& Sustainable Synthesis The Synthesis of Benzimidazoles and Quinoxalines from Aromatic Diamines and Alcohols by Iridium-Catalyzed Acceptorless Dehydrogenative Alkylation Toni Hille, Torsten Irrgang, and Rhett Kempe* [a] Abstract: Benzimidazoles and quinoxalines are important N-heteroaromatics with many applications in pharmaceuti- cal and chemical industry. Here, the synthesis of both classes of compounds starting from aromatic diamines and alcohols (benzimidazoles) or diols (quinoxalines) is re- ported. The reactions proceed through acceptorless dehy- drogenative condensation steps. Water and two equiva- lents of hydrogen are liberated in the course of the reac- tions. An Ir complex stabilized by the tridentate P^N^P ligand N 2 ,N 6 -bis(di-isopropylphosphino)pyridine-2,6-di- amine revealed the highest catalytic activity for both reactions. Benzimidazole and its derivatives are important com- pounds. [1–2] They are applied as pharmaceuticals [for example, the anthelmintic thiabendazole A (Scheme 1)] and as fungi- cides. [3] Furthermore, they are the basis of special chemicals for industrial applications such as chemical UVB filters, [4] pig- ments, [5] optical brighteners for coatings, [6] and thermo stable membranes for fuel cells. [7] Similar applications are known for quinoxalines. Quinoxaline derivatives are effective antineoplas- tics, [8] antivirals, [9] antidepressants, [10] antibiotics, [11, 12] as well as coccidiostats [13] (sulfaquinoxaline B, Scheme 1) in veterinary medicine and biocides [14] . In addition, they are used as dyes [15] and organic semi-conductors. [16] The acceptorless dehydrogenation is an elegant catalytic synthesis method. [17] It generates a valuable and easy to remove by-product namely H 2 . By doing so, a shift of the exist- ing equilibriums towards product formation can be facilitated. An especially attractive version of acceptorless dehydrogena- tion is its combination with condensation steps (acceptorless dehydrogenative condensations–ADC). Here, the liberation of H 2 is combined with the elimination of water (condensation). Recently, ADC has been used to react alcohols and amines to imines [18] and (synthetically very useful) to react alcohols and/ or diols and amines or amino alcohols to selectively substitut- ed N-heteroaromatic compounds, for instance, pyrroles [19] and pyridines [20] (Scheme 2). Here, we report on an Ir-catalyzed synthesis of benzimida- zoles and quinoxalines by ADC starting from benzene-1,2-dia- mines and aliphatic alcohols or 1,2-diols, respectively. Two equivalents of dihydrogen are eliminated in the course of the reactions. [21] Recently, several synthesis protocols for the syn- thesis of benzimidazoles, [22] based on o-diamines and primary [a] T. Hille, Dr. T. Irrgang, Prof. Dr. R. Kempe Lehrstuhl fɒr Anorganische Chemie II, UniversitȨt Bayreuth UniversitȨtsstrasse 30, NW I, 95440 Bayreuth (Germany) Fax: (+ 49) 921552157 E-mail : [email protected]Supporting information for this article is available on the WWW under http ://dx.doi.org/10.1002/chem.201400400. Scheme 1. Examples of an important benzimidazole (A) and quinoxaline (B). Scheme 2. State of the art in N-heterocycle synthesis through acceptorless dehydrogenative condensation reactions and the benzimidazole and qui- noxaline synthesis introduced here. Chem. Eur. J. 2014, 20,1–5 # 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1 && These are not the final page numbers! ÞÞ Communication DOI: 10.1002/chem.201400400
Transcript
& Sustainable Synthesis
The Synthesis of Benzimidazoles and Quinoxalines from AromaticDiamines and Alcohols by Iridium-Catalyzed AcceptorlessDehydrogenative Alkylation
Toni Hille, Torsten Irrgang, and Rhett Kempe*[a]
Abstract: Benzimidazoles and quinoxalines are importantN-heteroaromatics with many applications in pharmaceuti-cal and chemical industry. Here, the synthesis of bothclasses of compounds starting from aromatic diaminesand alcohols (benzimidazoles) or diols (quinoxalines) is re-ported. The reactions proceed through acceptorless dehy-drogenative condensation steps. Water and two equiva-lents of hydrogen are liberated in the course of the reac-tions. An Ir complex stabilized by the tridentate P^N^Pligand N2,N6-bis(di-isopropylphosphino)pyridine-2,6-di-amine revealed the highest catalytic activity for bothreactions.
Benzimidazole and its derivatives are important com-pounds.[1–2] They are applied as pharmaceuticals [for example,the anthelmintic thiabendazole A (Scheme 1)] and as fungi-cides.[3] Furthermore, they are the basis of special chemicals forindustrial applications such as chemical UVB filters,[4] pig-
ments,[5] optical brighteners for coatings,[6] and thermo stablemembranes for fuel cells.[7] Similar applications are known forquinoxalines. Quinoxaline derivatives are effective antineoplas-tics,[8] antivirals,[9] antidepressants,[10] antibiotics,[11, 12] as well ascoccidiostats[13] (sulfaquinoxaline B, Scheme 1) in veterinarymedicine and biocides[14] . In addition, they are used as dyes[15]
and organic semi-conductors.[16]
The acceptorless dehydrogenation is an elegant catalyticsynthesis method.[17] It generates a valuable and easy toremove by-product namely H2. By doing so, a shift of the exist-ing equilibriums towards product formation can be facilitated.An especially attractive version of acceptorless dehydrogena-tion is its combination with condensation steps (acceptorlessdehydrogenative condensations–ADC). Here, the liberation ofH2 is combined with the elimination of water (condensation).Recently, ADC has been used to react alcohols and amines toimines[18] and (synthetically very useful) to react alcohols and/or diols and amines or amino alcohols to selectively substitut-ed N-heteroaromatic compounds, for instance, pyrroles[19] andpyridines[20] (Scheme 2).
Here, we report on an Ir-catalyzed synthesis of benzimida-zoles and quinoxalines by ADC starting from benzene-1,2-dia-mines and aliphatic alcohols or 1,2-diols, respectively. Twoequivalents of dihydrogen are eliminated in the course of thereactions.[21] Recently, several synthesis protocols for the syn-thesis of benzimidazoles,[22] based on o-diamines and primary
[a] T. Hille, Dr. T. Irrgang, Prof. Dr. R. KempeLehrstuhl f�r Anorganische Chemie II, Universit�t BayreuthUniversit�tsstrasse 30, NW I, 95440 Bayreuth (Germany)Fax: (+ 49) 921552157E-mail : [email protected]
Supporting information for this article is available on the WWW underhttp ://dx.doi.org/10.1002/chem.201400400.
Scheme 1. Examples of an important benzimidazole (A) and quinoxaline (B).
Scheme 2. State of the art in N-heterocycle synthesis through acceptorlessdehydrogenative condensation reactions and the benzimidazole and qui-noxaline synthesis introduced here.
alcohols were developed. These protocols require additivesacting as hydrogen acceptors.[23–27] Furthermore, a photo-cata-lytic version of the reaction was reported.[28]
As a useful test reaction to find the optimal reaction condi-tions, we first investigated the formation of 2-phenyl-1H-ben-zo[d]imidazole (3 a) from benzene-1,2-diamine and benzyl alco-hol (Scheme 3). Benzene-1,2-diamine (1.0 equiv), benzyl alcohol
(1.1 equiv), and KOtBu (1.0 equiv) in diglyme under nitrogen at-mosphere (pressure tube closed with a bubble counter) wereused as starting parameters. A yield of 50 % within 24 h ata temperature of 110 8C was observed with a catalyst loadingof 1.0 mol % (catalyst 1 a, Table 1). In order to avoid dialkylationof the diamine, the diamine to alcohol ratio was altered andan excess of 1.43 equivalents of diamine could increase theyield up to 88 %. A comparison of THF (semipermeable mem-brane)[19a] and diglyme (pressure tube closed with a bubblecounter) both under nitrogen gas atmosphere revealed the
latter as the better solvent. The amount of base, which acceler-ates the alcohol oxidation and mediates the condensationstep, was optimized towards an efficient benzimidazole forma-tion. In the absence of base, a yield of only 20 % of 3 a was ob-served.
To make sure we were using the best catalyst, several P^N-ligand stabilized Ir-complexes and commercially available Ir-precursors and their combination with various phosphaneswere investigated as catalysts. The Ir-precursors [IrOMe(cod)]2
and [IrCl(cod)]2 (cod = 1,5-cyclooctadiene) themselves and incombination with several phosphane ligands did not give ac-ceptable yields of the product (Table 1, entries 11–18). Similarly,
Scheme 3. Test reaction for the optimization of the reaction conditions.
Table 1. Catalyst optimization for the benzimidazole synthesis.[a]
Entry Cat. complex Yield [%][b]
1 1 a 882 1 b 473 1 c 384 1 d 445 1 e 756 1 f 487 1 g 688 1 h 659 2 a 31
Ir complexes stabilized by bi-dentate P^N-ligands, which arevery efficient in borrowing hydrogen (or hydrogen auto-trans-fer) reaction,[29] failed as catalysts (Table 1, entries 9 and 10).
However, Ir complexes stabilized by tri-dentate P^N^P-li-gands gave significantly better conversions (Table 1, entries 1–8). Variation of ligand substituents revealed 1 a as the best cat-alyst for the benzimidazole synthesis discussed here. The re-leased gas was identified as hydrogen. After optimization ofthe reaction conditions and finding the best catalyst, webecame interested in exploring the substrate scope of the re-action (Table 2). Various functionalized benzyl alcohols and ali-phatic alcohols were used. Olefinic groups at the aliphatic alco-hol can be tolerated (Table 2, entry 8). With chloro-substitutedbenzyl alcohol a decrease of the yield is observed due tominor dehalogenation. Particularly good to very good isolatedyields were observed using aliphatic alcohols. Furthermore, wewere able to access methyl and methoxy-substituted diamines(Table 2, entries 9 and 10). The use of mono N-alkylated di-amines leads to N1-alkylated benzimidazoles in isolatedyields> 90 % (Table 2, entries 11 and 12). Based on the condi-tions we optimized, 1,2-diols were reacted with benzene-1,2-diamine. Instead of 2-hydroxyalkyl functionalized benzimida-zoles, quinoxalines[22e–g, r, 30] were formed. The catalyst system ef-ficiently oxidized both OH-groups of the diol, which condensemuch faster than benzimidazole formation is observed. In con-trast to earlier described transition metal-catalyzed syntheticroutes starting from o-phenylenediamines and diols[31] or a-hy-droxy ketones[32] which only work in the presence of a hydro-gen acceptor, we present a protocol with liberation of molecu-lar hydrogen to give quinoxalines. This new reaction was againoptimized regarding the reaction parameters. The performanceof selected catalysts is shown in Table 3.
The following parameters resulted from the optimization:a benzene-1,2-diamine to butane-1,2-diol ratio of 1.0 to 1.1,THF as solvent and 2.0 equivalents of KOtBu. Interestingly,a very low catalyst loading (0.06 mol % of 1 a) is needed. Thecatalysis runs nicely at 90 8C for 24 h in a reaction tube con-nected with a semi permeable membrane.[19a] The semi perme-able membrane selectively liberates H2 and allows working inTHF above the boiling point of the solvent. This synthesisroute permits the use of a variety of 1,2-diols (Table 4). The de-
sired products can be obtained in good to very good yields.Higher catalyst loadings were needed for disubstituted 1,2-diols (Table 4, entry 5) and monosubstituted 1,2-diols carryinga bulky substituent (Table 4, entry 6).
In conclusion, we developed a novel benzimidazole synthe-sis. Aromatic diamines and alcohols are connected by a con-densation step and by the liberation of two equivalents of H2.Iridium complexes stabilized by tridentate P^N^P-ligands arethe best catalysts for this reaction. If 1,2-diols are used, the oxi-dation of both alcohol functions and subsequent condensationforming quinoxalines is significantly faster than benzimidazoleformation.
Acknowledgements
This work was supported by the Deutsche Forschungsgemein-schaft (KE 756/23-1).
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Received: January 30, 2014Published online on && &&, 0000
The Synthesis of Benzimidazoles andQuinoxalines from Aromatic Diaminesand Alcohols by Iridium-CatalyzedAcceptorless DehydrogenativeAlkylation
Aromatic diamines were reacted withalcohols and diols to form benzimid-azoles or quinoxalines, respectively (seescheme). In the course of the reactions,water and two equivalents of hydrogen
gas were eliminated/liberated. An Ircomplex stabilized by a tridentateP^N^P ligand was found to be an effi-cient catalyst in these reactions.
