Letter Cite This: Org. Lett. 2018, 20, 138−141
pubs.acs.org/OrgLett
Organocatalytic C(sp3)−H Functionalization via Carbocation-Initiated Cascade [1,5]-Hydride Transfer/Cyclization: Synthesis of Dihydrodibenzo[b,e]azepines Shuai-Shuai Li,† Lan Zhou,† Liang Wang, Huaili Zhao, Liping Yu, and Jian Xiao* College of Chemistry and Pharmaceutical Sciences, Qingdao Agricultural University, Qingdao 266109, China S Supporting Information *
ABSTRACT: Carbocation-initiated cascade [1,5]-hydride transfer/cyclization and dimerization reactions were developed to synthesize dihydrodibenzo[b,e]azepine and octahydrodipyrroloquinoline derivatives in high yields. These redox-neutral C(sp3)−H functionalization-involving cascade reactions feature transition-metal-free, high atom- and step-economy, and environmental friendliness with water as the sole byproduct.
S
construct dihydrodibenzo[b,e]azepine skeletons is still in great demand. The cascade [1,5]-hydride transfer/cyclization for C(sp3)−H functionalization has received considerable attention owing to its environmental sustainability, atom economy, and high efficiency in the construction of heterocyclic compounds.6 This kind of reaction commonly leverages the tert-amino effect to generate the adjacent iminium ions via hydride transfer induced by unsaturated hydride-acceptor moieties, such as electron-poor alkene,7 aldehyde,8 imine,9 alkyne,10 and so on11 (Scheme 1, eq
even-membered nitrogen-containing heterocycles, such as 1benzazepines and dihydrodibenzo[b,e]azepines, are core structures of an array of natural products and commercial pharmaceuticals, such as competitive vasopressin receptor antagonist (tolvaptan), ACE inhibitor (benazepril), antidiuretics (fedovapagon), and antidepressants (mianserin and mirtazapine) (Figure 1).1 In particular, mirtazapine is one of the eight major
Scheme 1. Our Hypothesis for Hydride-Transfer Reaction
Figure 1. Representative examples of pharmaceuticals.
antidepressants available. Hence, the development of valuable methodology to construct 1-benzazepine and dihydrodibenzo[b,e]azepine skeletons is of great significance both academically and industrially. Recently, several novel strategies had been reported for the synthesis of 1-benzazepines,2 such as Pictet− Spengler reaction2a,b and NHC/Pd-catalyzed annulation.2c However, there is no doubt that the direct C−H functionalization-involving strategies are much more attractive owing to their intrinsic environmental sustainability and atom economy. In this context, the C−H functionalization-involving Mannich-type reaction3 and oxidative cross-coupling reaction4 were reported. However, all of these works were related to the synthesis of 1benzazepines, and few works had been reported for direct synthesis of dihydrodibenzo[b,e]azepine skeletons.5 Therefore, the development of C−H functionalization-involving methods to © 2017 American Chemical Society
1). Recently, Kim reported an internal redox reaction to synthesize the 1-benzazepines through [1,5]-hydride transfer/ 7-endo cyclization by use of a cyclopropane moiety as the hydride acceptor.12 Despite these significant advances, the types of hydride acceptors are still quite limited; thus, exploring the novel driving force to initiate cascade hydride transfer/cyclization for the construction of architecturally complex molecules remains a formidable challenge. As is well known, as an essential intermediate, carbocation has been widely utilized in organic synthesis.13 Recently, we have Received: November 10, 2017 Published: December 14, 2017 138
DOI: 10.1021/acs.orglett.7b03492 Org. Lett. 2018, 20, 138−141
Letter
Organic Letters
[b,e]azepine 2a in 34% and 25% yield, respectively (Table 1, entries 3 and 4). Surprisingly, the yield was improved to 73% when (+)-10-camphorsulfonic acid ((+)-CSA) was employed as a catalyst (Table 1, entry 5). Further screening indicated that the Lewis acid catalysts did not give good results (Table 1, entries 6 and 7). Notably, when the catalyst loading of (+)-CSA was increased to 30 mol %, the yield went up to 87% (Table 1, entry 8). Further increasing of the catalyst loading decreased the yield (Table 1, entry 9). Encouraged by the good result from (+)-CSA, subsequently, a series of the solvents were surveyed, and it was found that anhydrous DCE was the best solvent for this reaction (Table 1, entries 10−17). Notably, this reaction also worked in water, albeit with lower yield (Table 1, entry 11). Not surprisingly, lowering the reaction temperature to 80 °C gave rise to a drop in the yield from 93% to 57%, even prolonging the reaction time (Table 1, entry 18). Consequently, the use of 30 mol % of (+)-CSA in anhydrous DCE was chosen as the best reaction conditions. With the optimized reaction conditions in hand, the scope and generality of this cascade [1,5]-hydride transfer/cyclization reaction were next explored (Scheme 2). First, the substituents
disclosed a series of carbocation-involved reactions in which the key steps relied on the formation of carbocation intermediates under special conditions.14 Therefore, we envisaged that the in situ generated carbocation intermediate might act as an hydride acceptor to drive the formation of the active iminium intermediate via a feasible hydride-transfer process. Subsequently, the formed iminium ion can be intercepted by intramolecular nucleophilic attack to furnish the aza-heterocycles. To fulfill this goal, several considerable challenges need to be addressed: (1) the in situ generated carbocation must be stable enough to induce the α-H transfer of tertiary amine; (2) the nucleophile attacks the formed iminium ion selectively rather than the premier carbocation; and (3) the intramolecular nucleophilic attack must be faster than the intermolecular trap. As part of our research program aiming at establishing organocatalytic C(sp3)−H functionalization strategy for the construction of pharmaceutically important molecules,15 herein we disclose that, for the first time, the carbocation-initiated cascade [1,5]-hydride transfer/cyclization reaction to synthesize the pharmaceutically important dihydrodibenzo[b,e]azepine derivatives. This redox-neutral C(sp3)−H functionalization involving cascade reaction is environmentally friendly, releasing water as sole byproduct without the need for transition-metal catalyst. To verify our hypothesis, (3,5-dimethoxyphenyl)(2-(pyrrolidin-1-yl)phenyl)methanol 1a was selected as a model substrate to check the possibility of this reaction. Initially, various Brønsted acid and Lewis acid catalysts were screened in DCE at 100 °C to examine the reaction. When TFA and TsOH were used, only trace amount of products were observed (Table 1, entries 1 and 2). To our delight, the desired product was obtained when MsOH or TfOH was used as catalyst, giving dihydrodibenzo-
Scheme 2. Scope of [1,5]-Hydride Transfer/Cyclization Reaction
Table 1. Optimization of the Reaction Conditionsa
entry
catalyst
loading (mol %)
solvent
time (h)
yieldb (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17c 18d
TFA TsOH MsOH TfOH (+)-CSA Sc(OTf)3 Cu(OTf)2 (+)-CSA (+)-CSA (+)-CSA (+)-CSA (+)-CSA (+)-CSA (+)-CSA (+)-CSA (+)-CSA (+)-CSA (+)-CSA
20 20 20 20 20 20 20 30 40 30 30 30 30 30 30 30 30 30
DCE DCE DCE DCE DCE DCE DCE DCE DCE dioxane H2O MeCN toluene DMF DCM EtOH DCE DCE
10 10 10 10 10 10 10 10 10 10 96 10 10 10 10 10 10 48
trace trace 34 25 73 33 trace 87 79 51 34 47 56 trace 41 36 93 57
a
Reaction conditions: 1 (0.2 mmol), (+)-CSA (0.06 mmol) in 2 mL of distilled DCE at 100 °C under air. bAt 130 °C.
on the benzene ring linked to the nitrogen atom were investigated. Gratifyingly, both the electron-rich and electronpoor substituents were all well tolerated in this reaction, furnishing the corresponding products 2a−g in good to excellent yields (76−95%). The fluorine-containing electron-withdrawing group such as CF3 could afford the desired product 2b in good yield, implying the potential biological application of this system. Pleasingly, the position of the substituents did not influence the productivity significantly since the yields of different chlorinesubstituted products 2d−f were almost the same, with slightly
a Reaction conditions: 1a (0.2 mmol), catalyst in 2 mL of solvent at 100 °C under air. bIsolated yield after column chromatography. cUsing distilled DCE. dAt 80 °C.
139
DOI: 10.1021/acs.orglett.7b03492 Org. Lett. 2018, 20, 138−141
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Organic Letters lower yield for 2d. Although a methyl group gave the corresponding product 2g in excellent yield, however, the reaction did not occur when methyl group was changed to methoxyl group, which might be attributed to its strong electrondonating property. To our delight, quinoline skeleton as a subunit was also tolerated, albeit with comparatively lower yield. Next, a range of substrates containing five- or six-membered azacycles and acyclic amines were evaluated under the optimum conditions. As expected, octahydroisoindole similar to pyrrolidine was compatible with the conditions, affording the corresponding fused ring 2i in 65% yield. However, the substrates with six-membered azacycles exhibited relatively lower activity, providing the corresponding products 2j−l in comparatively lower yields and higher reaction temperature was indispensable. It was worth mentioning that these dihydro-dibenzo[b,e]azepine derivatives with six-membered azacycles were Mianserin analogues; thus, they held great potential for further antidepressant activity screening. Notably and importantly, the substrates incorporating acyclic amines, which were inapplicable in many hydride-transfer reactions, were suitable for this intramolecular redox reaction, affording the products 2m−p in moderate to good yields. This methodology provides an efficient synthetic route to dihydrodibenzo[b,e]azepine derivatives, which are privileged structures for medicinal chemistry. In order to further investigate the generality of this reaction, next we focused on modifying the benzyl alcohol moiety. Surprisingly, when the sole methoxyl substituent was installed to the benzene ring instead of two methoxyl groups, the expected cyclization product was not observed; however, the dimerization octahydrodipyrroloquinoline derivatives 3a,b were isolated in excellent yields after conditions screening (Scheme 3, see
Scheme 4. Proposed Reaction Mechanism
Scheme 3. Substrate-Controlled Cascade [1,5]-Hydride Transfer/Dimerization
Scheme 5. Investigation of Mechanism via Isotopic Experiments
Supporting Information). Even when the two methyl or one methyl and one methoxyl substituted substrates were tested, they still furnished the dimerization products 3c,d in excellent yields. The tetracyclic octahydrodipyrroloquinoline framework is a unique structure core in a wide range of biologically important alkaloid natural products, such as incargranine B and seneciobipyrrolidine.16 Despite its biological significance, only sporadic examples were reported related to the synthesis of octahydrodipyrroloquinoline framework.17 Furthermore, the investigation of the asymmetric version was conducted and only 18% ee was obtained (see the Supporting Information). The reaction pathway was proposed as follows. Initially, the substrate 1 underwent dehydration to form carbocation intermediate I, which induced the [1,5]-hydride transfer to afford iminium ion II. The subsequent intramolecular Pictet− Spengler type reaction with the iminium ion furnished the desired cyclization product 2 when the aromatic ring was electron-rich enough (Scheme 4, path A). Alternatively, the iminium ion II
obtained, and deuterium atom was observed at benzylic position of the product (100% deuteration). Therefore, the mechanism of intramolecular hydride transfer is fully proved. To illustrate the synthetic utility of this methodology, the functional group transformations of dihydrodibenzo[b,e]azepine product 2c were carried out (see Supporting Information). The methyl group could be easily removed with BBr3, providing the demethylation product in 81% yield. Moreover, Sonogashira coupling reaction of 2c with alkyne yielded the coupling product in 87% yield. In conclusion, we have developed a carbocation-initiated cascade [1,5]-hydride transfer/cyclization and cascade [1,5]hydride transfer/dimerization reaction to synthesize dihydrodibenzo[b,e]azepine and octahydrodipyrroloquinoline derivatives in high yields. This work demonstrated that the in situ generated carbocation can serve as a driving force to initiate a cascade hydride transfer involving process for the construction of architecturally complex molecules, which opens a new avenue for C(sp3)−H functionalization reactions. The biological importance of dihydrodibenzo[b,e]azepine and octahydrodipyrrolo-
could isomerize to the enamine intermediate III, which captured another molecule of II, giving rise to the intermediate IV. Lastly, the intramolecular Pictet−Spengler type reaction occurred to provide the dimerization product 3 (Scheme 4, path B). Path B followed the cascade [1,5]-hydride transfer/isomerization/ addition/Pictet−Spengler reaction process to furnish the octahydrodipyrroloquinoline framework. Remarkably, it was the electronic density of the benzene ring to decide which pathway would go. In addition, the formation of product 3c provided an evidence for carbocation as a driving force for [1,5]hydride transfer rather than O-alkylated quinone methides.11i To shed light on the mechanism, the deuterated substrate [D]1c was subjected to the reaction system with (+)-CSA as a catalyst (Scheme 5). As expected, the corresponding product [D]-2c was
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quinoline derivatives also highlight the future pharmaceutical application of this method.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b03492. Experimental procedures and characterization data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Shuai-Shuai Li: 0000-0001-7279-2885 Jian Xiao: 0000-0003-4272-6865 Author Contributions †
S.-S.L. and L.Z. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful for financial support from the NSFC (21702117, 21776148) and the Natural Science Foundation of Shandong Province for Distinguished Young Scholars (JQ201604) and General Project (ZR2016BM07, ZR2017BB005). We thank the Central Laboratory of Qingdao Agricultural University for NMR determination.
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REFERENCES
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DOI: 10.1021/acs.orglett.7b03492 Org. Lett. 2018, 20, 138−141
