CL-131101
Received: November 25, 2013 | Accepted: December 10, 2013 | Web Released: December 14, 2013
A Catalytic and Mechanistic Investigation of Optically Active Helical
Poly[3-(9-alkylfluoren-9-yl)propylene oxide]s in the Enantioselective Addition
of Ethylmagnesium Bromide to Aldehydes
Anlin Zhang, Nianfa Yang,* Liwen Yang,* and Dan Peng
Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education,
College of Chemistry, Xiangtan University, Hunan 411105, P. R. China
(E-mail: nfyang@mail.sdu.edu.cn)
Optically active helical poly[(S)-3-(9-alkylfluoren-9-yl)pro-
good yields and with high enantioselectivities up to 88% ee. We
hold that it is the helically chiral environment rather than the
point chirality that govern the enantioselective inducement of
the addition reaction.14 Moreover, to our knowledge, there is no
report about polymer-catalyzed asymmetric addition reaction of
ethylmagnesium bromide with aldehyde. Given this, the present
study takes the lead in research on helical polyethers as chiral
ligands in the asymmetric reaction of ethylmagnesium bromide
with aldehydes.
pylene oxide]s (poly-(S)-AFPOs) without additional stereogenic
units were used to induce the enantioselective addition of
ethylmagnesium bromide to aldehydes, giving up to 53% ee of
the products. It is the helically chiral environment rather than the
point chirality that governs the enantioselective inducement
of the addition reaction. Poly-(S)-AFPOs could be recovered
simply, post-treated conveniently and used repeatedly.
We investigated the ability of poly[(S)-3-(9-alkylfluoren-9-
yl)propylene oxide]s 4 to promote asymmetric addition of
ethylmagnesium bromide to aldehydes. Under the optimal
conditions, treatment of benzaldehyde with EtMgBr in the
presence of 4a, 4b, 4c, and 4d respectively, afforded (R)-1-
phenyl-1-propanol in 57-75% yields and 17-28% ee (Table 1,
Entries 1-4). The enantioselectivity of 4a was slightly higher
than that of 4b, 4c, and 4d. Then a series of sec-alcohols with
30%-52% ee and 52%-78% yield were obtained in the
asymmetric addition of ethylmagnesium bromide to aldehydes
(Table 1, Entries 5-14). In the presence of a catalytic amount of
4a (50 mol %), the addition of EtMgBr to the representative
aromatic aldehydes proceeded in moderate yields and with
enantioselectivities in the range of 30-42% ee (Table 1, Entries
5-12). However, in the case of sterically bulky aldehydes such
as 9-anthraldehyde and 1,1,1-triphenylpropanal, it gave two
significant amounts of addition products with up to 52% ee
(Table 1, Entries 13 and 14). That is to say, the addition of
EtMgBr to sterically bulky aldehydes afforded the products in
generally higher enantioselectivity than that observed with
ordinary aldehydes.
4a could be recovered simply by pouring the reaction
mixture into methanol at the end of the reaction and separating
the precipitated polymer via filtration. The recovered 4a could
be reused for many times to induce the asymmetric addition of
EtMgBr to aldehyde without losing its enantioselective induce-
ment ability. Table 2 shows the enantioselective inducing ability
of the recovered 4a for the asymmetric addition of EtMgBr to 9-
anthraldehyde. After 4a was reused six times, the ee of the
addition product did not change obviously.
The helical conformation is inherently chiral, and right- and
left-handed helices are exactly mirror images of each other;
therefore, they cannot be superimposed.1 Accordingly, if one
of the helices could be selectively synthesized, induced, or
constructed for molecules, supramolecules, oligomers, or poly-
mers, they should be optically active without any additional
configurationally chiral components. Inspired by sophisticated
biological helices that are of key importance for their elaborate
functions in living systems involving molecular recognition2 and
catalytic activity,3 chemists have challenged to develop artificial
helical polymers, supramolecules, and oligomers with a con-
trolled handedness, not only to mimic biological helices and
functions but also for their potential applications in materials
science, such as ferroelectric liquid crystals and nonlinear
optical materials, sensing specific molecules, the separation of
enantiomers, and asymmetric catalysis.4 So far, some helical
polymers have been applied to a variety of asymmetric catalytic
reactions. Such as palladium-catalyzed allylic substitution
reaction,5 asymmetric hydrogenation reaction,6 asymmetric
hydrogen-transfer reaction,7 epoxidation of chalcone deriva-
tives,8 asymmetric hydrosilylation of styrene,9 allylation reac-
tion of benzaldehyde with allyltrichlorosilane,10 the asymmetric
Henry reaction,11 and asymmetric aldol reaction.12 These static
and dynamic helical polymeric catalysts showed enantioselec-
tivity mainly based on their helical chirality, thus producing
optically active products with a modest enantiomeric excess.
Our previous research work has shown that helical poly[3-
(9-alkylfluoren-9-yl)propylene oxide]s (poly-AFPOs)13 had the
following advantages and characteristic: a) mild synthetic
conditions and high isolated yields; b) a highly stable helical
structure, even in solution, with no racemization or denaturation;
c) modifiable side chains onto which catalytically active sites
can be introduced without affecting the helical structure; d)
additional stereogenic units and multiactive coordination sites
such as N, P, and S are not included. Most nonracemic helical
polymers are unable to fulfill all of these requirements. Recently,
our group used poly-AFPOs as chiral ligands in the addition of
methyllithium to aldehydes, giving the products in moderate to
A plausible explanation for the stereochemical outcome of
the reaction inspired by our previous research13,14 can be based
on model transition states I-III depicted at Scheme 1. I is the
virtual steric structure of 4a simulated by “Chem 3D Ultra 8.0”
software, in which the black dot is oxygen atom. We speculate
that, when the solution of 4a is mixed with EtMgBr, the oxygen-
rounding hole should hold metal cation (Mg2+). In other words,
the metal cation (Mg2+) is embedded in the hole of one-handed
helical column of the polymer, leaving the negative alkyl ion
© 2014 The Chemical Society of Japan
Zhang, Anlin
