A 1H NMR and molecular modelling investigation of diastereotopic methylene hydrogen atoms

Article abstract of DOI:10.1002/mrc.1011

The ¹H NMR spectra of methyl 3-bromo-2-methylpropionate (1a) and the corresponding chloro compound (2a) show no long-range coupling between the methyl and methylene protons. In contrast, in the analogous dihalocompounds, methyl 2,3-dibromo-2-methylpropionate (1b) and methyl 2,3-dichloro-2-methylpropionate (2b), one of the methylene protons exhibits a large ⁴J_HH coupling (0.8 Hz) to the methyl group, but the other proton shows no observable splitting. This can be explained quantitatively by calculations of the conformational preferences in these compounds combined with the known orientation dependence of the ⁴J_HH couplings. One conformer predominates in the dihalo compounds 1b and 2b, and this is responsible for the ⁴J_HH coupling. In 1a and 2a all three conformers are populated and the ⁴J_HH couplings average to zero. The technique is a potentially general method of unambiguously assigning diastereotopic methylene protons. Copyright

Full text of DOI:10.1002/mrc.1011

MAGNETIC RESONANCE IN CHEMISTRY
Magn. Reson. Chem. 2002; 40: 279–283
A 1H NMR and molecular modelling investigation of
diastereotopic methylene hydrogen atoms
1
1
1
2∗
´
Claudio F. Tormena, Matheus P. Freitas, Roberto Rittner and Raymond J. Abraham
1
Physical-Organic Chemistry Laboratory, Instituto de Qu´ımica, Universidade Estadual de Campinas, CP 6154, 13083-970 Campinas, Sa˜ o Paulo, Brazil
The Chemistry Department, The University of Liverpool, PO Box 147, Liverpool L69 3BX, UK
2
Received 11 October 2001; Revised 10 December 2001; Accepted 14 December 2001
The 1H NMR spectra of methyl 3-bromo-2-methylpropionate (1a) and the corresponding chloro
compound (2a) show no long-range coupling between the methyl and methylene protons. In contrast,
in the analogous dihalocompounds, methyl 2,3-dibromo-2-methylpropionate (1b) and methyl 2,3-dichloro-
4
2-methylpropionate (2b), one of the methylene protons exhibits a large JHH coupling (0.8 Hz) to the
methyl group, but the other proton shows no observable splitting. This can be explained quantitatively by
calculations of the conformational preferences in these compounds combined with the known orientation
dependence of the 4JHHcouplings. One conformer predominates in the dihalo compounds 1b and 2b, and
4
4
this is responsible for the JHH coupling. In 1a and 2a all three conformers are populated and the JHH
couplings average to zero. The technique is a potentially general method of unambiguously assigning
diastereotopic methylene protons. Copyright  2002 John Wiley & Sons, Ltd.
KEYWORDS: 1H NMR spectroscopy; molecular modelling; diastereotopic hydrogen; halopropionates
alone, completely independent of any conformational iso-
merism, readily accounts for such non-equivalence. A strik-
ing example of this is the H spectrum of the fluoroacetate
INTRODUCTION
One of the most useful aspects of NMR spectroscopy is the
interpretation of spin–spin splitting patterns in terms of
the structural and configurational relationships among the
magnetic nuclei of the molecules investigated.1 Sanders and
Hunter2 in their guide for organic chemists, noted that many
chemists still believe that the two protons of a methylene
group in a flexible chain should always be equivalent. When
the methylene protons are non-equivalent, restricted rotation
is often invoked as the explanation. This response arises from
the misconception that rapid rotation of a CH2 group neces-
sarily leads to equalizing of the two proton chemical shifts.
This is of importance, as establishing the chemical equiva-
lence or non-equivalence of such protons can be an important
step in determining the structure of the compound by NMR.3
In principle, though not necessarily in practice, all
diastereotopic methylene protons are non-equivalent, even
when bond rotation is rapid on the NMR time scale.2 Thus, in
systems of the type A–CX2 –B, where A is a symmetric group
and B a group that lacks a plane of symmetry, the X nuclei are
internally diastereotopic and are therefore non-equivalent,
though accidental equivalence may occur. The origin of such
non-equivalence has usually been ascribed to differences in
conformer population,4a but investigations with chiral moi-
eties have shown that this is not the correct explanation.4b
Waugh and Cotton5 have noted that a symmetry argument
1
derivative of ˛-methyl benzyl alcohol. The CH2F moiety is an
ABX system (υA 4.900, υB 4.783 ppm; JAB 14.96, JAX 47.09, JBX
47.06 Hz).6 This is observed even though there is a twofold
barrier about the CH2F–CO2R bond with the populated con-
formations the cis and trans (F–C–C–O) forms with all the
atoms involved coplanar.7 Thus the non-equivalence of these
protons is simply due to the fact that they are diastereotopic
as a result of the chiral alcohol moiety of the ester.
Although many systems containing non-equivalent
methylene hydrogen atoms have been reported,8 there is
no unambiguous method of assigning the diastereotopic
protons in acyclic molecules. Here we present a method
of distinguishing and assigning such protons based on
molecular modelling plus the orientation dependence of the
4JHH coupling and illustrate the application of the method
to some halopropionate esters. The molecules investigated
are methyl 3-bromo-2-methylpropionate (1a), methyl 2,3-
dibromo-2-methylpropionate (1b) and the corresponding
chloro compounds (2a) and (2b).
O
O
X
O
X
O
X
ŁCorrespondence to: R. J. Abraham, The Chemistry Department,
The University of Liverpool, PO Box 147, Liverpool L69 3BX, UK.
E-mail: abrahamr@liv.ac.uk
Contract/grant sponsor: FAPESP; Contract/grant number:
2000/07692-5.
1a, X = Br
2a, X = Cl
1b, X = Br
2b, X = Cl
DOI: 10.1002/mrc.1011
Copyright  2002 John Wiley & Sons, Ltd.

280
C. F. Tormena et al.
The conformer energies of 0.0 kcal molꢀ1, 4.0 kcal molꢀ1 and
2.6 kcal molꢀ1 for IV, V and VI respectively show that con-
former IV predominates in solution (>95%). Again, the MM
RESULTS AND DISCUSSION
The 1H spectrum of 1a in CDCl3shows non-equivalent
methylene hydrogen atoms at υ 3.58 and 3.47 [Fig. 1(a)].
Both are doublet of doublets (JHH 10.0 and 6.7 Hz). In 1b
the two methylene hydrogen atoms are also non-equivalent
[Fig. 1(b)], but one of them has a geminal coupling and a
long-range coupling (4JHH D 0.81 Hz) with the methyl group,
whereas the other hydrogen is a doublet (2JHH D 9.80 Hz).
We suspected that this difference was due to differing
conformational preferences in the two molecules and used
modelling calculations to confirm this.
calculations agree with conformer energies of 0.0 kcal molꢀ1
,
2.6 kcal molꢀ1 and 3.2 kcal molꢀ1 for IV, V and VI respec-
tively. The much greater stability of conformer IV is clearly
due to the anti Br–C–C–Br orientation, whilst in conform-
ers V and VI the bromines are gauche to each other, thus
increasing the steric repulsion. Note that in all the con-
formations shown the carbonyl group of the ester is cis
to the central C–C bond and the ester methyl cis to the
carbonyl.
The conformer energies and populations were obtained
from both ab initio and molecular mechanics (MM) calcu-
lations using the Gaussian 98 and PCMODEL programs
respectively. The potential energy surfaces (PESs) for 1a
and 1b obtained from Gaussian 98 are shown in Fig. 2, with
the individual conformers in Fig. 3. There are three sta-
ble conformers for 1a [Fig. 2(a)] with similar energies and
populations. The calculated mole ratios are 0.14, 0.47 and
0.39 for conformers I, II and III respectively. The MM cal-
In 1b there is a coupling constant between one of the
methylene hydrogen atoms with the methyl group. Abraham
and coworkers9,10 have described the orientation dependence
of HH couplings over four saturated bonds as given by
Eqn (1), where ꢀ1 and ꢀ2 are the C–C–C–H dihedral angles
°
of the coupling path. Thus two dihedral angles of 180 lead to
large coupling constants (1 to 2 Hz), due to favourable orbital
overlap between the nuclei (the so-called W coupling).10 For
CH3 –C–C–H couplings, Eqn (1) reduces to Eqn (2).
culations agree with conformer energies of 1.3 kcal molꢀ1
,
0.0 kcal molꢀ1 and 0.5 kcal molꢀ1 for conformers I, II and III
respectively. For 1b there are also three possible conformers,
but now conformer IV is far more stable than the other two.
4JHH D cos2 ꢀ1 C cos2 ꢀ2 ꢀ 0.7
4JMeH D cos2 ꢀ ꢀ 0.2
ꢁ1ꢂ
ꢁ2ꢂ
a)
O
Br
O
b)
O
Br
O
Br
Figure 1. 1H NMR spectrum in CDCl3 at 300 MHz for: (a) 1a and (b) 1b.
Copyright  2002 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2002; 40: 279–283

NMR and modelling of diastereotopic methylene hydrogen atoms
281
conformer (IV) for 1b in both the vapour and solution, and
also that the long-range coupling constant is due to the
a)
−2914.218
−2914.220
−2914.222
−2914.224
−2914.226
−2914.228
−2914.230
−2914.232
°
antiperiplanar (180 ) orientation of the methyl group and Ha
in conformer IV.
In 1a the three stable conformers have almost the same
energy and dipole moments, thus they have approximately
equal populations in both the vapour and solution. If
the populations were equal (1 : 1 : 1), the averaged coupling
(4JMeH) between Ha,b and the methyl from Eqn (2) is 0.3 Hz.
This is just on the limit of resolution, and thus the observed
spectrum does not show any long-range coupling.
I
II
III
0
30 60 90 120 150 180 210 240 270 300 330 360
Dihedral angle Br-C-C-CH3/o
Methyl 2-chloro-2-methylpropionate (2a) and methyl 2,3-
dichloro-2-methylpropionate (2b) were also studied to see if
the same behaviour was observed in the chloro derivatives.
Chlorine is a smaller atom than bromine, and thus the
conformer energy differences should also be smaller, which
could stabilize the other conformations in the dichloro
compound. The PES for 2a and 2b show the same behaviour
as for the bromo compounds. The geometries and energies
were optimized and the results are given in Fig. 3(c) and (d).
The conformer mole fractions for 2a are 0.15, 0.50 and 0.35
for VII, VIII and IX respectively. The corresponding MM
conformer energies for 2a are 1.4 kcal molꢀ1, 0.0 kcal molꢀ1
and 0.4 kcal molꢀ1 for VII, VIII and IX respectively, and for
2b 0.0 kcal molꢀ1, 3.2 kcal molꢀ1 and 2.6 kcal molꢀ1 for X, XI
and XII respectively. The conformer energies for 2a are very
similar to those of 1a. In 2b the energy differences are not
so marked as in 2a, but, even so, the calculations predict
that conformer X with the anti ClÐ Ð ÐCl orientation is still
predominant (>95%).
b)
−5483.515
−5483.520
−5483.525
−5483.530
−5483.535
−5483.540
V
VI
IV
0
30 60 90 120 150 180 210 240 270 300 330 360
Dihedral angle Br-C-C-CH3/o
Figure 2. PESs at B3LYP/6-31g(d,p) level: (a) 1a and (b) 1b.
In the molecular fragment considered there are only
staggered conformations thus the Me–C–C–H dihedral
angle is ca 60 or 180 and the JMeH coupling is predicted
4
°
Thus, as expected, the NMR data for the chloro com-
pounds (Table 1) show very similar behaviour to the bromo
°
°
to be ca 0.8 Hz (ꢀ D 180 ) or 0.05 Hz (ꢀ D 60 ). There are two
possible conformations with antiperiplanar dihedral angles
compounds. In 2b the long-range coupling constant (4JMeH
)
°
of 180 (H–C–C–CH3) for 1b: IV and VI.
is 0.56 Hz in CDCl3 and 0.45 Hz in acetone. From Eqn (2) the
calculated coupling constants for Ha and Hb with the methyl
NMR experiments in solvents of varying polarities
(Table 1) were performed for 1a and 1b. The experimentally
observed coupling is the weighted time-averaged value of the
°
group in conformer X are 0.8 Hz (ꢀ D 175.8 ) for Ha and
4
°
0.16 Hz (ꢀ D 53.0 ) for Hb. This confirms that conformer X
couplings in the individual conformers. The JHH coupling
predominates in solution as well as in the vapour.
in 1b changes only slightly with solvent polarity (0.81 Hz in
CDCl3 to 0.65 Hz in DMSO-d6), indicating that there is no
large change in the conformer populations with solvent.11–13
Also, the calculated coupling constants for Ha and Hb with
the methyl group in IV are Ha 0.8 Hz and Hb 0.07 Hz using
In 2a the three stable conformers have almost the same
energy and dipole moments. Thus, the same analysis as for
4
1a is valid, to give the averaged JMeH coupling equal to
0.33 Hz.
°
°
These results show that small changes in the molecular
structure can affect the conformer populations, and thus
the calculated dihedral angles of 175.0 for Ha and 58.5
for Hb. This confirms the existence of one predominant
Table 1. Proton chemical shifts (υ, ppm) and coupling constants (Hz) for methyl 3-bromo-2-methylpropionate
(1a), methyl 2,3-dibromo-2-methylpropionate (1b) and the corresponding chloro compounds (2a) and (2b)
Compound
Ha
Hb
Hc
Me
2Jab
3Jac
3Jbc
3JꢁMe.H ꢂ
4JꢁMe.H ꢂ
c
a
1a CDCl3
1a DMSO
1b CDCl3
1b Ac-d6
1b DMSO
2a CDCl3
2a DMSO
2b CDCl3
2b DMSO
3.59
3.67
4.21
4.24
4.20
3.73
3.79
4.09
4.13
3.47
3.61
3.73
4.00
4.13
3.61
3.75
3.75
3.99
2.91
2.98
–
–
–
2.85
2.94
–
1.30
1.17
2.04
2.02
1.96
1.28
1.16
1.86
1.83
10.02
9.95
9.80
9.87
9.88
10.88
10.83
10.93
11.13
6.67
5.04
–
–
–
6.64
5.18
–
5.90
6.39
–
–
–
5.95
6.34
–
6.99
6.99
–
–
–
7.10
7.04
–
–
–
0.81
0.73
0.65
–
–
0.56
0.45
–
–
–
–
Copyright  2002 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2002; 40: 279–283

282
C. F. Tormena et al.
a)
Br
Ha
Hb
H3COOC
Ha
CH3
Hb
H3COOC
Hb
CH3
Br
H3COOC
Br
CH3
Ha
Hc
Hc
Hc
I
II
III
Erel (kcal mol−1
µ(D)
)
0.7
0.0
2.7
0.1
2.7
2.8
b)
Ha
Hb
Br
H3COOC
Hb
CH3
Br
H3COOC
Ha
CH3
Hb
H3COOC
Br
CH3
Ha
Br
Br
Br
IV
0
V
VI
Erel (kcal mol−1
µ(D)
)
4.0
2.6
1.3
2.1
3.3
c)
Cl
Ha
Hb
H3COOC
Ha
H3COOC
Cl
H3COOC
Hb
CH3
Hb
CH3
Ha
CH3
C l
Hc
VII
0.7
Hc
IX
Hc
VIII
Erel (kcal mol−1
µ(D)
)
0.0
2.7
0.2
2.9
2.6
d)
Hb
Ha
Cl
H3COOC
Hb
H3COOC
Ha
H3COOC
Cl
CH3
Hb
CH3
Ha
CH3
C l
Cl
Cl
Cl
X
0
XI
XII
1.7
Erel (kcal mol−1
µ(D)
)
2.6
1.4
2.0
3.4
Figure 3. Energies and dipole moments for stable rotamers at B3LYP/6-311CCg(2df,2p) level: (a) 1a; (b) 1b; (c) 2a; (d) 2b.
the NMR spectrum. The joint application of theoretical
calculations and NMR theory gives a complete explanation
of the behaviour of the long-range coupling constant in this
system and also provides a definitive assignment of the
diastereotopic protons.
Theoretical calculations
The calculations used both the MM PCMODEL program14
and the ab initio Gaussian 98 program.15 In the latter the
DFT/B3LYP method was used with the 6-31G(d,p) basis
set for the potential surface scan, and the energies and
geometries for the stable rotamers were optimized with the
6-311CCG(2df,2p) basis set.
EXPERIMENTAL
Syntheses
Spectra
Methyl 3-bromo-2-methylpropionate
The solvents (CDCl3, acetone-d6 and DMSO-d6) were
obtained commercially (Aldrich), stored over molecular
sieves and used without further purification. 1H NMR
spectra were obtained on a Varian Gemini spectrometer
operating at 300.06 MHz for 1H and 75.45 MHz for 13C.
Spectra were of ca 20 mg cmꢀ3 solutions, with a probe tem-
A solution of 18.7 g (0.187 mol) of washed and dried methyl
methacrylate in 100 ml of anhydrous diethyl ether was placed in
a 250 ml three-neck flask equipped with glass inlet tube for hydro-
gen bromide and magnetic stirring. The flask with its contents was
placed in an ice bath, and 16.6 g (0.206 mol) of anhydrous hydrogen
bromide was passed into the solution. After the hydrogen bromide
had been added, the flask was stoppered and allowed to stand
overnight at room temperature. After that, the reaction mixture was
washed with water (4 ð 50 ml) and dried with MgSO4. The solvent
was removed, and the desired product was vacuum distilled through
°
perature of ca 20 C , referenced to Me4Si. Typical conditions
were: 48 transients, spectral width 2500 Hz with 32k data
points and zero filled to 128k to give a digital resolution of
0.04 Hz.
a Vigreux column to give pure methyl 3-bromo-2-methylpropionate
16
(b.p. 84 C/37 mmHg) yield 5.0 g (15.0%). 1H NMR (300 MHz,
°
2
°
CDCl3, 20 C, Me4Si): υ 3.75 (s, 3H, OCH3), 3.59 (dd, 1H, JHH D 10.02,
Copyright  2002 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2002; 40: 279–283

NMR and modelling of diastereotopic methylene hydrogen atoms
283
2
3
3JHH D 6.67, CH2), 3.47 (dd, 1H, JHH D 10.02, JHH D 5.90, CH2),
REFERENCES
3
3
3
2.91 (ddd, 1H, JHH D 5.90, JHH D 6.67, JHH D 6.99, CH), 1.31 (d,
3H, JHH D 6.99, CH3), 13C NMR (75 MHz, CDCl3, Me4Si): 173.8
3
1. Ault A. J. Chem. Educ. 1970; 47: 813.
2. Sanders JKM, Hunter BK. Modern NMR Spectroscopy—a Guide
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11. Abraham RJ, Jones AD, Warne MA, Rittner R, Tormena CF. J.
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(C O), 52.1 (CH2Br), 42.1 (OCH3), 34.0 (CH), 16.3 (CH3).
Methyl 2,3-dibromo-2-methylpropionate
Methyl methacrylate 14.0 g (0.14 mol) in diethyl ether (60 ml) were
placed in a 100 ml three-neck flask equipped with a condenser,
addition funnel and magnetic stirrer. Bromine 22.4 g (0.14 mol) was
added dropwise over a period of 60 min. The reaction mixture
was washed with water (4 ð 50 ml) and dried with MgSO4. The
solvent was removed, and the desired product was vacuum distilled
through a Vigreux column to give pure methyl 3,2-dibromo-2-
methylpropionate (b.p. 86 C/10 mmHg) yield 13.0 g (49.0%). 1H
16
°
2
°
NMR (300 MHz, CDCl3, 20 C, Me4Si): υ 4.24 (dq, 1H, JHH D 9.80,
4JHH D 0.81, CH2), 3.84 (s, 3H, OCH3), 3.73 (d, 1H, JHH D 9.80,
2
CH2), 2.04 (d, 3H, JHH D 0.81, CH3), 13C NMR (75 MHz, CDCl3,
4
Me4Si): 169.2 (C O), 55.2 (CBr), 53.5 (CH2Br), 38.1 (OCH3), 26.4
(CH3).
12. Abraham RJ, Tormena CF, Rittner R. J. Chem. Soc. Perkin Trans. 2
1999; 1663.
13. Tormena CF, Rittner R, Abraham RJ, Basso EA, Pontes RM. J.
Chem. Soc. Perkin Trans. 2 2000; 2054.
Methyl 3-chloro-2-methylpropionate
The monochloro compound was prepared similarly to the corre-
sponding monobromo by using HCl instead of HBr. The com-
°
pound distilled at 66 C/30 mmHg yielded 4.8 g (18.8%) from 18.7 g
(0.187 mol) of methyl methacrylate. 1H NMR (300 MHz, CDCl3,
14. PCMODEL, Version 7. Serena Software: Bloomington, IN, USA.
15. Frisch MJ, Trucks CW, Schlegel HB, Scuseria GE, Robb MA,
Cheeseman JR, Zakrzewski VG, Montgomery JA, Stratmann RE,
Burant JC, Dapprich S, Millam JM, Daniels AD, Kudin KN,
Strain MC, Farkas O, Tomasi J, Barone V, Cossi M, Cammi R,
Mennucci B, Pomelli C, Adamo C, Clifford S, Ochterski J,
Petersson GA, Ayala PY, Cui Q, Morokuma K, Malick DK,
Rabuck AD, Raghavachavi K, Foresman JB, Ciolowski J,
Ortiz JV, Baboul AG, Stefanov BB, Liu G, Liashenko A,
Piskorz P, Komaromi I, Gomperts R, Martin RL, Fox DJ,
Keith T, Al-Laham MA, Peng CY, Nanayakkara A, Gonzalez C,
Challacombe M, Gill PMW, Johnson BG, Chen W, Wong MW,
Andres JL, Head-Gordon M, Replogle ES, Pople JA. Gaussian 98.
Gaussian Inc.: Pittsburgh, PA, 1998.
2
°
20 C, Me4Si): υ 3.73 (s, 3H, OCH3), 3.73 (dd, 1H, JHH D 10.88,
3JHH D 6.64, CH2), 3.60 (dd, 1 H, JHH D 10.88, JHH D 5.95, CH2),
2
3
3
3
3
2.86 (ddd, 1 H, JHH D 5.95, JHH D 6.64, JHH D 7.10, CH), 1.29
(d, 3H, 3JHH D 7.10, CH3). 13C NMR (75 MHz, CDCl3, Me4Si): 173.5
(C O), 52.0 (CH2Br), 45.8 (OCH3), 42.1 (CH), 15.1 (CH3).
Methyl 2,3-dichloro-2-methylpropionate
The dichloro compound waspreparedsimilarly to the corresponding
dibromo compound by using Cl2. The compound distilled at
°
67 C/5 mmHg yielded 12.5 g (39.3%) from 18.7 g (0.187 mol) of
1
°
methyl methacrylate. H NMR (300 MHz, CDCl3, 20 C, Me4Si): υ
2
4
4.09 (dq, 1H, JHH D 10.93, JHH D 0.56, CH2), 3.84 (s, 3H, OCH3),
2
4
3.75 (d, 1H, JHH D 10.93, CH2), 1.86 (d, 3H, JHH D 0.56, CH3).
13C NMR (75 MHz, CDCl3, Me4Si): 169.1 (C O), 65.4 (CH2Cl), 53.5
(CCl), 50.1 (OCH3), 24.9 (CH3).
16. Furniss BS, Hannaford AJ, Rogers V, Smith PWG, Tatchell AR.
Vogel’s Textbook of Practical Organic Chemistry, 4th ed. Longman
Inc.: New York, 1978.
Acknowledgements
We acknowledge FAPESP for financial support of this research, for
a scholarship (to M. P. F.) and a fellowship (to C. F. T.), and to CNPq
for a fellowship (to R. R.), and also CENAPAD-SP for the computer
facilities (Gaussian 98).
Copyright  2002 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2002; 40: 279–283