Anomeric effects versus steric hindrance to ionic solvation in protonated glucosylanilines and cyclohexylanilines

Article abstract of DOI:10.1021/ja035782l

The so-called reverse anomeric effect is the preference of cationic substituents for the equatorial position on a pyranose ring, but it is not consistent with current theories of molecular structure or with previous studies designed to test it. To probe this further, the N-protonation-induced shifts of the anomeric equilibrium in a series of N-(tetra-O-methylglucopyranosyl)anilines have been measured with high precision through an NMR titration method that compares basicities of α and β anomers in a mixture of the two. For comparison, the N-protonation-induced shifts of the cis/trans equilibrium in N-(4-tert-butylcyclohexyl)anilines have also been measured by this same method. In both series, there is a shift of the equilibrium toward equatorial upon N-protonation, consistent with steric hindrance to ionic solvation. This shift is smaller for the glucosylanilines than for the cyclohexylanilines, consistent with an enhancement of the normal anomeric effect that counters the steric hindrance and reduces the shift toward the equatorial β anomer. Moreover, the shift toward equatorial increases slightly but detectably with electron-withdrawing substituents on the cyclohexylaniline, which fine-tune the steric hindrance to ionic solvation. In contrast, the shift decreases for the glucosylanilines. This is consistent with an enhancement of the normal anomeric effect due to a more localized positive charge, rather than with a reverse anomeric effect. These results thus define the substituent dependence of the anomeric effect.

Full text of DOI:10.1021/ja035782l

Published on Web 06/28/2003
Anomeric Effects versus Steric Hindrance to Ionic Solvation
in Protonated Glucosylanilines and Cyclohexylanilines
Charles L. Perrin* and Joshua Kuperman
Contribution from the Department of Chemistry, UniVersity of California-San Diego,
La Jolla, California 92093-0506
Received April 24, 2003; E-mail: cperrin@ucsd.edu
Abstract: The so-called reverse anomeric effect is the preference of cationic substituents for the equatorial
position on a pyranose ring, but it is not consistent with current theories of molecular structure or with
previous studies designed to test it. To probe this further, the N-protonation-induced shifts of the anomeric
equilibrium in a series of N-(tetra-O-methylglucopyranosyl)anilines have been measured with high precision
through an NMR titration method that compares basicities of R and â anomers in a mixture of the two. For
comparison, the N-protonation-induced shifts of the cis/trans equilibrium in N-(4-tert-butylcyclohexyl)anilines
have also been measured by this same method. In both series, there is a shift of the equilibrium toward
equatorial upon N-protonation, consistent with steric hindrance to ionic solvation. This shift is smaller for
the glucosylanilines than for the cyclohexylanilines, consistent with an enhancement of the normal anomeric
effect that counters the steric hindrance and reduces the shift toward the equatorial â anomer. Moreover,
the shift toward equatorial increases slightly but detectably with electron-withdrawing substituents on the
cyclohexylaniline, which fine-tune the steric hindrance to ionic solvation. In contrast, the shift decreases
for the glucosylanilines. This is consistent with an enhancement of the normal anomeric effect due to a
more localized positive charge, rather than with a reverse anomeric effect. These results thus define the
substituent dependence of the anomeric effect.
effect (RAE).4 The term has had other connotations, but we
here restrict it to cationic substituents. This has raised consider-
Introduction
Reverse Anomeric Effect. The steric preference of a
substituent X for the equatorial position on a six-membered ring
is well-known. A quantitative measure of this preference is
designated as AX, the free-energy difference between axial and
equatorial conformers of a cyclohexane (eq 1).1 For substituents
bulker than hydrogen, this value is greater than zero. The
able controversy and skepticism, because the positive charge
ought to lower the energy of the σC-X* orbital and enhance the
stabilization of the axial form, not reverse it. Alternatively,
resonance form 1′ ought to contribute even more when the
substituent is cationic, because there is no longer the penalty
for charge separation.
According to one critical review of the RAE,5 the evidence
was in conflict. The first examples were R-glycosylpyridinium
ions, where the results could have been due merely to steric
repulsions. The clearest evidence for the RAE was the claim
that the equatorial preference in a xylosylimidazolium ion
exceeds what can be attributed to steric factors.6 The data,
obtained from NMR coupling constants, are reproducible and
not due to changes in solvent polarity, but the RAE was
nevertheless rejected in favor of steric effects.7 The RAE is
definitely not operative in protonated glucosylamines, with
NH2R+ groups of known steric contribution.8 It was therefore
AX ) GaxialX° - GequatorialX° )
-RT ln([axialX]/[equatorialX]) (1)
anomeric effect is the countertendency for an electronegative
X at C1 of a tetrahydropyran derivative (1) to take the axial
position.2 It is believed to be due to an nO-σC-X* delocalization
that stabilizes the axial form3 or, equivalently, to the contribution
of a charge-separated double-bond/no-bond resonance form (1′).
(2) Kirby, A. J. The Anomeric Effect and Related Stereoelectronic Effects at
Oxygen; Springer: New York, 1983. Juaristi, E.; Cuevas, G. The Anomeric
Effect; CRC Press: Boca Raton, FL, 1995. Graczyk, P. P.; Mikoajczyk,
M. Top. Stereochem. 1994, 21, 159. Thibaudeau, C.; Chattopadhyaya, J.
Stereoelectronic Effects in Nucleosides and Nucleotides and their Structural
Implications; Uppsala University Press: Uppsala, 1999.
When X is cationic, the equilibrium has been claimed to shift
toward equatorial, a phenomenon called the reverse anomeric
(3) Corte´s, F.; Tenorio, J.; Collera, O.; Cuevas, G. J. Org. Chem. 2001, 66,
2918.
(1) Hirsch, J. A. Top. Stereochem. 1967, 1, 199. Jensen, F. R.; Bushweller, C.
H. AdV. Alicycl. Chem. 1971, 3, 139. Bushweller, C. H. In Conformational
BehaVior of Six-Membered Rings: Analysis, Dynamics and Stereoelectronic
Effects; Juaristi, E., Ed.; VCH: New York, 1995; p 25.
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R. U. Pure Appl. Chem. 1971, 25, 527.
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(6) Paulsen, H.; Gyo¨rgydea´k, Z.; Friedmann, M. Chem. Ber. 1974, 107, 1590.
9
8846
J. AM. CHEM. SOC. 2003, 125, 8846-8851
10.1021/ja035782l CCC: $25.00 © 2003 American Chemical Society

Anomeric Effects in Protonated Glucosylanilines
A R T I C L E S
Scheme 1. Acid Dissociations of R and â
Tetramethylglucopyranosylanilinium Ions (R ) CH3)
concluded that coupling constants could not measure xylo-
sylimidazole populations reliably. Additional experimental
evidence is sparse,9 although geometric changes are consistent
with an enhanced normal anomeric effect, not a reverse one.10
Molecular orbital calculations are not conclusive,11 because it
is difficult to separate the RAE from steric effects and hydrogen
bonding, which also favor an equatorial form.
It is important to understand the conformational behavior of
sugar derivatives with cationic groups at the anomeric center.
Many biomolecules have cationic or protonatable heterocyclic
bases attached to a sugar, the most familiar being NAD+ and
the conjugate acids of nucleosides. Many other sugar derivatives
and analogues react only when protonated, and the conformation
of such activated intermediates is crucial for assessing stereo-
electronic effects.12 Moreover, understanding the conformational
behavior of such cations can guide stereospecific syntheses of
sugar derivatives.13 It must be noted though that our concern is
only with the endo anomeric effect, not the exo, which affects
the conformation about the exocyclic C1-X bond of the
unprotonated species and is operative in both R and â anomers.14
We therefore had reinvestigated glycopyranosylimidazoles.
The RAE would be manifested as an increase in the proportion
of the â anomer on N-protonation of an equilibrating mixture.
Although glycosylimidazoles are configurationally stable and
do not equilibrate,15 the increase can be evaluated indirectly,
because a corollary of the RAE is that the â anomer must be
more basic than the R. Instead, we found the opposite.16
Similarly, the greater basicity of R-glucosylamine, measured
by direct titration,17 can now be recognized as being inconsistent
with the RAE.
Scheme 2. Acid Dissociations of Cis and Trans
4-tert-Butylcyclohexylanilinium Ions (R ) (CH3)3C)
Glucosylanilines. This paper compares the anomeric equi-
libria of protonated and unprotonated tetra-O-methylglucosy-
lanilines (2, R ) CH3). Again, the RAE would be manifested
as an increase in the proportion of the â anomer on N-
protonation of an equilibrating mixture. In terms of Scheme 1,
+
this means that Ke would be greater than Ke. It must be
recognized that Scheme 1 constitutes a thermodynamic cycle.
It then follows that Ke+/Ke must equal the ratio of acidity
R
â
(7) Vaino, A. R.; Chan, S. S. C.; Szarek, W. A.; Thatcher, G. R. J. J. Org.
Chem. 1996, 61, 4514. Vaino, A. R.; Szarek, W. A. J. Org. Chem. 2001,
66, 1097.
constants, Ka /Ka . Thus, the RAE would equivalently be
manifested as a greater acidity of the R anomer or a greater
basicity of the â.
(8) Perrin, C. L.; Armstrong, K. B. J. Am. Chem. Soc. 1993, 115, 6825.
(9) Mikoajczyk, M.; Graczyk, P.; Wieczorek, M. W.; Bujacz, G. Angew. Chem.,
Int. Ed. Engl. 1991, 30, 578. Graczyk, P. P.; Mikolajczyk, M. Phosphorus,
Sulfur Silicon Relat. Elem. 1993, 78, 313. Juaristi, E.; Cuevas, G. J. Am.
Chem. Soc. 1993, 115, 1313. Thibaudeau, C.; Plavec, J.; Watanabe, K. A.;
Chattopadhyaya, J. J. Chem. Soc., Chem. Commun. 1994, 537. Jones, P.
G.; Kirby, A. J.; Komarov, I. V.; Wothers, P. D. J. Chem. Soc., Chem.
Commun. 1998, 1695.
(10) Kennedy, J.; Wu, J.; Drew, K.; Carmichael, I.; Serianni, A. S. J. Am. Chem.
Soc. 1997, 119, 8933. Alder, R. W.; Carniero, T. M. G.; Mowlam, R. W.;
Orpen, A. G.; Petillo, P. A.; Vachon, D. J.; Weisman, G. R.; White, J. M.
J. Chem. Soc., Perkin Trans. 2 1999, 589.
(11) Cramer, C. J. J. Org. Chem. 1992, 57, 7034. Chan, S. S. C.; Szarek, W.
A.; Thatcher, G. R. J. J. Chem. Soc., Perkin Trans. 2 1995, 45. Cramer, C.
J. J. Mol. Struct. (THEOCHEM) 1996, 370, 135. Ganguly, B.; Fuchs, B. J.
Org. Chem. 1997, 62, 8892. Cloran, F.; Zhu, Y. P.; Osborn, J.; Carmichael,
I.; Serianni, A. S. J. Am. Chem. Soc. 2000, 122, 6435.
(12) Deslongchamps, P. Stereoelectronic Effects in Organic Chemistry; Perga-
mon: Oxford, 1983. Sinnott, M. L. AdV. Phys. Org. Chem. 1988, 24, 113.
Perrin, C. L.; Arrhenius, G. M. L. J. Am. Chem. Soc. 1982, 104, 2839.
Perrin, C. L.; Nun˜ez, O. J. Am. Chem. Soc. 1986, 108, 5997. Perrin, C. L.;
Thoburn, J. D. J. Am. Chem. Soc. 1993, 115, 3140. Perrin, C. L.; Engler,
R. E. J. Am. Chem. Soc. 1997, 119, 585. Perrin, C. L.; Engler, R. E.; Young,
D. B. J. Am. Chem. Soc. 2000, 122, 4877. Perrin, C. L.; Young, D. B. J.
Am. Chem. Soc. 2001, 123, 4446. Perrin, C. L.; Young, D. B. J. Am. Chem.
Soc. 2001, 123, 4451. Alabugin, I. V.; Zeidan, T. A. J. Am. Chem. Soc.
2002, 124, 3175. Perrin, C. L. Acc. Chem. Res. 2002, 35, 28.
(13) West, A. C.; Schuerch, C. J. Am. Chem. Soc. 1973, 95, 1333. Bailey, W.
F.; Eliel, E. L. J. Am. Chem. Soc. 1974, 96, 1798. Ratcliffe, A. J.; Fraser-
Reid, B. J. Chem. Soc., Perkin Trans. 1 1990, 747. Mereyala, H. B.; Reddy,
G. V. Tetrahedron 1991, 47, 6435. Sun, L. H.; Li, P.; Zhao, K. Tetrahedron
Lett. 1994, 35, 7147. Hadd, M. J.; Gervay, J. Carbohydr. Res. 1999, 320,
61. Garcia, B. A.; Gin, D. Y. J. Am. Chem. Soc. 2000, 122, 4269. Wipf,
P.; Reeves, J. T. J. Org. Chem. 2001, 66, 7910. Che´ry, F.; Cassel, S.;
Wessel, H. P.; Rollin, P. Eur. J. Org. Chem. 2002, 171.
Steric hindrance to ionic solvation must also be taken into
account in comparing protonated and unprotonated glucosyla-
nilines. This phenomenon is well established, as in the basicities
of the methylated amines and the acidities of alcohols.18 For
imidazolyl, the steric contribution is small,19 because protonation
occurs at a distant nitrogen. Of course this cannot be general,
and steric hindrance to ionic solvation could strongly disfavor
protonation of the R anomer of 2.
The extent to which the positive charge of NH2Ar+ disfavors
an axial stereoisomer can be assessed with 4-tert-butylcyclo-
hexylanilines (3). A measure of steric hindrance to ionic
solvation is ∆A, the difference between the A values (eq 1) of
NH2Ar+ and NHAr substituents. This is equal to -RT ln(Ke
/
+
+
Ke), where Ke and Ke are equilibrium constants in Scheme 2.
Because this scheme likewise constitutes a thermodynamic
cis
cycle, ∆A must also equal the ratio of acidity constants, Ka
/
(14) Batchelor, R. J.; Green, D. F.; Johnston, B. D.; Patrick, B.; Pinto, B. M.
Carbohydr. Res. 2001, 330, 421.
(15) Bourne, E. J.; Finch, P.; Nagpurkar, A. G. J. Chem. Soc., Perkin Trans. 1
1972, 2202.
(16) Fabian, M. A.; Perrin, C. L.; Sinnott, M. L. J. Am. Chem. Soc. 1994, 116,
8398. Perrin, C. L.; Fabian, M. A.; Brunckova, J.; Ohta, B. K. J. Am. Chem.
Soc. 1999, 121, 6911.
(17) Blasko´, A.; Bunton, C. A.; Bunel, S.; Ibarra, C.; Moraga, E. Carbohydr.
Res. 1997, 298, 163.
(18) Aue, D. H.; Webb, H. M.; Bowers, M. T. J. Am. Chem. Soc. 1976, 98,
318. Bartmess, J. E.; Scott, J. A.; McIver, R. T., Jr. J. Am. Chem. Soc.
1979, 101, 6046.
(19) Perrin, C. L.; Fabian, M. A.; Armstrong, K. B. J. Org. Chem. 1994, 59,
5246.
9
J. AM. CHEM. SOC. VOL. 125, NO. 29, 2003 8847

A R T I C L E S
Perrin and Kuperman
Katrans. Thus, steric hindrance to ionic solvation would equiva-
lently be manifested as a greater acidity of the axial stereoisomer
or a greater basicity of the equatorial. For quantitative com-
parison, it must be recognized that this ∆A is an underestimate
of the steric contribution in glucosylanilines, because steric
effects in tetrahydropyrans are more severe than those in
cyclohexanes.20 Moreover, it is uncertain what effect the
2-methoxy in 2 might have.
to disprove the RAE, because earlier results cast considerable
doubt upon it, but rather to define quantitatively how substituents
modify the anomeric effect.
This study complements another on a similar series of
tetraacetylglucopyranosylanilines (2, R ) CH3CO).22 According
to NMR integrations, the anomeric equilibrium shows small
variations with aromatic substituent and solvent. Acidic condi-
tions shift the equilibrium toward the â anomer, but no
monotonic relationship could be detected between that shift and
the electron-withdrawing power of the substituent, and the shift
in the corresponding 5-thio analogues is toward the R anomer.
It was concluded that N-protonation of the parent glucosylaniline
leads to anomeric stabilization of the R anomer by ∼1 kcal/
mol, which is opposed by a steric destabilization of ∼1.2 kcal/
mol. The RAE was rejected, and the irregular variations were
ascribed to a combination of steric effects, electrostatic effects,
and orbital interactions. We now report NMR titration results
that provide a consistent assessment of both anomeric and steric
effects in a series of glucosylanilines, as well as the substituent
effects.
What makes the current study feasible is an NMR titration
method that measures relative basicities of closely related
molecules without the necessity of separating them.21 The ratio
R
â
trans
of acidity constants in Scheme 1 or 2 (Ka /Ka or Kacis/Ka
)
is obtained as the slope Kaax/Ka of a linearized plot (eq 2),
created from the observed chemical shifts (δax,δeq) of reporter
nuclei, measured after successive additions of aliquots of acid
to a mixture of the two bases. Also needed are the chemical
shifts of the neutral (δax ,δeq ) and protonated forms (δax ,δeq ),
which can be measured at the beginning and end of the titration
and which must differ appreciably if the nuclei are to be
appropriate as reporters. Because no pH measurement is
necessary, the method is capable of exceptionally high precision.
eq
0
0
+
+
Experimental Section
(δeq - δeq0)(δax+ - δax) )
Tetramethylglucopyranosylanilines (2). Glucosylanilines them-
selves were found to be unsuitable for these experiments, because they
hydrolyze in aqueous acid and they are insufficiently soluble in aprotic
solvents. Although the tetra-O-acetyl derivatives remain stable in
nonaqueous media, the acetyl signals mask the NMR signals of the
anomeric hydrogens. Tetra-O-methylglucosylanilines proved to be
suitable. The anomeric signals are sufficiently isolated, allowing
accurate assignment and measurement of the chemical shifts. It must
be admitted that the experiments were still complicated by decomposi-
tion and NMR line broadening, especially as the basicity of the aniline
decreased. Consequently, this study could not be extended to more
electron-withdrawing substituents, such as nitro.
In a typical procedure,23 tetra-O-methylglucose24 (1 mmol) and the
desired aniline (2 mmol) were dissolved in minimum ethanol and
refluxed for at least 4 h. The solution was cooled over ice, and the
resulting crystals were filtered off and rinsed with cold ethanol. Only
the para-methoxy product needed to be recrystallized. To minimize
hydrolytic decomposition during the subsequent titrations, water was
removed by lyophilization: mp 152-153° (p-BrC6H4), lit.25 154°, mp
97-98° (m-CH3OC6H4), mp 120-122° (p-FC6H4), mp 134-135°
(C6H5), lit.25 135°, mp 149-150° (p-CH3C6H4), lit.25 151°, mp 109-
110° (p-CH3OC6H4), lit.25 110°.
(Kaax/Kaeq)(δax - δax0)(δ + - δ ) (2)
eq
eq
Because Scheme 1 or 2 each constitutes a thermodynamic
cycle, the relative basicities then provide an indirect deter-
mination of the ratio Ke+/Ke. This ratio represents the extent to
which N-protonation shifts the equilibrium from axial toward
equatorial. For glucosylanilines (2), the ratio can be converted
to ∆∆GâfR°, the change upon N-protonation of the free-energy
difference ∆G° between R and â anomers (eq 3). Similarly,
the ratio for a tert-butylcyclohexylaniline (3) can be converted
to ∆A, the difference between A values (eq 1) of NH2Ar+ and
NHAr substituents (eq 4).
RT ln(Ke+/Ke) ) ∆∆GâfR° )
∆GNH Ar+° - ∆GNHAr° ) RT ln(KaR/Kaâ) (3)
2
RT ln(Ke+/Ke) ) ∆A ) ANH Ar+ - A
)
NHAr
2
RT ln(Kacis/Katrans) (4)
4-tert-Butylcyclohexylanilines (3). A procedure for cyclohexy-
lamines was adapted.26 4-tert-Butylcyclohexanone (3.08 g, 0.02 mol)
was combined with the appropriate aniline (0.04 mol) and catalytic
para-toluenesulfonic acid in benzene. The mixture was refluxed using
a Dean-Stark trap until no more water condensed. The benzene was
removed by distillation, and methanol, NaBH3CN (3.77 g, 0.06 mol),
and a small amount of bromocresol green were added. Concentrated
HCl was added dropwise until the solution remained pale yellow. The
mixture was then stirred for 20 h, diluted with water, made basic with
KOH, and extracted with ether. The organic layer was dried with K2-
CO3, and the solvent was removed at reduced pressure, yielding the
desired mixture of cis and trans 4-tert-butylcyclohexylanilines. The 1H
Moreover, it is possible to vary the substituents on the
aromatic ring. Electron-withdrawing substituents localize and
intensify the positive charge on the protonated nitrogen. This
permits a sensitive test of the consequences of N-protonation.
To the extent that a positive charge leads either to a RAE or to
an enhancement of the normal anomeric effect, and to the extent
that a positive charge creates steric hindrance to solvation, these
effects are expected to be larger with electron-withdrawing
substituents. A convenient measure of electron-withdrawing
power is the pKa of the corresponding ArNH3+. Thus, it is
possible not only to measure the effects of N-protonation on
the anomeric equilibrium in glucosylanilines (2) and on the cis/
trans equilibrium in tert-butylcyclohexylanilines (3), but also
to fine-tune the amount of proximal positive charge that is
generated by protonation. Indeed, our goal was not primarily
(22) Randell, K. D.; Johnston, B. D.; Green, D. F.; Pinto, B. M. J. Org. Chem.
2000, 65, 220.
(23) Ellis, G. P.; Honeyman, J. J. Chem. Soc. 1952, 2053.
(24) Zhao, K.; Sun, L.; Glazebnik, S.; Wang, H. Tetrahedron Lett. 1995, 17,
2953. Horning, E. C. Organic Syntheses Coll. Vol. III; Wiley: London,
1995; p 800.
(25) Baker, J. W. J. Chem. Soc. 1928, 1979.
(26) Hutchins, R. O.; Su, W.-Y.; Sivakumar, R.; Cistone, F.; Stercho, Y. P. J.
Org. Chem. 1983, 48, 3412.
(20) Franck, R. W. Tetrahedron 1983, 39, 3251.
(21) Perrin, C. L.; Fabian, M. A. Anal. Chem. 1996, 68, 2127.
9
8848 J. AM. CHEM. SOC. VOL. 125, NO. 29, 2003

Anomeric Effects in Protonated Glucosylanilines
A R T I C L E S
â
Table 1. KaR/Ka for Substituted Tetra-O-methylglucosylanilines
(2)
K R/K â
∆∆GâfR°, cal/mol
0
0
subst
δ
R , ppm
δ
â , ppm
a
a
p-Br
m-CH3O
p-F
5.16
5.21
5.15
5.21
5.16
5.12
4.46
4.52
4.46
4.53
4.49
4.44
1.61 ( 0.05
1.68 ( 0.06
1.69 ( 0.03
1.86 ( 0.04
2.09 ( 0.04
2.08 ( 0.03
281 ( 18
308 ( 22
312 ( 12
366 ( 14
437 ( 11
434 ( 8
H
p-CH3
p-CH3O
trans
Table 2. Kacis/Ka
for Substituted 4-tert-Butylcyclohexylanilines
(3)
0
0
subst
p-Cl
δ
cis , ppm
δ
trans , ppm
K cis/K trans
∆A, cal/mol
a
a
6.60a
3.62
3.58
3.54
6.56a
3.16
3.10
3.06
3.52 ( 0.03
3.21 ( 0.04
3.17 ( 0.03
3.07 ( 0.06
745 ( 6
692 ( 8
683 ( 6
665 ( 11
H
p-CH3
p-CH3O
Figure 1. Plot of log(KaR/Ka ) of tetramethylglucosylanilines (×) and log-
â
(Kacis/Katrans) of 4-tert-butylcyclohexylanilines (O) versus pKa of the
a For ortho H.
+
corresponding ArNH3
.
NMR spectrum of the parent (Ar ) phenyl) agrees with that of the
recrystallized trans isomer,27 except that it shows the presence of cis.
If product appeared to be sufficiently pure, recrystallization was avoided,
so as to retain the cis: mp 128° (p-ClC6H4), mp 108-109° (C6H5),
lit.28 108-109°, mp 124-126° (p-CH3C6H4), mp 69-72° (p-CH3-
OC6H4), lit.28 77-78°.
NMR Titrations. Acetonitrile was chosen as the solvent for these
titrations because the methylated sugars are soluble and stable, and
the H1 signals undergo measurable chemical-shift changes upon
protonation. Triflic acid-d was chosen because it is anhydrous and was
found to be strong enough to protonate the glucosylanilines.
trans stereoisomer and of the equatorial H1 in the cis are
designated δtrans and δcis, respectively.
Errors. Titrations of cyclohexylanilines gave Kacis/Katrans with
reasonable errors, as was judged from statistical analysis of the
fit to eq 2. Titrations of glucosylanilines were less reproducible,
R
â
and Ka /Ka was obtained as a weighted mean from replicate
titrations. In both cases, the errors reported in the tables are
remarkably low.
Substituent Effects. Figure 1 shows log(Kaax/Kaeq) for both
the tetramethylglucosylanilines (2) and the tert-butylcyclohexy-
lanilines (3), plotted against the pKa of the aniline itself.29 For
the former, the data can be fit to a straight line, with a slope of
+0.091 and with a correlation coefficient of 0.954. For the latter,
the slope is -0.037, and the correlation coefficient is 0.977.
Usually, low slopes lead to low correlation coefficients, but these
are respectably high.
Stock solutions of acid contained 0.23 mmol of CF3SO3D in 1.0
mL of acetonitrile-d3. In a typical titration, 0.023 mmol of a mixture
of tetramethylglucosylaniline or cyclohexylaniline stereoisomers was
dissolved in 0.7 mL of acetonitrile-d3 plus tetramethylsilane as internal
standard. For glucosylanilines, a trace of stock acid was added to
catalyze anomerization. Acid was then added to each NMR sample in
10-µL aliquots, and at least 10 additions were made. The chemical
shifts of an appropriate reporter nucleus in each of the two stereoisomers
were measured after each addition. Titrations were assumed to be
complete when further acid resulted in no change in chemical shifts.
Discussion
Configurational Stability. Both the tert-butylcyclohexyla-
nilines and the glucosylanilines are configurationally stable on
the NMR time scale. The former were synthesized as a mixture
of cis and trans stereoisomers, and they do not interconvert.
The latter were obtained as crystalline â anomers, and a trace
of acid was used to generate a small amount of R. Even though
the amount may change as the titration proceeds, concentrations
are never measured by signal integration. Instead, relative
basicities are measured by NMR titration. The thermodynamic
cycles of Schemes 1 and 2 then permit conversion to the change
of equilibrium on N-protonation. It is perhaps counterintuitive
that this change of equilibrium can be measured even though
the equilibrium is never established. The feasibility of this
“indirect” measurement is testimony to the power of thermo-
dynamics.
The observed lack of further change is evidence that triflic acid is
indeed strong enough to protonate the weakly basic tetramethylgluco-
sylanilines, even in a solvent of intermediate polarity. This avoids the
reliance on aqueous pKa’s of triflic acid and of the parent ArNH3+ for
judging that 1.5 equiv of triflic acid would ensure complete proton
transfer to such weakly basic glucosylanilines, even in so nonpolar a
solvent as CD2Cl2.22
Results
Titrations of Glucosylanilines. The KaR/Kaâ values obtained
from titrations of various tetra-O-methylglucosylanilines (2) are
reported in Table 1. Included are the initial chemical shifts of
the H1 reporter nuclei that were followed for each of the two
anomers, as well as the conversions to ∆∆GâfR° (eq 3).
Titrations of Cyclohexylanilines. Table 2 reports the Ka
RAE versus Enhancement of the Normal Anomeric Effect.
cis
/
R
â
The Ka /Ka and ∆∆GâfR° values in Table 1 are all positive,
meaning that the â anomer is more basic than the R. Equiva-
Katrans and ∆A (eqs 2,4) values obtained from titrations of 4-tert-
butylcyclohexylanilines (3). Included are the initial chemical
shifts of the reporter nuclei that were followed (H1 unless
otherwise noted). The chemical shifts of the axial H1 in the
+
lently, it follows from Scheme 1 that Ke is greater than Ke,
meaning that N-protonation shifts the anomeric equilibrium in
glucosylanilines toward â. This result would appear to support
the operation of the RAE. However, these values are comprised
(27) Brune, H. A.; Unsin, J.; Hemmer, R.; Reichhardt, M. J. Organomet. Chem.
1989, 369, 335.
(28) Bull, J. R.; Hey, D. G.; Meakins, G. D.; Richards, E. E. J. Chem. Soc. C
1967, 2077.
(29) Jencks, W. P.; Regenstein, J. Handbook of Biochemistry, 2nd ed.; Chemical
Rubber Co.: Cleveland, OH, 1970; p J-207.
9
J. AM. CHEM. SOC. VOL. 125, NO. 29, 2003 8849

A R T I C L E S
Perrin and Kuperman
of three possible contributions. There is a positive one from
the increased steric bulk of a protonated substituent relative to
the unprotonated, because of steric hindrance to ionic solvation.
There may be an additional positive contribution from the RAE.
Conversely, there may be a contribution from an enhancement
of the normal anomeric effect, which would shift the equilibrium
toward R and contribute negatively to ∆∆GâfR°. Only if the
observed ∆∆GâfR° exceeds the steric contribution could it be
more subtle than in the usual situation, where an equilibrium
or rate responds to substituents that stabilize or destabilize charge
that develops in the reaction. Here, both Kaax and Kaeq respond
nearly equally to substituents, because both reactions convert a
cation into a neutral, and the slope represents the difference
between the two responses. The substituents only fine-tune the
amount of proximal positive charge that is generated by
protonation.
concluded that the positive value is evidence for the RAE.
The opposite signs for the slopes in Figure 1 are a conse-
cis
quence of the opposing origins for Ka /Ka and Kacis/Katrans. In
both cases, an electron-withdrawing substituent localizes and
intensifies the positive charge on the nitrogen. For cyclohexy-
lanilines, this creates a requirement for additional solvation,
which is subject to steric hindrance, so Ke+/Ke increases, but
R
â
The operation of a steric effect is apparent from the Ka
/
trans
Ka
values in Table 2, which are uniformly positive. This
+
means that Ke is again greater than Ke or that N-protonation
shifts the equilibrium in these cyclohexylanilines also toward
trans. Alternatively, the positive ∆A values reflect a greater
effective steric bulk for a protonated aniline, which requires
solvation and is thus more stable when equatorial. It is not
necessary to specify the exact origin of this steric effect, or to
ascribe it to syn-axial repulsions.30 By whatever mechanism it
operates, this steric hindrance to ionic solvation must also be
operative in the glucosylanilines.
for glucosylanilines, the greater positive charge increases the
+
enhancement of the normal anomeric effect and decreases Ke
/
Ke. Moreover, the magnitude of the slope is greater for
glucosylanilines, showing that the anomeric effect is more
sensitive to the effective electronegativity of the substituent than
is steric hindrance to ionic solvation. This is evidence that
internal solvation by the 2-methoxy to both anomers of 2 is not
leveling the steric hindrance to solvation.
The ∆A values in the cyclohexylanilines provide an estimate
of the steric contribution to ∆∆GâfR° in the glucosylanilines.
Subtracting ∆A from the observed ∆∆GâfR° gives the anomeric
contribution, separated from the steric. The average from those
with substituents in common is -270 cal/mol. This is negative,
representing an N-protonation-induced shift of the equilibrium
toward R. Moreover, because steric effects in tetrahydropyrans
are more severe than those in cyclohexanes,20 this ∆A from
cyclohexylanilines underestimates the steric contribution, and
the magnitude of the anomeric shift toward R is even larger
than 270 cal/mol.
The key result is the qualitative one that ∆∆GâfR° in
glucosylanilines is smaller than ∆A in cyclohexylanilines. The
difference, corresponding to an estimate of the anomeric
contribution alone, is small but statistically highly significant.
Thus, N-protonation makes the equatorial isomer more favorable
for both, but to a greater extent for the cyclohexylaniline than
for the corresponding glucosylaniline. This is the exact opposite
of what would be expected from the RAE, and it is entirely
consistent with an enhancement of the normal anomeric effect.
Because anomeric effects represent the tendency of an elec-
tronegative substituent at C1 of a tetrahydropyran (1) to take
the axial position, this tendency ought to be enhanced on
protonation of that substituent, which augments its electrone-
gativity.
We therefore reaffirm the conclusion that there is no firm
evidence for the RAE. The enhancement of the normal anomeric
effect is consistent with the molecular orbital interpretation of
anomeric effects.3 It is inconsistent with an electrostatic interac-
tion between the charge on the protonated nitrogen and the
oxygen dipole, which in principle might stabilize the equatorial
form.5
Substituent Effects. The Kaax/Kaeq values in Tables 1 and 2
vary with the aniline substituent. This is clear from Figure 1,
where the electronic nature of the substituent is measured by
the basicity of the parent aniline. The slopes are quite small,
but they are measured reliably. Moreover, glucosylanilines and
cyclohexylanilines respond oppositely. This phenomenon is
This study goes beyond an earlier one on the anomeric
equilibrium in tetra-O-acetylglucosylanilines.22 Our results
reinforce their conclusion that N-protonation does not produce
a RAE but instead leads to an enhancement of the normal
anomeric effect. With the 5-thio analogues, this was clear,
because Ke ([â]/[R], as in Scheme 1) decreases on N-protonation.
For the glucosylanilines, Ke instead increases on N-protonation,
and there was no simple relationship between the basicity of
the aniline and the increase of Ke. Both Ke+ (in the protonated
forms) and Ke+/Ke were smaller for the parent glucosylaniline
than for either the para-methoxy or the para-nitro derivative.
This nonmonotonic behavior was attributed to a balance of
anomeric and steric effects, but it disagrees with our results,
because the thermodynamic cycle guarantees that the increase
of Ke on N-protonation must be equivalent to our KaR/Kaâ, which
Figure 1 shows varies regularly with basicity. Besides, the low
slope for cyclohexylanilines in Figure 1 demonstrates only a
small dependence of the steric effect on substituent. The data
in Table 2 show how difficult it is to measure these substituent
effects by NMR integrations, because the entire variation is from
a 3.1:1 ratio to a 3.5:1 ratio. It is also possible that proton transfer
to glucosylanilines was incomplete in nonpolar solvents,
especially to the less basic ones. The NMR titration method
provides consistent results that vary regularly with substitution.
Above it was asserted that electron-withdrawing substituents
localize and intensify the positive charge on the protonated
nitrogen. Certainly, they increase the amount of proximal
positive charge in both neutral aniline and protonated anilinium
ion, but is the increase really greater in the latter? Because
resonance can delocalize electron density from the nitrogen lone
pair of the former, electron-withdrawing substituents might
instead produce a greater increase of positive charge in the
neutral. However, the negative slope for cyclohexylanilines in
Figure 1 shows that electron-withdrawing substituents create a
greater steric hindrance to solvation, thus showing that they do
indeed increase the amount of proximal positive charge gener-
ated by N-protonation.
(30) Wiberg, K. B.; Hammer, J. D.; Castejon, H.; Bailey, W. F.; DeLeon, E.
L.; Jarret, R. M. J. Org. Chem. 1999, 64, 2085.
The data in Figure 1 show small deviations from the straight-
9
8850 J. AM. CHEM. SOC. VOL. 125, NO. 29, 2003

Anomeric Effects in Protonated Glucosylanilines
A R T I C L E S
line fits. Some of these are due to random scatter. Another
source is the inadequacy of aniline basicity in tracking the
intensification of positive charge on the nitrogen, because these
two phenomena reflect different mixes of inductive and
resonance contributions. Still another possibility to account for
deviations is curvature. There is no requirement for linearity,
which is a default relationship. Indeed, extrapolation would
suggest that the lines cross at pKa 6.6. This corresponds to a
hypothetical aniline with a substituent so electron-donating that
the enhancement of the anomeric effect vanishes and only steric
hindrance to solvation of a delocalized positive charge remains.
If so, the curves would not cross but converge instead.
steric hindrance and reducing the shift of the equilibrium toward
the equatorial â anomer. This is consistent with an enhancement
of the normal anomeric effect, rather than a RAE. We reaffirm
the conclusion that there is no firm evidence for this latter effect.
Moreover, the N-protonation-induced shift toward equatorial
increases slightly with electron-withdrawing substituents on the
cyclohexylaniline. This behavior is consistent with steric
hindrance to ionic solvation. In contrast, electron-withdrawing
substituents lead to a reduced shift toward equatorial (â) for
the glucosylanilines, again consistent with an enhancement of
the normal anomeric effect, and not with a reverse one. Both
the steric hindrance to ionic solvation and the enhancement of
the anomeric effect are greater when electron-withdrawing
substituents localize and intensify the positive charge on the
protonated nitrogen. These results thus define the dependence
on remote substituents of both steric hindrance to solvation and
the anomeric effect. It is remarkable that such a subtle
dependence on substituents, from fine-tuning of the positive
charge, can be detected.
Conclusions
Through a highly precise NMR titration method that is
applicable to a mixture of nonequilibrating stereoisomers, we
have succeeded in measuring the N-protonation-induced shifts
of the anomeric equilibrium in tetra-O-methylglucosylanilines
and of the cis/trans equilibrium in 4-tert-butylcyclohexylanilines.
Both equilibria shift toward the equatorial stereoisomer upon
Acknowledgment. This research was supported by NSF
Grants CHE94-20739 and CHE99-82103 and by Instrumentation
Grant CHE97-09183.
+
N-protonation (Ke > Ke in Schemes 1 and 2), a behavior
consistent with steric hindrance to ionic solvation. However,
the shift is smaller for the glucosylanilines than for the
cyclohexylanilines. Therefore, some effect is countering the
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