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Unimolecular and bimolecular transfer of N-substituents from pyridinium cations: Evidence for a clear mechanistic changeover
Tetrahedron Letters Vol.21, pp2697-2699 Q ~ergamon Press Ltd. 1980. Printed in Great Britain
UNIMOLECUI&R AND BIMOLECULAR TRANSFER OF N-SUBSTITUENTS FROM PYRIDINIUM CATIONS: EVIDENCE FOR A CLEAR MECHANISTIC CI-IANGEOVER Alan R. Katritzky,* School
Giuseppe
of Chemical
Musumarra, Kumars Sakizadeh, and Bratislav Jovanovic
Sciences,
University
Sayed M. M. El-Shafie,
of East Anglia,
Norwich,
NR4
7TJ.
in 2,4, 6-triphenylpyridiniums are transferred to piperidine, Summary -N-Substituents morpholine and pyridine by unimolecular and/or bimolecular processes in chlorobenzene These processes are quite distinct and afford no evidence for a mechanism solution. 1 and intermediate between % 5N2* l-Benzyl-2,4,6_triphenylpyridinium
perchlorate react6 with piperidine 1 The reaction was studied and 1 -benzylpiperidine.
2,4,6 -triphenylpyridlne
The second order
molar).
was not affected
significantly
At 100°
BF4-.
sulphoxide
The reactions
the rate on piperidine For the benzyl derivatives,
the reaction
order
component,
[piperidine].
40’ as the reaction expected:
of -ca
50 as the solvent
of N_-Substituted-Z,
(Fig.
l),
clearly
reaction
This
Some
proceeds
derivatives also occurs
also applies
to determine
of the resuIts
SN1 processes
Table
1.
almost (Figs,
entirely
2,3),
by a first
the kinetic
allyl,
dependence
order
order
process,
Me ) Bet -l
&I x lo5 (s&I) 2 Measurements
0.60
independent
second
of
analogue which was studied at The pattern
by SN2, while for secondary
to 40’
CH2CH:CH2
for
I 4. 04
1.26
(0.5
(0.1 refer
1-4.
process.
while there is a significant
to the I-p-methoxybenzyl
of
revealed
ie that
alkyl and e-methoxy-
are also important.
N_-Subst. -1
chloro-
with
First and second-order rate constants for reactions of l-substituted triphenylpyridiniums with piperidine in chlorabenzene at 1000 a
(1 mole
to
and p-methylbenzyl
by a second
p-substituted
!X2 x lo3
from
are shown in Figures
and also for the methyl,
alkyl and benzyl react
benzyl
varied
C104-
4,6-tripheoylpyridiniums
was very fast at higher temperatures.
primary
se?)
or by changing the anion from
were studied at 100’
and s-butyl
lf2 = 4, 94 x 10 -3 (1 molerl
and n-pentanol.
concentration.
compound
For the i-propyl
by a factor
of a series
in chlorobenzene
rate constants
by ionic strength’
the rate varied
benzene to dimethyl
piperidine
kinetically
at 100’ it shdwed first order dependence on SUbBtrate (over solution: -3 -3 - 10 molar) and first order dependence on piperidine (over range 10
in chlorobenzene -5 range 3x10 2 x 10-l
to give
(0.3
p-methoxybenzyl 2697
6.52 (0.3
derivative.
2,4,6-
henzyl Me0
CHMe2
CHMeEt
0.45
0. 14
0. 10
3.6
0.89
3.20
2698
First lc2 values
and second
increase
p-MeC,H,CH,
order
rate
in the series
constants
i-Pr
< s-Bu
< p-MaQC,H,CH,.
e-CH3C6H4CH2
are
The
not significantly
with
benzyl
derivative,
values
decreased
decreasing
SNl
for
nucleophiles
and SN2 reactions
produced
both with
SN2 reaction i-propyl,
i-propyl,
However,
s-butyl
in place
2).
for
i-propyl
that high
of e. g.
and with
pyridine.
reaction;
used,
s-Butyl
SN1 and SN2 reactions
the &2 of
the
the k2 values
values
l$
and
were
and p-methoxybenzyl supports
gave
independent
mechanism.
yields
These
of the expected
substitution
present
results
show that the bimolecular
derivatives.
We interpret
the corresponding
products
tetrafluoroborate
by unimoleculsr
between
For the
order
whereas
is accompanied
and p-methoxybenzyl
and
of piperidine.
l-i-propyl-2,4,6_triphenylpyridinium
3
benzyl
both unimolecular
which strongly
without an “intermediate”
<
and p-methoxybenzyl
in the expected
of the amine
(Fig.
allyl,
the
zero.
pyridine
as that found for i-propyl.
found for all substrates
s-butyl,
simultaneous
I).
As expected
< PhCH2
only bimolecular
morpholine)
of nucleophile
by reactions
piperidine
showed
1.
shown for CH3,
found for each of the nucleophfles:
We have shown separately are
from
of the nucleophilicity
and independebt
of the came form
values
in Table
(CH2CH;CH
as nucleophiles
also
(see Fig. were
in the order
>
gl
of the benzyl,
and pyridine
piperidine
processes
decreased
plots
these
nucleophilicity
bimolecular
constant
morpholine
(CH3
different
We next studied the reactions derivatives
are recorded
ionisation
for the
for
this as evidence
substrates
and the
amines
studied. The E&l to SN2 mechanistic
changeover
has been the subject
it has been ‘proposed
ionisation4
and alternatively that solvolyses generally proceed with solvent nucleophilic 5 . We believe that the above data provide in the case of l-(E-mathoxybenzyl)-,
l-i
-propyl-
clean
SNl
Table
2.
and 1-s-butyl-2,4.
6-triphenylpyridinium
to SN2 mechanistic
Pyridine
First and second order 2,4,6-triphenylpyridiniums
fi at
100’
-1
k2 x 103 (1 mole kl x 105 (set -1 1
c
particularly
proceed
clear
by initial
evidence
of a
changeover.
rate
Nucleophile
Morpholine
quite generally
con-
troversy;
assistance
both that substitutions
of considerable
set
-1
constants for reaction of l-substituted with morpholine and pyridine in chlorobenzen8
CsHgCH2 )
lc2 %I x x lo5 lo3
(set (1 mole -1 ) -1 set -1 )
except for
E-methoxybenzyl
2.37 (0.2
(0.0.032 1 at 40°.
i-l+
s-Bu
p- MeOC6H4 CH 2
0. 064
0.052
0.20
0.88
2, 9
4. 0
Co.04 4.0
< 0.01 0. 94
CAPTIONS
FOR FIGURES
Plots reactions
of pseudo first order
of 1 -substituted
solution with piperidine Fig.
1:
1-benzyl
Acknowledgement
rate constants
(sbs)
2,4,6-triphenylpyridinium (v),
morpholine
at 100’.
(0 ),
Fig.
-vs nucleophile tetrafluoroborates
and pyridine 2:
- We thank NATO for a grant
concentration
for
in chlorobenzene
(+ ).
1-i-propyl
at 1OOO.
(to G. M. ).
REFERENCES 111
A. R. Katritzky, and S.S. Thind,
J. B. Bapat, R. J. Blade, B. P. Leddy, J. C.S. Perkin I 4-18 (1979).
(2)
Unlike reactions as expected.
with anionic
(8)
J. Woyd and R. C. Patel,
(4)
R.A. Ibid,
(5)
-T. W. Bentley
nucleophiles
unpublished
for which large
(Received
in France ‘r4 March 1980)
Nie,
C. A. Ramsden,
salt effects
were found
work,
Sneen, Accounts Chem. Research 6, 46 (19’79); 2, 281 (1976) and references therein. and P. van R. Scbleyer,
P.-L.
Adv.
Phys.
Org.
cf.
D. J. McLennan,
Chem.
2,
15 (1877).
UNIMOLECUI&R AND BIMOLECULAR TRANSFER OF N-SUBSTITUENTS FROM PYRIDINIUM CATIONS: EVIDENCE FOR A CLEAR MECHANISTIC CI-IANGEOVER Alan R. Katritzky,* School
Giuseppe
of Chemical
Musumarra, Kumars Sakizadeh, and Bratislav Jovanovic
Sciences,
University
Sayed M. M. El-Shafie,
of East Anglia,
Norwich,
NR4
7TJ.
in 2,4, 6-triphenylpyridiniums are transferred to piperidine, Summary -N-Substituents morpholine and pyridine by unimolecular and/or bimolecular processes in chlorobenzene These processes are quite distinct and afford no evidence for a mechanism solution. 1 and intermediate between % 5N2* l-Benzyl-2,4,6_triphenylpyridinium
perchlorate react6 with piperidine 1 The reaction was studied and 1 -benzylpiperidine.
2,4,6 -triphenylpyridlne
The second order
molar).
was not affected
significantly
At 100°
BF4-.
sulphoxide
The reactions
the rate on piperidine For the benzyl derivatives,
the reaction
order
component,
[piperidine].
40’ as the reaction expected:
of -ca
50 as the solvent
of N_-Substituted-Z,
(Fig.
l),
clearly
reaction
This
Some
proceeds
derivatives also occurs
also applies
to determine
of the resuIts
SN1 processes
Table
1.
almost (Figs,
entirely
2,3),
by a first
the kinetic
allyl,
dependence
order
order
process,
Me ) Bet -l
&I x lo5 (s&I) 2 Measurements
0.60
independent
second
of
analogue which was studied at The pattern
by SN2, while for secondary
to 40’
CH2CH:CH2
for
I 4. 04
1.26
(0.5
(0.1 refer
1-4.
process.
while there is a significant
to the I-p-methoxybenzyl
of
revealed
ie that
alkyl and e-methoxy-
are also important.
N_-Subst. -1
chloro-
with
First and second-order rate constants for reactions of l-substituted triphenylpyridiniums with piperidine in chlorabenzene at 1000 a
(1 mole
to
and p-methylbenzyl
by a second
p-substituted
!X2 x lo3
from
are shown in Figures
and also for the methyl,
alkyl and benzyl react
benzyl
varied
C104-
4,6-tripheoylpyridiniums
was very fast at higher temperatures.
primary
se?)
or by changing the anion from
were studied at 100’
and s-butyl
lf2 = 4, 94 x 10 -3 (1 molerl
and n-pentanol.
concentration.
compound
For the i-propyl
by a factor
of a series
in chlorobenzene
rate constants
by ionic strength’
the rate varied
benzene to dimethyl
piperidine
kinetically
at 100’ it shdwed first order dependence on SUbBtrate (over solution: -3 -3 - 10 molar) and first order dependence on piperidine (over range 10
in chlorobenzene -5 range 3x10 2 x 10-l
to give
(0.3
p-methoxybenzyl 2697
6.52 (0.3
derivative.
2,4,6-
henzyl Me0
CHMe2
CHMeEt
0.45
0. 14
0. 10
3.6
0.89
3.20
2698
First lc2 values
and second
increase
p-MeC,H,CH,
order
rate
in the series
constants
i-Pr
< s-Bu
< p-MaQC,H,CH,.
e-CH3C6H4CH2
are
The
not significantly
with
benzyl
derivative,
values
decreased
decreasing
SNl
for
nucleophiles
and SN2 reactions
produced
both with
SN2 reaction i-propyl,
i-propyl,
However,
s-butyl
in place
2).
for
i-propyl
that high
of e. g.
and with
pyridine.
reaction;
used,
s-Butyl
SN1 and SN2 reactions
the &2 of
the
the k2 values
values
l$
and
were
and p-methoxybenzyl supports
gave
independent
mechanism.
yields
These
of the expected
substitution
present
results
show that the bimolecular
derivatives.
We interpret
the corresponding
products
tetrafluoroborate
by unimoleculsr
between
For the
order
whereas
is accompanied
and p-methoxybenzyl
and
of piperidine.
l-i-propyl-2,4,6_triphenylpyridinium
3
benzyl
both unimolecular
which strongly
without an “intermediate”
<
and p-methoxybenzyl
in the expected
of the amine
(Fig.
allyl,
the
zero.
pyridine
as that found for i-propyl.
found for all substrates
s-butyl,
simultaneous
I).
As expected
< PhCH2
only bimolecular
morpholine)
of nucleophile
by reactions
piperidine
showed
1.
shown for CH3,
found for each of the nucleophfles:
We have shown separately are
from
of the nucleophilicity
and independebt
of the came form
values
in Table
(CH2CH;CH
as nucleophiles
also
(see Fig. were
in the order
>
gl
of the benzyl,
and pyridine
piperidine
processes
decreased
plots
these
nucleophilicity
bimolecular
constant
morpholine
(CH3
different
We next studied the reactions derivatives
are recorded
ionisation
for the
for
this as evidence
substrates
and the
amines
studied. The E&l to SN2 mechanistic
changeover
has been the subject
it has been ‘proposed
ionisation4
and alternatively that solvolyses generally proceed with solvent nucleophilic 5 . We believe that the above data provide in the case of l-(E-mathoxybenzyl)-,
l-i
-propyl-
clean
SNl
Table
2.
and 1-s-butyl-2,4.
6-triphenylpyridinium
to SN2 mechanistic
Pyridine
First and second order 2,4,6-triphenylpyridiniums
fi at
100’
-1
k2 x 103 (1 mole kl x 105 (set -1 1
c
particularly
proceed
clear
by initial
evidence
of a
changeover.
rate
Nucleophile
Morpholine
quite generally
con-
troversy;
assistance
both that substitutions
of considerable
set
-1
constants for reaction of l-substituted with morpholine and pyridine in chlorobenzen8
CsHgCH2 )
lc2 %I x x lo5 lo3
(set (1 mole -1 ) -1 set -1 )
except for
E-methoxybenzyl
2.37 (0.2
(0.0.032 1 at 40°.
i-l+
s-Bu
p- MeOC6H4 CH 2
0. 064
0.052
0.20
0.88
2, 9
4. 0
Co.04 4.0
< 0.01 0. 94
CAPTIONS
FOR FIGURES
Plots reactions
of pseudo first order
of 1 -substituted
solution with piperidine Fig.
1:
1-benzyl
Acknowledgement
rate constants
(sbs)
2,4,6-triphenylpyridinium (v),
morpholine
at 100’.
(0 ),
Fig.
-vs nucleophile tetrafluoroborates
and pyridine 2:
- We thank NATO for a grant
concentration
for
in chlorobenzene
(+ ).
1-i-propyl
at 1OOO.
(to G. M. ).
REFERENCES 111
A. R. Katritzky, and S.S. Thind,
J. B. Bapat, R. J. Blade, B. P. Leddy, J. C.S. Perkin I 4-18 (1979).
(2)
Unlike reactions as expected.
with anionic
(8)
J. Woyd and R. C. Patel,
(4)
R.A. Ibid,
(5)
-T. W. Bentley
nucleophiles
unpublished
for which large
(Received
in France ‘r4 March 1980)
Nie,
C. A. Ramsden,
salt effects
were found
work,
Sneen, Accounts Chem. Research 6, 46 (19’79); 2, 281 (1976) and references therein. and P. van R. Scbleyer,
P.-L.
Adv.
Phys.
Org.
cf.
D. J. McLennan,
Chem.
2,
15 (1877).