Cobalt-catalyzed annulation of styrenes with α-bromoacetic acids

Article abstract of DOI:10.1039/d0ra09588e

We report a method for addition of α-bromophenylacetic acids to vinyl C-C bonds in styrenes to afford γ-lactones. Reactions employed a simple cobalt catalyst Co(NO₃)₂·6H₂O in the presence of dipivaloylmethane (dpm) ligand.

Full text of DOI:10.1039/d0ra09588e

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Cobalt-catalyzed annulation of styrenes with a-
bromoacetic acids†
Cite this: RSC Adv., 2021, 11, 5451
ab
*
Tung T. Nguyen,
ab Bao H. T. Ngo,ab Huy X. Le,ab Linh N. P. Vu,ab Tuong A. To,
ab
Anh N. Q. Phanab and Nam T. S. Phan
*
Received 11th November 2020
Accepted 23rd January 2021
We report a method for addition of a-bromophenylacetic acids to vinyl C]C bonds in styrenes to afford g-
lactones. Reactions employed simple cobalt catalyst Co(NO3)2$6H2O in the presence of
a
DOI: 10.1039/d0ra09588e
dipivaloylmethane (dpm) ligand. Many functionalities including halogen, ester, and nitro groups were
rsc.li/rsc-advances
compatible with reaction conditions. If a-bromoesters were used, vinylacetates were the major products.
g-Lactones are ubiquitously found in many natural products using a simple cobalt(II) catalyst (Scheme 1). The method has
and bioactive molecules.1 Among the available synthetic allowed for the use of a-halocarboxylic acids that are not limited
methods to obtain these ve-membered rings, those using to those containing quaternary centers.4d,4e
transition metals for functionalization of unsaturated carbon–
We rstly investigated the coupling of diphenylethylene 1a
carbon bonds, such as alkenes and alkynes, with oxygen- and a-bromophenylacetic acid 2a to afford the g-lactone 3aa.
containing nucleophiles have been substantially developed.2 Brief results and control experiments are shown in Table 1.7
Notable reports have used scarce noble metals and/or photo- Optimization was carried out with respect to cobalt salt, ligand,
redox conditions.3 Some examples employing abundant rst- base, and solvent. Using Co(acac)2 as catalyst for the annulation
row metals to obtain g-lactones from a-bromoacetic acids or afforded 3aa in 69% yield (entry 1). If a cobalt(III) precursor was
isosteric esters have been reported.4 A seminal method for employed, a trace amount of the lactone was detected (entry 2).
copper(0)-promoted hydrocarboxylation of alkenes with a- Since Co(OAc)2 was inferior to Co(acac)2 (entry 3), our hypoth-
iodocarboxylates was introduced by Metzger and Mahler.4a esis was that 1,3-diketone may play an important role in the
Following that, copper(I) salts have been developed for catalytic coupling.8 Running the reaction with Co(NO3)2$6H2O catalyst
annulation of a-bromoacetamides with styrenes.4b,4c Methods and acetylacetone ligand (L1) furnished 3aa in a comparable
for using low valent complexes of other non-noble metals such yield with respect to that of Co(acac)2 (entry 4). A better yield of
as those of Ni(0) or Fe(0) are known as well.4d,4e It should be the product was observed if dipivaloylmethane (dpm) ligand
noted that the reported transformations were presumably trig- (L2) was used (entry 5). Only 27% yield of 3aa was obtained in
gered by a homolytic cleavage of carbon–halogen bonds in a- case of an electron-poor 1,3-diketone (L3, entry 6). An unsym-
halocarboxylates.4d We envisioned that such an activation could metrical ligand bearing a phenyl moiety (L4) was also attemp-
be obtained if a suitable cobalt complex is used.
ted, furnishing the lactone 3aa in moderate yield (entry 7).
Using cobalt adducts in the organic transformations that Sodium carbonate was an inferior base to potassium carbonate
included alkyl halides was known.5 Most of the known examples (entry 8). To our surprise, adding a stoichiometric amount of
involved the formation of cobalt(I) species, thus usually DMF helped to increase the yield of 3aa (entry 9). 1,4-Dioxane
suffering from the presence of strong reductants and/or tailored could be used as the solvent for this coupling (entry 10).
ligands. Meanwhile, a direct elaboration of cobalt(II) complexes Notably, a small amount of by-product derived from 2a and
for incorporation of alkyl electrophiles is much rarer. It was CH3CN could be observed. Thus, pivalonitrile solvent was
hypothesized that a Co(II)/Co(III) catalytic cycle could be exploi-
ted since single-electron-transfer oxidation of Co(II) was
feasible.6 Herein we report our attempts to annulate styrenes
with a-bromophenylacetic acids for synthesis of g-lactones
aFaculty of Chemical Engineering, Ho Chi Minh City University of Technology
(HCMUT), 268 Ly Thuong Kiet, District 10, Ho Chi Minh City, Vietnam. E-mail:
tungtn@hcmut.edu.vn; ptsnam@hcmut.edu.vn
bVietnam National University Ho Chi Minh City, Linh Trung Ward, Thu Duc District,
Ho Chi Minh City, Vietnam
Scheme 1 Metal-catalyzed synthesis of g-lactones via activation of a-
† Electronic supplementary information (ESI) available. See DOI:
bromoacetic acids.
10.1039/d0ra09588e
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 5451–5455 | 5451

View Article Online
RSC Advances
Paper
Table 1 Studies of reaction conditionsa
Yield of 3aa
Entry
[Co]
Base
Solvent
(%)
1
2
Co(acac)2
Co(acac)3
Co(OAc)2
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
K2CO3
K2CO3
K2CO3
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
69
Trace
57
70
78
27
52
67
84
3
4b
5c
Co(NO3)2$6H2O
Co(NO3)2$6H2O
Co(NO3)2$6H2O
Co(NO3)2$6H2O
Co(NO3)2$6H2O
Co(NO3)2$6H2O
Co(NO3)2$6H2O
Co(NO3)2$6H2O
6d
7e
8c
9c,f
10c,f
11c,f
CH3CN
1,4-Dioxane
(CH3)3CCN
75
91
a
1a (0.1 mmol), 2a (0.2 mmol), [Co] catalyst (0.02 mmol), ligand (0.05
mmol), base (0.1 mmol), solvent (1 mL), at 140 ꢀC for 24 h. Yields of
3aa are GC yields using diphenyl ether internal standard. L1 was
used. L2 was used. L3 was used. L4 was used. DMF (0.1 mmol)
was added. Abbreviation: acac ¼ acetylacetonate.
b
c
d
e
f
attempted to avoid such addition (entry 11), affording 91% yield
of the lactone 3aa.
Scheme 2 Annulation of styrenes and a-bromo arylacetic acids.
Conditions: 1a–1j (0.5 mmol), 2a–2c (1.0 mmol), Co(NO3)2$6H2O (0.1
mmol), L2 (0.25 mmol), K2CO3 (0.5 mmol), DMF (0.5 mmol),
(CH3)3CCN (5 mL), at 140 ꢀC for 24 h. Yields are isolated yields. Dia-
stereomeric ratios (d.r.) were determined using 1H NMR spectrum.
Scope of the reaction with respect to reagents containing
C]C bonds was next studied. The results are shown in Scheme
2. Notably, some products were obtained in excellent diaster-
eoselectivities in comparison with two examples reported by
Nishihara and Iwasaki.4e A triphenyl-substituted g-lactone 3aa
was isolated in 85% yield. The simplest styrene coupled with a-
bromophenylacetic acid to furnish 3ba in 81% yield. Func-
tionalities including bromo (3ca), chloro (3da), ester (3ea), and
nitro (3fa, 3ga) groups were compatible with reaction condi-
tions. Some vinyl arenes containing polyaromatics were also
attempted. 2-Vinyl naphthalene was reacted to give the lactone
3ha in 88% yield. Cyclization of 9-vinylanthracene yielded
a trace amount of the desired product, somewhat implying the
importance of steric effect. Heterocycles, such as thiazole (3ia),
were competent substrates. Other a-bromoarylacetic acids such
as those containing chloro substituents could successfully
annulate with C]C bonds. Thus, the lactones (3hb, 3ac) were
obtained in moderate to good yields. At this moment, very
electron-poor styrenes (i.e. 3,5-bis(triuoromethyl)styrene) or
internal alkenes (i.e. indene) were not tolerant of reaction
conditions.
were relatively milder than those of a-bromophenylacetic acids,
presumably due to the presence of the quaternary carbon that
help cyclization step. Scope of styrenes is presented in Scheme
3. An excellent yield of the lactone (5a) was obtained in case of
diphenylethylene substrate. Reactions of molecules containing
bromo (5c), ester (5e), and nitro (5f) substituents occurred
without any difficulties. Similar to the iron-catalyzed method,4e
2-vinyl naphthalene was suitable for the annulation (5h). Vinyl
heteroarenes were competent substrates (5i). Electron-rich
arenes were also attempted, furnishing the lactones in good
yields (5k, 5l).
Using a-bromoacetates to couple with styrenes could yield
lactones.4d,4e However, only vinylacetates 7a–7c were obtained in
our conditions if similar substrates were employed (Scheme 4).
The results were indicative of a slow hydrolysis during reaction
course. It should be noted that the presence of quaternary
carbons in esters was crucial to afford the addition products.
To show the generality of our method, annulation of styrenes
with a-bromoisobutyric acid 4 was attempted. The conditions
5452 | RSC Adv., 2021, 11, 5451–5455
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
Scheme 3 Annulation of styrenes and a-bromoisobutyric acid. Conditions: styrenes (0.5 mmol), 4 (1.0 mmol), Co(NO3)2$6H2O (0.1 mmol), L2
(0.25 mmol), K2CO3 (0.5 mmol), (CH3)3CCN (5 mL), at 100 ꢀC for 24 h. Yields are isolated yields.
Scheme 4 Scope of a-bromoacetates. Conditions: 1a (0.5 mmol), a-bromoacetates 6a–6c (1.0 mmol), Co(NO3)2$6H2O (0.1 mmol), L2 (0.25
mmol), K2CO3 (0.5 mmol), DMF (0.5 mmol), (CH3)3CCN (5 mL), at 140 ꢀC for 24 h. Yields are isolated yields.
Based on previous studies,4d,4e a plausible mechanism was cyclization, in the presence of base, to eventually furnish the
proposed as that shown in Scheme 5. Homolytic cleavage of lactone 3aa. To support the hypothesis, some control experi-
C(sp3)–Br bond in 2a would afford the benzyl radical 8. Radical ments were carried out. Firstly, addition of radical quenchers
addition to 1a generated 9 followed by an oxidation that could such as TEMPO or BHT stopped the annulation, somewhat
afford a benzylic cation 10. The resulting species underwent conrming the involvement of radical species during the
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 5451–5455 | 5453

View Article Online
RSC Advances
Paper
Scheme 5 Mechanistic considerations.
reaction course. Secondly, running the reaction in absence of 1a
gave an anhydride 11 (detected by GC-MS). Such the dead-end
intermediate could be obtained via a homo-combination of
the radical 8 followed by a dehydration.
Notes and references
1 (a) E. J. Kang and E. Lee, Chem. Rev., 2005, 105, 4348; (b)
G. Lin, S. S. K. Chan, H. S. Chung and S. L. Li, Stud. Nat.
Prod. Chem, 2005, 32, 611.
2 N. T. Patil, R. D. Kavthe and V. S. Shinde, Tetrahedron, 2012,
68, 8079.
Conclusions
3 (a) N. G. Kundu and M. Pal, J. Chem. Soc., Chem. Commun.,
1993, 86; (b) H.-Y. Liao and C.-H. Cheng, J. Org. Chem.,
1995, 60, 3711; (c) R. C. Larock, M. J. Doty and X. Han, J.
Org. Chem., 1999, 64, 8770; (d) X.-J. Wei, D.-T. Yang,
L. Wang, T. Song, L.-Z. Wu and Q. Liu, Org. Lett., 2013, 15,
6054; (e) I. Triandallidi, M. G. Kokotou and C. G. Kokotos,
Org. Lett., 2018, 20, 36.
4 (a) J. O. Metzger and R. Mahler, Angew. Chem., Int. Ed.
Engl., 1995, 34, 902; (b) T. Nishikata, K. Itonaga,
N. Yamaguchi and M. Sumimoto, Org. Lett., 2017, 19,
2686; (c) Y. Lv, W. Pu, S. Mao, X. Ren, Y. Wu and H. Cui,
RSC Adv., 2017, 7, 41723; (d) D. Liu, S. Tang, H. Yi,
C. Liu, X. Qi, Y. Lan and A. Lei, Chem. Eur. J., 2014, 20,
15605; (e) M. Iwasaki, N. Miki, Y. Ikemoto, Y. Ura and
Y. Nishihara, Org. Lett., 2018, 20, 3848; (f) B. Wei,
K.-W. Li, Y.-C. Wu, S.-Q. Tong and R.-J. Song, Synthesis,
2020, 52, 3855.
In conclusion, we have developed a general method to couple
styrene C]C bonds with a-bromoarylacetic acids to afford g-
lactones. The transformation employed
precursor Co(NO3)2$6H2O facilitated by dipivaloylmethane
ligand. Reaction conditions were well compatible with a diver-
sied array of functional groups. Preliminary control experi-
ments supported a radical-involved mechanism that could
triggered by the cobalt-catalyzed activation of carbon–bromide
bond.
a simple cobalt
Conflicts of interest
There are no conicts to declare.
´
5 (a) A. Guerinot and J. Cossy, Acc. Chem. Res., 2020, 53,
Acknowledgements
1351; (b) K. Gao and N. Yoshikai, Acc. Chem. Res., 2014,
47, 1208.
We would like to thank the Vietnam National Foundation for
Science and Technology Development (NAFOSTED) for sup-
porting this research (under grant number 104.01-2019.44).
5454 | RSC Adv., 2021, 11, 5451–5455
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
6 For recent examples, see:(a) L. Grigorjeva and O. Daugulis, 8 For recent examples of the cobalt complex ligated by
Angew. Chem., Int. Ed., 2014, 53, 10209; (b) S. Maity,
R. Kancherla, U. Dhawa, E. Hoque, S. Pimparkar and
D. Maiti, ACS Catal., 2016, 6, 5493.
dipivaloylmethane, see:(a) T. T. Nguyen, L. Grigorjeva and
O. Daugulis, ACS Catal., 2016, 6, 551; (b) L. Lukasevics,
A. Cizikovs and L. Grigorjeva, Org. Lett., 2020, 22, 2720.
7 Please see the ESI† for full optimization studies.
© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 5451–5455 | 5455