Published on Web 07/12/2006
Highly Active Molybdenum-Alkylidyne Catalysts for Alkyne Metathesis:
Synthesis from the Nitrides by Metathesis with Alkynes
Robyn L. Gdula and Marc J. A. Johnson*
Department of Chemistry, UniVersity of Michigan, 930 North UniVersity AVenue, Ann Arbor, Michigan 48109-1055
Received November 25, 2005; E-mail: mjaj@umich.edu
Olefin metathesis has had an enormous impact on synthetic
chemistry over the past two decades, due to great improvements
in catalyst design.1-6 Although highly active homogeneous catalysts
for the analogous alkyne metathesis reaction have been known for
just as long, the latter process has seen less use until recently.7-15
Widespread application of alkyne metathesis in the areas of small
molecule, natural product, and polymer synthesis has apparently
been frustrated either by the inconvenience of syntheses of highly
active catalysts8 or by the incompatibility of sensitive substrates
with the somewhat harsher conditions required for the use of simpler
“in situ” catalysts.14,16 Very recently, several groups have reported
the formation of highly active catalysts derived from Mo(N[t-Bu]-
Ar)3 (1)8,10-13,17-20 and its analogue Mo(H)(η2-Me2CNAr)(N[i-Pr]-
Ar)2 (2).21-23 Depending on the ancillary ligand set, catalysts derived
from 1 and 2 display good functional group tolerance. Nevertheless,
the intermediate complexes 1 and 2 are synthetically challenging;
both are highly reactive air- and water-sensitive species capable of
reductively cleaving N2.17,21 We report herein the formation of the
exceptionally active alkyne metathesis catalyst EtCtMo(OC(CF3)2-
Me)3(DME) (3-DME; DME ) 1,2-dimethoxyethane) in high yield
from NtMo(OC(CF3)2Me)3 (4) or NtMo(OC(CF3)2Me)3(NCMe)
(4-NCMe) upon reaction with 3-hexyne at elevated temperature.
Complex 3-DME was first prepared two decades ago by Schrock,
who demonstrated its activity in alkyne metathesis reactions and
investigated its mechanism.24 Treatment of NtMo(OC(CF3)3)3-
(NCMe) (5-NCMe) with 3-hexyne and DME under similar condi-
tions likewise yields another highly active metathesis catalyst, EtCt
Mo(OC(CF3)3)3(DME) (6-DME). As the synthesis of 4, 4-NCMe,
and 5-NCMe is simple and straightforward, reaction of these
compounds with 3-hexyne is an exceptionally facile route to
multigram quantities of extremely active Mo-based alkyne metath-
esis catalysts.
As part of a study of the reactivity and relative stability of nitride
and alkylidyne complexes of the d-block elements, we recently
reported the synthesis of the terminal nitrido species 4 and 5-NCCH3
and degenerate N-atom exchange processes with and among nitriles
mediated by these complexes.25 A related tungsten complex, Nt
W(O-t-Bu)3 (7), is known to catalyze the same process; DFT
calculations support an azametallacyclobutadiene intermediate.26
Having established that the nitrido moiety in 4-NCMe and
5-NCMe is active in degenerate N-atom exchange with nitriles,
we next investigated reactions of these complexes with alkynes.
Among related tungsten compounds, NtW(OAr)3 (8; Ar ) 2,6-
i-Pr2C6H3) is formed rapidly and quantitatively when t-BuCt
W(OAr)3 (9) is treated with a nitrile, such as MeCN;27 similarly,
PhCtW(OC(CF3)Me2)3 (10) and PhCtW(OSi-t-BuMe2)3 (11) react
with aryl nitriles to afford NtW(OC(CF3)Me2)3 (12) and Nt
W(OSi-t-BuMe2)3 (13), respectively.28,29 These results indicate a
strong preference for nitride rather than alkylidyne ligation at this
metal center. However, periodic trends predict an eventual crossover
in the relative stability of alkylidyne and nitrido complexes as the
metal center becomes more electronegative.30
At room temperature, we do not observe alkylidyne formation
from compounds 4, 4-NCMe, or 5-NCMe upon reaction with nitriles
or symmetrical alkynes. However, when solutions of 4, 4-NCMe,
and 5-NCMe are exposed to unsymmetrical alkynes, products of
alkyne metathesis appear, albeit rather slowly. For example, at 92
°C, 10 mol % of 4 yielded 58% conversion to PhCtCPh in 19 h;
when 5-NCMe was used under the same conditions, conversion to
PhCtCPh was complete within this time period. Equilibrium was
not established in these reactions due to consumption of coproduced
MeCtCMe, which was polymerized under these conditions.
Polymerization of MeCtCMe is often seen with alkylidyne
complexes, such as 3-DME, for example.24 At 80 °C, mixtures
initially composed of equimolar 3-hexyne and 4-octyne approached
equilibrium with 3-heptyne to the extent of 71 and 92% after 3 h
with 5 mol % of 4 and 5-NCMe, respectively. Equilibrium was
reached soon thereafter. The reactions proceeded similarly rapidly
in CD2Cl2. Likewise, EtCtCMe gave rise to EtCtCEt and MeCt
CMe slightly more rapidly (again, MeCtCMe was ultimately
consumed). However, in THF, we observed no metathesis what-
soever of PhCtCMe or EtCtCMe. DME similarly inhibits
metathesis. In C6D6 with 3 equiv added DME, metathesis occurred
very slowly, with the first products being observed after 3 days
with 4 and after 24 h with 5-NCMe (6.7 mol % catalyst).
Although no alkylidyne complex is observed by NMR spectros-
copy under these conditions, these results suggest the possibility
that an alkylidyne complex is being formed from the nitrido
complex to a small extent. However, two initial observations rule
out reversible formation of alkylidyne complex in an equilibrium
that favors the nitrido complex. Unlike 9 and 10, EtCtMo(OC-
(CF3)2Me)3 (3),24 3-DME, and 6-DME fail to react with nitriles,
except to coordinate them reversibly. No terminal nitrido complexes
are formed in these cases. Furthermore, experiments involving
treatment of 4 and 5-NCMe with a mixture of excess alkyne (R-
CtC-R) and excess nitrile (R′-CtN) reveal no crossover of the
organic radicals (R, R′); use of unsymmetrical alkyne shows that
alkyne metathesis occurs, but without incorporation of the R′ group
from the nitrile.
Heating of solutions of 4, 4-NCMe, and 5-NCMe with 3-hexyne
and 1 equiv of DME to 95 °C in benzene results in complete
conversion to the corresponding propylidyne complexes over several
days. Thus, the alkylidyne complexes are formed irreVersibly from
the nitrido complexes, but with a large activation barrier. However,
DME is not required for this conversion but in fact inhibits the
reaction. When DME is not present in the initial mixture, conversion
to the propylidyne complex 3 is quantitative after only 14.5 h at
95 °C. DME is added to the solution in order to facilitate isolation
of the alkylidyne complexes in the form of the more crystalline
DME adducts 3-DME and 6-DME. Both preparations afford 3-DME
in approximately 60% yields on a multigram scale.
9
9614
J. AM. CHEM. SOC. 2006, 128, 9614-9615
10.1021/ja058036k CCC: $33.50 © 2006 American Chemical Society