Alkoxide-Catalyzed Hydrosilylation of Cyclic Imides to Isoquinolines

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

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Alkoxide-Catalyzed Hydrosilylation of Cyclic Imides to Isoquinolines via Tandem Reduction and Rearrangement Xiaoyu Wu, Guangni Ding, Liqun Yang, Wenkui Lu, Wanfang Li, Zhaoguo Zhang, and Xiaomin Xie* School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China

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ABSTRACT: An alkoxide-catalyzed hydrosilylation of cyclic imides to isoquinolines was realized via tandem reduction and rearrangement. Using TMSOK as the catalyst and (EtO)2MeSiH as the reductant, a series of cyclic imides containing different functional groups were reduced to the corresponding 3-aryl isoquinolines in moderate to good yields. The scenario of the reaction pathway was supposed to involve the reduction of imides to ω-hydroxylactams, which underwent rearrangement in the presence of a base catalyst, and then the carbonyl reduction, followed by siloxy elimination.

H

Scheme 1. Base-Catalyzed Hydrosilylation and Grabriel− Colman Reaction

ydrosilylation is an appealing method for the reduction of carbonyl compounds.1 Hydrosilanes are generally air- and moisture-stable hydride sources and their weaker Si−H bonds show significantly different polarization characteristics from C− H or H−H bonds.2 The use of organosilicon hydrides often provides a means of effecting reductions of organic substrates under very mild conditions with excellent functional group selectivity.3 Various catalysts have been explored for the chemo-, regio-, and/or stereoselective hydrosilylation of ketones, aldehydes, and carboxylic acid derivatives.1,3 Due to its higher environmental benignity and lower cost, Lewis base-catalyzed hydrosilylation of carbonyl compounds has drawn much attention.4 These catalysts show great potential to activate silanes via the formation of pentacoordinated hydrosilanide anion intermediates, which assist the transfer of silicon hydride to the carbon center.5 Furthermore, in the cases of basecatalyzed hydrosilylation, the verified alkalinity of the catalysts often leads to diversified reactivity and selectivity. The hydrosilylation of aryl-substituted acetamide generally gave amines.4i,j Exceptionally, Adolfsson’s group obtained enamines as the major products in tBuOK-catalyzed hydrosilylation of aryl-substituted acetamide.6a Alkoxide-catalyzed reduction of benzolactams with silanes afforded isoindoles. 6b These processes were realized by the addition of a Si−H bond to a carbonyl group and deprotonation. There are a few precedents regarding the base-catalyzed hydrosilylation of easily accessible cyclic imides whose reduction products represent an important class of building blocks that could be further synthetically elaborated.7 For example, the fluoride-catalyzed hydrosilylation of aryl imides with polymethylhydrosiloxane (PMHS) afforded aryl lactams.8a Using KOH as the catalyst, imides were selectively hydrosilylated to ωhydroxy lactams or lactams depending on the type of silanes.8b With regard to N-substituted phthalimides, base-promoted Grabriel−Colman rearrangement provided an important synthetic route toward 2,3-dihydro-1,4-isoquin-olinediones or their tautomer 1,4-dihydroxyisoquinolines (Scheme 1b).9 Accordingly, we envisioned that a tandem rearrangement and © XXXX American Chemical Society

hydrosilylation of such cyclic imides would afford an isoquinoline scaffold (Scheme 1c), which functions as an attractive Nheterocycle structure unit for functional materials, pharmaceuticals, agrochemicals, and ligand skeleton for catalysis.10 Specifically, isoquinoline derivatives have exhibited a diverse array of biological activities such as antitumor, anesthetic, and antibiotic properties.10c,d Therefore, much effort has been devoted to developing an efficient and inexpensive synthetic method to construct the isoquinoline skeleton. Until now, Pictet−Spengler reaction, Bischler−Napieralski reaction, and Pomernz−Fritsch reaction represent the most popular for the synthesis of isoquinolines or hydroisoquinolines.11 Recently, transition-metal-catalyzed cyclization between alkynes or nitriles and nitrogen compounds has emerged as an effective Received: July 21, 2018

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DOI: 10.1021/acs.orglett.8b02287 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters method to construct isoquinoline rings.12 Herein, we reported a simple and efficient procedure of base-catalyzed hydrosilylation of cyclic imides to isoquinolines via a tandem reduction and rearrangement. The hydrosilylation of N-4-methylbenzyl phthalimide (1a) was chosen as the model reaction to explore the possibility of the tandem reduction and rearrangement of cyclic imides to isoquinolines. tBuOK, which showed good catalytic activity for the hydrosilylation of amides to enamines, was initially selected as the catalyst.6 When 10 mol % of tBuOK and PhSiH3 (2.0 equiv) were used, the conversion of imide 1a was 95% after refluxing in dioxane for 24 h. However, the objective product 2a was observed in only 14% yield, together with 12% yield of the monoreduced product of the lactam (Tables 1 and S1, entry 1).

ineffective for the formation of 2a (Table 1, entry 10). Therefore, (EtO)2MeSiH was the optimal hydrosilane among the used hydrosilanes. Considering that the rearrangement of cyclic imides usually took place at high temperature, we carried out the reaction using (EtO)2MeSiH in a high boiling-point ethereal solvent such as diglyme. Indeed, the yield of isoquinoline 2a was increased to 65% at 120 °C (Table 1, entry 11). When the reaction temperature was increased further to 140 °C, the yield of 2a began to decrease (see Table S1, entry 17). Xylene and butyl ether were inferior for the tandem reaction. Examination of the reaction time disclosed that the reduction and rearrangement could complete within 6 h (see Table S1, entries 18−21). Next, we studied the reactivity of other base catalysts by using (EtO)2MeSiH in diglyme (Table 1, entries 14−18). The metal cations had a remarkable effect on the reactivity of the catalyst. Surprisingly, the yield of 2a plunged from 65% to 35% when t BuOK was replaced by tBuONa (Table 1, entry 11 vs entry 14). In the case of using CH3OK as the catalyst, 2a was obtained in 32% yield (Table 1, entry 15). Gratifyingly, the yield of 2a was increased to 76% when potassium trimethylsiloxide was used as the catalyst (Table 1, entry 16). The reaction catalyzed by KOH resulted in an increased yield of the lactam (Table 1, entry 17). K2CO3 was a totally ineffective catalyst for the production of 2a (Table 1, entry 18). Stoichiometric studies revealed that lower TMSOK loading led to depleted conversion, whereas higher loading of TMSOK and hydrosilane increased the yield of the lactam (see Table S1, entries 28−32). Omission of either the base or the hydrosilane resulted in no conversion of 1a at all (Table 1, entries 19 and 20). Therefore, the optimized reaction condition for the tandem reaction of reduction and rearrangement of imides to isoquinolines was (EtO)2MeSiH as silane, TMSOK (10 mol %) as catalyst, and diglyme as solvent. The scope of the base-catalyzed tandem reaction of reduction and rearrangement of N-benzyl phthalimide (1) to isoquinoline (2) was then explored under the optimized conditions. As shown in Scheme 2, the tandem reactions led to the efficient formations of a series of isoquinolines (2). Reactions of Nalkylbenzyl imides (1a−1d) gave the corresponding isoquinolines (2a−2d) in good yields ranging from 70 to 82%. However, 1e afforded 3-(β-naphthyl)isoquinoline (2e) in a lower yield of 54%. Substituents on the cyclic imides have significant influences on the tandem reaction outcome. The sterically hindered substituents at the ortho-position of the N-benzyl group decreased the reactivity to some extent. When the substituent at the nitrogen atom was changed from the βnaphthyl to an α-naphthyl group, the yield of corresponding isoquinoline (2f) dropped from 54% to 46%. In particular, the reaction of the imide (1h) with an o-tolyl group at the nitrogen atom did not occur until the reaction temperature increased to 150 °C under which 2h was obtained in 53% yield. The presence of electron-donating substituents on the imides increased the yields of the corresponding isoquinolines (2). 3-(4-Methoxyphenyl)-isoquinoline (2g) and 4-(isoquinolin-3-yl)-N,N-dimethyl-aniline (2j) were obtained in 81% and 82% yields, respectively. Notably, the reaction of the imide with an electronwithdrawing group at the N-benzyl group led to the corresponding isoquinoline (2k) in only 26% yield. This protocol showed good tolerance of halogens and allowed for the synthesis of various halogenated isoquinolines (2m−2p) in 40−79% yields. The tandem reaction afforded the isoquinoline with the cyano group (2l) in 34% yield without any reduction of the cyano group. Moreover, the reduction system is also

Table 1. Optimization Condition of the Base-Catalyzed Hydrosilylation of Cyclic Imides to Isoquinolinesa,b

entry

silane

1c 2d 3 4 5 6 7 8 9 10 11e 12e 13e 14e,f 15e,f 15e,f 16e,f 17e,f 18e,f 19e 20e

PhSiH3 Ph2SiH2 Ph3SiH Et3SiH (EtO)3SiH (MeO)3SiH (EtO)2MeSiH (MeO)2MeSiH PMHS (TMSO)2MeSiH (EtO)2MeSiH (EtO)2MeSiH (EtO)2MeSiH (EtO)2MeSiH (EtO)2MeSiH (EtO)2MeSiH (EtO)2MeSiH (EtO)2MeSiH (EtO)2MeSiH (EtO)2MeSiH

base

solvent

yield (%)

BuOK t BuOK t BuOK t BuOK t BuOK t BuOK t BuOK t BuOK t BuOK t BuOK t BuOK t BuOK t BuOK t BuONa EtOK CH3OK TMSOK KOH K2CO3

dioxane dioxane dioxane dioxane dioxane dioxane dioxane dioxane dioxane dioxane diglyme xylene (n-Bu)2O diglyme diglyme diglyme diglyme diglyme diglyme diglyme diglyme

14 50 4 N.R. 35 14 55 39 24 N.R. 65 27 27 35 55 32 76 46 0 N.R. N.R.

t

t

BuOK

a

Reaction conditions: 1a (1.0 mmol), silane (6.0 mmol), base (0.1 mmol), solvent (4.0 mL), 110 °C, 24 h. bConversion and yields are determined by 1H NMR analysis (internal standard: 4,4′-ditert-butyl1,1′-biphenyl). cSilane (2.0 mmol). dSilane (3.0 mmol). e120 °C. f Reaction time: 6 h.

When Ph2SiH2 was used as the reductant, the yield of isoquinoline 2a was increased to 50% yield (Table 1, entry 2). Unexpectedly, using Ph3SiH as the reductant afforded the lactam as the main product (Table 1, entry 3). Triethylsilane was inactive for the tandem reaction of reduction and rearrangement (Table 1, entry 4). Trialkoxy-silanes were inferior to Ph2SiH2, and the yields were much lower (Table 1, entries 5 and 6). Pleasingly, the yield of 2a was increased to 55% when (EtO)2MeSiH was used as the reductant (Table 1, entry 7). Replacing (EtO) 2 MeSiH with less sterically hindered (MeO)2MeSiH produced 2a in only 39% yield (Table 1, entry 8). (TMSO)2MeSiH, which has more steric hindrance, was B

DOI: 10.1021/acs.orglett.8b02287 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters Scheme 2. Base-Catalyzed Tandem Reaction of N-benzyl Phthalimide (1) to Isoquinoline (2)a,b

To gain insight into the reaction mechanism, we tried to synthesize isoquinoline 2a by a stepwise reaction of rearrangement and hydrosilylation. Inexplicably, Grabriel−Colman rearrangement of 1a did not occur when either a catalytic or stoichiometric amount of base was applied. Furthermore, the hydrosilylation of 4-hydroxy-3-(p-tolyl)isoquinolinone (M1), which was prepared by the reported procedure,13 also failed to give 2a under the standard conditions (Scheme 3A). These Scheme 3. Hydrosilylation of Possible Intermediates

observations suggested that the formation of the final isoquinoline may not involve a tandem process of rearrangement and reduction. As disclosed by ourselves, ω-hydroxyl lactams were often the intermediates for base-catalyzed hydrosilylation of cyclic imides. Then, the reactions of ω-hydroxyl lactams were attempted under the standard conditions. The ω-hydroxyl lactam with an electron-rich benzene ring (M2 and M4) could be smoothly converted to isoquinoline 2a and 2y in 63% and 69% yield, respectively (Scheme 3B). However, the tandem reaction of the ω-hydroxyl lactam with an electron-deficient benzene ring (M3) did not occur. These results were in accordance with the reactions of 1a, 1y, and 1z. These indicated ω-hydroxyl lactam derivatives may be the pivotal intermediates of the tandem reactions. Based on the above experiments, a plausible pathway for the tandem reaction was proposed (Scheme 4). First, the base-

a

Reaction conditions: N-benzyl phthalimide (1) (1.0 mmol), (EtO)2MeSiH (6.0 mmol), tBuOK (10.0 mol %), diglyme (4.0 mL), 120 °C, 6 h. bIsolated yields. c150 °C, 18 h. d1v (1.5 g, 5.0 mmol).

amenable to various imides derived from heterocycles, such as pyridine (2q, 2r, and 2s), furan (2t), and thiophene (2u). The reduction of the imides with an electron-rich furan group (1t) and thiophene (1u) proceeded smoothly to the corresponding isoquinolines (2t and 2u) in good yields. 2-Pyridinyl substituted isoquinoline (2s) was obtained in 64% yield. Further investigations showed that the electron-donating groups on the benzobenzene ring also increased the yield of the corresponding isoquinolines (2v−2y). 6,7-Dimethoxy-3-phenyl-isoquinoline (2v) was obtained in 87% yield. However, the regioselectivity of the catalytic system for the unsymmetrical imides was still very low. The reaction of N-benzyl-5-methyl phthalimide (1x) gave a mixture of 6-methyl-3-phenylisoquinoline (2x) and 7-methyl-3-phenyl-isoquinoline (2x′) in a 1.1/1.0 ratio. meta-Substituted phthalimide (1y) also produced two regioisomers of 2y and 2y′ in a 1/1.5 ratio. Regretfully, the tandem reaction was retarded largely by the electron-deficient benzobenzene ring of imides, such as N-benzyl-4,5-dichlorophthalic acid imide and N-benzyl-2,3-pyridinedicarboximide. In these cases, ω-hydroxyl lactams were obtained as main products. In addition, instead of benzyl at the nitrogen atom with an ester, amide, or allyl, the tandem reactions of imides could not occur. To evaluate the scalability of this facile tandem procedure, 1.5 g of 1v was subjected to the optimized conditions, and dimethoxysubstituted isoquinoline 2v was isolated in 72% yield.

Scheme 4. Proposed Mechanism of Alkoxide-Catalyzed Hydrosilylation of Cyclic Imides to Isoquinolines

activated silane via the formation of a pentacoordinated intermediate5 reduced the imide to form a ω-hydroxyl lactam derivative (I). Then, the deprotonation at α-methylene of the nitrogen atom (I) formed a carbanion (II), which underwent an intramolecular α-carbanion attack to the amide carbonyl to form an aziridine oxo-anion (III). The cleavage of the C−N bond in aziridine oxo-anion (III) occurred easily, which was initialized by the elimination of the siloxy group, whereby III was rearranged to 3-phenylisoquinolin-4(3H)-one (IV). The ketone carbonyl in IV was then hydrosilylated to form a silyl ether intermediate (V). Then, C3-deprotonation occurred under the basic conditions, which resembled the reaction pathway for the hydrosilylation of tertiary amides to enamines and benzolactams C

DOI: 10.1021/acs.orglett.8b02287 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters to isoindoles.6a After eliminating the siloxy group, the final product isoquinoline was generated. As indicated in Table 1 and Scheme 2, the bulkier base catalyst was beneficial to avoid the formation of lactam, and the presence of electron-donating groups at the N-benzylic group were helpful to promote the tandem reaction process. These two observations were in good accordance with the proposed involvement of intramolecular α-carbanion additions. In summary, we have developed an alkoxide-catalyzed tandem reaction of nucleophilic rearrangement and hydrosilylation of readily available phthalimides to isoquinolines. With TMSOK (10 mol %) as the catalyst and (EtO)2MeSiH as the reductant, various cyclic imides were effectively converted to the corresponding 3-aryl isoquinolines in moderate to good yields. This catalytic protocol showed a wide functional tolerance of halogens, naphthalene rings, and aromatic heterocycles.

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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.8b02287. Detailed optimization data for the reaction conditions, experimental procedures, and product characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhaoguo Zhang: 0000-0003-3270-6617 Xiaomin Xie: 0000-0002-5798-291X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (No. 21672143) and the Interdisciplinary Program of Shanghai Jiao Tong University (YG2017MS26) for financial support.

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REFERENCES

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DOI: 10.1021/acs.orglett.8b02287 Org. Lett. XXXX, XXX, XXX−XXX