{(E)- and {(Z)-2-[α,β-bis(methoxycarbonyl)vinyl]cyclopentadien-1-ylidene} triphenylphosphorane

Article abstract of DOI:10.1107/S0108270104006067

The Ramirez ylide undergoes electrophilic substitution with dialkyl acetylenedicarboxylates, yielding a mixture of the Z and E adducts. The crystal structure analyses of the two adducts formed using dimethylacetylene, viz. dimethyl (E)-and (Z)-1-[2-(triphenylphosphoranylidene)cyclopentadien-1-yl]ethylenedicarboxylate, both C₂₉H₂₅O₄P, explain an unusual chemical shift observed for the vinyl H atom of the Z adduct, which had previously precluded a definitive assignment of the isomers. In addition, the structures explain why only one of the isomers reacts further with acetylene esters to produce azulenes with a rare substitution pattern.

Full text of DOI:10.1107/S0108270104006067

organic compounds
Acta Crystallographica Section C
Crystal Structure
Communications
isomer by recrystallization. An X-ray crystallographic study
proved this to be, somewhat surprisingly, the Z adduct, (IIIa)
(Fig. 1).
ISSN 0108-2701
{(E)- and {(Z)-2-[a,b-bis(methoxy-
carbonyl)vinyl]cyclopentadien-1-
ylidene}triphenylphosphorane
Lee J. Higham, P. Gabriel Kelly, Helge Muller-Bunz and
È
Declan G. Gilheany*
The approximate planarity of the ®ve-membered ring and
the alkene function, together with the short distance between
Ê
atoms C2 and C6 [1.438 (2) A], suggests conjugation of the
Department of Chemistry, University College Dublin, Belfield, Dublin 4, Ireland
Correspondence e-mail: declan.gilheany@ucd.i.e
new alkene with the cyclopentadienyl ring. It is also clear that
the vinyl H atom is in an unusual location, placing it within the
bonding domain of the ylide bond (an ylide pocket), that
provides some explanation, together with the extensive
conjugation present, as to the unusual chemical shift of
4.71 p.p.m. We calculate the shortened contact distance
between atom P1 and the vinyl H atom attached to atom C9 to
Received 2 February 2004
Accepted 15 March 2004
Online 9 April 2004
The Ramirez ylide undergoes electrophilic substitution with
dialkyl acetylenedicarboxylates, yielding a mixture of the Z
and E adducts. The crystal structure analyses of the two
adducts formed using dimethylacetylene, viz. dimethyl (E)-
and (Z)-1-[2-(triphenylphosphoranylidene)cyclopentadien-1-
yl]ethylenedicarboxylate, both C29H25O4P, explain an unusual
chemical shift observed for the vinyl H atom of the Z adduct,
which had previously precluded a de®nitive assignment of the
isomers. In addition, the structures explain why only one of the
isomers reacts further with acetylene esters to produce
azulenes with a rare substitution pattern.
Ê
be 2.81 (2) A. This unusual H-atom position might be thought
of as a factor in the elongation of the C1ÐC2 bond
Ê
[1.4496 (19) A] relative to the C1ÐC5 bond length
Ê
[1.411 (2) A]. On the other hand, there is also substantial
elongation of the C1ÐC2 bond in the other adduct (see
below) and in the Ramirez ylide itself. The internuclear PÁ Á ÁH
Ê
distance is 2.71 A, calculated on the basis of an internuclear
Ê
CÐH distance of 1.08 A.
Comment
The ylide (cyclopentadien-1-ylidene)triphenylphosphorane
(often referred to as the Ramirez ylide), (I), is known to be
unreactive in the Wittig reaction because of the aromaticity
present in the ®ve-membered ring (Ramirez & Levy, 1957a,b).
Instead, as shown by Yoshida et al. (1971a,b, 1973), (I) reacts
with electrophiles, undergoing substitution at the 2-position.
In one case, an activated acetylene (diethyl acetylenedi-
carboxylate) was used, which gave a putative vinyl adduct,
(II), but the stereochemistry of the double bond was not
established. We had cause to reinvestigate this reaction and we
extended the study to other dialkyl acetylene esters [R = Me,
Et and tBu, i.e. (IV)]. The reactions typically gave two isomeric
adducts, one of which exhibits an unusual chemical shift
(4.7 p.p.m.) in the 1H NMR spectrum. This behaviour
precluded a de®nitive E/Z assignment of the isomers.
In addition, we have found that one isomer in each case
reacts with further acetylene ester to give azulenes with a rare
substitution pattern (Higham et al., 2004). It was clear that an
X-ray crystallographic study would identify the isomer
stereochemistry, shed light on the unusual chemical shift and
provide valuable information as to why only one isomer
should react to form azulenes. To this end, we reacted (I) with
dimethyl acetylenedicarboxylate and isolated the major
The spectroscopic data for the minor product were consis-
tent with a species formed quantitatively when dichloro-
methane solutions of (IIIa) were subjected to a source of UV
light for 96 h (this process could not be duplicated by
prolonged heating). Recrystallization from dichloromethane±
ethanol yielded orange crystals, which, when analysed by
X-ray diffraction, were found to be the E isomer, (IIIb)
(Fig. 2). In contrast to (IIIa), there is a high steric hindrance
inherent in (IIIb) which twists the alkene group and ®ve-
membered ring out of the same plane by 43.3 (2)ꢀ; the C2ÐC6
Ê
bond length increases to 1.464 (3) A, concomitant with the
associated loss of electron delocalization. The 1H NMR
spectrum supports this result in giving a chemical shift closer
to that expected for vinyl protons [6.29 p.p.m. compared with
4.71 p.p.m. in (IIIa)]. Pertinent bond lengths and angles for (I),
o308 # 2004 International Union of Crystallography
DOI: 10.1107/S0108270104006067
Acta Cryst. (2004). C60, o308±o311

organic compounds
(IIIa) and (IIIb) are given in Table 1 [see Ammon et al. (1973)
for comprehensive structural details of (I)].
more delocalized than in the parent. However, this conclusion
is not consistent with the lack of coplanarity between the ®ve-
membered ring and the vinyl substituent in (IIIb). It is also
noteworthy that the external C2ÐC1ÐP angle is wider in the
adducts [131.86 (11)ꢀ in (IIIa), 130.43 (14)ꢀ in (IIIb) and
125.3 (2)ꢀ in (I)].
It is striking that the bond lengths within the ®ve-membered
rings of (I), (IIIa) and (IIIb) are consistently unequal. Thus,
the C1ÐC2 and C4ÐC5 bonds are both elongated, while the
C3ÐC4 bond is truncated compared with the C1ÐC5 and
C2ÐC3 bonds. It is not clear why the bond-length inequalities
in the adducts (which might be expected) should also be
re¯ected in the parent, and we assume that this behaviour is a
coincidence. The PÐC1 bond lengths should be more infor-
mative, being related to the nature of the ylide bond, and it is
notable that, in the adducts, these bonds are elongated
We were also interested in the details of the conformation
of the ylide portion of the molecules. It is frequently observed
in X-ray studies that the carbanion substituents tend to take
up an orientation perpendicular to the plane of one of the P-
substituent bonds, referred to as the unique substituent. This
behaviour is illustrated in Fig. 3, the conformation being called
perpendicular. In addition, it is common that a deviation from
carbanion planarity occurs, of up to 20ꢀ from the plane
perpendicular to the plane containing the P C bond and the
unique substituent. Where there is a deviation, it is usually
towards the unique substituent, the PÐC bond length of
which is, in turn, lengthened. These observations are linked to
the electronic structure of a typical phosphonium ylide
(Gilheany, 1994), where there is overlap of the occupied p
orbital on the carbanion into an antibonding orbital of the
phosphorus substituent (called negative hyperconjugation).
Therefore, it is notable that in both adducts there is no such
deviation from planarity and no evidence that one of the PÐ
C(Ph) bonds is unique.
Ê
Ê
[1.7412 (14) A in (IIIa), 1.7357 (18) A in (IIIb) and
Ê
1.718 (2) A in (I)]. Ordinarily, this elongation would mean
(Gilheany, 1994) that the ylide carbanion in the adducts is
The bond lengths, X-ray photoelectron spectra and 1H
NMR coupling constants previously obtained for the Ramirez
ylide have been used to describe the amount of ylide±ylene
character present (86% ylide), but this two-component
bonding description has since fallen out of favour (Gilheany,
1994). It is suf®cient to note for (IIIa) and (IIIb) that the
increased PÐC1 bond length and C2ÐC1ÐP bond angle
suggest less ylene character than may be present in the parent
compound. The bond lengths and angles within the PPh3
group for the three compounds are as expected and unre-
markable.
Figure 1
The molecule of (IIIa), with displacement ellipsoids drawn at the 40%
probability level.
In conclusion, X-ray crystallographic studies of (IIIa) and
(IIIb) have allowed the stereochemical assignment of the vinyl
adducts. When diethyl and di-tert-butyl acetylenedicarboxyl-
ate were reacted with (I), a mixture of the Z and E isomers
1
were again obtained in each case. A H NMR resonance at
4.7 p.p.m. was observed in both reactions and assigned to the
Z adduct. Therefore, the adduct initially reported by Yoshida
et al. (1971a,b, 1973) can now con®dently be said to have Z
stereochemistry. Our mechanistic studies of the formation of
azulenes (Higham et al., 2004) reveal a step that is dependent
on a Wittig reaction, and this is only feasible for the E isomer,
(IIIb). For the Z isomer, (IIIa), this study illustrates why no
Figure 2
The molecule of (IIIb), with displacement ellipsoids drawn at the 40%
probability level.
Figure 3
The ylide bonding model, in which Ru represents the unique substituent.
ꢁ
Acta Cryst. (2004). C60, o308±o311
Lee J. Highham et al.
Two isomers of C29H25O4P o309

organic compounds
Data collection
reaction takes place, namely because no rotation of the vinyl
bond can bring the phosphorus and the requisite ꢀ-carbonyl
group into close enough proximity.
Bruker SMART CCD area-detector
diffractometer
' and ! scans
4261 independent re¯ections
3428 re¯ections with I > 2ꢄ(I)
Rint = 0.054
Absorption correction: multi-scan
(SADABS; Sheldrick, 2002)
Tmin = 0.856, Tmax = 0.985
65 646 measured re¯ections
ꢂmax = 25.0ꢀ
h = 17 ! 17
Experimental
k = 21 ! 21
l = 21 ! 21
For the preparation of (IIIa), dimethyl acetylenedicarboxylate
(0.43 g, 3.0 mmol) was added to a foil-enclosed solution of (I) (1.0 g,
3.0 mmol) in benzene (30 ml) and the mixture was stirred overnight
under nitrogen. The resulting yellow precipitate was ®ltered off and
stored. The ®ltrate was evaporated, yielding a solid that, when
Re®nement
Re®nement on F2
R[F2 > 2ꢄ(F2)] = 0.043
wR(F2) = 0.106
S = 1.06
4261 re¯ections
w = 1/[ꢄ2(F2o) + (0.0568P)2
+ 1.0912P]
where P = (F2o + 2Fc2)/3
(Á/ꢄ)max = 0.005
1
triturated with hot methanol, gave a ®ne yellow powder. H NMR
3
Ê
Áꢅmax = 0.37 e A
analysis showed that the two solids were identical and they were
therefore combined. Recrystallization from carbon tetrachloride±
ethanol yielded crystals suitable for X-ray analysis (yield 0.86 g, 60%;
m.p. 517±518 K). For the preparation of (IIIb), phosphorane (IIIa)
(1.0 g, 2.14 mmol) was dissolved in dichloromethane (40 ml) in a
50 ml Pyrex round-bottomed ¯ask under nitrogen. The ¯ask was
placed 15 cm from the UV source (300 W lamp) and irradiated for
96 h. The wine-coloured solution was evaporated and the resulting
brown±orange solid was recrystallized from dichloromethane±
ethanol in a darkened ¯ask, giving orange crystals (yield 0.9 g, 90%;
m.p. 507±508 K).
3
Ê
0.24 e A
407 parameters
All H-atom parameters re®ned
Áꢅmin
=
Table 1
Comparative geometric data for (IIIa), (IIIb) and (I).
Ê
Bond lengths (A)
(IIIa)
(IIIb)
(I)
PÐC1
1.7412 (14)
1.4496 (19)
1.411 (2)
1.409 (2)
1.389 (2)
1.411 (2)
1.8173 (15)
1.8045 (14)
1.8044 (15)
1.438 (2)
1.513 (2)
1.344 (2)
1.456 (2)
1.7357 (18)
1.436 (2)
1.393 (3)
1.388 (3)
1.376 (3)
1.416 (3)
1.8213 (18)
1.8074 (17)
1.7995 (18)
1.464 (3)
1.498 (3)
1.335 (3)
1.476 (3)
1.718 (2)
1.430 (3)
1.392 (4)
1.401 (4)
1.376 (4)
1.41 9(3)
±
±
±
±
±
±
±
C1ÐC2
C1ÐC5
C2ÐC3
C3ÐC4
C4ÐC5
P1ÐC12
P1ÐC18
P1ÐC24
C2ÐC6
C6ÐC7
C6ÐC9
C9ÐC10
Compound (IIIa)
Crystal data
3
C29H25O4P
Mr = 468.46
Dx = 1.259 Mg m
Mo Kꢁ radiation
Cell parameters from 5590
re¯ections
Monoclinic, P21=n
Ê
Ê
a = 13.7622 (15) A
b = 11.8849 (13) A
ꢂ = 2.3±26.3ꢀ
ꢃ = 0.14 mm
T = 293 (2) K
Bond angles (ꢀ)
(IIIa)
(IIIb)
(I)
1
Ê
c = 15.8788 (17) A
ꢀ = 107.914 (2)ꢀ
Ê
V = 2471.3 (5) A
Z = 4
C2ÐC1ÐP
C5ÐC1ÐP
C5ÐC1ÐC2
C1ÐC2ÐC3
C2ÐC3ÐC4
C2ÐC6ÐC7
C6ÐC9ÐC10
131.86 (11)
120.84 (11)
107.29 (12)
105.89(13)
109.65 (14)
114.76 (13)
124.08 (15)
130.43 (14)
122.47 (14)
107.08 (16)
106.18(16)
109.89 (18)
118.15 (16)
124.9 (2)
125.3 (2)
125.3 (2)
107.4 (2)
106.8 (2)
108.9(2)
±
3
Prism, brown
0.50 Â 0.40 Â 0.36 mm
Data collection
Bruker SMART CCD area-detector
diffractometer
' and ! scans
Absorption correction: multi-scan
(SADABS; Sheldrick, 2002)
Tmin = 0.877, Tmax = 0.950
43 496 measured re¯ections
6129 independent re¯ections
5100 re¯ections with I > 2ꢄ(I)
Rint = 0.027
±
ꢂ
max = 28.3ꢀ
h = 18 ! 18
k = 15 ! 15
l = 21 ! 21
Crystals of (IIIa) are monoclinic and space group P21/n was chosen
from the systematic absences. Crystals of (IIIb) are orthorhombic and
space group Pbca was chosen from the systematic absences. All H
atoms were located in a difference Fourier map and their positional
and isotropic displacement parameters were allowed to re®ne inde-
pendently. Re®ned CÐH distances in (IIIa) are in the range 0.89 (2)±
Ê
1.01 (2) A and re®ned CÐH distances in (IIIb) are in the range
Ê
0.89 (4)±1.05 (4) A, with individual standard uncertainties of between
Ê
0.02 and 0.04 A.
Re®nement
Re®nement on F2
R[F2 > 2ꢄ(F2)] = 0.048
wR(F2) = 0.127
S = 1.05
6129 re¯ections
w = 1/[ꢄ2(F2o) + (0.068P)2
+ 0.5453P]
where P = (F2o + 2Fc2)/3
(Á/ꢄ)max = 0.005
3
Ê
Áꢅmax = 0.41 e A
3
Ê
0.16 e A
407 parameters
All H-atom parameters re®ned
Áꢅmin
=
For both compounds, data collection: SMART (Bruker, 2001); cell
re®nement: SMART; data reduction: SAINT (Bruker, 2001);
program(s) used to solve structure: SHELXS97 (Sheldrick, 1990);
program(s) used to re®ne structure: SHELXL97 (Sheldrick, 1997);
molecular graphics: SHELXTL (Bruker, 2002) and ORTEPIII
(Burnett & Johnson, 1996); software used to prepare material for
publication: SHELXTL.
Compound (IIIb)
Crystal data
C29H25O4P
Mr = 468.46
Orthorhombic, Pbca
Ê
a = 14.8041 (9) A
Mo Kꢁ radiation
Cell parameters from 8862
re¯ections
ꢂ = 2.8±22.3ꢀ
1
Ê
Ê
The authors thank the National University of Ireland,
Maynooth, for a Scholarship (PGK) and the European
Commission for a Marie Curie Development Host Fellowship
HPMD-CT-2000-00053 (LH).
b = 18.0337 (12) A
ꢃ = 0.15 mm
T = 293 (2) K
c = 18.1151 (12) A
Ê
V = 4836.2 (5) A
3
Plate, orange
0.30 Â 0.25 Â 0.10 mm
Z = 8
Dx = 1.287 Mg m
3
ꢁ
o310 Lee J. Highham et al.
Two isomers of C29H25O4P
Acta Cryst. (2004). C60, o308±o311

organic compounds
Gilheany, D. G. (1994). Chem. Rev. 94, 1339±1374.
È
Gilheany, D. G. (2004). Chem. Commun. pp. 684±685.
Ramirez, F. & Levy, S. (1957a). J. Am. Chem. Soc. 79, 67±69.
Ramirez, F. & Levy, S. (1957b). J. Am. Chem. Soc. 79, 6167±6172.
Sheldrick, G. M. (1990). Acta Cryst. A46, 467±473.
È
Sheldrick, G. M. (1997). SHELXL97. University of Gottingen, Germany.
Sheldrick, G. M. (2002). SADABS. Version 2.03. University of Gottingen,
Germany.
Yoshida, Z., Yoneda, S. & Murata, Y. (1973). J. Org. Chem. 38, 3537±
Supplementary data for this paper are available from the IUCr electronic
archives (Reference: FA1050). Services for accessing these data are
described at the back of the journal.
Higham, L. J., Kelly, P. G., Corr, D. M., Muller-Bunz, H., Walker, B. J. &
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ꢁ
Acta Cryst. (2004). C60, o308±o311
Lee J. Highham et al.
Two isomers of C29H25O4P o311