Catalytic metathesis of conjugated diynes

Article abstract of DOI:10.1002/anie.201202101

The tungsten benzylidyne complex [PhC≡W{OSi(OtBu)₃} ₃] efficiently catalyzes the metathesis of conjugated diynes and ring-closing diyne metathesis (see scheme). Although this reaction implies C-C single-bond activation, ¹³C labeling studies reveal that it proceeds by classical alkylidyne group exchange and involves cleavage and formation of carbon-carbon triple bonds. Copyright

Full text of DOI:10.1002/anie.201202101

Angewandte
Chemie
DOI: 10.1002/anie.201202101
Diyne Metathesis
Catalytic Metathesis of Conjugated Diynes**
Sergej Lysenko, Jeroen Volbeda, Peter G. Jones, and Matthias Tamm*
Alkyne metathesis has recently witnessed a significant growth
in the number of homogeneous catalysts that are able to
efficiently promote the formation and cleavage of carbon–
carbon triple bonds.[1,2] All well-defined catalyst systems
reported to date can be regarded as variants of Schrock-type
molybdenum(VI) or tungsten(VI) alkylidyne complexes[3] in
With this highly active alkyne metathesis catalyst at hand,
we became interested in the metathesis of conjugated diynes
ꢀ À ꢀ
of the type RC C C CMe (2), which could potentially lead
[13]
ꢀ À ꢀ À ꢀ
to symmetric triynes RC C C C C CR if 2-butyne was
likewise formed. Therefore, we initially exposed 1,3-penta-
diyn-1-ylbenzene 2a (R = Ph) to our standard alkyne meta-
thesis (Table 1) by stirring a toluene solution (4 mL) of 2a
ꢀ À
which the metal–alkylidyne moiety, M C R, is supported by
alkoxide,[4] arlyoxide,[5] amido,[6] imidazolin-2-iminato,[7]
phosphoraneiminato,[8] or silanolate ligands.[9,10] With a few
exceptions,[11] alkyne metathesis usually requires the presence
[a]
ꢀ À ꢀ
Table 1: Diyne metathesis of R-C C C C-Me (2) catalyzed by 1.
Reagent
Product
R
Yield[b]
[%]
ꢀ
of internal alkynes, such as RC CMe (R = alkyl or aryl),
ꢀ À ꢀ
R-C C C C-R
ꢀ
ꢀ
affording the symmetric alkynes RC CR and MeC CMe. To
drive these equilibrium reactions to completion, the latter, 2-
butyne (b.p. = 278C), can be continuously removed from the
reaction mixtures under vacuum-driven conditions or, as
recently shown by Fꢀrstner and co-workers, advantageously
by addition of molecular sieves (5 ꢁ M.S.) to adsorb 2-
butyne.[9,12] For our part, the molecular-sieve-promoted
method was also clearly superior when alkyne homocoupling
and ring-closing alkyne metathesis (RCAM) reactions were
studied in the presence of catalytic amounts of the tri(tert-
butoxy)silanolate-supported tungsten benzylidyne complex
1 (Scheme 1).[10]
2a
2b
2c
3a
3b
3c
97
95
97
2d
3d
95
2e
2 f
3e
3 f
96
80[c]
[a] Conditions: Substrate (1.0 mmol), catalyst 1 (21 mg, 2 mol%),
toluene (4 mL), 5 ꢀ M.S. (1.0 g), room temperature, t=2 h. [b] Yield of
product isolated after filtration through alumina and purification by
column chromatography (if required). [c] The decreased yield arises from
the volatility of 3 f.
(1.0 mmol) and the catalyst 1 (2 mol%) for two hours at room
temperature in the presence of activated 5 ꢁ molecular
sieves. The conversion was monitored by gas chromatography
and, after full consumption of 2a, the reaction mixture was
filtered through alumina. Evaporation of the solvent afforded
a white solid, the spectroscopic characterization of which
indicated, much to our surprise, almost quantitative forma-
tion of 1,4-diphenyl-1,3-butadiyne (3a) instead of the antici-
pated triyne.[14] Clearly, the isolation of 3a requires the
formation of 2,4-hexadiyne (dimethyldiacetylene, DMDA)[15]
in equal amounts (Scheme 1), which is likely to be adsorbed
by the molecular sieves. Its formation could be unambigu-
ously confirmed by performing the metathesis of 2a at room
temperature under vacuum-driven conditions (0.1 mbar) in
1,2,4-trichlorobenzene solution. The reaction flask was con-
nected to a cold trap (liquid N2), and volatile DMDA was
collected by sublimation.[15] Under these conditions, 3a could
be isolated in 82% yield, indicating again the preferability of
the molecular-sieve-promoted method.
Scheme 1. Metathesis of diynes catalyzed by 1. For a definition of the
R groups, see Table 1; DMDA=dimethyldiacetylene.
[*] Dipl.-Chem. S. Lysenko, MSc J. Volbeda, Prof. P. G. Jones,
Prof. Dr. M. Tamm
Institut fꢁr Anorganische und Analytische Chemie, Technische
Universitꢂt Carolo-Wilhelmina
Hagenring 30, 38106 Braunschweig (Germany)
E-mail: m.tamm@tu-bs.de
Homepage: http://www.tu-braunschweig.de/iaac
[**] We wish to thank Dr. Christian Kleeberg, Danny Schlꢁns, and Priv.-
Doz. Dr. Jçrg Grunenberg for helpful discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201202101.
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At first glance, the conversion of 2a into the symmetric
diyne 3a and DMDA might suggest that the metathesis
À
proceeds by an exchange of the alkynyl groups by C C
activation[16] and cleavage of the internal carbon–carbon
single bond, similar to the mechanism established for the
titanocene-mediated and photocatalyzed cleavage and
recombination of disubstituted butadiynes.[17] In the presence
of the alkylidyne complex 1, however, such a mechanism is
clearly unlikely, and metathesis should proceed by exchange
of the alkylidyne groups by the formation of metallacyclobu-
tadiene intermediates according to the established mecha-
nism.[7c,18] To unequivocally preclude C C s-bond metathesis,
À
the 13C-labeled pentadiyne 2a-13C was synthesized from the
corresponding phenylacetylene, PhC 13CH, and subjected to
ꢀ
catalytic alkyne metathesis.[13] Under the conditions described
13
13
ꢀ
À ꢀ
above, the product PhC C C CPh (3a- C) was isolated in
high yield and shown by mass spectrometry and 13C NMR
spectroscopy to contain only one 13C atom residing exclu-
sively in the 2-position (Scheme 2).[19] Accordingly, the
absence of any detectable amounts of the doubly 13C-labeled
Scheme 3. Proposed catalytic cycle for the metathesis of diynes.
13
13
ꢀ
À
ꢀ
butadiyne PhC
C
C CPh rules out a mechanism involv-
ing alkynyl group exchange.
ates bearing the alkynyl groups exclusively in the a-positions.
As shown, this mechanism allows us to rationalize the
exclusive formation of the monolabeled diyne 3a-13C, while
DMDA-13C is adsorbed on 5 ꢁ M.S. It should be noted,
however, that an alternative catalytic cycle with b-alkynyl-
metallacyclobutadiene species can also account for these
observations (see the Supporting information).[14] At the
present stage, an unambiguous decision in favor of one of
these mechanisms, or another one altogether, is not possible
and requires additional experimental and theoretical sup-
port.[23]
To study the scope of the new diyne metathesis reaction,
para-(1,3-pentadiyn-1-yl)toluene (2b) and para-(1,3-penta-
diyn-1-yl)anisole (2c) were synthesized and subjected to the
standard metathesis (Table 1). For both substrates, the
reaction was completed within less than one hour, and
Figure 1 shows a representative conversion–time diagram
Scheme 2. Metathesis of the 13C-labeled diyne 3a-13C and formation of
13C-labeled phenylalkynylalkylidyne complexes.
To substantiate a possible mechanism for the observed
diyne metathesis, the 13C-labeled product 3a-13C was treated
with an equimolar amount of 1 in C6D6. The 13C NMR
spectrum indicated full conversion into the phenylalkynylal-
kylidyne complexes 4-a-13C and 4-b-13C and unlabeled
13
ꢀ
À ꢀ
ꢀ À
diphenylacetylene (tolane). The M C C C and M C
13
ꢀ
C C signals in the NMR spectrum are found at 252.3 and
95.8 ppm, with the tungsten satellites revealing coupling
constants of 1J(13C–183W) = 298.1 Hz and 2J(13C–183W) =
60.5 Hz, which is in very good agreement with the values
reported for the quinuclidine adduct of the related complex
[(tBuO)3W C C C Et].[20,21] Furthermore, the signals for Ca
ꢀ À ꢀ À
Figure 1. Conversion–time diagram for the metathesis of 2c. For
reaction conditions, see Table 1.
and Cb are flanked by carbon satellites, revealing a 1J(13C–13C)
coupling constant of 94.6 Hz. Isolation of 4 on a preparative
scale was however not achieved, as slow conversion into other
reaction products, presumably dinuclear species, was
observed.[20,22]
for the metathesis of 2c. It should be emphasized that
prolonged reaction times (t > 3 h) afford lower yields of 3b
and 3c, as trace amounts of other species such as the
corresponding mono- and triacetylenes are formed, as
indicated by GC/MS analysis and 13C NMR spectroscopy.
In view of the rapid formation of alkynylalkylidyne
species such as 4, we favor the mechanism outlined in
Scheme 3, which involves metallacyclobutadiene intermedi-
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Along with the arylpentadiynes 2a–2c, the homocoupling
of the aliphatic 3,5-heptadiyn-1-yl benzoates 2d and 2e was
studied under the same reaction conditions. The dibenzoates
3d and 3e were isolated as white crystalline solids in excellent
yield (Table 1), confirming the functional group tolerance
that had previously been established for catalyst 1.[10]
Compatibility with the trimethylsilyl group was also found
by cleanly converting 1-trimethylsilyl-1,3-pentadiyne (2 f)
into 1,4-bis(trimethylsilyl)butadiyne (3 f). The slightly re-
duced yield (80%) can be attributed to the volatility of 3 f and
its loss during solvent evaporation.
Admittedly, several arguments could be raised to question
the usefulness and applicability of the newly discovered diyne
metathesis reaction: 1) The preparation of the symmetric
diynes 3 (Table 1) by metathesis is not an atom-economic
transformation, as stoichiometric amounts of 2,4-hexadiyne
are formed; 2) the diyne substrates 2 have to be synthesized in
a rather elaborate manner, for example by treatment of
terminal alkynes with EtMgBr followed by reaction with
propargyl bromide and isomerization;[14] 3) copper-mediated
transformations, such as Glaser, Eglinton, or Hay coupling of
terminal alkynes usually provide a convenient and direct
access to symmetric diynes and polyynes.[24] In the latter
À
reactions, however, C C bond formation proceeds irrever-
sibly, which limits the development of high-yielding proce-
dures for the construction of macrocyclic diyne or oligo-
(diyne) scaffolds.[24,25] In contrast, the reversibility of carbon–
carbon triple-bond formation by catalytic ring-closing diyne
metathesis (RCDM) offers, similarly to conventional
RCAM,[7b] the possibility of synthesizing cyclodiynes under
thermodynamic control, providing the potential for increased
selectivity in macrocyclic product formation.[26]
Scheme 4. Ring-closing diyne metathesis (RCDM). Reaction condi-
tions: substrate (0.5 mmol), catalyst (21 mg, 4 mol%), toluene
(25 mL), 5 ꢀ M.S. (1.0 g), room temperature, t=16 h; yields of
isolated products: 90% (7), 80% (8). Ball-and-stick drawing of one of
twelve crystallographically independent molecules of 7.
Thus, the bis(diynes) 5 and 6 were prepared by esterifi-
cation or etherification of 3,5-heptadiyn-1-ol with adipoyl
dichloride or 1,4-di(bromomethyl)benzene, respectively, and
subjected to catalytic diyne metathesis under similar con-
ditions as described above, albeit at significantly higher
dilution (Scheme 4). In the case of 5, the cyclic adipate 7 was
isolated as a white crystalline solid in 90% yield. Character-
ization by NMR spectroscopy and gas chromatography
indicated the selective formation of monomeric 7, which
could be qualitatively confirmed by X-ray diffraction analysis.
However, the crystal structure suffers from modulation; the
apparent cell contains twelve independent molecules, and
refinement is unsatisfactory. The exact nature of the modu-
lation has not yet been established.
Under similar conditions, RCDM of 6 afforded the
bis(diyne) 8 as a crystalline white solid in 80% yield after
chromatographic purification. Single crystals of 8 suitable for
X-ray diffraction analysis were obtained from CH2Cl2/toluene
solution at 38C. The molecular structure is shown in
Figure 2,[14] confirming the formation of a [12.12]paracyclo-
phane.[27] The molecule has crystallographic inversion sym-
metry and contains two parallel, linear diyne moieties with
Figure 2. ORTEP diagram of 8 with ellipsoids set at 50% probability.
Selected bond lengths [ꢀ] and angles [8]: C1–C2 1.2037(17), C2–C3
1.3794(17), C3–C4 1.2029(17); C1-C2-C3 178.46(12), C2-C3-C4
177.34(11).
corresponding monomeric [12]paracyclophane is fully con-
sistent with the exclusive formation of a closely related
[10.10]paracyclophane under thermodynamically controlled
RCAM reaction conditions.[7b]
À
À
À
C1 C2, C2 C3, and C3 C4 bond lengths of 1.2037(17),
1.3794(17), and 1.2029(17) ꢁ, which is in good agreement
with the structures established for other bis(diyne)-bridged
cyclophanes.[28] Furthermore, the selective formation of
dimeric 8 and the absence of any detectable amounts of the
The results presented herein demonstrate the ability of
the tungsten benzylidyne complex 1 to promote the catalytic
metathesis of methyl-capped conjugated diynes, affording
symmetric diynes with a 1,4-butadiyne core. 13C-labeling
experiments provide clear evidence that these reactions
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[12] The possibility to run catalytic alkyne metathesis in the presence
of molecular sieves had already been previously addressed,
although 4 ꢁ M.S. were used; see: a) V. Huc, R. Weihofen, I.
Martin-Jimenez, P. Ouliꢂ, C. Lepetit, G. Lavigne, R. Chauvin,
New J. Chem. 2003, 27, 1412; b) V. Maraval, C. Lepetit, A.-M.
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[14] See the Supporting Information for full experimental details and
spectroscopic characterization of all compounds together with
a presentation of selected NMR spectra.
[15] E. F. Meyer, J. J. Meyer, J. Chem. Eng. Data 1986, 31, 273.
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proceed in a similar fashion to that established for conven-
tional metal-catalyzed alkyne metathesis, which involves
cleavage and formation of carbon–carbon triple bonds via
metallacyclobutadiene intermediates.[18] This procedure can
be adapted to the preparation of cyclodiynes by ring-closing
diyne metathesis (RCDM), and the reversibility of the
catalytic reaction steps provides the possibility of obtaining
a high degree of thermodynamic control and selectivity in
macrocyclic diyne and oligo(diyne) construction. Therefore,
we feel that this new method will be a useful and versatile
addition to the methods available in preparative acetylenic
chemistry,[29] for example in natural product and polymer
synthesis.[30,31]
Received: March 16, 2012
Published online: May 23, 2012
Keywords: alkyne metathesis · alkynes · cyclophanes · diynes ·
.
tungsten
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13
ꢀ
À ꢀ
doublets at 81.7 ppm for the 1,4-carbon atoms (C C C C)
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ꢀ
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