DOI: 10.1002/cssc.201402921
Full Papers
Bifunctional Silver(I) Complex-Catalyzed CO2 Conversion at
Ambient Conditions: Synthesis of a-Methylene Cyclic
Carbonates and Derivatives
Qing-Wen Song,[a] Wei-Qiang Chen,[b] Ran Ma,[a] Ao Yu,[b] Qiu-Yue Li,[a] Yao Chang,[a] and
Liang-Nian He*[a]
The chemical conversion of CO2 at atmospheric pressure and
room temperature remains a great challenge. The triphenyl-
phosphine complex of silver(I) carbonate was proved to be
a robust bifunctional catalyst for the carboxylative cyclization
of propargylic alcohols and CO2 at ambient conditions leading
to the formation of a-methylene cyclic carbonates in excellent
yields. The unprecedented performance of [(PPh3)2Ag]2CO3 is
presumably attributed to the simultaneous activation of CO2
and propargylic alcohol. Moreover, the highly compatible ba-
sicity of the catalytic species allows propargylic alcohol to
react with CO2 leading to key silver alkylcarbonate intermedi-
ates: the bulkier [(Ph3P)2AgI]+ effectively activates the carbon–
carbon triple bond and enhances O-nucleophilicity of the alkyl-
carbonic anion, thereby greatly promoting the intramolecular
nucleophilic cyclization. Notably, this catalytic protocol also
worked well for the reaction of propargylic alcohols, secondary
amines, and CO2 (at atmospheric pressure) to afford b-oxopro-
pylcarbamates.
Introduction
With increasing awareness of ever-growing CO2 levels in the
atmosphere, great efforts have been made to develop strat-
egies and technologies towards the reduction of carbon emis-
sions.[1] Indeed, CO2 fixation and conversion hold great prom-
ise for recycling CO2 into value-added products due to the
great potential of CO2 as an abundant, nontoxic, easily accessi-
ble, and sustainable C1 feedstock.[2] However, only a small pro-
portion of the total abundance of CO2 is currently being con-
sumed in industry; the main reason is that establishing catalyt-
ic and economical carbon neutral processes (with a high turn-
over number of a million) is challenging due to the thermody-
namic stability and kinetic inertness of CO2. In general, a highly
reactive metal reagent or catalyst, and/or nucleophile and high
pressure/temperature are required to activate and further in-
corporate CO2 into organic compounds.[3] The chemical trans-
formation of CO2 at atmospheric pressure and room tempera-
ture remains a great challenge, though great progress has
been made. In this context, the development of efficacious
processes using CO2 as chemical feedstock under mild condi-
tions (particularly low CO2 pressure, ideally at 1 bar) could be
still highly desirable.[4]
The catalytic carboxylative cyclization of propargylic alcohols
with CO2 shows great promise for the direct incorporation of
CO2 into high-value-added chemicals; namely a-alkylidene
cyclic carbonates, which are compounds incorporating a frame-
work that is important in natural products with potential bio-
activity,[5] and have a broad range of applications as intermedi-
ates in organic synthesis.[6] To date, several metal-free catalytic
systems, such as tertiary phosphine,[7] the N-heterocyclic car-
bene (NHC)/CO2 adduct,[8] bicyclic guanidine,[9] and the N-het-
erocyclic olefin/CO2 adduct[10] have been developed for the
preparation of a-alkylidene cyclic carbonates. However, high
CO2 pressure and high reaction temperatures are usually re-
quired for an efficient reaction. In this regard, further enhanced
reactivity and selectivity could depend on transition-metal cat-
alysts (containing Pd,[11] Co,[12] Cu,[13] and Ag)[14] capable of acti-
vating the carbon–carbon triple bond of propargylic alcohols.
Unfortunately, higher CO2 pressure or additional energy is
eventually inevitable for practical syntheses, despite the fact
that, according to the literature, relatively high selectivities
have been attained with metal catalysts so far. In 2007,
Yamada et al. reported on the silver-catalyzed procedure for
the carboxylative ring-closing reaction of internal propargylic
alcohols under low CO2 pressure in the presence of an organic
base, such as 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), which
presumably acts as a promoter for the formation of alkylcar-
bonic intermediate I (Scheme 1).[14a] However, relatively large
amounts of AgOAc and stoichiometric amounts of the organic
base were still required for a smooth reaction. Notably,
Yamada et al. achieved the enantioselective incorporation of
CO2 into chiral internal propargylic alcohols catalyzed by
[a] Dr. Q.-W. Song, Dr. R. Ma, Q.-Y. Li, Y. Chang, Prof. Dr. L.-N. He
State Key Laboratory and Institute of Elemento-Organic Chemistry
Collaborative Innovation Center of Chemical Science and
Engineering (Tianjin)
Nankai University
Tianjin, 300071 (PR China)
[b] W.-Q. Chen, Prof. A. Yu
College of Chemistry
Nankai University
Tianjin, 300071 (PR China)
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