(4R)-3-Allenyl-4-(diphenylmethyl)-oxazolidin-2-one, an unsubstituted allenamide

Article abstract of DOI:10.1107/S0108270104023121

The first X-ray structure of an unsubstituted allenamide, C ₁₉H₁₇NO₂, is reported. The solid-state phase supports the notion that a key minimum conformation of allenamides can be invoked to rationalize the observed stereochemical outcomes in many of our methodological studies employing allenamides. This minimum conformation involves two important factors, i.e. having approximate coplanarity between the planes of the oxazolidinone ring and the internal olefin, and having the allene moiety facing away from the carbamate carbonyl group. The C-N-C=C torsion angle that quantifies this approximate coplanarity between the plane of the oxazolidinone ring and that of the internal olefin, as determined from this crystallographic study, is -19.1 (2)°. A minimized structural calculation, which determined this angle to be -16.1°, is in close agreement. Additional structural features include a probable π-π interaction between the allene moiety and a benzene ring, and non-classical hydrogen bonding in the form of weak C-H...O interactions that are responsible for the formation of two-dimensional networks.

Full text of DOI:10.1107/S0108270104023121

organic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
In all our studies, there exists a uni®ed theme for the
stereochemical outcomes. We have proposed a preferred
conformation for these allenamides, in which the plane of the
internal ole®n is essentially coplanar with the oxazolidinone
(or imidazolidinone) ring, and in which the allene moiety is
facing away from the carbamate carbonyl, as shown in Model
A of the supplementary scheme. The coplanarity ensures that
the top face is blocked by the diphenyl moiety of Sibi's
auxiliary [that to which the allene is attached (Sibi et al.,
1995)], thus favoring reactivity at the bottom face, speci®cally
at the C2 C3 bond. The speci®c direction of the allene
moiety then promotes the observed diastereoselectivity. If
there were free rotation about the CÐN bond, we would
observe no diastereoselectivity in our investigations. However,
we have never provided any substantial evidence to support
our suggestion for a favored allenamide conformation.
Calculations were performed to determine the rotational
barrier of the allene moiety in relation to the oxazolidinone
ring for allenamide (7a) (Fig. 1). The calculations were derived
from the concerted rotation of the allene around the N1ÐC3
bond by 360ꢀ with 180 steps (energy minimization plots can be
found in the supplementary data). PM3 (Dewar et al., 1985)
semi-empirical calculations of the optimized geometries, using
SPARTAN (Wavefunction, 2002), indicated that the rota-
tional barrier is 2.89 kcal mol 1. Two minima were observed,
corresponding to having the allene moiety pointing away from
and toward the carbamate carbonyl group, with a difference
between the two of 1.70 kcal mol 1. The absolute minimum
conformation of the allenamide showed the allene facing away
from the carbonyl group, with a C6ÐN1ÐC3 C2 torsion
angle of 16.1ꢀ. Additionally, we calculated the rotational
barriers and the torsion angles [corresponding to the C6Ð
N1ÐC3 C2 torsion angle in (7a)] for chiral allenamides
(7b)±(7d) and found these values to be similar to those of
chiral allenamide (7a).
ISSN 0108-2701
(4R)-3-Allenyl-4-(diphenylmethyl)-
oxazolidin-2-one, an unsubstituted
allenamide
Michael R. Tracey, Tyler P. Grebe, William W. Brennessel
and Richard P. Hsung*
Department of Chemistry, University of Minnesota, 207 Pleasant St. SE, Minneapolis,
MN 55455-0431, USA
Correspondence e-mail: hsung@chem.umn.edu
Received 24 May 2004
Accepted 17 September 2004
Online 22 October 2004
The ®rst X-ray structure of an unsubstituted allenamide,
C19H17NO2, is reported. The solid-state phase supports the
notion that a key minimum conformation of allenamides can
be invoked to rationalize the observed stereochemical
outcomes in many of our methodological studies employing
allenamides. This minimum conformation involves two
important factors, i.e. having approximate coplanarity
between the planes of the oxazolidinone ring and the internal
ole®n, and having the allene moiety facing away from the
carbamate carbonyl group. The CÐNÐC C torsion angle
that quanti®es this approximate coplanarity between the plane
of the oxazolidinone ring and that of the internal ole®n, as
determined from this crystallographic study, is 19.1 (2)ꢀ. A
minimized structural calculation, which determined this angle
to be 16.1ꢀ, is in close agreement. Additional structural
features include a probable ꢀ±ꢀ interaction between the
allene moiety and a benzene ring, and non-classical hydrogen
bonding in the form of weak CÐHÁ Á ÁO interactions that are
responsible for the formation of two-dimensional networks.
Comment
Allenamides have emerged as attractive building blocks in
organic synthesis (Hsung et al., 2003). With vastly improved
stability and comparable reactivity relative to traditional
allenamines, allenamides can be utilized in a diverse array of
stereoselective methodologies (Achmatowicz & Hegedus,
2004; Huang et al., 2002; Ranslow et al., 2004; Xiong et al., 2000;
Wei et al., 2001). In our own work, we have speci®cally focused
on developing highly stereoselective hetero-[4+2]-cyclo-
additions [(1) ! (2); see supplementary material; Wei et al.,
1999; Berry et al., 2003; Rameshkumar & Hsung, 2003], oxy-
allyl cation [4+3]-cycloadditions [(1) ! (3a) or (3b);
Rameshkumar & Hsung, 2004; Xiong et al., 2001, 2003] and
epoxidations [(1) ! (4); Rameshkumar et al., 2002; Xiong et
al., 2002].
To further support our claim, we sought a single-crystal
X-ray structure of an allenamide. We found only one in the
Cambridge Structural Database (CSD; Version 5.25 of
November 2003; Allen, 2002) (CSD refcode XOGHAC; Gaul
et al., 2002), which was doubly substituted at the ꢁ position
(corresponding to C1 in the present structure) of the allene.
Since steric bulk on the allene in¯uences the conformation, we
were interested in the general case containing an unsub-
stituted allenamide. We therefore prepared the series of chiral
unsubstituted allenamides (7a)±(7d), and found that allen-
amide (7a) provided X-ray quality crystals.
o830 # 2004 International Union of Crystallography
DOI: 10.1107/S0108270104023121
Acta Cryst. (2004). C60, o830±o832

organic compounds
bromide (2.2 mmol). The precipitation of the sodium salt did not
affect the reaction. The mixture was stirred at room temperature for
16 h, after which the mixture was concentrated and redissolved in
ether (20 ml) and ®ltered through a small bed of celite. The solvent
was concentrated under reduced pressure, and the residue was
puri®ed by ¯ash silica-gel column chromatography (gradient solvent
system 0±20% EtOAc in hexane) to provide the desired propargyl
product in high yield (ꢁ90%). For the preparation of allenamide (7a),
KOtBu (20 mol%) was added to a homogeneous solution of the
propargyl product prepared above (1.8 mmol) in anhydrous tetra-
hydrofuran (20 ml) under nitrogen. The reaction mixture was stirred
at room temperature for 16 h and monitored by thin-layer chroma-
1
tography (50% EtOAc in hexane) or H NMR. After removing the
solvent under reduced pressure, the crude mixture was redissolved in
ether (20±50 ml) and ®ltered through a small bed of celite or basic
Al2O3 (25% EtOAc in hexane as eluant). The solvent was removed
under reduced pressure to provide pure allenamide in 50% yield over
the two steps. Single crystals suitable for X-ray diffraction were
grown over a period of one week at 263 K from a hexane±dichloro-
methane (1:1) mixture from which the dichloromethane slowly
evaporated. RF = 0.64 (50% EtOAc in hexane); m.p. 391±397 K; [ꢂ]D20
( ) 318.3 (c 0.12, CHCl3); [ꢂ]2D0 ( ) 264.4 (c 1.05, CH2Cl2); 1H NMR
(300 MHz, CDCl3): ꢃ 4.39 (dd, 1H, J = 3.6 and 8.7 Hz), 4.49 (dd, 1H,
J = 8.7 and 8.7 Hz), 4.64 (ddd, 1H, J = 3.6, 3.9 and 8.7 Hz), 4.72 (d, 1H,
J = 3.9 Hz), 5.36 (dd, 1H, J = 6.6 and 10.2 Hz), 5.43 (dd, 1H, J = 6.6 and
10.2 Hz), 6.86 (t, 1H, J = 6.6 Hz), 7.07±7.38 (m, 10H); 13C NMR
(75 MHz, CDCl3): ꢃ 49.3, 57.2, 64.9, 88.2, 96.1, 127.2, 127.7, 128.5,
128.7, 128.9, 129.2, 38.0, 139.7, 155.0, 201.4; IR (neat, cm 1): 3055 (m),
3031 (m), 2924 (m), 1960 (w), 1757 (s), 1458 (s), 1266 (s), 889 (m);
mass spectrum (EI): m/e (% relative intensity) 291 (53) M+, 167 (61),
165 (39), 152 (25), 124 (100), 115 (14); m/e calculated for C19H17NO2:
291.1259; found: 291.1260.
Figure 1
Minimized structures of allenamides (7a)±(7d).
Crystal data
3
C19H17NO2
Dx = 1.279 Mg m
Mo Kꢂ radiation
Cell parameters from 1550
re¯ections
Figure 2
The structure of allenamide (7a). Displacement ellipsoids are drawn at
the 50% probability level.
Mr = 291.34
Monoclinic, P21
a = 6.2531 (10) A
Ê
ꢅ = 2.3±24.8ꢀ
ꢆ = 0.08 mm
T = 173 (2) K
Ê
b = 13.738 (2) A
1
Ê
c = 8.8997 (14) A
As shown in Fig. 2, the allene fragment is facing away from
the carbamate carbonyl group. It is nearly coplanar with the
oxazolidinone ring, as indicated by the C6ÐN1ÐC3 C2
torsion angle of 19.1 (4)ꢀ, which is in good agreement with
the calculated value of 16.1ꢀ. The structural data also show
the close proximity of one of the phenyl rings to the top face of
ꢄ = 98.281 (3)ꢀ
3
Ê
V = 756.6 (2) A
Z = 2
Block, colorless
0.31 Â 0.26 Â 0.19 mm
Data collection
Bruker SMART CCD area-detector
diffractometer
! scans
Absorption correction: multi-scans
(SADABS; Blessing, 1995)
Tmin = 0.975, Tmax = 0.984
4418 measured re¯ections
1406 independent re¯ections
1231 re¯ections with I > 2ꢇ(I)
Rint = 0.034
Ê
the allene. The short C3Á Á ÁC15 distance [3.517 (4) A] suggests
ꢅmax = 25.0ꢀ
the presence of a ꢀ±ꢀ interaction and further supports our
notion that the diphenylmethyl moiety is within suf®cient
proximity to block the top face of the allene. All other
bond distances and angles are unexceptional (Table 1).
Intermolecular non-classical hydrogen bonding (CÐHÁ Á ÁO;
Table 2) is responsible for the formation of a two-dimensional
network parallel to the ab plane.
h = 7 ! 7
k = 16 ! 16
l = 6 ! 10
Table 1
Selected geometric parameters (A, ).
ꢀ
Ê
N1ÐC4
N1ÐC3
N1ÐC6
1.349 (3)
1.414 (4)
1.469 (3)
C1ÐC2
C2ÐC3
1.291 (5)
1.292 (4)
Experimental
C4ÐN1ÐC3
C4ÐN1ÐC6
C3ÐN1ÐC6
121.0 (2)
111.8 (2)
126.8 (2)
C1ÐC2ÐC3
C2ÐC3ÐN1
N1ÐC6ÐC7
175.6 (3)
126.6 (2)
111.3 (2)
For the preparation of N-propargylamides, NaH (60 wt% in mineral
oil, 2.2 mmol) was added to a homogeneous solution of Sibi's di-
benzylideneoxazolidinone (1.8 mmol) in anhydrous tetrahydrofuran
(15 ml) in small portions. The resulting suspension was stirred for
30 min at room temperature before the addition of propargyl
C6ÐN1ÐC3ÐC2
19.1 (4)
ꢂ
Acta Cryst. (2004). C60, o830±o832
Michael R. Tracey et al. C19H17NO2 o831

organic compounds
Re®nement
Supplementary data for this paper are available from the IUCr electronic
archives (Reference: SX1144). Services for accessing these data are
described at the back of the journal.
Re®nement on F2
R[F2 > 2ꢇ(F2)] = 0.034
wR(F2) = 0.079
S = 1.02
1406 re¯ections
207 parameters
H atoms treated by a mixture of
independent and constrained
re®nement
w = 1/[ꢇ2(Fo2) + (0.0403P)2
+ 0.0499P]
where P = (Fo2 + 2Fc2)/3
(Á/ꢇ)max < 0.001
3
Ê
References
Áꢈmax = 0.11 e A
3
Ê
0.13 e A
Áꢈmin
=
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ꢀ
Ê
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DÐHÁ Á ÁA
DÐH
HÁ Á ÁA
DÁ Á ÁA
DÐHÁ Á ÁA
C6ÐH6AÁ Á ÁO1i
1.00
0.95
2.44
2.56
3.427 (3)
3.400 (4)
168
148
C9ÐH9AÁ Á ÁO1ii
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È
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Ê
cement parameters [CÐH = 1.06 (4) and 1.04 (4) A]. All other H
atoms were placed geometrically and re®ned with relative isotropic
displacement parameters.
Data collection: SMART (Bruker, 2001); cell re®nement: SAINT
(Bruker, 2001); data reduction: SAINT; program(s) used to solve
structure: SHELXS97 (Sheldrick, 1997); program(s) used to re®ne
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publication: SHELXTL.
RPH is grateful for funding from the NSF (grant No. CHE-
0094005).
ꢂ
o832 Michael R. Tracey et al.
C19H17NO2
Acta Cryst. (2004). C60, o830±o832