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Transmetallation Versus Hydride Elimination

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Transmetallation Versus Hydride Elimination DOI: 10.1002/chem.201102678 Transmetallation Versus b-Hydride Elimination: The Role of 1,4- Benzoquinone in Chelation-Controlled Arylation Reactions with Arylboronic Acids Christian Skçld,*[a] Jonatan Kleimark,[b] Alejandro Trejos,[a] Luke R. Odell,[a] Sten O. Nilsson Lill,[b] Per-Ola Norrby,[b] and Mats Larhed[a] Abstract: The formation of an atypical, um, which is necessary to complete the complex. The association of BQ lowers saturated, diarylated, Heck/Suzuki, catalytic cycle, this electron-deficient the free-energy barrier for transmetal- domino product produced under oxida- alkene opens up a low-energy reaction lation of the s-alkyl complex to create tive Heck reaction conditions, employ- pathway from the post-insertion s-alkyl a pathway that is energetically lower ing arylboronic acids and a chelating than the oxidative Heck reaction path- vinyl ether, has been investigated by way. Furthermore, the calculations Keywords: arylation · CÀC cou- DFT calculations. The calculations showed that the reaction is made pling · density functional calcula- highlight the crucial role of 1,4-benzo- viable by BQ-mediated reductive elimi- tions · elimination · palladium · quinone (BQ) in the reaction. In addi- nation and leads to the saturated di- transmetallation tion to its role as an oxidant of palladi- arylatedACHTUNGRE product. Introduction step of the reaction, it was initially performed by using stoi- ACHTUNGRE chiometric amounts of Pd(OAc)2. More recent develop- The Mizoroki–Heck reaction[1] is a convenient and versatile ments have rendered the reaction catalytic by the re-oxida- method for the formation of carbon–carbon bonds through tion of Pd0 to PdII and today the oxidative Heck reaction is the vinylation or arylation of alkenes.[2] The reaction is per- an established and useful variant of the vinylic substitution formed by Pd0 catalysis in which the active PdII–vinyl or reaction.[4] –aryl intermediate is produced by oxidative addition of aryl One strategy to achieve regio- and stereoselectivity in the halides or pseudo-halides. Oxidative addition is followed by Mizoroki–Heck reaction has been by chelation-control, ex- migratory insertion of the alkene to produce a s-alkyl com- ploiting an auxiliary group to achieve a pseudo-intramolecu- plex and b-hydride elimination from this complex leads to lar reaction pathway.[2b,5] By using the nitrogen-based chelat- the unsaturated product. Finally, the catalytic cycle is com- ing vinyl ether 1 (Scheme 1), Hallberg and co-workers de- pleted from the hydridopalladium species by regenerating veloped a highly regioselective terminal substitution reac- Pd0 by reductive elimination in the presence of a base. The tion.ACHTUNGRE [5a] Whilst investigating a PdII catalytic protocol for the PdII-mediated version of the reaction with an organoboron reaction (Scheme 1), employing arylboronic acids as PdII– compound to form the crucial PdII–vinyl intermediate by aryl precursors, we found that the unprecedented, saturated, transmetallation was reported by Dieck and Heck in 1975.[3] diarylated product 4 was formed under certain reaction con- Because hydridopalladium is still formed in the terminal ditions (Scheme 1).[6] Subsequently, efficient protocols, which employ both electron-rich and -poor arylboronic acids, have been developed.[6,7] [a] Dr. C. Skçld, A. Trejos, Dr. L. R. Odell, Prof. M. Larhed Organic Pharmaceutical Chemistry Department of Medicinal Chemistry Uppsala University, BMC P.O. Box 574, SE-751 23 Uppsala (Sweden) Fax : (+46)18-4714474 E-mail: [email protected] [b] J. Kleimark, Dr. S. O. Nilsson Lill, Prof. P.-O. Norrby Department of Chemistry, University of Gothenburg Kemigrden 4, 8076, SE-412 96 Gçteborg (Sweden) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201102678. Re-use of this article is permitted in accordance with the Terms and Conditions set out at http://onlinelibrary.wiley.com/journal/10.1002/ACHTUNG- (ISSN)1521-3765/homepage/2111_onlineopen.html.TRENUG Scheme 1. 4714 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2012, 18, 4714 – 4722 FULL PAPER Chelation was shown to be vital for producing the saturat- Results and Discussion ed diarylated product because non-chelating alkyl vinyl ethers only yielded the Heck product. Interestingly, the This investigation was focused on the reaction pathway fol- choice of re-oxidant was also shown to be essential for the lowing the first transmetallation step because the resulting reaction outcome as only reactions with 1,4-benzoquinone PdII–Ph complex will be formed regardless of the subse- (BQ) as re-oxidant gave this atypical product. The forma- quent reaction path. Only neutral complexes were consid- tion of the saturated diarylated product 4 is consistent with ered, because no additives promoting the cationic pathway the interception of the s-alkyl complex formed after migra- (e.g., bidentate ligands) were present, and the reaction was tory insertion by a second transmetallation and finally prod- performed in a non-polar solvent (dioxane). Potential uct formation by reductive elimination, in essence the latter bridged dimers of the complexes were excluded from the ACHTUNGRE part of the Suzuki–Miyaura reaction (Scheme 2). However, computational study. The computationally simple Pd(OAc)2 was used in the calculations and this is an experimentally functional PdII source, although the preparative optimisation revealed trifluoroacetate as the optimal PdII source.[6,7] Unless otherwise stated the B3LYP-D3 free energies pre- ACHTUNGRE sented herein are given relative to Pd(OAc)2 associated to 1 (complex I, Figure 1) because this complex was considered to be a suitable starting point for the reaction. A dashed line in the free-energy profiles indicates that the specific pathway for the connection (e.g., reactant to transition state) was not investigated. Migratory insertion: Migratory insertions leading to both in- ternal and terminal phenylation were computationally inves- tigated, forming the s-alkyl complexes, which after subse- quent b-hydride elimination render 2 or 3, respectively. The transition state (TS) for internal phenylation (TS-a)was found to be 25 kJmolÀ1 higher in energy than the TS for ter- minal phenylation (TS-b). The free-energy profile is present- ed in Figure 1 and is in agreement with the experimental outcome of the reaction when the oxidative Heck product is Scheme 2. Hypothetic catalytic cycle for the production of 4 from 1 and formed, given that when chelation from the alkene is feasi- an arylboronic acid (outer circle). Simplified catalytic cycle for the oxida- ble, that is, no strongly coordinating bidentate ligand is pres- tive Heck reaction producing 3 is also shown (top half). ent in the reaction mixture, the terminal product is formed with a 3/2 selectivity of >50:1.[6] This is equivalent to the se- lectivity obtained with the Pd0-catalysed Mizoroki–Heck re- such a reaction pathway would require suppression of the action.[5a,11] Accordingly, the pathway leading to internal competing b-hydride elimination from the s-alkyl complex. phenylation was not further investigated. After migratory Suppression of this process has been achieved by allylic sta- insertion, the free energy of the s-alkyl complex is lowered bilisation of the s-alkyl complex in the similar diarylation of when the acetate counterion changes binding mode from conjugated terminal alkenes by using organostannanes as mono- to bidentate (IV to V, Figure 1). the aryl source.[8] Allylic stabilisation is not possible with 1 and some other mechanism must operate to suppress the b- h2-BQ association: The post-insertion complex V is a hydride elimination. This intrigued us and we decided to fur- square-planar PdII complex without any empty acceptor or- ther investigate this reaction. Thus, the aim of this study was bitals available for bonding. Thus, it will not coordinate ad- to investigate the novel reaction pathway from 1 to 4 ditional dative ligands. However, in this study we found that through the use of DFT calculations with a focus on eluci- complex V could associate with a h2-BQ ligand[12] to give dating the crucial role of BQ in the reaction. The reaction complex VI with an approximate trigonal-bipyramidal ge- can be seen as a combination of the oxidative Heck reaction ometry (Figure 2). This finding led us to investigate the and the Suzuki–Miyaura reaction, for which an extensive list nature of the Pd–BQ bond in more detail. A natural bond of theoretical investigations is available that have provided orbital (NBO) analysis[13] revealed a very strong donation– a profound mechanistic insight into these processes as well back-donation situation in the second-order perturbation as the Mizoroki–Heck reaction.[4c,9,10] analysis. In fact, the energy contribution calculated for the donation from BQ to palladium was similar in magnitude to that from each acetate oxygen atom, and about twice that of the donation (dative bond) from the nitrogen lone pair. In addition, the back donation from a filled d orbital on palla- Chem. Eur. J. 2012, 18, 4714 – 4722 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.chemeurj.org 4715 C. Skçld et al. path leading to (Z)-3. However, the difference in free energy for the pathway leading to the (E)- 3 was only 1 kJmolÀ1, which is in accord with the poor diaste- reoselectivity observed when the oxidative Heck product is formed.[6] For transmetallation, path- ways with the associated ace- tate to activate the boronic acid and pathways via Pd–OH com- plexes were investigated. The process of counterion exchange, association of the boronic acid and subsequent dihedral rota- tion was not calculated (i.e., from VII to the pre-transmetal- lation complex), because nu- merous pathways are possible and it is unlikely that the free- energy barriers for this process are higher than the competing b-hydride-elimination barrier of Figure 1.
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