A Bioinspired Catalytic Aerobic Functionalization of Phenols

Research Article pubs.acs.org/acscatalysis

A Bioinspired Catalytic Aerobic Functionalization of Phenols: Regioselective Construction of Aromatic C−N and C−O Bonds Kenneth Virgel N. Esguerra and Jean-Philip Lumb* Department of Chemistry, McGill University, Montreal, Quebec H3A 0B8, Canada S Supporting Information *

ABSTRACT: We report a bioinspired approach for the regioselective construction of both aromatic C−O and C−N bonds through the dehydrogenative coupling of phenols and aliphatic amines. Mechanistically, the process hinges on the ortho-oxygenation of phenols to ortho-quinones followed by aromatic C−N bond formation through a condensation/redox isomerization cascade. Overall, the process enables the regioselective formation of aromatic C−O and C−N bonds directly from C−H bonds under mild reaction conditions.

KEYWORDS: aerobic catalysis, ortho-quinone, phenol, amine, C−H functionalization, crossed-dehydrogenative coupling, amino-phenol

T

incorporation of heteroatoms using simple phenols and amines is an unresolved challenge of great interest. Herein, we describe an approach toward this aim that draws inspiration from two very different biosynthetic processes (Scheme 1). The first is the biosynthesis of melanin pigments, in which the Type III Cu-enzyme tyrosinase catalyzes the aerobic ortho-oxygenation of L-tyrosine 1 to L-dopaquinone 2 in the first and rate-limiting step of pigmentation (Scheme 1A).15 As a natural consequence of dearomatization, the meta-position of 1 becomes activated for nucleophilic attack by the pendant amine, leading to L-dopachrome 3 following tautomerization and reoxidation.16 This constitutes a regioselective means of oxidizing two aromatic C−H bonds, under conditions that generate water as the only byproduct.15b The second biosynthetic process is the production of collagen fibers, which capitalizes upon the innate reactivity of ortho-quinones and amines. In this case, an ortho-imino phenol 5 is produced within the active site of lysyl oxidase (cf. TTQ) following condensation and redox-isomerization (Scheme 1B).17 Although the immediate product of this coupling is ultimately hydrolyzed to the corresponding ortho-amino phenol 6 during enzymatic catalysis, we envisioned the initial steps leading to 5 as a uniquely efficient means of forming an aromatic C−N bond. In this manuscript, we detail our efforts to interface these two, unrelated biosynthetic processes into a 1-pot, sequential process (Scheme 2). This leads to an efficient synthesis of heteroatom-rich aromatic rings by a formal dehydrogenative coupling of phenols and

he importance of aromatic carbon−heteroatom bonds to the function of materials and biologically active compounds has motivated considerable efforts to improve the efficiency of their formation.1 In this context, catalytic aerobic dehydrogenative coupling reactions, in which aromatic C−O or C−N bonds are created directly from C−H, O−H, or N−H bonds, are desirable due to their inherent atom- and stepefficiency.2 Phenols are an attractive coupling partner for this class of reaction because of their well-established availability from petrochemical resources3 and increasing availability from biomass.4 They are also common structural motifs in natural products and organic materials.5,6 Nevertheless, their participation in dehydrogenative C−O and C−N coupling reactions is challenged by nonselective oxidation with a broad range of oxidants.7 Examples of dehydrogenative aryl ether formation are limited,8 with a noteworthy exception being Hay’s industrial polymerization of 2,6-dimethyl phenol.9 Likewise, the direct amination of phenols suffers from challenges that include limited scope, prefunctionalization of the nitrogen coupling partner, or the use stoichiometric quantities of a synthetic oxidant.10 There are only a handful of examples where aromatic C−N bonds are formed directly from C−H and N−H bonds,11 and there are even fewer that use O2 as the terminal oxidant.11a More generally, functionalization is typically not regioselective in the absence of blocking groups and results in derivatization of the ortho- and/or para-position;7b,f,12 direct functionalization of the meta-position remains difficult.13 These challenges are amplified under the oxidative conditions of dehydrogenative coupling, which often require protection of the hydroxyl group to avoid nonselective,14 radical-based polymerization.7 Thus, a direct, catalytic aerobic process for the regioselective © XXXX American Chemical Society

Received: February 9, 2017 Revised: March 29, 2017

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ACS Catalysis

Scheme 3. Classical Synthesis of ortho-Aminophenols

Scheme 1. (A) Tyrosinase-Mediated Dual Functionalization of L-Tyrosine; (B) Lysyl Oxidase Mediated Deamination of Lysine during Collagen Biosynthesis

(Scheme 4). For example, benzoxazole 12 or benzoxazinone 13 would result from either oxidative cyclization or redox-neutral lactonization of ortho-imino-phenols derived from benzylamine 11 or phenylglycine methyl ester 10, respectively. Alternatively, the use of 2,4-di-nitro-phenyl-hydrazine 14 would provide valuable “push-pull” azo-phenols (cf. 15), following installation of the azo-linkage by condensation and tautomerization. Finally, the use of dihydropyrrole 16 would afford N-arylated pyrrole 17, in a formal dehydrogenative coupling between a phenol and a pyrrole. As a means of developing optimized reaction conditions, we evaluated the dehydrogenative coupling of 4-tert-butyl phenol 7 and benzylamine 11, in a 1-pot sequential process (Table 1). We have previously reported the oxygenation of 7 using 4 mol % of Cu(CH3CN)4(PF6) (abbreviated as CuPF6), 5 mol % of di-tert-butyl-ethylenediamine (DBED) and O2 (2 atm), which returns a quantitative yield of coupled ortho-quinone 9 by way of an oxidative coupling between DBED-Cu(II)-semiquinone 8 and 7.23 In the current context, 9 was not isolated, and instead, one equivalent of 11 was simply added directly to the reaction mixture. This leads to benzoxazole 12 in 23% isolated yield following a cascade of condensation/redox isomerization/ cyclization and dehydrogenation (entry 1). Competitive pathways contributing to the low yield of 12 included the oxidation of 11 to benzyl imine 18, and a substitution of the phenoxyl group with benzyl amine to provide amino-ortho-quinone 19, consistent with the previous observations of Maumy and Capdeville (Scheme 5).24 Given that both of these deleterious pathways might result from a relatively slow dehydrogenation of dihydrobenzoxazole 12′ to benzoxazole 12 in the final step of the cascade, we investigated the effects of dessicants because we have previously observed improvements to the oxidative capacity of the DBED/CuPF6 system in their presence.23a Unfortunately, minimal improvements were observed in the presence of MgSO4 (entry 2), even with 2 equiv of 11 (entries 3−4). Upon increasing the catalyst loading to 8 mol % CuPF6 and 10 mol % DBED, we observed an encouraging increase in yield to 43% (entry 5). Further adjusting the ratio of DBED to Cu by raising the ligand loading to 15 mol % provided an additional increase in yield to 57% (entry 6), which was not improved by the addition of desiccants (entries 7−11) or increasing Cu loadings above 8 mol % (entries 10 and 11). Thus, entry 6 represents our optimized reaction conditions, which remain efficient for each of the additional nitrogen coupling partners. This provides benzoxazinone 13, azophenol 15, and N-arylated pyrrole 17 in yields of 85%, 56%, and 83%, respectively (Scheme 4). Notably, these conditions remain efficient on gram scale (10 mmol of 7), providing 13 from 7 and 10 in 82% yield. Our regiochemical assignment is based on a series of X-ray crystal structures (Figure 1) and is consistent with C−N coupling at the more electrophilic of the two carbonyls of quinone 9. These can be viewed as two

Scheme 2. Work Described Herein

amines that occurs at room temperature, using an earth-abundant and commercially available copper catalyst. An important feature of our proposed methodology is the versatile reactivity of ortho-imino-phenols, which have been extensively developed as electrophilic reagents18 and serve as key intermediates in heterocycle synthesis.19 In spite of their utility, current syntheses incur atom- and step-inefficiencies during the preparation of the corresponding ortho-aminophenol (Scheme 3).20 Representative procedures include the nitration and reduction of phenols,21 transition-metal catalyzed crosscoupling of a prefunctionalized aromatic ring,2 or nucleophilic aromatic substitution.22 In contrast, the tandem ortho-oxygenation/ condensation/redox isomerization approach proposed herein would offer a direct means for the synthesis of ortho-imino-phenols, under mild reaction conditions. Once formed, we would anticipate a range of downstream transformations, which could be guided by careful selection of the nitrogen coupling partner 3478

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Research Article

ACS Catalysis Scheme 4. Bioinspired Synthesis of Benzoxazoles, Benzoxazinones, Azophenols, and N-Aryl Pyrrolea

a Synthesis of 12: 7 (1 mmol), CuPF6 (8 mol %), DBED (15 mol %), O2 (1 atm), CH2Cl2 (0.1 M), 4 h, 23 °C, then 11 (2.0 equiv), O2 (1 atm), 2 h, 23 °C. Synthesis of 13: 7 (1 mmol), CuPF6 (8 mol %), DBED (15 mol %), O2 (1 atm), CH2Cl2 (0.1 M), 4 h, 23 °C, then 10 (2.0 equiv) in MeOH (5 mL), 4 h, 50 °C. Synthesis of 17: 7 (1.0 mmol), CuPF6 (8 mol %), DBED (15 mol %), O2 (1 atm), CH2Cl2 (0.1 M), 4 h, 23 °C, then 16 (2.0 equiv), 2 h, 23 °C. Synthesis of 15: 7 (1.0 mmol), CuPF6 (8 mol %), DBED (15 mol %), O2 (1 atm), CH2Cl2 (0.1 M), 4 h, 23 °C, then 14 (2.0 equiv), MeOH (3 mL), 12 h, 23 °C. Isolated yields are reported for each entry.

Table 1. Initial Studies on the Coupling of Phenol 6 and Amine 10a

entrya

CuPF6 (mol %)

DBED (mol %)

BnNH2 (equiv)

additive

yield of 12 (%)b

1 2 3 4 5 6 7 8 9 10 11

4 4 4 4 8 8 8 8 8 10 20

5 5 5 5 10 15 15 15 15 20 40

1.0 1.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0

-MgSO4 (3 equiv) -MgSO4 (3 equiv) --MgSO4 (3 equiv) M.S. (4 Å, 200 mg) Na2SO4 (3 equiv) ---

23 27 33 32 43 57 55 53 50 37 24

Scheme 5. Competitive Pathways in the Aerobic Coupling of 7 and 11

a

1

Conducted with 1.0 mmol of 7. bProduct yield is determined by H NMR using hexamethylbenzene as an internal standard.

independent π-systems, consisting of a vinylogous ester and an α, β-unsaturated ketone, with condensation occurring preferentially at the latter. As a result, the oxygen atom dervied from O2 during the ortho-oxygenation is ultimately replaced by nitrogen in the subsequent C−N coupling, providing a unique mechanism for the ortho-amination of phenols. Next, we evaluated the scope of mono- and disubstituted phenols with each of the amine coupling partners (Table 2).

Figure 1. Single crystal X-ray structure of 13 and selected products in Table 1 (entries 5 and 7). Hydrogens are omitted for clarity. Blue = nitrogen, red = oxygen, teal = carbon, yellow = fluorine.

Monosubstituted phenols bearing para- and meta- substituents readily couple with various amines to afford benzoxazoles, 3479

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ACS Catalysis Table 2. Scope of Phenolsa

Benzoxazole synthesis: phenol (1 mmol), CuPF6 (8 mol %), DBED (15 mol %), O2 (1 atm), CH2Cl2 (0.1 M), 4 h, 23 °C, then 11(2.0 equiv), O2 (1 atm), 2 h, 23 °C; Benzoxazinone synthesis: phenol (1 mmol), CuPF6 (8 mol %), DBED (15 mol %), O2 (1 atm), CH2Cl2 (0.1 M), 4 h, 23 °C, then 10 (2.0 equiv) in MeOH (5 mL), 4 h, 50 °C; N-Aryl pyrrole synthesis: phenol (1.0 mmol), CuPF6 (8 mol %), DBED (15 mol %), O2 (1 atm), CH2Cl2 (0.1 M), 4 h, 23 °C, then 16 (2.0 equiv), 2 h, 23 °C; Azophenol synthesis: phenol (1.0 mmol), CuPF6 (8 mol %), DBED (15 mol %), O2 (1 atm), CH2Cl2 (0.1 M), 4 h, 23 °C, then 14 (2.0 equiv), MeOH (3 mL), 12 h, 23 °C. Isolated yields are reported for each entry. a

benzoxazinones, N-aryl pyrroles or azophenols. Notably, phenols bearing enolizable protons (entries 1−3 and 8), aryl ethers (entries 4−6) and halogens (entry 6), as well as 2-napththols are all tolerated (entries 9 and 10). The sterically encumbered 2- and 4-positions of 3,5-diphenyl phenol are also readily functionalized to afford the corresponding benzoxazinones in good yields (entry 14). Finally, aromatic C−N coupling remains chemoselective in the presence of a ketone, as illustrated by the late-stage functionalization of estrone (entries 10 and 11). The functionalization of phenol (entry 15) and orthosubstituted phenols (entries 16−18) affords products in which three C−H bonds have been functionalized in the one-pot operation, including the installation of two aromatic C−O bonds and one aromatic C−N bond. Whereas the ortho-oxygenation of phenol provides a symmetrical ortho-quinone for which there are

no questions of regiochemistry in the amine coupling, the corresponding oxygenation of ortho-substituted phenols affords a 3,4,5-trisubstituted ortho-quinone, in which the carbonyls at C1 and C2 (quinone numbering) are differentiated by sterics (Table 2, inset). This directs C−N coupling to the more accessible C1 carbonyl to provide a single regioisomer of the product. An important feature of the current methodology is the efficiency with which multiple sites of simple phenols can be oxidized and activated for additional transformations under relatively mild reaction conditions. We believe that this sets the stage for an alternative approach to aromatic-heteroatom coupling reactions, for which we provide a preliminary proof of principal in Scheme 6. Standard oxygenative homocoupling of 4-tert-butyl phenol 7 to ortho-quinone 9 allows downstream conversion to either ortho-quinone 20 or para-quinone 22, by exposure to ethanol under basic or acidic conditions, respectively.25 3480

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ACS Catalysis Scheme 6. Synthesis of N-Aryl Pyrrole from ortho- and paraQuinonesa

ACKNOWLEDGMENTS

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REFERENCES

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N-aryl pyrrole synthesis: 20 or 21 (1 mmol), 16 (2.0 equiv), CH2Cl2 (0.1 M), 2 h, 23 °C.

Subsequent exposure of 20 or 22 to 16 provides regioisomeric N-aryl pyrroles 21 and 23, respectively, and completes a twostep sequence for the regioselective ortho- and metafunctionalization of 7, under mild conditions. More generally, this demonstrates the compatibility of our C−N coupling strategy with both ortho- and para-quinones. Notably, 21 and 22 are formed by the union of three nucleophiles (i.e., a phenol, an amine, and an alcohol) using nontraditional approaches for cross-coupling that hinge on the dearomatization of aromatic rings. Given the importance of these linkages to the applied chemical sciences, we anticipate numerous applications for this approach to cross-coupling. In summary, we have developed a bioinspired approach for the construction of aromatic C−O and C−N bonds through the regioselective coupling of simple phenols and aliphatic amines. Mechanistically, the method involves the in situ generation of ortho-quinones by aerobic dearomatization of phenols, enabling aromatic C−N and C−O bond formation to occur at room temperature. The reaction allows for the synthesis of four distinct nitrogen-containing product classes, all using a single, earth-abundant catalyst that operates at room temperature within hours. Such mild conditions for aromaticheteroatom bond formation are attractive and present a new opportunity to exploit aerobic catalysis for environmentally sensitive synthesis.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.7b00437. Experimental procedures and characterization data including NMR spectra, IR, HRMS, and crystallographic data (PDF) X-ray data for compound 13 (CIF) X-ray data for benzoxazole of Table 2, entry 4 (CIF) X-ray data for benzoxazole of Table 2, entry 6 (CIF)

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Financial support was provided by the Natural Sciences and Engineering Council of Canada (NSERC, Discovery Grant to J.-P.L.), the Fonds de Recherche du QuébecNature et Technologies (FRQNT, Team Grant to J.-P.L.), the FRQNT Center for Green Chemistry and Catalysis at McGill University, the NSERC CREATE Program in Green Chemistry at McGill University, and the McGill University-Canadian Institutes of Health Drug Development Trainee Program (doctoral fellowship to K.V.N.E.). We wish to thank Dr. Thierry Maris (University of Montreal) for help with X-ray crystallography.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jean-Philip Lumb: 0000-0002-9283-1199 Notes

The authors declare no competing financial interest. 3481

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