A reductive recycle strategy for the facile synthesis of molybdenum(VI) alkylidyne catalysts for alkyne metathesis

Article abstract of DOI:10.1039/b212405j

A convenient synthesis of trisamido molybdenum(VI) alkylidyne complexes has been developed, in which the key step is the addition of a geminal dichloride to a trisamido molybdenum(III) complex in the presence of magnesium to continuously recycle unwanted side product 4, selectively generating the desired alkylidyne complexes in high yield.

Full text of DOI:10.1039/b212405j

A reductive recycle strategy for the facile synthesis of molybdenum(VI)
alkylidyne catalysts for alkyne metathesis†
Wei Zhang, Stefan Kraft and Jeffrey S. Moore*
Roger Adams Laboratory, Departments of Chemistry, Materials Science & Engineering, University of
Illinois at Urbana-Champaign, Urbana, IL 61801, USA. E-mail: js-moore@uiuc.edu; Fax: 1-217-244-8068;
Tel: 1-217-244-4024
Received (in Cambridge, UK) 17th December 2002, Accepted 13th February 2003
First published as an Advance Article on the web 27th February 2003
A convenient synthesis of trisamido molybdenum(VI) alkyli-
dyne complexes has been developed, in which the key step is
the addition of a geminal dichloride to a trisamido molybde-
num(III) complex in the presence of magnesium to con-
tinuously recycle unwanted side product 4, selectively
generating the desired alkylidyne complexes in high yield.
Given that Fürstner’s method directly produces alkylidyne 5,
which is the precursor to Cummins’ active trialkoxyalkylidyne
2, an expedient synthesis of complex 2 could involve redirecting
the reaction in eqn. 2 to favor production of HCMo(N[t-
Bu]Ar)3. We anticipated that a reducing agent could selectively
react with monochloride 4 returning it to starting complex 3,
while leaving complex 5 unaffected. In the presence of excess
methylene chloride, the net result would be a reductive recycle
approach to selectively generate the desired complex 5 in one
pot (eqn. 3). This concept was demonstrated by the following
experiment. When a 0.1 M solution of triamide 3 was treated
with 13CH2Cl2 (10 fold excess) in THF-d8 (ferrocene as internal
standard), a 2+1 mixture of monochloride 4 and methylidyne
5-13C was obtained in 97% yield by 1H NMR integration of the
t-Bu peaks at 24.41 ppm and 1.46 ppm. Addition of 54 mg
magnesium (2.2 mmol) followed by stirring for 45 min at room
temperature, resulted in the clean formation of methylidyne
5-13C ( > 90% yield) as evidenced by 1H and 13C NMR. Neither
the starting triamide 3 nor monochloride 4 could be detected. A
small amount of N-tert-butyl aniline from hydrolysis was the
only detectable side product.
Despite the tremendous impact of alkene metathesis on organic
and polymer synthesis over the last decade,1 the analogous
alkyne metathesis chemistry is a less developed synthetic
method. The alkyne metathesis catalysts that have been most
widely used for natural product synthesis and the preparation of
phenylene ethynylene polymers are based on molybdenum or
tungsten complexes.2 Unfortunately, the most readily available
of these catalysts are a mixture of catalytic and non-catalytic
species that require high temperatures for metathesis activity
with limited functional group tolerance. In this communication
we outline a convenient synthesis of trisamido molybde-
num(VI) alkylidyne complexes and their conversion into highly
active alkyne metathesis catalysts.
The first homogeneous catalytic alkyne metathesis dates back
more than 25 years when Mortreux and Blanchard discovered
that Mo(CO)6 and various phenols metathesize diphenylacety-
lenes at 160 °C.3 The active form of this catalyst remains
unknown, and in spite of high temperature and limited
functional group tolerance, Mori3b and Bunz2d have used the
reaction extensively for both small molecule and polymer
synthesis. The first well-defined alkyne metathesis catalyst was
the tungsten alkylidyne complex (t-BuO)3WC(t-Bu) developed
by Schrock in the early 1980’s.4 This catalyst can metathesize
alkynes under milder conditions (25–60 °C), but thio ethers,
amines or crown ether segments are not tolerated. Schrock, and
more recently Cummins, have reported that analogous trialk-
oxymolybdenum(VI) carbyne complexes (R3O)3MoCR1(2) are
highly active alkyne metathesis catalysts even at room tem-
perature,5 but their scope has not been reported in detail.
Although trialkoxy alkylidyne 2 can be conveniently obtained
from alcoholysis of the metathesis inactive triamide 1 (eqn. 1),
the tedious and multistep nature of the synthesis of starting
complex 15,6 presents a practical limitation to this potentially
useful catalyst. A related alkyne metathesis catalyst was
developed by Fürstner who combined readily available7
(2)
(3)
Given the feasibility of the reductive recycle approach, a
preparatively useful procedure was developed. Several reducing
agents and solvent combinations were examined but none of
them were as successful as magnesium in THF. Zinc–copper
couple, manganese and samarium slowly produced 5 at 50 °C,
but additional unknown products formed under these condi-
tions. At 70 °C, monochloride 4 decomposes quickly which
makes the reductive recycle strategy not feasible at elevated
temperature. The superior reductive efficiency exhibited by
magnesium prompted us to select this reducing agent for further
exploration.
We next examined the scope of the reaction with various
geminal dihalides. The reaction was not successful when
attempted with benzal chlorides. In contrast, 1,1-dichloroethane
(15 equiv.) reacted with molybdenum triamide 3 to generate
ethylidyne 6 (eqn. 3) in very good yield. When triamide 3 was
treated with 1,1-dichloropropane (15 equiv.) in the presence of
magnesium, a large quantity of a side product, presumably
derived from a Grignard reaction, unexpectedly formed instead
of desired product. This byproduct was eliminated by lowering
the quantity of 1,1-dichloropropane to two-fold relative to
Mo[N(t-Bu)Ar]3 (Ar
= 3,5-C6H3Me2) (3) with dichloro-
methane to produce a mixture of ClMo[N(t-Bu)Ar]3 (4) and
HCMo[N(t-Bu)Ar]3 (5) (eqn. 2). This mixture provided alkyne
metathesis activity at 80 °C with good functional group
compatibility.8,9 The catalytically active species is derived from
monochloride 4 whereas methylidyne 5 is almost metathesis
inactive.10
(1)
† Electronic supplementary information (ESI) available: spectral data. See
http://www.rsc.org/suppdata/cc/b2/b212405j/
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CHEM. COMMUN., 2003, 832–833
This journal is © The Royal Society of Chemistry 2003

molybdenum triamide 3. Under these conditions, the desired
propylidyne 7 was obtained cleanly and in high yield.11 The X-
ray crystal structure12 of propylidyne 7 (Fig. 1) revealed a
characteristic Mo–C triple bond distance of 1.735(2) Å,5 in
accord with the observed 13C NMR chemical shift of 302.6 ppm
for the carbyne carbon (C37). The X-ray analysis of 7 also
showed the close packing of the amido ligands on one side of
this molecule. The aryl rings adopt approximately three-fold
symmetry. Thereby a pocket is formed around the Mo center
which shields the central metal quite efficiently.
The authors would like to thank Dr Scott Wilson for help on
X-ray structure analysis. This work was supported by National
Science Foundation under Grant No. NSF CHE 00-91931.
Notes and references
1 For reviews see: T. M. Trnka and R. H. Grubbs, Acc. Chem. Res., 2001,
34, 18; A. Fürstner, Angew. Chem., Int. Ed., 2000, 39, 3012; R. R.
Schrock, Tetrahedron, 1999, 55, 8141.
2 A. Fürstner and G. Seidel, Angew. Chem., Int. Ed., 1998, 37, 1734; A.
Fürstner, O. Guth, A. Rumbo and G. Seidel, J. Am. Chem. Soc., 1999,
121, 11108; A. Fürstner and A. Rumbo, J. Org. Chem., 2000, 65, 2608;
K. Weiss, A. Michel, E.-M. Auth, U. H. F. Bunz, T. Mangel and L.
Mullen, Angew. Chem., Int. Ed., 1997, 36, 506.
3 A. Mortreux and M. Blanchard, J. Chem. Soc., Chem. Commun., 1974,
786; N. Kaneta, T. Hirai and M. Mori, Chem. Lett., 1995, 1055; H. C. M.
Vosloo and J. A. K. du Plessis, J. Mol. Catal. A: Chem., 1998, 133,
205.
4 R. R. Schrock, D. N. Clark, J. Sancho, J. H. Wengrovius, S. M. Rocklage
and S. F. Pedersen, Organometallics, 1982, 1, 1645; M. L. Listemann
and R. R. Schrock, Organometallics, 1985, 4, 74; R. R. Schrock,
Polyhedron, 1995, 14, 3177; R. R. Schrock, Acc. Chem. Res., 1986, 19,
342.
5 L. G. McCullough and R. R. Schrock, J. Am. Chem. Soc., 1984, 106,
4067; L. G. McCullough, R. R. Schrock, J. C. Dewan and J. C. Nurdzek,
J. Am. Chem. Soc., 1985, 107, 5987; J. S. Murdzek and R. R. Schrock,
in Carbyne Complexes, VCH: New York, 1988; Y.-C. Tsai, P. L.
Diaconescu and C. C. Cummins, Organometallics, 2000, 19, 5260.
6 J. C. Peters, A. L. Odom and C. C. Cummins, Chem. Commun., 1997,
1995; J. B. Greco, J. C. Peters, T. A. Baker, W. M. Davis, C. C.
Cummins and G. Wu, J. Am. Chem. Soc., 2001, 123, 5003; T. Agapie,
P. L. Diaconescu and C. C. Cummins, J. Am. Chem. Soc., 2002, 124,
2412.
Fig. 1 X-Ray crystal structure of complex 7. Anisotropic displacements are
drawn at 50% probability, hydrogen atoms are omitted for clarity. Selected
bond lengths in Å and bond angles in °. Mo–N (aver.) 1.981(3), Mo–C37
1.735(2), C38–C37–Mo 177.4(3), C37–Mo–N 102.51(11).
7 Complex 3 is synthesized in four steps from commercially available
MoCl5 See: F. Stoffelbach, D. Saurenz and R. Poli, Eur. J. Inorg. Chem.,
2001, 2699 and ref 9.
8 A. Fürstner, C. Mathes and C. W. Lehmann, J. Am. Chem. Soc., 1999,
121, 9453.
Given that a preparatively useful synthesis of trisamido
molybdenum alkylidynes was in hand, we next wanted to
demonstrate that these complexes are useful precursors for
alkyne metathesis. Alcoholysis of 15 mg molybdenum(VI)
propylidyne complex 7 (22 µmol) with 3 equiv. of a,a,a-
trifluoro-o-cresol or perfluoro-tert-butyl alcohol in toluene
generated a trialkoxymolbdenum(VI) propylidyne complex that
was used directly for metathesis studies. When 28 mg of
benzoate ester 8 (90 µmol) was added to this catalyst solution,
the equilibrium ratio of 4+1 was reached in 22 h at room
temperature.
In summary, we developed a practically useful method to
synthesize trisamido molybdenum(VI) alkylidyne complexes
based on a reductive recycle approach. This method provides a
convenient, large-scale preparation of trisamido molybde-
num(VI) alkylidyne complexes in pure form. Alcoholysis of
molybdenum(VI) propylidyne 7 with a,a,a-trifluoro-o-cresol
or perfluoro-tert-butyl alcohol provide active catalysts for
alkyne metathesis at room temperature. The scope of this
reductive recycle strategy is still under investigation to evaluate
its feasibly toward the synthesis and isolation of trialk-
oxymolybdenum(VI) alkylidyne catalysts for alkyne met-
athesis.
9 A. Fürstner, C. Mathes and C. W. Lehmann, Chem. Eur. J., 2001, 7,
5299.
10 Carbyne complex 5 is inert to metathesis as demonstrated by Fürstner
using stoichiometry studies on the mixture of 4 and 5. Using pure 5 we
have followed up on this experiment and conclude that 5 is catalytically,
inactive. Also see: ref 4: R. R. Schrock, D. N. Clark, J. Sancho, J. H.
Wengrovius, S. M. Rocklage and S. F. Pedersen, Organometallics,
1982, 1, 1645; R. R. Schrock, Polyhedron, 1995, 14, 3177.
11 Preparation of molybdenum(VI) propylidyne 7: all manipulations were
performed in an inert argon atmosphere. To a solution of molybdenum
triamide 3 (0.5053 g, 0.81 mmol) in THF (18 mL) was added
CH3CH2CHCl2 (160 µL, 1.6 mmol) resulting in a color change from red
to dark amber. Magnesium turnings (0.2412 g, 10.1 mmol) were added
and the resulting mixture was stirred for 1.5 h at room temperature. The
solvent was removed in vacuo, and the residue was redissolved in
pentane (25 mL). The solid precipitate was removed by filtration and the
filtrate was concentrated in vacuo. The product was obtained as a light
yellow powder in 91% yield (492 mg). 1H NMR (d8-THF, 500 MHz,
280 °C): d 6.72 (3H, s), 5.73 (6H, s), 3.53 (2H, q, J, = 7.5Hz), 2.09
(18H, s), 1.46 (3H, t, J, = 8.0Hz), 1.27 (27H, s); 13C NMR (d8-THF, 125
MHz): d 302.6, 151.4, 137.5, 131.0, 128.0, 61.3, 45.7, 34.0, 21.7, 14.8
(coalesce of different isomers was observed when temperature gradually
raised from 280 °C to room temperature); MS (EI): m/z (%): M+ 665.4
(13), 610.3 (15), 552.2 (8), 490.2 (7), 162.1 (100), 121.1 (40); HR-MS
(EI) (C39H59MoN3): calcd 665.3607, found 665.3605; elemental
analysis cald (%) for C39H59MoN3 (665.83): C 70.35, H 8.93, N 6.31;
found C 69.75, H 8.84, N 6.25%.
12 Crystal data, for 7: Suitable single crystals for the X-ray diffraction
study were grown from n-pentane. C39H59MoN3, M
monoclinic, space group P21/c, a = 19.144(10), b = 10.513(6), c =
19.324(11) Å, b = 104.075(9)°. V, = 3772(4) Å3, Z, = 4, rcalcd
= 665.83,
=
1.172 g cm23; m (Mo–Ka) = 0.376 mm21, T, = 193(2) K, 39471
reflections measured, 6919 unique (Rint = 0.0446) which were used in
all calculations. The final wR(F2), was 0.0844 (all data). CCDC 200432.
See http://www.rsc.org/suppdata/cc/b2/b212405j/ for crystallographic
data in .cif or other electronic format.
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