The reaction of 1-(2,4,6-trimethylphenyl)-1,2-dihydrophosphinine 1-oxides with dimethyl acetylenedicarboxylate; a [4+2] or a [2+2] cycloaddition?

Article abstract of DOI:10.1039/b010080n

The reaction of dimethyl acetylenedicarboxylate (DMAD) with 3- and 5-methyl-1-aryl-1,2-dihydrophosphinine oxides (6a and 6b, respectively) obtained by the two-step ring enlargement of 2,5-dihydro-1H-phosphole oxide 4 followed different routes. Isomer 6a entered into a [4+2] cycloaddition with DMAD giving, although in low yield, phosphabicyclooctadiene 7, while 6b reacted with the acetylene moiety according to a recently discovered [2+2] protocol to afford spirocyclic oxaphosphete 8. The reaction of isomers 6a and 6b with N-phenylmaleimide under forcing conditions furnished the expected Diels-Alder cycloadducts (10a and 10b, respectively). Hence, depending on the reactant, isomer 6b displayed a dual reactivity.

Full text of DOI:10.1039/b010080n

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The reaction of 1-(2,4,6-trimethylphenyl)-1,2-dihydrophosphinine
1-oxides with dimethyl acetylenedicarboxylate;
a [4؉2] or a [2؉2] cycloaddition?
1
György Keglevich,a* Ágnes Gyöngyvér Vaskó,a András Dobó,b Krisztina Ludányib and
László To´´ k e c
a Department of Organic Chemical Technology, Budapest University of Technology and
Economics, 1521 Budapest, Hungary
b Hungarian Academy of Sciences, Chemical Research Center, 1525 Budapest, Hungary
c Research Group of the Hungarian Academy of Sciences at the Department of Organic
Chemical Technology, Budapest University of Technology and Economics, 1521 Budapest,
Hungary
Received (in Cambridge, UK) 11th December 2000, Accepted 23rd March 2001
First published as an Advance Article on the web 18th April 2001
The reaction of dimethyl acetylenedicarboxylate (DMAD) with 3- and 5-methyl-1-aryl-1,2-dihydrophosphinine
oxides (6a and 6b, respectively) obtained by the two-step ring enlargement of 2,5-dihydro-1H-phosphole oxide 4
followed different routes. Isomer 6a entered into a [4ϩ2] cycloaddition with DMAD giving, although in low yield,
phosphabicyclooctadiene 7, while 6b reacted with the acetylene moiety according to a recently discovered [2ϩ2]
protocol to afford spirocyclic oxaphosphete 8. The reaction of isomers 6a and 6b with N-phenylmaleimide under
forcing conditions furnished the expected Diels–Alder cycloadducts (10a and 10b, respectively). Hence, depending
on the reactant, isomer 6b displayed a dual reactivity.
empirical calculations.13 It is a challenge for us to explore the
Introduction
scope and limitations of this cycloaddition reaction giving an
entry to valuable oxaphosphetes that are the unsaturated
derivatives of the well-known Wittig intermediates, oxaphos-
phetanes.14 In this paper, we discuss how the P-2,4,6-trimethyl-
phenyl substituent affects the reactivity of the double-bond
isomers of the dihydrophosphinine oxide in cycloaddition
reactions.
The 1,2-dihydrophosphinine oxides are excellent dienes in
Diels–Alder reactions leading to 2-phosphabicyclo[2.2.2]octene
derivatives1–8 that are precursors of low-coordinate fragments,
methylenephosphine oxides [YP(O)CH2, Y = Ph, RO] useful in
phosphorylations.2–5,7–11 It was, however, surprising to find that
whilst the reaction of the phenyldihydrophosphinine oxides
(1, Ar = Ph) with dimethyl acetylenedicarboxylate (DMAD)
afforded the phosphabicyclooctadiene oxides 2 expected,2 the
cycloaddition of the 2,4,6-triisopropylphenyl derivative (1,
Ar = 2,4,6-triisopropylphenyl) with DMAD took place accord-
ing to a [2ϩ2] protocol to furnish spirocyclic oxaphosphate
312,13 (Scheme 1). This was the first case in which the cyclo-
Results and discussion
The model compounds, the dihydrophosphinine oxides (6a
and 6b) were synthesised by the two-step ring enlargement
of dihydrophosphole oxide 4. According to our procedure
elaborated for the ring expansion of other dihydrophosphole
oxides,15–17 dichlorocarbene generated in a liquid–liquid two-
phase system was added onto the double bond of the starting
compound 4, resulting in the formation of 3-phosphabicyclo-
[3.1.0]hexane 3-oxide 5 as a mixture of two diastereomers (5a
and 5b) (Scheme 2). The diastereomers 5a and 5b were separ-
ated by repeated column chromatography; stereostructure of
the isomers 5a and 5b was substantiated on the basis of stereo-
3
specific JPC couplings18,19 detected for C-6 of the adducts 5.
3
The JPC coupling of 8.4 Hz suggested the trans disposition of
the P-aryl substituent and the dichlorocyclopropane ring (5a),
while the value of 15.5 Hz confirmed structure 5b. In the second
step, the dichlorocyclopropane ring of adducts 5a and 5b was
opened up thermally to afford dihydrophosphinine oxide 6 as
an 80–20% mixture of double-bond isomers 6a and 6b (Scheme
2). All new compounds (5a, 5b, 6a and 6b) were characterised
by 31P, 13C and 1H NMR, as well as mass spectroscopic
methods. The 13C NMR assignments were confirmed by spectra
obtained by the Attached Proton Test (APT) technique. The
elemental composition was confirmed in all cases by high-
resolution mass spectrometry (HR-MS).
Scheme 1
addition of the P᎐O group with an acetylene moiety had been
observed to take place. The unusual reactivity of the P᎐O group
᎐
᎐
that is obviously the consequence of the presence of the
electron-releasing P-aryl substituent was investigated by semi-
In the first place, an 80–20% mixture of dihydrophosphinine
oxides 6a and 6b was treated with DMAD in boiling toluene for
1062
J. Chem. Soc., Perkin Trans. 1, 2001, 1062–1065
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b010080n

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mass spectroscopy. The δP-value of 26.4 matched well the value
of 24.0 reported for the P-2,4,6-triisopropylphenyl analogue.13
1
The 13C NMR spectrum was convincing in revealing four JPC
couplings (61.0–107.7 Hz) due to the pentavalent penta-
coordinate phosphorus atom. The other spectral parameters of
product 8 also showed close resemblance to those of other
oxaphosphetes.13 The elemental composition of compound 8
was confirmed by HR-MS.
It is an interesting and even surprising observation that the
two double-bond isomers (6a and 6b) display different reactiv-
ity in reaction with DMAD; isomer 6a reacts in the usual [4ϩ2]
fashion, while the cycloaddition of isomer 6b follows a [2ϩ2]
protocol. As a consequence of the steric hindrance due to the
2,4,6-trimethylphenyl group, the Diels–Alder reactivity of the
3-methyldihydrophosphinine oxide 6a is suppressed. This is
well demonstrated by the fact that, although phosphabicyclo-
octadiene 7 is derived from the major dihydrophosphinine
isomer (6a), its yield was only 9%. At the same time, the
electron distribution in the -MeCH᎐CH–P(O)- moiety of
᎐
double-bond isomer 6b enables the P=O group to take part in a
[2ϩ2] cycloaddition reaction. In the few examples of the [2ϩ2]
cycloadditions described, the critical role of the electron-
releasing 2,4,6-triisopropylphenyl substituent was clearly estab-
lished.13 In dihydrophosphinine oxide 6b, the electron-releasing
ability of the trimethylphenyl ring needs to be completed by the
effect of a conjugated methyl group to favour the [2ϩ2]
cycloaddition.
The formation of 2,4,6-trimethylphenyl derivative 8 is
obviously an extension of the recently described reaction.
Further work to utilise other phosphine oxides, as well as
to carry out ab initio calculations to evaluate the mechanism,
is in progress. The reactivity of the oxaphosphetes is also to be
studied; the possibility for rearrangement of the oxaphos-
phetes to the corresponding phosphoranes (i.e., 8 to 9) will be
explored.
Scheme 2
7 days. 31P NMR spectroscopy of the crude mixture showed the
presence of a major (89%) and a minor (11%) component at
δP 26.3 and 41.9, respectively. After separation by repeated
column chromatography, the species at δP 42.0 was assigned as
the 2-phosphabicyclo[2.2.2]octa-5,7-diene 7, while the compon-
ent with δP 26.4 was identified as the spirocyclic oxaphosphete
8 (Scheme 3). The structure of the phosphabicyclooctadiene 7
Finally, we wished to test the [4ϩ2] reactivity of dihydro-
phosphinine oxide isomers 6a and 6b in reaction with N-phenyl
maleimide (NPMI). The diene components (6a and 6b) were
consumed only after a prolonged reaction time (7 days at 110
ЊC). Starting from the 4 : 1 mixture of isomers 6a and 6b, the
expected phosphabicyclooctenes 10a and 10b were obtained in
a 22 : 78% ratio after flash column chromatography (Scheme 4).
As can be seen from the change in the isomeric ratio, the
reactivity of the 3-methyldihydrophosphinine oxide 6a is much
more suppressed than that of the 5-methyl isomer 6b. Much of
6a may have undergone polymerisation as was suggested by the
presence of the insoluble material formed. The Diels–Alder
cycloadducts 10a and 10b obtained in 23% yield were charac-
terised by 31P, 13C and 1H NMR, as well as mass spectroscopic
methods. The 13C NMR assignment was confirmed by the APT
technique. Spectral parameters of products 10a and 10b showed
close resemblance to analogous derivatives described earlier.7,8
The elemental composition of cycloadducts 10a and 10b was
supported by HR-FAB.
Scheme 3 Non-systematic numbering for spiro-system 8.
1
was confirmed by 31P and H NMR, as well as EI, including
high-resolution electron impact (HR-EI) mass spectroscopy.
The P-phenyl analogues described earlier exhibited 31P NMR
chemical shifts in the range of 38.5–42.9.2 The only olefinic
1
signal, at δH 6.58 (J1 = J2 = 6.2 Hz), in the H NMR spectrum
It can be seen that both double-bond isomers 6a and 6b could
enter into a [4ϩ2] cycloaddition with NPMI, which is a better
dienophile than DMAD. These cycloadditions, especially that
of isomer 6a, were not, however, too efficient due to the steric
hindrance brought about by the presence of the trimethyl-
phenyl substituent.
of 7 represented C6–H. The mass spectroscopic fragmentation
involving the loss of the bridging moiety was also characteristic
of the earlier phosphabicyclooctadienes.9 The structure of iso-
meric oxaphosphete 8 obtained in 86% yield was supported by
31P, 13C and 1H NMR, as well as fast-atom bombardment (FAB)
J. Chem. Soc., Perkin Trans. 1, 2001, 1062–1065
1063

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43), 119 (Ar, 30); HR-MS, Mϩfound = 316.0562. C15H19Cl2OP
requires M, 316.0551 for the 35Cl isotopes.
5b: Yield 0.65 g (8%); 31P NMR (CDCl3) δ 79.1; 13C NMR
(CDCl3) δ 20.6 (C4Ј-Me), 21.2 (J = 5.0, C1-Me), 23.0 (J = 3.7,
C2Ј-Me), 34.0 (J = 66.7, C4), 35.8 (J = 8.4, C1), 36.3 (J = 7.1,
C5), 40.1 (J = 66.8, C2), 72.2 (J = 15.5, C6), 128.6 (J = 89.3, C1Ј),
129.8 (J = 10.8, C3Ј), 139.7 (J = 10.2, C2Ј), 141.1 (J = 1.6, C4Ј);
1H NMR (CDCl3) δ 1.75 (s, 3H, C4Ј-Me), 2.28 (s, 3H, C1-Me),
2.55 (s, 6H, C2Ј-Me); MS, m/z (rel. int.) 316 (Mϩ, 19%), 301
(M Ϫ 15, 21), 281 (M Ϫ 35, 90), 245 (281 Ϫ 36, 100), 165
(ArPO Ϫ H, 39), 119 (Ar, 33); HR-MS, Mϩfound = 316.0559.
C15H19Cl2OP requires M, 316.0551 for the 35Cl isotopes.
4-Chloro-3- and -5-methyl-1-(2,4,6-trimethylphenyl)-1,2-
dihydrophosphinine 1-oxides 6a and 6b
A sample of 0.71 g (2.24 mmol) of dichlorocarbene adduct 5a
was heated at 135 ЊC in a vial for 1 h until the evolvement of
hydrochloric acid ceased. The crude product so obtained was
purified by column chromatography (as above) to give 0.35 g
(56%) of the product as an 80–20% mixture of double-bond
isomers 6a and 6b. A similar result was obtained by thermolysis
of the 72–28% mixture of 5a and 5b. MS, m/z (rel. int.) 280
Scheme 4 Non-systematic numbering scheme for adducts 10a, 10b.
To summarise our results, the 3-methyl-1-(2,4,6-trimethyl-
phenyl)-1,2-dihydrophosphinine oxide 6a displays a suppressed
[4ϩ2] reactivity with both DMAD and NPMI. At the same
time, the 5-methyl counterpart 6b encounters a dual reactivity;
a [2ϩ2] cycloaddition was observed with DMAD, but the usual
[4ϩ2] reaction took place with NPMI.
(Mϩ, 82%), 245 (M Ϫ 35, 100), 119 (Ar, 24); HR-MS, Mϩ
=
found
280.0798. C15H18ClOP requires M, 280.0784 for the 35Cl
isotope.
6a: 31P NMR (CDCl3) δ 18.8; 13C NMR (CDCl3) δ 21.0 (C4Ј-
Me), 23.3 (C2Ј-Me), 23.5 (J = 4.8, C3-Me), 37.5 (J = 68.8, C2),
121.7 (J = 91.8, C6), 124.0 (J = 20.2, C3), 124.7 (J = 104.9, C1Ј),
131.2 (J = 11.8, C3Ј), 131.5 (J = 9.6, C4), 140.6 (C5), 142.0 (C4Ј),
Experimental
1
1
The 31P, 13C and H NMR spectra were taken on a Bruker
142.9 (J = 10.8, C2Ј); H NMR (CDCl3) δ 2.10 (s, 3H, C3-Me),
2
DRX-500 instrument operating at 202.4, 125.7 and 500 MHz,
respectively. Chemical shifts are downfield relative to 85%
H3PO4 or SiMe4 (TMS). J-Values are given in Hz. Mass spectra
were obtained on a MS-902 or on a ZAB-2SEQ spectrometer
at 70 eV. The IR spectrum of compound 8 was measured on a
Perkin-Elmer 1600 spectrometer with a Fourier transformer.
3-Methyl-1-(2,4,6-trimethylphenyl)-2,5-dihydro-1H-phos-
phole 1-oxide 4 was prepared from 1-chloro-3-methyl-2,5-
dihydro-1H-phosphole20 and 2,4,6-trimethylphenylmagnesium
bromide followed by oxidation as described for the synthesis
of other aryl-dihydrophosphole oxides.21 31P NMR (CDCl3)
δ 60.8; MS, m/z (rel. int.) 234 (Mϩ, 100%), 219 (M Ϫ 15, 32),
119 (Ar, 48).
2.30 (s, 3H, C4Ј-Me), 2.51 (s, 6H, C2Ј-Me), 6.36 (dd, JPH
=
3JHH = 12.8, 1H, C6-H), 6.79 (dd, JPH = 34.9, JHH = 12.8, 1H,
3
3
C5-H).
6b: 31P NMR (CDCl3) δ 17.5; 13C NMR (CDCl3) δ 21.0
(C4Ј-Me), 23.3 (C2Ј-Me), 25.1 (J = 13.2, C5-Me), 31.9 (J = 68.7,
C2), 121.3 (J = 95.4, C6), 123.8 (J = 10.1, C3), 145.8 (C5).
Cycloaddition of dihydrophosphinine oxides 6a and 6b with
DMAD
The solution of 0.46 g (1.64 mmol) of the 4 : 1 isomeric mixture
of dihydrophosphinine oxides 6a and 6b and 0.25 ml (2.03
mmol) of DMAD in 6 ml of toluene was stirred at the boiling
point for 7 days. The crude mixture obtained after evaporation
of the volatile components in vacuo was refined by flash column
chromatography (silica gel; 3% methanol in chloroform). 31P
NMR showed the presence of 89% of 8 and 11% of 7. The
components were separated by repeated column chromato-
graphy using the adsorbent and the eluant as above to give 0.12
g (86% based on 6b) of oxaphosphete 8 and 0.05 g (9% based
on 6a) of phosphabicyclooctadiene 7. The purity of the latter
species (7) was 95% according to NMR.
6,6-Dichloro-1-methyl-3-(2,4,6-trimethylphenyl)-3-phospha-
bicyclo[3.1.0]hexane 3-oxides 5a and 5b
To a solution of 6.0 g (25.6 mmol) of dihydrophosphole 4
and 1.10 g (4.84 mmol) of benzyltriethylammonium chloride
(TEBAC) in 120 ml of abs. chloroform was added dropwise
a solution of 44 g (1.10 mol) of sodium hydroxide in 48 ml
of water. The mixture was stirred and heated for 4 h. After
filtration and separation, the organic phase was made up to
its original volume and 1.10 g (4.84 mmol) of TEBAC was
added. The reaction mixture was treated with a second
portion of aq. sodium hydroxide as above. Flash column
chromatography of the crude product obtained after evapor-
ation of the organic phase (silica gel; 3% methanol in chloro-
form) afforded the product as a 72–28% mixture of isomers 5a
and 5b in 84% yield. The isomers were separated by repeated
column chromatography using the same adsorbent and eluant,
as above.
8: 31P NMR (CDCl3) δ 26.4; 13C NMR (CDCl3)† δ 16.7
(J = 17.8, C6-Me), 21.2 (C4Ј-Me), 23.1 (J = 5.8, C2Ј-Me), 28.6
(J = 61.0, C9), 51.0 (MeO), 51.9 (MeO), 73.9 (J = 107.7, C3),
119.9 (J = 14.0, C8), 122.1 (J = 93.2, C1Ј), 122.8 (J = 84.8, C5),
131.1 (J = 12.1, C3Ј), 140.3 (J = 13.9, C7), 142.0 (J = 11.0, C2Ј),
142.7 (C4Ј), 155.3 (J = 14.3, C6), 167.0 (J = 14.6, C᎐O), 167.7
᎐
(J = 15.8, C᎐O), 182.9 (J = 6.2, C4); 1H NMR (CDCl ) δ 2.12 (s,
᎐
3
3H, C6-Me), 2.27 (s, 3H, C4Ј-Me), 2.59 (s, 6H, C2Ј-Me), 3.05 (dd,
J1 = 18.0, J2 = 9.8, 1H, C9-H), 3.60 (s, 3H, OMe), 3.79 (s, 3H,
OMe), 5.03 (dd, J1 = 18.4, J2 = 13.0, 1H, C9-H), 6.51 (s, 1H,
C8-H), 6.66 (d, J = 22.9, 1H, C5-H), 6.90 (s, 2H, ArH); IR
(film) 2952, 1731, 1445, 1083, 756 cmϪ1; FAB, 423 (M ϩ H);
HR-FAB, (M ϩ H)ϩfound = 423.1060. C21H25ClO5P requires
m/z, 423.1128 for the 35Cl isotope.
5a: Yield 1.7 g (21%); 31P NMR (CDCl3) δ 80.7; 13C NMR
(CDCl3) δ 20.9 (C4Ј-Me), 21.9 (J = 7.3, C1-Me), 23.5 (J = 3.4,
C2Ј-Me), 33.4 (J = 64.5, C4), 36.0 (J = 9.5, C1), 37.3 (J = 7.6, C5),
38.8 (J = 64.5, C2), 71.7 (J = 8.4, C6), 127.5 (J = 85.9, C1Ј), 131.1
1
1
(J = 11.0, C3Ј), 141.5 (J = 10.7, C2Ј), 141.6 (J = 2.1, C4Ј); H
7: 31P NMR (CDCl3) δ 42.0; H NMR (CDCl3) δ 6.58 (d,
NMR (CDCl3) δ 1.48 (s, 3H, C4Ј-Me), 2.29 (s, 3H, C1-Me), 2.57
(s, 6H, C2Ј-Me); MS, m/z (rel. int.) 316 (Mϩ, 10%), 301 (M Ϫ 15,
6), 281 (M Ϫ 35, 100), 245 (281 Ϫ 36, 52), 165 (ArPO Ϫ H,
† Non-systematic numbering scheme.
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J. Chem. Soc., Perkin Trans. 1, 2001, 1062–1065

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J1 = J2 = 6.2, C6-H); MS, m/z (rel. int.) 422 (Mϩ, 2%), 211
References
[M Ϫ MeO Ϫ ArP(O)CH2, 100]; HR-FAB, (M ϩ H)ϩ
=
found
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2, 283.
2 L. D. Quin, J.-S. Tang, G. S. Quin and Gy. Keglevich, Heteroatom
Chem., 1993, 4, 189.
423.1089. C21H25ClO5P requires m/z, 423.1128 for the 35Cl
isotope.
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Sulfur Silicon Relat. Elem., 1995, 106, 155.
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Cycloaddition of dihydrophosphinine oxides 6a and 6b with
NPMI
A similar reaction of 1.0 g (3.57 mmol) of the isomeric
dihydrophosphinine oxides 6a and 6b and 0.71 g (4.10 mmol)
of NPMI in 10 ml of toluene furnished 0.37 g (23%) of
phosphabicyclooctene 10 as a 22–78% mixture of isomers 10a
and 10b after repeated column chromatography carried out as
above. MS, m/z (rel. int.) 453 (Mϩ, 73%), 438 (M Ϫ 15, 35),
418 (M Ϫ 35, 100), 91 (55); HR-FAB, (M ϩ H)ϩfound
=
454.1271. C25H26ClNO3P requires m/z, 454.1339 for the 35Cl
isotope.
10a: 31P NMR (CDCl3) δ 41.7; 13C NMR (CDCl3)† δ 21.1
(C4Ј-Me), 23.5 (J = 4.2, C4-Me), 24.5 (J = 3.6, C2Ј-Me), 33.6
(J = 77.1, C3), 39.1 (J = 3.2, C7), 40.8 (J = 60.9, C1), 44.6
(J = 7.4, C4), 49.4 (J = 10.9, C8), 121.1 (J = 5.0, C6), 126.5 (C3Љ),a
129.1 (C4Љ), 129.4 (C2Љ),a 131.1 (J = 11.3, C3Ј), 140.5 (J =
10.1, C5), 141.2 (J = 11.3, C2Ј), 141.8 (J = 1.9, C4Ј), 174.4
(C9), 176.3 (J = 15.1, C11),a may be reversed; 1H NMR (CDCl3)
3
δ
1.74 (d, J = 3.0, C4-Me), 5.96 (dd, JPH = 3JHH = 6.0,
C6-H).
10b: 31P NMR (CDCl3) δ 42.3; 13C NMR (CDCl3)† δ 18.4
(J = 2.3, C6-Me), 21.1 (C4Ј-Me), 24.5 (J = 3.6, C2Ј-Me), 33.2
(J = 75.9, C3), 39.7 (J = 1.9, C7), 42.9 (J = 7.4, C4), 44.9
(J = 60.4, C1), 45.5 (J = 11.1, C8), 128.6 (J = 12.2, C6), 126.5
(C3Љ),b 129.1 (C4Љ), 129.4 (C2Љ),b 131.4 (J = 11.4, C3Ј), 140.6
(J = 10.4, C5), 141.4 (J = 10.0, C2Ј), 142.2 (J = 1.9, C4Ј), 175.6
(C9), 176.5 (J = 15.3, C11),b may be reversed; 1H NMR (CDCl3)
δ 1.47 (d, J = 2.0, C6-Me), 1.62 (s, C4Ј-Me), 2.30 (s, o-Me), 2.66
(s, o-Me).
19 Gy. Keglevich, Gy. M. Keseru´´ , H. Forintos, Á. Szöllo´´ sy, K. Ludányi
and L. To´´ ke, J. Chem. Soc., Perkin Trans. 1, 1999, 1801.
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Acknowledgements
We thank the Ministry of Higher Education (FKFP, Grant
No. 363/1999) and the Hungarian Scientific Research Fund
(OTKA, Grant No. T 029039) for financing this project.
J. Chem. Soc., Perkin Trans. 1, 2001, 1062–1065
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