Recyclable Helical Poly(phenyl isocyanide)-Supported l -Proline Catalyst for Direct Asymmetric Aldol Reaction in Brine
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Recyclable Helical Poly(phenyl isocyanide)‑Supported l‑Proline Catalyst for Direct Asymmetric Aldol Reaction in Brine Chonglong Li1,2 · Jihai Wang1 · Huiyun Ding1 Received: 14 July 2020 / Accepted: 21 August 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract A novel helical poly(phenyl isocyanide) bearing Boc protected l-proline pendants (poly-1m) was designed and synthesized. Removed the protecting Boc groups on the l-proline pendants led to the formation of helical polymer poly-2m, which showed high optical activity owing to the preferred right-handed helix of polyisocyanide main chain. Optically active helical poly-2m showed excellent catalytic ability on asymmetric aldol reaction. Helical polymer catalysts exhibited enhanced stereoselectivity in aldol reaction compared to small molecule l-proline. Under the optimized aldol reaction condition, the enantiomeric excess (ee) and diastereomeric ratio (dr) values of the aldol reaction product were respectively up to 90% and > 20/1. Moreover, the helical polyisocyanide catalyst Poly-2m can be easily recovered and reused in the aldol reaction for at least five cycles with maintained its activity and stereoselectivity. Graphic Abstract Enantioselective aldol reaction catalyzed by poly-2m.
C N
Ar PEt3 Pd Cl PEt3
Ar HN
O
N
Ar
n
N
TFA, CH2Cl2
o
THF, 55 C, 15 h
HN
O
N Boc
N Boc
r.t. 12 h
HN
poly-2m
Right-handed Helix
H 3
Right-handed Helix
O
O
O
R'
OH
Cat. 10 mol% Brine, -10 oC
4
O N H
Ar = H3CO
poly-1m
1
n
R' 1
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10562-020-03369-8) contains supplementary material, which is available to authorized users. Extended author information available on the last page of the article
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C. Li et al.
Keywords Helical polymer · Supported-catalyst · Asymmetric aldol reaction · Stereoselectivity
1 Introduction
chemistry and materials science, such as chiral packing materials for asymmetric catalysis [8–11], separating enantiomers in chromatography [12–15], chiral selectors [16–18], and chirality materials [19–22]. Now, many optically active helical polymers have been synthesized by directed polymerizing monomers with needed functional groups. Poly(phenyl isocyanide) is a better helical polymer because of environmental stability and wide applications [23–26]. A new type of asymmetric chiral polymer-supported catalyst can be prepared by introducing the catalytic group into the helical polyisocyanate. In this contribution, we designed and synthesized of a novel helical poly(phenyl isocyanide) bearing l-proline pendant (poly-2m). The poly-2m had good stability and expanded the scope than previously reported references [27, 28]. The polymer-supported catalyst exhibited significant enhancement on stereoselectivity in the aldol reaction compared to small molecule l-proline. The polymer-supported catalyst has the advantages of mild reaction conditions, easy separation from the reaction system, high reuse r
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