Substituent effects on 15N and 13C NMR chemical shifts of 5-phenyl-1,3,4-oxathiazol-2-ones
987
O–
O
O
O
N
O
N
O
N
Y–
Y
Y
S+
S+
S
I
II
III
Figure 1. Resonance effects in 5-phenyl-1,3,4-oxathiazol-2-ones.
solvent. 15N data were acquired at natural abundance. 15N
chemical shifts were measured directly using a standard
inverse gated decoupling pulse sequence (zgig), while 13C
chemical shifts were measured using a standard power gated
decoupling pulse sequence (zgpg30), both from the Bruker
pulse sequence library.
Spectral windows were set at 240 ppm for 13C and
500 ppm for 15N. For 15N acquisitions, a total of ca. 15 k scans
of 16 k data points were collected and then zero-filled to 32 k
points prior to Fourier transformation (FT). The recycle delay
(D1) was set at 5 s, and total acquisition time per sample was
ca. 21 h. 13C acquisitions collected a total of ca. 300 scans
of 32K data points and then zero-filled to 64K points prior
to FT. The recycle delay (D1) was set at 2 s. Chemical shift
measurement accuracy for all experiments was estimated at
š0.1 ppm.
carbonyl group (structure III). As the 1,3,4-oxathiazol-2-one
ring becomes increasingly electron-deficient, the remaining
ring bonds, N-4/C-5 and C-5/O-1, show a bond order
increase that is significant in magnitude.9 Although this
increase, particularly for N-4/C-5, is not what might be
expected from a simple resonance picture (Fig. 1), it is
consistent with an overall ring contraction arising from
a lowering (by induction) of the C-5 orbital energy,
thus enhancing the ꢄ character of the N-4/C-5 (and C-
5/O-1) bonds. Moreover, this pattern was observed in
our studies on 3-phenylisoxazoles.1 However, unlike the
3-phenylisoxazole ring, which attenuates both electron-
donating and -withdrawing substituents, the bond orders in
the 1,3,4-oxathiazol-2-one ring system change monotonically
over the range of Y groups we studied. For this reason, all of
the chemical shifts (15N and 13C) of the oxathiazol-2-one ring
system are more profoundly influenced by the nature of the
phenyl substituent.
Computational methods
All computations were carried out using the Gaussian 03
program10 andemployedtheB3LYPfunctional.11 Substituted
5-phenyl-1,3,4-oxathiazole geometries were optimized in the
gas phase using the very tight convergence criteria (the 6-
311CCg(2d,2p) basis set was used for all models) and with
an ultrafine integration grid. Frequency calculations were
performed to demonstrate that the structures represented the
minimum energy (no imaginary frequencies were observed).
15N and 13C chemical shifts were calculated using the GIAO
method12–15 and were referenced to the calculated chemical
shift of nitromethane (ꢀ153.31 ppm) or tetramethylsilane
(183.37) optimized at the same level of theory. Wiberg Bond
indices16 and Natural Population Analysis Charges17 were
calculated using NBO 3.1.18
EXPERIMENTAL
Melting points were determined on a MelTemp apparatus.
Extracts were dried over Na2SO4, and solvents were
removed by rotary evaporation at reduced pressure. Product
purities were determined by gas chromatography–mass
spectrometry analysis on a Hewlett Packard HP 6890
system equipped with a HP-5MS cross-linked diphenyl
(5%) dimethyl (95%) polysiloxane capillary column (30 m ð
0.25 mm ð 0.25 µm film), a 5973 mass selective detector, and
a HP Kayak XA computer.
Compounds
Preparation of 5-phenyl-1,3,4-oxathiazol-2-ones (1a–1h)
5-Phenyl-1,3,4-oxathiazol-2-ones were prepared by a stan-
dard procedure employing treatment of substituted ben-
zamides with chlorocarbonylsulfenyl chloride; compounds
1a–1h have been reported previously.5 They were purified by
recrystallization from an appropriate solvent; their physical
constants and spectral data matched literature values.
Acknowledgements
This work was supported by a Senior Scientist Mentor Initiative
Award from The Camille and Henry Dreyfus Foundation, Inc.
(Award SI-00-031 to J.H.M.). Computer resources provided by
Haverford College and the National Computer Science Alliance
(Grant CHE050033N to M.H.S.) are gratefully acknowledged.
Additional support was provided by the Williams College and
Haverford College Faculty Research Funds. We thank Deborah
Morandi for assistance with preparation of the manuscript.
NMR spectroscopy
NMR spectra were measured at 298 K with a Bruker Avance
DRX 500 MHz NMR spectrometer operating at frequencies
of 500.630 (1H), 125.884 (13C), and 50.748 (15N) using a
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1
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°
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Copyright 2007 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2007; 45: 985–988
DOI: 10.1002/mrc