16 Nucleophilic Polymerization of Heterocycles ALFRED KREUTZBERGER Scientific Laboratory, Ford Motor Co., Dearborn, Mich.
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Investigations aimed at employing properties of
the unique
heterocycles to incorporate
such
rings into polymeric chains are reported.
The
most prominent
characteristic
of
an
aromatic
heterocycle of the pyridine type is the tendency of the hetero atom to withdraw π-electron density from the ring.
Particularly, the C atoms adjacent
to the hetero atom become thereby positively charged and thus represent the sites of attack by nucleophilic agents.
The principle of nucleophilic
polymerization of heterocycles is applied in the use of bifunctional nucleophiles.
A romatic heterocyclic rings show a great many similarities i n behavior to carbocyclic compounds. Particularly marked is the pronounced stability of nuclei of both systems during many chemical reactions. In addition to these similarities, the presence of hetero atoms entails certain behavior differences typical of hetero cyclic systems. W h i l e the chemistry of aromatic carbocyclic systems w i t h regard to formation of polymers is fairly w e l l established, relatively little is known about the suitability of heterocyclic rings for polymer formation. Thus, it has been the purpose of these investigations to evaluate typical heterocyclic behavior w i t h respect to the possibilities of incorporating heterocyclic rings directly into polymeric chains. E x c l u d e d from these considerations have been such heterocyclic systems i n w h i c h substituents attached to the ring are instrumental i n polymer formation. Examples of this type are resin formation from melamine w h i c h is based essentially on the chemistry of aromatic amino groups, or the synthesis of fibers from vinylpyridines typifying the chemistry of ethylenic double bonds. In contrast to these examples, the present discussion is restricted to the hetero cyclic r i n g as such. Depending on the nature of the attack on an aromatic nucleus, reagents are classified as electrophilic, nucleophilic, or radical. T h e fact that most chemists associate the term "aromatic substitution" w i t h nitration, sulfonation, or azo coupling of the benzene ring demonstrates the predominance of the electrophilic substitution type i n the carbocyclic series. However, the presence of a hetero atom i n the ring changes the electron distribution i n such a manner that nucleo philic substitutions gain more importance. That this situation is identical w i t h the effect exercised b y certain substituents attached to a carbocyclic system can 208 PLATZER; POLYMERIZATION AND POLYCONDENSATION PROCESSES Advances in Chemistry; American Chemical Society: Washington, DC, 1962.
KREUTZBERGER
Nucleophylic Polymerization of Heterocycles
209
be demonstrated b y a comparison of two concrete cases. T h e chlorine atom i n chlorobenzene ( I ) , under ordinary conditions, cannot be replaced b y the amino group when ammonia is used as a typical nucleophilic agent. N o t until very rigid conditions are applied w i l l this reaction proceed ( I ) , (Equation 1 ) . 25'γ Aqueous N H ,
r ^ ^ h
a
CI
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1
6 12 hours, 180-200° C., steel autoclave, presence of copper compounds
[|
J
(1) NH2
**
T h e chlorine atom i n I, however, can be activated b y the introduction of a nitro group into the same ring, thereby considerably facilitating the substitution of C I in II b y N H (22) ( E q u a t i o n 2 ) . 2
Saturated alcoholic N H , 3
5 hours, 100° C , presence of Κ I, no autoclave necessary
CU-H
^i—
Τ
N0
NH
2
In the light of modern electronic theory, the nitro group i n I I has made the molecule accessible to nucleophilic attack due to its negative resonance effect (-R effect). It thus withdraws ττ-electron density from the ring so as to impart alternate charges on the conjugated system. Ortho a n d para positions thereby become amenable to attack b y nucleophilic agents. This situation is represented by structure III w i t h respect to the ortho position.
T h e indicated electronic shift w o u l d result i n a species ( I V ) containing a positively charged ortho position w h i c h w o u l d n o w accept the unshared pair of electrons from the attacking nucleophilic agent, the amine, to form the transition state ( V ) . T h e latter finally eliminates one mole of hydrogen chloride w i t h forma tion of the end product ( V I ) ( E q u a t i o n 3). Η I ΙΪΝΓ—R Θ
Η Cl
-HCl
Ν
ο
xQ e IV N
There exists a striking analogy i n the behavior of carbocyclic nitro compounds and aromatic heterocycles and it has therefore been concluded that the hetero atom—e.g., nitrogen i n pyridine—effects a similar electron displacement as does the nitro group i n nitrobenzene. I n the heterocyclic series, the structure parallel to o-chloronitrobenzene ( I I I ) w o u l d be that of 2-chloropyridine ( V I I ) . PLATZER; POLYMERIZATION AND POLYCONDENSATION PROCESSES Advances in Chemistry; American Chemical Society: Washington, DC, 1962.
210
ADVANCES IN CHEMISTRY SERIES
Clilci Ν VII
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That pyridine species with decreased electron densities at positions adjacent to the nitrogen atom make significant contributions to the resonance h y b r i d even i n the ground state has been shown b y dipole measurements (12, 14). Interaction w i t h a nucleophilic agent is consequently expected to follow a mechanism analogous to that involving o-chloronitrobenzene ( E q u a t i o n 4 ) . Η I
iN-R 1
Η „
Ν θ
H®
I
-CI
-HCl
—
ίί-ΛΓΗ Ν \ρι Θ
VIII
I
V
Ν
IX
« N—R
(4)
Χ
T h e hetero ring atom i n 2-chloropyridine ( V I I ) thus facilitates the reaction w i t h ammonia i n a manner similar to that of the nitro group i n o-chloronitroben zene (II) (6) ( E q u a t i o n 5 ) . Anhydrous N H 3 , ff^l
5 hours, 2 2 0 ° C ,
Π
presence of Z n C l , no autoclave necessary^ 2
fvj'
I
N
(5)
R
]S
VIII In principle, the activating power of a hetero atom equals approximately that of a nitro group (13). A n increase of hetero atoms i n one a n d the same ring is ex pected to facilitate nucleophilic substitution even more. Replacement of V I I b y a 2,6-dihalopyridine a n d of N H b y a bifunctional nucleophilic agent i n E q u a t i o n 5 represents the basis for a general nucleophilic polymerization process. It is obvious, however, that hydrogen halide as a b y product is disadvantageous i n polymer formation because of its potential tendency to cleave bonds again. T h e nucleophilic substitution mechanism provides a bypass to this situation. A s can be seen from Equations 3 a n d 4, chlorine leaves the transition states ( V a n d I X ) , respectively, as an anion. T h u s , the problem w o u l d be to attach to the ring a suitable anion other than chlorine w h i c h , upon leaving the ring, w o u l d combine w i t h a proton to form an inert entity. 3
T h e trichloromethyl group appears to meet these requirements. T h i s group is k n o w n to leave as an anion (2) i n nucleophilic substitution reactions involving compounds of the type of chloral, trichloroacetic acid, a n d trichloroacetone, a n d subsequently to combine w i t h a proton to form chloroform. T w o fundamental experiments demonstrate clearly the activation of the C C 1 group b y the hetero ring atom. W h i l e there is no reaction when ammonia is 3
a
+ N H gas, room temperature, 3
CCI3
VIII
+NH ι
Ν
CC1
IX
U ^ 1 _ _
(6) N
H
no CCI3 cleavage
N^^N 11
II
4 hours
3
3
N ^ N
gas,
r o o m temperature,
11
2 hours
Il
^
*
ι
H\
J — N H
Ν
2
χ
PLATZER; POLYMERIZATION AND POLYCONDENSATION PROCESSES Advances in Chemistry; American Chemical Society: Washington, DC, 1962.
K
)
KREUTZBERGER
211
Nucleophylic Polymerization of Heterocycles
passed into α,α,α-trichlorotoluene at room temperature ( E q u a t i o n 6 ) , facile nucleophilic attack occurs o n 2-trichloromethyl-s-triazine under the same condi tions ( E q u a t i o n 7 ) . 2-Trichloromethyl-s-triazine ( I X ) also appeared to be a suitable model com pound for substitution studies w i t h aqueous ammonia. However, the surprising result was obtained that I X undergoes ring cleavage rather than substitution. Concentrated as w e l l as dilute aqueous ammonia cleaves the r i n g i n I X at room temperature to form trichloroacetamide ( E q u a t i o n 8 ) . E v e n water alone brings about this ring cleavage, but the reaction time i n this case is about 1 week.
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N ^ N
H N
+ 5 % Aqueous N H ,
2
3
ix
°xi
This tendency to ring cleavage seems to be limited to the u n - a n d monosubstituted s-triazine ring, for trisubstituted s-triazine derivatives are stable under the same conditions. Moreover, they are amenable to nucleophilic attack b y aqueous amines, when carrying suitable substituents—e.g., the trichloromethyl group. T h e first two substituents i n 2,4,6-tris( trichloromethyl )-s-triazine ( X I I ) may thus be replaced rather easily by the N H a n d N H C H groups through the action of aqueous ammonia or aqueous methylamine, respectively ( E q u a t i o n 9) (20). 2
ÇC1
ÇCI3
3
X T
Ν
^L. N
3
coned, aqueous N H , room temperature
T
3
X
C l C - i l J~-CCk
ÇCI3
1 N ^ N
3
J-NHR
' C1,C^
1 N ^ N
Aqueous N H ,
Ν
aqueous C H N H , 3
2
Ν R =HorCH
°
RHN—II
^L-NHR Ν XII room temperature VC* " XIII R = HorCH However, the attempt to replace the last C C 1 group i n X I I by the same reaction resulted i n displacement b y an O H group ( E q u a t i o n 1 0 ) . or
3
1 2 Q
o
>
A
r
r
u
w
( 9 )
u
3
3
3
CC1 I
3
N ^ N h
OH I
Aqueous N H , 6 to 8 hours, 120° C .
3
N ^ M
o r
I
C1 C—^ ^ — C C I 3 Ν XII
aqueous C H N H , 3
j{
A
2
(
m
RHN—< & NHR * Ν XIV R = H or C H This reaction clearly shows the competition between two nucleophiles, amine and water. Furthermore, it suggests that under certain conditions other suitable O H - c o n t a i n i n g nucleophilic agents m a y attack the ring at the C C l - b o n d e d site. O n the other hand, it is to be concluded that substitution b y N H R of the last C C 1 group on the ring can be accomplished only i n OH-free media. This idea has been verified b y the use of solvents like chloroform, dioxane, acetonitrile, a n d Ν,Ν-dimethylformamide ( D M F ) (Equation 11) ( J O ) . 3
several hours, 120° C .
3
3
3
CCI
3
Ν
C1C—IL 3
Gaseous C H N H 3
J—CC1
Ν XII
NHR
Gaseous N H in D M F , 4 hours, 1 6 5 ° C .
3
3
2
2.5 hours, 165° C .
in D M F ,
Ν
V
N
^ RHN—^_.
