358
J.R. Garabatos-Perera, H. Butenscho¨n / Journal of Organometallic Chemistry 693 (2008) 357–360
unique, highly efficient ligands for a large number of tran-
sition metal catalyzed transformations such as C–C cou-
pling reactions (Suzuki–Miyaura, Heck, Stille, Sonogashira,
Negishi and enolate coupling) and C–O and C–N bond for-
mation reactions [13].
became possible by microwave irradiation (250 W,
150 ꢁC, 8 bar) [16]. In particular, the reaction of 4-methyl-
bromobenzene (12) gave a fourfold higher yield than with
conventional heating, and the reaction time was reduced
from 24 h to 1 h. For 4-chlorobenzonitrile (11) the yield
increased from 92% to 95% with a reduction in reaction
time from 72 h to 1 h. The highest yield (97 %) was
achieved by using 4-bromoacetophenone (16) under micro-
wave irradiation (entry 13). Initial experiments showed
Pd(dba)2 to be a better palladium source than Pd(OAc)2
(entries 1–3).
PtBu2
tBu
Ph
Ph
Ph
Ph
Fe
Ph
PtBu2
P
tBu
tBu
R
Suzuki–Miyaura coupling reactions were first reported
in 1981 [17] and have since then been further developed
by modifications in the metal catalyst, the phosphane
ligand, and reaction conditions such as solvent or the intro-
duction of microwave heating [18–22]. Some of the results
shown in Table 1 compare quite well with those of other
groups. For example, the reaction with 4-bromoacetophe-
none (16, entry 13) gives 97% yield, while the reaction
achieved 91% in a phosphane free, microwave heated reac-
tion. Similarly, the reaction of 4-methoxybromobenzene
(14, entry 11, 88%) compares to 86% under such conditions
[21]. Although more sophisticated diphosphine ligands give
better results in some cases [19], the results obtained for 4-
chlorobenzonitrile in the presence of 7 particularly with
aryl chloride 11 compare very well with those obtained
e.g. by Hierso with a much more complicated ferrocene-
based phosphane [23] or by Buchwald with 2-(di-tert-buty-
lphosphino)biphenyl [15].
R: electron withdrawing or
electron delivering group
ino, m orp-Position
5
3
4
The extraordinary activity of palladium catalysts
formed with these electron rich and sterically bulky phos-
phanes in coupling reactions involving aryl chlorides has
been explained by an increased propensity of the more elec-
tron rich catalyst to an oxidative addition of the aryl halide
and an easier decomplexation of one phosphane ligand
finally resulting in the formation of the catalytically active
Pd(0)L species (L = phosphane) [14]. The so far limited
structural diversity of electron rich and sterically bulky
monophosphanes causes major interest in the synthesis of
novel bulky and electron rich monophosphanes.
In our earlier work we had developed a procedure to
incorporate the sterically bulky and electron rich triferroce-
nylmethyl fragment by reaction of the triferrocenylmethyl
carbenium ion with nucleophiles. The carbenium ion was
easily generated by treatment of triferrocenylmethanol (6)
with triphenylmethyl tetrafluoroborate [12]. According
to this procedure treatment of the triferrocenylcarbeni-
um with lithiated methyldiphenylphosphane gave
phosphane 7 in 70 % yield. 7 was characterized spectro-
scopically.
In conclusion, we have presented the synthesis of (2,2,2-
triferrocenylethyl)diphenylphosphane and demonstrated
its suitability as a ligand in the palladium catalyzed
Suzuki–Miyaura cross-coupling reaction with several aryl
bromides and 4-chlorobenzonitrile. Improved yields and
shorter reaction times were realized under microwave
irradiation.
1. Experimental
PPh2
OH
+
–
1. Ph C BF
3
4
1.1. (2,2,2-Triferrocenylethyl)diphenylphosphane (7)
2. Ph PCH Li
2
2
Fe
Fe
Fe
Fe
THF
70 %
At 25 ꢁC, a well stirred suspension of triferrocenylmeth-
anol (6) (0.666 g, 1.1 mmol) in 50 mL of anhydrous diethyl
ether was treated with Ph3CBF4 (0.377 g, 1.1 mmol), and
the solution was allowed to react until no starting material
remained (TLC, CH2Cl2). The green precipitate was
washed three times with 3 · 50 mL of anhydrous DEE
each, dissolved in 50 mL of anhydrous THF and cooled
to ꢀ78 ꢁC. To this solution (lithiomethyl)diphenylphos-
phane borane complex, which was obtained from methyldi-
phenylphosphane borane complex (0.466 g, 2.1 mmol),
s-BuLi (1.86 mL, 1.8 M in DEE, 1.0 mmol) and 9.32 mL
of THF [24], was added. The reaction mixture was allowed
to warm to 25 ꢁC, and after 1 h 40 mL of Et2NH was
added. After stirring for 2 d at 25 ꢁC the volatiles were
removed at reduced pressure. The remaining solid was
washed with anhydrous CH2Cl2 until no product remained
Fe
Fe
6
7
Phosphine 7 was tested in the Suzuki–Miyaura coupling
reaction of phenylboronic acid and selected electron poor
and as well as electron rich arylbromides and chlorides.
Results are summarized in Table 1.
The results indicate that phosphane 7 promotes the
Suzuki–Miyaura coupling of aryl bromides and chlorides.
Best results were achieved with electron poor aryl halides
9–11. More electron rich aryl halides such as bromoben-
zene (8) or 4-methylbromobenzene (12) give poorer yields.
Higher yields and significantly shorter reaction times