A new method for the formation of anti-apicophilic (O-cis) spirophosphoranes - Kinetic studies on the stereomutation of O-cis arylspirophosphoranes to their O-trans isomers

Article abstract of DOI:10.1002/ejoc.200600031

P-H spirophosphorane 4 bearing two Martin ligands was converted to dianions (5a-n), bearing R groups, with excess organolithium reagents (RLi). Subsequent oxidation with I₂ at ambient temperature gave anti-apicophilic (O-cis) spirophosphoranes

Full text of DOI:10.1002/ejoc.200600031

FULL PAPER
DOI: 10.1002/ejoc.200600031
A New Method for the Formation of Anti-apicophilic (O-cis)
Spirophosphoranes – Kinetic Studies on the Stereomutation of O-cis
Arylspirophosphoranes to Their O-trans Isomers
Kazumasa Kajiyama,[a][‡] Miki Yoshimune,[a] Satoshi Kojima,[a] and Kin-ya Akiba*[‡‡]
Keywords: Anti-apicophilicity / Oxidation / Kinetics / Arylspirophosphorane / Pseudorotation
P–H spirophosphorane 4 bearing two Martin ligands was
converted to dianions (5a–n), bearing R groups, with excess
organolithium reagents (RLi). Subsequent oxidation with I2
at ambient temperature gave anti-apicophilic (O-cis) spiro-
phosphoranes 2a–n. All anti-apicophilic phosphoranes 2a–n
isomerized irreversibly to the O-trans spirophosphoranes 3a–
n, respectively. For 2,6-dialkylphenyl derivatives 2k–n, the
barrier for pseudorotation between enantiomers 2-RP and 2-
Sp was rather high, and thus the interconversion could not
be observed on the NMR timescale. The activation param-
eters for the pseudorotation of the triisopropylphenyl (TIP)
derivative 2n to 3n were almost identical with those of nBu
derivative 2b to 3b, i.e., ∆H‡ = 21.3 kcalmol–1, ∆S‡ = –9.4 eu.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
torial substituents via a square pyramidal (SP) structure
with the remaining equatorial substituent being the pivot.
There are two fundamental methods for raising the stereo-
mutation barrier between stable isomers interconverting via
multiple numbers of BPR steps. One is to increase as much
as possible the difference in apicophilicity between the api-
cal and equatorial substituents. This should lead to in-
creased relative stability of the more stable isomers. The
other is the incorporation of small rings (five members or
less) into the compound to introduce into the multi-step
BPR process strained and thus highly unstable intermedi-
ates in which the small ring should span between two equa-
torial sites.
By incorporating the Martin ligand, which utilizes the
Thorpe–Ingold effect,[6] we have recently succeeded in the
first isolation and characterization of anti-apicophilic O-cis
alkylphosphoranes 2a–c, which are high energy configura-
tional isomers of ordinary O-trans phosphoranes 3a–c, in
which the two oxygen atoms occupy the apical positions.
The synthesis of 2a–c was accomplished via thermal cycli-
zation reactions of P–H (apical) monocyclic phosphoranes
1a–c (Scheme 1) in pyridine.[7] In order to gain a compre-
Introduction
Interest in the chemistry of hypervalent[1] 10–P–5[2] phos-
phoranes stems from studies aimed at elucidating the reac-
tion mechanism of processes such as phosphoryl transfer
and hydrolysis of phosphates in biological systems. Most
stable phosphoranes assume trigonal bipyramidal (TBP)
structures in which there are two distinctive sites, the apical
and the equatorial sites. The equatorial bonds can roughly
be described as involving the sp2 orbitals of the P atom,
while the apical bonds can be explained with the concept
of the three-center four-electron bond (3c–4e).[3] Since the
electrons of the apical bonds can be delocalized into the
apical substituents, electronegative substituents are pre-
ferred in these apical sites to relieve electron density upon
the central atom. This gives rise to the concept of apicophil-
icity.[1,4] The site preference of the substituents is also de-
pendent on steric bulk, that is, the bulky substituents are
generally preferred in the less congested equatorial sites rel-
ative to the apical sites.
The fast stereomutation of the phosphoranes is reason-
ably explained by the Berry pseudorotation (BPR) pro-
cess,[5] which is based on bond bending. In this mechanism,
the two apical substituents exchange places with two equa-
[a] Department of Chemistry, Graduate School of Science, Hiro-
shima University,
1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan
[‡] Present address: Department of Chemistry, School of Science,
Kitasato University,
1-15-1 Kitasato University, Sagamihara, Kanagawa 228-8555,
Japan
[‡‡] Present address: Advanced Research Center for Science and
Engineering, Waseda University,
3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan
E-mail: akibaky@waseda.jp
Scheme 1.
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K. Kajiyama, M. Yoshimune, S. Kojima, K.-y. Akiba
FULL PAPER
Scheme 2.
hensive understanding of this unique group of compounds
en route to unveiling the whole multi-step BPR process, we
decided to look into O-cis compounds with an aryl mono-
Results and Discussion
The dianions 5, formed in-situ by the treatment of P–H
dentate ligand. Because of the electronegative nature of aryl (equatorial) spirophosphorane 4 with excess organolithium
groups compared with alkyl groups, we envisioned that the reagents, were oxidized by I2 in Et2O to afford O-cis phos-
preparation would be troublesome. Indeed, P–H (apical) phoranes 2 at room temperature (Scheme 2).[8] Although
compounds 1, which are precursors to the O-cis compounds the quantitative generation of the dianions 5a–c could be
could not be obtained initially. In order to establish a achieved with ca. 3 equiv. of aliphatic lithium reagents, it
milder method for the preparation of O-cis compounds, we was necessary to apply reagents in larger excesses in the
briefly examined the use of oxidizing reagents. As a result, case of aromatic lithium reagents (typically 6 equiv.) to af-
we found that the preparation of O-cis 2 could be achieved ford the dianions 5d–n. The 31P NMR chemical shifts as-
by I2 oxidation of the dianion 5, which is generated by the signable to O-cis 2, O-trans 3, and dianions 5 in the reaction
reaction of an excess of organolithium reagents with P–H mixture are shown in Table 1. The phosphorus resonances
phosphorane 4 (Scheme 2) without the need to isolate inter- of the dianions 5 appeared as broad singlets, and the chemi-
mediate P–H (apical) phosphoranes 1a–c.[7b,9] This has en- cal shifts of these species were similar to those observed for
abled us to isolate and characterize the first anti-apicophilic 10–P–4 species[10] despite the presence of bond switching
phosphorane with a monodentate aryl group. Herein we de- equilibria of P–O bonds via either 8–P–3 or 12–P–5 species.
scribe details of the generation of O-cis 2 by I2 oxidation of As previously reported for alkyl-substituted 2a–c and 3a–c,
5, and the effects of substituents on the monodentate aryl 31P NMR signals for 2d–n were at lower field than those
ring of O-cis arylphosphoranes on the stereomutation pro- for 3d–n in the aryl-substituted series. In the case of O-
cess, along with a kinetic study on the stereomutation of O- cis 2h, two phosphorus resonances were observed, due to
cis 2k–n to O-trans 3k–n.
rotational isomers about the phosphorus–carbon bond of
Table 1. 31P NMR chemical shifts of O-cis 2, O-trans 3, and dianion 5, and the ratio of 2 and 3 after oxidation.
31P NMR chemical shift in Et2O [δ]
Ratio of O-cis 2/O-trans 3
Entry
R
2
3
5
directly after
after 30 min
1
2
3
4
5
6
7
8
a
b
c
d
e
f
g
h
i
j
k
l
m
n
methyl
n-butyl
tert-butyl
phenyl
4-tert-butylphenyl
4-methoxyphenyl
4-(dimethylamino)phenyl
2-methylphenyl
4-methoxy-2,6-dimethylphenyl
–6.3
–3.5
7.7
–8.6
–8.5
–8.3
–9.2
–5.6, –8.8
–3.6
–22.6
–18.8
–9.8
–35.5
–23.1
Ͼ99:Ͻ1
Ͼ99:Ͻ1
Ͼ99:Ͻ1
27:73
71:29
72:28
96:4
Ͼ99:Ͻ1
Ͼ99:Ͻ1
Ͻ1:Ͼ99
Ͻ1:Ͼ99
Ͻ1:Ͼ99
25:75
23:77
53:47
66:34
54:66
–10.1
[a]
–31.9
–32.5
–33.1
–33.8
–26.6
–26.5
–25.6
–26.4
–26.2
–25.8
–25.7
[a]
[a]
[a]
87:13
–15.4
–13.0
–11.0
–9.2
–9.8
–10.8
–10.8
Ͼ99:Ͻ1
Ͼ99:Ͻ1
Ͼ99:Ͻ1
Ͼ99:Ͻ1
Ͼ99:Ͻ1
Ͼ99:Ͻ1
Ͼ99:Ͻ1
9
10
11
12
13
14
4-(dimethylamino)-2,6-dimethylphenyl –3.4
2,6-dimethylphenyl (DMP)
2,4,6-trimethylphenyl (TMP)
2,4,6-triethylphenyl (TEP)
2,4,6-triisopropylphenyl (TIP)
–4.7
–3.9
–3.1
–2.7
62:38
63:37
Ͼ99:Ͻ1
[a] The dianionic species could not be observed.
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Studies on the Stereomutation of O-cis Arylspirophosphoranes
FULL PAPER
the 2-methylphenyl monodentate group (Table 1, run 8). for the alkyl derivatives 2a–c right after oxidation (Table 1,
The two alkyl groups in the 2- and 6-positions of the mono- entries 1–3). In the case of phenyl derivative 2d, however,
dentate aryl group were also observed separately in 2i–n, the O-cis isomer was the minor product, and pseudorota-
thus indicating that at least on the NMR timescale, the P– tion to O-trans 3d was complete within 30 min at room tem-
C bond rotation for the monodentate group is slow. The perature (Table 1, entry 4), probably due to the higher apic-
corresponding rotational barrier for 3h was also high, as ophilicity of the phenyl group compared with alkyl groups
evident from the presence of four CF3 signals in the 19F in general.[11] In order to raise the BPR barrier by raising
NMR spectrum.
the energies of species such as B-RP and B-SP in a typical
In the room-temperature 19F NMR spectra of the O-cis multi-step BPR process such as in Figure 2, compounds 2e–
alkyl derivatives 2a–c, signals for the four anisochronous g, bearing electron donating groups (tBu, OMe, NMe2,
trifluoromethyl groups were observed as a pair of quartets respectively) in the 4-positions of the monodentate aryl
due to fast interconversion between enantiomers 2-RP and groups were examined. Although the O-cis isomer became
2-SP by a single step pseudorotation with the R group as the major product upon oxidation, this perturbation was
the pivot (Figure 1).[5,7] However, for the O-cis 2,6-dialk- not enough to allow isolation or detailed characterization.
ylphenyl derivatives, 19F NMR spectra of the crude prod- The incorporation of one methyl group at an ortho position
ucts after workup at room temperature showed either three of the monodentate aryl group (2h) was also not sufficient
(2i–l) or four signals (2m, n), indicating the presence of a (Table 1, entry 8). However, the use of bulky aryl groups
slower corresponding BPR process for the 2,6-dialkylphenyl with two ortho substituents[12] led to the exclusive formation
derivatives. This should be a consequence of the steric hin- of the O-cis isomer, and these products (2i–n) were found
drance exerted by the ortho alkyl groups as evident from to have sufficient lifetimes for characterization by 1H NMR
the decelerated P–C bond rotation (vide supra).
(Table 1, entries 9–14). Especially in the case of the TIP
The ratios of O-cis 2 and O-trans 3 in Et2O at room tem- derivative, the O-cis isomer 2n could be purified by silica
perature were determined by 31P NMR spectroscopy imme- gel chromatography (yield 71%) and further characterized
diately after the addition of I2 and 30 min later (Table 1). by single crystal X-ray analysis (vide infra). The 1,3,5-
Unlike the thermal method that required elevated tempera- tris(tBu)phenyl group was found to be too bulky to be in-
1
tures, the exclusive generation of O-cis 2 could be observed corporated. The small JP-C value of 44.5 Hz for 2n (com-
Figure 1. Interconversion between enantiomers 2-RP and 2-SP by a single step pseudorotation. The groups designated exo and endo
cannot interconvert with this process.
Figure 2. A multi-step pseudorotation process between the most stable isomers, O-trans 3-SP and 3-RP.
Eur. J. Org. Chem. 2006, 2739–2746
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K. Kajiyama, M. Yoshimune, S. Kojima, K.-y. Akiba
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Figure 3. The ORTEP drawings of 3l (left), 2n (middle), and 3n (right) showing the thermal ellipsoids at the 30% probability level. All
the hydrogens have been omitted for clarity.
1
pared with 125.2 and 115.9 for the other two JP-C values ordinate species.[13] Using this method, Holmes has demon-
of equatorial carbons) clearly indicates the presence of strated that the structural distortions of nonmetalated spi-
coupling between P and an apical carbon.
rophosphoranes follow a BPR process from TBP towards
The X-ray structures of O-cis 2n and O-trans 3l and 3n SP geometry in which the monodentate substituent occu-
are depicted in Figure 3, with selected structural parameters pies the axial site and the bidentate substituents occupy the
listed in Table 2. O-trans 3l and 3n assumed slightly dis- four basal sites. The percent distortions estimated by the
torted TBP structures and were quite symmetric, with the dihedral angle method using unit bond lengths[13,14] for
apical P–O and equatorial P–C bond lengths of the Martin compounds structurally determined in this study are given
ligands being practically the same. While the sum of the in Table 3. If a distortion is along a certain Berry coordi-
three equatorial bond angles of 358.1° for O-cis 2n was ne- nate, the sum of percent distortion from TBP to RP(SP)
arly identical to the ideal value of 360°, the apical bond (RP = rectangular pyramid) and that from RP(SP) to TBP
(168.9°) was more distorted than that of O-trans 3n should be nearly 100%, as seen for O-trans 3b (R = nBu)
(178.8°). In the crystal structure of O-cis 2n, the apical and 3l (R = 2,4,6-trimethylphenyl) with C19 as the pivot.
bonds of the bidentate groups were significantly longer than An analysis revealed that the TBP geometry of O-cis 2c
the corresponding equatorial bonds [P1–O1(apical): 1.777 (R = tBu) was distorted toward the transition state of the
Ͼ P1–O2(equatorial): 1.678 Å and P1–C10(apical): 1.885 Ͼ interconversion between 2c-RP and 2c-SP with the mono-
P1–C1(equatorial): 1.818 Å] as anticipated from the differ- dentate group as the pivot, whereas the distortion of O-cis
ence in the bonding scheme between the apical and equato- 2b (R = nBu) was along the BPR coordinate towards B
1
rial bonds and coinciding nicely with JP–C values (vide su- with C10, a carbon of a bidentate group, as the pivot. This
pra).
is consistent with the barrier for the interconversion of
enantiomers of 2b being higher than that for enantiomers
of 2c.[7b] As for 2n and 3n, the sums of percent distortion
from TBP to SP and that from SP to TBP are 102%, but
only one equatorial angle (O2–P1–C1 for 2n: 114.0° and
C1–P1–C10 for 3n: 115.8°) is smaller than the ideal value
of 120°. Similar distortion has been found in a few phos-
phoranido complexes.[15] Additionally, the equatorial P–O
bond of 2n (1.678 Å) was somewhat longer than those of
alkyl derivatives 2b (1.658 Å) and 2c (1.652 Å), despite the
apical P–O bond length of 2n (1.777 Å) being comparable
to those of the alkyl derivatives 2b (1.773 Å) and 2c
(1.771 Å).[7] Thus, the distortion from TBP geometry of 2n
and 3n does not follow any pseudorotation process. This is
Table 2. Selected bond lengths and angles for 2n, 3n, and 3l.
3l
2n
3n
Bond lengths [Å]
P1–O1
P1–O2
P1–C1
P1–C10
P1–C19
1.754(1)
1.754(1)
1.826(2)
1.826(2)
1.837(2)
1.777(1)
1.678(1)
1.818(2)
1.885(2)
1.865(2)
1.757(3)
1.763(3)
1.820(3)
1.825(4)
1.845(4)
Bond angles [°]
O1–P1–O2
O1–P1–C1
O1–P1–C10
O1–P1–C19
O2–P1–C1
O2–P1–C10
O2–P1–C19
C1–P1–C10
C1–P1–C19
C10–P1–C19
175.5(1)
87.0(1)
82.4(1)
86.8(1)
178.8(2)
87.1(2)
92.8(2)
91.6(2)
91.8(2)
87.4(2)
89.3(2)
115.8(2)
123.6(2)
120.6(2)
90.8(1)
92.3(4)
90.8(1)
87.0(1)
92.3(4)
122.9(1)
118.6(4)
118.6(4)
168.9(1)
86.9(1)
114.0(1)
86.5(1)
123.0(1)
98.3(1)
121.1(1)
98.8(1)
Table 3. Percent distortion (%) of phosphorane structures.
pivot: C19
pivot: C1
TBP to RP RP to TBP TBP to SP SP to TBP
2b
3b
2c
3l
2n
3n
20
17
13
10
15
8
96
83
88
90
103
106
20
17
13
10
15
8[a]
85
108
104
105
87
Holmes has proposed a method of determining the de-
gree of structural distortion of pentacoordinate compounds
from TBP towards the transition state structure of square
pyramidal structure (SP) on the BPR coordinate, based
94[a]
upon calculations involving internal angles of the pentaco- [a] Pivot: C10.
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Studies on the Stereomutation of O-cis Arylspirophosphoranes
FULL PAPER
clearly due to the bulkiness of the TIP group which makes as those for the nBu derivative 2b (∆H‡ = 21.8 kcalmol–1
the approach of the other two equatorial ligands, which is and ∆S‡ = –9.0 eu),[7] indicating that significant modifica-
required for pseudorotation to proceed, difficult. Thus, the tion is required to make the apicophilicity of aryl groups
structural distortion is in good agreement with the high comparable with alkyl groups. The use of AcOH as solvent
barrier for the pseudorotation between enantiomers of 2n did not lead to a significant increase in rate. Therefore it is
compared with 2b or 2c. The pseudorotation of the TIP- safe to believe that the stereomutation process is of BPR in
substituted bis(4,4Ј-dimethyl-2,2Ј-biphenylyl)phosphorane nature, without contribution from pathways involving
has also been reported not to follow the simplest Berry bond-dissociation and recombination as rate-determining
pseudorotation process with the TIP group as the pivot.[12] steps. This is in agreement with previously reported stereo-
The stereomutation rates of O-cis 2k–n to O-trans 3k–n mutation between diastereomeric O-trans stiboranes.[16] It
were measured in toluene (273–293 K for 2k–m to 3k–m, must be pointed out that there is a possibility that the turn-
313–333 K for 2n to 3n) by monitoring the 19F NMR sig- stile mechanism is the actual pathway.[17] Unfortunately, we
nals of the change in ratio of 2 and 3. All of the measured presently have no experimental means to judge the validity
processes followed first-order kinetics. In the case of the of this often-proposed mechanism.
pseudorotation of O-cis TIP phosphorane 2n to O-trans 3n,
the rates in acetic acid were also measured. The Eyring plot
of the rates turned out to be linear (Figure 4, Figure 5),
which implies that there are no competing pathways for the
stereomutation. Representative rates and activation param-
eters are shown in Table 4. The stereomutation of 2n was
found to be much slower than that for 2k–m with the differ-
ence in ∆G‡ between 2n (∆G‡ = 24.1 kcalmol–1) and 2l (∆G‡
= 22.2 kcalmol–1) being 1.9 kcalmol–1 at 293 K. On the
other hand, the difference in stereomutation rate among
2k–m in toluene was very small but observable, and in
agreement with expectations. The rate was smallest for the
most sterically hindered triethylphenyl-substituted 2m, and
coming in second was 2l, which has three (one more than
2k) electron-donating methyl groups on the monodentate
Figure 5. Eyring plot for the stereomutation of O-cis 2n to O-trans
3n in toluene and in acetic acid at 313–333 K.
aryl group. Incidentally, the activation parameters for the
stereomutation in toluene of the TIP derivative 2n (∆H‡ =
21.3 kcalmol–1 and ∆S‡ = –9.4 eu) were essentially the same
In conclusion, the generation of anti-apicophilic O-cis ar-
ylspirophosphoranes 2d–n at room temperature could be
achieved by the I2 oxidation of the dianions 5 formed by
the treatment of P–H (equatorial) spirophosphorane 4 with
excess aryllithium reagents. On the basis of the O-cis 2/O-
trans 3 ratios in the reaction mixtures determined by 31P
NMR analyses, the stabilities of the O-cis arylphosphoranes
towards stereomutation to O-trans arylphosphoranes
seemed to be more dependent on steric bulk than the elec-
tronic character of the monodentate aryl substituents. The
19F NMR spectra showed the isomerization between
enantiomers of O-cis arylphosphoranes to be slower than
that of O-cis alkylphosphoranes. Only O-cis TIP phos-
phorane 2n could be isolated in the cis series. The crystal
structures of O-cis 2n and O-trans 3l and 3n could be deter-
mined by X-ray structural analyses, and the TIP derivatives
2n and 3n were verified to be geometrical isomers. The per-
Figure 4. Eyring plots for the stereomutation of O-cis 2k–m to O-
trans 3k–m in toluene at 273–293 K.
Table 4. Representative rates and activation parameters for the permutation of O-cis 2k–n to O-trans 3k–n.[a]
Entry
R
Solvent
k [s–1] (T [K])
∆G‡ [kcal mol–1] (T [K])
∆H‡ [kcalmol–1]
∆S‡ [eu]
1
2
3
4
5
DMP
TMP
TEP
TIP
k
l
m
n
n
toluene
toluene
toluene
toluene
acetic acid
(2.61 0.05)×10–4 (293)
(1.81 0.01)×10–4 (293)
(1.60 0.02)×10–4 (293)
(6.26 0.07)×10–4 (333)
(5.98 0.13)×10–4 (333)
21.9 (293)
22.2 (293)
22.2 (293)
24.4 (333)
24.5 (333)
20.4 1.0
20.0 0.5
20.4 0.7
21.3 0.7
19.5 0.8
–5.4 3.6
–7.5 1.8
–6.4 2.4
–9.4 2.0
–15.0 2.5
TIP
[a] Error is given as standard deviation.
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K. Kajiyama, M. Yoshimune, S. Kojima, K.-y. Akiba
FULL PAPER
and then I2 (1.65 g, 6.50 mmol) was added. The mixture was stirred
for 1 h at –78 °C. The resulting solution was washed with aq.
Na2S2O3 (2× 30 mL) and brine, the organic layer was dried with
anhydrous MgSO4, and the solvents were evaporated. The resulting
crude solid was recrystallized from acetonitrile to afford 2c[7b] as
colorless crystals (529 mg, 48%).
cent distortion along the Berry pseudorotation coordinate
estimated by the dihedral angle method suggested that the
TBP structures of 2b and 2c were distorted towards SP ge-
ometry, leading to the enantiomer of 2b and an intermedi-
ate en route to O-trans 3c, respectively. As for the TIP deriv-
atives 2n and 3n, the distortion from TBP geometry did not
follow any pseudorotation process due to the extremely
large steric repulsion between the TIP group and the other
equatorial ligands. Kinetic measurements of the O-cis to O-
trans pseudorotation process demonstrated that the acti-
vation parameters for TIP derivative 2n were identical with
those for nBu derivative 2b.
4-Bromo-N,N,3,5-tetramethylaniline:[17] Benzyltrimethylammonium
tribromide[20] was added to a solution of N,N-3,5-tetramethylani-
line (2.00 mL, 12.2 mmol) in CH2Cl2 (50 mL) and MeOH (20 mL)
at room temperature to give an orange-colored solution. The solu-
tion was stirred for 5 min until it became discolored. The solvent
was evaporated in vacuo and 10% KOH aq. was added to neu-
tralize. The mixture was extracted with Et2O (3× 50 mL), the com-
bined organic layer was dried with anhydrous K2CO3, and then the
solvent was removed. The crude product was distilled to afford a
colorless liquid (2.19 g, 79 %). Bp 86 °C (0.15 Torr). 1H NMR
(CDCl3): δ = 6.52 (s, 2 H), 2.95 (s, 6 H), 2.43 (s, 6 H) ppm.
Experimental Section
Melting points were measured with a Yanaco micro melting point
apparatus and are uncorrected. 1H NMR (400 MHz), 13C NMR
(100 MHz), 19F NMR (376 MHz), and 31P NMR (162 MHz) spec-
tra were recorded on a JEOL EX-400 spectrometer. 1H NMR
chemical shifts (δ) are given in ppm downfield from internal Me4Si
or from residual chloroform-d (δ = 7.26). 13C NMR chemical shifts
(δ) are given in ppm from chloroform-d (δ = 77.0). 19F NMR chem-
General Procedure for the Synthesis of Arylphosphoranes: To a solu-
tion of ArBr (2.72 mmol) in Et2O (5 mL) was added nBuLi
(1.46 mL, 2.33 mmol, 1.62  in hexane) at –78 °C. Except for the
lithiation of anisole derivatives, the mixture was warmed to room
temperature and stirring was continued for 3 h or 12 h (for aniline
derivatives). 4-Bromoanisole and 4-bromo-3,5-dimethylanisole
were lithiated at 0 °C for 8 h. To the solution of ArLi was added a
solution of 4 (0.388 mmol) in Et2O (5 mL) at –78 °C, and stirring
was continued at room temperature for 1 h, followed by the ad-
dition of I2 (2.33 mmol). Within five min, the resulting solution
was washed with aq. Na2S2O3, and the mixture was extracted with
Et2O (20 mL×3). The collected organic layer was dried with anhy-
drous MgSO4, and the solvent was evaporated in vacuo. The crude
product was purified by PTLC (silica gel, hexane:CH2Cl2 = 3:1),
followed by recrystallization from hexane/CH2Cl2 to give 3 (or 2n)
as colorless crystals.
ical shifts (δ) are given in ppm downfield from external CFCl3. 31
P
NMR chemical shifts (δ) are given in ppm downfield from external
85% H3PO4. Elemental analyses were performed on a Perkin–El-
mer 2400 CHN elemental analyzer. All reactions were carried out
under N2. Et2O was freshly distilled from Na-benzophenone. Ace-
tic acid was distilled from P2O5. Toluene was distilled from CaH2.
4-Bromo-3,5-dimethylanisole,[18] bromo-2,4,6-triethylbenzene,[12]
bromo-2,4,6-triisopropylbenzene,[12] and phosphorane 4[19] were
prepared according to published procedures. Preparative thin layer
chromatography was carried out on plates of Merck silica gel 60
GF254.
[TBPY-5-11]-1-(2-Methyl)phenyl-3,3,3Ј,3Ј-tetrakis(trifluorometh-
yl)-1,1Ј-spirobi[3H,2,1,λ5-benzoxaphosphole] (3h): 57%; m.p. 155 °C
(sublimation). 1H NMR (CDCl3): δ = 8.64–8.58 (m, 2 H), 7.80–
[TBPY-5-12]- and [TBPY-5-11]-1-Methyl-3,3,3Ј,3Ј-tetrakis(trifluoro-
methyl)-1,1Ј-spirobi[3H,2,1,λ5-benzoxaphosphole] (2a and 3a): To a
solution of 4 (3.01 g, 5.83 mmol) in Et2O (50 mL) was added MeLi
(17.0 mL, 17.3 mmol, 1.02  in Et2O) at 0 °C, and the mixture was
stirred for 3 h at room temperature. I2 (4.50 g, 17.7 mmol) was then
added to the solution at –78 °C, and the mixture was stirred for
1 h. The resulting mixture was washed with aq. Na2S2O3 (120 mL)
and brine (100 mL), dried with anhydrous MgSO4, and the solvent
was evaporated in vacuo with cooling (acetone/CO2 bath). The re-
sulting crude solid was washed with n-hexane to afford white solid
2a[7b] (2.64 g, 83%) containing a small portion (8%) of 3a.[7b]
3
7.74 (m, 6 H), 7.40 (dd, 3JHP = 18.1 Hz, JHH = 7.8 Hz, 1 H), 7.40
(t, 3JHH = 7.8 Hz, 1 H), 7.05–7.02 (m, 2 H), 2.29 (s, 3 H) ppm. 19F
4
4
NMR (CDCl3): δ = –74.9 (q, JFF = 9.7 Hz, 3F), –75.0 (q, JFF
=
4
4
9.7 Hz, 3F), –75.7 (q, JFF = 9.7 Hz, 3F), –75.8 (q, JFF = 9.7 Hz,
3F) ppm. 31P NMR (CDCl3): δ = –27.5 ppm. C25H15F12O2P: calcd.
C 49.52, H 2.49; found C 49.34, H 2.31.
[TBPY-5-11]-1-(2,6-Dimethyl-4-methoxy)phenyl-3,3,3Ј,3Ј-tetrakis-
(trifluoromethyl)-1,1Ј-spirobi[3H,2,1,λ5-benzoxaphosphole] (3i):
1
27%; m.p. 172 °C (sublimation). H NMR (CDCl3): δ = 8.48–8.44
4
(m, 2 H), 7.72–7.68 (m, 6 H), 6.40 (d, JHP = 5.4 Hz, 2 H), 3.74 (s,
[TBPY-5-12]-1-Butyl-3,3,3Ј,3Ј-tetrakis(trifluoromethyl)-1,1Ј-spirobi-
[3H,2,1,λ5-benzoxaphosphole] (2b): To a solution of 4 (3.09 g,
5.99 mmol) in Et2O (50 mL) was added nBuLi (11.7 mL,
18.0 mmol, 1.52  in hexane) at 0 °C, and then the solution was
stirred for 3 h at room temperature. The solution was cooled to
–78 °C, and then I2 (4.60 g, 18.1 mmol) was added. The mixture
was stirred for 1 h at –78 °C, and the resulting solution was washed
with aq. Na2S2O3 (2× 50 mL) and brine (2× 50 mL). The organic
layer was dried with anhydrous MgSO4 and concentrated in vacuo.
The resulting crude product was washed with n-hexane to afford
2b[7b] as a white solid (3.12 g, 91%).
4
3 H), 2.22 (s, 6 H) ppm. 19F NMR (CDCl3): δ = –74.2 (q, JFF
=
9.8 Hz, 6F), –75.7 (q, 4JFF = 9.8 Hz, 6F) ppm. 31P NMR (CDCl3):
δ = –27.3 ppm. C27H19F12O3P: calcd. C 49.86, H 2.94; found C
49.91, H 2.56.
[TBPY-5-11]-1-(4-Dimethylamino-2,6-dimethyl)phenyl-3,3,3Ј,3Ј-tet-
rakis(trifluoromethyl)-1,1Ј-spirobi[3H,2,1,λ5-benzoxaphosphole] (3j):
1
9%; m.p. 147 °C (sublimation). H NMR (CDCl3): δ = 8.48–8.46
(m, 2 H), 7.71–7.66 (m, 6 H), 6.17 (s, 2 H), 2.89 (s, 6 H), 2.22 (s, 6
H) ppm. 19F NMR (CDCl3): δ = –67.8 (q, 4JFF = 9.8 Hz, 6F), –69.3
4
(q, JFF = 9.8 Hz, 6F) ppm. 31P NMR (CDCl3): δ = –26.6 ppm.
C28H22F12NO2P: calcd. C 50.69, H 3.34, N 2.11; found C 50.40, H
3.08, N 1.84.
[TBPY-5-12]-1-(1,1-Dimethyl)ethyl-3,3,3Ј,3Ј-tetrakis(trifluorometh-
yl)-1,1Ј-spirobi[3H,2,1,λ5-benzoxaphosphole] (2c): To a solution of
4 (1.01 g, 1.95 mmol) in Et2O (20 mL) was added tBuLi (4.00 mL,
6.48 mmol, 1.6  in pentane) at 0 °C, and the solution was stirred
for 3 h at room temperature. The solution was cooled to –78 °C,
[TBPY-5-12]-1-(2,6-Dimethyl)phenyl-3,3,3Ј,3Ј-tetrakis(trifluoro-
1
methyl)-1,1Ј-spirobi[3H,2,1,λ5-benzoxaphosphole] (2k): H NMR
3
(CDCl3): δ = 7.82 (d, JHH = 7.3 Hz, 1 H), 7.65–7.62 (m, 3 H),
2744
www.eurjoc.org
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2006, 2739–2746

Studies on the Stereomutation of O-cis Arylspirophosphoranes
7.53–7.50 (m, 4 H), 7.18 (dd, 4JHP = 12.2 Hz, 3JHH = 7.3 Hz, 1 H),
FULL PAPER
(d, JCP = 19.7 Hz), 130.6, 130.4 (d, JCP = 7.2 Hz), 130.3 (d, JCP
9.3 Hz), 125.5 (d, JCP = 12.4 Hz), 125.0 (d, JCP = 7.4 Hz), 123.6
(d, JCP = 14.0 Hz), 122.7 (q, JCF = 287.6 Hz), 122.3 (q, JCF
=
5
3
4
7.10 (dt, JHP = 2.0 Hz, JHH = 7.3 Hz, 1 H), 6.95 (dd, JHP
=
3
8.3 Hz, JHH = 7.3 Hz, 1 H), 2.72 (s, 3 H), 2.17 (s, 3 H) ppm. 19F
=
4
NMR (CDCl3): δ = –72.9 (q, JFF = 9.8 Hz, 3F), –75.0 (br. s, 6F),
287.6 Hz), 122.2 (d, JCP = 17.6 Hz), 121.8 (q, JCF = 287.6 Hz),
121.7 (q, JCF = 287.6 Hz), 81.4 (sept, JCF = 31.3 Hz), 79.7 (sept,
JCF = 31.3 Hz), 33.7 (d, JCP = 1.6 Hz), 32.9 (d, JCP = 5.7 Hz), 31.7
(d, JCP = 5.2 Hz), 25.4, 24.6, 24.4 (d, JCP = 1.6 Hz) ppm. 19F NMR
4
–75.9 (q, JFF = 9.7 Hz, 3F) ppm. 31P NMR (CDCl3): δ = –5.7.
[TBPY-5-11]-1-(2,6-Dimethyl)phenyl-3,3,3Ј,3Ј-tetrakis(trifluoro-
methyl)-1,1Ј-spirobi[3H,2,1,λ5-benzoxaphosphole] (3k): 68%; m.p.
(CDCl3): δ = –74.1 (q, JFF = 9.8 Hz, 3F), –74.2 (q, 4JFF = 9.8 Hz,
4
1
122 °C (sublimation). H NMR (CDCl3): δ = 8.49–8.46 (m, 2 H),
3F), –74.3 (q, 4JFF = 9.8 Hz, 3F), –74.6 (q, 4JFF = 9.8 Hz, 3F) ppm.
31P NMR (CDCl3): δ = –2.8 ppm. C33H31F12O2P: calcd. C 55.16,
H 4.35; found C 55.16, H 4.09.
5
3
7.74–7.70 (m, 6 H), 7.05 (dt, JHP = 2.4 Hz, JHH = 7.3 Hz, 1 H),
4
3
6.86 (dd, JHP = 6.8 Hz, JHH = 7.3 Hz, 2 H), 2.21 (s, 6 H) ppm.
19F NMR (CDCl3): δ = –74.1 (q, 4JFF = 9.8 Hz, 6F), –75.8 (q, 4JFF
= 9.8 Hz, 6F) ppm. 3 1 P NMR (CDCl3 ): δ = –27.3 ppm. [TBPY-5-11]-1-(2,4,6-Triisopropyl)phenyl-3,3,3Ј,3Ј-tetrakis(tri-
C26H17F12O2P: calcd. C 50.34, H 2.76; found C 50.29, H 2.52.
fluoromethyl)-1,1Ј-spirobi[3H,2,1,λ5-benzoxaphosphole] (3n): A
solution of 2n (50 mg, 0.07 mmol) in Et2O (3 mL) was heated at
60 °C for 12 hours. The solvent was removed in vacuo and the resi-
due was recrystallized from hexane/CH2Cl2 to afford 3n quantita-
[TBPY-5-12]-1-(2,4,6-Trimethyl)phenyl-3,3,3Ј,3Ј-tetrakis(trifluoro-
1
methyl)-1,1Ј-spirobi[3H,2,1,λ5-benzoxaphosphole] (2l): H NMR
3
(CDCl3): δ = 7.81 (d, JHH = 7.3 Hz, 1 H), 7.64–7.60 (m, 3 H),
1
tively (50 mg, 0.07 mmol, 99%). Mp 144 °C. H NMR (CDCl3): δ
4
4
7.56–7.42 (m, 4 H), 6.84 (d, JHP = 5.8 Hz, 1 H), 6.76 (d, JHP
=
4
= 8.45–8.40 (m, 2 H), 7.71–7.65 (m, 6 H), 6.88 (d, JHP = 6.4 Hz,
7.8 Hz, 1 H), 2.67 (s, 3 H), 2.22 (s, 3 H), 2.13 (s, 3 H) ppm. 19F
3
3
2 H), 3.74 (sept, JHH = 6.8 Hz, 2 H), 2.79 (sept, JHH = 6.8 Hz, 1
4
NMR (CDCl3): δ = –74.5 (q, JFF = 9.7 Hz, 3F), –75.1 (br. s, 6F),
3
3
H), 1.21 (d, JHH = 6.8 Hz, 6 H), 1.17 (d, JHH = 6.8 Hz, 6 H),
–76.1 (q, JFF = 9.7 Hz, 3F) ppm. 31P NMR (CDCl3): δ = –5.2.
4
0.46 (d, JHH = 6.8 Hz, 3 H) ppm. 13C NMR (CDCl3): δ = 149.5
3
[TBPY-5-11]-1-(2,4,6-Trimethyl)phenyl-3,3,3Ј,3Ј-tetrakis(trifluoro-
methyl)-1,1Ј-spirobi[3H,2,1,λ5-benzoxaphosphole] (3l): 64%; m.p.
(d, JCP = 3.7 Hz), 148.7 (d, JCP = 14.7 Hz), 136.8 (d, JCP
=
1
20.2 Hz), 136.4 (d, JCP = 11.0 Hz), 135.0 (d, JCP = 178.3 Hz),
1
1
175 °C (sublimation). H NMR (CDCl3): δ = 8.49–8.45 (m, 2 H),
133.2 (d, JCP = 3.7 Hz), 133.1 (d, JCP = 154.4 Hz), 131.2 (d, JCP
7.72–7.66 (m, 6 H), 6.67 (d, 4JHP = 6.4 Hz, 2 H), 2.19 (s, 3 H), 2.18
= 14.7 Hz), 124.8 (d, JCP = 14.7 Hz), 122.7(d, JCP = 16.5 Hz), 122.7
4
(s, 6 H) ppm. 19F NMR (CDCl3): δ = –74.2 (q, JFF = 9.8 Hz,
(q, JCF = 288.6 Hz), 122.3 (q, JCF = 288.6 Hz), 82.1 (sept, JCF
=
4
6F), –75.7 (q, JFF = 9.8 Hz, 6F) ppm. 31P NMR (CDCl3): δ =
31.2 Hz), 33.7, 31.3 (d, JCP = 5.5 Hz), 23.8, 23.6 ppm. 19F NMR
4
(CDCl3): δ = –74.0 (q, JFF = 9.8 Hz, 6F), –74.8 (q, 4JFF = 9.8 Hz,
–27.0 ppm. C27H19F12O2P: calcd. C 51.12, H 3.02; found C 51.02,
H 2.90.
6F) ppm. 31P NMR (CDCl3): δ = –2.8 ppm. C33H31F12O2P: calcd.
C 55.16, H 4.35; found C 54.99, H 4.19.
[TBPY-5-12]-1-(2,4,6-Triethyl)phenyl-3,3,3Ј,3Ј-tetrakis(trifluoro-
1
methyl)-1,1Ј-spirobi[3H,2,1,λ5-benzoxaphosphole] (2m): H NMR
X-ray Crystal Structure Determination of 3l, 2n, and 3n: Crystals
suitable for X-ray structure determination were mounted on a Mac
Science MXC3 diffractometer and irradiated with graphite-mono-
chromated Cu-Kα radiation (λ = 1.54178 Å) for data collections.
Lattice parameters were determined by least-square fitting of 31
reflections for all compounds with 31° Ͻ 2θ Ͻ 35° for 3l and with
54° Ͻ 2θ Ͻ 60° for 2n and 3n. Data were collected with the 2θ/ω
scan mode. All data were corrected for absorption[21] and extinc-
tion.[22] The structures were solved by a direct method and refined
by full-matrix least-squares methods with the TeXsan program.[23]
All non-hydrogen atoms were refined with anisotropic thermal pa-
rameters. All hydrogen atoms were included in the refinement on
calculated positions (length of C–H: 1.0 Å) riding on their carrier
atoms with isotropic thermal parameters.
(CDCl3): δ = 7.96–7.90 (m, 1 H), 7.88–7.84 (m, 3 H), 7.58–7.52 (m,
4 H), 7.39 (d, JHP = 6.4 Hz, 1 H), 7.36 (d, JHP = 6.4 Hz, 1 H),
4
4
3
3
2.96–2.64 (m, 6 H), 1.42 (t, JHH = 7.3 Hz, 3 H), 1.32 (t, JHH
=
7.3 Hz, 3 H), 1.29 (t, 3JHH = 7.3 Hz, 3 H) ppm. 19F NMR (CDCl3):
4
4
δ = –73.7 (q, JFF = 9.8 Hz, 3F), –74.6 (q, JFF = 9.8 Hz, 3F),
–75.1 (q, 4JFF = 9.8 Hz, 3F), –75.2 (q, 4JFF = 9.8 Hz, 3F) ppm. 31
NMR (CDCl3): δ = –3.7 ppm.
P
[TBPY-5-11]-1-(2,4,6-Triethyl)phenyl-3,3,3Ј,3Ј-tetrakis(trifluoro-
methyl)-1,1Ј-spirobi[3H,2,1,λ5-benzoxaphosphole] (3m): 16%. M.p.
1
157 °C (sublimation). H NMR (CDCl3): δ = 8.52–8.44 (m, 2 H),
4
2
7.76–7.64 (m, 6 H), 6.76 (d, JHP = 6.4 Hz, 2 H), 2.79 (dq, JHH
=
=
3
2
3
14.6 Hz, JHH = 7.3 Hz, 2 H), 2.74 (dq, JHH = 14.6 Hz, JHH
3
3
7.3 Hz, 2 H), 2.54 (q, JHH = 7.3 Hz, 2 H), 1.17 (t, JHH = 7.3 Hz,
3 H), 0.71 (t, JHH = 7.3 Hz, 6 H) ppm. 19F NMR (CDCl3): δ =
3
CCDC-281932 (for 3l), -154409 (for 2n), and -154410 (for 3n) contain
the supplementary crystallographic data (CIF) for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
–74.1 (q, 4JFF = 9.8 Hz, 6F), –75.1 (q, 4JFF = 9.8 Hz, 6F) ppm. 31
P
NMR (CDCl3): δ –26.6 ppm. C30H25F12O2P: calcd. C 53.27, H
3.72; found C 53.29, H 3.53.
[TBPY-5-12]-1-(2,4,6-Triisopropyl)phenyl-3,3,3Ј,3Ј-tetrakis(tri-
fluoromethyl)-1,1Ј-spirobi[3H,2,1,λ5-benzoxaphosphole] (2n): 71%.
Mp 157 °C (decomp.). H NMR (CDCl3): δ = 7.80–7.75 (m, 2 H),
Kinetic Measurements of the Pseudorotation of 2k–n: Crude samples
(ca. 5 mg) after work-up of the mixtures of 2k and 3k (1:1), 2l and
3l (1:1), and 2m and 3m (3:2) or a sample of 2n (ca. 5 mg) dissolved
in freshly distilled solvent (0.5–0.6 mL) were sealed separately in
NMR tubes under N2. 19F NMR spectra were measured in variable
1
7.66–7.63 (m, 2 H), 7.57–7.52 (m, 2 H), 7.47–7.42 (m, 1 H), 7.20
4
(dd, 3JHP = 11.7 Hz, 3JHH = 7.8 Hz, 1 H), 7.08 (dd, JHP = 4.4 Hz,
4JHH = 1.9 Hz, 1 H), 6.95 (dd, 4JHP = 8.8 Hz, 4JHH = 1.9 Hz, 1 H), temperature mode (error within 1 °C). The observed temperatures
3
3
1
3.94 (sept, JHH = 6.8 Hz, 1 H), 3.39 (sept, JHH = 6.8 Hz, 1 H),
were calibrated with the H NMR chemical shift difference of sig-
3
3
2.83 (sept, JHH = 6.8 Hz, 1 H), 1.41 (d, JHH = 6.8 Hz, 3 H), 1.36 nals of neat 1,3-propanediol (high temperature region) and MeOH
3
3
(d, JHH = 6.8 Hz, 3 H), 1.21 (d, JHH = 6.8 Hz, 6 H), 1.04 (d,
(low temperature region). The kinetic data were analyzed by as-
3JHH = 6.8 Hz, 3 H), 0.39 (d, JHH = 6.8 Hz, 3 H) ppm. 13C NMR suming irreversible first-order kinetics using the equation ln(c0/c)
3
1
(CDCl3): δ = 149.1 (d, JCP = 9.3 Hz), 149.1, 142.7 (d, JCP
44.5 Hz), 142.3 (d, JCP = 21.7 Hz), 137.5 (d, JCP = 115.9 Hz),
136.9 (d, JCP = 19.7 Hz), 134.2 (d, JCP = 125.2 Hz), 133.4 (d, JCP
=
= kT, in which c0 = concentration of reactant at t = 0, c = concen-
tration of reactant at arbitrary intervals. The activation enthalpies
and entropies were calculated according to the transition state
theory of Eyring by linear regression.
1
1
= 2.1 Hz), 133.2 (d, JCP = 16.6 Hz), 130.9 (d, JCP = 15.5 Hz), 130.7
Eur. J. Org. Chem. 2006, 2739–2746
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2745

K. Kajiyama, M. Yoshimune, S. Kojima, K.-y. Akiba
FULL PAPER
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Acknowledgments
The authors are in debt to Dr. Shiro Matsukawa for providing tech-
nical assistance. The authors are grateful to Central Glass Co. Ltd.
for a generous gift of hexafluorocumyl alcohol. Partial support of
this work through Grants-in-Aid for Scientific Research (Nos.
06740488 and 07740501) provided by the Ministry of Education,
Science, and Culture of the Japanese Government is heartily ac-
knowledged.
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Received: January 16, 2006
Published Online: April 3, 2006
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