Direct Kinetic Evidence for the Active Anionic Palladium(0) and

Aug 29, 2017 - Direct Kinetic Evidence for the Active Anionic Palladium(0) and Palladium(II) Intermediates in the Ligand-Free Heck Reaction with Aroma...
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Direct Kinetic Evidence for the Active Anionic Palladium(0) and Palladium(II) Intermediates in the Ligand-Free Heck Reaction with Aromatic Carboxylic Anhydrides Alexander F. Schmidt,* Anna A. Kurokhtina, Elizaveta V. Larina, Elena V. Yarosh, and Nadezhda A. Lagoda Department of Chemistry of the Irkutsk State University, K. Marx Str.,1, 664003 Irkutsk, Russia ABSTRACT: Differential selectivity in the competitive Heck reaction with aromatic carboxylic anhydrides has been found to depend on the nature of both the cation and anion of a salt used as an additive for the catalytic system. This fact indicates that the anions present in the system are involved in active anionic species.

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avoided.7−9 As opposed to most kinetic studies, we applied the differential selectivity of competing reactions as the main evaluated parameter. In contrast to catalytic activity that is connected to the reaction rate and measured traditionally during kinetic investigations or to integral selectivity based on the relative yields of formed products, the differential selectivity study unambiguously reveals possible changes in the nature of active species. As opposed to integral selectivity, the differential selectivity is based on the ratio of the rates of products formed instead of the ratio of their yields. This feature results in this kinetic parameter becoming independent of the amount of active species (for extensive substantiation see ref 10). Therefore, any changes in differential selectivity in contrast to catalytic activity and integral selectivity unambiguously indicate changes in the active species nature.

INTRODUCTION Recent advances have caused the role that anionic palladium intermediates play in catalytic cross-coupling cycles to be revisited. On the basis of the results of model experiments, Amatore and Jutand proposed earlier that anionic complexes of Pd(0) participate in the oxidative addition of aryl halides in the Heck reaction in the presence of a catalytic system containing phosphines.1,2 Subsequent excellent work by Hartwig et al.3 compared the integral selectivity calculated as the ratio of the yields of α and β regioisomers formed in the stoichiometric reaction of anionic Pd complexes with alkenes and the catalytic Heck coupling. It was demonstrated that, even when a phosphine-containing catalyst was used, phosphine-free anionic Pd intermediates participated in the step where the alkene reacted (i.e., canonical coordination/insertion of alkenes into Pd(II) complexes). Hartwig et al. also demonstrated, using a similar approach, phosphine-free active Pd intermediates containing small anionic ligands in the C−H cleavage step of direct arylation of benzene4 related to the Heck reaction. However, the nature of the Pd complexes participating in the step where aryl halides reacted (i.e., canonical oxidative addition to Pd(0) complexes) was not discussed. While some reports have speculated on the role of anionic Pd intermediates in cross-coupling reactions, there is almost no experimental evidence supporting such cases.5,6 In this study, we attempted to elucidate the nature of the catalytic intermediates in the Heck reaction with aromatic carboxylic anhydrides under ligand-free conditions. We believe this is a very attractive methodology in comparison to the traditional Heck reaction. Notably, an external base is not required for the reaction to proceed. Stoichiometric amounts of endogenous halide ions that would normally form under general Heck coupling are also © XXXX American Chemical Society

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RESULTS AND DISCUSSION Two types of competing experiments were carried out: reactions where two aromatic carboxylic anhydrides or two alkenes competed were used to investigate the nature of the active species of Pd(0) (in the anhydride oxidative addition step) and Pd(II) (in the alkene coordination/insertion step) (parts a and b of Scheme 1, respectively). In such experiments, the values of differential selectivity of the reactions were estimated using the phase trajectories of the competing reactions.10,11 Because the phase trajectory is the dependence on the concentration of the product of one of the competing substrate conversion reactions vs the product of another one, the slope of the phase trajectory represents the ratio of the rates Received: June 30, 2017

A

DOI: 10.1021/acs.organomet.7b00496 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

two carboxylic anhydrides (Scheme 1a) had no influence on the phase trajectories (Figure 1a). This result is consistent with the

Scheme 1. Competing Heck Reactions Using (a) Two Aromatic Carboxylic Anhydrides and (b) Two Alkenes

of competing reactions: i.e., it is definitely connected with the differential selectivity of the reaction.10 Therefore, plotting phase trajectories obtained in different competition experiments using the same pair of competing substrates makes it possible to directly track changes in differential selectivity by a simple comparison of the phase trajectories obtained in these experiments. It should be pointed out that any conjecture about the nature of active species in the reactions (Scheme 1) is based on the data obtained from traditional Heck reactions with aryl halides. However, the results obtained for the reaction with aryl halides cannot be applied directly to the reaction with carboxylic reagents because of the fundamental differences in substrate nature. It is well-known that the classical Heck reaction conditions (with aryl halides) produce large amounts of endogenous halide ions because of aryl halide conversion. These ions significantly influence the catalysis.12,13 On the other hand, in the Heck reaction with aromatic carboxylic anhydrides the addition of an inorganic halide salt increases the catalytic activity significantly.7 Further evidence of a strong effect of inorganic salts upon catalytic activity was reported.14 The nature of the halide ion (also supplied by the inorganic salt) can influence the composition of active complexes in phosphine-containing catalytic systems by substituting the benzoate ligands, as revealed by ex situ NMR probes of the reaction mixture.15 When PdCl2(PPh3)2 was selected as the catalyst precursor, it was observed that only Pd complexes containing chlorine existed in the reaction mixture over the course of the reaction, while a large excess of benzoate anions was formed owing to carboxylic anhydride conversion. Using the analogy of the phosphine-containing catalytic system, it is logical to suppose that the active Pd intermediates in our phosphine-free catalytic system with halide salt additives may also include halide ions. Therefore, their differential selectivity should depend on the nature but not on the amount of halide ions observed (considering that the amount of a salt used is sufficient to form Pd complexes saturated by halide ions). It follows from the data obtained that using different concentrations of LiCl for the competing reactions involving

Figure 1. Phase trajectories of competing arylation (a) of styrene by benzoic and 4-methoxybenzoic anhydrides using 0.5 mmol (◆), 2 mmol (□), and 3 mmol (×) of LiCl and (b) of styrene and n-butyl acrylate by benzoic anhydride using 0.5 mmol (◆), 1.4 mmol (●), and 2 mmol (△) of LiCl. Solid curves are plotted on the basis of integral equations assuming a first-order reaction in competing substrates (see the Experimental Section).

assumption about catalysis through halide-containing Pd complexes in the step where two anhydrides competed: i.e., their oxidative addition to Pd(0) complexes. The effect of LiCl concentration on the phase trajectories when two alkenes competed (Scheme 1b and Figure 2b) was also not observed, indicating the independence of the differential selectivity of the catalytic cycle step where two alkenes competed, i.e. in the coordination/insertion steps of alkenes, of the concentrations of chloride ions. It should be pointed out that the catalytic activity of the reaction at different concentrations of LiCl substantially varied. However, the invariability of differential selectivity allowed drawing an unequivocal conclusion about the identical nature of the active species. Therefore, the change in catalytic activity at varying LiCl concentrations is caused solely by the changes in amount of active species saturated by halide ions rather than changes in its nature. The opportunity to draw such a conclusion demonstrates the fundamental advantage of differential selectivity in comparison with the results obtained in catalytic activity investigations. Additionally, neither reaction time profiles nor the phase trajectories varied when sodium benzoate or benzoic acid additives were used, indicating the independence of both the catalytic activity and B

DOI: 10.1021/acs.organomet.7b00496 Organometallics XXXX, XXX, XXX−XXX

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Scheme 2. Formation of Tight Ion Pairs of the Anionic Complexes Conforming with the Influence of the Natures of both Anions and Cations on the Differential Selectivities of Common Intermediates in the (a) Competitive Oxidative Addition to Pd(0) Complexes and (b) Competitive Coordination/Insertion to Pd(II) Complexes in the Heck Reaction

that, when NaCl was used instead of LiCl, the phase trajectories of the competing reaction of two aromatic carboxylic anhydrides did not coincide, which reveals the strong influence of the alkali metal on the differential selectivity. Unfortunately, a weaker influence of the salt cation nature on the differential selectivity was observed when bromide salts were used (Figure 2a). However, it should be pointed out that the coincidence of the phase trajectories could be observed under both conditions: when the nature of the active species was maintained constant and when the reaction selectivity depended too weakly on the parameters. 10 However, the discrepancy of the phase trajectories when LiCl and NaCl were used unambiguously pointed to the influence of alkali metal on the reaction selectivity in the step when two aromatic carboxylic anhydrides competed: i.e., in oxidative addition. Since the alkali-metal cation cannot directly coordinate with the Pd center, the most reasonable explanation of such an influence on the relative reactivity of active species becomes possible when these species are anionic through electrostatic interaction in the ion pairs formed. Therefore, the rationale for the influence of both the cation and anion of the inorganic salt on the reaction selectivity when two anhydrides compete can be the anionic nature of active Pd species in the oxidative addition step. Analogous patterns were observed when two alkenes competed (Scheme 1b and Figure 2b). The phase trajectories of the reactions using NaBr and NaCl additives did not coincide, indicating the dependence of the differential selectivity of the catalytic cycle step (where two alkenes competed) on the nature of the halide ion: i.e., the entrance of halide anion in the active Pd(II) complex that participates in the coordination/insertion steps of alkenes. At the same time, the use of LiCl instead of NaCl and NaBr instead of NBu4Br unambiguously indicated the strong influence of the salt cation that cannot directly coordinate with Pd(II) on the differential selectivity of the reaction and, consequently, on the nature of the active species. Therefore, the active Pd complexes in the catalytic cycle step where alkene molecules competed, i.e., coordination/insertion, are also anionic. It was earlier revealed that the nature of both the cation and anion of the salt used influenced the catalytic activity in the Heck reaction with

Figure 2. Phase trajectories of competing arylation (a) of styrene by benzoic and 4-methoxybenzoic anhydrides using 0.5 mmol of LiCl (◆), NaCl (△), NaBr (●), LiBr (○), and NBu4Br (■) and (b) of styrene and n-butyl acrylate by benzoic anhydride using 0.5 mmol of LiCl (◆), NaBr (○), NaCl (■), and NBu4Br (▲). Solid curves are plotted on the basis of integral equations assuming a first-order reaction in competing substrates (see the Experimental Section).

the differential selectivity respectively on the presence of the additives and consistent with the results of an ex situ NMR study of phosphine-containing catalytic systems.15 The results obtained (Figure 1) are consistent with the previous assumption about catalysis through halide-containing Pd complexes. However, that assumption was based on the results of model experiments only: i.e., not under real catalytic conditions.15 In contrast to the influence of the salt amount, the variation of the salt nature changed the phase trajectories of competing Heck reactions using the same catalyst precursor (Figure 2a,b): i.e., the differential selectivity of the reaction was sensitive to the added salt nature. Furthermore, the differential selectivity was dependent on both the nature of the halide ion and the nature of the cation present in the salt. For instance, when NaCl was used instead of NaBr in the competing reaction of benzoic and 4-methoxybenzoic anhydrides (Figure 2a), the phase trajectories of the reaction did not coincide. Thus, the strong influence of the nature of the halide ion in the inorganic salt on the differential selectivity was revealed in the competing reactions involving anhydrides (Scheme 2a). This pattern unequivocally indicates the entrance of the halide anion in the active Pd complex and its participation in the catalytic cycle step where two anhydrides competed: i.e., their oxidative addition to Pd(0) complexes. It also follows from Figure 2a C

DOI: 10.1021/acs.organomet.7b00496 Organometallics XXXX, XXX, XXX−XXX

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differential selectivity is evidence for the anionic nature of active Pd intermediates forming tight ion pairs with cations in the course of the reaction. Thus, the data obtained point to the reaction proceeding through both the Pd(0) and Pd(II) anionic species present in the reaction catalytic cycle.

carboxylic anhydrides, while the integral selectivity of the reaction (i.e., the ratio of α and β regioisomers of the reaction product, Scheme 1) was influenced by the nature of the halide ion only.14 However, these data on catalytic activity as well as on integral selectivity did not allow us to draw a conclusion about the anionic character of the active species. The catalytic activity patterns do not allow making such conclusions because the catalytic activity could vary owing to any change in the amount of the active species when salt cation and anion concentrations are varied: for instance, due to their different solubilities and/or degrees of dissociation. In contrast, the measurement of differential selectivity of the reaction on competing substrates helped us unambiguously establish for the first time that the properties of active palladium complexes formed using a ligandless (i.e., without use of phosphine additives) catalytic system are determined not only by the anions present in the Pd coordination sphere but also by the cations that cannot coordinate with Pd(0) and Pd(II) directly. On the basis of the known Pd chemistry, this situation becomes possible if just anionic (neither neutral nor cationic) Pd complexes exist. The alkali metal and ammonium cations can influence the reaction selectivity, for instance, by forming tight ion pairs of the type [Pd(0) or Pd(II) anionic complex]−− [cation]+ (parts a and b of Scheme 2). It should be noted that, in spite of existing evidence for the Mizoroki−Heck reaction occurring through a homogeneous mechanism only by participation of Pd molecular complexes,10,12 it is impossible to exclude the hypothesis about of catalysis of the reaction with aromatic carboxylic anhydrides proceeding on the surface of Pd colloidal particles formed in situ. However, according to this assumption the data obtained allow us to draw a conclusion about the anionic character of active Pd species situated on the surface of colloidal particles. There was no doubt about the existence of anionic Pd(II) complexes in cross-coupling reactions with aryl halides using ligand-free catalytic systems, where a large amount of endogenous halide anions is present.12,13 Such complexes were observed under real catalytic conditions in the Heck reaction using ex situ EXAFS or mass spectrometry16,17 as well as by in situ UV−vis spectroscopy.18 However, the participation of anionic Pd(II) complexes in the coordination/insertion step was demonstrated on the basis of a comparative study of integral selectivities of stoichiometric and real catalytic reactions only.3 Only indirect evidence for the participation of anionic Pd(0) complexes in the catalytic cycle as active species in the oxidative addition step was published until recently;1,2 for the ligand-free catalytic systems, such data were absent in the literature previously. The influence of cations on the differential selectivity when two carboxylic anhydrides compete (Figure 2a) for reaction with Pd(0) compounds (Scheme 2a) to our knowledge is the first experimental evidence for the direct involvement of anionic Pd(0) complexes in catalysis. In summary, the data indicate that the differential selectivity of competing Heck reactions with aromatic carboxylic anhydrides depends on the nature of both the cation and halide anions. The dependence of differential selectivity under competition of either aromatic carboxylic anhydrides or alkenes on the nature of the salt anion together with its independence of the salt concentration unambiguously indicate the presence of these anions in the coordination sphere of active Pd(0) and Pd(II) intermediates in the catalytic cycle. The effect of cations being unable to directly coordinate with the Pd center on

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EXPERIMENTAL SECTION

General Considerations. The quantitative compositions of the samples were determined using gas chromatography (GC) (HP 4890 instrument fitted with a flame ionization detector (FID) and 15 m HP5 methyl phenyl siloxane capillary column) and GC/mass spectrometry (MS) (Shimadzu GC-MS QP-2010 Ultra, ionization energy of 70 eV, 0.25 μm × 0.25 mm × 30 m GsBP-5MS column, He as the carrier gas) analyses. The recorded mass spectra were compared with those available in the literature (Wiley, NIST, and NIST05 comparison libraries). Samples for GC and GC-MS analyses were collected at different reaction time points. To estimate the reproducibility of the data, each experiment was performed three times. Mizoroki−Heck Reaction Using Two Competing Aromatic Carboxylic Anhydrides. Benzoic and 4-methoxybenzoic anhydrides (2.5 mmol each), styrene (5 mmol), PdCl2 (0.08 mmol, 1.6 mol %), LiCl (0.5−3 mmol), and naphthalene (1 mmol) as an internal standard for GC and GC-MS analyses were added to 5 mL of Nmethyl-2-pyrrolidone (NMP) in a glass reactor equipped with a magnetic stir bar and a septum inlet. The reactor was placed into a preheated oil bath (140 °C), and the reaction mixture was stirred. To investigate the influence of the halide salt nature on the differential selectivity, LiCl was substituted by NaCl, NaBr, LiBr, and NBu4Br (0.5 mmol). Mizoroki−Heck Reaction Using Two Competing Alkenes. Styrene and n-butyl acrylate (2.5 mmol each), benzoic anhydride (5 mmol), PdCl2 (0.08 mmol, 1.6 mol %), LiCl (0.5−2 mmol), and naphthalene (1 mmol) as an internal standard for GC and GC-MS analyses were added to 5 mL of NMP in a glass reactor equipped with a magnetic stir bar and a septum inlet. The reactor was placed into a preheated oil bath (140 °C), and the reaction mixture was stirred. To investigate the influence of the halide salt nature on the differential selectivity, LiCl was substituted by NaCl, NaBr, and NBu4Br (0.5 mmol). Fitting of the Experimental Data Obtained in the Competitive Reactions. The relative reactivities of the two competing substrates could easily be determined assuming a firstorder reaction in each substrate.10 In this case, the rate ratio of competing reactions is expressed by the equation

k [X ][S2] rP2 k [S2] = 2 com = 2 rP1 k1[X com][S1] k1 [S1] where [S1] and [S2] are the concentrations of the two competing substrates and [Xcom] is the concentration of the common catalytic cycle intermediate reacting with the substrates. Then

k [X ][S2] rP2 k [S2] d[S2]/dt = = 2 com = 2 rP1 d[S1]/dt k1[X com][S1] k1 [S1] k [S2] d[S2] = 2 d[S1] k1 [S1] [S2]

∫[S2]

0

k d[S2] = 2 [S2] k1

[S1]

∫[S1]

0

d[S1] [S1]

⎛ [S2] ⎞ k 2 ⎛ [S1] ⎞ ln⎜ ⎟ = ln⎜ ⎟ k1 ⎝ [S1]0 ⎠ ⎝ [S2]0 ⎠ where [S1]0 and [S2]0 are the initial concentrations of S1 and S2, respectively. The integral dependence of [S1] on [S2] will be D

DOI: 10.1021/acs.organomet.7b00496 Organometallics XXXX, XXX, XXX−XXX

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Organometallics [S1] =

[S1]0 k rel

([S2]0 )

(5) Schroeter, F.; Soellner, J.; Strassner, T. ACS Catal. 2017, 7, 3004−3009. (6) Guest, D.; Menezes Da Silva, V. H.; De Lima Batista, A. P.; Roe, S. M.; Braga, A. A. C.; Navarro, O. Organometallics 2015, 34, 2463− 2470. (7) Stephan, M. S.; Teunissen, A. J. J. M.; Verzijl, G. K. M.; de Vries, J. G. Angew. Chem., Int. Ed. 1998, 37, 662−664. (8) Goossen, L. J.; Rodríguez, N. Chem. Commun. 2004, 10, 724− 725. (9) Goossen, L. J.; Paetzold, J. Angew. Chem., Int. Ed. 2002, 41, 1237−1241. (10) Schmidt, A. F.; Kurokhtina, A. A.; Larina, E. V. Catal. Sci. Technol. 2014, 4, 3439−3457. (11) Schmidt, A. F.; Kurokhtina, A. A.; Larina, E. V. Kinet. Catal. 2012, 53, 84−90. (12) Schmidt, A. F.; Al Halaiqa, A.; Smirnov, V. V. Synlett 2006, 2006, 2861−2878. (13) de Vries, J. G. Dalton Trans. 2006, 421−429. (14) Shmidt, A. F.; Smirnov, V. V. Kinet. Catal. 2000, 41, 743−744. (15) Shmidt, A. F.; Smirnov, V. V. Kinet. Catal. 2002, 43, 195−198. (16) Evans, J.; O’Neill, L.; Kambhampati, V. L.; Rayner, G.; Turin, S.; Genge, A.; Dent, A. J.; Neisius, T. J. Chem. Soc., Dalton Trans. 2002, 2207−2212. (17) de Vries, A.; Parlevliet, H. M. F. J.; Schmieder-van de Vondervoort, L.; Mommers, J. H. M.; Henderickx, H. J. W.; Walet, M. A. M.; de Vries, J. G. Adv. Synth. Catal. 2002, 344, 996−1002. (18) Schmidt, A. F.; Al-Halaiqa, A.; Smirnov, V. V.; Kurokhtina, A. A. Kinet. Catal. 2008, 49, 638−643.

[S2]k rel

where krel is the relative reactivity of the two competing substrates (i.e., k1/k2). Assuming that S1 and S2 convert into the products P1 and P2 only, it is properly valid that

[P1] = [S1]0 − [S1] [P2] = [S2]0 − [S2] Then the integral dependence of [P1] on [P2] on the phase trajectories will be

⎛ [S1]0 ⎞ ⎟⎟([S2]0 − [P2])k rel [P1] = ⎜⎜[S1]0 − ([S2]0 )k rel ⎠ ⎝ The krel values of the competitive reactions (Table 1) were estimated by minimizing deviations between the calculated and

Table 1. Calculated krel Values of the Competitive Reactions krel

figure

anhydrides

LiCl (0.5) LiCl (2.0) LiCl (3.0)

4.4

Figure 1a

alkenes

LiCl (0.5) LiCl (1.4) LiCl (2.0)

0.16

Figure 1b

anhydrides

LiCl (0.5) LiBr (0.5) NaBr (0.5) NaCl (0.5) NBu4Br (0.5)

4.4 3.2 2.6 2.3 2.5

Figure 2a

alkenes

LiCl (0.5) NaCl (0.5) NaBr (0.5) NBu4Br (0.5)

0.16 0.36 0.13 0.08

Figure 2b

competing substrates

additive (mmol)

observed product concentrations by standard nonlinear function minimization methods (Microsoft Excel 2002, Solver addin).

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AUTHOR INFORMATION

Corresponding Author

*E-mail for A.F.S.: [email protected]. ORCID

Alexander F. Schmidt: 0000-0002-1881-7620 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Government Assignment from the Ministry of Education and Science of the Russian Federation (No. 4.9489.2017/8.9) and Russian Foundation for Basic Research (16-29-10731 ofi_m).

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REFERENCES

(1) Amatore, C.; Came, E.; Jutand, A.; M’Barki, M. A.; Meyer, G. Organometallics 1995, 14, 5605−5614. (2) Amatore, C.; Jutand, A. J. Organomet. Chem. 1999, 576, 254−278. (3) Carrow, B. P.; Hartwig, J. F. J. Am. Chem. Soc. 2010, 132, 79−81. (4) Tan, Y.; Hartwig, J. F. J. Am. Chem. Soc. 2011, 133, 3308−3311. E

DOI: 10.1021/acs.organomet.7b00496 Organometallics XXXX, XXX, XXX−XXX