agent and conditions would be the major advantages of such
a method, with the aim of forming unstable Cp2Zr(II) species
in the presence of a substrate.
In each case, 1 mmol of Cp2ZrCl2, 0.66 mol of the metal,
and 2 mmol of diphenylacetylene were used. This stoichi-
ometry corresponded to the hypothetical transformations as
depicted in Scheme 1 (eq a). The reactions were carried out
as described previously at room temperature for 4 h. When
using La or Ce,13 the reaction mixtures turned deep red after
45 min and 1 h, and the dimer 1a was formed in 78 and
60% yields, respectively. In contrast, the use of Nd13 proved
to be inefficient. The reaction did not occur even by warming
the reaction mixture to 50 °C.
In the next experiments, 0.86 mmol of mischmetall was
used to reduce 1 mmol of Cp2ZrCl2. Since only about 80-
85% of mischmetall has to be considered to take part in the
reaction, this corresponded to the stoichiometric amount of
the active reductant (Ce, La) according to Scheme 1 (eq a).
Two alternative experimental approaches have been devel-
oped and can be used to efficiently form a zirconocene
equivalent. In the first, the color turned red within 10 min,
when the mixture of mischmetall powder, Cp2ZrCl2, and
diphenylacetylene in THF was warmed at 50 °C. The reaction
was next carried out at room temperature for the additional
4 h. In the second, a rapid color change (5-10 min) was
induced at room temperature by the addition of a few crystals
of iodine, and the reaction was then continued as previously.
By using the two experimental protocols above, the diene
1a was obtained, respectively, in 84 and 86% yields.14
By applying the latter procedure, several reactions have
been demonstrated to occur smoothly under mild conditions
(Scheme 2). Similarly to diphenylacetylene, also di(n-
Herein, we report on a practical and efficient experimental
protocol for the formation of a zirconocene equivalent by
employing mischmetall as reductant. Mischmetall, an alloy
of rare earths (Ce 48-50%, La 32-34%, Nd 13-14%, Pr
4-5%, and ca. 1% others), is a cheap material, commercially
available as ingots which can easily be powdered with a
rasp.7 After scraping, the mischmetall powder can be stored
under argon for several weeks. The use of mischmetall in
various synthetically useful reactions has recently been
reviewed.8 Mischmetall has been employed as a co-reductant
in Sm(II)-mediated Barbier and Grignard reactions,9 as well
as pinacol coupling reactions,10 or in SmI2/Pd(0)-catalyzed
coupling of allylic esters with ketones.11 Interestingly, in the
absence of another metal component, it appeared to be rather
unreactive toward esters, ketones, and the majority of alkyl
and aryl halides. These features as well as its availability
made mischmetall an interesting candidate for generating
Cp2Zr(II) species from Cp2ZrCl2.
Initially, when the mixture of mischmetall powder (1-2
mmol,12 based on its average molecular weight of 140
gmol-1) and Cp2ZrCl2 (1 mmol) in THF (5 mL) was stirred
under argon at room temperature for about 1 h, there was a
change of color from pale yellow to deep red. By carrying
out the reaction in the presence of 2 mmol of diphenylacetyl-
ene for 4 h, the dimer, (E,E)-1,2,3,4-tetraphenyl-1,3-buta-
diene (1a), was obtained after a hydrolytic workup, in
moderate yield of about 50%. Furthermore, the deuterated
analogue 1b was obtained after deuteriolysis, thus demon-
strating the formation of zirconacyclopentadiene intermediate
(A) (Scheme 1, eq b). To determine the specific reactivities
Scheme 2a
Scheme 1a
a Ln ) Mischmetall (Ce, La, Nd, Pr).
of the main mischmetall components, Ce, La, and Nd, the
reactions using pure lanthanide metals were next performed.
a Reaction conditions: THF, 2 crystals of I2, Cp2ZrCl2, Ln (0.86
(7) Caution: mischmetall is pyrophoric and sparks can come out from
ingot when scraped.
mmol). PMP ) 4-methoxyphenyl; PMB ) 4-methoxybenzyl.
(8) Lannou, M.-I.; He´lion, F.; Namy, J.-L. Tetrahedron 2003, 59, 10551-
10565.
(9) Di Scala, A.; Garbacia, S.; He´lion, F.; Lannou, M.-I.; Namy, J.-L.
Eur. J. Org. Chem. 2002, 2989-2995.
propyl)acetylene coupled to afford the dimer 1c in 93% yield.
Interestingly, the title reagent proved to be compatible with
the use of terminal alkynes. Thus, the reaction with phenyl-
(10) He´lion, F.; Lannou, M.-I.; Namy, J.-L. Tetrahedron Lett. 2003, 44,
5507-5510.
(11) Me´de´gan, S.; He´lion, F.; Namy, J.-L. Eur. J. Org. Chem. 2005,
4715-4722.
(12) Mischmetall ingots were purchased from Fluka or Alfa Aesar-
Johnson Matthey (minimum rare earth content is 99.0%).
(13) La, Ce, and Nd ingots (Aldrich, 99.9% purity) were powdered with
a rasp under Ar.
2946
Org. Lett., Vol. 8, No. 14, 2006