The electronic effects of bulky aryl substituents on low coordinated phosphorus atoms in diphosphenes and phosphaalkenes by functionalization at the para position

Article abstract of DOI:10.1246/bcsj.78.1110

The electronic effects of bulky aryl substituents on low coordinated phosphorus atom(s) in diphosphenes and phosphaalkenes have been examined by means of functionalization at the para position of the substituents. Bulky bromobenzenes bearing an electron-d

Full text of DOI:10.1246/bcsj.78.1110

1110 Bull. Chem. Soc. Jpn., 78, 1110–1120 (2005)
ꢀ 2005 The Chemical Society of Japan
The Electronic Effects of Bulky Aryl Substituents
on Low Coordinated Phosphorus Atoms in Diphosphenes
and Phosphaalkenes by Functionalization at the Para Position
ꢀ
Subaru Kawasaki, Akitake Nakamura, Kozo Toyota, and Masaaki Yoshifuji
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba, Sendai 980-8578
Received October 29, 2004; E-mail: yoshifj@mail.tains.tohoku.ac.jp
The electronic effects of bulky aryl substituents on low coordinated phosphorus atom(s) in diphosphenes and phos-
phaalkenes have been examined by means of functionalization at the para position of the substituents. Bulky bromo-
benzenes bearing an electron-donating or electron-withdrawing group at the para position were prepared as precursors
of the substituents and some of them were applied to preparations of the corresponding diphosphenes and fluorenylidene-
(phenyl)phosphines bearing low-coordinated phosphorus atom(s). Electronic perturbation by the para-substituent in the
system was indicated by UV–vis spectra and 31P NMR chemical shifts. These observations can be attributed to effects
induced by electronic properties of the bulky aryl substituent(s) on the lone pair of the low-coordinated phosphorus atom.
Kinetic stabilization with bulky aryl substituents such as
drance around the reactive center, and the kinetic stabilizing
ability is expected to be conserved. In other words, the molec-
ular design of para-modification overcomes the problems with
ortho-modification. Furthermore, the electronic effect of the
para-substituent may be analyzed by Hammett values.9 It is
expected that the modified bulky aryl substituents will work
as useful tools for exploring reaction mechanisms by using
linear free energy relationships.10
ꢀ
2,4,6-tri-t-butylphenyl (abbreviated to Mes ),1 2,4,6-tris[bis-
(trimethylsilyl)methyl]phenyl,2 and meta-terphenyl ligand3
plays an important role in modern heavier main group element
chemistry.4 Kinetic stabilization has been successfully applied
to studies on multiple bonds between the heavier main group
elements: This stabilization method has been supposed not to
perturb the electronic character and is superior to thermody-
namic stabilization, at least for the purpose of studying the
genuine electronic character of such bonds.1,5
Results and Discussion
However, thermodynamic stabilization (or destabilization)
and tuning of reactivity and electronic character are still attrac-
tive from the viewpoint of applications in material science. For
example, if the electronic effect of the substituent is well un-
derstood, it may be possible to control the reactivity of the
multiple bonds. In addition, knowledge about the structure–
property relationships may become a useful guide for the
design of new materials.
Preparations of Bulky Bromobenzenes Bearing a Func-
tional Group at the Para Position. At first, we prepared
bulky bromobenzene derivatives 1a–g (Chart 1) as possible
precursors of low coordinated phosphorus compounds. Al-
though the synthesis of 1a starting from 2,6-di-t-butyl-4-nitro-
aniline was reported by Rundel in 1968,11 the route contained
some elaborate separation processes.12 Compound 1b was pre-
pared by direct bromination of 1,3,5-tri-t-butylbenzene with
bromine in trimethyl phosphate.13 As for the other bromoben-
zenes, however, similar bromination of other 1-functionalized
3,5-di-t-butylbenzenes such as 3,5-di-t-butylanisole or 3,5-di-
t-butylaniline gave undesired 2-bromo derivatives. Thus, we
developed an alternative synthetic method for 1a and 1c, start-
ing from 2a14 and 2c15 via compounds 3–5 (Scheme 1): Cyan-
ation of 2a,c gave 3a,c, which were methylated to give 4a,c.
Although some diphosphenes1 bearing an electron-donating
or an electron-withdrawing bulky substituent were reported,6
there has been no systematic study. On the other hand, we
and others have studied modifications of the bulky aryl sub-
stituents at the ortho position(s)7 and we applied the modified
bulky substituents to the studies of multiply bonded phospho-
rus compounds, focusing on intramolecular interactions. We
thus prepared various compounds, such as dithioxophos-
phoranes,7a,e,h diselenoxophosphoranes,7a,e,h and phospha-
alkenes.7i However, such modifications often led to decreases
in the degree of steric hindrance or insufficient stabilization
for systematic studies on electronic properties. In the present
t-Bu
R
Br
t-Bu
ꢀ
paper, we report on modifications of the bulky Mes group
as well as preparation of diphosphenes and phosphaalkenes
bearing the modified substituents: an electron-donating group
or electron-withdrawing group was introduced into the para
position while retaining both ortho-t-butyl groups.8
1a: R = H; b: R = t-Bu; c: R = OMe;
d: R = NMe2; e: R = CO2Me; f: R = CN;
g: R = I; h: R = Br; i: R = Li; j: R = CO2H;
k: R = CONH2; l: R = CF3
Such modifications do not decrease the degree of steric hin-
Chart 1.
Published on the web June 6, 2005; DOI 10.1246/bcsj.78.1110

S. Kawasaki et al.
Bull. Chem. Soc. Jpn., 78, No. 6 (2005) 1111
Br
Br
Br Br Br NC
Br CN NC
Br CN
t-Bu
t-Bu
ref.17
t-Bu
t-Bu
a
b
Br
1h
NMe2
1d
R
R
3a,c
R
4a,c
2a: R = H
c: R = OMe
a
CHO Br CHO
Br
Br
Br
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
c
d
d
e
f
R
R
Li
1i
I
5a,c
1a,c
1g
Scheme 1. Preparations of bulky bromobenzenes. a)
KCN, 18-crown-6, KI, MeCN; b) MeI, KOH, DMSO; c)
DIBAH, C6H6, then 10% H2SO4; d) H2NNH2 H2O,
b
Br
Br
ꢁ
t-Bu
t-Bu
t-Bu
t-Bu
KOH, triethylene glycol.
Reduction of 4a,c with DIBAH (diisobutylaluminium hydride)
formed 5a,c, which were then subjected to Wolff–Kishner
reduction (the Huang–Minlon method) to afford 1a,c.
A similar synthetic route to 1d was then investigated, start-
ing from 2-bromo-5-(dimethylamino)-1,3-dimethylbenzene
(6).16 However, an attempted bromination of 6 with NBS
failed to form 2-bromo-1,3-bis(bromomethyl)-5-(dimethyl-
amino)benzene. We thus prepared 1d from 1h (Scheme 2) as
previously reported.17
CO2H
1j
CO2Me
1e
c
Br
Br
t-Bu
t-Bu
t-Bu
t-Bu
CONH2
CN
1k
1f
Novel bromobenzenes 1e,f bearing electron-withdrawing
groups, as well as bromoiodobenzene 1g, were also prepared
from 1h17 (Scheme 2). Regioselective lithiation of 1h with bu-
tyllithium, followed by treatment with an excess amount of dry
ice, afforded carboxylic acid 1j in 70% yield. The compound
1j was then converted to the methyl ester 1e by reaction with
iodomethane in the presence of K2CO3. Moreover, 1j was con-
verted into amide 1k, which was then dehydrated with thionyl
chloride to give nitrile 1f.
Scheme 2. Preparations of bulky bromobenzenes. a) n-BuLi,
THF; b) CO2(s), then aq HCl; c) SOCl2, CHCl3, aq NH3;
d) I2; e) MeI, K2CO3, DMF; f) SOCl2.
t-Bu
R
t-Bu
1a,c,d
P
P
t-Bu
Attempted preparation of 2-bromo-1,3-di-t-butyl-5-(trifluo-
romethyl)benzene (1l) by using trialkyl(trifluoromethyl)silane
was unsuccessful. Compound 1g was subjected to Fuchikami’s
conditions;18 however, the desired compound was not obtained
in good yield.
a,b
R
t-Bu
8c,d
(8aref.12
)
c
t-Bu
R
PCl2
Preparations of Diphosphenes and Fluorenylidene-
(phenyl)phosphines. As some para-functionalized bulky bro-
mobenzenes were obtained, we applied these substituents to
diphosphenes as steric protecting groups (Scheme 3). Lithia-
tion of bromobenzenes 1a,c,d with butyllithium followed by
reaction with PCl3 gave dichlorophosphines 7a,12 7c, and
7d.17 Dichlorophosphines 7c [31P NMR (THF–C6D6) ꢀP
153.3] and 7d were treated with Mg turnings in THF. para-
Methoxy substituted diphosphene 8c was obtained in 12%
yield. Although the formation of 8d was observed by 31P NMR
spectroscopy [ꢀP (CDCl3) 500.6], it turned out to be air and
moisture sensitive and an attempted isolation of 8d was unsuc-
cessful. Unfortunately, attempted conversion of bromoben-
zenes 1e,f, bearing electron-withdrawing groups to the corre-
sponding diphosphenes 8e,f failed, due to unsuccessful prepa-
rations of dichlorophosphines 7e,f, respectively.19
t-Bu
t-Bu
d,e
P
7a: R = H
c: R = OMe
d: R = NMe2
R
t-Bu
9a,c,d
Scheme 3. Syntheses of 8c,d and 9a,c,d. a) n-BuLi, THF; b)
PCl3; c) Mg, THF; d) 9-trimethylsilyl-9-fluorenyllithium,
THF; e) TBAF, THF.
dation of the properties of fluorenylidene(phenyl)phosphines.
Reactions of 7c,d with 9-trimethylsilyl-9-fluorenyllithium,
followed by treatment with tetrabutylammonium fluoride
(TBAF), gave phosphaalkenes 9c,d. Compound 9a was also
obtained by a similar method, whereas 9b was prepared ac-
cording to a method reported in the literature.20 Because at-
Our interest was then focused on the preparation and eluci-

1112 Bull. Chem. Soc. Jpn., 78, No. 6 (2005)
Functionalized Diphosphenes and Phosphaalkenes
tempted preparations of 7e,f were unsuccessful, as mentioned
above, we sought an alternative approach (i.e., quaternaliza-
tion of the amino group) for the introduction of an electron-
withdrawing group into fluorenylidene(phenyl)phosphines.
As shown in Scheme 4, 9d was allowed to react with MeI or
MeOTf in C6D6 to form the corresponding ammonium salts
singlet signals at ꢀ ¼ 4:22 (3.90) are assignable to the NþMe3
protons. It should be mentioned that methylation on phospho-
rus will lead to a doublet in the 1H NMR spectrum. The
31P NMR signal for 9m,n appeared in the normal phospha-
alkene region; this fact also indicates that the methylation
occurred on the nitrogen atom.
Structures of Diphosphenes and Fluorenylidene-
(phenyl)phosphines. Results of X-ray crystal analysis of
the diphosphene 8c have been reported in a previous commu-
nication.8a Structures of the fluorenylidene(phenyl)phosphines
9b–d were unambiguously determined by X-ray crystallogra-
phy (Figs. 1–3). Attempted analyses of compounds 9a,m,n
have been unsuccessful because of lack of crystals suitable
for X-ray analyses.
1
9m or 9n, respectively. In the H NMR spectra of 9m (9n),
MeI
or TfOMe
t-Bu
9d
P
X
t-Bu
NMe3
Tables 1 and 2 list some important bond lengths and angles
for 8b,c and 9b–d, respectively. The P=P bond length for 8c is
slightly longer than that for 8b. The P–Cipso bonds between the
9m: X = I
n: X = OTf
Scheme 4. Preparation of 9m,n.
ꢀ
two diphosphenes are close in value (1.860–1.869 A). The P–
P–C bond angles in 8c (av. 99.6ꢂ) are smaller than those in 8b
Fig. 1. Molecular structure of 9b showing the atomic label-
ing scheme with thermal ellipsoids (50% probability).
Fig. 3. Molecular structure of 9d showing the atomic label-
ing scheme with thermal ellipsoids (50% probability).
ꢀ
Table 1. Selected Bond Lengths (A), Bond Angles (deg),
and Dihedral Angles (deg) for Diphosphenes
R
t-Bu
C(7)
t-Bu
P(1) P(2) t-Bu
C(1)
t-Bu
8b: R = t-Bu
c: R = OMe
R
Length or Angle
8baÞ
8cbÞ
P(1)=P(2)
P(1)–C(1)
P(2)–C(7)
P(1)–P(2)–C(7)
P(2)–P(1)–C(1)
C(1)–P(1)–P(2)–C(7)
2.034(2)
1.862(2)
1.862(2)
102.8(1)
102.8(1)
172.2(1)
2.043(1)
1.860(4)
1.869(4)
98.7(1)
100.5(1)
165.0(2)
Fig. 2. Molecular structure of 9c showing the atomic label-
ing scheme with thermal ellipsoids (50% probability).
a) Data taken from Ref. 1a. b) Data taken from Ref. 8a.

S. Kawasaki et al.
Bull. Chem. Soc. Jpn., 78, No. 6 (2005) 1113
(102.8ꢂ). Although the deviation from planarity is relatively
large for the P=P ꢁ system in 8c [torsion angle for C–P=P–
C: 165.0(2)ꢂ], the P=C ꢁ systems in 9b,c are nearly planar
and 9d deviates from planarity only slightly [C(14)–P(1)–
C(1)–C(13): 175.0(4)ꢂ]. The P=C bond lengths for 9b–d are
31P NMR: the order of the ꢀP values is a (488.7) < b
(490.0) < c (495.9) < d (500.6) for diphosphenes and m
(238.3), n (238.8) < a (254.8) < b (256.5) < c (258.0) < d
(264.8) for the fluorenylidene(phenyl)phosphine series. In con-
trast, the effect of substituent on the 13C NMR chemical shift
for the P=C carbon is very small among compounds 9.
Figure 4 shows the relation between the 31P NMR chemical
shifts and the Hammett ꢂ values of the para-substituents. Both
compounds 8 and 9 show good correlation with similar corre-
lation factors.
ꢀ
similar within the difference of 0.008 A. Again the P–Cipso
bonds are very close in value among the three fluorenylidene-
(phenyl)phosphines and the C–P–C angle becomes slightly
smaller as the electron-donating ability of the para substituent
increases. The change in the C–P–C angles may indicate that
electron-donating groups decrease the degree of hybridization
of the phosphorus atom. As for the other bond lengths and an-
gles concerning the P=C ꢁ system, relations are unclear be-
tween the geometrical values and the electron-donating abili-
ties of the substituents.
Properties of Diphosphenes and Fluorenylidene(phenyl)-
phosphines. Table 3 shows 31P NMR and UV–vis data for
8a–d, 9a–d, and 9m,n. Increasing electron-donating ability
of the para-substituent causes a shift to lower field in
In the case of UV–vis spectra (Figs. 5–7), the electron-do-
nating substituents at the para position cause bathochromic
shifts of the absorption band of longest wavelength (compared
with the spectra of 8a and 9a), although red shifts are also ob-
served in the case of trimethylammonium derivatives 9m,n.
Contrary to this, absorption bands of shorter wavelengths are
less affected by the para substitution. The bands of longest
wavelength of diphosphenes are generally assigned to the
ꢀ
n ! ꢁ transition and the bands of longest wavelength for
ꢀ
The results indicate that the para substituents strongly affect
8a–c are also assignable to the n ! ꢁ transition (see below).
ꢀ
Table 2. Selected Bond Lengths (A), Bond Angles (deg),
and Dihedral Angles (deg) for Fluorenylidene(phenyl)-
phosphines
C(13)
t-Bu
P(1) C(1)
C(2)
t-Bu
C(14)
R
9b: R = t-Bu
c: R = OMe
d: R = NMe2
Length or Angle
9b
9c
9d
P(1)=C(1)
P(1)–C(14)
C(1)–C(2)
C(1)–C(13)
1.701(6)
1.832(8)
1.47(1)
1.693(4) 1.695(5)
1.833(4) 1.834(6)
1.483(6) 1.480(7)
1.484(10) 1.491(6) 1.475(7)
P(1)–C(1)–C(13)
P(1)–C(1)–C(2)
C(14)–P(1)–C(1)
C(14)–P(1)–C(1)–C(13) ꢃ177:9ð6Þ
118.1(6)
135.0(6)
105.0(4)
119.4(3) 119.9(4)
135.7(3) 134.9(4)
104.6(2) 103.9(2)
178.0(3) 175.0(4)
Fig. 4. Relations between 31P NMR chemical shift and
Hammett ꢂ value for compounds 8 and 9. R: correlation
factor.
Table 3. Selected 31P NMR (162 MHz, CDCl3), 13C NMR (100 MHz, CDCl3), and UV–Vis Data for 8a–d, 9a–d, and
9m,n
aÞ
Compd
ꢀP
ꢀC (1JPC/Hz)bÞ
ꢃ/nm (log ")
8a
8b
8c
8d
488.7cÞ
490.0eÞ
495.9
—
—
—
—
290 (4.03)c,dÞ
284 (4.19)d,fÞ
291 (4.21)dÞ
340 (3.85)
340 (3.89)
346 (3.75)
450 (3.00)
460 (3.13)
477 (3.12)
500.6
9a
9b
9c
9d
9m
9n
254.8
256.5hÞ
258.0
264.8
238.3
238.8iÞ
170.3 (42.2)
170.3 (42.7)
171.2 (43.2)
171.3 (44.2)
171.6 (42.3)
171.6 (42.3)iÞ
266 (4.40)gÞ
256 (4.30)gÞ
266 (4.42)gÞ
267 (4.59)gÞ
268 (4.41)dÞ
267 (4.43)dÞ
275 (4.45)
275 (4.32)
275 (4.46)
275 (4.61)
276 (4.44)
276 (4.47)
362 (4.25)
365 (4.05)
369 (4.30)
361 (4.27)
366 (4.21)
366 (4.24)
418 (sh, 3.52)
374 (4.28)
376 (4.21)
376 (4.25)
443 (3.25)
a) Relative to external 85% H3PO4. b) Signal due to the P=C carbon of compound 9. c) Data taken from Ref. 12.
d) Measured in CH2Cl2. e) Data taken from Ref. 21. f) Data taken from Ref. 1a. g) Measured in hexane. h) Data taken
from Ref. 6i, see also Ref. 20. i) Measured in CD2Cl2.

1114 Bull. Chem. Soc. Jpn., 78, No. 6 (2005)
Functionalized Diphosphenes and Phosphaalkenes
Fig. 7. UV–vis spectra for fluorenylidene(phenyl)phos-
phines 9a,d,m,n in hexane.
Fig. 5. UV–vis spectra for diphosphenes 8b,c in CH2Cl2.
of the effect depends on the contribution of the 3p atomic
orbital of the phosphorus atom in the nonbonding molecular
orbital.
A correlation between the chemical shift in NMR spectros-
ꢀ
copy and the n ! ꢁ absorption wavelength in UV–vis spec-
troscopy has been noted in some cases.23a In the cases of com-
pounds 8a–d and 9a–d, the wavelength of the absorption band
of longest wavelength correlates with the ꢀP value. This fact
indicates that the electron-donating groups at the para position
ꢀ
affect both the n ! ꢁ transition and the chemical shift. The
ꢀ
the energy level of the n orbital is increased by the electron-do-
observed red shift of the n ! ꢁ transition may indicate that
nating groups (see below) and that the energy gap decreases
between the n orbital and the ꢁ orbital, because the ꢁ
ꢀ
ꢀ
orbital containing the phosphorus atom does not seem to be
affected so much due to the geometrical reason mentioned
above. Any decrease in the energy gap leads to a low field shift
of ꢀ value of multinuclear NMR, because the energy gap ꢁE is
in inverse proportion to the deshielding paramagnetic term ꢂp
in the shielding constant ꢂ.23b,c
Fig. 6. UV–vis spectra for fluorenylidene(phenyl)phos-
phines 9a–d in hexane.
ꢀ
the n ! ꢁ transition of the diphosphenes 8, whereas the
ꢀ
effect to the ꢁ ! ꢁ is small. As for the absorption of
In contrast, amino group-substituted phosphaalkene Me2N–
P=C(SiMe3)Ph (ꢀP 248) shows a shift to higher field, com-
pared with MeO–P=C(SiMe3)Ph (ꢀP 305) or t-Bu–P=C-
(SiMe3)Ph (ꢀP 290 and 328, mixture of isomers).24 In this case,
it is likely that the directly bound heteroatom (nitrogen or oxy-
gen) affects the P=C ꢁ-system by interaction of the hetero-
fluorenylidene(phenyl)phosphines, Bickelhaupt et al. reported
the UV–vis spectra of (fluorenylidene)(mesityl)phosphine
or (fluorenylidene)(xylyl)phosphine; however, the bands of
ꢀ
longest wavelength have not been assigned to either n ! ꢁ
ꢀ
or ꢁ ! ꢁ .22a
ꢀ
We postulate that these absorption bands overlap each other
in the case of 9. The red shift observed in 9b–d (compared to
that of 9a) suggests, taking the results of 8 into account, that
atom lone pair with the P=C ꢁ orbital rather than with the
n orbital.
Theoretical calculations for compounds 9a,9c,9d [HF/6-
31+G(d) level] and model compounds 10a,c,d [where t-Bu
was replaced by Me (Chart 2); calculated at the B3LYP/6-
31G(d) level], were carried out in order to gain insight into
the molecular orbitals (Table 4). Because of the geometrical
requirement mentioned above, optimizations were done start-
ing from structures in which the phosphorus ꢁ bond and the
aromatic ꢁ-system of the bulky aryl substituent are nearly per-
pendicular. In the cases of 9a,c,d and 10a,c,d, the HOMO cor-
responds to a nonbonding orbital while the LUMO and the
ꢀ
the para substituents also affect the n ! ꢁ transition of 9,
ꢀ
whereas the effect to the ꢁ ! ꢁ transition is small. Due to
ꢀ
the red shift, in the case of 9d, the n ! ꢁ transition (ꢃmax
ꢀ
443 nm) is well separated from the ꢁ ! ꢁ transition. It
should be mentioned that the phosphorus ꢁ bond and the aro-
matic ꢁ-system of the bulky aryl substituent are nearly orthog-
onal due to the steric hindrance of the ortho t-butyl groups in
both compounds 8 and 9, making the effect of the para-sub-
ꢀ
stituent on the ꢁ ! ꢁ transition less effective. In other
ꢀ
words, the electron-donating group affects mainly the electron-
ic states of the phosphorus lone pair (n orbital) and the degree
HOMOꢃ1 correspond to ꢁ and ꢁ orbitals, respectively.
The energy level of the HOMO is increased by electron-donat-

S. Kawasaki et al.
Bull. Chem. Soc. Jpn., 78, No. 6 (2005) 1115
ꢀ
Table 4. Results of Calculation: Optimized P=C Bond Length (A), Selected MO levels (eV), LUMO–
HOMO Energy Gap (eV), and 31P NMR Chemical Shift (ꢀP) of 9aÞ and 10bÞ
cÞ
Compd
P=Ccalcd
LUMO
HOMO
HOMOꢃ1
ꢁLUMO{HOMO
ꢀP(calcd)
9a
9c
9d
10a
10c
10d
10o
10p
1.6674
1.6680
1.6683
1.7026
1.7032
1.7035
1.7044
1.7043
0.85
0.89
0.98
ꢃ2:34
ꢃ2:28
ꢃ2:17
ꢃ4:72
ꢃ4:68
ꢃ7:75
ꢃ7:70
ꢃ7:59
ꢃ5:83
ꢃ5:75
ꢃ5:06
ꢃ7:98
ꢃ7:95
ꢃ7:90
ꢃ7:87
ꢃ7:603
ꢃ5:87
ꢃ5:78
ꢃ5:67
ꢃ8:16
ꢃ8:11
8.60
8.59
8.57
3.49
3.47
2.89
3.26
3.27
269dÞ
279dÞ
263dÞ
270eÞ
272eÞ
269eÞ
199eÞ
198eÞ
a) Calculated at the HF/6-31+G(d) level. b) Calculated at the B3LYP/6-31G(d) level. c) Relative to
PMe3 (ꢀP ꢃ 62), calculated by the GIAO method. d) Calculated at the HF/6-31G(d) level. e) Calcu-
lated at the B3LYP/6-31+G(d,p) level.
ters and a structure–property relationship was demonstrated. It
was revealed that electron-donating groups at the para position
increase the reactivity of the low coordinated phosphorus com-
pounds, although the phosphorus ꢁ-bond and the aryl groups
are nearly orthogonal. Most of the previously reported sterical-
ly protecting groups are designed not to perturb the electronic
character of the multiple bonds of the heavier main group
elements; however, our present study revealed that well
designed bulky aryl substituents can protect the unstable bond
and change its electronic character and reactivity at the same
time. This basic understanding may become a useful guide
for new protecting groups and lead to the development of
new materials25 or catalysts.26 The results reported here will
lead to prediction and control of the properties and reactivity
of phosphaalkenes and diphosphenes, as well as other com-
pounds containing multiple bonds between the heavier main
group elements.
Me
P
R
Me
10a: R = H
c: R = OMe
d: R = NMe2
o: R = N+(H)Me2
p: R = N+Me3
Chart 2.
ing groups (9d > 9c > 9a). The calculated order of the
HOMO–LUMO gaps [9d < 9c < 9a] corresponds to the ten-
dency of the red-shift in the UV–vis spectra. GIAO calcula-
tions of 31P NMR chemical shifts for 9a,c,d and 10a,c,d
showed ꢀP to be in the 260–270 region, which is in relatively
good agreement with the experimental data, however, the sug-
gested order of the chemical shift [ꢀP (9c) (lower field) > ꢀP
(9a) > ꢀP (9d) (higher field)] is inconsistent with the experi-
mental results. Probably, the differences among the three com-
pounds are small and beyond the accuracy of the method. In
the cases of 10o and 10p (Chart 2) as models for 9m and
9n, respectively, the HOMO is a ꢁ orbital while the HOMOꢃ1
corresponds to a nonbonding orbital. The calculated HOMO–
LUMO gaps for 10o,p correspond to the experimentally
observed slightly red-shifted absorption band of the longest
wavelength in the UV–vis spectra for 9m,n (compared to
9a) and the calculated ꢀP corresponds to up-field shift in the
31P NMR spectra, although the effects of counter anion and
solvent were not taken into account in these calculations.
Experimental
Melting points were measured on a Yanagimoto MP-J3 micro
melting point apparatus and are uncorrected. NMR spectra were
recorded on a Bruker Avance-400 or a Bruker AM-600 spectrom-
eter. UV spectra were measured on a Hitachi U-3210 spectrome-
ter. IR spectra were obtained on a Horiba FT-300 spectrometer.
MS spectra were taken on either a JEOL HX-110 or a Hitachi
M-2500S spectrometer. FT-ICR-MS spectra were measured on a
Bruker APEX III spectrometer. X-ray diffraction data were col-
lected on a Rigaku R-AXIS IV diffractometer. Reactions were
performed under an argon atmosphere, unless otherwise specified.
2-Bromo-1,3-bis(cyanomethyl)benzene (3a). To a solution
of 2a (10.5 g, 30.6 mmol) in MeCN (170 mL) were added 18-
crown-6 (2.0 g, 7.63 mmol), KCN (5.76 g, 88.5 mmol), H2O
(20 mL), and KI (0.3 g, 1.8 mmol) at room temperature. After be-
ing stirred for 24 h, the reaction mixture was diluted with water
and extracted with EtOAc. The organic extracts were washed with
brine, and then dried (MgSO4) and concentrated. The residue was
purified by column chromatography (silica gel, chloroform) to af-
ford 3a (6.94 g, 96% yield): Colorless solid, mp 102–104 ꢂC;
1H NMR (400 MHz, CDCl3) ꢀ 7.56–7.45 (m, 3H), 3.88 (s, 4H);
13C{1H} NMR (100 MHz, CDCl3) ꢀ 132.0 (o-arom.), 130.2 (m-ar-
om.), 129.0 (p-arom.), 125.4 (Br–C), 117.2 (CN), 26.0 (CH2); IR
(KBr) 3016, 3008, 2922, 2247, 1462, 1429, 1402, 1030, 766
cmꢃ1; MS (EI, 70 eV) m=z (rel intensity) 236 (Mþ + 2; 58),
234 (Mþ; 60), 209 (Mþ ꢃ CN + 1; 48), 207 (Mþ ꢃ CN ꢃ 1;
Conclusion
In summary, we have developed synthetic routes to some
bulky bromobenzenes bearing electron-donating substituents
or electron-withdrawing substituents at the para position.
Some low-coordinated phosphorus compounds bearing such
substituents were prepared and the para-substituent effect
was evaluated by NMR and UV–vis spectroscopies as well
as by theoretical calculations. The electronic effects of the
para-substituent were evaluated in terms of Hammett parame-

1116 Bull. Chem. Soc. Jpn., 78, No. 6 (2005)
Functionalized Diphosphenes and Phosphaalkenes
49), 155 (Mþ ꢃ Br; 100), and 128 (Mþ ꢃ Br ꢃ CN ꢃ 1; 48).
Found: m=z 233.9799. Calcd for C10H7BrN2: M, 233.9792.
Found: C, 50.86; H, 3.13; Br, 34.04; N, 11.64%. Calcd for
C10H7BrN2: C, 51.09; H, 3.00; Br, 33.99; N, 11.92%.
2-Bromo-1,3-bis(1-cyano-1-methylethyl)-5-methoxybenzene
(4c). Compound 3c (5.33 g, 20.1 mmol) was allowed to react with
MeI in the presence of KOH to give 4c (6.3 g, quant.): Colorless
crystals, mp 149.5–151 ꢂC; 1H NMR (400 MHz, CDCl3) ꢀ 7.05 (s,
2H, arom.), 3.87 (s, 3H, OMe), 1.95 (s, 12H, Me); 13C{1H} NMR
(100 MHz, CDCl3) ꢀ 159.2, 142.2, 123.7, 114.8, 113.6, 56.0, 38.6,
28.7; IR (KBr) 2991, 2944, 2844, 2231, 1710, 1589, 1462, 1403,
1296, 1251, 1211, 1186, 1051, 1020, 877, 849, 731 cmꢃ1; MS
(EI, 70 eV) m=z 322 (Mþ + 2; 99), 320 (Mþ; 100), 280 (Mþ
ꢃ Me ꢃ CN + 1; 20), 278 (Mþ ꢃ Me ꢃ CN ꢃ 1; 20), 252
(Mþ ꢃ CMe2CN; 18), and 250 (Mþ ꢃ CMe2CN ꢃ 2; 17). Found
m=z 320.0520. Calcd for C15H17BrN2O: 320.0524.
2-Bromo-1,3-bis(1-cyano-1-methylethyl)benzene (4a). To a
solution of 3a (11.3 g, 48 mmol) in DMSO (155 mL) was added
KOH (37.5 g, 0.66 mol) with cooling using an ice bath. After 15
ꢂ
min of stirring at 0 C, MeI (24 mL, 0.38 mol) was added to the
reaction mixture. The reaction mixture was allowed to warm to
ambient temperature, and then was stirred for 40 h. When the re-
action mixture was diluted with Et2O, precipitates formed; they
were collected by filtration. The filtrate was washed with water
and brine, combined with a solution of the precipitates dissolved
in EtOAc, and then dried (MgSO4) and concentrated. The residue
was purified by column chromatography (silica gel, hexane–
EtOAc, 4ꢂ:1) to afford 4a (9.7 g, 69% yield): Colorless solid, mp
155–157 C; 1H NMR (400 MHz, CDCl3) ꢀ 7.52–7.39 (m, 3H),
1.95 (s, 12H); 13C{1H} NMR (100 MHz, CDCl3) ꢀ 141.1 (o-
arom.), 128.6 (p-arom.), 127.6 (m-arom.), 124.9 (Br–C), 123.8
(CN), 38.3 (CMe2CN), 28.9 (Me); IR (KBr) 2993, 2231, 1475,
1456, 1396, 1255, 1155, 1022, 796, 727 cmꢃ1; MS (EI, 70 eV)
m=z 292 (Mþ + 2; 79), 290 (Mþ; 80), 277 (Mþ ꢃ Me + 2;
24), 275 (Mþ ꢃ Me; 24), 250 (Mþ ꢃ Me ꢃ CN + H + 2; 98),
and 248 (Mþ ꢃ Me ꢃ CN + H; 100). Found: m=z 290.0421.
Calcd for C14H15BrN2: M, 290.0419. Found: C, 57.27; H, 5.20;
N, 9.41%. Calcd for C14H15BrN2: C, 57.75; H, 5.19; N, 9.62%.
2-Bromo-1,3-bis(1-formyl-1-methylethyl)benzene (5a). Un-
der an argon atmosphere, to a solution of 4a (9.686 g, 33.3 mmol)
in dry benzene (420 mL) was added a solution of 83.7 mmol of
DIBAH (0.93 M solution in hexane) at 0 ꢂC. The reaction mixture
was then allowed to warm to ambient temperature and was stirred
for 2 h. The reaction mixture was quenched carefully with water;
then 10% sulfuric acid was added to it and the resulting mixture
was stirred for 2 h. The mixture was worked up using benzene
and brine. The organic phase was dried (MgSO4) and concentrat-
ed. The residue was purified by column chromatography (silica
gel, CHCl3) to provide 5a (9.088 g, 92% yield): Colorless solid;
1H NMR (400 MHz, CDCl3) ꢀ 9.78 (s, 2H, CHO), 7.49–7.42
(m, 3H), 1.55 (s, 12H); 13C{1H} NMR (100 MHz, CDCl3) ꢀ
203.3 (CHO), 144.9 (o-arom.), 128.7 (p-arom.), 128.7 (m-arom.),
125.4 (Br–C), 53.2 (CMe2CHO), 24.3 (Me); IR (KBr) 2981, 2976,
2945, 2931, 1707, 1456, 1390, 1367, 1255, 1201, 1159, 1101,
1011, 845, 798, 725 cmꢃ1. Weak molecular ion peak in EI-MS.
2-Bromo-1,3-di-t-butylbenzene (1a). A mixture of 5a (482.2
mg, 1.623 mmol), triethylene glycol (40 mL), KOH (551 mg), wa-
ter (3.8 mL), and hydrazine monohydrate (1.6 mL) was heated at
2-Bromo-1,3-bis(1-formyl-1-methylethyl)-5-methoxybenzene
(5c). Compound 4c (6.0 g, 18.8 mmol) was allowed to react with
DIBAH to give crude 5c. The crude product was purified by flash
column chromatography (silica gel, hexane–EtOAc, 3:1) to fur-
nish 5c (6.1 g, quant.): Colorless crystals, mp 98–100 ꢂC; 1H NMR
(400 MHz, CDCl3) ꢀ 9.78 (s, 2H, CHO), 6.99 (s, 2H, arom.), 3.88
(s, 3H, OMe), 1.54 (s, 12H, Me); 13C{1H} NMR (100 MHz,
CDCl3) ꢀ 203.0 (CHO), 159.6, 145.9, 115.7, 114.5, 55.9, 53.2,
24.3; IR (KBr) 2979, 2943, 2881, 2803, 2707, 1710, 1587,
1463, 1400, 1298, 1213, 1126, 1053, 1022, 897, 849, 727 cmꢃ1
;
MS (EI, 70 eV) m=z 328 (Mþ + 2; 6), 326 (Mþ; 6), 299
(Mþ ꢃ CHO + 1; 3), 297 (Mþ ꢃ CHO; 3), and 247 (Mþ ꢃ Br;
100). Found m=z 326.0521. Calcd for C15H19BrO3: 326.0518.
2-Bromo-1,3-di-t-butyl-5-methoxybenzene (1c). Compound
5c (2.0 g, 6.1 mmol) was allowed to react with hydrazine mono-
hydrate in the presence of KOH to give 1c (873 mg, 48% yield):
Colorless crystals, mp 59–60.5 ꢂC; 1H NMR (400 MHz, CDCl3) ꢀ
7.02 (s, 2H, arom.), 3.85 (s, 3H, OMe), 1.63 (s, 18H, t-Bu);
13C{1H} NMR (100 MHz, CDCl3) ꢀ 157.9 (MeO–C), 151.0 (o-ar-
om.), 115.9 (Br–C), 112.7 (m-arom.), 55.5 (OMe), 38.7 (CMe3),
31.2 (CMe3); IR (KBr) 2935, 2869, 2827, 1587, 1458, 1390,
1288, 1217, 1066, 1010, 869, 847 cmꢃ1; MS (EI, 70 eV) m=z
300 (Mþ + 2; 99), 298 (Mþ; 100), 285 (Mþ ꢃ Me + 2; 32),
283 (Mþ ꢃ Me; 33), and 57 (t-Buþ; 52). Found m=z 298.0934.
Calcd for C15H23BrO: 298.0932.
4-Bromo-3,5-di-t-butylbenzoic Acid (1j). Under an argon
atmosphere, to a solution of 1h (1.04 g, 3 mmol) in THF (10
mL) was added 3.2 mmol of n-BuLi (1.6 M solution in hexane)
at ꢃ78 ꢂC. The mixture was added dropwise to an excess amount
of crushed dry ice via a cannula. The cooling bath was removed
and the solution was allowed to warm to room temperature. After
stirring for an additional 30 min, the reaction was quenched by the
addition of 1 M hydrochloric acid and the mixture was extracted
with EtOAc. The organic phase was washed with water and brine,
dried (Na2SO4), and concentrated. The residue was purified by
flash column chromatography (silica gel, hexane–EtOAc, 10:1
to 2:1) to give 1j (656 mg, 70% yield): Colorless solid, mp
ꢂ
130 C for 2 h. The flask was then equipped with a Dean–Stark
ꢂ
trap and the reaction mixture was heated at 200 C for 4 h. After
being cooled to room temperature, the mixture was worked up us-
ing Et2O and brine. The organic layer was dried (MgSO4) and
concentrated. Removal of the solvent in vacuo afforded 1a11,12
(307.0 mg, 70% yield).
ꢂ
1
150–152 C; H NMR (400 MHz, CDCl3) ꢀ 11.8 (brs, 1H), 7.56
(s, 2H), 6.87 (s, 2H), 1.48 (s, 18H); 13C{1H} NMR (100 MHz,
CDCl3) ꢀ 179.5, 149.6, 130.0, 128.7, 124.1, 37.4, 32.4; IR
(KBr) 3456, 3113, 3089, 3026, 2983, 2960, 1700, 1568, 1394,
1369, 1286, 1253, 1205, 1083, 933, 860, 796 cmꢃ1; MS (EI, 70
eV) m=z 314 (Mþ + 2; 8), 312 (Mþ; 9), 299 (Mþ ꢃ Me + 2;
34), 297 (Mþ ꢃ Me; 35), 281 (Mþ ꢃ OH ꢃ O + 2; 72),
279 (Mþ ꢃ OH ꢃ O; 77), 253 (Mþ ꢃ CO2H ꢃ Me + 1; 81),
251 (Mþ ꢃ CO2H ꢃ Me ꢃ 1; 82), and 130 (Mþ ꢃ Br ꢃ CO2H
ꢃ t-Bu ꢃ 1; 100). Found m=z 312. 0713. Calcd for C15H21BrO2:
312.0725.
2-Bromo-1,3-bis(cyanomethyl)-5-methoxybenzene
(3c).
Similarly to 3a, 2c (26.8 g, 71.9 mmol) was allowed to react with
KCN to give 3c (12.0 g, 63% yield): Colorless crystals, mp 155–
ꢂ
1
157 C; H NMR (400 MHz, CDCl3) ꢀ 7.10 (s, 2H, arom.), 3.88
(s, 3H, OMe), 3.86 (s, 4H, benzyl); 13C{1H} NMR (100 MHz,
CDCl3) ꢀ 159.8 (MeO–C), 132.7 (m-arom.), 116.9, 115.9, 115.2,
56.2, 26.1; IR (KBr) 2944, 2840, 2252, 1585, 1457, 1428, 1324,
1261, 1191, 1163, 1064, 1020, 945, 926, 846 cmꢃ1; MS (EI, 70
eV) m=z 266 (Mþ + 2; 98) and 264 (Mþ; 100). Found m=z
263.9905. Calcd for C11H9BrN2O: 263.9898.
Methyl 4-Bromo-3,5-di-t-butylbenzoate (1e). To a solution
of 1j (217 mg, 0.7 mmol) in DMF (2 mL) were added K2CO3

S. Kawasaki et al.
Bull. Chem. Soc. Jpn., 78, No. 6 (2005) 1117
(117 mg, 0.84 mmol) and MeI (0.1 mL, 1.6 mmol). After being
stirred for 1.5 h, the reaction mixture was diluted with Et2O.
The combined organic layer was washed with water and brine.
The organic layer was dried (MgSO4) and concentrated. The res-
idue was purified by flash column chromatography (silica gel,
hexane–EtOAc, 10:1) to furnish 1e as a yellow viscous oil (157
ane) to give 854 mg (73% yield) of 1g: Colorless solid, mp 42–44
ꢂC; 1H NMR (400 MHz, CDCl3) ꢀ 7.51 (s, 2H), 1.68 (s, 18H);
13C{1H} NMR (100 MHz, CDCl3) ꢀ 154.8, 129.6, 123.2, 97.8,
39.8, 32.0; IR (KBr) 2962, 2871, 1556, 1479, 1396, 1371, 1240,
1197, 1116, 987, 865 cmꢃ1; MS (EI, 70 eV) m=z 396 (Mþ + 2;
45), 394 (Mþ; 45), 254 (Mþ ꢃ I ꢃ Me + 2; 23), 252 (Mþ
ꢃ I ꢃ Me; 23), 227 (Mþ ꢃ I ꢃ 3Me + 5; 49), 225 (Mþ
ꢃ I ꢃ 3Me + 3; 49), and 131 (Mþ ꢃ I ꢃ Br ꢃ t-Bu; 100).
Found m=z 393.9822. Calcd for C14H20BrI: 393.9791.
1
mg, 69% yield). 1e: H NMR (400 MHz, CDCl3) ꢀ 7.54 (s, 2H),
3.85 (s, 3H), 1.38 (s, 18H); 13C{1H} NMR (100 MHz, CDCl3) ꢀ
172.6, 149.9, 130.6, 128.6, 123.8, 52.0, 37.2, 32.2; IR (KBr)
2962, 2907, 1734, 1568, 1486, 1398, 1365, 1255, 1207, 1143,
1076, 865, 850, 788, 769 cmꢃ1; MS (EI, 70 eV) m=z 328
(Mþ + 2; 12), 326 (Mþ; 12), 313 (Mþ ꢃ Me + 2; 33), 311
(Mþ ꢃ Me; 34), 297 (Mþ ꢃ OMe + 2; 43), 295 (Mþ ꢃ OMe;
45), 281 (Mþ ꢃ OMe ꢃ O + 2; 98), 279 (Mþ ꢃ OMe ꢃ O;
100), 253 (Mþ ꢃ CO2Me ꢃ Me + 1; 81), and 251 (Mþ
ꢃ CO2Me ꢃ Me ꢃ 1; 67). Found m=z 326.0895. Calcd for
C16H23BrO2: 326.0881.
1,2-Bis(2,6-di-t-butyl-4-methoxyphenyl)diphosphene (8c).
Under an argon atmosphere, to a solution of 1c (866 mg, 2.91
mmol) in THF (8 mL) was added 2.99 mmol of n-BuLi (1.6 M
solution in hexane) at ꢃ78 ꢂC. After 1 min of stirring at ꢃ78
ꢂC, PCl3 (0.76 mL, 8.5 mmol) was added to the reaction mixture.
The coolingꢂbath was removed and the solution was allowed to
warm to 60 C. After 90 min of stirring at that temperature, vol-
atiles were removed under vacuum. The resulting crude dichloro-
phosphine 7c was used in the following reaction without further
purification. The residue was dissolved in THF (8 mL), and Mg
turnings (70 mg, 2.92 mmol) were added to the solution at 0
ꢂC. The resulting mixture was stirred for 1 h at room temperature.
The solution was diluted with hexane, filtered to remove inorganic
salts, and then concentrated. The residue was purified by flash col-
umn chromatography (silica gel, hexane–EtOAc, 100:1) to give a
mixture of compounds. The mixture was further purified by GPC
(toluene) ꢂto give 90 mg of 8c (12% yield): Orange prisms, mp
4-Bromo-3,5-di-t-butylbenzamide (1k). To a solution of 1j
(70 mg, 0.22 mmol) in CHCl3 (1 mL) was added SOCl2 (1
mL). After 2 h of stirring at 70 ꢂC, volatiles were removed in
vacuo. To the residue was added aqueous 30% NH3 (1 mL),
and the resulting mixture was stirred at ambient temperature for
6 h. The reaction mixture was extracted with CHCl3. The organic
layer was washed with water and brine. The organic layer was
dried (MgSO4) and concentrated. The residue was purified by
flash column chromatography (silica gel, hexane–EtOAc, 10:1
to 1:2) to furnish 1k (26.2 mg, 38% yield): Colorless crystals,
mp 128–130 ꢂC; 1H NMR (400 MHz, CDCl3) ꢀ 7.54 (s, 2H),
6.24 (s, 1H), 5.74 (s, 1H), 1.49 (s, 18H); 13C{1H} NMR (100
MHz, CDCl3) ꢀ 175.2, 150.0, 133.1, 129.0, 123.6, 37.7, 32.8;
IR (KBr) 2956, 2929, 1643, 1566, 1469, 1369, 1251, 1095,
1021, 1014, 931, 891, 863, 791, 758, 702, 699 cmꢃ1; MS (EI,
70 eV) m=z 313 (Mþ + 2; 69), 311 (Mþ; 69), 296 (Mþ
ꢃ NH2 + 1; 63), 294 (Mþ ꢃ NH2 ꢃ 1; 48), 281 (Mþ ꢃ NH2
ꢃ O + 2; 100), 279 (Mþ ꢃ NH2 ꢃ O; 96), 254 (Mþ ꢃ t-Bu;
78), and 252 (Mþ ꢃ t-Bu ꢃ 2; 78). Found m=z 311.0874. Calcd
for C15H22BrNO: 311.0885.
4-Bromo-3,5-di-t-butylbenzonitrile (1f). A solution of 1k
(26.2 mg, 84ꢂmmol) in SOCl2 (0.8 mL) was stirred for 3 h in an
oil bath (75 C). Volatiles were removed in vacuo, and then the
residue was purified by flash column chromatography (silica gel,
hexane–EtOAc, 10:1) to furnish 1f (25.3 mg, quant.): Yellow
plates, mp 59–61 ꢂC; 1H NMR (400 MHz, CDCl3) ꢀ 7.53 (s,
2H), 1.57 (s, 18H); 13C{1H} NMR (100 MHz, CDCl3) ꢀ 158.3,
128.6, 128.0, 120.9, 108.7, 32.2, 30.5; IR (KBr) 2960, 2863,
2222, 1732, 1564, 1475, 1404, 1367, 1255, 1207, 1124, 1076,
1022, 931, 862, 798 cmꢃ1; MS (EI, 70 eV) m=z 295 (Mþ + 2;
32), 293 (Mþ; 23), 280 (Mþ ꢃ Me + 2; 35), 278 (Mþ ꢃ Me;
34), 238 (Mþ ꢃ t-Bu + 2; 19), 236 (Mþ ꢃ t-Bu; 18), and 182
(Mþ ꢃ Br ꢃ 2Me ꢃ 2; 100). Found m=z 293.0780. Calcd for
C15H20BrN: 293.0779.
1
165–169 C (decomp.); H NMR (400 MHz, CD2Cl2) ꢀ 7.14 (s,
4H), 3.56 (s, 6H), 1.41 (s, 36H); 13C{1H} NMR (100 MHz,
CD2Cl2) ꢀ 159.3, 152.6, 115.2, 108.6, 55.4, 35.3, 31.6;
31P{1H} NMR (162 MHz, CDCl3) ꢀ 495.9; IR (KBr) 2969,
2911, 2864, 1585, 1552, 1462, 1411, 1386, 1286, 1218, 1195,
1062, 931, 860, 842, 741 cmꢃ1; UV (hexane) ꢃmax nm (log ")
290 (4.21), 347 (3.74), 478 (3.10); MS (EI, 70 eV) m=z 500
(Mþ; 16), 251 (ArylPþ + 1; 84), and 249 (ArylPþ ꢃ 1; 100).
Found m=z 500.3000. Calcd for C30H46O2P2: 500.2973.
1,2-Bis[2,6-di-t-butyl-4-(dimethylamino)phenyl]diphosphene
(8d). Compound 1d (634 mg, 2.03 mmol) was converted to
7d, which was allowed to react with magnesium turnings to give
crude 8d. Attempted isolation of 8d was unsuccessful:
31P{1H} NMR (162 MHz, CDCl3) ꢀ 500.6.
Typical Procedure for the Preparation of 9. Under an argon
atmosphere, to a solution of 1 (1 mmol) in THF (2 mL) was added
1.06 mmol of n-BuLi (1.6 M solution in hexane) at ꢃ78 ꢂC. After
this mixture was stirred for 1 min at the temperature, PCl3 (0.25
mL, 2.9 mmol) was added to it. The cooling bath was then re-
moved and the solution was slowly warmed to 60 ꢂC. After 90
min of stirring at that temperature, volatiles were removed under
vacuum. The resulting crude dichlorophosphine was used for the
next reaction without further purification. A THF solution of 9-tri-
methylsilyl-9-fluorenyllithium was prepared at ꢃ78 ꢂC by the ad-
dition of 1.06 mmol of n-BuLi (1.6 M solution in hexane) to a so-
lution of 9-(trimethylsilyl)fluorene (238 mg, 1 mmol) in THF (2
mL). This solution was added dropwise to the solution of the di-
2-Bromo-1,3-di-t-butyl-5-iodobenzene (1g). Under an argon
atmosphere, to a solution of 1h (1.04 g, 3.0 mmol) in THF (6 mL)
was added 3.0 mmol of n-BuLi (1.6 M solution in hexane) at ꢃ78
ꢂC. The mixture was added dropwise to a solution of I2 (2.0 g, 7.9
mmol) in THF (2 mL). The cooling bath was removed and the so-
lution was warmed to room temperature. After being stirred for an
additional 30 min, the reaction mixture was quenched with satu-
rated aqueous NaHCO3 and Na2S2O3. The reaction mixture was
diluted with hexane, and washed with water and brine. The organ-
ic layer was dried (MgSO4) and concentrated in vacuo. The resi-
due was purified by flash column chromatography (silica gel, hex-
ꢂ
chlorophosphine at ꢃ78 C. The reaction mixture was allowed to
warm to room temperature, and then 3 mmol of TBAF (1 M solu-
tion in THF) were added. After 1.5 h of stirring, the reaction mix-
ture was diluted with EtOAc, and then washed with water and
brine. The organic layer was dried (MgSO4) and concentrated.
The residue was purified by flash column chromatography (silica
gel, hexane–EtOAc) to give 9.
9a (86% yield): Yellow powder, mp 162–164 ꢂC; 1H NMR
(400 MHz, CDCl3) ꢀ 8.28 (dd, 1H, J ¼ 7:1, 4.4 Hz), 7.64 (d,

1118 Bull. Chem. Soc. Jpn., 78, No. 6 (2005)
Functionalized Diphosphenes and Phosphaalkenes
1H, J ¼ 7:0 Hz), 7.58–7.53 (3H, m), 7.47 (dd, 1H, J ¼ 9:0, 6.7
Hz), 7.37–7.29 (m, 2H), 7.17 (t, 1H, J ¼ 7:4 Hz), 6.78 (dd, 1H,
J ¼ 7:5, 7.5 Hz), 5.29 (dd, 1H, J ¼ 7:7, 2.7 Hz), 1.46 (s, 18H);
13C{1H} NMR (100 MHz, CDCl3) ꢀ 170.3 (d, J ¼ 42:2 Hz,
P=C), 155.2, 143.3 (d, J ¼ 26:8 Hz), 139.8 (d, J ¼ 10:0 Hz),
138.8 (d, J ¼ 10:0 Hz), 138.5 (d, J ¼ 9:2 Hz), 138.4 (d, J ¼
14:5 Hz), 129.2, 128.6 (d, J ¼ 6:9 Hz), 128.5 (d, J ¼ 6:2 Hz),
127.3, 127.3 (d, J ¼ 6:9 Hz), 126.7 (d, J ¼ 7:6 Hz), 125.9,
121.0, 120.7, 119.8, 119.4, 119.3, 38.5, 33.0; 31P{1H} NMR
(162 MHz, CDCl3) ꢀ 254.8; IR (KBr) 2952, 2896, 1570, 1438,
1357, 1240, 1199, 1143, 800, 771, 731 cmꢃ1; UV (hexane)
ꢃmax nm (log ") 237 (4.55), 266 (4.40), 275 (4.45), 362 (4.25);
MS (EI, 70 eV) m=z 384 (Mþ; 27), 219 (ArylPþ + 1; 100), 165
(Fluorenylþ; 28), and 57 (t-Buþ; 9). Found m=z 384.2010. Calcd
for C27H29P: 384.2007.
CDCl3) ꢀ 171.6 (d, J ¼ 42:3 Hz, P=C), 158.9, 148.3, 143.7 (d,
J ¼ 62:7 Hz), 142.7 (d, J ¼ 27:8 Hz), 140.3 (d, J ¼ 10:9 Hz),
138.6 (d, J ¼ 14:6 Hz), 138.2 (d, J ¼ 17:5 Hz), 129.6 (d, J ¼
5:8 Hz), 129.3 (d, J ¼ 7:3 Hz), 127.7 (d, J ¼ 2:1 Hz), 127.4 (d,
J ¼ 2:9 Hz), 125.6 (d, J ¼ 7:3 Hz), 120.9 (d, J ¼ 24:8 Hz),
120.0, 116.3, 58.4, 39.6, 32.9 (d, J ¼ 6:5 Hz); 31P{1H} NMR
(162 MHz, CDCl3) ꢀ 238.3; IR (KBr) 2952, 1738, 1577, 1473,
1442, 1398, 1363, 1246, 1198, 1130, 1043, 943, 902, 773, 729
cmꢃ1; UV (CH2Cl2) ꢃmax nm (log ") 268 (4.41), 276 (4.44),
366 (4.21), 376 (4.21).
[3,5-Di-t-butyl-4-(9-fluorenylidenephosphino)phenyl]trimeth-
ylammonium Trifluoromethanesulfonate (9n). In an NMR
tube, MeOTf (0.2 mL, 1.8 mmol) was added to a solution of 9d
(2.1533 mg, 5 mmol) in C6D6 (0.5 mL). The mixture was shaken
for 1 min, upon which the solution turned light yellow. Volatiles
were removed in vacuo and the residue was washed with hexane 3
times to give 2.0 mg of 9n (68% yield): Yellow powder, mp 250–
252 ꢂC; 1H NMR (400 MHz, CD2Cl2) ꢀ 8.25 (dd, 1H, J ¼ 7:6, 4.0
Hz), 7.80 (s, 2H), 7.67–7.61 (m, 2H), 7.42–7.33 (m, 2H), 7.25 (dd,
1H, J ¼ 7:6, 7.6 Hz), 6.81 (dd, 1H, J ¼ 7:6, 7.6 Hz), 5.23 (d, 1H,
J ¼ 8:0 Hz), 3.90 (s, 9H), 1.51 (s, 18H); 13C NMR (100 MHz,
CD2Cl2) ꢀ 171.9 (d, J ¼ 42:3 Hz), 158.9, 147.3, 143.7 (d, J ¼
62:7 Hz), 142.7 (d, J ¼ 27:0 Hz), 140.3 (d, J ¼ 10:3 Hz), 138.5
(d, J ¼ 14:6 Hz), 138.2 (d, J ¼ 17:5 Hz), 121.0, 120.8, 120.0,
116.2, 58.0, 39.4, 32.3 (d, J ¼ 7:2 Hz); 31P{1H} NMR (162
MHz, CD2Cl2) ꢀ 238.8; IR (KBr) 2858, 1738, 1577, 1481,
1442, 1268, 1155, 1029, 775, 731 cmꢃ1; UV (CH2Cl2) ꢃmax nm
(log ") 267 (4.43), 276 (4.47), 366 (4.24), 376 (4.25).
9b20 (42% yield): UV (hexane) ꢃmax nm (log ") 238 (4.48), 266
(4.30), 275 (4.32), 365 (4.05).
9c (38% yield): Orange crystals, mp 205–206.5 ꢂC; 1H NMR
(400 MHz, CDCl3) ꢀ 8.27 (dd, 1H, J ¼ 7:2, 4.4 Hz), 7.64 (d,
1H, J ¼ 7:2 Hz), 7.57 (d, 1H, J ¼ 7:6 Hz), 7.37–7.28 (m, 2H),
7.18 (dd, 1H, J ¼ 7:6, 7.6 Hz), 7.14 (s, 2H), 6.84 (dd, 1H,
J ¼ 7:6, 7.6 Hz), 5.56 (dd, 1H, J ¼ 8:0, 6.8 Hz), 3.96 (s, 3H),
1.44 (s, 18H); 13C{1H} NMR (100 MHz, CDCl3) ꢀ 171.2 (d, J ¼
43:2 Hz, P=C), 160.4, 156.8, 143.2 (d, J ¼ 26:8 Hz), 139.7 (d,
J ¼ 10:3 Hz), 138.8 (d, J ¼ 17:1 Hz), 138.5 (d, J ¼ 14:5 Hz),
129.2 (d, J ¼ 56:5 Hz), 128.5 (d, J ¼ 6:2 Hz), 128.4 (d, J ¼ 6:9
Hz), 127.7 (d, J ¼ 5:6 Hz), 127.7, 126.8 (d, J ¼ 7:6 Hz), 121.0,
120.7, 119.8, 119.4, 119.3, 112.2, 55.6, 38.6, 32.8; 31P{1H} NMR
(162 MHz, CDCl3) ꢀ 258.0; IR (KBr) 2966, 2931, 2891, 2860,
1589, 1438, 1284, 1097, 1066, 1026, 804, 771, 727 cmꢃ1; UV
(hexane) ꢃmax nm (log ") 239 (4.61), 266 (4.42), 275 (4.46),
369 (4.30); MS (EI, 70 eV) m=z 414 (Mþ; 18) and 249
(ArylPþ ꢃ 1; 100). Found 414.2104. Calcd for C28H31OP:
414.2113.
Theoretical Calculation. Molecular orbital calculations were
performed using the Gaussian 03 program.27 Geometry optimiza-
tions were done first at the RHF/STO-3G level starting from ei-
ther the co-ordinates obtained from the crystal structure analyses
of 9c,d or co-ordinates modified from those of 9c,d, in the cases
of 9a, 10a,c,d,o,p. The geometries were further optimized at the
RHF/6-31+G(d) or RB3LYP/6-31G(d) levels.
ꢂ
1
9d (14% yield): Red crystals, mp 191–193 C; H NMR (400
MHz, CDCl3) ꢀ 8.32 (dd, 1H, J ¼ 7:8, 4.4 Hz), 7.67 (d, 1H, J ¼
7:2 Hz), 7.60 (d, 1H, J ¼ 7:6 Hz), 7.37–7.31 (m, 2H), 7.20 (dd,
1H, J ¼ 8:0, 8.0 Hz), 7.00 (s, 2H), 6.88 (dd, 1H, J ¼ 7:2, 7.2
Hz), 5.67 (dd, 1H, J ¼ 8:0, 7.2 Hz), 3.03 (s, 6H), 1.48 (s, 18H);
13C{1H} NMR (100 MHz, CDCl3) ꢀ 171.3 (d, J ¼ 44:2 Hz,
P=C), 155.8, 151.2, 143.5 (d, J ¼ 26:2 Hz), 139.5 (d, J ¼ 10:2
Hz), 138.9 (d, J ¼ 16:8 Hz), 138.5 (d, J ¼ 14:5 Hz), 128.3 (d, J ¼
2:8 Hz), 128.2 (d, J ¼ 3:3 Hz), 127.3 (d, J ¼ 3:7 Hz), 127.2,
127.1, 127.1, 123.7 (d, J ¼ 56:1 Hz), 121.0, 120.7, 119.8, 119.2
(d, J ¼ 2:3 Hz), 111.0, 41.1, 38.7, 32.9 (d, J ¼ 6:8 Hz);
31P{1H} NMR (162 MHz, CDCl3) ꢀ 264.8; IR (KBr) 3097,
2964, 1593, 1481, 1437, 1338, 1252, 1182, 1047, 989, 844, 775,
735 cmꢃ1; UV (hexane) ꢃmax nm (log ") 239 (4.50), 267 (4.59),
275 (4.61), 374 (4.28); MS (EI, 70 eV) m=z 427 (Mþ; 1), 262
(ArylPþ ꢃ 1; 80), and 206 (ArylPþ ꢃ t-Bu; 100). Found m=z
427.2426. Calcd for C29H34NP: 427.2429.
[3,5-Di-t-butyl-4-(9-fluorenylidenephosphino)phenyl]trimeth-
ylammonium Iodide (9m). To a solution of 9d (3.0405 mg, 7.1
mmol) in C6D6 (0.5 mL) in an NMR tube, was added MeI (0.2 mL,
3.2 mmol). The mixture was warmed at 50 ꢂC for 2 h, and the so-
lution turned light yellow. Volatiles were removed in vacuo, and
the residue was washed with hexane 3 times to give 2.1 mg of 9m
(52% yield): Yellow powder, mp 168 ꢂC (decomp.); 1H NMR
(400 MHz, CDCl3) ꢀ 8.20 (dd, 1H, J ¼ 8:0, 4.0 Hz), 7.85 (s,
2H), 7.62–7.55 (m, 2H), 7.38–7.28 (m, 2H), 7.19 (dd, 1H,
J ¼ 7:6, 7.6 Hz), 6.77 (dd, 1H, J ¼ 7:6, 7.6 Hz), 5.14 (d, 1H, J ¼
8:0 Hz), 4.22 (s, 9H), 1.51 (s, 18H); 13C{1H} NMR (100 MHz,
Crystal Data for 9b. C31H37P, Mr ¼ 440:61. Orthorhombic,
space group Pca21 (#29), a ¼ 16:276ð3Þ, b ¼ 9:396ð2Þ, c ¼
16:794ð3Þ A, V ¼ 2568:4ð8Þ A , Z ¼ 4, ꢄ ¼ 1:139 g cmꢃ3, ꢅ ¼
1:23 cmꢃ1; R1 ¼ 0:078 (1766 data, I > 2ꢂðIÞ), R ¼ 0:179 (all
data), Rw ¼ 0:180 (all data), GOF ¼ 2:29. 1854 unique reflections
with 2ꢆ ꢄ 50:0ꢂ were recorded on an imaging plate diffractometer
(Mo Kꢇ radiation, graphite monochrometer) at ꢃ153 ꢂC. The
structure was solved by direct methods and expanded using Fou-
rier techniques. The non-hydrogen atoms were refined anisotropi-
cally. Hydrogen atoms were included but not refined. Crystallo-
graphic data have been deposited at the Cambridge Crystallo-
graphic Data Centre (No. CCDC-246318).
ꢀ
ꢀ 3
Crystal Data for 9c. C28H31OP, Mr ¼ 414:53. Monoclinic,
space group C2 (#5), a ¼ 16:958ð4Þ, b ¼ 8:963ð6Þ, c ¼
ꢂ
ꢀ
ꢀ 3
15:997ð3Þ A, ꢈ ¼ 107:98ð1Þ , V ¼ 2312ð1Þ A , Z ¼ 4, ꢄ ¼
1:190 g cmꢃ3, ꢅ ¼ 1:36 cmꢃ1; R1 ¼ 0:068 (1953 data, I >
2ꢂðIÞ), R ¼ 0:175 (all data), Rw ¼ 0:171 (all data), GOF ¼
3:19. 1966 unique reflections with 2ꢆ ꢄ 50:0ꢂ were recorded on
an imaging plate diffꢂractometer (Mo Kꢇ radiation, graphite mono-
chrometer) at ꢃ153 C. The structure was solved by direct meth-
ods and expanded using Fourier techniques. The non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were includ-
ed but not refined. Crystallographic data have been deposited at
the Cambridge Crystallographic Data Centre (No. CCDC-246319).
Crystal Data for 9d. C29H34NP, Mr ¼ 440:61. Orthorhombic,
space group Pca21 (#29), a ¼ 16:323ð6Þ, b ¼ 9:415ð2Þ, c ¼
16:117ð4Þ A, V ¼ 2476ð1Þ A , Z ¼ 4, ꢄ ¼ 1:146 g cmꢃ3, ꢅ ¼
ꢀ
ꢀ 3

S. Kawasaki et al.
Bull. Chem. Soc. Jpn., 78, No. 6 (2005) 1119
1:27 cmꢃ1; R1 ¼ 0:059 (1800 data, I > 2ꢂðIÞ), R ¼ 0:099 (all da-
ta), Rw ¼ 0:119 (all data), GOF ¼ 1:31. 1976 unique reflections
with 2ꢆ ꢄ 50:0ꢂ were recorded on an imaging plate diffractometer
(Mo Kꢇ radiation, graphite monochrometer) at ꢃ120 ꢂC. The
structure was solved by direct methods and expanded using Fou-
rier techniques. The non-hydrogen atoms were refined anisotropi-
cally. Hydrogen atoms were included but not refined. Crystallo-
graphic data have been deposited at the Cambridge Crystallo-
graphic Data Centre (No. CCDC-246320).
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(2003).
This work was supported in part by some Grants-in-Aid
for Scientific Research (Nos. 13304049, 14044012, and
13640522) from the Ministry of Education, Culture, Sports,
Science and Technology.
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