The Diels-Alder reaction of diphenyl(1,2-propadienyl)phosphine oxide and diphenyl(1-propynyl)phosphine oxide with cyclopentadiene

Article abstract of DOI:10.1080/10426500490468128

The Diels-Alder reaction of diphenyl(1,2-propadienyl)phosphine oxide 1 and diphenyl(1-propynyl)phosphine oxide 2 with cyclopentadiene is reported. 1 reacts smoothly at room temperature in the presence of one equivalent of aluminum trichloride to give the corresponding adducts endo 3a and exo 3b (90:10 ratio) whose structure was attributed on the basis of their ¹³C NMR spectra, whereas 2 is a poor dienophile, affording the corresponding adduct in low yield even under harsh conditions.

Full text of DOI:10.1080/10426500490468128

Phosphorus, Sulfur, and Silicon, 179:1323–1328, 2004
Copyright ꢀC Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500490468128
THE DIELS-ALDER REACTION OF
DIPHENYL(1,2-PROPADIENYL)PHOSPHINE OXIDE
AND DIPHENYL(1-PROPYNYL)PHOSPHINE OXIDE
WITH CYCLOPENTADIENE
Michel Maffei
Laboratoire des Organo-Phosphor e´ s, Marseille, France
(
Received August 21, 2003; accepted October 21, 2003)
The Diels-Alder reaction of diphenyl(1,2-propadienyl)phosphine oxide
1
and diphenyl(1-propynyl)phosphine oxide 2 with cyclopentadiene is
reported. 1 reacts smoothly at room temperature in the presence of one
equivalent of aluminum trichloride to give the corresponding adducts
endo 3a and exo 3b (90:10 ratio) whose structure was attributed on the
basis of their 13C NMR spectra, whereas 2 is a poor dienophile, afford-
ing the corresponding adduct in low yield even under harsh conditions.
Keywords: Cyclopentadiene; Diels-Alder reaction; diphenyl(1,2-
propadienyl)phosphine; diphenyl(1-propynyl) phosphine oxide;
organophosphorus dienophiles
The Diels-Alder reaction is a powerful method for the synthesis of bi-
cyclic compounds. In this area, the use of organophosphorus dienophiles
1
2,3
is well documented. However, cycloaddition reactions of allenic
4,5
or acetylenic dienophiles bearing a phosphorus containing electron
withdrawing group has been much less studied.
Herein are reported the Diels-Alder reactions of diphenyl(1,2-
propadienyl)phosphine oxide 1 and diphenyl(1-propynyl)phosphine ox-
ide 2 with cyclopentadiene.
FORMULAE 1 and 2
Address correspondence to Michel Maffei, Laboratoire des Organo-Phosphor e´ s (UMR
6
009 du CNRS), Case 552, Facult e´ des Sciences de Saint J e´ r oˆ me, Avenue Escadrille
Normandie-Ni e´ men, 13397 Marseille Cedex 20, France. E-mail: michel.maffei@univ.u-
mrs.fr
3
1
323

1
324
M. Maffei
3
A recent article describes the Diels-Alder reaction of 1 with cy-
◦
clopentadiene (85 h at 80 C) to yield a single isomer in 64% yield, whose
endo structure was assigned on the basis of NMR data.
In contrast with this report, 1 was found to react smoothly with
cyclopentadiene at room temperature in the presence of one equivalent
of aluminum trichloride, giving rise to a (90:10) mixture of endo 3a and
exo 3b isomers.
SCHEME 1
The endo isomer was isolated after two recrystallizations from
toluene, whereas treatment of the same 90:10 endo/exo mixture with
sodium ethoxide led to epimerization at the carbon α to phosphorus
and afforded a 20:80 mixture of 3a/3b from which spectral data for exo
3
b were deduced. These structures were unambiguously attributed by
1
3
examination of C NMR data, using the characteristic values of three
bond carbon-phosphorus coupling constants which depend on the con-
figuration, and follow a Karplus relation.1
b,6
Thus, for the endo isomer 3a, the dihedral angle PC2-C1C7 is close to
80 , so the JPC7 coupling constant is at a maximum (10.8 Hz). In the
◦
3
1
case of the exo isomer 3b, JPC7 = 0, which is consistent with a dihedral
◦
angle PC2-C1C7 close to 90 . Therefore, the major isomer produced in
the AlCl3-promoted Diels-Alder reaction is endo 3a, according to pre-
1
b,c
vious results described with related dienophiles.
These data also
3
suggest that the major isomer produced in the thermal cycloaddition
might be the exo rather than the endo isomer.
In contrast with 1, diphenyl(1-propynyl)phosphine oxide 2 was found
to be a poor dienophile. Its synthesis was carried out in 73% yield by pro-
5
totropic isomerization of 1, by analogy with acetylenic phosphonates,
upon reaction with a catalytic amount of potassium tert-butoxide in
THF. Its low reactivity as dienophile (Scheme 2) was exemplified by the
different experiments listed in Table I.
The conversion of 2 into 4 was rather low in the presence of one equiv-
alent of aluminum trichloride at room temperature (entries 1 and 2),
or in refluxing toluene (entry 3) or xylene (entry 5), and did not greatly

Reaction of Diphenyl(1,2-Propadienyl)Phosphine Oxide
1325
SCHEME 2
increase on prolonged heating (compare entries 3 and 4). Even perform-
ing the reaction in a steel bomb under rather harsh conditions did not
promote a complete reaction, whereas several by products arising from
decomposition of 2 were formed. However, from this last experiment
(
entry 7), the adduct 4 could eventually be isolated in 17% yield.
These miscellaneous results are consistent with the generally low
reactivity of acetylenic organophosphorus dienophiles reported in the
literature: only dienophiles possessing a second electron withdrawing
group (phosphonate,4
tory results.
a,4b,4e
ester, sulfone or sulfoxide, ) give satisfac-
4c
4d
The use of adducts 3 in synthesis is currently under progress.
EXPERIMENTAL
THF was distilled from sodium and dichloromethane from calcium hy-
dride prior to use. Cyclopentadiene was prepared by cracking of dicy-
1
13
31
clopentadiene immediately prior to use. H, C, and P NMR spectra
were recorded in CDCl3 solutions on a Bruker AC 200 spectrometer.
TABLE I Diels-Alder Reaction Between 2 and Cyclopentadiene Under
Different Conditions.
Entry
Conditionsa
Time (h)
Conversion of 2 into 4 (%)b
1
2
3
4
5
6
7
AlCl3 (1 equiv.), room temperature
ibid.
Refluxing toluene
ibid.
24
48
24
48
24
24
4
20
35
20
35
40
Refluxing xylene
◦
◦
c
Sealed tube, 140 C, no solvent
55
70
d
c
Steel bomb, 220 C, 90 bars
aCyclopentadiene (2 equiv. with respect to 2) was added every 12 h, except for entries
6
and 7.
b
Determined by 31P NMR analysis of the crude product.
c
Several by products were formed.
d
Dicyclopentadiene was used.

1
326
M. Maffei
1
Chemical shifts were measured from chloroform (δ = 7.26 ppm for H
13
and δ = 77 ppm for C). 85% H3PO4 as external standard was used
3
1
for P chemical shifts. Elemental analyses were performed by the Ser-
vice de Microanalyse de la Facult e´ de St. J e´ r oˆ me. Melting points were
measured on a B u¨ chi apparatus and are uncorrected. Diphenyl(1,2-
7
propadienyl)phosphine oxide 1 was prepared as previously reported.
(
(3-Methylene)bicyclo[2.2.1]hept-5-en-2yl)diphenyl
Phosphine Oxide 3
◦
To a cooled (0 C) suspension of aluminum trichloride (3.85 g, 28.9 mmol)
in dichloromethane (5 mL) under argon was added dropwise a solution
of 1 (7 g, 29.1 mmol) in dichloromethane (25 mL), and the mixture
was allowed to reach room temperature with stirring (ca. 1 h). The
◦
brown yellow solution was cooled again to 0 C, and cyclopentadiene (4 g,
6
0 mmol) was added dropwise via syringe. The mixture was stirred at
room temperature for 12 h, cyclopentadiene (4 g) was added, and the
solution was stirred for another 12 h. After cooling to 0 C, the mix-
◦
ture was quenched by dropwise addition of saturated NH4Cl (20 mL),
diluted with dichloromethane (20 mL), and the layers were decanted.
The organic layer was washed with 1N HCl (2 × 10 mL), saturated
NaHCO3 (10 mL), brine (20 mL), and dried over MgSO4. Filtration and
removal of solvent afforded a yellowish solid which was recrystallized
from toluene/cyclohexane (1:1) to yield 7.3 g (82%) of a white solid, m.p.
◦
31
1
57–161 C. P NMR analysis showed the presence of two signals cor-
responding to 3a and 3b in a 90:10 ratio. C20H19OP : calcd. C 78.43, H
6
.21; found C 78.63, H 6.38.
A portion of this mixture was recrystallized twice from toluene to
◦
afford pure 3a, m.p. 170 C.
1
Endo 3a. (numbering refers to Scheme 1) H NMR: 1.47(d, J = 9 Hz,
H, H7a); 1.57 (d, J = 9 Hz, 1H, H7b); 3.12, 3.26 (2 s br., 1H each, H1 and
1
H4); 3.57 (m br., 1H, H2); 4.26 and 4.96 (2 s br., 1H each, H8a and H8b);
6
.02 (d, J = 2.5 Hz, H5 or H6); 6.05(d, J = 2.5 Hz, H5 or H6); 7.42–7.49
(
7
1
m, 6H aromatics); 7.77–7.90 (m, 4H aromatics). 13 C NMR: 45.0 (d, J =
8 Hz, C2); 45.4 (s, C1 or C4); 51.1 (d, J = 10.8 Hz, C7); 53.1 (s, C1 or C4);
06.9 (s, C8); 128.2, 128.5, 131.5, 131.6, 132.5, 134.3 (aromatics); 134.4
3
1
(
2
d, J = 3.8 Hz, C5 or C6); 133.7 (s, C5 or C6); 146.5 (s, C3).
P NMR:
9.1.
Exo 3b. To a solution of sodium ethoxide in ethanol (prepared by
adding 500 mg of sodium metal in 10 mL of absolute ethanol) was added
◦
1
g of the 3a/3b mixture, and the mixture was stirred at 40 C overnight.
After cooling to room temperature, the red solution was quenched with
0 mL of saturated NH4Cl, and the mixture was concentrated in vacuo.
1

Reaction of Diphenyl(1,2-Propadienyl)Phosphine Oxide
1327
Dichloromethane (20 mL) was added, the layers were decanted and the
organic layer was washed with water, brine, and dried over MgSO4.
Filtration and removal of solvent afforded a yellowish solid which was
recrystallized from toluene to afford a (20:80) mixture of 3a/3b (m.p.
◦
1
1
1
55–156 C) from which spectral data for 3b were deduced. H NMR:
.44 (d, J = 8.3 Hz, H7a); 2.24 (d, J = 8.7 Hz, H7b); 2.93, 3.10, 3.27 (3
br.s., 1H each, H1, H2, H4); 4.06, 4.98 (2 br.s., 1H each, H8a and H8b);
6
.23 (s, 2H, H5 and H6); 7.43–7.49 (m, 6H aromatics); 7.81–7.88 (m, 4H
1
3
aromatics).
C NMR: 43.0 (d, J = 67 Hz, C2); 45.2 (s, C1 or C4); 47.3
(
s, C7); 51.9 (s, C1 or C4); 108.1 (s, C8); 128.5, 128.7, 130.8, 131.3, 131.5,
1
1
33.8 (aromatics); 137.1 (d, J = 16 Hz, C5 or C6); 137.9 (s, C5 or C6);
46.2 (s, C3). 31 P NMR: 29.8.
Diphenyl(1-propynyl)phosphine Oxide 2
To a solution of 1 (4.8 g, 20 mmol) in THF (20 mL) was added solid potas-
sium tert-butoxide (78 mg, 0.7 mmol) at room temperature. The mixture
turned immediately deep red while a slight heat evolution was noticed.
After stirring at room temperature for one hour, 1N HCl (10 mL) was
added and most of the THF was removed in vacuo. Dichloromethane
(
100 mL) was added, and the layers were decanted. The organic layer
was washed with brine (20 mL) and dried over MgSO4. Filtration
and removal of solvent left a red oil which solidified upon standing
at room temperature overnight. This was recrystallized from ethyl
acetate/cyclohexane (1:1) to yield 3.5 g (73%) of a pale yellow solid,
◦
1
m.p. 91–92 C. H NMR: 2.05 (d, J = 3.5 Hz, 3H); 7.46 (m, 6H); 7.81 (m,
1
3
4
H).
C NMR: 4.9 (d, J = 3.0 Hz, CH3); 74.4 (d, J = 176 Hz); 105.5
31
(
d, J = 31.8 Hz); 128.5, 130.8, 132.0, 135.2 (aromatics). P NMR: 6.4.
C15H13OP : calcd. C 75.00, H 5.42; found C 75.05, H 5.51.
Diphenyl(3-methyl-norborna-2,5-dienyl)phosphine
Oxide 4
A mixture of 2 (2.4 g, 10 mmol) and dicyclopentadiene (4 g, 30 mmol)
◦
was heated at 220 C under 90 bar with shaking for 4 h in a steel
3
1
bomb. P NMR analysis of the reddish very viscous oil thus obtained
showed the presence of a major signal (δ = 23.9 ppm, 70%), unreacted
2
(10%), and several other products (δ = 22 ppm, 27 ppm, 28.7 ppm,
and 32.3 ppm). This mixture was subjected to column chromatogra-
phy. Initially, petroleum ether (400 mL) was used as eluent to separate
the cyclopentadiene. Further elution with ether (400 mL) yielded a un-
separable mixture of 4, unreacted 2, and byproducts, and final elution
with ethyl acetate (500 mL) gave 520 mg (17%) of pure 4 as a very

1
328
M. Maffei
1
viscous yellowish oil that did not solidify. H NMR: (numbering refers
to Scheme 1) 1.98 (d, J = 0.7 Hz, 3H, H8); 2.06–2.12 (m, 2H, H7); 3.49,
3
.57 (2 br.s., 1H each, H1 and H4); 6.63, 6.72 (2 br.s., 1H each, H5 and
13
H6); 7.40–7.77 (m, 10H). C NMR: 17.2 (d, J = 3.6 Hz, CH3); 53.9 (d,
J = 12.5 Hz, C1 or C4); 59.0 (d, J = 12.7 Hz, C1 or C4); 71.0 (d, J = 5 Hz,
C7); 127.9, 128.4, 130.8, 131.1 (aromatics); 140.1 (d, J = 2 Hz, C5 or C6);
1
42.9 (s, C5 or C6). 31 P NMR: 23.9. C20H19OP : calcd. C 78.43, H 6.21.
+
Found C 78.71, H 6.44. MS (ESI) : m/z = 307 [M + H ].
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