Dynamic 1H NMR study of the barrier to rotation about the C-N bond in primary carbamates and its solvent dependence

Article abstract of DOI:10.1021/jo061301f

The dynamic ¹H NMR study of some primary carbamates in the solvents CDCl₃ and CD₃COCD₃ between 183 and 298 K is reported. The free energies of activation, thus obtained (12.4 to 14.3 kcal mol^-1), were attributed to the conformational isomerization about the N-C bond. These barriers to rotation show solvent dependence in contrast to the tertiary analogues and are lower in free energy by ca. 2-3 kcal mol^-1.

Full text of DOI:10.1021/jo061301f

1
Dynamic H NMR Study of the Barrier to
than in comparable amides because conjugation between the
nitrogen lone pair and the carbonyl group is reduced by the
same but competing effects between the oxygen lone pairs and
the same carbonyl group. However, one of the oxygen lone pairs
can be donated into the carbonyl π system, and thus, it can
partially compensate the loss of π conjugation with the nitrogen
lone pair when C-N bond rotation takes place.
Rotation about the C-N Bond in Primary
Carbamates and Its Solvent Dependence
,
†
Ali Reza Modarresi-Alam,* Parisa Najafi,
Mohsen Rostamizadeh, Hossein Keykha,
Hamid-Reza Bijanzadeh, and Erich Kleinpeter
†
‡
§
Carbamates are of particular interest due to their usefulness
Department of Chemistry, UniVersity of Sistan & Baluchestan,
Zahedan, Iran, Department of Chemistry, Tarbiat Modarress
UniVersity, P. O. Box 14115-175, Tehran, Iran, and
Department of Chemistry, UniVersity of Potsdam,
2
2-24
23-26
in various industries
fungicides and pesticides), in the pharmaceuticals industry
as drug intermediates, and in the polymer industry2
synthesis of polyurethane and also in peptide syntheses. In
addition, among the various amine-protecting groups, carbam-
ates are commonly used due to their chemical stability toward
acids, bases, and hydrogenation. In particular, the pharmaco-
logical activity and the importance of syn and anti rotamers of
carbamates in their biological acitivities motivated detailed
investigations of the energetic properties of these systems as
as agrochemicals
(herbicides,
23,24,27
3,24
in the
P. O. Box 60 15 53, D-14415 Potsdam, Germany
27
modaresi@hamoon.usb.ac.ir
2
8
ReceiVed June 23, 2006
1
,12,15,29
conformational switches in molecular devices.
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(
1
The dynamic H NMR study of some primary carbamates
457-463.
in the solvents CDCl
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3
and CD
3
COCD
3
between 183 and 298
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(12.4 to 14.3 kcal mol ), were attributed to the conforma-
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rotation show solvent dependence in contrast to the tertiary
analogues and are lower in free energy by ca. 2-3 kcal
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-21
The free energies of activation in
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Tarbiat Modarress University.
University of Potsdam.
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10.1021/jo061301f CCC: $37.00 © 2007 American Chemical Society
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208
J. Org. Chem. 2007, 72, 2208-2211
Published on Web 02/20/2007

1
TABLE 1. Dynamic H NMR Data for the Primary Carbamates 1a-d in CDCl3 and CD3COCD3
chemical shifts
δ (ppm)
lowest temp
reached (K)
∆Gq b
∆∆Gq c
a
kc (s-1
(kcal mol-1)
(kcal mol-1)
entry
compd
solvent
∆ν (Hz)
Tc, °C (K)
)
1
2
3
4
5
6
7
8
1a
1a
1b
1b
1c
1c
1d
1d
CD3COCD3d
CDCl3
CD3COCD3
CDCl3
CD3COCD3
CDCl3
CD3COCD3
CDCl3
6.749, 6.585
5.726, 4.868
6.484, 6.299
6.003, 5.592
6.953, 6.628
5.619, 5.221
7.309, 6.958
7.036, 5.296
183
233
233
233
233
233
203
213
82.0
429.0
92.5
205.5
162.5
199.0
175.5
870.0
13 (286)
3 (276)
18 (291)
8 (281)
29 (302)
-3 (270)
33 (306)
27 (300)
182.04
952.38
205.35
456.21
360.75
441.78
389.61
1931.10
13.8
12.4
13.9
13.0
14.2
12.5
14.3
13.1
1.4
0
0.9
0
1.7
0
1.2
0
a
Chemical shift of the protons of NH2 group. Margin of error ( 0.2 kcal mol-1. c ∆∆G ) diffence in the free energy of activation in solvents CD3COCD3
b
q
d
q
and CDCl3. Same result obtained in completely dry acetone-d6; thus, trace water in the normal NMR solvent is of no influence on ∆G .
NMR measurements, both in the gas phase and in solution,
have shown that the rotational barrier in amides is strongly
affected by the medium.2
,7,15-17,30-34
Nevertheless, a different
behavior was observed for tertiary carbamates if studied by
dynamic NMR spectroscopy in solution: Actually, it has been
found that the rotational barrier proved to be apparently
1
5-17
insensitive to the solvent polarity.
Recently, we reported
1
dynamic H NMR spectroscopy studies of various carbamates
and other nitrogen-containing organic compounds.
4,33-37
To the best of our knowledge, quantitative experimental
studies concerning conformational variations, occurring in
primary carbamates, have not yet been undertaken. Thus, it is
the major aim of this paper to determine the barriers to rotation
of some primary carbamates by dynamic NMR spectroscopy
and to answer the question of if, in contrast to tertiary
carbamates, their barriers to rotation will be solvent dependent.
The carbamates 1a-d were prepared from alcohols and
phenols 2, sodium cyanate 3, and trichloroacetic acid in the solid
FIGURE 1. Variable-temperature 1H NMR study of the primary
carbamate 1a in CDCl
3
.
3
8
state (cf. Scheme 1).
The rate constants, k, for the present dynamic process in the
carbamates 1a-d were calculated at the coalescence tempera-
ture (Tc) employing the Gutowsky-Holm equation (kc )
SCHEME 1. Synthesis of the Carbamates 1a-d
-
1/2 3,14,34,37,39,40
π∆ν/2 ).
Assuming the transmission coefficient,
q
κ, to be unity, the free energies of activation (∆G ) were
calculated according to the Eyring equation (∆G ) RTc[ln Tc
q
3
,14,34,37,39,40
-
ln kc + 23.76]).
Barriers to rotation, thus obtained for the carbamates 1a-d,
are given in Table 1 and proved to be very similar if compared
in the same solvent. However, the free energies of activation
were found to be solvent dependent; i.e., in CD COCD they
1
Results of the dynamic H NMR study of the carbamates
1
a-d are shown in Table 1. Gradual cooling of the samples
3
3
1
-1
broadens the H NMR signals of the hydrogens of the NH2
group, which coalesce and then, at lower temperatures, split
into two signal of perfectly equal intensity. For example, the
are about 1-1.7 kcal mol greater than in CDCl as the solvent.
3
In addition, a number of conclusions can be drawn from the
dynamic NMR data in Table 1, and they will be compared with
results reported for similar compounds in the literature (cf.
Scheme 2).
1
variable-temperature H NMR spectra of 1a in CDCl3 are given
in Figure 1; exchange broadening of the NH2 protons in 1a-d
were employed for determining the C,N barriers to rotation.
The carbamates 1a-d studied reveal barriers to rotation which
-
1
are 2-3 kcal mol lower than in N-alkyl-substituted carba-
(
30) Wibreg, K. B.; Rush, D. J. J. Am. Chem. Soc. 2001, 123, 2038-
8
-11,14-19
mates;
in other words, the electron-donating alkyl
2
046.
(
31) Drakenberg, T.; Dahlquist, K. J.; Forsen, J. J. Phys. Chem. 1972,
substituents increase the contribution of the canonical form B
in Scheme 3 and hence the barrier to rotation. Electron-
withdrawing groups, on the other hand, e.g., in aryl-substituted
7
6, 2178-2183.
(
32) LeMaster, C.B.; True, N.S. J. Phys. Chem. 1989, 93, 1307-1311.
(33) Dabbagh, H. A.; Modarresi-Alam, A. R. J. Chem. Res., Synop. 2000,
q
4
4-45.
tert-butyl N-methyl-N-arylcarbamates 4, decrease ∆G (from
(34) Dabbagh, H. A.; Modarresi-Alam, A. R. J. Chem. Res., Synop. 2000,
1
90-192.
(
35) Dabbagh, H. A.; Modarresi-Alam, A. R.; Tadjarodi, A.; Taeb, A.
(38) Modarresi-Alam, A. R.; Rostamizadeh, M.; Najafi, P. Turk. J. Chem.
2006, 30, 269-276.
Tetrahedron 2002, 58, 2621-2625.
36) Modarresi-Alam, A. R.; Khamooshi, F. Synth. Comun. 2004, 34,
29-135.
37) Modarresi-Alam, A. R.; Keykha, H.; Khamooshi, F.; Dabbagh, H.
A. Tetrahedron 2004, 60, 1525-1530.
(
(39) G u¨ nther, H. NMR Spectroscopy, 2nd ed.; Wiley: New York, 1995;
Chapter 9.
1
(
(40) Oki, M. Application of Dynamic NMR Spectroscopy to Organic
Chemistry; VCH Publishers: New York, 1985.
J. Org. Chem, Vol. 72, No. 6, 2007 2209

SCHEME 2
SCHEME 3. Restricted Rotation in Primary Carbamatesa
a
Ground-state GS (A), N-lone pair delocalization (B), and transition
states (TS1 and TS2).
SCHEME 4
3.2 kcal mol in the 4-MeO to 10.7 kcal mol-1 in the 4-CN
-
1
1
19
derivative).
In contrast to the influence of N-substituents in 4, carbamates
a-d show little substituent dependence. Indeed, it is found
1
that in 1a-d the rotational barrier about the N-C bond proves
to be apparently insensitive to both electronic and steric effects
of the various substituents at oxygen (cf. Table 1). In the
carbamates 1c and 1d, however, carrying aromatic substituents
on oxygen, obviously one of the oxygen lone pairs is less
capable of being donated into the carbonyl group than in the
case of aliphatic substituents; in other words, the O-aryl
substituent withdraws π-electron density, and thus, more
π-electron density is available for delocalization with the
carbamate carbonyl. Consequently, 1c and 1d have little higher
barrier to rotation than the aliphatic analogues. Similar results
have been reported for N,N-dimethylaryl carbamates 5 (cf.
Scheme 2) by Yamagami et al.14
q
increase in ∆G with higher solvent polarity must be attributed
to different solvation of GS and TS1/TS2 and that more polar
solvents preferentially stabilize the more polar ground state
relative to the less polar rotational transition states.
et al. and Rablen et al. also calculated the dipole moments
of GS and TS1/TS2; in DMF 7 and DMA 6a, for example,
transition states have dipole moments lower than the ground
states, while in cyclohexyl N,N-dimethylcarbamate 10 (CDMC)
and MDMC 6b TS2 has the larger dipole moment compared
1
5-17
Lectka
1
5
7,16
17,21
with TS1s and GSs.
Indeed, theoretical calculations showed
that there are differences in the dipole moment of GS and
TS1/TS2 and that they are similar for amides and carbamates;
thus, a similar response could be expected for both systems.
The lower total molecular dipole moment of carbamates relative
to amides was pointed out as the main factor for the insensitivity
of their rotational barrier to solvent polarity. The results
confirmed that MDMC 6b is insensitive to the bulk solvent
polarity, probably as a result of the relatively small molecular
dipole moment. In the following, we will propose another
mechanism to explain the solvent effect on the rotational barrier
in primary carbamates like 1.
The total dipole moment does not seem to be the main factor
for the solvent dependence of the rotational barrier in carbamates
1a-d; the dipole moments of three primary carbamates, namely
methyl carbamate, ethyl carbamate, and 2-methyl-2-pentyl
carbamate, are 2.40, 2.59, and 2.64 D, respectively,4 almost
equal with the dipole moments of the tertiary carbamates
Larger differences in the barrier to rotation were obtained in
q
the two different solvents (cf. Table 1); ∆∆G values are well
above the experimental error and almost the same as obtained
for amides. For instance, the rotational barriers in N,N-
dimethylacetamide (DMA) 6a and in N,N-dimethylformamide
-
1
(
DMF) 7 were found to increase by 2.4 and 1.4 kcal mol ,
respectively, on going from the gas phase to acetonitrile solution
(
the same results were obtained for the amides 6a, 8a, and 9a
7,15-17
in Scheme 2).
This result implies that solvation or
hydrogen bonding, or both, proved to be more important in
carbamates 1: If the polarity of the solvent increased on going
from less polar CDCl3 (ꢀ ) 4.9) to the more polar CD3COCD3
(
ꢀ ) 20.7) the barriers to rotation also increased by 1-1.7 kcal
-
1
mol . Thus, the question arises why the dynamic behavior of
primary carbamates 1a-d shows almost the same solvent
dependence as amides in contrast to the tertiary N-substituted
analogues which proved independent.
2,43
1
5
16
44
MDMC 6b (2.51, 2.55 and 2.57 D, respectively) and
1
7
Theoretical and experimental studies have been published to
explain both the amide and the carbamate isomerization
CDMC 10 (2.4 D ). In fact, this is not consistent with a
rotational process that involves a relatively dipolar ground state
(due to resonance contributor B) and less polar transition states
TS1 or TS2 as in amides.
2,7,14-17,30,41
processes in the gas phase and in solution.
There
are two possible transition states for the rotational process, TS1
and TS2, where the lone pair of the pyramidalized nitrogen can
be anti or syn to the carbonyl oxygen (cf. Scheme 3). Theoretical
calculations at the 6-31G* level of theory of DMA (6a) have
q
The observed solvent effects on ∆G in carbamates 1 can be
discussed also in terms of hydrogen bonding: If the NH2 group
in primary carbamates is subject to hydrogen bonding, resonance
form B will be strengthened; thus, the double bond character
in the C-N bond can be increased (cf. Schemes 3 and 4). In
the acetone solutions, the NH2 protons of carbamates may be
-
1
shown that TS1 is more stable than TS2 by 4.1 kcal mol and
that both transition states have a lower dipole moment than the
1
5,41
ground state.
Thus, it was concluded for amides that the
2
210 J. Org. Chem., Vol. 72, No. 6, 2007

strongly bonded to the carbonyl oxygen in the acetone molecule,
while in chloroform solution they are left relatively free. In other
words, the main difference can be due to the hydrogen bond
accepting ability of acetone relative to chloroform. The carbonyl
oxygen of acetone donates stronger hydrogen bond to more
acidic hydrogens of NH2 in the ground state (form B) rather
than less acidic hydrogens of NH2 in TS1 or/and TS2 as shown
in Scheme 4. Therefore, the rotational barrier about the C-N
bond in carbamates 1a-d is expected to increase in acetone.
Hydrogen-bonding effects on the carbonyl oxygen of acetone
and other carbonyl oxygen atoms even with a C-H group
proton acceptor compared with the carbonyl group of amides.
Dilution in dioxane is expected to easily break self-associated
species and shift the N signal upfield.
1
4
54
In carbamates 1a-d, in both solvents, the two NH2 proton
signals are shifted further downfield when the temperature is
lowered (cf. Figure 1 and Supporting Information); the effect
proved to be greater for the more downfield N-H proton,
resulting in a gradual increase in ∆ν in CDCl3 but not so in
acetone. The downfield shift of the proton signal is usually
employed as a probe of hydrogen bonding. Thus, in CDCl3 (the
nonpolar solvent), it seems likely that there is an equilibrium
of carbamate-carbamate and carbamate-chloroform via hy-
drogen bonding and dipolar-dipolar interactions. However, if
these interactions were strong, a marked increase in the rotation
barriers must have been observed in CDCl3 but not in acetone.
Also for secondary carbamates, no dimerization or self-
association in acetone and chloroform was reported, the latter
45-50
(C-H-OdC) are well-established.
A similar effect appears
to be operative in hydrogen bonding of secondary and primary
amides such as formamide, acetamide, and N-methyacetamide
in solvents dioxane, formaldehyde, acetone, and methyl propyl
ketone (MPK), respectively.3
1,51-57
A second question is raised here: Do the primary carbamates
1
0,19-21,43,44,58
1a-d show aggregation as self-association (dimer and/or
being a nonpolar solvent,
but the formation of
polymer)? Since primary amides are excellent proton donors,
as well as proton acceptors, they are strongly associated in
solutions via intermolecular hydrogen bonds; this influences the
complexes of carboxylic acid moieties via hydrogen bonding
19-21
such as 11 in Scheme 2 was demonstrated.
However, Garcia
and co-workers reported the formation of a cyclic dimers of
q
47,51,53-55
45
barrier to C,N rotation (∆G ) which is slightly increased.
hydroxamic acids in acetone solution. Both hydroxamic acids
For studying self-association, the use of a nonpolar solvent is
desirable. Dioxane was employed since this is a relatively weak
and their derivatives are stronger acids (pKa 8-10) than the
corresponding amides and carbamates. Therefore, the tendency
for the formation of dimers of hydroxamic acids by hydrogen
bonding is strong but acetone cannot replace the carbonyl group
of one of the hydroxamic acid molecules.
(
41) Gao, J. J. Am. Chem. Soc. 1993, 115, 2930-2935.
(42) Lee, C. M.; Kumler, W. D. J. Am. Chem. Soc. 1961, 83, 4596-
4
600.
For more quantitative discussions on specific interactions of
individual solvents with primary carbamates, more theoretical
and experimental information should be obtained. The current
data are sufficient to emphasize, however, that the barrier to
internal rotation about the N-C bond in the primary carbamates
1a-d are higher in acetone than in chloroform and they proved
to serve as significantly useful probe for the investigation of
solvent effects and the presence of hydrogen bonds.
(
(
(
43) Oki, M.; Nakanish, H. Bull. Chem. Soc. Jpn. 1971, 44, 3148-3151.
44) Bandekar, J.; Okuzumi, Y. THEOCHEM 1993, 281, 113-122.
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3
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M.; Secent, M. L.; Nino, A.; Munoz, C. Inorg. Chem. 2005, 44, 2908-
2
5
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Acknowledgment. The Sistan & Baluchestan University
Graduate Council supported this research. We thank one of the
reviewers for pointing to the possible effect of trace water on
the rotational barriers.
7
(
(
5
277-5282.
(
(
(
(
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1
Supporting Information Available: Variable-temperature H
NMR spectra of 1a-d in different solvents. This material is
available free of charge via the Internet at http://pubs.acs.org.
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077-1081.
JO061301F
(
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