Cobalt-catalyzed acceptorless dehydrogenative coupling of aminoalcohols with alcohols: direct access to pyrrole, pyridine and pyrazine derivatives

Abstract

Here, the first example is reported of a new, molecularly defined SNS-cobalt(II) catalyst for the acceptorless dehydrogenative coupling (ADC) of unprotected amino alcohols with secondary alcohols leading to pyrrole and pyridine derivatives.

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Transition-metal-catalyzed acceptorless dehydrogenation (AD) and hydrogen autotransfer (HA) reactions have been attracting much interest in recent times, in large part due to their excellent step-economy and high atom-efficiency.^1,2 These strategies play a crucial role in activating inert chemical bonds, such as the O–H bond of alcohols and the N–H bond of amines without pre-functionalization. In particular, catalytic acceptorless dehydrogenative coupling (ADC) reactions provide a direct approach for the construction of diverse heterocyclic compounds from easily available starting materials such as alcohols, amines, and unsaturated systems with the liberation of H₂ and H₂O.

Pyrrole constitutes one of the most important N-heterocyclic motifs and is ubiquitous in natural products, drug intermediates, agrochemicals, dyes, and functional materials.^3,4 Given their importance, the development of efficient strategies for the synthesis of pyrroles from simple feedstock chemicals is a prime focus in contemporary science. The classical approach to pyrrole synthesis involves the well-established Knorr,⁵ Paal–Knorr,⁶ and Hantzsch⁷ methods. However, direct and sustainable access to pyrroles from simple alcohols is appealing, since alcohols can be derived from abundantly available lignocellulosic biomass by hydrogenolysis.⁸

In 2013, Kempe and co-workers demonstrated the first direct synthesis of pyrroles from amino alcohols and secondary alcohols efficiently catalyzed by iridium(III)-complexes.⁹ Subsequently, Milstein¹⁰ and Saito¹¹ reported a conceptually similar strategy using a ruthenium(II)-catalyst. A ruthenium-catalyzed three-component reaction of benzylic ketones, vicinal diols, and amines to access pyrroles has been reported by Beller and co-workers.¹² However, the majority of the ADC reactions depend on the precious and less-abundant 4d and 5d transition-metals.^1,2 In recent times, intensive efforts have been devoted to the design of novel catalysts based on non-precious metals,^13–15 in particular, cobalt(II)-complexes in various AD/HA reactions.¹⁵ Based on Crabtree's original work with a Ru catalyst,¹⁶ Milstein and co-workers reported the synthesis of N-substituted pyrroles from aliphatic diols and amines using a moleculary defined PNN-Co(II) pincer complex (Scheme 1).^15h

Scheme 1 Direct synthesis of pyrrole via an ADC strategy.

Importantly, it should be noted that all of the catalysts in AD/HA reactions possess (electron-rich) phosphine ligands. Despite the tremendous success of phosphine ligands in homogeneous catalysis, their preparation is often non-trivial, requiring handling under an inert atmosphere, needing multi-step syntheses, etc. As a consequence, the phosphine ligands are expensive and can be challenging to make on a large scale, therby hindering sustainable development.¹⁷ Of late, Gusev and co-workers showed that a sulfur-based ligand is a possible extension to replace the phosphine ligand and worked efficiently in the hydrogenation of esters.^17a

Intrigued by these findings, we began to explore the potential of a simple, phosphine ligand-free Co(II)-complex as a precatalyst for the preparation of diverse N-heterocycles. Initially, new sulphur-based SNS-cobalt(II) pincer complexes 1–2 have been prepared. The reaction of L1 and L2 with CoCl₂ in MeOH at room temperature results in the formation of the corresponding Co-complexes 1–2, in 77% and 72% yields, respectively.¹⁸ Complexes 1–2 were characterized by elemental analysis, MALDI-TOF, EPR, IR and magnetic measurements.

A structural determination of 1 indicated a dimeric structure.¹⁹ The dimeric molecules made up of hexacoordinated Co(II) ions are joined by two bridging chloride ligands. The neutral ^EtSNS ligand is coordinated to the cobalt center in the typical tridentate mode, with a S1–Co1–S2 angle of 161.29(9)°. Selected bond distances and angles are given in the caption of Fig. 1. Remarkably, complexes 1 and 2 catalyze the dehydrogenative coupling of unprotected 1,2- and 1,3-amino alcohols with secondary alcohols in an efficient manner that enables the direct synthesis of 1H-pyrroles, and pyridines (or quinolines), respectively.²⁰ This reaction involves the consecutive C–N and C–C bond formation with the liberation of hydrogen gas and water.

Fig. 1 X-ray crystal-structure analysis of 1 with 50% probability of thermal ellipsoids. Hydrogen atoms (except N–H) were omitted for clarity. Selected bond lengths [A°] and angles [°]: Co(1)–N(1) 2.124(7), Co(1)–S(1) 2.544(2), Co(1)–S(2) 2.555(2), Co(1)–Cl(1) 2.351(2), Co(1)–Cl(2) 2.548(2), Co(1)–Cl(3) 2.441(2), S(1)–Co(1)–S(2) 161.29(9), Cl(1)–Co(1)–Cl(2) 178.32(8), Cl(1)–Co(1)–N(1) 94.82(19), Co(1)–Cl(2)–Co(2) 95.74(8), S(1)–Co(1)–N(1) 81.58(18), S(2)–Co(1)–N(1) 82.39 (18).

The reaction of 1-amino-3-methylbutan-2-ol (3c) with 1-phenylethanol was chosen as a model system for the acceptorless dehydrogenative coupling to form 1H-pyrroles (see ESI,† for optimization studies). Thus, refluxing an equimolar amount of 1,2-amino alcohol (1 mmol), 1-phenylethanol (1 mmol) and KO^tBu (1 mmol) in m-xylene in the presence of 2.5 mol% complex 1 gave 69% yield of 2-isopropyl-5-phenyl-1H-pyrrole (5c). In the same reactions, complex 2 yielded 67% of 5c. Analysis of the gas phase by gas chromatography (GC) revealed the formation of dihydrogen. Indeed, increasing the ratio of 1,2-amino alcohol : 1-phenylethanol to 1 : 2 slightly enhanced the yield of the desired 1H-pyrrole to 77%. Next, the solvent effect on the reaction was examined. Thus, the yield of 5c in m-xylene (77%) was higher than in toluene (47%) or THF (32%) or in n-octane (52%). Various bases such as KHMDS (67%), KH (71%), and NaO^tBu (63%) were also employed under similar catalytic conditions and gave relatively decent yields of 5c. Notably, under similar reaction conditions, a Ru(II)-catalyst RuCl₂(PPh₃)[HN(C₂H₄SEt)₂]¹⁸ derived from the ^EtSNS ligand did not perform better than the present cobalt(II) catalysts.

With the optimized reaction conditions in hand, the present cobalt-catalyzed dehydrogenative annulation of amino alcohols with secondary alcohols was then deployed in the synthesis of diverse 1H-pyrroles. The scope of the present cobalt catalysis was probed with 1-phenylethan-1-ol as the benchmark substrate and different β-aminoalcohols. As shown in Scheme 2, our strategy is compatible with various β-aminoalcohols and secondary alcohols, affording unsymmetrical 2,5-disubstituted 1H-pyrroles in excellent yields (up to 89% yield).

Scheme 2 Direct synthesis of 1H-pyrroles via dehydrogenative annulation reaction. ^aReaction conditions: 0.25 mmol of 3, 0.5 mmol of 4, 2.5 mol% of catalyst 1, KO^tBu (0.26 mmol), and 2 mL of m-xylene heated at reflux under argon atm for 24 h. ^bAll reported isolated yields are the average of two independent experiments.

Interestingly, the reaction proceeded successfully with both aliphatic and aromatic unprotected β-aminoalcohols and gave the desired 1H-pyrroles in moderate to good yields (58–86%). The impact of varying the substituents on the secondary alcohol coupling partner was assessed using L-valinol as a benchmark substrate. The straightforward 1H-pyrrole formation reaction proceeded in excellent yields with electron-donating substituents on the 1-phenylethan-1-ol and led to the desired dehydrogenated products 5h, 5j and 5l in good yields. On the other hand, the presence of an electron-withdrawing substituent (–Cl) on the phenyl ring resulted in a moderate yield of the corresponding 1H-pyrrole (product 5i in 70%). Interestingly, the reaction of 1-(4-nitrophenyl)ethan-1-ol with 2-amino-3-methylbutan-1-ol gave 5k in 67% isolated yield, where hydrogenation of the nitro group (by the in situ generation of hydrogen gas) was observed. Notably, an aliphatic alcohol (e.g. 2-decanol) underwent dehydrogenative coupling successfully and gave the expected product 5n in 45% isolated yield. Under the optimal conditions, 1-phenylethan-1-ol reacted with 2-amino-4-methylpentan-1-ol and led to 5o in good yield (74%).

After having established that β-aminoalcohols and secondary alcohols have been successfully coupled via ADC to form 1H-pyrroles under our cobalt catalytic process, we explored the use of γ-aminoalcohols as coupling partners to access pyridine and/or quinoline derivatives. Pyridines and quinolines are other important classes of N-heteroarenes with interesting bioactivity and their synthesis is of industrial significance.²¹ Notable progress has been made in the direct synthesis of pyridine and quinoline derivatives based on the ADC strategy using noble metal (Ir and Ru) based complexes.^10,22,23 Very recently, Kirchner and co-workers reported the manganese-catalyzed efficient synthesis of quinolines by reaction of 2-aminobenzyl alcohols and secondary alcohols.²⁴ Indeed, the ADC approach marks a substantial departure from the classical Hantzch-like dihydropyridine synthesis²⁵ and offers a concise and regiospecific access to pyridine or quinoline derivatives. Interestingly, a series of secondary alcohols have suitably assembled with γ-aminoalcohols to afford excellent product selectivity and good isolated yields of the desired pyridine derivatives under optimal conditions. This acceptorless dehydrogenation leads to aromatization and the condensation step deoxygenates the alcohol component.

As shown in Scheme 3, different aryl systems with varying electronic properties and alkyl groups have been regioselectively introduced, giving rise to the C-2 substituted pyridines. All of the pyridine derivatives (7a–f) were isolated in moderate to good yields (60–83%). More pleasingly, C-2 substituted quinolines (7g–k) were also prepared involving dehydrogenative cyclization of 2-aminobenzyl alcohol with various secondary alcohols using our established protocol in good yields (up to 87%). Thus, the present phosphine-free cobalt(II) complex displayed remarkable activity in the direct synthesis of various 2-substituted pyridines and quinolines.

Scheme 3 Direct synthesis of pyridines and quinolines via dehydrogenative annulation reaction. ^aReaction conditions: 0.25 mmol of 6, 0.5 mmol of 4, 2.5 mol% of catalyst 1, KO^tBu (0.26 mmol), and 2 mL of m-xylene heated at reflux under argon atm for 24 h. ^bIsolated yields.

Pyrazines are biologically important organic compounds and have potential applications in the chemical industry.^26,27 Gratifyingly, treatment of β-aminoalcohol with an equivalent amount of KO^tBu in the presence of a catalytic amount of 1 selectively led to pyrazine (8) with the liberation of hydrogen gas and water (Scheme 4). The present cobalt-catalyzed direct pyrazine synthesis was tested for gram-scale synthesis, and it worked smoothly and gave 8 in 68% (1.02 g) isolated yield. This result implies that our elegant cobalt-based catalytic system has potential for the large-scale production of pyrazine under operationally simple, benign conditions.

Scheme 4 Synthesis of pyrazine (8) from β-aminoalcohol.

In summary, the first phosphine ligand-free, non-precious base-metal catalyst²⁸ that enables the direct synthesis of N-heterocyclic compounds (1H-pyrroles, pyridines, quinolines, and pyrazine) from amino alcohols and alcohols as key starting materials with the liberation of hydrogen gas and water as the sole by-products has been described. The present expedient strategy makes use of a new, air-stable molecularly defined SNS-cobalt(II) complex.

This research is supported by the EMR/2015/30 and SERB (SB/FT/CS-065/2013). SPM, VGL, MKS and JR thank CSIR for the fellowship. We thank Mr Saibal Bera for X-ray analysis and Dr Kumar Vanka for helpful suggestions.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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18. See the ESI†.
19. To avoid the disorder in the structure, the chemical occupancy of C16A and C16B has been assigned as 0.5 and the CCDC No of complex 1 is 1555768.
20. Complex 1 also catalyzes N-alkylation of amines with alcohols by hydrogen autotransfer strategy. S. P. Midya and E. Balaraman, to be published.
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28. A triazine-based electron-rich PN₅P-Mn(I)-complex catalyzed synthesis of pyrrole has been reported. However, it is noteworthy to mention that the relative Co-complex is inactive and failed to yield the 1H-pyrrole, see: F. Kallmeier, B. Dudziec, T. Irrgang and R. Kempe, Angew. Chem., Int. Ed., 2017, 56, 7261.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1555768. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7cc07427a

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