3-(Triphenylphosphoranylidene)-pentane-2,4-dione and diethyl 2-(triphenylphosphoranylidene)-malonate

Article abstract of DOI:10.1107/S0108270105015064

The title ylides, 3-(triphenylphosphoranylidene)pentane-2,4-dione, C ₂₃H₂₁O₂P, (I), and diethyl 2- (triphenylphosphoranylidene)malonate, C₂₅H₂₅O ₄P, (II), differ in the conformations adopte

Full text of DOI:10.1107/S0108270105015064

organic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
possible intermolecular interactions in the solid state. Diacyl-
phosphorane conformations should be similar to those of the
keto esters.
ISSN 0108-2701
3-(Triphenylphosphoranylidene)-
pentane-2,4-dione and diethyl
2-(triphenylphosphoranylidene)-
malonate
Fernando Castaneda,a* Christian Aliaga,a Clifford A.
Ä
b
c
d
Â
Bunton, Marõa Teresa Garland * and Ricardo Baggio
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,
bDepartment of Chemistry and Biochemistry, University of California, Santa Barbara,
The scheme above depicts a diacyl ylide that can adopt
various conformations depending on the orientations of the
carbonyl groups relative to the P atom. The methyl signal in
the 1H NMR spectrum in CHCl3 is a sharp singlet over a wide
temperature range. In ylide solutions, these groups are
therefore equivalent or are equilibrating rapidly on the NMR
timescale, although equilibration is slow in monoacyl ylides
(Wilson & Tebby, 1972). These differences are consistent with
energy barriers for equilibration from ab initio computations
(CastanÄeda, Recabarren et al., 2003; Bachrach, 1992). Con-
former (b), with both carbonyl O atoms syn with respect to the
P atom, has favorable interactions between anionoid O atoms
and cationoid P atoms, and this structure is supported by
chemical evidence (Cooke & Goswami, 1973). However, in a
near planar ylide moiety, there will be methyl±methyl repul-
sions. The anti±anti coplanar conformation, (c), should be
electrostatically disfavored because of dipole repulsions
between the carbonyl groups and possible steric repulsions
between methyl and phenyl groups.
c
Â
Â
CA 931060, USA, Departamento de Fõsica, Facultad de Ciencias Fõsicas y
Â
Matematicas, Universidad de Chile, Casilla 487-3, Santiago de Chile, Chile, and
d
Â
Â
Â
Â
Departamento de Fõsica, Comision Nacional de Energõa Atomica, Av. Gral Paz
1499, 1650 Buenos Aires, Argentina
Correspondence e-mail: fcastane@ciq.uchile.cl, mtgarlan@dfi.uchile.cl
Received 22 March 2005
Accepted 11 May 2005
Online 23 July 2005
The title ylides, 3-(triphenylphosphoranylidene)pentane-2,4-
dione, C23H21O2P, (I), and diethyl 2-(triphenylphosphoranyl-
idene)malonate, C25H25O4P, (II), differ in the conformations
adopted by their extended ylide moieties. In (I), one carbonyl
O atom is syn and the other is anti with respect to the P atom,
the ylide group is nearly planar, with a maximum PÐCÐ
(C O) angle of 18.2 (2)ꢀ, and the PÐC, CÐC and C
O
bond lengths are consistent with electronic delocalization
involving the O atoms. In (II), both carbonyl O atoms are anti
and the ester groups are twisted out of the plane of the near
trigonal ylide C atom, reducing delocalization, the largest PÐ
CÐ(C O) angle being 30.2 (2)ꢀ.
The crystal structures of two ylides are discussed here, viz.
3-(triphenylphosphoranylidene)pentane-2,4-dione, (I), and
diethyl 2-(triphenylphosphoranylidene)malonate, (II), stabi-
lized by diketo and diester groups, respectively (Figs. 1 and 2).
Comment
For acyl(alkoxycarbonyl)phosphoranes (keto esters), the
conformations are similar in solution and in the solid state,
with the keto and alkoxy O atoms oriented towards the
phosphorus and the acyl groups in the ylide plane, thus
allowing extended ylide resonance (Abell & Massy-Westropp,
1982; Abell et al., 1988, 1989). Classical structures of stabilized
ylides are accordingly written with a double bond between the
ylide C atom and the stabilized group (Bachrach & Nitsche,
1994). We have shown elsewhere (CastanÄeda, Terraza et al.,
2003) that acyl(alkoxycarbonyl)phosphoranes, Ph3P
C(COR0)CO2R, can adopt a near planar preferred confor-
mation that allows extensive electronic delocalization and
favorable interactions between the cationoid P atom and the
keto and alkoxy O atoms, both in the solid state and in solu-
tion. The preferred conformations result from both attractive
and repulsive intramolecular interactions in solution, and
The structural results are consistent with previous NMR,
chemical and computational evidence (Castan
Äeda, Terraza et
al., 2003) regarding differences in the solid state and in solu-
tion. Selected bond lengths and angles are presented in
Tables 1 and 3, with hydrogen-bond and short-contact
geometry in Tables 2 and 4.
o496 # 2005 International Union of Crystallography
DOI: 10.1107/S0108270105015064
Acta Cryst. (2005). C61, o496±o499

organic compounds
The two ylides share a number of common features, in
particular a slightly distorted tetrahedral arrangement around
the P atom, as observed for stabilized keto±ester phosphorus
ylides (CastanÄeda, Terraza et al., 2001). The PÐC bonds [P1Ð
C1 is clearly sp2-hybridized, the sum of the bond angles being
essentially 360ꢀ [359.9 (5) and 359.6 (5)ꢀ, respectively].
A distinctive feature that differentiates the two structures is
the disposition of the C O groups; in (I), one carbonyl O
atom is syn and the other is anti with respect to the P atom,
while in (II), both O atoms are positioned anti. Though neither
of the two extended ylide groups is planar, the carbonyl
groups in (I) deviate less from the plane de®ned by the nearly
trigonal ylide C atom than those in (II), the maximum absolute
value for the PÐCylideÐC O torsion angles being 18.2 (2)ꢀ
for (I) and 30.2 (2)ꢀ for (II). This structural disposition favors
an extensive electronic delocalization in the nearly planar
ylide system in (I) and the interaction of one carbonyl O atom
with the cationoid P atom, and minimizes intramolecular
interference involving methyl groups.
Ê
C1 = 1.7521 (18) and 1.748 (3) A for (I) and (II), respectively]
are longer than typical double bonds, because of the ylidic
resonance, and intermediate between commonly accepted
Ê
values for single and double bonds (PÐC = 1.80±1.83 A and
Ê
C = 1.66 A; Howells et al., 1973). In both ylides, ylide atom
P
Both structures have several intra- and intermolecular non-
bonding interactions, which, despite being weak, have
profound effects on both the molecular and the packing
conformations. The intramolecular interactions and contacts
are mainly of the CÐHÁ Á Áꢀ type, involving methyl H atoms
and phenyl groups. In (I), methyl atom C5 interacts with a
Ê
phenyl ring, with a H5BÁ Á ÁCg1 distance of 2.79 A and a
Ê
C5Á Á ÁCg1 distance of 3.541 (3) A (Fig. 1; Cg1 is the centroid of
the C11/C21/C31/C41/C51/C61 ring). In (II), the CÁ Á Áꢀ(arene)
contacts involve methyl atom C7, with a C7Á Á ÁCg2 distance of
Ê
3.984 (6) A (Fig. 2; Cg2 is the centroid of the C12/C22/C32/
C42/C52/C62 ring). The ®rst of these interactions has an
important effect on the molecular geometry, through the
reduction of the C11ÐP1ÐC13 angle to 103.31 (8)ꢀ, sensibly
smaller than expected for a strictly tetrahedral P atom, and
with the concomitant opening of the C1ÐP1ÐC12 angle
[114.62 (9)ꢀ; Fig. 1].
There are also several short PÁ Á ÁO contacts of different
types, as a result of conformational differences in the two
structures; in (I), such contacts involve carbonyl atom O1,
which is syn to phosphorus and thus favored for this type of
Figure 1
The molecular structure of (I), showing the atomic numbering scheme,
and intermolecular (single broken lines) and intramolecular (double
broken lines) contacts. H atoms, except phenyl H atoms involved in
hydrogen bonds, have been omitted for clarity. Displacement ellipsoids
are drawn at the 30% probability level. For symmetry codes, refer to
Table 2.
Ê
interaction [P1Á Á ÁO1 = 2.767 (2) A]. The corresponding O
atoms (O2 and O4) in (II) are oriented away from phosphorus
and are therefore prohibited from this kind of contact but
Ê
have a short O2Á Á ÁO4 intramolecular distance of 2.816 (3) A.
The syn-oriented alkoxy atoms O1 and O3 (relative to phos-
phorus) are oriented near phosphorus [PÁ Á ÁO = 3.028 (3) and
Ê
2.775 (3) A, respectively]. Intermolecular interactions are
mainly CÐHÁ Á ÁO hydrogen bonds involving phenyl donors
and carbonyl acceptors (Tables 2 and 4). However, the most
important effects are those that allow (or forbid) electronic
delocalization.
In (I), the distortions from planarity of the extended ylide
group (as induced by non-bonding interactions) are not
extremely severe; the PÐCÐC O torsion angles (Table 1)
suggest some degree of coplanarity and, concomitantly, ylide
resonance involving both acyl moieties. This interpretation
correlates with the IR carbonyl stretching frequencies in KBr
(1600 and 1557 cm 1), which are similar to those in CHCl3
(1601 and 1540 cm 1). In (II), however, they appear to favor
the out-of-plane geometry of the ylide and carbonyl moieties,
and the consequent decrease in ylide resonance. The out-of-
Figure 2
The molecular structure of (II), showing the atomic numbering scheme,
and intermolecular (single broken lines) and intramolecular (double
broken lines) contacts. H atoms, except phenyl H atoms involved in
hydrogen bonds, have been omitted for clarity. Displacement ellipsoids
are drawn at the 30% probability level. For symmetry codes, refer to
Table 4.
ꢁ
Acta Cryst. (2005). C61, o496±o499
Castaneda et al. C23H21O2P and C25H25O4P o497
Ä

organic compounds
Table 1
Selected geometric parameters (A, ) for (I).
plane torsion angles (Table 3) are consistent with the IR
carbonyl stretching frequencies (1711 and 1632 cm 1), indi-
cating that in the solid state only one carbonyl group is
suf®ciently close to coplanarity with respect to the ylide
moiety to participate signi®cantly in electronic delocalization.
It is often assumed that in the search for a balance between
opposing ylide resonance and non-bonding interactions it is
the former that dominates conformations of stabilized
phosphorus ylides, and structures are usually written with a
formal double bond between the ylide and acyl C atoms. This
structural assumption appears to be valid for (I) both in the
crystal and in solution, and for the keto esters, but it is not
valid for (II). To this extent, (II) behaves differently from the
other ylides stabilized by keto and ester groups as a result of
interactions in the solid state of carbonyl atoms O2 and O4
with H atoms of neighboring ylides. The presence (or absence)
of such interactions is therefore a major factor controlling
conformation both in the crystalline state and in solution.
ꢀ
Ê
P1ÐC1
1.7521 (18)
1.8109 (18)
1.8159 (18)
1.8182 (18)
1.243 (2)
O2ÐC4
C1ÐC2
C1ÐC4
C2ÐC3
1.228 (2)
1.444 (3)
1.449 (3)
1.507 (3)
P1ÐC11
P1ÐC12
P1ÐC13
O1ÐC2
C1ÐP1ÐC11
C1ÐP1ÐC12
C1ÐP1ÐC13
C11ÐP1ÐC12
C11ÐP1ÐC13
110.03 (8)
114.62 (9)
109.22 (8)
107.92 (9)
103.31 (8)
C12ÐP1ÐC13
C2ÐC1ÐP1
C4ÐC1ÐP1
C2ÐC1ÐC4
111.14 (8)
110.80 (13)
125.20 (14)
123.94 (17)
P1ÐC1ÐC2ÐO1
18.2 (2)
P1ÐC1ÐC4ÐO2
164.61 (16)
Table 2
Hydrogen-bond and short-contact geometry (A, ) for (I).
ꢀ
Ê
DÐHÁ Á ÁA
DÐH
HÁ Á ÁA
DÁ Á ÁA
DÐHÁ Á ÁA
C31ÐH31Á Á ÁO2i
C63ÐH63Á Á ÁO2ii
C51ÐH51Á Á ÁO1iii
0.93
0.93
0.93
2.59
2.47
2.52
3.422 (3)
3.183 (2)
3.376 (3)
149
134
153
Experimental
Symmetry codes: (i) x; y ‡ 21; z 12; (ii) x ‡ 1; y; z; (iii) x; y ‡ 1; z.
Compound (I) was prepared by reaction of 1-(triphenylphosphor-
anylidene)propan-2-one with acetic anhydride (Chopard et al., 1965)
(yield 80%, m.p. 437±438 K from ethyl acetate/cyclohexane, 1:1).
Compound (II) was prepared by Horner & Oediger (1958) from
triphenylphosphine dichloride and diethyl malonate in a basic
medium (56% yield), but we report here a simpler synthesis by
transylidation. A solution of ethyl chloroformate (39 mmol) in dry
benzene (10 ml) was added slowly to (carboethoxymethylene)-
triphenylphosphorane (60 mmol) in dry benzene (100 ml) under a
dry atmosphere at room temperature. After 1 h, (carboethoxy-
methyl)triphenylphosphonium chloride separated from the stirred
solution as a white solid. The ®ltered solvent was evaporated to give
an oil which was crystallized from ethyl acetate (yield 85%, m.p. 379±
380 K).
Compound (II)
Crystal data
C25H25O4P
Mr = 420.42
Mo Kꢂ radiation
Cell parameters from 25
re¯ections
Monoclinic, P21=c
ꢃ = 7.5±12.5ꢀ
Ê
a = 12.572 (14) A
1
Ê
b = 9.022 (12) A
ꢄ = 0.15 mm
T = 295 (2) K
Ê
c = 19.52 (2) A
ꢁ = 90.12 (2)ꢀ
V = 2214 (4) A
Rectangular prism, colorless
0.35 Â 0.20 Â 0.15 mm
3
Ê
Z = 4
Dx = 1.261 Mg m
3
Compound (I)
Data collection
Crystal data
Siemens R3m diffractometer
!/2ꢃ scans
h = 0 ! 14
3
C23H21O2P
Mr = 360.37
Dx = 1.264 Mg m
Mo Kꢂ radiation
Cell parameters from 25
re¯ections
k = 10 ! 0
4069 measured re¯ections
3880 independent re¯ections
3138 re¯ections with I > 2ꢅ(I)
Rint = 0.027
l = 23 ! 23
Monoclinic, P21=c
2 standard re¯ections
every 98 re¯ections
intensity decay: 1.8%
Ê
a = 14.9365 (18) A
ꢃ = 7.5±12.5ꢀ
ꢄ = 0.16 mm
T = 295 (2) K
Ê
b = 9.6588 (13) A
1
Ê
c = 13.7975 (16) A
ꢃ
max = 25.1ꢀ
ꢁ = 107.907 (9)ꢀ
3
Ê
V = 1894.1 (4) A
Z = 4
Rectangular prism, colorless
0.40 Â 0.24 Â 0.20 mm
Table 3
Selected geometric parameters (A, ) for (II).
ꢀ
Ê
Data collection
Siemens R3m diffractometer
!/2ꢃ scans
h = 17 ! 16
P1ÐC1
P1ÐC11
P1ÐC12
P1ÐC13
O1ÐC2
O1ÐC3
O2ÐC2
1.748 (3)
1.828 (3)
1.819 (4)
1.814 (3)
1.363 (3)
1.482 (4)
1.214 (4)
O3ÐC5
O3ÐC6
O4ÐC5
C1ÐC2
C1ÐC5
C3ÐC4
1.372 (3)
1.438 (3)
1.214 (3)
1.453 (3)
1.440 (4)
1.466 (5)
k = 11 ! 0
3480 measured re¯ections
3325 independent re¯ections
2782 re¯ections with I > 2ꢅ(I)
Rint = 0.013
l = 0 ! 16
2 standard re¯ections
every 98 re¯ections
intensity decay: 1.5%
ꢃmax = 25.0ꢀ
Re®nement
C1ÐP1ÐC11
C1ÐP1ÐC12
C1ÐP1ÐC13
C11ÐP1ÐC12
C11ÐP1ÐC13
C12ÐP1ÐC13
108.60 (15)
115.14 (16)
111.66 (11)
108.71 (12)
106.69 (11)
105.68 (13)
C2ÐO1ÐC3
C5ÐO3ÐC6
P1ÐC1ÐC2
P1ÐC1ÐC5
C2ÐC1ÐC5
117.4 (2)
116.4 (2)
122.95 (19)
117.87 (17)
118.8 (2)
Re®nement on F2
R[F2 > 2ꢅ(F2)] = 0.040
wR(F2) = 0.106
S = 1.05
3325 re¯ections
w = 1/[ꢅ2(F2o) + (0.0657P)2
+ 0.5296P]
where P = (F2o + 2Fc2)/3
(Á/ꢅ)max = 0.004
3
Ê
Áꢆmax = 0.22 e A
3
Ê
0.26 e A
237 parameters
H-atom parameters constrained
Áꢆmin
=
P1ÐC1ÐC2ÐO2
149.8 (2)
P1ÐC1ÐC5ÐO4
170.8 (2)
ꢁ
o498 Castaneda et al.
C23H21O2P and C25H25O4P
Acta Cryst. (2005). C61, o496±o499
Ä

organic compounds
Table 4
Hydrogen-bond and short-contact geometry (A, ) for (II).
We acknowledge the Spanish Research Council (CSIC) for
providing us with a free-of-charge license to the CSD system.
ꢀ
Ê
DÐHÁ Á ÁA
C53ÐH53Á Á ÁO2iv
Symmetry codes: (iv) x ‡ 1; y
DÐH
HÁ Á ÁA
DÁ Á ÁA
DÐHÁ Á ÁA
Supplementary data for this paper are available from the IUCr electronic
archives (Reference: GG1259). Services for accessing these data are
described at the back of the journal.
0.93
2.36
3.213 (6)
152
1
2
;
z ‡ 12.
References
Re®nement
Abell, A. D., Clark, B. M. & Robinson, W. T. (1989). Aust. J. Chem. 42, 1161±
1167.
Re®nement on F2
R[F2 > 2ꢅ(F2)] = 0.047
wR(F2) = 0.129
S = 1.10
3880 re¯ections
272 parameters
H-atom parameters constrained
w = 1/[ꢅ2(F2o) + (0.059P)2
+ 1.0148P]
where P = (F2o + 2Fc2)/3
(Á/ꢅ)max = 0.003
3
Ê
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3
Ê
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Áꢆmin
=
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SHELXL97
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Ê
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Ê
Ê
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ꢁ
Acta Cryst. (2005). C61, o496±o499
Castaneda et al. C23H21O2P and C25H25O4P o499
Ä