Mechanism of Palladium-Catalyzed Alkylation of Aryl Halides with

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Mechanism of Palladium-Catalyzed Alkylation of Aryl Halides with Alkyl Halides through C−H Activation: A Computational Study Ling Zhu, Yuan-Ye Jiang,* Xia Fan, Peng Liu, Bao-Ping Ling, and Siwei Bi* School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, People’s Republic of China

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S Supporting Information *

ABSTRACT: Pd-catalyzed C(sp3)−H activation/alkylation of 2-tert-butylaryl halides with alkyl halides and CH2Br2 represents an advantageous strategy for the C−H functionalization with halogens as traceless directing groups. Several possible mechanisms were proposed for the reactions, but no further evidence was available to judge their relative feasibilities. Herein, a mechanistic study was performed with the aid of density functional theory (DFT) methods. Calculations indicate that the coupling of aryl bromides with alkyl chlorides is likely to generate alkylated benzocyclobutenes via aryl−Br oxidative addition on Pd(0) catalysts, C(sp3)−H activation, alkyl−Cl oxidative addition, aryl−alkyl reductive elimination, aryl−H activation, and aryl−C(sp3) reductive elimination. The coupling of aryl iodides with CH2Br2 is likely to generate indane derivatives via aryl−I oxidative addition, C(sp3)−H activation, alkyl−Br oxidative addition, aryl−CH2Br reductive elimination, alkyl−Br oxidative addition, C(sp3)−alkyl reductive elimination, and reduction of palladium dibromide complexes by amines. By comparison, the metathesis of alkyl chlorides on Pd(II) intermediates and the pathway involving palladium carbene intermediates are found to be less favored. Meanwhile, the coordination of in situ generated salts KI, KBr, and KHCO3 to palladium complexes, which has been less considered in previous mechanistic studies, is found to lead to more energetically favored pathways in most of the steps. Finally, the oxidative addition of alkyl halides generating Pd(IV) intermediates or the reduction of palladium dibromide complexes by amines, rather than the previously proposed C(sp3)−H activation, is found to be the rate-determining step in the two types of coupling reactions. This result does not go against the reported primary kinetic isotope effect (KIE) based on intramolecular competition reactions because the C(sp3)−H activation is irreversible according to our calculations. C(sp3) bonds,15 most of the reported cases are limited to intramolecular cyclization reactions. In contrast, earlier work from Dyker realized intermolecular aryl−aryl bond formation associated with intramolecular Ar−H and C(sp3)−H activation.16 Thereafter, Buchwald,17 Hoshi,18 and co-workers reported the intermolecular aryl−C(sp3) bond coupling of 2,4,6-triisopropylbromobenzene and arylboronic acids. In 2016, Martin et al. reported the cross-coupling of 2-tertbutyl-substituted aryl bromides with diazo compounds, by which one C(sp3)−C(sp3) bond and one aryl−C(sp3) bond can be constructed in the tandem process.19 In the same year, Zhang and co-workers realized the alkylation/Heck reaction20 and the dual Ar−C(sp3) bond formation21 of 2-iodobiphenyl. Very recently, Zhang et al. made progress and achieved the cross-couplings of 2-tert-butyl-substituted aryl halides with more robust alkyl chlorides and CH2Br2 to produce orthoalkylated benzocyclobutenes and indane derivatives (Scheme 1).22 Two aryl−C(sp3) bonds are formed in the couplings with alkyl chlorides, and one C(sp3)−C(sp3) bond and one aryl− C(sp3) bond are formed in the couplings with CH2Br2. In contrast to couplings with arylboronic acids17,18 or diazo ester derivatives,19 the presence of alkyl halides possibly

1. INTRODUCTION Transition-metal-catalyzed C−H activation/functionalization is a highly atom economical synthetic strategy without traditional substrate preactivation and has shown great potential in the synthesis of bioactive molecules, pharmaceuticals, natural products, and industrial materials.1 In this type of reaction, directing groups are frequently employed to improve reaction selectivity for the substrates containing several C−H bonds with close reactivities and to facilitate the activation of inert C−H bonds.2 Nevertheless, some of the directing groups are difficult to remove and/or derivatize, and thus the practical values of the relevant approaches are limited. In this context, considerable attention has been paid to the C−H activation enabled by traceless directing groups3 such as carboxylic acids,4 silicon tethers,5 S-containing groups,6 benzyl,7 acyl,8 boron ester,9 carbonate,10 aldehyde,11 hydroxyl,12 and various Nsubstituents.13 As one of the most common functional groups, halogens direct Pd-catalyzed C−H activation through the oxidative addition on palladium(0) complexes to form a Pd−C bond. Owing to the versatility of Pd−C bonds in organometallic chemistry, the C−halide bond can become traceless by converting into other types of chemical bonds after C−H activation. Although this strategy was demonstrated to be effective for the construction of aryl−C(sp2)14 and aryl− © XXXX American Chemical Society

Received: March 29, 2018

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DOI: 10.1021/acs.organomet.8b00185 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

To enrich the knowledge of Pd-catalyzed C−H activation with traceless directing groups, a DFT study on the Pdcatalyzed alkylation of aryl halides with alkyl halides was performed herein. We found that the couplings with alkyl chlorides and CH2Br2 both prefer the oxidative addition/ reduction elimination mechanisms involving Pd(IV) intermediates. Meanwhile, the in situ generated salts were found to facilitate most of the elementary steps of the catalytic cycles. Furthermore, oxidative addition of alkyl halides to generate Pd(IV) intermediates was found to be the rate-determining step rather than C−H bond cleavage, and the related energy profile also explains the observed primary KIE effect.

Scheme 1. Pd-Catalyzed Alkylation of Aryl Halides with (a) Alkyl Chlorides and (b) CH2Br2a

2. COMPUTATIONAL METHODS

a

DMF = N,N-dimethylformamide, DMA = N,N-dimethylacetamide.

A mechanistic study was performed by using the Gaussian09 program.29 Geometry optimization was conducted in the gas phase by using the DFT method B3LYP30 and a mixed basis set: i.e., LANL2DZ31 with extra polarization functions32 for Pd (ζ(f) = 1.472), Br (ζ(d) = 0.428), and I (ζ(d) = 0.289) and 6-31G(d) for the rest of the atoms. At the same level of theory, frequency analysis was performed to get thermodynamic corrections and also to check whether optimized structures are minima or saddle points. Intrinsic reaction coordinate (IRC) analysis33 was also performed to confirm that the optimized transition states connect with correct intermediates. On the basis of the optimized structures, solution-phase single-point energies were calculated by using the DFT method M06L34 associated with the mixed basis set (SDD35 for Pd, Br, and I and 6-311+G(d,p) for the rest of the atoms), the solvation model SMD,36 and an ultrafine grid.37 N,N-Dimethylformamide (DMF) was used as the solvent for the couplings with alkyl chlorides, and N,Ndimethylacetamide (DMA) was used as the solvent for the couplings with CH2Br2 in the solution-phase single-point energy calculations. A value of 1.9 kcal/mol, calculated from RT ln(Csol/Cgas), was added to the Gibbs free energies of all species to address the standard state change from 1 atm to 1 M at 298.15 K because the concentration was increased from 1/24.5 mol/L to 1 mol/L. The solution-phase singlepoint energies added by Gibbs free energy corrections and 1.9 kcal/ mol, i.e. solution-phase Gibbs free energies referring to 1 M and 298.15 K, were used for the following mechanistic discussion.

22

generates Pd(IV) intermediates in Zhang’s reactions (path A, Scheme 2) whereas some Pd(IV) complexes are known to be unstable23 and are generated by treating Pd(II) complexes with stronger oxidants24 or via electrochemical oxidation.25 As an alternative, the metathesis of Pd(II) intermediate B with alkyl chlorides was also proposed (path B).22 On the other hand, two different pathways were proposed for the couplings with CH2Br2. One involves reductive elimination and oxidative addition (path C), and the other proceeds via carbene intermediate (path D). Furthermore, a primary KIE effect was observed for both of the alkylation reactions, and C−H activation was accordingly proposed to be the rate-determining step. Previous computational mechanistic studies on the transition-metal-catalyzed C−H activation assisted by traceless directing groups paid more attention to N-substituents26 and carboxylic acids.27 In more relevant cases, several groups including ours reported computational studies on Pd-catalyzed C−H functionalization of aryl iodides, but these cases did not involve the chemistry of alkyl halides.28 Therefore, no direct evidence is available to clarify the detailed mechanism of the halogen-directed Pd-catalyzed alkylation through C−H activation.22

Scheme 2. Proposed Mechanisms for Pd-Catalyzed Alkylation of Aryl Halides with Alkyl Halides

B


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Figure 1. Calculated solution-phase Gibbs free energy profile of the aryl−Br oxidative addition, C(sp3)−H activation, and alkyl−Cl oxidative addition in the Pd-catalyzed alkylation of 1 with 2 (in kcal/mol).

Figure 2. Optimized structures of selected transition states. Bond lengths are given in angstroms.

3. RESULTS AND DISCUSSION 3.1. Mechanism of Alkylation with Alkyl Chlorides. The aryl bromide 1 and alkyl chloride 2 was chosen as the model reactants for the mechanistic study on the reactions producing benzocyclobutene derivatives (Figure 1). The catalyst precursor Pd(OAc)2 is expected to be reduced to a Pd(0) complex to start the catalytic cycle according to the literature,22,38 and thus the complex Pd[P(o-tol)3]2 (CP1) was considered as the reference state of the palladium catalyst. The ligand exchange of CP1 with 1 releases the bulky phosphane ligand P(o-tol)3 and generates CP2, from which facile oxidative addition occurs via TS1 and generates CP3 with an energy barrier of 17.5 kcal/mol. Attempts to locate the oxidation addition transition state containing two P(o-tol)3 always led to the dissociation of one P(o-tol)3, possibly due to the steric effect of P(o-tol)3.39 In CP3, the aryl is located trans to the phosphine ligand and the isomer CP4 is more stable. CP3 can facilely isomerizes to CP4 through the Y-shaped TS2. Meanwhile, all efforts to locate the oxidative addition transition state directly generating CP4 led to either TS1 or TS2.

Through a relaxed energy surface scan, the oxidative addition transition state directly generating CP4 is estimated to be less stable than TS1 by only 1.5 kcal/mol, and thus we do not exclude this possibility (Figure S1). Thereafter, the anion exchange of CP4 with K2CO3 generates KBr and CP7, from which inner-sphere carbonate-assisted C(sp3)−H activation occurs via TS3. Despite this common type of C−H activation process,40 the transition state with the presence of KBr, i.e. TS4, was found to be more favored by 2.2 kcal/mol. Because the KCO3− moiety does not directly coordinate to the palladium center, TS4 is close to outer-sphere C−H activation that was found to be favored over inner-sphere C−H activation in some previous studies.41 TS4 has a shorter O1−H1 bond and a longer C1−H1 bond in comparison with TS3, whereas the Pd−C1 bond lengths differ little in the two transition states (Figure 2). According to the acidities,42 the heterolytic cleavage of the C1−H1 bond is expected to be more endergonic than that of the O1−H1 bond, meaning that TS4 should be less stable than TS3 if only the C1−H1 and O1−H1 bond lengths are considered. In this case, we proposed a C


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Figure 3. Calculated solution-phase Gibbs free energy profile of the last three steps in Pd-catalyzed alkylation of 1 with 2 (in kcal/mol).

Figure 4. Calculated solution-phase Gibbs free energy profile of the aryl−I oxidative addition, C(sp3)−H activation, isomerization, and alkyl−Br oxidative addition in Pd-catalyzed alkylation of 3 with CH2Br2 (in kcal/mol).

stronger interaction between the PdII complex and the bromide ligand in comparison to that between PdII and KHCO3, which is reflected in the calculated reaction energies for the coordination of KHCO3 and KBr to Pd(II) complexes (Scheme S1) and is responsible for the lower relative energy of TS4. After TS4, CP8 is formed and further generates the complex CP9 by combining with alkyl chloride 2. Thereafter, the

C(sp3)−Cl bond cleavage occurs via the tetragonal-pyramidal transition state TS5 with an energy barrier of 30.6 kcal/mol. In TS5, the alkyl group R is located at the axial position and a K− Cl bond is also partially formed between the alkyl chloride and the KBr·KHCO3 moiety. In contrast, the tetragonal-pyramidal transition state TS6, in which the phosphine ligand is located at the cis position of the aryl ligand, is less favored by 2.9 kcal/ mol. We also considered other possible oxidative addition D


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Figure 5. Calculated solution-phase Gibbs free energy profile of aryl−C(sp3) reductive elimination, C(sp3)−Br oxidative addition, C(sp3)−C(sp3) reductive elimination, and catalyst regeneration in Pd-catalyzed alkylation of 1 with CH2Br2 (in kcal/mol).

12.9 kcal/mol to afford CP13. The C(sp3)−C(sp2) reductive elimination occurs via TS9 and generates the strained product pro via a higher energy barrier of 25.1 kcal/mol. It is interesting to see that the replacement of KBr·KHCO3 with P(o-tol)3 makes the above three steps (via TS10, TS11, and TS12, respectively) more kinetically difficult by 1.2, 2.1, and 7.0 kcal/mol, respectively. 3.2. Mechanism of Alkylation with CH2Br2. In this section, the mechanism of the alkylation of aryl iodide 3 with CH2Br2 to generate indane derivatives was considered. As shown in Figure 4, the alkylation of 3 with CH2Br2 proceeds via oxidative addition of the aryl−I bond (TS13), isomerization (TS14), C(sp3)−H activation (TS16), and oxidative addition of the alkyl−Br bond (TS17), which are similar to the steps in the coupling of 1 and 2. It is found that the presence of the salt KI in the C(sp3)−H activation step (TS16) is more favorable in comparison to the case with the absence of KI (TS15). However, in the oxidative addition of the alkyl−Br bond, the salt seems to be less important, as the pathway via TS18 is less favored than that via TS17 by 1.2 kcal/mol. This phenomenon is different from the case of TS5 and TS6 (Figure 1). It is known that the K−Cl bond dissociation energy is greater than that of K−Br by about 13 kcal/mol.44 Accordingly, it is might be understandable that the intrinsically weaker interaction between the K+ in the KI·KHCO3 moiety and the Br− of CH2Br2 makes the coordination of the salt KI· KHCO3 less important in comparison to the electron-donating phosphine ligand for the oxidative addition of the alkyl−Br bond. We also investigated the direct Ar−C reductive

transition states, but they are energetically higher than TS5 by 8.0−21.1 kcal/mol (Scheme S2). Similar to these results, recently Kwong, Lin, and co-workers also proposed that a salt rather than the monodentate phosphine ligand is coordinated to palladium in the oxidative addition of Ar−Br bond on Pd(II) complex.43 In addition to the pathway involving Pd(IV) intermediates, we considered the metathesis of R−Cl and Pd− Ar bonds from CP9 but failed to locate the corresponding transition state. To evaluate the energy demand of the metathesis process, a relaxed energy surface scan was performed. The scan of the Ar−R bond leads to a SN2 process even though we set the Cl atom close to the Pd atom at the starting point. Meanwhile, this process leads to an electronic energy increase of over 45 kcal/mol with reference to CP9 (Figure S2). In addition, we have considered the Ar−C bond reductive elimination from CP8. This process can still generate the final product via the following oxidative addition of R−Cl and Ar−C bonds (Scheme S3). However, this process is kinetically less favored than the pathway via TS5 by 1.5 kcal/ mol. On the basis of these results, we propose that the oxidative addition/reductive elimination pathway is more plausible. CP10 is formed after TS5 and has a relative energy close to that of CP8. This phenomenon possibly results from the close electronegativity of Pd and C atoms, which makes the Pd center in CP10 not too electron deficient. Thereafter, fast C(sp3)−C(sp2) reductive elimination occurs via TS7 to generate CP11 (Figure 3). CP11 undergoes ligand exchange with K2CO3 to generate CP12, from which the facile C(sp2)− H bond activation occurs via TS8 with an energy barrier of E


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Organometallics Scheme 3. Previously Reported KIE Study for Alkylation with (a) Alkyl Halides and (b) CH2Br222

loading amount of NEt3 is 2.0 equiv in the experiments.22 According to the above calculations and experimental observations, we speculate that it is NEt3 which reduces the PdBr2Ln complex to the Pd(0) complex in the couplings with CH2Br2. The transformation of CP27 into CP1 involving the C−H activation of NEt3 from CP27, the C−Br reductive elimination to generate CP1 and Et2NCHBrCH3, and the elimination of Et2NCHBrCH3 by K2 CO 3 to generate Et2NCHCH2 was considered (Figure S3). The overall Gibbs free energy barrier and Gibbs free energy change of the reaction CP27 + 2 K2CO3 + NEt3 → CP1 + 2 KBr + 2 KHCO3 + Et2NCHCH2 are 29.3 and −17.6 kcal/mol, respectively, which are acceptable for the experimental temperature (80 °C). Nevertheless, to the best of our knowledge, no further experimental results are available to clarify the detailed mechanism and products of this reduction, and the present results only indicate that the catalyst regeneration is operative under the experimental conditions but is not necessarily the real mechanism. 3.3. Rationale of Primary KIE Effect. To explore the ratedetermining steps of the alkylation with alkyl halides or CH2Br2, Zhang et al. performed a KIE study (Scheme 3). According to the observed primary KIE effect, they proposed that C−H bond cleavage is the rate-determining step in the two types of alkylation reactions. In contrast to this proposal, calculations indicate that the oxidative addition of alkyl halide 2 (via TS5) is kinetically slower than the C−H bond cleavage (via TS4) by 5.7 kcal/mol in the alkylation with alkyl halides, meaning that the former step is the rate-determining step. Similarly, in the alkylation with CH2Br2, the oxidative addition of CH2Br2 (via TS17) is found to be slower than the C−H activation (via TS16) and the catalyst regeneration by 1.7 and 6.9 kcal/mol, respectively, also supporting the oxidative addition and catalyst regeneration as possible rate-determining steps. It should be pointed out that Zhang et al.’s KIE study was based on the intramolecular competition between the C−H and C−D bonds in the single substrate. In this type of reaction, when the C−H bond cleavage is irreversible, the kinetic differences between C−H and C−D bond cleavage still differentiate the generation rate of undeuterated and deuterated products even if the C−H bond cleavage is not the rate-determining step, leading to a primary KIE effect.45 Consistent with this situation, the calculated relative energies of the C−H activation transition states (TS4 and TS16) are

elimination from CP21, but it is still less favored than TS17, possibly due to the weaker C−Br bond (Scheme S3). From the Pd(IV) complex CP23, direct reductive elimination could readily occur via TS19 to afford CP24 (Figure 5). In addition, the ligand exchange of CP23 with KI· KHCO3 generates CP28, from which irreversible reductive elimination occurs via TS22 with an overall energy barrier lower by 5.3 kcal/mol. By comparison, the formation of palladium carbene complexes from CP22 is found to be highly endergonic. The located palladium carbene complexes have relative energies that are higher than that of TS22 by over 30 kcal/mol (Scheme S4), indicating that this mechanism is impossible. Different from the following transformations after CP11, one reactive benzylic C−Br bond is available after TS19 and TS22, and thus fast oxidative addition of the C−Br bond could occur via TS20 and TS23, respectively. The presence of the salt leads to an underlying energy profile via TS23, and the elementary energy barrier is 10.3 kcal/mol with reference to CP30, lower than the elementary energy barrier from CP24 to TS20 by 7.3 kcal/mol. On the other hand, the competitive ortho aryl C−H activation via TS23-CH was considered but is kinetically slower than the C−Br bond oxidative addition via TS23 by 3.9 kcal/mol. In the next C−C reductive elimination on Pd(IV) intermediates, the coordination of electrondonating phosphine ligands becomes slightly superior again, as the pathway via TS21 has a lower elementary energy barrier in comparison to those via TS24 and TS25. When all the results above are taken into account, the overall energy barrier of the pathway with salt participation (via TS22, TS23, and TS24) is 14.9 kcal/mol (from CP31 to TS24), and that of the pathway with no salt participation (via TS19, TS20 and TS21) is 17.6 kcal/mol (from CP24 to TS20), showing that the former is more favored. The indane product pro2 is generated after the C(sp3)− C(sp3) reductive elimination stage along with the formation of a PdBr2Ln complex (e.g., CP27). To start the next catalytic cycle, the Pd(II) complex is required to be reduced to a Pd(0) complex. The direct elimination from CP27 to generate Br2 and CP1 was calculated to be highly endergonic by 58.9 kcal/ mol, and we excluded this possibility. Meanwhile, although it is known that Pd(OAc)2 can be reduced to a Pd(0) complex by phosphine ligands,38 only 20 mol % of P(o-tol)3 was used in the reaction, and thus other reagents should be mainly responsible for the catalyst reduction. It was observed that adding NEt3 increases the yield of the indane product and the F


Organometallics

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21473100, 21702119, and 21603116) and Natural Science Foundation of Shandong Province (No. ZR2017QB001).

higher than those of the transition states of the following steps, meaning that the C(sp3)−H bond cleavage is irreversible in both of the alkylation reactions. Therefore, our conclusion that the oxidative addition of alkyl halides to generate Pd(IV) intermediates or that the catalyst regeneration from CP27 to CP1 is the rate-determining step does not go against the reported KIE studies.

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4. CONCLUSION The mechanism of Pd-catalyzed alkylation of o-tBu-substituted aryl halide derivatives with alkyl halides and CH2Br2 through C−H activation was investigated by DFT methods. We found the couplings of o-tBu-substituted aryl bromides with alkyl chlorides generate ortho-alkylated benzocyclobutene derivatives through path A shown in Scheme 2. The couplings of otBu-substituted aryl iodides with CH2Br2 generate orthoalkylated benzocyclopentene derivatives through path C shown in Scheme 2. By comparison, the metathesis mechanism and carbene-involved mechanism were found to be less possible. Meanwhile, we found that salts can not only act as bases to promote C−H activation but also possibly cooperate with Pd catalysts to lead to more kinetically favored pathways. Furthermore, calculations indicate that the oxidative addition of the alkyl−halide bond to generate Pd(IV) intermediates or the catalyst regeneration from CP27 to CP1, rather than the previously proposed C(sp3)−H activation, is the ratedetermining step in the two types of alkylation reactions. This result does not go against the experimentally observed primary KIEs because the relevant C−H cleavage is irreversible and the KIEs were measured on the basis of intramolecular competition experiments. In all, this paper clarifies the detailed mechanism of Pd-catalyzed alkylation of aryl halides with alkyl halides via C−H activation and provides new insights into the relevant rate-determining steps and in situ generated salts.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00185.

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Article

Calculated reaction energies for the coordination of salts to Pd(II) complexes, relaxed energy surface scan, disfavored oxidative transition states, competitive Ar− C reductive elimination on Pd(II) complexes, relative Gibbs free energies of palladium carbene intermediates, reductive catalyst regeneration, and calculated energies (in hartrees) of all structures presented (PDF) Cartesian coordinates of the calculated structures (XYZ)

AUTHOR INFORMATION

Corresponding Authors

*E-mail for Y.-Y.J.: [email protected]. *Email for S.B.: [email protected]. ORCID

Yuan-Ye Jiang: 0000-0002-4763-9173 Siwei Bi: 0000-0003-3969-7012 Notes

The authors declare no competing financial interest. G


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