Article pubs.acs.org/Organometallics
Mechanism of Intramolecular Rhodium- and Palladium-Catalyzed Alkene Alkoxyfunctionalizations Sai Vikrama Chaitanya Vummaleti,† Miasser Al-Ghamdi,†,§ Albert Poater,‡ Laura Falivene,† Jessica Scaranto,§ Dirk J. Beetstra,§ Jason G. Morton,§ and Luigi Cavallo*,† †
KAUST Catalysis Center, Physical Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Saudi Arabia ‡ Institut de Química Computacional i Catàlisi and Departament de Química, Universitat de Girona, Campus Montilivi, 17071 Girona, Catalonia, Spain § SABIC CRI, Fundamental Catalysis, Thuwal 23955-6900, Saudi Arabia S Supporting Information *
ABSTRACT: Density functional theory calculations have been used to investigate the reaction mechanism for the [Rh]-catalyzed intramolecular alkoxyacylation ([Rh] = [RhI(dppp)+] (dppp, 1,3bis(diphenylphosphino)propane) and [Pd]/BPh3 dual catalytic system assisted intramolecular alkoxycyanation ([Pd] = PdXantphos) using acylated and cyanated 2-allylphenol derivatives as substrates, respectively. Our results substantially confirm the proposed mechanism for both [Rh]- and [Pd]/ BPh3-mediated alkoxyfunctionalizations, offering a detailed geometrical and energetical understanding of all the elementary steps. Furthermore, for the [Rh]-mediated alkoxyacylation, our observations support the hypothesis that the quinoline group of the substrate is crucial to stabilize the acyl metal complex and prevent further decarbonylation. For [Pd]/BPh3-catalyzed alkoxycyanation, our findings clarify how the Lewis acid BPh3 cocatalyst accelerates the only slow step of the reaction, corresponding to the oxidative addition of the cyanate O−CN bond to the Pd center.
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INTRODUCTION One of the relevant topics in catalysis is the development of protocols more effective than those currently available to the industry. This is particularly important in the synthesis of heterocycles,1 because the relatively small production volumes allow advances in catalysis to be rapidly adopted by the industry, with reduced economic and technological impact. Among variously functionalized heterocyclic structures, substituted dihydrobenzofurans occupy a relevant position because they constitute the core skeleton found in a broad array of biologically active natural and pharmacological molecules.1,2 Many of the proposed classical approaches, which employ intramolecular SN2 reactions, suffer from the major drawback of only one bond formed during the ring closure. In recent years, transition metal catalyzed alkene difunctionalization reactions3 provided one of the most powerful strategies for the synthesis of dihydrobenzofurans and other related oxygen heterocycles. These reactions enable the formation of carbon−carbon/ carbon−heteroatom bonds in a single step from simple alkene precursors.4,5 Among the possible reported reactivity patterns, the one shown in Scheme 1, consisting in the intramolecular insertion of a CC double bond into the O−FG bond, is of particular interest since it can lead to a variety of useful products. In this context, two new synthetic methods for metalcatalyzed alkene difunctionalization reactions have been © 2015 American Chemical Society
Scheme 1. Synthetic Strategy for the Synthesis of Heterocycles, Based on Intramolecular Alkene Alkoxyfunctionalization
reported, viz., the Rh-catalyzed intramolecular alkoxyacylation of acylated 2-allylphenol derivatives6,7 and the Pd-catalyzed intramolecular alkoxycyanation of cyanated 2-allylphenol derivatives,8 respectively (Scheme 2). These methods involve the activation of the C−O bond of the substrate by the catalyst and finally lead to the formation of dihydrobenzofuran derivatives containing ketones or nitriles as functional groups.9 From a mechanistic perspective, both the intramolecular alkoxyacylation and alkoxycyanation reactions appear to proceed through the reaction pathway shown in Scheme 3.10 In the first step of the catalytic cycle, the O−FG (FG = functional group) bond undergoes oxidative addition to the low-valent metal catalyst to generate a metal alkoxide intermediate.11 The next step corresponds to the migratory insertion of the alkene into the M−O bond of the metal Received: September 5, 2015 Published: November 13, 2015 5549
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stationary points was performed by analytical frequency calculations. For better energetics, energies were re-evaluated via single-point calculations on the BP86/SVP geometries with the triple-ζ plus one polarization function basis set proposed by Ahlrichs (TZVP keyword in Gaussian)17 using the M06 functional.18 Solvent effects were estimated with the polarizable continuum solvation model PCM using 1,2-dicholorethane (for alkoxyacylation reaction) and tetrahydrofuran (for alkoxycyanation reaction) as solvents.19 With this M06/TZVP electronic energy in solvent, zero point energy and thermal corrections were included from the gas-phase frequency calculations at the BP86/ SVP level.20 To check for an impact of dispersion interactions on geometries, we focused on the key step corresponding to oxidative addition of the substrate to the Rh catalyst, from intermediate [Rh]-I to the following transition state TS-I of Figure 1. Geometry optimization was performed adding the Grimme D3 dispersion term. The forming R−C and Rh−O bonds and the breaking C−O bonds are within 0.01 Å from the same distances obtained in the absence of the dispersion term (see Table 1), indicating that dispersion interactions have a negligible impact on the geometry of these systems. Further, the energy difference of 5.04 kcal/mol between transition state TS-I and intermediate [Rh]-I evaluated using the BP86 geometries, as in Figure 1, is increased by only 0.22 kcal/mol, for a final value of 5.26 kcal/mol, when BP86-D3 geometries, ZPEs, thermal corrections and entropy effects are used.
Scheme 2. Schematic Representation of the Experimental Procedure for the Rh-Catalyzed Intramolecular Alkene Alkoxyacylation and Pd-Catalyzed Alkene Alkoxycyanationa
a
cod = 1,5-cyclooctadiene; dba = dibenzylideneacetone.
Scheme 3. Mechanism of Intramolecular Alkene Alkoxyfunctionalization Reactionsa
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a
RESULTS AND DISCUSSION Figure 1 reports the energy profile for the Rh-catalyzed oxyacylation reaction with acylated 2-allylphenol derivative 1a as the substrate. The geometry of the key transition states is reported in Figure 2. The reaction starts from the positively charged Rh(cod)2 precatalyst species [Rh(cod)2]+. In the presence of dppp, a cod ligand of [Rh(cod)2]+ can be replaced to give the [Rh(dppp)(cod)]+ species. Displacement of a cod ligand is highly exothermic, with a release of 27.2 kcal/mol. In principle, also the second cod ligand might be replaced by another dppp ligand, but, for the sake of simplicity, we considered replacement of the cod ligand of [RhI(dppp)(cod)]+ by the substrate 1a, leading to the coordination intermediate [Rh]-I ([Rh] = [RhI(dppp)+]), in which the metal coordinates to the quinoline N atom of substrate 1a. The substrate-bound intermediate [Rh]-I is 13.5 kcal/mol above the [Rh(dppp)(cod)]+ and 1a. Then, we explored oxidative addition of the acyl C−O bond to the Rh metal, leading to the acyl metal complex [Rh]-II-Cis. This step requires the overcoming of a barrier of 5.1 kcal/mol above [Rh]-I (or an overall barrier of 18.6 kcal/mol above [Rh(dppp)(cod)]+ species and 1a). Bearing in mind that the experiments indicated the presence of chelating groups bearing a heteroatom fundamental to achieve reactivity,9b for instance, the quinoline moiety, we investigated the oxidative addition step for a hypothetical substrate 1a′, in which the N atom of quinoline is substituted by a −CH group (see Figure S1 in the Supporting Information, SI).21 In agreement with the experiments and with the initial hypothesis on the beneficial role of quinoline, our results show that the hypothetical acyl metal complex [Rh]-II′Cis is 18.9 kcal/mol less stable when compared with complex [Rh]-II-Cis of Figure 1. Additionally, from complex [Rh]-IICis, the CO dissociation step is kinetically much more demanding than from the complex [Rh]-II′-Cis (43.6 vs 22.4 kcal/mol, respectively). This confirms that coordination of quinoline assists the oxidative addition and would indeed prevent the decarbonylation by stabilizing the acyl metal complex.22
FG = functional group, M = metal.
alkoxide intermediate.12 Finally, the formation of the C−FG bond by reductive elimination gives the desired functionalized heterocyclic products and regenerates the metal catalyst. Despite the elegance of the two methods, they both suffer from some drawbacks. In the case of the Rh-catalyzed reaction, good yields are obtained only with substrates presenting a substituted alkene functionality, and the aromatic moiety must be a quinoline; see Scheme 2. As for the Pd-catalyzed reaction, a Lewis acid such as BPh3 has to be used to trigger the reaction. In this context, we believe that understanding the details of the whole reaction mechanism could help the future development of this versatile strategy. To this end we performed a DFT analysis and we investigated both the Pd and Rh systems within a single and consistent computational protocol, to provide an illuminating comparison. We believe that our work offers interesting insight into these systems, since we addressed both the working catalyst and substrate, as well as those structural modifications of the substrate that result in no catalytic activity.
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COMPUTATIONAL DETAILS
All the DFT calculations were performed at the GGA level with the Gaussian09 set of programs,13 using the BP86 functional of Becke and Perdew.14 The electronic configuration of the molecular systems was described with the split-valence plus one polarization function basis set of Ahlrichs for H, B, C, N, O, and P (SVP keyword in Gaussian).15 For Rh and Pd we used the small-core, quasi-relativistic Stuttgart−Dresden effective core potential, with an associated valence basis set (SDD keywords in Gaussian09).16 Geometry optimizations were performed without symmetry constraints, and the characterization of the located 5550
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Figure 1. Computed stationary points for the Rh-catalyzed intramolecular alkoxyacylation reaction with the acylated 2-allylphenol derivative as a substrate (free energies in solution are given in kcal/mol).
II-Cis. The next step corresponds to migratory insertion of the alkene into the Rh−O bond (ring-closure step), leading to stable intermediate [Rh]-III. This is a rather low energy step, with a barrier of only 4.9 kcal/mol. The penultimate step corresponds to the C−C bond formation of the reductive elimination step, leading to intermediate [Rh]-IV, with the product 2a coordinated to the [Rh] center. This step is predicted to be the rate-determining step with an estimated barrier of 21.7 kcal/mol. As a final step, [Rh]-IV would release the product 2a, regenerating the [Rh] catalyst, ready to coordinate the second molecule of 1a, and thus closing the catalytic cycle. To summarize, the studied [Rh]-mediated alkoxyacylation reaction mechanism is thermodynamically favorable and kinetically facile when considering the remarkably high temperature of 150 °C needed experimentally to achieve a yield of 2a of at least 90%. For terms of comparison, we now turn to characterize the reaction mechanism presented in Scheme 3 for the [Pd]/BPh3catalyzed intramolecular alkoxycyanation reaction with the cyanate 2-allylphenol derivative (1b) as a substrate, and the corresponding energy profile is shown in Figure 3. The geometry of the key transition structures is presented in Figure 4. Synthetically, the reaction starts from the neutral Pdxantphos species ([Pd]), M. Coordination of substrate 1b to the Pd atom in M via the BPh3 cocatalyst activated the CN bond, leading to intermediate [Pd]-I, which lies 8.5 kcal/mol below M (for the sake of simplicity, we considered the substrate already bound to the BPh3 cocatalyst).23 The next step corresponds to the oxidative addition of the O−CN bond of the substrate to the Pd atom, leading to the stable intermediate [Pd]-II. This step is predicted to be the rate-determining step, with an estimated barrier of 14.2 kcal/mol. From an energy
Table 1. Value of Forming and Breaking Bonds in Transition State TS-I of Figure 1, with and without Dispersion Interactions Added to the BP86 Functional (Distances in Å) level of theory
Rh−C
Rh−O
C−O
BP86 BP86-D3
2.132 2.130
2.261 2.271
1.681 1.687
Figure 2. Chemical structures of computed high-energy transition states for the Rh-catalyzed intramolecular oxyacylation reaction: (a) [Rh]-I-II-Cis corresponds to acyl C−O bond cleavage and (b) [Rh]III-IV corresponds to C−C bond formation (selected distances are in Å; the imaginary frequencies characterizing the transition states are given in brackets).
Next, isomerization of [Rh]-II-Cis leads to the less stable intermediate [Rh]-II-Trans, which lies 6.3 kcal/mol above [Rh]-II-Cis and requires a barrier of 9.2 kcal/mol above [Rh]5551
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Figure 3. Computed stationary points for the Pd/BPh3-catalyzed intramolecular alkoxycyanation reaction with the cyanate 2-allylphenol derivative as the substrate (free energies in solution are given in kcal/mol).
closure step), leading to intermediate [Pd]-IV. This is a rather low energy step, with a barrier of only 0.6 kcal/mol. The final step corresponds to the formation of a new C−C bond ([Pd]IV → [Pd]-V) with an estimated barrier of only 12.0 kcal/mol. The calculated lower barrier heights for palladium when compared with rhodium are quite reasonable as the oxycyanation reaction with Pd is carried out experimentally at a temperature of 60 °C to achieve a yield of at least 90%.
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CONCLUSIONS In summary, we have reported a theoretical study describing the mechanism for the [Rh]-mediated intramolecular alkoxyacylation and [Pd]/BPh3-mediated intramolecular alkoxycyanation reactions using DFT calculations. Our results suggest that the investigated mechanism is favored thermodynamically, and it is kinetically easy for both rhodium and palladium catalysts. For the Rh-catalyzed reaction the rate-determining step corresponds to the reductive elimination step (with a barrier of 21.7 kcal/mol), with the initial oxidative addition presenting a barrier only 3.1 kcal/mol lower. On the other hand, for the Pd-catalyzed reaction the rate-determining step corresponds to the oxidative addition step (with a barrier of 14.2 kcal/mol), with the final reductive elimination step presenting a barrier only 2.0 kcal/mol lower. Overall, for the Pd-catalyzed reaction, the presented energy profile is extremely smooth, with the estimated barriers well below 15 kcal/mol. Calculations also indicated that the presence of chelating groups such as quinoline in the substrate is fundamental to stabilize the Rh-acyl complex via coordination of the quinoline N atom to the Rh cener and by suppressing decarbonylation. In the case of the Pd-based reaction, the BPh3 cocatalyst is fundamental to facilitate the only energy-demanding step, which is the oxidative addition of the O−CN bond to the Pd center.
Figure 4. Chemical structures of computed high-energy transition states for Pd/BPh3-catalyzed intramolecular oxycyanation reaction: (a) [Pd]-I-II corresponds to acyl C−O bond cleavage and (b) [Pd]IV-V corresponds to C−C bond formation (selected distances are in Å; the imaginary frequencies characterizing the transition states are given in brackets).
point of view, [Pd]-II lies 14.4 kcal/mol below [Pd]-I. It is worth mentioning here that to better understand the cooperative catalysis of [Pd]/BPh3 we studied the oxidative addition step in the absence of the BPh3 cocatalyst. Our results show that the predicted barrier of the oxidative addition step is ∼10.0 kcal/mol higher in energy than the corresponding barrier in the presence of BPh3. This observation supports the role of BPh3 as crucial for the oxidative addition of O−CN bonds to the Pd metal by covalent coordination to the cyano group of the substrate 1b.24 Next, structural rearrangement of [Pd]-II leads to the less stable intermediate [Pd]-III, which lies 11.6 kcal/mol above [Pd]-II. The barrier for this step is predicted to be 9.7 kcal/mol above [Pd]-II. The next step corresponds to migratory insertion of the alkene into the Pd−O bond (ring5552
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Referring to the catalytic cycle of Scheme 3, our results suggest that in both cases insertion of the CC moiety into the metal−O bond is an extremely easy step, with barriers only around 10 kcal/mol. The initial oxidative addition of the O-FG group to the metal is a very easy step, with a barrier only around 5 kcal/mol for the Rh system, while it is slightly more demanding, with a barrier around 14 kcal/mol for the Pd system. On the contrary, reductive elimination of the product is energetically more expensive for the Rh system, with a barrier around 22 kcal/mol, while it is smooth for the Pd system, with a barrier around 12 kcal/mol. In conclusion, our work highlights that for these systems the intramolecular alkoxyfunctionalization strategies developed present rather smooth energy profiles, and it can be used as a guideline for developing new strategies in the field.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00749. Role of quinoline in the oxidative addition step; Cartesian coordinates and energies of all the species discussed in this work (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS S.V.C.V. and L.F. thank SABIC for a fellowship Award. A.P. thanks the Spanish MINECO for project CTQ2014-59832-JIN and the European Commission for a Career Integration Grant (CIG09-GA-2011-293900). This research was supported by the King Abdullah University of Science and Technology.
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