discotics
Pizzanes are
discotics with an aromatic core like benzene and naphthalene. They were
discovered at the
SATOCONOR.COM
J.G. van der Galiën ‘Coupling Pizzane Mesogens On
Poly(acryloylchloride)’ 2.1. (2003)
Proposal
SATOCONOR.COM Journal of Organic Chemistry
Coupling Pizzane Mesogens On Poly(acryloylchloride)3
Two possible
methods for obtaining new liquid crystalline polymers for use in LCD’s and
LED’s
By Johan Gerard van der Galiën
For comments e-mail: johan.van.der.galien@satoconor.com
Version 1.2. December 11, 2003 (Version 1.0. from April 22, 2003)
Introduction
It should be
possible to couple certain discotic liquid crystals, discovered at the
Method 1:
The ten-step 11-aminoundecanoic acid route
I will
begin with the synthesis of the spacer molecule. This is a three-step process
that starts with the commercial available 11-aminoundecanoic acid (1a.),
which gives a spacer molecule of the same length Kouwer used. As a matter of
fact 1a. is used on large industrial scale for the production of
nylon-11 (by a condensation reaction at high temperature and pressure). So it
should be readily available.
Because
protection of the amino group is needed, as I will explain later, this compound
is turned in to N-maleonyl-11-aminoundecanoic acid (2a.) by a
published reaction (90% yield) of 1a. with 7-exo-oxohimic
anhydride.4 This cheap bicyclic reagent can be easily obtained from
an exo-Diels-Alder addition between furan and maleic anhydride.4
The alkene double bond of compound 2a. needs to be hydrogenated to 2b. by means
of a rhodium-complex as a catalyst. I presume this hydrogenation reaction does
not affect the conjugated carbonyl groups of the N-maleonyl moiety and the
carboxylic functionality because the rhodium-catalyst specifically targets an
alkene double bond. The hydrogenation of the double bond is necessary because
there is possibility that the corresponding acylchloride moiety, with I will
prepare later on, will attack it. Though normally this only happens in the
presence of peroxides. But to make matters worse the alkene double bond of the
maleonyl moiety would then certainly give polymerization and cyclisation under
the Friedel-Crafts condition I will use later on. The next step is to
functionalise the carboxylic acid moiety of 2b., so it can be coupled to
the central ring of the mesogen precursor, with thionylchloride to an
acylchloride (3a.). This is the reason why protection of the amino group
is required, as I mentioned earlier. Without protection the amino-acylchloride
would undergo polymerization and cyclisation!
The next
step is to couple the spacer molecule (3a.) on to the mesogen precursor:
The 1,2,3,4,5,6,7-heptamethylnaphthalene (4a.) central ring moiety. The
mesogen precursor 4a. can be synthesized according to a literature
procedure.5 This coupling is done by Friedel-Crafts acylation.
After the
coupling I can get rid off the imide protective group because it is no longer
necessary. This can be done by base or acid catalyzed hydrolysis. If 5a.
is a hard nut to crack then there is always nitrous acid treatment to yield the
free amine 6a..
The methyl
groups need to be functionalized for the coupling of the p-alkoxyphenoxymethyl
side chains to make the mesogen. This functionalisation is done by free radical
bromation. But this would also certainly attack the α positions of the
carbonyl moiety of 6a.. Because there is a high degree of keto-enol
tautomerisation to be expected from a carbonyl adjacent to an aromatic ring!
And this can lead to a Hell-Volhard-Zellinski kind of side reaction
whereby the α-positions are also bromated. Thus the α-positions need
to be protected this can be done by transforming the carbonyl to a ketal with
hydrochloric acid and trimethyl orthoformate (a commercial available reagent)
yielding amino ketal 7a..
I chose
for trimethyl and not for triethyl formate because the carbonyl could be to
sterically hindered, being adjacent to an aromatic ring with al those methyl
groups. As a matter of fact steric hindrance could be no problem at all because
there is only one methyl group in the direct neighborhood (two ring carbon
atoms away). The next nearest is three ring carbons away because of the ring
junction! Maybe this reaction stops at the hemiacetal, but if this can be
isolated I can use this as well. The bromation then needs to be conducted under
severe moist free circumstances under nitrogen atmosphere. Either way with
ketal or hemiacetal the keto-enol tautomerisation is no longer a problem
because it has stopped. But about the bromation reaction there is another side
reaction to consider the bromation of the methylene groups of the spacer
moiety.
But it
should be possible in my opinion to selectively monobromate, in a free radical
reaction, which can be done with several reagents, the methyl-groups of 7a.
to monobromomethyl groups (8a.) and not affect the methyleen-groups of
the spacer-moiety of 7a. because:
·
Methyleen-groups in propane have a hydrogen
abstraction ratio of 220 (the ratio of the methyl groups in ethane is set to
For steric reasons I suspect that no
dibromomethyl and tribromomethyl groups will form. This is supported by the fact
that Kok et al. did monobromation of the methylgroup of 4a. with Br2/CCl4
under UV-illumination. (Severe free radical conditions!).1
After the
bromation the ketal group of 8a. can be removed yielding mesogen
precursor 9a.. This can be done by acid catalyzed alcoholysis. I do not
think that solubility will be a problem in this step, one can always use
mixtures of solvents with for instance methanol.
The next
step is attaching the seven p-alkoxyphenoxy side chains. This is more or less a
literature procedure because Kok et al. have performed it on
1,2,3,4,5,6,7-heptabromomethylnaphthalene.1 (An analogue of 9a..)
It would of course be interesting to know whether the obtained 10a. is
also a liquid crystal for p-alkoxy is p-hexyloxy en p-heptyloxy. If this is the
case then most likely the on the polymer coupled 10a. will also be
mesogen side chains.
The free
amine mesogen can then be coupled to poly(acryloylchloride) giving the
structure 11a.. This coupling reaction with an amine is not described by
Kouwer.2 He uses mixtures of alcohols (methanol and one or two
different hydroxy-mesogens) and only achieves 95% reaction of the pendant
acylchloride groups of poly(acryloylchloride). Maybe it is a good idea to use
mixtures of amines because this reaction is highly exothermic (Thus must have a
very great enthalpy and should go for nearly 100%.)
Method 2:
The five-step 1,12-diaminododecane route
The first
step is to hydroxylate the mesogen precursor
1,2,3,4,5,6,7-heptamethylnaphthalene (1c.) with trifluorperacetic acid
in an aromatic electrophilic substitution reaction. This mono hydroxylation
should go in high yield because of the seven methyl groups the activated
naphthol (2c.), through the introduction of one hydroxy group, is not
susceptible for further attack. Aromatic substrates with no or less alkyl
groups give mixtures poly hydroxy products.
2c. is then
ready for the coupling with the commercial available spacer molecule
1,12-diaminododecane (3c.) by means of a Bucherer reaction. This
aromatic nucleophilic substitution reaction with sodium bisulfite, should be
conducted diluted and with an excess of 3c. whereby the 2c. is
added gradually to the reaction mixture. This because of suppressing the side
reaction that the secundair amine product (4c.) couples again with 2c..
From this point
on the route is essentially the same as Method 1. Thus bromation followed by
coupling the p-alkoxyphenoxy side chains and the coupling on to the polymer. Of
course in this scheme there is no need of protecting α positions of the
carbonyl during bromation as a ketal! About the aliphatic nucleophilic
substitution can there also be a competitive aromatic nucleophilic substitution
of the diaminododecane moiety? I do not think so because R-NH- is a
very poor leaving group, Ar-O- is only an moderate nucleophile, the
naphthalene ring is not activated for aromatic nucleophilic substitution and
also the Ar-NH-R bond is severely sterically hindered.
-o0o-
1)
Kok D.M., Wynberg H. ‘New discotic benzene and naphthalene derivatives with
columnar mesophases’ Mol. Cryst. Liq. Cryst 129 53-60 (1985)
2)
Kouwer P.H.J. ‘Mesophase formation in discotic liquid crystalline polymers’ Dissertation
TU Delft (2002) Can be downloaded from the internet as .pdf file for
Acrobat Reader.
3)
Chemistry from: March J. ‘Advanced organic chemistry: Reactions, mechanisms and
structure’ Third Edition, Wiley Interscience (1984) and Ternay A.L. jr.
‘Contemporary Organic Chemistry’ Second Edition, WB Saunders Company (1979)
4)
Ondruš V., Fišera L., Bradac V. ‘On the use of water as a solvent – simple and
short one-step synthesis of maleimides’ Published on the internet.
5)
Krysin A.P., Bodoev N.V., Koptyug V.A. Russ. J. Org. Chem. 13
1290 (1977)