Cobalt-catalyzed multisubstituted allylation of the chelation-assisted C–H bond of (hetero)arenes with cyclopropenes

Abstract

Cyclopropenes are highly strained three-membered carbocycles, which offer unique reactivity in organic synthesis. Herein, Cp*Co^III-catalyzed ring-opening isomerization of cyclopropenes to cobalt vinylcarbene has been utilized for the synthesis of multisubstituted allylarenes via directing group-assisted functionalization of C–H bonds of arenes and heteroarenes. Employing this methodology, various substituents can be introduced at all three carbons of the allyl moiety with high selectivity. The important highlights are excellent functional group tolerance, multisubstituted allylation, high selectivity, gram scale synthesis, removable directing group, and synthesis of cyclopenta[b]indoles. In addition, a potential cobaltocycle intermediate was identified and a plausible mechanism is also proposed.

---

Introduction

Allylated arenes and heteroarenes are prevalent structural motifs found in various natural products and biologically important molecules.¹ Synthetically, substituted allyl groups serve as an excellent handle to increase the complexity of a molecule with simple synthetic operations. These allyl groups are traditionally introduced into arenes via either the Friedel–Crafts allylation of electron rich arenes² or reaction of prefunctionalized organometallic reagents with allyl electrophiles.³ These methods have only limited substrate scope and require sensitive organometallic reagents. In contemporary organic synthesis, traditional methods are wisely replaced with the transition metal catalyzed direct allylation of directing group-assisted C–H bonds of (hetero)arenes.⁴ In the past few decades, precious transition metals such as Pd,⁵ Ru,⁶ Rh,⁷ Ir,⁸etc.⁹ were efficiently utilized for the allylation of various C–H bonds. Recently, Cp*Co^III based catalysts have emerged as efficient alternatives to the precious metal catalysts in allylation¹⁰ and other C–H bond functionalization reactions.¹¹ In addition, due to the small size and hard nature of cobalt catalysts, complementary reactivity was observed in a number of other transformations. All these allylation methods utilize either allylic alcohol derivatives or vinyl-epoxides, -cyclic carbonates and -cyclopropanes, allenes and others (Scheme 1a). Most of these transformations offer access to simple allylated and mono-substituted allylated arenes. Nevertheless, methods for introduction of allyl group containing substitution at allylic and alkene carbons are rather limited because most of these reactions involve an initial 1,2-migratory insertion of a C–metal bond into the alkene of the allylating reagent, which is highly controlled by the steric factor. Hence, it is highly desirable to develop a complementary approach for the multisubstituted allylation of C–H bonds utilizing a unique allylating reagent, which would involve an unconventional mode of allylation.

Scheme 1 Transition metal-catalyzed C–H bond allylation and reaction of cyclopropenes.

Substituted cyclopropenes constitute an important class of building blocks in organic synthesis.¹² Due to their high ring strain energy, cyclopropenes demonstrated versatile and exceptional reactivity, which are efficiently exploited in various transformations. In general, the reactivity of cyclopropenes can be classified into two types: (1) reaction of alkenes without ring-opening such as 1,2-addition and carbometallation, and (2) reaction accompanied by ring opening¹³ such as cycloisomerization, metathesis and reaction of vinyl carbenes (Scheme 1b). On the other hand, application of cyclopropenes in transition metal-catalyzed C–H bond functionalization of (hetero)arenes is in its infancy.¹⁴ The known transformations involve cyclopropanation of arenes via addition to alkene¹⁵ or cycloisomerization to heterocycles employing Rh(III)-based catalysts¹⁶ (Scheme 1c). However, transition metal-catalyzed ring-opening of cyclopropenes to vinylcarbenes¹⁷ and its application in C–H bond functionalization is yet to be explored.

We have been extensively involved in the in situ generation of metalcarbenes from various precursors¹⁸ and its application in the construction of various complex carbo- and heterocycles through C–H bond functionalization.¹⁹ In continuation of our work on metalcarbenes and encouraged by the unique reactivity of cyclopropenes, we envisioned the utilization of cyclopropenes as a non-diazo precursor of vinylcarbenes in transition metal-catalyzed C–H bond functionalization for the synthesis of allylated (hetero)arenes. Due to the high reactivity of cyclopropenes, we intended to introduce substituents at all three carbons of the allyl moiety, which is otherwise difficult to achieve. We herein report the Cp*Co^III-catalyzed multisubstituted allylation of directing group-assisted C–H bonds of (hetero)arenes, such as indoles and pyrroles, exploiting substituted cyclopropenes as the allylating reagent.²⁰ This method represents the first example of multisubstituted allylation of (hetero)arenes, where substituents are introduced at all three carbons of the allyl group with high selectivity control.

Results and discussion

N-Heterocycles, particularly indoles, are of great synthetic interest due to their relevance in biological systems. Hence, we started our investigation with allylation of indole derivatives. First, we examined the reaction of N-pyridylindole 1a and 3,3-diphenylcyclopropene 2a as the model reaction in the presence of [Cp*CoI₂CO] catalyst. Based on the initial optimization (see the ESI† for more details), reaction of 1a and 2a in the presence of 10 mol% of [Cp*CoI₂CO], 20 mol% of AgSbF₆ and 10 mol% of NaOAc in DCE at 100 °C for 1.5 h afforded the expected allylated product 3a in 91% yield (Table 1, entry 1). In the absence of either [Cp*CoI₂CO] or AgSbF₆, allylation was not observed. Although 1a was recovered from the reaction mixture, 2a underwent intramolecular rearrangement to the indene derivative and was isolated in high yields (Table 1, entry 2). A similar result was observed when NaOAc was removed (Table 1, entry 3). These observations suggest that the reaction is catalyzed by Cp*Co^III, and AgSbF₆ and NaOAc are essential for the observed activity of the catalyst. Replacement of NaOAc with other bases such as KOAc, NaOPiv and AgOAc gave product 3a in lower yield, reiterating the importance of NaOAc (Table 1, entries 4–6). Screening of solvents revealed that polar solvents, such as DMF, DMSO, and CH₃CN that can chelate with metals, are not suitable for the present transformation (Table 1, entry 7). Also use of non-polar aromatic solvents such as toluene was not beneficial (Table 1, entry 8). Reducing the catalyst loading to 5 mol% or use of the Lewis acidic catalyst [Cp*Co(CH₃CN)₃](SbF₆)₂ in the absence of AgSbF₆ gave the product 3a in reduced yield (Table 1, entries 1 and 9). Thus, 10 mol% of [Cp*CoI₂CO], 20 mol% of AgSbF₆ and 10 mol% of NaOAc in DCE at 100 °C were chosen as optimal conditions for the allylation of C–H bonds of indole 1 with cyclopropenes 2.

Table 1 Cp*Co^III catalysed allylation of the C–H bond of 1a with cyclopropene 2a^a

Entry	Conditions	Yield^b (%)
a Reaction conditions: 1a (1.0 equiv.), 2a (2 equiv.), [Cp*CoI2CO] (10 mol%), DCE, 100 °C, 1–2 h. b All are isolated yields. c With 5 mol% of [Cp*CoI2CO].
1	Standard conditions	91 (69)^c
2	Absence of Cp*CoI2CO or AgSbF6	NR
3	Without NaOAc	5
4	KOAc instead of NaOAc	60
5	NaOPiv instead of NaOAc	56
6	AgOAc instead of NaOAc	51
7	DMF, DMSO and CH3CN instead of DCE	NR
8	Toluene instead of DCE	28
9	With [Cp*Co(CH3CN)3](SbF6)2 instead of the catalyst and AgSbF6	47

---

After successfully establishing the allylation of the C–H bond with cyclopropene, the scope and limitation of the strategy were investigated. To begin with, various substituted indole derivatives were examined under the optimized conditions with cyclopropene 2a (Scheme 2). 5-Alkyl, aryl and halo substituted indole derivatives were readily allylated under the optimized conditions to furnish products 3a–3g in excellent yields. Electron-donating (methoxy) group was well tolerated under the reaction conditions to afford the allylated product 3h in 82% yield in 2 h. On the other hand, electron-withdrawing (cyano) group also showed reasonable compatibility and gave the vinylated product 3j in relatively lower yield after 4 h as a mixture of E : Z isomers in 2 : 1 ratio. But the allyloxy group underwent deprotection under the reaction conditions and the corresponding hydroxy product 3i was isolated.

Scheme 2 Cp*Co(III)-catalysed multisubstituted allylation of chelation-assisted C–H bonds with cyclopropenes. ^‡From the corresponding allyl ether. ^†Ratio of stereoisomers.

Similar results were observed with indole substituted at the 4-position, and synthesis of allyated products 3k–3m was achieved in good yield. Importantly, sterically congested 7-ethylindole and 3-methyl indole derivatives underwent a smooth reaction to give the allylated products 3n and 3p in relatively low yields (66 and 52% yields, respectively), and the latter was isolated as a 2 : 1 mixture of allylated and vinylated products. Furthermore, a 7-azaindole derivative having an additional coordination site also furnished the allylated product 3o in 81% yield. Subsequently, the effect of substituents on the pyridine directing group was examined. The 4-methylpyridine derivative effectively directed the allylation to provide the product 3q in 64% yield. But the electron deficient 5-nitropyridine derivative showed poor directing ability and the formation of product 3r was observed in 30% yield. Interestingly, replacement of pyridine with pyrimidine as the directing group, which can be readily removed after the reaction, furnished the product 3s in excellent yield.

Successively, the scope and limitations of cyclopropenes were explored. Symmetrically 3,3-disubstituted cyclopropenes on reaction with 1a under the optimized conditions afforded the products 3t and 3u in excellent yields. Use of unsymmetrical 3,3-diaryl, 3-aryl-3-heteroaryl and 3-alkyl-3-aryl cyclopropenes also afforded the corresponding allylated products 3v–3y in excellent yields as a mixture of E/Z isomers with variable ratio. Interestingly, symmetrical cyclopropenes derived from dialkyl ketones, such as dicyclohexyl ketone, cyclododecanone and 4-phenylcyclohexanone, also showed high level of compatibility under the optimized conditions to deliver allylated products 3z, 3aa and 3ab in good yields. Replacement of symmetrical disubstituted cyclopropene with unsymmetrical 1,3-diphenylcyclopropene furnished the product 3ac in 92% yield with complete E-selectivity. Consequently, trisubstituted cyclopropenes were examined. Formation of allylated product 3ad having substituents at allylic and alkene carbons in moderate yield from 1,3-dimethyl-3-phenylcyclopropenes was observed. Similar observations were noted with 1,2,3-triarylcyclopropenes containing substituents at the alkene carbon, where allylated products 3ae–3ah were obtained in moderate yields. This observation suggests that the steric factor in cyclopropene significantly affects the outcome of the reaction.

Subsequently, studies were directed to understand the importance of the directing group. Thus, bisindole having different substituents at nitrogen, methyl and pyridyl, respectively, selectively afforded the allylation at pyridyl substituted indole (3ai), which supported the importance of the directing group. It is important to note that formation of C7-allylated product 3aj was observed in 33% yield when N-pyridylindoline was treated with 2a under the optimized conditions. This result further expands the scope of allylation to the C7–H bond of the indole derivative.

Having successfully studied the allylation of indole derivatives, we next focused on the allylation of pyrroles and arenes using pyridine and pyrazole as the directing group, respectively. The reaction of N-pyridylpyrrole 4a with 2a in the presence of [Cp*Co^III] under the optimized conditions furnished the allylated product 6a in 82% yield (Scheme 3). Similarly, other substituted allylpyrroles 6b–6d were obtained in moderate to good yields. Next, N-phenylpyrazole reacted efficiently with 3a under the optimized conditions and afforded vinylated product 7a in 67% yield as the sole product, instead of the corresponding allylated product. Similar results were obtained with other substituted N-arylpyrazole derivatives and gave vinylated products 7b–7e in good yields.

Scheme 3 Reaction of 2a with N-pyridylpyrrole 4 and N-arylpyrazole 5.

After having shown the wide substrate scope, the synthetic application of the developed strategy was investigated. Gram scale synthesis of 2a and 1a furnished the allylated product 3a in comparable yield, suggesting that the reaction can be readily scaled up and the methodology is applicable to the large-scale preparation of allylated derivatives (Scheme 4a). Reaction of 3s with NaOEt furnished 2-allylated indole 9 in good yield, revealing that the employed directing group could be readily removed, and the method can be used for further synthetic manipulations (Scheme 4b).

Scheme 4 Synthetic utility. (a) Gram scale reaction. (b) Removal of directing group. (c) Synthesis of cyclopenta[b]indoles. (d) One-pot synthesis of cyclopenta[b]indole.

Cyclopenta[b]indoles are important structural motifs widely found in bioactive natural products and pharmaceutically interesting compounds.²¹ We next envisioned the conversion of allylated products 3 to the corresponding cyclopenta[b]indoles 10. Thus, reaction of 3a with 30 mol% of BF₃·OEt₂ in DCE afforded the product 10a in 81% yield (Scheme 4c). Subsequently, various substituted cyclopenta[b]indoles 10b–j were achieved in moderate to good yields in the presence of BF₃·OEt₂. Importantly, Cp*Co^III-catalyzed allylation and Lewis acid mediated cyclization were successfully integrated to afford cyclopenta[b]indole 3a in 68% yield in one-pot (Scheme 4d).

We also performed preliminary investigation to understand the possible mechanism of the developed transformation. The initial competitive experiment with electronically different indoles revealed preference to electron-rich indole derivatives (Scheme 5a). This suggests the possible electrophilic metalation pathway for C–H bond functionalization with Cp*Co^III. On the other hand, electronically different cyclopropenes showed only very slight preference to electron-rich cyclopropenes, which might be due to the high reactivity of cyclopropenes. Next, a deuterium exchange experiment was performed to support the formation of the Co–C bond through C–H bond functionalization. Treatment of 1a with CD₃OD under the optimized conditions, in the absence of cyclopropene, led to the recovery of 1a with a significant amount of deuteration at C2- and C7-position suggesting the potential reversible formation of the Co–C bond (Scheme 5b).

Scheme 5 Preliminary mechanistic investigation. (a) Competitive experiment. (b) Deuterium exchange experiment. (c) Stoichiometric reaction.

Next, we focused our attention on isolating the possible transition state of C–H metalated species. Thus, equimolar ratios of 1a and Cp*CoI₂CO were treated in the presence of 2 equiv. of AgSbF₆ and 1 equiv. of NaOAc in DCE at 100 °C for 6 h. We noticed a clear change in the color of the reaction mixture and the formation of a solid. Isolation and characterization of the formed solid through ¹H NMR showed that all five methyl groups of Cp* are not equal and one of them appeared as a doublet. Consequently, single crystal X-ray analysis of the isolated solid confirmed the formation of pyridinium salt 11 in 59% yield as a single diastereomer (Scheme 5c).²² The formation of 11 can be explained through the initial insertion, reductive elimination followed by protonation of the resultant allyl Co-species.

To isolate the potential intermediate, the stoichiometric reaction was performed at a reduced temperature (65 °C). Various attempts to isolate the complex formed was unsuccessful. Hence, the formed intermediate was analyzed by ESI-HRMS, which showed the presence of only complex 12. The mass spectrum was in complete agreement with the simulated spectra of complex 12 (see the ESI† for more details). Successively, treatment of the reaction mixture with cyclopropene 2a furnished the allylated product 3a in good yield. Interestingly, heating of complex 12 to 100 °C afforded the pyridinium salt 11 in 42% yield. These observations suggest complex 12 as a potential intermediate of the present allylation and formation of pyridinium salt 11.

Based on the preliminary mechanistic investigation, the plausible mechanism for the multisubstituted allylation is proposed (Scheme 6). The catalytic reaction starts with the formation of A from the pre-catalyst Cp*CoI₂CO and AgSbF₆/NaOAc through exchange of ligands. Reaction of active species A with indole derivative 1 would furnish the cyclometallated species Bvia base-assisted electrophilic C–H bond functionalization. Formation of vinylcarbene D could be explained through the initial coordination of cyclopropene 2 to B followed by rearrangement. Subsequently, 1,1-migratory insertion would give intermediate E, which on protodemetallation would give the product 3 and regenerate the active catalyst A. On the other hand, based on the substrate, intermediate E could exist as allyl-Co species E′, which on protonation would give the vinylated product. As seen in the substrate scope, a C3-allylated product could be formed via the intermolecular reaction of vinylcarbene with cyclometallated species B, which makes the C3-position more nucleophilic.

Conclusions

In conclusion, we have developed an efficient and general multisubstituted allylation of chelation-assisted C–H bond of (hetero)arenes in the presence of Cp*Co^III catalyst and substituted cyclopropenes as an efficient allylating reagent. The allylation occurs through ring-opening isomerization of cyclopropenes to vinylcarbenes followed by migratory insertion, which allows introduction of substituents in all three carbons of the allyl moiety. Various functional groups were well tolerated to afford diversely substituted allyated indoles, pyrroles and arenes in good yields and selectivity. Importantly, gram scale synthesis, removal of the directing group and synthetic manifestation to cyclopenta[b]indoles demonstrated the synthetic applicability of the present methodology. Furthermore, a potential metalated intermediate was identified through a stoichiometric reaction, which paved way to the plausible mechanism.

Scheme 6 Plausible mechanism.

Data availability

All the data have been included in the ESI.†

Author contributions

K. R. performed all experiments. Both authors contributed to the conception of the experiments, discussion of the results and preparation of manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank DST-SERB, New Delhi, India (Project No.: SB/SJF/2020-21/15) for financial support through SwarnaJayanti Fellowship. K. R. thanks University Grants Commission (UGC), New Delhi for fellowship.

Notes and references

1. (a) H. Y. Sohn, K. H. Son, C. S. Kwon, G. S. Kwon and S. S. Kang, Phytomedicine, 2004, 11, 666–672; (b) J. Du, Z.-D. He, R.-W. Jiang, W.-C. Ye, H.-X. Xu and P. P.-H. But, Phytochemistry, 2003, 62, 1235–1238; (c) G. Ni, Q.-J. Zhang, Z.-F. Zheng, R.-Y. Chen and D.-Q. Yu, J. Nat. Prod., 2009, 72, 966–968.
2. (a) R. M. Roberts and A. A. Khalaf, Friedel–Crafts Alkylation Chemistry: A Century of Discovery, M. Dekker, 1984; (b) M. Niggemann and M. J. Meel, Angew. Chem., Int. Ed., 2010, 49, 3684–3687.
3. (a) Y. Kiyotsuka, H. P. Acharya, Y. Katayama, T. Hyodo and Y. Kobayashi, Org. Lett., 2008, 10, 1719–1722; (b) C. Spino, M.-C. Tremblay and C. Gobdout, Org. Lett., 2004, 6, 2801–2804; (c) N. Harrington-Frost, H. Leuser, M. I. Calaza, F. F. Kneisel and P. Knochel, Org. Lett., 2003, 5, 2111–2114.
4. (a) N. K. Mishra, S. Sharma, J. Park, S. Han and I. S. Kim, ACS Catal., 2017, 2821–2847; (b) S. Dutta, T. Bhattacharya, D. B. Werz and D. Maiti, Chem, 2021, 7, 555–605.
5. (a) S. Y. Lee and J. F. Hartwig, J. Am. Chem. Soc., 2016, 138, 15278–15284; (b) S. Bae, H.-L. Jang, H. Jung and J. M. Joo, J. Org. Chem., 2015, 80, 690–697; (c) S. Fan, F. Chen and X. Zhang, Angew. Chem., Int. Ed., 2011, 50, 5918–5923; (d) T. K. Achar, X. Zhang, R. Mondal, M. S. Shanavas, S. Maiti, S. Maity, N. Pal, R. S. Paton and D. Maiti, Angew. Chem., Int. Ed., 2019, 58, 10353–10360.
6. (a) S. Oi, Y. Tanaka and Y. Inoue, Organometallics, 2006, 25, 4773–4778; (b) M. Kim, S. Sharma, N. K. Mishra, S. Han, J. Park, M. Kim, Y. Shin, J. H. Kwak, S. H. Han and I. S. Kim, Chem. Commun., 2014, 50, 11303–11306; (c) R. Manikandan, P. Madasamy and M. Jeganmohan, Chem.–Eur. J., 2015, 21, 13934–13938; (d) G. S. Kumar and M. Kapur, Org. Lett., 2016, 18, 1112–1115; (e) X. Wu and H. Ji, Org. Lett., 2018, 20, 2224–2227; (f) X.-Q. Hu, Z. Hu, A. S. Trita, G. Zhang and L. J. Gooßen, Chem. Sci., 2018, 9, 5289–5294.
7. (a) A. S. Tsai, M. Brasse, R. G. Bergman and J. A. Ellman, Org. Lett., 2011, 13, 540–542; (b) R. Zeng, C. Fu and S. Ma, J. Am. Chem. Soc., 2012, 134, 9597–9600; (c) B. Ye and N. Cramer, J. Am. Chem. Soc., 2013, 135, 636–639; (d) H. Wang, N. Schröder and F. Glorius, Angew. Chem., Int. Ed., 2013, 52, 5386–5389; (e) Z.-T. Jiang, J. Huang, Y. Zeng, F. Hu and Y. Xia, Angew. Chem., Int. Ed., 2021, 60, 10626–10631; (f) J. Xia, L. Kong, X. Zhou, G. Zheng and X. Li, Org. Lett., 2017, 19, 5972–5975; (g) S.-S. Zhang, J.-Q. Wu, Y.-X. Lao, X.-G. Liu, Y. Liu, W.-X. Lv, D.-H. Tan, Y.-F. Zeng and H. Wang, Org. Lett., 2014, 16, 6412–6415; (h) S. E. Korkis, D. J. Burns and H. W. Lam, J. Am. Chem. Soc., 2016, 138, 12252–12257; (i) Z. Qi, L. Kong and X. Li, Org. Lett., 2016, 18, 4392–4395; (j) G. Zhu, W. Shi, H. Gao, Z. Zhou, H. Song and W. Yi, Org. Lett., 2019, 21, 4143–4147.
8. (a) Y. J. Zhang, E. Skucas and M. J. Krische, Org. Lett., 2009, 11, 4248–4250; (b) R. Yabe, Y. Ebe and T. Nishimura, Synthesis, 2021 DOI:10.1055/a-1477-7059.
9. (a) T. Yao, K. Hirano, T. Satoh and M. Miura, Angew. Chem., Int. Ed., 2011, 50, 2990–2994; (b) N. Barsu, D. Kalsi and B. Sundararaju, Chem.–Eur. J., 2015, 21, 9364–9368; (c) N. Kaplaneris, T. Rogge, R. Yin, H. Wang, G. Sirvinskaite and L. Ackermann, Angew. Chem., Int. Ed., 2019, 58, 3476–3480; (d) S. Ali, J. Huo and C. Wang, Org. Lett., 2019, 21, 6961–6965.
10. (a) D.-G. Yu, T. Gensch, F. de Azambuja, S. Vásquez-Céspedes and F. Glorius, J. Am. Chem. Soc., 2014, 136, 17722–17725; (b) T. Gensch, S. Vásquez-Céspedes, D.-G. Yu and F. Glorius, Org. Lett., 2015, 17, 3714–3717; (c) D. Zell, Q. Bu, M. Feldt and L. Ackermann, Angew. Chem., Int. Ed., 2016, 55, 7408–7412; (d) Y. Suzuki, B. Sun, K. Sakata, T. Yoshino, S. Matsunaga and M. Kanai, Angew. Chem., Int. Ed., 2015, 54, 9944–9947; (e) Y. Bunno, N. Murakami, Y. Suzuki, M. Kanai, T. Yoshino and S. Matsunaga, Org. Lett., 2016, 18, 2216–2219; (f) D. Kalsi, R. A. Laskar, N. Barsu, J. R. Premkumar and B. Sundararaju, Org. Lett., 2016, 18, 4198–4201; (g) K. Ramachandran and P. Anbarasan, Eur. J. Org. Chem., 2017, 2017, 3965–3968; (h) H. Wang, M. M. Lorion and L. Ackermann, ACS Catal., 2017, 7, 3430–3433; (i) L. Kong, S. Yu, G. Tang, H. Wang, X. Zhou and X. Li, Org. Lett., 2016, 18, 3802–3805; (j) D. Zell, V. Müller, U. Dhawa, M. Bursch, R. R. Presa, S. Grimme and L. Ackermann, Chem.–Eur. J., 2017, 23, 12145–12148; (k) S. Y. Choi, H. D. Kim, J.-U. Park, S.-a. Park and J. H. Kim, Org. Lett., 2019, 21, 10038–10042; (l) R. Tanaka, I. Tanimoto, M. Kojima, T. Yoshino and S. Matsunaga, J. Org. Chem., 2019, 84, 13203–13210; (m) U. Dhawa, C. Tian, W. Li and L. Ackermann, ACS Catal., 2020, 10, 6457–6462.
11. (a) T. Yoshino and S. Matsunaga, Adv. Synth. Catal., 2017, 359, 1245–1262; (b) Z.-H. Guan, M. Usman, Z.-H. Ren and Y.-Y. Wang, Synthesis, 2017, 49, 1419–1443; (c) M. Moselage, J. Li and L. Ackermann, ACS Catal., 2015, 498–525; (d) J. Ghorai and P. Anbarasan, Asian J. Org. Chem., 2019, 8, 430–455; (e) S. Wang, S.-Y. Chen and X.-Q. Yu, Chem. Commun., 2017, 53, 3165–3180.
12. (a) M. Rubin, M. Rubina and V. Gevorgyan, Chem. Rev., 2007, 107, 3117–3179; (b) Z.-B. Zhu, Y. Wei and M. Shi, Chem. Soc. Rev., 2011, 40, 5534–5563; (c) M. R. Wilson and R. E. Taylor, Angew. Chem., Int. Ed., 2013, 52, 4078–4087; (d) L. Dian and I. Marek, Chem. Rev., 2018, 118, 8415–8434; (e) A. Archambeau, F. Miege, C. Meyer and J. Cossy, Acc. Chem. Res., 2015, 48, 1021–1031.
13. (a) T. Nakano, K. Endo and Y. Ukaji, Chem.–Asian J., 2016, 11, 713–721; (b) D. T. H. Phan and V. M. Dong, Tetrahedron, 2013, 69, 5726–5731; (c) S. Ma and J. Zhang, J. Am. Chem. Soc., 2003, 125, 12386–12387; (d) H. Zhang, B. Wang, K. Wang, G. Xie, C. Li, Y. Zhang and J. Wang, Chem. Commun., 2014, 50, 8050–8052; (e) F. Miege, C. Meyer and J. Cossy, Angew. Chem., Int. Ed., 2011, 50, 5932–5937; (f) A. Archambeau, F. Miege, C. Meyer and J. Cossy, Angew. Chem., Int. Ed., 2012, 51, 11540–11544; (g) D. Zhang, Z. Kang, J. Liu and W. Hu, iScience, 2019, 14, 292–300; (h) P. C. Young, M. S. Hadfield, L. Arrowsmith, K. M. Macleod, R. J. Mudd, J. A. Jordan-Hore and A.-L. Lee, Org. Lett., 2012, 14, 898–901; (i) L. H. Phun, J. Aponte-Guzman and S. France, Angew. Chem., Int. Ed., 2012, 51, 3198–3202.
14. T. A. Shah, P. B. De, S. Pradhan, S. Banerjee and T. Punniyamurthy, Chem.–Asian J., 2019, 14, 4520–4533.
15. (a) N. Semakul, K. E. Jackson, R. S. Paton and T. Rovis, Chem. Sci., 2017, 8, 1015–1020; (b) Y. Luo, H.-L. Teng, M. Nishiura and Z. Hou, Angew. Chem., Int. Ed., 2017, 56, 9207–9210; (c) T. K. Hyster and T. Rovis, Synlett, 2013, 24, 1842–1844.
16. (a) H. Zhang, K. Wang, B. Wang, H. Yi, F. Hu, C. Li, Y. Zhang and J. Wang, Angew. Chem., Int. Ed., 2014, 53, 13234–13238; (b) X. Wang, A. Lerchen, C. G. Daniliuc and F. Glorius, Angew. Chem., Int. Ed., 2018, 57, 1712–1716.
17. (a) H. Zhang, B. Wang, H. Yi, Y. Zhang and J. Wang, Org. Lett., 2015, 17, 3322–3325; (b) M. J. González, J. González, L. A. López and R. Vicente, Angew. Chem., Int. Ed., 2015, 54, 12139–12143; (c) R. J. Ross, R. Jeyaseelan and M. Lautens, Org. Lett., 2020, 22, 4838–4843.
18. (a) D. Yadagiri and P. Anbarasan, Chem. Sci., 2015, 6, 5847–5852; (b) D. Yadagiri, A. C. S. Reddy and P. Anbarasan, Chem. Sci., 2016, 7, 5934–5938; (c) A. C. S. Reddy, V. S. K. Choutipalli, J. Ghorai, V. Subramanian and P. Anbarasan, ACS Catal., 2017, 7, 6283–6288; (d) A. C. S. Reddy and P. Anbarasan, J. Catal., 2018, 363, 102–108; (e) D. Yadagiri, M. Chaitanya, A. C. S. Reddy and P. Anbarasan, Org. Lett., 2018, 20, 3762–3765; (f) S. Rajasekar and P. Anbarasan, Org. Lett., 2019, 21, 3067–3071; (g) A. C. S. Reddy, P. M. Reddy and P. Anbarasan, Adv. Synth. Catal., 2020, 362, 801–806; (h) P. M. Reddy, K. Ramachandran and P. Anbarasan, J. Catal., 2021, 396, 291–296.
19. (a) D. Yadagiri and P. Anbarasan, Org. Lett., 2014, 16, 2510–2513; (b) M. Chaitanya, D. Yadagiri and P. Anbarasan, Org. Lett., 2013, 15, 4960–4963; (c) S. Rajasekar, D. Yadagiri and P. Anbarasan, Chem.–Eur. J., 2015, 21, 17079–17084; (d) J. Ghorai, A. C. S. Reddy and P. Anbarasan, Chem.–Eur. J., 2016, 22, 16042–16046; (e) J. Ghorai, A. C. S. Reddy and P. Anbarasan, Chem.–Asian J., 2018, 13, 2499–2504.
20. J. Foerstner, A. Kakoschke, D. Stellfeldt, H. Butenschön and R. Wartchow, Organometallics, 1998, 17, 893–896.
21. (a) K. Stratmann, R. E. Moore, R. Bonjouklian, J. B. Deeter, G. M. L. Patterson, S. Shaffer, C. D. Smith and T. A. Smitka, J. Am. Chem. Soc., 1994, 116, 9935–9942; (b) J. Nakazawa, J. Yajima, T. Usui, M. Ueki, A. Takatsuki, M. Imoto, Y. Y. Toyoshima and H. Osada, Chem. Biol., 2003, 10, 131–137; (c) H. Chen, J. Bai, Z.-F. Fang, S.-S. Yu, S.-G. Ma, S. Xu, Y. Li, J. Qu, J.-H. Ren, L. Li, Y.-K. Si and X.-G. Chen, J. Nat. Prod., 2011, 74, 2438–2445.
22. CCDC2091505†.

---

Footnote

† Electronic supplementary information (ESI) available. CCDC2091505. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1sc03476f

---