Mo[N(t-Bu)(Ar)]3 complexes as catalyst precursors: In situ activation and application to metathesis reactions of alkynes and diynes [4]

Full text of DOI:10.1021/ja991340r

J. Am. Chem. Soc. 1999, 121, 9453-9454
9453
Scheme 1
Mo[N(t-Bu)(Ar)]3 Complexes As Catalyst Precursors:
In Situ Activation and Application to Metathesis
Reactions of Alkynes and Diynes
Alois Fu¨rstner,* Christian Mathes, and Christian W. Lehmann
Max-Planck-Institut fu¨r Kohlenforschung
D-45470 Mu¨lheim/Ruhr, Germany
ReceiVed April 26, 1999
A strongly renewed interest in alkyne metathesis can presently
be noticed within the organic as well as the polymer chemists
community.1,2 Schrock-type alkylidyne complexes as well as less
defined species prepared in situ e.g. from Mo(CO)6 and phenol
additives constitute adequate catalysts for these purposes.2 In this
context we have reported the first synthesis of macrocyclic
cycloalkynes via ring-closing metathesis (RCM) of diyne sub-
strates catalyzed by (tBuO)3WtCCMe3 (1).3,4 In view of the great
preparative potential of this new transformation we have embarked
into a systematic study of its scope, as it seems to supplement
the well-established RCM reaction of dienes.5,6
that [(Ar)(tBu)N]3MoCl (3)9 and the terminal alkylidyne complex
[(Ar)(tBu)N]3MotCH (4)10,11 constitute major components of this
Inspired by publications of Cummins et al. on molybdenum
complexes of the general type Mo[N(tBu)(Ar)]3 (2),7,8 which
activate the triple bond of molecular nitrogen in a stoichiometric
fashion, we investigated the reactivity of such compounds toward
alkynes. Although 2 (Ar ) 3,5-dimethylphenyl) itself does not
effect any metathesis event, we noticed a strongly endothermic
process upon dissolving complex 2 in CH2Cl2; the resulting
mixture efficiently catalyzes a metathetic coupling of different
aliphatic as well as aromatic alkynes (Scheme 1). CH2Cl2 can be
used as the solvent, but the addition of ≈25 equiv (not optimized)
of CH2Cl2 per mol of 2 to a solution in toluene turned out to be
equally effective.
mixture. Although we were unable so far to separate the individual
compounds by crystallization, authentic 3 can be conveniently
obtained by reaction of 2 and Cl2 in ether solution (eq 1). To the
best of our knowledge, this is the first example of a trisamido-
molybdenum(IV) chloride reported in the literature. An X-ray
analysis of this compound (Figure 1)9 shows the close packing
of the amido ligands on one side of the molecule. Thereby a
pocket is formed around the Mo center that seems to shield the
central metal quite efficiently. This congested arrangement may
explain some of the favorable chemical properties of 3 (vide infra).
Activation of complex 2 with CH2Cl2 and evaporation of all
volatiles affords a very sensitive solid residue that contains
different molybdenum species. MS and NMR inspection indicates
(1) For the first alkyne metathesis using a defined alkylidyne catalyst see:
Wengrovius, J. H.; Sancho, J.; Schrock, R. R. J. Am. Chem. Soc. 1981, 103,
3932.
(2) For a short review see: Bunz, U. H. F.; Kloppenburg, L. Angew. Chem.,
Int. Ed. Engl. 1999, 38, 478. For applications see: (b) Weiss, K.; Michel, A.;
Auth, E. M.; Bunz, U. H. F.; Mangel, T.; Mu¨llen K. Angew. Chem., Int. Ed.
Engl. 1997, 36, 506. (c) Pschirer, N. G.; Bunz, U. H. F. Tetrahedron Lett.
1999, 40, 2481. (d) Kloppenburg, L.; Song, D.; Bunz, U. H. F. J. Am. Chem.
Soc. 1998, 120, 7973. (e) Mortreux, A.; Blanchard, M. J. Chem. Soc., Chem.
Commun. 1974, 786. (f) Villemin, D.; Cadiot, P. Tetrahedron Lett. 1982, 23,
5139.
In view of our previous experiences in ring closing diyne
metathesis effected by complex 13 and of the generally accepted
mechanism involving metal alkylidyne intermediates,12 it seemed
reasonable to assume that alkylidyne 4 present as a major
component in the crude mixture constitutes the catalytically
relevant species. Control experiments, however, using either pure
3 or its bromo analogue 5 (eq 2) revealed that these inorganic
compounds catalyze alkyne metathesis as efficiently as the “in
situ” mixture formed form 2 and CH2Cl2 (cf. Table 1). In line
with this observation, we noticed that treatment of 2 with halide
(3) Fu¨rstner, A.; Seidel, G. Angew. Chem. 1998, 110, 1758; Angew. Chem.,
Int. Ed. Engl. 1998, 37, 1734.
(4) For the preparation and applications of 1 see: (a) Schrock, R. R.; Clark,
D. N.; Sancho, J.; Wengrovius, J. H.; Rocklage, S. M.; Pedersen, S. F.
Organometallics 1982, 1, 1645. (b) Freudenberger, J. H.; Schrock, R. R.;
Churchill, M. R.; Reingold, A. L.; Ziller, J. W. Organometallics 1984, 3,
1563. (c) Schrock, R. R. Polyhedron 1995, 14, 3177.
(5) For recent reviews on RCM of alkenes see the following for leading
references: (a) Grubbs, R. H.; Chang, S. Tetrahedron 1998, 54, 4413. (b)
Fu¨rstner, A. Top. Catal. 1997, 4, 285. (c) Schuster, M.; Blechert, S. Angew.
Chem., Int. Ed. Engl. 1997, 36, 2036. (d) Fu¨rstner, A. Top. Organomet. Chem.
1998, 1, 37.
(6) For recent macrocycle syntheses by RCM from our laboratory see: (a)
Fu¨rstner, A.; Langemann, K. J. Am. Chem. Soc. 1997, 119, 9130. (b) Fu¨rstner,
A.; Ackermann, L. Chem. Commun. 1999, 95. (c) Fu¨rstner, A.; Picquet, M.;
Bruneau, C.; Dixneuf, P. H. Chem. Commun. 1998, 1315. (d) Fu¨rstner, A.;
Koch, D.; Langemann, K.; Leitner, W.; Six, C. Angew. Chem., Int. Ed. Engl.
1997, 36, 2466. (e) Fu¨rstner, A.; Langemann, K. Synthesis 1997, 792. (f)
Fu¨rstner, A.; Mu¨ller, T. J. Org. Chem. 1998, 63, 424. (g) Fu¨rstner, A.; Gastner,
T.; Weintritt, H. J. Org. Chem. 1999, 64, 2361.
(9) Compound 3: MS (70 eV): m/z 661 (3, M+), 604 (24, M+ - C4H9),
548 (29, 604 - C4H8), 492 (100, 548 - C4H8). Crystallographic data: red
plates, monoclinic P21, a ) 10.7575(6) Å, b ) 11.1362(6) Å, c ) 14.8699-
(8) Å, â ) 94.480(2)°, T ) 100 K, Z ) 2, wR2 ) 0.102, R ) 0.055. For
further details see the Supporting Information.
(10) Although the NMR analysis of the crude material is hampered by the
paramagnetic nature of admixed open-shell molybdenum species, the very
characteristic signals of 4 were unequivocally identified, thus corroborating
the MS data. Compound 4: 13C NMR (125 MHz, C6D6): δ 287.0 ppm (Mot
CH), 1H NMR (600 MHz, C6D6) δ 5.66 ppm (MotCH); positive cross-peak
1
between these signals in H, 13C correlation experiments. MS (70 eV): m/z
639 (9, M+), 582 (30), 526 (34), 470 (12). The admixed paramagnetic species
give rise to the following signals in the 1H NMR spectra: δ 23.74 (br), 6.44
(s), 6.35 (s), -1.02 (br), -1.41 (br), -4.95 (br).
(7) Cummins, C. C. Chem. Commun. 1998, 1777. (b) Laplaza, C. E.;
Cummins, C. C. Science 1995, 268, 861. (c) Laplaza, C. E.; Johnson, A. R.;
Cummins, C. C. J. Am. Chem. Soc. 1996, 118, 709. (d) Laplaza, C. E.; Johnson,
M. J. A.; Peters, J. C.; Odom, A. L.; Kim, E.; Cummins, C. C.; George, G.
N.; Pickering, I. J. Am. Chem. Soc. 1996, 118, 8623.
(8) Preparation: Laplaza, C. E.; Odom, A. L.; Davis, W. M.; Cummins,
C. C. J. Am. Chem. Soc. 1995, 117, 4999.
(11) Complex 4 has been prepared by Cummins by a different route: Peters,
J. C.; Odom, A. L.; Cummins, C. C. Chem. Commun. 1997, 1995.
(12) The mechanism via alkylidyne- and metallacyclobutadiene complexes
has originally been proposed by: Katz, T. J.; McGinnis, J. J. Am. Chem. Soc.
1975, 97, 1592. See also ref 1.
10.1021/ja991340r CCC: $18.00 © 1999 American Chemical Society
Published on Web 09/23/1999

9454 J. Am. Chem. Soc., Vol. 121, No. 40, 1999
Communications to the Editor
Table 2. RCM Catalyzed by [(Ar)(tBu)N]3MoCl (3) (10 mol %)
a
Generated in Situ from Complex 2 and CH2Cl2
Figure 1. Molecular structure of 3. Selected bond length (Å) and angles
(deg): Mo-N1 1.963(3), Mo-N2 1.963(3), Mo-N3 1.952(3), Mo-Cl
2.350(1), N1-Mo-Cl 97.4(1), N2-Mo-Cl 102.7(1), and N3-Mo-Cl
101.2(1). Note that all three amido groups have planar geometry.
Table 1. Ring Closing Metathesis Effected by Complex 2
Activated in Situ by Different Additives
entry
solvent
toluene
additivea
CH2Cl2
yield (%)
1
2
3
4
5
6
7
8
9
CH2Cl2
80
81
82
70
84
84
78
81
75
70
79
a All reactions are carried out in toluene at 80 °C for 20-48 h.
CHCl3
CCl4
uates the effective Lewis acidity of the metal center and prevents
coordination of potential donor substrates onto the catalytically
active template. In contrast to complex 1, however, catalyst 3 is
sensitive toward “acidic” protons such as those of secondary
amides, whereas tertiary amides are fully compatible and deliver
the corresponding cyclic products in excellent yield (entry 8).
Therefore catalysts 1 and 3 are complementary with respect to
their tolerance toward certain functional groups. Note, however,
that esters, isolated double bonds, and silyl ethers are compatible
with both of them.3
toluene
toluene
toluene
toluene
toluene
toluene
toluene
CH2Br2
CH2I2
C6H5CHCl2
C6H5CH2Cl
Me3SiCl
10
11
[(Ar)(tBu)N]3MoCl (3)b
[(Ar)(tBu)N]3MoBr (5)c
a
≈25 equiv of the respective additive relative to 2 are used. b Pure
3 was used as the catalyst instead of 2 + additive. c Pure 5 was used
as the catalyst instead of 2 + additive.
Because of the excellent performance of complex 2 in this new
field of application, we developed an improved synthesis of the
required sterically demanding amine ligand (t-Bu)(Ar)NH based
on the palladium-catalyzed amination reaction pioneered by
Buchwald.15,16 For details see the Supporting Information.
In short, we believe that compound 2 and its derivatives 3 and
5 are useful and versatile reagents for advanced organic synthesis.
Further studies will be disclosed in the near future.
sources other than CH2Cl2 also leads to the generation of active
species in solution (Table 1). This raises interesting questions as
to how such halide species trigger a catalytic cycle and what
intermediates along the ensuing alkyne metathesis pathway may
possibly look like.13
Although these mechanistic aspects are not yet clear,14 the
catalyst formed in situ as well as pure 3 or 5 are excellent tools
from the preparative point of view (Table 2). Not only do they
effect the formation of macrocyclic cycloalkynes of different ring
sizes, but they also tolerate functional groups which completely
shut down the catalytic actiVity of the tungsten alkylidyne catalyst
1 preViously used.2,3 This is true for thioethers (entry 3), basic
nitrogen atoms (entry 6), and polyether chains (entry 7). This
favorable property is tentatively ascribed to the crowded coor-
dination sphere around the Mo(IV) center in 3 endowed by the
bulky amide substituents (cf. Figure 1). The steric shielding atten-
Acknowledgment. Financial support by the MPG, the DFG (Leibniz
program), and the Fonds der Chemischen Industrie is gratefully acknow-
ledged.
Supporting Information Available: A procedure for ring closing
diyne metathesis, large scale preparation of (tBu)(Ar)NH, NMR spectra
of all new cycloalkynes, and X-ray structure of compound 3 (PDF). This
material is available free of charge via the internet at http://pubs.acs.org.
JA991340R
(13) It has been suggested that pathways that do not involve metal
alkylidyne intermediates may also be operative in alkyne metathesis, cf.:
Kaneta, N.; Hirai, T.; Mori, M. Chem. Lett. 1995, 627. (b) Kaneta, N.; Hikichi,
K.; Asaka, S.-I.; Uemura, M.; Mori, M. Chem. Lett. 1995, 1055.
(14) Another interesting question concerns the formation of the terminal
alkylidyne complex 4 from 2 and CH2Cl2.
(15) Wolfe, J. P.; Wagwaw, S.; Buchwald, S. L. J. Am. Chem. Soc. 1996,
118, 7215.
(16) For a related synthesis of similar ligands see: Johnson, A. R.;
Cummins, C. C. Inorg. Synth. 1998, 32, 123.