Facile [3+2] dimerization and formal dehydrogenative coupling mode of a cyanophenylphosphaallene

Article abstract of DOI:10.1016/j.tet.2007.07.063

Due to a facile head-to-tail [3+2] dimerization, even a sterically demanding group such as the Mes^* (2,4,6-tri-tert-butylphenyl) group around the P{double bond, long}C{double bond, long}C moiety did not allow us to isolate 3-(4-cyanophenyl)-1-(

Full text of DOI:10.1016/j.tet.2007.07.063

Tetrahedron 63 (2007) 10246–10252
Facile [3D2] dimerization and formal dehydrogenative
coupling mode of a cyanophenylphosphaallene
a
a
b
c
Shigekazu Ito,a, Sou Hashino, Noboru Morita, Masaaki Yoshifuji, Daisuke Hirose,
*
Masae Takahashic, and Yoshiyuki Kawazoe
*
c
aDepartment of Chemistry, Graduate School of Science, Tohoku University, Aoba, Sendai 980-8578, Japan
bDepartment of Chemistry, The University of Alabama, Tuscaloosa, AL 35487-0336, USA
cInstitute for Material Research, Tohoku University, Katahira 2-1-1, Aoba, Sendai 980-8577, Japan
Received 4 June 2007; revised 18 July 2007; accepted 20 July 2007
Available online 26 July 2007
This paper is dedicated to the memory of the late Professor Yoshihiko Ito
Abstract—Due to a facile head-to-tail [3+2] dimerization, even a sterically demanding group such as the Mes* (2,4,6-tri-tert-butylphenyl)
group around the P]C]C moiety did not allow us to isolate 3-(4-cyanophenyl)-1-(2,4,6-tri-tert-butylphenyl)-1-phosphaallene from the
elimination reaction of 2-bromo-3-(4-cyanophenyl)-1-(2,4,6-tri-tert-butylphenyl)-1-phosphaprop-1-ene with DBU (1,8-diazabicyclo-
[5.4.0]undec-7-ene), and the corresponding 1,4-diphosphafulvene containing cyano groups was obtained and characterized. Theoretical
studies on the [3+2] dimerization of phosphaallene characterize possible intermediates affording 1,4-diphosphafulvenes and also suggest
the cyano group effect to facilitate the saturation of the P]C double bonds. On the other hand, 1,2-bis(4-cyanophenyl)-3,4-bis[(2,4,6-tri-
tert-butylphenyl)phosphinidene]cyclobutene was obtained from 2-bromo-3-(4-cyanophenyl)-3-trimethylsiloxy-1-(2,4,6-tri-tert-butyl-
phenyl)-1-phosphaprop-1-ene together with the 3-(4-cyanophenyl)-1-phosphaallene.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
dimerization of 1 accompanying a [1,2]-H shift (Chart 1).
We recently succeeded in the formal synthesis of 2 as air-
stable crystalline compounds from 14–6 and revealed that
one of the diphosphafulvenes 2 possesses considerable
electron-donating property to afford the corresponding
charge-transfer (CT) complex with 7,7,8,8-tetracyanoquino-
dimethane (TCNQ).4 On the other hand, by utilizing
P-heterocyclic carbene–titanocene or zirconocene com-
plexes, Le Floch and co-workers independently reported
the synthesis of 1,4-diphosphafulvenes7 and advanced to
the preparation of a cation radical derived from 1,4-diphos-
phafulvene.8 These findings promise to open new research
fields in organoelectronics and spintronics based on the
chemistry of P-heterocyclic compounds.
A number of common N-heterocyclic compounds play im-
portant roles in science and technology. Recently, cyclic or-
ganic compounds containing phosphorus atom(s) have also
attracted considerable attention in connection with the de-
velopment of novel functional materials.1,2 As an approach
to the synthesis of structurally and functionally intriguing
P-heterocyclic systems, we have paid considerable attention
to the reactivity of the kinetically stabilized phosphaallenes
1 as a fundamental cumulative molecule containing an
sp2-type phosphorus. The chemistry of low-coordinated
phosphorus compounds has intensively developed following
the establishment of the kinetic stabilization technique
utilizing sterically encumbered substituents to inhibit satura-
tion of the multiple bonds of phosphorus,3 which facilitates
the use of phosphaalkenes as reagents for novel molecular
transformation.
Together with 1,4-diphosphafulvene derivatives, 3,4-diphos-
phinidenecyclobutenes (DPCBs) 3 are obtained from phos-
phaallene through dehydrogenative homocoupling (Chart
One of the intriguing P-heterocyclic systems prepared from
1 is 1,4-diphosphafulvenes 2 via a formal head-to-tail [3+2]
R
Mes*
H
R
R
P
R
P
P
C
C
Mes*
Mes*
P
P
Keywords: Phosphaalkenes; Phosphorus heterocycles; Oligomerization;
DFT calculations.
* Corresponding authors. Tel.: +81 22 795 6560; fax: +81 22 795 6562 (S.I);
Tel.: +81 22 215 2481; fax: +81 22 215 2052 (M.T.); e-mail addresses:
shigeito@mail.tains.tohoku.ac.jp; masae@imr.edu
Mes*
Mes*
1
R
Mes* = 2,4,6-t-Bu3C6H2
Chart 1.
2
3
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.07.063

S. Ito et al. / Tetrahedron 63 (2007) 10246–10252
10247
1). We previously described several DPCB formations from
the corresponding phosphaallene and alkyllithium, along
with the generation of hydrogen.6b,6c DPCBs are available
as rigid P2 ligand systems with strong p-accepting charac-
ters that provide unique synthetic catalysts.9,10
2b, probably including the (RP,RP) and (SP,SP) isomers,
was characterized based on spectroscopic data and was com-
parable to 2a.13
R
Mes*
H
R
DBU
6
In this paper we report new findings concerning the experi-
mental and theoretical investigations of [3+2] dimerization
of 1. In the course of studies on phosphaallenes, we found
that an electron-withdrawing cyano group contributes to de-
stabilize the phosphaallene skeleton and to facilitate [3+2]
dimerization. The [3+2] dimerization of phosphaallene
was assessed by DFT calculations on model compounds to
depict the reaction pathways from 1 to 2. Redox properties
as well as one-electron oxidation processes of 1,4-diphos-
phafulvenes are also discussed. In addition, we report the
preparation of a novel DPCB derivative bearing cyano
groups by the use of 1-bromo-2-(2,4,6-tri-tert-butyl-
phenyl)-2-phosphaethenyllithium.6b,6c,10,11
+
Mes*
Mes*
P
P
C
C
P
DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene
R
1
2
R = Ph (a)
R = 4-NCC6H4 (b)
78%
0%
38%
not isolated
Scheme 2.
A similar instability of phosphacumulenes in spite of the
presence of the bulky Mes* group was previously reported.
Markl and co-workers found head-to-tail [2+2] P]C di-
merizations of phosphabutatriene derivatives containing
electron-withdrawing aryl groups at the 4-position,14
which is comparable to the case of [3+2] dimerization of
1b not allowing the isolation of 1b in the presence of
DBU.
€
2. Results and discussion
2.1. Synthesis and [3D2] dimerization of phosphaallenes
2.2. DFT calculations
As described in our previous reports,4–6 the reactivity of
phosphaallene is strongly influenced by substituent(s) at
the 3-position. In the course of our synthetical research
based on the chemistry of phosphaallenes, we found that
the presence of a cyano-containing substituent enhances re-
activity including [2+3] dimerization to inhibit the isolation
of the desired phosphaallene in spite of the presence of the
bulky Mes* group.
To illustrate the reaction pathways of the head-to-tail [3+2]
dimerization of phosphaallene, we performed DFT calcula-
tions on this system starting with compound 7 as the model
compound of 1. The calculated structures for 7, 8, and 9 (the
model compounds for 1, 2, and the geometrical isomer of
2) showed comparable parameters with the experimental
6c
data of 1aSi 2a,4 and meso-5-adamantylidene-1,2,3,4-
tetraphenyl-1,4-diphosphafulvene,7 respectively (Chart 2)
(see Supplementary data). The energy difference between
8 and 9 is very small, and thus explains the isolation of
both the anti and syn isomers of hexaphenyl-1,4-diphospha-
fulvene.7
2,2-Dibromo-1-phosphaethene 412 was allowed to react with
butyllithium to generate 5, and was subsequently treated
with benzyl bromide4 or 4-cyanobenzyl bromide to afford
the corresponding 2-bromo-1-phosphapropene 6 in moder-
ate yields. The cyano group seemed to be inactive to the
lithium intermediate 5 (Scheme 1).
Figure 1 depicts the calculated reaction pathways for [3+2]
dimerization of 7 to 8 with three intermediary structures.
As shown in Figure 2a, the metric parameters of the transi-
tion state T17–8 of the highest energy barrier indicate that
the terminal phosphorus and carbon approach each other
to cause C–H bond breaking for the [1,2]-H shift and elonga-
tion of the P]C bond in molecule A. The terminal C atom of
molecule A maintains the sp2 configuration and the migrat-
ing H atom shows closer distance to the inner C atom rather
than to the terminal carbon interacting with the phosphorus
of molecule B. The dihedral angle of H1–P1–C1–H2 is
ꢀ175.8ꢁ, suggesting that the migrating H atom interacts
with the p orbital perpendicular to the P]C moiety. The
HOMO and LUMO of phosphaallene15 would play key roles
in choosing one of the concerted [3+2] cyclizations to the
five-membered ring skeleton. On the other hand, the struc-
ture of T17–8 (Fig. 2a) indicates steric repulsion between
Mes*
Br
Mes*
Br
Br
Mes*
Br
Li
BrCH2R
n-BuLi
P
C
P
C
P C
CH2R
4
5
6
a: R = Ph
b: R = 4-NCC6H4
Scheme 1.
We next examined base-induced HBr elimination of 6 with
DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) as shown in
Scheme 2. To obtain 1a as a single product from 6a, DBU
treatment was found suitable to give the desired 1a in a mod-
erate yield. On the other hand, 6b showed considerably
different reaction characteristics from 6a in the presence
of DBU. When 6b was mixed with DBU, we observed the
31P NMR signal due to 1b [dP (C6D6) 80.2] in the reaction
mixture; however, the signal due to 6b disappeared in solu-
tion during the workup procedures, and 2b was obtained as
an isolable product as red-orange crystals. Therefore, 1b
would be considerably unstable and would dimerize in the
presence of DBU at room temperature. The structure of
H
H
H
Mes*
SiMe3
Ph
H
H
P
H
H
P
P
C
C
P
C
C
P
P
7
8
9
1aSi
Chart 2.

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S. Ito et al. / Tetrahedron 63 (2007) 10246–10252
Figure 1. Computationally characterized head-to-tail [3+2] dimerization procedures of phosphaallene 7 affording 8 (B3LYP/aug-cc-pVTZ). Values in bold face
correspond to relative energies (kcal/mol). Values in parentheses display Gibbs free energies (kcal/mol at 298.15 K). Values in square brackets show dipole
moments (Debye).
the H atom on phosphorus of molecule A (H1) and the termi-
nal CH2 group of molecule B (C4, H5, H6), which prevents
facile cyclization to the five-membered ring in spite of
simultaneous dual P,C bond formation. Figure 2b displays
the Mulliken charges of T17–8 together with that of 7. The
terminal P and C atoms of molecule A are more negative
and the migrating H atom shows increased positive charge,
whereas molecule B shows a polarized structure [HPꢀ–
C+]CH2] in comparison with 7. Such polarized structure
of phosphaallene was suggested in previously reported com-
putations.15 The calculation for the reaction from 7 to 8
indicates that conformational changes from intermediary
T27–8 to T37–8 including p-electron delocalization are re-
quired. Figure 3 shows the calculated reaction pathways
from 7 to 9. In contrast to the case of the reaction from 7
to 8, no intermediates such as T27–8 were obtained for the
change from T17–9 to 9. However, the parameters of T17–9
are comparable with T17–8 (the imaginary frequency mode
of both with T17–8 and with T17–9 is mainly the [1,2]-H
shift), indicating non-permitted overlap between the central
C atoms to separate the left P]C and the right C]C moie-
ties. Nevertheless, in the case of T17–9, the simultaneous P,C
bond formation to 9 would not be prevented due to the ab-
sence of the steric repulsion discussed on the structure of
T17–8. Thus, orientation of the substituents around the
P]C]C moiety would control the dimerization mecha-
nism leading to the selective formation of 8 or 9. The high
potentials of T17–8 and T17–9 might be reduced in the
presence of bases facilitating the [1,2]-H migration. Those
activation energies for the [3+2] dimerization might be
overcome by the presence of a base to accelerate the
polarized structure, as shown in Figure 2. Furthermore,
phosphaallenes bearing an electron-withdrawing aryl
substituent would easily dimerize to the corresponding
1,4-diphosphafulvene, and indeed isolation of phospha-
allene 1b in the presence of DBU was practically impossible
(Scheme 2).16
spectrum, the absorptions of 2b show considerable red shifts
compared with 2a (256, 417 nm)4, probably due to the re-
duction of the HOMO–LUMO gap. Cyclic voltammetric
analyses of 2b, on the other hand, indicate lower stability
of the generating cationic species [Eopx +0.67 V, +1.30 V
(sh), +1.58 V] (Fig. 4), whereas 2a shows a reversible oxida-
tion potential (E1/2+0.53 V).4 In the cathodic processes, the
reduction potentials of 2b [Erped ꢀ0.63 V (sh), ꢀ0.77 V] are
comparable to 2a [Erped ꢀ0.68 V]. Despite the reduced
electron-donating property, 2b and TCNQ formed
a charge-transfer complex that was confirmed by qualitative
observation of TCNQ anion radical (see Supplementary
data). Effects of the cyano groups of 2b on the 1,4-diphos-
phafulvene moiety are too complicated to characterize the
photo- and electrochemical properties precisely, and there-
fore more detailed analyses are required to clarify the
substituent effects.
2.4. Preparation of novel DPCB derivatives
carrying cyanophenyl groups
3,4-Diphosphinidenecyclobutenes (DPCBs) 3 are alterna-
tive intriguing dimeric structures of phosphaallenes with
loss of two H atoms. This dehydrogenative homocoupling
of 1b would provide the corresponding 1,2-bis(4-cyano-
phenyl)-3,4-diphosphinidenecyclobutene, but unfortunately
1b was too unstable to isolate under the conditions in
Scheme 2. Therefore, we examined an alternative synthesis
of the CN-bearing DPCB derivative according to our recent
procedures starting from 4.10,11
Dibromophosphaethene 4 was allowed to react with butyl-
lithium, 4-cyanobenzaldehyde, and chlorotrimethylsilane
successively to afford the 2-bromo-1-phosphapropene 10b
in a moderate yield. According to the case of 10a,11 10b
was subsequently treated with butyllithium and 1,2-dibro-
moethane, and the corresponding DPCB 3b was obtained
as yellow crystals. The 31P NMR data of 3b showed
a lower-field chemical shift (dP 187) compared with 3a (dP
170), due to the presence of the cyano groups (Scheme 3).
Other NMR spectroscopic data of 3b confirm the
DPCB structure. On the other hand, mass spectra and ele-
mental analysis of 3b indicate inclusion of oxygen in the
crystalline state, although attempted X-ray crystallography
2.3. Physicochemical properties of 1,4-diphospha-
fulvenes
Although novel 1,4-diphosphafulvene 2b exhibits similar
AB signals to 2a in 31P NMR, several different physico-
chemical properties were observed. In the UV absorption

S. Ito et al. / Tetrahedron 63 (2007) 10246–10252
10249
Figure 3. Calculated intermediate for head-to-tail [3+2] dimerization of 7
affording 9 (B3LYP/aug-cc-pVTZ). Values in bold face correspond to rela-
tive energies (kcal/mol). Values in parentheses display Gibbs free energies
(kcal/mol at 298.15 K). Values in square brackets show dipole moments
(Debye).
3. Conclusion
A 3-(4-cyanophenyl)-1-phosphaallene 1b, an intermediary
generated from a 2-bromo-1-phosphapropene 6b, showed
facile [3+2] dimerization in the presence of DBU affording
2b. This is in sharp contrast to the case of 1a, which was suc-
cessfully prepared from 6a and DBU. The DFT calculations
characterize several plausible intermediary structures and
pathways in the reaction from 1 to 2 and also indicate the cy-
ano group effect to facilitate the dimerization. The physical
properties of the cyano-containing 1,4-diphosphafulvene 2b
exhibited several different characteristics from those of 2a.
The formal dehydrogenative homocoupling product of 1b,
a 3,4-diphosphinidenecyclobutene 3b, was prepared accord-
ing to the recent DPCB synthetic procedures together with
the cyano-bearing phosphaallene 1b. Thus, from the effects
of the cyanophenyl groups, several intriguing properties of
phosphaallenes were derived in relation to the chemistry of
˚
Figure 2. (a) Optimized geometry of T17–8 (bond lengths: A; bond angles:
degree). (b) Mulliken charges of T17–8 and 7.
to investigate the O2 absorption has thus far been
unsuccessful. In addition to 3b, a small amount of the
cyanophenylphosphaallene 1b was obtained in Scheme 3,
indicating the decisive role of DBU to give the 1,4-
diphosphafulvene 2b in Scheme 2. In fact, a mixture of the
isolated 1b and DBU afforded 2b (see Section 4). Although
the detailed reaction mechanism from 10 to 3 has yet to be
clarified, it is plausible that the products in Scheme 3 do
not facilitate the [3+2] dimerization affording 1,4-diphos-
phafulvene 2. Thus, the findings described in Schemes 2
and 3 would provide suitable processes for obtaining the
[3+2] dimerization and the dehydrogenative homocoupling
of 1, respectively.
Figure 4. Cyclic voltammogram of 2b. Conditions: 1 mM in dichloro-
methane; supporting electrolyte: 0.1 M tetrabutylammonium perchlorate
(TBAP); working electrode: glassy carbon; counter electrode: platinum
wire; reference electrode: Ag/AgCl (E1/2 (ferrocene/ferricinium)¼+0.60 V)
at 20 ꢁC; scan rate: 50 mV sꢀ1
.

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S. Ito et al. / Tetrahedron 63 (2007) 10246–10252
R
P
R
P
Mes*
Br
Br
Mes*
Br
1) n-BuLi
1) n-BuLi
P
C
P C
2) 0.5 BrCH2CH2Br
2) RCHO
3) Me3SiCl
CH R
Mes*
Mes*
4
Me3SiO
10
3
a: R = Ph
b: R = 4-NCC6H4
a: R = Ph
b: R = 4-NCC6H4
Scheme 3.
1,4-diphosphafulvene and DPCB derivatives, including ap-
plication for electron-transfer processes and catalysis.
30 min. The mixture was allowed to warm up to room tem-
perature and the solvent was removed in vacuo. The residue
was extracted with hexane and the organic layer was purified
by column chromatography (SiO2, hexane) to afford 1a17
(173 mg, 78%).
4. Experimental section
4.1. General methods
4.4. Reaction of 6b with DBU
All reactions were carried out under an argon atmosphere by
means of the standard Schlenk techniques. All solvents em-
ployed were dried by appropriate methods. 1H, 13C{1H} and
31P{1H} NMR spectra were recorded on a Bruker Avance
400 spectrometer with Me4Si (1H, 13C) and H3PO4 (31P), re-
spectively, as internal or external standard. Mass spectra
were recorded on a Bruker APEX3 spectrometer. Electro-
chemical analyses were performed with a BAS-50W voltam-
metric analyzer. Compound 4 was prepared by the
procedures described in the literature.12
To a solution of 6b (64.6 mg, 0.133 mmol) in THF (5 mL)
was added DBU (0.140 mmol) at 0 ꢁC and the mixture
was stirred for 30 min. The mixture was allowed to warm
up to room temperature and the solvent was removed in va-
cuo. The residue was extracted with hexane and an aliquot of
the solution was monitored by 31P NMR to observe a signal
due to 1b [dP 80.2 (C6D6)] together with 2b in a 1:2 ratio.
The peak of 1b disappeared within 1 h. The organic layer
was purified by column chromatography (SiO2, hexane/
EtOAc 10:1) to afford 2b (21 mg, 38%). Orange-red powder
1
(hexane), mp 233–234 ꢁC (decomp.); H NMR (400 MHz,
4.2. Preparation of 6b
CDCl3) d 1.31 (9H, s, p-t-Bu), 1.31 (9H, s, p-t-Bu), 1.54
(18H, s, o-t-Bu) 1.58 (18H, s, o-t-Bu), 6.40–6.44 (1H,
3
To a solution of 4 (400 mg, 0.893 mmol) in THF (25 mL)
was added butyllithium (0.90 mmol, 1.4 M solution in
hexane, 1 M¼1 mol dmꢀ3) at ꢀ78 ꢁC and after stirring
for 10 min 4-cycanobenzyl bromide (9.0 mmol) in THF
(5 mL) was added. After stirring for 15 min, the mixture
was allowed to warm up to room temperature and the solvent
was removed in vacuo. The residue was extracted with hex-
ane and the organic layer was purified by column chromato-
graphy (SiO2, hexane) to afford 6b (190 mg, 47%).
Colorless needles (EtOH), mp 114–116 ꢁC; 1H NMR
(400 MHz, CDCl3) d 1.33 (9H, s, p-t-Bu), 1.51 (18H, s,
m, CH), 6.44 (2H, d, JHH¼8.2 Hz, C6H4), 6.88 (1H,
2
3
dd, JPH¼37.8 Hz, JPH¼12.2 Hz, CH), 7.09 (2H, d,
3
3JHH¼8.2 Hz, C6H4), 7.14 (2H, d, JHH¼8.2 Hz, C6H4),
7.38 (2H, d, 3JHH¼8.2 Hz, C6H4), 7.45 (2H, d, 4JPH¼2.4 Hz,
4
Mes*), 7.51 (2H, d, JPH¼2.4 Hz, Mes*); 13C{1H} NMR
(101 MHz, CDCl3) d 31.5 (s, p-CMe3), 31.6 (s, p-CMe3),
33.8 (s, o-CMe3), 34.3 (d, JPC¼4.7 Hz, o-CMe3), 35.3 (s,
4
3
p-CMe3), 35.4 (s, p-CMe3), 40.0 (d, JPC¼4.3 Hz,
3
o-CMe3), 40.2 (d, JPC¼5.1 Hz, o-CMe3), 108.1 (s, CN),
110.6 (s, CN), 119.3 (s, Carom), 119.8 (s, Carom), 124.2 (s,
m-Mes*), 124.8 (s, m-Mes*), 125.7 (d, JPC¼57.0 Hz,
1
3
1
o-t-Bu), 4.23 (2H, d, JPH¼18.3 Hz, CH2), 7.41–7.67 (6H,
ipso-Mes*), 126.4 (d, JPC¼60.1 Hz, ipso-Mes*), 127.5
m, arom); 13C{1H} NMR (101 MHz, CDCl3) d 31.8 (s,
p-CMe3), 33.2 (d, 4JPC¼6.7 Hz, o-CMe3), 33.5 (s, p-CMe3),
(dd, JPC¼6.0, 2.9 Hz, CHarom), 127.8 (dd, JPC¼6.0 Hz,
4
3
4JPC¼2.9 Hz, CHarom), 130.0 (dd, 1JPC¼9.9 Hz,
2
1
38.3 (s, o-CMe3), 50.1 (d, JPC¼34.5 Hz, CH2), 119.3 (s,
2JPC¼4.4 Hz, ]C), 131.5 (dd, JPC¼46.2, 23.4 Hz, ]C),
CN), 122.6 (s, m-Mes*), 130.6 (s, p-C6H4), 132.1 (s,
m-C6H4), 132.5 (s, o-C6H4), 137.8 (d, JPC¼53.0 Hz,
131.8 (s, CHarom), 132.2 (s, CHarom), 140.6 (t, 2JPC¼3.5 Hz,
1
1
2
]CH), 142.2 (dd, JPC¼22.6 Hz, JPC¼10.3 Hz, ]CH),
152.5 (s, p-Mes*), 153.4 (s, p-Mes*), 159.7 (s, o-Mes*),
159.9 (s, o-Mes*), 159.9 (s, Carom), 160.0 (s, Carom);
31P{1H} NMR (162 MHz, CDCl3) d 54.5 (d, 2JPP¼35.2 Hz),
3
ipso-Mes*), 144.0 (d, JPC¼12.7 Hz, ipso-C6H4), 151.5 (s,
3
p-Mes*), 153.5 (d, JPC¼2.2 Hz, o-Mes*), 161.9 (d,
1JPC¼61.0 Hz, P]C); 31P{1H} NMR (162 MHz, CDCl3)
d 253.6; ESI-MS calcd for C27H3579BrNP+Na 506.1583,
found m/z 506.1582. Elemental analysis (%) calcd
for C27H35BrNP (484.45) C 66.94, H 7.28, N 2.89; found
C 66.78, H 7.23, N 2.95. A trace amount (<5%) of the
25.5 (d, JPP¼35.2 Hz); UV (CH2Cl2) lmax (3ꢂ10ꢀ3) 256
2
(33.6), 461 (4.7) nm; ESI-MS calcd for C54H68N2P2+Na
829.4750, found m/z 829.4747. Elemental analysis (%) calcd
for C54H68N2P2$H2O (825.09) C 78.61, H 8.55, N 3.40;
found C 78.71, H 8.52, N 3.42.
3
geometrical isomer of 6b (dP 249.2, JPH¼9.1 Hz) was
included.
4.5. Preparation of 10b
4.3. Reaction of 6a with DBU
To a solution of 4 (553 mg, 1.23 mmol) in THF (20 mL) was
added butyllithium (1.25 mmol) at ꢀ100 ꢁC and the mixture
was stirred for 15 min. The reaction mixture was mixed with
a solution of 4-cyanobenzaldehyde (1.25 mmol) in THF
To a solution of 6a (270 mg, 0.588 mmol) in THF (15 mL)
was added DBU (1,8-diazabicyclo[5.4.0]undec-7-ene,
0.882 mmol) at 0 ꢁC and the mixture was stirred for

S. Ito et al. / Tetrahedron 63 (2007) 10246–10252
10251
2
(7 mL) at ꢀ100 ꢁC and stirred for 15 min. After warming up
to room temperature, the mixture was treated with chloro-
trimethylsilane (1.30 mmol) and was stirred for 1 h. The
volatile materials were removed in vacuo and the residue
was extracted with dichloromethane. The organic layer
was concentrated and the residue was recrystallized from
hexane to afford 10b (392 mg, 56%). Colorless powder,
o-CMe3), 111.1 (s, CN), 112.0 (d, JPC¼10.6 Hz, ]CH),
119.3 (s, s, p-C6H4), 122.7 (s, m-Mes*), 128.8 (s, m-
C6H4), 129.5 (s, ipso-C6H4), 132.6 (s, o-C6H4), 139.4 (d,
1JPC¼11.1 Hz, ipso-Mes*), 150.7 (s, p-Mes*), 154.3 (d,
1
3JPC¼3.6 Hz, o-Mes*), 240.4 (d, JPC¼25.0 Hz, P]C);
31P{1H} NMR (162 MHz, CDCl3) d 78.5; ESI-MS calcd
for C27H34NP+Na 426.2321, found m/z 426.2320.
1
mp 151–153 ꢁC; H NMR (400 MHz, CDCl3) d 0.19 (9H,
s, SiMe3), 1.33 (9H, s, p-t-Bu), 1.39 (9H, s, o-t-Bu), 1.49
(9H, s, o-t-Bu), 5.74 (1H, d, 3JPH¼12.8 Hz, CH), 7.38 (1H,
4.7. Reaction of 1b with DBU
3
s, Mes*), 7.41 (1H, s, Mes*), 7.60 (2H, d, JHH¼8.4 Hz,
To a solution of 1b (10 mg, 25 mmol) in THF (3 mL) was
added DBU (30 mmol) and the mixture was concentrated
in vacuo. After 2 h, the residue was monitored by 31P
NMR to observe 2b solely.
3
C6H4), 7.62 (2H, d, JHH¼8.4 Hz, C6H4); 13C{1H} NMR
(101 MHz, CDCl3) d 0.52 (s, SiMe3), 31.7 (s, p-CMe3),
4
4
33.0 (d, JPC¼6.8 Hz, o-CMe3), 33.1 (d, JPC¼7.1 Hz,
o-CMe3), 35.4 (s, p-CMe3), 38.2 (s, o-CMe3), 38.4 (s,
o-CMe3), 79.8 (d, 2JPC¼38.5 Hz, CH), 111.7 (s, CN), 119.4
(s, Carom), 122.6 (s, m-Mes*), 127.8 (s, CHarom), 132.1 (s,
4.8. DFT calculations
1
CHarom), 136.5 (d, JPC¼54.1 Hz, ipso-Mes*), 147.6 (d,
Computations were performed with the Gaussian03 (revi-
sion C.02) quantum chemical program package.18 In the
DFT calculations, the functional of Becke3–Lee–Yang–
Parr (B3LYP)19,20 was employed. Geometry optimizations
and harmonic vibrational frequency calculations were car-
ried out with the B3LYP methods using aug-cc-pVTZ basis
set. The harmonic vibrational frequencies of all stationary
points were computed to characterize them as minima (all
frequencies are real) or transition states (TSs; only one imag-
inary frequency). The reaction pathways were verified by
IRC calculations.
3JPC¼11.1 Hz, Carom), 151.5 (s, p-Mes*), 153.4 (d,
2
2JPC¼2.3 Hz, o-Mes*), 153.9 (d, JPC¼2.3 Hz, o-Mes*),
1
165.8 (d, JPC¼64.0 Hz, P]C); 31P{1H} NMR (162 MHz,
CDCl3) d 258.1; ESI-MS calcd for C30H4379BrNPOSi+Na
594.1927, found m/z 594.1925. Elemental analysis (%) calcd
for C27H35BrNPOSi (572.63) C 62.92, H 7.57, N 2.45; found
C 63.14, H 7.41, N 2.45.
4.6. Preparation of 3b and 1b
To a solution of 10b (0.165 mg, 0.288 mmol) in THF
(10 mL) was added butyllithium (0.588 mmol) at ꢀ100 ꢁC
and the mixture was stirred for 15 min. Subsequently 1,2-di-
bromoethane (0.14 mmol) was added to the reaction mix-
ture. The mixture was allowed to warm up to room
temperature and stirred for 4 h. The volatile materials were
removed in vacuo and the residue was extracted with di-
chloromethane. The organic layer was concentrated in vacuo
and the residue was washed with hexane to give 3b (60 mg,
52%). The hexane solution was concentrated and the residue
was purified by column chromatography (SiO2, hexane/
EtOAc 100:1) to give 1b (14 mg, 12%). 3b: yellow powder
Acknowledgements
This work was supported in part by Grants-in-Aid for Scien-
tific Research (Nos. 13303039 and 14044012) from the Min-
istry of Education, Culture, Sports, Science and Technology,
Japan, and the Program Research, Center for Interdisciplin-
ary Research, Tohoku University. The authors would like to
express their gratitude to the crew for the support of super-
computers at the Institute for Material Research, Tohoku
University. One of the authors (S.I.) acknowledges the Sup-
port for Young Researchers, Graduate School of Science,
Tohoku University.
1
(hexane), mp 297–300 ꢁC (decomp.); H NMR (400 MHz,
CDCl3) d 1.40 (18H, s, p-t-Bu), 1.53 (36H, s, o-t-Bu), 6.56
(4H, d, JHH¼8.3 Hz, C6H4), 7.10 (4H, d, JHH¼8.3 Hz,
3
3
C6H4), 7.35 (4H, s, Mes*); 13C{1H} NMR (101 MHz,
4
CDCl3) d 32.0 (s, p-CMe3), 33.7 (d, JPC¼2.0 Hz, o-
Supplementary data
CMe3), 35.6 (s, p-CMe3), 38.7 (s, o-CMe3), 111.6 (s, CN),
118.8 (s, CHarom), 122.4 (s, m-Mes*), 128.7 (s, Carom),
Supplementary data associated with this article can be found
in the online version, at doi:10.1016/j.tet.2007.07.063.
1
131.9 (s, CHarom), 134.6 (d, JPC¼27.1 Hz, ipso-Mes*),
136.1 (s, Carom), 151.4 (s, p-Mes*), 153.3 (pt, (2JPC+3JPC)/
2¼6.8 Hz, C]C), 155.4 (s, o-Mes*), 174.7 (dd,
1JPC¼17.2, 8.7 Hz, P]C); 31P{1H} NMR (162 MHz,
CDCl3) d 187.4; EI-MS m/z (rel intensity) 804 (M+;
100%); ESI-MS calcd for C54H66P2+Na 827.4593, found
m/z 827.4591; calcd for C27H34NP+Na+4O 891.4390, found
m/z 891.4381. Elemental analysis (%) calcd for
C54H66N2P2+2O2 (869.06) C 74.63, H 7.65, N 3.22; found
C 74.18, H 7.90, N 3.40. 1b: Colorless solid, mp 140–
References and notes
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