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Scheme 2 Plausible mechanism for production of cyclic carbonates from
olefins and NaHCO3.
Notably, disubstituted olefins were also amenable substrates for
this transformation with good product yield (2q, 2r). In some
cases, it was necessary to use DMF–H2O as the solvent instead of
acetone–H2O to improve the reaction efficiency (2f, 2p). All the
products were converted into the corresponding carbonates with
no or little epoxide formation, indicating the high versatility of the
microwave-assisted transformation utilizing sodium bicarbonate
as the C1 source.
A plausible mechanism was proposed based on the product
distribution study (Scheme 2). Bromohydrin 3a was generated first
with stoichiometric NBS and olefins followed by deprotonation of
the hydroxyl group to deliver anion 5a and the in situ generation of
CO2. The subsequent intramolecular cyclization of 5a (path b)
is more favoured than the intermolecular cycloaddition with CO2
(path a), which resulted in the formation of a significant amount of
epoxide 4a within a relatively short reaction period. Two reaction
pathways may contribute to the conversion of epoxide 4a to cyclic
carbonate 2a including regeneration of intermediate 5a (path c) and
direct transformation to achieve cyclic product 2a (path d).9a
An efficient microwave-assisted one-pot synthesis of cyclic
carbonates starting from olefins has been achieved with a wide
substrate scope. Compared to conventional heating methods,
microwave heating provides remarkably better selectivities and
yields of desired products. NaHCO3 proved to be an excellent
substitute for CO2 gas, thus avoiding the need for use of
compressed CO2. This is convenient for lab scale experiments
and provides a potentially viable approach for exploiting microwave
heating at the industrial scale.15 This method is also well poised
toward green processing due to the use of environmentally friendly
solvents such as the acetone–water mixtures.
Scheme 1 Substrate scope.
optimized conditions, the starting olefins were converted com-
pletely into bromohydrins within 10 minutes. As the reaction
proceeded further, these bromohydrins were converted to epoxides
and cyclic carbonates, and epoxides themselves were transformed
to cyclic carbonates at the same time. Under microwave irradiation,
the maximum epoxide yield was obtained after approximately 1
hour, and the concentration decreased thereafter. Eventually, all
epoxide intermediates were converted to cyclic carbonate products
in the microwave reactor. Evidently, microwave irradiation
accelerated the transformation of epoxides to carbonates, probably
due to ‘‘hot spots’’ generated by enhanced microwave absorption at
the surfaces of the insoluble NaHCO3 particles.6b,7
This project was supported by the DOE (DE-FOA-0000253),
the National Science Foundation of China (No. 61372043) and
973 Programs (No. 2013CB328905).
Using the optimal conditions in the microwave apparatus, we
investigated olefins bearing different functional groups (Scheme 1).
All electron-rich aromatic olefins (2c, 2d, 2i), electron-deficient
aromatic olefins (2e–2h) and terminal aliphatic olefins (2j–2p) were
converted to the corresponding cyclic carbonates in moderate to
good yields. The microwave-assisted conditions not only tolerate
functionalities such as halide, trifluoromethyl, ether, nitrile, and
ester, but also allowed for conversion of even more challenging
substrates such as aldehyde, ketone, and alcohol that had potential
to undergo cyclization under basic conditions (2h, 2m–2o).
Notes and references
1 (a) R. N. Gedye, F. Smith and K. Westaway, Tetrahedron Lett., 1986,
27, 279; (b) R. J. Giguere, T. L. Bray, S. M. Duncan and G. Majetich,
Tetrahedron Lett., 1986, 27, 4945.
2 (a) V. Polshettiwar and R. S. Varma, Chem. Soc. Rev., 2008, 37, 1546;
(b) C. O. Kappe, Chem. Soc. Rev., 2008, 37, 1127; (c) J. J. Kiddle, J. Am.
Chem. Soc., 2006, 128, 1771; (d) A. David, Nature, 2003, 421, 571.
3 (a) Microwave Assisted Organic Synthesis, ed. D. M. P. Mingos, Black-
well Publishing, Oxford, 2005; (b) Microwaves in Organic Synthesis,
ed. A. Loupy, Wiley-VCH Verlag GmbH, Weinheim, 2nd edn., 2006;
(c) Microwaves in Organic and Medicinal Chemistry, ed. C. O. Kappe
and A. Stadler, Wiley-VCH Verlag GmbH, Weinheim, 2005.
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Chem. Commun., 2014, 50, 3245--3248 | 3247