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Article Cite This: ACS Omega 2018, 3, 1614−1620
To “Rollover” or Not? Stereoelectronically Guided C−H Functionalization Pathways from Rhodium−Abnormal NHC Intermediates Champak Dutta, Debasish Ghorai,† and Joyanta Choudhury* Organometallics & Smart Materials Laboratory, Department of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhopal 462066, India S Supporting Information *
ABSTRACT: Rollover C−H activation with transition-metal complexes has been found to be a difficult but viable pathway to functionalize potentially chelating molecules, which are otherwise reluctant to react further. However, selective rollover or nonrollover C−H activation pathway depends on the stereoelectronic demand of the associated organometallic intermediate(s). The presented work addresses the above issue on abnormal N-heterocyclic carbene (NHC) platform. Catalytic reactions of pyridine-imidazolium substrates with internal alkynes have been selectively guided toward either rollover or nonrollover C−H functionalization route via fulfilling the steric and electronic demands of the relevant rhodium(III)−abnormal NHC metallacyclic intermediates.
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INTRODUCTION Transition-metal-catalyzed C−H bond activation functionalization of organic molecules is a vast and versatile research area.1 Obviously, numerous chemical strategies have been developed over the years to achieve such versatility via exploiting various ways of activating the C−H bonds within organic molecules. A recent addition to the existing strategies is “rollover C−H activation” protocol,2 the catalytic version of which has been applied successfully to functionalize strongly chelating (bidentate/tridentate) organic molecules.3 The basic problem associated with rollover C−H activation in chelating molecules is facile formation of a stable metallachelate, which is resistant to reopen via decomplexation, thus inhibiting the activation of the C−H bond (Figure 1a). Hence, examples of successful catalytic protocols involving rollover C−H activation functionalization are still limited.3 These include C−H cyanation of arylimidazo[1,2-α]pyridines (by the Song and Hao group),3a C−H diamination of arylpyridines (by the Lu group)3b and purines (by the Chang group),3c and annulation of 2phenylimidazo[1,2-a]pyridines (by the Li group).3d Later, the Chang group developed elegant protocols for rollover functionalization of pyridine ligand-based strong bidentate (2,2′-bipyridines) and tridentate (2,2′:6′,2″-terpyridines) molecules.3e,f In parallel, recently, our group has addressed an interesting Cp*Rh-catalyzed switchable rollover C−H functionalization of imidazolium ring-containing organic molecules, which are potentially strong chelators based on pyridine− normal N-heterocyclic carbene (nNHC) bidentate motif (Figure 1b).3g Through this chemistry, one can achieve a variety of fused cationic imidazopyridinium structures in just © 2018 American Chemical Society
one step. Notably, cationic, ring-fused N-heterocycles represent an important class of organic compounds recognized to be of significant interest in diverse research areas, including bioactivity,4 materials chemistry,5 supramolecular chemistry,6 etc. As evident from the above-mentioned rollover C−H functionalization strategies, stereoelectronic factors operating at the catalytic metal center play a pivotal role for the success in overcoming such a high activation barrier rollover process to initiate the subsequent C−H bond cleavage step. For instance, ligand trans effect, intramolecular H-bonding, and waterassisted H-bonding were proposed as the driving force (Figure 1a).3a−f After the success with pyridine−nNHC moieties,3g the question of stereoelectronically controlled “switchable” rollover strategy on an abnormal NHC (aNHC)−pyridine bidentate chelating motif has been addressed in this work (Figure 1c). aNHC, being a stronger σ-donor than nNHC, enhances nucleophilicity of the bound metal center of a complex, resulting in acid sensitivity and demetalation of the corresponding complexes.7 Hence, selective functionalization of these moieties remains a challenge. To the best of our knowledge, stoichiometric or catalytic functionalization of M− aNHC motif is still unknown. Herein, we not only show catalytic functionalization of this motif with internal alkynes but also demonstrate on how the relevant steric and electronic factors of a cyclometalated intermediate can trigger the reaction Received: November 24, 2017 Accepted: January 26, 2018 Published: February 7, 2018 1614
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Scheme 1. Examples of Nonrollover Alkenylation and Rollover Annulation Catalysis
Figure 1. “Rollover” vs “nonrollover” C−H activation functionalization.
pathway toward either pyridine rollover, followed by C−C annulation, or direct alkenylation without any rollover step (Figure 1c).
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RESULTS AND DISCUSSION Our investigation started with the reaction between 2,3dimethyl-1-(pyridin-2-yl)-1H-imidazol-3-ium hexafluorophosphate (1a) (0.11 mmol) and diphenylacetylene (2a) (0.1 mmol) in the presence of [Cp*RhIIICl2]2 (0.003 mmol) as catalyst precursor, NaOAc (0.5 mmol) as base, and AgOTf (0.25 mmol) as oxidant in dichloroethane (DCE) at a temperature of 110 °C under argon atmosphere. This reaction afforded the nonrollover-alkenylated product 3a in 86% isolated yield after 24 h (Scheme 1). The structure of 3a was confirmed unambiguously by NMR, electrospray ionization-high-resolution mass spectrometry, and X-ray diffraction (XRD) techniques. At room temperature, the yield was only 49%, whereas the reaction under oxygen atmosphere and in the absence of AgOTf did not furnish any product. Furthermore, optimization studies (see Supporting Information, SI) showed that the presence of catalyst and NaOAc was required in such reactions. To broaden the scope of this catalytic protocol at the aNHC platform, further reactions of other various heteroarenesubstituted imidazolium salts with different internal alkynes were carried out to get alkenylated products (3b−d) in moderate to good yields, as shown in Scheme 1. Details of the imidazolium salts (1a−f) and internal alkynes (2a−e) are
provided in the Supporting Information. Different Nsubstituents, such as benzyl, in the imidazolium substrate 1615
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ACS Omega provided good yield (80%) of the corresponding product (3e) with 4-octyne (2b). The reaction with substituted pyridyl systems featured interesting results depending on the stereoelectronic nature of the substituent and alkyne. Thus, with OMe substituent adjacent to the pyridine nitrogen atom, the reaction provided only alkenylated products with both aliphatic and aromatic alkynes (3f, 86%; 3g, 76%). However, quinoline-substituted imidazolium substrate yielded alkenylated products (3h, 79%; 3i; 73%) with aliphatic alkynes, such as 4-octyne and 1-phenyl1-butyne, but annulated product (4f, 88%) with diphenylacetylene. Reaction of methyl-substituted system with aliphatic alkyne, 4-octyne, provided alkenylated product in minor amount (3j, 35%) and annulated product in major quantity (4a, 60%). The selectivity was completely switched to annulated product (4b, 92%) when aromatic alkyne, diphenylacetylene, was employed. With substantial steric bias, tert-butyl substituent resulted in annulated products (4c, 79%; 4d, 84%; 4e, 80%) regardless of the nature of the incoming alkyne electrophile. Finally, when a very much electrondeficient alkyne was used as internal alkyne partner, rolloverannulated product (4g, 75%) was observed even with unsubstituted imidazolium moiety. It was apparent that the above switchable nonrollover versus rollover C−H functionalization catalysis at the aNHC platform was due to a collective output from the electronic effect of incoming alkyne and steric and directing effects generated by the pyridine substituent adjacent to the donor nitrogen atom. In this regard, recent elegant examples of switchable selectivity in C−H activation/ functionalization chemistry based on controlling the nature of organometallic intermediates within the catalytic cycle are noteworthy.8 As seen from Scheme 1, without any substituent, no rollover was observed for pyridine moiety except when the alkyne was very much electron-deficient 4,4′-dinitrodiphenyl acetylene (4g). Ligation of such an electron-deficient alkyne to the metal center of the metallacycle intermediate prompted it to meet its electronic demand, which could be offered by Rh−C bond formation via rollover C−H activation. For the case of tert-butyl substitution, immediate rollover intermediate formation was expected owing to mainly steric effect. Hence, all products were of annulated type for this substituent. Any less electron-deficient alkynes, such as 4-octyne or 1-phenyl-1butyne, were not sufficient to push the rollover process in most of the other cases. The above stereoelectronically guided switchable nonrollover versus rollover C−H functionalization behavior prompted us to look into the mechanistic aspect of this process. To gain insight into the probable reaction steps involved in the catalytic cycle, a series of control experiments were performed. First and foremost, isolation of a series of aNHC−pyridine nonrollover rhodium(III) intermediates 5a−d was accomplished in excellent yields (80−89%) from the reaction of the corresponding imidazolium substrates (1) and [Cp*RhIIICl2]2 in DCE under inert atmosphere using NaOAc as base (Scheme 2a). All of these intermediates were characterized by NMR spectroscopy and mass spectrometry analyses (see SI). Moreover, the molecular structures of 5a and 5d were also determined by single-crystal X-ray diffraction analyses (see SI for details). Intermediacy of these complexes into the alkenylation catalytic cycle was confirmed by successfully performing catalytic as well as stoichiometric reactions of a representative complex 5a with different alkynes to obtain the desired nonrollover-alkenylated products in good yields
Scheme 2. Control Studies for Alkenylation Reaction
(Scheme 2 and Supporting Information). For the catalytic reaction, proton (H+) from acetic acid (AcOH) formed after the deprotonation of the imidazolium C−H bond of the 1616
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process. Interestingly, the above rollover process was also found to proceed with the other intermediates (5c and 5d) in the presence of AgOTf and NaOAc (see Supporting Information for details), although the catalytic reaction yielded different products (either nonrollover-alkenylated or rollover-annulated) from the corresponding imidazolium substrates (see Scheme 1). This fact suggested that not only there exists an equilibrium between the nonrollover and rollover rhodium(III) intermediates under the reaction condition, but also the final reaction toward either the nonrollover alkenylation or rollover annulation pathway is decided by the energy barrier of the overall reaction (or the rate-determining step), which was guided by the steric and electronic influence from the substituents present on the imidazolium and alkyne substrates. Detailed computational studies will be undertaken in future to examine this aspect. In fact, the reversibility of the rollover process was checked by adding an equivalent amount of AcOH into the solution containing the rollover intermediate (Scheme 3b). The regeneration of the starting nonrollover aNHC− pyridine coordinated motif confirmed the high feasibility of such reversibility in the actual catalytic reaction. The formation of the rollover-annulated product 4b from the stoichiometric reaction of the intermediate 5b with aromatic alkyne in the presence of AgOTf and NaOAc (Scheme 3c) confirmed the existence of a reaction sequence involving rollover C−H activation/cyclometalation at the coordinated pyridine ring, followed by alkyne insertion and reductive elimination. Intermediate 5b was also found to be active in a control catalytic experiment for the annulation reaction (Scheme 3d), reconfirming its intermediacy in the catalysis. On the basis of the results of the mechanistic investigation described above and related previous works,3g,9 we proposed a catalytic cycle as depicted in Scheme 4. The pathway involves the initial formation of an aNHC−pyridine-coordinated rhodacycle intermediate 5, which can transform into a solvento complex 5′ after halide abstraction by AgOTf. The incoming alkyne coordinates (Anr) and inserts into the Rh−C(aNHC) bond to form intermediate Bnr, which finally gives away the alkenylated product 3 via protodemetalation in the presence of in situ generated AcOH (or adventitious H2O). On the other hand, intermediate 5 can undergo pyridine rollover C−H activation/cyclometalation in the presence of NaOAc after halide abstraction by AgOTf, giving rise to intermediate 6. To 6, alkyne coordinates (Ar) and inserts into Rh−C(Py) bond to form a seven-membered intermediate Br. The intermediate Br undergoes reductive elimination to furnish the annulated product 4 and Rh(I) species, which is oxidized back to Rh(III) by AgOTf to continue the catalytic cycle. Observation of a silvery layer on inner walls of the reaction vessel because of the presence of elemental silver is in accordance with the reductive elimination step. A detailed mechanistic investigation will be the subject of future study.
substrate with NaOAc or adventitious H2O present in the reaction mixture could have promoted the required protodemetalation step to furnish the alkenylated product 3b (Scheme 2b). The successful stoichiometric formation of the alkenylated product 3b from the intermediate 5a both in the absence (Scheme 2c) and presence (Scheme 2d) of added AcOH was the basis for the above speculation. To understand the rollover process, the representative intermediate 5b was reacted with stoichiometric amounts of AgOTf and NaOAc in CD3CN at 45 °C for 30 min. The analysis of the 1H NMR spectrum suggested the rollover C−H activation/cyclometalation at the pyridine ring (Scheme 3a, and Supporting Information). A similar control experiment but in the absence of NaOAc did not result such rollover cyclometalation, suggesting the crucial role of NaOAc as base for this Scheme 3. Control Studies for Annulation Reaction
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CONCLUSIONS In a nutshell, this work exhibited a switchable C−H bond functionalization of pyridine-imidazolium chelating substrates to nonrollover alkenylation or rollover annulation products with internal alkynes on an exclusive abnormal Rh−NHC platform. Mechanistic investigation, including isolation, characterization, and control reactions of several organometallic rhodacyclic intermediates, highlighted the crucial role of stereoelectronic effects exerted by the substrate and alkyne backbones in the process of this bimodal reactivity. The 1617
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ACS Omega Scheme 4. Proposed Catalytic Cycle Based on Control Experiments and Previous Reports9
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ACKNOWLEDGMENTS This work was financially supported by DST-SERB (grant no. EMR/2016/003002) and IISER Bhopal. C.D. and D.G. acknowledge doctoral and postdoctoral fellowships from IISER Bhopal, respectively.
mechanistic path was a result of a collective output of contributions from steric at the metallacycle involved and the electronic nature of the incoming alkyne electrophile.
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ASSOCIATED CONTENT
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S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b01846. General methods and materials; general procedure for the synthesis of imidazolium salts (1a−k); general procedure for the nonrollover alkenylation reactions; table of imidazolium salts and internal alkynes used; optimization studies; experimental characterization data of the products (3a−j); general procedure for the rollover annulation reactions; experimental characterization data of the products (4a−g); mechanistic studies; controlled studies: NMR tube experiment; characterization data for imidazolium salts; characterization data for alkenylated products; characterization data for annulated products; characterization data for aNHC prerollover intermediates (PDF) Single-crystal X-ray diffraction data of the compounds corresponding to CCDC nos. 1587268 (3a), 1587267 (4f), 1431209 (5a), and 1587269 (5d) (ZIP)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Joyanta Choudhury: 0000-0003-4776-8757 Present Address †
Institut für Organische und Biomolekulare Chemie, GeorgAugust-Universität, Tammannstraße 2, 37077 Goettingen, Germany (D.G.). Notes
The authors declare no competing financial interest. 1618
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DOI: 10.1021/acsomega.7b01846 ACS Omega 2018, 3, 1614−1620
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DOI: 10.1021/acsomega.7b01846 ACS Omega 2018, 3, 1614−1620
