Virtual Issue Posts on Organocatalysis - American Chemical Society

Editorial pubs.acs.org/acscatalysis

Virtual Issue Posts on Organocatalysis: Design, Applications, and Diversity

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of stereochemistry, this selection of articles encompasses novel catalytic systems that operate under the unique modes of catalysis of simple organic molecules. It is all about multiple expressions of organocatalysis. Chiral secondary amine catalysis is a mainstay in the field of organocatalysis, and its vast synthetic utility has been demonstrated extensively. A remarkable example is its application to the synthesis of prostaglandin analogues reported by Aggarwal and co-workers (Scheme 1).4,5 The researchers conducted L-proline-catalyzed asymmetric dimerization of succinaldehyde followed by intramolecular aldol reaction of a β-hydroxy aldehyde intermediate under the influence of dibenzylammonium trifluoroacetate to produce bicyclic lactol. Importantly, adding charcoal to the reaction mixture improved the purification process of the lactol that was subsequently converted into acetal and lactone. Since the lactone was crystalline, simple recrystallization enhanced the optical purity. From these key intermediates, the ophthalmologic drugs Latanoprost and Bimatoprost were assembled in a very concise manner. Proline and proline amide catalysis is believed to proceed through a mechanism where the secondary amine forms an enamine with aldehyde or ketone and the acidic proton provides hydrogen bonding to activate the electrophile. Considering this mechanism and the importance of the acidity of the hydrogen-bond donor unit in attaining higher activity and better stereocontrol, Li, Cheng, and co-workers determined equilibrium acidities of proline and proline amide derivatives in DMSO by an overlapping indicator method (Scheme 2).6 In addition, the relationship between acidities of the catalysts and stereoselectivity was explored with a typical aldol reaction of acetone with p-nitrobenzaldehyde, providing a useful set of guidelines for designing new proline-derived catalysts. α,α-Diphenylprolinol trimethylsilyl ether (1), first developed by Jørgensen7 and Hayashi,8 has been utilized for various stereoselective α-functionalization of aldehydes.9 Boeckman and co-workers reported the enantioselective α-hydroxymethylations of aldehydes catalyzed by 110 and, in a paper published in The Journal of Organic Chemistry, they described detailed investigations on this transformation, which enabled the identification of critical reaction parameters directly associated with improved reaction profiles (Scheme 3).11 This general and scalable aldol reaction generated a lactol intermediate that was efficiently derivatized to the corresponding β-hydroxycarboxylic acid by Pinnick oxidation and δ-hydroxy-α,β-unsaturated ester via Wittig olefination, respectively. In another application of 1, Chen and co-workers successfully utilized 1 for the resolution of racemic secondary nitroallylic alcohols through Michael addition of aldehydes and consecutive acetalization sequences, establishing the five contiguous stereogenic centers on the fully substituted

he use of small organic molecules as catalysts for selective organic transformations is a relatively new field in catalysis and synthetic chemistry. Over the last century, organic molecules have been utilized sporadically for promoting specific target transformations, as exemplified by the Hajos−Parrish reaction.1 However, it is only since the late 1990s that the potential of organic catalysts for solving important synthetic problems has been clearly demonstrated. Especially after two landmark contributions in 2000,2 the field, termed “organocatalysis,” has grown rapidly, being established as a third class of catalysis in addition to the previously accepted enzymatic catalysis and organometallic catalysis.3 The swift growth is largely due to numerous advantages organocatalysis offers for the chemical synthesis community. The catalyst molecules are often inexpensive, and it is easy to prepare both enantiomers from readily available natural and unnatural chiral pools, leaving ample possibility for structural modification. Since organic molecules are generally insensitive to oxygen and moisture, organocatalytic reactions can be performed under an aerobic atmosphere without special care for reagent and solvent drying, which significantly simplifies experimental operations. Compared to metal-based catalysts, fewer toxicity issues tend to be associated with small organic molecules, enhancing the safety of catalysis not only in academic research but also in industrial applications. The obvious merits are the ease of removal of the catalyst from the reaction waste and virtually no risk of minuscule contamination of the catalyst-derived impurities in the final products. These salient features of organocatalysis are beneficial for the efficient catalyst and reaction development with substantially reduced costs, continuously attracting new researchers into this field. Organocatalysis covers various modes of catalysis, including common Brønsted and Lewis acid−base catalysis, nucleophilic catalysis, and redox catalysis, and is evolving to exploit the synergy with organometallic catalysis, enzymatic catalysis, and photochemical catalysis. In this new ACS Select virtual issue (http://pubs.acs.org/page/vi/organocatalysis), we have chosen 24 articles published in 2014 and 2015 in ACS Catalysis, Organic Letters, The Journal of Organic Chemistry, and the Journal of the American Chemical Society, which serve to illustrate exciting discoveries within organocatalysis, the inherent potential of the field, and its diversity. These include ever-expanding utility of the enamine catalysis of chiral secondary and primary amines, new applications of the Brønsted acid catalysis of structurally related, axially chiral phosphoric acids, and immense potential of the reactivity and selectivity of bifunctional acid−base catalysts. The importance of ion-pairing in organocatalysis is also featured by the anionbinding catalysis and phase-transfer catalysis, followed by the cutting edge of the catalysis relying on the nucleophilic property of nitrogen-containing heterocycles. In these fine contributions, we often read how computational studies play a crucial role in deeper understanding of the mechanism and even lend power to the catalyst design. Aside from the control © XXXX American Chemical Society

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Scheme 1. L-Proline-Catalyzed Aldol Reaction for the Synthesis of Prostaglandin Analogues. Adapted from Ref 5

Scheme 2. pKa of Proline and Proline Amide Derivatives in DMSO. Adapted from Ref 6

Scheme 3. Enantioselective α-Hydroxymethylation of Aldehydes. Adapted from Ref 11

Scheme 4. Kinetic Resolution of Racemic Secondary Nitroallylic Alcohols. Adapted from Ref 12

tetrahydropyranol products (Scheme 4).12 The less-reactive (S)-nitroallylic alcohols were recovered in a highly enantioenriched form. Diastereomeric, homochiral 3-aminopyrrolidin carboxylate methyl esters can be prepared by means of a copper(I)catalyzed [3 + 2] cycloaddition between azomethine ylides and nitroalkenes.13 According to the article by Cossı ́o and coworkers, these densely substituted, cyclic secondary amines belonged to the L-series of natural amino acids and served as effective catalysts for the asymmetric conjugate addition of ketones to nitroalkenes (Scheme 5).14 Interestingly, however, the reaction with exo-L-2 produced the conjugate adduct with stereochemistry opposite to that obtained with L-proline as a

catalyst, while endo-L-2 afforded a product with the same sense of asymmetric induction observed with L-proline, although less stereoselective than the exo congener. Gröger, Berkessel, and co-workers employed L-histidine as a catalyst for the asymmetric cross-aldol reaction of readily available isobutanal and glyoxylate, and subsequent stereoselective reduction with alcohol dehydrogenase (ADH), which can be carried out at high substrate concentrations with in situ removal of acetone, led to spontaneous ring closure to give (R)pantolactone of high enantiomeric purity (Scheme 6).15,16 This chemoenzymatic three-step process allows one-pot-like operation, offering a practical route to (R)-pantolactone, a key 6981


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Scheme 5. Michael Addition with Diastereomeric L-Proline Ester Derivatives as Catalysts. Adapted from Ref 14

near stoichiometric amount of the aldehyde was crucial for preserving the catalyst activity throughout the reaction. Notably, the addition of BINOL resulted in a substantial increase in enantioselectivity. The resulting (R)-1-arylmethyl1,2,3,4-tetrahydroisoquinolines bearing protecting groups on the nitrogen and catechol moieties of the tetrahydroisoquinoline ring system were readily converted to a variety of alkaloids, highlighting the versatility of this approach. The axially chiral but structurally different (R)-STRIP smoothly catalyzed a dearomatizing redox coupling of aryl hydrazines with ketones to furnish highly enantioenriched 1,4diketones as reported by List and co-workers (Scheme 8).21 This unique transformation was proposed to proceed through [3,3]-sigmatropic diaza-Cope rearrangement of the protonated ene hydrazine intermediate A to give diimine B. The generation of the quaternary carbon center at this stage was due to the presence of the ortho-alkyl substituents at the hydrazine framework, which prevent rearomatization to the corresponding indole. Furthermore, the reaction conditions, similar to those established for the asymmetric indolizations,22 were mild enough for the diimine to undergo a hydrolysis rather than the possible alkyl shift, affording the desired 1,4-diketones. One of the effective strategies for enhancing the catalytic performance of this class of chiral Brønsted acids is to link two phosphoric acid components. Shi and co-workers showed that chiral bis-phosphoric acid 3 was the catalyst of choice for the asymmetric 1,3-dipolar cycloaddition of azomethine ylides, derived from N-benzylisatins and diethyl 2-aminomalonate,

Scheme 6. Practical Chemoenzymatic Route to (R)Pantolactone. Adapted from Ref 16

industrial intermediate for the production of (R)-pantothenate, better known as vitamin B5. Triggered by the pioneering studies by Akiyama17 and Terada,18 research on the exploitation of chiral phosphoric acid catalysis has experienced explosive growth and is still expanding its synthetic potential.19 As such, Hiemstra and co-workers demonstrated the effectiveness of (R)-TRIP as a catalyst for the enantioselective Pictet−Spengler reaction of N-(o-nitrophenylsulfenyl (Nps))-2-arylethylamines with arylaldehydes (Scheme 7).20b Thorough optimization revealed that the use of magnesium sulfate as a drying agent in combination with a

Scheme 7. Organocatalytic Approach to Optically Active 1-Benzyl-1,2,3,4-tetrahydroisoquinoline Alkaloids. Adapted from Ref 20b

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provides a reliable tool for stereoselective access to a variety of β- and γ-substituted tetralones. The combination of thiourea with tertiary amines represents a common structural motif of Brønsted acid−base bifunctional catalysts. Fan and co-workers developed a chiral amine-thiourea 5-catalyzed, asymmetric tandem aminolysis/aza-Michael addition reaction of spirocyclic para-dienoneimides, inspired by the structural features of natural Apocynaceae alkaloids (Scheme 11).27 The stereochemical outcome of this transformation was established through the enantioselective desymmetrization after initial aminolysis, and the resulting enantioenriched, functionalized hydrocarbazole served as a valuable building block for the synthesis of natural products (+)-limaspermidine and (+)-deethylibophyllidine. Other than the ureas, squaramide is often employed as the hydrogen-bond-donor component of bifunctional catalysts. Ghorai and co-workers delineated a strategy for the enantioselective synthesis of 1- and 3-substituted isochromans based on the cinchona alkaloid-derived squaramide 6-catalyzed, intramolecular oxa-Michael addition of the alkoxyboronate bearing a chalcone appendage, generated in situ through the reduction of the starting aldehyde with pinacolborane activated by the tertiary amine moiety of 6 (Scheme 12).28b The utility of chiral isochroman products was visualized by the straightforward derivatization into (+)-sonepiprazole, the D4 receptor antagonist, without considerable loss of the optical purity. The structurally similar, quinine-based squaramide catalyst 7a promoted the enantioselective Michael addition of dimethyl malonate to nitroalkenes more efficiently on water (brine) than in organic solvents such as dichloromethane because of the hydrophobic hydration effect.29 Song and Bae, who reported this rate acceleration, further observed that differences in catalyst structure significantly impacted reactivity in the Michael addition. Simply switching the catalyst from 7a to more hydrophobic, dihydroquinine-derived squaramide 7b, which has an ethyl group at C3, brought a substantial increase in reaction rate, resulting in the quantitative formation of the Michael adduct with comparable enantiomeric excess within 30 min (Scheme 13).30 This system was capable of accommodating 2-substituted dimethyl malonates as nucleophiles. Seidel, Schreiner, and co-workers conducted in-depth experimental and computational studies aimed at gaining insight into the mechanism of the acylative kinetic resolution of racemic benzylic amines under the dual catalysis of 4dimethylaminopyridine (DMAP) and chiral thiourea derivatives (Scheme 14).31,32 Thorough evaluation of various reaction variables confirmed the optimal conditions and amide-thiourea

Scheme 8. Brønsted Acid-Catalyzed Dearomatizing Redox Cross Coupling Reaction. Adapted from Ref 21

with methyleneindolinones to give structurally complex bisspirooxindoles possessing three contiguous stereogenic centers, including two quaternary centers, in high yield with rigorous diastereo- and enantiocontrol (Scheme 9).23 The origin of the excellent stereoselectivity could be ascribed to the structural feature of 3 and its powerful dual hydrogen-bonding activation of the substrates. Bifunctional organocatalysts, which simultaneously recognize and activate both electrophiles and nucleophiles, have garnered significant attention largely because higher catalytic activity and precise stereocontrol are often expected with broad reaction manifolds, and thus, they are actively studied and developed.24 Along this line, Nagasawa and co-workers developed structurally flexible guanidine-bisurea bifunctional catalysts of type 4 and found that exposure of tetralone-derived βketoesters to cumene hydroperoxide (CHP) in the presence of 4 gave the α-hydroxylation products with excellent enantioselectivity (Scheme 10).25 In order to elucidate the mechanism, density functional theory calculations were carried out. The identified transition state (TS-S) structure revealed that the substrate enolate interacts with guanidine and urea components of the catalyst, while CHP coordinates with the remaining urea moiety, and then the oxidation occurs from the si-face.26 On the basis of the deep consideration of this transition state model, they further designed an oxidative kinetic resolution of racemic, β- and γ-substituted β-ketoesters derived from tetralone, which indeed proceeded with a high level of selectivity under conditions similar to those used for hydroxylation with 4 as a requisite catalyst. This method

Scheme 9. Asymmetric 1,3-Dipolar Cycloaddition Catalyzed by Bis-phosphoric Acid. Adapted from Ref 23

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Scheme 10. Guanidine-Bisurea Bifunctional Catalysis for Asymmetric Hydroxylation of β-Ketoesters. Adapted from Ref 26

Scheme 11. Catalytic Asymmetric Tandem Reaction for the Synthesis of Hydrocarbazole Alkaloids. Adapted from Ref 27

Scheme 12. Intramolecular Oxa-Michael Reaction of Alkoxyboronates for Asymmetric Synthesis of Isochromans. Adapted from Ref 28b

8 was found to provide the highest selectivity factor. The key reactive intermediate of this reaction was a chiral ion pair C consisting of a benzoylpyridinium cation and a benzoate anion that was captured by 8 through the hydrogen-bonding from two N−H protons of the thiourea moiety. Importantly, the amide N−H proton was not involved in such anion binding. Rather, it intramolecularly interacted with the sulfur atom to enhance the acidity of the thiourea unit, thereby positively impacting anion binding.

The use of structurally defined chiral cations for the precise control of reactive anionic species offers a powerful strategy for the development of catalytic stereoselective molecular transformations. The outstanding contribution to this area is the development of chiral dicationic bis-guanidinium salts 9 as effective catalysts for the asymmetric oxidation of electrondeficient alkenes with potassium permanganate (KMnO4), described by Tan, Wang, and Zong in the Journal of the American Chemical Society (Scheme 15).33 Treatment of 2-aryl acrylates with KMnO4 in the presence of 9a yielded 6984


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disubstituted α,β-unsaturated trifluoromethylketones afforded syn-dihydropyranones with high levels of relative and absolute stereocontrol (Scheme 16).36 The [4 + 2] cycloaddition

Scheme 13. Bifunctional Organocatalysis on Water: Effect of Catalyst Hydrophobicity on Reactivity. Adapted from Ref 30

Scheme 16. Asymmetric NHC-Catalyzed Redox [4 + 2] Cycloaddition. Adapted from Ref 36

Scheme 14. Dual-Catalysis Anion-Binding Approach to Kinetic Resolution of Amines. Adapted from Ref 32 proceeded via the generation of azolium enolate and was general with respect to the substituents on both reactants, which facilitated the asymmetric synthesis of a wide range of triand tetrasubstituted trifluoromethyl dihydropyranones. Bressy, Chuzel, and co-workers illustrated the prominent features of chiral isothiourea 11 as a nucleophilic organocatalyst in the development of an efficient, highly enantioselective acylative desymmetrization of acyclic meso 1,3-diols having πunsaturations (Scheme 17).37 This reaction involved not only the desymmetrization of the starting diol but also the catalystcontrolled chiroablative kinetic resolution of the desymmetrized product, which amplified its enantiopurity by recognizing and transforming the minor enantiomer into a symmetric, achiral diester. The presence of the carbon−carbon double bonds in the products was exploited in postfunctionalizations, leading to the asymmetric synthesis of (−)-diospongin A. Chiral bipyridine N,N′-dioxides have been recognized as effective Lewis base catalysts, particularly for the asymmetric allylation and propargylation of aldehydes with appropriate silyl nucleophiles. In the catalytic cycle of these transformations, formation of a hexacoordinate silicon intermediate leads to the stereodetermining transition state that adopts a closed, chairlike geometry D (Scheme 18). Wheeler and co-workers utilized the Automated Alkylation Reaction Optimizer for N-oxides (AARON)38 to identify the transition state structures required to predict stereoselectivity of benzaldehyde allylation catalyzed by 18 different axially chiral bipyridine N,N′-dioxides.39 The predicted enantiomeric excesses were in reasonable agreement with experiment for most of the catalysts. This study also

dihydroxylation products with high enantioselectivity, where the addition of 20 wt % aqueous potassium iodide was critical. In the oxidation of trisubstituted enoates with acetic acid as a requisite additive and 9b as a catalyst, enantioenriched 2hydroxy-3-oxocarboxylic esters were obtained in good-to-high yield. Under these conditions, both E- and Z-isomers of the enoates were transformed to the same enantiomer. N-heterocyclic carbenes (NHCs) occupy a unique place in the organocatalysis field, enabling distinct modes of Umpolung reactivity.34 In NHC-catalyzed redox chemistry, acyl azoliums and azolium enolates can be generated from α-functionalized aldehydes.35 In an article published in ACS Catalysis, Smith and co-workers described that the treatment of aminoindanolderived NHC precatalyst 10 with cesium carbonate in either dichloromethane or THF with α-aroyloxyaldehydes and α,β-

Scheme 15. Chiral Bis-guanidinium Catalysis for Enantioselective Oxidation of Alkenes with KMnO4. Adapted from Ref 33

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Scheme 17. Isothiourea-Catalyzed Desymmetrization of Acyclic meso-1,3-Diols. Adapted from Ref 37

Continuing their study on the development of catalytic asymmetric Wittig reactions,41 Werner and co-workers demonstrated the feasibility of a base-free catalytic Wittig reaction (Scheme 20).42 Readily available tributylphosphine

Scheme 18. Selectivity Prediction and Catalyst Design by AARON. Adapted from Ref 39

Scheme 20. Base-Free Catalytic Wittig Reaction. Adapted from Ref 42

revealed that transition states based on the ligand configuration E were often strongly favored. Moreover, this powerful computational toolkit was used for assessing the performance of the bipyridine N,N′-dioxides as catalysts for the propargylation of benzaldehyde with allenyltrichlorosilane, identifying two catalyst structures that should exert reasonable stereoselectivity. The potential and diversity of organocatalysts are often seen in the development of catalytic transformations that are unique in their own right, aside from stereochemical control. Yeung and co-workers disclosed that the bromolactonization of olefinic carboxylic acids was efficiently catalyzed by methyl 2methyl-1H-indol-3-carboxylate (Scheme 19).40 This novel

(PBu3) acted as an efficient catalyst and initially underwent Michael addition to diethyl maleate to form a zwitterionic enolate intermediate. The subsequent [1,2]-proton-shift generated the corresponding ylide that reacted with aldehyde to give the succinate product with high E-selectivity. The liberated phosphine oxide was reduced in situ by the silane to regenerate the phosphine catalyst. Walsh and co-workers shed light on the potential of sulfenate anions as organocatalysts,43 and in a paper published in Organic Letters, they illustrated the advantage of employing tert-butyl phenyl sulfoxide as a traceless precatalyst for the in situ generation of phenyl sulfenate anion by the action of potassium tert-butoxide (KOBut), which catalyzed the coupling of benzylic halides to trans-stilbenes (Scheme 21).44 Since the byproduct formed from the generation of the sulfenate anion through E2 elimination is gaseous isobutylene, this protocol circumvents the formation of a catalytic quantity of inseparable unsymmetrical stilbene impurity, caused by the use of benzyl phenyl

Scheme 19. Indole-Catalyzed Bromolactonization of Olefinic Acids. Adapted from Ref 40

Scheme 21. Catalysis of Sulfenate Anion for the Coupling of Benzylic Halides. Adapted from Ref 44

transformation can be performed in nonpolar solvents such as heptane and cyclohexane under solid−liquid biphasic condition with N-bromosuccinimide (NBS) as a bromine source. The actual electrophilic brominating species is the indole incorporating a bromine at the C3 carbon; the succinimide, a bromine carrier, can be recovered by simple filtration after completion of the reaction. This method’s advantage is that it is applicable to the preparation of basesensitive bromolactones. 6986


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sulfoxide as a precatalyst in the previous procedure,43 ensuring the high-yielding production of various trans-stilbenes. In a significant departure from their studies on the properties and functions of the phenalenyl cations possessing an empty nonbonding molecular orbital (NBMO),45 S. K. Mandal, Datta, P. K. Mandal, and co-workers utilized the phenalenyl cation as an organic Lewis acid (Scheme 22).46 9-Methoxy-1-ethox-

co-workers evaluated the catalytic activity of various ambiphilic phosphine-borane derivatives in the hydroboration of CO2 with catechol borane or BH3·SMe2 to give methoxyboranes. Their findings revealed that catechol derivatives such as 15a and 15b· Scheme 24. Hydroboration of CO2 with Phosphine-Boranes as Catalysts. Adapted from Ref 49

Scheme 22. Phenalenyl Cations as Organic Lewis Acid Catalysts. Adapted from Ref 46

yphenalenium tetrafluoroborate (12) displayed high catalytic activity in the ring-opening reaction of 2-(phenoxymethyl)oxirane with N-methylaniline under neat conditions to afford the desired β-hydroxy amine in 80% yield. The origin of the observed reactivity could be ascribed to the initial noncovalent interaction between the empty NBMO of the cationic unit of 12 and the amine lone pair, which increased the acidity of the N−H functionality to enable activation of the epoxide. Then, the second molecule of amine attacked the electrophilic carbon of the epoxide in this complex. This finding provides strong implications for the design of cationic organic Lewis acid catalysts. Suginome, Ohmura, and Morimasa reported that 4,4′bipyridines functioned as effective catalysts for the diboration of various sterically hindered pyrazine derivatives (Scheme 23).47 The catalytic activity of 4,4′-bipyridines was critically

BH3 were most reactive (Scheme 24).49 It was significant that 13 C-labeling experiments strongly suggested the formaldehyde adduct 15·CH2O was the active species responsible for efficient reduction of CO2. In the reaction with BH3·SMe2 as a reductant, 15b·CH2O exhibited turnover frequencies of up to 873 h−1. In summary, organocatalysis is an active research field that continues to expand and diversify, fueled by the many scientists carrying out new catalyst and reaction development. I encourage you to read about these exciting discoveries of catalyst reactivity and selectivity in the virtual issue, which is intended to outline the current state and future outlook of organocatalysis. One article might even stimulate your next scientific challenge!

Takashi Ooi, Associate Editor

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Scheme 23. 4,4′-Bipyridine-Catalyzed Diboration of Pyrazine Derivatives. Adapted from Ref 47

Nagoya University

AUTHOR INFORMATION

Notes

Views expressed in this editorial are those of the author and not necessarily the views of the ACS.

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REFERENCES

(1) Hajos, Z. G.; Parrish, D. R. J. Org. Chem. 1974, 39, 1615−1621. (2) (a) List, B.; Lerner, R. A.; Barbas, C. F., III J. Am. Chem. Soc. 2000, 122, 2395. (b) Ahrendt, K. A.; Borths, C. J.; MacMillan, D. W. C. J. Am. Chem. Soc. 2000, 122, 4243. (3) (a) Comprehensive Enantioselective Organocatalysis; Dalko, P. I., Ed.; Wiley-VCH: Weinheim, Germany, 2013. (b) Topics in Current Chemistry: Asymmetric Organocatalysis; List, B., Ed.; Springer-Verlag: Berlin, Germany, 2009. (4) Coulthard, G.; Erb, W.; Aggarwal, V. K. Nature 2012, 489, 278− 281. (5) Prévost, S.; Thai, K.; Schützenmeister, N.; Coulthard, G.; Erb, W.; Aggarwal, V. K. Org. Lett. 2015, 17, 504−507. (6) Li, Z.; Li, X.; Ni, X.; Cheng, J.-P. Org. Lett. 2015, 17, 1196−1199. (7) Marigo, M.; Wabnitz, T. C.; Fielenbach, D.; Jørgensen, K. A. Angew. Chem., Int. Ed. 2005, 44, 794−797. (8) Hayashi, Y.; Gotoh, H.; Hayashi, T.; Shoji, M. Angew. Chem., Int. Ed. 2005, 44, 4212−4215. (9) Meninno, S.; Lattanzi, A. Chem. Commun. 2013, 49, 3821−3832.

dependent on the substituent at the C2 position, and dichlorosubstituted 13 exhibited the highest efficiency. This diboration reaction relies on the distinct mode of activation of nonpolar B−B bond in bis(pinacolato)diboron, which is executed via reductive addition to 13 to generate a dearomatized intermediate, and subsequent oxidative boryl transfer to pyrazine to furnish the desired product. This catalytic σ-bond activation system offers a viable tool for achieving otherwise difficult organic transformations. Building on their discovery of a highly active phosphineborane catalyst for CO2 reduction,48 Fontaine, Bouhadir, and 6987


ACS Catalysis

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