Rhenium-Catalyzed Dehydrogenative Coupling of Alcohols and

Jan 29, 2019 - An efficient synthesis of quinolines, pyrimidines, quinoxalines, pyrroles, and aminomethylated aromatic compounds catalyzed by a well-d...
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Rhenium-Catalyzed Dehydrogenative Coupling of Alcohols and Amines to Afford Nitrogen-Containing Aromatics and More Matthias Mastalir,† Mathias Glatz,† Ernst Pittenauer,‡ Günter Allmaier,‡ and Karl Kirchner*,† †

Institute of Applied Synthetic Chemistry and ‡Institute of Chemical Technologies and Analytics, Technische Universitat Wien, Getreidemarkt 9, A-1060 Vienna, Austria

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ABSTRACT: An efficient synthesis of quinolines, pyrimidines, quinoxalines, pyrroles, and aminomethylated aromatic compounds catalyzed by a welldefined Re(I) PNP pincer complex is described. All reactions proceed with liberation of dihydrogen and elimination of water. Under optimized reaction conditions a wide range of organic functional groups are tolerated. This study demonstrates that rhenium catalysts are performing extremely well in dehydrogenative processes with considerably lower catalyst loadings and shorter reaction times when compared to analogous Mn(I) complexes.

T

he acceptorless dehydrogenation (AD) of alcohols1 is an atom-economical and oxidant-free process to yield ketones and aldehydes with concomitant release of dihydrogen. In most cases, these carbonyl compounds are catalytically generated in situ where they are then transformed into organic compounds such as nitrogen-containing aromatics, e.g., pyridines, pyrroles, pyrimidines, quinolines, pyrazines, quinoxalines, as well as amino-methylated aromatic molecules, imines, and amines. The subsequent reactions are organic condensation reactions generating merely water as a nontoxic byproduct. It is thus not surprising that this methodology has emerged as a powerful tool for the benign construction of valuable intricate organic molecules. In recent years, many efficient homogeneous catalysts based on both precious and nonprecious metals such as Ru,2 Rh,3 Ir,4 Os,5 Mn,6 Fe,7 Co,8 and Ni9 have been developed for AD of alcohols in conjunction with subsequent condensation reactions involving mostly amines. Molecularly defined rhenium complexes, in contrast to manganese,10 have been less explored in (de)hydrogenation catalysis involving alcohols.11 In this article, we report on the activity of a well-defined Re(I) PNP hydride complex as catalyst for the acceptorless dehydrogenation of alcohols to give, in conjunction with amines, pyrroles, pyrimidines, quinolines, quinoxalines, and aminomethylated aromatic molecules (Scheme 1). First, rhenium catalysts Re1−Re4 (Figure 1) were investigated for the coupling of benzyl alcohol with 4-toluidine (1.4 equiv) as a model reaction in toluene (3 mL) in both closed and open vials. The results are summarized in Table 1. The products were analyzed by 1H and 13C{1H} NMR spectroscopy. In all cases, isolated yields after purification by column chromatography are presented. Conversions were typically 5−10% higher due to the formation of small amounts of side products (ketones, aldehydes, and aldol condensation products). Re1−Re3 (0.02 mol % based on alcohol) were used © XXXX American Chemical Society

Scheme 1. Rhenium-Catalyzed Formation of Imines, NHeteroaromatics, and Aminomethylated Aromatics

Figure 1. Re(I) PNP pincer complexes screened as catalysts (R = iPr).

as catalysts in a closed system at 120 °C in the presence of 10 mol % of t-BuOK as base and afforded 1-(phenyl)-N-(4tolyl)methaneimine (1) together with substantial amounts of N-(benzyl)-4-methylaniline (2) (Table 1, entries 1−3). The best results could be achieved with catalyst Re3 affording 1 and 2 in 69 and 27% isolated yields. Catalyst Re4 bearing NMe linkers was catalytically inactive, and no reaction took place Received: January 4, 2019

A

DOI: 10.1021/acs.orglett.9b00034 Org. Lett. XXXX, XXX, XXX−XXX

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Table 1. Catalyst Screening for the Amination and Imination of Benzyl Alcohol with 4-Toluidine as a Benchmark Systema,b

entry

cat.

cat. loading [mol %]

T [°C]

t [h]

yield 1 [%]

yield 2 [%]

conv. [%]

1 2 3 4 5 6 7 8

Re1 Re2 Re3 Re4 Re3c Re3d Re3c Re3c

0.02 0.02 0.02 0.02 0.02 0.02 0.05 0.02

120 120 120 120 120 120 100 120

8 8 8 8 8 8 8 6

59 67 69

21 26 27

91 98 97

96 34 96 98

57

98 97 98 99

a

Reaction conditions: 1.0 mmol of benzyl alcohol, 1.2 mmol of 4-toluidine, 0.1 mmol of t-BuOK, 3 mL of toluene. bIsolated yields, purified via column chromatography. cOpen vial. d5 bar hydrogen pressure.

required an increase of the catalyst loading to 0.05 mol % in order to achieve high yields (Table 1, entry 7). Finally, when the reaction with Re3 was performed in 6 h, 1 was also formed in 96% isolated yield (Table 1, entry 8). With catalyst Re3 as the most efficient catalyst in the series, we then applied this methodology to other AD processes combined with subsequent condensation reactions to yield quinolines, pyrimidines, quinoxalines, pyrroles, amino-methylated naphthols, and phenols. In order to demonstrate the efficiency and versatility of the Rh(I) PNP pincer catalyst Re3 four representative examples of each of these transformations were investigated (Tables 2−7). A series of quinolines were prepared from 2-aminobenzhydrol and four secondary alcohols with a catalyst loading of 0.2 mol %, 50 mol % of t-BuOK in toluene as solvent at 140

(Table 1, entry 4) which clearly emphasizes the important role of the acidic NH moieties for the catalytic reaction. It seems thus likely that the reaction proceeds via an outer-sphere hydride transfer and reversible PNP ligand deprotonation/ protonation as reported recently for the analogous Mn(I) PNP hydride complex.11f When the reaction with Re3 was performed in an open system, 1 was formed selectively in 96% isolated yield (Table 2, entry 5). On the other hand, performing the reaction with Re3 under a hydrogen pressure of 5 bar yielded a mixture of 1 and 2 in 34 and 57% isolated yields, respectively (Table 1, entry 6), underlining that the Re(I) complex Re3, in analogy to the analogous Mn(I) complex reported previously,6a is not an efficient hydrogenation catalyst. Lowering the temperature to 100 °C Table 2. Synthesis of Quinolines Using 2-Aminobenzhydrol and Secondary Alcoholsa,b

Table 3. Three-Component Pyrimidine Syntheses from Benzamidine and Primary and Secondary Alcoholsa,b

a

a

Reaction conditions: 1.0 mmol of benzamidine, 1.2 mmol of prim. alcohol, 1.2 mmol of sec. alcohol, 0.75 mmol of t-BuOK, and 3 mL of toluene. bIsolated yields.

Reaction conditions: 1.0 mmol of 2-aminobenzhydrol, 1.5 mmol of sec. alcohol, 0.5 mmol of t-BuOK, and 3 mL of toluene. bIsolated yields. B

DOI: 10.1021/acs.orglett.9b00034 Org. Lett. XXXX, XXX, XXX−XXX

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Table 6. Synthesis of Pyrroles Using 1,2-Aminoalcohols and Secondary Alcoholsa,b

a

Reaction conditions: 1.0 mmol of 1,2-phenylenediamine, 1.2 mmol of 1,2-diole, 0.5 mmol of t-BuOK, and 3 mL of toluene. bIsolated yields.

a

Table 5. Synthesis of Pyrroles from Hexane-2,5-diole and Primary Aminesa,b

Table 7. Aminomethylation of Aromatic Compounds from Methanol and Secondary Aminesa,b

Reaction conditions: 1.0 mmol of 1,2-aminobenzyl, 1.3 mmol of sec. alcohol, 0.7 mmol of t-BuOK, and 3 mL of toluene. bIsolated yields.

a

Reaction conditions: 1.0 mmol of hexane-2,5-diole, 1 mmol of amine, 0.5 mmol of t-BuOK, and 3 mL of toluene. bIsolated yields. a

Reaction conditions: 1.0 mmol of 2-napthol or phenol, 1.5 mmol of MeOH, 1.1 mmol of t-BuOK, 1.1 mmol of sec. amine, and 4 mL of toluene. bIsolated yields.

°C, and a reaction time of 8 h in closed vials. The reaction occurs at its less substituted position and is regioselective with respect to the carbonyl compounds which are formed in situ (Table 2). All quinolines were isolated in good to excellent yields (83−90%). Imidines via a three-component process were also studied. Under comparable conditions as outlined above for the preparation of quinolines but with a catalyst loading of 1 mol %, four different pyrimidines were synthesized. Variation of the primary and secondary alcohols resulted in the formation of compounds 7−10 in 80−92% isolated yields (Table 3). The entire transformation is an efficient, selective, and high-yielding single-step procedure. The treatment of 1 equiv of 1,2-phenylenediamine and various aliphatic and aromatic 1,2-diols (1.5. equiv) in the presence of catalyst Re3 (0.05 mol %) and base (50 mol % tBuOK) at 120 °C for 6 h in a closed system afforded quinoxalines 11−14 as the products in 67−84% isolated yield (Table 4).

We then prepared a representative series of N-substituted pyrroles derived from hexane-2,5-diol and primary amines (Table 5) as well as 2,5-disubstituted pyrroles obtained from 1,2-aminoalcohols and secondary alcohols (Table 6). In the first case the reaction requires 0.1 mol % of catalyst, base (70 mol % of t-BuOK), a temperature of 140 °C, and a reaction of 6 h, while the latter proceeds at 160 °C and 6 h well with 0.2 mol % of catalyst and the same amount of base. Finally, Re3 (0.5 mol %) in the presence of base (110 mol % t-BuOK) at 140 °C for 6 h in a closed system efficiently catalyzes the aminomethylation of activated aromatic compounds with MeOH as a sustainable C1 building block. Again, this process is a three-component reaction (Table 7). 2Naphtol and two phenols were treated with N-benzyl-Nmethylamine, pyrrolidine, and piperidine, respectively, and 1.5 equiv of MeOH to afford the respective amino-methylated products in high isolated yields. C

DOI: 10.1021/acs.orglett.9b00034 Org. Lett. XXXX, XXX, XXX−XXX

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In sum, this study describes an efficient and practical synthesis of substituted quinolines, pyrimidines, quinoxalines, pyrroles, and amino-methylated aromatic compounds catalyzed by a well-defined hydride Re(I) PNP pincer complex. In contrast to isoelectronic Mn(I) PNP pincer catalysts,6 considerably lower catalyst loadings (the costs of Re(I) and Mn(I) PNP precatalysts is essentially the same) as well as shorter reaction times could be applied in most cases. A mechanism which involves an outer-sphere hydride transfer and reversible PNP ligand deprotonation/protonation is proposed since Re4 bearing NMe rather than NH linkers is catalytically inactive for this process. Quinolines can be assembled in a regioselective fashion from 2-aminobenzylalcohols and secondary alcohols. A series of pyrimidines are obtained via a three-component process utilizing benzamidine and two different alcohols; quinoxalines are prepared by reacting 1,2-phenylenediamine with aliphatic and aromatic 1,2diols; while pyrroles are afforded via two different routes utilizing either hexane-2,5-diol and primary amines or 1,2aminoalcohols and secondary alcohols. Aminomethylations were performed with naphtols and phenols in conjunction with amines and methanol as C1 building blocks. The selective C− C and C−N bond formations involve liberation of molecular hydrogen (acceptorless dehydrogenation) and elimination of H2O (condensation). Under optimized conditions the presence of a wide range of typical organic functional groups is tolerated. This study clearly demonstrates that rhenium catalysts are capable of performing extremely well in dehydrogenative processes with considerably lower catalyst loadings and shorter reaction times when compared to analogous Mn(I) complexes. In analogy to Mn(I), also the Re(I) PNP complex Re3 is not an efficient hydrogenation catalyst which includes also hydrogen borrowing (hydrogen autotransfer) processes.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b00034. Experimental details, MS data, and 1H, 13C{1H}, and 19 1 F{ H} NMR spectra of all organic compounds (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: (+43) 1 58801 163611. Fax: (+43) 1 58801 16399. ORCID

Karl Kirchner: 0000-0003-0872-6159 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the Austrian Science Fund (FWF) is gratefully acknowledged (Project No. P29584−N28). REFERENCES

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DOI: 10.1021/acs.orglett.9b00034 Org. Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.orglett.9b00034 Org. Lett. XXXX, XXX, XXX−XXX