COMMUNICATION
Prins-type polymerization using ionic liquid hydrogen fluoride saltsw
Shinsuke Inagi, Yuta Doi, Yuichiro Kishi and Toshio Fuchigami*
Received (in Cambridge, UK) 10th February 2009, Accepted 13th March 2009
First published as an Advance Article on the web 3rd April 2009
DOI: 10.1039/b902713k
We successfully carried out Prins-type polymerization for the
first time to prepare 4-fluorinated tetrahydropyran-containing
polymers in a highly stereoselective manner.
to a monomer solution in CH2Cl2 at room temperature. The
reaction was quenched by the addition of water. The separated
organic layer was repeatedly washed with NaHCO3 (aq) and
water. Purification by reprecipitation into CH3OH afforded
a light-yellow powder. Table 1 summarizes the results
of polymerization conducted in various mixed solvents. In
the case of using neat HF salt (entry 1), the purified polymer
was partially soluble in CHCl3. The insoluble part probably
Recently, numerous reports on Prins cyclization have been
published because it attracts much attention as a facile
synthetic method to prepare tetrahydropyran (THP) deriva-
tives, which are components of sugar derivatives, natural
products, and so on.1 Under acidic conditions, cyclization of
homoallyic alcohols with aldehydes (and ketones) gives THP
having a substituent such as hydroxide, halide, or acetate
at the 4-position.2 Very recently, we reported a facile and
highly efficient synthesis of 4-fluorinated THP in ionic liquid
hydrogen fluoride (HF) salts (Et3NÁnHF or Et4NFÁnHF).3,4
These HF salts played roles as a non-volatile reaction medium,
a Brønsted acid, and a fluorine source in this system. In
addition, it was found that the fluorine atom was introduced
into THP with high stereoselectivity.
consists of
a higher molecular weight polymer and/or
a partially cross-linked polymer.7 On the other hand, the
addition of a co-solvent (CH2Cl2) suppressed the production
of insoluble polymer in this polymerization system. Entry 3
(CH2Cl2–Et4NFÁ5HF = 50 : 50) provided a soluble polymer
in moderate yield. The gel permeation chromatography (GPC)
analysis of polymer 2a gave a unimodal profile (Mn = 3300,
Mw/Mn = 1.7, where Mn and Mw are the number- and
weight-average molecular weight, respectively). Lower HF
ratio resulted in the decrease of the polymer product yield
(entries 4 and 5).
To the best of our knowledge, there have been no reports on
polymerization based on a Prins cyclization reaction to date.
Furthermore, only a few reports on synthesis of THP-containing
synthetic polymers by ring-opening polymerization of bicyclic
compounds5 and radical cyclopolymerization of an acrylate
ether dimer6 have been published. Herein, we wish to report
the first example of polymer synthesis via Prins cyclization
by using homoallylic alcohols having an aldehyde moiety
(its acetal form) as monomers in ionic liquid HF salts. The
resulting polymers include a 4-fluorinated THP unit in a highly
stereoselective manner through selective fluorocyclization.
Monomers (1), composed of an homoallylic alcohol and an
aldehyde moiety, were prepared with the aldehyde moiety
protected. The acetal group of the monomers would turn into
an activated aldehyde in the presence of HF salts (Scheme 1).
A steric restriction of the p-phenylene spacers between the
alcohol and the aldehyde moieties prevents an intramolecular
reaction, but favors an intermolecular reaction towards
polymerization. Then the formation of six-membered ring,
followed by nucleophilic substitution of the fluoride ion at the
4-position of THP unit leads to 4-fluorinated THP-containing
polymers. In a previous report, Prins cyclization was success-
fully performed when Et4NFÁ5HF containing a high content
of hydrogen fluoride was used as a HF salt.3 Polymerization of
the monomers was carried out by the addition of Et4NFÁ5HF
The structure of 2a was determined by 1H-, 13C- and
19F-NMR.
A comparison with the model compound,
4-fluoro-2,6-diphenyltetrahydropyran, gave information of
the fine structure of 2a. A characteristic coupled signal at
5 ppm indicated the selective introduction of a fluorine atom
onto the 4-position of the THP ring3 as shown in Fig. 1a. The
small peaks at 10–7.5 ppm are assigned to the protons derived
from a benzaldehyde at the terminal. A couple of peaks at
around À94 ppm observed in the 19F NMR spectrum (Fig. 1b)
indicates the introduction of fluorine to the 4-position of
2,6-diphenyl THP with an all-cis form.3 Elemental analysis
data also support the structure of this polymer.8 It should be
noted that the stereoselective fluorocyclization by the Prins-
type reaction successfully proceeded even in polymerization.
As a counter experiment, we investigated the Prins-type
9
polymerization in BF3ÁOEt2–CH2Cl2 without HF salts.
Although the polymerization proceeded, the 1H NMR
spectrum of the obtained polymer indicated the complex
mixture of 4-fluorinated THP and 4-hydroxylated THP.10 In
this case, undesired nucleophilic attack by the generated water
occurred, whereas the protonation of water prevents such side
reactions to afford the predominantly fluorinated product in
the case of using HF salts.3 Hence, the use of HF salt as the
acid and fluorine source was found to be the most ideal way to
produce the well-defined polymer.
Polymerization of monomer 1b having a tetrafluorobenzene
ring under the optimized conditions (Table 1, entry 3) did not
afford any polymeric products (Table 2, entry 1). Instead, the
deprotection of the acetal group in 1b was observed
solely. This result indicated that the acidity of HF salt
was not enough to start the cyclization due to the lowered
Department of Electronic Chemistry, Tokyo Institute of Technology,
4259, Nagatsuta, Midori-ku, Yokohama, 226-8502, Japan.
E-mail: fuchi@echem.titech.ac.jp; Fax: +81 45-924-5406;
Tel: +81 45-924-5406
w Electronic supplementary information (ESI) available: Experimental
section, 13C NMR spectrum of 2a and 19F NMR spectrum of 2b. See
DOI: 10.1039/b902713k
ꢀc
This journal is The Royal Society of Chemistry 2009
2932 | Chem. Commun., 2009, 2932–2934
Inagi, Shinsuke
