Photocatalytic decarboxylative [2 + 2 + 1] annulation of 1,6-enynes with N-hydroxyphthalimide esters for the synthesis of indene-containing polycyclic compounds

Meng-Jie Jiao , Dan Liu , Xiu-Qin Hu * and Peng-Fei Xu *
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China. E-mail: xupf@lzu.edu.cn

Received 22nd September 2019 , Accepted 16th October 2019

First published on 16th October 2019


An efficient photoredox-mediated [2 + 2 + 1] cascade annulation of 1,6-enynes with N-hydroxyphthalimide esters was reported for the synthesis of a series of indene-containing polycyclic frameworks through a sequence of radical addition, 5-exo-dig cyclization, 1,n-H shift and 5-endo-trig cyclization. This protocol exhibits excellent functional group tolerance for establishing spiro and non-spiro polycyclic architectures under mild conditions. Moreover, this transformation could also be achieved smoothly via a simple phosphine/iodide-based photoredox system.


Indene-containing polycyclic skeletons are significant motifs in natural products and drug molecules exhibiting promising applications in fighting against human immunodeficiency virus infections and cardiovascular diseases (Scheme 1).1 Access to these important cyclopenta[a]indene structures has been implemented mostly in a stepwise fashion.2 Due to both of their biomedical value and synthetic challenge, chemists have been trying to develop more efficient methods for the construction of these scaffolds, and the most attractive one of these methods is the cascade cyclization.3
image file: c9qo01166h-s1.tif
Scheme 1 Indene-containing polycyclic compounds.

One of the common synthetic strategies for constructing these structures is the [2 + 2 + m] annulation of 1,n-enynes (n = 6 and 7) with various m-atom units to form complex carbocyclic and heterocyclic compounds with high atom and step economy.4 Over the past decades, numerous cascade annulations have been reported under transition-metal-catalyzed5 and photocatalytic6 conditions. In 2016, Li and co-workers7 reported a copper-promoted oxidative tandem reaction of aniline-linked 1,7-enynes to synthesize acrylamide-fused [6.6.5] or [6.6.6.6] polycyclic architectures. Subsequently, their group8 accomplished a [2 + 2 + 2] annulation of 1,n-enynes for the synthesis of 7,8-dihydrophenanthridine-6,9(5H,6aH)-diones, in which azobis(alkyl nitrile)s acted as the radical sources (Scheme 2a). Besides, a pioneering study was described by Tu's group,9a who used a dual C(sp3)–H activation and bifunctionalization protocol to realize the domino spirocyclization of 1,7-enynes (Scheme 2b). In these approaches, metal catalysts, oxidants and high temperature are required, therefore, the exploration of milder and more efficient synthetic strategies for the construction of such indene-containing carbocyclic scaffolds is still highly desirable.


image file: c9qo01166h-s2.tif
Scheme 2 The common synthetic strategies of [2 + 2 + m] annulation.

Visible-light photoredox catalysis has been recognized as a powerful platform for the development of new synthetic strategies in organic chemistry. In this field, the construction of complex molecules via a redox-neutral mechanism that requires no external components to turn over the photocatalytic cycle has attracted great attention.10 Alkyl carboxylic acids are stable, nontoxic and naturally abundant feedstocks. The direct oxidative decarboxylation of carboxylic acids as well as their derivatization to activated esters to be reaction reagents has extended their applications in C–C bond formation. N-Hydroxyphthalimide esters (NHP esters) have been proved to be a versatile radical precursor with redox active properties in the functionalization of C(sp2)–H bonds and C(sp)–H bonds.11,13a–d Recently, various radical-mediated reactions of NHP esters, which could produce carbon-centered alkyl moieties via single-electron reduction/decarboxylation in the presence of a nickel catalyst12 or a photoredox catalyst,13 have been achieved for the formation of diverse C–X (X = C, N, B, Si, and S) bonds, such as alkynylation,12a,13d alkenylation,12b,13e arylation,12c acylation,12f alkylation,13c borylation13f and other transformations.14 This strategy is expected to avoid the use of catalytic silver and stoichiometric persulfate oxidants.15 Furthermore, the applications of NHP esters in cyclization reactions have also been studied.16 Inspired by these important advances, we hypothesized that the photoredox catalytic cascade cyclization of 1,6-enynes could be realized in a one-pot fashion based on that NHP esters could be decomposed into the corresponding alkyl radicals (Scheme 2c).

Initially, the photocatalytic [2 + 2 + 1] annulation between 1,6-enyne 1a and tetrahydropyran-substituted NHP ester 2a was tested. To our delight, when carried out under irradiation with blue LEDs for 12 hours in DMF at 25 °C, the reaction yielded product 3aa in 62% yield using fac-Ir(ppy)3 as the photocatalyst (Table 1, entry 1). Considering the effect of acidic additives in activating NHP esters,13c the optimization of the reaction conditions was conducted by the addition of different protonic acids such as TfOH, TsOH·H2O and TFA (entries 2–5). The results revealed that all of these acidic additives improved the reaction efficiency and TFA turned out to be more efficient than other acids. In addition, water could also promote the formation of 3aa (entries 6 and 7). The screening of other photocatalysts such as Ru(bpy)3Cl2·6H2O, Ir(ppy)2(dtbbpy)PF6 and cat-PMP17 showed that fac-Ir(ppy)3 is the optimal catalyst (entries 8–11). However, replacing DMF with DMSO or acetone resulted in decreased yields, and the reaction did not occur in MeCN or CH2Cl2 (entries 11–14). Finally, control experiments indicated that photocatalyst and visible light irradiation were essential in this transformation (entries 16 and 17).

Table 1 Optimization of the reaction conditionsa

image file: c9qo01166h-u1.tif

Entry Photocatalyst Additive Solvent 3aa (%)
a Reaction conditions: 1a (0.10 mmol), 2a (0.30 mmol), photocatalyst (2 mol%) and additive (0.20 mmol) in solvent (2 mL) at room temperature under nitrogen with blue LED irradiation for 12 h. b Isolated yield. c TFA (0.10 mmol). d H2O (50 equiv.). e Photocatalyst (1 mol%). f In the dark.
1 Ir(ppy)3 None DMF 62
2 Ir(ppy)3 TfOH DMF 77
3 Ir(ppy)3 TsOH·H2O DMF 72
4 Ir(ppy) 3 TFA DMF 82
5c Ir(ppy)3 TFA DMF 79
6 Ir(ppy)3 H2O DMF 66
7d Ir(ppy)3 H2O DMF 68
8 Ru(bpy)3Cl2·6H2O TFA DMF 11
9 Ir(ppy)2(dtbbpy)PF6 TFA DMF 63
10 Cat-PMP TFA DMF Trace
11e Ir(ppy)3 TFA DMF 76
12 Ir(ppy)3 TFA DMSO 61
13 Ir(ppy)3 TFA CH2Cl2 0
14 Ir(ppy)3 TFA CH3CN 0
15 Ir(ppy)3 TFA Acetone 9
16 None TFA DMF 0
17f Ir(ppy)3 TFA DMF 0


Based on the optimized conditions, we next investigated the substrate scopes of this decarboxylative cascade reaction. A broad range of functional groups including ether (3aa), difluoride (3ab), amide (3ac), ketone (3ad) and alkene (3ak) on the aliphatic ring exhibited excellent tolerance in the spirocyclization reaction. As shown at the top of Scheme 3, the chemical structure of 3aa was confirmed by X-ray single-crystal analysis.18 Additionally, it was found that various cyclic alkyl esters ranging from four-membered to seven-membered cycles could be used as substrates with 1a in this novel and efficient protocol to build spirocyclopenta[a]indene skeletons (3ae–3ah). Next, α-branched ester derived from commercially available 2-tetrahydrofuroic acid was also a suitable source for α-C functionalization to get 3aj in 52% yield with a good diastereomeric ratio (10.5[thin space (1/6-em)]:[thin space (1/6-em)]1). Moreover, indene-fused [6.5.5.5.6] cyclic molecule (3ai) could be obtained from substrate 2i. Finally, we were pleased to find that several alkanoic esters could be used as substrates to produce tricyclic products (3al and 3am).


image file: c9qo01166h-s3.tif
Scheme 3 The scope of N-hydroxyphthalimide esters.

Meanwhile, the scope of 1,6-enynes was explored, as shown in Scheme 4. Substrates 1 bearing either electron-rich or electron-deficient groups at the C5 or C6 position on the aromatic ring of the 1-allyl-2-ethynylbenzene exhibited good reactivities, and the corresponding products 3ba–3ea were obtained in 65% to 83% yields. When the large sized naphthyl-substituted 1f was used, the reaction still achieved a 42% yield (3fa). Generally, the target products could also be produced from 1,6-enynes bearing methyl, fluoro and chloro groups at the p-tolyl moiety (3ga–3ia). Acrylonitrile (1j) and benzyl acrylate (1k) were viable substrates as well to assemble 3ja and 3ka, respectively. Furthermore, enynes with a thiophen-2-yl substituent or other alkyl substituents at the terminal alkynes could also be converted into 3la–3oa.


image file: c9qo01166h-s4.tif
Scheme 4 The scope of 1,6-enynes.

Considering the commercial and ecological factors, we then investigated the feasibility of using a phosphine/iodide-based photoredox catalytic system19 in this decarboxylative [2 + 2 + 1] annulation process (Scheme 5a). It was found that the combination of triphenylphosphine and sodium iodide could catalyze the reaction, and 3aa was obtained in the yield of 60%. Next, a gram-scale reaction was conducted to deliver 3aa in synthetic yield under standard conditions, which demonstrated the potential application of the reaction (Scheme 5b).


image file: c9qo01166h-s5.tif
Scheme 5 The applicability of this reaction and control experiments.

To gain further insights into the plausible mechanism, a series of control experiments were conducted. As shown in Scheme 5c, the transformation did not proceed when TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy), a radical scavenger, was added to the reaction system; however, the adduct 4 of TEMPO and alkyl radical was detected by HRMS. Additionally, the production of 5 might explain the generation of the radical when stoichiometric 1,1-diphenylethylene was used as a radical-trapping reagent. On the other hand, the 50% yield of 3aa in the same system showed the high reactivity of 1,6-enyne 1a. Based on the above experimental results, the reaction may proceed by a radical pathway.

A proposed mechanism for the photocatalytic radical domino carbocyclization of 1,6-enynes with NHP esters is depicted in Scheme 6. Initially, photocatalyst fac-IrIII(ppy)3 is irradiated to the excited state. The protonated cationic intermediate 2a′ undergoes fragmentation and decarboxylation through a single-electron transfer (SET) with the excited species [fac-IrIII(ppy)3]* image file: c9qo01166h-t1.tif to obtain carbon-centered radical A, which is added to the C–C double bond of 1a. After 5-exo-dig cyclization, an intramolecular 1,5-H atom shift occurs due to the reactivity and instability of vinylic radical B, and 5-endo-trig cyclization then occurs to give intermediate D. Next, a single-electron oxidation event of intermediate D produces cationic E and regenerates the photocatalyst. Alternatively, quantum yield measurements (Φ = 12.7) reveal that the reaction may proceed by a chain propagation process, and thus intermediate D can also oxidize 2a′ to A (see the ESI).20 Finally, intermediate E is transformed into the polycyclic target 3aavia further deprotonation.


image file: c9qo01166h-s6.tif
Scheme 6 A proposed reaction mechanism.

Conclusions

In summary, we have developed a novel and synthetically valuable protocol for the synthesis of cyclopenta[a]indene and fluorene analogues via the photoredox-catalyzed [2 + 2 + 1] annulation reaction of 1,6-enynes with N-hydroxyphthalimide esters. Furthermore, the phosphine/iodide-based visible-light-mediated system can also be used in the reaction and a gram-scale reaction has been conducted successfully to exhibit the potential application of this method. Overall, this cascade reaction features wide substrate scopes, mild reaction conditions and exceptional functional group tolerance.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful to the NSFC (21632003, 21871116 and 21572087), the key program of Gansu province (17ZD2GC011) and the “111” program from the MOE of P. R. China for financial support.

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  18. CCDC 1935061 (3aa) contains the supplementary crystallographic data for this paper.
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  20. For more details, see the ESI..

Footnote

Electronic supplementary information (ESI) available: Synthetic procedures, characterisation data and copies of spectra of all compounds. CCDC 1935061 (3aa). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9qo01166h

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