2
14
CASSIDY ET AL.
1
dimethylmalonate and very low field H NMR signals in hydrogen cis-cyclohexane
1
,2-dicarboxylate dissolved in aprotic solvents. We further report the observation
1
of downfield H NMR signals characteristic of low barrier hydrogen bonding in
aqueous acetone solutions of highly strained vicinal and geminal dicarboxylic acids
at Ϫ50ЊC.
EXPERIMENTAL PROCEDURES
Materials. cis-Cyclohexane 1,2-dicarboxylic anhydride, 2,2-dimethylmalonic acid,
maleic acid, and trans-cyclohexane 1,2-dicarboxylic acid were purchased from Al-
drich and succinic acid from Fisher. All were recrystallized before use, and their
1
purity was verified by H NMR. Solutions of tetrabutylammonium hydroxide (1.0
M) in methanol (Aldrich or Sigma) in sealed vials were opened under nitrogen.
Anhydrous CHCl and CDCl (Aldrich) in sealed vials were opened in a dry box
3
3
and protected from moisture throughout the experiments. Acetonitrile-d (CD CN)
3
3
dimethylsulfoxide-d (DMSO-d ), and tetrahydrofuran-d (THF-d ) were purchased
6
6
8
8
from Aldrich in boxes of sealed ampoules.
Preparation of tetrabutylammonium salts of dicarboxylic acids and analysis by
H NMR. cis-Cyclohexane 1,2-dicarboxylic acid was prepared by reaction of the
1
anhydride with 1.0 M acetic acid in an ice bath overnight. The solvent was removed
1
in vacuo, and the compound was recrystallized from water. H NMR (500 MHz)
of cis-cyclohexane 1,2-dicarboxylic acid in DMSO-d , ambient temperature, recycle
6
time ϭ 1 s: ͳ 12.03 (2H, s); 2.67 (2H, m); 1.86 (2H, m); 1.66 (2H, m); 1.37 (4H, m).
The tetrabutylammonium (NBu ) salts of cis- and trans-cyclohexane 1,2-dicarbox-
4
ylic acid, succinic acid, 2,2-dimethylmalonic acid, and succinic acid were prepared
by dissolving the purified acids in methanol, adding one equivalent of tetrabutylam-
monium hydroxide from a 1.0 M stock solution, and stirring for 20 min. The solvent
was removed by rotary evaporation in vacuo, and the salt was dried under vacuum.
1
Preliminary H NMR samples were prepared at 15 mM in anhydrous CDCl or
3
DMSO-d and diluted to 1.5 or 0.5 mM. Samples of 15 mM hydrogen cis-cyclohexane
6
1
1
,2-dicarboxylate were also prepared in acetonitrile-d and THF-d . H NMR (500
3
8
MHz) of 15 mM hydrogen cis-cyclohexane 1,2-dicarboxylate in CDCl , ambient
3
temp., recycle time ϭ 1s: ͳ 19.75 (1H, s); 3.29 (8H, t, NBu CH ’s); 2.74 (2H, m);
4
2
2
1
.00 (4H, m); 1.66 (8H, q, NBu CH ’s); 1.44 (8H, m, NBu CH ’s); 1.44 (2H, m);
4 2 4 2
1
.00 (12H, t, NBu CH ’s). H NMR of 15 mM hydrogen 2,2-dimethylmalonate in
4
3
CDCl : ͳ 19.6 (1H, s); 3.26 (8H, t, NBu CH ’s); 1.16 (8H, m, NBu CH ’s), 1.44
3
4
2
4
2
(
8H, m, NBu CH ’s and 6H (acid 2-CH ’s)); 1.04 (12H, t, NBu CH ’s).
4 2 3 4 3
Preparation of O-deuterated 2,2-dimethylmalonate salt. The O-deuterated tetrabu-
tylammonium hydrogen 2,2-dimethylmalonate was prepared by dissolving hydrogen
,2-dimethylmalonic acid in D O, freezing the sample, and drying under vacuum.
2
2
Exchange of deuterium into the sample was verified by the disappearance of the
1
1
2
H NMR signal for –COOH in the H NMR spectrum. The H NMR spectrum was
obtained on the 500 MHz DMX Bruker NMR spectrometer through the BroadBand
2
channel with the H lock channel detuned. A 1% solution of CDCl in CHCl was
3
3
used as an external reference. The low-field deuterium signal was not observable