A R T I C L E S
Zhang et al.
substrate compatibility, and temperature required for catalytic
activity. The catalysts that have been most widely used for
natural product synthesis and preparations of phenylene
ethynylene polymers are based on molybdenum or tungsten
complexes.2 The most readily available of these catalysts is a
poorly characterized species.5 High temperature (g130 °C) is
required for metathesis activity, and limited functional group
tolerance is exhibited. In a preliminary communication, we
reported a reductive recycle strategy for the convenient synthesis
of alkyne metathesis catalysts having activity at room temper-
ature.6 In this paper, systematic studies on the scope and
limitations of these trialkoxymolybdenum(VI) alkylidyne com-
plexes are reported. Most importantly, for internal alkynes
unsymmetrically substituted with an alkyl and an aryl substitu-
ent, we have found that the metathesis activity depends
significantly on the structure of the alkyl substituent. The origin
of the reactivity difference of substrates with various alkynyl
substituents is discussed.
with monochloride 2 returning it to starting complex 1, while
leaving complex 3 unaffected. In the presence of excess
methylene chloride, the net result is a reductive recycle
approach to selectively generate complex 3 in one pot (eq 2).
Through this strategy, trisamidomolybdenum(VI) ethyli-
dyne 4 and propylidyne 5 were prepared in high yield and
purity by the treatment of triamide 1 with 1,1-dichloroethane
(15 equiv) or 1,1-dichloropropane (2 equiv), respectively, in
the presence of magnesium.10 The X-ray crystal structure6 of
propylidyne 5 revealed a characteristic Mo-C triple bond
distance of 1.735(2) Å,11 in accordance with the observed 13C
NMR chemical shift of 302.6 ppm for the carbyne carbon. The
X-ray analysis of 5 also showed the close packing of the amido
ligands on one side of the molybdenum atom with the aryl rings
adopting approximately C3 symmetry. Alcoholysis of complex
5 with 3 equiv of phenol (e.g., R,R,R-trifluoro-o-cresol) or
alcohol (e.g., perfluoro-tert-butyl alcohol) generated a catalyti-
cally active species, presumed to be trialkoxymolybdenum(VI)
propylidyne, which was then directly applied to metathesis
studies.12
Results
Recently Fu¨rstner reported that the combination of readily
available7 Mo[N(t-Bu)Ar]3 (Ar ) 3,5-C6H3Me2) with methylene
chloride provides a mixture of ClMo[N(t-Bu)Ar]3 (2) and
HCMo[N(t-Bu)Ar]3 (3) (eq 1).8 Upon examing various reducing
Using amide 6 as the substrate (eq 3), the metathesis
reaction with complex 5 and R,R,R-trifluoro-o-cresol was
conducted in various solvents at room temperature in order to
survey the dependence of catalytic activity on solvent. Com-
pound 6 was selected because it represented a moderately
functionalized substrate. Reactions performed in chloroform and
toluene produced nearly quantitative yields of dimer 7 in about
30 min. Precipitation of 7 drove the reaction to completion.
Clearly the secondary amide functionality of 6 did not interfere
with catalyst activity. In acetonitrile or tetrahydrofuran, the
reaction was slower but still proceeded to 76% conversion within
8 h at room temperature. However, when the reaction was
conducted in acetone, only 40% conversion was achieved,
accompanied by catalyst decomposition as evidenced by the
formation of a black precipitate. No dimer was formed in N,N-
dimethylformamide or methanol. Metathesis of 3-propynyl
agents,9 we found that magnesium is able to selectively react
(5) (a) Mortreux, A.; Blanchard, M. J. Chem. Soc., Chem. Commun. 1974,
786-787. (b) Kaneta, N.; Hirai, T.; Mori, M. Chem. Lett. 1995, 1055-
1056. (c) Vosloo, H. C. M.; du Plessis, J. A. K. J. Mol. Catal. A: Chem.
1998, 133, 205-211. (d) Recently, Grela reported the use of 2-fluorophenol
as a cocatalyst with Mo(CO)6 that is simple and of wider applicability.
See: Grela, K.; Ignatowska, J. Org. Lett, 2002, 4, 3747-3749.
(6) Zhang, W.; Kraft. S.; Moore, J. S. Chem. Commun. 2003, 832-833.
(7) Complex 1 is synthesized in four steps from commercially available MoCl5.
Cummins discovered and pioneered the synthesis and study of Mo[N(t-
Bu)Ar]3; see: (a) Laplaza, C. E.; Odom, A. L.; Davis, W. M.; Cummins,
C. C.; Protasiewicz, J. D. J. Am. Chem. Soc. 1995, 117, 4999-5000. (b)
Cummins, C. C. Chem. Commun. 1998, 1777-1786. (c) Johnson, A. R.;
Cummins, C. C.; Gambarotta, S. Inorg. Synth. 1998, 32, 123-132. (d) Tsai,
Y.-C.; Stephens, F. H.; Meyer, K.; Mendiratta, A.; Gheorghiu, M. D.;
Cummins, C. C. Organometallics 2003, 22, 2902-2913. Poli developed a
convenient synthesis of MoCl3(thf)3; see: (e) Stoffelbach, F.; Saurenz, D.;
Poli, R. Eur. J. Inorg. Chem. 2001, 2699-2703.
(10) Triamide 1 was treated with 2 equiv of 1,1-dichloropropane instead of 15
equiv in order to minimize a side product which presumably arises from a
Grignard reaction. Also see ref 6.
(11) (a) McCullough, L. G.; Schrock, R. R. J. Am. Chem. Soc. 1984, 106, 4067-
4068. (b) McCullough, L. G.; Schrock, R. R.; Dewan, J. C.; Murdzek, J.
C. J. Am. Chem. Soc. 1985, 107, 5987-5998. (c) Murdzek, J. S.; Schrock,
R. R. Carbyne Complexes; VCH: New York, 1988. (d) Tsai, Y.-C.;
Diaconescu, P. L.; Cummins, C. C. Organometallics 2000, 19, 5260-5262.
(e) Blackwell, J. M.; Figueroa, J. S.; Stephens, F. H.; Cummins, C. C.
Organometallics 2003, 22, 3351-3353.
(12) The isolation of pure trialkoxymolybdenum(VI) catalyst was unsuccessful,
and the attempts to acquire MS and elemental analysis data for R,R,R-
trifluoro-p-cresol Mo(VI) propylidyne and p-nitrophenol catalyst failed.
R,R,R-Trifluoro-p-cresol Mo(VI) propylidyne was characterized by 1H, 13C,
and 19F NMR. See Supporting Information.
(8) Fu¨rstner, A.; Mathes, C.; Lehmann, C. W. J. Am. Chem. Soc. 1999, 121,
9453-9454.
(9) Several combinations of reducing agents and solvent were examined, but
magnesium in THF is the most effective. A zinc-copper couple, manga-
nese, and samarium slowly produced 3 at 50 °C, but unknown side products
also formed. At 70 °C, monochloride 2 decomposed quickly which makes
the reductive recycle strategy not applicable at high temperature. Also see
ref 6.
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330 J. AM. CHEM. SOC. VOL. 126, NO. 1, 2004