Marius Lutz

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Document DEVICE REPORTERCLutzetal MS Chem - accepted version - RC
This document is the Accepted Manuscript version of a Published Work that appeared in final form in Chem, copyright © Cell Press after peer review and technical editing by the publisher. This manuscript version is made available under the CC-BY-NC-ND 4.0 license To access the final edited and published work see
Shuttle Arylation by Rh(I) Catalyzed Reversible Carbon­Carbon Bond Activation of Unstrained Alcohols
Marius D. R. Lutz,1 Valentina C. M. Gasser,1 and Bill Morandi1,2,*
1Laboratory of Organic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg 3, 8093 Zürich, Switzerland 2Lead Contact *Correspondence: [email protected]
The advent of transfer hydrogenation and borrowing hydrogen reactions paved the way to manipulate simple alcohols in previously unthinkable manners and circumvent the need for hydrogen gas. Analogously, transfer hydrocarbylation could greatly increase the versatility of tertiary alcohols. However, this reaction remains unexplored because of the challenges associated with the catalytic cleavage of unactivated C­C bonds. Herein, we report a rhodium(I)-catalyzed shuttle arylation cleaving the C(sp2)­C(sp3) bond in unstrained triaryl alcohols via a redox-neutral carbon elimination mechanism. A selective transfer hydrocarbylation of substituted (hetero)aryl groups from tertiary alcohols to ketones was realized, employing benign alcohols as latent C-nucleophiles. All preliminary mechanistic experiments support a reversible -carbon elimination/migratory insertion mechanism. In a broader context, this novel reactivity offers a new platform for the manipulation of tertiary alcohols in catalysis.
C­C activation, rhodium, shuttle catalysis, ketone, alcohol, -carbon elimination, transitionmetal catalysis

INTRODUCTION Catalytic reversible reactions have found widespread use in the chemical community, as highlighted by the broad range of applications that alkene metathesis offers across the molecular sciences.1­4 Another prominent example is the transfer hydrogenation reaction, in which secondary alcohols and ketones are interconverted by formally shuttling a molecule of hydrogen, in lieu of using the hazardous gas (Scheme 1A, top).5 Many variants of this concept have been developed over the years, including borrowing hydrogen reactions that transiently dehydrogenate alcohols to construct new C­C bonds.6­11 The analogous transfer hydrocarbylation reaction using tertiary alcohols would provide exciting new avenues for the synthesis and modification of these species; however, reversibly cleaving and reforming a strong C­C bond represents a far more considerable challenge (Scheme 1A, bottom). Methods to directly activate C­C bonds are attractive, as the substrates of interest can be functionalized without the need to introduce reactive functional handles, such as halides, and some have been strategically used in total synthesis.12 When compared to the significant progress made in the functionalization of inert C­H bonds, catalytic reactions to cleave strong C­C bonds selectively are scarce.13,14 Reasons for their low reactivity compared to C­ H bonds can be found in their kinetic inertness, as the -orbital is highly directional and does not allow for significant overlap with the d-orbitals of transition metals.15,16 Recently, several strategies to oxidatively cleave unstrained C­C bonds using directing groups have been devised,14,15,17­20 however, there are few reports of unbiased systems that proceed via redox-neutral -carbon elimination.16,18,21,22 While strategies to activate propargylic and allylic alcohols are well known, the activation of an unstrained C(sp2)­C(sp3) bond remains challenging. We and others have reported cross-couplings of ,-disubstituted arylmethanols with alkene electrophiles via directed C­C bond activation forging new C­C bonds.23­28 Despite these, reports of selective C(sp2)­C(sp3) bond cleavage of alcohols without using a directing group are rare and limited to specific substrate scaffolds designed to relieve internal strain or gain aromaticity.29,30 A notable exception is a Pd-catalyzed biaryl coupling of ,-disubstituted arylmethanols and aryl bromides reported by Miura.31,32 Continuing our interest in developing catalytic reversible reactions,33­37 we sought to develop a shuttle catalysis reaction to interconvert tertiary alcohols and ketones via reversible C­C bond cleavage (Scheme 1A, bottom). Such a reaction would parallel the widely employed transfer hydrogenation reaction between alcohols and ketones, with the key difference being that a C­C bond would be cleaved and reformed instead of a C­H bond. Besides the inherent challenge of catalyzing the cleavage of a C­C bond in a traditionally inert alcohol substrate (Scheme 1B, top), it is worth noting that there are only a few examples of transition metal-catalyzed aryl additions onto unactivated (i.e., aliphatic) ketones, even when employing traditional organoboron nucleophiles (Scheme 1B, bottom).38­46 Reasons for the limited reports of insertions of ketones into late transition metal-carbon bonds include the high bond strength of the C=O -bond, increased steric hinderance compared to terminal monosubstituted -olefins, and the less favorable interaction between a soft late transition metal center and an alkoxide.47 Thus, the development of a shuttle catalysis approach to ketone arylation is contingent upon addressing two significant challenges in organometallic chemistry, namely the activation of unbiased CC bonds and the productive migratory insertion of a MR species into ketone substrates. A shuttle arylation protocol overcoming these challenges would open new research avenues that take advantage of
reversible -carbon elimination to parallel the ingenious reactions developed in the area of transfer hydrogenation and borrowing hydrogen reactions.5­7,11 It would also pave the way for catalytic alternatives to highly reactive stoichiometric Grignard and organolithium additions. Crucial to unlocking the desired reactivity is the identification of a suitable catalyst that can lower the activation barrier of the key transition state for C­C bond cleavage, which is identical to the one for C­C bond formation by virtue of the microscopic reversibility principle (Scheme 1B, right). Herein, we describe a reversible catalytic shuttle arylation reaction between triaryl alcohols and aliphatic ketones, by cleaving and reforming strong C­C bonds (Scheme 1C). The reaction does not require directing groups or destabilization of the alcohol substrates and proceeds in the presence of a range of polar functional groups. Various aliphatic cyclic and linear ketones, including pharmaceuticals, could be arylated in good yields using this method.

The Bigger Picture
Carbon-carbon bonds are omnipresent in chemicals encountered in everyday life, including biomolecules, many polymers, and pharmaceuticals. To enable the valorization of renewable feedstocks and recycling of industrially relevant polymers, these stable C­C bonds have to be selectively cleaved, which is a remaining challenge in synthetic organic chemistry. Previous methods have limited applicability and rely on substrate bias.
In this work, we describe a reversible transfer hydrocarbylation reaction to interconvert alcohols and ketones by cleaving and reforming strong C­C bonds. Using this method, benign alcohols can serve as alternatives to stoichiometric and highly reactive organometallic reagents to form value-added alcohol products from ketones. The work described herein represents a step toward valorizing renewable alcohols and paves the way to develop a platform for the creative manipulation of tertiary alcohols in catalysis.

Scheme 1. Context of this work
(A) Transfer hydrogenation is a widely employed reaction to interconvert 1° and 2° alcohols and carbonyl compounds. An analogous transfer hydrocarbylation of 3° alcohols is unknown. (B) Two main challenges, namely activation of the C(sp2)­C(sp3) bond in the alcohol and productive insertion into the ketone, have to be overcome to unlock the desired reactivity. Our strategy is to lower the barrier of the key transition state that occurs during both key steps. (C) This work: Development of a catalytic shuttle arylation reaction between 3° alcohols and ketones.
Catalyst evaluation and reaction development
Inspired by stoichiometric -carbon elimination studies from rhodium(I) alkoxide complexes by Hartwig and co-workers,48­50 we envisaged that tuning the steric environment around the

metal center to stabilize reactive, low coordinated complexes could unlock a catalytic transfer arylation tolerating a range of unbiased scaffolds. We began our studies with triphenylmethanol (1a) and 4,4'-difluorobenzophenone (2) as model substrates in the presence of [Rh(cod)Cl]2 and a weak base in toluene at 110 °C. Initially, a panel of ligands was evaluated, and several conditions promoted benzophenone (3a) formation by -carbon elimination but only trace amounts of alcohol product 4 were observed. After extensive reaction optimization, suitable conditions for the transfer of the phenyl group were identified
(see Section S2 for details). NHC ligands effected -carbon elimination of 1a towards benzophenone (3a) and facilitated productive insertion of the aryl group into ketone 2. Among all NHC ligands tested, the electron-rich and sterically demanding ligand IPr*OMe was identified as the optimal ligand for this transformation (Scheme 2A).51 Interestingly, the major product was not triaryl alcohol 4, but diaryl ketone 5, which is formed through
subsequent -aryl elimination of the more electron-withdrawing group from the transiently generated alcohol 4 over the course of the reaction. After 48 h nearly full conversion of both alcohols 1a and 4 towards the ketones 3a and 5, and (fluoro)benzene was observed. In an effort to avoid downstream reactions of the desired alcohol product, we next sought ways to take advantage of this unique reactivity to develop a synthetically attractive reaction. We reasoned that using an aliphatic ketone as acceptor would be beneficial because: 1) the tertiary alcohol product would likely not have the ability to scramble through cleavage of a strong C(sp3)­C(sp3) bond, preventing undesired crossover reactions; 2) the overall transfer reaction would likely be exothermic due to the formation of a conjugated ketone by-product as driving force (DFT predicts G = -4.0 kcal mol-1), while no deconjugation of the acceptor substrate (aliphatic ketone) would take place (see Section S7). Indeed, the reaction of 1a and 4,4-dimethylcyclohexanone (6a) worked efficiently and reached full conversion with only marginal excess (1.5 equiv.) of the donor (Scheme 2B). The desired alcohol product 7a was obtained in excellent yield, while 1a was converted into ketone 3a and benzene as a side product (by protodemetalation). Control experiments confirmed the essential role of the catalyst and a weak base. Sterically encumbered NHC ligands resulted in the highest yield, but other ligands also effected product formation. Notably, the reaction could even be conducted at 0.5 mol% catalyst loading, albeit with prolonged reaction time.

Scheme 2. Reaction development
(A) Proof of concept of a catalytic reversible shuttle arylation reaction. Reaction conditions: 1a (0.10 mmol), 2 (0.10 mmol), [Rh(cod)Cl]2 (2.5 mol%), IPr*OMe  HBF4 (5 mol%), KOtBu (5 mol%) and K3PO4 (1 equiv.) in toluene (0.2 M) at 125 °C for 24 h. GC-FID yields using ndodecane as an internal standard. (B) Catalytic aryl transfer to aliphatic ketones. Reaction conditions: 1a (0.15 mmol), 6a (0.10 mmol), [Rh(cod)Cl]2 (2.5 mol%), IPr*OMe  HBF4 (5 mol%), KOtBu (5 mol%) and K3PO4 (1 equiv.) in toluene (0.2 M) at 125 °C for 24 h. aGC-FID yields using n-dodecane as an internal standard. bIsolated yield after purification. c72 hours.
Substrate scope
After finding the optimal conditions, we investigated the generality and utility of this transformation. A range of cyclic (6a-b) and linear (6d-e) aliphatic ketones were efficiently arylated to afford the corresponding tertiary alcohols in good yields (Scheme 3A). Acetophenone could also be arylated, albeit the formation of product 7c was reversible, leading to a diminished yield. Sterically hindered ketones were unreactive under the reaction conditions, which might indicate that bulky ketones are unable to approach the sterically encumbered catalyst (Table S12). Several functional groups were found to be compatible with the reaction conditions, such as ketal-protected ketones (7f), protected amines (7g), ethers (7h), and sulfones (7i). Notably, substrates containing sensitive functional groups towards Grignard and organolithium reagents underwent the arylation at the ketone selectively, leaving carboxylic esters (7k-l), amides (7m), and Weinreb amides (7n) untouched. Cross-coupling handles such as aryl silanes (7j) and chlorides (7o) remained unchanged, offering the potential for orthogonal synthetic manipulation. The arylation also

proved to be effective in the presence of a heterocyclic residue (7p). Electrophilic moieties, such as alkyl tosylates (7q) and bromides (7r), and even epoxides (7s) were tolerated, revealing the mildness of this method. Protic functionalities such as alcohols did not interfere with the reaction, furnishing diol 7t as a mixture of diastereomers. Drug molecules, such as nabumetone and pentoxifylline, underwent arylation in good yields (7u-v), demonstrating the potential for late-stage derivatization of bioactive compounds. To further showcase the scalability and robustness of this reaction, the transfer arylation of 1a and 6d was conducted on 10 mmol scale with a reduced catalyst loading of 1 mol% (Scheme 3B). The alcohol product 7d' was obtained in 70% yield after purification. The second product, benzophenone (3a'), was recovered in 77% with respect to 1a, or 116% based on the limiting reagent 6d. Subsequently, we turned our focus to the alcohol scope. We first investigated several symmetrical triaryl alcohols 1 (Scheme 4A). A range of alkyl-substituted alcohols (8a-c) and a 2-naphthyl group (8d) were suitable donor molecules, affording the desired alcohols in good to excellent yield. Moreover, several common functional groups were tolerated, such as fluoro (8e), chloro (8f), methoxy (8g), trifluoromethyl (8h, 8j), and trifluoromethoxy (8i) groups. The reaction was efficient with both electron-rich and -deficient donors, albeit the yield of 8g and 8j was limited due to fast consumption of the alcohol starting material via protodemetalation. Notably, heterocyclic scaffolds could be readily used and allowed the transfer of 1,3-benzodioxolane (8k), benzofurane (8l-m), and morpholine (8n) moieties. Beyond triaryl alcohols, we investigated the propensity of other alcohols to undergo carbon elimination (Scheme 4B). 1,1-Diphenylethanol (1p) afforded product 9a in moderate yield, because the formation of the by-product acetophenone is reversible (7c, vide supra). Excitingly, a 2° alcohol (1q) could be engaged to afford the product 9b in a promising yield, showing potential for productive -carbon elimination in the presence of a competing hydride. We surmise that rapid and reversible -hydride elimination occurs under the reaction conditions, yet the final formation of the desired tertiary alcohol kinetically traps the occasionally formed -carbon elimination intermediate, hence slowly driving the mixture towards the desired product. This result thus suggests the possibility to merge complex sequences of reversible -hydride and -carbon eliminations, opening new avenues for the manipulation of alcohols in catalysis.

Scheme 3. Scope of ketone acceptors aRatio determined by 1H NMR of the crude reaction mixture.

Scheme 4. Scope of alcohol donors
Preliminary mechanistic studies After demonstrating the synthetic versatility of this protocol, we performed experiments to investigate the reaction's reversibility. To confirm that -carbon elimination is reversible for triaryl alcohols 1, diaryl ketone 3b was added to the reaction mixture of 1a and 6a (Scheme 5A). Incorporation of both the phenyl group (red) and the p-tolyl group (blue) into 6a was observed in a 4.9 : 1 ratio, together with a mixture of all three possible diaryl ketones, confirming that addition into diaryl ketone 3b is reversible and has a similar rate to the addition into aliphatic ketone 6a.

To evaluate the reversibility of the reaction in the case of (dialkyl)aryl alcohols 7, alcohol trans-7b and benzophenone (3a) were subjected to the reaction conditions (Scheme 5B). Notably, isomerization to the more stable cis-isomer and small amounts of ketone 6b were
observed, highlighting that -aryl elimination from (dialkyl)aryl alcohols also takes place, albeit the reaction rate is much lower than for triaryl alcohols. To gain insight into the electronic preference of the reaction, intermolecular competition experiments with two symmetrical triaryl alcohols were carried out (Table S15). Interestingly, both electron-rich and -deficient substituted aryl groups were preferentially cleaved compared to phenyl, with no correlation between the electronic character and the observed selectivity. Notably, a similar trend was observed in Pd-catalyzed -aryl elimination.52 The interpretation of these results is, however, complicated by the partial reversibility of the addition of one aryl group to another diaryl ketone by-product leading to crossover products (see Scheme 5A). To reduce the impact of these effects, the initial rates using different triaryl alcohols and an excess of ketone acceptor were independently measured and plotted against the Hammett parameter + (Scheme 5C).53 In this case, a linear-free-energy relationship with a negative slope was observed ( = -0.62, R2 = 0.91). Based on the obtained  value and the better correlation with the + parameter, we can conclude that there is a partial positive charge buildup on the benzylic carbon in the turnover-limiting step of the reaction. We next independently synthesized the complex [Rh(IPr*OMe)(cod)Cl] (10) that is presumedly formed in situ (Scheme 5D). The complex was catalytically competent, effectively giving the same yield of product. The crystal structure obtained by X-ray diffraction of the air-stable complex illustrates the steric encumbrance of the NHC ligand around the substrate binding site. The steric influence of the ligand was parameterized by its buried volume (Vbur).54 While the buried volume of IPr*OMe is slightly larger than that of IPr at 3.5 Å radius (Vbur = 37.2% versus 33.6%), its reach extends further in the periphery (Vbur = 46.4% versus 38.6% at 5.5 Å).

Scheme 5. Mechanistic experiments

(A) Cross-over experiment. (B) Reversibility of  -carbon elimination from aliphatic alcohols. (C) Hammett study. (D) Precatalyst synthesis, catalytic competence and crystal structure. Based on these preliminary observations, we propose the following reversible catalytic cycle (Scheme 6). Ligand exchange of complex I (10) with the alcohol substrate followed by dissociation of a cod ligand creates the presumed catalytically active species II. As in the case of -hydride elimination, an empty coordination site is a requisite for -carbon elimination to occur. The coordinatively unsaturated Rh center in II likely interacts with one of the aryl rings in a 2-fashion, as has been shown in X-ray structures of related complexes,48 thereby facilitating the C­C bond cleavage. Aryl complex III then undergoes reversible ketone exchange to form complex IV that in turn undergoes migratory insertion into the ketone to form alkoxide complex V. Every step in the catalytic cycle is reversible according to our findings. Finally, alcohol exchange closes the cycle.
Scheme 6. Proposed catalytic cycle Conclusion In summary, we have developed a catalytic shuttle arylation reaction that cleaves unactivated C(sp2)­C(sp3) bonds in unstrained alcohols and arylates unactivated aliphatic ketones. This is the first example of a reversible transfer hydrocarbylation reaction that parallels the ubiquitous transfer hydrogenation reaction of secondary alcohols and carbonyl acceptors. Using this method, triaryl alcohols can be used as latent nucleophilic aryl sources for traditionally challenging catalytic additions to ketones. Preliminary mechanistic studies point towards reversible C­C bond activation as a key factor in enabling this process. In a broader context, this novel reactivity offers numerous opportunities for the creative use of tertiary alcohols in synthesis which parallel major achievements made in transfer hydrogenation and borrowing hydrogen reactions.

EXPERIMENTAL PROCEDURES Resource Availability Lead Contact Further information and requests for resources should be directed to and will be fulfilled by the Lead Contact, Bill Morandi ([email protected]).
Materials Availability All unique/stable reagents generated in this study are available from the Lead Contact without restriction.
Data and Code Availability Crystallographic data for 7a, cis-7b, and 10 have been deposited in the Cambridge Crystallographic Data Center (CCDC) under accession numbers CCDC: 2035749, 2035750 and 2035751. These data can be obtained free of charge from the CCDC at All other data are available from the Lead Contact upon reasonable request.
SUPPLEMENTAL INFORMATION Supplemental Information can be found online at xxxx.
ACKNOWLEDGMENTS We thank the European Research Council (ERC Grant 757608 ShuttleCat), the Swiss National Science Foundation (SNF 184658), and the ETH Zürich for financial support. M.D.R.L. is grateful for funding from the German Academic Scholarship Foundation. We thank the NMR service, the Molecular and Biomolecular Analysis Service (MoBiAS) and X-ray departments of ETH Zürich for technical assistance. We thank the Morandi group members for critical proof reading of the manuscript.
AUTHOR CONTRIBUTIONS M.D.R.L. and B.M. conceived the project. M.D.R.L. discovered and developed the reaction; M.D.R.L. and V.C.M.G. conducted the synthetic studies. All authors contributed to the writing and editing of the manuscript.
DECLARATION OF INTERESTS The authors declare no competing interests.

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