Oxidatively Induced Reductive Elimination: Exploring the Scope and

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Oxidatively Induced Reductive Elimination: Exploring the Scope and Catalyst Systems with Ir, Rh, and Ru Complexes Jinwoo Kim, Kwangmin Shin, Seongho Jin, Dongwook Kim, and Sukbok Chang J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 14, 2019

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Journal of the American Chemical Society

Oxidatively Induced Reductive Elimination: Exploring the Scope and Catalyst Systems with Ir, Rh, and Ru Complexes Jinwoo Kim,†,‡,§ Kwangmin Shin,‡,§ Seongho Jin,†,‡ Dongwook Kim,‡ and Sukbok Chang*,‡,† †Department ‡Center

of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, South Korea

for Catalytic Hydrocarbon Functionalization, Institute for Basic Science (IBS), Daejeon 34141, South Korea

Supporting Information ABSTRACT: Direct conversion of C–H bonds into C–C bonds is a promising alternative to the conventional cross-coupling reactions, thus giving rise to a wide range of efficient catalytic C–H functionalization reactions. Among the elementary stages in the catalytic C–C bond formation, reductive elimination constitutes a key step of the catalytic cycle, and, therefore, extensive studies have been made to facilitate this process. In this regard, oxidation on the metal center of a post-transmetalation intermediate would be an appealing approach. Herein, we have explored the substrate scope, catalyst systems, and oxidation tools to prove that the oxidatively induced reductive elimination (ORE) plays a critical role in the product-releasing C–C bond formation. Notably, we have demonstrated that ORE broadly operates with a series of half-sandwich d6 Ir(III)-, Rh(III)-, and Ru(II)-aryl complexes. We have described that the metal center oxidation of the isolable post-transmetalation intermediates by means of chemical- or electro-oxidation can readily deliver the desired arylated products upon reductive elimination even at ambient temperature. Computation studies delineated the thermodynamics of the reductive elimination, where the activation barriers are shown to be significantly reduced upon increasing the oxidation states of the intermediates. We were also successful to corroborate this ORE in the corresponding Rh-methyl complex. In addition, catalytic conditions were optimized to incorporate this mechanistic understanding into the Ir, Rh, and Ru-catalyzed C–C bond formations under mild conditions. Rh(III), and isoelectric Ru(II) systems in regard to the oxidatively induced reductive elimination process.9-11 It was revealed that ORE operates commonly in each metal system, eventually leading to mild catalytic C–H arylation reactions.12 Significantly, the key post-transmetalation intermediates of Ir-, Rh-, and Ru-complexes were isolated and fully characterized by X-ray diffraction analysis. Cyclic voltammetry studies and DFT calculations strongly support the experimental results that oxidation of those transmetalation intermediates does facilitate the otherwise demanding reductive elimination process. An analogous mechanistic study on the C–H methylation was also performed on a Rh system to prove that the high-valent pathway also operates in this case. Finally, we have demonstrated that the ORE pathway can be applied to the Ir, Rh, and Ru catalytic systems under mild conditions.

INTRODUCTION Transition metal-catalyzed C–H bond activation and subsequent oxidative C–C bond formation using organometallic carbon nucleophiles has emerged as a promising alternative to the conventional cross-coupling procedures,1 leading to a more straightforward synthetic route to introduce carbon moieties.2 On the basis of the well-established elementary steps in palladium catalysis in particular, the C–C bond formation has been postulated to proceed via transmetalation followed by direct reductive elimination (Scheme 1a, top).3 On the other hand, an alternative high-valent pathway, where the reductive elimination is enabled by oxidation of the metal center of key post-transmetalation intermediate, has been appreciated in catalysis recently (Scheme 1a, bottom).4 It is noteworthy in that oxidative promotion of otherwise challenging reductive elimination process has long been documented albeit mainly in a stoichiometric manner.5,6 In this context, we recently reported that the oxidatively induced reductive elimination (ORE) pathway operates in the Cp*Ir(III)-catalyzed C(sp2)–H arylation with arylsilanes by the action of suitable oxidants (Scheme 1b).7 Along with our continuous interests in the intermediacy of high-valent species in C–H functionalizations,8 this finding called into further question whether ORE can be extended and generalized into other catalyst systems. Herein, we present detailed studies on the exploration of substrate scope, catalyst systems, and oxidation tools (Scheme 1c). In particular, we have investigated the comparative behaviors of Ir(III),

RESULT AND DISCUSSION At the outset of this study, we initially performed computational analysis to validate the feasibility of the envisioned oxidatively induced reductive elimination of the presupposed transmetalated intermediates (Figure 1). As the model post-transmetalation complexes, we envisioned to investigate Cp*Ir(III), Cp*Rh(III), and (pcymene)Ru(II)-aryl complexes, presumably obtainable from a cyclometalation of benzo[h]quinoline (1a) and subsequent transmetalation of an aryl group (Ir-Ar, RhAr, and Ru-Ar, respectively). DFT calculations on these presupposed species were first carried out to see whether oxidation can take place on the metal center. Indeed, the complex Ir-Ar revealed a metal-centered HOMO, and, 1

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moreover, expulsion of one electron from this neutral species provided metal-centered β-LUMO, thereby suggesting that the first oxidation of the iridacycle occurs at the Ir-metal center. Moreover, the corresponding analogues derived from Cp*Rh(III) and (p-cymene)Ru(II) (Rh-Ar and Ru-Ar) also exhibited the similar behavior on the metal-centered HOMO and β-LUMO configuration in

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the neutral and singly-oxidized cationic species, respectively. These computational results suggest that the one-electron oxidation of the presumed Rh- and Ru-aryl complexes occurs indeed at the metal center. The orbital analysis revealed that the HOMO of each transmetalation intermediate constitutes nonbonding metal d orbitals.

Scheme 1. Oxidatively induced reductive elimination for d6 Ir-, Rh-, and Ru-catalyzed C–C bond formation. (a) Plausible reaction pathways for TM-catalyzed C–C bond formation with aryl/alkyl nucleophiles

(b) Previous work: Oxidatively induced Ir-catalyzed C–H arylation using arylsilane O

DG

R [M'] C H

DG C

Mn

Me

Ar-Si(OEt)3 cat. Cp*IrIII

NHR H

cat. [M]

Me

Me

via 'Direct reductive elimination' (generally proposed)

Me Ir

O

THF, 25 oC

RHN

[Ox.]

DG C

NHR Ar

Isolated

C R

R

O heat X

Ar

DG

L

Me

III

Me

Me

Facile R.E. Through high-valent Ir species

L Mn+m R

Me

[Ox.]

Me

DG

via 'Oxidatively induced reductive elimination' (still elusive)

Ir C

Me

IV

Ar

(c) This work: Mechanistic studies on Ir-, Rh-, and Ru-catalyzed C–H aryl-/alkylation via 'oxidatively induced reductive elimination' pathway DG H

ligand

Ar B(OR)2 or + H3C B(OR)2

[M] DG

DG

C

R

m+

ligand [Ox.]

n

DG

R

Ir

Me Facile R.E. at r.t.

ligand

M

Rh

R R = Ar

Characterized

DG

Ru M=

R

C

High E barrier

DG

[O]

n

M

n+m

M

R

C

DG

m = 1 or 2

R

 Isolation & characterization of intermediates  ORE in d6 Ir, Rh, and Ru systems  Theoretical rational with DFT calculation  Application to catalytic conditions

Figure 1. Preliminary DFT calculation study on model substrates. (a) Kohn-Sham orbital plots of HOMO of the postulated post-transmetalation intermediates (left) and β-LUMO of the singly-oxidized cationic species (right). Isovalue = 0.1. (b) Orbital diagram of Ir-aryl species Ir-Ar. (c) NBO charge change upon oxidation of intermediate Ir-Ar. For orbital analysis and NBO charge change for Rh and Ru analogues, see the Supporting Information. m = 1, -LUMO

m = 0, HOMO

(a)

(b)

m+

N

dx2-y2

- e-

Ir(III+m)

dz2

CF3 Ir-Ar

m+

N

- e-

LAr LBQ

dxz

z

N

Rh(III+m)

y

HOMO-9

x

HOMO-11

N

(c) 0.103

m+

Ru(II+m)

Ar C

Ir

CF3 Rh-Ar

N

CF3

HOMO

dxy dyz

-e

0.294

-0.053 N

-

IrIII

- e-

-0.030 N

IrIV

CF3

CF3 Ru-Ar

-0.064

2


CF3 0.014

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their 13C NMR chemical shift at δ 147.5 and δ 166.4 ppm (doublet, 1JRh-C = 36.7 Hz) in CD2Cl2, respectively. Encouraged by the successful isolation of Ir- and Rharyl species, we also attempted to capture an isoelectric (p-cymene)Ru(II)-aryl analogue. We were delighted to see that the proposed transmetalation also operates albeit under slightly modified conditions, providing the desired aryl ruthenacycle Ru-Ar in 77% yield (Scheme 3b). 13C NMR signal of the transmetalated aryl ipso carbon appeared at δ 170.0 ppm. It should be mentioned that the transmetalation of in situ generated aryl borate did not take place in the absence of copper species, leading us to assume that Cu-aryl species forms first and subsequently transfers its aryl moiety to the metallacycles (M-OTFA, M: Ir, Rh, and Ru) to produce the desired posttransmetalation intermediates (M-Ar, M: Ir, Rh, and Ru).14

Furthermore, the NBO charge of each metal center was calculated to be significantly increased upon oxidation (from 0.103, 0.058, and -0.211 to 0.294, 0.232, and 0.142 for Ir, Rh, and Ru species, respectively) while that of two metal-bonded carbons was relatively less sensitive to the oxidation, further substantiating the metal-centered oxidation in those model complexes (See the Supporting Information for details). Scheme 2. complexes.

Preparation

of

pre-transmetalation

(a)

[Cp*MCl2]2 +

N 1a (2.0 eq.)

AgOTFA (4.0 eq.) Li2CO3 (2.0 eq.)

M

N

CH2Cl2 (0.03 M) 25 °C, 24 h M = IrIII 86% RhIII 60%

OTFA

M = Ir Ir-OTFA Rh Rh-OTFA

(b)

[(p-cymene)RuCl2]2 +

1a

(2.0 eq.)

Above conditions

N

60%

Scheme 3. Preparation of Ir-, Rh- and Ru-aryl species.

Ru OTFA

(a)

Ru-OTFA

N

After obtaining the theoretical insights, we next attempted to capture experimentally the postulated key transmetalation intermediates. As the first step, metallacycles were prepared from stoichiometric cyclometalation reactions of Ir, Rh, and Ru metal precursors with benzo[h]quinoline (Scheme 2),13 and the corresponding neutral metallacyclic compounds IrOTFA, Rh-OTFA, and Ru-OTFA were isolated in reasonable yields. As the next step, stoichiometric transmetalation of aryl group was tried. Among various arylating agents examined, we were pleased to find that arylboronic esters were readily transmetalated to the metallacycles. (4-Trifluoromethyl)phenyl neopentylglycol borate (2a) as a representative coupling partner was found to smoothly react with metallacycles Ir-OTFA and Rh-OTFA in the presence of Cu(OAc)2 (0.5 equiv) and ammonium fluoride promoter (TBAF, 2.0 equiv) to form the corresponding post-transmetalation intermediates IrAr and Rh-Ar in 77% and 89% yields, respectively (Scheme 3a). Significantly, both complexes were stable upon exposure to air for a few weeks. The ipso carbon of the transmetalated aryl group of Ir-Ar and Rh-Ar showed (a)

F3C

M OTFA

M = Ir Ir-OTFA Rh Rh-OTFA (b)

F3C

N

Estimates

THF, 25 oC, 12 h M = IrIII 77% RhIII 89% O B 2a O

OTFA

N

M CF3

M = Ir Ir-Ar Rh Rh-Ar

Me (2.0 eq.) Me

Cu(OAc)2 (50 mol %) tBuOK (2.0 eq.)

Ru

N

Ru

THF, 25 oC, 12 h 77%

CF3

Ru-OTFA

Ru-Ar

The structure of all three transmetalated metal-aryl complexes were unambiguously determined by single crystal X-ray diffraction analyses (Figure 2). In Ir-Ar, the distance between the Ir center and ipso carbon of the transmetalated aryl group is 2.050(2) Å, being similar to that of the previously reported analogous benzamide complex (2.050 Å).7 Bond length of the metal–C(aryl) bond of the corresponding Rh and Ru species is 2.035(2) Å and 2.071(2) Å, respectively, and that between metal center and benzo[h]quinolinyl carbon is in a similar range for all three metal complexes.

(b)

Ir–C(Ar) 2.050(2) Å Ir–C(BQ) 2.055(2) Å C(Ar)-Ir-C(BQ) 85.67(9)°

O Me B (2.0 eq.) Me 2a O Cu(OAc)2 (50 mol %) (nBu)4NF·xH2O (2.0 eq.)

(c)

Estimates Rh–C(Ar) 2.0349(19) Å Rh–C(BQ) 2.0478(18) Å C(Ar)-Rh-C(BQ) 85.85(7)°

Estimates Ru–C(Ar) 2.071(2) Å Ru–C(BQ) 2.059(2) Å C(Ar)-Ru-C(BQ) 86.21(8)°

Figure 2. ORTEP diagrams of (a) aryl iridacycle Ir-Ar, (b) aryl rhodacycle Rh-Ar, and (c) aryl ruthenacycle Ru-Ar. (50% probability ellipsoids for each species). Hydrogen atoms were omitted for clarity. 3



The C–M–C angles are 85.67(9)°, 85.85(7)°, and 86.21(8)° for M = Ir, Rh, and Ru, respectively, revealing a pseudo-octahedral geometry. Unlike Ir-Ar and Rh-Ar, the aryl group of Ru-Ar is slightly bent toward the pcymene ligand showing M–C(ipso)–C(-CF3) = 166.3o.15 It should be emphasized that, to our best knowledge, RhAr represents the first example of such isolated transmetalated species obtained from Cp*Rh(III) derivatives while Rh(III)-catalyzed direct C–H arylation with aryl nucleophiles has been known.9d,f,g,16 Likewise, to our best knowledge, transmetalation of an aryl group on the cyclometalated Ru(II) center has not been documented despite of the widely reported Ru-catalyzed biaryl coupling reactions.9a,k

and Ru-aryl complexes, we attempted to validate our initial working hypothesis that oxidation of the posttransmetalation complexes can facilitate the desired reductive elimination to release the C–C coupled product. Referring to the known oxidation potential of AgI (E1/2 = 0.41 V vs. Fc/Fc+ in THF), it was predicted that a silver(I) oxidant can generate the postulated high-valent metallacycle species from the isolated Ir-, Rh-, and Ruaryl intermediates.19 Indeed, when complexes Ir-Ar, RhAr, and Ru-Ar were treated with AgOTFA (2.2 equiv) in THF solvent at room temperature, reductive elimination occurred smoothly in all cases, and the desired arylated product 3a was provided in 70%, 97%, and 92% yield, respectively. In stark contrast, reductive elimination did not take place even at 80 °C under oxidant-free conditions, while the post-transmetalation species remained intact. Importantly, the use of 1.0 equiv of AgOTFA gave rise to 3a in 64%, 69%, and 74% yields from Ir-Ar, Rh-Ar, and Ru-Ar, respectively, implying that the reductive elimination is induced by single-electron oxidation.

(a) N

Ru

N

Rh

N

Epa(Ru-Ar) (0.164)

Epa(Rh-Ar) (0.331)

0.2

CF3

E1/2(Ag/Ag+) (0.41)

Epa(Ir-Ar) (0.406)

Potential(V vs. Fc/Fc+, 0.1

Ir

CF3

CF3

0.3

Table 1. Stoichiometric ORE of Ir, Rh, and Ru-aryl species in the presence of Ag(I) oxidant.

THF) 0.4

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0.5

(b)

Cp* or p-cymene N 20 A

Cp* or p-cymene

Mn

AgOTFA Ar n

THF T, 12 h

N

M(n+m) Ar

Ar

N

3a

III

M = Ir , RhIII, RuII Ar = (4-CF3)C6H4

m+

m = 1 or 2

heat, 80 °C

Figure 3. (a) Comparison of the first Epa values for Ir, Rh, and Ru aryl species in THF. (b) Narrow potential window CV of Ir, Rh, and Ru aryl species around the first oxidation potential. (scan rate = 800 mV/s)

Entry

Ag (equiv)

T (°C)

1 2 3

2.2 1.0 -

25 25 80

Yield (%)a Ir-Ar Rh-Ar Ru-Ar 70 97 92 64 60 74 n.d.b n.d. n.d.

Yield was determined by 1H NMR spectroscopy using 1,1,2,2tetrachloroethane as an internal standard. bn.d., not detected. a

For a quantitative evaluation on the current oxidative acceleration of the reductive elimination from the posttransmetalation intermediates, we performed DFT calculations (Figure 4). As anticipated, the activation barrier (ΔGRE‡) was calculated to be decreased dramatically upon oxidation of the metal centers. For instance, the postulated reductive elimination requires 41.2, 19.1, and 5.6 kcal/mol starting from IrIII, IrIV, and IrV-aryl precursors, respectively. Likewise, ΔGRE‡ of RhAr and Ru-Ar was also significantly reduced upon metallic oxidation of the transmetalation complexes. In the case of Rh-Ar, it was shown to change from 28.9 kcal/mol (RhIII) to 9.4 kcal/mol (RhIV), thus suggesting a facile reductive elimination from the supposed Rh(IV) precursor even at ambient temperature.20 The energy barrier for the reductive elimination from Ru-Ar was also mitigated upon oxidation: ΔGRE‡ = 33.9, 14.8, and 8.9 kcal/mol from RuII, RuIII, and RuIV species, respectively. These computational estimations are fully consistent with the experimental results in Table 1.

To investigate redox behavior of the obtained posttransmetalation complexes, a series of cyclic voltammetry studies were conducted (Figure 3, and also see the Supporting Information for details). Cyclic voltammogram of Ir-Ar showed the first quasi-reversible redox event at E1/2 = 0.325 V (Epa = 0.406 V, scan rate = 800 mV/s), which was tentatively assigned to the IrIII/IrIV redox couple.7 The second irreversible oxidation was measured at Epa = 0.821 V (800 mV/s), which was assigned to the oxidation of IrIV species to IrV. On the other hand, CV of Rh-Ar displayed a single irreversible oxidation peak at Epa = 0.331 V (800 mV/s).17 Finally, CV of Ru-Ar revealed a single oxidation wave at Epa = 0.164 V (800 mV/s), indicating that this complex tends to be more easily oxidized compared to the Ir and Rh counterparts. At this stage, it is not clear whether these irreversible redox events are related to single-electron transfer or two-electron oxidation due to the potential inversion.18 Having identified the redox behaviors of the Ir-, Rh-

4


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TS-RE

TS-RE

GRE‡

M-Pdt

GRE

M-Ar

(a) Cp* or p-cymene

RhIII(28.9)

TS-RE CF3

C–C Bond length of the transtion state

Reductive elimination exothermicity (c)

IrIV(19.1) RuIII(14.8)

M-Ar

Mn+m

RuII(33.9)

Mn+m

N

N

(b)

IrIII(41.2)

m+

M-Pdt

M-Ar

Activation energy for the reductive elimination

‡ m+

Cp* or p-cymene

RuIV(8.9)

RhIV(9.4)

CF3 m = 0 to 2

IrV(5.6) RhV(0.6)

Figure 4. Representative data of calculated energy profiles for reductive elimination from Ir, Rh, and Ru-aryl intermediates with different oxidation states. (a) The activation barriers for reductive elimination from neutral, cationic, and dicationic Ir, Rh, and Ru-aryl species. Note its decreasing tendency along the increase in the metal oxidation state. (b) Energy difference between each metal-aryl complex and the reductive elimination product. (c) The distance between ipso carbons of benzo[h]quinoline and (4-trifluoromethyl)phenyl moiety in each transition state. See the Supporting Information for the detail. pathway where reductive elimination takes place right after the transmetalation, we propose an alternative highvalent pathway in the presupposed Ir-, Rh-, and Rucatalyzed C–H arylation. We surmised that if each mechanistic step, cyclometalation, transmetalation, and reductive elimination, takes place in concordance with each other, the catalytic cycle will successfully operate to provide C–H arylated product.

Upon oxidation of the metal center, the reductive elimination process was calculated to become exothermic with more negative ΔGRE (Figure 4b). Along with this, the distance between two ipso carbons (benzo[h]quinolinyl and transmetalated aryl moieties) in the transition state was more elongated at the higher oxidation, thus leading to more reactant-like transition state structure according to the Hammond postulate (Figure 4c). With the above computational results altogether that reduction in ΔGRE‡, reversal in the exothermicity, and change in the transition state geometry, it strongly supports that the reductive elimination is indeed promoted both kinetically and thermodynamically upon oxidation.

Table 2. Catalytic C–H arylation via oxidatively induced reductive elimination pathway.a 25~50 °C

N

Scheme 4. Catalytic cycle for the oxidatively induced reductive elimination.

O B

+ X

1a

2a 2b 2c

1a

Mn

N

L

Mn

L

n-1

Mn-2

Transmetalation ArB(OR)2, CuII 3a RE

3a

N

Low-valent pathway

RE N

Mn

L Ar

Ar = (4-CF3)C6H4

L

Mn+1

Ar

N

X =CF3 CH3 Br

3a-c

X

Cyclometalation

[Ox.]

M

IrIII RhIII RuII

O

[Ox] High-valent pathway

[Cp*IrCl2]2 [Cp*RhCl2]2 [(p-cymene)RuCl2]2 [(p-cymene)RuCl2]2

T (°C) 25 25 25 50

Yield (%) 75 75 38b 60b

Br Br Br

[Cp*IrCl2]2 [Cp*RhCl2]2 [(p-cymene)RuCl2]2

25 25 50

66 81 65b

CH3 CH3 CH3

[Cp*IrCl2]2 [Cp*RhCl2]2 [(p-cymene)RuCl2]2

25 25 50

95 90 16b

Entry

X

Catalyst

1 2 3 4

CF3 CF3 CF3 CF3

5 6 7 8 9 10

Reaction conditions: 1a (0.10 mmol), 2 (2.0 equiv), catalyst (5 mol %), AgNTf2 (20 mol %), Cu(OAc)2 (50 mol %), AgF (2.2 equiv) in THF/TFE (1:1, 0.5 mL), 12 h. Yields were determined by 1H NMR spectroscopy using 1,1,2,2-tetrachloroethane as the internal standard. bAg2O (2.2 equiv) and Cu(OTf)2 (50 mol %) were used instead of AgF and Cu(OAc)2

a

Based on the above mechanistic descriptions, we envisaged that oxidatively induced reductive elimination (ORE) would be operative even in the catalytic C–H arylation of benzo[h]quinoline with arylboronic esters (Scheme 4). Instead of the generally accepted low-valent 5



eventually giving rise to a broadly applicable C–H arylation procedure in catalytic cycle. We thus examined direct C–H arylation on various substrates using arylboronic ester coupling partner with Cp*Ir(III) as the representative catalyst system (Scheme 5).9e,23 Pleasingly, a wide range of substituted phenyl boronic esters were smoothly reacted with benzo[h]quinoline (1a) at room temperature irrespective their electronic variation (3a–3d). Pyridyl and amide moieties were found to be effective directing groups (3e–3f). Olefinic C–H arylation proceeded stereoselectively under the present conditions (3g). Quinoline N-oxide was arylated exclusively at the C8 position at ambient conditions to afford the corresponding product 3h in high yield. Substrates bearing weak coordinating groups participated efficiently in the C–H arylation but at slightly higher temperature (50 °C). For instance, α-tetralone was arylated selectively at the peri position in reasonable yield (3i). Biologically relevant chromone was also effectively arylated to give 3j in 81% yield. In the same line, propiophenone and 1acetylcyclohenene were reacted with aryl borate under the present Ir catalytic conditions to afford 3k and 3l, respectively. Notably, this reaction represents the first example of Ir-catalyzed direct C–H arylation using arylboronic esters. In addition, the Ir catalyst system displayed high compatibility with diverse directing groups for the C–H arylation when compared to the corresponding Rh or Ru systems. It should be mentioned that Sorensen et al. reported an Ir(III)-catalyzed C–H alkylation of aromatic aldehydes using alkylboron reagents.24 Our current working hypothesis that product-releasing reductive elimination is enabled by the oxidation of metal center of transmetalated intermediates led us next to surmise that external electric potential may also induce the same ORE process.4h,6b,25 As a preliminary study, we briefly performed an electrolysis of Ir-Ar in 0.3 M nBu4NPF6 electrolyte solution (Scheme 6). To minimize side reactions presumably arising from excessive potential, we arbitrarily chose the external potential as an average value of the first anodic peak Epa (50 mV/s) and the onset potential Eonset. We were delighted to find that the desired reductive elimination of Ir-Ar did occur by the electrooxidation to afford the C–H arylated product 3a in quantitative yield. Without the electric potential, no conversion was observed even in the prolonged time.26

To see the feasibility of the postulated high-valent pathway in catalytic systems, we preliminarily examined C–H arylation of benzo[h]quinoline (1a) with representative aryl borates (2) by employing each Ir, Rh, and Ru catalyst in the presence of silver(I) oxidant (Table 2). Reaction of 1a with 2a (X = CF3) took place smoothly at room temperature with Cp*Ir(III) or Cp*Rh(III) catalysts in the presence of Cu(OAc)2 transmetalating reagent, providing 75% yield of 3a in each case (entries 1–2). On the other hand, the same reaction with (pcymene)Ru(II) catalyst was a bit sluggish (entry 3), thus requiring slightly elevated temperature (50 °C) to obtain 3a in 60% by using Ag2O and Cu(OTf)2 instead of AgF and Cu(OAc)2 (entry 4).21 Variation on the substituents of arylboronic esters was then briefly examined (entries 5– 10). p-Bromophenyl moiety was transferred without difficulty under the catalytic conditions using Ir, Rh, and Ru catalysts, yielding 66%, 81%, and 65% of 3c (entries 8–10). On the other hand, in case of p-tolyl boronic ester 2b, Ru catalyst was less effective than Ir or Rh systems 3c (entries 8–10).22 Scheme 5. Effect of directing groups in the Cp*Ircatalyzed C–H arylation with ArB(OR)2.a

DG + H

Ar

O B

1 Substrate

[Cp*IrCl2]2 (5 mol %) AgNTf2 (20 mol %) AgF (2.2 eq.) Cu(OAc)2 (50 mol %)

DG

PhCF3/TFE, 25 oC

O 2

Ar 3

Product

Substrate

Product

X N

1a

N+

N

H

N+ H O1h 3a X = CF3 3b Me 3c Br 3d Cl

93% 98% 96% 98%

H

O 3i: 72%b

1i H

H 1e

CF3

tBu

H 1f

N H 3f: 77%

N H H 1g

tBu

3j: 81%b

H 1k

tBu

CF3 O

O

CF3

N H

O

tBu

O

O

nPr

1j

O N H

O

CF3

O O

3e: 69%

O

CF3

O

N

N

O-

3h: 84%

3k: 59%b

CF3

Scheme 6. Preliminary study for electrolytic reductive elimination through oxidatively induced pathway.

O

O

nPr 3g: 53%

CF3

Page 6 of 11

H 1l

3l: 62%b

CF3

N

Reaction conditions: 1 (0.10 mmol), 2 (2.0 equiv), catalyst (5 mol %), AgNTf2 (20 mol %), Cu(OAc)2 (50 mol %), and AgF (2.2 equiv) in PhCF3/TFE (1:1, 0.5 mL) for 12 h at 25 oC. Isolated yields are reported. bRun at 50 °C. a

N

Ir CF3 Ir-Ar

We subsequently wondered whether arylboronic esters can also be transmetalated to metallacycles obtainable from substrates bearing various directing groups, thus

0.3 M TBAPF6, THF:TFE=1:1 const. E 25 °C, 6 h

3a

CF3

E > Eonset quant No potential not detected

Encouraged by the successful demonstration on the oxidatively induced reductive elimination in the above catalytic C–H arylation with Ir, Rh, and Ru catalyst 6


Page 7 of 11

systems, we next wondered whether the similar highvalent pathway would work also in a C–H methylation reaction.27 We first attempted to synthesize a metallacycle methyl complex for a model study.28 Pleasingly, when Rh-OTFA was treated with 2,5,5-trimethyl-1,3,2dioxaborinane (4) in the presence of tBuOK at room temperature, a cyclometalated rhodium-methyl complex Rh-Me was formed and isolated in 48% yield (Scheme 7). The transmetalated methyl group displayed characteristic doublets in 1H and 13C NMR spectra at  -0.40 (2JH-Rh = 2.40 Hz) and  -1.94 (1JC-Rh = 27.94 Hz) ppm in CD2Cl2, respectively. This Rh-Me complex was observed to be stable under air exposure for several weeks. It is interesting that, unlike the aryl group transfer to RhOTFA (Scheme 3a), copper additive was not required in the transmetalation of the methyl group. Although the exact mechanism of this methyl transmetalation is not fully understood at the present stage, we tentatively assume that it may be attributed to the relatively electronrich property of the methyl group.16,29 It should be mentioned that the transfer of a methyl group from boronic esters to a Cp*-rhodacycle has not been reported, and to our best knowledge, this represents the first example of isolating a rhodacycle methyl complex.

Me was performed (Figure 6). As anticipated, the energy barrier for this reductive elimination from Rh-Me was found to be dramatically reduced upon increasing the oxidation state (ΔGRE‡ = 28.5, 8.6, and 0.8 kcal/mol for RhIII, RhIV, and RhV, respectively). As a result, it implies that the C–C bond formation from RhIII-Me is challenging but that it will be much more facile at the higher oxidation states. The reductive elimination process becomes more exothermic as the oxidation state of the rhodium center of Rh-Me increases, suggesting that as in the case of Rh-Ar, the product-forming reductive elimination is more feasible thermodynamically and kinetically upon oxidation. In accordance with the Hammond postulate, the transition states for the reductive elimination also display longer C(sp2)–C(methyl) distance as the oxidation state of the metal center increases, being reminiscence to the reactant metallacycle.

Scheme 7. Preparation of Rh-Me using methyl boronic ester.

N

O Me Me B 4 Me (2.0 eq.) O tBuOK (2.0 eq.)

Rh OTFA

Rh-OTFA

THF, 25 oC, 12 h 48%

Rh

N

Me

Rh-Me Estimates

N

Rh–CH3 2.175(6) Å Rh–C(sp2) 2.085(6) Å  C(sp2)-Rh-C(Me) 85.1(4)°

Rh Me

Figure 5. ORTEP diagram of methyl rhodacycle Rh-Me. (50% probability ellipsoids for each species). Hydrogen atoms were omitted for clarity.

Rh-Me

Structural analysis of the newly prepared Rh-Me complex was next carried out with measurement of its electrochemical behavior. The solid structure of Rh-Me was determined by a single crystal XRD analysis (Figure 5). The Rh–C(methyl) bond length is 2.175(6) Å, slightly longer than Rh–C(aryl) distance of the aryl counterpart (Figure 2b), and the C(sp2)–M–C(methyl) angle of Rh-Me is 85.1(4)°, thus displaying a pseudo-octahedral pianostool structure. The cyclic voltammogram of Rh-Me showed an irreversible oxidation at Epa = 0.159 V (800 mV/s), being significantly lower when compared to its aryl counterpart (Rh-Ar, 0.331 V), implying that metallic oxidation of Rh-Me would be more feasible than that of Rh-Ar species (See the Supporting Information for details). An orbital analysis on Rh-Me revealed that oxidation indeed will occur at the metal center of this transmetalated methyl complex as in case of its aryl analogue (Rh-Ar). However, at the current stage, it is not clear whether this irreversible peak represents an oneelectron transfer or two-electron oxidation. We next wondered whether the metallic oxidation of Rh-Me will also promote the desired carbon-methyl bond forming reductive elimination. For the quantitative estimation, computational analysis on the postulated reductive elimination from various oxidation states of Rh-

‡ n+

N

Rh(III+n)+ CH3

N

Rh(III+n)-Me

n+

Rh(III+n)+ N

CH3

Rh(I+n)+ CH3 Rh(I+n)-Pdt

TS-RE

TS-RE(31) (28.5)

G (kcal/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


RhI-Pdt (24.5) RhIII-Me (0.0)

RhIV-Me (0.0)

TS-RE(42) (8.6)

RhII-Pdt (-8.9)

TS-RE(53) (0.8) RhV-Me (0.0)

RhIII-Pdt (-32.3)

Figure 6. Calculated energy profiles for reductive elimination from Rh-Me in various oxidation states. See the Supporting Information for the detailed calculation results. Next, we briefly examined a stoichiometric ORE experiment (Scheme 8). Consistent with the computational prediction, reductive elimination of Rh-Me proceeded smoothly at room temperature by the action of chemical oxidant, AgOTFA (2.2 equiv) herein, giving rise to 10-methylbenzo[h]quinoline 5 in quantitative yield. When 1 equiv of silver oxidant was employed, 5 was 7



obtained in 64% yield under otherwise identical conditions. Again, we assume that this result suggests that the reductive elimination of Rh-Me proceeds via the formation of singly-oxidized RhIV-Me intermediate. No background reductive elimination was detected from RhMe under thermal conditions (80 oC) without silver oxidant. We also observed that the mechanistic understanding was readily applied to the Rh-catalyzed C– H methylation process under mild conditions (Scheme 8b).

The Supporting Information is available free of charge on the ACS Publication website Experimental procedures, analysis data and NMR spectra for products, and details of the computational studies including coordinates of all the species discussed (PDF) Crystallographic data of Ir-Ar, Rh-Ar, Ru-Ar, Ir-Me, Rh-Me, and Ru-Me (CIF)

AUTHOR INFORMATION Corresponding Authors *[email protected]

Scheme 8. Oxidatively induced reductive elimination from Rh-Me intermediate using Ag(I) oxidant under stoichiometric and catalytic conditions. (a)

ORCID Sukbok Chang: 0000-0001-9069-0946

w/o Ag(I), 80 °C

Author Contributions §J. K. and K. S. contributed equally.

n+

AgOTFA (x eq.)

Rh

N

N

CH3 THF 25 °C, 12 h

Rh(III+n)

Notes The authors declare no competing financial interest.

N

CH3

Me

5

[Ag] (eq.) Yield n = 1 or 2

Rh-Me (b)

N 1a

+ H 3C

O B

2.2 1.0

[Cp*RhCl2]2 (5 mol %) AgNTf2 (20 mol %) AgF (2.2 eq.) O

4 (2.2 eq.)

THF:TFE = 1:1 25 °C, 12 h, 85%

5

98% 64%

ACKNOWLEDGEMENTS

N

REFERENCES

This research was supported by the Institute for Basic Science (IBS-R010-D1) in Korea.

CH3

(1) Applications II: Transition Metal Compounds in Organic Synthesis 2, Hiyama, T., Ed. Comprehensive Organometallic Chemistry III From Fundamentals to Applications; Crabtree, R. H.; Mingos, D. M. P. Elsevier Science: 2007; Vol. 11. (2) Handbook of C-H Transformations: Applications in Organic Synthesis, Dyker, G., Ed. Wiley-VCH: 2005. (3) (a) Chen, X.; Engle, K. M.; Wang, D. H.; Yu, J. Q. Palladium(II)-catalyzed C−H activation/C−C cross-coupling reactions: versatility and practicality. Angew. Chem., Int. Ed. 2009, 48, 5094-5115. (b) Jana, R.; Pathak, T. P.; Sigman, M. S. Advances in Transition Metal (Pd,Ni,Fe)-Catalyzed CrossCoupling Reactions Using Alkyl-organometallics as Reaction Partners. Chem. Rev. 2011, 111, 1417-1492. (c) Bedford, R. B. How Low Does Iron Go? Chasing the Active Species in FeCatalyzed Cross-Coupling Reactions. Acc. Chem. Res. 2015, 48, 1485-1493. (4) (a) Chen, X.; Li, J.-J.; Hao, X.-S.; Goodhue, C. E.; Yu, J.-Q. Palladium-Catalyzed Alkylation of Aryl C–H Bonds with sp3 Organotin Reagents Using Benzoquinone as a Crucial Promoter. J. Am. Chem. Soc. 2006, 128, 78-79. (b) King, A. E.; Brunold, T. C.; Stahl, S. S. Mechanistic Study of Copper-Catalyzed Aerobic Oxidative Coupling of Arylboronic Esters and Methanol: Insights into an Organometallic Oxidase Reaction. J. Am. Chem. Soc. 2009, 131, 5044-5045. (c) Wang, X.; Lu, Y.; Dai, H.-X.; Yu, J.Q. Pd(II)-Catalyzed Hydroxyl-Directed C−H Activation/C−O Cyclization: Expedient Construction of Dihydrobenzofurans. J. Am. Chem. Soc. 2010, 132, 12203-12205. (d) Hoyt, J. M.; Sylvester, K. T.; Semproni, S. P.; Chirik, P. J. Synthesis and electronic structure of bis(imino)pyridine iron metallacyclic intermediates in iron-catalyzed cyclization reactions. J. Am. Chem. Soc. 2013, 135, 4862-4877. (e) Neufeldt, S. R.; Seigerman, C. K.; Sanford, M. S. Mild Palladium-Catalyzed C–H Alkylation Using Potassium Alkyltrifluoroborates in Combination with MnF3. Org. Lett. 2013, 15, 2302-2305. (f) Cassani, C.; Bergonzini, G.; Wallentin, C.-J. Active Species and Mechanistic Pathways in Iron-Catalyzed C–C Bond-Forming Cross-Coupling Reactions.

CONCLUSION In conclusion, we have proved that the oxidatively induced reductive elimination (ORE) can be significantly extended to Ir-, Rh-, and Ru-catalyst systems for the C–H arylation. Arylboronic esters were efficiently transmetalated to afford the key post-transmetalation intermediates which were fully characterized to include X-ray structural analysis for the first time. In parallel with computational studies and electrochemical analyses on the isolated transmetalated species, experimental observations confirmed that metallic oxidation dramatically changes the potential energy surface, thus eventually allowing otherwise inaccessible productreleasing reductive elimination to operate under mild conditions. The current approach based on the high-valent reductive elimination pathway was also successfully applied to the catalytic conditions with various cyclometalating substrates. In addition, a transmetalated Rh-methyl intermediate was isolated for the first time, and its analogous behavior toward ORE was also investigated. The present study clearly demonstrates that ORE is a convincing strategy to enable a catalytic system by facilitating the key C–C bond-forming stage. We anticipate that our current results would provide further insights on the elementary stages in coupling reactions to eventually offer an opportunity for exploring previously inaccessible catalytic reactions. ASSOCIATED CONTENT Supporting Information 8


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Catalyzed Mild and Broad C−H Arylation of Arenes and Alkenes with Aryldiazonium Salts Leading to the External Oxidant-Free Approach. J. Am. Chem. Soc. 2015, 137, 8584-8592. (c) She, Z.; Wang, Y.; Wang, D.; Zhao, Y.; Wang, T.; Zheng, X.; Yu, Z. X.; Gao, G.; You, J. Two-Fold C–H/C–H Cross-Coupling Using RhCl3·3H2O as the Catalyst: Direct Fusion of N(Hetero)arylimidazolium Salts and (Hetero)arenes. J. Am. Chem. Soc. 2018, 140, 12566-12573. (24) For Ir-catalyzed C–H alkylation using alkyl boronate, see: Chen, X.-Y.; Sorensen, E. J. Ir(III)-catalyzed ortho C–H alkylations of (hetero)aromatic aldehydes using alkyl boron reagents. Chem. Sci. 2018, 9, 8951-8956. (25) (a) Dudkina, Y. B.; Mikhaylov, D. Y.; Gryaznova, T. V.; Tufatullin, A. I.; Kataeva, O. N.; Vicic, D. A.; Budnikova, Y. H. Electrochemical Ortho Functionalization of 2-Phenylpyridine with Perfluorocarboxylic Acids Catalyzed by Palladium in Higher Oxidation States. Organometallics 2013, 32, 4785-4792. (b) Saito, F.; Aiso, H.; Kochi, T.; Kakiuchi, F. PalladiumCatalyzed Regioselective Homocoupling of Arenes Using Anodic Oxidation: Formal Electrolysis of Aromatic Carbon– Hydrogen Bonds. Organometallics 2014, 33, 6704-6707. (c) Grayaznova, T. V.; Dudkina, Y. B.; Islamov, D. R.; Kataeva, O. N.; Sinyashin, O. G.; Vicic, D. A.; Budnikova, Y. Н. Pyridinedirected palladium-catalyzed electrochemical phosphonation of C(sp2)–H bond. J. Organomet. Chem. 2015, 785, 68-71. (d) Bu, Q.; Gońka, E.; Kuciński, K.; Ackermann, L. Cobalt-Catalyzed Hiyama-type C–H Activation with Arylsiloxanes: Versatile Access to Highly-Ortho-Decorated Biaryls. Chem. Eur. J. [Online early access]. DOI: 10.1002/chem.201806114. (e) Gao, X.; Wang, P.; Zeng, L.; Tang, S.; Lei, A. Cobalt(II)-Catalyzed Electrooxidative C–H Amination of Arenes with Alkylamines. J. Am. Chem. Soc. 2018, 140, 4195-4199 (26) (a) Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J. L. Acid-Assisted Reductive Elimination as a Route to Platinum(II) Products from Platinum(IV) Tris(pyrazolyl)borate Reagents. Organometallics 2000, 19, 3854-3866. (b) Rocchigiani, L.; Fernandez-Cestau, J.; Budzelaar, P. H. M.; Bochmann, M. Reductive Elimination Leading to C–C Bond Formation in Gold(III) Complexes: A Mechanistic and Computational Study. Chem. Eur. J. 2018, 24, 8893-8903 (27) Wang, B.; Li, C.; Liu, H. Cp*Rh(III)-Catalyzed Directed C–H Methylation and Arylation of Quinoline N-Oxides at the C8 Position Adv. Synth. Catal. 2017, 359, 3029-3034. (28) (a) Werner, H.; Werner, R. Aromaten(phosphan)metallKomplexe, IX. Synthese und Elektrophilie dikationischer (Aromaten)metall(II)-Komplexe des Typs [C6R6’M(PMe3)2C2H3R]2+ (M = Ru, Os). Chem. Ber. 1985, 118, 4543-4552. (b) McGhee, W. D.; Bergman, R. G. Synthesis of an (η3-allyl)(hydrido)iridium complex and its reactions with arenes and alkanes. Sequential intermolecular carbon-hydrogen oxidative addition and hydride-to-alkene migratory insertion reactions. J. Am. Chem. Soc. 1988, 110, 4246-4262. (c) Alaimo, P. J.; Arndtsen, B. A.; Bergman, R. G. Alkylation of Iridium via Tandem Carbon–Hydrogen Bond Activation/Decarbonylation of Aldehydes. Access to Complexes with Tertiary and Highly Hindered Metal–Carbon Bonds. Organometallics 2000, 19, 2130-2143. (29) Partyka, D. V. Transmetalation of Unsaturated Carbon Nucleophiles from Boron-Containing Species to the Mid to Late d-Block Metals of Relevance to Catalytic C–X Coupling Reactions. (X = C, F, N, O, Pb, S, Se, Te) Chem. Rev. 2011, 111, 1529-1595.

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Oxidatively Induced Reductive Elimination: Exploring the Scope and

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