Practical new silyloxy-based alkyne metathesis catalysts with optimized activity and selectivity profiles

Article abstract of DOI:10.1021/ja104800w

Triphenylsilanolate ligands were found to impart excellent reactivity and outstanding functional group tolerance on molybdenum alkylidyne complexes, which catalyze alkyne metathesis reactions of all sorts. The active species either can be obtained in high yield by adaptation of the established synthesis routes leading to Schrock alkylidynes or can be generated in situ from the molybdenum nitride complex 11, which itself is readily accessible in large quantity from inexpensive sodium molybdate. Complexation of the active silanolate complexes 12 and 24 with 1,10-phenanthroline affords complexes 15 and 25, respectively, which are stable in air for extended periods of time. Although these phenathroline adducts are per se unreactive vis-a-vis alkynes, catalytic activity is conveniently restored upon exposure to MnCl₂. Therefore, the practitioner has the choice of different alkyne metathesis (pre)catalysts, which are easy to handle yet broadly applicable and exceedingly tolerant. A host of representative inter- as well as intramolecular alkyne metathesis reactions, including applications to a considerable number of bioactive and, in part, labile natural products, shows the remarkable scope of these new tools. Moreover, it was found that the addition of molecular sieves (5 A ≤ 4 A > 3 A) to the reaction mixture significantly improves the chemical yields while simultaneously increasing the reaction rates. This benefit is ascribed to effective binding of 2-butyne, which is released as the common byproduct in reactions of alkynes bearing a methyl end-cap. Thus, alkyne metatheses can now be performed at ambient temperature with neither the need to apply vacuum to drive the conversion nor recourse to tailor-made substrates. The structures of representative examples of this new generation of alkyne metathesis catalysts in the solid state were determined by X-ray analysis.

Full text of DOI:10.1021/ja104800w

Published on Web 07/22/2010
Practical New Silyloxy-Based Alkyne Metathesis Catalysts
with Optimized Activity and Selectivity Profiles
Johannes Heppekausen, Robert Stade, Richard Goddard, and Alois Fu¨rstner*
Max-Planck-Institut fu¨r Kohlenforschung, D-45470 Mu¨lheim/Ruhr, Germany
Received June 2, 2010; E-mail: fuerstner@kofo.mpg.de
Abstract: Triphenylsilanolate ligands were found to impart excellent reactivity and outstanding functional
group tolerance on molybdenum alkylidyne complexes, which catalyze alkyne metathesis reactions of all
sorts. The active species either can be obtained in high yield by adaptation of the established synthesis
routes leading to Schrock alkylidynes or can be generated in situ from the molybdenum nitride complex
11, which itself is readily accessible in large quantity from inexpensive sodium molybdate. Complexation
of the active silanolate complexes 12 and 24 with 1,10-phenanthroline affords complexes 15 and 25,
respectively, which are stable in air for extended periods of time. Although these phenathroline adducts
are per se unreactive vis-a`-vis alkynes, catalytic activity is conveniently restored upon exposure to MnCl2.
Therefore, the practitioner has the choice of different alkyne metathesis (pre)catalysts, which are easy to
handle yet broadly applicable and exceedingly tolerant. A host of representative inter- as well as
intramolecular alkyne metathesis reactions, including applications to a considerable number of bioactive
and, in part, labile natural products, shows the remarkable scope of these new tools. Moreover, it was
found that the addition of molecular sieves (5 Å g 4 Å . 3 Å) to the reaction mixture significantly improves
the chemical yields while simultaneously increasing the reaction rates. This benefit is ascribed to effective
binding of 2-butyne, which is released as the common byproduct in reactions of alkynes bearing a methyl
end-cap. Thus, alkyne metatheses can now be performed at ambient temperature with neither the need to
apply vacuum to drive the conversion nor recourse to tailor-made substrates. The structures of representative
examples of this new generation of alkyne metathesis catalysts in the solid state were determined by X-ray
analysis.
Scheme 1. Basic Mechanism of Alkyne Metathesis Catalyzed by
Metal Alkylidynes
Introduction
A major reason for the overwhelming success of olefin
metathesis is the ready availability of catalysts that combine
high activity with a good to excellent tolerance toward functional
groups other than alkenes.1,2 As some of these key catalysts
are modular, well accessible, and easy to handle, this transfor-
mation was rapidly embraced by the synthetic community and
has profoundly changed the way contemporary organic and
polymer chemistry are practiced.3
this scrambling process was swiftly discerned7 and experimen-
tally proven (Scheme 1).8,9 The active catalysts were shown to
be Schrock alkylidyne complexes,10 which either can be
generated in situ or, preferentially, are administered to the
reaction mixtures as structurally well-defined, preformed species.
Despite this excellent background knowledge, it was only after
a considerable lag period11 that the potential of alkyne metathesis
Compared to the omnipresence of alkene metathesis, the
related metathesis of alkynes is much less commonly used.4,5
Initially described as early as 1968,6 the principle underlying
(1) Schrock, R. R.; Murdzek, J. S.; Bazan, G. C.; Robbins, J.; DiMare,
M.; O’Regan, M. J. Am. Chem. Soc. 1990, 112, 3875.
(5) (a) Fu¨rstner, A. In Handbook of Metathesis; Grubbs, R. H., Ed.; Wiley-
VCH: Weinheim, 2003; Vol. 2, p 432. (b) Davies, P. W. In Handbook
of Cyclization Reactions; Ma, S., Ed.; Wiley-VCH: Weinheim, 2009;
Vol. 1, p 599.
(2) (a) Nguyen, S. T.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc.
1993, 115, 9858. (b) Schwab, P.; Grubbs, R. H.; Ziller, J. W. J. Am.
Chem. Soc. 1996, 118, 100.
(3) (a) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18. (b)
Fu¨rstner, A. Angew. Chem., Int. Ed. 2000, 39, 3012. (c) Connon, S. J.;
Blechert, S. Angew. Chem., Int. Ed. 2003, 42, 1900. (d) Nicolaou,
K. C.; Bulger, P. G.; Sarlah, D. Angew. Chem., Int. Ed. 2005, 44,
4490. (e) Deiters, A.; Martin, S. F. Chem. ReV. 2004, 104, 2199. (f)
Hoveyda, A. H.; Zhugralin, A. R. Nature 2007, 450, 243.
(6) Pennella, F.; Banks, R. L.; Bailey, G. C. Chem. Commun. 1968, 1548.
(7) Katz, T. J.; McGinnis, J. J. Am. Chem. Soc. 1975, 97, 1592.
(8) Wengrovius, J. H.; Sancho, J.; Schrock, R. R. J. Am. Chem. Soc. 1981,
103, 3932.
(9) Churchill, M. R.; Ziller, J. W.; Freudenberger, J. H.; Schrock, R. R.
Organometallics 1984, 3, 1554.
(4) (a) Fu¨rstner, A.; Davies, P. W. Chem. Commun. 2005, 2307. (b) Zhang,
W.; Moore, J. S. AdV. Synth. Catal. 2007, 349, 93. (c) Schrock, R. R.;
Czekelius, C. AdV. Synth. Catal. 2007, 349, 55. (d) Mortreux, A.;
Coutelier, O. J. Mol. Catal. A: Chem. 2006, 254, 96.
(10) (a) Schrock, R. R. Chem. ReV. 2002, 102, 145. (b) Schrock, R. R.
Angew. Chem., Int. Ed. 2006, 45, 3748. (c) Schrock, R. R. J. Chem.
Soc., Dalton Trans. 2001, 2541. (d) Schrock, R. R. Acc. Chem. Res.
1986, 19, 342.
9
10.1021/ja104800w  2010 American Chemical Society
J. AM. CHEM. SOC. 2010, 132, 11045–11057 11045

A R T I C L E S
Heppekausen et al.
was recognized and explored.12,13 The fact that nonterminal
alkynes are required as the substrates may be one reason for
the smaller impact compared with olefin metathesis,14,15 yet a
growing number of applications in the recent literature to natural
product synthesis,4 coordination chemistry,16 and material
science17,18 show that this inherent drawback can be (partly)
compensated for or even turned to advantage by the excellent
selectivity of alkyne metathesis, as well as by the possibility of
further elaborating the acetylenic products primarily formed in
a diverse fashion by a host of different postmetathetic
transformations.19-24
Alkyne metathesis was originally discovered using a hetero-
geneous catalyst composed of tungsten oxide on silica which
was operative only at 200-450 °C.6,25 Shortly thereafter,
Mortreux and co-workers showed that homogeneous mixtures
comprising Mo(CO)6 and resorcinol (or other phenols) in high-
boiling solvents are active at more manageable temperatures
(ca. 130-160 °C).26 Despite many rounds of optimization of
the molybdenum source, the phenol additive, the solvent, and
the reaction conditions, this simple and cheap system has not
reached the level of activity and selectivity that better-defined
(pre)catalysts can offer.27-29 It was the advent of well-defined
d0-alkylidyne complexes of tungsten,8,30,31 molybdenum,32,33 and
rhenium34,35 that set new standards in the field. Among them,
(11) For early applications to fairly unfunctionalized substrates, see refs
26, 27 and the following: (a) Villemin, D.; Cadiot, P. Tetrahedron
Lett. 1982, 23, 5139. (b) Kaneta, N.; Hirai, T.; Mori, M. Chem. Lett.
1995, 627. (c) Kaneta, N.; Hikichi, K.; Asaka, S.; Uemura, M.; Mori,
M. Chem. Lett. 1995, 1055.
(23) In addition to the established chemistry of alkynes, recent advances
in the carbophilic activation of triple bonds with noble metal catalysts
hold particular promise in this regard; see: (a) Fu¨rstner, A.; Davies,
P. W. Angew. Chem., Int. Ed. 2007, 46, 3410. (b) Gorin, D. J.; Toste,
D. Nature 2007, 446, 395. (c) Jime´nez-Nu´n˜ez, E.; Echavarren, A. M.
Chem. Commun. 2007, 333. (d) Hashmi, A. S. K. Chem. ReV. 2007,
107, 3180. (e) Fu¨rstner, A. Chem. Soc. ReV. 2009, 38, 3208.
(24) The established alkyne metathesis catalysts do not react with alkenes,
whereas alkene metathesis catalysts can attack alkynes. For an early
application exploiting this orthogonal selectivity profile, see: Fu¨rstner,
A.; Dierkes, T. Org. Lett. 2000, 2, 2463.
(12) Fu¨rstner, A.; Seidel, G. Angew. Chem., Int. Ed. 1998, 37, 1734.
(13) Fu¨rstner, A.; Guth, O.; Rumbo, A.; Seidel, G. J. Am. Chem. Soc. 1999,
121, 11108.
(14) (a) Terminal alkynes are mainly polymerized in the presence of the
standard catalysts, cf.: Bray, A.; Mortreux, A.; Petit, F.; Petit, M.;
Szymanska-Buzar, T. J. Chem. Soc., Chem. Commun. 1993, 197. (b)
Moreover, terminal alkynes are known to degrade Schrock alkylidynes
via formation of deprotiometallacyclobutadiene intermediates, which
could be unambiguously characterized, cf. ref 32a and the following:
McCullough, L. G.; Listemann, M. L.; Schrock, R. R.; Churchill,
M. R.; Ziller, J. W. J. Am. Chem. Soc. 1983, 105, 6729.
(25) For further studies on heterogeneous catalysts for alkyne metathesis,
see ref 53 and the following: (a) Mortreux, A.; Blanchard, M. Bull.
Soc. Chim. Fr. 1972, 1641. (b) Moulijn, J. A.; Reitsma, H. J.;
Boelhouwer, C. J. Catal. 1972, 25, 434. (c) Mortreux, A.; Petit, F.;
Blanchard, M. J. Mol. Catal. 1980, 8, 97.
(15) For a report claiming metathesis of terminal alkynes by a modified in
situ catalyst, see: Coutelier, O.; Nowogrocki, G.; Paul, J.-F.; Mortreux,
A. AdV. Synth. Catal. 2007, 349, 2259.
(16) (a) Bauer, E. B.; Szafert, S.; Hampel, F.; Gladysz, J. A. Organome-
tallics 2003, 22, 2184. (b) Bauer, E. B.; Hampel, F.; Gladysz, J. AdV.
(26) (a) Mortreux, A.; Blanchard, M. J. Chem. Soc., Chem. Commun.
1974, 786. (b) Mortreux, A.; Dy, N.; Blanchard, M. J. Mol. Catal.
1975/76, 1, 101. (c) Mortreux, A.; Delgrange, J. C.; Blanchard, M.;
Lubochinsky, B. J. Mol. Catal. 1977, 2, 73. (d) Mortreux, A.; Petit,
F. Tetrahedron Lett. 1978, 4967.
ˇ
Synth. Catal. 2004, 346, 812. (c) Kotora, M.; Nee`as, D.; St`ıpnie`ka, P.
Collect. Czech. Chem. Commun. 2003, 68, 1897. (d) Bobula, T.;
ˇ
Hudlicky, J.; Nova´k, P.; Gyepes, R.; Cisaøova, I.; St`ıpnie`ka, P.; Kotora,
M. Eur. J. Inorg. Chem. 2008, 3911. (e) Sato, M.; Watanabe, M. Chem.
Commun. 2002, 1574.
(27) (a) Du Plessis, J. A. K.; Vosloo, H. C. M. J. Mol. Catal. 1991, 65,
51. (b) Vosloo, H. C. M.; du Plessis, J. A. K. J. Mol. Catal. A: Chem.
1998, 133, 205. (c) Bencheick, A.; Petit, M.; Mortreux, A.; Petit, F.
J. Mol. Catal. 1982, 15, 93. (d) Pschirer, N. G.; Bunz, U. H. F.
Tetrahedron Lett. 1999, 40, 2481. (e) Brizius, G.; Bunz, U. H. F. Org.
Lett. 2002, 4, 2829. (f) Sashuk, V.; Ignatowska, J.; Grela, K. J. Org.
Chem. 2004, 69, 7748.
(17) (a) Weiss, K.; Michel, A.; Auth, E.-M.; Bunz, U. H. F.; Mangel, T.;
Mu¨llen, K. Angew. Chem., Int. Ed. Engl. 1997, 36, 506. (b)
Kloppenburg, L.; Song, D.; Bunz, U. H. F. J. Am. Chem. Soc. 1998,
120, 7973. (c) Zhang, X.-P.; Bazan, G. C. Macromolecules 1994, 27,
4627. (d) Krouse, S. A.; Schrock, R. R. Macromolecules 1989, 22,
2569. (e) Bunz, U. H. F.; Kloppenburg, L. Angew. Chem., Int. Ed.
1999, 38, 478. (f) Bly, R. K.; Dyke, K. M.; Bunz, U. H. F. J.
Organomet. Chem. 2005, 690, 825. (g) Brizius, G.; Kroth, S.; Bunz,
U. H. F. Macromolecules 2002, 35, 5317. (h) Zhang, W.; Moore, J. S.
Macromolecules 2004, 37, 3973. (i) Brizius, G.; Pschirer, N. G.;
Steffen, W.; Stitzer, K.; zur Loye, H.-C.; Bunz, U. H. F. J. Am. Chem.
Soc. 2000, 122, 12435. (j) Carnes, M.; Buccella, D.; Siegrist, T.;
Steigerwald, M. L.; Nuckolls, C. J. Am. Chem. Soc. 2008, 130, 14078.
(28) For a direct comparison of a Mortreux-type catalyst with more defined
catalysts, see refs 13 and 46d. For an example in which an attempted
alkyne metathesis with a Mortreux-type catalyst failed because of a
competing reaction of the substrate with the phenol additive, see: Ma,
J.; Ku¨hn, B.; Hackl, T.; Butenscho¨n, H. Chem.sEur. J. 2010, 16, 1859.
(29) In most cases the nature of the active catalyst in solution is unknown,
even though it might be guessed on the basis of the extensive
knowledge of the chemistry of metal alkylidynes. Therefore, the
expressions “catalyst” and “precatalyst” are not rigorously distin-
guished in this publication.
(30) (a) Schrock, R. R.; Clark, D. N.; Sancho, J.; Wengrovius, J. H.;
Rocklage, S. M.; Pedersen, S. F. Organometallics 1982, 1, 1645. (b)
Pedersen, S. F.; Schrock, R. R.; Churchill, M. R.; Wasserman, H. J.
J. Am. Chem. Soc. 1982, 104, 6808. (c) Freudenberger, J. H.; Schrock,
R. R.; Churchill, M. R.; Rheingold, A. L.; Ziller, J. W. Organometallics
1984, 3, 1563. (d) Listemann, M. L.; Schrock, R. R. Organometallics
1985, 4, 74. (e) Tonzetich, Z. J.; Lam, Y. C.; Mu¨ller, P.; Schrock,
R. R. Organometallics 2007, 26, 475.
ˇ
(18) (a) Miljanic´, O. S.; Vollhardt, K. P. C.; Whitener, G. D. Synlett 2003,
29. (b) Hellbach, B.; Gleiter, R.; Rominger, F. Synthesis 2003, 2535.
(c) Johnson, C. A.; Lu, Y.; Haley, M. M. Org. Lett. 2007, 9, 3725.
(d) Ge, P.-H.; Fu, W.; Herrmann, W. A.; Herdtweck, E.; Campana,
C.; Adams, R. D.; Bunz, U. H. F. Angew. Chem., Int. Ed. 2000, 39,
3607. (e) Pschirer, N. G.; Fu, W.; Adams, R. D.; Bunz, U. H. F. Chem.
Commun. 2000, 87.
(19) Ring-closing alkyne metathesis in combination with Lindlar-type
semireduction provides a stereoselective entry into macrocyclic (Z)-
alkenes that remain difficult to make by conventional RCM; cf. refs
12, 13. For recent advances toward highly Z-selective alkene metathesis
catalysts, see: Jiang, A. J.; Zhao, Y.; Schrock, R. R.; Hoveyda, A. H.
J. Am. Chem. Soc. 2009, 131, 16630.
(31) (a) Beer, S.; Brandhorst, K.; Hrib, C. G.; Wu, X.; Haberlag, B.;
Grunenberg, J.; Jones, P. G.; Tamm, M. Organometallics 2009, 28,
1534. (b) Beer, S.; Hrib, C. G.; Jones, P. G.; Brandhorst, K.;
Grunenberg, J.; Tamm, M. Angew. Chem., Int. Ed. 2007, 46, 8890.
(c) Beer, S.; Brandhorst, K.; Grunenberg, J.; Hrib, C. G.; Jones, P. G.;
Tamm, M. Org. Lett. 2008, 10, 981.
(20) Ring-closing alkyne metathesis in combination with trans-hydrosily-
lation and protodesilylation provides a stereoselective entry into
macrocyclic (E)-alkenes, see: (a) Fu¨rstner, A.; Radkowski, K. Chem.
Commun. 2002, 2182. (b) Lacombe, F.; Radkowski, K.; Seidel, G.;
Fu¨rstner, A. Tetrahedron 2004, 60, 7315.
(32) (a) McCullough, L. G.; Schrock, R. R.; Dewan, J. C.; Murdzek, J. C.
J. Am. Chem. Soc. 1985, 107, 5987. (b) McCullough, L. G.; Schrock,
R. R. J. Am. Chem. Soc. 1984, 106, 4067.
(21) For the formation of an aromatic heterocycle by an RCAM/alkyne
activation sequence, see: Fu¨rstner, A.; Castanet, A.-S.; Radkowski,
K.; Lehmann, C. W. J. Org. Chem. 2003, 68, 1521.
(33) (a) Tsai, Y.-C.; Diaconescu, P. L.; Cummins, C. C. Organometallics
2000, 19, 5260. (b) Blackwell, J. M.; Figueroa, J. S.; Stephens, F. H.;
Cummins, C. C. Organometallics 2003, 22, 3351.
(22) For the formation of 1,3-dienes by an alkyne metathesis/enyne cross-
metathesis sequence, see: (a) Fu¨rstner, A.; Larionov, O.; Flu¨gge, S.
Angew. Chem., Int. Ed. 2007, 46, 5545. (b) Fu¨rstner, A.; Flu¨gge, S.;
Larionov, O.; Takahashi, Y.; Kubota, T.; Kobayashi, J. Chem.sEur.
J. 2009, 15, 4011. (c) Groaz, E.; Banti, D.; North, M. Eur. J. Org.
Chem. 2007, 3727.
(34) Weinstock, I. A.; Schrock, R. R.; Davis, W. M. J. Am. Chem. Soc.
1991, 113, 135.
(35) Chabanas, M.; Baudouin, A.; Cope´ret, C.; Basset, J.-M. J. Am. Chem.
Soc. 2001, 123, 2062.
9
11046 J. AM. CHEM. SOC. VOL. 132, NO. 32, 2010

Practical New Silyloxy-Based Alkyne Metathesis Catalysts
A R T I C L E S
the tungsten neopentylidyne complex 1 found the broadest use
These successes notwithstanding, all available catalystssexcept
for Mortreux’s in situ recipe and its variantsssuffer from high
sensitivity toward oxygen, moisture, and, in the case of complex
6,48 even molecular nitrogen. The necessary expert knowledge
in handling such compounds may be another reason why alkyne
metathesis has not yet become more popular. Convinced by the
power of this transformation, however, our group is committed
to change this situation by developing catalysts that retain the
excellent selectivity of complexes such as 1-6 while being
significantly more user-friendly. Ideally, the next generation of
alkyne metathesis catalysts should be applicable to complex and
polysubstituted targets while being cheap, easy to make, and
air stable. In a recent Communication we described a first step
toward this goal;49 outlined below, we present a full account of
our work in this area, which led to a set of new catalysts that
meet these stringent criteria very well.
and is now commercially available.8,30
Results and Discussion
Molybenum Nitrido Complexes with Ancillary Silanolate
Ligands. Inspiration for the development of novel user-friendly
yet effective alkyne metathesis catalysts was provided by
Johnson and co-workers, who reported that certain nitride
complexes of molybdenum and tungsten endowed with fluori-
nated alkoxide ligands react with sacrificial alkynes to generate
metal alkylidynes in situ (Scheme 2).50 However, the preparation
of 7 as a prototype precursor complex requires the use of azides
as well as the handling of air-sensitive intermediates.
In an attempt to find a more convenient entry point, we
investigated if the same intermediate 8 could be generated by
alcoholysis of the much more accessible precursor 1151 with
hexafluoro-tert-butanol (14). Even though the mixture of 11 and
Early on, this catalyst was shown to induce certain alkyne
metathesis reactions, even at ambient temperature, within
minutes. 1 turned out to be compatible with surprisingly many
functional groups, although donor sites such as basic nitrogen,
divalent sulfur, polyether fragments, or various heterocycles
block its activity, presumably by coordination to the fairly Lewis
acidic tungsten center.12,13 Moreover, carbonyl groups may react
with 1 and analogues in a Wittig-like manner.36 Subsequent
variations of the ligand sphere, as represented, for example, in
complex 2, allowed the activity to be improved even further.31
The related molybdenum alkylidynes are generally considered to
be less reactive but may exhibit an increased functional group
tolerance. This fact is usually explained by the lower Lewis acidity
of Mo(6+) compared to W(6+), though the chosen set of ancillary
ligands must be taken into consideration.32 Catalytically active and
highly tolerant molybdenum species can also be prepared in situ
by reacting the now also commercial trisamido complex 6 with
CH2Cl2 or other gem-dihalides.37-39 Their remarkable selectivity
profile is evident from many applications right through to the total
synthesis of structurally complex and, in part, labile target
molecules, including epothilone A and C,38,40 the latrunculins,41
prostaglandin E2 1,15-lactone,42 cruentaren A,43 amphidinolide
V,22a,b sophorolipid lactone,44 and myxovirescin.45-47
(45) Fu¨rstner, A.; Bonnekessel, M.; Blank, J. T.; Radkowski, K.; Seidel,
G.; Lacombe, F.; Gabor, B.; Mynott, R. Chem.sEur. J. 2007, 13,
8762.
(46) For other applications to natural product chemistry from our laboratory,
see: (a) Fu¨rstner, A.; Seidel, G. J. Organomet. Chem. 2000, 606, 75.
(b) Fu¨rstner, A.; Rumbo, A. J. Org. Chem. 2000, 65, 2608. (c)
Aguilera, B.; Wolf, L. B.; Nieczypor, P.; Rutjes, F. P. J. T.; Overkleeft,
H. S.; van Hest, J. C. M.; Schoemaker, H. E.; Wang, B.; Mol, J. C.;
Fu¨rstner, A.; Overhand, M.; van der Marel, G. A.; van Boom, J. H. J.
Org. Chem. 2001, 66, 3584. (d) Fu¨rstner, A.; Stelzer, F.; Rumbo, A.;
Krause, H. Chem.sEur. J. 2002, 8, 1856. (e) Song, D.; Blond, G.;
Fu¨rstner, A. Tetrahedron 2003, 59, 6899. (f) Ghalit, N.; Poot, A. J.;
Fu¨rstner, A.; Rijkers, D. T. S.; Liskamp, R. M. Org. Lett. 2005, 7,
2961.
(36) Freudenberger, J. H.; Schrock, R. R. Organometallics 1986, 5, 398.
(37) Fu¨rstner, A.; Mathes, C.; Lehmann, C. W. J. Am. Chem. Soc. 1999,
121, 9453.
(47) For applications by other groups, see: (a) Vintonyak, V. V.; Cala`,
M.; Lay, F.; Kunze, B.; Sasse, F.; Maier, M. E. Chem.sEur. J. 2008,
14, 3709. (b) Vintonyak, V. V.; Maier, M. E. Angew. Chem., Int Ed.
2007, 46, 5209. (c) Vintonyak, V. V.; Maier, M. E. Org. Lett. 2007,
9, 655. (d) Doyle, D.; Murphy, P. V. Carbohydr. Res. 2008, 343, 2535.
(e) Kraft, P.; Berthold, C. Synthesis 2008, 543. (f) Groothuys, S.; van
den Broek, S. A. M. W.; Kuijpers, B. H. M.; IJsselstijn, M.; van Delft,
F. L.; Rutjes, F. P. J. T. Synlett 2008, 111. (g) Schulz, S.; Yildizhan,
S.; Stritzke, K.; Estrada, C.; Gilbert, L. E. Org. Bioorg. Chem. 2007,
5, 3434. (h) Ghalit, N.; Rijkers, D. T. S.; Liskamp, R. M. J. J. Mol.
Catal. A: Chem. 2006, 254, 68. (i) Chan, J.; Jamison, T. F. J. Am.
Chem. Soc. 2004, 126, 10682. (j) IJsselstijn, M.; Aguilera, B.; van
der Marel, G. A.; van Boom, J. H.; van Delft, F. L.; Schoemaker,
H. E.; Overkleeft, H. S.; Rutjes, F. P. J. T.; Overhand, M. Tetrahedron
Lett. 2004, 45, 4379.
(38) Fu¨rstner, A.; Mathes, C.; Lehmann, C. W. Chem.sEur. J. 2001, 7,
5299.
(39) (a) Zhang, W.; Kraft, S.; Moore, J. S. J. Am. Chem. Soc. 2004, 126,
329. (b) Zhang, W.; Kraft, S.; Moore, J. S. Chem. Commun. 2003,
832.
(40) Fu¨rstner, A.; Mathes, C.; Grela, K. Chem. Commun. 2001, 1057.
(41) (a) Fu¨rstner, A.; De Souza, D.; Parra-Rapado, L.; Jensen, J. T. Angew.
Chem., Int. Ed. 2003, 42, 5358. (b) Fu¨rstner, A.; Turet, L. Angew.
Chem., Int. Ed. 2005, 44, 3462. (c) Fu¨rstner, A.; Kirk, D.; Fenster,
M. D. B.; A¨ıssa, C.; De Souza, D.; Mu¨ller, O. Proc. Natl. Acad. Sci.
U.S.A. 2005, 102, 8103. (d) Fu¨rstner, A.; De Souza, D.; Turet, L.;
Fenster, M. D. B.; Parra-Rapado, L.; Wirtz, C.; Mynott, R.; Lehmann,
C. W. Chem.sEur. J. 2007, 13, 115. (e) Fu¨rstner, A.; Kirk, D.; Fenster,
M. D. B.; A¨ıssa, C.; De Souza, D.; Nevado, C.; Tuttle, T.; Thiel, W.;
Mu¨ller, O. Chem.sEur. J. 2007, 13, 135.
(48) For a review on the activation of small molecules by complex 6, see:
Cummins, C. C. Chem. Commun. 1998, 1777.
(42) (a) Fu¨rstner, A.; Grela, K.; Mathes, C.; Lehmann, C. W. J. Am. Chem.
Soc. 2000, 122, 11799. (b) Fu¨rstner, A.; Grela, K. Angew. Chem., Int.
Ed. 2000, 39, 1234.
(49) Bindl, M.; Stade, R.; Heilmann, E. K.; Picot, A.; Goddard, R.; Fu¨rstner,
A. J. Am. Chem. Soc. 2009, 131, 9468.
(50) (a) Geyer, A. M.; Wiedner, E. S.; Gary, J. B.; Gdula, R. L.; Kuhlmann,
N. C.; Johnson, M. J. A.; Dunietz, B. D.; Kampf, J. W. J. Am. Chem.
Soc. 2008, 130, 8984. (b) Gdula, R. L.; Johnson, M. J. A. J. Am. Chem.
Soc. 2006, 128, 9614.
(43) (a) Fu¨rstner, A.; Bindl, M.; Jean, L. Angew. Chem., Int. Ed. 2007, 46,
9275. (b) Bindl, M.; Jean, L.; Herrmann, J.; Mu¨ller, R.; Fu¨rstner, A.
Chem.sEur. J. 2009, 15, 12310.
(44) Fu¨rstner, A.; Radkowski, K.; Grabowski, J.; Wirtz, C.; Mynott, R. J.
Org. Chem. 2000, 65, 8758.
(51) Chiu, H.-T.; Chuang, S.-H.; Lee, G.-H.; Peng, S.-M. AdV. Mater. 1998,
10, 1475.
9
J. AM. CHEM. SOC. VOL. 132, NO. 32, 2010 11047

A R T I C L E S
Heppekausen et al.
Scheme 2. Metal Nitride/Metal Alkylidyne Interconversion as an
Alternative Concept for the in Situ Generation of Alkyne
Metathesis Catalystsa
Scheme 4a
a Reactions and conditions: (a) Ph3SiOH (3 equiv), toluene; (b) 1,10-
phenanthroline, 82%; (c) MnCl2, toluene, 80-100 °C.
a Cf. ref 50.
that carefully dried 1,10-phenanthroline leads to the nicely
crystalline complex 15, which seems to be indefinitely stable
on the benchtop without any precautions whatsoever (Scheme
4).56 However, judging from the lack of catalytic activity in
the metathesis of 1-phenyl-1-propyne as the model reaction, the
bidentate ligand does not seem to come off the metal template
to any noticeable extent at temperatures below 110 °C.
Gratifyingly, though, the catalytic activity can be restored
upon treatment of 15 with metal salts that are able to form stable
complexes with phenanthroline, including MnCl2, FeCl2, FeCl3,
CoCl2, CuCl2, ZnCl2, MgCl2, and NiCl2 (see the Supporting
Information). Among them, MnCl2 is preferred for practical
reasons, because this salt is cheap, benign, nontoxic, readily
available, hardly Lewis acidic, and nonhygroscopic; hence,
commercial samples can be used as such without further drying.
The activation of complex 15 with MnCl2 can be performed
either prior to the addition of the substrate or in its presence.
As expected, the combination of 15/MnCl2 shows roughly the
same activity and selectivity profile as the pyridine complex
13, since both systems are thought to release an identical active
fragment, i.e., complex 12. Tables 1-3 provide a detailed survey
of the performance of the 15/MnCl2 system in a host of inter-
and intramolecular alkyne metathesis reactions (see below).
In view of its impressive scope and unparalleled stability,
complex 15 is considered a highly practical yet broadly
applicable precatalyst for alkyne metathesis. Although its
reactivity after treatment with MnCl2 or related salts is clearly
lower than that of preformed Schrock alkylidynes such as 1-5
or the powerful precursor complex 6, the handling of 15 is trivial
and the functional group tolerance uncompromised, rivaling or
even surpassing that of 1-6; note that storage on the benchtop
in air is totally inconceiVable for 1-6 or any other structurally
defined alkyne metathesis catalyst known to date, and even the
pyridine adduct 13 will eventually degrade in moist air. An
optimized synthesis (see experimental details in the Supporting
Information) allows multigram amounts of 15 to be prepared
from inexpensive Na2MoO4.
Scheme 3. Preparation of a New Metathesis Precatalyst That Is
Air-Stable for Limited Periods of Timea
a Conditions: (a) TMSCl, 1,2-dimethoxyethane (DME), reflux; (b)
LiHMDS, hexane, 64% (over both steps); (c) Ph3SiOH (3 equiv), toluene,
80 °C; (d) pyridine (5 equiv), 81% (over both steps). Cf. ref 49.
14 was devoid of any appreciable catalytic activity, we were
pleased to observe that the use of triphenylsilanol resulted in a
catalytically active mixture.49,52,53 Complex 12 was readily
identified as the active component (Scheme 3). Although 12
itself is hydrolytically labile and needs to be handled under an
inert atmosphere, the corresponding pyridine adduct 13 turned
out to be sufficiently stable to be weighed in air (Scheme 3).49
At temperatures around 80 °C, however, solutions of this adduct
in toluene exhibit appreciable catalytic activity, most likely by
slow decomplexation of the pyridine. The fact that the presence
of the N-heterocyclic ligand does not quench the catalytic
activity of the molybdenum core may explain why this particular
precatalyst, which has recently been made commercially avail-
able, was found compatible with many other polar groups (ester,
ketone, amide, carbamate, sulfonate, nitro, ether, thioether, silyl
ether, alkene, acetal, glycoside, thiazole, pyridine, thiophene,
etc.); only epoxides, aldehydes, and acid chlorides were found
to react stoichiometrically with the nitride function of 12 and
hence consume the catalyst.49,54,55
Although 13 combines favorable chemical attributes with a
reasonable stability, it will eventually hydrolyze and hence must
still be stored under an inert atmosphere. In an attempt to find
an even more robust alternative, several ligands other than
pyridine were tested. During this screening process, it was found
The reason why even the only weakly Lewis acidic MnCl2
is capable of pulling the phenanthroline ligand off the Mo(6+)
template may be found in the structure of complex 15 in the
solid state. As can be seen from Figure 1, the strongly distorted
octahedral coordination geometry brings the bulky Ph3SiO-
groups in a mer arrangement. The N(1)tMo-N(3) angle of
(52) For alkyne metathesis catalysts modified by different silanols, see:
(a) Cho, H. M.; Weissman, H.; Wilson, S. R.; Moore, J. S. J. Am.
Chem. Soc. 2006, 128, 14742. (b) Villemin, D.; He´roux, M.; Blot, V.
Tetrahedron Lett. 2001, 42, 3701.
(54) For a recent application of complex 13 in alkaloid total synthesis,
see: Smith, B. J.; Sulikowski, G. A. Angew. Chem., Int. Ed. 2010, 49,
1599.
(53) For alkyne metathesis catalysts grafted onto silica, see: (a) Coutelier,
O.; Gauvin, R. M.; Nowogrocki, G.; Tre´bosc, J.; Delevoye, L.;
Mortreux, A. Eur. J. Inorg. Chem. 2007, 5541. (b) Cho, H. M.;
Weissman, H.; Moore, J. S. J. Org. Chem. 2008, 73, 4256. (c)
Chabanas, M.; Baudouin, A.; Cope´ret, C.; Basset, J.-M. J. Am. Chem.
Soc. 2001, 123, 2062. (d) Weissman, H.; Plunkett, K. N.; Moore, J. S.
Angew. Chem., Int. Ed. 2006, 45, 585. (e) Gauvin, R. M.; Coutelier,
O.; Berrier, E.; Mortreux, A.; Delevoye, L.; Paul, J.-F.; Mame`de, A.-
S.; Payen, E. Dalton Trans. 2007, 3127. (f) Merle, N.; Taoufik, M.;
Nayer, M.; Baudouin, A.; Le Roux, E.; Gauvin, R. M.; Lefebvre, F.;
Thivolle-Cazat, J.; Basset, J. M. J. Organomet. Chem. 2008, 693, 1733.
(55) For the acylation chemistry of terminal metal nitrides, see the following
for leading references: (a) Curley, J. J.; Sceats, E. L.; Cummins, C. C.
J. Am. Chem. Soc. 2006, 128, 14036. (b) Clough, C. R.; Greco, J. B.;
Figueroa, J. S.; Diaconescu, P. L.; Davis, W. M.; Cummins, C. C.
J. Am. Chem. Soc. 2004, 126, 7742. (c) Figueroa, J. S.; Piro, N. A.;
Clough, C. R.; Cummins, C. C. J. Am. Chem. Soc. 2006, 128, 940.
(d) Sarkar, S.; Abboud, K. A.; Veige, A. S. J. Am. Chem. Soc. 2008,
130, 16128. and literature cited therein.
(56) A year-old batch shows unchanged appearance and catalytic activity.
9
11048 J. AM. CHEM. SOC. VOL. 132, NO. 32, 2010

Practical New Silyloxy-Based Alkyne Metathesis Catalysts
A R T I C L E S
Scheme 5a
a Reagents and conditions: (a) 5-decyne, toluene-d8, 100 °C, cf. text.
Scheme 6a
Figure 1. Structure of Mo(tN)(OSiPh3)3(phen) (15) in the solid state
(N1-Mo1-N2 ) 91.2(1)°). Disordered solute toluene is omitted for clarity;
phen )1,10-phenanthroline.
160.24(7)° deviates significantly from 180° and the Mo atom
lies 0.33(1) Å above the plane defined by N(2), O(1), O(2) and
O(3). Importantly, the Mo-N(3) distance of 2.512(2) Å is much
larger than the distance from the metal to the phenanthroline
nitrogen atom in the equatorial plane (Mo-N(2) 2.292(2) Å).
The fact that the second N-atom of the chelate ligand obviously
binds very weakly may explain why 12, even in the presence
of excess pyridine, affords only the monoadduct 13 (Scheme
3). Two of the three Mo-O-Si angles are surprisingly obtuse
(Mo-O(1)-Si(1) 165.26(8)°, Mo-O(2)-Si(2) 168.04(8)°, Mo-
O(3)-Si(1) 145.78(8)°), which likely reduces the congestion in
the periphery. The MotN(1) bond (1.659(2) Å) is slightly
longer than that observed in the pyridine adduct 13 (1.653(2)
Å)49 and near the longer end of reported MotN distances
(1.563-1.688 Å,57 with the exception of one outlier of 1.786
Å).58
Molybdenum Alkylidyne Complexes with Ancillary Silano-
late Ligands. As already mentioned, inspiration for the
development of the novel silanolate complexes presented here
was the work of Johnson et al., who showed that certain metal
nitride species can convert into the corresponding alkylidyne
complexes on reaction with a nonterminal alkyne (Scheme
2).50 An analogous metathetic exchange process is believed
to account for the catalytic activity exhibited by nitrides 12,
13, and 15. Yet, on NMR inspection of a mixture comprising
(Ph3SiO)3MotN (12)59 and 5-decyne (2 equiv) in toluene-
d8, we were not able to detect appreciable amounts of
valeronitrile (17), which must form in a quantity equal to
that of the presumed alkylidyne complex 16 (Scheme 5). Only
after prolonged heating of the mixture in a sealed tube (100
°C, 6 d) were small amounts of 17 discernible, in addition
to massive polymerization of the mixture (see the Supporting
Information).
a Reagents and conditions: (a) Ph3SiOLi (3 equiv), Et2O, -40 °C f rt,
then MeCN, 85%; (b) PhLi, Et2O, reflux; (c) NMe4Br, H2O, 52% (over
both steps); (d) oxalyl bromide, CH2Cl2, -78 °C f -15 °C; (e) Br2, 1,2-
dimethoxyethane (dme, 5 equiv), CH2Cl2, -78 °C f rt, 88% (over both
steps); (f) Ph3SiOK (4 equiv), toluene; (g) Et2O, 92%.
The striking inefficiency of the crucial nitride f alkylidyne
exchange compared with the excellent catalytic performance
of 15 in our preparative experiments (Tables 1-3) implies that
the small amounts of alkylidynes, such as 16, formed in the
mixture must be superbly active. We sought to clarify this aspect
by preparing the hitherto unknown molybdenum alkylidynes
of the general structure (Ph3SiO)3MotCR and derivatives
thereof. This goal was readily attained by adaptation of the
established entry routes to related d0-alkylidynes (Scheme 6).
Specifically, the desired neopentylidyne complex 19·MeCN was
obtained by ligand exchange of the corresponding trichloride
18, which, in turn, was prepared according to the route
previously reported by the Schrock group.32 Much more efficient
overall was the synthesis of the corresponding benzylidyne
analogue 24·Et2O, which was formed analogously from cheap
Mo(CO)6 via the known building block 22.60,61 As described
in the Supporting Information, 24·Et2O can be obtained in only
three operations, as the intermediates 21 and 23 need not be
isolated.
(57) (a) Chisholm, M. H.; Davidson, E. R.; Pink, M.; Quinlan, K. B. Inorg.
Chem. 2002, 41, 3437. (b) Ritleng, V.; Yandulov, D. V.; Weare,
W. W.; Schrock, R. R.; Hock, A. S.; Davis, W. M. J. Am. Chem. Soc.
2004, 126, 6150.
(58) Dilworth, J. R.; Dahlstrom, P. L.; Hyde, J. R.; Zubieta, J. Inorg. Chem.
Acta 1983, 71, 21.
(60) (a) Fischer, E. O.; Maasbo¨l, A. Chem. Ber. 1967, 100, 2445. (b)
Fischer, E. O.; Maasbo¨l, A. Ger. Offen. DE 1214233, 1966; Chem.
Abstr. 1966, 65, 12474.
(61) (c) Mayr, A.; McDermott, G. A. J. Am. Chem. Soc. 1986, 108, 548.
(d) McDermott, G. A.; Dorries, A. M.; Mayr, A. Organometallic 1987,
6, 925.
(59) This experiment was deliberately performed with 12 generated in situ
from 11 and Ph3SiOH, as such a solution is free of any N-donor ligand
that might retard the metathetic nitride/alkylidyne interconversion by
complexation to the metal center.
9
J. AM. CHEM. SOC. VOL. 132, NO. 32, 2010 11049

A R T I C L E S
Heppekausen et al.
Figure 4. Metathetic conversion of 1-phenyl-1-propyne to tolane catalyzed
by 1 mol % of either Mo(tCPh)(OSiPh3)3(Et2O) (24· Et2O) (blue triangles)
or (tBuO)3WtCtBu (1) (red circles) in toluene at ambient temperature and
ambient pressure. The yields were determined by GC using biphenyl as an
internal standard.
Mo(1)-O(4)-Si(4) 149.0(3)°). This suggests that the Si-O-Mo
groups serve as hinges which, through facile bending, allow
for optimal use of the available space in the periphery. The
potassium cation escorts the molybdate core by binding to the
fourth silanolate oxygen, which is more pyramidalized than
the other oxygen atoms in order to make its lone pairs accessible
for binding to the cation (Mo(1)-O(1)-Si(1) 134.5°). The
MotC distances of the two independent molecules of 23 at
1.747(6) and 1.756(5) Å are slightly longer than the equivalent
distance of the corresponding neutral analogue 19·MeCN
(1.741(2) Å; see below).
Figure 2. Structure of the ate-complex [(Ph3SiO)4MotCPh]- [K+ ·(dme)]
(23) in the solid state. Only one of the two independent molecules in the
unit cell is depicted for clarity; dme )1,2-dimethoxyethane.
Workup of the crude mixtures by evaporation of the solvent
and trituration of the residues with MeCN or Et2O delivered
the neutral alkylidyne complexes 19 and 24, respectively, in
the form of the corresponding adducts. Their NMR data were
in excellent agreement with the proposed structures. The
constitution of 19·MeCN was unambiguously confirmed by
crystal structure analysis (Figure 3), which revealed a distorted
square pyramidal coordination geometry about the Mo center.
The MotC(1) bond length of 1.741(2) Å is slightly shorter than
in the ate-complex 23. Interestingly, the acetonitrile ligand does
not bind in a strictly linear fashion, as evident from the
Mo-N(1)-C(98) angle of only 166.31(15)°; moreover, the
CtN triple bond is extended to 1.145(3) Å as a result of
the coordination to the high-valent metal center (reference bond
length for RCtN, 1.136 Å).64 However, no spontaneous
metathesis takes place between the ligated nitrile and the
alkylidyne unit as a consequence of the still largely end-on rather
than side-on coordination mode of the CtN triple bond.
As expected for prototype Schrock alkylidynes, the aceto-
nitrile and ether adducts of 19 and 24 are both air- and
moisture-sensitive. Even though they do not provide any
advantage over other alkylidynes in terms of handling, they
turned out to be exquisitely active alkyne metathesis catalysts
which are operative at low loadings and distinguished by an
outstanding selectivity profile. Whereas the preparative details
are outlined in a later section of this paper, the plot for the
conversion of 1-phenyl-1-propyne to tolane catalyzed by
24 · Et2O against the rate recorded for the classical tungsten
neopentylidyne (tBuO)3WtCCMe3 (1), which defines the
standard in the field, clearly demonstrates the extraordinary
performance of the novel catalyst (Figure 4). Specifically,
Figure 3. Structure of the complex Mo(tCtBu)(OSiPh3)3(MeCN)
(19·MeCN) in the solid state (C1-Mo1-N1 94.4(1)°). Two of the phenyl
groups (at C21 and C81) are disordered over two positions.
Interestingly, we noticed that the primary product formed
during the ligand-exchange process of 2262 and Ph3SiOK was
the ate-complex 23, in which four rather than three silanolates
are covalently bound to the molybdenum center (Figure 2).63
This may be taken as an indication for the poor donor capacity
of the silanolates, which allows the molybdenum to retain an
appreciable Lewis acidity, which is essential for high activity
in alkyne metathesis.32 Even though the Ph3SiO unit seems very
bulky, the complex accommodates four of them in a square
pyramidal environment about the metal center, with the alky-
lidyne forming the apex and the silanolates the basal plane. The
bond angles of the individual Si-O-Mo units differ quite
significantly from each other and can be almost linear
(Mo(1)-O(3)-Si(3) 177.8(3)°; Mo(1)-O(2)-Si(2) 151.8(2)°;
(62) Complex 22 was prepared according to ref 60. A detailed procedure
is described in the Supporting Information.
(63) This ate-complex crystallizes from the reaction mixture even if only
3 equiv of Ph3SiOK is added to complex 22.
(64) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen, A. G.;
Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1.
9
11050 J. AM. CHEM. SOC. VOL. 132, NO. 32, 2010

Practical New Silyloxy-Based Alkyne Metathesis Catalysts
A R T I C L E S
Scheme 7a
a Reagents and conditions: (a) 1,10-phenanthroline, toluene/Et2O, 81%;
(b) MnCl2, toluene, 80 °C.
the equilibrium is reached after 25 min at ambient temper-
ature; at this point, complex 1 gave less than 5% conversion.
Although molybdenum alkylidynes are generally considered
less active than their tungsten counterparts,31,32,65 these data
show that the triphenylsilanolate ligands impart a truly
remarkable activity onto the operative molybdenum alkyli-
dyne unit, rendering 24 · Et2O one of the most active
metathesis catalysts known to date. This, in turn, corroborates
the view that even small amounts of (Ph3SiO)3MotCR
generated in situ from (Ph3SiO)3MotN · L (L ) pyridine,
phenanthroline) and an appropriate alkyne can account for
the excellent results obtained with such robust precatalyst
systems. Actually, we currently believe that nitride complexes
of the type (Ph3SiO)3MotN · L act as a reservoir, from which
tiny amounts of active alkylidyne species are constantly
released during the reaction. This may explain why solutions
of 13 and 15 retain catalytic activity at 80 °C over the course
of days,49 whereas all known metal alkylidynes degrade when
kept under such forcing conditions for prolonged periods of
time.
In an attempt to transfer the stabilizing effect of 1,10-
phenanthroline from the metal nitride to the metal alkylidyne
series, the crude reaction mixture formed upon ligand exchange
of Br3MotCPh·dme (22) with Ph3SiOK was added to a solution
of carefully dried 1,10-phenanthroline in toluene. The desired
adduct 25 could be isolated from this mixture in good yield
(Scheme 7). Its structure, as determined by X-ray crystal-
lography (Figure 5), closely resembles the structure of the
isolobal nitride complex 15. Once again, a strongly distorted
coordination geometry about the metal center is observed,
characterized by a significant deviation of the C(1)tMo· · ·N(1)
axis (168.34(11)°) from linearity. The Mo · · ·N bond lengths
are again very uneven (2.244(2) versus 2.408(2) Å), with the
distance to the apical nitrogen being much longer. The alkyli-
dyne bond length MotC(1) (1.761(3) Å) exceeds those in the
corresponding square pyramidal nitrile adduct 19 · MeCN
(1.7410(17) Å) and the ate-complex 23 (1.747(6)/1.756(5) Å).
Importantly, the 1,10-phenanthroline ligand stabilizes this
particular Schrock alkylidyne to the extent that complex 25 is
stable in air for hours;66,67 however, complex 25 per se does
not react with 1-phenyl-1-propyne to any appreciable extent
under conditions in which the corresponding ether adduct
24·Et2O leads to very fast conversions. Gratifyingly, though,
addition of commercial MnCl2 (1 equiv relative to 25) and
heating of the reaction mixture for 30 min to about 80 °C
restores an outstanding performance; the resulting catalyst
solution containing complex 24 is then active even at ambient
temperature. The ligand swap from molybdenum to manganese
can easily be monitored by precipitation of MnCl2 ·phen and
Figure 5. Top: Structure of Mo(tCPh)(OSiPh3)3(phen) (25) in the solid
state. The 1,10-phenanthroline ligand is disordered over two positions in
the ligand plane (one conformation shown). Bottom: Core of the complex
showing the distorted coordination geometry about the Mo center (green);
O, red; N, blue.
was confirmed by mass spectrometry. Within experimental error,
the resulting solutions are largely equipotent to those containing
authentic 24·Et2O (see Tables 1-3 for details) and are able to
metathesize most alkyne substrates within unprecedentedly short
periods of time. Likewise, the functional group tolerance is
extraordinary (see below). Therefore, we conclude that complex
25 is an excellent compromise between the desirable activity,
selectivity, and practicality aspects associated with alkyne
metathesis.
The user may hence choose among (i) the air-sensitive
Mo(tCPh)(OSiPh3)3(Et2O) (24·Et2O), which is superbly active
and selective; (ii) the phenanthroline adduct 25 thereof, which,
after activation with MnCl2, is similarly potent but can be
handled in air, even though it still needs to be stored under
argon; and (iii) the totally air-stable nitride complex 15, which
is exceptionally user-friendly. The preexisting alkylidyne unit
present in 24 and 25 ensures that these complexes are active
under notably mild conditions even at very low loadings (see
(67) (a) Addition of hydrotris(3,5-dimethyl-1-pyrazolyl)borate to
(dme)Cl3WtCPh leads to an air-stable but catalytically inactive
Schrock alkylidyne; see: Blosch, L. L.; Abboud, K.; Boncella, J. M.
J. Am. Chem. Soc. 1991, 113, 7066. (b) An analogous complex derived
from (dme)Br3WtCPh was described as moderately air-stable but
sensitive to moisture; see: Jeffery, J. C.; McCleverty, J. A.; Mortimer,
M. D.; Ward, M. D. Polyhedron 1994, 13, 353.
(65) For a computational study, see: Zhu, J.; Jia, G.; Lin, Z. Organometallics
2006, 25, 1812.
(66) Crystalline samples of 25 seem partly intact after several days in air.
9
J. AM. CHEM. SOC. VOL. 132, NO. 32, 2010 11051

A R T I C L E S
Heppekausen et al.
Scheme 8a
a Reagents and conditions: (a) PhLi, Et2O, reflux; (b) NMe4Br, H2O,
55% (over both steps); (c) oxalyl bromide, CH2Cl2, -78 °C f -15 °C;
(d) Br2, 1,2-dimethoxyethane (dme, 5 equiv), CH2Cl2, -78 °C f rt, 85%
(over both steps); (e) Ph3SiOK (4 equiv), toluene; (f) 1,10-phenanthroline,
toluene, 79%.
Figure 6. Structure of the ate-complex [(Ph3SiO)4WtCPh]- [K+ ·(phen)]
(29) in the solid state.
below), whereas nitride 15 first needs to be converted into an
operative alkylidyne in situ, a process that mandates higher
loadings of the precatalyst and heating of the mixture to about
80 °C. Whatever precatalyst one might prefer, however, all
systems are available in quantity from cheap and commercially
available starting materials, lead to excellent preparative results,
and compare favorably with all established alkyne metathesis
catalysts (see below). Complex 25 is much more stable and
practical than any other catalytically active Schrock alkylidyne
known to date. Therefore, we believe that these systems,
collectively, represent a significant step forward in our quest
for truly user-friendly yet generally applicable alkyne metathesis
catalysts.
Scheme 9. Formation of 2-Butyne as a Generic Byproduct in the
Reaction of Alkynes End-Capped with a Methyl Group, As
Exemplified by the Conversion of 1-Phenyl-1-propyne to Tolane
zation seems to prevail, this particular complex is not relevant
in the present context; its reaction behavior as well as further
attempts to secure neutral tungsten alkylidynes of the general
type [(Ph3SiO)3WtCR]·L are subject of a separate investiga-
tion.
Tris(triphenylsilyloxy)tungsten Alkylidyne Complexes. To
study the generality of the concept, the corresponding tungsten
alkylidynes endowed with triphenylsilanolate ancillary ligands
were targeted. To this end, the known benzylidyne tribromide
complex 2760,61 was subjected to ligand exchange with Ph3SiOK
in toluene, which furnished the corresponding ate-complex 28
(Scheme 8). In contrast to its molybdenum analogue 23,
however, treatment with 1,10-phenanthroline in toluene did not
afford a neutral adduct but merely replaced the dme ligated to
the potassium counterion to give the surprisingly robust ate-
complex 29. X-ray diffraction proved that the phenanthroline
ligand binds to the potassium escort rather than the tungsten
center (Figure 6). Overall, the structure of 29 in the solid state
resembles the molybdenum counterpart 23, featuring again three
obviously flexible metal-O-Si moieties, which manage the
bulk in the periphery by appropriate bending. The WtC(1) bond
length (1.758(2) Å) is identical within the error limits to the
corresponding bond length in the molybdenum species 23
(1.747(6)/1.756(5) Å).
Beneficial Effect of 5 Å Molecular Sieves on Alkyne
Metathesis. A priori, the metathetic scrambling of a pair of
alkynes leads to an equilibrium, which needs to be shifted to
one side in order to make the reaction preparatively useful. If
one of the products is an alkyne of low molecular weight (2-
butyne, 3-hexyne, etc.), this goal is usually accomplished by
driving this compound out of the mixture.
Therefore, alkynes with capping methyl substituents are the
most common substrates, and the reactions are usually per-
The corresponding neutral tungsten alkylidyne species could
not, so far, be generated upon trituration of 28 or 29 with Et2O
nor on reaction with TMSCl. Not unexpectedly, the catalytic
activity of these stable ate complexes themselves is marginal.
Even though 1-phenyl-1-propyne slowly gets consumed, only
small amounts of tolane (e25%) were detected. As polymeri-
Figure 7.
Metathesis of 1-phenyl-1-propyne catalyzed by
Mo(tCPh)(OSiPh3)3(Et2O) (24·Et2O, 1 mol %) in toluene in the absence
(blue triangles) or in the presence (blue squares) of powdered MS 5 Å at
ambient temperature. Comparison with the plot obtained upon pretreatment
of the catalyst with MS 5 Å, which was removed prior to the addition of
the substrate (red circles). The yields were measured by GC against biphenyl
as internal standard.
(68) (a) Zhang, W.; Moore, J. S. J. Am. Chem. Soc. 2004, 126, 12796. (b)
Zhang, W.; Moore, J. S. J. Am. Chem. Soc. 2005, 127, 11863. (c)
Zhang, W.; Brombosz, S. M.; Mendoza, J. L.; Moore, J. S. J. Org.
Chem. 2005, 70, 10198.
9
11052 J. AM. CHEM. SOC. VOL. 132, NO. 32, 2010

Practical New Silyloxy-Based Alkyne Metathesis Catalysts
A R T I C L E S
formed at elevated temperatures and/or by applying gentle
vacuum (Scheme 9). However, the solubility of 2-butyne in
hydrocarbon solvents is non-negligible, and the use of reduced
pressure may lead to undesirable changes in the concentration
of the mixture, as the solvent will eventually also distill off.
Moreover, 2-butyne may be consumed by certain alkyne
metathesis catalysts in a competitive polymerization process.39a
To avoid such problems, Moore and co-workers have developed
a “precipitation-driven” setup for alkyne metathesis;68 though
highly productive, this approach requires tailor-made starting
materials and is less favorable from the viewpoint of atom
economy.
Since catalysts of increased activity, such as 24·Et2O, allow
the reaction to be performed at ambient temperature, the problem
of possible 2-butyne (bp ) 27 °C) accumulation in solution
becomes more pressing. This is evident from Figure 7, which
shows that the metathesis of 1-phenyl-1-propyne effected by
24·Et2O (1 mol %) in toluene is fast but levels off at about
50% conversion after 20 min when carried out at ambient
temperature and atmospheric pressure (blue triangles).
We reasoned that 2-butyne might also be removed from the
mixture with the aid of an appropriate molecular sieve (MS).69
Since activated MS 5 Å is capable of adsorbing C4 hydrocar-
bons,70 the reaction was repeated in the presence of 2 mg of
MS 5 Å per µmol of released 2-butyne.71 The corresponding
plot (blue squares) in Figure 7 shows the dramatic effect of
this additive on the reaction rate as well as the conversion, which
is essentially complete within 5 min under otherwise identical
conditions. This outcome cannot be explained by simple removal
of moisture from the mixture, as the equilibrium is not affected
by adventitious water. This notion is corroborated by the fact
that MS 3 Å has no significant effect on the reaction, even
though it is known to bind water effectively; MS 4 Å is slightly
less efficient than MS 5 Å, which is the preferred additive (see
Supporting Information).
The data in Figure 7 also show that added MS 5 Å not only
improves the conversion but also significantly accelerates the
reaction rate. This effect became particularly evident when a
solution of 24·Et2O in toluene was stirred in the presence of
powdered MS 5 Å for 30 min and the molecular sieves were
then removed from the mixture prior to the addition of the
substrate. In this case, the equilibrium is reached in <5 min
(Figure 7, red circles), which is much faster than the reaction
without this “preactivation”. It is believed that the MS 5 Å helps
liberate the required free coordination site at molybdenum by
absorbing the only weakly bound diethyl ether ligand.72,73
unless stated otherwise.75 Consequently, we recommend the use
of MS 5 Å in future alkyne metathesis reactions independent
of the chosen catalyst and hope that this convenient setup will
make the preparation of tailor-made substrates largely unnecessary.
Survey of the Preparative Scope of the New Alkyne
Metathesis Catalysts. With a set of new (pre)catalysts and an
optimized procedure for alkyne metathesis at hand, we were
committed to explore the scope of this methodology in detail.
To this end, a representative set of alkyne homometathesis,
alkyne cross-metathesis (ACM),74 and ring-closing alkyne
metathesis (RCAM) reactions was investigated, as well as a
prototype cyclooligomerization. Even though not all substrates
were tested with all available catalysts, care was taken to
accumulate a sufficiently large data set for direct comparison.
Furthermore, particular attention was paid to investigating the
compatibility of the novel catalysts with polar functional groups,
since this aspect is key for future applications in advanced
organic synthesis and material science.
The results compiled in Tables 1-3 deserve further comment.
With very few exceptions, complexes 15, 24·Et2O, and 25
behaved similarly well, giving good to excellent results with a
diverse set of functionalized substrates. Although it may not
be surprising that the isolated yields tended to be highest when
the preformed alkylidyne 24·Et2O was employed, the very stable
molybdenum nitride 15 also performed remarkably well. The
price to be paid for the use of this fully air stable precatalyst,
however, is a larger loading (generally 10 mol %), a higher
reaction temperature (usually 80 °C),75 and longer reaction
times.
As expected, the data compiled in the tables also show that
the alkylidyne complex 24·Et2O and the much more robust
phenanthroline adduct 25 thereof, after activation with MnCl2,
by and large lead to the same preparative results, although
slightly higher loadings were applied in the latter case. However,
it is emphasized that the catalyst loading has not been optimized
for each entry reported here. In view of the spectacular reactivity
of 24 (see Figure 7), we are optimistic that loadings well below
the 1-2 mol % generally used during this investigation can be
reached by proper adjustment of the reaction parameters, in
particular since the half-life of 24·Et2O in toluene-d8 was found
to be ∼30 h at ambient temperature (see Supporting Informa-
tion). In fact, a single experiment, in which 1-phenyl-1-propyne
was exposed to only 0.1 mol % of 24·Et2O and the usual loading
of MS 5 Å, reached 95% conversion (GC) after 2 h reaction
time. Likewise, the same model substrate was quantitatively
converted to tolane within e20 min by 1 mol % of complex
24·Et2O, even at -10 °C. To the best of our knowledge, this is
the lowest temperature at which an alkyne metathesis has so
far been successfully performed, but it does not seem to mark
the lower limit for our new catalyst.
In any case, this simple measure of adding powdered MS 5
Å to the reaction mixture turned out to be general and was
applied to all preparative experiments compiled in Tables 1-3,
The compatibility of the new catalysts with various functional
groups is outstanding. Esters, ethers, various silyl ethers,
(69) Alkyne metathesis reactions using Mortreux-type catalyst mixtures
have previously been run in the presence of MS 4 Å in order to remove
traces of moisture. However, it was reported later that carefully dried
solvents make the addition of MS 4 Å unnecessary. No effect of this
additive on the conversion was noticed; see: (a) Huc, V.; Weihofen,
R.; Martin-Jimenez, I.; Oulie´, P.; Lepetit, C.; Lavigne, G.; Chauvin,
R. New J. Chem. 2003, 27, 1412. (b) Maraval, V.; Lepetit, C.;
Caminade, A.-M.; Majoral, J.-P.; Chauvin, R. Tetrahedron Lett. 2006,
47, 2155.
(72) Control experiments, in which the metathesis of 1-phenyl-1-propyne
was carried out in Et2O, THF, or CH2Cl2 as the reaction medium, did
not go to complete conversion, even in the presence of molecular
sieves. Reactions in THF were much slower than those in the other
solvents investigated.
(73) Notably, the metathesis of 1-phenyl-1-propyne induced by the mo-
lybdate complex 23 (1 mol%) in toluene in the presence of MS 5 Å
proceeded with the same rate as that induced by 24·Et2O/MS 5 Å.
(74) Fu¨rstner, A.; Mathes, C. Org. Lett. 2001, 3, 221.
(70) MS 5 Å is slightly more effective than MS 4 Å, whereas the use of
MS 3 Å has little effect, if any. Since MS 3 Å, however, is an efficient
trap for water, this comparison shows that the observed rate accelera-
tion and the improved yields are not caused by ensuring a rigorously
dry medium.
(71) As expected, powdered MS 5 Å is more effective than the use of
pellets, and about 2 mg/µmol butyne turned out to be optimal.
(75) As the reactions with complex 15 have to be performed at higher
temperature, addition of molecular sieves is usually not necessary and
sometimes even disadvantageous due to possible side reactions at this
temperature.
9
J. AM. CHEM. SOC. VOL. 132, NO. 32, 2010 11053

A R T I C L E S
Heppekausen et al.
Table 1. Intermolecular Alkyne Metathesis Reactions in the Presence of 5 Å Molecular Sieves
a 15 (10 mol %), MnCl2 (10 mol %), MS 5 Å, toluene, 80 °C, 30 min, then addition of the substrate and reaction at 80 °C unless stated otherwise.
b 24·Et2O, (2 mol %), toluene, ambient temperature, MS 5 Å. c 25 (5 mol %), MnCl2 (5 mol %), toluene, 80 °C, 30 min; then addition of the substrate
and MS 5 Å, and reaction at ambient temperature. d At 50 °C. e At 100 °C. f Cf. text; NR ) no reaction.
thioethers, sulfonates, amides, carbamates, ketones, acetals,
epoxides (see below), nitro groups, and trifluoromethyl groups
are generally well tolerated, even if they are oriented toward
the reacting alkyne (Table 2, entry 12). Likewise, a nitrile was
also found at least kinetically stable in the presence of all three
novel molybdenum silanolate complexes, despite being a
possible substrate for metathesis (Table 1, entry 13). Moreover,
compounds containing various types of aromatic heterocycles
(pyridine, thiophene, thiazole, carbazole) known to interfere with
the activity of the classical tungsten alkylidyne 113,38 were
metathesized without problems. Chiral centers next to enolizable
carbonyl groups were not racemized. An elimination-prone
primary tosylate (Table 1, entry 10) as well as an acid- and
base-sensitive aldol substructure (Table 2, entries 7) also
remained intact. Although the orthogonal character of alkene
and alkyne metathesis has previously been recognized,24 the
rigorous distinction of the catalysts between the π-systems of
alkynes and olefins is noteworthy: olefins are inert, independent
of whether they are mono-, di-, or trisubstuted, terminal, internal,
or conjugated to a carbonyl group. Likewise, only the acetylene
motif of a 1,3-enyne will undergo productive metathesis (Table
2, entries 11 and 12, and Table 3, entry 2), and even highly
base- and acid-sensitive skipped 1,4-enynes posed no problems
whatsoever (Table 2, entries 9 and 10).
This excellent profile is particularly evident from a host of
applications to compounds bearing more than one polar group
(Table 2). We have largely relied upon substrates previously
used in this laboratory for the total synthesis of bioactive natural
products, including homoepilachnene (entry 6)13 and epothilone
C (entry 7).38,40 The product shown in entry 10 leads to ent-
amphidinolide V;22a,b this example is particularly noteworthy,
as it contains reactive vinylepoxide, hydroxyepoxide, allylic
ether, and skipped enyne motifs packed in a dense array, in
addition to an ester and a silyl ether group. Entry 12 shows
9
11054 J. AM. CHEM. SOC. VOL. 132, NO. 32, 2010

Practical New Silyloxy-Based Alkyne Metathesis Catalysts
A R T I C L E S
Table 2. Intramolecular Alkyne Metathesis Reactions
a 15 (10 mol %), MnCl2 (10 mol %), toluene, 80 °C, 30 min, then addition of the substrate and reaction at 80 °C unless stated otherwise; no MS
added, cf. ref 75. b 24·Et2O (2 mol %), toluene, ambient temperature, MS 5 Å. c 25 (5 mol %), MnCl2 (5 mol %), toluene, 80 °C, 30 min, then addition
of the substrate and MS 5 Å and reaction at ambient temperature. d At 80 °C. e With 4-5 mol % of catalyst.
9
J. AM. CHEM. SOC. VOL. 132, NO. 32, 2010 11055

A R T I C L E S
Heppekausen et al.
Table 3. Alkyne Cross-Metathesis Reactions
a 15 (10 mol %), MnCl2 (10 mol %), MS 5 Å, toluene, 80 °C, 30 min, then addition of the substrate and reaction at 100 °C unless stated otherwise.
b 24·Et2O (2 mol %), toluene, ambient temperature, MS 5 Å. c 25 (5 mol %), MnCl2 (5 mol %), toluene, 80 °C, 30 min, then addition of the substrate
and MS 5 Å and reaction at ambient temperature. d Only low conversion.
another highly adorned case, in which a ketone, ester, amide,
two different acetals, a chelation-prone ether, seven chiral
centers, and a 1,3-enyne entity are present; this particular product
was instrumental for our synthesis of the antibiotic myxovires-
cin.45 Equally instructive is entry 8, which shows the key
intermediate en route to the highly cytotoxic F-ATPase inhibitor
cruentaren A.43 Whereas attempted ring closure of this densely
substituted diyne substrate with the aid of the tungsten alkyli-
dyne 1 resulted only in the cleavage of the -OTHP group due
to the Lewis acidity of this complex, 24·Et2O afforded the 12-
membered cycloalkyne in excellent yield. These applications
are complemented by two additional examples from an ongoing
synthesis project in this laboratory, which will be reported in
separate publications in due time.76 They are included in Table
2 to further illustrate the exceptional power of the new
alkylidyne complex 24·Et2O in forming even particularly fragile
(entry 9) as well as fairly strained compounds (entry 11). It is
of note that the product displayed in entry 9 could not be made
at all with the tungsten alkylidyne 1, whereas complex 6 gave
variable results and required high loadings (20-40 mol %), most
likely due to competing side reactions with the very reactive
oxirane ring of this particular compound. Apparently, the Mo
center in 24·Et2O is not sufficiently Lewis acidic to cause any
damage, and the silanolates do not engage in epoxide opening
due to their low nucleophilicity. From the preparative viewpoint,
it is also important to note that the RCAM reactions displayed
in Table 2 were performed on scales ranging from a few
milligrams to several grams of product (see the Supporting
Information).
Another relevant case is the cyclooligomerization of the bis-
propynylated carbazole derivative depicted in Table 2, entry
13. The resulting macrocycle is of interest in material science
and was previously best prepared by “precipitation-driven”
alkyne metathesis in trichlorobenzene that required, however,
a specially designed starting material and a highly sensitive
complex of type 5 (81%,68 61%,77). We were pleased to see
that the new procedure, benefitting from the ability of MS 5 Å
to drive the conversion of a propynylated substrate to completion
at ambient temperature, was at least equally productive,
independent of which of the newly prepared catalysts was
chosen (81-83%).
nitride species and hence destroy the activity of complexes 13
and 15. Importantly, however, these reactive groups seem to
be tolerated by the preformed alkylidyne 24 · Et2O (Table 2,
entries 9 and 10). Whether this compatibility is due to the fact
that 24·Et2O reacts at ambient temperature, whereas 13 and 15
need heating, remains to be studied. Likewise, ketones posed
no problem with catalysts 24·Et2O and 25 (Table 1, entries 7
and 12; Table 2, entry 12) but seem to interfere with the nitride
complex 15. Although the product of intermolecular metathesis
of the acetophenone derivative shown in Table 1, entry 7, was
detected (<40%), full conversion could not be reached in this
particular case. Aldehydes, in contrast, seem to mark the limit
of the current methodology. Attempts to metathesize a propy-
nylated benzaldehyde derivative were unsuccessful because the
catalysts quickly got destroyed (Table 1, entry 6). Ongoing work
in this laboratory studies the stoichiometric reaction behavior
of these complexes in more detail.
Conclusions
Although alkyne metathesis may never reach the breadth of
alkene metathesis because of a smaller substrate base, the
potential of this transformation is nevertheless significant, as
witnessed by a rapidly increasing number of applications to
sophisticated targets. To fully explore its scope, however, it is
mandatory to make the required catalysts more readily available
and user-friendly. To this end, we present a new generation of
alkyne metathesis (pre)catalysts that are optimized for activity
on the one hand and practicality on the other. Specifically, the
readily available Schrock alkylidyne complex 24·Et2O consti-
tutes one of the most active catalysts known to date, but retains
an outstanding tolerance for functional groups. The only weakly
donating triphenylsilanolate ligands impart a well-balanced level
of Lewis acidity onto the d0-molybdenum center, which is
required for high catalytic activity yet is not high enough to
endanger polar substituents. At the same time, the sheer size of
the Ph3Si residues prevents more than one alkyne from binding
to the metal center and hence disfavors competing polymeri-
zation pathways while likely facilitating the cycloreversion of
the metallacyclobutadiene intermediates. In contrast to the
previously used alkoxides or fluorinated alkoxides, the bulk of
the Ph3SiO- unit seems more “flexible”, as bending of the
Mo-O-Si angle is facile. “Stretching” of this hinge provides
the necessary space about the molybdenum and, as a conse-
quence, prevents substrate-binding from becoming a limiting
factor.
Only a few limitations of the new molybdenum complexes
have so far been encountered. As noticed previously for the
pyridine adduct 13,49 epoxides do react with the catalysts and
(76) Hickmann, V.; Alcarazo, M.; Fu¨rstner, A. J. Am. Chem. Soc. 2010, in
press (doi: 10.1021/ja104796a).
(77) Zhang, W.; Cho, H. M.; Moore, J. S. Org. Synth. 2007, 84, 177.
Although 24·Et2O itself is air- and moisture-sensitive, the
corresponding phenanthroline adduct 25 is stable in air for hours.
9
11056 J. AM. CHEM. SOC. VOL. 132, NO. 32, 2010

Practical New Silyloxy-Based Alkyne Metathesis Catalysts
A R T I C L E S
While solutions of this complex in toluene are inactive per se,
reaction of 25 with MnCl2 releases 24 into the solution by a
ligand swap and thereby restores superb performance. The
combination 25/MnCl2 is therefore considered a practical entry
point into all kinds of alkyne metathesis reactions.
Even more facile is the manipulation of the corresponding
phenanthroline nitrido complex 15, which seems indefinitely
stable on the benchtop. 15 is accessible in multigram
quantities from inexpensive starting materials. It is again by
phenanthroline transfer to MnCl2 that an active template is
released from this precatalyst, which needs, however, yet to
undergo an exchange of nitride for alkylidyne to become
active. Although this process requires heating to about 80
°C, the combination 15/MnCl2 performed very well in a
representative number of inter- and intramolecular alkyne
metathesis reactions.
pores. This operationally simple measure drives the conver-
sion, allows the reactions to be conducted at ambient
temperature and atmospheric pressure, and makes the use of
designer substrates unnecessary.
Acknowledgment. Generous financial support by the MPG and
the Fonds der Chemischen Industrie is gratefully acknowledged.
We thank Dr. Martin Bindl, Dr. Eike Heilmann, and Dr. Daniel A.
Clark for preliminary studies; Dr. A. Jantsch for preparing the
substrate shown in Table 1, entry 12; V. Hickmann and Dr. K.
Micoine for performing the reactions displayed in Table 2, entries
9 and 11; and the NMR, X-ray, and chromatography departments
of our Institute for excellent support.
Supporting Information Available: Experimental section
including spectroscopic data of all compounds, further informa-
tion about the structure of the complexes in the solid state, and
additional screening data. This material is available free of
charge via the Internet at http://pubs.acs.org.
Finally, we report that the addition of 5 Å molecular sieves
exerts a pronounced effect on the reaction rate as well as on
the conversion in metathesis reactions of alkynes bearing a
methyl end-cap. This beneficial influence is ascribed to the
removal of 2-butyne from the mixture by absorption into the
JA104800W
9
J. AM. CHEM. SOC. VOL. 132, NO. 32, 2010 11057