Et2SiH2 assisted the selective dimerization of terminal alkynes catalyzed by Cp*2UMe2

Article abstract of DOI:10.1016/S0022-328X(02)02136-8

A practical approach has been developed for the catalytic synthesis of short oligomers, dimers and/or trimers of terminal alkynes. The method allows control of the extent and, in some cases, the regiospecificity in the catalyzed oligomerization of terminal alkynes promoted by bis(pentamethylcyclopentadienyl)uranium dimethyl complex (Cp*₂U(CH₃)_2, Cp= C₅Me₅). The metallocene precursor is known to promote the simultaneous production of a large number of differently sized oligomers in the presence of terminal alkynes. However, the addition of a specific secondary silane ensures the selective synthesis of short oligomers.

Full text of DOI:10.1016/S0022-328X(02)02136-8

Journal of Organometallic Chemistry 670 (2003) 97Á107
/
www.elsevier.com/locate/jorganchem
Et SiH assisted the selective dimerization of terminal alkynes
2
2
ꢀ
catalyzed by Cp UMe
2
2
Ji Quan Wang, Moris S. Eisen *
Department of Chemistry, Institute of Catalysis Science and Technology, Technion, Israel Institute of Technology, Haifa 32000, Israel
Received 10 October 2002; accepted 14 November 2002
Abstract
A practical approach has been developed for the catalytic synthesis of short oligomers, dimers and/or trimers of terminal alkynes.
The method allows control of the extent and, in some cases, the regiospecificity in the catalyzed oligomerization of terminal alkynes
promoted by bis(pentamethylcyclopentadienyl)uranium dimethyl complex (Cp2
ꢀ
U(CH ) , Cp*ꢁC Me ). The metallocene precursor
/
3 2
5 5
is known to promote the simultaneous production of a large number of differently sized oligomers in the presence of terminal
alkynes. However, the addition of a specific secondary silane ensures the selective synthesis of short oligomers.
#
2003 Elsevier Science B.V. All rights reserved.
Keywords: Organoactinide; Alkyne complexes; Dimerization of alkynes; Oligomerization of alkynes; Catalysis; Silane
1
. Introduction
the means to control the selectivity (extent of oligomer-
ization) of the products has been to proceed through the
addition of amines [6]. It is noteworthy to point out that
catalysts (early and late transition metals) for the highly
regioselective specific dimerization of terminal alkynes
have been reported but provide no means to generate or
to control the formation of higher oligomers [7,8].
Recently [9], we have shown that organoactinide com-
Tailoring stoichiometric and catalytic reaction to
obtain selective, and if possible regiospecific, products
is a challenging goal in modern organometallic chem-
istry. In many cases, mild and easily controlled reactions
have been obtained [1,2]. Besides the many successes,
controlling the selectivity and regiospecificity of a
catalytic oligomerization/polymerization remains a chal-
lenge [3]. Dihydrogen, for instance, is added to cut the
plexes of the type (C Me ) AnMe (Anꢁ
/Th, U) were
5
5 2
2
found effective precatalysts for the oligomerization of
t
growing polymeric chains in the catalytic ZieglerÁ
/Natta
terminal alkynes. Bulky acetylenes, such as BuCÅ
was dimerized (Eq. (1)) producing the head-to-tail
CH, was trimerized (Eq. (2))
/
CH
polymerization reaction [4]. In the polymerization of
alkenes by cationic Group 4 metallocenes, the counter-
ion affects the molecular weight of the observed poly-
mers [5]. For the catalytic oligomerization of alkynes,
dimer, whereas TMSCÅ
/
specifically towards the head-to-tail-to-head trimer,
respectively [10].
ð1Þ
*
Corresponding author. Tel.: ꢂ
/
972-4-829-2680; fax: ꢂ972-4-823-3735.
/
0022-328X/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 3 2 8 X ( 0 2 ) 0 2 1 3 6 - 8

98
J.Q. Wang, M.S. Eisen / Journal of Organometallic Chemistry 670 (2003) 97Á107
/
ð2Þ
For non-bulky terminal alkynes, the oligomerization
reported a convenient principle for controlling the
chemoselectivity in the oligomerization of terminal
alkynes. The strategy uses an acidic chain-transfer agent
that does not end-up in the product (in contrast to, e.g.
leads to a mixture of dimers to decamers with no chemo-
or regioselectivity among different oligomers [10]. The
proposed mechanistic scenario for the oligomerization
of terminal alkynes is presented in Scheme 1. This
mechanism is based on kinetic experiments and trapping
intermediate species such as I and II. The rate-determin-
H ), and does not require subsequent elimination from
2
the product to release the unsaturated oligomer (e.g. in
contrast to ethene oligomerization by metallocene
catalysts or magnesium reagents) [11]. Controlling the
oligomerization was achieved by adding an amine into
the catalytic cycle, without altering the turnover fre-
quencies as compared with the non-controlled process
[6]. The drawback of this strategy is that it worked
ing step, in the catalytic trimerization of TMSCÅ
/CH
was found to be the elimination of the trimer from the
bis(dieneyne)thorium complex II. This result indicates
that both rates: (i) s-bond metathesis between the
actinide-carbyls and the alkyne and (ii) insertion of the
triple bond into the metal acetylide moiety (steps 1Á
Scheme 1), are much faster than the rate for the
protonolysis the metalÁdialkenyl complex (step 4).
/
3 in
satisfactorily with the thorium complex Cp2
it did not work satisfactorily with the corresponding
ꢀ
ThMe , but
2
t
/
uranium complex (besides for the bulky BuCÅ
/
H), due
A fascinating conceptual question regarded the ability
to tailor the proposed mechanism in such a fashion so as
to induce the formation of a specific dimer or trimer and
thus preventing the formation of higher oligomers. That
is, to obstruct pathways 3 and 4 in Scheme 1, obligating
the catalyst to follow routes 2 and 5. We recently
to the differences in the corresponding enthalpies of the
protonolysis reactions. The proposed mechanism, as
proposed for the controlling oligomerization for the
thorium complex with amines, is described in Scheme 2.
In addition to the reactivity of organoactinides as
described above, we have recently shown that orga-
Scheme 1. Proposed scenario for the oligomerization of terminal alkynes promoted by organoactinide complexes.

J.Q. Wang, M.S. Eisen / Journal of Organometallic Chemistry 670 (2003) 97Á
/107
99
Scheme 2. Proposed mechanism for the controlled oligomerization of terminal alkynes promoted by the thorium complex Cp
of primary amines.
ꢀ
ThMe
2
in the presence
noactinides are active catalysts towards the hydrosilyla-
tion of terminal alkynes with primary silanes [12]. At
room temperature, three major products were obtained
as presented in Eq. (3):
vinylsilane without any trace formation of the other two
hydrosilylation isomers (geminal or cis). At high tem-
perature (65 8C), however, the chemoselectivity and the
regioselectivity of the vinylsilanes formed in the orga-
ð3Þ
Interestingly, regardless of the alkyl substituents and
the metal center, the major product in the hydrosilyla-
tion reaction at room temperature was the trans-
noactinide-catalyzed hydrosilylation of terminal alkynes
with PhSiH , were found to be different, as presented in
Eq. (4):
3
ð4Þ

100
J.Q. Wang, M.S. Eisen / Journal of Organometallic Chemistry 670 (2003) 97Á107
/
Due to the fact that no hydrosilylation products of
vinylsilanes were obtained, even under the presence of
2.2. Controlled catalytic oligomerization of HCÅ
/CR by
Cp UMe in the presence of Et SiH
ꢀ
2
2
2
2
large excess amounts of PhSiH , it seems plausible to
3
conclude that secondary silanes would be acidic enough
to induce a rapid protonolysis of growing oligomers of
terminal alkynes without inducing the corresponding
additional hydrosilylation reaction. Therefore, here we
present the activity of the organoactinide complex
2
.2.1. General
In a typical procedure, alkyne and silane were added
to an NMR tube containing 10 mg (0.0186 mmol) of the
catalyst in ca. 0.6 ml of solvent (THF-d or C D ) by
8
6
6
Cp UMe2 in the presence of the secondary silane
ꢀ
2
vacuum transfer in a high vacuum line. The sealed tube
was heated in an oil bath (oil temperatureꢁ100 8C) for
4 h. After 100% conversion of the alkyne (detected by
Et SiH for the selective dimerization of terminal
2
2
/
alkynes. The selectivity control, which refers to the
number of the different oligomers obtained, could be
achieved by considering the difference in the calculated
2
1
H-NMR), the organic products were vacuum trans-
1
13
ferred to another NMR tube and identified by H, C,
Cosy, Noesy, CÃH correlation NMR spectroscopy and
GCÁMS. Different geometrical isomers were identified
by comparing the signals of clean isomers [6][8b][9,10].
bond-disruption energies between an actinideÁ
/alkenyl-
/
and an actinideÁsilane-bond, and the combination of
/
/
the non-selective catalytic pathways with individual
stoichiometric reactions. This strategy provides an
additional
oligomerizationÁ
route
to
polymerization reactions of terminal
control,
in
principle,
/
2.2.2. Dimerization of 3-methyl-1-butyne by Cp2
and Et SiH
ꢀ
UMe2
alkynes producing specific enynes for further chemical
transformations [13].
2
2
According to the general procedure described above,
.08 ml (0.78 mmol) of 3-methyl-1-butyne was dimerized
0
with 0.1 ml (0.76 mmol) of Et SiH to a mixture of the
2
2
2
. Experimental
geminal head-to-tail dimer (47.5%), trans head-to-head
dimer, the alkene (23.7%) and the diethylsilyl isopropy-
lacetylene (23.8%) as the major products. Additional
trace amounts (5%) of a mixture of trimers up to
pentamers were also observed.
2
.1. Materials and methods
All manipulations of air-sensitive materials were
performed with the rigorous exclusion of oxygen and
moisture in flamed Schlenk-type glassware on a dual
manifold Schlenk line, or interfaced to a high vacuum
2.2.3. Trimerization of 1-hexyne by Cp2
Et SiH
ꢀ
UMe and
2
ꢃ5
(
Atmospheres’ glove box with a medium capacity
10
Torr) line, or in a nitrogen-filled ‘Vacuum
2
2
As described above, 0.08 ml (0.68 mmol) of 1-hexyne
was catalytically trimerized (100%) with 0.09 ml (0.86
mmol) of Et SiH to the tail-to-head-to-tail trimer.
recirculator (1Á2 ppm O ). Argon and nitrogen were
/
2
purified by passage through a MnO oxygen-removal
˚
column and a Davison 4 A molecular sieve column.
Ether solvents were distilled under argon from sodium
benzophenone ketyl. Hydrocarbon solvents (THF-d8,
2
2
n
Additional trace amounts (up to 1%) of BuCÅ
/
CSiEt H
2
and various trimers were observed in the GCÁ
/
MS.
6.02 (s, 1H, H CÄ
/
1
H-NMR (200 MHz, C D ) dꢁ
/
/
6
6
2
2
benzene-d ) were distilled under nitrogen from NaÃ
/
K
CCH Ä
C), 4.92 (d, 1H, JHH
2.10Á2.50 (m, 6H, CH CH CH CH ), 1.25Á
/
C), 5.21 (d, 1H, JHHꢁ
/
1.95 Hz, H CÄ
/
CCHÄ
CCHÄC),
1.65 (m,
1.03 (m, 9H, CH CH -
6
2
2
alloy. All solvents for vacuum line manipulations were
stored in vacuo over NaÃK alloy in resealable bulbs.
Acetylenic compounds (Aldrich) were dried and stored
ꢁ
/
1.95 Hz, H CÄ
/
/
2
/
/
/
2
2
2
3
12H, CH CH CH CH ), 0.85Á
/
2
2
2
3
2
2
˚
over activated molecular sieves (4 A), degassed and
freshly vacuum distilled. Silane (Aldrich) was dried over
CH CH ).
1
2
3
3
C-NMR (50 MHz, C D ) dꢁ
/
149.8 (s, H CÄ
/
CCHÄ
/
6
6
2
1
NaÃ
/
K alloy and stored over activated molecular sieves
C), 137.8 (d, JCHꢁ
H CÄCCHÄC), 118.0 (t,
CCHÄC), 99.9 (s, C ÅCCH ), 83.7 (s, CÅ
/
154.1 Hz, H CÄ
/
CCHÄ
/
C), 125.8 (s,
2
1
˚
4 A). Cp UMe [14] was prepared according to
ꢀ
(
/
/
J
CHꢁ
/
156.4 Hz, H C Ä
/
2
2
2
2
published procedures. NMR spectra were recorded on
a Bruker AM 200 and Bruker AM 400 spectrometers.
/
/
/
CCH ), 43.0
2
2
(t, CH CH CH CH ), 40.4 (t, CH CH CH CH ), 38.4
2
2
2
3
2
2
2
3
1
Chemical shifts for H-NMR and C-NMR are refer-
13
(t, CH CH CH CH ), 34.9 (t, CH CH CH CH ), 34.4
2 2 2 3 2 2 2 3
enced to internal solvent resonances and are reported
MS experiments were
(t, CH CH CH CH ), 30.6 (t, CH CH CH CH ), 25.7
2 2 2 3 2 2 2 3
relative to tetramethylsilane. GCÁ
/
(t, CH CH CH CH ), 25.4 (t, CH CH CH CH ), 22.6
2 2 2 3 2 2 2 3
conducted in a GCÁMS (Finnigan Magnum) spectro-
/
(t, CH CH CH CH ), 17.0 (q, CH ), 16.9 (q, CH ),
2 2 2 3 3 3
meter. NMR experiments were conducted in Teflon
valve-sealed tubes (J-Young) after vacuum transfer of
the liquids in a high-vacuum line.
16.5 (q, CH3).
High-resolution mass spectrum: Calc. for C H
30
1
8
ꢂ
[M ]: 246.4398. Found: 246.4387.

J.Q. Wang, M.S. Eisen / Journal of Organometallic Chemistry 670 (2003) 97Á
/107
101
2
Cp UMe and Et SiH
.2.4. Dimerization of 3,3 dimethyl-1-butyne by
ꢀ
3. Results and discussion
2
2
2
2
t
As described above, 0.08 ml (0.64 mmol) of BuCÅ
CH was catalytically dimerized (100% conversion after
4 h) with 0.08 ml (0.64 mmol) of Et SiH to a mixture
/
The goal of this investigation was to examine the
scope, chemoselectivity and the regioselectivity for the
oligomerization of terminal alkynes controlled by the
addition of a secondary silane catalyzed by the uranium
2
2
2
of the trans head-to-head dimer (15.3%), geminal head-
to-tail dimer (38.5%) and the corresponding alkene
complex Cp2UMe . This study represents an extension
ꢀ
2
(44.2%) as the major products. Trace amounts of the
silylalkyne and oligosilanes (2%) were also observed.
of, and comparison to, our previous investigation of the
oligomerization of terminal alkynes promoted by orga-
noactinides, and to the controlled oligomerization of
ꢀ
alkynes promoted by Cp2ThMe2 in the presence of
amines [6].
2
Cp UMe and Et SiH
.2.5. Trimerization of trimethylsilylacetylene by
ꢀ
2
2
2
2
As described above, 0.08 ml (0.56 mmol) of TMSCÅ
/
CH was catalytically trimerized (100% conversion after
3.1. Reaction and scope of the controlled oligomerization
2
to-head-to-head
4 h) with 0.07 ml (0.56 mmol) of Et SiH to the head-
2 2
trimer
(TMS)CH Ä
/
CH CH Ä
/
The organoactinide complex (C Me ) UMe reacts
5
a
b
c
5 2
2
C(TMS)CÅ
/
C(TMS) (95%), the corresponding TMSCÅ
/
with terminal alkynes in the presence of secondary
silanes, yielding small oligomers depending on the
nature of the alkyne. In addition to the oligomers, the
corresponding silylacetylene was always present at the
end of the reaction in small amounts. These results, in
general, are in contrast to the oligomerization chemo-
and regioselectivity that have been observed under the
same conditions in the absence of a controlling agent.
The initial reaction of (C Me ) UMe with an alkyne, in
CSiEt H (0.5%) and the three possible geminal head-to-
tail dimer (2.2%), trans head-to-tail dimer (0.7%) and cis
head-to-tail dimer (1.6%).
2
1
(
TMS)CH Ä
/
CH CH Ä
/
C(TMS)CÅ
/
C(TMS):
H-
ꢁ
ꢁ
a
b
c
3
3
NMR (200 MHz, C D ) dꢁ
4:1 Hz; J
1:6 Hz; H ), 5.87 (d, 1H, J
H, CÅ
(
/
7.34 (dd, 1H,
J
6
6
H Hb
a
3
1
1
9
ꢁ11:6 Hz; H ), 6.85 (d, 1H, J
H H
c
b
H Hb
c
b
3
ꢁ14:1 Hz; H ), 0.18 (s,
c
H H
a
a
b
/
CSiCH ), 0.17 (s, 18H, CÄ
/CSiCH3).
3
5
5 2
2
1
3
TMS)CH Ä
/
CH CH Ä
/
C(TMS)CÅC(TMS):
/
C-
the presence of the silane, yields the bis-acetylide
complex. Neither mono- nor bis-silyl complexes were
observed even when a large excess of silane was used.
The chemo- and regioselectivity products of the con-
a
b
c
1
NMR (50 MHz, C D ) dꢁ
/
149.7 (d, J ꢁ154:8 Hz;
6
1
6
CHa
CH ), 146.8 (d, JCHb ꢁ153:4 Hz; CH ), 139.9 (d,
a
b
1
JCHc ꢁ135:5 Hz; CH ), 131.6 (s, CÄ
/
C(TMS)CÅ
C), 107.7 (s, CÄC(TMS)CÅ
C(TMS)), 2.7 (q, JCHꢁ117.6 Hz, ÅCTMS), 0.4 (q,
C(TMS)).
/
C),
a
i
n
t
1
08.2 (s, CÄ
/
C(TMS)C Å
/
/
/
trolled oligomerization of PrCÅ
CH, and (TMS)CÅCH with Et SiH are presented in
Eqs. (5)Á(8):
/
CH, BuCÅ
2
/
CH, BuCÅ
/
1
/
/
/
2
1
JCH
ꢁ
/
115.6 Hz, 2ꢄ
/
CÄ
/
/
ð5Þ
ð6Þ

102
J.Q. Wang, M.S. Eisen / Journal of Organometallic Chemistry 670 (2003) 97Á107
/
ð7Þ
ð8Þ
i
For PrCÅ
/
CH (Eq. (5)), the controlling reaction is
Moreover, this result indicates the presence of a
uranium-hydride intermediate species responsible for
highly chemo- and regioselective affording the exclusive
formation of the geminal dimer, in addition to the
corresponding alkene and the silylalkyne. The formation
of the alkene indicates that the secondary silane is acting
as a source of hydrogen to reduce the triple bond, as
already encountered in the hydrosilylation of terminal
alkynes promoted by similar organoactinides [12].
the catalytic reduction of the alkyne.
n
For BuCÅ
/
CH (Eq. (6)), oneexclusivetrimerisformed.
The specific trimer is formed through the regioselective
addition of the terminal alkyne to the organometallic
moiety (M-A), which theoretically is responsible for the
formation of the geminal isomers (Eq. (9)):
ð9Þ

J.Q. Wang, M.S. Eisen / Journal of Organometallic Chemistry 670 (2003) 97Á
/107
103
This result indicates that the insertion of the second
alkyne into the metalla-eneyne complex M-A (which is
presumably in the preferred stereochemistry due to the
bulky substituent), and the subsequent trimer proto-
nolysis, from the M-AA moiety to yield the trimer AA,
are much faster than either the insertion of an alkyne
into a possible intermediate complex M-B (yielding
complexes M-BB and/or M-CC), and/or the protono-
lysis of any of the three complexes M-B, M-BB and M-
CC, by either the alkyne or the silane, eliminating the
dimer B or any of the two trimers BB or CC, respectively
dimers are much faster than a second alkyne insertion.
Therefore, the larger amount of the geminal dimer as
compared with that of the trans isomer indicates that
the insertion of the first alkyne into the uranium
acetylide, forming the geminal organometallic moiety,
is about twice as fast as the formation of the corre-
sponding trans organometallic fragment. In addition to
the dimers, alkene is obtained as the major product with
the concomitant trace formation of the corresponding
silylalkyne and oligosilanes. The formation of an alkene
implies once more, the presence of a hydride organour-
anium complex that will be presumably obtained by the
(
see Eq. (10)) [15].
ð10Þ
t
In the controlled reaction of the bulky BuCÅ
/
CH (Eq.
7)), both the geminal and the trans dimers are obtained.
reaction between the starting bis-acetylide complex and
the silane (Eq. (11)). Hence, equal amounts of the
silylalkyne and alkene are theoretically expected as
(
Of interest is the comparison to the non-controlled
reaction in which the geminal dimer was found to be the
only product, whereas in the controlled process with
primary amines, the trans isomer was the exclusively
obtained compound. Since no trimers were obtained, it
seems plausible to conclude that the protonolysis of the
i
detected for PrCÅ
/
CH. The formations of oligosilanes
indicate that both Et SiH and silylalkyne may act as
2
2
the hydride source, reducing the final concentration of
the latter [16]:
ð11Þ

104
J.Q. Wang, M.S. Eisen / Journal of Organometallic Chemistry 670 (2003) 97Á107
/
2 2
Scheme 3. Plausible scenario for the controlled oligomerization of terminal alkynes catalyzed by Cp UMe in the presence of a secondary silane.
ꢀ
2 2 2 2
Scheme 4. Plausible mechanism for the hydrogenation of terminal alkynes by Et SiH promoted by Cp UMe .
ꢀ

J.Q. Wang, M.S. Eisen / Journal of Organometallic Chemistry 670 (2003) 97Á
/107
105
When performing the uranium-controlled oligomer-
ization of TMSCÅCH (Eq. (8)), the head-to-head-to-
head trimer was regioselectively formed, in addition to
small amounts of the three possible dimers and the
corresponding silylalkyne. As already observed for other
alkynes, the first insertion of the alkyne prefers a
geminal stereochemistry, presumably due to steric
3.2. Catalytic reaction scope and mechanism
/
The results presented here are for the catalytic
oligomerization of terminal alkynes controlled by a
secondary silane producing dimers or trimers with the
absence of higher oligomers, as compared with the non-
controlled catalytic cycle (tetramersÁ/decamers), demon-
reasons. The second insertion for TMSCÅ
/
CH was
strating the ability to tailor the extent of oligomerization
ꢀ
UMe2
2
found to be in opposite regiochemistry to that observed
n
catalyzed by the organouranium complex Cp
for BuCÅ
/
CH. This result is expected and in agreement
CH, as
[19]. This approach is based on an acidic chain-transfer
mechanism as a competing reaction, modifying the non-
selective oligomerization mechanism towards small
oligomers. Using this strategy, the chain-transfer re-
agent does not end-up in the product (no hydrosilylation
product) and, therefore, does not require subsequent
elimination from the product to release the unsaturated
oligomer. Interestingly, the formed trimers contain the
with the regiochemistry insertion of TMSCÅ
/
found in the non-controlled reaction or in the controlled
oligomerization promoted by amines. This is due to the
appreciable electronic character of the TMS group,
stabilizing a partial carbanion charge at the a-position
and a partial carbocation at the b-position (Eq. (12))
[
9,17]:
ð12Þ
Because of the stereochemistry of the insertion of
same regiochemistry as the dimer complex before it was
protonolyzed, indicating that the rate for releasing the
dimer from the organometallic complex with the regio-
chemistry of M-B (Eq. (10)) is much slower than the rate
of alkyne insertion into the metal complex with the
regiochemistry M-A and subsequent protonolysis to the
trimer (Eq. (9)). This result also corroborates that the
insertion of the alkyne yielding, either M-A or M-B is
not reversible [15].
olefins or alkynes into alkyl-organoactinide bonds are
known to proceed through a four-centered transition
state in the dimerization of terminal alkynes, the
geminal and/or the trans isomers are expected (Eq.
(
12)). The formation of the cis isomer in the controlled
oligomeriation of TMSCÅCH indicates that a fast
/
isomerization of the trans isomer must occur before
the protonolysis of the dimer (Eq. (13)). Interestingly, a
similar isomerization has been observed in the oligo-
merization of terminal alkynes promoted by a cationic
organouranium complex [18]:
Since the stereochemical approach of either silane or
alkyne is by a side approach [20], it seems plausible that
for bulky alkyne substituents, the steric hindrance
ð13Þ

106
J.Q. Wang, M.S. Eisen / Journal of Organometallic Chemistry 670 (2003) 97Á107
/
imparted by the pentamethylcyclopentadienyl rings
prevents a second alkyne to insert, but allows the rapid
protonolysis (the electronic effect of TMS is much
stronger than the steric effect tolerating the trimer
formation). A plausible scenario for the controlled
oligomerization of terminal alkynes is described in
Scheme 3.
obtained organometallic silylacetylide. A detailed under-
standing of the thermodynamics of the single steps in the
desired reactions was the key to ‘‘designing’’ the
catalytic cycle. Studies using different protonolytic
sources are under investigation.
The mechanism presented in Scheme 3 consists of a
sequence of well-established elementary reactions, such
Acknowledgements
as the insertion of acetylene into an MÃ
/
C s-bond and
This research was supported by The Israel Science
Foundation, administered by The Israel Academy of
Sciences and Humanities under contract 83/01-1, and by
the Fund for the Promotion of Research at the
Technion. J.Q.W. thanks the Technion for a postdoc-
toral fellowship.
s-bond metathesis. The precatalyst, (C Me ) UMe , in
5
5 2
2
the presence of a secondary silane and alkyne is
converted to the bis-acetylide complex C (step 1).
Complex C, which is the catalytic resting form of the
complex under the catalytic conditions, reacts rapidly
with one equivalent of silane, yielding the mono-
acetylide mono-silane uranium complex D (step 2).
Complex D reacts with an incoming alkyne, as the
rate-limiting step, producing complex E (step 3). The
formulation of complex E, instead of the bis-alkenyl
complex (the insertion of two alkynes into complex C) is
based on different ratios obtained for dimer formations
in the non-assisted silane mechanism. This is confirmed
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t
(
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BuCÅ
/
ꢀ
UMe produced both dimers, whereas
2
in the absence of silanes only the geminal dimer is
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Scheme 3 and their formation is presented in Scheme 4.
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5
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(
(
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1
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Cp UMe , by using a selected silane. This has led to
ꢀ
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For TMSCÅCH, a different regiochemistry is obtained due to the
/
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(
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TMSCÅ
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CH and CH3CÅCH. Different polarizations were found
/
[
for both groups showing the large effect of the substituent on the
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[
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[
4
[19] For a preliminary result, see: M.S. Eisen, The XVIIIth Interna-
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(