2,4,6-trialkylphenyl-2H-phospholes from slightly aromatic 1H-phospholes and their use in [4 + 2] cycloaddition reactions

Article abstract of DOI:10.1002/hc.10151

1-(2,4,6-Trialkylphenyl) phosphates 1a,b possess a moderate aromatic character. Despite of that they underwent a sigmatropic rearrangement at 150°C to afford 2H-phospholes 2a,b which by trapping with tolane, or in reaction with another unit of 2 gave [4 +

Full text of DOI:10.1002/hc.10151

Heteroatom Chemistry
Volume 14, Number 4, 2003
2,4,6-Trialkylphenyl-2H-phospholes from
Slightly Aromatic 1H-Phospholes and Their
Use in [4 + 2] Cycloaddition Reactions
Gyo¨rgy Keglevich,1 Rena´ta Farkas,1 T´ımea Imre,2 Krisztina Luda´nyi,2
3
4
´
Aron Szo¨llo˝sy, and La´szlo´ To˝ke
1Department of Organic Chemical Technology, Budapest University of Technology
and Economics, 1521 Budapest, Hungary
2Chemical Research Center, Hungarian Academy of Sciences, 1525 Budapest, Hungary
3Department of General and Analytical Chemistry, Budapest University of Technology
and Economics, 1521 Budapest, Hungary
4Research Group of the Hungarian Academy of Sciences, Budapest University of Technology
and Economics, 1521 Budapest, Hungary
Received 13 November 2002; revised 2 December 2002
On the one hand, phospholes are widely used as
ABSTRACT:
1-(2,4,6-Trialkylphenyl)phospholes
1a,b possess a moderate aromatic character. Despite
of that they underwent a sigmatropic rearrangement
at 150◦C to afford 2H-phospholes 2a,b which by
trapping with tolane, or in reaction with another
unit of 2 gave [4 + 2] cycloadducts 3a,b, or in a
reversible reaction, dimer 6, respectively. Dedimer-
ization of species 6 at 150◦C in the presence of
tolane, or at 0◦C under oxidative circumstances, led
to 1-phosphanorbornadiene 3a, or phosphole oxide
ligands in transition-metal complexes, on the other
hand, their chemistry offers many points of inter-
est, such as the 1H-phosphole → 2H-phosphole sig-
matropic rearrangement explored by Mathey [3].
For phenylphospholes, lacking aromaticity due to
the pyramidal character of the phosphorus atom,
the overlap between the ꢀ-dienic system and the ꢁ-
orbital of the exocyclic P Ph bond promotes the mi-
gration of the Ph-group. The 2H-phospholes gener-
ated at 160◦C were trapped by alkynes and alkenes
or dimerized in [4 + 2] cycloadditions to yield 1-
phosphanorbornene derivatives [3–7]. An important
practical application of the reaction under discus-
sion is the synthesis of 2,2ꢁ-bis-(1-phosphanorborn-
adienyl) (BIPNOR) which is an efficient biphos-
phine for asymmetric catalysis [8]. Proton [1,5]
shifts in P-unsubstituted 1H-phospholes and subse-
quent dimerizations were also observed to take place
[9].
ꢀ
C
dimer 8, respectively.
2003 Wiley Periodicals, Inc.
Heteroatom Chem 14:316–319, 2003; Published online
in Wiley InterScience (www.interscience.wiley.com). DOI
10.1002/hc.10151
INTRODUCTION
Phospholes, perhaps the most representative family
of P-heterocycles are of considerable interest [1,2].
RESULTS AND DISCUSSION
Correspondence to: Gyo¨rgy Keglevich; e-mail: keglevich@oct.
bme.hu.
Contract grant sponsor: OTKA.
2,4,6-Trialkylphenylphospholes introduced recently
[10–13] form a special class of phospholes, as
they have some aromatic character due to the
Contract grant number: T 042479.
2003 Wiley Periodicals, Inc.
c
ꢀ
316

2,4,6-Trialkylphenyl-2H-phospholes from Slightly Aromatic 1H-Phospholes 317
planarization of the P-pyramid. For 1-(2,4,6-
triisopropylphenyl)phosphole (1a), the Bird-index
of aromaticity [14] was found to be 40.4, which
entered, although not too efficiently, into an aro-
matic electrophilic substitution reaction [15]. At the
same time, 1a, as well as the 2,4-di-tert-butyl-6-
methylphenyl phosphole (1b), could also be involved
in Diels–Alder cycloadditions [16]. The dual reac-
tivity of the arylphospholes 1a,b is also reflected
in our present finding that they undergo a sigma-
tropic Ar[1,5] rearrangement on heating at 150◦C
in toluene, in a sealed tube. The intermediate 2H-
phospholes 2a,b were trapped by reaction with
tolane to furnish 1-phosphanorbornadienes 3a,b.
To obtain stable products, the phosphines (3a,b)
were oxidized to the corresponding phosphine ox-
ides (4a,b) (Scheme 1). Product 4b was isolated as
a 52:48 mixture of two rotamers (41b and 42b). The
phosphanorbornadienes (3a, 4a, and 4b) were iden-
SCHEME 2
Thermal treatment of arylphosphole 1a in the
1
tified by 31P, 13C, and H NMR, as well as HRMS
absence of tolane led to the formation of dimer 6
(Scheme 2). Formally, the dimer may be derived by
the Diels–Alder reaction of 2H-phosphole 2a with
species 5 formed by another, this time a H[1,5] sig-
matropic rearrangement. The structure of 6 was con-
firmed by 31P, 13C, and 1H NMR spectral parameters,
as well as HRMS. As regards the stereostructure of
6, no direct proof is available at present. The 31P NMR
(δP 34.4 and −17.2 1 JPP = 216.6 Hz) together with the
13C NMR data obtained for 6 match those reported
for the analogous P-phenyl derivative [7] that was
believed to have the rings in the exo-fusion. At the
same time, reversible formation of dimer 6 (see next
paragraph) substantiated rather the endo form, such
as an analogous species with relatively long and weak
P P and C C bonds at the junction of the dimer [17].
Evaluation of the geometry of the highly sensitive cy-
cloadduct will require ab initio quantum chemical
calculations.
data. 13C NMR spectral parameters of compound 4a
closely resembled those of an analogue described
earlier [5].
It was interesting to find that the 1-(2,4,6-tri-tert-
butylphenyl-)3-methylphosphole exhibiting a Bird-
index of 56.5, which is comparable with that of pyr-
role (59), resisted the pericyclic reaction attempted.
This is probably caused by the fact that the ꢀ-system
of the phosphole moiety is not available due to the
electron delocalization. Hence, it can be concluded
that the extent of aromaticity controls the reactivity
in the 1H- to 2H-phosphole rearrangement.
Reversibility for the formation of dimer 6 was
proved first by its reaction with tolane, as it led
to the formation of cycloadduct 3a (Scheme 3). At
150◦C, the dimer was decomposed to two units of
2H-phosphole (2a and 5) (Scheme 2) entering into
reaction with tolane. Mathey also described the re-
versible formation of a series of 2H-phosphole cy-
cloadducts [6,9,18]. It is the novel and quite surpris-
ing observation of ours that precursor 6 was also
dedimerized on treatment at 0◦C with hydrogen per-
oxide to afford phosphole oxide intermediate 7 lead-
ing spontaneously to its dimer 8 (Scheme 3).
SCHEME 1
On the basis of its 31P and 13C NMR spectral pa-
rameters, the phosphole oxide dimer 8 obtained after
purification by column chromatography was identi-
cal with an authentic sample described by us earlier
[10]. It is noted, however, that the reaction under

318 Keglevich et al.
∗
ꢁ
19.5, C2), 146.6 (J = 28.1, C6), 148.2 (C2 ), 148.3
∗
∗
∗
ꢁ
ꢁ
(C4 ), 149.4 (C6 ), 152.8 (C5), 161.2 (C3), tentative
assignment; M + H = 479).
Phosphine 3a (0.96 g, ∼2.0 mmol) in 40 ml of
chloroform was treated with 0.70 ml (∼6.2 mmol)
of 30% hydrogen peroxide with intensive stirring
at 0◦C. After addition was complete, the contents
of the flask were allowed to warm to 25◦C and the
stirring was continued for 1 h. The mixture was
extracted with 2×15 ml of water and the organic
phase was dried (Na2SO4). The crude product ob-
tained after evaporating the solvent was purified by
column chromatography (silica gel, 3% methanol in
chloroform) to afford 0.60 g (61%) of phosphine ox-
ide 4a. 31P NMR (CDCl3) δ 52.0; 13C NMR δ 21.2
(J = 16.1, C4 Me), 22.8 (CHCH3), 23.8 (CHCH3),
24.1 (CHCH3), 24.2 (CHCH3), 24.6 (CHCH3), 25.3
(CHCH3), 31.4 (o-CHMe2), 31.5 (o-CHMe2), 34.4 (p-
SCHEME 3
CHMe2), 46.9 (J = 29.0, C4), 69.6 (J = 68.4, C7),
discussion was accompanied by a series of side reac-
tions. The complex composition of the mixture pre-
vented the isolation and identification of the minor
components.
∗
ꢁ
139.3 (J = 76.3, C2), 146.4 (J = 3.2, C2 ), 147.1 (J =
∗
ꢁ
ꢁ
3.5, C6 ), 147.7 (J = 74.5, C6), 148.7 (C4 ), 154.1 (J =
16.7, C5), 162.0 (J = 15.9, C3),∗ may be reversed; H
1
NMR δ 0.74 (d, J = 6.8, 3H, CHCH ), 1.09 (d, J = 6.7,
To summarize our results, we found that 2,4,6-
trialkylphenylphospholes with not too high extent
of aromaticity (with a Bird-index around 40) un-
derwent the 1H-phosphole → 2H-phosphole sigmat-
ropic rearrangement. The latter species were trapped
in Diels–Alder reaction with tolane, or in one case,
the 2H-phosphole was dimerized in a [4 + 2] fashion.
As the dimerization is reversible, the cycloadduct
served as the precursor of 2H-phosphole in reaction
with tolane, but on treatment with hydrogen perox-
ide, the dimer was the source of the corresponding
1H-phosphole.
3
3H, CHCH ), 1.26 (d, J = 6.9, 6H, CH(CH )2), 1.29 (d,
3
3
J = 6.7, 3H, CHCH ), 1.41 (d, J = 6.9, 3H, CHCH3),
3
1.45 (s, 3H, C4 CH3), 2.68 (q, J = 6.8, 1H, CHMe2),
2.86–2.97 (m, 3H, CHMe2, CH2), 3.25 (q, J = 6.7, 1H,
CHMe2), 7.14 (d, J ∼ 39, CH , overlapped by the
aromatic signals); HRFAB (M + H)+found = 495.2734,
C34H40OP requires 495.2817.
The mixture of 41b (δP 53.2, 52%) and 42b (δP
52.0, for both species (M + H)+ = 495) was prepared
in a similar way.
Preparation of Dimer 6
EXPERIMENTAL
Same procedure was repeated with phosphole 1a,
but without adding tolane. Evaporation of toluene
gave a residue (∼0.6 g) that was extracted with ace-
tone to leave 0.20 g (33%) of 6 as a thick oil. All op-
erations were performed under nitrogen. 31P NMR
1
The 31P, 13C, and H NMR spectra were taken on a
Bruker DRX-500 spectrometer operating at 202.4,
125.7, and 500 MHz, respectively. Chemical shifts
are downfield relative to 85% H3PO4 or TMS. The
couplings are given in hertz. Mass spectrometry was
performed on a ZAB-2SEQ instrument.
(CDCl3) δ 34.4 (P1) and −17.2 (P2), JPP = 216.6; 13C
1
NMR δ 24.2 (broad signal, CH(CH3)2), 25.2 (C5 Me),
a 25.8 (C7 Me), a 34.4 (broad signal, CHMe2), 43.4
(J = 20.2, C10), 43.7 (J = 4.0, C5), 55.6 (J = 26.0, C6),
Generation of 2a and Its Trapping with Tolane
ꢁ
60.5 (J1 = 3.6, J2 = 6.1, C7), 120.7 (Cꢂ ), b 120,9 (Cꢂ ),
ꢁ
A mixture of 0.6 g (2.0 mmol) of phosphole 1a and
0.37 g (2.1 mmol) of tolane in 15 ml of toluene
was degassed by nitrogen and heated in a sealed
tube at 150◦C for 4 days. The solvent was evap-
orated to give 0.96 g (∼100%) of cycloadduct 3a.
31P NMR (CDCl3) δ 5.9; 13C NMR δ 21.9 (C4 Me),
23.5 (CHCH3), 23.9 (CHCH3), 25.0 (CH(CH3)2), 25.1
(CHCH3), 26.0 (CHCH3), 31.3 (p-CHMe2), 35.1 (o-
CHMe2), 67.8 (C4), 70.6 (J = 5.6, C7), 138. 5 (J =
b 121.0 (Cε), b 121.2 (Cε ), b 132.0 (J = 12.9, Cꢃ), c
ꢁ
132.2 (J = 15.3, Cꢃ ), c 141.0 (J = 32.1, C3), 144.6
(J1 = J2 = 5.4, C4), 146.1 (J = 33.7, C9), 146.7 (Cꢄ,
ꢁ
ꢁ
ꢁ
Cꢄ ), d 147.3 (Cꢅ, Cꢅ ), d 147.7 (Cδ, Cδ ), 151.5 (J = 8.2,
1
C8), a–d tentative assignment; H NMR δ 1.06–1.33
(m, 39H, 6CH(CH )2 + CH3), 1.62 (s, 3H, CH3), 5.82
3
(d, J = 10.0, 1H, CH ), 5.98 (dd, J1 = 3.9, J2 = 7.9,
1H, CH ); HRMS, M+found = 600.3891, C40H58P2 re-
quires 600.4014.

2,4,6-Trialkylphenyl-2H-phospholes from Slightly Aromatic 1H-Phospholes 319
[2] Quin, L. D. In Phosphorus-Carbon Heterocyclic
Chemistry: The Rise of a New Domain; Mathey,
F. (Ed.); Pergamon: Amsterdam, 2001; Ch. 4.2.1–
4.2.2.
[3] Mathey, F.; Mercier, F. C R Acad Sc Paris, Se II b, 1997,
324, 701.
[4] Mathey, F.; Mercier, F.; Charrier, C. J Am Chem Soc
1981, 103, 4595.
[5] Laporte, F.; Mercier, F.; Ricard, L.; Mathey, F. Bull Soc
Chim Fr 1993, 130, 843.
[6] Goff, P.; Mathey, F.; Ricard, L. J Org Chem 1989, 54,
4754.
[7] Mercier, F., Mathey, F. J Organomet Chem 1984, 263,
55.
[8] Mathey, F.; Mercier, F.; Robin, F.; Ricard, L. J
Organomet Chem 1998, 577, 117.
[9] Charrier, C.; Bonnard, H.; Lauzon, G.; Mathey, F. J
Am Chem Soc 1983, 105, 6871.
[10] Keglevich, Gy.; Quin, L. D.; Bo¨cskei, Zs.; Keseru˝,
Gy. M.; Kalgutkar, R.; Lahti, P. M. J Organomet Chem
1997, 532, 109.
Dedimerization of 6
Dimer 6 (0.20 g, 0.33 mmol) in 10 ml of degassed
toluene was heated with 0.12 g (0.67 mmol) of tolane
at 150◦C in a sealed tube for 4 days. Evaporation of
the solvent left ∼100% of 3a with δP 5.8.
The reaction of 6 with hydrogen peroxide was
performed as described above for the 3a → 4a trans-
formation to give dioxide 8 in 21% yield. 31P NMR
3
(CDCl3) δ 57.2 and 80.5, JPP = 38.0 (δP lit. [10],
3
56.4 and 80.1, JPP = 38.1); 13C NMR (CDCl3) δ 18.7
(J = 16.8, C3 Me),∗ 19.1 (C5 Me),∗ 43.3 (J1 = 74.6,
J2 = 12.8, C7a), 47.9 (J = 60.0, C7), 51.8 (J1 = J2 =
13.3, C3a), 52.4 (J = 64.2, C4), C∗6∗, 130.5 (J = 97.9,
C2), 135.5 (J = 12.2, C5), 157.0 (J1 = 23.9, J2 = 9.3,
∗
C3), may be reversed, ∗∗overlapped in the range of
= 119.8–124.6 (δC lit. [10], 13C NMR (CDCl3) δ 18.9
(J = 16.7, C3 Me),∗ 19.3 (C5 Me),∗ 43.5 (J1 = 74.8,
J2 = 12.8, C7a), 48.0 (J = 60.0, C7), 51.9 (J1 = J2 =
13.1, C3a), 52.6 (J = 64.5, C4), C∗6∗, 130.7 (J = 97.9,
C2), 135.8 (J = 12.2, C5), 157.1 (J1 = 23.9, J2 = 9.1,
C3), ∗may be reversed, ∗∗overlapped in the range of =
119.9–124.5); 1H NMR (CDCl3) δ 1.61 (s, 3H, C5 Me),
2.1 (s, 3H, C3 Me), 6.18 (d, J = 12.4, 1H, C6 H), 6.25
(d, J = 23.8, 1H, C2 H) (δH lit. [10], 1H NMR (CDCl3)
δ 1.61 (s, 3H, C5 Me), 2.0 (s, 3H, C3 Me), 6.17 (d,
J = 12.4, 1H, C6 H), 6.23 (d, J = 23.9, 1H, C2 H));
MS, 632 (M+).
[11] Quin, L. D.; Keglevich, Gy.; Ionkin, A. S.; Kalgutkar,
R.; Szalontai, G. J Org Chem 1996, 61, 7801.
[12] Keglevich, Gy.; Bo¨cskei, Zs.; Keseru˝, Gy. M.;
´
Ujsza´szy, K.; Quin, L. D. J Am Chem Soc 1997, 119,
5095.
[13] Keglevich, Gy. In Targets in Heterocyclic Systems,
Attanasi, O.; Spinelli, D. (Eds.); Italian Society of
Chemistry (in press); 2002, Vol. 6.
[14] Bird, C. W. Tetrahedron 1990, 46, 5697.
[15] Keglevich, Gy.; Chuluunbaatar, T.; Dajka, B.; Dobo´,
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A.; Szo¨llo˝sy, A.; To˝ke, L. J Chem Soc, Perkin Trans 1,
2000, 2895.
[16] Keglevich, Gy.; Nyula´szi, L.; Chuluunbaatar, T.;
Bat-Amgalan, N.; Luda´nyi, K. Imre, T.; To˝ke, L. Tetra-
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[17] Lauzon, G.; Charrier, C.; Bonnard, H.; Mathey, F.;
Fischer, J.; Mitschler, A. J Chem Soc, Chem Commun
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