Pushing the Limits with Squaramide-Based Organocatalysts in Cyclic Carbonate Synthesis

Article abstract of DOI:10.1021/acscatal.7b00475

Squaramides are presented as modular, easy to optimize, and effective catalysts for the conversion of epoxides and carbon dioxide into cyclic organic carbonates (COCs). The catalytic potential of these squaramides, in combination with a suitable halide nucleophile, is particularly noted when internal epoxides are examined as substrates, and their transformation into disubstituted COCs marks a rare case of an effective organocatalyst for these challenging conversions. Control experiments support the mechanistic view that the squaramides are predominantly involved in the stabilization of intermediate oxo and carbonato anions which, after their formation, are able to displace a bromide nucleophile from an initially formed 1:1 assembly comprising the squaramide host.

Full text of DOI:10.1021/acscatal.7b00475

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Article
Pushing the Limits with Squaramide-based
Organocatalysts in Cyclic Carbonate Synthesis
Sergio Sopeña, Eddy Martin, Eduardo C. Escudero-Adán, and Arjan W. Kleij
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00475 • Publication Date (Web): 11 Apr 2017
Downloaded from http://pubs.acs.org on April 11, 2017
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ACS Catalysis
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Pushing the Limits with Squaramide-Based
Organocatalysts in Cyclic Carbonate Synthesis
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Sergio Sopeña,† Eddy Martin,† Eduardo C. Escudero-Adán† and Arjan W. Kleij*,†,‡
†Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and
Technology (BIST), Av. Països Catalans 16, 43007 – Tarragona, Spain.
‡Catalan Institute of Research and Advanced Studies (ICREA), Pg. Lluís Companys 23, 08010 –
Barcelona, Spain.
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ABSTRACT: Squaramides are presented as modular, easy to optimize and effective catalysts
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for the conversion of epoxides and carbon dioxide into cyclic organic carbonates (COCs). The
catalytic potential of these squaramides, in combination with a suitable halide nucleophile, is
particularly noted when internal epoxides are examined as substrates and their transformation
into disubstituted COCs marks a rare case of an effective organocatalyst for these challenging
conversions. Control experiments support the mechanistic view that the squaramides are
predominantly involved in the stabilization of intermediate oxo- and carbonato-anions which,
after their formation, are able to displace a bromide nucleophile from an initially formed 1:1
assembly comprising the squaramide host.
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Keywords: carbon dioxide, cyclic carbonates, homogenous catalysis, organocatalysis,
squaramides
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ACS Catalysis
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ꢀ INTRODUCTION
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Small molecule activation,1 and particularly carbon dioxide (CO2) conversion, continues to
challenge the synthetic communities to devise more efficient and sustainable catalysis protocols.
CO2 is a renewable carbon feed stock that has shown potential to provide an alternative for some
fossil fuel based chemical synthesis,2 though generally there still exists a requirement for new
catalyst development to expand on the portfolio of organic compounds that are derived from this
waste molecule.3 Despite the high kinetic stability, catalysis has manifested itself as the primary
technology for the conversion and fixation of CO2. Among the most widely studied reactions that
utilize CO2 as a substrate are those that lead to either cyclic4 or poly-carbonates,5 using cyclic
ethers as reaction partners. Both categories of these organic carbonates have attracted interest
from academic and industrial communities, and various commercial processes have now been
developed.6
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The synthesis of cyclic organic carbonates (COCs) has witnessed a spectacular progress over
the last decade with the field being clearly dominated by metal-catalyzed approaches.7 Indeed,
homogeneous metal catalysis has demonstrated to solve a series of important challenges in the
area including the use more challenging cyclic ether substrates such as internal di-8 and
trisubstituted epoxides,9 and oxetanes.10 Recently, metal-free approaches have been presented as
sustainable alternatives for metal-catalyzed formation of COCs.11 Intrinsically, organocatalytic
activation of the cyclic ether substrates is less powerful compared to metal-based conversions: as
a result, poorer overall kinetics and substrate scope are typically observed requiring thus
significantly higher catalyst loading and/or reaction temperatures. Among the substrate
activation strategies, hydrogen-bonding (HB) has been prevalent and has delivered a simple
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