Novel Z-selective head-to-head dimerization of terminal alkynes catalyzed by lanthanide half-metallocene complexes

Article abstract of DOI:10.1021/ja027595d

Various terminal alkynes have been cleanly dimerized into the corresponding head-to-head (Z)-enynes by use of the half-metallocene lutetium alkyl complexes Me₂Si(C₅Me₄)(NAr)Lu(CH₂SiMe₃)(THF) (Ar = Ph, C₆H₃Me₂-2,6, C₆H₂Me₃-2,4,6) as catalysts. Aromatic C-Cl, C-Br, and C-I bonds, which are known to be extremely susceptible to reductive cleavage by transition metals, survived in the present reactions. The corresponding dimeric alkynide species [Me₂Si(C₅Me₄)(NAr)Lu(μ-CCR)]₂ are thought to be the true catalysts, some of which have been isolated and structurally characterized. These alkynide species were thermally stable and soluble at the reaction temperatures (80-110 °C), but they precipitated upon cooling to room temperature after completion of the reaction. Therefore, this catalyst system works homogeneously but can be separated and reused, thus constituting the first example of a recyclable catalyst system for the dimerization of terminal alkynes and also the first example of (Z)-selective head-to-head dimerization of aromatic terminal alkynes. Copyright

Full text of DOI:10.1021/ja027595d

Published on Web 01/14/2003
Novel Z-Selective Head-to-Head Dimerization of Terminal Alkynes Catalyzed
by Lanthanide Half-Metallocene Complexes
Masayoshi Nishiura,† Zhaomin Hou,*,† Yasuo Wakatsuki,† Taeko Yamaki,†,‡ and Takeshi Miyamoto‡
Organometallic Chemistry Laboratory, RIKEN (The Institute of Physical and Chemical Research), Hirosawa 2-1,
Wako, Saitama 351-0198, Japan, and Department of Chemistry, Faculty of Science, Kitasato UniVersity,
Kitazato 1-15-1, Sagamihara, Kanagawa 228-8555, Japan
Received July 9, 2002 ; E-mail: houz@postman.riken.go.jp
Table 1. Dimerization of Phenylacetylene Catalyzed by
Catalytic dimerization of terminal alkynes is an atom economic
and straightforward method for the synthesis of conjugated enynes,
which are important building blocks for organic synthesis and
significant components in various biologically active compounds.1
In particular, the regio- and stereoselective head-to-head (Z)-
dimerization of terminal alkynes is of special importance and
interest, because the resulting (Z)-enynes are key units found in a
variety of naturally occurring anticancer antibiotics.1b Although
various transition metal2 and f-element catalysts3 are known to
catalyze the dimerization of terminal alkynes, in most cases a
mixture of regio (head-to-head vs head-to-tail)- and stereo (E/Z-
head-to-head)-isomers was obtained, while (Z)-selective head-to-
head dimerization of terminal alkynes has hardly been achieved.2-4
None of the previously reported catalyst systems were found to be
recyclable. We report here a novel organolanthanide catalyst system
for the regio- and stereoselective head-to-head (Z)-dimerization of
terminal alkynes. In addition to its excellent selectivity, this catalyst
system can be recovered and reused.
In the course of our studies on lanthanide half-metallocene
complexes,5,6 we examined the reactivity of a series of half-
metallocene alkyl complexes such as 5-10 toward phenylacetylene
(Table 1).7 In sharp contrast with the previously reported catalyst
systems,2,3 these complexes, in particular, the lutetium anilido
complexes 7-9, showed extremely high head-to-head (Z)-selectivity
for the dimerization of phenylacetylene, which yielded solely the
(Z)-dimer 2a (entries 3-5, Table 1). As far as we are aware, this
is the first example of exclusive formation of a (Z)-enyne compound
in the dimerization of an aromatic alkyne.2-4
Lanthanide Half-Metallocene Complexesa
product distributionb
2a:3a:4a:olym
b
entry
cat.
time/h
conversion/%
1
2
3
4
5
6
5
6
7
8
9
20
18
18
5
5
17
85
69
>99
>99
>99
89
89:0:6:5
92:0:0:8
100:0:0:0
100:0:0:0
100:0:0:0
72:0:13:15
10
a Conditions: phenylacetylene (1 mmol), cat. (0.02 mmol), in C6D6 (0.45
1
mL) at 80 °C. b Determined by H NMR and GC-MS.
Table 2. Dimerization of Terminal Alkynes Catalyzed by 8a
c
temp
(°C)
time
(h)
convn
(%)
selectc
(%)
entry
R
solventb
C6D6
toluene-d8
1
2
3
Ph (a)
4-MeC6H4 (b)
4-MeOC6H4 (c) toluene-d8 +
80
110
110
5
2
2
>99
>99
>99
>99
>99
>99
THFd
4
5
6
7
8
4-CF3C6H4 (d)
4-ClC6H4 (e)
4-BrC6H4 (f)
4-IC6H4 (g)
2-ClC6H4 (h)
2-BrC6H4 (i)
n-C6H13 (j)
toluene-d8
C6D6
C6D6
C6D6
C6D6
110
80
80
80
80
2
2
2
2
3
97
>99
>99
>99
>99
>99
>99
95e
>99
>99
>99
>99
>99
95f
9
10
C6D6
THF-d8
80
100
3
14
a Conditions: substrate (1 mmol), 8 (0.02-0.05 mmol). b 0.45 mL.
c Determined by 1H NMR and GC-MS. d THF: 5 equiv per 8. In THF-free
toluene, 2,4-bis(4-methoxyphenyl)but-1-ene-3-yne (33%, head-to-tail dimer)
was also formed. e 1,4,6-Tris[4-(trifluoromethyl)phenyl]hex-1-yne-3,5-diene
(5%) was also formed. f 2-Hexyldec-1-ene-3-yne (5%, head-to-tail dimer)
was also formed.
Complex 8 was then chosen as a catalyst to examine the
dimerization of various terminal alkynes. Some of the representative
results are summarized in Table 2. The aromatic C-Cl (entries 5
and 8), C-Br (entries 6 and 9), and C-I bonds (entry 7), which
are known to be extremely susceptible to reductive cleavage by
transition metals, survived in the present reactions to afford
exclusively the corresponding (Z)-enynes 2e-i, a new class of enyne
building blocks that would be useful for the architecture of further
large molecules. A novel solvent effect on the regioselectivity was
also observed. The dimerzation of 4-methoxyphenylacetylene (1c)
in pure toluene gave a 67:33 mixture of the head-to-head and head-
to-tail dimers, whereas that in the presence of a small amount of
THF (ca. 5 equiv per 8) yielded solely the head-to-head (Z)-dimer
2c (entry 3, Table 2). Similarly, the dimerization of 1-octyne (1j)
in toluene-d8 gave the head-to-tail dimer in 76% yield, while that
† Riken.
‡ Kitasato University.
9
1184
J. AM. CHEM. SOC. 2003, 125, 1184-1185
10.1021/ja027595d CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
an alkyne, must play a critically important role in the present (Z)-
selective dimerization.9,11,12 This is in sharp contrast with the
analogous reactions catalyzed by lanthanide metallocene or ben-
zamidate-ligated catalysts, in which the addition of an alkynide to
an alkyne took place in an “intramolecular” fashion at a monomeric
alkynide/alkyne intermediate and thus always yielded the (E)-enyne
products whenever the head-to-head reaction occurred.3
Scheme 2
Supporting Information Available: Experimental details, X-ray
data for 9 and 11 (CIF), and spectral data for all new compounds (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
References
(1) (a) Trost, B. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 259. (b) Nicolaou,
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W. K.; Grosche, M.; Reisinger, C.-P.; Weskamp, T. J. Organomet. Chem.
2001, 617-618, 616. (c) Ohmura, T.; Yorozuya, S.; Yamamoto, Y.;
Miyaura, N. Organometallics 2000, 19, 365. (d) Nishibayashi, Y.;
Yamanashi, M.; Wakiji, I.; Hidai, M. Angew. Chem., Int. Ed. 2000, 39,
2909. (e) Qu¨, J.-P.; Masui, D.; Ishii, Y.; Hidai, M. Chem. Lett. 1998,
1003. (f) Trost, B. M.; Sorum, M. T.; Chan, C.; Harms, A. E.; Ru¨hter, G.
J. Am. Chem. Soc. 1997, 119, 698. (g) Yi, C. S.; Liu, N. Organometallics
1996, 15, 3968. (h) Bianchini, C.; Frediani, P.; Masi, D.; Peruzzini, M.;
Zanobini, F. Organometallics 1994, 13, 4616. (i) Jun, C.-H.; Lu, Z.;
Crabtree, R. H. Tetrahedron Lett. 1992, 33, 7119.
(3) For examples, see: (a) Duchateau, R.; van Wee, C. T.; Teuben, J. H.
Organometallics 1996, 15, 2291. (b) Schaverien, C. J. Organometallics
1994, 13, 69. (c) Evans, W. J.; Keyer, R. A.; Ziller, J. W. Organometallics
1993, 12, 2618. (d) Heeres, H. J.; Nijhoff, J.; Teuben, J. H. Organome-
tallics 1993, 12, 2609. (e) Forsyth, C. M.; Nolan, S. P.; Stern, C. L.;
Marks, T. J. Organometallics 1993, 12, 3618. (f) Heeres, H. J.; Teuben,
J. H. Organometallics 1991, 10, 1980. (g) den Haan, K. H.; Wielstra, Y.;
Teuben, J. H. Organometallics 1987, 6, 2053. (h) Haskel, A.; Straub, T.;
Dash, A. K.; Eisen, M. S. J. Am. Chem. Soc. 1999, 121, 3014. (i) Dash,
A. K.; Gourevich, I.; Wang, J. Q.; Wang, J.; Kapon, M.; Eisen, M. S.
Organometallics 2001, 20, 5084.
(4) Z)-Selective head-to-head dimerization of aliphatic alkynes catalyzed by
ruthenium catalysts was recently reported.2e However, these catalysts were
not suitable for the dimerization of aromatic alkynes.
(5) Recent reviews: (a) Arndt, S.; Okuda, J. Chem. ReV. 2002, 102, 1953.
(b) Hou, Z.; Wakatsuki, Y. J. Organomet. Chem. 2002, 647, 61. (c) Hou,
Z.; Wakatsuki, Y. Coord. Chem. ReV. 2002, 231, 1.
in THF-d8 afforded the head-to-head Z-dimer (2j) in 95% yield
(entry 10, Table 2).
All of these reactions were homogeneous at the reaction
temperatures (80-110 °C). When the reaction mixtures were cooled
to room temperature, however, the corresponding lutetium alkynide
species precipitated as crystalline powders. In the case of phenyl-
acetylene, single crystals of 11 were obtained, which adopted a
dimeric structure via the phenylacetylide bridges as shown by an
X-ray analysis.8,9 The activity and selectivity of 11 for the
dimerization of phenylacetylene were almost the same as those of
the alkyl complex 8 under similar conditions,10 which strongly
suggests that the true catalysts in the present systems are alkynide
species and more importantly the alkynide catalyst species can be
recovered and reused. A much higher recovery yield for the less
substituted (and thus less soluble) anilido complex 12 (93%) was
achieved than for 11 (65%) by evaporation of the reaction solvent
and hexane extraction of the residue. The recovered 11 or 12 was
identical to that freshly prepared by the reaction of 8 or 9 with
phenylacetylene, respectively, and could therefore be reused without
loss of activity and selectivity (Scheme 1).10
A possible reaction mechanism is shown in Scheme 2. Acid-
base reaction between an alkyl complex and a terminal alkyne
should easily give a dimeric alkynide species such as 11 or 12.
Coordination of an alkyne to a metal center of the dimeric alkynide
species could afford A or B by breaking one of the two alkynide
bridges.11 Attack of the terminal alkynide to the coordinated alkyne
in A (path a) should give C, which upon deprotonation reaction
with another molecule of alkyne would release the (Z)-enyne
product 2 and regenerate the alkynide catalyst species. On the other
hand, the addition of the terminal alkynide to the alkyne in B (path
b) would give D, which upon reaction with another molecule of
alkyne could yield the head-to-tail dimer 4 and regenerate the
alkynide species similarly.12 Apparently, a dimeric intermediate such
as A, which leads to “intermolecular” addition of an alkynide to
(6) (a) Hou, Z.; Zhang, Y.; Tezuka, H.; Xie, P.; Tardif, O.; Koizumi, T.;
Yamazaki, H.; Wakatsuki, Y. J. Am. Chem. Soc. 2000, 122, 10533. (b)
Hou, Z.; Koizumi, T.; Nishiura, M.; Wakatsuki, Y. Organometallics 2001,
20, 3323. (c) Tardif, O.; Hou, Z.; Nishiura, M.; Koizumi, T.; Wakatsuki,
Y. Organometallics 2001, 20, 4565. (d) Hou, Z.; Zhang, Y.; Tardif, O.;
Wakatsuki, Y. J. Am. Chem. Soc. 2001, 123, 9216. (e) Hou, Z.; Zhang,
Y.; Nishiura, M.; Wakatsuki, Y. Organometallics 2003, 22, 129.
(7) Complexes 5-9 (new compounds) were synthesized by the reactions of
Ln(CH2SiMe3)3(THF)2 with 1 equiv of Me2Si(C5Me4H)NHR′, according
to a procedure reported for the synthesis of 10. See: Arndt, S.; Voth, P.;
Spaniol, T. P.; Okuda, J. Organometallics 2000, 19, 4690.
(8) See the Supporting Information for details.
(9) The dimeric alkynide complexes 11 and 12 were very thermally stable,
which remained unchanged after being heated at 150 °C in toluene-d8
overnight in the absence of an alkyne. This is in striking contrast with
what was observed previously in the case of the analogous metallocene
or benzamidate-ligated lanthanide alkynide complexes, which rapidly
decomposed or coupled into trienediyl derivatives upon heating.3a,c-e
(10) A small amount of THF (2 equiv per 11 or 12) was required to achieve
the exclusive head-to-head (Z)-dimerization of phenylacetylene, because
11 or 12 is a THF-free complex, while 8 or 9 bears a THF ligand. This
result is in agreement with the THF effect observed above.
(11) The 13C NMR spectrum of the analogous yttrium (I ) 1/2) alkynide
complex [Me2Si(C5Me4)(NPh)Y{µ-CCC6H4(C5H11-n)-4}(THF)]2 in C6D6
(containing 1 equiv of THF per Y) showed a triplet at both 22 °C (δ
138.7, 1JYC ) 23 Hz) and 80 °C (δ 142.2, 1JYC ) 23 Hz) for the R-alkynide
carbon, suggesting that the µ-alkynide-bridged dimeric structure in this
type of complex is considerably strong. Addition of 2 equiv of 4-(n-
pentyl)phenylacetylene to this solution at 80 °C afforded quantitatively
the corresponding Z-dimerization product in 3 h, while the triplet for the
R-alkynide carbon remained almost unchanged after the reaction.
(12) Path a could be a sterically favored process, while path b might be
electronically preferred especially when R is an alkyl or electron-donating
group. In the presence of THF, A might be preferred to B because of
crowdedness caused by the THF molecules (ligands) around the metal
centers. Further studies are in progress to clarify the mechanism.
JA027595D
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J. AM. CHEM. SOC. VOL. 125, NO. 5, 2003 1185