10.1002/asia.201800534
Chemistry - An Asian Journal
FULL PAPER
Based on Fenton's reaction[24] and the previous works[18,25,26], a
catalytic cycle involving a single electron transfer is proposed
here (Scheme 1). Fe(OTf)3 is reduced to iron (II) (A) by alkyl
radicals or vinylarenes. Iron (II) species A transfers an electron
to perester to form iron (III) species (B) and an alkyl radical. The
alkyl radical reacts with styrene to form benzylic radical C. The
benzylic radical (C) is oxidized by iron (III) species (B) to a
carbocation (D) and regenerates iron (II) (A). The carbocation
(D) then undergoes deprotonation to deliver the desired product.
Yuan, and Xinqiang Fang from our institute for suggestions and
help.
Keywords: iron catalysis • vinylic C-H alkylation • radical • alkyl
diacyl peroxides • peresters
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Conclusions
This method disclosed effective utilizations of alkyl peresters
and alkyl diacyl peroxides as the general primary, secondary,
and tertiary alkylating reagents in vinylic C-H alkylation of
vinylarenes, dienes and 1,3-enynes. This catalytic reaction is
operationally simple, and is compatible with a broad range of
functionalities including carboxyl, boronic acid, methoxy, ester,
amino and halides. Several late stage functionalization of natural
products and drug molecules was conducted to demonstrate the
synthetic applications of this reaction. This protocol provided a
facile approach to some olefins that are difficult to access
previously.
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Experimental Section
General procedure for Table 2a: vinylic C-H alkylation of styrene with t-
butyl peresters: Styrene 1a (52 mg, 0.5 mmol, 1 equiv.), Fe(OTf)3 (2.5 mg,
1 mol%) and THF (2 mL) were added into a dried Schlenk tube
(connecting to a water cooling condenser and N2) with a stirring bar. After
perester 2b-2n (0.5 mmol, 1.0 equiv.) was added, the schlenk tube was
inserted in an oil bath. The oil bath was heated from room temperature to
reflux. After 30 min and 60 min, the second batch (0.5 mmol, 1.0 equiv.)
and third batch of perester (0.5 mmol, 1.0 equiv.) were added, separately.
After 5 hours, the reaction mixture was cooled to ambient temperature
and concentrated by rotary evaporation under reduced pressure. The
residue was purified by fast column chromatography on silica gel
(petroleum ether) to yield 3.
[5]
General procedure for Table 2b: Styrene 1a (52 mg, 0.5 mmol, 1 equiv),
diacyl peroxide 4 (0.75 mmol, 1.5 equiv), Fe(OTf)3 (12.6 mg, 5 mol%)
and THF (2 mL) were added into a flame dried Schlenk tube with a
stirring bar and the reaction tube was inserted into an oil bath which was
heated to 90 oC. The reaction solution was bubbling for 3-5 min then
slightly boiling. After 3 hours, the reaction mixture was cooled to ambient
temperature and concentrated by rotary evaporation under reduced
pressure. The residue was purified by column chromatography on silica
gel (petroleum ether) to yield 5.
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Acknowledgements
We thank NSFC (grant no. 21672213), Strategic Priority
Research Program of the Chinese Academy of Sciences (Grant
No. XDB20000000), The 100 Talents Program, “The 1000 Youth
Talents Program”, Haixi Institute of CAS (CXZX-2017-P01) for
financial support. We also thank Professor Weiping Su, Daqiang
G.-Z. Wang, R. Shang, Y. Fu, Org. Lett. 2018, 20, 888.
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