Catalytic addition of terminal alkynes to carbodiimides by half-sandwich rare earth metal complexes

Article abstract of DOI:10.1021/ja0560027

The catalytic addition of terminal alkynes to carbodiimides has been achieved for the first time by use of half-sandwich rare earth metal complexes, such as {Me₂Si(C₅Me₄)(NPh)}Y(CH₂SiMe₃)(THF)₂

Full text of DOI:10.1021/ja0560027

Published on Web 11/10/2005
Catalytic Addition of Terminal Alkynes to Carbodiimides by Half-Sandwich
Rare Earth Metal Complexes
Wen-Xiong Zhang, Masayoshi Nishiura, and Zhaomin Hou*
Organometallic Chemistry Laboratory, RIKEN (The Institute of Physical and Chemical Research), Hirosawa 2-1,
Wako, Saitama 351-0198, Japan, and PRESTO, Japan Science and Technology Agency (JST), Japan
Received August 31, 2005; E-mail: houz@riken.jp
Nucleophilic addition of organometallic reagents “RM” to
carbodiimides (R′NdCdNR′) to give the corresponding amidinates
“(R′NC(R)NR′)M” is a well established process. The amidinate
units have long been used as ancillary ligands for stabilization of
various metal complexes, including those of early transition metals
and lanthanides.1-4 However, catalytic transformation of a metal
amidinate species has not been reported previously. Hydrolysis of
a metal amidinate species could be a convenient, albeit stoichio-
metric route to amidines R′NdC(R)NHR′. However, propiol-
Figure 1. ORTEP drawing of 3. Selected bond lengths (Å): Y(1)-N(1)
amidines R′NdC(CtCR)(NHR′) which contain a conjugated C-C
triple bond could hardly be obtained in this way because of their
high sensitivity to hydrolysis.5 On the other hand, although addition
2.393(5), Y(1)-N(2) 2.339(5), N(1)-C(1) 1.322(7), N(2)-C(1) 1.325(7),
C(1)-C(2) 1.449(8), C(2)-C(3) 1.182(8).
Scheme 1 . Formation of an Yttrium Propiolamidinate and Its
Reaction with Phenylacetylene
of terminal alkyne C-H bonds across carbodiimides could, in
principle, provide a straightforward, atom-economic route to
propiolamidines, such a catalytic reaction has hardly been explored.6
In fact, although various amidines have been reported, little
information about propiolamidines could be found in the literature,
and in particular, isolated propiolamidines remained illusive.5,6
We report here that half-sandwich rare earth metal complexes
bearing silylene-linked cyclopentadienyl-amido ligands can act as
an excellent catalyst for the addition of terminal alkynes to
carbodiimides, which yields efficiently the corresponding N,N′-
disubstituted propiolamidines. A rare earth metal amidinate species
has been confirmed to be a true catalytic species in this process.
As far as we are aware, this is the first example of catalytic
transformation of a metal amidinate species and also the first
example of efficient preparation of well-defined propiolamidines,5,6
a new family of amidines which may show unique reactivity that
differs from those of the previously known amidines.7
The alkyl complex 1a also showed high catalytic activity under
similar conditions (Table 1, entry 1). THF seemed to be a better
solvent than benzene or toluene for this reaction (Table 1, entry
2). The Yb (1c) and Lu (1d) complexes were also effective, although
their activity was slightly lower than that of the Y analogue 1a
(Table 1, entries 4-6). Formation of a phenylacetylene homodimer-
ization product was not observed under the present conditions,
although these complexes were active for phenylacetylene homo-
coupling in the absence of a carbodiimide.8
A wide range of terminal alkynes could be used for this catalytic
cross-coupling reaction. The reaction was not affected by either
electron-withdrawing or -donating substituents or their positions
at the phenyl ring of an aromatic alkyne (Table 1, entries 9-18).
The aromatic C-Cl (entries 12 and 14) and C-Br (entries 13 and
15) bonds survived in the present reactions. Heteroatom-containing
alkynes, such as pyridylacetylenes (entries 19 and 20), were also
applicable. In the case of an alkyl alkyne (entry 21), the reaction
became a little bit slower probably owing to its weaker acidity.
N,N′-Diaryl-substituted carbodiimides were not suitable for this
reaction, probably because the resulting amidines are too acidic.
The tris(alkyl) complex Y(CH2SiMe3)3(THF)2 showed no catalytic
activity under the same conditions, suggesting that the Cp-amido
ligands also play an important role.
The reaction of {Me2Si(C5Me4)(NPh)}Y(CH2SiMe3)(THF)2 (1a)
with 1 equiv of phenylacetylene in toluene yielded quantitatively
the corresponding phenylacetylide 2 (Scheme 1).8 Nucleophilic
addition of 2 to 1,3-di-tert-butylcarbodiimide took place rapidly at
80 °C to give the propiolamidinate complex 3 (Scheme 1 and Figure
1). A reaction between 3 and phenylacetylene was not observed at
room temperature in toluene-d8. However, when a 1:1 mixture of
3 and phenylacetylene was heated to 80 °C, the propiolamidine 4b
and the phenylacetylide 2 were formed almost quantitatively. More
remarkably, catalytic formation of 4b was achieved when excess
phenylacetylene and 1,3-di-tert-butylcarbodiimide (1:1) were added
to 3 in toluene-d8 at 80 °C. These results are in striking contrast
with what was observed previously for other amidinate complexes,
such as {PhC(NSiMe3)2}2YCH(SiMe3)2,4n in which the amidinate
ligands remained intact even in the presence of an excess amount
of terminal alkynes at high temperatures. Although a large number
of amidinate complexes of various metals have been reported,1-4
catalytic transformation of an amidinate species is, to the best of
our knowledge, unprecedented.
The use of an isolated amidinate complex, such as 3, was not
necessarily required for the present catalytic cross-coupling reaction.
9
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J. AM. CHEM. SOC. 2005, 127, 16788-16789
10.1021/ja0560027 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Catalytic Addition of Terminal Alkynes to Carbodiimidesa
demonstrate that an amidinate unit, though being often used as an
ancillary ligand for various organometallic complexes, can itself
participate in a catalytic reaction under appropriate conditions.
Acknowledgment. This work was partly supported by a Grant-
in-Aid for Scientific Research on Priority Areas (No. 14078224,
“Reaction Control of Dynamic Complexes”) from the Ministry of
Education, Culture, Sports, Science and Technology of Japan and
by the Natural Science Foundation of China (20328201).
temp
C)
time
(h)
yieldb
(%)
entry
R
R′
cat.
solvent
(
°
Supporting Information Available: Experimental details, X-ray
data for 3, 4g, and 4i, and scanned NMR spectra of all products (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
1a C6D6
80
80
80
3
1
1
4a (98)c
4a (>99)c
4a (98)c
1a THF-d8
1b THF-d8
1a THF-d8
1c THF-d8
1d THF-d8
80 0.5 4a (93)c
80 0.5 4a (84)c
80 0.5 4a (76)c
References
t-Bu 1a toluene 110
Cy 1a THF 80
t-Bu 1a toluene 110
Cy
i-Pr
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
4b (94)
4c (95)
4d (94)
4e (96)
4f (96)
4g (95)
4h (93)
4i (97)
4j (94)
4k (97)
4l (97)
4m (95)
4n (98)
4o (98)
4p (70)
(1) Review: Barker, J.; Kilner, M. Coord. Chem. ReV. 1994, 133, 219-300.
(2) For examples of main group metal amidinate complexes, see: (a) Rowley,
C. N.; DiLabio, G. A.; Barry, S. T. Inorg. Chem. 2005, 44, 1983-1991.
(b) Foley, S. R.; Zhou, Y.; Yap, G. P. A.; Richeson, D. S. Inorg. Chem.
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A. R.; Mulvey, R. E.; Bowlings, R. B. J. Am. Chem. Soc. 1998, 120,
7816-7824.
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Meerendonk, W. J.; Schro¨der, K.; Brussee, E. A. C.; Meetsma, A.; Hessen,
B.; Teuben, J. H. Eur. J. Inorg. Chem. 2003, 427-432. (b) Keaton, R. J.;
Jayaratne, K. C.; Henningsen, D. A.; Koterwas, L. A.; Sita, L. R. J. Am.
Chem. Soc. 2001, 123, 6197-6198. (c) Kondo, H.; Yamaguchi, Y.;
Nagashima, H. J. Am. Chem. Soc. 2001, 123, 500-501. (d) Kondo, H.;
Yamaguchi, Y.; Nagashima, H. Chem. Commun. 2000, 1075-1076. (e)
Do¨hler, T.; Go¨rls, H.; Walther, D. Chem. Commun. 2000, 945-946. (f)
Yamaguchi, Y.; Nagashima, H. Organometallics 2000, 19, 725-727. (g)
Koterwas, L. A.; Fettinger, J. C.; Sita, L. R. Organometallics 1999, 18,
4183-4190. (h) Whitener, G. D.; Hagadorn, J. R.; Arnold, J. J. Chem.
Soc., Dalton Trans. 1999, 1249-1255. (i) Averbuj, C.; Eisen, M. S. J.
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Meyer, T. Y. Organometallics 1999, 18, 4417-4420.
4-MeC6H4
4-CF3C6H4
4-MeOC6H4
4-ClC6H4
4-BrC6H4
2-ClC6H4
2-BrC6H4
2-MeC6H4
3-MeC6H4
4-F-3-MeC6H3 i-Pr
3-Py
2-Py
CH3(CH2)4
1a THF
1a THF
80
80
t-Bu 1a toluene 110
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
1a THF
1a THF
1a THF
1a THF
1a THF
1a THF
1a THF
1a THF
80
80
80
80
80
80
80
80
i-Pr
Cy
i-Pr
1a toluene 110
a Conditions: terminal alkynes, 2.07 mmol; carbodiimides, 2.01 mmol;
catalyst, 0.06 mmol; solvent, 5 mL, unless otherwise noted. b Isolated yield.
c Conditions: terminal alkynes, 0.35 mmol; carbodiimides, 0.34 mmol;
catalyst, 0.01 mmol. Yields were determined by 1H NMR using 1,3,5-
trimethylbenzene as an internal standard.
(4) For examples of lanthanide amidinate complexes, see: (a) Bambirra, S.;
Bouwkamp, M. W.; Meetsma, A.; Hessen, B. J. Am. Chem. Soc. 2004,
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Commun. 2004, 2742-2743. (c) Villiers, C.; Thue´ry, P.; Ephritikhine,
M. Eur. J. Inorg. Chem. 2004, 4624-4632. (d) Richter, J.; Feiling, J.;
Schmidt, H.-G.; Noltemeyer, M.; Bru¨ser, W.; Edelmann, F. T. Z. Anorg.
Allg. Chem. 2004, 630, 1269-1275. (e) Bambirra, S.; Leusen, D. v.;
Meetsma, A.; Hessen, B.; Teuben, J. H. Chem. Commun. 2003, 522-
523. (f) Zhang, J.; Ruan, R.; Shao, Z.; Cai, R.; Weng, L.; Zhou, X.
Organometallics 2002, 21, 1420-1424. (g) Luo, Y.; Yao, Y.; Shen, Q.;
Sun, J.; Weng, L. J. Organomet. Chem. 2002, 662, 144-149. (h) Bambirra,
S.; Meetsma, A.; Hessen, B.; Teuben, J. H. Organometallics 2001, 20,
782-785. (i) Aubrecht, K. B.; Chang, K.; Hillmyer, M. A.; Tolman, W.
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J. R.; Brussee, E. A. C.; Meetsma, A.; Hessen, B.; Teuben, J. H.
Organometallics 2000, 19, 3197-3204. (k) Doyle, D.; Gun’ko, Y. K.;
Hitchcock, P. B.; Lappert, M. F. J. Chem. Soc., Dalton Trans. 2000, 4093-
4097. (l) Schmidt, J. A. R.; Arnold, J. Chem. Commun. 1999, 2149-
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R.; Whitener, G. D.; Shafir, A.; Arnold, J. Organometallics 1999, 18,
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Scheme 2. A Possible Mechanism of Catalytic Addition of
Terminal Alkynes to Carbodiimides
A catalytic cycle for the present cross-coupling reaction is shown
in Scheme 2. The acid-base reaction between a half-sandwich rare
earth metal alkyl and a terminal alkyne should yield straightfor-
wardly an alkynide species such as A.8 Nucleophilic addition of
the alkynide species to a carbodiimide would afford the amidinate
species B, which on abstraction of a proton from another molecule
of alkyne would yield the corresponding amidine and regenerate
the alkynide A. The isolation of 3 and its reaction with phenyl-
acetylene to give 2 and 4b (see Scheme 1) strongly support this
mechanism.
(5) Although formation of propiolamidines by hydrolysis of alkali metal
propiolamidates was reported, the spectral data provided could not be
unambiguously assigned to the claimed amidines. See: Fujita, H.; Endo,
R.; Aoyama, A.; Ichii, T. Bull. Chem. Soc. Jpn. 1972, 45, 1846-1852. It
is now rather skeptical that the moisture-sensitive propiolamidines could
survive the given hydrolysis conditions. Under similar conditions, we
obtained an amide product rather than an amidine. See Supporting
Information for more details.
(6) It was previously claimed that addition of a terminal alkyne to a
carbodiimide was achieved in the presence of Co2(CO)8/H4Ru4(CO)12
.
However, the spectral data provided for the product were obviously
indicative of an amide, but were mistakenly assigned to an amidine. See:
Schmidt, G. F.; Su¨ss-Fink, G. J. Organomet. Chem. 1988, 356, 207-
211. See also Supporting Information for more details.
In summary, the catalytic addition of terminal alkynes to
carbodiimides has been achieved for the first time by use of half-
sandwich rare earth metal complexes as a catalyst, which offers a
straightforward, atom-economic route to N,N′-disubstituted
propiolamidines, a new family of amidines that were difficult to
access by other means. Moreover, the results observed in this work
(7) (a) Boyd, G. V. In The Chemistry of Amidines and Imidates; Patai, S.,
Rappoport, Z., Eds.; Wiley: New York, 1991; Vol. 2, Chapter 8. (b)
Greenhill, J. V.; Lue, P. Prog. Med. Chem. 1993, 30, 203-326.
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Am. Chem. Soc. 2003, 125, 1184-1185. (b) Nishiura, M.; Hou, Z. J. Mol.
Catal. A 2004, 213, 101-106.
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