Catalytic Double [2 + 2] Cycloaddition Relay Enabled C–C Triple Bond

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

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Catalytic Double [2 + 2] Cycloaddition Relay Enabled C−C Triple Bond Cleavage of Yne−Allenones Heng Li,† Wen-Juan Hao,† Mian Wang,‡ Xue Qin,† Shu-Jiang Tu,*,† Peng Zhou,§ Guigen Li,§,∥ Jianyi Wang,*,‡ and Bo Jiang*,†

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†

School of Chemistry & Materials Science, Jiangsu Key Laboratory of Green Synthetic Chemistry for Functional Materials, Jiangsu Normal University, Xuzhou 221116, P. R. China ‡ Medical College, Guangxi University, Nanning 530004, P.R. China § Institute of Chemistry & BioMedical Sciences, Nanjing University, Nanjing 210093, P. R. China ∥ Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409-1061, United States S Supporting Information *

ABSTRACT: An unusual catalytic double [2 + 2] cycloaddition relay reaction of yne−allenones with unactivated alkenes and alkynes has been achieved, which enabled C−C triple-bond cleavage to access more than 60 examples of functionalized phenanthren-9-ols with generally good yields. This reaction provides a regioselective and practical method for the construction of carbocyclic ring systems with a high degree of functional group compatibility. Aside from surveying the scope of this transformation, mechanistic details of this process are provided by conducting systematic theoretical calculations.

C

cyclobutenes as a new cycloaddition partner and, therefore, presents the possibility for further intermolecular [2 + 2] cycloaddition with unactivated alkenes. Due to the strong ring tension of the tetracyclic intermediates, C−C bond cleavage is easily accomplished to access the expected phenanthren-9-ol products 3. We carefully performed systematic theoretical calculations on this reaction for predicting the feasibility of the transformation process (see the Supporting Information). We then validated the computational results by conducting the reaction of yne−allenones with unactivated alkenes catalyzed by BF3·Et2O (Scheme 1a). Exchanging alkenes for the unactivated alkynes 4 resulted in the corresponding aromatized phenanthren-9-ols 5 with generally good yields and high regioselectivities (Scheme 1b). The current protocol would represent the unprecedented organocatalytic bicyclization strategy for the regioselective synthesis of new phenanthren-

ycloaddition reactions have proven to be the most straightforward and atom-economical synthetic strategy to assemble functionalized cyclic rings containing both heterocyclic and isocyclic skeletons, especially for fourmembered rings, from alkenes in the organic community.1 For synthesis of four-membered rings of cyclobutanes or cyclobutenes, thermal, photoinduced or/and transition metalcatalyzed [2 + 2] cycloaddition reactions of the two alkenes or between an alkene and an alkyne or an allenoate have been well developed as one of the most powerful routes.2−5 Several elegant and seminal works have been successfully presented by the groups of Narasaka,6 Yoon,7 Hsung,8 Brown,9 Loh,10 and others. On the contrary, the analogous organocatalytic [2 + 2] cycloaddition constitutes a much more difficult task, especially for unactivated alkenes or alkynes,11 due to the fact that their cycloaddition partners must match their frontier molecular orbitals. In this regard, a double [2 + 2] cycloaddition relay cascade of yne−allenones with unactivated alkenes and alkynes will offer convenient access to the functionalized phenanthren9-ols, and to the best of our knowledge, there is no literature report on utilization of a catalytic double [2 + 2] cycloaddition relay to realize the C−C triple bond breaking/rearranging.12 Recently, our laboratory disclosed an intramolecular [2 + 2] cycloaddition of benzene-linked yne−allenones affording the cyclobuta[a]naphthalen-4-ols via 1,4-radical additions.13 In our efforts to extend the utility of this transformation, we became interested in developing a double [2 + 2] cycloaddition relay by combining yne−allenones with unactivated alkenes to complete the CC triple bond cleavage. A Lewis acid catalyzed intramolecular [2 + 2] cycloaddition between the distal allenic double bond and the alkyne moiety renders © XXXX American Chemical Society

Scheme 1. Catalytic Synthesis of Phenanthren-9-ols

Received: June 13, 2018

A

DOI: 10.1021/acs.orglett.8b01841 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

cycloaddition relay reaction by examining yne−allenone and alkene components, and the results are illustrated in Scheme 2.

9-ols by merging double [2 + 2] cycloaddition relay with C−C bond cleavage under the mild conditions. Herein, we report our efforts to elaborate on this attractive transformation. At the outset of our investigation, yne−allenone 1a and 1,1diphenylethylene (2a) were chosen as the model substrates to attempt the reaction with 15 mol % of BF3·Et2O in CH3CN at 50 °C (Table 1). However, the reaction of 1a with 2a in a 1:2

Scheme 2. Scope of Double [2 + 2] Cycloaddition Relay Reaction of Yne−Allenones with Alkenesa

Table 1. Optimization of Reaction Conditions for 3aa

entry

ratio (1a/ 2a)

cat. (mol %)

solvent

T (°C)

yieldb (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

1:2 1:2 1:2 1:2 1:2 1:2 1:1.5 1:2.5 1:1.5 1:1.5 1:1.5 1:1.5 1:1.5 1:1.5 1:1.5

BF3·Et2O (15) BF3·Et2O (15) BF3·Et2O (15) BF3·Et2O (15) BF3·Et2O (15) BF3·Et2O (15) BF3·Et2O (15) BF3·Et2O (15) Cu(OTf)2 (15) AgOTf (15) Y(OTf)3 (15) BF3·Et2O (15) BF3·Et2O (15) BF3·Et2O (10) BF3·Et2O (20)

CH3CN THF DMF 1,4-dioxane toluene DCE DCE DCE DCE DCE DCE DCE DCE DCE DCE

50 50 50 50 50 50 50 50 50 50 50 40 60 50 50

0 32 0 0 0 78 84 70 78 58 25 44 70 68 80

a

Reaction conditions: Benzene-tethered yne−allenone (1a, 0.3 mmol, 1.0 equiv), 1,1-diphenylethylene (2a, x mmol), catalyst (y mol %), solvent (5.0 mL), air, 6 h. bIsolated yield based on 1a.

mol ratio did not proceed under the above conditions (entry 1). Luckily, the reaction in THF solvent worked smoothly, affording the desired product 3a in 32% yield (entry 2). These results reveal that the reaction solvent is very sensitive to the success of the transformation. With this preliminary result in hand, further optimization of the reaction conditions was subsequently conducted. The use of N,N-dimethylformamide (DMF), 1,4-dioxane, and toluene as reaction media completely suppressed the reaction process (entries 3−5). Gratifyingly, the desired product 3a could be generated in 78% yield when the reaction was performed in DCE (entry 6). Decreasing the substrate ratio is beneficial to the transformation, as the substrate ratio of 1a and 2a in 1:1.5 gave a higher yield of 84% (entry 7). In sharp contrast, fine-tuning the substrate ratio to 1:2.5 resulted in a lower conversion (entry 8). Other common Lewis acid catalysts, including Cu(OTf)2, AgOTf, and Y(OTf)3), were then screened, and all of them could promote the conversion into product 3a but provided lower yields as compared with BF3·Et2O (entries 9−11). The reaction gave a relatively lower conversion when the reaction temperature was adjusted to either 40 or 60 °C (entries 12 and 13). In addition, an attempt to decrease the catalytic amount of Et2O·BF3 to 10 mol % was found to have a detrimental impact on the reaction yield (entry 14). A similar outcome was also observed when the Et2O·BF3 loading was increased to 20 mol % (entry 15). With the above optimal conditions in hand (Table 1, entry 7), we went on to explore the generality of this [2 + 2]

a (i) Reaction conditions: Benzene-tethered yne−allenones (1, 0.3 mmol, 1.0 equiv), alkenes (2, 0.45 mmol), BF3·Et2O (15 mol %), DCE (5.0 mL), 50 °C, air, 6 h; (ii) Isolated yields in parentheses based on 1.

Yne−allenones with diverse functionalities were first investigated in combination with 1,1-diphenylethylene (2a) under the optimal conditions. Both electron-poor and electron-rich groups at different positions of the arylalkynyl moiety (R1) can all tolerate this catalytic system, regioselectively delivering the corresponding products 3b−3k in 60% −96% yields. Diverse substituents, such as fluoro (1b), chloro (1c and 1d), bromo (1e), methyl (1f and 1g) ethyl (1h), tert-butyl (1i) and methoxy (PMP = p-methoxyphenyl, 1j), were suitable for this transformation. The presence of sterically more demanding substituents on the alkyne moiety, for example, the ochlorophenyl and 1-naphthyl (1-Np) groups, furnished phenanthren-9-ol products 3d and 3k in reasonable yields. The electronic nature of the substituents on the internal aryl ring of substrates 1 was then systematically investigated, and it was found that the reaction worked well with various functional groups attached by either C4 or C5 of the internal arene ring. The variant of substituents, including chloro, fluoro, methyl, and methoxy, would be compatible in the present reaction protocol, and the corresponding phenanthren-9-ols B

DOI: 10.1021/acs.orglett.8b01841 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

tethered yne−allenones 1bb turned out to be a favorable component, offering product 5r in 53% yield. Furthermore, the reaction can be extended to different aryl terminal alkynes with different electronic properties and substitution patterns, and the corresponding products 5s−ee were afforded in 46−87% yields. In addition, 2-ethynylthiophene still showed a higher reactivity, giving access to the corresponding 2-thienylsubstituted product 5ff in good yield. It should be noted that this is the first example of [2 + 2] cycloaddition relay/C−C bond cleavage for the synthesis of these multifunctionalized phenanthren-9-ols in a highly regioselective manner. The structures of the products 3h and 5s were determined by X-ray diffraction analysis. We performed a density functional theory (DFT) study to find the most plausible reaction pathway.14 All technical details are given in the SI. By combining the DFT study with the previous literature reports,10,13 a reasonable mechanism is proposed in Schemes 4 and 5. Yne-allenone 1a first undergoes

3l−z in 48%−84% yields were produced with high regioselectivity. Interestingly, the pyridine-tethered yne− allenones 1aa−bb could be successfully converted into 3aa and 3bb in 54% and 68% yields, respectively. Next, the scope with respect to the unactivated alkene component was evaluated. Symmetrically disubstituted terminal alkenes were proven to be suitable substrates, thus enabling a metal-free bicyclization approach to produce the corresponding phenanthren-9-ols 3cc−ff with 88%−97% yields. As anticipated, unsymmetrical disubstituted terminal alkenes underwent the transformation smoothly to afford 3gg and 3hh in 81% and 74% yields, respectively. In addition, styrene was also workable under the conditions. Notably, E-1-phenylpropylene as a representative internal alkene was accommodated with the mild reaction conditions, leading to the desired product 3jj in 68% yield. To expand the utility of this methodology, we directed our efforts to probing the reaction scope by replacing alkenes with unactivated terminal alkynes. To our delight, the reaction between benzene-linked yne−allenones 1 and alkynes worked equally well by fine-tuning the reaction conditions (10 mol % Et2O·BF3, 40 °C), leading to structurally diverse phenanthren9-ols (5a−ff) with generally good yields and high regioselectivity (Scheme 3). Similar to the above protocol of alkene transformations, yne−allenones 1 bearing either electronically neutral, poor, or rich substituents on both the arylalkynyl moiety and the internal arene ring were readily engaged in these transformations, providing an alternative approach to access densely decorated phenanthren-9-ols. The pyridine-

Scheme 4. Plausible Reaction Pathways

Scheme 3. Scope of Double [2 + 2] Cycloaddition Relay Reaction of Yne−Allenones with Alkynesa

an intramolecular [2 + 2] cycloaddition to offer adduct intermediate A,15 which is exothermic by 41.6 kcal/mol with an energy barrier of 21.8 kcal/mol. The intermediate A then undergoes an intermolecular [2+ 2] cycloaddition with 1,1diphenylethylene (2a) to give intermediate B, which is endothermic by 7.9 kcal/mol with an energy barrier of 22.3 kcal/mol. This energy barrier is not difficult to overcome, showing that the intermolecular [2 + 2] cycloaddition of A with 2a is a reasonable process. The following internal C−C bond breaking between two four-membered rings occurs very easily (with a low energy barrier of 4.4 kcal/mol and exothermic by 6.8 kcal/mol). In this process, the increased NBO charges of C1 and C2 atoms can be mainly compensated by C3, C4, C5, and C6 atoms and the orbital of C2 atom delocalize to orbital of C4 atom, showing that the broken C1 and C2 atoms are mainly stabilized by the phenyl ring A and C4 and C6 atom (shown in Table S1 and Figure S1). The subsequent BF3·Et2O-catalyzed proton-transfer of D to the final phenanthren-9-ol 3a is exothermic by 32.1 kcal/mol with an energy barrier of 7.3 kcal/mol, in which Et2O mainly plays a proton-transfer bridge role and the electron-rich O5 atom can be stabilized by BF3. The strong exothermic effect may be the main driving force for the double [2 + 2] cycloaddition relay. The presence of solvent DCE makes these processes more favorable. In summary, starting from yne−allenones and unactivated unsaturated hydrocarbons, we have designed and developed a conceptually new approach for the regioselective formation of phenanthren-9-ols with generally good yields by means of catalytic double [2 + 2] cycloaddition relay enabled C−C bond cleavage. A DFT study suggested that the reaction progress includes a double [2 + 2] cycloaddition relay, C−C

a

(i) Reaction conditions: Benzene-tethered yne−allenones (1, 0.3 mmol, 1.0 equiv), alkynes (4, 0.45 mmol), BF3·Et2O (10 mol %), DCE (5.0 mL), 40 °C, air, 12 h; (ii) isolated yields in parentheses based on 1. C

DOI: 10.1021/acs.orglett.8b01841 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters Scheme 5. Free Energy Profiles for Double [2 + 2] Cycloaddition Relay Reaction of Yne-Allenones with Alkenesa

a

Free energies in the solution phase are given in parentheses.

(YQ2015003), the NSF of Jiangsu Province (BK20160212), and the Qing Lan Project of Jiangsu Education Committee.

bond cleavage, and proton transfer. The protocol features high atom economy, annulation efficiency, and functional group tolerance as well as mild reaction conditions, providing a direct and general synthetic method for constructing these carbocyclic ring systems. Further investigation and application of this double-cycloaddition strategy is underway in our laboratory.

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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.8b01841. Experimental procedures and spectroscopic data for all new compounds 3a−3jj and 5a−5ff (PDF) Accession Codes

CCDC 1820509−1820510 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Guigen Li: 0000-0002-9312-412X Jianyi Wang: 0000-0003-3917-8327 Bo Jiang: 0000-0003-3878-515X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the NSFC (Nos. 21472071 and 21602087), the PAPD of Jiangsu Higher Education Institutions, the Outstanding Youth Fund of JSNU D

DOI: 10.1021/acs.orglett.8b01841 Org. Lett. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.orglett.8b01841 Org. Lett. XXXX, XXX, XXX−XXX