Photocatalytic Umpolung Synthesis of Nucleophilic π-Allylcobalt Complexes for Allylation of Aldehydes

Complexes for Allylation of Aldehydes

Research Article

Photocatalytic Umpolung Synthesis of Nucleophilic π‑Allylcobalt

Caizhe Shi, Fusheng Li, Yuqing Chen, Shuangjie Lin, Erjun Hao, Zhuowen Guo, Urwa Tul Wosqa,

Dandan Zhang, and Lei Shi*

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sı Supporting Information

ABSTRACT: The concept of “umpolung” reactivity of π-allylmetal

complexes has been developed as a powerful method for the

allylation of aldehydes. This paper describes the photocatalytic

umpolung strategy for the synthesis of nucleophilic allylcobalt

complexes through a single-electron-transfer (SET) process. This

strategy enables the metallaphotoredox allylation of carbonyls with

allyl acetate using organic N,N-diisopropylethylamine as the

terminal reductant bypassing the use of a stoichiometric amount

of metals. Ultraviolet−visible spectroscopy was used to monitor the

redox changes of cobalt in the reaction.

KEYWORDS: π-allylcobalt complexes, photoredox catalysis, allylation, umpolung, alcohol

omoallylic alcohols and their derivatives are essential

synthetic intermediates for the preparation of numerous

Metallaphotoredox catalysis is a new and rapidly growing

17−25

research ﬁeld.

Recently, the combination of photoredox

natural products and medicine. The concept of “umpolung”

reactivity of π-allylmetal complexes has been developed as a

powerful approach for carbonyl allylation with high regio- and

and cobalt catalysis has been developed as a distinct new option

in the ﬁeld of metallaphotoredox catalysis and has found

26−44

numerous applications in synthetic chemistry.

Given that

−5

diastereo-selectivity control (Scheme 1a).

Such “reverse

photoredox processes can directly modify the oxidation state of

transition metals by single electron transfer (SET), we wonder

whether the photoredox system can convert an electrophilic π-

reactivity” of π-allylmetal complexes can be controlled by the

modiﬁcation of their electronic environment through the use of

additives and ligands. For example, π-allylpalladium complexes

are known to be electrophilic and can react with a wide range of

nucleophiles (Tsuji−Trost reaction), while nucleophilic π-

allylpalladium complexes are also well documented to catalyze

III

allylcobalt species to a nucleophilic π-allylcobalt species. If so,

a novel dual photoredox and cobalt-catalyzed allylation of

−

carbonyls based on a 2e (oxidative addition) and a 1e (SET)

relay process is anticipated (Scheme 1b). In this paper, we report

the ﬁrst photocatalytic generation of nucleophilic π-allylcobalt

complexes for carbonyl allylation through an umpolung strategy.

We selected 4-methoxybenzaldehyde 1a and allyl acetate as

the standard substrates to optimize the reaction conditions

the umpolung allylation of carbonyls and imines. However, an

excess of metal reductants such as Et Zn and Et B is usually

required for such a “reactivity switch” process.

Cobalt is an earth-abundant low-cost ﬁrst-row transition

metal and generally of low toxicity. π-Allylcobalt complexes

(

Table 1). Following the rapid screening of diﬀerent reaction

show similar “amphiphilic character” in organic reactions. Mita

and Sato reported the nucleophilic addition of π-allylcobalt/

conditions, we were able to identify the conditions of CoBr (10

mol %), the photocatalyst Ir(ppy) (dtbbpy)PF (Ir-1), 4,4′-di-

Xantphos complexes to ketones and carbon dioxide using excess

tert-butyl-2,2′-dipyridyl L3 (10 mol %), N,N-diisopropylethyl-

amine (DIPEA) (3 equiv), and allyl acetate (3 equiv) mixed in

dimethylformamide (DMF) at room temperature. The obtained

AlMe as the reductant, in which the low-valent π-allylcobalt

complexes generated through reductive elimination were

proposed to be the key intermediates. Co-catalyzed allylic

alkylation and amination were reported by the groups of

Kojima/Matsunaga and Li, respectively, in which the key

Received: December 5, 2020

Revised: February 10, 2021

Published: February 22, 2021

III

electrophilic π-allylcobalt species was generated in situ via

oxidative addition of allyl acetate to a low-valent Co

complex.

0,11

However, the photocatalytic conversion of

electrophilic π-allylcobalt to nucleophilic π-allylcobalt remains

12−16

an understudied topic.

2021 American Chemical Society

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ACS Catal. 2021, 11, 2992−2998

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Research Article

III

Scheme 1. (a) Concept of “Umpolung” Reactivity of π-Allylmetal Complexes and (b) Conversion of Electrophilic π-AllylCo to

Nucleophilic π-AllylCo via SET

mixture was irradiated with a 450 nm LED lamp for 32 h and

aﬀorded the product 2a in 58% yield (entry 1). Various solvents

such as acetonitrile, tetrahydrofuran, and dichloroethane were

tested, generally producing low yields in the 10−20% range. The

use of Hantzsch ester as an organic electron donor instead of

DIPEA resulted in no product (entry 2). Further screening of

such as indomethacin, probenecid, and lithocholic acid

derivatives, and the desired products 3a−3c were obtained in

a decent yield.

The regio- and diastereo-selectivity of the new catalysis

system was further explored using cinnamyl alcohol as the

coupling substrate. It was found that tert-butyl cinnamyl

carbonate 4 is more reactive than cinnamyl acetate for the

allylation of various aldehydes, as shown in Table 3. Only

branched products 5a−5l were obtained in favor of trans-

diastereoselectivity with moderate to good yields. The highest

diastereoselectivity (d.r. > 20:1) was achieved with cyclo-

hexanecarboxaldehyde (5h), albeit in somewhat low yield. The

trans-diastereoselectivity can be elucidated based on the

Zimmerman−Traxler transition state, where the oxygen atom

of the carbonyls coordinates with the cobalt. The aryl group of

the π-allylcobalt complexes and the R group of the aldehyde

preferentially adopt an equatorial orientation, resulting in the

generation of an anti-isomer of the corresponding homoallylic

various photocatalysts, including 4CzIPN, [Ir(dF(CF )-

ppy) (dtbbpy)]PF₆(Ir-2), and Ru(bpy) (PF ) (Ru-1),

6 2

revealed that they are less eﬃcient for allylation (entries 3−5).

Diﬀerent ligands such as 4,7-diphenyl-1,1-phenanthroline L1,

,10-phenanthroline L2, and 1,2-bis(diphenylphosphino)-

ethane L4 (entries 6−8), were tested as replacements of L3.

These ligands were less eﬃcient for catalysis. Further testing of

various cobalt salts showed that CoSO ·H O gave the best

results and increased the yield up to 80% (entries 9−13).

Additive eﬀects were tested but did not increase the yield

(

entries 14 and 15). Additionally, the reaction did not occur in

the absence of either light, Co, DIPEA, or photocatalyst (entries

6−19). The reaction is sensitive to low light intensity, H O,

alcohols.

and high oxygen concentration, as shown by the condition-

The scalability of the reaction was determined by the

synthesis of 5a in a gram-scale under standard conditions.

Gratifyingly, product 5a was obtained in decent 83% isolated

yield. The use of earth-abundant cobalt, wide compatibility with

various sensitive functional groups, and easy to operate

conditions of this reaction are appealing features for laboratory

and industrial applications.

based sensitivity testing (Supporting Information S7).

After optimizing the reaction conditions, we evaluated the

scope of aldehydes for the allylation reactions (Table 2). It was

clear that the reactions worked smoothly with lots of aromatic

and aliphatic aldehydes producing various trans-alcohols in

generally decent yields. The substituent eﬀect of the aromatic

ring has little eﬀect on the allylation reaction. Both the electron-

rich groups and electron-withdrawing groups, including

methoxy 2a and ﬂuorine 2l, gave homoallylic alcohols in

reasonable yields. The reaction of sterically hindered mesitalde-

hyde 2e was also successful. Heteroatoms did not impede the

catalytic cycle, including oxygens 2g and 2u and the protected

amine 2x. Besides, various heteroarenes, which are widely used

as core structures in medicinal synthesis, were also suitable

substrates for this photoredox and cobalt catalysis, including

furans 2q and 2r and thiophene 2p. Due to the extremely mild

photocatalytic conditions, reactions showed excellent compat-

ibility with many synthetically important and valuable functional

groups that include the free alcohol 2j, alkene 2w, ether 2h,

bromobenzene 2k, and chlorobenzene 2o. Allylation of the acid-

sensitive citronellal 2w substrate was also successful without

forming a cyclization byproduct. The synthetic potential of this

dual photoredox and cobalt catalysis was demonstrated by the

late-stage functionalization of complicated structure molecules,

Stern−Volmer luminescence quenching studies were con-

ducted (Supporting Information S2) and showed that DIPEA

can readily quench the excited photocatalyst Ir-1 at the rate of

−1 −1

3.42 × 10 L M s . Then, a series of experiments were

performed to probe the diﬀerent π-allylcobalt species in the

catalytic cycle. First, CoBr , L3, and Ir-1 were added to a

solution of DIPEA in DMF (Figure 1C(a)). The resulting

mixture was exposed to 450 nm LED irradiation. In the ﬁrst SET

process, the color of the reaction changed from green to blue

(Figure 1C(b)). The ultraviolet−visible (UV−vis) spectro-

scopic analysis of the reaction was also performed. A broad

absorption band at 550−700 nm gradually appeared within 30

min, indicating the formation of Co 7 species, which is

27,46,47

consistent with previous reports (Figure 1A(b)).

Then,

the allyl acetate was added to the resulting reaction system in the

dark, and the color changed to brown (Figure 1C(c)). At the

same time, UV−vis spectroscopic analysis of the reaction

revealed the rapid disappearance of absorption of Co 7, and a

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Table 1. Optimization of the Reaction Conditions

entry

variation from standard conditions

yield (%)

none

HE instead of DIPEA

Ir-2 instead of Ir-1

Ru-1 instead of Ir-1

4CzIPN instead of Ir-1

L1 instead of L3

trace

L2 instead of L3

L4 instead of L3

Co(BF ) ·6H O instead of CoBr

Co(OAc ) ·4H O instead of CoBr

2 2 2 2

Co(NO ) ·4H O instead of CoBr

Co(acac) instead of CoBr₂

CoSO ·H O instead of CoBr

CoSO ·H O, CF COOH

CoSO ·H O, K CO

no Co catalyst

no DIPEA

no photocatalyst

no light

Reaction conditions: 1a (0.5 mmol scale). Yields were determined by H NMR spectroscopy vs an internal standard.

new strong absorption band appeared at 380−550 nm,

photocatalyst under irradiation. In fact, when the reaction was

further exposed to irradiation, the absorption band of the

III

suggesting the formation of π-allylcobalt 8 via the oxidative

III

addition of allyl acetate to a low-valent Co complex (Figure

proposed Co 8 disappeared and a new absorption band

B(c)). The resulting 8 is known to react with various

appeared at 360−450 nm, indicating the generation of π-

10,11

nucleophiles but not with aldehydes.

π-allylcobalt can be further reduced to π-allylcobalt by a

It was reported that

allylcobalt 9 (Figure 1B(d)). Thus, the allylation indeed

III

occurred with irradiation and 2a was smoothly obtained (Figure

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Table 2. Scope of Aldehydes in Allylation Reactions

Table 3. Scope of Substituted Allyl Carbonates in Allylation

Reactions

Reaction conditions: 1a (0.5 mmol scale). Yields were determined

by H NMR spectroscopy vs an internal standard. Diastereomeric

ratios were determined by H NMR spectroscopy. tert-Butyl prenol

carbonate was used.

According to previous reports in the literature and the results

of this study, we tentatively propose a catalytic cycle, as shown in

Figure 2. Initial excitation of the Ir-1 photocatalyst produces the

III

red

III

photoexcited *Ir-1 species. The *Ir-1 catalyst (E₁[*Ir /

Ir ] = +0.66 V vs SCE in CH CN) is reduced by DIPEA (E

DIPEA) = +0.65 V) via SET to produce a reducing Ir-1

(

•

species and [DIPEA] . Reduction of the ligand-coordinated

red

III

Ln−Co 6 by Ir-1 (E [Ir /Ir ] = −1.51 V vs SCE in

III

CH CN) aﬀords Ln−Co 7 species and Ir-1 . Then, Ln−Co 7

reacts with the allylic acetate by oxidative addition and generates

III

the electrophilic π-allylcobalt 8 species that is readily reduced

by another Ir-1 photocatalyst to the key nucleophilic π-

allylcobalt 9 species. The addition of 9 to aldehydes forms a

new C−C bond in the alkoxycobalt product. Hydrolysis of the

Reaction conditions: 1a (0.5 mmol scale). Yields were determined

oxygen−Co bond releases Ln−Co 6 and the alcohol product.

by H NMR spectroscopy vs an internal standard.

Finally, Ir-1 can regenerate Ln−Co 7 for the next catalytic cycle.

CONCLUSIONS

■

C(d)). This ﬁnding indicated that the nucleophilic π-

In summary, this work reports a novel photocatalytic allylation

of carbonyls through dual cobalt and photoredox catalysis. This

eco-friendly photoredox reaction enables the allylation of

numerous aldehydes with easily available allyl acetate using

organic DIPEA as the terminal reductant and bypassing the use

of the stoichiometric amount of metals. The success of the

allylcobalt 9 (dark orange) was formed via SET and most

likely is the key active intermediate toward the aldehydes.

48,49

addition, the quantum yield was determined to be 0.012,

suggesting that the radical-chain mechanism is not likely

(Supporting Information S4).

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ACS Catal. 2021, 11, 2992−2998

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Figure 1. UV −vis spectroscopy studies of diﬀerent π -allylcobalt species in the reaction (for details, please see Supporting Information S8).

Research Article

Figure 2. Proposed catalytic cycle.

reaction relies on the generation of nucleophilic allylcobalt^I^I

Experimental details, characterization data of compounds,

complexes based on the photocatalytic umpolung strategy.

and NMR spectra (PDF)

ASSOCIATED CONTENT

■

AUTHOR INFORMATION

■

sı Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acscatal.0c05330.

Corresponding Author

Lei Shi − Zhang Dayu School of Chemistry, State Key

Laboratory of Fine Chemicals, Dalian University of

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Organomagnesium Reagents Prepared through Halogen-Metal Ex-

Research Article

Technology, Dalian, Liaoning 116024, China; Henan Key

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Laboratory of Organic Functional Molecules and Drug

Innovation, Key Laboratory of Green Chemical Media and

Reactions, Ministry of Education, School of Chemistry and

Chemical Engineering, Henan Normal University, Xinxiang,

Henan 453007, China; orcid.org/0000-0002-2644-1168 ;

Email: shilei17@dlut.edu.cn

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Authors

Caizhe Shi − Zhang Dayu School of Chemistry, State Key

Laboratory of Fine Chemicals, Dalian University of

Technology, Dalian, Liaoning 116024, China

Fusheng Li − Zhang Dayu School of Chemistry, State Key

Laboratory of Fine Chemicals, Dalian University of

Technology, Dalian, Liaoning 116024, China

Yuqing Chen − Zhang Dayu School of Chemistry, State Key

Laboratory of Fine Chemicals, Dalian University of

Technology, Dalian, Liaoning 116024, China

Shuangjie Lin − Zhang Dayu School of Chemistry, State Key

Laboratory of Fine Chemicals, Dalian University of

Technology, Dalian, Liaoning 116024, China

Erjun Hao − Henan Key Laboratory of Organic Functional

Molecules and Drug Innovation, Key Laboratory of Green

Chemical Media and Reactions, Ministry of Education, School

of Chemistry and Chemical Engineering, Henan Normal

University, Xinxiang, Henan 453007, China

Zhuowen Guo − Zhang Dayu School of Chemistry, State Key

Laboratory of Fine Chemicals, Dalian University of

Technology, Dalian, Liaoning 116024, China

Urwa Tul Wosqa − Zhang Dayu School of Chemistry, State Key

Laboratory of Fine Chemicals, Dalian University of

Technology, Dalian, Liaoning 116024, China

Dandan Zhang − Zhang Dayu School of Chemistry, State Key

Laboratory of Fine Chemicals, Dalian University of

Technology, Dalian, Liaoning 116024, China

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(

14) Schwarz, J. L.; Schafers, F.; Tlahuext-Aca, A.; Luckemeier, L.;

Complete contact information is available at:

https://pubs.acs.org/10.1021/acscatal.0c05330

Glorius, F. Diastereoselective Allylation of Aldehydes by Dual

Photoredox and Chromium Catalysis. J. Am. Chem. Soc. 2018, 140,

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Author Contributions

C.S. and F.L. contributed equally. The manuscript was written

through contributions of all authors.

(15) Li, F.-S.; Chen, Y.-Q.; Lin, S.-J.; Shi, C.-Z.; Li, X.-Y.; Sun, Y.-C.;

Guo, Z.-W.; Shi, L. Visible-light-mediated Barbier Allylation of

Aldehydes and Ketones via Dual Titanium and Photoredox Catalysis.

Org. Chem. Front. 2020, 7, 3434−3438.

Funding

(

16) Li, F.-S.; Lin, S.-J.; Chen, Y.-Q.; Shi, C.-Z.; Yan, H.-P.; Li, C.-C.;

The project was supported by the Open Research Fund of

School of Chemistry and Chemical Engineering, Henan Normal

University (no. 2020YB03), Key Scientiﬁc Research Projects of

Higher Education Institutions in Henan Province (no.

Wu, C.; Lin, L.-Q.; Duan, C.-Y.; Shi, L. Photocatalytic Generation of π -

allyltitanium Complexes via Radical Intermediates. Angew. Chem., Int.

Ed. 2021, 60, 1561−1566.

(

17) Twilton, J.; Le, C. C.; Zhang, P.; Shaw, M. H.; Evans, R. W.;

1A150002), LiaoNing Revitalization Talents Program (no.

MacMillan, D. W. C. The Merger of Transition Metal and

Photocatalysis. Nat. Rev. Chem. 2017, 1, 52.

(18) Tellis, J. C.; Kelly, C. B.; Primer, D. N.; Jouffroy, M.; Patel, N. R.;

Molander, G. A. Single-Electron Transmetalation via Photoredox/

Nickel Dual Catalysis: Unlocking a New Paradigm for sp -sp Cross-

XLYC1907126), and Provincial College Student Innovation and

Entrepreneurship Training Program Support Project (no.

020101410101010339).

Notes

Coupling. Acc. Chem. Res. 2016, 49, 1429−1439.

20) Gentry, E. C.; Knowles, R. R. Synthetic Applications of Proton-

The authors declare no competing ﬁnancial interest.

19) Skubi, K. L.; Blum, T. R.; Yoon, T. P. Dual Catalysis Strategies in

Photochemical Synthesis. Chem. Rev. 2016 , 116 , 10035 −10074.

Coupled Electron Transfer. Acc. Chem. Res. 2016 , 49 , 1546 −1556.

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Chem. Rev. 2016, 116, 10075−10166.

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ACKNOWLEDGMENTS

We thank WATTCAS CHEM-TECH Co., Ltd. for kindly

providing the photoreactor.

■

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