Plausible Rh(V) Intermediates in Catalytic CâH Activation Reactions

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Plausible Rh(V) intermediates in catalytic C–H activation reactions Suhelen Vásquez-Céspedes, Xiaoming Wang, and Frank Glorius ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03048 • Publication Date (Web): 20 Nov 2017 Downloaded from http://pubs.acs.org on November 20, 2017

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Plausible Rh(V) Intermediates in Catalytic C–H Activation Reactions Suhelen Vásquez-Céspedes, Xiaoming Wang and Frank Glorius* *Westfälische Wilhelms-Universität Münster, Organisch-Chemisches Institut, Corrensstrasse 40, 48149, Germany. ABSTRACT. Catalysis by defined transition metal complexes has captivated the attention of the scientific community over the last decades. The well-documented utility of Rh(III) complexes in C–H activation reactions have enabled the development of a plethora of new catalytic methods. High valent transition metal species in C–H activation reactions was first predicted in palladiumbased transformations. From those early studies, it was apparent that differences in reactivity and selectivity could be expected. By analogy, potential higher valent Rh(V) complexes could represent a new approach in C–H activation reactions and offer different opportunities to improve and broaden the current state of the art in the field.

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Ts N3 [Rh(III)]

N O R

O

X N

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O O O I

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R1 V Y DG Rh X

Existence? Different reactivity/selectivity? C H Activation?

Rh(V) as plausible intermediates

KEYWORDS. C–H Activation, Rhodium (V), Rhodium (III), High-valent transition metal catalysis, Transition metal-catalyzed reactions. 1. Introduction Catalysis by well-defined transition metal complexes has captivated the attention of the scientific community over the last decades. Particularly, transition metal-catalyzed C–H activation

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reactions have become an active and ever growing research area.1 The use of half-sandwich Cpderivatives transition metal complexes of the group 9 has extensively contributed in the discovery and development of a plethora of new catalytic transformations. On a high note, the use of well-defined Ir(III), Rh(III) and Co(III) complexes has not only played a fundamental role in the development of C–H activation reactions but also has contributed to the better understanding of reaction mechanisms.2 Mostly employed with electron-rich Cp-derived ligands, the potential of these d6 half sandwich complexes to access higher oxidation states represents an interesting point in catalytic C–H activation reactions that is yet not well explored or understood. Although experimental evidence has so far proven elusive, several mechanistic proposal including detailed computational studies have provocatively suggested the presence of higher valent Ir(V) and Rh(V) complexes as intermediates in catalytic reaction pathways. Herein, we present an overview of the latest studies towards the identification and understanding of higher valent Rh(V) intermediates in C–H activation reactions and their possible implication in the development of new transformations that exploit and benefit from highly oxidized catalytic intermediates. 1.1 Overview of C–H activation by means of high valent metal complexes of Pd(IV), Ni(IV) and Au(III) The presence of high valent transition metal intermediates as catalytically active species for C–H activation was first proposed for palladium-based reactions and mechanistically investigated by the Sanford group.3 From their studies, it was apparent that differences in reactivity and selectivity could be expected.4 High oxidation state metal complexes of Pd(IV), Pd(III), Ni(IV) and Au(III) have been investigated in the last years as complementary catalysts to their corresponding commonly employed lower valent complexes (Pd(0), Pd(II), Ni(II), Au(I)).5,6 Theoretical and experimental evidence has been gathered supporting a change in reactivity and selectivity towards C–H activation reactions. For example, the reductive elimination step from Pd(II) complexes to form C–halogen bonds is thermodynamically disfavored and kinetically slow whereas this step has been shown to be accelerated by higher valent palladium catalysts.5c Ni(IV) catalysis has had a notably increase in interest.7,8 New evidence supporting the possibility of using well-defined Ni(IV) complexes in catalytically relevant reactions has recently appeared. Studies have demonstrated that Ni(IV) intermediates can undergo different organometallic reactions, such as transmetallation, and offer the possibility to outcompete a fast reductive elimination process allowing access to a potential variety of cleverly designed Ni(II)/Ni(IV) catalytic cycles.8b It has also been very recently demonstrated that well-defined Ni(IV) complexes can accomplish C–H bond activation by electrophilic metallation,8c a behavior that has also been implicated at Ni(III) centers.9 Well-defined Au complexes possess an established difference in selectivity for C–H activation reactions of arenes. Electron-rich Au(I) complexes display a preference for the direct C–H metalation of the most acidic proton of electron-poor (hetero)arenes. On the contrary, Au(III) species seem to preferentially activate the most reactive C–H bond by means of electrophilic aromatic substitution reactions of electron-rich (hetero)arenes.10 From these comparisons it could be expected that ligand-stabilized Rh(V) species could have the potential to open up new reaction pathways to expand the scope and understanding of Rh-catalyzed C–H activation reactions. Rh(V) species lead to the possibility of a more feasible reductive elimination step enabling the use of more challenging coupling partners and the development of new transformations, furthermore Rh(V) species would also


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become highly electrophilic in nature, forging new alternatives to explore C–H activation processes by electrophilic pathways (Scheme 1).

Scheme 1. Reactivity of high valent Pd(IV) (A), Ni(IV) (B) and Au(III) (C) complexes in catalytic C–H activation reactions and possible Rh(V) complexes (D). 2. Rhodium(III)-catalyzed C–H activation reactions: a general mechanistic introduction Since the isolation of the [Cp*RhCl2]2 by Maitlis,11 its first use in catalytic C–H activation reactions by Satoh and Miura12 and a number of key contributions by the groups from Jones,13 Fagnou,14 Glorius,15 Ellman,1b Li,16 Rovis,17 and Cramer,18 among others, the potential of the Cpderived Rh(III) complexes to achieve a plethora of catalytic transformations has been wellrecognized and explored.19 The initial mechanistic steps are arguably similar in most cases: first the starting unreactive 18 electrons complex becomes an active catalytic species generally by treatment with a halogen acceptor such as silver(I) salts. Coordination to a Lewis base directs the metal center to a specific C–H bond allowing metalation to take place. The fate of the formed metallacycle will depend on the reaction conditions and the coupling partner.20 Although many of the reported Rh(III) catalyzed transformations are carried out under redox-neutral conditions,1w plenty of other widely known and explored reactions include the use of (super)stoichiometric oxidants. On a general mechanistic overview of oxidative reactions, after metalation and subsequent initial functionalization of the coupling partner, the Rh(III) complex undergoes a reductive elimination process to form the corresponding new bond and release Rh(I). These species are regenerated to Rh(III) by reaction with the oxidant. A general Rh(III)/Rh(I)/Rh(III) reaction manifold is usually invoked in these transformations (Scheme 2A). However, the presence of oxidants, either from the cleavage of a covalent bond in the directing group (internal oxidant) or from an external source, as reaction additives, could lead to an alternative reaction pathway where the Rh(III) catalyst could access a higher valent Rh(V) intermediate (Scheme 2B). The potential presence of such intermediates in C–H activation reactions has been proposed in the literature in the last decade.


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Scheme 2. General mechanistic overview of Rh(III)-catalyzed oxidative-type of reactions involving two different proposed catalytic manifolds. A) Rh(III)/Rh(I)/Rh(III) process. B) Rh(III)/Rh(V)/Rh(III) process. 2.1 Previous studies on putative Rh(V) complexes 2.1.1 Silyl hydride complexes During the past 30 years a series of reports has hinted at the possibility of stable and isolable Rh(V) complexes from reactions that include treatment with silanes. Maitlis and coworkers obtained an isolable complex when treating [Cp*RhCl2]2 with triethylsilane in the presence of triethylamine.21a-c Originally characterized by mass spectrometry along with 1H, 13C, 29Si and 103 Rh NMR, the complex seemed to be in accordance with a Rh(V) species, however, the reactivity was more reminiscent of a more electron-rich Rh(III) center than the corresponding highly electrophilic Rh(V). The same group extended the investigation into these Rh(V) complexes through the synthesis of a series of dihydrobis(triethylsilyl)pentamethylcyclopentadienyl rhodium complexes by the reaction of [Cp*RhCl2]2 with triethylsilane to obtain 1 (Scheme 3).21d Characterization by X-ray diffraction and neutron diffraction showed a structure where the two triethylsilyl ligands were located trans in the basal plane. The hydrides were found to be trans to each other and cis to the silyl ligands. The calculated Si–H distance in the complex, by neutron diffraction, was found to be 2.27 Å indicating an unlikely significant direct bonding of the triethylsilyl group with the hydride, suggesting the presence of the silyl group as two separated ligands in contrast to a η2-silane. The authors concluded that the structural characterization of the complex allowed them to identify it as a Rh(V) complex (2). However, the authors remarked as unexpected the stability and reactivity of 2. Only under forcing reaction conditions the complex was found to react with neutral ligands such as triethyl phosphate, tert-butyl isocyanide or carbon monoxide. Yet, it reacted much easily with different electrophiles, contrary to the expected reactivity of a highly electron-deficient Rh(V) complex.21d These results, although representing the first examples of plausible Rh(V) complexes, did not present an unambiguously assigned higher valent Rh(V) complex. Following studies, as will be commented in the next paragraphs, demonstrated different Si–H interactions of the ligands suggesting the likelihood of the described Rh(V) complexes to exist as Rh(III) species.


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Scheme 3. Synthesis of plausible Cp*Rh(V) species by Maitlis and coworkers. The silicon–hydrogen bond activation by Cp*Rh(III) complexes was further investigated by the groups of Bergman and Brookhart.22 They prepared a series of silyl complexes from the reaction of a cationic Cp*Rh(III) complex 3 with triphenyl-, trimethyl- or triethylsilane (Scheme 4). The authors studied the reaction using low temperature NMR. From a range of temperatures of -80 to -40 °C, they were able to only observe the species 4 from the activation of the Si–H bond along with a release of methane when the reaction was carried out in the presence of 1.0 equivalent of triphenylsilane. After warming to -30 °C, a distinct new complex was detected and characterized by the authors as the rhodium complex 5. Analysis by 29Si NMR allowed the detection of 29Si satellites with a JSi–H of 84 Hz, characteristic of η2-silane complexes,23 suggesting an interaction with the Rh(III) metal from a σ Si–H ligand. However, when the study was carried out using 5.0 equivalents of triphenylsilane at room temperature, a different complex was detected (6a). No 29 Si satellites where observed for the new species and the authors were unable to characterize the species as the Rh(V) complex (6a) or a fluxional hydridosilane Rh(III) complex (6b). When studying the corresponding alkylsilane complexes equivalent to 6, the authors also found 29Si satellites with JSi–H values that seemed to be in accordance with η2-silane complexes, suggesting the presence of σ Si–H ligand and a Rh(III) metal center in preference to Rh(V), although the obtained data did not allow for a definitive assignment of the oxidation state of the metal center.

Scheme 4. Synthesis and studies of potential Cp*Rh(V) complexes by Bergman and Brookhart. Similar studies were also carried out by the groups of Vyboishchikov and Nikonov.24 They investigated formal Rh(V) cyclopentadienylsilyl hydride complexes CpRh(SiMe3)2H2, CpRh(SiMe3)3H, CpRh(SiMe3)2(SiEt3)H, and [Cp(Me3P)Rh(SiMe3)2H]+ along with [Cp*Rh(SiMe3)2(PMe3)H]+ by means of DFT calculations, Mayer bond indices and Si–H coupling constants. The authors concluded that in the case of the complex CpRh(SiMe3)2H2, the calculated structure with the minimum energy seemed to be consistent with a Rh(V) silyl hydride complex, especially considering that the four different alternative Si–H bond distances were calculated to be in the range of 2.284–2.358 Å, values normally considered to represent nonbonding interactions.25 However, calculation of the JSi–H by means of quantum chemistry indicated the possibility of residual Si–H bonding in the complex. Complexes CpRh(SiMe3)3H and CpRh(SiMe3)2(SiEt3)H presented delocalized Si–H bond interactions of the silyl groups and the hydride. Also when comparing [Cp(Me3P)Rh(SiMe3)2H]+ and [Cp*Rh(SiMe3)2(PMe3)H]+,


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the presence of stronger Si–H bond interactions was calculated for the Cp* complex in comparison with the unsubstituted analogue. The authors concluded that all the analyzed complexes had degrees of Si–H interaction between the ligands and, therefore, none of the complexes was a formal representation of a Rh(V) metal center. 2.1.2 Boryl complexes The group from Hartwig and coworkers developed a regiospecific C–H borylation of alkanes employing Cp*Rh(III) complexes in combination with B2pin2 as the coupling partner.26 In their breakthrough report in 2000, high valent Rh(V) species were suggested as possible intermediates in the transformation (Scheme 5).26a The authors suggested that an oxidative addition of borane or diboron compounds into the catalyst could lead to hydridorhodium boryl 7 or rhodium bis(boryl)complexes 8. Evidence supporting such intermediates was found by 11B NMR in combination with the study of stoichiometric reactions using rhodium silyl hydride complexes in the presence of bispinacolborane, however, characterization of an isolated and pure rhodium boryl complex proved challenging.

Scheme 5. Direct C–H borylation of alkanes with Cp*Rh(III) and suggested intermediates as proposed by the Hartwig group. Detailed mechanistic studies on the transformation were reported by the group in 2005 and 2010.26b,c The intermediacy of formal Rh(V) complexes was thoroughly investigated by the synthesis of the complexes 7 and 8. X-ray data of 7 showed that the complex adopts a trans fourlegged piano stool structure in the solid state. Solution NMR spectroscopy studies suggested B– H bonding interactions in their structure, a result that was also observed for the rhodium complex 8. The authors concluded that for complex 7, a significant B–H bonding was obtained and unsymmetrical interactions were obtained for each of the B–H pair in the complex, which is more accurately represented as a Rh(III) complex. In the case of complex 8, computational methods suggested the intermediacy of a symmetric Cs Rh(V) complex with equivalent B–H distances, however, the complex was uphill in energy (1.3 kcal/mol) when compared to the unsymmetrical rhodium complex 8 that was, according to the data, assigned as a Rh(III) species. 2.2 Studies on plausible Cp*Rh(V) species by the addition of oxidants As an alternative, forming higher valent Cp-derived Rh(V) species could be achieved by direct oxidation of well-defined Rh(III) complexes. The groups from Templeton and Jones envisioned the reaction of complex 9 with the oxygen atom transfer reagent 2-tert-


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butylsulfonyliodosylbenzene (sPhIO, 10) and investigated the process by 1H NMR (Scheme 6).27 They observed the unexpected formation of a new κ2 2-(2-pyridyl)phenoxide ligand and the coordination of a second molecule of sPhIO to the metal center as a neutral ligand (11). Dissociation of the sPhIO led to a dimeric species 12. The complex 11 could be reacted with a large excess of 2-phenylpyridine, hydrogen peroxide and acetic acid to give 2-(2-pyridyl)phenol. The catalytic version of the transformation was proposed to be limited by the degradation of the Cp* ligand due to the oxidant sPhIO, forming a number of unidentified species as determined by 1 H NMR. A similar degradation under oxidative conditions was observed for an analogue Cp*Ir(III) complex.28 The authors did not observe evidence for the formation of Rh(V) oxo species or any indication of oxidation to Cp*Rh(V) species.27

Scheme 6. Studies on the oxidation of Cp*Rh(III) complexes by Templeton and Jones. Templeton and coworkers continued their study on the reaction of Cp*Rh(III) complexes with oxidants (Scheme 7).29 In the reaction of the Cp*Rh complex 13 with sPhIO the authors did not observe the transfer of the oxygen from the sPhIO to the Cp*Rh complex 13 to give a different species than 14, however, the consumption of the starting materials and an inefficient mass balance led them to propose a plausible degradation of the Cp*Rh complex. No evidence for a higher oxidation rhodium(V) species was found.

Scheme 7. Oxidation attempts of Cp*Rh(III) complexes by Templeton and coworkers. 3. Catalytic C–H activation reactions with proposed Rh(V) intermediates In the area of Rh(III) catalyzed C–H activation, two strategies have been employed to access the hypothetical Rh(V) intermediates. On the one hand, the use of oxidative directing groups such as hydroxyacetamide or hydroxybenzamide derivatives have allowed to propose reaction mechanisms where the formation of a Rh(V) nitrene species can be imagined. On the other hand, reactions in the presence of oxidative coupling partners such as hypervalent iodine compounds have also promoted the examination of potential Rh(V) intermediates. Although, to the best of our knowledge, no unambiguously assigned Cp*Rh(V) complex has been isolated to date, a series of studies by means of DFT calculations have offered strong support towards Rh(V) species as intermediates in the reaction mechanisms.


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3.1 Proposed Rh(V) intermediates formed from oxidizing directing groups The use of internal oxidants in C–H activation reactions catalyzed by rhodium complexes represented an important step forward in the area, reducing the amount of oxidants added and potentially enabling milder reaction conditions.30 The reports by the groups of Fagnou31 and Glorius32 showcased the use of a N–O bond as an internal oxidant in Cp*Rh(III) catalyzed C–H activation reactions and led to numerous examples that demonstrated the potential of internal oxidazing directing groups in Rh-catalyzed C–H activation. In 2012, Xia and coworkers33 used DFT calculations to investigate the divergence in product formation in the reaction of olefins with two different benzamide derivatives bearing distinct N–OR groups as internal oxidants, as reported by the Glorius group (Scheme 8). After C–H activation and olefin insertion, the generated 7-membered metallacycle 15 can undergo different reaction pathways that are dependent of the identity of the N–OR derivative. When N–OMe was employed, the reaction led to the olefinated product 16a via a sequence of β-H elimination and reductive elimination forming a Rh(I) intermediate (15a) which was reoxidized to Rh(III) by cleavage of the N–OMe bond. When using the N–OPiv moiety, coordination from the pivaloyl group stabilizes the Rh(III) metallacycle enabling the formation of a Rh(V) nitrene intermediate (15b) through a 5membered ring transition state. Reductive elimination would form the product 16b and regenerate the Rh(III) species.

Scheme 8. Divergent product formation involving the potential generation of a Rh(V) intermediate. In 2013, the group from Rovis and coworkers developed the Cp*Rh(III)-catalyzed synthesis of γlactams from benzamide derivatives and diazo compounds (Scheme 9).34 The mechanism of the reaction was studied by means of DFT calculations by Xia and coworkers.35 After C–H activation, diazo insertion and migration, the six-membered intermediate 17 could undergo a reductive elimination to form the C–N bond, however, the energy barrier for this step was calculated to be over 45 kcal/mol. The alternative formation of a Rh(V)–nitrenoid species 18 by pivalate migration and subsequent reductive elimination presented a more favorable energy barrier of 9.2 kcal/mol. A third pathway involving 1,3-allyl migration that includes a dearomatization of the phenyl ring from the diazo compound was also calculated and was found to possess a high energy barrier of 36.7 kcal/mol, making this step very unlikely.


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Scheme 9. Synthesis of γ-lactams by Rovis and coworkers with possible Rh(III) and Rh(V) intermediates The group from Cui and coworkers developed a Cp*Rh(III)-catalyzed synthesis of azepinones 19 by the reaction of benzamide derivatives and vinyldiazo compounds (Scheme 10).36 After C–H activation and 1,1-migratory insertion (20), the authors proposed a 1,3-allylic migration to generate the key intermediate 21, followed by a reductive elimination step to form the new C–N bond. The reaction mechanism was further investigated by Xia and coworkers.35 According to the calculations, the reductive elimination from 20 is energetically demanding, suggesting the direct C–N bond formation from a Rh(III) species (20) is not possible and other steps should occur in a more favorable way prior to the heterocyclization. The DFT calculations showed a favorable pathway for the formation of a Rh(III) intermediate (21) after the carbene insertion step and subsequential 1,3-allylic migration. The possible reductive eliminations from 21 to yield the corresponding C–N bond formation were calculated and found to be energetically unfavorable (>30 kcal/mol). Alternatively, a pivalate migration and formation of the Rh(V)– nitrenoid 22 gives a reductive elimination step with a small energy barrier of 6.3 kcal/mol leading to the cyclic product 19, obtained after a protodemetalation step.

Scheme 10. Synthesis of azepinones by Cui and coworkers with potential Rh(III) and Rh(V) intermediates. A divergent product outcome was observed by Cui and coworkers in the Rh(III)-catalyzed C–H activation/cyclization of different benzamide derivatives with methylenecyclopropanes (MCP) (Scheme 11).37 After the insertion of the MCP, the formed seven-membered ring can either undergo a pivalate migration to yield the cyclopropane product 23 or a β-C elimination to give the 8-membered lactam derivative 24. The group from Xia and coworkers carried out a DFT calculation into the reaction mechanism to account for the possibility of the reductive elimination


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step occurring from a Rh(V) intermediate.38 In the work published by Cui and coworkers, a dependence on the identity of the N–OR group was experimentally observed, where the reaction proved efficient with N–OPiv but no product was observed with N–OMe. The DFT results by Xia and coworkers highlighted the generation of a Rh(V)–nitrenoid intermediate from pivalate migration as a key aspect to accomplish the C–N bond formation step (25 and 26). It was calculated that the reductive elimination from the Rh(III) intermediates (27 and 28) was unlikely due to activation barriers of over 30 kcal/mol, making a process involving a Rh(III)/Rh(I)/Rh(III) pathway thermodynamically disfavored. The authors also found a high energy demanding migration of the methoxyl group from the N–OMe moiety to form the Rh(V) intermediate, explaining the experimentally observed lack of reactivity.

Scheme 11. Lactam synthesis by Cui and coworkers with different Rh(III) and Rh(V) intermediates by DFT calculations. The Cp*Rh(III)-catalyzed reaction of N-phenoxyacetamides with alkynes to give orthohydroxyphenylenamides or benzofurans was reported by the groups of Liu and Lu in 2013 (Scheme 12).39 The authors favored a Rh(III)/Rh(I)/Rh(III) reaction manifold involving a reductive elimination to form the C–O bond and a Rh(I) species (29). A subsequential N–O bond cleavage was proposed to oxidize the Rh(I) to a Rh(III) species (30) and no alternative pathway for a possible Rh(V) intermediate was discussed. The mechanistic study of the transformation was investigated by the groups of Houk and Wu and a comparison between the Rh(III)/Rh(I) and the Rh(III)/Rh(V) manifolds was undertaken.40 The authors found that after C–H activation and alkyne insertion, the seven-membered metallacycle (31) can either undergo a reductive elimination involving a Rh(III)/Rh(I)/Rh(III) catalytic cycle (29 and 30) or an oxidative addition to the N–O bond for a Rh(III)/Rh(V)/Rh(III) manifold (32). The first proposed catalytic cycle involves a high energy barrier of 32 kcal/mol whereas the formation of a Rh(V)–nitrenoid species (32) was found to be more favorable due to a lower barrier of 24.6 kcal/mol. The authors also found that the N–O bond cleavage led to a ring contraction of the metallacycle facilitating the formation of the Rh(V)–nitrenoid intermediate (32). The Natural Bond Orbital (NBO) analysis of the process from intermediates 31 to 32 showed a change on the NBO charge of the rhodium center from 0.217 to 0.387 along with an increase in the electron binding energy from 85.8 to 86.9 eV, both results supporting the oxidation of Rh(III) to Rh(V) during the catalytic cycle.


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Scheme 12. Synthesis of ortho-hydroxyphenylenamides and benzofurans by Lu and Liu and Rh(III)/Rh(V) pathway favored by DFT calculations. In 2014 the groups from Li41 and Chang42 independently reported the functionalization of Noxide quinolines with alkynes featuring an oxygen atom transfer process (Scheme 13). The authors considered the possibility of either a Rh(III)/Rh(I)/Rh(III) or a Rh(III)/Rh(V)/Rh(III) reaction mechanism in their transformations. A computational study of the reaction and the role of the catalyst on the N–O bond cleavage was undertaken by the groups of Li and Lan.43 After the formation of the cyclometallated species 33, the possible formation of a Rh(V)-oxo (34) complex was considered, however, the calculated energy for the formation of the oxo complex was 45.8 kcal/mol which rendered this process unlikely to occur in the investigated transformation. The authors concluded that the preferred reaction path involved a reductive elimination step from a Rh(III) oxazinoquinolinium complex (33) to give a Rh(I) complex (35) that was reoxidized to Rh(III) after cleavage of the N–O bond forming intermediate 36, giving the corresponding product after protodemetallation.


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Scheme 13. Functionalization of quinolines N-oxides as reported by the groups of Chang and Li. The Cp*Rh(III)-catalyzed reaction of N-phenoxyacetamides with cyclopropenes to form a series of 2H-chromenes was developed by the Wang group in 2014 (Scheme 14).44 The proposed reaction mechanism did not consider the formation of a Rh(V) intermediate but a redox-neutral Rh(III) manifold seemed to be favored by the authors. Three computational studies were reported by the groups of Xia and Li to investigate the reaction mechanism.45 The tricyclic metallacycle 37 is formed after C–H activation and cyclopropene insertion. Two alternative pathways could lead to the cyclic product, specifically β-C elimination where the reaction would be considered redox-neutral (38) or N–O cleavage with the generation of a Rh(V) intermediate (39). Interestingly, contradictory results were observed in the reported calculation by both groups. In their first report, Xia and coworkers found that the opening of the cyclopropyl ring by β-C elimination was energetically more favorable than the N–O cleavage to obtain a Rh(V) species, supporting a redox-neutral Rh(III)-catalyzed transformation.45a However, the group from Li and coworkers found a divergent result where their DFT calculations (using the same M06 functional and similar basis sets) indicated a preferential N–O bond cleavage with formation of a Rh(V) intermediate as the most energetically accessible pathway.45b The authors highlighted the need of the Rh(V) intermediate 39 to facilitate the subsequent ring opening process to lead to the Rh(III) species 39a that would give the final product through an electrocyclization step. The discrepancy between both results was studied again by the Xia group focusing on the different substitution patterns of the cyclopropene derivative that seemed to be the point of divergence of both studies. Notably, the resulting DFT calculations did not show a substantial difference regarding the cyclopropene derivative and the authors indicated that their data preferentially supported a redox-neutral Rh(III) process for the investigated reaction.45c


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Scheme 14. Synthesis of 2H-chromenes by Wang and coworkers and DFT-calculated Rh(III) and Rh(V) intermediates. Further studies related to the reactivity of N-phenoxyacetamides as oxidizing directing groups were conducted by the groups of Li and Liao (Scheme 15).46 They carried out the mechanistic investigation of the Cp*Rh(III)-catalyzed reaction of N-phenoxyacetamides with two different coupling partners (N-tosylhydrazones or styrenes) according to the experimental work reported by the groups of Liu and Lu47a and Wang.47b After insertion of the hydrazone, the results from the study of the mechanism involving the N-tosylhydrazone substrates supported the formation of a Rh(V)–nitrenoid intermediate (40) due to the cleavage of the N–O bond of the directing group. These species would easily isomerize to the more stable Rh(III) intermediate 40a that after rearomatization and protodemetalation would give the olefinated products 40b. Other plausible pathways involving a Rh(III)/Rh(I) and a redox-neutral Rh(III) mechanism, via intermediate 41, were also calculated by the authors, however, the formation of the Rh(V)– nitrenoid intermediate 40 was the most favored pathway. In the case of the styrenes as coupling partners, the results did not suggest the formation of a Rh(V) species and favored a Rh(III) intermediate after β-H elimination. The authors concluded that the obtained differences in the reaction pathways for the same oxidizing directing group lie in the structures of the corresponding Rh(V)–nitrenoid intermediates, where a higher stabilization is obtained for the Rh(V) intermediate of the N-tosylhydrazone given its facile reduction to a Rh(III) species via tautomerization.


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Scheme 15. Synthesis of ortho-substituted phenols by Wang and the groups of Li and Liao. A Rh(V)/Rh(III) pathway was favored by DFT calculations. The Rovis group developed a Rh(III)-catalyzed cyclopropanation48a and carboamination48b reaction using N-enoxyphthalimides and alkenes as coupling partners (Scheme 16). The difference in the reaction outcome was found to be related to the reaction solvent, where 2,2,2trifluoroethanol (TFE) would lead to the cyclopropanation product whereas the use of methanol as solvent produced the carboaminated product. The authors studied the reaction mechanism and explained the difference in product as methanol would lead to the formation of a bidentate directing group due to opening of the phthalimide, forming a bicyclic Rh(III) intermediate (42) after the olefin insertion. The authors proposed a Rh(III)/Rh(I)/Rh(III) reaction pathway for their transformation. In the case of the cyclopropanation reaction, a migratory insertion of the olefin would lead to a Rh(III) intermediate (43) that undergoes an intramolecular carborhodation through a 3-exo-trig cyclization to form a key Rh-complex with a cyclopropane unit. The reaction mechanisms of both transformations were investigated independently by the groups of Li49a and Liu and Chen49b by means of DFT calculations. Both groups reached the conclusion that a more energetically favorable Rh(III)/Rh(V)/Rh(III) reaction manifold is likely taking place in both transformations. The calculated key cyclic Rh(V) intermediates 44 and 45 were shown to have a lower energy barrier than other calculated intermediates corresponding to a Rh(III)/Rh(I)/Rh(III) reaction manifold.

Scheme 16. Cyclopropanation and carboamination reactions by Rovis and coworkers with proposed key reaction intermediates and DFT calculated Rh(V) species. As part of their research into Cp*Rh(III) catalyzed Wagner-Meerwein-type rearrengements,50a the Glorius group developed an intramolecular amide transfer in N-phenoxyacetamides under mild and neutral conditions (Scheme 17).50b The authors found the reaction to be co-catalyzed by a strained olefin in a type of transformations reminiscent of the Catellani reaction. A series of experimental and theoretical studies indicated the importance of the co-catalyst to promote the N–O cleavage of the N-phenoxyacetamides and the transfer of the amide moiety through a proposed Rh(V) intermediate (46). As a highlight of the developed methodology, the authors successfully applied their reaction for the late stage modification of natural products and a


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ACS Catalysis

marketed drug such as clofoctol. The experimental mechanistic investigation suggested a stepwise pathway that first cleaves the N–O bond and then forms the C–N bond of the product through a likely key Rh(V) intermediate (46). Computational studies showed that in the most energetically favorable pathway, the Rh(III) species (47) adds to the N–O bond in a formal oxidative process giving a Rh(V) nitrenoid species (46) which adds to the ortho-carbon of the substrates by a low energy barrier of 7.2 kcal/mol. Alternatively, a concerted pathway without the involvement of a Rh(V) intermediate was found to be energetically disfavored due to a high barrier of 41.6 kcal/mol.

Scheme 17. Rh/strained olefin-cocatalyzed intramolecular C–H bond amination by Glorius and coworkers where a potential Rh(V) intermediate plays a key role in the transformation. The groups of Li and Wang developed the synthesis of dihydroisoquinolin-1(2H)-ones under Cp*Rh(III) catalysis with benzamides and 2,2-difluorovinyl tosylates as coupling partners (Scheme 18).51 The authors investigated the reaction mechanism with a series of experiments and theoretical calculations by DFT. The formation of the cyclic difluorinated products, 48, was determined to be the end result of a preferential C–N bond formation over competing β-F elimination. The results of the DFT calculations showed that coordination of the carboxyl oxygen of the pivaloyl group to the metal center enabled an intramolecular oxidation to Rh(V) by migration of the -OPiv group to give the intermediate 49. A low energy barrier of 11.9 kcal/mol was calculated for this process and NBO analysis indicated a diminished NBO charge on the metal along with an increase in electron binding energy, the latter two results in accordance with the generation of a Rh(V) intermediate. Alternatively, the authors also investigated the possibility of a redox-neutral process. In that pathway a reductive elimination and C–N bond formation occurs, where a formal β-oxygen elimination takes place to trigger the migration of the -OTs group to form the Rh(III) species 50. However, this pathway was found to have a higher activation energy than the Rh(V) route.


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Scheme 18. Synthesis of fluorinated dihydroisoquinolin-1(2H)-ones and Rh(V) intermediate favored by DFT calculations. Very recently, the group from Feng and coworkers described an interesting effect of fluorine substituents on a formal hydroarylation of alkynes and aryl migration reaction to obtain fluorine containing alkenyl-substituted anilines (Scheme 19).52 The authors initially envisioned a possible aryl migration promoted by the formation of a Rh(V)-nitrenoid complex, obtained after the oxidative cleavage of the N–OPiv bond. The reaction mechanism was proposed to involve an initial C–H activation and regioselective alkyne insertion to form a seven-membered rhodacycle (51). Consecuent -OPiv migration would generate a Rh(V)-nitrenoid species (52) that could promote a dearomative ipso attack of the arene group to form 53. The authors proposed that the regeneration of aromaticity would be achieved after nucleophilic addition of an alcohol to form the six-membered Rh(III) species 54. Protodemetallation of such Rh(III) species would lead to the corresponding products.

Scheme 19. Cp*Rh(III)-catalyzed fluorinated aniline synthesis and aryl migration via a Rh(V)nitrenoid as proposed by Feng and coworkers. 3.2 Proposed Rh(V) intermediates formed from coupling partners 3.3.1 Aryl halides and halogenating reagents as coupling partners


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In 2012, the Glorius group proposed the intermediacy of a Rh(V) species in catalytic C–H activation showcasing a cross-dehydrogenative aryl–aryl coupling (Scheme 20).53 Based of the reported results, the authors proposed that the 2-fold C–H activation process observed in the reaction could proceed by two alternative reaction mechanisms. After the first C–H activation, the Rh(III) intermediate 55 could coordinate bromobenzene in a η2-manner and lead to two different pathways. On one hand, in the Rh(III)/Rh(I)/Rh(III) mechanism, the coordinated complex 55 would undergo C–H activation by concerted metalation-deprotonation or σ-bond metathesis affording 56, giving the product after reductive elimination. The active catalytic species would be regenerated after a reoxidation process. On the other hand, in a Rh(III)/Rh(V)/Rh(III) mechanism, oxidative addition of the C–H bond of the arene to complex 55 would form a Rh(V) hydride species (57) leading to the product by reductive elimination.

Scheme 20. Aryl–aryl dehydrogenative coupling by Glorius and coworkers. In 2012, the Glorius group developed an ortho-directed bromination and iodination of benzamides and other different classes of aromatic compounds using Cp*Rh(III) as catalyst and N-halosuccinimides as the coupling partner.54a Although no detailed mechanistic study was given by the authors, two different reaction mechanisms were proposed (Scheme 21). In one case, the potential oxidative addition of the halogenating reagent into the Rh(III) catalyst would lead to a Rh(V) species (58) that undergoes a reductive elimination to yield the corresponding products and regenerate the active catalytic species. Alternatively, a redox-neutral SN2-type reaction was also proposed as the productive pathway (59) to yield the observed products. Following their previous studies, the Glorius group developed a site-selective bromination of electron-rich heteroarenes, using similar conditions as their previous results and employing Nbromophthalimide as the halogenating reagent.54b The possible involvement of a Rh(V) species in the reaction mechanism was also acknowledged in their research.


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Scheme 21. Directed ortho-halogenation of benzamides by Glorius and coworkers with proposed Rh(III) and Rh(V) reaction intermediates. The group from Bai, Liu, Lan and coworkers carried out a computational study of the Rh(III)catalyzed ortho-bromination of arenes as reported by Glorius.55 The authors discussed the possible oxidative addition of the N-bromosuccinimide into a cationic Cp*Rh(III) complex to form a Cp*Rh(V) intermediate (58) that would realize the C–H activation step from the corresponding arene with a benzamide as directing group, however, this mechanism was ruled out due to an unfavorable activation energy of 38.9 kcal/mol. Hence, a redox-neutral Rh(III) pathway was found to be preferable for the ortho-bromination of benzamides. Nevertheless, employing other directing groups such as 2-pyridil led to an interesting point highlighted by the authors. The computational studies suggested a change in reaction mechanism where an oxidative addition of the NBS to the Rh(III) complex would lead to a more favorable Rh(V) intermediate in the reaction pathway. Along the same lines, the Glorius group recently reported the ortho-chlorination of different arenes and heteroarenes using 1,3-dichloro-5,5-dimethylhydantoin (DCDMH) as the chlorinating agent and [Cp*RhCl2]2 as the catalyst.56 Interestingly, DFT calculations of the reaction mechanism found large free-energy barriers of 24.4–29.9 kcal/mol for the formation of a Rh(V) intermediate from the oxidative addition of the chlorinating reagent to the Rh(III) species and hence, favored a substitution based mechanism (formal SN2-type) for this chlorination transformation. 3.2.2 Azides, 1,2,4-dioxazoles and anthranil as coupling partners Chang and coworkers developed a series of elegant methods to realize the directed C–H amidation of arenes using azides as amide sources under rhodium catalysis (Scheme 22).57 A thorough mechanistic investigation was conducted based on the two most important proposed steps of the reaction mechanism: i) its reaction with the azide and the generation of a metal– nitrene species, and ii) the release of the product and catalyst regeneration.58 Based on their previous results, the authors proposed a concerted and a stepwise pathway for the formation of the C–N bond from azide complex 60. Notably, the main difference from both pathways was related to the oxidation state of the metal center and how it would change to a higher Rh(V) species (61) in a stepwise pathway, whereas a redox-neutral Rh(III) intermediate would be obtained from the concerted route via transition state 62. DFT calculations indicated that the formation of the C–N bond of the product proceeds through a stepwise process that involves the


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generation of a rhodium–nitrenoid intermediate. The step was calculated to be exergonic by 4.3 kcal/mol and the formed Rh–nitrenoid intermediate was found to have a double bond character with a bond length of 1.85 Å. This highly oxidized species undergoes a facile reductive amido insertion through a three-centered transition state to give a Rh(III)–amido intermediate, a process that was found to be thermodynamically favorable. The authors also conducted a Natural Bond Orbital (NBO) analysis for the metal center during the C–N bond forming step and found a change in the electron density of the rhodium center (corresponding to a NBO charge change from 0.523 to 0.745) and an increase of the electron binding energy (89.3 to 90.5 eV), both results in accordance with the proposed change of the oxidation state of Rh(III) to Rh(V). DFT calculations for the proposed transition state of the concerted and redox-neutral pathway indicated a likely inaccessible energetic barrier of 43.6 kcal/mol. The combination of experimental and theoretical results allowed the authors to rule out the concerted reaction pathway and support a stepwise route with the formation of a Rh(V)–nitrenoid species 61 as the most likely mechanistic pathway for the amidation reaction.

Scheme 22. Directed C–H amination of arenes by Chang and coworkers with potential Rh(V) intermediate favored by DFT calculations. Following their studies of the mechanistic aspects of the Cp*Rh(III)-catalyzed amidation reaction, the groups from Chang and Musaev carried out the experimental and DFT investigation of the comparative reactivity of Cp*M(III) complexes (M = Ir, Rh, Co) in a directed C–H amination reaction of benzamides with organic azides.2b The authors proposed a general mechanistic overview of the reaction consisting of four key steps (63–66) which featured the formation of a nitrene intermediate after coordination of the azide and N2 release (Scheme 23). An intramolecular insertion into the M–C bond and protodemetalation would yield the aminated product. Computational studies on the reaction mechanism showed the formation of the nitrene intermediate to be exergonic by 11.9 kcal/mol, likely due to the release of N2 and the formation of the Rh=NPh species (65). Furthermore, the nitrene intermediate posses a double bond character with σ donation from the nitrogen to the metal center and back-donation (π* bond) from the rhodium center to the NPh fragment, indicating an oxidation of the metal center from Rh(III) to Rh(V). The subsequential insertion of the nitrenoid into the C–Rh bond (66) was found to be exergonic by 41.1 kcal/mol and with a small energy barrier of 7.5 kcal/mol. The authors indicated that the nitrene insertion step was thermodynamically and kinetically favored due to the instability of the Rh=NPh intermediate and the reduction of the Rh(V) nitrene species to the Rh(III) amido complex. The alternative mechanistic pathway where no net change in the Rh(III) species would be obtained was found to be highly endergonic by approximately 34 kcal/mol and a very unlikely alternative for the studied transformation.


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O N H

RN3

+

O

[Cp*MCl2]2 (4 mol%) AgSbF6 (16 mol%) 1,1,2,2-TCE, T, t

t BuHN

O

Product t BuHN

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N H NHR M T yield Ir 65 °C 93% Rh 110 °C 54% Co 110 °C 10%

Cp* M(III)

O

RN3 63

t BuHN

O

Cp* M(III) N R

t BuHN

Cp*

O

M(III) N R N2

66

64 t BuHN

O Cp* M(V) N

R

N2

65

Scheme 23. Comparative studies of Cp*M(III) complexes by Chang and Musaev. Continuing with their effort to improve directed C–H amination reactions, Chang and coworkers developed a new amidating reagent, the 1,4,2-dioxazole derivatives (67) that proved superior to coupling partners such as azides in C–H activation reactions.59 The group carried out a thorough study of the reaction mechanism using Cp*Ir(III) and Cp*Rh(III) complexes to obtain a better understanding of the preferential and faster reactivity of such amidating reagents with the Ir(III) complexes.59d The authors proposed two alternative reaction pathways involving a redox-neutral Rh(III) process or a Rh(III)/Rh(V)/Rh(III) manifold (Scheme 24). In path A, a concerted SN2type attack of the metal center along with CO2 cleavage, occurring at the same time, would lead to the expected product via the transition state 68. Alternatively, a stepwise process would lead to the formation of a M=N bond, giving the Rh(V)–imido species 69. A migratory insertion followed by protodemetalation would afford the corresponding products. DFT calculations indicated that the Rh–nitrenoid formation (69), along with carbon dioxide extrusion, might correspond to the rate determining step of the process. Kinetic experiments seems to support the DFT calculations as a first order was found for 1,4,2-dioxazolone in the reaction. The alternative path A was found to be much higher in energy (30.8 kcal/mmol for 68) which led the authors to disregard it as a possible reaction pathway.


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Scheme 24. Mechanistic alternatives for the C–H amidation of arenes with 1,4,2-dioxazoles by Chang and coworkers. Li and coworkers developed the use of anthranil (70) as coupling partner for the directed sp2 and sp3 C–H amination of arene derivatives (Scheme 25).60a The novel reaction showcased a high atom economy and furthermore, the introduction of two functional groups in the products in one step. The authors studied the reaction mechanism by means of experimental and computational studies. The transformation was proposed to proceed through the formation of a Rh(V) nitrenoid intermediate (71), from the cleavage of the N–O bond of the anthranil, as the pathway with the lowest energy. Anthranil was further used as an amination reagent by other groups in Cp*Rh(III)-catalyzed C–H activation of differnent substates.60

Scheme 25. C–H amination of arene derivatives as developed by Li, Lan and coworkers. 3.2.3 Hypervalent iodine compounds as coupling partners A number of examples with the use of hypervalent iodine compounds as coupling partners in C– H activation reactions have been reported in the last years. The group of Li and coworkers developed the ortho-alkynylation of (hetero)arenes using Cp*Rh(III) and Cp*Ir(III) complexes as catalysts and 1-[(triisopropylsilyl)-ethynyl]-1,2-benziodoxol-3(1H)-one (TIPS-EBX, 72) as the coupling partner (Scheme 26).61a For the Rh(III)-catalyzed system, the authors conducted a


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series of mechanistic studies to investigate the type of interaction between the catalyst and the hypervalent iodine compound. Although no strong conclusion regarding the intermediacy of a Rh(V) species was obtained, three different reaction mechanisms were proposed. Addition of the hypervalent iodine compound to the Rh(III) metallacycle 73 would result in a Rh(V) compound 74 that leads to the observed product after reductive elimination. Alternatively, the authors proposed a Rh(III)/Rh(I)/Rh(III) and a redox neutral Rh(III) catalytic cycle by intermediate 75 in a formal SN2-type reaction leading to product formation, however, both proposals were less supported by experimental evidence and previous literature. In three other similar approaches, the same group also proposed a Rh(V) intermediate in the directed alkynylation of aldehydes, azomethineylides and the C–H alkynylation of 2-pyridones, using TIPS-EBX as the coupling partner.61b–d

Scheme 26. Directed ortho-alkynylation of arenes by Li and coworkers with proposed Rh(III) and Rh(V) intermediates. Similarly to the studies from Li and coworkers, the groups from Loh62 and Glorius63 independently reported the Cp*Rh(III)-catalyzed alkynylation of alkenes using TIPS-EBX as the coupling partner. Both groups proposed the formation of a Rh(V) species as a possible intermediate in the reaction mechanism. Along the same lines, a number of other alkynylation reactions that employ hypervalent iodine compounds as coupling partners have also been proposed to occur via a Rh(V) intermediate.64 Loh and coworkers developed the chelate-assisted amidation of sp2 and sp3 C–H bonds using amidobenziodoxolones as amidating reagent under Cp*Rh(III) catalysis (Scheme 27).65 The reaction mechanism was proposed to proceed through two different pathways, both involving the formation of a Rh(V) intermediate in the form of 76 or the nitrene 77.


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Scheme 27. Directed ortho-amidation of arenes and alkenes by Loh and coworkers with proposed Rh(V) intermediates. 4. Conclusion and outlook The potential use of higher valent Rh(V) complexes represents a new approach in C–H activation reactions and offer different opportunities to improve and broaden the current state of the art in the field. It is expected that a better understanding of the reaction mechanisms and the intermediates involved would enable the development of more efficient catalytic processes. As discussed, plenty of examples have recently appeared in the literature that propose the presence of catalytically active Rh(V) intermediates in C–H activation reactions. However, it remains uncertain whether unambiguously assigned Rh(V) complexes could be isolated and hence, allow its direct use in catalytic processes. Studies on silyl and boryl complexes of Cp-derived rhodium catalysts have experimentally and theoretically demonstrated stable interactions among the ligands that prevent them from becoming higher valent Rh(V) species. Hence, to the best of our knowledge, the isolation and characterization of a Cp-derived Rh(V) complex has not been described in the literature. However, a number of different computational studies have shown plenty of support towards the existence of such higher valent intermediates, especially in the formation of Rh(V)-nitrenoid species. Yet, considering that Rh(V) species could be extremely fast reactive intermediates in solution, their characterization and further isolation represent an unfathomable task. Arguably, the use of tools such as cyclic voltammetry or 103Rh NMR spectroscopy along with the use of strong electron-donating ligands66 that can potentially stabilize highly electrophilic Rh(V) species, might represent valid alternatives to explore the synthesis and characterization of such intermediates. On an engaging fact, it has recently been showed by our group and others that considering a highly electrophilic Rh(V) species as intermediate in the reaction mechanism can lead to interesting and potentially unexpected transformations. We expect that this perspective allows the general audience to have a better understanding of the current findings related to higher valent rhodium intermediates in Rhcatalyzed C–H activation reactions and to consider such species as potential alternatives to explore new directions in the C–H activation field.

AUTHOR INFORMATION


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Corresponding Author *[email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The authors acknowledge financial support from the Deutsche Forschungsgemeinschaft (Leibniz Award). The authors are grateful to Tobias Knecht (WWU Münster) for helpful discussions. ABBREVIATIONS DFT, density functional theory; Cp, cyclopentadiene; Cp*, pentamethylcyclopentadiene; rt, room temperature; Ad, adamantyl; B(ArF)4, tetrakis[3,5-bis(trifluoromethyl)phenyl]borate; RE, reductive elimination; EWG, electron withdrawing group; Het, heteroarene.

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