A new crystalline zwitterionic product from the reaction of Bu3P and DMAD

Article abstract of DOI:10.1002/ejoc.200390101

Tributylphosphane reacts with DMAD to give the crystalline adduct 2 which is stabilised by extensive delocalisation and whose formation involves an unusual rearrangement. The isomeric structure 7 for the previously reported adduct with triphenylphosphane is supported by ¹³C NMR spectroscopy. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.

Full text of DOI:10.1002/ejoc.200390101

SHORT COMMUNICATION
A New Crystalline Zwitterionic Product from the Reaction of Bu3P and
DMAD
R. Alan Aitken,*[a] Stephen J. Costello,[a] Alexandra M. Z. Slawin,[a][‡] and Neil J. Wilson[a]
Keywords: Rearrangements / Zwitterions / Phosphanes
Tributylphosphane reacts with DMAD to give the crystalline
adduct 2 which is stabilised by extensive delocalisation and
whose formation involves an unusual rearrangement. The
isomeric structure 7 for the previously reported adduct with
triphenylphosphane is supported by 13C NMR spectroscopy.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2003)
Introduction
Results and Discussion
We recently reported that tributylphosphane, DMAD
and carbonyl sulfide react in a ratio of 2:2:1 to give Bu3PS
and the novel zwitterionic product 1, whose structure was
proved beyond doubt by X-ray diffraction.[1] At the same
time it was mentioned that, by adding CS2 to a solution of
Bu3P and DMAD, the major 1:1:2 adduct 3[2] was accom-
panied by a minor product 2 which we suspected to be the
thione analogue of 1 based on the available spectroscopic
data (Scheme 1). Further work has now revealed that this
same product can also be isolated in low yield from the
reaction of Bu3P and DMAD with COS (5%) or PhNCS
(8%) and, more significantly, also with PhNCO (5%) and
even PhNCNPh (7%), thus clearly ruling out the thione
structure. We now report the isolation of this new product
in pure form, its unambiguous characterisation and struc-
ture determination, and a plausible mechanism for its
formation.
Compound 2 was obtained in low yield by stirring a solu-
tion of Bu3P and DMAD in CH2Cl2 at room temp. for 24 h
followed by evaporation of the solvents and chromato-
graphic purification. It exhibits very similar spectroscopic
data to 1, with an essentially identical 31P NMR shift (2:
δP ϭ ϩ36.1 ppm; 1: δP ϭ ϩ36.2 ppm) and the only signific-
1
ant differences from the H and 13C NMR spectroscopic
data of 1 being the shift of one of the four OMe signals
from the range δH ϭ 3.7Ϫ4.0 ppm down to δH ϭ 3.25 ppm,
and the apparent absence of one ester carbonyl signal. The
interaction of phosphanes with DMAD in various ways has
been known for a long time. Almost 50 years ago Horner
and Hoffmann suggested structure 4 for the 2:1 adduct
formed with triethylphosphane,[3] while, depending upon
the reacting ratios, triphenylphosphane can form either the
2:1 adduct initially thought to be 5[4] but later corrected to
6,[5] together with the isomeric ylide 7,[6] or the 1:2 adduct
8.[7]
Scheme 1
[a]
School of Chemistry, University of St. Andrews,
North Haugh, St. Andrews, Fife, KY16 9ST, UK
E-mail: raa@st-and.ac.uk
amzs@st-and.ac.uk
[‡]
To receive any enquiries regarding the X-ray structure
determination
Eur. J. Org. Chem. 2003, 623Ϫ625  2003 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 1434Ϫ193X/03/0204Ϫ0623 $ 20.00ϩ.50/0
623

R. A. Aitken, S. J. Costello, A. M. Z. Slawin, N. J. Wilson
SHORT COMMUNICATION
Although analytical and mass spectrometric data con-
firmed our adduct 2 to have the formula Bu3P·2DMAD,
the highly informative 31P coupled 13C NMR spectroscopic
data were not in agreement with structures corresponding
to 4Ϫ8 but were indicative of a close analogue of 1. The
problem was finally solved when crystals of 2 suitable for
X-ray diffraction were obtained. The resulting structure is
shown in Figure 1 and is simply the analogue of 1 with the
CO2Me group on C(2) replaced by OMe.
Scheme 2
Figure 1. X-ray structure of the zwitterionic compound 2 showing
˚
crystallographic numbering scheme; selected bond lengths (A) and
angles (°): P(1)ϪC(2) 1.853(4), P(1)ϪC(21) 1.796(5), C(1)ϪC(2)
1.569(5), C(1)ϪO(1) 1.227(4), C(2)ϪC(3) 1.522(5), C(3)ϪC(4)
1.376(5), C(3)ϪC(7) 1.442(5), C(4)ϪC(5) 1.418(5), C(4)ϪC(5)
1.418(5), C(4)ϪC(9) 1.495(9), C(5)ϪC(1) 1.426(5), C(5)ϪC(11)
1.433(6); C(1)ϪC(2)ϪC(3) 103.0(3), C(2)ϪC(3)ϪC(4) 107.2(3),
In view of the structure found for 2 we thought it worth-
while checking that the compound obtained by Tebby and
co-workers did indeed have the structure 7 and was not in-
stead the triphenyl analogue of 2. The compound was read-
ily prepared by reacting Ph3P with DMAD in boiling di-
ethyl ether followed by chromatographic separation of 6
and 7. The 13C NMR spectrum confirmed that structure 7
is correct for the minor product, the key difference being
the large phosphorus coupling (JP,C ϭ 119 Hz) observed for
the doubly bonded ylide carbon as compared to the smaller
value (JP,C ϭ 64 Hz) for the singly bonded carbon of 2. The
spectrum also revealed a high extent of polarisation of the
ring double bond (δ ϭ 87.0, 160.0 ppm) as expected for a
β,γ-unsaturated ylide structure. It seems likely that the ylide
7 also results from a 1,2-methoxy shift in the intermediate
corresponding to 13 but, perhaps because of the greater
steric bulk of the Ph3P group, it occurs in the opposite sense
to give the product with OMe on the other side of the car-
bonyl group to phosphorus.
C(3)ϪC(4)ϪC(5)
114.0(3),
C(4)ϪC(5)ϪC(1)
107.9(3),
C(5)ϪC(1)ϪC(2) 107.0(3)
As in the case of 1, this is stabilised by extensive de-
localisation. Thus, C(3)ϪC(4), C(4)ϪC(5) and C(5)ϪC(1)
are all intermediate between the normal double and single
bond lengths and the fact that C(3)ϪC(7) and C(5)ϪC(11)
are significantly shorter than C(4)ϪC(9) indicates parti-
cipation of the C(3) and C(5) ester groups in the delocalis-
ation. The delocalised nature of the allyl anion as shown in
structures 2A/2B is also reflected in the 13C NMR chemical
shifts of δC ϭ 93.5 and 101 ppm for C-2 and C-4, while C-
3 gives a signal in the carbonyl region at just over δ ϭ
160 ppm.
We rationalise the formation of 2 by the sequence of reac-
tions shown in Scheme 2 where, as in the formation of 1,[1]
tributylphosphane first reacts with two molecules of
DMAD to give the intermediate 9 corresponding to
Horner’s adduct 4. Rather than interact with the added het-
erocumulene, this may undergo cyclisation to give the res-
onance-stabilised cyclobutenide structure 10. A further
cyclisation gives the strained bicyclo[2.1.0]pentene interme-
diate 11 which then undergoes a 1,2-shift of the methoxy
group to afford 2 directly. Alternatively the process may in-
volve the isomeric adduct 12 which can cyclise directly to
the five-membered ring intermediate 13 which then un-
dergoes a 1,2-methoxy shift to afford 2.
Experimental Section
Melting points were recorded on a Reichert hot-stage microscope
and are uncorrected. Infra red spectra were recorded as Nujol mulls
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
624
Eur. J. Org. Chem. 2003, 623Ϫ625

A New Crystalline Zwitterionic Product from the Reaction of Bu3P and DMAD
as reference for 31P. Chemical shifts are reported in ppm to high
0.0603, RW ϭ 0.1314 for 1993 reflections with I Ͼ 2σ(I) and 317
SHORT COMMUNICATION
frequency of the reference and coupling constants J are in Hz. variables. Data were recorded on a Bruker SMART diffractometer
˚
Mass spectra were obtained on an AEI/Kratos MS-50 spectrometer
using chemical ionisation with isobutane as ionising gas.
with graphite-monochromated Mo-Kα radiation (λ ϭ 0.71073 A).
The structure was solved by direct methods and refined using full-
matrix least-squares methods.
Preparation of Adduct 2: A Solution of tributylphosphane (2.02 g,
10 mmol) and DMAD (2.84 g, 20 mmol) in dichloromethane
(50 mL) was stirred at room temp. for 20 h and then the solvents
were evaporated. Flash-column chromatography of the residue
(SiO2, hexane/ethyl acetate, 1:1) gave mainly hydrolysis products
such as Bu3PO, dimethyl fumarate and unchanged DMAD but a
minor fraction was found to be 5-methoxy-5-tributylphosphonio-
2,3,4-tri(methoxycarbonyl)cyclopent-2-en-1-on-4-ide (2; 0.42 g,
9%) as yellow prisms, m.p. 143Ϫ144 °C. IR (Nujol): ν˜ ϭ 1735
cmϪ1, 1658, 1513, 1347, 1100, 1017, 918, 785, 759, 725, 630. 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, 5-OMe), 3.70 (s, 3 H), 3.77 (s, 3 H), 3.95 (s,
3 H) ppm. 13C NMR: δ ϭ 13.2 (d, J ϭ 1 Hz, Bu C-4), 16.5 (d, J ϭ
43 Hz, Bu C-1), 24.05 (d, J ϭ 15 Hz, Bu C-3), 24.14 (d, J ϭ 5 Hz,
Bu C-2), 50.6 (OMe), 50.8 (OMe), 52.1 (OMe), 53.6 (d, J ϭ 13 Hz,
5-OMe), 85.4 (d, J ϭ 64 Hz, C-5), 93.5 (d, J ϭ 2 Hz, C-4), 101.1
(C-2), 162.1 and 163.5 (each d, J ϭ 1 Hz, 2-CO and C-3), 165.1 (d,
J ϭ 10 Hz, 4-CO), 167.5 (3-CO), 188.5 (d, J ϭ 3 Hz, C-1) ppm.
31P NMR: δ ϭ ϩ36.1 ppm. MS (CI): m/z (%) ϭ 487 (5) [MHϩ],
455 (7), 285 (88), 253 (50), 219 (19), 203 (100). C25H39O8P (486.58):
calcd. C 59.24, H 8.08; found C 59.43, H 8.35.
CCDC-187176 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge
Crystallographic Data Centre, 12, Union Road, Cambridge
CB2 1EZ, UK; Fax: (internat.) ϩ44-1223/336-033; E-mail:
deposit@ccdc.cam.ac.uk].
Preparation of Adduct 7: Reaction of Ph3P with 2 equiv. of DMAD
in boiling diethyl ether following the method of Tebby and co-
workers[6] gave, after chromatographic separation, compound 6
(38%; 31P NMR: δ ϭ ϩ49.5 ppm) and compound 7 (13%) as yellow
crystals, m.p. 224Ϫ226 °C (ref.,[6] 222Ϫ224 °C). 1H NMR: δ ϭ 3.10
(s, 3 H, 5-OMe), 3.42 (s, 3 H), 3.67 (s, 3 H), 3.78 (s, 3 H), 7.45Ϫ7.70
(m, 15 H) ppm. 13C NMR: δ ϭ 51.0 (OMe), 51.7 (OMe), 52.3 (2
ϫ OMe), 70.6 (d, J ϭ 119 Hz, PϭC), 87.0 (d, J ϭ 13 Hz, C-4),
107.4 (d, J ϭ 10 Hz, C-5), 121.9 (d, J ϭ 93 Hz, Ph C-1), 128.9 (d,
J ϭ 13 Hz, Ph C-3), 133.2 (Ph C-4), 134.0 (d, J ϭ 10 Hz, Ph C-2),
160.0 (d, J ϭ 14, C-3), 162.2, 166.1 and 168.0 (3 ϫ CO2Me), 194.9
(d, J ϭ 8 Hz, CO) ppm. 31P NMR: δ ϭ ϩ11.7 ppm.
[1]
R. A. Aitken, P. Lightfoot, N. J. Wilson, Eur. J. Org. Chem.
2001, 35Ϫ37.
The same product was also obtained by stirring a solution of Bu3P
(10 mmol) and DMAD (20 mmol) in dichloromethane (50 mL) in
the presence of a heterocumulene (10 mmol) at room temp. for 24 h
followed by evaporation of the solvents and chromatography as
above. The yields obtained were: CS2 0.14 g (3%); COS 0.24 g (5%);
PhNCO 0.24 g (5%), PhNCS 0.39 g (8%), PhNϭCϭNPh 0.34 g
(7%).
[2]
R. A. Aitken, S. V. Raut, G. Ferguson, Tetrahedron 1992, 48,
8023Ϫ8030.
[3]
L. Horner, H. Hoffmann, Angew. Chem. 1956, 68, 473Ϫ485.
[4]
A. W. Johnson, J. C. Tebby, J. Chem. Soc. 1961, 2126Ϫ2130.
[5]
N. E. Waite, J. C. Tebby, R. S. Ward, D. H. Williams, J. Chem.
Soc. (C) 1969, 1100Ϫ1104.
N. E. Waite, J. C. Tebby, R. S. Ward, M. A. Shaw, D. H. Willi-
[6]
ams, J. Chem. Soc. (C) 1971, 1620Ϫ1622.
Crystal Data for 2: C24H39O8P, M ϭ 486.52, yellow prism, crystal
dimensions 0.19 ϫ 0.1 ϫ 0.1 mm, monoclinic, space group P21/c,
[7]
M. A. Shaw, J. C. Tebby, R. S. Ward, D. H. Williams, J. Chem.
Soc. (C) 1967, 2442Ϫ2446.
Received October 1, 2002
[O02538]
˚
a ϭ 8.4495(1), b ϭ 35.479(1), c ϭ 9.1735(3) A, β ϭ 99.527(1)°,
3
V ϭ 2712.1(1) A , Z ϭ 4, Dc ϭ 1.192 Mg·mϪ3, T ϭ 293 K, R ϭ
˚
Eur. J. Org. Chem. 2003, 623Ϫ625
625