Preferred conformations of stabilized phosphorus ylides

Article abstract of DOI:10.1080/10426500390228657

The stabilized phosphorus ylides, Ph₃P=(CO.R′)CO.OR; 1, R=Et, R′=CH₂P⁺Ph₃; 2, R=R'′=Me; 3, R=Et, R′=Me; 4, R=Prⁱ; R′=Me; 5, R=Bu^t; R′=Me, adopt a near planar conformation in the crystal whi

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Preferred Conformations of Stabilized Phosphorus
Ylides
Fernando Castañeda a , Claudio A. Terraza a , Clifford A. Bunton b , Nicholas D. Gillitt b
María T. Garland a
&
a University of Chile , Santiago, Chile
b University of California , Santa Barbara, Santa Barbara, California
Published online: 23 Aug 2006.
To cite this article: Fernando Castañeda , Claudio A. Terraza , Clifford A. Bunton , Nicholas D. Gillitt & María T. Garland
(2003) Preferred Conformations of Stabilized Phosphorus Ylides, Phosphorus, Sulfur, and Silicon and the Related Elements,
178:9, 1973-1985, DOI: 10.1080/10426500390228657
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Phosphorus, Sulfur, and Silicon, 178:1973–1985, 2003
ꢀC
Copyright Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500390228657
PREFERRED CONFORMATIONS OF STABILIZED
PHOSPHORUS YLIDES
Fernando Castan˜eda,a Claudio A. Terraza,a
Clifford A. Bunton,b Nicholas D. Gillitt,b and Mar´ıa T. Garlanda
University of Chile, Santiago, Chilea and University of
California, Santa Barbara, Santa Barbara, Californiab
The stabilized phosphorus ylides, Ph3P C(CO.Rꢁ)CO.OR; 1, R Et,
Rꢁ CH2P+Ph3; 2, R Rꢁ Me; 3, R Et, Rꢁ Me; 4, R Pri; Rꢁ Me; 5,
R But; Rꢁ Me, adopt a near planar conformation in the crystal which
allows extensive electronic delocalization. The keto and alkoxylic oxy-
gens are oriented and align favorably with the cationoid phosphorus.
These conformations bring methyl hydrogens in the ester residue into
proximity with the face of a phenyl group and lead to π-shielding and
upfield shifts of the 1H NMR signals of 3 over a wide temperature range
(−50–95◦C) in (CD3)2CO, CDCl3 and DMSOd-6. Geometries of 2 and
3, optimized by using the HF 3-21 (G∗) or 6-31 (G∗) basis sets, are very
similar to those in the crystal, but semiempirical treatments generate
structures in which either the ester or keto moiety is twisted out of plane.
Keywords: Conformation; NMR spectra; optimized structures;
phosphorus ylides
Conformations of stabilized phosphorus ylides (shown in Scheme 1)
depend on the balance between ylidic delocalization involving the P C
and C O bonds and nonbonding dipole-dipole and steric interactions.1,2
Electronic delocalization favors coplanarity, but steric repulsions have
the opposite effect. Crystalline ylides 1–5 are approximately coplanar
in the ylidic moiety (refer to Scheme 1),3–5 reflecting the role of de-
localization. The keto oxygen is oriented toward the cationoid phos-
phorus, and the ester moiety adopts the Z-(trans) conformation, as in
simple carboxylic esters.6 The presence of the phosphonium ionic center
The authors thank the CEPEDEQ, Facultad de Ciencias Qu´ımicas y Farmace´uticas,
Universidad de Chile for instrumental facilities.
Address correspondence to Fernando Castan˜eda, Departamento de Qu´ımica Orga´nica
y Fisicoqu´ımica, Facultad de Ciencias Qu´ımicas y Farmace´uticas, Universidad de Chile,
Casilla 233, Santiago, Chile. E-mail: xramirez@ciq.uchile.cl
1973

1974
F. Castan˜eda et al.
SCHEME 1 Conformations of stabilized keto-ester phosphorus ylides 1–5.
and CHCl3 in crystalline 1,4 or the original cocrystallization of 3 with
acetic acid,3b do not affect structures, which are apparently controlled
by bonding and nonbonding interactions in the ylidic residue. Confor-
mations with both carbonyl oxygens syn to phosphorus are excluded by
x-ray crystallography and would not explain the observed NMR spectral
evidence. 3–5
In crystalline 1 and 3 the methyl of the ethoxy group is close to the
face of a phenyl group and in solution shielding significantly decreases
1
the H NMR chemical shift of the methyl hydrogens.4 Shielding had
been observed earlier for some keto-ester ylides3a,e,f in solution and
for a 2-triphenylphosphoranylidene succinic acid derivative7 where the
methyl of an ethoxy group could be close to the face of a phenyl group.
These observations indicate that simple 1H NMR spectroscopy can pro-
vide evidence on ylidic conformation in solution. Nishio et al.8 give many
examples of compounds in which a methyl group is in close proximity
to the face of an aromatic group. They conclude that these interactions
may be energetically favorable and involve weak, nonclassical, hydro-
gen bonding.8,9 However, in some examples cited, interactions in other
parts of the molecule may control the overall geometry and this could
be the situation in our ylides.
Interactions between cationoid phosphorus and carbonyl oxygen are
important in monoacyl ylides with significant barriers to rotation about
the formal single bond between ylidic and carbonyl carbons.2,10 Ylide
monoesters can adopt conformations with the carbonyl oxygen syn or
anti to phosphorus (refer to Scheme 2).11 As with the monoacyl deriva-
tives there are significant rotational barriers, but in the absence of large
steric interactions, conformers do not have very different free energies.
SCHEME 2 Conformations of ylide monoesters.

Stabilized Phosphorus Ylides
1975
Conformations in the crystal are well defined, but may differ from
those in solution, which can be monitored by examining the NMR
spectra.1,2,10 We follow this general approach with ylides 1–5, typically
with CDCl3 as solvent, although acetone-d6 and DMSO-d6 were used in
studying temperature effects. Conformations often depend on temper-
1
ature and solvent composition, and we used H NMR spectroscopy to
examine the conformation of 3 over a range of conditions. We also com-
pared conformations of 2 and 3 from x-ray crystallography with those
from geometrical optimization12,13 because our preliminary work on an
otherwise similar diacyl ylides 6, PH3P C (CO·CH3)2, indicates that it
has different conformations in the crystal and in solution.
RESULTS AND DISCUSSION
NMR-Spectroscopy
Initially 1H NMR spectra were obtained in CDCl3 at 25◦C, but we var-
ied both temperature and solvent and examined the spectrum of 3 in
acetone-d6 and DMSO-d6 at low and high temperatures respectively.
Signals were sharp in all conditions (down to −50◦C), with no indica-
tion of isomeric mixtures. Complete NMR data are in the Experimental
section.
Chemical shifts of the keto CH3 in 2–5 are almost independent of
other substituents, but those of β-CH3 of the alkoxy group are lowered
by π-shielding relative to δ O-CH2-CH3 of 1.26 ppm for ethyl acetate14b
(see Table I). The upfield shift is most evident with the ethoxy deriva-
tives, 14 and 3, and persists with the iso-propoxy derivative, 4, but
to a lesser extent, because the effect is averaged over six rather than
three hydrogens. We saw only limited evidence of π-shielding with the
TABLE I Methyl 1H NMR Chemical Shifts and Carbonyl Stretching
Frequencies of Keto-Ester Phosphorus Ylides 1–5. Ph3P C(CO
Rꢁ)CO2Ra
ν, cm−1
R
Rꢁ
δ(R), ppm
δ(Rꢁ), ppm Ester Ketone
1
2
3
4
5
CH2CH3
CH3
CH2CH3
CH(CH3)2
C(CH3)3
CH2P+Ph3
0.57
3.10
0.63
0.75
1.03
1653
1681
1654
1652
1662
1562
1560
1555
1552
1543
CH
2.49
2.46
2.50
2.47
3
CH
3
CH
3
CH
3
aNMR spectra in CDCl3 at 25◦C and IR spectra in a KBr disk.

1976
F. Castan˜eda et al.
t-butoxy derivative, 5, where any effect would be averaged over nine hy-
drogens, although in the crystal the t-butoxy residue of 5 is close to the
face of a phenyl group.5 The 1H chemical shifts of the t-butyl groups of
t-BuOH and t-BuOMe are 1.28 and 1.19 ppm, respectively, in CDCl3, 14
and are higher than that in 5, although they should be lower in terms of
electronic substituent effects (see Table I). The 1H NMR signal of OCH3
of the methyl ester, 2, also is shifted upfield, by ca. 0.5 ppm (see Table I)
relative to simple methyl esters (the corresponding chemical shift for
methyl acetate is 3.67 ppm14b).
These spectra indicate that the alkoxylic moiety and the keto oxygen
are syn- to phosphorus which reduces dipole repulsions between the
carbonyl groups and allows favorable interactions between phosphorus
and the keto and alkoxylic oxygens as in the crystal.3,5 The chemical
shifts of the keto CH3 are approximately 2.5 ppm (see Table I) and there
is no indication of shielding by a phenyl group, consistent with the keto
oxygen being oriented toward phosphorus.
1
Temperature effects (from −50 to 95◦C) on the H NMR spectrum
of 3 in the various solvents are much very smaller (see Table II) than
those observed for monoacyl and monoester ylides, indicating a strong
preference for a single average conformation. Signals are sharp, even at
low temperatures, consistent with little hindrance to rotation about the
TABLE II Variable Temperature 1H NMR Spectral Data of Ethyl
2-Triphenylphosphoranylidene-3-oxobutyrate (3).
Ph3P C(CO CH3) CO2CH2CH3
Chemical shifts; ppm
Line widths; Hz
CO·OCH2CH3
CO CH3
CH3
CO·OCH2CH3
CO CH3
CH3
Solvent temp. ◦
C
CH3
CH2
CH3
DMSO-d6
95
0.61
0.57
0.55
0.53
0.50
3.59
3.57
3.57
3.54
3.54
2.27
2.27
2.27
2.27
2.27
0.80
0.86
1.22
1.13
1.20
1.13
1.22
1.42
1.45
1.43
65
50
35
20
Acetone-d6
17.7
10
0.61
0.59
0.57
0.55
0.54
0.52
0.50
0.48
3.63
3.62
3.61
3.61
3.60
3.59
3.59
3.59
2.31
2.31
2.31
2.31
2.31
2.31
2.31
2.31
0.86
0.88
0.78
0.79
0.91
1.14
1.33
1.69
1.04
1.09
1.08
1.12
1.13
1.32
1.36
1.51
0
−10
−20
−30
−40
−50

Stabilized Phosphorus Ylides
1977
ylidic or other bonds.2 Line widths of CH2-CH3 and COCH3 are higher
in DMSOd-6 than in acetoned-6, due to differences in solvent viscosi-
ties and decrease modestly with increasing temperature (see Table II).
Chemical shifts of CH2 CH3 and COCH3 depend only slightly on tem-
perature and solvent, but those of CH2 CH3 increase with increasing
temperature, probably because the increase in mobilities within the
ylide decreases π-shielding by the phenyl groups.
Structural Optimization
We used an ab initio quantum mechanical treatment of structures of
2 and 3,12,13 and compared them with those in the crystal.5 Initially
we carried out a conformational search with MMFF parameters and
followed it with semiempirical AM1 or MNDO/d treatments which
gave structures with one carbonyl group approximately coplanar with
the P C bond and the other approximately orthogonal to it, which
were very different from those in the crystal.5 However, approximately
planar ylidic structures with both carbonyl groups anti- to phosphorus
were disfavored, probably by dipolar repulsions. Those with both keto
and ester carbonyl groups oriented towards phosphorus were also
disfavored. Some of these structures are inconsistent with the observed
π-shielding of the alkoxylic CH3 and the lack of shielding of the keto
CH3 signals (see Tables I and II). All differ markedly from those in
the crystal. It appears that these semiempirical treatments do not
account for the balance between ylidic delocalization and nonbonding
interactions.
Structures of 2 and 3 optimized by using the HF 3-21 (G∗), followed by
the 6-31 (G∗) basis set, are very similar to those in the crystal and struc-
tures from the two basis sets are identical to the eye.5 We started the
calculations from different initial structures, e.g., an MMFF best confor-
mation followed by AM1 or MNDO/d predicted structures, or an initial
input geometry with keto and ester carbonyl groups respectively syn-
and anti to phosphorus. These independent calculations gave almost
identical heats of formation enthalpies, but with small differences in
C P phenyl torsional angles. As a test we took an ab initio derived struc-
ture of 3, and applied the AM1 treatment, which gave a nonplanar struc-
ture, but the subsequent ab initio treatment restored the near planar
ylidic structure almost identical to that from the original computation.
Structures optimized in ab initio calculations allow extended elec-
tronic delocalization and orientation of oxygen lone pairs toward
cationoid phosphorus and no inter-carbonyl dipolar repulsions, as in
the crystal.5 Structures in solution apparently are mobile, with rel-
atively rapid rotation about the P C bond2 and sharp, temperature

1978
F. Castan˜eda et al.
independent, 1H NMR signals (see Table II). Small changes in geometry
probably do not significantly affect formation enthalpies. These calcu-
lations neglect interactions with the solvent which seem to be relatively
unimportant.
Structures in the Crystal and in Solution
Conformations in solution depend on a balance between electronic de-
localization, and nonbonding dipole-dipole and steric interactions, but
packing forces can be important in the crystal. Interactions with sol-
1
vents are relatively unimportant, based on the similar H NMR spec-
tra in different solvents and differences between CHCl3, (CH3)2SO, and
(CH3)2C O as hydrogen bond donors/acceptors15 (see Tables I and II).
Structures in the crystal generally can be defined unambiguously,
but conformers may interconvert rapidly in solution, and relatively slow
methods, such as NMR spectroscopy, then provide only averaged geome-
tries. Structures from theoretical calculations are limited by the rigor
of the basis set, the time required for the calculation, and allowance for
solute-solvent interactions.13,16
◦
˚
TABLE III Selected Computed and X-Ray Geometric Parameters (A, ) for
Methyl 2-Triphenylphosphoranylidene-3-oxobutyrate (2).
a
Ph3P C(CO CH3) CO2CH3
˚
Bond lengths (A)
P1 C1
P1 C(C)
P1 C(A)
P1 C(B)
C1 C2
C1 C5
1.74
1.81
1.82
1.81
1.44
1.43
[1.747(2)]
[1.802(2)]
[1.815(2)]
[1.819(2)]
[1.428(3)]
[1.441(3)]
C2-O4
C2 C3
C5 O7
C5 O6
O6 C8
1.23
1.52
1.22
1.37
1.45
[1.243(2)]
[1.510(3)]
[1.212(2)]
[1.349(3)]
[1.435(3)]
Bond angles (◦)
C1 P1 C1C
C1 P1 C1A
C1C P1 C1A
C1 P1 C1B
C1C P1 C1B
Torsion angles (◦)
P1 C1 C2 O4
115
116
106
110
107
[115.61(10)]
[108.97(10)]
[106.92(9)]
[111.13(10)]
[105.66(10)]
C1A P1 C1B
C2 C1 C5
C2 C1 P1
C5 C1 P1
103
121
118
120
[108.24(9)]
[123.05(19)]
[113.18(15)]
[123.63(16)]
10.6
[12.0(3)]
P1 C1 C5 O7
. . . .
166
[162.47(19)]
[2.971(2)]
˚
Contact distances (A)
. . . .
P1
O4
2.94
3.16
[2.806(2)]
P1
O6
2.84
. . . .
C8(H)
Ph
aX-ray crystallographic values5 are in parentheses with the estimated SD, and posi-
tions are numbered as in Figure 1. (Reproduced by permission from the International
Union of Crystallography.)

Stabilized Phosphorus Ylides
1979
◦
˚
TABLE IV Selected Computed and X-Ray Geometric Parameters (A, ) for
Ethyl 2-Triphenylphosphoranylidene-3-Oxobutyrate (3).
a
Ph3P C(CO CH3) CO2CH2CH3
˚
Bond lengths (A)
P1 C1
P1 C(A)
P1 C(B)
P1 C(C)
C1 C2
C1 C5
C2 O4
1.73 [1.757(3)]
1.81 [1.804(3)]
1.80 [1.817(3)]
1.81 [1.821(3)]
1.45 [1.434(4)]
1.43 [1.444(4)]
1.23 [1.248(4)]
C2 C3
C5 O7
C5 O6
O6 C8
C8 C9
1.52 [1.507(4)]
1.22 [1.197(4)]
1.38 [1.352–1.360(8)]
1.46 [1.457–1.466(8)]
1.53 [1.494–1.497(10)]
Bond angles (◦)
C1 P1 C1A
C1 P1 C1B
C1A P1 C1B
C1 P1 C1C
C1A P1 C1C
Torsion angles (◦)
P1 C1 C2 O4
115
113
109
111
100
[109.67(14)] C1B P1 C1C
[113.78(14)] C2 C1 C5
[107.20(15)] C2 C1 P1
[114.37(15)] C5 C1 P1
[110.18(15)]
107 [101.19(14)]
121 [123.5(3)]
117 [109.9(2)]
118 [126.4(3)]
11
[1.7(4)]
P1 C1 C5 O7 178 [170.7(3)]
˚
Contact distances (A)
. . . .
. . . .
O6
P1
O4
2.95 [2.699(4)]
3.7
P1
2.70 [3.014–3.026(7)]
. . . .
C9(H)
Ph
aX-ray crystallographic values5 are in parentheses with the estimated SD, and posi-
tions are numbered as in Figure 2. (Reproduced by permission from the International
Union of Crystallography.)
Calculated geometries of 2 and 3 are similar to those in the crystal,
and bond angles and lengths are compared in Tables III and IV.
The numbering of the atoms in the calculated structures, as shown
in Figures 1 and 2, corresponds to those used in describing crystal
structures.5 In comparing computed geometries with those in the crys-
tal, we round off computed lengths and distances with the second dec-
imal place and angles to the nearest degree, because of computational
. . .
. . .
limitations, and contact distances C8(H) Ph and C9(H) Ph, are ap-
proximate and are based on computed distances to phenyl carbon atoms.
1
The H NMR spectra of 1–5, with the upfield shifts of CH3 of the
alkoxylic groups and the essentially constant chemical shifts of the
keto CH3 in various conditions, are consistent with the crystal struc-
tures, although they do not provide evidence regarding values of the
P1 C1 C2 O4 torsional angles. The close proximities of CH3 of the
alkoxylic groups to phenyl and the π-shielding agree with the re-
sults of Nishio et al.,8,9 although they may be directed by the elec-
tronic delocalization and interactions of the keto and ester groups with
phosphorus rather than by C H/π interactions. There should be a fa-
vorable interaction between a keto oxygen and a phosphonium ion

1980
F. Castan˜eda et al.
FIGURE 1 The molecular structure of methyl 2-triphenylphosphoranylidene-
3-oxobutyrate (2) showing calculated and crystal geometries. For the crys-
tal structure hydrogen atoms have been omitted for clarity. (Reproduced by
permission from the International Union of Crystallography.)
FIGURE 2 The molecular structure of ethyl 2-triphenylphosphoranylidene-3-
oxobutyrate (3) showing calculated and crystal geometry. For the crystal struc-
ture hydrogen atoms have been omitted for clarity. (Reproduced by permission
from the International Union of Crystallography.)

Stabilized Phosphorus Ylides
1981
. . .
in 1 (refer to Scheme 1) where, in the crystal, the P O distance is
2.9 A.
4
˚
In the crystal, the ester C O bond (C5-O7) is consistently shorter
5
˚
than the keto C O bond (C2 O4) (by 0.023–0.051 A), and this short-
ening, although not its magnitude, is predicted by calculations (see
Tables III and IV). Bond angles at the ylidic C1 deviate from the strictly
trigonal 120◦ due to a balance between steric compressions involving
phenyl and alkoxy groups, and C3 and O7, and favorable interactions
5
˚
between P1 and O4 and O6, where interatomic distances are ca. 2.8 A.
These variations are predicted qualitatively for 2, but the calculated
C5 C1 P1 bond angle of 3 is low (see Table IV). We note a problem in
refining the geometry of the ethoxy residue in crystalline 3.5 Predicted
distances of the methyl hydrogen of an alkoxylic group to the face of a
˚
phenyl group (ca. 3.6 A) are consistent with the observed upfield NMR
shifts (see Tables I, III and IV).
Predicted bond lengths and angles are similar to those in the crys-
tal and differences in torsional angles of the phenyls and contact
distances are due to limitations in the calculations or to packing
forces in the crystal. The sharp signals in the NMR spectra indi-
cate that structures are mobile in solution, with free rotation about
P C and C CH3 bonds on the NMR timescale. Thus structures in
the crystal and averaged structures in solution could differ provided
that conformational barriers are low in solution (see Tables I and
II).2–4 The situation is different for monoacyl ylides where barriers are
high.10
Carbonyl Stretching Vibrations
The IR carbonyl stretching frequencies of the ester and keto residues
are relatively insensitive to substituents (see Table I). Ylides undergo
a large number of stretching and bending vibrations, and, for simplic-
ity, we examined those of the carbonyl groups of 2 and 3 by taking the
structures obtained by using the 3-21 (G∗) or 6-31 (G∗) basis sets and
identifying the motions by using PM3 semiempirical parameters,12,13,17
which give for 2, ν = 1980 and 1873 cm−1 for the ester and keto car-
bonyl groups respectively. The corresponding values for 3 are 1974 and
1875 cm−1. This procedure indicates that stretch of the keto carbonyl
groups is not strongly coupled with other motions, except for bending of
the bonds to the carbonyl carbon, but that of the ester carbonyl groups
is coupled strongly with bending of the O C OR residues. This proce-
dure predicts frequencies which are too high by factors of 1.19–1.22,12
and the factors are approximately 1.32 if we use the AM1 rather than
the PM3 basis set in identifying motions.

1982
F. Castan˜eda et al.
CONCLUSIONS
In the crystal and in solution ylides 1–5 adopt conformations with the
alkoxylic group, but not the keto methyl group, oriented toward phos-
phorus. The near planarity of the ylidic moiety in the crystal permits
extensive electronic delocalization and favorable interactions between
cationoid phosphorus and the keto and alkoxylic oxygens. These struc-
tures allow π-shielding of CH3 of the alkoxylic groups due to their
directed orientations, with the possibility of mildly favorable C H/π
interactions.8,9
Interactions with solvents used in crystallizations appear not to af-
fect conformations in the crystal3,4 or in solution (see Tables III and
IV), although CHCl3 is a hydrogen-bond donor and DMSO interacts
with cationoid centers.15
Structures of 2 and 3 optimized by using the HF 3-21 (G∗) and 6-31
(G∗) basis sets are similar to those in the crystal. However, but molec-
ular mechanics and semiempirical treatments fail by giving structures
in which either the keto or ester group is twisted out of plane.
EXPERIMENTAL
Synthesis
The acylalkoxycarbonyl-ylides 2–5 were prepared by conventional
transylidation methods.1,5 The phosphonium-ylide salt 1 was ob-
tained by reaction of ethoxycarbonylmethylenetriphenyl phosphorane
with ethoxycarbonylmethyltriphenylphosphonium bromide.18 Com-
mercially available solvents for NMR measurements were used without
further purification except that where as necessary trace acid in CDCl3
was removed by Al2O3 or NaHCO3. The phosphorus-carbon coupling
constants are very similar to those seen earlier in other phosphorus
ylides.19
A solution of acyl chloride (20 mmol) in 5 cm3 of dry C6H6 was
added slowly to the alkoxycarbonylmethylenetriphenylphosphorane
(40 mmol) and 100 cm3 of dry C6H6 under a dry atmosphere. The
stirred solution was maintained at room temperature for 30 min to
allow a white crystalline solid to separate. After filtration of alkoxy-
carbonylmethyltriphenylphosphonium chloride, the solvent was evap-
orated to give the alkyl-2-triphenylphosphoranylidene-3-oxobutyrates,
2–5, as oils which were crystallized from ethyl acetate.
Methyl 2-Triphenylphosphoranylidene-3-oxobutyrate (2). 68% from
methoxy carbonylmethylenetriphenylphosphorane and acetyl chloride,

Stabilized Phosphorus Ylides
1983
m.p. 151–152◦C, lit. 151–152◦C20; νmax (KBr)/cm−1 1681 (CO2CH3) and
1560 (COCH3)). 1H-NMR δppm (300 MHz; CDCl3): 2.49 (3H, s, CO CH3),
3.10 (3H, s, O CH3), 7.4–7.75 (15H, m, aromatic); 13C-NMR δppm (75.4
MHz; CDCl3): 29.2 (d, 3JP C6.6, CO CH3), 49.5 (O CH3), 71.2 (d,
1
3
1JP C111.9, P C), 126.6 (d, JP
93.3, C6H5), 128.5 (d, JP
12.1,
C
C
4
2
C6H5), 131.6 (d, JP 3.2, C6H5), 133.1 (d, JP 9.9, C6H5), 168.5 (d,
C
C
2JP 15.3, CO2Me), 195.4 (d, 2JP 3.3, COMe). 31P-NMR δppm (121.4
C
C
MHz; CDCl3): 17.9.
Ethyl 2-Triphenylphosphoranylidene-3-oxobutyrate (3). 79%, m.p.
169–171◦C, lit. 169–171◦C;21 νmax (KBr)/cm−1 1654 (CO2Et) and 1555
(COCH3)). 1H-NMR δppm (300 MHz; CDCl3): 0.63 (3H, t, J 7.1,
O CH2 CH3), 2.46 (3H, s, COCH ), 3.7 (2H, q, J 7.1, O CH CH3),
3
2
7.5 (15H, m, aromatic). 13C-NMR δppm (75.4 MHz; CDCl3): 14.1
(O CH2 CH3), 29.7 (d, 3JP C 6.85, CO CH3), 58.7 (O CH2 CH3), 70.9
(d, 1JP C111.6, P C), 126.8 (d, 1JP C 93.65, C6H5), 128.4 (d, 3JP C 12.45,
4
2
C6H5), 131.4 (d, JP 2.9, C6H5), 133.0 (d, JP 9.8, C6H5), 168.1 (d,
C
C
2
2JP
15, CO2Et), 195.4 (d, JP
4.1, COMe). 31P-NMR δPPM (121.4
C
C
MHz; CDCl3): 17.6.
i-Propyl-2-triphenylphosphoranylidene-3-oxobutyrate (4). 89% from
i-propoxy carbonylmethylenetriphenylphosphorane and acetyl chlo-
ride, m.p. 186–187◦C; νmax (KBr)/cm−1 1652 (CO2Pri) and 1552
(COMe)). 1H-NMR δppm (300 MHz; CDCl3): 0.75 (6H, d, J 6.2,
O CH(CH3)2), 2.50 (3H, s, COCH3), 4.6–5.1 (1H, m,
J 6.2,
O CH(CH3)2), 7.4–8.0 (15H, m, aromatic). 13C-NMR δppm (75.4 MHz;
CDCl3): 21.5 (CO2CH(CH3)2), 29.1 (d, 3JP
6.6, COCH3), 65.6
C
(CO2CH(CH3)2), 71.7 (d, 1JP
111.9, P C), 126.7 (d, 1JP
93.9,
C
C
3
4
C6H5), 128.5 (d, JP
12.1, C6H5), 131.6 (d, JP
3.3, C6H5), 133.2
C
C
2
2
2
(d, JP 9.8, C6H5), 167.5 (d, JP C15.3, CO2Pri), 195.2 (d, JP 3.3,
C
C
COMe). Anal. Calcd. for C25H25O3P: C, 74.24; H, 6.23. Found: C, 74.36;
H, 6.32.
t-Butyl-2-triphenylphosphoranylidene-3-oxobutyrate (5). (69%) from
tert-butoxy carbonylmethylenetriphenylphosphorane and acetyl chlo-
ride, m.p. 177–178◦C; νmax (KBr)/cm−1 1662 (CO2But) and 1543
1
(COCH3)). H-NMR δppm (300 MHz; CDCl3): 1.03 (9H, s, OC(CH3)3),
2.47 (3H, s, CO CH3), 7.3–8.0 (15H, m, aromatic). 13C-NMR δppm (75.4
3
MHz; CDCl3): 28.0 (CO2C(CH3)3), 29.75 (d, JP 7.5,CO CH3), 55.0
C
1
3
(CO2C(CH3)3), 70.4 (P C), 127.1 (d, JP 91.3, C6H5) 128.4 (d, JP
C
C
12.8, C6H5), 131.3 (d, 4JP C 3.8, C6H5), 133.0 (d, 2JP C 9.0, C6H5), 167.6
(d, JP 14.3, CO2But), 195.0 (d, JP 3.8, COMe). Anal. Calcd. for
2
2
C
C
C26H27O3P: C, 74.63; H, 6.50. Found: C, 74.55; H, 6.40.

1984
F. Castan˜eda et al.
Spectra
1H NMR spectra were recorded on Bruker DRX 300 or Varian INOVA
500 spectrometers with tetramethylsilane (TMS) as internal reference
and 13C and 31P NMR spectra were recorded on the Bruker spectrometer
and are referred to TMS or external 85% H3PO4, respectively, with δ,
ppm., and J, Hz. Infrared spectra were obtained with a KBr disk on a
Bruker IFS 56 FT spectrometer.
Structural Optimization
Calculations were made with Spartan Plus 2.0 (Wavefunction)
software.12
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