Preparation and properties of phosphaethynes bearing bulky aryl groups with electron-donating substituents at the para position

Article abstract of DOI:10.1021/jo049571q

Phosphaethynes bearing a 2,6-di-tert-butyl-4-(dimethylamino)phenyl, 2,6-di-tert-butyl-4-methoxyphenyl, or 2,6-di-tert-butylphenyl group were prepared. A ³¹P NMR spectroscopic investigation of the chemical shifts indicated that electron-donating

Full text of DOI:10.1021/jo049571q

P r ep a r a tion a n d P r op er ties of P h osp h a eth yn es Bea r in g Bu lk y
Ar yl Gr ou p s w ith Electr on -Don a tin g Su bstitu en ts a t th e
P a r a P osition
Kozo Toyota, Subaru Kawasaki, and Masaaki Yoshifuji*
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba, Sendai 980-8578, J apan
yoshifj@mail.tains.tohoku.ac.jp
Received March 16, 2004
Phosphaethynes bearing a 2,6-di-tert-butyl-4-(dimethylamino)phenyl, 2,6-di-tert-butyl-4-methoxy-
phenyl, or 2,6-di-tert-butylphenyl group were prepared. A 31P NMR spectroscopic investigation of
the chemical shifts indicated that electron-donating groups at the para position cause shifts to a
higher field. Bathochromic shifts caused by the electron-donating groups were apparently observed
in UV-vis spectra. The structure of 2-[2,6-di-tert-butyl-4-(dimethylamino)phenyl]-1-phosphaethyne
was analyzed by X-ray crystallography.
In tr od u ction
perturbed multiple bonds is now a subject of interest in
heteroatom chemistry.
Understanding and controlling the multiple bonds
between the heavier main group elements is one of the
most attractive and important subjects in modern main-
group chemistry.1 In general, such multiple bonds are
highly reactive but several methodologies for stabilization
of such reactive bonds have been developed. Thermody-
namic stabilization by electronic perturbation has been
used in studies of multiple bonds between carbon and
heavier heteroatoms,2 and more recently, intramolecular
donor-stabilization3 and kinetic stabilization4 have been
effectively applied to multiple bonds between the heavier
heteroatoms. In this context, the systematic comparison
of properties of electronically perturbed multiple bonds
with those of kinetically stabilized, but less electronically
We and others have studied properties of low-coordi-
nated phosphorus compounds5 such as diphosphenes,4b,c
phosphaethenes,2,4a,6 and phosphaethynes,7,8 as well as
those in conjugation or cumulative systems such as phos-
phapolyenes9 or phosphaallenes,10 respectively. Research
on conjugated systems containing phosphorus-carbon
double bonds has been developed both experimentally
and theoretically; however, conjugated phosphorus-
carbon triple bonds have scarcely been studied. Since the
(5) (a) Regitz, M.; Scherer, O. J . Multiple Bonds and Low Coordina-
tion in Phosphorus Chemistry; Georg Thieme Verlag: Stuttgart,
Germany, 1990, and references therein. (b) Dillon, K. B.; Mathey, F.;
Nixon, J . F. Phosphorus: The Carbon Copy; J ohn Wiley: Chichester,
UK, 1998. (c) Yoshifuji, M. J . Organomet. Chem. 2000, 611, 210.
(6) For example: (a) Issleib, K.; Schmidt, H.; Wirkner, C. Z. Anorg.
Allg. Chem. 1982, 488, 75. (b) Yoshifuji, M.; Toyota, K.; Inamoto, N.
Tetrahedron Lett. 1985, 26, 1727.
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(b) Kroto, H. W.; Nixon, J . F.; Simmons, N. P. C.; Westwood, N. P. C.
J . Am. Chem. Soc. 1978, 100, 446. (c) Hopkinson, M. J .; Kroto, H. W.;
Nixon, J . F.; Simmons, N. P. C. Chem. Phys. Lett. 1976, 42, 460. (d)
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10.1021/jo049571q CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/01/2004
J . Org. Chem. 2004, 69, 5065-5070
5065

Toyota et al.
first generation of parent phosphaethyne by Gier,7a
generation or preparation of several carbon- or hetero-
atom-substituted phosphaethyne derivatives such as
fluoro-,7b alkyl-,7c-e vinyl-,7f ethynyl-,7g cyano-,7h trimeth-
ylsilyl-,7i or diisopropylamino-substituted phospha-
ethynes7k,l have been reported. In contrast to the alkyl-
phosphaethynes, studies on aryl-substituted phospha-
ethynes8 have been limited, despite the expectation that
aryl moieties may interact more effectively than alkyl
groups do and that introduction of functional groups is
easier than that of the alkyl groups. To the best of our
knowledge, the preparations of only four metal-free
arylphosphaethynes (i.e., phenyl-,8a mesityl-,8b 2,4,6-tri-
tert-butylphenyl-,8c and 2,4,6-tri-tert-pentylphenyl-substi-
tuted8d phosphaethynes) have been reported so far.
Although a theoretical calculation concerning (2,6-di-
methylphenyl)phosphaethyne was reported,8e this phos-
phaethyne has not been experimentally realized.
It should be noted that substituents on the aromatic
rings are limited to three simple alkyl groups (i.e.,
methyl, tert-butyl, and tert-pentyl) in the reported
arylphosphaethynes, and no heteroatom-containing group
has been introduced as a substituent into the aromatic
ring, probably because of relatively high reactivities of
arylphosphaethynes. However, sterically hindered aryl
substituents containing some functional groups are ex-
pected to afford new arylphosphaethynes as stable com-
pounds and will help to gain insight into the structure-
properties relationship of arylphosphaethynes. It may
also help to develop applications of the electronically
perturbed phosphaethynes and/or their metal complexes
in the field of materials science and as catalysts in
synthetic organic chemistry. For this purpose, we de-
signed new para-functionalized bulky aryl substituents
simultaneously containing tert-butyl groups at the ortho
positions: We report here the preparation and properties
of kinetically stabilized and electronically perturbed
phosphaethynes 2b,c bearing para-functionalized bulky
aryl groups as well as para-nonsubstituted congener 2a .
These new arylphosphaethynes may help in understand-
ing the nature of the interaction between the P-C triple
bond and the aromatic π-system.
Rundel12a starting from 4-nitro-2,6-di-tert-butylaniline
was found to be rather inefficient due to formation of
byproducts and an elaborate separation process.12b There-
fore we prepared 1c as follows: 3,5-xylidine was bromi-
nated with NBS in acetonitrile,13 to give 4-bromo-3,5-
xylidine14 (98% yield), which was then converted to 2,5-
dibromo-1,3-dimethylbenzene (3)15 by Sandmeyer reaction
with use of CuBr/HBr (38% yield). Bromination of 3 with
NBS in CCl4 gave compound 4 (55% yield), which was
then cyanated with KCN to form 5 (90% yield). Treat-
ment of 5 with MeI and KOH in DMSO gave 6 in 87%
yield. Reduction of 6 to the corresponding dialdehyde 7
by DIBAL, followed by Wolff-Kishner reduction (the
Huang-Minlon method) afforded 3,5-dibromo-1,3-di-tert-
butylbenzene (1e) (20% yield based on 6). Regioselective
amination of 1e with benzophenone imine in the presence
of palladium catalyst16 afforded 1f (97% yield), and 1f
was then hydrolyzed to 1g12 with hydrochloric acid (57%
yield). N-Methylation of 1g by treatment with Formalin
and NaBH4 gave the bromobenzene 1c in 84% yield.
Although we could obtain compound 1c as described
above via 1e (as mentioned above, 1e was prepared from
commercially available 3,5-xylidine in 7 steps in 3%
overall yield), this route has the following disadvantage:
Workup of the Sandmeyer reaction providing 3 was
tedious due to the separation of the copper byproducts
from 3 and the reproducibility was unsatisfactory (yields
were ca. 20-40%). Alternatively, we found that the direct
bromination of 1a (prepared from commercially available
2-bromo-m-xylene) in trimethyl phosphate also gave 1e
as a sole product in 63% yield (6 steps from the starting
2-bromo-m-xylene in 12% overall yield) without formation
of the possible byproduct 2,4-dibromo-1,3-di-tert-butyl-
benzene (Scheme 1). It should be noted that an attempted
direct bromination of 2-bromo-m-xylene under similar
conditions resulted in the formation of a mixture of m-
and p-dibromo derivatives, probably because the steric
hindrance of the tert-butyl group plays an important role
in the regioselective bromination of 1a . Compared with
Rundel’s preparative method12a for 1c (3 steps from
4-nitro-2,6-di-tert-butylaniline), our present route re-
quires 9 steps from 2-bromo-m-xylene to 1c via 1e;
however, our method is reliable and the workup is easy
for each step, making large scale preparation possible (1.7
g of 1c was prepared from 2-bromo-m-xylene within 2
weeks).
(10) (a) Yoshifuji, M.; Toyota, K.; Shibayama, K.; Inamoto, N.
Tetrahedron Lett. 1984, 25, 1809. (b) Yoshifuji, M.; Toyota, K.; Inamoto,
N. J . Chem. Soc., Chem. Commun. 1984, 689. (c) Karsch, H. H.; Ko¨hler,
F. H.; Reisacher, H.-U. Tetrahedron Lett. 1984, 25, 3687. (d) Appel,
R.; Fo¨lling, P.; J osten, B.; Siray, M.; Winkhaus, V.; Knoch, F. Angew.
Chem., Int. Ed. Engl. 1984, 23, 619. (e) Ma¨rkl, G.; Sejpka, H.; Dietl,
S.; Nuber, B.; Ziegler, M. L. Angew. Chem., Int. Ed. Engl. 1986, 25,
1003. (f) Ma¨rkl, G.; Kreitmeier, P. Angew. Chem., Int. Ed. Engl. 1988,
27, 1360.
(11) (a) Toyota, K.; Kawasaki, S.; Nakamura, A.; Yoshifuji, M. Chem.
Lett. 2003, 32, 430. See also: (b) Toyota, K.; Nakamura, A.; Yoshifuji,
M. Chem. Commun. 2002, 3012. (c) Toyota, K.; Matsushita, Y.;
Shinohara, N.; Yoshifuji, M. Heteroat. Chem. 2001, 12, 418.
(12) (a) Rundel, W. Chem. Ber. 1968, 101, 2956. See also: (b)
Yoshifuji, M.; Niitsu, T.; Shiomi, D.; Inamoto, N. Tetrahedron Lett.
1989, 30, 5433.
(13) Fox, G. J .; Hepworth, J . D.; Paskins, K. N. Org. Synth. 1976,
55, 20.
(14) Courtin, A.; von Tobel, H.-R. Helv. Chim. Acta 1977, 60, 1994.
(15) Blanksma, J . J . Recl. Trav. Chim. Pays-Bas 1906, 25, 165.
(16) Mann, G.; Hartwig, J . F.; Driver, M. S.; Ferna´ndez-Rivas, C.
J . Am. Chem. Soc. 1998, 120, 827.
Resu lts a n d Discu ssion
P r ep a r a tion of Bu lk y Br om oben zen es. The start-
ing bulky bromobenzenes, 2-bromo-1,3-di-tert-butylben-
zene (1a ) and 2-bromo-1,3-di-tert-butyl-5-methoxyben-
zene (1b), were prepared as described in a previous
report.11a The method for the preparation of 1c by
5066 J . Org. Chem., Vol. 69, No. 15, 2004

Phosphaethynes Bearing Bulky Aryl Groups
SCHEME 1a
SCHEME 2
phaethynes such as 2b,c, by contribution of the resonance
shown in Scheme 2.
These tendencies in the δP and J PC values are consis-
1
tent with the cases of phosphaethynes HCtP, t-BuCtP,
and i-Pr2NCtP (it should be noted that experimental
data for methoxy- or dimethylamino-substituted phos-
phaethynes are not available because these compounds
have not been prepared). As for the 31P NMR chemical
shift, the order of the δP values is i-Pr2NCtP (δP -99.6)
< t-BuCtP (δP -69.2) < HCtP (δP -32).7d,k The order
of the 1J PC values is also i-Pr2NCtP (14.7 Hz) < t-BuCtP
(38.5 Hz) < HCtP (56.0 Hz),7d,k which matches the order
in the arylphosphaethynes (2c < 2b < 2d < 2a ), although
the differences between the values are smaller in the case
of 2.
Concerning the stabilities or reactivities of the phos-
phaethynes, i-Pr2NCtP slowly decomposes above -20 °C,
while t-BuCtP is stable at room temperature, under
inert atmosphere. In this case, however, steric hindrance
around the triple bond seems to have a large effect on
the stability, and indeed the parent HCtP polymerizes
above -124 °C. In the case of the phenylogues 2a -d ,
their relative reactivities may be less affected by the
steric hindrance around the triple bond, because the
degree of hindrance is similar among these compounds,
so we can observe the almost purely electronic effect of
the para substituents.
The dimethylamino-substituted arylphosphaethyne 2c
decomposed within a week at room temperature in air.
On the other hand, 2d , as well as 2a , was extremely
stable and no significant decomposition was observed
even after 1 month under similar conditions. This high
reactivity of 2c may also indicate the perturbation caused
by the p-amino group (Scheme 2), which increases the
nucleophilic activity of the phosphorus atom. In fact, 2c
decomposed in the presence of methyl triflate in C6D6 at
room temperature, while 2d was stable in the presence
of methyl triflate and no significant change was observed
after several days.
As for the UV spectra of 2a -d (Table 1, see also Figure
1 in the Supporting Information), a bathochromic shift
of the π-π* transition and an increase of ꢀ were observed
with the increase of electron-donating ability, similar to
the cases of para-substituted phenylacetylene derivatives
and benzonitrile derivatives: this fact also is consistent
with the electron delocalization caused by the electron-
donating group as shown in Scheme 2.
a
Reagents and conditions: (a) Br2, P(O)(OMe)3, 63% yield; (b)
n-BuLi, THF, then PCl3; (c) LDA, CHBr3, THF [9a : 47% yield
based on 1a ; 9b: 64% yield based on 1b; 9c: not isolated]; (d)
NiBr2(PPh3)2, PPh3, Zn, n-Bu4N+I-, THF [2a : 24% yield based
on 9a ; 2b: 30% yield based on 9b; 2c: 32% yield based on 1c].
TABLE 1. 31P NMR (162 MHz), 13C NMR (100 MHz), a n d
UV-Vis Da ta for 2a -d
δC(CtP)b (1J PC/Hz)
λ
max/nm (log ꢀ)c
a
compd
R
δP
2a
H
37.2
168.3 (54.0)
168.9 (52.4)
171.4 (49.8)
168.7 (53.2)
315 (4.03), 330 (3.94)
330 (4.22)
377 (4.50)
2b
2c
OMe 32.4
NMe2 32.1
t-Bu
2d d
34.4
320 (4.17), 334 (4.09)
a
b
c
In CDCl3, relative to external 85% H3PO4. In CDCl3. In
CH2Cl2. Data taken from ref 8c (31P NMR, 101.25 MHz; 13C
NMR, 62.9 MHz).
d
P r ep a r a tion a n d P r op er ties of P h osp h a eth yn es.
Having obtained the bromobenzenes 1a -c, we then
prepared phosphaethynes 2a -c as follows (Scheme 1):
Bromobenzenes 1a -c were converted to the correspond-
ing phosphonous dichlorides 8a ,12b 8b,11 and 8c [8c: not
isolated, 31P{1H} NMR (162 MHz, THF-C6D6) δP 155.7],
which were allowed to react with bromoform and LDA17
to give dibromomethylenephosphines 9a-c [9a: δP (CDCl3)
269.2; 9b: δP (CDCl3) 270.8; 9c (not isolated, due to its
instability in air): δP (C6D6) 276.3]. Dehalogenation
reactions of 9a -c via 1,2-shift18 with NiBr2(PPh3)2 as
catalyst18a gave phosphaethynes 2a -c.
Table 1 shows selected spectral data for 2a -c, together
with those for 2d .8c 31P NMR signals of 2 shift to a lower
field [order of the δP values: c (NMe2) < b (OMe) < d
(t-Bu) < a (H), larger positive values correspond to the
lower field] when the electron donating ability of the
para-functional group decreases.19 The values of chemical
shifts (δC) of the sp carbons in 13C NMR spectra are very
close among the four phosphaethynes. Differences in the
values of spin-spin coupling constants (1J PC) are also
small; however, they become slightly smaller as the
electron-donating ability of the para-functional groups
increases (i.e., again the order is c < b < d < a ). This
Although attempted X-ray crystallographic analyses of
2a ,b failed due to the poor quality of single crystals for
the analysis, the structure of 2c was determined by X-ray
crystallography (the reflections were collected at -70
°C).20 Figure 1 shows an ORTEP21 drawing of the
molecular structure of 2c. The P(1)-C(1)-C(2) portion
1
tendency in J PC may indicate that the PC bond order
decreases, in electron donating group-substituted arylphos-
(17) Goede, S. J .; Bickelhaupt, F. Chem. Ber. 1991, 124, 2677.
(18) (a) Sugiyama, H.; Ito, S.; Yoshifuji, M. Angew. Chem., Int. Ed.
2003, 42, 3802. (b) Appel, R.; Immenkeppel, M. Z. Anorg. Allg. Chem.
1987, 553, 7. (c) Yoshifuji, M.; Niitsu, T.; Inamoto, N. Chem. Lett. 1988,
1733.
(19) For example: Swain, C. G.; Lupton, E. C., J r. J . Am. Chem.
Soc. 1968, 90, 4328.
(20) Crystallographic data have been deposited at the Cambridge
Crystallographic Data Centre (No. CCDC-227150).
(21) J ohnson, C. K. Oak Ridge National Laboratory Report, ORNL-
TM-5138, Oak Ridge, TN, 1976.
J . Org. Chem, Vol. 69, No. 15, 2004 5067

Toyota et al.
from hexane gave 11.0 g (55% yield) of 4: Colorless crystals,
mp 96-98 °C; 1H NMR (400 MHz, CDCl3) δ 7.57 (2H, s, arom.)
and 4.59 (4H, s, CH2Br); 13C{1H} NMR (100 MHz, CDCl3) δ
140.6, 134.4, 125.7, 121.8, and 33.1 (CH2Br); IR (KBr) 3129,
3069, 1533, 1431, 1263, 1209, 1111, 1024, 980, 872, and 731
cm-1; MS (70 eV) m/z (rel intensity) 426 (M+ + 8; 4), 424 (M+
+ 6; 16), 422 (M+ + 4; 25), 420 (M+ + 2; 17), 418 (M+; 4), 345
(M+ - Br + 6; 33), 343 (M+ - Br + 4; 98), 341 (M+ - Br + 2;
100), 339 (M+ - Br; 35), 264 (M+ - 2Br + 4; 18), 262 (M+
-
2Br + 2; 32), 260 (M+ - 2Br; 17), 183 (M+ - 3Br + 2; 13), 181
(M+ - 3Br; 14), and 102 (M+ - 4Br; 49). Found: m/z 421.7165.
Calcd for C8H679Br281Br2: M, 421.7165.
2,5-Dibr om o-1,3-bis(cya n om et h yl)ben zen e (5). To a
solution of 4 (42 g, 0.10 mol) in acetonitrile (500 mL) were
added 18-crown-6 (5.3 g, 0.02 mol), KCN (14.0 g, 0.22 mol),
water (40 mL), and a small amount of KI at room temperature.
After being stirred for 24 h, the reaction mixture was worked
up with water and AcOEt. The organic extracts were washed
with brine, dried over MgSO4, and concentrated. Column
chromatography (SiO2) of the residue provided 28.2 g (90%
yield) of 5: Colorless crystals, mp 134-136 °C; 1H NMR (400
MHz, CDCl3) δ 7.68 (2H, s, arom.) and 3.87 (4H, s, CH2CN);
13C{1H} NMR (100 MHz, CDCl3) δ 133.7, 133.0, 124.1, 122.7,
116.4, and 25.8 (CH2); IR (KBr) 3166, 3104, 2991, 2935, 2249,
F IGURE 1. Molecular structure of 2c, showing the atomic
labeling scheme with thermal ellipsoids (50% probability).
Hydrogen atoms are omitted for clarity. Some selected bond
lengths (Å) and angles (deg): P(1)-C(1), 1.533(3); C(1)-C(2),
1.446(4); N(1)-C(5), 1.382(4); P(1)-C(1)-C(2), 178.7(3); C(5)-
N(1)-C(16), 120.5(3); C(5)-N(1)-C(17), 120.2(3); C(16)-N(1)-
C(17), 117.9(3).
1728, 1566, 1427, 1404, 1236, 1130, 1026, 923, and 849 cm-1
;
MS (70 eV) m/z (rel intensity) 316 (M+ + 4; 6), 314 (M+ + 2;
13), 312 (M+; 6), 289 (M+ - CN + 3; 9), 287 (M+ - CN + 1;
19), 285 (M+ - CN - 1; 10), 235 (M+ - Br + 2; 48), 233 (M+
- Br; 47), 208 (M+ - Br - CN + 1; 27), 206 (M+ - Br - CN
- 1; 29), 154 (M+ - 2Br; 60), and 127 (M+ - 2Br - CN - 1;
100). Found: m/z 311.8892. Calcd for C10H6Br2N2: M, 311.8898.
is almost linear with an angle of 178.7(3)°; the value is
close to that for 2d 22 [177.0°, measured at 25 °C]. The
PtC bond length in 2c [1.533(3) Å] is slightly longer than
that for 2d [1.516(13) Å]. The C(1)-C(2) distance of 2c
[1.446(4) Å] is longer than that for 2d [1.427 Å]. These
facts indicate that the structure of the P(1)-C(1)-C(2)
moiety for 2c slightly differs from that for 2d in the
crystal. However, the degree of the substituent effect on
geometrical change seems to be small, compared with
that on UV absorption.
2,5-Dibr om o-1,3-bis(1-cyan o-1-m eth yleth yl)ben zen e (6).
To a solution of 5 (28.2 g, 89.8 mmol) in dimethyl sulfoxide
(200 mL) was added KOH (30 g) with cooling by using an ice
bath. Iodomethane (34 mL, 540 mmol) was then added at 0
°C. The reaction mixture was allowed to warm to room
temperature and stirred for 3 days. When the reaction mixture
was diluted with Et2O, precipitates formed. The precipitate
was separated by filtration and the filtrate was washed with
water and brine. The precipitate was dissolved in AcOEt and
the combined organic phase was dried over MgSO4 and
concentrated. Column chromatography (SiO2) of the residue
provided 32.3 g (87% yield) of 6: Colorless crystals, mp 163-
Con clu sion s
1
165 °C; H NMR (400 MHz, CDCl3) δ 7.57 (2H, s, arom.) and
In summary, we have prepared sterically protected
arylphosphaethynes 2b ,c, bearing electron-donating
groups at the para positions, as well as the para-
nonsubstituted 2a . Comparison of the spectroscopic data
was carried out keeping the two tert-butyl groups at the
ortho positions. The 31P NMR chemical shifts, spin-spin
1.91 (12H, s, Me); 13C{1H} NMR (100 MHz, CDCl3) δ 143.0,
130.7, 123.7, 123.0, 122.9, 38.2, and 28.7; IR (KBr) 2981, 2924,
2231, 1737, 1562, 1461, 1394, 1248, 1184, 1117, 1016, 870, and
733 cm-1; MS (70 eV) m/z (rel intensity) 372 (M+ + 4; 18), 370
(M+ + 2; 34), 368 (M+; 18), 357 (M+ - Me + 4; 9), 355 (M+
-
Me + 2; 22), 353 (M+ - Me; 10), 330 (M+ - Me - CN + 3; 48),
328 (M+ - Me - CN + 1; 100), 326 (M+ - Me - CN - 1; 48),
302 (M+ - 3Me - CN + 5; 38), 300 (M+ - 3Me - CN + 3; 77),
298 (M+ - 3Me - CN + 1; 40), 264 (M+ - Br - CN + 1; 23),
and 262 (M+ - Br - CN - 1; 24). Found: m/z 367.9518. Calcd
for C14H14Br2N2: M, 367.9524.
1
coupling constants J PC, and the UV spectra of 2b,c
suggest the contribution of resonance shown in Scheme
2. However, the contribution of the betaine form does not
seem to be large enough, at least in the crystals, to
shorten the bond distance between the sp-hybridized
carbon atom and the ipso-carbon atom of the Mes* group.
Further studies on structure-properties relationships
and reactivities of the phosphaethynes, as well as those
of phosphaethenes and diphosphenes, are in progress.
2,5-Dibr om o-1,3-d i-ter t-bu tylben zen e (1e). Meth od A:
Under an argon atmosphere, to a solution of 6 (32.3 g, 87.3
mmol) in dry benzene (200 mL) was added 218 mmol of DIBAH
(0.93 M solution in hexane) at 0 °C. The reaction mixture was
allowed to warm to ambient temperature and stirred for 2 h.
The mixture was carefully quenched with water, then 10%
sulfuric acid was added. The resulting mixture was stirred for
6 h, diluted with CHCl3, and filtered through Celite. The
filtrate was washed with water and brine, and the organic
phase was dried over MgSO4 and concentrated to give crude
7. The crude 7 thus obtained was dissolved in triethylene glycol
(100 mL) and hydrazine monohydrate (8.9 g, 178 mmol) was
added. The resulting mixture was heated at 130 °C for 2 h.
The mixture was allowed to cool to room temperature and
KOH (12 g, 214 mmol) was added. Then the flask was equipped
with a Dean-Stark trap and the mixture was heated to 150
°C for 3 h to remove water under slightly reduced pressure.
Exp er im en ta l Section
2,5-Dibr om o-1,3-bis(br om om eth yl)ben zen e (4). To a
solution of 3 (12.6 g, 47.7 mmol) in CCl4 (50 mL) were added
NBS (17.0 g, 95.5 mmol) and AIBN (0.4 g, 2.4 mmol). The
resulting mixture was refluxed for 2 h and cooled to room
temperature. Insoluble succinimide was filtered off and the
solution was concentrated. Recrystallization of the residue
(22) Arif, A. M.; Barron, A. R.; Cowley, A. H.; Hall, S. W. J . Chem.
Soc., Chem. Commun. 1988, 171.
5068 J . Org. Chem., Vol. 69, No. 15, 2004

Phosphaethynes Bearing Bulky Aryl Groups
The reaction mixture was heated at 200 °C for 2 h. The
resulting mixture was allowed to cool to room temperature,
diluted with hexane, and washed with water and brine. The
organic layer was dried over MgSO4 and concentrated. Column
chromatography (SiO2, hexane) of the residue provided 6.9 g
(20% yield) of 1e. Meth od B: To a solution of 1a (10 g, 37.1
mmol) in trimethyl phosphate (40 mL) was added Br2 (9.5 mL,
185 mmol) and the resulting mixture was stirred at 90 °C for
10 h. Then the mixture was quenched with aq NaHCO3 and
aq Na2S2O3. The organic layer was diluted with hexane and
washed with water and brine. The organic phase was dried
over MgSO4 and concentrated in vacuo. Column chromatog-
raphy (SiO2, hexane) of the residue provided 1e (8.1 g, 63%
yield). 1e: Colorless crystals, mp 54-56 °C; 1H NMR (400
MHz, CDCl3) δ 7.50 (2H, s, arom.) and 1.59 (18H, s, t-Bu); 13C-
{1H} NMR (100 MHz, CDCl3) δ 152.0, 129.8, 124.1, 121.8, 38.7,
and 31.1 (d, 1J PC ) 40.1 Hz, PdC); IR (KBr) 2964, 2920, 2877,
1554, 1477, 1373, 1244, 1198, 1007, 868, and 841 cm-1; MS
(70 eV) m/z (rel intensity) 350 (M+ + 4; 13), 348 (M+ + 2; 26),
346 (M+; 14), 335 (M+ - Me + 4; 15), 333 (M+ - Me + 2; 31),
331 (M+ - Me; 16), 307 (M+ - 3Me + 6; 9), 305 (M+ - 3Me +
4; 19), 303 (M+ - 3Me + 2; 10), and 57 (t-Bu+; 100). Found:
m/z 345.9926. Calcd for C14H20Br2: M, 345.9932.
Typ ica l P r oced u r e for th e P r ep a r a tion of 9. Under an
argon atmosphere, to a solution of 1 (15.4 mmol) in THF (50
mL) was added 17.6 mmol of butyllithium (1.6 M solution in
hexane) at -78 °C. After 5 min of stirring, PCl3 (45.8 mmol)
was added at -78 °C. The solution was then allowed to warm
to 60 °C and stirred for 90 min. The volatiles were removed
under vacuum, and then THF (30 mL) and bromoform (17.2
mmol) were added. The resulting mixture was allowed to cool
to -98 °C and a THF (20 mL) solution of LDA (35.4 mmol)
was added. The reaction mixture was then stirred for 15 min
at -98 °C and allowed to warm to room temperature. The
reaction mixture was diluted with hexane and washed with
water and brine. The organic layer was dried over MgSO4 and
concentrated. Column chromatography or recrystallization of
the residue gave 9a ,b . Compound 9c was used in the next
reaction without further purification, because of the instability.
1
9a : Colorless powder, mp 107-109 °C; H NMR (400 MHz,
CDCl3) δ 7.43 (2H, d, 3J ) 8.4 Hz, arom.), 7.34 (1H, t, 3J ) 8.4
Hz, arom.), and 1.53 (18H, s, t-Bu); 13C{1H} NMR (100 MHz,
CDCl3) δ 154.0 (d, J PC ) 2.3 Hz), 142.9 (d, J PC ) 57.6 Hz),
129.7, 127.1 (d, J PC ) 84.1 Hz), 125.8, 38.1, and 33.0 (d, J PC
)
6.9 Hz); 31P{1H} NMR (162 MHz, CDCl3) δ 269.2; IR (KBr)
2946, 2915, 2866, 1571, 1468, 1389, 1362, 1242, 1192, 1142,
1041, 798, 748, and 721 cm-1; MS (70 eV) m/z (rel intensity)
392 (M+ + 2; 1), 379 (M+ - Me + 4; 3), 377 (M+ - Me + 2; 6),
375 (M+ - Me; 3), 313 (M+ - Br + 2; 41), 311 (M+ - Br; 42),
283 (M+ - Br - 2Me + 2; 28), 281 (M+ - Br - 2Me; 29), 231
(M+ - Br2C - 1; 100), 217 (M+ - Br2C - Me; 83), and 175
(M+ - Br2C - t-Bu; 47). Found: m/z 391.9720. Calcd for
C15H2179Br81BrP: M, 391.9728. 9b: Colorless powder, mp 113-
115 °C; 1H NMR (400 MHz, CDCl3) δ 6.99 (2H, s, arom.), 3.87
(3H, s, OMe), and 1.52 (18H, s, t-Bu); 13C{1H} NMR (100 MHz,
CDCl3) δ 160.5, 156.0 (d, J PC ) 2.8 Hz), 134.3 (d, J PC ) 56.7
Hz), 128.4 (d, J PC ) 86.3 Hz), 111.8, 55.3 (OMe), 38.3, and 33.0
(d, J PC ) 6.8 Hz); 31P{1H} NMR (162 MHz, CDCl3) δ 270.8; IR
(KBr) 2958, 2868, 1591, 1556, 1462, 1392, 1361, 1287, 1221,
1198, 1061, 931, 864, 840, and 719 cm-1; MS (70 eV) m/z (rel
intensity) 424 (M+ + 4; 7), 422 (M+ + 2; 15), 420 (M+; 8), 343
(M+ - Br + 2; 6), 341 (M+ - Br; 6), 249 (M+ - Br2C - 1; 30),
and 193 (M+ - Br2C - t-Bu; 100). Found: m/z 419.9864. Calcd
for C16H23Br2OP: M, 419.9853.
(4-Br om o-3,5-di-ter t-bu tylph en yl)(diph en ylm eth ylen e)-
a m in e (1f). To a 50-mL two-neck flask were added 1e (2.0 g,
5.75 mmol), Pd(OAc)2 (27 mg, 0.11 mmol), DPPF (103 mg, 0.19
mmol), and NaOt-Bu (670 mg, 6.98 mmol) under argon, using
Schlenk technique. The mixture was suspended in toluene (23
mL) and 1 mL (5.96 mmol) of benzophenone imine was added.
The reaction mixture was vigorously stirred at 95 °C for 10 h
and was allowed to cool to room temperature. The mixture was
diluted with AcOEt and washed with water and brine. The
organic layer was dried over Na2SO4 and concentrated. Column
chromatography (SiO2, hexane-AcOEt 100:1) of the residue
provided 2.5 g (97% yield) of 1f: Yellow powder, mp 128-130
1
°C; H NMR (400 MHz, CDCl3) δ 7.85-7.10 (10H, m, arom.),
6.80 (2H, s, arom.), and 1.44 (18H, s, t-Bu); 13C{1H} NMR (100
MHz, CDCl3) δ 168.9 (CdN), 149.8, 149.3, 140.1, 138.1, 137.0,
132.9, 131.3, 130.5, 129.8, 129.7, 129.0, 128.7, 128.7, 128.5,
120.8, 119.5, 38.5, and 31.2; IR (KBr) 2952, 2918, 2877, 1614,
1577, 1446, 1389, 1313, 1273, 1205, 1008, 973, 877, 773, and
696 cm-1; MS (70 eV) m/z (rel intensity) 449 (M+ + 2; 47), 447
(M+; 45), 352 (M+ - Br - Me - 1; 21), 312 (M+ - Br - t-Bu
+ 1; 59), and 165 (Ph2C+ - 1; 100). Found: m/z 447.1564. Calcd
for C27H30BrN: M, 447.1562.
Typ ica l P r oced u r e for th e P r ep a r a tion of 2. Under
argon atmosphere, a mixture of 9 (1 mmol), NiBr2(PPh3)2 (0.1
mmol), PPh3 (0.2 mmol), Zn (1 mmol), and tetrabutylammo-
nium iodide (1 mmol) in THF (5 mL) was stirred for 24 h under
reflux. After cooling, the reaction mixture was diluted with
hexane and washed with water and brine. The organic layer
was dried over MgSO4 and concentrated. Column chromatog-
raphy (SiO2, hexane) of the residue afforded the desired
phosphaethyne 2. 2a : Colorless powder, mp 70-71 °C; 1H
NMR (400 MHz, CDCl3) δ 7.32 (2H, d, 3J ) 8.0 Hz, arom.),
5-Am in o-2-br om o-1,3-d i-ter t-bu tylben zen e (1g). To a
solution of 1f (2.5 g, 5.58 mmol) in THF (10 mL) was added 1
M hydrochloric acid (13 mL). The mixture was refluxed for 1
h, then made alkaline with 2 M aq NaOH, diluted with CHCl3
and washed with brine. The organic layer was dried over Na2-
SO4 and concentrated in vacuo. Column chromatography (SiO2,
hexane-AcOEt 10:1 to 4:1) of the residue provided 900 mg
(57% yield) of 1g.
3
7.23 (1H, t, J ) 8.0 Hz, arom.), and 1.71 (18H, s, t-Bu); 13C-
1
{1H} NMR (100 MHz, CDCl3) δ 168.3 (d, J PC ) 54.0 Hz, CP),
157.5 (d, J PC ) 6.3 Hz), 129.2 (d, J PC ) 5.7 Hz), 127.9 (d, J PC
2-Br om o-1,3-d i-ter t-b u t yl-5-(d im et h yla m in o)b en zen e
(1c). To a stirred mixture of concentrated sulfuric acid (1 mL),
35% formalin (5 mL), and THF (5 mL) were added slowly a
slurry of 1g (900 mg, 3.2 mmol) and NaBH4 (2.6 g, 68.7 mmol)
in THF (30 mL) at 0 °C. The cooling bath was removed, and
the reaction mixture was allowed to warm to ambient tem-
perature and stirred for an additional 1 h. The reaction was
then quenched by addition of water and the resulting mixture
was extracted with CHCl3. The organic phase was washed with
water and brine, dried over Na2SO4, and concentrated. The
crude product was passed through a silica gel column (hex-
ane-AcOEt 4:1) to give 830 mg (84% yield) of 1c: 1H NMR
(400 MHz, CDCl3) δ 6.87 (2H, s, arom.), 3.00 (6H, s, NMe2),
and 1.64 (18H, s, t-Bu); 13C{1H} NMR (100 MHz, CDCl3) δ
150.0, 148.9, 112.2, 111.9, 41.2, 38.7, and 31.3; IR (KBr) 3008,
2977, 2946, 1595, 1483, 1414, 1396, 1342, 1269, 1203, 1064,
) 21.8 Hz), 124.4 (d, J PC ) 1.6 Hz), 37.1, and 31.1 (d, J PC
)
1.9 Hz); 31P{1H} NMR (162 MHz, CDCl3) δ 37.2; UV (CH2Cl2)
315 (log ꢀ 4.03) and 330 nm (3.94); IR (KBr) 3052, 3008, 2861,
2723, 1520, 1461, 1387, 1360, 1250, 1203, 802, and 748 cm-1
;
MS (70 eV) m/z (rel intensity) 232 (M+; 11), 217 (M+ - Me;
21), 199 (M+ - P - 2; 31), 175 (M+ - t-Bu; 98), 161 (M+
-
t-Bu - Me + 1; 98), and 143 (M+ - t-Bu - P - 1; 100). Found:
m/z 232.1376. Calcd for C15H21P: M, 232.1381. 2b: Pale yellow
oil; 1H NMR (400 MHz, CDCl3) δ 6.85 (2H, s, arom.), 3.86 (3H,
s, OMe), and 1.69 (18H, s, t-Bu); 13C{1H} NMR (100 MHz,
1
CDCl3) δ 168.9 (d, J PC ) 52.4 Hz, CP), 160.1 (d, J PC ) 5.6
Hz), 159.4 (d, J PC ) 5.6 Hz), 120.9 (d, J PC ) 21.8 Hz), 110.2,
55.4 (OMe), 37.4, and 31.0 (d, J PC ) 1.5 Hz); 31P{1H} NMR
(162 MHz, CDCl3) δ 32.4; UV (CH2Cl2) 330 nm (log ꢀ 4.22); IR
(neat) 2978, 2925, 2771, 1595, 1533, 1460, 1396, 1290, 1222,
1070, 873, 848, and 764 cm-1; MS (70 eV) m/z (rel intensity)
262 (M+; 100), 247 (M+ - Me; 29), 205 (M+ - t-Bu; 40), and
191 (M+ - t-Bu - Me + 1; 27). Found: m/z 262.1481. Calcd
997, 838, and 725 cm-1. Found: m/z 311.1243. Calcd for C16H26
-
BrN: M, 311.1249.
J . Org. Chem, Vol. 69, No. 15, 2004 5069

Toyota et al.
for C16H23OP: M, 262.1487. 2c: Yellow prisms, mp 96-98 °C;
1H NMR (400 MHz, CDCl3) δ 6.63 (2H, s, arom.), 3.07 (6H, s,
NMe), and 1.71 (18H, s, t-Bu); 13C{1H} NMR (100 MHz, CDCl3)
13304049, 14044012, and 13640522) from the Ministry
of Education, Culture, Sports, Science and Technology,
J apan.
1
δ 171.4 (d, J PC ) 49.8 Hz, CP), 159.4 (d, J PC ) 5.6 Hz), 149.6
(d, J PC ) 4.7 Hz), 116.1 (d, J PC ) 21.7 Hz), 107.9, 40.4 (NMe),
37.6, and 31.1 (d, J PC ) 1.9 Hz); 31P{1H} NMR (162 MHz,
CDCl3) δ 32.1; UV (CH2Cl2) 377 nm (log ꢀ 4.50); IR (KBr) 3037,
2954, 2715, 1599, 1525, 1487, 1421, 1338, 1261, 1203, 1057,
989, 841, and 756 cm-1; MS (70 eV) m/z (rel intensity) 275
(M+; 43), 218 (M+ - t-Bu; 35), 190 (M+ - t-Bu - 2Me + 2; 99),
and 162 (M+ - 2t-Bu + 1; 100). Found: m/z 275.1797. Calcd
for C17H26NP: M, 275.1803. 2d : UV (CH2Cl2) 320 (log ꢀ 4.17)
and 334 nm (4.09).
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures for the preparation of 4-bromo-3,5-xylidine and 3, UV-
vis spectra of 2a -d (Figure 1), UV-vis spectra of p-MeO-
C6H4CtCH,p-(Me2N)C6H4CtCH,p-MeOC6H4CN,and p-(Me2N)-
C6H4CN (Figure 2), 13C NMR spectra for compounds 1e, 1f,
2a -c, 4-6, 9a ,b, and X-ray data in CIF format for 2c. This
material is available free of charge via the Internet at
http://pubs.acs.org.
Ack n ow led gm en t. This work was supported in part
by the Grants-in-Aid for Scientific Research (Nos.
J O049571Q
5070 J . Org. Chem., Vol. 69, No. 15, 2004