5028 Organometallics, Vol. 26, No. 20, 2007
Ogasawara et al.
binap catalyst gave the axially chiral allene (R)-4en12 of 57%
ee in 56% yield (Scheme 7). On the assumption that there is no
exchange process between anti-12 and syn-12, the reaction of
(Z)-2e with 3n′ catalyzed by Pd/(R)-binap should form (S)-4en,
in which the relative positions of the Ph group and the Me group
on the allenic terminal carbon are reversed compared with the
allenic product from (E)-2e. As predicted from Scheme 6,
however, the product from (Z)-2e was also (R)-4en with nearly
identical enantiopurity (58% ee, 61% yield).
Table 2. Preparation of Allenes 4 Catalyzed by Pd Complex
1a
entry
diene 2
nucleophile 3
time (h)
yield of 4b (%)
1
2
3
4
5
6
7
8
9
2a
2a
2a
2b
2c
2d
(Z)-2e
(E)-2e
(Z)-2e
2f
3m
3n
3o
3o
3o
3m
3m
3n
3n
3n
18
24
24
24
36
4
24
72
36
6
84 (4am)
88 (4an)
70 (4ao)
79 (4bo)
76 (4co)
91 (4dm)
95 (4em)
35 (4en)c
29 (4en)
90 (4en)
Conclusions
10
a Reaction was carried out with 2 (1.0 mmol) and 3 (1.1 mmol) in THF
In summary, we have determined a solid-state structure of
an alkylidene-π-allylpalladium complex for the first time. While
the Pd complex reacts with a stabilized nucleophile at the
unsubstituted terminal of the η3-allylic moiety to give an allene,
a Grignard reagent attacks at the Pd center to give a conjugated
diene. The nucleophile-dependent selectivity is consistent with
previously reported allene versus diene selectivity in alkylidene-
π-allylpalladium-mediated reactions. The Pd complex was found
to be a good catalyst precursor for the reactions between
2-bromo-1,3-dienes and stabilized nucleophiles, giving allenes.
The dynamic process of forming the alkylidene-π-allylpalladium
complexes from 2-bromo-1,3-dienes and Pd(0) was also clari-
fied.
b
in the presence of 1 (0.02 mmol; 2 mol %). Isolated yield by silica gel
c
chromatography. Remaining substrate (E)-2e was recovered in 44% yield.
exclusively; no conjugated dienes were detected. In our original
report of the reaction,2a it was found that the choice of phosphine
ancillary ligand was important to gain high activity of the
palladium catalyst. The best choice was the triarylbisphosphine
dpbp,11 and a Pd species with dppb, which is the phosphine
ligand in 1, showed poor performance. As a consequence, the
catalyst 1 showed lower catalytic activity, and relatively higher
temperature and longer time were required for the reactions in
Table 2.
A reaction between (E)-2e and 3n was especially sluggish
and was incomplete even after 72 h (entry 8). An analogous
reaction with the isomeric (Z)-2e was also slow (entry 9). On
the other hand, the triflate 2f, which is isostructural to (E)-2e,
was much more reactive, and a reaction with 3n was complete
within 6 h (entry 10). Oxidative addition of either 2e or 2f to a
Pd(0) species gives an equilibrium mixture of the σ-dienylpal-
ladium 9 and the exo-alkylidene-π-allylpalladium 10 (Scheme
5). Triflate anion is a weaker σ-donor than bromide, and thus
replacement of the anion in the Pd intermediates from bromide
to triflate should drive the equilibrium toward 10. This may be
the origin of the different reactivity between 2e and 2f. Because
a nucleophile attacks 10 (not 9) to give an allene in the final
step of the catalytic process, the triflate substrate 2f is more
reactive than the bromide substrate 2e.
Experimental Section
General Procedures. All anaerobic and/or moisture-sensitive
manipulations were carried out with standard Schlenk techniques
under predried nitrogen or with glovebox techniques under prepu-
rified argon. H NMR (at 400 MHz) and 13C NMR (at 101 MHz)
1
chemical shifts are reported in ppm downfield of internal tetram-
ethylsilane. 31P NMR (at 162 MHz) chemical shifts are externally
referenced to 85% H3PO4. Tetrahydrofuran was distilled from
benzophenone-ketyl under nitrogen prior to use. Dichloromethane
was distilled from CaH2 under nitrogen prior to use. [PdCl(π-
allyl)]2,13 NaBArF ,8 2-bromo-1,3-dienes (2a,2a 2b,1c 2c,1a 2d,7 and
4
2e2c), and 1,3-dien-2-yl triflate 2f2d were prepared according to the
reported methods. All other chemicals were obtained from com-
mercial sources.
Preparation of [Pd(η3-CH2CMeCdCMe2)(dppb)]‚B[C6H3-3,5-
(CF3)2]4 (1). A mixture of [PdCl(η3-C3H5)]2 (84.5 mg, 462 µmol/
Pd), dppb (200 mg, 469 µmol), and Na[CMe(CO2Me)2] (82.0 mg,
488 µmol) was placed in a Schlenk flask, and to this was added
dry THF (5 mL) under nitrogen. The mixture was stirred at room
temperature for 20 min, then Me2CdCBr-CMedCH2 (2d, 98.0
mg, 560 µmol) was added to the flask by means of a syringe. After
stirring the reddish-orange solution for 5 h at room temperature,
all the volatiles were removed under reduced pressure. To the
Preparation of [Pd(η3-CH2CHCdCMePh)(dppb)]BArF
4
(11). Another alkylidene-π-allylpalladium complex, [Pd(η3-CH2-
CHCdCMePh)(dppb)]BArF (11), was prepared as a pale
4
yellow, viscous oil from 3-bromo-4-phenyl-1,3-pentadiene (2e)
by the method shown in Scheme 1. Since complex 11 possesses
two different substituents on the exo-alkylidene carbon, it exists
as a mixture of two isomers (see Scheme 6). Complex 11
prepared from (E)-2e consists of anti- and syn-isomers in a 95:5
molar ratio. No exchange was detected between the two isomers.
It was found that the relative configuration of the PhMeCdC
moiety in 2e did not affect formation of 11; that is, (Z)-2e
also afforded 11 with the identical isomeric distribution
(anti-11/syn-11 ) 95:5). Apparently, there is a rapid exchange
process between syn- and anti-intermediates 12, which have a
bromide counteranion, and they reach thermodynamic equilib-
rium prior to termination of the exchange by NaBArF4 treatment.
The observations shown in Scheme 6 imply that the difference
between E- and Z-isomers of the bromodiene substrate might
show no influence in the alkylidene-π-allylpalladium-mediated
asymmetric allene synthesis. A reaction of (E)-2e with Cs-
[C(NHAc)(CO2Et)2] (3n′) at 30 °C in the presence of a Pd/(R)-
residue were added NaBArF (470 mg, 530 µmol) and CH2Cl2 (5
4
mL), and the mixture was stirred over night at room temperature.
The precipitates were removed by filtration, and the filtrate was
evaporated to dryness under reduced pressure. The yellow residue
was chromatographed on silica gel with CHCl3 as an eluent to give
the complex (655 mg, 95% yield) as a pale yellow, viscous oil.
Recrystallization from Et2O/pentane gave colorless, prismatic
1
crystals. H NMR (CDCl3): δ 0.95 (s, 3H), 1.55-1.89 (m, 4H),
1.83 (s, 3H), 1.96 (s, 3H), 2.33-2.51 (m, 4H), 3.05 (d, JHP ) 8.6
Hz, 1H), 4.03 (d, JHP ) 5.1 Hz, 1H), 7.23-7.28 (m, 2H), 7.34-
7.56 (m, 19H), 7.59-7.63 (m, 1H), 7.71-7.77 (m, 10H). 31P{1H}
NMR (CDCl3): δ 15.3 (d, JPP ) 46.3 Hz), 30.7 (d, JPP ) 46.3
(12) The specific rotation value for 4en of 58% ee is [R]26.0 -38.7 (c
1.26, CHCl3). The absolute configuration of 4en was deduced by the Lowe-
Brewster rule; see: (a) Lowe, G. Chem. Commun. 1965, 411. (b) Brewster,
J. H. Top. Stereochem. 1967, 2, 1.
D
(11) dpbp ) 2,2′-bis(diphenylphosphino)-1,1′-biphenyl. See: Ogasawara,
M.; Yoshida, K.; Hayashi, T. Organometallics 2000, 19, 1567, and
references therein.
(13) Tatsuno, A.; Yoshida, T.; Otsuka, S. Inorg. Synth. 1979, 19, 220.
Ogasawara, Masamichi
