Pinacol rearrangement in the polymer backbone: A new class of reactive polymers with condensed benzopinacol units in the main chain

Article abstract of DOI:10.1016/S0040-4039(99)02309-6

Novel reactive polymers based on condensed benzopinacol dimethyl ether moieties in the main chain were synthesized and their acid-catalyzed pinacol rearrangement in the polymer backbone proceeded to afford poly(benzopinacolone)s quantitatively with elimination of methanol. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(99)02309-6

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 1433–1437
Pinacol rearrangement in the polymer backbone: a new class of
reactive polymers with condensed benzopinacol units in the main
chain
Hideyuki Otsuka, Iji Onozuka and Takeshi Endo ∗
Research Laboratory of Resources Utilization, Tokyo Institute of Technology, Nagatsuta-cho, Midori-ku, Yokohama 226-8503,
Japan
Received 4 November 1999; revised 1 December 1999; accepted 3 December 1999
Abstract
Novel reactive polymers based on condensed benzopinacol dimethyl ether moieties in the main chain were
synthesized and their acid-catalyzed pinacol rearrangement in the polymer backbone proceeded to afford
poly(benzopinacolone)s quantitatively with elimination of methanol. © 2000 Elsevier Science Ltd. All rights
reserved.
Keywords: polymers; rearrangements; naphthalenes.
Tetraarylpinacol derivatives have been of interest owing to their potential physical properties and
reactivities based on their central carbon–carbon bonds, diols and/or aromatic rings. For instance, it
has been known that tetraarylpinacols undergo thermal1 and photolytic dissociation.2 It is also well
known that benzopinacol derivatives (1) work as ‘iniferter’ for controlled radical polymerisation.3
Focusing on diols in benzopinacol, it was found that oxidation of benzopinacol gives two molecules of
benzophenone.4 On the other hand, Olah and co-workers have recently reported that various benzopinacol
derivatives undergo pinacol rearrangement or convert to condensed aromatics, and the proportion of the
products is dependent on the added acids.5
Since the parent benzopinacol is not so stable under basic conditions, an alternative condensed
benzopinacol such as 1,2-diaryl-1,2-acenaphthenediol (2, R=H) is useful for modification due to its high
stability against a strong base. In fact, in the case of condensed benzopinacol, it is possible to introduce
substituents into the hydroxyl groups by using alkali metal hydride.
∗
Corresponding author. Tel: +81-45-924-5244; fax: +81-45-924-5279; e-mail: tendo@res.titech.ac.jp (T. Endo)
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(99)02309-6
tetl 16237

1434
Since condensed benzopinacols can be regarded as precursors for various functional systems, it may
be possible to construct novel reactive polymers by incorporating such units into the polymer chain.
In other words, various reactions can take place in the polymer backbone if condensed benzopinacol
moieties are introduced into the polymer chain.6 To realize a novel reactive polymer system based
on benzopinacol in the backbone, we designed polymers containing 1,2-diaryl-1,2-acenaphthenediol
moieties in the main chain by polycondensation of a bisphenol-type monomer with bifunctional acyl
chlorides. Among various reactions of benzopinacol derivatives, pinacol rearrangement is known to
proceed as an intramolecular reaction.7 Here, we preliminarily communicate the synthesis of polymers
containing condensed benzopinacol and their complete rearrangement to poly(benzopinacolone).
The bisphenol-type monomer 5 was prepared according to Scheme 1. Acenaphthenequinone was
treated with two equivalents of a Grignard reagent derived from p-bromophenylbenzyl ether to afford
the benzopinacol derivative 3. Diol 3 was methylated by methyl iodide under basic conditions and
elimination of the benzyl groups was carried out using hydrogen catalyzed by palladium–carbon to give
5 in 94% yield. Since the second nucleophilic attack of the Grignard reagent to the diketone moiety in
acenaphthenequinone occurs from the opposite side of the first aromatic ring, the obtained monomer 5
has ‘anti’ orientation.
Scheme 1.
Polyesters 74–78 were obtained by the interfacial polycondensation of the obtained monomer 5 with
alkyl diacyl chlorides 6n in the presence of quaternary ammonium salts as a phase-transfer catalyst
(Scheme 2, Table 1). The polycondensation proceeded after mixing the monomers at room temperature
within several minutes to afford white fibrous flakes of polyester (74–78). The polymers 74–78 were
successfully obtained from both dispersions and the homogeneous solutions of reagents in good yields.
Gel permeation chromatographical (GPC, HLC-8020) analyses indicated that the obtained polymers
have the number–average molecular weight (Mn) of approximately 10 000 and the molecular weight
distribution (Mw/Mn) of 1.36–1.44.
Scheme 2.
The structure of polymers 7n was supported by their NMR (JEOL Lambda-500) and IR (JASCO
FT/IR-5300) spectra. Fig. 1A illustrates the representative 1H NMR spectrum of polymer 74. The signals

1435
Table 1
Polycondensation of monomer 5 with diacyl chloridesa
assigned to the naphthalene protons are observed at 7.37, 7.57 and 7.86 ppm, by doublet, triplet and
doublet, respectively. As shown in Fig. 2A, the IR spectrum of 74 has an absorption signal at 1753 cm−1
which is assignable to C_O group.
Fig. 1. Partial 1H NMR spectra of polymers 74 and 104. Asterisks (*) mark bands of end groups
As a model compound for pinacol rearrangement in the polymer backbone, 8 was treated with
trifluoroacetic acid (TFA) in benzene. Although the hydroxyl groups (pinacol) in 8 were methylated,
the reaction proceeded to give 9 quantitatively after 30 min accompanied with elimination of methanol
(Scheme 3).
Under the same conditions, pinacol rearrangement of prepolymers 74–78 proceeded completely
(Scheme 4, Table 2) to give the corresponding polymers with an Mn of approximately 10 000 and
Mw/Mn of 1.36–1.43 without the formation of the low-molecular weight products. In all cases, no
significant change in Mn and Mw/Mn was observed through the rearrangement. The structure of the

1436
Fig. 2. Partial IR spectra of polymers 74 and 104
Scheme 3.
obtained poly(benzopinacolone)s 104–108 was determined by spectroscopic methods. Fig. 1B illustrates
1
the representative H NMR spectrum of polymer 104. The signals assigned to the naphthalene protons
are all split because of dissymmetry, compared with its prepolymer 74. In the 13C NMR spectrum,
the generated carbonyl carbon (pinacolone) is observed at 203.36 ppm. As shown in Fig. 2B, the IR
spectrum of 104 has absorption signals at 1755 cm−1 and 1721 cm−1 assignable to C_O stretching not
only in ester but also in benzopinacolone. These spectroscopic data clearly reveal that acid-catalyzed
pinacol rearrangement in the polymer backbone proceed to give poly(benzopinacolone) quantitatively
with elimination of methanol.
In summary, we have demonstrated that novel reactive polymers based on condensed benzopi-
nacol dimethyl ether moieties in the main chain were synthesized and their reactions leading to
poly(benzopinacolone) proceeded completely. Since various reactions of benzopinacol have been report-

1437
Scheme 4.
Table 2
Pinacol rearrangement of polymers 7n
a
ed so far, the poly(benzopinacol)s are useful not only as acid-responsive polymers but also as synthetic
intermediates (reactive polymers). Further studies are currently in progress in this laboratory.
References
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