviates the energy penalty for the C−N bond stretch, lowering the intrinsic barrier for oxidative addition. These synergistic effects also differentiate the target C−N bond from the adjacent C−N bonds of the glutarimide substituent, preventing side reactions involving undesired C−N bond cleavage. Thus, the inert nature of the glutarimide group is an additional important factor that contributes to the high reactivity of the N-glutarimide amides toward the designed C−N bond activation. For the chemoselectivity between ketone and decarbonylated biaryl, the formation of ketone product is kinetically favorable due to the faster C−C reductive elimination, while ketone is thermodynamically less stable than the decarbonylated biaryl products. These mechanistic insights will shed light on the future design of amide C−N bond activations and related synthetic applications.



with the in situ generated Ni(0) active catalyst A, the oxidative addition breaks the amide C−N bond and generates the LnNi(acyl)(amino) intermediate B. B undergoes the transmetalation with boronic acid to form the LnNi(acyl)Ar2 intermediate C, and subsequent decarbonylation generates the LnNiAr1Ar2 intermediate D. The C−C reductive elimination of D produces the biaryl product and regenerates the nickel(0) active catalyst A. Alternatively, C can directly undergo the C(acyl)−C(aryl) reductive elimination to produce the ketone product. We first studied the proposed catalytic cycle using the experimental substrates, 1-benzoylpiperidine-2,6-dione and 2naphthaleneboronic acid. The free energy changes of the most favorable pathway leading to the biaryl product are shown in Figure 1. From the most stable nickel−amide complex 1, an isomerization occurs to generate the preoxidative addition intermediate 2. Subsequent oxidative addition via a fivecentered transition-state TS3 requires a barrier of 9.6 kcal· mol−1 as compared to the intermediate 1, leading to the PCy3Ni(acyl)(amino) intermediate 4. This exceptionally low barrier for C−N cleavage corroborates the high reactivity of the N-glutarimide amides. Notably, the N-glutarimide amides have very low nN−π*(acyl) resonance, which correlates with the amide reactivity in cross-coupling reactions according to Szostak’s recent study12c and is one of the factors that facilitates the oxidative addition via C−N cleavage (vide infra). From 4, the coordination with sodium carbonate is quite exergonic, and the signals responsible for the nickel−carbonate complex without sodium cations are detected in the stoichiometric electrospray ionization mass spectrometry experiments by Szostak and co-worker.3a 5 further reacts with 2-naphthaleneboronic acid to generate the stable intermediate 6, and the sodium carbonate in 6 acts as an intramolecular base to facilitate the transmetalation via TS7. The PCy3Ni(acyl)(naphthyl) intermediate 9 then undergoes the decarbonylation

COMPUTATIONAL METHODS

All density functional theory (DFT) calculations were performed by Gaussian 09 program.16 The geometry optimizations were conducted using the B3LYP functional,17 with LANL2DZ basis set18 for nickel and 6-31G(d) basis set for the other atoms. To confirm whether each optimized stationary point is an energy minimum or a transition state, as well as evaluate the zero-point vibrational energy and thermal corrections at 298 K, the vibrational frequencies were computed at the same level of theory as for the geometry optimizations. On the basis of the gas-phase optimized structures, the single-point energies and solvent effects were evaluated with the M06 functional,19 SDD basis set20 for nickel, and 6-311+G(d,p) basis set for the other atoms. The solvation energies were calculated using the self-consistent reaction field with the CPCM implicit solvent model.21 Fragment distortion and interaction energies were calculated with the M06 functional, SDD basis set for nickel, and 6-311+G(d,p) basis set for the other atoms, without the inclusion of solvation energy corrections. The details of distortion/interaction analysis are provided in the Supporting Information. The 3D diagrams of computed species were generated using CYLView.22



RESULTS AND DISCUSSION Proposed Catalytic Cycle. On the basis of previous mechanistic studies on the nickel-catalyzed cross-coupling 15523

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Figure 1. DFT-computed Gibbs free energy changes of the most favorable pathway of Ni/PCy3-catalyzed decarbonylative Suzuki-Miyaura coupling between the N-glutarimide amide 14 and 2-naphthaleneboronic acid.

via TS10, and the subsequent C(sp2)−C(sp2) reductive elimination via TS13 eventually produces the biaryl product 15 and regenerate intermediate 1 for the next catalytic cycle. On the basis of the calculated free energy changes of the whole catalytic cycle, the resting state is the pretransmetalation intermediate 6, and the rate-limiting step is the C(sp2)−C(sp2) reductive elimination with an overall barrier of 27.2 kcal·mol−1 (6 to TS13). This significant barrier explains the requirement for high temperature in experiment.3a To further validate this mechanistic model, we calculated the free energy barriers of a number of substituted substrates and compared the computed barriers with the competition experiments from Szostak’s work3a (Figure S1). The satisfying consistency between the computational and experimental results provides additional support for the proposed mechanism. Our calculations also showed that the aliphatic N-glutarimide amide has a higher overall barrier for the same Suzuki-Miyaura coupling (Figure S2). Origins of the High Reactivity of N-Glutarimide Amides in the Ni-Mediated C−N Bond Activation. To understand the origins of the exceptional reactivity of Nglutarimide amides in the Ni-mediated C−N bond cleavage, we compared the N-glutarimide amide 14 to the N-Bn-N-Boc benzamide 16 and the N-Me-N-Ph benzamide 21, which represents three typical types of amides that are generally employed in the Ni-catalyzed cross couplings.1 The free energy changes of the Ni/PCy3-mediated C−N bond activation of the three amides are included in Figure 2. The overall C−N bond activation barrier of 14 is only 9.6 kcal·mol−1, while those of 16 and 21 are more than 6.0 kcal·mol−1 higher. In addition, the

oxidative addition with 14 is significantly more exergonic than the other two amides. Both the kinetics and thermodynamics highlight the distinctive reactivities of the N-glutarimide amides. We also compared the C−N bond activation of the Nglutarimide amide 14 to that of the corresponding Nsuccinimide amide, and this N-succinimide amide has a C−N bond activation barrier of 13.0 kcal/mol (Figure S3). These computed barriers of amide C−N bond activation agree well with the observed reactivities of amides in Ni/PCy3-catalyzed Suzuki-Miyaura coupling under the same conditions (Scheme 3).3a The thermodynamics of the oxidative addition depends highly on the coordinating N-substituents of amide. The carbonyl group of 14 provides an additional strong Ni−O interaction in the postoxidative addition intermediate 4, which stabilizes this intermediate and makes the corresponding C−N cleavages exergonic. For the Boc-activated amide 16, similar Ni−O interaction exists in the intermediate 20, leading to the exergonic oxidative addition. While for the anilide 21, no such Ni−O interaction exists in tricoordinated nickel intermediate 20, and thus the oxidative addition is endergonic by 5.7 kcal· mol−1. To elucidate the effects on the kinetic barriers of C−N bond activation, we decomposed the overall barrier to two parts: the free energy of isomerization from the most stable nickel−amide complex to the preoxidative addition intermediate (e.g., 1 to 2, labeled in green, Figure 3), and the intrinsic barrier of oxidative addition from the preoxidative addition intermediate to the C− N cleavage transition state (e.g., 2 to TS3, labeled in yellow, Figure 3). These comparisons reveal two crucial factors that 15524

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Figure 2. DFT-computed free energy changes of Ni/PCy3-mediated C−N activation of three typical types of amides. Free energies are in kcal·mol−1.

contribute to the exceptional C−N bond activation reactivities of N-glutarimide amides: the twisting C−N bond and the coordinating N-substituents. The comparisons between the N-glutarimide amide 14 and the Boc-activated amide 16 suggest that the geometric twisting can dramatically lower the intrinsic barrier of C−N cleavage. Both 14 and 16 have a similar free energy of isomerization (1.9 and 3.8 kcal·mol−1, Figure 3) because both the corresponding preoxidative addition intermediates (2 and 18) have similar stabilizing nickel−oxygen (carbonyl) interactions. These carbonyl substituents, glutarimidyl group of 14 and Boc group of 16, essentially act as directing groups to facilitate the generation of the reactive intermediates 2 and 18. While both the amides 14 and 16 contain the directing carbonyl groups, the cleaving C−N bond of N-glutarimide amide 14 is intrinsically weaker than that of N-Boc amide 16. The geometric twisting of 14 removes the nN−π*(acyl) resonance of the amide C−N bond, making it a single C(acyl)−N bond. This mitigates the energy penalty to distort the corresponding C−N bond in TS19, leading to the significantly lower intrinsic C−N cleavage barrier (7.7 kcal·mol−1 of 14 vs 13.1 kcal·mol−1 of 16). The results of distortion/interaction analysis24,25 on TS3 and TS19 are included in the Supporting Information (Figures S4 and S5). In addition, the comparison between 14 and 21 highlights the importance of coordinating leaving group for the formation of preoxidative addition intermediates. The glutarimidyl group of N-glutarimide amides provides the nickel−oxygen interaction that stabilizes the preoxidative addition intermediate 2. In contrast, such directing effect is not present in the N-methylN-phenylbenzamide 21, and the intermediate 23 has the amide C−N bond coordinating to nickel. This weak coordination

Scheme 3. Comparisons of the Amide Reactivities in Ni/ PCy3-Catalyzed Suzuki-Miyaura Coupling3a

Figure 3. Components of the overall Ni/PCy3-mediated C−N bond activation barriers of the three amides.

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Figure 4. DFT-computed structures, free energies, and distortion/interaction analysis of the competing transition states of the Ni/PCy3-mediated bond activations of the N-glutarimide amide 14. Energies are in kcal·mol−1.

Figure 5. DFT-computed free energy changes of the competing pathways for the ketone formation and biaryl formation from resting state 6.

activation transition states are shown in Figure 4. The cleavage of the twisted amide C−N bond via TS3 requires a barrier of only 9.6 kcal·mol−1, while the C−N bond of the glutarimidyl group requires a 17.2 kcal·mol−1 barrier for cleavage via TS26, and the barrier of the C(aryl)−C(carbonyl) bond activation via TS28 is 23.0 kcal·mol−1. Therefore, there is a strong chemoselectivity toward the cleavage of the desired amide

destabilizes 23 and leads to the high isomerization energy from 22 to 23, eventually increasing the overall C−N bond activation barrier. Origins of Chemoselectivity of Bond Activations. We next explored the origins of chemoselectivity of the Ni/PCy3mediated bond activations of N-glutarimide amide 14. The optimized structures and free energies of the competing bond 15526

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Journal of the American Chemical Society C−N bond, which is consistent with the experimental observations that only this amide C−N bond was cleaved.3a In addition, the high cleavage barriers for the C−C and C−N bonds of the glutarimide group (via TS26 and TS28) highlight the inert nature of this substituent, and the stability of Nsubstituents is critical in achieving the high reactivity of the desired C−N bond activation. To understand the origins of this chemoselectivity, the distortion/interaction analysis was applied on the competing transition states (Figure 4). Comparing the two C−N bond activation transition states, TS3 and TS26, the difference of ΔEint is the leading cause for the chemoselectivity. ΔEint(TS3) is −44.6 kcal·mol−1 while the ΔEint(TS26) is −18.6 kcal·mol−1. This is due to the lack of nickel−oxygen interaction in TS26, which again highlights the importance of the directing effect of the glutarimidyl group. Comparing the C−N and C−C bond activation transition states, TS3 and TS28, both the ΔEdist and ΔEint disfavor the C−C bond activation transition-state TS28. The ΔEint favors TS3 because of the favorable nickel−oxygen interaction in this transition state. In addition, the C−N bond of N-glutarimide amide 14 is essentially a single C(acyl)−N bond due to the geometric twisting,26 which is much weaker as compared to the C(aryl)−C(carbonyl) bond. The stretch of the strong C−C bond leads to the large ΔEdist for TS28, contributing to the high barrier of corresponding C−C bond activation. Origins of the Chemoselectivity of Decarbonylation versus Carbonyl Retention. To understand the control of the competition between decarbonylation and carbonyl retention, we studied the free energy changes of the formations of ketone and biaryl products from the resting state 6 (Figure 5). The formation of the biaryl product (blue pathway) has been discussed above, and the reductive elimination via TS13 is the rate-limiting step of this pathway. Alternatively, the PCy3Ni(acyl)(aryl) intermediate 9 can undergo a direct C(aryl)−C(acyl) reductive elimination via TS30 to generate the ketone product 31 (red pathway). The formation of ketone is kinetically favorable because of the faster reductive elimination (TS30 vs TS13), while the ketone product is thermodynamically less stable as compared to the biaryl product (31 vs 15). In addition, it is noteworthy that the ketone product 31 is less stable than the resting state of the catalytic cycle 6. Therefore, lowering the reaction temperature cannot allow the substantial formation of ketone product because of the equilibrium between 31 and 6, which is consistent with the experimental results.3a Only when the reaction condition is sufficient enough to overcome the high kinetic barrier for the biaryl formation (27.2 kcal·mol−1, 6 to TS13), the irreversible C(aryl)−C(aryl) does reductive elimination occur to produce the observed biaryl product. This mechanistic rationale is supported by several related experimental observations.3b,27 Since ketone is the kinetic product, if a Ni-catalyzed cross coupling with N-glutarimide amides can occur under mild conditions, the ketone formation should be observed instead of the biaryl formation. Indeed, Szostak and co-workers showed that the Ni-catalyzed Negishi coupling with the same N-glutarimide amides can occur under mild conditions, and this reaction produces the ketone products3b (Scheme 4a). More importantly, our computations essentially reveal a background reaction in which the nickel catalyst promotes the decarbonylation of biaryl ketones to generate biaryls (from 31 to 15) through an unexpected C(aryl)−C(acyl) bond activation of ketone (via TS30). These

Scheme 4. (a) Experimental Results That Support the Formation of Ketone Product in Ni-Catalyzed Cross Coupling with N-Glutarimide Amides;3b (b) Ni-Mediated Decarbonylation of Aryl Ketone27

predictions are validated by Chatani’s recent breakthroughs on the Ni-catalyzed decarbonylation of biaryl ketones27 (Scheme 4b). A Ni-mediated C(aryl)−C(acyl) bond activation occurred,28 and subsequent decarbonylation and reductive elimination produced the decarbonylated biaryl products. The additional calculations with the experimental NHC ligand, IMesMe, also showed a surmountable barrier for the decarbonylation of phenyl naphthyl ketone (27.3 kcal/mol, Figure S6), which provides direct comparisons to the reported experimental results.27 In addition, Szostak’s experimental studies showed that the Ni/PCy3-catalyzed decarbonylative Suzuki-Miyaura coupling with N-glutarimide amides can tolerate acetyl group.3a These results suggest that the alkyl aryl ketones are significantly less reactive toward the proposed Ni-catalyzed decarbonylation as compared to the biaryl ketones. To further validate the hypothesized in situ decarbonylation, we studied the Ni/PCy3mediated decarbonylation of methyl naphthyl ketone, and compared the results with those of phenyl naphthyl ketone (Scheme 5). The overall barrier with phenyl naphthyl ketone is 5.8 kcal·mol−1 lower than that with methyl naphthyl ketone (26.6 vs 32.4 kcal·mol−1), which is consistent with the fact that acetyl group is tolerated in this reaction.3a The increment of overall decarbonylation barrier with methyl naphthyl ketone is mainly contributed by the substituent effects on the C−C bond activation and reductive elimination. The C−C bond activation is 13.8 kcal·mol−1 endergonic with phenyl naphthyl ketone (31 to 9), while that with methyl naphthyl ketone is endergonic by 17.1 kcal·mol−1 (32 to 34). This is due to the change of dNi-π*acyl interaction in the LNi(acyl)(aryl) intermediates (9 and 34); the benzoyl group of 9 is a stronger π acceptor than the acetyl group of 34, which stabilizes the intermediate 9 through the favorable dNi-π*acyl interaction.23c In addition, the reaction with methyl naphthyl ketone requires the C(sp3)−C(sp2) reductive elimination via TS37, which is intrinsically more difficult compared with the C(sp2)−C(sp2) reductive elimination via TS13.29 Therefore, the decarbonylation reactivity of alkyl aryl ketones is significantly lower than that of biaryl ketones.



CONCLUSIONS In summary, the reactivities and selectivities of N-glutarimide amides in Ni-catalyzed Suzuki-Miyaura coupling have been elucidated with DFT calculations. Two critical factors lead to the unusually high reactivity of N-glutarimide amides in Ni15527

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transformation was confirmed by Chatani’s recent experimental studies.27

Scheme 5. DFT-Computed Free Energy Changes of the Ni/ PCy3-Mediated Decarbonylation of Phenyl Naphthyl Ketone (a) and Methyl Naphthyl Ketone (b)



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b09482. Additional computational results; coordinates and energies of DFT-computed stationary points (PDF)



AUTHOR INFORMATION

Corresponding Author

*[email protected] (X.H.) ORCID

Xin Hong: 0000-0003-4717-2814 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS Financial support from Zhejiang University, the Chinese “Thousand Youth Talents Plan”, and the “Fundamental Research Funds for the Central Universities” is gratefully acknowledged. Calculations were performed on the National Supercomputing Center in Shenzhen and the high-performance computing system at the Department of Chemistry, Zhejiang University.



REFERENCES

(1) For reviews, see: (a) Meng, G.; Szostak, M. Org. Biomol. Chem. 2016, 14, 5690. (b) Meng, G.; Shi, S.; Szostak, M. Synlett 2016, 27, 2530. (c) Liu, C.; Szostak, M. Chem. - Eur. J. 2017, 23, 7157. (d) Dander, J. E.; Garg, N. K. ACS Catal. 2017, 7, 1413. (e) Gao, Y.; Ji, C.-L.; Hong, X. Sci. China: Chem. 2017, DOI: 10.1007/s11426-0179025-1. (2) (a) Hie, L.; Nathel, N. F. F.; Shah, T. K.; Baker, E. L.; Hong, X.; Yang, Y.-F.; Liu, P.; Houk, K. N.; Garg, N. K. Nature 2015, 524, 79. (b) Weires, N. A.; Baker, E. L.; Garg, N. K. Nat. Chem. 2016, 8, 75. (c) Baker, E. L.; Yamano, M. M.; Zhou, Y.; Anthony, S. M.; Garg, N. K. Nat. Commun. 2016, 7, 11554. (d) Hie, L.; Baker, E. L.; Anthony, S. M.; Desrosiers, J.-N.; Senanayake, C.; Garg, N. K. Angew. Chem., Int. Ed. 2016, 55, 15129. (e) Simmons, B. J.; Weires, N. A.; Dander, J. E.; Garg, N. K. ACS Catal. 2016, 6, 3176. (f) Simmons, B. J.; Hoffmann, M.; Hwang, J.; Jackl, M. K.; Garg, N. K. Org. Lett. 2017, 19, 1910. (g) Dander, J. E.; Baker, E. L.; Garg, N. K. Chem. Sci. 2017, 8, 6433. (3) For nickel-catalyzed transformations involving N-glutarimide amides, see: (a) Shi, S.; Meng, G.; Szostak, M. Angew. Chem., Int. Ed. 2016, 55, 6959. (b) Shi, S.; Szostak, M. Chem. - Eur. J. 2016, 22, 10420. (c) Shi, S.; Szostak, M. Org. Lett. 2016, 18, 5872. (d) Shi, S.; Szostak, M. Synthesis 2017, 49, 3602. (4) For other related transformations involving N-glutarimide amides, see: (a) Meng, G.; Szostak, M. Angew. Chem., Int. Ed. 2015, 54, 14518. (b) Meng, G.; Szostak, M. Org. Lett. 2015, 17, 4364. (c) Hu, F.; Lalancette, R.; Szostak, M. Angew. Chem., Int. Ed. 2016, 55, 5062. (d) Liu, C.; Meng, G.; Szostak, M. J. Org. Chem. 2016, 81, 12023. (e) Meng, G.; Szostak, M. Org. Lett. 2016, 18, 796. (f) Liu, C.; Achtenhagen, M.; Szostak, M. Org. Lett. 2016, 18, 2375. (g) Liu, C.; Meng, G.; Liu, Y.; Liu, R.; Lalancette, R.; Szostak, R.; Szostak, M. Org. Lett. 2016, 18, 4194. (h) Meng, G.; Shi, S.; Szostak, M. ACS Catal. 2016, 6, 7335. (i) Liu, C.; Liu, Y.; Liu, R.; Lalancette, R.; Szostak, R.; Szostak, M. Org. Lett. 2017, 19, 1434. (j) Liu, Y.; Shi, S.; Achtenhagen, M.; Liu, R.; Szostak, M. Org. Lett. 2017, 19, 1614. (k) Meng, G.; Lei, P.; Szostak, M. Org. Lett. 2017, 19, 2158. (l) Lei, P.; Meng, G.; Shi, S.; Ling, Y.; An, J.; Szostak, R.; Szostak, M. Chem. Sci. 2017, 8, 6525. (m) Liu, C.; Szostak, M. Angew. Chem., Int. Ed. 2017, 56, 12718.

mediated C−N bond activation, the coordinating N-substituents, and the geometric twisting. The coordinating glutarimidyl substituent acts as a directing group, which facilitates the formation of the reactive intermediate for the C−N bond cleavage. This nickel−oxygen(carbonyl) interaction also allows the formation of the tetra-coordinated nickel intermediate after the oxidative addition, resulting in the exergonic C−N bond cleavage. In addition, the geometric twisting removes the planar acyl−nitrogen conjugation of amide, and the twisting C−N bond of N-glutarimide amides is essentially a single N−C(acyl) bond.26 This lowers the energy required for the C−N bond stretch during the oxidative addition; thus, the intrinsic barrier of C−N bond cleavage with N-glutarimide amides is low. For the chemoselectivity of decarbonylation, a thermodynamic versus kinetic control of product formation was discovered. The biaryl ketones have a lower barrier of formation than the decarbonylated biaryl products, while the formation of biaryl ketones is thermodynamically less favorable. The calculations suggest that the nickel catalyst is able to promote the decarbonylation of biaryl ketones through a unexpected C−C bond activation pathway. This proposed 15528

DOI: 10.1021/jacs.7b09482 J. Am. Chem. Soc. 2017, 139, 15522−15529

Article

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DOI: 10.1021/jacs.7b09482 J. Am. Chem. Soc. 2017, 139, 15522−15529