Selective Oligomerization and [2 + 2 + 2] Cycloaddition of Terminal Alkynes from Simple Actinide Precatalysts

Article abstract of DOI:10.1021/acs.organomet.5b00455

A catalyzed conversion of terminal alkynes into dimers, trimers, and trisubstituted benzenes has been developed using the actinide amides U[N(SiMe₃)₂]₃ (1) and [(Me₃Si)₂N]₂An[κ²-(N,C)-CH₂Si(CH₃)N(SiMe₃)] (An = U (2), Th (3)) as precatalysts. These complexes allow for preferential product formation according to the identity of the metal and the catalyst loading. While these complexes are known as valuable precursors for the preparation of various actinide complexes, this is the first demonstration of their use as catalysts for C-C bond forming reactions. At high uranium catalyst loading, the cycloaddition of the terminal alkyne is generally preferred, whereas at low loadings, linear oligomerization to form enynes is favored. The thorium metallacycle produces only organic enynes, suggesting the importance of the ability of uranium to form stabilizing interactions with arenes and related π-electron-containing intermediates. Kinetic, spectroscopic, and mechanistic data that inform the nature of the activation and catalytic cycle of these reactions are presented. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.organomet.5b00455

Article
pubs.acs.org/Organometallics
Selective Oligomerization and [2 + 2 + 2] Cycloaddition of Terminal
Alkynes from Simple Actinide Precatalysts
†
‡
,‡
,†
Rami J. Batrice, Jamie McKinven, Polly L. Arnold,* and Moris S. Eisen*
†
Schulich Faculty of Chemistry, Institute of Catalysis Science and Technology, Technion − Israel Institute of Technology, Technion
City, 32000, Haifa, Israel
‡
EaStCHEM School of Chemistry, University of Edinburgh, Joseph Black Building, The King’s Building, Edinburgh EH9 3FJ, United
Kingdom
*
S Supporting Information
ABSTRACT: A catalyzed conversion of terminal alkynes into
dimers, trimers, and trisubstituted benzenes has been
developed using the actinide amides U[N(SiMe ) ] (1) and
3
2 3
2
[
(
(Me Si) N] An[κ -(N,C)-CH Si(CH )N(SiMe )] (An = U
3 2 2 2 3 3
2), Th (3)) as precatalysts. These complexes allow for
preferential product formation according to the identity of the metal and the catalyst loading. While these complexes are known
as valuable precursors for the preparation of various actinide complexes, this is the first demonstration of their use as catalysts for
C−C bond forming reactions. At high uranium catalyst loading, the cycloaddition of the terminal alkyne is generally preferred,
whereas at low loadings, linear oligomerization to form enynes is favored. The thorium metallacycle produces only organic
enynes, suggesting the importance of the ability of uranium to form stabilizing interactions with arenes and related π-electron-
containing intermediates. Kinetic, spectroscopic, and mechanistic data that inform the nature of the activation and catalytic cycle
of these reactions are presented.
INTRODUCTION
■
The reactivity of sp-hybridized carbon is a core aspect of
organic chemistry. Alkynes have been extensively studied as
adaptable synthons in modern synthesis, and such reactions
1
have given rise to a versatile array of organic products: for
2
3
4
5
example, enynes, ketones, diynes, metal acetylides, and
6
substituted benzenes. In particular, organic enynes and
trisubstituted benzenes have attracted considerable attention7
in recent years owing to their ubiquity in natural products,
8
9
supramolecules, and medicinal compounds. Since the first
reports of the palladium-mediated dimerization of alkynes to
10
enynes by Trost, and the nickel-promoted cycloaddition of
11
alkynes to benzenes by Reppe and Schweckendick, several
advancements have been made utilizing numerous metal
complexes to catalyze these processes. To date, a vast array
of transition metals have been applied to achieve alkyne
catalysis toward enynes and substituted arenes, including metals
from each group of the transition block and many main-group
Figure 1. Previously reported oligomerization and cyclotrimerizations
mediated by transition-metal, main-block-element, and lanthanide
complexes.
12
elements (Figure 1).
Although many of these systems are highly developed,
several shortcomings exist, including the use of expensive metal
catalysts, elaborate ligand frameworks, and limited regio- and
chemoselectivities. Moreover, no single system to our knowl-
edge has been reported that is capable of switching catalytic
activity to perform both oligomerization and cyclotrimerization
selectively.
Some complexes of the f elements have emerged as adept
catalysts in numerous organic transformations and have drawn
considerable attention owing to their unique properties. The
lanthanides have been shown to facilitate oligomerization of
13
alkynes and, in some rare examples, their cyclotrimeriza-
14
tion; however, the latter process requires heterobimetallic
15
systems. The organometallic chemistry of the early actinides
has given rise to impressive and novel structures and
16
reactivities and has been further applied to effect challenging
17
chemical transformations, including hydroaminations, hydro-
Received: May 27, 2015
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.5b00455
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Article
1
8
19
alkoxylations, hydrosilylations, the polymerization of
kinetic parameters operative in the actinide-mediated oligome-
rization and cyclotrimerization of terminal alkynes. Herein, we
have carried out a systematic study of the catalyzed
oligomerization of a range of terminal alkynes using the
2
0
21
dienes, esters, and epoxides, and various small-molecule
2
2
activations. Previously, the oligomerization of terminal
alkynes has been shown to proceed when mediated by various
organoactinide complexes, namely of the type (C Me ) AnMe
2
actinide amides U[N(SiMe ) ] (1) and [(Me Si) N] An[κ -
5
5 2
2
3 2 3
3
2
2
(
An = U, Th), and a mechanism was proposed for this process
(N,C)-CH Si(CH ) N(SiMe )] (An = U (2), Th (3)) as
precatalysts.
2
3 2
3
23
mediated by this class of catalyst (Scheme 1). The mechanism
Scheme 1. Proposed Mechanism for the Catalytic
Oligomerization of Terminal Alkynes Mediated by
Organoactinide Complexes
RESULTS AND DISCUSSION
■
We present the reaction scope and selectivity of alkyne
oligomerization and cyclization reactions catalyzed by the
actinide amide complexes shown in Figure 2. In addition,
experimentally derived mechanistic studies, as well as kinetic
and thermodynamic calculations derived from experimental
data, are provided.
a
Figure 2. Amido actinide precatalysts used in the oligomerization and
cyclotrimerization of terminal alkynes.
The actinide amido complexes U[N(SiMe ) ] (1) and
3
2 3
2
[
(Me Si) N] An[κ -(N,C)-CH Si(CH ) N(SiMe )] (An = U
3 2 2 2 3 2 3
a
(2), Th (3)) react with terminal alkynes to yield dimers,
trimers, or trisubstituted benzenes as the major products, with
small quantities of alkene being generated for selected
experiments (Figure 3). The distribution of products was
found to be strongly dependent on the nature of the metal
center and the reacting alkyne (Table 1).
Ligands are omitted, and only one of the two reactive sites is shown
for clarity.
of oligomerization does not require reductive elimination to
liberate the organic products, indicative of a conservation of the
metal oxidation state for this cycle.
The selectivity of these complexes for the oligomerization
process was further enhanced by the addition of a primary
amine which serves as a proton source, enabling rapid cleavage
of the metallodimer and thus improving control over the
number of insertions of the alkyne. This resulted in a high
chemoselectivity toward the formation of, chiefly, dimers for
the majority of substrates studied. A similar chemoselectivity
was found when the substrate was allowed increased access to
the metal by the use of the ansa-bridged cyclopentadienyl
n
ligand in [Me Si(C Me ) ]Th Bu , rapidly and selectively
2
5
4 2
2
yielding organic dimers. Investigation of the cationic uranium
complex [U(NEt ) ][BPh ] in the oligomerization of terminal
2
3
4
alkynes has given rise to comparable oligomerization products;
however, the isolation of the π-alkynyl uranium complex
t
2
t
[
(Et N) U(CC Bu)(η -HCC Bu)][BPh ] represented the
2
2
4
first example of such a species, raising the conceptual question
as to whether other uranium complexes may exhibit this
24
behavior (Scheme 2).
The goal of this investigation was to study the combined
effects of coordinative unsaturation and an open metal sphere
and to examine the scope, chemoselectivity, regioselectivity,
metal center influence, mechanism, and thermodynamic and
Figure 3. Accessible products from the catalytic reaction of actinide
amides with terminal alkynes. The R substituent is n-butyl (a), tert-
butyl (b), trimethylsilyl (c), or phenyl (d).
Scheme 2. Reaction of tert-Butylacetylene with a Cationic Uranium Complex Generating the π-Alkynyl Uranium
B
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Article
a
Table 1. Results for the Catalytic Oligomerization/Cyclotrimerization of Terminal Alkynes by Complexes 1−3
yield (%)
b
entry
catalyst (amt (mol %))
R
conversion (%)
4
5
6
7
8
9
10
11
12
1
2
3
4
1 (1)
nBu
nBu
nBu
nBu
nBu
nBu
tBu
tBu
tBu
tBu
tBu
88
100
88
91
13
96
5
2
29
1
1
42
1
1 (10)
2 (1)
15
22
2
2 (10)
3 (1)
100
86
41
37
c
5
92
93
22
7
7
6
3 (10)
1 (1)
99
c
7
87
40
55
22
54
43
46
11
21
8
1
22
1
7
23
5
c
8
1 (10)
2 (1)
100
77
c
9
60
9
3
6
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
2 (10)
3 (1)
100
70
23
2
14
c
41
14
50
14
39
3 (10)
1 (1)
tBu
97
c
c
SiMe3
SiMe3
SiMe3
SiMe3
SiMe3
SiMe3
Ph
42
22
57
2
17
41
2
1 (10)
2 (1)
100
97
32
34
22
27
8
25
43
32
19
38
2 (10)
3 (1)
100
64
23
19
35
41
3 (10)
1 (1)
87
d
d
d
d
d
d
,
,
,
,
e
e
e
e
92
25
48
35
40
63
49
40
51
1 (10)
2 (1)
Ph
100
96
Ph
2
17
2 (10)
3 (1)
Ph
100
99
Ph
91
77
9
3 (10)
Ph
100
15
8
a
b
Product percentages are ratios of converted substrate. Reactions were run for 72 h at 75 °C in C D . R = substituent of the corresponding RC
6
6
c
d
e
CH. Traces of larger oligomers. Products and distributions determined by HPLC-MS. Remaining product contains dimers up to tetramers.
Reactivity and Selectivity of Alkyne Oligomerization
and Cyclotrimerization Catalyzed by Complexes 1−3.
Most notably, [2 + 2 + 2] cycloaddition is only observed when
the uranium catalysts 1 and 2 are used. This is of particular
interest, as the observed cycloaddition is largely found in late-
to excellent conversion (≥86%) to yield the geminal dimer 4a
(Figure 3), produced from the head-to-tail insertion almost
exclusively (≥91%) (Table 1, entries 1, 3, and 5), with a small
amount of trisubstituted benzenes (≤3%) additionally being
produced.
Conversely, and of particular note, when uranium catalysts 1
and 2 were used at high loading (10%) (Table 1, entries 2 and
4), the major products formed were found to be the
trisubstituted benzenes. The oligomerization of 1-hexyne
mediated by complex 1 results in a quantitative conversion of
monomer to cyclotrimers, 29% of 1,2,4-tri-n-butylbenzene
25
transition-metal systems, with few examples found in early-
transition-metal chemistry and no previously reported instances
26
in actinide systems. A further unusual aspect of the results in
Table 1 is the alkene product, arising formally from alkyne
hydrogenation. The in situ generation of a uranium hydride is
proposed to account for this result. In this study, the
regioselectivity of the migratory insertion is described according
to the disposition of the alkyne R substituent into the metal−
carbon bond, with the head-to-head mode of insertion giving
rise to a trans-configured enyne or a head-to-tail insertion
producing the gem enyne (Scheme 3).
(
10a) and 42% of 1,3,5-tri-n-butylbenzene (11a), involving the
second migratory insertion of alkyne (Scheme 1, step c),
followed by cyclization to give the arene product. Unexpect-
edly, the remaining 15% of the product was identified as 1-
hexene (12a), a hydrogenation product. The uranium metal-
lacycle 2 similarly gave a quantitative conversion, yielding cyclic
trimers as the major products (41% of 10a and 37% of 11a),
with the remaining 22% of product converted to 1-hexene
Oligomerization/Cyclotrimerization of 1-Hexyne by Com-
plexes 1−3. In the reaction with 1-hexyne, the use of a low
catalyst loading (1%) for each of the metal complexes gives rise
(
12a). The results for 1% catalyst loading with 1 and 2 were
Scheme 3. Possible Pathways for Insertion of Terminal
Alkyne into a Metal−Acetylide Bond Yielding (a) Head-to-
Tail and (b) Head-to-Head Insertion Products
very similar to those for the use of thorium catalyst 3, with 4a
and 5a predominating, but with benzenes 10a and 11a
completely absent. Unlike 1 and 2, the product distribution
was nearly identical at a 10% loading of 3.
The observed preference toward the head-to-tail insertion
shown by catalysts 1−3 is explained by the minimal steric
hindrance imparted by the n-butyl group of the alkyne, allowing
the terminal CH moiety to enter the coordination sphere of the
metal toward the thermodynamically favored regioselectivity of
27
insertion. The high yield of dimer generated from this
substrate illustrates the lower energetic pathway of protolytic
C
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Article
cleavage in comparison to migratory insertion into the
actinide−vinyl bond. This is corroborated by the small
quantities of product formed from the additional insertion
into this intermediate, evident by the formation of only small
amounts of trimer 6a (≤7%), similarly occurring from a head-
to-tail insertion. At 10% catalyst loading in complexes 1 and 2,
it was seen that the [2 + 2 + 2] cycloaddition was favored. It is
worth noting that the 1,3,5-trisubstituted benzenes are only
accessible through two consecutive head-to-tail insertions
toward the metallotrimer (Figure 3, 9′) according to the
proposed mechanism (Scheme 1), whereas the 1,2,4-trisub-
stituted benzene may be formed by a head-to-head insertion in
either of the two migratory insertion steps, allowing for three
possible intermediates to this product, (Figure 3, 6′, 7′, or 8′).
Furthermore, the higher degree of cyclization at high loading in
uranium is proposed to arise from a bimetallic mechanism,
owing to the larger ratio of metal to alkyne in solution (vide
infra). A bimetallic cyclization is additionally lent credence by
(Table 1, entries 11 and 12, respectively). At 1 mol % loading,
moderate conversions are observed (70%) with the production
of equimolar amounts of dimers 4b and 5b (41 and 43%,
respectively). Conversely, at high catalyst loading, the thorium
metallacycle (3) provides a 97% conversion with 46% of trans
dimer (5b) as the major product and an additional 14% of gem
dimer (4b) with 39% of trimer (7b) from a head-to-head-to-
head insertion.
In consideration of the reactivity with tert-butylacetylene, it is
first apparent that the steric bulk of the alkyne inhibits
sequential insertions during the catalytic cycle, as is shown by
the modest conversions for these experiments (Table 1, entries
7, 9, and 11). In addition, the presence of the alkene 12b in
roughly equimolar amounts to the cyclotrimers, yet in
considerable excess to catalyst, is informative that, while a
uranium hydride species may be formed upon activation of the
precatalyst, an additional mechanism must be operative in the
catalytic cycle to regenerate the uranium hydride (vide infra). In
addition, the catalytic reaction using complex 1 shows the
propensity of the uranium complex to undergo rapid insertion
into the gem-metallaenyne. Moreover, the absence of trimer 9b
indicates rapid cyclization of the metallatrimer being energeti-
cally favored over protonolytic cleavage.
recent work describing a stoichiometric uranium-mediated
Ad
bimetallic C−C coupling of alkynes, yielding [{(( ArO) N)-
3
IV
2
2
28
U } (μ-η :η -1,2-(CH) -cyclopentane)]. For complex 1, the
2
2
consecutive head-to-tail insertion is found to be prevalent, as
made apparent by the larger amount of the symmetric arene
1
1a, whereas complex 2 generates a larger quantity of the 1,2,4-
Uranium complex 2 shows remarkable results in the catalysis
with this substrate. At high catalyst loading, head-to-head
insertion emerges as the dominant pathway to product 5b and
elucidates the rigidity of the metal acetylide moiety; however,
after the first insertion has occurred, the resulting metal−vinylic
bond provides less steric encumbrance, allowing for the head-
to-tail mode of insertion for the incoming monomer. This can
be assumed to be manifest from the flexibility imparted in
tri-n-butylbenzene. This is indicative of different active catalytic
species despite the remarkable similarity between the
precatalyst structures.
Oligomerization/Cyclotrimerization of tert-Butylacetylene
by Complexes 1−3. To investigate the steric effects of the
substrate, tert-butylacetylene emerged as the obvious contrast
to n-butylacetylene (νeff 1.24 and 0.68, respectively). The
reduction of alkyne to the corresponding alkene was observed
to the greatest extent for this substrate (Table 1, entries 8 and
29
2
moving from an sp to sp hybridization on the coordinating
carbon atom. Performing the reaction at a 1 mol % catalyst
loading revealed interesting chemical reactivity; the uranium
complex 2 at 10 mol % loading favors head-to-head insertion
yielding dimer 5b. However, at 1% loading, this experiment
showed an obvious reversal in regioselectivity, yielding 60% of
the gem dimer 4b and only 22% of trans dimer 5b. The reversal
in regioselectivity is an outstanding feature in this reaction and,
to the best of our knowledge, the first example of such a
reversal arising from variable catalyst loading. We suggest that,
at high concentration of the sterically cumbersome tert-
butylacetylene, the formation of a π-alkynyl complex similar
1
8
0). At 1 mol % loading, the uranium catalyst 1 gave an overall
7% conversion of tert-butylacetylene to both dimers 4b and 5b
(
22 and 40% yields, respectively) and trimer 7b (21%) which is
obtained by head-to-head insertion into 5′ and subsequent
protonolysis (Table 1, entry 7). This precatalyst at 10 mol %
loading follows a similar behavior, yielding a slightly larger
quantity of cyclotrimerization product, providing the trans
dimer 5b as the major product (55%) (Table 1, entry 8).
Only a 77% monomer conversion was achieved using catalyst
2
at 1 mol % loading with a marked decrease in chemo- and
24
regioselectivity; 60 and 22% of dimers 4b and 5b, respectively,
were formed in the reaction mixture (Table 1, entry 9). Using
the metallacycle 2 at 10% loading generated the lowest amount
of the cycloaddition products for any of the uranium catalysts
used at high loadings (8% total of arene) but did provide
quantitative conversion of the terminal alkyne (Table 1, entry
to one previously found in the literature is generated, favoring
head-to-tail insertion. While unexpected, this finding allows for
the tuning of the regioselectivity of the oligomerization of tert-
butylacetylene by simple alteration of the catalyst loading.
The study of the thorium metallacycle 3 in the oligomeriza-
tion of tert-butylacetylene proved more straightforward,
showing preference for head-to-head insertion. Interestingly,
when the catalyst load is decreased to 1%, head-to-head
insertion is still seen as the major mechanistic pathway;
however, the approximate equimolar concentrations of gem and
trans dimer (41 and 43%, respectively) suggests an equilibrium
process of insertion into the thorium acetylide. Despite this
product ratio, the head-to-head insertion remains the most
favorable when the regioselectivity toward the production of
trimer 7b (14%) is considered.
10). This experiment yielded 54% of the trans dimer 5b, as well
as 14% of alkene 12b and 23% of trimer 6b; however, an
interesting feature of this reaction is the observed reversal in
regioselectivity on comparison of the dimers formed in the 1
and 10% catalyst loading runs using precatalyst 2. Such a
reversal in regioselectivity shows a previously unseen behavioral
pattern in either f-group or transition-metal catalysis which is
dictated by the applied catalyst loading. The change of
regioselectivity is highly suggestive of migratory insertion into
the uranium acetylide existing in an equilibrium process.
Use of the thorium metallacycle precatalyst 3 provided the
greatest selectivity toward the oligomerization of tert-
butylacetylene. At 1 and 10% catalyst loadings, the two possible
dimers and trimer 7b were produced as the major products
Oligomerization/Cyclotrimerization of (Trimethylsilyl)-
acetylene by Complexes 1−3. The trimethylsilyl moiety has
been computationally and experimentally determined to be an
30
approximate steric equivalent to methyl, allowing for a
predominantly electronic analysis of its reactivity. As the
D
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inductive effects of a trimethylsilyl moiety result in stabilization
yield (Table 1, entry 18). The presence of this trimer as the
major product indicates the high level of electronic influence
over the second insertion into the gem-dimer intermediate 4c′.
The observed activity for this catalyst−substrate combination,
however, was one of the few examples in which a moderate
substrate conversion was observed (87%) at high catalyst
loading. These data show a marginal preference toward head-
to-tail insertion into the active thorium−alkyne bond. Further
insertions appear to be governed by the electronic properties of
the alkyne, resulting in competitive head-to-head insertion and
protonolysis.
31
of α-carbanions and β-carbocations, a head-to-head mode of
insertion is expected to be favored. Given the considerable
Lewis acidity of the metal center and the electronic nature of
the substrate, a conceptual question was formed as to whether
steric or electronic control would dominate in the observed
regioselectivity (Figure 4).
Oligomerization/Cyclotrimerization of Phenylacetylene by
Complexes 1−3. In order to study the behavior of
comparatively electron-poor alkenesin contrast to the
examples detailed abovephenylacetylene was selected for
study. Owing to the electron deficiency of the alkyne, a
preference for cyclization is expected, owing to an increased π-
back-bonding interaction. In addition, the minimal steric
hindrance imparted by the phenyl ring (νPh = 0.57) would
in theory pose a minimal steric barrier to this process. Equation
1 shows the anticipated preference for head-to-tail insertion, on
the basis of the bond polarization of this substrate.
Figure 4. Expected polarization of (trimethylsilyl)acetylene toward an
actinide catalyst in a migratory insertion step.
Furthermore, owing to the electron-rich nature of the alkyne,
it was theorized that a π-alkynyl complex would be more likely
to form to the metal center, resulting in a higher selectivity
toward cyclization. Examination of the reactions shows that the
average reactivity of the actinide complexes toward this
monomer is the lowest of the alkynes studied (Table 1, entries
29
13−18), arising from the formation of a strong π-alkynyl
complex to the metal center, in turn increasing the enthalpic
barrier of insertion. However, this same π-bonding interaction
is thought to give rise to the relatively high level of
cycloaddition seen for the uranium complexes. Catalyst 1 at
low loading (Table 1, entry 13) reveals thermodynamic control
of insertion, reflected by the 50% of gem dimer (4c) produced.
In addition, the formation of tris(trimethylsilyl)benzene and
the absence of linear trimer implies that a rapid cyclization
occurs with a lower energetic barrier in comparison to
protonolysis. Increasing the catalyst loading to 10 mol %
results in cyclotrimer formation as the dominant pathway (57
and 41% of 1,2,4- and 1,3,5-tris(trimethylsilyl)benzene,
respectively), with less than 2% of larger oligomers formed
according to GC-MS analysis (Table 1, entry 14). On
consideration of the ratio of the arene products, the preference
of head-to-tail insertion is still found to be substantial, as is
illustrated by the quantity of symmetric cyclotrimer 11c
generated.
Complex 2 at 1% and 10% catalyst loadings shows a
propensity toward a similar head-to-tail insertion, with superior
regioselectivity to 1, with dimer 4c, trimer 9c, and cycloaddition
product 11c observed as the dominant products (Table 1,
entries 15 and 16). The degree of cyclization is significantly
diminished in comparison to 1. A 1% loading of complex 2
generated the gem dimer 5c in 32% yield; however, a significant
amount of trimer 9c is generated (38%). With respect to 1,
complex 2 gives diminished selectivity at high catalyst loading.
Although high chemoselectivity toward the formation of only
dimers or cyclotrimers is apparent, the regioselectivity is less
favorable on comparison of the product distributions. The lack
of any linear trimer in this reaction informs us that, similar to
the behavior seen using catalyst 1, the metallotrimer
intermediate cyclizes rapidly and preferentially.
When either uranium catalyst was used, a minimum of 75%
of cyclized product was observed (Table 1, entries 19−22),
with up to 97% trisubstituted benzene being produced
according to HPLC-MS (Table 1, entry 20). In addition, a
strong preference toward head-to-tail insertion is evident by the
production of the 1,3,5-triphenylbenzene species 11d as the
major product in the cyclizations. A 1% catalyst loading in
thorium complex 3 achieved near-quantitative conversion of
phenylacetylene, yielding 91% of gem dimer (Table 1, entry
2
3). Similarly, the 10% loading experiment generated the gem
dimer as the major product, albeit with a notable decrease in
regioselectivity. The results from the study of this substrate
strongly support the previously stated hypothesis of the
regioselectivity of insertion.
Cyclotrimerization of 1,6-Heptadiyne Mediated by Ura-
nium Complexes 1 and 2. The hypothesis of a bimetallic
mechanism was probed by employing 1,6-heptadiyne in an
effort to isolate the bis(indane) product 13 (eq 2).
Mechanistic Studies of Alkyne Oligomerization and
Cyclotrimerization Catalyzed by 1−3. Comparative studies
between complexes 2 and 3, respectively, are highly evocative of
the importance of the 5f electrons in the observed catalysis;
however, the data from this study, as well as previous catalytic
The thorium metallacycle 3 catalysis of this substrate shows a
product distribution suggestive of multiple, competitive
processes. At low catalyst loading, 3 gives 64% conversion,
yielding gem dimer 4c as the minor product in 22% yield with
trans dimer 5c and trimer 8c being generated in 43 and 35%
yields, respectively (Table 1, entry 17). At 10% catalyst loading,
a similar regioselectivity of products is observed; however, the
trimer 8c was produced instead as the major product in 41%
studies using Cp*AnMe , fail to provide sufficient information
2
to dismiss effects which may arise from the difference in ionic
radii. That the actinide-mediated oligomerization of terminal
alkynes toward enynes is a sequential alkyne insertion into an
23
An−acetylide bond has been well established. However, given
that the comparative reactivities between uranium(III) and
E
DOI: 10.1021/acs.organomet.5b00455
Organometallics XXXX, XXX, XXX−XXX

Organometallics
(IV) were as yet unreported and that cyclotrimerization
reactions are rare in early-transition-metal chemistry and are
entirely absent from actinide chemistry, further studies were
carried out to determine the activation pathway of complexes
Article
-
(IV) acetylide complex. The aforementioned studies have
shown that the first equivalent of alkyne protolytically cleaves
the thorium−carbon bond of the metallacyclobutane, resulting
in the formation of a tris(amido)thorium acetylide complex.
The remaining amides were subsequently protolytically cleaved
by additional alkyne, producing the thorium(IV) acetylide and
3 equiv of bis(trimethylsilyl)amine as determined by NMR.
Mechanistic Investigation of the Catalytic Oligomeriza-
tion of 1-Hexyne by Complexes 1−3. The operative
mechanism is proposed to occur in a manner analogous to
that shown in Scheme 1. Activation of the precatalyst occurs to
generate the active uranium or thorium acetylide complex (vide
supra). Given the observed chemo- and regioselectivities, in
conjunction with the occurrence of preferential formation of
dimers or trimers on the basis of catalyst loading, the migratory
insertion of alkyne is proposed to be an equilibrium process
(Scheme 1, step a). The first insertion into the metal−alkyne
bond generates the metallodimer; from this intermediate, two
divergent pathways are accessible, either σ-bond metathesis to
produce the organic dimer and regenerate the active catalyst
1
−3 and key steps in the mechanism of [2 + 2 + 2]
cycloaddition.
Mechanism of Activation. In order to ascertain the
1
mechanism of activation, H NMR, fluorescence, C60 radical
trapping, and MALDI-TOF experiments were utilized. Further
study was prudent when considering the observed reduction of
the alkyne, a process rarely seen in actinide chemistry in the
32
absence of a reducing agent. In the study of the activation of
1
catalyst 1 by 1-hexyne, it was observed by H NMR
spectroscopy that protonolysis of two of the available three
bis(trimethylsilyl)amide ligands occurs, forming a bis(acetylide)
uranium complex. Moreover, the presence of the alkene
product 12, arising from insertion of alkyne into an
intermediate uranium hydride species, suggests that the initial
activation involves a one-electron oxidation of two uranium
centers across the C−H bond of alkyne substrate (eq 3), readily
(
step d) or additional insertion to generate the metallotrimer
intermediate (step b). Similar experiments with catalyst
loadings in the range of 0.5−5 mol % provided comparable
product distributions. If the mole ratio of precatalyst was
increased to 10 mol % or greater (up to 20 mol % as was
investigated in this study), the chemoselectivity tended toward
the formation of cyclotrimers.
The kinetics of the oligomerization of 1-hexyne were studied
1
33
using H NMR spectroscopy at 75 °C and monitored by the
accessible due to the low oxidation potential of uranium(III).
appearance of one or both of the vinylic proton signals of the
geminal dimer (δ 5.37 and 5.08 ppm). The rate equation for
the oligomerization mediated by any of the three precatalysts
was found to be first order in precatalyst and first order in
alkyne, giving rise to the rate equation
Accordingly, fluorescence data were used to ascertain the
oxidation state of the metal after activation with alkyne. The
emission spectrum obtained was found to be in agreement with
recent studies characterizing the metal-based fluorescence of
the uranium(IV) oxidation state (see Figure S1 in the
34
Supporting Information).
∂p
=
kobs[catalyst][alkyne]
It was initially assumed that the uranium(IV) metallacycle
would maintain the same formal charge; however, it has been
previously established that uranium(IV) complexes can be
reduced to U(III) through the homolytic cleavage of a U−alkyl
∂t
(5)
Experimentally derived thermodynamic parameters from
Eyring and Arrhenius plots (see Figures S3−S8 in the
Supporting Information) were utilized to determine the
35
bond such as from the tert-butyl group shown in eq 4.
⧧
⧧
enthalpy (ΔH ), entropy (ΔS ), and energy of activation
a
(
E ) of oligomerization. From the derived data using catalyst 1,
relatively high enthalpy and energy of activation are observed
−
1
(
27.6(2) and 28.3(2) kcal mol , respectively), and an
approximately neutral entropy of activation of −0.5(6) eu is
n
found. Use of deuterium-labeled alkyne ( BuCCD) reveals a
In order to substantiate the possibility of a radical reaction,
EPR studies were utilized using C as a radical trapping agent.
To a toluene solution of complex 2 with 2 equiv of fullerene
was added an excess of 1-hexyne, and the EPR spectrum was
acquired immediately after addition and after 3 h of heating at
primary kinetic isotope effect (KIE = 2.85), corroborating the
protonolysis as the rate-determining step of the catalytic cycle
(Scheme 1, step d). The dimerization process mediated by
complex 2 is similarly accompanied by high enthalpy (21.2(2)
6
0
−
1
−1
kcal mol ) and activation energies (21.9(2) kcal mol ),
however, with a highly negative entropy of activation (−19.8(7)
eu), indicating a highly ordered transition state, supporting a
four-centered transition state of migratory insertion. These
parameters mediated by the thorium metallacycle 3 lie between
those of the two uranium complexes; the enthalpy and energy
75 °C. It was seen that, after the sample was heated, a
monomeric fullerenyl radical was produced, as detected by EPR
spectroscopy (see Figure S2 in the Supporting Information).
Given this observation, accurate determination of the active
catalytic species was not possible; however, it was additionally
found that the +4 oxidation state is conserved.
−
1
of activation were calculated as 26.1(3) and 26.8(3) kcal mol ,
respectively, with a negative entropy supporting an ordered
transition state (−8.5(8) eu).
Although the reaction of the thorium metallacycle 3 with
stoichiometric amounts of phenylacetylene was previously
3
6
studied, no investigation had been performed when additional
alkyne was introduced. After reaction with an excess of alkyne,
it was seen by NMR spectroscopy that all amides were
displaced, resulting in the formation of a homoleptic thorium-
Mechanistic Investigation of the Catalytic Cyclotrimeriza-
tion of (trimethylsilyl)acetylene by Complex 1. The novel
actinide-mediated catalytic cyclotrimerization was thoroughly
investigated in this study. The thermodynamic and kinetic data
F
DOI: 10.1021/acs.organomet.5b00455
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
a
Scheme 4. Proposed Mechanism for the Catalytic Cyclotrimerization of Terminal Alkynes Promoted by Complexes 1 and 2
a
Ligands are omitted for clarity.
obtained here, as well as variable catalyst loading experiments,
provide information which elucidates the mechanism of [2 + 2
An additional question which remained was the determi-
nation of the involvement of the conjugate acid (HN(SiMe ) )
3
2
+
2] cycloaddition (Scheme 4).
arising from protonolysis of the actinide precatalyst. In previous
studies, it was seen that the addition of external amine did not
The ability of the uranium centers to cyclize the metal-
38
laoligomers, a reaction absent from thorium catalysis, shows the
variability in reactivity of these two metals arising from the
aid in improving the selectivity of uranium-mediated catalysis
but did result in control for short oligomers using the thorium
37
analogue. To a 1 mol % catalyst solution (1−3) was added 30
differing electronic structures. The high catalyst loading of 1
and 2 required to effect cyclization suggests that arene
formation likely occurs by a bimetallic process, yielding a
uranium-arene intermediate. The catalytic cycle shown retains
the +4 oxidation state of the uranium center. Protonolysis by an
external alkyne yields the substituted benzene and regenerates
the active uranium acetylide catalyst. When experiments were
performed with 20 mol % catalyst loading, cyclotrimer was still
found to the major product in the reaction mixture; however,
the reaction rate was accelerated proportionally owing to the
first-order behavior of the catalyst. Similar to the experiments
performed with 1-hexyne, decreasing the catalyst loading below
equiv of HN(SiMe ) , and the reaction was followed by
3 2
monitoring product formation at 75 °C. It was observed that
the addition resulted in no discernible difference in the product
distribution, and therefore the amine is not an active
component of the catalytic cycle.
The study of 1,6-heptadiyne cyclization further aided in
supporting the proposed mechanistic cycle. According to
Scheme 5, the migratory insertion of 1,6-heptadiyne occurs
through a uranium acetylide intermediate (step a), followed by
an intramolecular insertion into the resulting uranium−vinylic
bond (step b). A critical step in the reaction is the cyclization of
the internal alkyne, whereby an additional 1 equiv of the
uranium catalyst is required to form a π complex and distort the
1
0 mol % (in the range of 0.5−5 mol %) increased the
propensity of oligomer formation.
2
alkyne toward sp geometry to facilitate the insertion (steps c
The experimental kinetic and thermodynamic data for the
cyclotrimerization of (trimethylsilyl)acetylene mediated by
complex 1 is first order in catalyst and alkyne, suggesting that
the cyclization of the metallatrimer (Scheme 4, step 4) occurs
rapidly and is not rate-limiting. Deuterium labeling of the
and d). Following this, protolytic cleavage generates the
intermediate monoindane product (step e). This cycle is
repeated on the terminal alkyne from this step to generate the
desired bis(indane) product 14.
In describing the formation of alkene, it was necessary to
determine how uranium(III) is regenerated from the uranium-
alkyne (Me SiCCD) revealed a primary kinetic isotope effect
3
(
KIE = 2.77), identifying that the aryl−alkynide exchange
(
IV) active catalyst during the catalytic cycle from complexes 1
protonolysis is the slow step of the reaction (Scheme 4, step 5).
The experimentally derived thermodynamic parameters reveal a
moderate enthalpy and energy of activation (11.3(1) and
and 2, as more reduction product was generated than is
possible with the assumption of a stoichiometric activation
pathway (eq 3). This recurring reactivity pattern can be found
in Table 1 (entries 2, 4, 7, 8, and 10) and suggests a unforeseen
side reaction; given the divergent activation pathways observed
by complex 1 versus complex 2 and considerations of the
catalytic mechanism, it is proposed that uranium(III) is
−
1
1
2.0(1) kcal mol , respectively); however, the large negative
⧧
entropy (ΔS = −39.5(3) eu) is most notably in agreement
with a highly ordered transition state which would be expected
for the cyclization process.
G
DOI: 10.1021/acs.organomet.5b00455
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Organometallics
Article
Scheme 5. Proposed Mechanism for the First Cyclization of
exposed a combination of metal center, catalyst loading, and
substrate influences which allow for tuning of the regio- and
chemoselectivity of the formed products. The electron-deficient
and -rich alkynes phenylacetylene and (trimethylsilyl)acetylene,
respectively, show a strong preference toward cyclization when
either uranium catalyst is used. It is seen that the thorium
metallacycle shows a high degree of regioselectivity, generating
only two major oligomeric products when 1-hexyne or
phenylacetylene is used, or three products when tert-
butylacetylene or (trimethylsilyl)acetylene is used.
a
1
,6-Heptadiyne
A comparative study between the two uranium catalysts used
showed that, although the precatalysts share similar character-
istics, the activation and nature of the active species are
different in each case, giving rise to different regio- and
chemoselectivities of oligomerization and cyclization products.
The production of arenes as a result of catalytic cyclo-
trimerization by the uranium precatalysts 1 and 2 provides
considerable evidence for the significance of the actinide 5f
electrons in structure, bonding, and reactivity. The use of these
simple amido actinide complexes in other new chemical
transformations is under further investigation.
a
The final product is obtained by a repetition of this cycle.
produced by a minor reaction pathway by bond homolysis of
the uranium(IV) arene intermediate, generating the uranium-
EXPERIMENTAL SECTION
■
Materials and Methods. All manipulations of air-sensitive
materials were performed with the rigorous exclusion of oxygen and
moisture in flamed Schlenk-type glassware or J. Young Teflon valve
sealed NMR tubes on a dual-manifold Schlenk line interfaced to a
(
III) complex and an arene radical which terminates at the
neutral trisubstituted benzene (Scheme 6).
Scheme 6. Suggested Mechanism of Radical Formation of
the Arene and Regeneration of Uranium(III)
−5
high-vacuum (10 Torr) line or in a nitrogen-filled Innovative
Technologies glovebox with a medium-capacity recirculator (1−2 ppm
of O ). Argon and nitrogen were purified by passage through a MnO
2
oxygen-removal column and a Davison 4 Å molecular sieve column.
The hydrocarbon solvents benzene-d (Cambridge Isotopes), toluene
6
(
Bio-Lab), toluene-d (Cambridge Isotopes), and THF (Aldrich) were
8
distilled under nitrogen from Na/K alloy. 1-Hexyne, tert-butylacety-
lene, (trimethylsilyl)acetylene, and phenylacetylene were purchased
from ABCR, degassed, and stored under nitrogen over 4 Å molecular
sieves, and transferred under vacuum immediately prior to use. The
The generated uranium(III) species may then undergo the
previously proposed one-electron-oxidation process to generate
the aforementioned uranium hydride 1b, followed by insertion
and protolytic cleavage by alkyne to generate alkene 12 and the
uranium active catalyst. Radical trapping experiments were
performed by the addition of a fullerene solution at the half-life
of the reaction using both catalysts 1 and 2. The formation of
an organic fullerenyl radical was detected by EPR spectroscopy,
consistent with the aforementioned process.
3
9
2
actinide complexes U[N(SiMe ) ] (1) and [(Me Si) N] An[κ -
3
2
3
3
2
2
5
5
(N,C)−CH Si(CH ) N(SiMe )] (An = U (2), Th (3)) were
prepared according to published methods.
2
3
2
3
NMR spectra were recorded on a Bruker Avance 300, Bruker
Avance III 400, or Bruker Avance 500 spectrometer. Chemical shifts
1
for H NMR are referenced to internal protio solvent and reported
relative to tetramethylsilane. ESR spectra were recorded on a Bruker
EMX-10/12 X-band (ν = 9.4 GHz) digital spectrometer. Spectra of
fullerene radicals were recorded at a microwave power of 0.06 mW and
2
0 kHz magnetic field modulation of 0.02 G amplitude. Digital field
CONCLUSIONS
■
resolution was 4096 points per spectrum, allowing all hyperfine
splitting to be measured directly with an accuracy better than 0.01 G.
Spectral processing and simulation were performed with Bruker WIN-
EPR and SimFonia Software. MALDI-TOF LD+ and LD− experi-
ments were performed on a Waters MALDI Micromass MX
spectrometer using the standard Micromass 96-well matrix along
with fullerene, which has a higher ability for light absorption and
ionization than Ag. Mass analysis was performed in the reflectron
mode in the region between m/z 300 and 3000. GC/MS analyses were
carried out on a Thermo Scientific ITQ series GC-Ion Trap MS
System or a single-quadrupole Waters ZMD instrument. LC/MS
analyses were performed on a Thermo Scientific LCQ Fleet Ion Trap
Mass Spectrometer with a reverse-phase octadecyl HPLC-MS column
and acetonitrile/water as eluent, with a photodiode array detector.
Electronic spectra were acquired on a Shimadzu UV-1800
spectrophotometer, and fluorescence measurements were performed
on a Jobin Yvon (Fluorolog-3) Fluorometer.
In this study we have introduced a class of actinide amide
catalyst which allows for a control of regioselective oligome-
rization or cyclotrimerization of terminal alkynes. Of these, the
latter [2 + 2 + 2] cycloaddition process is unprecedented, a
surprising result given the large expected negative enthalpy of
cyclization associated with the formation of aromatic species. In
the linear oligomerization, a combination of migratory
insertions and protolytic cleavages comprise the catalytic
cycle. Mechanistic studies suggest a bimetallic cyclization of
an intermediate metallatrimer, followed by protonolysis which
liberates the organic arene product. Additionally, a radical
cyclization is experimentally supported as a minor mechanistic
pathway that generates a uranium(III) moiety capable of
forming a uranium(IV) hydride, which results in the reduction
of alkyne to alkene.
General Procedure for Actinide-Mediated Alkyne Oligome-
In addition, we have carried out systematic studies varying
the steric and electronic effects of the substrate which has
rization/Cyclotrimerization. In a typical experiment, 0.5 mL of an
∼30 mM solution of catalyst in benzene-d was transferred to a J.
6
H
DOI: 10.1021/acs.organomet.5b00455
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Young Teflon-sealed NMR tube. Either 10 or 100 equiv of terminal
alkyne was transferred to the same tube, which was then sealed,
shaken, and heated to 75 °C for 72 h to ensure completion of the
(q). Oligomerization of ethynyltrimethylsilane by complex 3
48
(1%): 214 μL of ethynyltrimethylsilane; 64% conversion; 4c (22%);
5
4
9
c (43%); 8c (35%), (see reaction o).
1
(r). Oligomerization of ethynyltrimethylsilane by complex 3
reaction. The samples were analyzed as crude reaction mixtures by H,
48
1
3
(10%): 21 μL of ethynyltrimethylsilane; 87% conversion; 4c (27%);
C, and two-dimensional NMR spectroscopy, and the data were
4
9
5
c (32%); 8c (41%), (see reaction o).
s). Oligomerization of phenylacetylene by complex 1 (1%): 165
μL of phenyacetylene; 92% conversion; dimers (5%), MS (ESI+) m/z
compared to literature values for the organic products. Following this,
samples were quenched by the addition of a few drops of methanol,
filtered through a short Celite plug, and analyzed by GC/MS or LC/
MS methods on the reaction mixtures. Product ratios were determined
(
+
+
2
(
(
05.16 ([M + H] ), 126.94 (H CC(Ph)(CC) ); trimers (7%), MS
2
+
+
ESI+) m/z 306.37 (M ), 205.10 (H CC(Ph) (CHCPh) ); 10d
1
2
by either H NMR spectroscopy or UV−vis spectroscopy interfaced to
+
+
25%), MS (ESI+) m/z 306.34 (M ), 229.18 (Ph C H ); 11d (63%),
2
6
3
the LC/MS system.
+
+
MS (ESI+) m/z 306.33 (M ), 229.12 (Ph C H ).
2
6
3
(
a). Oligomerization of 1-hexyne by complex 1 (1%): 172 μL of 1-
(t). Oligomerization of phenylacetylene by complex 1 (10%): 17
41
1
hexyne; 88% conversion; 4a (91%); 6a (5%); H NMR (300 MHz,
C D ) δ 6.39 (s, 1H), 5.00 (d, J = 1.0 Hz, 1H), 4.93 (d, J = 1.0 Hz,
μL of phenyacetylene; 100% conversion; dimers (2%), MS (ESI+) m/
+
+
6
6
z 205.17 ([M + H] ), 126.30 (H CC(Ph)(CC) ); 10d (48%), MS
2
+
+
1
1
H), 2.54 (t, J = 6.0 Hz, 2H), 2.46 (t, J = 9.0 Hz, 4H), 1.41 (m, 2H),
(
ESI+) m/z 306.11 (M ), 229.23 (Ph C H ); 11d (49%), MS (ESI+)
m/z 306.39 (M ), 229.26 (Ph C H ).
2 6 3
4
2
+
+
.34 (m, 6H), 1.22 (m, 4H), 0.90 (t, J = 7.2 Hz, 9H); 10a (2%); 11a
2
6
3
42
(
1%).
b). Oligomerization of 1-hexyne by complex 1 (10%): 17 μL of 1-
(u). Oligomerization of phenylacetylene by complex 2 (1%): 165
(
μL of phenyacetylene; 96% conversion; dimers (19%), MS (ESI+): m/
41
42
42
+
+
hexyne; 100% conversion; 4a (13%); 10a (29%); 11a (42%);
1
z 204.16 (M ), 127.01 (H CC(Ph)(CC) ); trimers (5%), MS (ESI
2
1
+
+
2a (15%); H NMR (300 MHz, C D ) δ 5.77 (ddt, 1H), 5.11−4.95
+): m/z 306.34 (M ), 204.16 (H2CC(Ph) (CHCPh) ); 10d (35%),
MS (ESI+): m/z 306.30 (M ), 229.23 (Ph C H ); 11d (40%), MS
(ESI+): m/z 306.31 (M ), 229.20 (Ph
6
6
+
+
(
m, 2H), 1.96 (q, J = 6.0 Hz, 2H), 1.41−1.27 (m, 4H), 0.88 (t, J = 6.0
Hz).
c). Oligomerization of 1-hexyne by complex 2 (1%): 172 μL of 1-
2
6
3
+
+
C
H
3
).
2
6
(
(v). Oligomerization of phenylacetylene by complex 2 (10%): 17
41
hexyne; 88% conversion; 4a (96%); 6a (2%) (see reaction a); 10a
μL of phenyacetylene; 100% conversion; dimers (2%), MS (ESI+) m/
42
42
+
+
(
1%); 11a (1%).
d). Oligomerization of 1-hexyne by complex 2 (10%): 17 μL of 1-
z 205.25 ([M + H] ), 126.91 (H
2
CC(Ph)(CC) ); 10d (48%), MS
+
+
(
ESI+) m/z 306.30 (M ), 229.25 (Ph C H ); 11d (49%), MS (ESI+)
m/z 306.37 (M ), 229.36 (Ph C H ).
(
2
6
3
42
42
+
+
hexyne; 100% conversion; 10a (41%); 11a (37%); 12a (22%) (see
2
6
3
(
w). Oligomerization of phenylacetylene by complex 3 (1%): 165
reaction b).
51 52
μL of phenyacetylene; 99% conversion; 4d (91%); 5d (9%).
(x). Oligomerization of phenylacetylene by complex 3 (10%): 17
μL of phenyacetylene; 100% conversion; 4d (77%); 5d (15%);
(
e). Oligomerization of 1-hexyne by complex 3 (1%): 172 μL of 1-
41
hexyne; 86% conversion; 4a (92%); 6a (7%) (see reaction a).
f). Oligomerization of 1-hexyne by complex 3 (10%): 17 μL of 1-
5
1
52
(
5
3
41
12d (8%).
hexyne; 99% conversion; 4a (93%); 6a (7%) (see reaction a).
g). Oligomerization of tert-butylacetylene by complex 1 (1%):
Preparative Reaction of 1,6-Heptadiyne with Complexes 1.
In a thick-walled Schlenk tube with a J. Young Teflon stopcock was
placed 0.11 g (153 μmol) of complex 1 in 15 mL of toluene. A 175 μL
portion (0.141 g, 1.53 mmol) of 1,6-heptadiyne was added to the
reaction mixture, followed by sealing the tube, evacuating the head
space, and heating to 75 °C for 24 h. The reaction mixture was then
cooled to room temperature and quenched by the addition of water.
The organic layer was separated, and the aqueous layer was extracted
three times with 10 mL portions of ethyl ether and then three 10 mL
portions of ethyl acetate. The organics were combined, dried over
(
4
3
1
(
85 μL of tert-butylacetylene ; 87% conversion; 4b (22%); 5b
40%); 7b (21%); H NMR (300 MHz, C D ) δ 6.06 (d, J = 15 Hz,
44
1
6 6
1
1
H), 5.63 (d, J = 15 Hz, 1H), 5.46 (s, 1H), 1.47 (s, 9H), 1.44 (s, 9H),
45 46 47
.25 (s, 9H); 10b (8%); 11b (1%); 12b (7%).
h). Oligomerization of tert-butylacetylene by complex 1 (10%):
(
4
4
1
9 μL of tert-butylacetylene; 100% conversion; 5b (55%); 11b
46
47
(
22%); 12b (23%).
i). Oligomerization of tert-butylacetylene by complex 2 (1%):
(
4
3
1
85 μL of tert-butylacetylene; 77% conversion; 4b (60%); 5b
sodium sulfate (Na SO ), filtered, and evaporated to dryness. The
44
1
2
4
(
22%); 6b (9%); H NMR (300 MHz, C D ) δ 5.90 (d, J = 1.8 Hz,
6 6
crude product was purified by preparative thin-layer chromatography
with n-hexane as eluent, yielding the mono- and bis(indane) products
1
9
H), 5.86 (d, J = 1.8 Hz, 1H), 5.80 (s, 1H), 1.23 (s, 18H), 1.18 (s,
45 46 47
H); 10b (3%); 11b (1%); 12b (5%).
j). Oligomerization of tert-butylacetylene by complex 2 (10%):
22% based on 1,6-heptadiyne). The 1H NMR spectra of these
(
(
compounds were found to be in agreement with previous literature
4
4
5
4
1
9 μL of tert-butylacetylene; 100% conversion; 5b (54%); 6b (23%),
data.
45 46 47
Synthesis of nBuCCD. 1-Hexyne (20 mL, 174 mmol) was
(
see reaction i); 10b (6%); 11b (2%); 12b (16%).
k). Oligomerization of tert-butylacetylene by complex 3 (1%):
(
syringed into a thick-walled Schlenk tube containing a 1.6 M solution
4
3
n
1
85 μL of tert-butylacetylene; 70% conversion; 4b (41%); 5b
of BuLi in hexane (90 mL, 144 mmol) at −95 °C. The mixture was
44
(
43%); 7b (16%), (see reaction g).
l). Oligomerization of tert-butylacetylene by complex 3 (10%):
warmed slowly to 0 °C and stirred for 30 min. The mixture was then
warmed to room temperature, and the volatiles were removed in vacuo
to yield a white solid. The flask was cooled to −85 °C, and a under
(
4
3
19 μL of tert-butylacetylene; 97% conversion; 4b (14%); 5b
44
(
46%); 7b (39%), (see reaction g).
m). Oligomerization of ethynyltrimethylsilane by complex 1
nitrogen flush a large excess of D O (50 mL) was slowly added by
2
(
syringe. The tube was sealed and slowly warmed to 0 °C and the
mixture stirred vigorously for 10 min until all solids dissolved. The
mixture was separated, and the organic layer was distilled under
4
8
(1%): 214 μL of ethynyltrimethylsilane; 42% conversion; 4c (50%);
49
42
42
5
c (11%); 10c (22%); 11c (17%).
n). Oligomerization of ethynyltrimethylsilane by complex 1
10%): 21 μL of ethynyltrimethylsilane; 100% conversion; 10c
(
nitrogen. The distillate was run through a plug of MgSO , redistilled,
4
n
(
and stored over 4 Å molecular sieves, yielding 18 mL of BuCCD.
42
42
2
(
58%); 11c (42%).
o). Oligomerization of ethynyltrimethylsilane by complex 2
H NMR: δ 1.85 ppm. No signal for the terminal alkyne proton (C
1
(
CH) was found in the H NMR.
4
8
(1%): 214 μL of ethynyltrimethylsilane; 97% conversion; 4c (32%);
Synthesis of Me SiCCD. (Trimethylsilyl)acetylene (10 mL, 70
3
49
50
1
5
c (8%); 8c (19%); 9c (38%); H NMR (300 MHz, C D ) δ 7.03
mmol) was syringed into a thick-walled Schlenk tube containing a 1.6
6
6
4
2
n
(
m, 1H), 6.90 (m, 1H), 6.84 (s, 1H), 0.10 (s, 27H); 10c (2%); 11c
M solution of BuLi in hexane (37 mL, 59 mmol) at −95 °C. The
42
(
2%).
p). Oligomerization of ethynyltrimethylsilane by complex 2
mixture was warmed slowly to 0 °C and stirred for 30 min. The
mixture was then warmed to room temperature, and the volatiles were
removed in vacuo to yield a white solid. The flask was cooled to −85
(
4
8
(10%): 21 μL of ethynyltrimethylsilane; 100% conversion; 4c (34%);
49
42
42
5
c (25%); 10c (23%); 11c (19%).
°C, and under a nitrogen flush an excess of D O (20 mL) was slowly
2
I
DOI: 10.1021/acs.organomet.5b00455
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
added by syringe. The tube was sealed and slowly warmed to 0 °C and
the mixture stirred vigorously for 10 min until all solids dissolved. The
mixture was separated, and the organic layer was distilled under
AUTHOR INFORMATION
Corresponding Authors
E-mail: polly.arnold@ed.ac.uk.
E-mail: chmoris@tx.technion.ac.il.
■
*
*
nitrogen. The distillate was run through a plug of MgSO , redistilled,
4
and stored over 4 Å molecular sieves, yielding 8 mL of Me SiCCD.
3
2
1
H NMR: δ 2.32 ppm. H NMR spectroscopy showed less than 1% of
Author Contributions
the terminal alkyne proton (CCH).
The manuscript was written through contributions of all
authors./All authors have given approval to the final version of
the manuscript.
EPR Studies of Oligomerization of 1-Hexyne with Com-
plexes 1 and 2. Preparation of samples was performed inside an
inert-atmosphere glovebox. Stock solutions of reagents were prepared
in vials within the glovebox and added to J. Young Teflon-sealed NMR
Notes
The authors declare no competing financial interest.
−1
tubes. A 1 mL portion of fullerene stock solution (2 mg mL in
toluene) was added to 45 μL of 1-hexyne followed by the addition of
ACKNOWLEDGMENTS
100 μL of a 40 mM stock solution of complexes 1 or 2 or by the
■
addition of the fullerene solution to a similar catalytic solution after the
sample reacted with heating for 1 day. Samples were heated to 75 °C
followed by acquisition of the EPR spectrum immediately after
addition and after completion of the reaction, showing the presence of
a fullerene-trapped organic radical. MALDI-TOF LD+ analysis was
performed on the crude sample to determine the molecular weight of
the fullerene-trapped species.
Kinetic Studies of 1-Hexyne Oligomerization and Trimethy-
lacetylene Cyclotrimerization. In a typical experiment, an NMR
sample was prepared as described above (see General Procedure for
Actinide-Mediated Alkyne Oligomerization). Experiments for reagent-
order determination were performed at variable concentrations of
catalyst or alkyne, spanning 1 order of magnitude concentration
differences while the other reagent concentration was constant. The
sample tube was inserted into the probe of a Bruker Avance 300
spectrometer that had been previously set to the desired temperature
This work was supported by the Israel Science Foundation
administered by the Israel Academy of Science and Humanities
under Contract No. 78/14, the Technion-Israel Institute of
Technology, the University of Edinburgh, EaStCHEM School
of Chemistry, and the EPSRC (U.K.). We also thank COST
Action CM1006 for making this collaboration possible and Dr.
Boris Tumanskii for his generous assistance with EPR
spectroscopy.
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DOI: 10.1021/acs.organomet.5b00455
Organometallics XXXX, XXX, XXX−XXX