organic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
in Tables 1 to 8. Ylides (I) to (V) have a nearly tetrahedral P
atom with helically arranged phenyl rings. The PhA phenyl
rings bisect the planes formed by the ylidic C atoms and the
carbonyl groups, while the PhC phenyl rings present their
faces to the alkoxy groups.
ISSN 0108-2701
Methyl, ethyl, isopropyl and tert-butyl
3-oxo-2-(triphenylphosphoranyl-
idene)butyrates, a common pattern for
a preferred conformation
Fernando Castaneda,a Claudio A. Terraza,a Maria Teresa
Ä
Garland,b Clifford A. Buntonc and Ricardo F. Baggiod*
a
Â
Â
Â
Departamento de Quõmica Organica y Fisicoquõmica, Facultad de Ciencias
Â
Â
Quõmicas y Farmaceuticas, Universidad de Chile, Casilla 233, Santiago, Chile,
b
Â
Â
Â
Departamento de Fõsica, Facultad de Ciencias Fõsicas y Matematicas, Universidad
de Chile, Casilla 487-3, Santiago, Chile, cDepartment of Chemistry and Biochem-
istry, University of California, Santa Barbara, CA 93106, USA, and dDepartamento de
Â
Â
Â
Â
Fõsica, Comision Nacional de Energõa Atomica, Buenos Aires, Argentina
Correspondence e-mail: baggio@cnea.gov.ar
The PhÐP C and PhÐPÐPh angles, which are close to
109ꢁ, indicate a slightly distorted tetrahedral environment
around the P centres, arguing against dsp3-P hybridization.
The major deviations from normal values are observed for the
C1CÐP1ÐC1B angles in the ethyl [101.2 (1)ꢁ], isopropyl
[101.0 (2)ꢁ] and tert-butyl [100.2 (1)ꢁ] derivatives, probably as
a result of a steric compression between the PhC rings and the
alkoxy groups, which pushes PhC toward PhB. The sums of the
angles about the ylidic C atom are, in all cases, consistent with
a near trigonal planar geometry and sp2 hybridization.
Received 24 July 2000
Accepted 3 November 2000
The crystal structures of four alkyl 3-oxo-2-(triphenyl-
phosphoranylidene)butyrates, where the alkyl group is methyl
(C23H21O3PÁ0.5C6H6), (II), ethyl (C24H23O3P), (III), isopropyl
(C25H25O3P), (IV), or tert-butyl (C26H27O3P), (V), show all of
them to have the same conformation. They present a
tetrahedral P atom and an sp2 ylidic C atom, with the carbonyl
groups adopting anti conformations with respect to the keto
groups located close to the P atom. PÐCÐCÐO torsion
angles, bond lengths and angles indicate an effective electronic
delocalization toward the keto groups. In each case, one H
atom of the alkoxy group is close to one of the phenyl rings.
These preferred conformations are evaluated as the result of
attractive and repulsive intramolecular interactions.
The PÐC1 bond lengths, which are in the range 1.740 (2)±
Ê
1.757 (3) A, are between the commonly accepted values for a
Ê
single (ꢂ1.80±1.83 A; Howells et al., 1973) and a double bond
Ê
(ꢂ1.63±1.73 A; Howells et al., 1973) as a consequence of
Comment
Crystalline (3-carboethoxy-3-triphenylphosphoranylidene-2-
oxopropanyl)triphenylphosphonium bromide, (I), has the
1
conformation shown in the scheme below and its H NMR
spectrum shows that, in solution, the ethoxylic methyl-H
atoms are diamagnetically shielded by a phenyl ring (Bara-
hona et al., 1998). Similar shielding has been observed with
other non-ionic ylides, indicating that the preferred confor-
mation of (I) is not dictated by the ionic phosphonium residue
(Bachrach & Nitsche, 1994; Bacaloglu et al., 1995; Abell et al.,
1982; Abell & Massy-Westropp, 1982). We have therefore
examined the crystal structures of four non-ionic ylides with
different alkoxy groups, i.e. with R = CH3, (II), CH2CH3, (III),
CH(CH3)2, (IV), and C(CH3)3, (V). One of these compounds,
(III), has already been described by Abell et al. (1989) and is
almost isostructural with (IV).
Figure 1
The molecular structure of (II) showing the atom-numbering scheme.
Displacement ellipsoids are drawn at the 40% probability level and H
atoms have been omitted for clarity.
The molecular structures of compounds (II)±(V) are shown
in Figs. 1 to 4, respectively, and selected dimensions are given
ꢀ
180 # 2001 International Union of Crystallography
Printed in Great Britain ± all rights reserved
Acta Cryst. (2001). C57, 180±184
Castaneda, Fernando
