Novel metal-free Lewis acid catalysis by phosphonium salts through hypervalent interaction

Article abstract of DOI:10.1016/j.tet.2005.09.075

Phosphonium salts as a novel metal-free Lewis acid catalyst can be considered as organocatalysts. The introduction of a five-membered dioxaphosphacycle to the phosphonium salt is critical to its function as a Lewis acid catalyst for the Diels-Alder reacti

Full text of DOI:10.1016/j.tet.2005.09.075

Tetrahedron 62 (2006) 401–409
Novel metal-free Lewis acid catalysis by phosphonium salts
through hypervalent interaction
*
Masahiro Terada and Mitsuhiro Kouchi
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba-ku, Sendai 980-8578, Japan
Received 5 July 2005; revised 30 August 2005; accepted 16 September 2005
Available online 12 October 2005
Abstract—Phosphonium salts as a novel metal-free Lewis acid catalyst can be considered as organocatalysts. The introduction of a five-
membered dioxaphosphacycle to the phosphonium salt is critical to its function as a Lewis acid catalyst for the Diels–Alder reaction. The key
to the successful catalysis by the phosphonium salt is the utilization of hypervalent bonding as a strategic interaction for the generation of an
active species.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
Over the past half-century, organophosphorus compounds
have been extensively utilized in organic synthesis ranging
from equimolar to catalytic usage.1 In particular, their
benefits have been widely appreciated in the field of
organometallic chemistry and they have become
indispensable for versatile organic transformations
catalyzed by transition metal complexes. In recent years,
the challenges associated with the use of organophosphorus
compounds without metal salts as catalysts have received
considerable interest2 from the viewpoint of organo-
catalysis.3 Among them, the Morita–Baylis–Hillman
reaction has been extensively investigated.2d,4,5 Catalytic
processes have been successfully accomplished by taking
advantage of the nucleophilic or Lewis basic nature of
organophosphorus compounds.6 However, little attention
has been paid to catalysis on the basis of the electrophilic,
and hence the Lewis acidic nature of pentavalent
organophosphorus compounds. Herein, we report the use
of phosphonium salts as novel Lewis acid catalysts, which
offers a new entry to a rapidly growing area in
organocatalysis.3 The key to the success of the catalysis is
the utilization of hypervalent bonding as a strategic
interaction for the generation of an active species7,8 by the
phosphonium salt (Scheme 1).9
Scheme 1. Hypervalent interaction between phosphonium salt and Lewis
base.
electronic structures with formal valence shell electrons
over octet, are well investigated in terms of their unique
structure and reactivity.11 A hypervalent bond is formed by
adding an unshared electron pair of a Lewis base (LB) to a
cationic organophosphorus compound, a phosphonium salt,
where the lower lying s* orbital of the PC–G bond accepts
those electrons.12 The hypervalent bond thus formed is
stabilized when an electron-withdrawing substituent (G)
occupies the apical position of a trigonal bipyramidal (TBP)
arrangement.
2. Results and discussion
2.1. Preparation of phosphonium salts
The formation of pentacoordinate organophosphorus com-
pounds, namely, hypervalent compounds,10 which have
As illustrated in Figure 1, several phosphonium salts (1–3)
bearing mono- or bicyclic structures have been synthesized.
In order to stabilize the hypervalent bonding,13 we
introduced an oxygenated functionality as the electron-
withdrawing group. 1–3 were prepared from the corre-
sponding hydroxy phosphine oxide (4) or phosphinate (5)
Keywords: Hypervalent compounds; Lewis acid; Phosphorus; Organoca-
talyst; Diels–Alder reaction.
*
Corresponding author. Tel./fax: C81 22 795 6602;
e-mail: mterada@mail.tains.tohoku.ac.jp
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.09.075

402
M. Terada, M. Kouchi / Tetrahedron 62 (2006) 401–409
Interestingly enough, the spectra of biphenol-derived
phosphonium salts (3a,b) did not show any chemical shift
change even though the salts (3) had a similar dioxa-
phosphacycle to 2. These NMR studies indicate clearly that
the five-membered 1,3,2-dioxaphosphacycle is the key
structural factor for gaining coordination ability.18 Detailed
analysis of the 1H NMR spectra of the 2/DMF complex gave
further information on the coordination mode of the
phosphonium salts (2) with DMF (Fig. 2). When bicyclic
phosphonium salt (2a) was exposed to 3.0 equiv of DMF,
two sets of DMF signals were observed along with upfield
shifts of the signals in the aromatic region (Fig. 2b). One set
of DMF signals appeared significantly downfield compared
with the original shift (Fig. 2e vs b). The other set was
assigned to uncoordinated DMF because of little chemical
shift change. Furthermore, the aromatic region of 2a showed
a more complex shift pattern (Fig. 2a vs b). 2D NMR
experiments of the 2a/DMF complex revealed that the
catechol moiety was separated into four signals whereas the
dibenzophosphole moiety retained symmetry.19 These
chemical shift changes strongly suggest that phosphonium
Figure 1. Chemical structures of phosphonium salts (1–3) prepared.
and trifluoromethanesulfonic anhydride at room tempera-
˚
ture for 1 h in the presence of 4 A molecular sieves (MS) as
a desiccant and a base to trap the trifluoromethanesulfonic
acid generated (Scheme 2).14 The generation of phos-
phonium salts (1–3) was confirmed by 31P NMR analysis, in
which significant downfield shifts were observed.15 During
this transformation, the chemical shift pattern observed in
1
the H NMR spectra changed in a symmetric fashion. This
can be ascribed to the formation of phosphonium salts with
tetrahedral configuration.
Scheme 2. Preparation of phosphonium salts (1–3) from 4 or 5.
2.2. Coordination studies
Initially, we attempted 31P NMR analysis to estimate the
interaction between phosphonium salts (1–3) and dimethyl-
formamide (DMF) as a representative carbonyl Lewis base
in CDCl3. It is noteworthy that not only the ring size but also
the substituent on the phosphorus atom is essential for the
coordination of DMF to phosphonium salts (1–3). Phos-
phonium salts (1a,b) derived from the Martin ligand, which
is known to stabilize a hypervalent species,16 exhibited no
Figure 2. 1H NMR analysis of phosphonium salts (2) with or without DMF
in CDCl3 (30 mM solution). Solid circles (C) indicate catechol moiety.
Hollow circles (B) indicate aromatic substituents of phosphorus atom:
(a) 2a; (b) 2a with DMF (3.0 equiv); (c) 2b; (d) 2b with DMF (3.0 equiv);
(e) DMF.
1
chemical shift change in the H and 31P NMR spectra. In
contrast, marked upfield shifts were observed in the spectra
of catechol-derived phosphonium salts (2a,b).17

M. Terada, M. Kouchi / Tetrahedron 62 (2006) 401–409
403
Table 1. Diels–Alder reaction catalyzed by various phosphonium saltsa
Figure 3. Coordination mode of DMF/phosphonium salt (2a) complex.
Entry
Phosphonium salt
Yield (%)
salt (2a) arranged into the TBP configuration upon
coordination with DMF and one of the two oxygen atoms
of the catechol moiety as well as the carbonyl oxygen of
DMF occupied the apical position of the TBP arrangement
(Fig. 3). In fact, marked NOE was observed at the C3-proton
of the catechol moiety upon irradiation of the formyl proton
of DMF, in contrast to little NOE detected at the
dibenzophosphole moiety. On the other hand, only one set
of DMF signals was observed (Fig. 2e), when monocyclic
phosphonium salt (2b) was exposed to excess DMF
(3.0 equiv). As seen in the spectra for 2a, signals assigned
to the aromatic region shifted upfield but showed significant
broadening (Fig. 2c vs d). It is obvious that the
complexation between 2b and DMF is in equilibrium within
the NMR timescale at room temperature.
1
2
3
4
5
6
1a
1b
2a
2b
3a
3b
—
2a
2b
2c
2d
2e
2f
34
7
75
41
No reaction
Trace
Trace
91
88
89
Trace
91
85
7b
8c
9c
10c
11c
12c
13c
a The reactions were carried out under the conditions shown in Section 4
unless otherwise noted. Phosphonium salts were prepared from 4 or 5
(10 mol%) and Tf2O (10 mol%).
b The reaction was carried out in the presence of Tf2O (10 mol%) without
phosphonium salts.
c Phosphonium salts were prepared from phosphinate (5) (10 mol%) and
Tf2O (15 mol%).
2.3. Lewis acid catalysis by phosphonium salts
In the next phase of our investigation, we focused on Lewis
acid catalysis by the phosphonium salts. In order to estimate
the catalytic activity, we chose the Diels–Alder reaction as
the probe reaction (Table 1). a,b-Unsaturated amides (6)
were employed as the dienophile because NMR experi-
ments have indicated that phosphonium salts, in particular,
catechol derivatives (2), function as an activator of amide
functionality. The Diels–Alder reaction of unsaturated
amide (6a) with cyclopentadiene (7) catalyzed by
10 mol% phosphonium salts (1–3), which were prepared
in situ from 4 or 5 (10 mol%) and Tf2O (10 mol%) in the
to its low solubility in a halogenated solvent. Again, the
importance of the five-membered 1,3,2-dioxaphosphacyclic
structure in achieving high catalytic efficiency should be
emphasized.
Table 2. Diels–Alder reaction of amide dienophiles (6)a
˚
presence of MS 4 A, was carried out at 0 8C for 4 h. As
expected, the catechol-derived phosphonium salts (2) gave
Diels–Alder product (8a) in moderate to good chemical
yield (entries 3 and 4).20 By contrast, under similar reaction
conditions, phosphonium salts bearing the Martin ligand (1)
and biphenol derivatives (3) gave 8a in low chemical yield
(entries 1 and 2) and in trace amounts (entries 5 and 6),
respectively. These results are consistent with the tendency
Entry
2
6
Conditions
Yield
(%)
Endo/exo
1
of the coordination ability observed in H and 31P NMR
studies of DMF/phosphonium salt mixtures. Thus, higher
catalytic activity is attained with stronger coordination
ability of the phosphonium salt to the Lewis base. An
increase in the amount of Tf2O from 1.0 to 1.5 equiv with
respect to the amount of the starting phosphinate (5)
increased the chemical yield (entries 8 and 9). It was
confirmed by control experiments that Tf2O (10 mol%) did
not catalyze the reaction at all in the absence of 5 (entry 7).
The phosphonium salts (2c to 2f) derived from dialkyl-
phosphinate also worked well (entries 10–13) except for the
five-membered phosphacarbocyclic derivative (2d) (entry
10). The absence of rate acceleration in the case of 2d is due
1
2
3
4
2a
2e
2a
2e
2a
2e
—
—
6b
6b
6c
6c
6d
6d
6d
6d
0 8C, 2 h
0 8C, 2 h
K20 8C, 1 h
K20 8C, 4 h
0 8C, 4 h
0 8C, 4 h
Room temperature, 17 h 82
0 8C, 4 h 74
99
99
99
87
31
46
O99:!1
O99:!1
O99:!1
O99:!1
1:5.3
1:6.7
1.9:1
1:3.6
5
6
7b
8c
a The reactions were carried out under the conditions shown in Section 4
unless otherwise noted. Phosphonium salts were prepared from
phosphinate (5) (10 mol%) and Tf2O (15 mol%).
b The reaction was carried out in the absence of a catalyst.
c
˚
TfOH (5 mol%) was employed as the catalyst without MS 4 A.

404
M. Terada, M. Kouchi / Tetrahedron 62 (2006) 401–409
As listed in Table 2, phosphonium salts 2a and 2e are
applicable to the reaction of several dienophiles (6) with
amide functionality.21 When Z-dienophile (6b, 6c) was
employed, either 2a or 2e gave the product (8b, 8c) in high
chemical yield with extremely high endo selectivity (entries
1–4). On the other hand, in the reaction with E-dienophile
(6d), exo-8d was obtained predominantly in moderate
chemical yield (entries 5 and 6).22 The exo selectivity thus
achieved is in contrast to the reaction yielding endo-8d as
the major product under thermal conditions (entry 7). The
difference in endo/exo selectivity suggests that phos-
phonium salts activate the amide carbonyl functionality of
dienophile (6d). In the transition state, the amide moiety
coordinates with phosphonium salts oriented to the endo
direction to increase secondary orbital interactions, which
leads to the formation of exo-8d as the major stereoisomer.
In order to rule out the possibility of the catalysis by
adventitious acids, we employed TfOH as a catalyst and
observed the endo/exo selectivity (entry 8). The TfOH
catalyst exhibited lower exo selectivity than that of
phosphonium salts (2a and 2e). The differences in endo/
exo selectivity thus observed clearly indicate that the Diels–
Alder reaction of the dienophiles (6) bearing an amide
functionality is accelerated by phosphonium salts.
Analytical thin-layer chromatography (TLC) was per-
formed on Merck precoated TLC plates (silica gel 60
GF254, 0.25 mm). Flash column chromatography was
performed on silica gel 60 N (spherical, neutral, 100–
210 mm; Kanto Chemical Co., Inc.). All reactions were
carried out under a nitrogen (N2) atmosphere in dried
glassware. All substrates were purified by column chroma-
tography or distillation prior to use. Dichloromethane and
THF were supplied from Kanto Chemical Co., Inc. as
‘Dehydrated solvent system’. Chloroform-d1 was dried over
˚
activated molecular sieves 4 A and used under nitrogen
˚
atmosphere. Molecular sieves 4 A activated powder was
purchased from Aldrich (Catalogue no. 23,366-8) and
activated at 300 8C for 3 h under reduced pressure prior to
use. Other solvents and other simple chemicals were
purchased and used as such. 5-Chloro-5H-benzo[b]-
phosphindole 5-oxide was prepared by the literature
method.23 Z-dienophiles (6a–c)24 and E-dienophiles (6d)25
were prepared according to the literature procedure,
respectively.
4.2. Preparation of hydroxy phosphine oxide (4) and
phosphinate (5)
4.2.1. General procedure for the synthesis of hydroxy
phosphine oxide (4). To a stirred solution of n-butyllithium
(12.4 mL of an 1.6 M n-hexane solution, 22 mmol) was
added TMEDA (0.66 mL, 4.4 mmol). The mixture was
stirred at room temperature for 15 min until it become
cloudy. The mixture was then cooled to 0 8C and
1,1,1,3,3,3-hexafluoro-2-phenyl-2-propanol (1.68 mL,
10 mmol), dissolved in 1.5 mL of THF, was then added
dropwise to the mixture. After being stirred for 30 min, the
ice bath was removed and the mixture was stirred for 12 h at
ambient temperature. Diarylphosphinic chloride (9 mmol),
dissolved in 5 mL of THF, was added dropwise to the
solution of lithium reagent at K78 8C. After being stirred
for 30 min at K78 8C, the reaction mixture was gradually
warmed up to room temperature and stirring was continued
for additional 8 h at ambient temperature. The mixture was
quenched by adding 30 mL of saturated NH4Cl solution.
Following extraction with ethyl acetate (3!30 mL), the
combined organic layers were washed with 0.5 M HCl
solution and brine. The resultant organic phase was dried
over MgSO4 and concentrated under reduced pressure to
give crude product (4). Purification by silica-gel column
chromatography and recrystallization gave pure product (4)
as a colorless solid in 60–70% yield.
3. Conclusion
In summary, we have demonstrated novel Lewis acid
catalysis by phosphonium salts in which the five-membered
dioxaphosphacycle is critical for the salt to function
efficiently as a Lewis acid catalyst. The Diels–Alder
reaction of a,b-unsaturated amides was markedly acceler-
ated by a catalytic amount of catechol-derived phosphonium
salt to afford the products in high chemical yield. We have
also concluded that hypervalent interaction is essential for
activating the amide functionality based on observations of
the interaction between phosphonium salt and the Lewis
base.
4. Experimental
4.1. General
Infrared spectra were recorded on a Shimazu FTIR-8200PC
spectrometer. H NMR spectra were recorded on a JEOL
1
GSX-270 (270 MHz) or Bru¨ker AM-600 (600 MHz)
spectrometer. Chemical shifts are reported in ppm from
the solvent resonance as the internal standard. Data are
reported as follows: chemical shift, integration, multiplicity
(sZsinglet, dZdoublet, tZtriplet, qZquartet, quinZ
quintet, sexZsextet, brZbroad, mZmultiplet) and
coupling constants (Hz). 13C NMR spectra were recorded
on a JEOL GSX-270 (67.8 MHz) spectrometer with
complete proton decoupling. Chemical shifts are reported
in ppm from the solvent resonance as the internal standard.
31P NMR spectra were recorded on a Bru¨ker Avance 400
(162 MHz) spectrometer with complete proton decoupling.
Chemical shifts are reported in ppm from H3PO4 resonance
as the external standard. Mass spectra analysis was
performed at the Instrumental Analysis Center for
Chemistry, Graduate School of Science, Tohoku University.
4.2.1.1. 2-(5-Oxido-5H-benzo[b]phosphindol-5-yl)-
a,a-bis(trifluoromethyl)benzenemethanol (precursor of
1a). 1H NMR (270 MHz, CDCl3): d 7.17–7.30 (2H, m), 7.44
(2H, td, JZ7.3, 3.8 Hz), 7.65–7.65 (3H, m), 7.83 (2H, dd,
JZ7.3, 3.2 Hz), 7.97–8.04 (3H, m), 10.22 (1H, br s); 13C
NMR (67.8 MHz, CDCl3): d 79.40 (quin, JF–Z29.5 Hz,
JP–Z2.4 Hz), 121.42 (d, JP–Z10.8 Hz), 123.04 (q, JF–Z
287.8 Hz), 129.48 (d, JP–Z13.8 Hz), 129.74 (dd, JP–Z
8.8 Hz, JF–Z3.9 Hz), 129.78 (d, JP–Z11.3 Hz), 130.34 (d,
JP–Z94.3 Hz), 131.57 (d, JP–Z111.5 Hz), 131.42 (d, JP–Z
8.9 Hz), 132.40 (d, JP–Z1.4 Hz), 132.89 (d, JP–Z14.7 Hz),
134.02 (d, JP–Z2.4 Hz), 137.99 (d, JP–Z5.4 Hz), 141.71 (d,
JP–Z23.5 Hz); 31P NMR (162 MHz, CDCl3): d 46.2; IR
(KBr): 3082, 2925, 1442, 1271, 1245, 1205, 1193, 1164,

M. Terada, M. Kouchi / Tetrahedron 62 (2006) 401–409
405
1151, 1130, 954, 931, 856, 759, 729, 715, 704, 551 cmK1
;
1103, 943, 932, 748, 729, 546 cmK1; HRMS (ESI) Calcd for
HRMS (ESI) Calcd for C21H14F6O2P ([MCH]C) 443.0636.
Found 443.0630.
C18H16O3P ([MCH]C) 311.0837. Found 311.0832.
4.2.2.3. Dibutylphosphinic acid 2-hydroxyphenyl
ester (precursor of 2c). H NMR (270 MHz, CDCl3): d
1
4.2.1.2. 2-(Diphenylphosphinyl)-a,a-bis(trifluoro-
methyl)benzenemethanol (precursor of 1b). 1H NMR
(270 MHz, CDCl3): d 7.24 (1H, dd, JZ16.2, 7.6 Hz), 7.39
(1H, t, JZ7.6 Hz), 7.44–7.66 (9H, m), 7.98 (1H, t, JZ
5.7 Hz), 10.57 (1H, br s); 13C NMR (67.8 MHz, CDCl3): d
79.51 (quind, JF–Z29.0 Hz, JP–Z2.5 Hz), 122.88 (q,
JF–Z289.3 Hz), 128.69 (d, JP–Z12.3 Hz), 128.73 (d,
JP–Z13.3 Hz), 130.30 (dd, JP–Z8.3 Hz, JF–Z3.9 Hz),
130.85 (d, JP–Z95.8 Hz), 131.96 (d, JP–Z2.5 Hz), 131.23
(d, JP–Z109.0 Hz), 132.11 (d, JP–Z9.8 Hz), 132.57 (d,
JP–Z2.5 Hz), 136.03 (d, JP–Z12.7 Hz), 138.07 (d, JP–Z
3.9 Hz); 31P NMR (162 MHz, CDCl3): d 46.3; IR (KBr):
3043, 2925, 1436, 1267, 1240, 1195, 1153, 1126, 954, 933,
866, 758, 727, 696, 542 cmK1; HRMS (ESI) Calcd for
C21H16F6O2P ([MCH]C) 445.0792. Found 445.0787.
0.89 (6H, t, JZ7.3 Hz), 1.40 (4H, sext, JZ7.3 Hz), 1.50–
1.78 (4H, m), 1.82–1.99 (4H, m), 6.76–6.85 (1H, m), 6.94
(1H, d, JZ7.8 Hz), 6.99–7.07 (2H, m), 9.09 (1H, br s); 13C
NMR (67.8 MHz, CDCl3): d 23.41 (d, JP–Z1.0 Hz), 23.56,
23.72 (d, JP–Z15.3 Hz), 26.71 (d, JP–Z86.9 Hz), 119.85 (d,
JP–Z1.5 Hz), 120.37 (d, JP–Z0.1 Hz), 121.80 (d, JP–Z
4.4 Hz), 126.29 (d, JP–Z0.9 Hz), 139.28 (d, JP–Z9.8 Hz),
147.94 (d, JP–Z2.9 Hz); 31P NMR (162 MHz, CDCl3): d
67.4; IR (neat): 3074, 2958, 2933, 1494, 1460, 1294, 1242,
1164, 1101, 931, 923, 752, 732 cmK1; HRMS (ESI) Calcd
for C14H24O3P ([MCH]C) 271.1463. Found 271.1458.
4.2.2.4. o-[(1-Oxido-1-phospholanyl)oxy]phenol (pre-
cursor of 2d). H NMR (270 MHz, CDCl3): d 1.72–2.09
1
(8H, m), 6.80–6.87 (1H, m), 6.94 (1H, d, JZ8.1 Hz), 7.04–
7.12 (2H, m), 8.77 (1H, br s); 13C NMR (67.8 MHz, CDCl3):
d 22.91 (d, JP–Z12.8 Hz), 23.36 (d, JP–Z87.9 Hz), 120.05
(d, JP–Z1.5 Hz), 120.61 (d, JP–Z1.5 Hz), 121.88 (d, JP–Z
4.4 Hz), 126.67 (d, JP–Z1.5 Hz), 139.37 (d, JP–Z10.3 Hz),
148.17 (d, JP–Z2.9 Hz); 31P NMR (162 MHz, CDCl3): d
91.3; IR (KBr): 3109, 1508, 1458, 1290, 1274, 1234, 1168,
1099, 920 cmK1; HRMS (ESI) Calcd for C10H14O3P ([MC
H]C) 213.0681. Found 213.0675.
4.2.2. General procedure for the synthesis of hydroxy
phosphinate (5). To a stirred solution of diol (1.8 mmol) in
CH2Cl2 (5 mL) was added triethylamine (0.56 mL,
4 mmol). The mixture was cooled to 0 8C and the
corresponding phosphinic chloride (2 mmol), dissolved in
1.5 mL of CH2Cl2, was added dropwise to the mixture.
After being stirred for 30 min, the ice bath was removed and
the mixture was stirred for 3 h at ambient temperature. The
mixture was quenched by adding 10 mL of water. Following
extraction with CH2Cl2 (3!15 mL), the combined organic
layers were washed with 0.5 M HCl solution and saturated
NaHCO3 solution. The resultant organic phase was dried
over MgSO4 and concentrated under reduced pressure to
give a crude product (5). Purification by silica-gel column
chromatography and recrystallization gave pure product (5)
as a colorless solid in 45–83% yield.
4.2.2.5. o-[(1-Oxido-1-phosphorinanyl)oxy]phenol
(precursor of 2e). H NMR (270 MHz, CDCl3): d 1.36–
1
1.41 (1H, m), 1.78–2.18 (9H, m), 6.79–6.88 (1H, m.), 6.99–
7.11 (3H, m), 9.06 (1H, s); 13C NMR (67.8 MHz, CDCl3): d
23.76 (d, JP–Z5.9 Hz), 25.77 (d, JP–Z8.9 Hz), 26.17 (d,
JP–Z82.5 Hz), 120.04, 120.40, 121.80 (d, JP–Z4.4 Hz),
126.51 (d, JP–Z1.4 Hz), 138.76 (d, JP–Z9.8 Hz), 148.15 (d,
JP–Z3.0 Hz); 31P NMR (162 MHz, CDCl3): d 60.8; IR
(KBr): 3070, 2953, 1516, 1460, 1294, 1242, 1184, 1163,
1103, 916, 825, 760 cmK1; HRMS (ESI) Calcd for
C11H16O3P ([MCH]C) 227.0837. Found 227.0832.
4.2.2.1. o-[(5-Oxido-5H-benzo[b]phosphindol-5-yl)-
oxy]phenol (precursor of 2a). 1H NMR (270 MHz,
CDCl3): d 6.70–6.80 (2H, m), 7.08–7.16 (2H, m) 7.39
(2H, td, JZ7.6, 4.1 Hz), 7.58–7.67 (4H, m), 7.82 (2H, dd,
JZ7.8, 4.1 Hz), 8.60 (1H, s); 13C NMR (67.8 MHz, CDCl3):
d 119.67 (d, JP–Z1.5 Hz), 120.66 (d, JP–Z1.5 Hz), 121.37 (d,
JP–Z12.7 Hz), 122.27 (d, JP–Z3.9 Hz), 126.10 (d, JP–Z
139.5 Hz), 126.85 (d, JP–Z1.5 Hz), 129.30 (d, JP–Z11.9 Hz),
129.45 (d, JP–Z15.3 Hz), 134.41 (d, JP–Z2.5 Hz), 139.46 (d,
JP–Z9.8 Hz), 140.47 (d, JP–Z30.0 Hz), 148.08 (d, JP–Z
2.9 Hz);31PNMR (162 MHz, CDCl3):d 48.9; IR(KBr):3177,
1590, 1515, 1460, 1291, 1238, 1224, 1179, 1132, 1106, 936,
928, 832, 759, 749, 727, 552 cmK1; HRMS (ESI) Calcd for
C18H14O3P ([MCH]C) 309.0681. Found 309.0685.
4.2.2.6. o-[(1-Oxido-1-phosphepanyl)oxy]phenol (pre-
cursor of 2f). H NMR (270 MHz, CDCl3): d 1.73–2.22
1
(12H, m), 6.78–6.84 (1H, m), 6.95 (1H, d, JZ8.4 Hz), 7.01–
7.09 (2H, m), 9.09 (1H, br s); 13C NMR (67.8 MHz, CDCl3):
d 20.68 (d, JP–Z1.5 Hz), 28.44 (d, JP–Z83.1 Hz), 29.54,
119.95 (d, JP–Z0.1 Hz), 120.41 (d, JP–Z1.0 Hz), 122.04 (d,
JP–Z3.9 Hz), 126.41 (d, JP–Z1.5 Hz), 139.02 (d, JP–Z
9.8 Hz), 148.20 (d, JP–Z2.9 Hz); 31P NMR (162 MHz,
CDCl3): d 73.9; IR (KBr): 3074, 2931, 1508, 1458, 1380,
1290, 1240, 1197, 1178, 1153, 937, 918, 781 cmK1; HRMS
(ESI) Calcd for C12H18O3P ([MCH]C) 241.0994. Found
241.0988.
4.2.2.2. Diphenylphosphinic acid 2-hydroxyphenyl
ester (precursor of 2b). H NMR (270 MHz, CDCl3): d
1
6.66–6.72 (1H, m), 6.85 (1H, d, JZ8.1 Hz), 6.96–7.05 (2H,
m), 7.45–7.62 (6H, m), 7.89 (4H, dd, JZ12.7, 7.0 Hz), 9.00
(1H, s); 13C NMR (67.8 MHz, CDCl3): d 119.70 (d, JP–Z
1.5 Hz), 120.41 (d, JP–Z1.0 Hz), 122.38 (d, JP–Z4.4 Hz),
126.38 (d, JP–Z1.5 Hz), 128.65 (d, JP–Z137.1 Hz), 128.74
(d, JP–Z13.2 Hz), 131.86 (d, JP–Z10.8 Hz), 133.11 (d,
JP–Z3.0 Hz), 139.03 (d, JP–Z9.8 Hz), 148.04 (d, JP–Z
3.0 Hz); 31P NMR (162 MHz, CDCl3): d 39.1; IR (KBr):
3123, 1518, 1460, 1439, 1292, 1240, 1215, 1176, 1134,
4.2.2.7. 20-[(5-Oxido-5H-benzo[b]phosphindol-5-
yl)oxy][1,10-biphenyl]-2-ol (precursor of 3a). 1H NMR
(270 MHz, CDCl3): d 6.14 (1H, s), 6.84–6.97 (3H, m), 7.23–
7.35 (7H, m), 7.52 (2H, t, JZ7.3 Hz), 7.63 (2H, dd, JZ7.3,
3.8 Hz); 13C NMR (67.8 MHz, CDCl3): d 117.24, 120.84 (d,
JP–Z9.8 Hz), 121.10, 121.68 (d, JP–Z3.4 Hz), 125.84,
126.08 (d, JP–Z2.0 Hz), 127.43 (d, JP–Z139.5 Hz), 128.81
(d, JP–Z8.8 Hz), 129.25, 129.41 (d, JP–Z2.0 Hz), 129.54
(d, JP–Z1.4 Hz), 131.08, 131.63 (d, JP–Z3.9 Hz), 132.42

406
M. Terada, M. Kouchi / Tetrahedron 62 (2006) 401–409
(d, JP–Z1.5 Hz), 133.89 (d, JP–Z2.4 Hz), 140.09, 140.53,
153.53; 31P NMR (162 MHz, CDCl3): d 42.9; IR (KBr):
3134, 1560, 1508, 1440, 1222, 1205, 1188, 1130, 1068, 920,
758, 748 cmK1; HRMS (ESI) Calcd for C24H18O3P ([MC
H]C) 385.0994. Found 385.0988.
m), 7.82–7.90 (4H, m), 8.03–8.16 (6H, m); 31P NMR
(162 MHz, CDCl3): d 97.2.
4.3.5. 2,2-Dibutyl-1,3,2-benzodioxaphospholium salt
with trifluoromethanesulfonic acid (2c). 1H NMR
(270 MHz, CDCl3): d 0.91 (6H, t, JZ7.3 Hz), 1.48 (4H,
td, JZ14.6, 7.3 Hz), 1.59–1.74 (4H, m), 3.13 (4H, dd, JZ
15.9, 9.2 Hz), 7.23–7.28 (2H m), 7.32–7.37 (2H, m); 31P
NMR (162 MHz, CDCl3): d 142.6.
4.2.2.8. 20-[(5-Oxido-5H-benzo[b]phosphindol-5-
yl)oxy][1,10-binaphthalen]-2-ol (precursor of 3b). 1H
NMR (270 MHz, CDCl3): d 6.43 (1H, dd, JZ11.3,
7.6 Hz), 6.91 (1H, d, JZ8.4 Hz), 6.99–7.14 (3H, m),
7.24–7.41 (6H, m), 7.45–7.57 (3H, m), 7.62–7.69 (3H, m),
7.86 (1H, d, JZ8.1 Hz), 7.96 (1H, d, JZ6.8 Hz), 7.99 (1H,
d, JZ8.4 Hz), 8.09 (1H, d, JZ8.9 Hz); 13C NMR
(67.8 MHz, CDCl3): d 117.24, 120.03 (d, JP–Z2.0 Hz),
120.57, 120.71 (d, JP–Z13.3 Hz), 121.18 (d, JP–Z12.8 Hz),
123.54, 124.47, 124.90 (d, JP–Z4.4 Hz), 125.93, 126.44 (d,
JP–Z93.4 Hz), 126.60, 127.66 (d, JP–Z22.6 Hz), 128.12 (d,
JP–Z1.0 Hz), 128.65 (d, JP–Z8.2 Hz), 129.18 (d, JP–Z
4.4 Hz), 129.39, 129.40 (d, JP–Z11.8 Hz), 130.12, 130.84
(d, JP–Z1.4 Hz), 131.84 (d, JP–Z1.5 Hz), 133.90 (d, JP–Z
2.0 Hz), 133.96 (d, JP–Z2.4 Hz), 140.18 (d, JP–Z2.9 Hz),
140.62 (d, JP–Z2.4 Hz), 146.68 (d, JP–Z9.4 Hz), 152.59;
31P NMR (162 MHz, CDCl3): d 44.1; IR (KBr): 3195, 1624,
1593, 1438, 1230, 1203, 1132, 1070, 985, 956, 840, 817,
756, 721 cmK1; HRMS (ESI) Calcd for C32H22O3P ([MC
H]C) 485.1307. Found 485.1301.
4.3.6. Spiro[1,3,2-benzodioxaphospholium-2,20-phos-
pholanium] salt with trifluoromethanesulfonic acid
(2d). Not available due to low solubility of 2d in CDCl3.
4.3.7. Spiro[1,3,2-benzodioxaphospholium-2,20-phos-
phorinanium] salt with trifluoromethanesulfonic acid
(2e). H NMR (270 MHz, CDCl3): d 1.93–2.02 (2H, m),
1
2.27–2.45 (4H, m), 3.07 (2H, t, JZ6.7 Hz), 3.14 (2H, t, JZ
6.7 Hz), 7.23–7.30 (2H, m), 7.32–7.39 (2H, m); 31P NMR
(162 MHz, CDCl3): d 137.7.
4.3.8. Spiro[1,3,2-benzodioxaphospholium-2,20-phos-
phepanium] salt with trifluoromethanesulfonic acid
(2f). H NMR (270 MHz, CDCl3): d 1.88–1.90 (4H, m),
1
2.11–2.25 (4H, m), 3.13 (2H, t, JZ6.5 Hz), 3.19 (2H, t, JZ
6.5 Hz), 7.20–7.27 (2H, m), 7.29–7.36 (2H, m); 31P NMR
(162 MHz, CDCl3): d 146.8.
4.3. 1H and 31P NMR analysis of phosphonium salt (1–3)
4.3.9. Spiro[dibenzo[d,f][1,3,2]dioxaphosphepinium-5H-
dibenzophospholium] salt with trifluoromethanesulfonic
˚
To a suspension of an activated MS 4 A (100 mg: activated
1
acid (3a). H NMR (270 MHz, CDCl3): d 7.38–7.43 (2H,
at 300 8C for 3 h under reduced pressure) and hydroxy
phosphineoxide (4) or hydroxy phosphinate (5) (0.03 mmol)
in anhydrous deuterated chloroform (0.9 mL) was added
trifluoromethanesulfonic anhydride (0.03 mmol) under
nitrogen atmosphere at room temperature. After stirring
for 1 h at ambient temperature, the resultant suspension was
m), 7.49–7.74 (8H, m), 7.77–7.82 (2H, m), 8.04 (2H, tt, JZ
7.8, 1.4 Hz), 8.26 (2H, dd, JZ7.8, 5.4 Hz); 31P NMR
(162 MHz, CDCl3): d 83.0.
4.3.10. Spiro[dinaphto[2,1-d:10,20-f][1,3,2]dioxaphos-
phepinium-5H-dibenzophospholium] salt with trifluoro-
methanesulfonic acid (3b). 1H NMR (270 MHz, CDCl3): d
7.30 (2H, m), 7.46–7.50 (4H, m), 7.53–7.61 (4H, m), 7.69
(2H, ddd, JZ8.1, 5.7, 2.4 Hz), 8.04 (2H, t, JZ7.8 Hz), 8.15
(2H, d, JZ8.4 Hz), 8.27 (2H, dd, JZ7.8, 5.4 Hz), 8.32 (1H,
d, JZ9.2 Hz); 31P NMR (162 MHz, CDCl3): d 83.5.
˚
centrifuged to separate MS 4 A and the supernatant solution
was replaced to an NMR tube under nitrogen atmosphere.
1
The sample thus prepared was measured by H and 31P
NMR at room temperature.
4.3.1. 1,3-Dihydro-3,3-bis(trifluoromethyl)spiro[5H-
dibenzophospholium-2,1-benzoxaphospholium] salt
with trifluoromethanesulfonic acid (1a). 1H NMR
(270 MHz, CDCl3): d 7.52–7.63 (4H, m), 7.95–8.02 (2H,
m), 8.06–8.26 (5H, m), 8.59 (1H, dd, JZ12.4, 7.6 Hz); 31P
NMR (162 MHz, CDCl3): d 92.3.
4.4. Typical procedure for the Diels–Alder reaction
˚
To a suspension of activated MS 4 A (100 mg: activated at
300 8C for 3 h under reduced pressure) and hydroxy
phosphinate (5) (0.06 mmol) in anhydrous dichloromethane
(1.8 mL) was added trifluoromethanesulfonic anhydride
(0.06 or 0.09 mmol) under nitrogen atmosphere at room
temperature. The mixture was stirred for 1 h at ambient
temperature and then cooled to the indicated temperature. A
dienophile (6) (0.6 mmol) was added to the suspension at
that temperature. After stirring for 30 min, a ca. 3.0 M
solution of cyclopentadiene (7) (ca. 1.5 mmol) in dichloro-
methane was added to the suspension. Stirring was
continued under the indicated conditions and the
reaction mixture was quenched with sodium bicarbonate.
Conventional workup followed by silica-gel column
chromatography purification gave pure product (8).
4.3.2. 1,3-Dihydro-1,1-diphenyl-3,3-bis(trifluoromethyl)-
2,1-benzoxaphospholium salt with trifluoromethane-
sulfonic acid (1b). H NMR (270 MHz, CDCl3): d 7.73–
1
7.81 (4H, m), 7.88–7.97 (7H, m), 8.10–8.25 (2H, m), 9.11–
9.17 (1H, m); 31P NMR (162 MHz, CDCl3): d 92.7.
4.3.3. Spiro[5H-dibenzophospholium-1,3,2-benzodioxa-
phospholium] salt with trifluoromethanesulfonic acid
1
(2a). H NMR (270 MHz, CDCl3): d 7.36–7.42 (2H, m),
7.51–7.56 (2H, m), 7.64 (2H, tdd, JZ7.6, 4.1, 1.1 Hz),
7.97–8.14 (6H, m); 31P NMR (162 MHz, CDCl3): d 100.5.
4.3.4. 2,2-Diphenyl-1,3,2-benzodioxaphospholium salt
with trifluoromethanesulfonic acid (2b). 1H NMR
(270 MHz, CDCl3): d 7.33–7.39 (2H, m), 7.45–7.55 (2H,
4.4.1. 2-endo-Methoxycarbonyl-3-endo-(pyrrolidin-1-
yl)carbonylbicyclo[2.2.1]hept-5-ene (8a). 1H NMR

M. Terada, M. Kouchi / Tetrahedron 62 (2006) 401–409
407
(270 MHz, CDCl3): d 1.14 (1H, d, JZ8.1 Hz), 1.23 (1H, d,
JZ8.6 Hz), 1.69 (2H, td, JZ13.5, 6.5 Hz), 1.70–1.79 (2H,
m), 2.91 (1H, br s), 2.96 (1H, br s), 3.06 (2H, dd, JZ10.0,
3.2 Hz), 3.14–3.21 (3H, m); 3.30 (1H, t, JZ6.5 Hz), 3.37
(3H, s), 6.04 (1H, dd, JZ8.1, 2.7 Hz), 6.10 (1H, dd, JZ8.1,
2,7 Hz); 13C NMR (67.8 MHz, CDCl3): d 23.60, 25.54,
45.15, 45.55, 45.63, 45.69, 47.42, 47.88, 48.03, 60.69,
133.32, 134.85, 169.80, 172.22; IR (neat): 3462, 2972,
2949, 2871, 1739, 1643, 1434, 1352, 1340, 1251, 1195,
1149, 1076, 1041, 914, 723, 707 cmK1; HRMS (ESI)
Calcd for C14H20NO3 ([MCH]C) 250.1443. Found
250.1438.
2937, 1726, 1641, 1552, 1458, 1382, 1325, 1276, 1242,
1213, 1172, 1033, 873, 698 cmK1; HRMS (ESI) Calcd for
C14H22NO3 ([MCH]C) 252.1600. Found 252.1594.
4.5. Stereochemical assignment of 8d
4.5.1. Transformation to hexahydro-6-iodo-2-oxo-N-
propyl-3,5-methano-2H-cyclopenta[b]furan-7-carboxa-
mide (9). To a stirred solution of 8d (40.0 mg, 0.16 mmol:
one diastereomer obtained from the major product of
Table 2 entry 7) in methanol (1.0 mL) was added dropwise
2 M NaOH solution (0.5 mL) at 0 8C. After being stirred for
15 min, the ice bath was removed and the mixture was
stirred for 1 h at room temperature. The resultant mixture
was quenched by adding 1 M HCl solution (2 mL).
Following extraction with CH2Cl2 (4!15 mL), the com-
bined organic layers were concentrated under reduced
pressure to give crude carboxylic acid. The crude material
thus obtained was used for the next lactonization without
further purification. To a stirred solution of crude carboxylic
acid in CH2Cl2 (1.0 mL) at room temperature was added
NaHCO3 (67.2 mg, 0.8 mmol), water (2.0 mL), KI (265 mg,
1.60 mmol), and I2 (142 mg, 0.56 mmol) in this order. After
being stirred for 3 h at ambient temperature, the reaction
mixture was quenched by adding saturated Na2S2O3
solution. Following extraction with CH2Cl2 (3!10 mL),
the combined organic layers were dried over Na2SO4 and
concentrated under reduced pressure to give crude lactone
(9). Purification by silica-gel column chromatography gave
pure lactone (9) (52.1 mg, 92% form 8d), 1H NMR
(270 MHz, CDCl3): d 0.91 (3H, t, JZ7.6 Hz), 1.51 (2H,
sex, JZ7.6 Hz), 2.28 (2H, m), 2.61 (1H, s), 2.84 (1H, s),
3.05 (1H, d, JZ3.5 Hz), 3.14–3.21 (3H, m), 3.85 (1H, d, JZ
1.6 Hz), 5.12 (1H, d, JZ4.9 Hz), 5.91 (1H, br s); 13C NMR
(67.8 MHz, CDCl3): d 11.40, 22.72, 28.66, 35.11, 41.52,
41.77, 46.21, 51.55, 51.69, 88.89, 169.45, 178.49; IR (neat):
3327, 2964, 2933, 1781, 1651, 1539, 1458, 1346, 1305,
1247, 1172, 1116, 1006, 985, 912, 840, 736 cmK1; HRMS
(ESI) Calcd for C12H16INNaO3 ([MCNa]C) 372.0067.
Found 372.0067.
4.4.2. 2-endo-Methoxycarbonyl-3-endo-(N-propylcarba-
moyl)bicyclo[2.2.1]hept-5-ene (8b). H NMR (270 MHz,
1
CDCl3): d 0.85 (3H, t, JZ7.6 Hz), 1.29 (1H, d, JZ8.4 Hz),
1.39–1.47 (3H, m), 3.04–3.12 (4H, m), 3.19–3.20 (2H, m),
3.54 (3H, s), 5.68 (1H, br s), 6.10 (1H, dd, JZ5.7, 3.0 Hz),
6.44 (1H, dd, JZ5.7, 3.0 Hz); 13C NMR (67.8 MHz,
CDCl3): d 11.24, 22.68, 41.13, 45.64, 47.11, 48.92, 49.05,
50.21, 51.29, 133.36, 136.47, 171.43, 173.44; IR (KBr):
3321, 3003, 2952, 2877, 1720, 1647, 1556, 1440, 1336,
1263, 1226, 1197, 1151, 1035, 792, 690, 615 cmK1; HRMS
(ESI) Calcd for C13H20NO3 ([MCH]C) 238.1443. Found
238.1438.
4.4.3. 2-endo-Methoxycarbonyl-3-endo-(N-phenylcarba-
moyl)bicyclo[2.2.1]hept-5-ene (8c). H NMR (270 MHz,
1
CDCl3): d 1.35 (1H, d, JZ8.6 Hz), 1.50 (1H, dt, JZ8.6,
1.6 Hz), 3.18 (1H, br s), 3.19 (1H, br s), 3.23 (1H, dd, JZ
10.3, 1.6 Hz), 3.39 (1H, dd, JZ10.3, 3.0 Hz), 3.54 (3H, s),
6.22 (1H, dd, JZ5.7, 3.0 Hz); 6.55 (1H, dd, JZ5.7, 3.0 Hz),
7.05 (1H, t, JZ7.3 Hz), 7.26 (2H, t, JZ7.3 Hz), 7.46 (2H, d,
JZ7.8 Hz), 7.63 (1H, br s); 13C NMR (67.8 MHz, CDCl3):
d 45.76, 47.40, 49.12, 49.35, 51.03, 51.59, 119.67,
123.96, 128.81, 133.49, 136.76, 137.97, 170.09, 173.44;
IR (KBr): 3342, 1716, 1685, 1600, 1542, 1490, 1442,
1307, 1255, 1213, 1176, 754, 696 cmK1; HRMS (ESI)
Calcd for C16H18NO3 ([MCH]C) 272.1287. Found
272.1281.
4.4.4. 2-endo-Ethoxycarbonyl-3-exo-(N-propylcarba-
moyl)bicyclo[2.2.1]hept-5-ene (endo-8d). 1H NMR
(270 MHz, CDCl3): d 0.88 (3H, t, JZ7.3 Hz), 1.16–1.25
(3H, m), 1.41–1.56 (3H, m), 1.82 (1H, d, JZ7.0 Hz), 2.32
(1H, d, JZ4.9 Hz), 2.99 (1H, s), 3.15–3.23 (4H, m), 4.02–
4.14 (2H, m); 5.99 (3H, br s), 6.06 (1H, dd, JZ5.4, 2.4 Hz),
6.21 (1H, dd, JZ5.4, 3.0 Hz); 13C NMR (67.8 MHz,
CDCl3): d 11.28, 14.17, 22.84, 41.22, 44.76, 46.76, 47.55,
48.42, 49.41, 60.63, 135.30, 137.87, 173.91, 174.11; IR
(neat): 3311, 2970, 2939, 1733, 1645, 1544, 1458, 1315,
1269, 1209, 1190, 1116, 1033, 864, 729, 698 cmK1; HRMS
(ESI) Calcd for C14H22NO3 ([MCH]C) 252.1600. Found
252.1594.
Acknowledgements
The authors are grateful to Central Glass Co., Ltd for the
generous gifts of hexafluorocumyl alcohol and trifluoro-
methanesulfonic anhydride. This work was partially
supported by Grants-in-Aid for Scientific Research (Nos.
14340227 and 15036210) from the Ministry of Education,
Culture, Sports, Science and Technology of Japan and the
Asahi Glass Foundation.
4.4.5. 2-exo-Ethoxycarbonyl-3-endo-(N-propylcarba-
moyl)bicyclo[2.2.1]hept-5-ene (exo-8d). 1H NMR
(270 MHz, CDCl3): d 0.85–0.92 (3H, m), 1.17–1.31 (3H,
m), 1.43–1.55 (4H, m), 2.54 (1H, dd, JZ4.9, 1.4 Hz), 3.08–
3.20 (5H, m), 4.12–4.19 (2H, m), 5.95 (1H, br s), 6.17–6.21
(2H, m); 13C NMR (67.8 MHz, CDCl3): d 11.29, 14.19,
22.87, 41.13, 45.45, 46.66, 47.60, 48.37, 49.67, 60.93,
135.69, 136.37, 172.53, 175.03; IR (KBr): 3317, 2970,
Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tet.2005.09.
075. This comprises a scanned image of the 1H NMR
spectra of phosphonium salts (1), table of 31P NMR
chemical shifts of 1–5, and scanned image of 2D (COSY)
and NOE spectra of 2a/DMF complex.

408
M. Terada, M. Kouchi / Tetrahedron 62 (2006) 401–409
7. Several restricted examples of the Lewis acid catalysis by
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transformation of 4 or 5 to phosphonium salts (1–3); 1a: d 92.3

M. Terada, M. Kouchi / Tetrahedron 62 (2006) 401–409
409
(from d 46.2); 1b: d 92.7 (from d 46.3); 2a: d 100.5 (from d
48.9); 2b: d 97.2 (from d 39.1); 2c: d 142.6 (from d 67.4); 2d:
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60.8); 2f: d 146.8 (from d 73.9); 3a: d 83.0 (from d 42.9); 3b: d
83.5 (from d 44.1).
marked upfield shifts were observed in the 31P NMR spectrum,
and a significant broadening of the peak assigned to
phosphonium salt (2a) was observed in the 1H NMR spectrum.
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conditions except for the absence of phosphonium salt.
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the endo-stereoisomer by IR measurement [C]O(lactone);
1781 cmK1].
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