Mechanistic Study of Copper-Catalyzed Decarboxylative CâN Cross

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Mechanistic Study of Copper-Catalyzed Decarboxylative C−N CrossCoupling with Hypervalent Iodine Oxidant Yi-Nuo Yang, Ju-Long Jiang,* and Jing Shi* Hefei National Laboratory for Physical Sciences at the Microscale, iChEM, CAS Key Laboratory of Urban Pollutant Conversion, Anhui Province Key Laboratory of Biomass Clean Energy, Department of Chemistry, University of Science and Technology of China, Hefei 230026, People’s Republic of China S Supporting Information *

ABSTRACT: Copper-catalyzed directed decarboxylative C−N cross-coupling, which is promoted by a hypervalent iodine oxidant, provides a new strategy for the site-selective formation of aliphatic C−N bonds. Despite the great synthetic potential, the mechanism of this reaction and especially the origin of the radical species still remain controversial. To resolve this problem, herein density functional theory (DFT) calculations have been employed to elucidate the mechanistic details of this reaction. As a result, a comprehensive reaction pathway involving IIII−O bond heterolysis, single electron transfer (SET), hydrogen atom transfer (HAT), decarboxylation, proton transfer, and reductive elimination is reported. Meanwhile, analyzing the necessity of the directing groups in realizing the site selectivity, we found that the chelation of the directing group to the Cu(III) center can remarkably facilitate the proton transfer process.

1. INTRODUCTION Transition-metal-catalyzed decarboxylative cross-coupling, as an eco-friendly and user-friendly strategy, has been a powerful tool to construct chemical bonds in the past few decades.1 As an important resource for decarboxylation, nonactivated aliphatic carboxylic acids are abundant in natural products2 and therefore have received broad attention in decarboxylative cross-coupling reactions recently.3−6 For instance, MacMillan and co-workers systematically constructed a series of aliphatic Csp3−C bonds by visible-light-induced decarboxylation.4 In a nonphotocatalytic system, a silver-catalyzed decarboxylative strategy has been reported by the Li group to construct a variety of aliphatic Csp3−X bonds under oxidative conditions (X = Cl, F, N3).6 Generally, the decarboxylation of aliphatic carboxylic acids proceeds via a radical mechanism.1a,c However, the high reactivity of the generated alkyl radical results in a low site selectivity when two or more coupling sites are available. To achieve controllable site selectivity, our group introduced picolinamide (PA) as the directing group and realized the first directed decarboxylative C−N bond formation very recently7 (Scheme 1). To delve into the possible reaction mechanism, experimental efforts have been made in our group’s previous work.7 Experiments using TEMPO as a radical scavenger have confirmed that the reaction undergoes a radical mechanism, since the alkyl radical generated via decarboxylation has been captured and characterized. Furthermore, no decarboxylative species was obtained in the control experiments when Cu catalyst or PhIO was absent, indicating that the decarboxylation process should be promoted by both the catalyst and the © XXXX American Chemical Society

Scheme 1. Cu-Catalyzed Decarboxylative C−N CrossCoupling Reaction

oxidant. Nonetheless, it is still unclear how the decarboxylation occurs and why the catalyst and oxidant are necessary in this step. To clarify these problems, a comprehensive theoretical study has been carried out in this work. According to preceding works, it was proposed that the aliphatic carboxylic acids can be directly oxidized into the alkyl radicals by a transition metal via an SET process under heating conditions (Scheme 2a).3a,b,5a−c,6 However, this possibility has been ruled out by our aforementioned control experiments. On this basis, we proposed that the carboxylic acid can initially combine with PhIO to generate a carboxyl iodine(III) intermediate such as A, and A can subsequently react with the Cu catalyst to realize the Received: February 6, 2017

A

DOI: 10.1021/acs.organomet.7b00095 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

the thermodynamic corrections of enthalpy and Gibbs free energy. For every transition state, IRC analysis was conducted to verify the connection between the right reactant and product.14,15 On the basis of the optimized structures, single-point energy calculations were performed with a mixed basis set of SDD for I and Cu and 6311+G(d,p) for other atoms. For the SET steps, Marcus theory was used to evaluate their activation barrier.16 To avoid confusion, in the following figures and schemes, when a bracket is used, such a species was calculated as a complex. When a plus or a curved arrow is used, species were calculated as separated species. Due to the complexity of this system, the number of reactants often changes in some steps during the catalytic cycle. To avoid the negative impact from the overestimation of entropic effects in these steps and ensure the reliability of our calculation,17 Gibbs free energy, corrected Gibbs free energy (by Truhlar’s quasi-harmonic correction18), and enthalpy values have all been calculated here. Herein, enthalpy values are used for discussions,19,20 while Gibbs free energy and corrected Gibbs free energy are shown in the Supporting Information as reference. 2.2. Model Reaction. In accordance with our experiments, the model reaction used in calculations involves Int1 as the substrate, Cu(OTf)2 as the catalyst, DMAP as the ligand, PhIO as the oxidant, and DCM as the solvent. Considering that PhIO exists as a zigzag polymer and has poor solubility in nonpolar solvents,21 we used dimeric (PhIO)2 as the oxidant to simulate the polymeric (PhIO)n.

Scheme 2. Different Mechanistic Proposals: (a) TransitionMetal-Oxidized Decarboxylation; (b, c) Oxidant PhIO and Cu Catalyst Collaborative Decarboxylation

decarboxylation and the following C−N coupling (Scheme 2b). However, a controversy about the mechanism of the carboxyl iodine(III) intermediates still remains, as the RCOO−I bond can be cleaved either homolytically8 (Scheme 2b, path 1) or heterolytically9 (Scheme 2b, paths 2 and 3). More specifically, the Wang group suggested a mechanism involving an aforementioned heterolytic cleavage, which is followed by an SET process between I(III) cation and rhenium catalyst.9a Thereafter, the alkyl radical produced from path 1 to 3 can be trapped by the Cu catalyst and the generated adduct then undergoes a series of transformations, including chelation with the PA directing group, hydrogen capture, and reductive elimination to generate the final products (Scheme 2b). In addition, I−O bond homolysis can also be induced by Cu(I) in some trifluoromethylative reactions.10 Therefore, path 4 was also considered to evaluate this possibility (Scheme 2c). On the basis of the discussions referenced above, several questions from the theoretical perspective can be raised here. (i) Which cleavage pattern of the I−O bond does the carboxyl iodine(III) complex prefer, homolysis or heterolysis? (ii) What role does the Cu catalyst play in this reaction? (iii) How could the PA directing group control the site selectivity? Understanding the mechanistic details in depth, which can be achieved by an extensive theoretical investigation, is essential to clarifying the questions mentioned above. Furthermore, we hope that this study would benefit researchers working on the reactions involving hypervalent iodine reagents or the decarboxylative cross-coupling reactions.

3. RESULTS AND DISCUSSION In this study, we first examined the coordination pattern of Cu catalyst (section 3.1), followed by an investigation on the aforementioned possible pathways (section 3.2). After that, careful comparisons of these pathways were performed to generate the most favorable one (section 3.3). 3.1. Equilibrium of Cu(I) and Cu(II) Complexes. A Cu(I)/Cu(III) catalytic cycle is proposed for the decarboxylative C−N cross-coupling in Scheme 2.22 The active Cu(I) can be initially produced either through the disproportionation of Cu(II) or via the reduction of Cu(II) by a nucleophile.23,24 In consideration of the multichelating sites in the carboxylic acid substrate Int1, we investigated the coordination patterns between Int1 and free Cu(I)/Cu(II) species25 in solution (Scheme 3). Nonetheless, the results clearly showed that the coordination of 1 to both Cu(I) and Cu(II) is unfavorable. In comparison to the substrate Int1, DMAP is a better ligand due to its stronger electron-donating ability. Accordingly, the catalytic cycle involving Cu starts with a DMAP-coordinated Cu species. 3.2. Detailed Mechanisms. 3.2.1. Direct I−O Bond Homolysis Mechanism (Path 1). We first examined the feasibility of homolytic cleavage of the I−O bond. As shown in Figure 1, the substrate Int1 can react with the dimer (PhIO)2 to deliver Int2 through the proton-transfer transition state TS12,26 and the latter undergoes an isomerization to form the carboxyl iodine(III) intermediate Int3. It is a continuously exothermic process with an enthalpy decrease of 24.5 kcal/mol from Int1 to Int3. Afterward, the homolytic cleavage of the I− O bond requires the spin state of intermediate Int3 changing from a singlet to a triplet, which could be achieved by the minimum energy crossing point (MECP) on the potential energy surface (PES). Since the enthalpy change from Int3 to

2. COMPUTATIONAL DETAILS 2.1. Computational Method. All calculations were conducted with the Gaussian 09 package11 and carried out by the B3LYP density functional12 with SMD solvation model (solvent = dichloromethane).13 A mixed basis set of LANL2DZ for I and Cu and 631+G(d) for other atoms was used in unrestricted geometry optimization. The frequency analysis was calculated at the same level of theory to confirm the nature of stationary points and obtain B


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Organometallics Scheme 3. Relative Energies of Different Cu(I)/Cu(II) Complexes (kcal/mol)

Scheme 4. Enthalpy Change for the Heterolysis in Path 2 (kcal/mol)

intermediate Int3 mentioned above. Notably, as shown in Figure 2, a deprotonation process (Int19 to Int9) is involved in the heterolytic mechanism, which results in the formation of a conjugated acid of DMAP (+HDMAP). Moreover, +HDMAP is also formed in some acid−base equilibria (see the Supporting Information for more details). Therefore, we considered this conjugated acid in the following pathways. Illustrated as path 2 in Scheme 4, the direct heterolysis of the I−OX bond is strongly endothermic by 58.1 kcal/mol (Int3 → Int11 + XO−). Alternatively, an acid (+HDMAP)-promoted heterolysis of the O−I bond has been located with a significantly lowered enthalpy change (ΔHInt10→Int11 = +32.5 kcal/mol), which is still high and could hardly happen under such reaction conditions. Accordingly, this pathway can be excluded (the complete transformations ofpath 2 are shown in the Supporting Information). Then, we turn to the RCOO−I bond heterolysis mechanism (path 3 in Figure 2). The enthalpy change of direct RCOO−I bond heterolysis is +25.1 kcal/mol (Int3 → Int14 + 1-H), which is much lower than that of the I−OX bond in path 2, indicating that the RCOO−I bond is much more easily cleaved than the I−OX bond. This result is in accordance with the optimized structure of intermediate Int3 that the RCOO−I bond is longer than the I−OX bond by 0.24 Å (Figure 3). Similarly to path 2, the possibility of +HDMAP-assisted RCOO−I bond heterolysis was also considered, and a more favorable enthalpy change of +10.3 kcal/mol (Int13 → Int14 + Int1-L) was obtained. To calculate the enthalpy barrier of this cleavage, a relaxed scan of I−O distance in Int13 was conducted. The scan results showed that the electronic energy increases monotonously as the I−O distance increases (see the scan curve in the Supporting Information), demonstrating that the enthalpy change can reliably represent the enthalpy barrier from Int13 to Int14. After the heterolysis of the RCOO−I bond, the formed Int14 can be reduced by Cu(I) species via a single electron transfer (SET) process. In comparison to CuIL2, the SET between CuIL3 and Int14 is much easier and thus generates the neutral intermediate Int15 (ΔH⧧Int14→Int15 = +0.5 kcal/mol, ΔHInt14→Int15 = −20.4 kcal/mol). Once it receives an electron from Cu(I), the I−O distance in Int15 is elongated to 3.95 Å (Figure 3), suggesting a simultaneous cleavage of the I− O bond. Meanwhile, an oxygen radical species is formed and the spin density on oxygen is calculated to be 0.86.

Figure 1. Enthalpy profile for path 1 (kcal/mol).

the homolytic cleavage product Int4 is already 29.2 kcal/mol, an even higher enthalpy associated with the MECP can be therefore expected. On the other hand, as long as the homolytic cleavage of the I−O bond occurs, the generated carboxyl radical Int4 would be immediately decarboxylated to produce the alkyl radical Int5.26 This conclusion is inconsistent with our control experiment that decarboxylative product cannot be obtained without Cu catalyst. According to the control experiment and calculation results, we conclude that the direct homolytic cleavage of I−O bond is difficult to occur (the following transformations of path 1 are shown in the Supporting Information). 3.2.2. I−O Bond Heterolysis Mechanism (Paths 2 and 3). In this section, the possibility of the I−O bond heterolysis is evaluated and discussed. According to the mechanism reported by the Wang group,9a we investigated the heterolytic cleavage for both the I−OX bond (path 2 in Scheme 4) and the RCOO−I bond (path 3 in Figure 2) in the carboxyl iodine(III) C


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Figure 3. Optimized structures of selected intermediates and transition states.

captured by the Cu(II) species to form the Cu(III) intermediate Int17, followed by a chelation of the PA directing group to generate Int18. Due to the dissociation of a strongly bound DMAP ligand in the above process, the enthalpy of Int18 is obviously higher than that of Int17 by 18.1 kcal/mol. Starting from Int18, the chelation of the amine to the Cu center leads to the release of a DMAP ligand, and the latter moves to form a hydrogen bond with the nearby N−H, as can

It is noteworthy that the high reactivity of the oxygen radical in Int15 subsequently leads to a hydrogen atom transfer (HAT) process with carboxylic acid Int1-L to deliver the carboxyl radical Int4. Immediately, a decarboxylation takes place in Int4 and accordingly yields the alkyl radical Int5. The enthalpy change associated with the transformation from Int15 to Int5 is −5.4 kcal/mol, and the driving force is believed to be the release of CO2. Later on, the alkyl radical Int5 can be D


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Organometallics


intramolecular PA group to form Int23. Subsequently, the generated Int23 can easily go through an HAT process via TS24−25-t with the oxygen radical species formed in Int20-t and produce the triplet intermediate Int25-t (Figure 3). Thereafter, a Cu-assisted decarboxylation happens in Int25-t with an activation barrier of 9.5 kcal/mol, therefore generating the alkyl radical Int26-t. Upon the formation of Int26-t, it can be captured by the nearby Cu(II) to deliver the Cu(III) complex Int9 with a highly exothermic process of 20.0 kcal/ mol. Finally, the final product TM is formed via the aforementioned reductive elimination process. Throughout this pathway, the relative enthalpies of the intermediates and transition states are systematically much higher than those of path 3 (ΔΔH = +13.0 kcal/mol), and therefore the possibility of path 4 could be excluded. 3.3. Discussions about the Favorable Path 3 Mechanism. As can be seen in section 3.2, four typical pathways have been examined in this work. For path 1, the difficulty in direct I−O homolysis of Int3 gives rise to a high enthalpy change up to +29.2 kcal/mol. In combination with the control experiment, path 1 is believed to be unfavorable under the reaction conditions. Meanwhile, the I−OX heterolysis mechanism (path 2) is also excluded due to the high enthalpy demands of +32.5 kcal/mol in the heterolysis step. For the Cu-mediated I−O homolytic mechanism (path 4), the relative enthalpies are systematically higher than that of path 3. In addition to paths 1−4 mentioned above, an alternative pathway involving decarboxylation of carboxylate (RCOO−) rather than the carboxylic acid (RCOOH) was also considered. However, the much higher enthalpy barrier (over 40 kcal/mol) indicates that this pathway is unfavorable (see the Supporting Information for more details). Therefore, this reaction occurs favorably via the RCOO−I bond heterolysis mechanism (path 3). On the basis of the above results and discussions, we propose an I−O bond heterolysis mechanism as shown in Scheme 5. This pathway starts with the formation of the adduct Int3,

be seen in Int19. This ligand exchange process from Int18 to Int19 has been confirmed by a relaxed scan of Cu−N(DMAP) distance with an electronic energy barrier of +6.3 kcal/mol (see the scan curve in the Supporting Information). Thereafter, considering the fact that DMAP can also play a role as a the base, a proton transfer process can easily take place with Int19 via IntTS19-926 (Figure 3). Therefore, the more stable planar tridentate Cu(III) intermediate Int9 is produced, accompanied by a highly exothermic process of −20.6 kcal/mol (Int19 to Int9). For this deprotonation process (Int18 → Int9), another traditional concerted metalation−deprotonation (CMD) transition state was also considered (TS-CMD in Figure 2) and has been shown to be unfavorable (see the Supporting Information for more details). Finally, a reductive elimination can happen in Int9 with a relatively small barrier of only +10.9 kcal/mol (via TS9-TM) and thus generate the final C−N coupling product TM. This step is remarkably exothermic by 36.1 kcal/mol (Int9 to TM) and provides the driving force of the entire pathway. Overall, the enthalpy barrier of path 3 is +25.9 kcal/mol (Int5 to 18-scan), which is reasonable under the reaction conditions. 3.2.3. Cu-Mediated I−O Bond Homolysis Mechanism (Path 4). In this section, our focus has been diverted to a more specific pathway in which a Cu-mediated homolytic cleavage of the I−O bond is involved.10 As shown in Figure 4, the carbonyl oxygen in Int3 can coordinate to Cu(I) species and thus form Int20. Then, the Cu(I) in Int20 can reduce the hypervalent iodine indirectly to generate the triplet intermediate Int20-t (through MECP), where the I−O bond has already been cleaved, and the carboxyl Cu(II) complex Int21 is formed. In this Cu(I)-mediated I−O homolysis process, an enthalpy barrier of over +17.4 kcal/mol is required (Int3 → Int20-t), which is much higher than the heterolysis in path 3 (ΔH⧧Int13→Int14 = +10.3 kcal/mol. Afterward, the vacant site of Cu(II) in Int21 can either be fulfilled with a free DMAP in solution to produce the stable intermediate Int22 (ΔHInt21→Int22 = −23.2 kcal/mol) or be ligated by the E


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4. CONCLUSION In this work, detailed theoretical investigations have been carried out to elucidate the mechanism of this copper-catalyzed decarboxylative C−N coupling reaction. The favorable pathway consists of protic acid assisted I−O bond heterolysis, SET, HAT, decarboxylation, proton transfer, and reductive elimination processes. Throughout the results and discussions, several conclusions have been made here. (a) In this work, the heterolytic cleavage of the RCOO−I bond is much easier with the help of +HDMAP, which is formed in the proton-transfer step, whereas the homolytic pathways need to overcome higher enthalpy barriers and thus show less feasibility. (b) The Cu catalyst plays a complicated role in this reaction, including participation in the SET step (Cu(I) → Cu(II)), scavenger of the alkyl radical (Cu(II) → Cu(III)), and the realization of reductive elimination (Cu(III) → Cu(I)). (c) The PA directing group significantly stabilizes the proton transfer step, which is the rate-determining step, and that is the origin of the site selectivity. We hope that the present results can provide some mechanistic information for this specific field in organic chemistry.

Scheme 5. Overall Mechanistic Proposal for the Favorable Pathway (Path 3)

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ASSOCIATED CONTENT

S Supporting Information *

which can undergo an +HDMAP-assisted RCOO−I bond heterolysis to produce the cationic iodine(III) intermediate Int14. Then, an SET process occurs between Int14 and Cu(I) species to form the oxygen radical Int15, followed by an HAT process to generate the carboxyl radical Int4. Thereafter, Int4 can be easily decarboxylated to produce the alkyl radical Int5, which will be subsequently trapped by Cu(II) species and then undergo a proton transfer process. Finally, the generated Cu(III) intermediate Int9 goes through a reductive elimination to offer the target product TM. Furthermore, to clarify how the PA directing group controls the site selectivity, we investigated two specific steps (proton transfer and reductive elimination) with the PA group replaced by a nondirected Bz group (Scheme 6). In comparison to the

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00095. Additional discussions and energies of optimized structures (PDF) Cartesian coordinates of the calculated structures (XYZ)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for J.-L.J.: [email protected]. *E-mail for J.S.: [email protected]. ORCID

Jing Shi: 0000-0003-0180-4265 Notes

Scheme 6. Nondirected Bz Group Involved Proton Transfer and Reductive Elimination

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the NSFC (21572214, 21572212, 21325208, U1530262) and CAS (YZ201563). The numerical calculations in this paper have been done on the supercomputing system in the Supercomputing Center of University of Science and Technology of China.

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corresponding results in Scheme 6, Bz group involved proton transfer demands an enthalpy barrier of +28.4 kcal/mol, which is 2.5 kcal/mol higher than that of the PA involved process. However, the Bz group involved reductive elimination happens more easily with a small enthalpy barrier of only +1.1 kcal/mol. With the PA group, a planar tridentate ligand is formed and can remarkably help to stabilize the Cu(III) center. Therefore, the enthalpy barrier of the proton transfer step is lowered due to PA’s stabilization, whereas the later reductive elimination becomes more difficult, since it has to destroy this planar tridentate geometry (illustrated as TS9-TM in Figure 3; the planar tridentate ligand is deformed). Accordingly, the stabilization of the PA directing group in the proton transfer step can account for the site selectivity.

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