Zirconium complexes incorporating diaryldiamidoferrocene ligands: Generation of cationic derivatives and polymerization activity towards ethylene and 1-hexene

Article abstract of DOI:10.1016/S0020-1693(02)01300-2

Palladium-catalyzed coupling of ArX (ArX=MesBr, 2,6-dichloroiodobenzene and 9-bromonaphtalene) with 1,1′-diaminoferrocene afforded a series of substituted diamide ligands Fc(NHAr)₂ (1). A Zr-dibenzyl complex 2 of the mesityl-substituted ligand was synthesized using ZrBn₄, and its ability to act as precatalyst for olefin polymerization was investigated. Abstraction of one benzyl group using B(C₆F₅)₃ resulted in the zwitterionic species [LZrBn][BnB(C₆F₅)₃] (3), which reacted slowly (approximately 30 min) with ethylene to afford the monoinsertion product [LZrCH₂CH₂CH₂Ph][BnB(C₆F ₅)₃]. Further insertions of ethylene were slower and led, after 2 h, to the di- and tri-insertion products. Use of [Ph₃C][B(C₆F₅)₄] as an activator resulted in a competent 1-hexene polymerization catalyst affording high molecular weight poly-1-hexene with a PDI of 1.3-1.4.

Full text of DOI:10.1016/S0020-1693(02)01300-2

Inorganica Chimica Acta 345 (2003) 216Á220
/
www.elsevier.com/locate/ica
Zirconium complexes incorporating diaryldiamidoferrocene ligands:
generation of cationic derivatives and polymerization activity towards
ethylene and 1-hexene
Alexandr Shafir a, John Arnold b,*
a Department of Chemistry, University of California, Berkeley, CA, USA
b The Chemical Sciences Division, Lawrence Berkeley National Laboratory, 526 Latimer Hall, Berkeley, CA 94720-1460, USA
Received 7 June 2002; accepted 2 September 2002
Dedicated to Professor R.R. Schrock
Abstract
Palladium-catalyzed coupling of ArX (ArXꢀMesBr, 2,6-dichloroiodobenzene and 9-bromonaphtalene) with 1,1?-diaminoferro-
/
cene afforded a series of substituted diamide ligands Fc(NHAr)2 (1). A Zr-dibenzyl complex 2 of the mesityl-substituted ligand was
synthesized using ZrBn4, and its ability to act as precatalyst for olefin polymerization was investigated. Abstraction of one benzyl
group using B(C6F5)3 resulted in the zwitterionic species [LZrBn][BnB(C6F5)3] (3), which reacted slowly (approximately 30 min) with
ethylene to afford the monoinsertion product [LZrCH2CH2CH2Ph][BnB(C6F5)3]. Further insertions of ethylene were slower and led,
after 2 h, to the di- and tri-insertion products. Use of [Ph3C][B(C6F5)4] as an activator resulted in a competent 1-hexene
polymerization catalyst affording high molecular weight poly-1-hexene with a PDI of 1.3Á1.4.
/
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Olefin polymerization; Zirconium complexes; Amidoferrocene complexes
1. Introduction
We recently reported the synthesis of the silylated
diaminoferrocene Fc(NHSiMe3)2, from which, a series
of Ti and Zr complexes were prepared [15]. We also
communicated the results of our study of the activation
Diamide donor ligands are among the most actively
investigated non-Cp ligand systems for catalytic olefin
of
Fc(NSiMe3)2TiMe2
with
B(C6F5)3
and
polymerization with Group 4 metals [1Á10]. Our own
/
[Ph3C][B(C6F5)4] (TB) which highlighted the unusually
short interaction between the iron and the electron-
deficient Ti center in the m-Cl dimer [Fc(NSi-
Me3)2TiCl][B(C6F5)4] [16]. When Fc(NSiMe3)2TiMe2 is
activated with TB, the resulting cation reacts with 1-
hexene, however only short-chain oligomers are pro-
duced whose olefinic end groups are suggestive of a b-H
elimination termination pathway.
With the goal of producing more competent 1-hexene
polymerization catalysts, we prepared a series of aryl-
substituted Fc ligands. Aryl-substituted diamines are
well known to act as supporting ligands for active olefin-
polymerization catalysts [2,4,5,17]. In this paper we
report the synthesis of aryl-substituted ligands through
recent efforts have been focused on developing ferro-
cene-based diamide ligands. The ferrocene backbone
provides for a relatively rigid coordination environment,
with a well-defined directionality of the nitrogen donor
vectors. The system, however, is not completely inflex-
ible, allowing for partial relief of cyclic strain through
the opening of the CpÃ
/
Cp dihedral angle by as much as
128. In addition, further stabilization is possible by
distortions [11] that serve to bring the electron-rich
ferrocene group in close proximity to electron-deficient
cationic metal centers [12Á14].
/
a Pd-mediated CÃ
/
N bond formation. In addition, a Zr
* Corresponding author. Fax: ꢁ
E-mail address: arnold@socrates.berkeley.edu (J. Arnold).
/
1-508-437 7852
dibenzyl complex was prepared and its ability to
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 1 3 0 0 - 2

A. Shafir, J. Arnold / Inorganica Chimica Acta 345 (2003) 216Á
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217
function as an olefin polymerization catalyst was
studied.
yield. Sterically more demanding 2,6-diisopropyl-bro-
mobenzene and 2,4,6-tris-t-bu-bromobenzene failed to
couple to any noticeable extent under these conditions.
Furthermore, attempts to couple 1,1?-dibromoferrocene
with aniline proved unsuccessful.
The 1H NMR spectra of both 1b and 1c exhibited the
ferrocene pattern consisting of two pseudo-triplets.
Compounds derived from 1a will be discussed in this
report, while 1b and 1c will be the subject of a future
publication.
The Zr-dibenzyl complex 2 was obtained by reacting
1a with Zr(CH2Ph)4 in toluene (Eq. (2)). The resulting
complex was isolated from a toluene/pentane mixture as
a red crystalline air- and moisture-sensitive solid in
moderate yields. The 1H NMR spectrum of the product
is consistent with C2v symmetry and, as in the free
ligand, contains two ferrocenyl pseudo-triplets.
2. Results and discussion
The functionalized diamine 1a was synthesized
through Pd0-mediated cross-coupling of 1,1?-diamino-
ferrocene with mesityl bromide (Eq. (1)) following a
procedure developed by Buchwald and coworkers [18]
This procedure has already been applied by Schrock et
al. to the synthesis of mesityl-substituted diamido-amino
donor ligands [2].
ð1Þ
The reaction was complete after 8 h of heating at
100 8C and the pure product was obtained following
recrystallization from Et2O. The H NMR spectrum of
1
1a is typical of 1,1?-disubstituted ferrocenes, containing
two pseudo-triplets for the ferrocene backbone in
addition to the typical resonances associated with the
NH and the mesityl groups. Related phenyl-substituted
aminoferrocenes have been reported by others [19,20].
ð2Þ
Interestingly, while crude, unrecrystallized 1a reacted
readily with Zr(CH2Ph)4, the pure crystalline diamine
appeared to react sluggishly, if at all. The culprit in this
unusual behavior was traced to the presence of a small
amount of NaOtBu that was present in the crude ligand
from the cross-coupling step. Apparently then, a trace of
base is necessary in order to catalyze the protonolysis
reaction [21].
In typical fashion, the strong Lewis acid B(C6F5)3
reacted cleanly with 2 via benzyl abstraction to form the
zwitterionic p-complex 3 (Eq. (3)). The compound is
soluble in benzene, suggesting a close ion-pairing in
solution.
Encouraged by the ease of the CÃ/N bond formation
in the case of MesBr, we decided to probe the effective-
ness of the catalyst with other aryl halide reagents.
Schrock et al. reported that 2,6-dichlorobromobenzene
is an effective arylating agent under similar conditions
[3]. In our system, the corresponding product 1b was
obtained using 2,6-dichloroiodobenzene, although 2
weeks of heating to 100 8C were required to drive the
reaction to completion. Similarly, the use of 9-bromo-
napthalene upon heating the reaction mixture to 100 8C
for 40 h led to the formation of the diamine 1c in 80%
ð3Þ

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A. Shafir, J. Arnold / Inorganica Chimica Acta 345 (2003) 216Á220
/
A single sharp resonance at ꢂ
/
12.5 ppm was observed
new sets of resonances appeared corresponding to di-
and tri-insertion products.
The reaction mixture was hydrolyzed with 1 M HCl
in the 11B NMR spectrum, confirming the presence of a
tetrahedral borate environment. The symmetry of the
resulting species is reduced from C2v to Cs as evidenced
by the presence of four ferrocenyl resonances (4.09, 3.93,
3.65 and 3.43 ppm) in the 1H NMR spectrum. This
splitting pattern is typical of the ferrocenyl ABCD spin
system, indicating that each Cp-bound proton resides in
a unique chemical environment (note that the two Cp
rings are still related by a mirror plane). Two new benzyl
and the resulting solution was analyzed by GCÁ
addition to propylbenzene (m/zꢀ120, 66%), the mix-
ture contained pentyl- and heptylbenzenes (m/zꢀ148
(20%) and 176 (4%), respectively).
/MS. In
/
/
The use of [Ph3C][B(C6F5)4] (TB), rather than
B(C6F5)3, as a co-catalyst often leads to an increased
activity of the resulting catalytic species. Indeed, while
the 2/B(C6F5)3 combination did not yield a viable 1-
hexene polymerization catalyst, addition of 1-hexene to
a mixture of 2 and TB in chlorobenzene led to an
exothermic reaction which, after quenching with acid,
afforded poly-1-hexene.
resonances*
doublet 2.96 ppm*
the boron-bound methylene groups, respectively. Re-
stricted rotation of the mesityl groups around the NÃ
/a sharp singlet at 3.20 ppm and a broad
/were assigned to the Zr-bound and
/C
bond is evidenced by the presence of three mesityl
methyl and two aromatic mesityl CH resonances. The
aromatic benzyl borate resonances are shifted upfield
(6.35 ortho, 6.24 meta and 5.79 para) in the typical
manner observed for p-bound arene ligands. Coordina-
tion of a benzylborate anion to cationic Ti and Zr
centers is well documented and leads to the stabilization
of an otherwise extremely electron-deficient metal center
Up to 400 equiv. of 1-hexene were consumed, yielding
poly-1-hexene with molecular weights of up to 20 000
with relatively low polydispersities (PDIꢀ
/
1.3Á1.4).
/
Addition of more than approximately 400 equiv. of 1-
hexene does not lead to higher molecular nor higher
production of the polymer. This finding, along with the
fact that no olefinic resonances were observed in the 1H
NMR, leads us to believe that the catalyst deactivation
occurs through the CH activation of a mesityl methyl
group, a situation encountered by the Schrock group
with a related mesityl-functionalized amido system [3].
[22Á25].
/
Although stable in C6D6 for several days, the cation 3
1
decomposed upon attempted isolation. The H NMR
spectrum of the solid isolated from C6H6/pentane shows
two doublets (at 0.35 and 2.95 ppm) resulting from a
Indeed, combining 2 and TB in C6D5Cl at ꢂ30 8C led,
/
upon warming to 0 8C, to a mixture of two highly
asymmetric products, both of which appear to contain
an activated methyl group as part of a metallacycle with
zirconium. In both compounds the methylene hydrogen
pair of diastereotopic methylene hydrogens (JHH
Hz). Attempts to characterize the material further have
been unsuccessful, nevertheless, the pattern is consistent
ꢀ12.3
/
with CÃ/H bond activation of one of the mesityl methyl
atoms (mesÃ
/
CH2Ã/Zr) are diastereotopic and are found
groups to form a Zr-metallacycle, and we note that
related reactions have been observed previously [3,26].
Solutions of 3 did not react with 1-hexene even upon
at significantly different shifts (2.70 and 0.03 ppm). The
scope and mechanism of these, and related, reactions is
presently under active investigation.
1
heating to 60 8C for several hours, as judged by H
NMR spectroscopy. In the case of ethylene, however, 3
reacted slowly at room temperature as shown in Eq. (4).
Thus, injection of approximately 4 equiv. of ethylene
into an NMR tube containing a solution of 3 in C6D5Cl
led after 30 min to the consumption of 1 equiv. of
ethylene and a complete conversion of 3 to the mono-
insertion product 4.
3. Experimental
3.1. General considerations
Standard Schlenk-line and glove box techniques were
used unless otherwise indicated. Pentane, C6H5CH3,
C6H6 and Et2O were purified by passage through a
column of activated alumina and degassed with Ar prior
to use. PhCl was distilled from CaH2. Fc(NH2)2 [27],
ZrBn4 [28], B(C6F5)3 [29] and [Ph3C][B(C6F5)4] [30] were
prepared according to published procedures. The Pd
catalyst (rac-BINAP)Pd(dba) was prepared and used
according to a procedure established by Buchwald and
ð4Þ
coworkers [18]. MesBr was distilled from CaH2. GCÁ
/
The methylene hydrogens of the newly formed g-
phenylpropyl ligand appeared as multiplets at 2.72, 2.61
and 1.64 ppm. Four new ferrocene multiplets were also
present, as well as a new set of mesityl resonances.
Further insertion of ethylene was slow, but after 2 h,
MS measurements were done using an Agilent 6890
Series GC system equipped with an Agilent 5973N Mass
Detection System. C6D6 was vacuum-transferred from
Na/benzophenone. C6D5Cl was vacuum transferred
from CaH2. All 1H, and 13C{1H} and 11B NMR spectra

A. Shafir, J. Arnold / Inorganica Chimica Acta 345 (2003) 216Á
/220
219
1
crystals of 1c (530 mg, 62%). H NMR (C6D6): d 7.67
were recorded on a Bruker DRX 500 MHz spectro-
meter. 1H NMR chemical shifts in C6D6 are given
relative to C6D5H (7.16 ppm). Due to the presence of a
(d, 2H), 7.38Á7.17 (m, 10H), 6.98 (m, 2H), 5.40 (s, 2H,
/
NH), 4.19 (‘t’, 4H, Fc), 3.94 (‘t’, 4H, Fc) ppm.
1
large number of peaks in the aromatic region, the H
NMR shifts in C6D5Cl are given relative to an internal
standard Ph3CCH2Ph (3.85 ppm for CH2), which was
either generated during a reaction or added to the
reaction mixture. 13C{1H} NMR spectra are relative to
C6D6 (128.3 ppm). 11B NMR spectra are referenced to
3.2.4. [Fc(NMes)2]ZrBn2 (2)
Diamine 1a (2.42 g, 5.35 mmol), ZrBn4 (2.73 g, 6.00
mmol) and NaOtBu (20 mg) were combined in a flask
and 40 ml of C6H5CH3 was added. The resulting
mixture was stirred for 12 h, at which point the solvent
was removed and the solid was extracted with C6H5CH3
an external BF3ÁEt2O in C6D6 standard (0.0 ppm).
/
Elemental analyses were determined at the Microanaly-
tical Laboratory of the College of Chemistry, University
of California, Berkeley.
(2ꢃ
C5H12 was added to induce crystallization. The mixture
was then cooled to ꢂ40 8C for 12 h and the resulting
/
30 ml). The solution was concentrated to 10 ml and
/
red crystalline product was isolated by filtration. Yield:
1
1.95 g, 51%. H NMR (C6D6): d 7.06 (t, 4H), 6.85 (m,
3.2. Synthesis of complexes
6H), 6.70 (d, 4H), 4.06 (‘t’, 4H, Fc), 3.85 (‘t’, 4H, Fc),
2.38 (s, 12H, o-Me), 2.32 (s, 4H, ZrCH2), 2.18 (s, 6H, p-
Me) ppm. 13C{1H} NMR (C6D6): d 149.2 (Ar), 145.9
(Ar), 134.8 (Ar), 134.5 (Ar), 130.2 (Ar), 130.0 (Ar), 127.6
(Ar), 123.1 (Ar), 93.5 (ipso-Fc), 69.8 (Fc), 69.6 (Fc), 69.5
3.2.1. Fc(NHMes)2 (1a)
A 100 ml flask was charged with Fc(NH2)2 (2.00 g,
9.26 mmol), NaOtBu (2.60 g, 27 mmol), and (BI-
NAP)Pd(dba) (96 mg, 100 mmol, 0.5 mol%). Toluene
(30 ml) and MesBr (3.05 ml, 20.0 mmol) were added and
the mixture was heated to 100 8C for 10 h. The reaction
mixture was allowed to cool and the volatiles were
removed under vacuum. Extraction with CH2Cl2, filtra-
tion and removal of the solvent resulted in 3.65 g of red
crystalline product (87%). Analytically pure compound
was obtained by recrystallization of the product from
(ZrÃ
/
CH2), 21.2 (p-Me), 20.5 (o-Me) ppm. Anal. Calc.
for C42H44FeN2Zr: C, 69.69; H, 6.13; N, 3.87. Found:
C, 69.38; H, 6.48; N, 3.63%.
3.2.5. Observation of
{[Fc(NMes)2]ZrBn}{BnB(C6F5)3} (3)
A solution of 2 (17 mg, 23 mmol) in 0.3 ml of C6D6
was added to a solution of B(C6F5)3 (12 mg, 23 mmol) in
0.3 ml of C6D6. A dark red solution was formed
immediately. 1H NMR (C6D6): d 7.379 (m, 2H, m-
Ph), 7.193 (d, 2H, o-Ph), 7.092 (t, 1H, p-Ph), 6.859 (s,
2H, m-Mes), 6.798 (s, 2H, m-Mes), 6.358 (d, 2H, o-Ph),
6.261 (m, 2H, m-Ph), 5.797 (t, 1H, p-Ph), 4.086 (m, 2H,
Fc), 3.926 (m, 2H, Fc), 3.653 (m, 2H, Fc), 3.433 (m, 2H,
1
Et2O (2.70 g, 64%). H NMR (C6D6): d 6.78 (s, 4H,
MesH), 4.05 (s, 2H, NH), 3.80 (‘t’, 4H, Fc), 3.78 (‘t’,
4H, Fc), 2.17 (s, 6H, Me), 2.12 (s, 12H, Me) ppm.
13C{1H} NMR (C6D6): d 140.1 (Ar), 132.7 (Ar), 131.3
(Ar), 130.2 (Ar), 108.5 (ipso-Fc), 65.0 (Fc), 60.7 (Fc),
21.2 (p-Me), 19.0 (o-Me) ppm. Anal. Calc. for
C28H32FeN2: C, 74.33; H, 7.13; N, 6.19. Found: C,
74.31; H, 7.27; N, 6.08%.
Fc), 3.197 (s, 2H, ZrÃ
/
CH2), 2.963 (br d, 2H, BÃ
/CH2),
2.227 (s, 6H, Me), 2.213 (s, 6H, Me), 1.950 (s, 6H, Me)
ppm. 13C{1H} NMR (C6D6): d 152.1 (Ar), 149.7 (Ar),
136.2 (Ar), 134.5 (Ar), 133.5 (Ar), 130.8 (Ar), 129.8 (Ar),
129.7 (Ar), 129.3 (Ar), 129.1 (Ar), 128.6 (Ar), 128.5 (Ar),
126.3 (Ar), 125.6 (Ar), 123.9 (Ar), 91.5 (ipso-Fc), 75.3
3.2.2. Fc[NH(2,6-Cl2C6H3)]2 (1b)
A 50 ml flask was charged with Fc(NH2)2 (300 mg,
1.39 mmol), NaOtBu (355 mg, 3.70 mmol), (BI-
NAP)Pd(dba) (14 mg, 14 mmol, 0.5 mol%) and 2,6-
dichloroiodobenzene (791 mg, 2.90 mmol). Toluene (15
ml) was added and the mixture was heated to 100 8C
for 14 days. The volatile fraction was removed and the
product was extracted with CH2Cl2. Removal of the
solvent afforded 420 mg of 1b as a red powder (60%). 1H
NMR (C6D6): d 6.92 (d, 4H), 6.27 (t, 2H), 5.28 (s, 2H,
NH), 4.01 (‘t’, 4H, Fc), 3.82 (‘t’, 4H, Fc) ppm.
(Fc), 74.7 (Fc), 68.5 (Fc), 67.6 (Fc), 63.1 (ZrÃ
(BÃ
CH2) (chemical shift inferred from a 13CÁ1
HMQC experiment), 20.5 (Me), 19.9 (Me) ppm. 11B
NMR (C6D6): d ꢂ12.5 ppm.
/
CH2), 36
/
/
H
/
3.2.6. Observation of
{[Fc(NMes)2]ZrCH2CH2CH2Ph}{BnB(C6F5)3} (4)
Ethylene gas (2.0 ml) was injected through a septum,
via syringe, into an NMR tube containing a solution of
2 (17 mg, 23 mmol) and B(C6F5)3 (12 mg, 23 mmol) in 0.6
ml of C6D5Cl. Progress of the reaction was monitored
3.2.3. Fc[NH(9-Nap)]2 (1c)
A 50 ml flask was charged with Fc(NH2)2 (400 mg,
1.85 mmol), NaOtBu (444 mg, 4.63 mmol), (BI-
NAP)Pd(dba) (20 mg, 20 mmol, 0.5 mol%) and 9-
bromonaphtalene (780 mg, 3.80 mmol). Toluene (15
ml) was added and the mixture was heated to 100 8C
for 40 h. After cooling to room temperature the solution
1
by H NMR spectroscopy. After 30 min, 1 equiv. of
ethylene was found to be consumed and conversion of 3
1
to 4 was complete. H NMR (C6D5Cl): d 7.2Á
/6.8 (m,
7H, Ar), 6.35 (t, 2H, Ar), 4.37 (m, 2H, Fc), 4.04 (m, 2H,
Fc), 3.88 (m, 2H, Fc), 3.56 (m, 2H, Fc), 2.72 (m, 2H,
was filtered and cooled to ꢂ40 8C affording yellow
/

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A. Shafir, J. Arnold / Inorganica Chimica Acta 345 (2003) 216Á220
/
[8] A.D. Horton, K.L. Von Hebel, J. De With, Macromol. Symposia
173 (2001) 123.
CH2), 2.61 (m, 2H, CH2), 2.28 (s, 3H, Me), 2.18 (s, 3H,
Me), 2.10 (s, 3H, Me), 1.64 (m, 2H, CH2) ppm.
[9] F.G.N. Cloke, T.J. Geldbach, P.B. Hitchcock, J.B. Love, J.
Organomet. Chem. 506 (1996) 343.
3.3. Polymerization of 1-hexene
[10] G.R. Giesbrecht, A. Shafir, J. Arnold, Chem. Commun. (2000)
2135.
[11] The electronic origin of these interactions is unclear at present.
What may be related distortions in phenylene diamine systems
have been attributed to p-interactions, either from the nitrogens
or the phenyl ring. For pertinent references see: (a) J.M. Boncella,
S.Y.S. Wang, D.D. Vanderlende, R.L. Huff, K.A. Abboud, W.M.
Vaughn, J. Organomet. Chem. 530 (1997) 59;
In a typical procedure, a solution of 2 (34 mg, 47
mmol) and [Ph3C][B(C6F5)4] (42 mg, 45 mmol) was
prepared in PhCl (7.0 ml). After stirring the mixture
for 5 min, 1-hexene was added via syringe. After stirring
for an additional 30 min, the mixture was quenched with
2 ml of 1 M HCl. The volatile components were
removed under vacuum and the residue was extracted
with THF; after filtration, the solvent was again
removed to yield poly-1-hexene as a very viscous gum.
The resulting polymer was characterized using GPC
calibrated using polystyrene standards.
(b) A. Galindo, A. Ienco, C. Mealli, New J. Chem. 24 (2000) 73;
(c) K. Aoyagi, P.K. Gantzel, K. Kalai, T.D. Tilley, Organome-
tallics 15 (1996) 923.
[12] D. Seyferth, B.W. Hames, T.G. Rucker, M. Cowie, R.S. Dickson,
Organometallics 2 (1983) 472.
[13] M. Sato, M. Sekino, M. Katada, S. Akabori, J. Organomet.
Chem. 377 (1989) 327.
[14] M. Sato, K. Suzuki, H. Asano, M. Sekino, Y. Kawata, Y.
Habata, S. Akabori, J. Organomet. Chem. 470 (1994) 263.
[15] A. Shafir, M.P. Power, G.D. Whitener, J. Arnold, Organome-
tallics 20 (2001) 1365.
4. Supplementary material
[16] A. Shafir, J. Arnold, J. Am. Chem. Soc. 123 (2001) 9212.
[17] P.E. O’Connor, D.J. Morrison, S. Steeves, K. Burrage, D.J. Berg,
Organometallics 20 (2001) 1153.
The material is available from the authors on request.
[18] J.P. Wolfe, S. Wagaw, S.L. Buchwald, J. Am. Chem. Soc. 118
(1996) 7215.
[19] M. Herberhold, M. Ellinger, W. Kremnitz, J. Organomet. Chem.
241 (1983) 227.
Acknowledgements
[20] U. Siemeling, O. Kuhnert, B. Neumann, A. Stammler, H.G.
Stammler, B. Bildstein, M. Malaun, P. Zanello, Eur. J. Inorg.
Chem. (2001) 913.
[21] We note that mixing ZrBn4 with NaOtBu in toluene affords a
precipitate of Na[ZrBn4(OtBu)], which has been characterized by
NMR spectroscopy and elemental analysis. This complex also
acts to catalyze the reaction between 1a and ZrBn4. Full details
will be reported separately.
This research was supported by the U.S. Department
of Energy under Contract No. DE-AC03-76SF00098.
We also thank Professor K.P.C. Vollhardt for use of his
GCÁMS.
/
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