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On the relative basicities of imidazole and benzimidazole.

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Angew. Chem. Suppl.
This manuscript is
to be cited as
Angew. Chem. Suppl.
7983,411-418
7983,411-418
Dieses Manuskript ist
zu zitieren als
-
-
0 Verlag Chemie GmbH. D-6940Weinhelm. 1983
0721 -42271831040404118025010
The sign and magnitude of this effect
/14/
Jose Elguero, RObeLt Flammang and Andre Maquestiau
Since the proton affinity of imidazole
-6.3 kcal.moie-'1.
15
a reasonable value for benzrmrdazole 3 would
be in the range 228 to 230 kcal.mole-' / 1 7 / .
~
Although there are slight differences in the literature values
It has been shown /20/ that the charge of the acidic proton
2
(the symmetry of cations %and
(Table 1) it has been clearly established that imidazole Lais more basic
about 1.5 pK
in agreement With experimental
and for other similar pairs of bases, also measured by Mautner / 1 5 / :
(AH =
2 by
LS
pyridme-quinolme I A H = -5.6 kcal .mole-') and pyrazine-qulnoxallne
223.3 kcal.mole-I /16/,
than benzimidazole
2b
_y
results for the couple cyclopentadrene~indene( 4 H r -6.6 kcal.mole-I)
O n the relative basicities of imidazole and benzimidazole
Javier Catalan'.
-
la
2a
lb
units. The number of references
1s
simplifies the problem in this case)
well correlated with the acidLty. The higher electronic charge den-
sity of the acidic proton of benzimidazole cation
proves the rnterest that these two biologically important molecules have
aroused.
&
(q
pared with the corresponding charge of imldazolium ion
= 0.823) com-
2 (g
=
0.811)
shows that the first is the weaker acid.
- The charge of the basic nitrogen (N-3) 1s thLs homologous series
of neutral derivatives 1s related to their basicities /21/. The e l e c u o nic charge densities of the N-3 atoms of benzimidazole & ( g
and imidazole
(q
= 5.266)
= 5.239) Show that the first one is the more basic.
The three criteria coincide with the predrction that benzrmidazole
in the vapour phase ought to be more basic than imidazole. In order to
ascertain this theoretical conclusion we have determined by mass specDI. J. Catalan
trometry the intrinsic relative proton affinities of both compounds
Departamento de Quimica Fisica y Quimica CuBntica, Facultad de Ciencias,
using Cooks' method /22.23/.
A
chemical ionization experrrnent was carried
C-XIV, Unrversidad Autdnoma de Madrid, Cantoblanco, Madrid-34 (Spain1
out on a benzimidazole-imidazole mixture using isobutane as the reacting
Dr. J. Elquero, InStitUtO de Quimica Msdica, CSIC, Juan de la Cierva
gas (reversel geometry ma55 spectrometer). The MIKE spectrum (unlmolecu-
3. Madrid-6 (Spaml
lar fragmentationel of the benzim~dazole-imida2Oleprotonated d m e r
Dr. R. Flamang, Prof. Dr.
A.
(m/z 181) showed protonated benzimidazole 2
& (m/r 119) formation to be
Maquestla",
40 times greater than protonated imidazole & ( m / r 69)( E q . 2 ) . From
Laboratoire de Chimie Organique, Universite de 1'Etat d Mons, 19,
this ratio ,
Avenue MaiStriaU, 1000 Mons (Belgium)
we
conclude that benzimidazole has a greater proton affi-
nity than imidazole 2. AS expected /23/, the interaction of the drmer
- 411 -
-
Table 1. Basicity of imidazole and benzimidazole IpK values in water)
Imrdazole L
Benzimidamle
-
with a collision gas increases somewhat the relative abundance of protoDated m i d a z o l e (Zb/lb
2
413
13): the CID/MIKE method is less sensitive to
=
proton affinities differences,but it has the advantage Of enhancrnq the
:
5.53 (20°Cl
5.48 (25OC) /9/
1938
:
6.95 (25OCI /1/
1948
1955
:
7.09 (25OC) / 2 /
1951
:
/8/
/lo/
1962
:
1.20 (20°C) /3/
1958
:
5.53 (2OoC)
1966
:
7.10 (ZO'C)
/4/
1960
:
5.58 (25OC) /11/
1968
:
7.00 (20°C)
/5/
1962
:
5.68 (20°C)
1910
:
7.12
125°C1
/6/
1964
:
5.52
1911
:
1 . 2 2 (25OC) /7/
1971
:
5.77 (25OC)
(30°C)
intensity of metastable peaks due to the protonated molecules.
/3/
/12/
/
21
[Eq-
/7/
A theoretical analysis Of the acid-base Strength of 1.3-diaroles
leads to the following results:
p"a
.2,4,5-Tdeh
I
Having established experimentally and sustained theoretically the
-
fact that the lntrlnsx basicity of imidazole 1 s lower than that of ben8-
zrmtdazole, all the previous qualitative explanations for the opposite
conc1usion. based on p~
SLOD
values /7,24,25/, must be =elected. The lnver-
of basicities in water still has to be explained.
The first explanation that comes to mind is a steric effect to the
solvation (by water molecules) in the benzimidarolium ion
u
,
20
8.1
2, but
the
fact that ASo for both acid-base equillbrla 1s almost identical / 7 /
83
82
84
524
526
528
530
out this hypothesis.
A
Situation similar to that observed / I S /
azines (pyridine, pyridazme, pyrimrdme, pyrazinel
1%
in
rules
the
neither to be ex-
pected. In this last case the attenuation of the basicity by thP aqueous
all of them show that the intrinsic basicity (proton affinity) of imidais lower than that of benzimidazole 2 . This conclusion w a s drawn
zole
from the following considerations, corresponding to different theoretical approaches to proton affinities
-
:
The energy variation for the isodesmic reaction, Eq. 1, AE =
-7.1 kcal.moleC',
points to an activating effect on the heterocyclic
solvent follows the variation of the dipole moment of the neutral molecule. Thus, in o u r case, a phenomenon of general. Unspecific, solvation can
also be excluded since both heterocycles have similar dipole moments /26/
To account for the 8-10 kcal.mole-I difference / 2 8 / between the
gas phase and aqueous solution, two phenomena may be called for. O n e of
them, probably the one that COntrlbiiteS in a lesser extent.
five-membered ring caused by the benzenic ring with a concomitant I"-
!thening of the hydrogen bonds going from imidazole a
Crease of basicity.
benrimrdazole 2 ( q
(4
=
= 5.266) and from benzimidarolium ion
0.823) to imidazolium ion
2
(g
=
15
a streng-
5.2391 to
2
(qH =
0.811). both effects stabilizing
the left part of Eq. 1. The second and most important one 1s thc polarizability effect /29/ which
- 412 -
IS
-
greater in cation
414
-
2
than m cation
-
benzenlc r m g , effect 'umt dlssaoears
lb, due to the &-condensed
almost completely / 3 0 / ~n water solutron when the positive charge
/17/
2s
dispersed, through the hydrogen bonds, towards the solvent molecules
(specific solvation).
planar. Mlecul;lr
geometries have been totally optimized Using the INDO method /31/ and
the technique developed by Rinaldr
e./32/.
g
I"
PI,
!0.02°,
and
a.".,
Imidazole
5.0754
(1.6008)
ImidazoleHI Benzrmidazole
5.0011
11.48291
5.1307
(1.66901
5.0480
11.5332)
C-2
3.7972
(1.01561
3.7438
(0.91751
3.7511
(0.94271
3.7026
10.8524)
N-3
5.2397
(1.19071
5.0011
ii.4a291
5.2661
11.2204)
5.0480
(1.53321
c-4
3.9214
11.06841
3.9156
( 1 .osa31
3.9988
11.01651
3.9775
(1.0045)
c-5
3.9795
11.12461
3.9156
11.05831
3.9861
11.0274)
3.9493
10.9718)
3.9755
11.0182)
3.9493
10.97181
4.0083
(1.0435)
3.9775
(1.0045)
__
__
BenzimidaZoleH'
-3a
__
-~
3.9262
11.0416)
3.9067
11.0642)
-7a
__
._
3 -8998
11.0207)
3.9067
(1.06421
H-1
0.9013
0.8114
0.8978
0.8231
H-2
1.0384
0.9315
1.0432
0.9475
H-3
__
0.8114
~-
0.8231
C.L. LlOtta, E.M. Perdue, H.P.
/20/
J.
Catalan, A. Macias, J . Chen. Sac. Perkin Trans. 2 , 1979, 1632;
-
/21/ J . Catalan, 0 . MO, P. Perez, M. YaReZ, J. Am. Chem. SOC., 1 2 119791
6520.
/22/ R.G. Cooks, T.L. Kruger, J . Am. Chem. Soc., 99 11977) 1279.
MCLUCkey, D. Cameron, R.G. Cooks, J . Am. Chem. Soc., 103 119811
/23/ S . A .
1313.
/24/
K.
Hofmann, Imidazole and Its DerIvativeS, Part I, Interscience-
John Wiley, New York, 1953, p . 250.
/ 2 5 / P.N. Preston, rn Benzimidazoles and Congeneric Compounds. Part 1
(P.N. Preston, D.R. Smith. G. Tennant, EdS.1. Interscience-John
Wiley, New Y a k , 1981, p. 79
/26/ Calculated values, la, 3.96 D,
/27/
/27/
/28/
in droxanr
:
3 , 4.07
1Eq. 1 ) .
119711 3914; J . F . Wolf, J.L.M. Abboud, R.W. Taft. J. Org. Chem.,
11977) 3316 and references cited therein.
/30/ R.W. Taft, Kinetics
Of
Ion-Molecule Reactions 1P. Aus~OOS,Ed.),
Plenum Press. New York, 1978. R.W. Taft, M. Taagepera, J.L.M. abboud,
Wolf, D.J. DeFrees, W.J. Hehre, J.E. Bartmess, R.T. McIver J r . ,
0.9343
1.0247
0.9825
J.F.
0.9343
1.0325
0.9878
J . Am.
H-6
__
__
__
__
1.0332
0.9878
1.0260
0.9825
Chem. Soc., 100 (19781 7765.
/31/ J.A. Pople. D.L. Beveridge, Approximate Molecular Orbital Theory,
McGrau-Hill, New York, 1978.
415 -
-
source and with a small collision cell in the second field free reqion
/33/.
1975. 1675
/29/ J.J. Brauman, J.M. Riveros, L.K. Blair, J. Am. Chem. S o c . , 93
1.0358
The mas* spectrometry experiments weze carried out on a Varian Mat
3.79 D; experimental values
AHoIaq) = 2 kcal.mole-I /7/ and AElgas1 = - 7.1 kcal.mole-l
1.0114
311 A reversed geometry mass spectrometer equlpped with a dual CI/EI ion
&,
D, La, 3.97 D.
P. Mauret, J.P. Fayet, M. Fabre, B u l l . SOC. Chm. Fr.,
H-4
-
(1974)
Hopkins. J. Am. Chem. Soc.,
W.J. Hehre. M. Taagepera. R.W. Taft, R.D. Topsom. J. Am. Chem. Soc.,
H-5
H-7
M.T. Bowers, Gas Phase 10" Chemistry, Vol. 2 , Academic
103 (1981) 1344 and references therein.
N- 1
__
has a very different basicity i n aqueous
7308.
the following table.
Atom
c-7
/19/
respectively.
-
_.
Rue,
/la/]
= 9.11 / 1 9 / ) .
Press, New York, 1979, p. 1.
Convergence 2s assumed
to be reached when changes m bond iengths, angles and total energy
values are within +0.0007
C-6
Solution ( p K
/ i s / D.H.
All systems studied theoretically are assumed tobe
Charges are reported
A compound with comparable proton affinity, the 4-aminopyridine
1229 kcal.mole-I
417 -
2 (19721
/ 3 2 / D. Rinaldi, J.L. Rivail. Compt. Rend. Acad. Sc1. (Cl,
1664.
/33/ A. Maquestrau.
Y.
Van Haverbeke, C. De Meyer, R. Flamnang, J . Per-
l a x , B u l l . Soc. Chm. Belg., @ 11976) 69.
Received November 22, 1 9 8 2 ,
revlsed February 8,
/1/ A.H.M. Kirby, A. Neuberger, Brochem. J . .
/2/
32
11938) 1146.
B.L. Mickel, A.C. Andrews, J. Am. Chem. Soc.,
/ 3 / C.J. Hawkms, D.D. Perrin. J. Chem. Soc.,
/4/
S.P.
/5/
E.
3
(19551 5291.
1962, 1351.
Datta. A.K. Grzybowski, J. Chem. SOC. 1B1, 1966, 136.
M n z a l e z , R. Jacquier, Bull. Soc. Chim. Fr.,
1968,
5006.
/ 6 / G.W. Jameson, J.M. Lawlor, J. Chem. SOC. (61,"0. 53.
/ 7 / M.J. Blais, 0. Enea, G. Berthon, Thermchim. Acta,
/ 8 / A. Albert, R. Goldacre. J . Phillips, J . Chem. S o c . .
/9/
M.T. Davies, P. Mamdlis, V. Petrov, 6 . Sturgeon, J . Pharm. Pharmacol., 3
__
/lo/
(19511 420.
D.J. B r O m , ____
J. Chem. SOC., 1958, 1974.
/11/ T.J. Lane, K.P. Quinlan, J. Am. Chem. S o c . ,
/12/ D.J. Rabiger, M.M. JOulli6, J . O r g . Chem.,
/13/
(19771 335.
1948. 2240.
82 (19601 2995.
29 (1964) 476.
A E 1kcal.male-I) correspond to the algebraic sum Elimidazole) -
EIX-Imidazolel - E1X-InidazoleH').
EIImldazoleH')
where El) are
the total energies corresponding to the optimized geometrLeS for the
-
studied molecules. In a first approximation. A E will represent
the enthalpie variation for the isodesmic reaction Of proton transfer I ~ H *+ x-Im
Im + X - I ~ H +
.
/14/ Calculated from Fig. 4 of M. Meot-Ner (Mautnerl, J . PhyS. Chem.,
84 (19801 2716.
/15/
M. Meot-Ner
IMautner), J . Am. Chem.
-
/16/
D.H.
SOC.,
E l (19791 2396.
A u e , O r a l communication at the "Euchem conference on I o n i c In
Chemistry
:
Gaseous vs Solvated Ions", Lido dl 05tia (Roma), September,
1982
-
416
-
-
418
-
1983 / Z
206 S /
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