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

The first 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 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 [pi]-[pi] 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.

S1. 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 et al., 2004; Huang et al., 2002; Ranslow et al., 2004; Xiong et al., 2000; Wei et al., 2001). In our own work, we have specifically focused on developing highly stereoselective hetero [4 + 2] cycloadditions (1 → 2; see supplementary materials) (Wei et al., 1999; Berry et al., 2003; Rameshkumar et al., 2003), oxyallyl cation [4 + 3] cycloadditions (1 → 3a or 3 b) (Rameshkumar et al., 2004; Xiong et al., 2001, 2003) and epoxidations (1 → 4) (Rameshkumar et al., 2002; Xiong et al., 2002).

In all our studies, there exists a unified theme for their stereochemical outcomes. We have proposed a preferred conformation for these allenamides in which the plane of the internal olefin 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 Scheme 1. 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, specifically at the C2═C3 bond. The specific direction of the allene moiety then promotes the observed diastereoselectivity. If there were free rotation about the C—N 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); these calculations? were derived from the concerted rotation of the allene around the N1—C3 bond by 360° 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 rotational 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, 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, with a C6—N1—C3═C2 torsion angle of −16.1°. Additionally, we calculated the rotational barriers and the torsion angles (corresponding to the C6—N1—C3═C2 torsion angle in 7a) for chiral allenamides 7 b–7 d and found these values to be similar to those of chiral allenamide 7a.

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 gamma position (corresponding to C1 in our structure) of the allene. Since steric bulk on the allene influences the conformation, we were interested in the general case containing an unsubstituted allenamide. We therefore prepared the series of chiral unsubstituted allenamides 7a–7 d, and found that allenamide 7a provided X-ray quality crystals for which a structural determination was performed.

As shown in Fig. 2, the allene fragment is facing away from the carbamate carbonyl. It is nearly co-planar with the oxazolidinone ring as indicated by the C6—N1—C3═C2 torsional angle of −19.1 (4)°, which is in good agreement with the calculated result of −16.1°. The structural data also show the close proximity of one of the phenyl rings to the top face of the allene. The short C3···C15 distance of 3.517 (4) Å suggests the presence of a π–π interaction and further supports our notion that the diphenyl moiety is within sufficient proximity to block the top face of the allene. All other bond distances and angles are unexceptional. Intermolecular non-classical hydrogen bonding (C—H···O; Table 2) is responsible for the two-dimensional network parallel to the ab plane.

S2. Experimental

For the preparation of N-propargyl amides, to a homogeneous solution of Sibi's dibenzylidene oxazolidinone (1.8 mmol) in anhydrous tetrahydrofuran (15 ml) was added NaH (60wt% in mineral oil, 2.2 mmol) in small portions. The resulting suspension was stirred for 30 min at room temperature before the addition of propargyl 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 into ether (20 ml) and filtered through a small bed of celite. The solvent was concentrated under reduced pressure, and the residue was purified by flash 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, to a homogeneous solution of the propargyl product prepared above (1.8 mmol) in anhydrous tetrahydrofuran (20 ml) was added KOtBu (20 mol%) under nitrogen. The reaction mixture was stirred at room temperature for 16 h and monitored by TLC (50% EtOAc in hexane) or ¹H NMR. After removing the solvent under reduced pressure, the crude mixture was redissolved in ether (20–50 ml) and filtered 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% over the two steps. Single crystals suitable for X-ray diffraction were grown over a one week period at 263 K from a hexane–dichloromethane (1:1) mixture in which the dichloromethane slowly evaporated. R_f = 0.64 (50% EtOAc in hexane); m.p. 391–397 K; [α]_D²⁰ (-) 318.3 (c 0.12, CHCl₃); [α]_D²⁰ (-) 264.4 (c 1.05, CH₂Cl₂); ¹H NMR (300 MHz, CDCl₃): δ 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): δ 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.

S3. Refinement

Because of the lack of significant anomalous scattering, Friedel equivalents were merged upon final refinement. The absolute configuration was assigned on the basis of known and unchanging chiralities of precursor molecules (Scheme 1). H atoms on the terminal end of the allene moeity (C1) were found from a difference Fourier map and refined with individual isotropic displacement parameters. All other H atoms were placed geometrically and refined with relative isotropic displacement parameters.

Computing details

Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Bruker, 2000); software used to prepare material for publication: SHELXTL.

Figures

Scheme 1. Synthesis of allenamides. Figure 1. Minimized structures of allenamides 7a–7 d. Figure 2. Structure of allenamide 7a. Displacement ellipsoids are drawn at the 50% probability level. The second view highlights the coplanarity between the planes of the oxazolidinone ring and the internal olefin.

(7a)

Crystal data

C19H17NO2	F(000) = 308
Mr = 291.34	Dx = 1.279 Mg m^−3
Monoclinic, P21	Mo Kα radiation, λ = 0.71073 Å
a = 6.2531 (10) Å	Cell parameters from 1550 reflections
b = 13.738 (2) Å	θ = 2.3–24.8°
c = 8.8997 (14) Å	µ = 0.08 mm^−1
β = 98.281 (3)°	T = 173 K
V = 756.6 (2) Å^3	Block, colorless
Z = 2	0.31 × 0.26 × 0.19 mm

Data collection

Bruker SMART CCD area-detector diffractometer	1406 independent reflections
Radiation source: normal-focus sealed tube	1231 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.034
ω scans	θmax = 25.0°, θmin = 2.3°
Absorption correction: multi-scans (SADABS; Blessing, 1995)	h = −7→7
Tmin = 0.975, Tmax = 0.984	k = −16→16
4418 measured reflections	l = −6→10

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.034	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.079	H atoms treated by a mixture of independent and constrained refinement
S = 1.02	w = 1/[σ^2(Fo^2) + (0.0403P)^2 + 0.0499P] where P = (Fo^2 + 2Fc^2)/3
1406 reflections	(Δ/σ)max < 0.001
207 parameters	Δρmax = 0.11 e Å^−3
1 restraint	Δρmin = −0.13 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
O1	0.2813 (3)	0.34256 (16)	0.2904 (3)	0.0605 (6)
O2	0.5406 (3)	0.34983 (14)	0.4936 (2)	0.0514 (5)
N1	0.6177 (3)	0.27542 (15)	0.2879 (2)	0.0375 (5)
C1	0.8930 (6)	0.2308 (3)	−0.0383 (4)	0.0550 (8)
H1A	0.967 (6)	0.287 (3)	−0.095 (4)	0.089 (12)*
H1B	0.943 (6)	0.161 (3)	−0.059 (5)	0.090 (12)*
C2	0.7485 (5)	0.24623 (19)	0.0490 (3)	0.0410 (6)
C3	0.5943 (4)	0.2571 (2)	0.1300 (3)	0.0439 (7)
H3A	0.4509	0.2524	0.0782	0.053*
C4	0.4641 (5)	0.32383 (19)	0.3496 (3)	0.0454 (7)
C5	0.7700 (4)	0.33048 (19)	0.5217 (3)	0.0442 (7)
H5A	0.8116	0.3022	0.6240	0.053*
H5B	0.8535	0.3911	0.5139	0.053*
C6	0.8113 (4)	0.25813 (18)	0.3993 (3)	0.0348 (6)
H6A	0.9460	0.2747	0.3563	0.042*
C7	0.8180 (4)	0.15216 (17)	0.4590 (3)	0.0332 (6)
H7A	0.6836	0.1417	0.5057	0.040*
C8	1.0100 (4)	0.13262 (18)	0.5827 (3)	0.0354 (6)
C9	1.0122 (5)	0.04589 (18)	0.6632 (3)	0.0434 (7)
H9A	0.8921	0.0030	0.6451	0.052*
C10	1.1882 (5)	0.0213 (2)	0.7698 (3)	0.0535 (8)
H10A	1.1892	−0.0388	0.8225	0.064*
C11	1.3627 (5)	0.0840 (3)	0.7997 (3)	0.0566 (8)
H11A	1.4833	0.0671	0.8726	0.068*
C12	1.3596 (5)	0.1706 (2)	0.7233 (3)	0.0546 (8)
H12A	1.4777	0.2143	0.7450	0.066*
C13	1.1868 (4)	0.1952 (2)	0.6149 (3)	0.0438 (7)
H13A	1.1883	0.2552	0.5619	0.053*
C14	0.8155 (4)	0.07853 (16)	0.3323 (3)	0.0351 (6)
C15	0.6343 (5)	0.0217 (2)	0.2869 (3)	0.0484 (7)
H15A	0.5127	0.0268	0.3393	0.058*
C16	0.6284 (6)	−0.0426 (2)	0.1655 (4)	0.0623 (9)
H16A	0.5033	−0.0810	0.1352	0.075*
C17	0.8022 (6)	−0.0502 (2)	0.0906 (4)	0.0633 (10)
H17A	0.7968	−0.0926	0.0057	0.076*
C18	0.9860 (6)	0.0031 (2)	0.1371 (4)	0.0591 (9)
H18A	1.1087	−0.0039	0.0863	0.071*
C19	0.9925 (5)	0.06699 (19)	0.2577 (3)	0.0460 (7)
H19A	1.1203	0.1033	0.2895	0.055*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0467 (12)	0.0635 (13)	0.0726 (15)	0.0191 (11)	0.0126 (11)	0.0178 (12)
O2	0.0600 (12)	0.0453 (11)	0.0519 (12)	0.0115 (10)	0.0188 (10)	−0.0024 (9)
N1	0.0385 (12)	0.0356 (11)	0.0375 (14)	0.0069 (9)	0.0022 (10)	0.0024 (10)
C1	0.064 (2)	0.062 (2)	0.0373 (18)	−0.0090 (16)	0.0023 (15)	−0.0020 (15)
C2	0.0484 (16)	0.0404 (14)	0.0304 (14)	−0.0006 (12)	−0.0072 (13)	0.0015 (11)
C3	0.0420 (15)	0.0465 (15)	0.0391 (16)	0.0065 (12)	−0.0084 (13)	0.0004 (13)
C4	0.0505 (16)	0.0361 (14)	0.0519 (18)	0.0098 (13)	0.0155 (14)	0.0113 (13)
C5	0.0560 (16)	0.0377 (15)	0.0396 (16)	0.0005 (13)	0.0088 (13)	−0.0023 (12)
C6	0.0382 (14)	0.0365 (13)	0.0291 (14)	0.0010 (11)	0.0026 (11)	0.0001 (11)
C7	0.0334 (12)	0.0352 (13)	0.0318 (14)	0.0002 (10)	0.0072 (11)	0.0020 (10)
C8	0.0422 (14)	0.0390 (13)	0.0253 (13)	0.0035 (11)	0.0056 (11)	−0.0017 (11)
C9	0.0604 (18)	0.0354 (13)	0.0341 (15)	0.0060 (12)	0.0058 (13)	−0.0018 (12)
C10	0.084 (2)	0.0437 (15)	0.0334 (16)	0.0235 (16)	0.0089 (15)	0.0044 (13)
C11	0.0572 (19)	0.072 (2)	0.0370 (17)	0.0201 (17)	−0.0048 (14)	−0.0020 (16)
C12	0.0471 (16)	0.079 (2)	0.0357 (17)	−0.0038 (15)	−0.0015 (13)	0.0033 (16)
C13	0.0432 (15)	0.0546 (16)	0.0325 (16)	−0.0015 (13)	0.0020 (12)	0.0058 (13)
C14	0.0405 (14)	0.0315 (13)	0.0313 (14)	0.0032 (11)	−0.0015 (11)	0.0036 (10)
C15	0.0528 (17)	0.0405 (15)	0.0492 (18)	−0.0061 (14)	−0.0019 (14)	0.0030 (13)
C16	0.073 (2)	0.0430 (17)	0.063 (2)	−0.0055 (16)	−0.0165 (18)	−0.0071 (16)
C17	0.092 (3)	0.0468 (17)	0.0444 (19)	0.0178 (17)	−0.0135 (19)	−0.0126 (14)
C18	0.075 (2)	0.060 (2)	0.0426 (18)	0.0171 (17)	0.0088 (16)	−0.0060 (15)
C19	0.0515 (17)	0.0472 (16)	0.0383 (16)	0.0064 (13)	0.0031 (13)	−0.0037 (13)

Geometric parameters (Å, º)

O1—C4	1.215 (3)	C8—C13	1.397 (4)
O2—C4	1.350 (4)	C9—C10	1.387 (4)
O2—C5	1.445 (3)	C9—H9A	0.9500
N1—C4	1.349 (3)	C10—C11	1.385 (5)
N1—C3	1.414 (4)	C10—H10A	0.9500
N1—C6	1.469 (3)	C11—C12	1.369 (5)
C1—C2	1.291 (5)	C11—H11A	0.9500
C1—H1A	1.06 (4)	C12—C13	1.383 (4)
C1—H1B	1.04 (4)	C12—H12A	0.9500
C2—C3	1.292 (4)	C13—H13A	0.9500
C3—H3A	0.9500	C14—C19	1.379 (4)
C3—C15	3.517 (4)	C14—C15	1.387 (4)
C5—C6	1.524 (4)	C15—C16	1.392 (4)
C5—H5A	0.9900	C15—H15A	0.9500
C5—H5B	0.9900	C16—C17	1.359 (5)
C6—C7	1.549 (3)	C16—H16A	0.9500
C6—H6A	1.0000	C17—C18	1.376 (5)
C7—C14	1.514 (3)	C17—H17A	0.9500
C7—C8	1.531 (3)	C18—C19	1.382 (4)
C7—H7A	1.0000	C18—H18A	0.9500
C8—C9	1.389 (4)	C19—H19A	0.9500
C4—O2—C5	108.8 (2)	C13—C8—C7	123.7 (2)
C4—N1—C3	121.0 (2)	C10—C9—C8	120.6 (3)
C4—N1—C6	111.8 (2)	C10—C9—H9A	119.7
C3—N1—C6	126.8 (2)	C8—C9—H9A	119.7
C2—C1—H1A	124 (2)	C11—C10—C9	120.4 (3)
C2—C1—H1B	121 (2)	C11—C10—H10A	119.8
H1A—C1—H1B	115 (3)	C9—C10—H10A	119.8
C1—C2—C3	175.6 (3)	C12—C11—C10	119.4 (3)
C2—C3—N1	126.6 (2)	C12—C11—H11A	120.3
C2—C3—H3A	116.7	C10—C11—H11A	120.3
N1—C3—H3A	116.7	C11—C12—C13	120.8 (3)
O1—C4—N1	127.5 (3)	C11—C12—H12A	119.6
O1—C4—O2	122.6 (3)	C13—C12—H12A	119.6
N1—C4—O2	109.9 (2)	C12—C13—C8	120.6 (3)
O2—C5—C6	105.2 (2)	C12—C13—H13A	119.7
O2—C5—H5A	110.7	C8—C13—H13A	119.7
C6—C5—H5A	110.7	C19—C14—C15	118.3 (3)
O2—C5—H5B	110.7	C19—C14—C7	120.8 (2)
C6—C5—H5B	110.7	C15—C14—C7	120.9 (2)
H5A—C5—H5B	108.8	C14—C15—C16	120.8 (3)
N1—C6—C5	99.7 (2)	C14—C15—H15A	119.6
N1—C6—C7	111.3 (2)	C16—C15—H15A	119.6
C5—C6—C7	111.5 (2)	C17—C16—C15	119.8 (3)
N1—C6—H6A	111.3	C17—C16—H16A	120.1
C5—C6—H6A	111.3	C15—C16—H16A	120.1
C7—C6—H6A	111.3	C16—C17—C18	120.2 (3)
C14—C7—C8	109.97 (19)	C16—C17—H17A	119.9
C14—C7—C6	112.1 (2)	C18—C17—H17A	119.9
C8—C7—C6	113.1 (2)	C17—C18—C19	120.1 (3)
C14—C7—H7A	107.1	C17—C18—H18A	119.9
C8—C7—H7A	107.1	C19—C18—H18A	119.9
C6—C7—H7A	107.1	C14—C19—C18	120.7 (3)
C9—C8—C13	118.2 (2)	C14—C19—H19A	119.7
C9—C8—C7	118.0 (2)	C18—C19—H19A	119.7
C1—C2—C3—N1	148 (4)	C14—C7—C8—C13	112.4 (3)
C4—N1—C3—C2	152.3 (3)	C6—C7—C8—C13	−13.7 (3)
C6—N1—C3—C2	−19.1 (4)	C13—C8—C9—C10	−1.6 (4)
C3—N1—C4—O1	13.2 (4)	C7—C8—C9—C10	175.8 (2)
C6—N1—C4—O1	−174.2 (3)	C8—C9—C10—C11	1.3 (4)
C3—N1—C4—O2	−167.4 (2)	C9—C10—C11—C12	0.1 (4)
C6—N1—C4—O2	5.2 (3)	C10—C11—C12—C13	−1.2 (5)
C5—O2—C4—O1	−171.3 (2)	C11—C12—C13—C8	0.9 (4)
C5—O2—C4—N1	9.2 (3)	C9—C8—C13—C12	0.5 (4)
C4—O2—C5—C6	−19.2 (3)	C7—C8—C13—C12	−176.8 (2)
C4—N1—C6—C5	−16.1 (3)	C8—C7—C14—C19	−56.1 (3)
C3—N1—C6—C5	156.0 (2)	C6—C7—C14—C19	70.5 (3)
C4—N1—C6—C7	101.7 (2)	C8—C7—C14—C15	124.6 (2)
C3—N1—C6—C7	−86.2 (3)	C6—C7—C14—C15	−108.7 (3)
O2—C5—C6—N1	20.3 (2)	C19—C14—C15—C16	−2.4 (4)
O2—C5—C6—C7	−97.2 (2)	C7—C14—C15—C16	176.9 (2)
N1—C6—C7—C14	58.6 (3)	C14—C15—C16—C17	0.2 (4)
C5—C6—C7—C14	169.0 (2)	C15—C16—C17—C18	2.0 (5)
N1—C6—C7—C8	−176.4 (2)	C16—C17—C18—C19	−1.9 (5)
C5—C6—C7—C8	−66.1 (3)	C15—C14—C19—C18	2.4 (4)
C14—C7—C8—C9	−65.0 (3)	C7—C14—C19—C18	−176.8 (2)
C6—C7—C8—C9	169.0 (2)	C17—C18—C19—C14	−0.3 (4)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
C6—H6A···O1^i	1.00	2.44	3.427 (3)	168
C9—H9A···O1^ii	0.95	2.56	3.400 (4)	148

Symmetry codes: (i) x+1, y, z; (ii) −x+1, y−1/2, −z+1.