Formation and structure of a novel zwitterionic phosphoniocyclopentenonide from Bu3P, DMAD and COS

Article abstract of DOI:10.1002/1099-0690(200101)2001:1<35::aid-ejoc35>3.0.co;2-x

Tributylphosphane, DMAD and carbonyl sulfide react in a 2:2:1 ratio to give Bu₃PS and the crystalline adduct 2 which is stabilised by extensive delocalisation; the sulfur analogue 5 has also been obtained.

Full text of DOI:10.1002/1099-0690(200101)2001:1<35::aid-ejoc35>3.0.co;2-x

SHORT COMMUNICATION
Formation and Structure of a Novel Zwitterionic Phosphoniocyclopentenonide
From Bu3P, DMAD and COS
R. Alan Aitken,*[a] Philip Lightfoot,[a][‡] and Neil J. Wilson[a]
Keywords: Cycloadditions / Zwitterion / Phosphanes / Cumulenes
Tributylphosphane, DMAD and carbonyl sulfide react in a
is stabilised by extensive delocalisation; the sulfur analogue
2:2:1 ratio to give Bu3PS and the crystalline adduct 2 which
5 has also been obtained.
Introduction
crystalline organophosphorus compound (12%; δP ϭ
ϩ36.2) to which we assign the zwitterionic structure 2. Ana-
lytical and spectroscopic data showed the product to have
the composition Bu3P·CO·2DMAD and to have four differ-
ent CO2Me groups. The 13C NMR spectroscopic data were
particularly informative, with the magnitude of the coup-
lings to phosphorus being fully in agreement with the struc-
ture 2. The high frequency of the signal for C-4, which
brings it into the region of the ester carbonyls, and the low
frequencies for C-3 and C-5 point to the delocalised nature
of the allylic anion.
In order to confirm the structure and obtain a measure
of the extent of delocalisation we carried out an X-ray
structure determination; the resulting structure is shown in
Figure 1. The existence of 2 as a stable compound is due to
the extensive possible delocalisation of the negative charge
and the bond lengths provide support for this. Thus,
C(2)ϪC(3), C(3)ϪC(4) and C(4)ϪC(5) are all intermediate
between the normal double and single bond lengths and
the fact that C(3)ϪC(8) and C(5)ϪC(12) are significantly
shorter than C(4)ϪC(10) indicates participation of the C(3)
and C(5) ester groups in the delocalisation.
Some time ago we reported that tributylphosphane, car-
bon disulfide and DMAD react in a ratio of 1:1:2 to give
the stabilised ylide 1.[1,2] More recently we have described
the interaction of tributylphosphane and CS2 with strained
double bonds leading to a variety of synthetically useful
cycloaddition processes.[3Ϫ6] In an attempt to extend the
scope of this work we have investigated the reaction of tri-
butylphosphane and carbonyl sulfide with DMAD and re-
port here the formation and structure of a novel zwitter-
ionic product (Scheme 1).
Scheme 1
We rationalise the formation of 2 by the sequence of reac-
tions shown in Scheme 2, where tributylphosphane first re-
acts with two molecules of DMAD in a well precedented
process[8] to give the intermediate 3, which may then attack
COS. Electrocyclisation could give a six-membered ring in-
termediate which may then be desulfurised by a second
equivalent of phosphane. Alternatively, the mechanism may
involve a seven-membered ring intermediate as shown.
Results and Discussion
We initially examined the addition of tributylphosphane
to a solution of COS in CH2Cl2 at Ϫ78 °C but, in contrast
to the case of CS2, where the deep red 1:1 adduct is formed
immediately, no colour developed and, upon evaporation,
the phosphane was recovered unchanged. This is in agree-
ment with an early report that COS does not form an ad-
duct with trialkylphosphanes.[7] However, addition of
DMAD to the cold solution of COS and tributylphosphane
led to immediate development of a deep red colour which
gradually faded to pale yellow upon warming to room tem-
perature over several hours. Evaporation of the solution fol-
lowed by chromatography of the residue led to isolation of
tributylphosphane sulfide (15%; δP ϭ ϩ43.2) and a new
Consideration of the structure of 2 shows that it is noth-
ing more than an adduct between tributylphosphane and
tetramethyl cyclopentadienonetetracarboxylate, and this
suggested a possible alternative synthesis. On grounds of
polarisation, addition of nucleophiles to
a
cyclo-
pentadienone would normally be expected to occur at the
3-position to give, in this case, the isomeric compound 4
(Scheme 3). However, it is clear that there is more oppor-
tunity for delocalisation of the negative charge in 2 and,
although 4 might be kinetically favoured, 2 is likely to be a
more thermodynamically stable product. In an attempt to
confirm this we prepared tetramethyl cyclopentadienonete-
[a]
School of Chemistry, University of St. Andrews,
North Haugh, St. Andrews, Fife, KY16 9ST, UK
E-mail: raa@st-and.ac.uk
[‡]
To receive any enquiries regarding the X-ray structure
determination: E-mail: pl@st-and.ac.uk
Eur. J. Org. Chem. 2001, 35Ϫ37
WILEY-VCH Verlag GmbH, 69451 Weinheim, 2001
1434Ϫ193X/01/0101Ϫ0035 $ 17.50ϩ.50/0
35

R. A. Aitken, P. Lightfoot, N. J. Wilson
SHORT COMMUNICATION
˚
Figure 1. X-ray structure of the zwitterionic compound 2 showing crystallographic numbering scheme; selected bond lengths (A) and
angles (°): P(1)ϪC(1) 1.870(7), P(1)ϪC(18) 1.796(7), C(1)ϪC(2) 1.57(1), C(2)ϪO(1) 1.220(8), C(2)ϪC(3) 1.40(1), C(3)ϪC(4) 1.42(1),
C(3)ϪC(8) 1.45(1), C(4)ϪC(5) 1.37(1), C(4)ϪC(10) 1.52(1), C(5)ϪC(1) 1.51(1), C(5)ϪC(12) 1.45(1); C(1)ϪC(2)ϪC(3) 107.1(8),
C(2)ϪC(3)ϪC(4) 108.3(9), C(3)ϪC(4)ϪC(5) 113.6(8), C(4)ϪC(5)ϪC(1) 107.3(8), C(5)ϪC(1)ϪC(2) 103.3(7)
Scheme 3
It is clear that the different behaviour of COS and CS2
in these reactions is due to the failure of the former to form
an adduct with tributylphosphane, which is then free to in-
teract with DMAD to give 3. It occurred to us that the CS2
reaction might be induced to follow a similar path by add-
ing the reagents together in a different order and indeed
this proved to be the case. Thus, when a solution of two
equivalents each of Bu3P and DMAD in CH2Cl2 was
stirred at 0 °C for 1 h and then one equivalent of CS2 was
added and the mixture stirred at room temperature for 20 h,
the corresponding phosphoniocyclopentenethionide 5 (3%)
was formed together with Bu3PS. It seems likely that this
type of reaction may also occur with other types of hetero-
cumulene and this is currently under investigation.
Experimental Section
Scheme 2
Melting points were recorded on a Reichert hot-stage microscope
tracarboxylate by the reported method[9] and treated it with
and are uncorrected. Infra red spectra were recorded as Nujol mulls
an excess of tributylphosphane. Unfortunately, however, re-
action with the phosphane was unable to compete with the
extremely facile Diels Alder dimerisation of the cyclo-
pentadienone and no new phosphorus-containing products
were formed.
on a PerkinϪElmer 1420 instrument. NMR spectra were obtained
1
for H at 300 MHz and for 13C at 75 MHz using a Bruker AM300
instrument, and for 31P at 121 MHz using a Varian Gemini 2000
instrument. All spectra were run on solutions in CDCl3 with in-
1
ternal Me4Si as reference for H and 13C and external 85% H3PO4
36
Eur. J. Org. Chem. 2001, 35Ϫ37

Formation and Structure of a Novel Zwitterionic Phosphoniocyclopentenonide
SHORT COMMUNICATION
as reference for 31P. Chemical shifts are reported in ppm to high
frequency of the reference and coupling constants J are in Hz.
Mass spectra were obtained on an AEI/Kratos MS-50 spectrometer
by electron impact at 70 eV.
3 H), 3.95 (s, 3 H). Ϫ 13C NMR: δ ϭ 13.2 (Bu C-4), 16.5 (d, J ϭ
43 Hz, Bu C-1), 24.0 (d, J ϭ 16 Hz, Bu C-3), 24.2 (d, J ϭ 6 Hz,
Bu C-2), 50.7 (OMe), 50.8 (OMe), 52.1 (OMe), 53.7 (d, J ϭ 12 Hz,
OMe), 85.5 (d, J ϭ 64 Hz, C-2), 93.5 (C-3), 101.1 (C-5), 162.2,
163.6, 165.2 (d, J ϭ 10 Hz), 167.5 (2 C), 188.6 (d, J ϭ 2 Hz, C-1).
Preparation of 2: A solution of carbonyl sulfide (ca. 0.50 g, 8 mmol)
was prepared by bubbling carbonyl sulfide into dichloromethane
(25 mL) at Ϫ78 °C and to this was added DMAD (2.27 g,
16 mmol) at Ϫ78 °C. Tributylphosphane (3.23 g, 16 mmol) was ad-
ded to give a red colour. The solution was evaporated and flash
column chromatography of the red oil (SiO2, ethyl acetate) gave
two fractions.
Ϫ
31P NMR: δ ϭ ϩ36.1. Ϫ MS (CI): m/z (%) ϭ 486 (7) [MHϩ
Bu], 456 (18), 424 (100), 395 (54).
Ϫ
Crystal data for 2: C25H39O9P, M ϭ 514.55, yellow plate, crystal
dimensions 0.35 ϫ 0.15 ϫ 0.03 mm, monoclinic, space group P21/
˚
c (#14), a ϭ 8.369(5), b ϭ 37.561(8), c ϭ 9.21(1) A, β ϭ 101.50(7)°,
˚
V ϭ 2837(3) A3, Z ϭ 4, Dc ϭ 1.205 Mg mϪ3, T ϭ 293 K, R ϭ
0.103, RW ϭ 0.095 for 1848 reflections with I Ͼ 3σ(I) and 317
2,3,4,5-Tetra(methoxycarbonyl)-5-(tributylphosphonio)cyclopent-
variables. Data were collected on a Rigaku AFC7S diffractometer
˚
2-en-1-on-4-ide (2) was obtained as yellow crystals (0.48 g, 12%),
with graphite-monochromated Mo-Kα radiation (λ ϭ 0.71069 A).
The structure was solved by direct methods (SIR92) and refined by
full-matrix least-squares methods.
˜
m.p. 184Ϫ192 °C. Ϫ IR (CH2Cl2): ν ϭ 3055 cmϪ1, 2987, 2411,
1
2306, 1713, 1422, 1363, 1268, 1224, 897. Ϫ H NMR: δ ϭ 0.95 (t,
J ϭ 7 Hz, 9 H), 1.4Ϫ1.6 (m, 12 H), 2.3Ϫ2.45 (m, 6 H), 3.68 (s, 3
H), 3.72 (s, 3 H), 3.76 (s, 3 H), 3.95 (s, 3 H). Ϫ 13C NMR: δ ϭ
13.4 (Bu C-4), 17.9 (d, J ϭ 45 Hz, Bu C-1), 24.3 (d, J ϭ 16 Hz, Bu
C-3), 24.9 (d, J ϭ 5 Hz, Bu C-2), 50.9 (OMe), 51.4 (OMe), 52.4
(OMe), 53.5 (OMe), 63.6 (d, J ϭ 52 Hz, C-2), 96.7 (d, J Ͻ 2 Hz,
C-3), 102.2 (C-5), 162.7, 163.2 (d, J ϭ 8 Hz), 163.8, 167.0, 167.1
(C-4), 183.7 (C-1). Ϫ 31P NMR: δ ϭ ϩ36.2. Ϫ MS (70 eV): m/z
(%) ϭ 514 (92) [Mϩ], 483 (75), 455 (65), 411 (30), 312 (75), 281
(70), 76 (100). Ϫ C25H39O9P (514.6): calcd. C 58.35, H 7.64; found
C 58.54, H 7.70.
Crystallographic data (excluding structure factors) for the structure
included in this paper have been deposited with the Cam-
bridge Crystallographic Data Centre as supplementary publication
no. CCDC-139580. Copies of the data can be obtained free of
charge on application to CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK [Fax: (internat.)
deposit@ccdc.cam.ac.uk].
ϩ 44-1223/336-033; E-mail:
[1]
R. A. Aitken, G. Ferguson, S. V. Raut, J. Chem. Soc., Chem.
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Tributylphosphane sulfide (0.28 g, 15%): 1H NMR: δ ϭ 0.96 (t,
J ϭ 3 Hz, 9 H), 1.38Ϫ1.64 (m, 12 H), 1.77Ϫ1.90 (m, 6 H). Ϫ 31P
NMR: δ ϭ ϩ43.2.
[2]
[3]
[4]
[5]
[6]
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40, 1061Ϫ1064.
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Preparation of 5: A Solution of tributylphosphane (1.42 g, 7 mmol)
and DMAD (1.00 g, 7 mmol) in dichloromethane (50 mL) was
stirred at 0 °C for 1 h. Carbon disulfide (0.53 g, 7 mmol) was added
dropwise whereupon the solution immediately turned red. The so-
lution was allowed to stir for 20 h at room temp. and then evapor-
ated. Flash column chromatography (SiO2, petroleum/diethyl ether/
ethyl acetate, 1:1:1) gave mainly the known adduct 1 but a minor
fraction was found to be 2,3,4,5-tetra(methoxycarbonyl)-5-(tribu-
tylphosphonio)cyclopent-2-en-1-thion-4-ide (5) as a brown oil
(50 mg, 3%). Ϫ 1H NMR: δ ϭ 0.94 (t, J ϭ 7 Hz, 9 H), 1.4Ϫ1.8
(m, 12 H), 2.15Ϫ2.30 (m, 6 H), 3.25 (s, 3 H), 3.70 (s, 3 H), 3.77 (s,
[7]
[8]
[9]
R. C. Cookson, J. B. Henstock, J. Hudec, B. R. D. Whitear, J.
Chem. Soc. C 1967, 1986Ϫ1993.
Received August 28, 2000
[O00440]
Eur. J. Org. Chem. 2001, 35Ϫ37
37