Theoretical Studies on Intramolecular CâH Amination of Biaryl Azides

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Theoretical Studies on Intramolecular C−H Amination of Biaryl Azides Catalyzed by Four Different Late Transition Metals Qi Zhang, Caihong Wu, Lixin Zhou, and Juan Li* Department of Chemistry, Jinan University, Guangzhou, Guangdong 510632, People’s Republic of China S Supporting Information *

ABSTRACT: A systematic DFT study was performed to examine how catalysts with different metals influence the C−H amination mechanism of biaryl azides. The mechanisms of (cod)Ir(OMe), RuCl3(DME) (DME = CH3OCH2CH2OCH3, a solvent molecule), Rh2(O2CCF3)4, and ZnI2 were investigated in our study. The calculations indicated that the C−H amination reactions mainly proceeded through a stepwise mechanism regardless of the metal center (Ru, Ir, Rh, or Zn). The energetic span (δE) model proposed by Shaik et al. has been applied to reveal the kinetic behavior of the four catalytic cycles. The results indicate that the ruthenium species exhibits a higher catalytic performance than the other three. The investigation of magnetic properties suggests that no matter what the metal center wasIr, Ru, Rh, or Znthe C−N bond formation step is pseudoelectrocyclization with an orbital disconnection on the nitrogen atom (N1) in the pseudopericyclic transition structure.

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INTRODUCTION In recent years, in comparison to the traditional cross-couplingbased methods where extra steps are needed for preactivation of the aromatic rings, C−H bond functionalization has received increased attention as an alternative strategy for indole synthesis due to the fact that it offers improved atom- and step-economy.1 Metal-catalyzed C−H amination via nitrene insertion represents a general strategy for direct functionalization of C−H bonds with the potential to control selectivity.2 In recent years, there has been growing interest in the use of organic azides as the nitrogen source in the amination of C−H bonds because the nitrogen gas byproduct is nonhazardous to the environment.3 Metal catalysts such as Ru,4,5 Rh,6 Co,7 Ir,8 Zn,6e and Fe9 have been reported to display potent activity toward the formation of new carbon−nitrogen bonds from vinyl or aryl C−H bonds. While there are thorough studies of the mechanism of thermal or photolytic nitrene formation from o-biaryl azides,10−12 mechanistic studies of metal nitrenes generated from aryl azides are less common. The theoretical studies on the mechanism detailed for the Ru-catalyzed C−H amination reaction was explored by the Jia group, which indicated the reaction involves three steps: nitrogen liberation, formation of the C−N bond, and rearrangement through a 1,2hydrogen shift.4 Driver and co-workers provided experimentalbased mechanistic information for C−H bond azide amination catalyzed by dirhodium complexes,6a which is nearly duplicated with theoretical reaction mechanisms for ruthenium complexes reported by the Jia group.4 However, several important questions remain to be answered about the mechanism for Ru- and Rh-catalyzed C−H amination © 2013 American Chemical Society

reactions. Driver and co-workers demonstrated that the two biaryl azide phenyl rings are actively involved in the nitrogen liberation step, while the Jia group proposed the nitrogen liberation step occurs without largely disturbing the conjugation of the two phenyl rings. As mentioned by the Jia group,4 the mechanism of the C−N bond formation step was determined by the two resonance structures of the metal−nitrene intermediate (Scheme 1). The electrocyclization was proposed Scheme 1

as the dominant mechanism for the Ru-catalyzed systems. Driver also suggested that C−N formation proceeds through an electrocyclization mechanism for the Rh-catalyzed systems. Furthermore, Driver proposed that the π orbital of the CN moiety acts as a nucleophile during the electrocyclization mechanism. In fact, the nitrogen p lone pair orbital is also likely to display nucleophilic behavior. In other words, the C−N formation step may follow a pseudoelectrocyclization mechanism. In recent publications, different types of pericyclic Received: August 3, 2012 Published: January 11, 2013 415

dx.doi.org/10.1021/om300746z | Organometallics 2013, 32, 415−426

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Scheme 2

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reactions have been studied to investigate possible pseudopericyclic versions: electrocyclic reactions, cycloadditions, sigmatropic rearrangements, cheletropic reactions, and group transfers/eliminations.13,14 Pseudopericyclic reactions were originally defined by Lemal et al. as a concerted electronic rearrangement through a cyclic array of atoms, where the participation of nonbonding orbitals, orthogonal to the bonding ones, yields one or more disconnections in the cyclic array of the overlapping orbitals.13a Except for a localized orbital method14a,i originating from the definition of pseudopericyclic reactions, the identification of the pericyclic/pseudopericyclic nature of reaction mechanisms has relied on computation of the magnetic properties14b,c,e,15 or other diagnostics such as the electron fluctuation,14f the electron localization function (ELF),14g,16 or the ellipticity of the electron density.14h Additionally, the mechanisms for other metal-catalyzed C−H bond aminations have not yet been studied. To examine how different metal catalysts influence the reaction mechanism and continue efforts aimed at understanding how metal complexes catalyze C−H bond amination, we extend our research to reactions with other late transition metal complexes using a theoretical approach. We have chosen two metal complexes as the objects of our study, ZnI2 and [(cod)Ir(OMe)]2 (where cod = η4-cyclooctadiene), both of which have been reported to catalyze the amination of the C−H bond in the literature.6,8 In this paper, we set out to investigate the details of the four metal-catalyzed C−H bond amination reaction mechanisms with the aid of density functional theory (DFT) calculations. The pericyclic or pseudopericyclic cyclization nature of the C− N formation step can be determined on the basis of magnetic properties and molecular orbit analysis.14a−c,e,i,15 Through our studies, we hope to answer the following questions: For each of the four metal complexes, does the catalyzed C−H bond amination reaction operate through a stepwise or concerted mechanism? If the stepwise mechanism is operative, which of the steps is rate-determining? Which reaction mechanism is responsible for the C−N formation step for the four systems (Ir, Ru, Rh, or Zn), electrocyclization or pseudoelectrocyclization? Our hope is that a systematic study of the factors influencing the mechanism of metal-catalyzed C−H bond amination reactions will lead to a fundamental understanding of these factors that will assist experimental efforts in finding better catalysts.

COMPUTATIONAL DETAILS

Molecular geometries of the complexes were optimized at the Becke3LYP level of density functional theory.17 Frequency calculations at the same level of theory were also performed to identify all the stationary points as minima (zero imaginary frequencies) or transition states (one imaginary frequency) and to provide free energies at 298.15 K. Intrinsic reaction coordinate (IRC)18 analysis was executed to confirm that all stationary points were smoothly connected to each other. Zn, I, Cl, Rh, Ir, and Ru atoms were described using the LANL2DZ basis set including a double-valence basis set with the Hay and Wadt effective core potential (ECP).19 Polarization functions were added for the transition metals Zn (ζf = 3.031), Rh (ζf = 1.350), Ir (ζf = 0.938), and Ru (ζf = 1.235)20 and for atoms directly bonded to the metal center, Cl (ζd = 0.514) and I (ζd = 0.289).21 Polarization functions were also added for C (ζd = 0.600) and for the H atom (ζp = 1.100) involved in the C−H bond-breaking process.21 The 6-31G22 basis set was used for other atoms. The natural bond orbital (NBO) program23 was also used to obtain the natural population of atoms. The magnetic properties nucleus-independent chemical shift (NICS), magnetic susceptibility (χ), and magnetic susceptibility anisotropy (χanis) were calculated at different points along the IRC. In the magnetic susceptibility and NICS calculations, the NMR shielding tensors were computed with a larger basis set (6311+G(2d,2p)). Nucleus-independent chemical shift was calculated using the GIAO (gauge-independent atomic orbital) method,24 but this method does not provide information about magnetic susceptibility, so χ and χanis were calculated using the IGAIM (individual gauges for atoms in molecules) method.25 All calculations were performed with the Gaussian 03 packages.26

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RESULTS AND DISCUSSION Before going into the details of the mechanistic aspects, we first describe the model used in our calculations. We consider the mechanism of metal nitrenes generated from biaryl azides catalyzed by four metal complexes. To illustrate the detailed mechanism of C−N bond formation, the RuCl3-catalyzed system was also optimized as described in the Computational Details; that is, we add a d polarization shell to the three azide N atoms with an exponent of 0.864, following the computational methods reported by the Jia group.4 We assume that the active species in RuCl 3 -catalyzed reactions in DME (CH3OCH2CH2OCH3) is related to the metal fragment RuCl3(DME), as proposed by Jia et al.4 Rhodium(II) carboxylates are verified as efficient catalysts to participate in amination of C−H bonds compared to other congeneric catalysts.6c,d In order to facilitate the calculation, we selected Rh2(O2CCF3)4 as a model catalyst in the calculations for the 416


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Figure 1. Energy profiles calculated for the C−H amination reaction catalyzed by (cod)Ir(OMe) (a) and RuCl3 (b) in the gas phase. The calculated relative free energies and electronic energies (in parentheses) are given in kcal/mol. Also shown are the turnover frequency (TOF) determining intermediate (TDI) and TOF determining transition state (TDTS) according to the energetic span model for the stepwise mechanism.

rhodium(II) carboxylate. For the catalytically competent species for [(cod)Ir(OMe)]2, it should be reasonable for us to consider the (cod)Ir(OMe) fragment as the active species that coordinates to the azide substrate initiating the reaction,

which is consistent with the results reported by Stradiotto et al.27 The intramolecular addition of alkyl- or arylamine-tethered unactivated olefins catalyzed by [Ir(cod)Cl]2 reported by Stradiotto et al. demonstrated that the activation energy for 417


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Figure 2. Energy profiles calculated for the C−H amination reaction catalyzed by Rh2(O2CCF3)4 (a) and ZnI2 (b). The calculated relative free energies and electronic energies (in parentheses) are given in kcal/mol. Also shown are the turnover frequency (TOF) determining intermediate (TDI) and TOF determining transition state (TDTS) according to the energetic span model for the stepwise mechanism.

Table 1. Selected Bond Distances (Å) Calculated for 1Ir, 1Ru, 1Rh, and 1Zna

a

The atoms C1, C2, C3, N1, and N2 are labeled.

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Table 2. Selected Bond Distances (Å) Calculated for TS(1‑2)Ir, TS(1‑2)Ru, TS(1‑2)Rh, and TS(1‑2)Zna

a


Table 3. Selected Bond Distances (Å) Calculated for 2Ir, 2Ru, 2Rh, and 2Zna

a


Table 4. Selected Bond Distances (Å) Calculated for TS(2‑3)Ir, TS(2‑3)Ru, TS(2‑3)Rh, and TS(2‑3)Zna

a


barrier when examining a linear-transit approach. In summary, (cod)Ir(OMe), RuCl3(DME), Rh2(O2CCF3)4, and ZnI2 were used as active catalysts in the calculations. Stepwise versus Concerted Mechanism. As an illustration, Scheme 2 shows that the azide complex 1 is transformed into nitrene complex 2 via nitrogen liberation; then 2 can be rearranged to give complex 4 through two possible pathways: a one-step concerted insertion of nitrene via transition state TS24 or a two-step stepwise process with C−N bond formation giving an intermediate 3 followed by a 1,2proton shift. The energetics of both the stepwise and concerted pathways for the coupling reaction were calculated. The energy profiles for both of the two possible reaction pathways are shown in Figures 1 and 2 for four metal-catalyzed systems. Our calculations are based on the gas-phase model. In Figures 1 and 2, the relative free energies (ΔG) and electronic energies (ΔE, in parentheses) are given. To account for the effect of entropy, we use the free energies rather than the electronic energies for our discussion since the reactions studied here involve two or more molecules. Tables 1−4 display the calculated geometric

Scheme 3

Scheme 4

fragmentation of [Ir(cod)Cl]2 is unlikely to invoke a significant barrier, as there was no indication for the existence of such a

Table 5. Selected Bond Distances (Å) Calculated for 1′Ir, 1′Ru, 1′Rh, and 1′Zna

a

The atoms C1, C2, C3, N1, and N2 are labeled. 419


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Table 6. Selected Bond Distances (Å) Calculated for TS(1‑2)′Ir, TS(1‑2)′Ru, TS(1‑2)′Rh, and TS(1‑2)′Zna

a


Table 7. Selected Bond Distances (Å) Calculated for 2′Ir, 2′Ru, 2′Rh, and 2′Zna

a


exergonic by 4.1, 12.0, 10.0, and 6.3 kcal/mol, respectively, suggesting that the coordination process occurs easily. As indicated in Figure 1, the one-step concerted insertion from 2Ir→4Ir and 2Ru→4Ru, which require barriers of 34.6 and 26.9 kcal/mol, are much less favorable than the two-step process with rate-determining barriers of 23.2 and 16.1 kcal/ mol calculated for the 2Ir→3Ir and 2Ru→3Ru steps, respectively. Likewise, the 2Rh→4Rh and 2Zn→4Zn steps require free energy barriers of 19.8 and 16.4 kcal/mol, respectively, as seen from Figure 2. The barriers from 2Rh→3Rh and 2Zn→3Zn are 5.1 and 3.0 kcal/mol, respectively. Clearly, the barrier is too high for the one-step concerted process to be energetically feasible. Given these results, the stepwise mechanism should be operative in the C−H bond amination reaction catalyzed by (cod)Ir(OMe),

Scheme 5

structures for the species involved. The initial coordination of phenylazide to Ir, Ru, Rh, and Zn was calculated to be

Figure 3. Spatial plots of the highest occupied molecular orbitals for the transition states TS(2−3)Ir, TS(2−3)Ru, TS(2−3)Rh, and TS(2−3)Zn. 420


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Figure 4. Variation of isotropic magnetic susceptibility (χ) along the path for the ruthenium-, iridium-, rhodium-, or zinc-catalyzed system, respectively (relative to reactant).

RuCl3(DME), Rh2(O2CCF3)4, or ZnI2. Therefore, we will consider the energetically preferred stepwise mechanism for the C−H bond amination reactions in the following discussion. First, we discuss the iridium- and ruthenium-catalyzed C−H bond amination reaction. As shown in Figure 1a and b, nitrogen is liberated from the azide complex 1Ir or 1Ru to give the iridium or ruthenium nitrene complex 2Ir or 2Ru through transition state TS(1−2)Ir or TS(1−2)Ru, respectively. The overall free energy barrier for the formation of 2Ir or 2Ru was calculated to be 14.5 or 11.0 kcal/mol, respectively. The C−N bond is then formed through transition state TS(2−3)Ir or TS(2−3)Ru with a barrier of 23.2 or 16.1 kcal/mol, followed by a 1,2-proton shift with a barrier of 12.7 or 12.4 kcal/mol, respectively. In 4Ir or 4Ru, the Ir−N or Ru−N bond has the longest distance (2.247 or 2.206 Å) of the investigated species, respectively, a typical amine-tometal dative bond. An attempt to locate a transition state from 1Ir→3Ir or 1Ru→3Ru failed, and only TS(1−2)Ir or TS(1−2)Ru was obtained. These results indicate that direct conversion of 1Ir→ 3Ir and 1Ru→3Ru without involving the nitrene complex has a very high barrier and can be ruled out. We also studied the C−H bond amination reaction catalyzed by Rh2(O2CCF3)4 and ZnI2, as shown in Figure 2a and b, respectively. Nitrogen is liberated from the azide complex 1Rh

or 1Zn via the transition state TS(1−2)Rh or TS(1−2)Zn to give the nitrene complex 2Rh or 2Zn, respectively. The barriers for the nitrogen liberation step are calculated to be 20.4 and 22.7 kcal/ mol in the corresponding rhodium- and zinc-catalyzed systems, respectively. Compared with TS(1−2)Ir and TS(1−2)Ru (Tables 1−4), TS(1−2)Rh and TS(1−2)Zn have longer N1−N2 distances (1.645 and 1.688 Å for the Rh and Zn species, respectively, versus 1.572 and 1.582 Å for Ir and Ru), indicating that the rhodium and zinc systems have later transition states, consistent with the relative barriers of the nitrogen liberation step. The C3−N1 bond distances of TS(2−3)Rh and TS(2−3)Zn (2.192 and 2.343 Å, respectively) differ from those of 2Rh and 2Zn slightly (2.755 and 2.828 Å). This implies that the C−N formation step takes place easily, which is verified by the low free energy of activation of only 5.1 or 3.0 kcal/mol for the 2Rh→3Rh or 2Zn→ 3Zn step, respectively (Figure 2a or b). In contrast, the TS(2−3)Ir and TS(2−3)Ru represent much later transition states for the iridium- and ruthenium-catalyzed systems, as seen by tracking the C3−N1 bond distances; the relative C3−N1 bond in TS(2−3)Ir or TS(2−3)Ru (2.007 or 1.966 Å) is substantially shorter in 2Ir or 2Ru (2.993 or 2.877 Å) than in the Rh or Zn systems. The 1,2-hydrogen shift step gives the 4Rh and 4Zn products with overall barriers of 13.8 and 12.5 kcal/mol, respectively. In 421


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Figure 5. Variation of anisotropic magnetic susceptibility (χanis) along the path for the ruthenium-, iridium-, rhodium-, or zinc-catalyzed system, respectively (relative to reactant).

addition, we were again not able to find a transition state from 1Rh to 3Rh or 1Zn to 3Zn. As can be seen in Figures 1 and 2, the differences in the barriers to the nitrogen liberation or C−N bond formation steps among the four metal-catalyzed systems correlate well with their endothermicity difference. Nitrogen liberation from the azide complexes, 1Ir and 1Ru, to give the nitrene complexes, 2Ir and 2Ru, occurs with a small barrier (14.5 and 11.0 kcal/ mol) and a large exergonicity (−42.3 and −45.6 kcal/mol), respectively. However, the C−N bond formation step (2Ru→ 3Ru) has a higher barrier and is slightly exergonic by 0.7 kcal/ mol. Relative to RuCl3(DME), the barrier for the 2Ir→3Ir step is higher and switches from exothermic and exergonic to endothermic and endergonic (1.6 kcal/mol). Our DFT calculations of the proposed reaction mechanism for Rh2(O2CCF3)4 and ZnI2 indicate that the 1Rh→2Rh and 1Zn→2Zn steps require free energy barriers of 20.4 and 22.7 kcal/mol and are exergonic by 16.9 and 8.9 kcal/mol, respectively. For the 2Rh→3Rh and 2Zn→3Zn transitions, the free energy barriers were calculated to be 5.1 and 3.0 kcal/mol with an exergonicity of 22.5 and 29.2 kcal/mol, respectively. Overall, the important step in the reaction is the nitrogen liberation step. Once the metal−nitrene intermediate is formed,

it undergoes nearly barrierless C−N formation in the rhodiumand zinc-catalyzed systems. According to the energetic span model developed by Shaik et al.,28 for the rhodium- and zinc-catalyzed systems, the nitrogen liberation transition states TS(1−2)Rh and TS(1−2)Zn are the TDTS, while the initial reactants 1Rh and 1Zn form the TOFdetermining intermediate (TDI), which results in a energetic span of 20.4 and 22.7 kcal/mol. In contrast, the energetic span of the iridium- and ruthenium-catalyzed C−H bond amination reaction is calculated to be 23.2 and 16.1 kcal/mol. This means that the ruthenium species exhibit higher catalytic performance than the other three. Detailed Mechanism for the Nitrogen Liberation Process. As shown in Tables 1−4, we found that the changes in bond lengths of C1−N1 and C1−C2 were larger in the 1Rh→ 2Rh and 1Zn→2Zn steps than in the 1Ru→2Ru and 1Ir→2Ir steps, indicating that the phenyl ring containing the azide substituent is actively involved in the nitrogen liberation step for the rhodium- and zinc-catalyzed systems (Scheme 3). The mechanism of the nitrogen liberation process for iridium- and ruthenium-catalyzed systems is similar to that shown in Scheme 4. The C1−N1 bond is shortened from 1Ru→2Ru and 1Ir→2Ir 422


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Figure 6. Variation of NICS along the path for the ruthenium-, iridium-, rhodium-, or zinc-catalyzed system, respectively.

(1.466 → 1.356 Å and 1.453 → 1.376 Å, respectively) because the nitrogen center hybridization changes from sp3 to sp2. In support of the proposed mechanisms above, the atomic charges and bond orders (which are Wiberg bond indices29 and a measure of bond strength) were obtained by using the natural bond orbital analysis.30 The NBO charge analysis shows that the C2 atom charge changes from −0.046 (1Rh) and −0.053 (1Zn) to 0.022 (TS(1−2)Rh) and 0.030 (TS(1−2)Zn). Clearly, the partial charge associated with the C2 atom in general changes from negative to positive, consistent with the notion that the phenyl ring containing the azide substituent is actively involved in the nitrogen liberation step for the rhodium- and zinccatalyzed systems (Scheme 3). In contrast, the charge of the C2 atom changes from −0.060 (1Ir) and −0.043 (1Ru) to −0.026 (TS(1−2)Ir) and −0.030 (TS(1−2)Ru). For the Ir and Ru systems, the partial charge on the C2 atom only becomes less negative in the transition state, corresponding to Scheme 4. Additionally, the calculated charges on the C3 atom in TS(1−2)Rh and TS(1−2)Zn are −0.195 and −0.180, respectively, indicating that the phenyl ring containing the C3 atom is not heavily involved in the nitrogen liberation step.

As shown in Tables 1−4, the changes in Rh−N and Zn−N bond length from 1Rh→2Rh and 1Zn→2Zn (2.204 → 1.980 Å and 2.218 → 2.026 Å, respectively) are smaller than the changes in Ru−N and Ir−N bond length from 1Ru→2Ru and 1Ir→2Ir (2.168 → 1.846 Å and 2.092 → 1.757 Å, respectively). Indeed, the NBO analysis also indicates that the change in Wiberg bond index (bond order) calculated for the Rh−N1 and Zn−N1 bonds when going from 1Rh→2Rh and 1Zn→2Zn (0.20 → 0.62 and 0.08 → 0.19, respectively) is smaller than that calculated for the Ru−N1 and Ir−N1 bond change from 1Ru→ 2Ru and 1Ir→2Ir (0.28 → 1.13 and 0.41 → 1.38, respectively), indicating that the back-bonding interactions are significant in 2Ir and 2Ru. Moreover, the bond order data suggest that the mechanism of the nitrogen liberation process for the rhodiumand zinc-catalyzed systems is similar to that shown in Scheme 3, and that for the iridium- and ruthenium-catalyzed systems is consistent with Scheme 4. To examine if different substrates affect the proposed mechanisms above, we studied the nitrogen liberation step of the substrate vinyl azides. As shown in Tables 5−7, the double bond containing the azide substituent is still actively involved in the nitrogen liberation step for the rhodium- and zinc-catalyzed 423


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systems. Moreover, the changes in bond lengths of C1−C2 from 1′Rh→2′Rh and from 1′Zn→2′Zn were more obvious than those from 1Rh→2Rh and from 1Zn→2Zn due to the loss of the conjugation of the phenyl ring for vinyl azides, which further confirms the proposed mechanisms above for the nitrogen liberation step for four transition metal-catalyzed systems. The reason that the mechanism of the nitrogen liberation process for rhodium- and zinc-catalyzed systems is different from that of the iridium- and ruthenium-catalyzed systems can be explained as follows. In the four active catalysts (Rh2(O2CCF3)4, ZnI2, (cod)Ir(OMe), and RuCl3(DME)), ruthenium and iridium bear ligands with electron-donating properties, while metal zinc and rhodium bear ligands with electron-withdrawing properties. The electron-donating ligand connected to the late transition metals ruthenium and iridium facilitates internal electron transfer from the metal to the N1 atom in the corresponding nitrene complexes (2Ru and 2Ir) and thereby makes back-bonding interactions dominant in Ru−N and Ir−N. In contrast, the electron-withdrawing ligand connected to rhodium and zinc hinders this internal electron transfer in the corresponding nitrene complexes (2Rh and 2Zn) and leads to weak back-bonding interactions in the Rh−N and Zn−N bonds. Detailed Mechanism for the Formation of the C−N Bond. As mentioned in the Introduction, the mechanism of the C−N bond formation step was determined by the two resonance structures of the metal−nitrene intermediate (Scheme 1). We can consider Im1 as being formed via electrophilic attack of nitrene on the aromatic ring, while Im2 is formed via an electrocyclic or pseudoelectrocyclic reaction. In the structures calculated for 3Ir, 3Ru, 3Rh, and 3Zn, the two sixmembered carbon rings each have two short and four long bonds and the geometry around the N center is approximately planar (with a sum of angles around N of 359.7°, 356.4°, 355.6°, and 358.1°, respectively). These structural parameters indicate that the resonance structure Im2 is intact for 3Ir, 3Ru, 3Rh, and 3Zn, suggesting electrocyclization or pseudoelectrocyclization as the dominant mechanism for these four transition metal-catalyzed systems. To further support the electrocyclization or pseudoelectrocyclization mechanism, we also introduced a CH2 moiety to eliminate the possibility of having this mechanism as shown in Scheme 5. Our DFT calculations showed that the barrier for the formation of intermediates 3Ir-CH2, 3Ru-CH2, 3Rh-CH2, and 3Zn-CH2, is higher in energy by 11.5, 16.0, 12.5, and 11.4 kcal/ mol than that of the corresponding intermediates 3Ir, 3Ru, 3Rh, and 3Zn, respectively. An NBO charge analysis also showed that the N1 atoms in 2Ir, 2Ru, 2Rh, and 2Zn carry a negative charge (−0.489, −0.145, −0.330, and −0.619, respectively), which indicates that the N1 atom could attack the carbon atom nucleophilically. The calculations indicate that no matter what the metal center isIr, Ru, Rh, or Znthe electrocyclization or pseudoelectrocyclization mechanism should be responsible for formation of the C−N bond. Next, we executed a more detailed study of the mechanistic characteristics to determine the plausibility of the pseudoelectrocyclic path. Two criteria have been suggested to assess the electrocyclic/pseudoelectrocyclic nature of a reaction. As shown in Figure 3, orbital analysis shows that the four transition structures, TS(2−3)Ru, TS(2−3)Ir, TS(2−3)Rh, and TS(2−3)Zn, have a single orbital disconnection at the N1 atom bearing lone electron pairs. According to the definition of Birney,13c a single orbital disconnection may, but will not

always, lead to a pseudoelectrocyclization transition structure, which is in open conflict with Lemal’s original definition,13a on the basis of whether, in fact, a single disconnection is enough to produce a pseudoelectrocyclization. Therefore, the magnetic susceptibility (χ), magnetic susceptibility anisotropy (χanis) and nucleus-independent chemical shift were used to distinguish pseudoelectrocyclization from a normal electrocyclization. Figures 4 and 5 show the variation of magnetic susceptibility and magnetic susceptibility anisotropy during the electrocyclization process for the four metal-catalyzed systems. At different points along the reaction path we calculated the magnetic susceptibility and magnetic susceptibility anisotropy. Each curve displayed no noticeable minima near the transition states (TS(2−3)Ru, TS(2−3)Ir, TS(2−3)Rh, and TS(2−3)Zn). On the basis of this behavior, these reactions should be classified as pseudoelectrocyclization. Contrary to magnetic susceptibility, another way of measuring pseudoelectrocyclization is by means of the NICS index, defined by Schleyer as the negative of the magnetic shielding,31 which can be evaluated at a point, avoiding some of the problems associated with global properties such as χ. In his paper,31,32 Schleyer chose to evaluate NICS at the geometrical center of the ring or at the ring critical point as obtained from Bader’s atoms-in-molecules theory.33 Moreover, he prevented obtaining NICS in the molecular plane due to the contributions of the σ bonds to the shielding and suggested calculating it out of the plane, more precisely at 1 Å above or below the plane of the aromatic molecule.34 As a consequence, we opted to calculate NICS at three different points: the ring critical point and 1 Å above and below the ring critical plane. The results are shown in Figure 6. Near the transition states (TS(2−3)Ru, TS(2−3)Ir, TS(2−3)Rh, and TS(2−3)Zn), the profiles of four reactions do not exhibit any minimum in any of the points considered. We can conclude that, as obtained from NICS, these reactions are classified as pseudoelectrocyclization. Therefore, our calculations indicate that no matter what the metal center isRu, Ir, Rh, or Znthe pseudoelectrocyclization mechanism should be responsible for the C−N formation step.

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CONCLUSIONS The C−H amination reaction mechanism of biaryl azides was theoretically studied with the aid of DFT calculations at the B3LYP level. We chose four metal complexes as the objects of our study, (cod)Ir(OMe), RuCl3(DME), Rh2(O2CCF3)4, and ZnI2. The calculations show that the C−H amination reaction mainly proceeds through a stepwise mechanism: nitrogen liberation followed by the formation of a C−N bond and then 1,2-hydrogen shift. The efficiency of the different catalytic systems (in the order Ru > Rh > Zn > Ir) is determined by the recently developed energetic span model. For the nitrogen liberation step, there are two possible reaction mechanisms. Our calculations indicate that the phenyl ring containing the azide substituent is actively involved in this step for the rhodium- and zinc-catalyzed systems. However, for the iridiumand ruthenium-catalyzed systems, the nitrogen liberation step occurs without substantially disturbing the conjugation of the phenyl ring containing the azide substituent. The difference in mechanistic results is likely related to the fact that ruthenium and iridium are bound to electron-donating ligands, while rhodium and zinc are connected to electron-withdrawing ligands. The different ligand properties likely promote the 424


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mechanistic differences for the formation of the various intermediate nitrenoid/nitrene complexes. On the basis of the investigation of magnetic properties, the calculated results indicate that the pseudoelectrocyclization mechanism should be responsible for the C−N bond formation step, which also shows an orbital disconnection on the nitrogen atom (N1) in the pseudopericyclic transition structure, regardless of the metal center (Ru, Ir, Rh, or Zn).

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S Supporting Information *

This material is available free of charge via the Internet at http://pubs.acs.org.

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Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundations of China (Grant No. 21103072), the Natural Science Foundation of Guangdong Province (Grant No. S2012010008758), and Fundamental Research Funds for the Central Universities (Grant No. 21612404).

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REFERENCES

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