Reduction of molybdenum(V) chloride with various reducing metals: Reactivity correlations with the descendant Lewis acids

Article abstract of DOI:10.1246/cl.2000.140

Reactivity of low-valent molybdenum prepared from MoCl₅ with various reducing metals in DME, was dependent on the reducing metals in the order of Al > Sn, In > Zn, Mg, Li in the case of cyclotrimerization of alkynes. This order is parallel to the acidity of the descendant Lewis acids.

Full text of DOI:10.1246/cl.2000.140

140
Chemistry Letters 2000
Reduction of Molybdenum(V) Chloride with Various Reducing Metals:
Reactivity Correlations with the Descendant Lewis Acids
Ryuichiro Hara,* Qiaoxia Guo, and Tamotsu Takahashi*
Catalysis Research Center and Graduate School of Pharmaceutical Sciences, Hokkaido University and CREST, Science and Technology
Corporation (JST), Sapporo 060-0811
(Received October 26, 1999; CL-990913)
Reactivity of low-valent molybdenum prepared from
MoCl5 with various reducing metals in DME, was dependent
on the reducing metals in the order of Al > Sn, In > Zn, Mg, Li
in the case of cyclotrimerization of alkynes. This order is par-
allel to the acidity of the descendant Lewis acids.
Low-valent molybdenum species have attracted attention
with their diverse coordination patterns.1 Reduction of MoCl5
(or more precisely described as a dimer, Mo2Cl10) with main
group metals as the reducing agent seems to be the most con-
venient way to prepare the low-valent molybdenum species.
During the course of our study on the reaction of low valent
molybdenum compounds, we found novel correlation between
reactivity of the molybdenum compounds and acidity of the
descendant Lewis-acid of the reducing metals. We would like
to report here that the reactivity of low-valent molybdenum
compounds prepared from MoCl5 with various reducing metals
in DME.2
The comparison study of various reducing metals for the
reduction of molybdenum pentachloride in the case of
cyclotrimerization of 1-octyne (eq. 1) is shown in Table 1 (entry
1–6). The procedure is as follows; a mixture of MoCl5 (1 mmol)
and 5 mL of DME was treated with reducing metals (Al, Sn, In,
1 mmol; Zn, Mg, 1.5 mmol; Li, 3 mmol)3 and heated at 50 °C
for 3 h. To the reaction mixture was added 1-octyne (3 mmol)
and then the mixture was stirred at 50 °C for 6 h. The reaction
was quenched with 3M HCl aq. and organic products were
extracted with Et2O. The products were analyzed by GC.
In the cyclotrimerization of 1-octyne using the MoCl5/Al
combination, 1,2,4-tri-n-hexylbenzene (1) and 1,3,5-tri-n-
hexylbenzene (2) were obtained in 90% combined yield in a
ratio of 8:1 (entry 1). Moderate yields and selectivities were
obtained when In or Sn was used for the reduction of MoCl5
(entry 2,3). Unexpectedly, strong reducing metals such as Zn,
Mg and Li (entry 4–6) gave poor yields and poor selectivities.
This result indicates that the yields are roughly in the order of
Al > Sn, In > Zn, Mg, Li. Interestingly, this result is closely
parallel to the acidity of the descendant Lewis acids, AlCl3 >
SnCl4 > InCl3 > ZnCl2 > MgCl2 > LiCl.4 This suggests that the
metal-chloride moiety such as Al-Cl, Sn-Cl and In-Cl has a
cooperative effect with the reduced molybdenum in the
cyclotrimerization of alkynes.
for MoCl5 in trimerization of alkynes. Therefore, for other
alkynes such as phenylacetylene and trimethylsilylacetylene,
Al, Sn and In were used (Table 2). For phenylacetylene, Al,
Sn and In gave comparable results (entry 1–3); however, In
gave the best regioselectivity (13:1). For trimethylsilylacety-
lene, because of its sterically demanding substituent, 1,3,5-
tris(trimethylsilyl)benzene was predominantly obtained (entry
4–6). The ratio varies from the case of Al (1:9) to that of In
(1:2), again it differs from one metal to another.
In a reaction of 3 equiv of 1,7-octadiyne (3) with 1 equiv
of MoCl5 and 1 equiv of Al at 50 °C for 1 h, partially cyclized
dimer 4 was obtained in 89% yield (eq. 2). A prolonged reac-
It is clear that Al, Sn and In are suitable reducing metals
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
141
References and Notes
1
The group 6 metal complexes whose charges on the metal
centers are from -2 to +6 are widely covered, see
"Comprehensive Organometallic Chemistry," ed by G.
Wilkinson, F. G. A. Stone, E. W. Abel, Pergamon, Oxford
(1982), Vol. 3, Chap. 27.1-27.2, pp. 1079–1253;
"Comprehensive Organometallic Chemistry II," ed by E.
W. Abel, F. G. A. Stone, G. Wilkinson, Pergamon, Oxford
(1995), Vol. 5, Chap. 3–8, pp. 155–549; "Comprehensive
Coordination Chemistry," ed by G. Wilkinson, R. D.
Gillard, J. A. McCleverty, Pergamon, Oxford (1987), Vol.
3, Chap. 36.1–36.6, pp. 1229–1444.
2
For general reviews on cyclotrimerization reactions, see N.
E. Schore, "[2+2+2] Cycloadditions," in "Comprehensive
Organic Synthesis," ed by B. M. Trost, I. Fleming,
Pergamon, Oxford (1991), Vol. 5, pp. 1129; D. B.
Grotjahn, "Transition Metal Alkyne Complexes:
Transition Metal-catalyzed Cyclotrimerization" in
"Comprehensive Organometallic Chemistry II," ed by E.
W. Abel, F. G. A. Stone, G. Wilkinson, Pergamon, Oxford
(1995), Vol. 12, pp. 741.
tion time (6 h) at 80 °C afforded trimer 5 in 46% along with
undefined higher oligomers.
Because the cyclotrimerization stage is more sterically
demanding than the preceding dimerization, we expected
cross-trimerization of α,ω-diyne and another molecule of
alkyne. Therefore, 1,7-octadiyne (3) was treated at 50 °C for 1
h with a stoichiometric amount of MoCl5/Al and an alkyne was
added to the reaction mixture (eq. 3). The results are shown in
3
4
Al, Sn, Zn: powder; In: small particles; Mg: turnings; Li:
slices.
1H-NMR chemical shift differences (Dd) of crotonalde-
hyde on complexation with various Lewis acids were reex-
amined in order to evaluate the Lewis acidity of AlCl3,
SnCl4, InCl3 and ZnCl2 by exactly following the proce-
dures described in, R. F. Childs, D. L. Mulholland, and A.
Nixon, Can. J. Chem., 60, 801 (1982). From the results
given below, we may safely conclude that the order of
Lewis acidity is AlCl3 > SnCl4 > InCl3 > ZnCl2. ∆δ
(CD2Cl2) (H1, H2, H3, H4 of crotonaldehyde, respectively):
AlCl3: -0.23, +0.77, +1.32, +0.45 (-0.20, +0.76, +1.23,
+0.47, lit.); SnCl4: +0.02, +0.49, +0.86, +0.28 (+0.02,
+0.50, +0.87, +0.29, lit.); InCl3: -0.02, +0.18, +0.37,
+0.08; ZnCl2: -0.02, +0.09, +0.13, +0.03.
Table 3. With 1-octyne, 2-n-hexyl-5,6,7,8-tetrahydronaphtha-
lene (6a) was obtained in moderate yield of 41%. Small
amounts of homo-coupling compounds were detected. It was
striking that disubstituted alkynes could be used as the coun-
terpart. Therefore, with 4-octyne and dimethyl acetylenedicar-
boxylate (DMAD), 2,3-di-n-propyl- and 2,3-bis(methoxycar-
bonyl) derivatives (6b and 6c) were obtained in 51% and 49%
yield, respectively. The yields are not yet satisfactory; howev-
er, this result suggests that the mechanism for the trimerization
of alkynes by the MoCl5/Al system is explained by a stepwise
cyclization presumably involving molybdacyclopentadiene.5
We have, so far, no definite information on the structure
of the MoCl5/Al mixture; however, we assume an associative
structure with µ-bridging chlorine atoms as shown in Figure. 1.
5
S. A. R. Knox, R. F. D. Stansfield, F. G. A. Stone, M. J.
Winter, and P. Woodward, J. Chem. Soc., Chem.
Commun., 1978, 221; S. Slater and E. L. Muetterties,
Inorg. Chem., 19, 3337 (1980); S. Slater and E. L.
Muetterties, Inorg. Chem., 20, 946 (1981); M. Green, P.
A. Kale, and R. J. Mercer, J. Chem. Soc., Chem.
Commun., 1987, 375; M. J. Chetcuti, L. A. DeLiberato, P.
E. Fanwick, and B. E. Grant, Inorg. Chem., 29, 1295
(1990).
6
7
A. Greco, F. Pirinoli, and G. Dall'asta, J. Organomet.
Chem., 60, 115 (1973); For MoCl(η3-C3H5)(η6-
arene)(Cl2AlEt), see; M. L. H. Green and J. Knight, J.
Chem. Soc., Dalton Trans., 1974, 311; M. L. H. Green and
J. Knight, J. Chem. Soc., Dalton Trans., 1976, 213.
F. A. Cotton, J. L. Hubbard, D. L. Lichtenberger, and I.
Shim, J. Am. Chem. Soc., 104, 679 (1982); P. A. Agaskar,
F. A. Cotton, D. R. Derringer, G. L. Powell, D. R. Root,
and T. J. Smith, Inorg. Chem., 24, 2786 (1985); F. L.
Campbell, III, F. A. Cotton, and G. L. Powell, Inorg.
Chem., 24, 4384 (1985); P. A. Agaskar and F. A. Cotton,
Inorg. Chem., 25, 15 (1986).
This type of association between a low-valent Mo and an
Al(III) halide is known in a few cases.6 In addition, tetranu-
clear form Mo2Cl4(AlCl3)2 should be also considered as Cotton
and co-workers reported in a related study.7
In summary, we found a relation between the reactivity of
low-valent molybdenum and the reducing metals.
Characterization of MoCl5/metal composites and elucidation of
the role of reducing metals in the trimerization reactions of
alkynes must await further studies.