Formation of an η3-allyl and a cyclic γ-hydroxyalkyl complex of molybdenum from reaction of MoH2(η5-cyclopentadienyl)2 with homoallyl alcohol

Article abstract of DOI:10.1016/j.jorganchem.2003.12.037

The reaction of Cp₂MoH₂ (Cp=η⁵ -C₅H₅) with homoallyl alcohol in the presence of a protonic acid afforded a cationic η³-crotyl molybdenum complex and a cyclic α-methyl-γ-hydroxypropyl molybdenum complex. This reaction proceeds via the stepwise formation of the cyclic complex, followed by formation of the η³-crotyl complex.

Full text of DOI:10.1016/j.jorganchem.2003.12.037

Journal of Organometallic Chemistry 689 (2004) 1025–1028
www.elsevier.com/locate/jorganchem
Formation of an g3-allyl and a cyclic c-hydroxyalkyl complex
of molybdenum from reaction of MoH2(g5-cyclopentadienyl)2
with homoallyl alcohol q
a,
a
a
Makoto Minato *, Susumu Hiratsuka , Ryoko Sekimizu ,
a,
b
Takashi Ito *, Kohtaro Osakada
a
Department of Materials Chemistry, Graduate School of Engineering, Yokohama National University,
79-5 Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan
b
Research Laboratory of Resources Utilization, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan
Received 29 October 2002; accepted 24 December 2003
Abstract
The reaction of Cp2MoH2 (Cp ¼ g5-C5H5) with homoallyl alcohol in the presence of a protonic acid afforded a cationic g3-crotyl
molybdenum complex and a cyclic a-methyl-c-hydroxypropyl molybdenum complex. This reaction proceeds via the stepwise for-
mation of the cyclic complex, followed by formation of the g3-crotyl complex.
Ó 2004 Published by Elsevier B.V.
Keywords: Molybdenum; Hydrido complex; Homoallyl alcohol
1. Introduction
such as HCl, HI, CF3COOH, or TsOH (TsO ¼ p-
CH3(C6H4) SO3) afford cationic g3-allyl molybdenum
complexes and cyclic c-hydroxypropyl molybdenum
complexes [3]. In this paper, we report the extension of
these studies to the reaction of 1 with homoallyl alcohol
(3-buten-1-ol), leading to the formation of a cationic g3-
crotyl molybdenum complex and a cyclic a-methyl-
c-hydroxypropyl molybdenum complex.
The oxidative addition of allylic substrates such as
allyl halides, allyl alcohols, allyl carbonates, and allyl
acetates to low-valent metal complexes is a fundamental
organometallic transformation [1]. Nucleophilic attack
on the g3-allyl ligand which is coordinated to transition
metals is one of the most important reactions in organic
synthesis. In contrast to the allylic substrates, their ho-
mologs, homoallylic substrates such as homoallylic al-
cohols have found little use in synthesis because of their
lack of reactivity. Homoallylic alcohols are formed
regiospecifically by a number of routes [2]. Therefore,
given the useful method for activating homoallylic al-
cohols by metal complexes, they would be employed as
the key intermediates for synthetic purposes.
2. Results and discussion
The reaction between Cp2MoH2 (1) and homoallyl
alcohol was undertaken as the extension of our previous
work on the reactions of 1 with allyl alcohols. Thus the
reaction was run in methanol at 50 °C (Eq. (1)).
We previously reported that the reactions of allyl
alcohols with bis(cyclopentadienyl)-molybdenum dihy-
dride Cp2MoH2 (1) in the presence of protonic acid HX,
+
+
X-
HX
X-
+
Cp2Mo
Cp2MoH2
+
Cp2Mo
OH
O
H
q Supplementary data associated with this article can be found, in the
online version, at doi:10.1016/j.jorganchem.2003.12.037.
* Corresponding authors. Fax: +81-45-339-3933.
1
X = TsO
3
2
E-mail address: minato@ynu.ac.jp (M. Minato).
ð1Þ
0022-328X/$ - see front matter Ó 2004 Published by Elsevier B.V.
doi:10.1016/j.jorganchem.2003.12.037

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M. Minato et al. / Journal of Organometallic Chemistry 689 (2004) 1025–1028
1H NMR analysis of the crude precipitates showed
ꢀ
value expected for a C–C single bond (1.54 A) and is
ꢀ
there appear to be two species present. On the basis of
the similarity of this data to the 1H NMR spectra for the
cationic g3-allyl molybdenum complexes and the cyclic
c-hydroxypropyl molybdenum complexes [3], the prod-
ucts were tentatively assigned to the cyclic a-methyl-
c-hydroxypropyl molybdenum complex (2) and the
cationic g3-crotyl molybdenum complex (3). In the
present reaction, these were observed in a 2.0:1.0 ratio
according to their respective Cp signals at 5.30 (2) and
5.42 (3) ppm. The solubility of 2 in THF is higher than
that of 3, which facilitates the isolation of 2. Crystalli-
zation from THF/diethyl ether afforded 2 as dark-red
crystals. Characterization of 2 was achieved by spec-
troscopy as well as by X-ray crystallographic analysis.
Its IR spectrum displays the strong band at 2969 cmꢀ1
assignable to the stretching of the O–H group coordi-
nated to the central molybdenum atom. In the 1H NMR
spectrum measured in CD3OD, besides the singlet sig-
nals due to Cp protons at d 5.30 and 5.15 ppm, doublet
of doublets (d ¼ 3:78, 1H, J ¼ 6:5, 7.8 Hz, OCH),
multiplet (d ¼ 3:10, 1H, OCH), multiplet (d ¼ 2:60, 1H,
MoCH), doublet (d ¼ 1:50, 3H, J ¼ 6:5 Hz, MoCH-
(CH3)), and multiplet (d ¼ 1:23, 2H, MoCH(CH3)CH2)
signals are, respectively, observed. The accurate struc-
tural details of 2 were determined by single-crystal X-ray
crystallography [4]. The X-ray diffraction analysis
reveals that 2 exists as a discrete cation and anion. The
view of the molecular geometry of the cation is shown in
Fig. 1.
rather close to a full double bond (1.34 A). Approximate
sp2 hybridization at C1 is also reflected in the C2–C1–
C3 bond angle of 125°. Furthermore, the C1–C2 dis-
2
3
ꢀ
tance is 1.49 A, as expected for a single C(sp )–C(sp )
bond. However, 13C NMR data for 2 poses a rather
puzzling situation. The 13C resonances for the C1 and
C3 carbon atoms appear at d 31.9 and 41.8 ppm, re-
spectively; these values seem to be close to a C–C sat-
urated system. Therefore, we interpret this to mean that
the canonical form A shown in Eq. (2) makes a non-
ꢀ
trivial contribution. The C4–O1 bond length (1.45 A) is
appropriate for a C(sp3)–O single bond.
+
CH2
CH2
Cp2Mo
Cp2Mo
+
ð2Þ
O
H
O
H
A
The g3-crotyl complex (3) was characterized by
NMR and IR spectrum. The IR spectrum of 3 shows the
characteristic medium-intensity absorption at 3081 cmꢀ1
assignable to the stretching of the C–H bonds of the Cp
ligands. The 1H and 13C NMR signals were identified by
comparing the spectrum data with those reported for the
related g3-crotyl molybdenum complex [5]. In the H
1
NMR spectrum, 3 exhibits two sharp Cp singlets at d
5.42 and 5.19 ppm. The CH3 doublet of the crotyl ligand
appears at d 1.87 ppm (3H, J ¼ 9:1 Hz). The g3-allyl
protons resonate at d 4.23 (central, m, 1H), 3.22–3.05
(internal and terminal-syn, m, 2H), and 1.72 (terminal-
anti, m, 1H). The 13C NMR spectrum shows the g3-
crotyl resonances at d 92.6 (central), 64.8 (terminal),
33.5 (internal), and 21.0 (CH3) ppm.
The molecular structure contains the five-membered
ring consisting of Mo–C–C–C–O atoms: the ring is
roughly planar, having a sum of internal angles 536.08°.
The most unusual feature of the structure is the C1–C3
ꢀ
bond distance (1.28 A). It is abnormally shorter than the
In the reactions of allyl alcohols with 1, the ratio of
the resultant g3-allyl and the cyclic c-hydroxypropyl
molybdenum complexes was found to change drastically
depending on the nature of acids used. Hence the reac-
tions of 1 with homoallyl alcohol in the presence of
other acids were examined. The dependence of the ratio
2/3 on the kind of protonic acid HX in the reaction was
1
examined by monitoring the H NMR signal intensities
of the Cp resonances of the crude reaction products and
the results are summarized in Table 1. As noted, pref-
erence of the formation of 2 to 3 increases as the acidity
of HX increases; this tendency is compatible with the
results for the allyl alcohol system.
Table 1
Dependence of the ratio 2/3 on the protonic acid
HX
Reaction conditions Yield (2 + 3) (%) 2/3
ꢀ
Fig. 1. Molecular structure of 2. Selected bond lengths (A) and angles
CF3COOH
TsOH
CF3SO3H
50 °C, 6 h
50 °C, 6 h
50 °C, 6 h
98
91
95
1.3/1.0
2.0/1.0
24.5/1.0
(°): Mo1–O1 2.18(1), Mo1–C1 2.27(3), C1–C2 1.49(3), C1–C3 1.28(4),
C3–C4 1.39(4), O1–C4 1.45(2); O1–Mo1–C1 74.0(8), Mo1–C1–C3
112(2), C1–C3–C4 124(3), O1–C4–C3 111(2), Mo1–O1–C4 115(1).

M. Minato et al. / Journal of Organometallic Chemistry 689 (2004) 1025–1028
1027
To our knowledge, the formation of an g3-allyl
complex and/or a cyclic c-hydroxypropyl complex by
the reaction of a metal complex with homoallyl alcohol
is entirely unprecedented. The present results show the
course of the reaction of 1 with homoallyl alcohol is
quite similar to that with the allyl alcohol system. In
fact, we found that the compounds 2 and 3 are also
synthesized independently from 1 and crotyl alcohol (2-
buten-1-ol). Hence, one of the plausible explanation for
the formation of 2 and 3 from homoallyl alcohol is that
homoallyl alcohol undergoes rapid isomerization to give
crotyl alcohol; then it reacts exclusively with 1. It is well
known that transition metal hydride complexes are able
to catalyze olefin rearrangement [6]. However, detailed
the Mo–H bond occurs affording 2. These pathways are
quite similar to those observed in the allyl alcohol system.
In the case of homoallyl alcohol, both a five- and a six-
membered ring will be formed, but the five-membered
ring is only preferred. This phenomenon would be ex-
plained by more favorable entropy factors leading to a
five-membered ring [7], although other explanation would
also be offered. Five-membered ring closure seems to give
the more stable cation 2 in which the molybdenum atom
bearing positive charge is bound to the secondary carbon
atom. As mentioned above, when 2 was treated with
TsOH, it readily transformed into 3. Moreover, we con-
firmed that this reaction does not proceed without acid at
all. Thus, 2 is dehydrated with acid to give the interme-
diate g1-allyl molybdenum complex; then isomerization
involving g1-allyl to g3-allyl interconversion occurs to
yield 3. In this dehydration step, Xꢀ group of the acid HX
would act as a base which removes the b-proton from the
intermediate 5. The relative basicity of the Xꢀ group
seems to affect the rate of the reaction with stronger base
reacting faster. Therefore, CF3SOꢀ3 group will be less
likely to react with 5 than CF3COꢀ2 and TsOꢀ. These
considerations are consistent with the experimental ob-
servations that preference of the formation of 2 to 3 in-
creases as the acidity of HX increases.
1
inspection of the reaction mixture using GLC and H
NMR revealed that there is no indication for the for-
mation of crotyl alcohol.
Following observations provide insight into the
mechanism of this reaction. Monitoring of the reaction
by 1H NMR spectroscopy showed that 2 is first formed;
then it is converted to 3. Thus, 2 is assumed to be the
intermediate species in the formation of 3. To prove the
feasibility of this path, the reaction of 2 with excess of
TsOH in MeOH was examined. We found that com-
pound 2 is completely converted into 3 at room tem-
perature within 10 h. Based on these observations and
the previous results concerning the allyl alcohol system,
the plausible mechanism for the reaction between 1 and
homoallyl alcohol is proposed in Scheme 1.
3. Experimental
The trihydrido cation formed initially by protonation
of 1 releases hydrogen to give the reactive monohydrido
complex 4 [3]. After coordination of homoallyl alcohol to
the complex, cyclization via intramolecular insertion into
Unless otherwise noted, all manipulations were con-
ducted using standard Schlenk techniques under purified
argon or nitrogen. Commercially available reagent
grade chemicals were used as such without any further
OH
HX
-H2
[Cp2MoH3]+X-
Cp2MoH2
Cp2MoHX
1
4
+
+
H
HX
X-
Cp2Mo
Cp2Mo
Cp2Mo
X
X-
O
O
H
H
O
H
2
5
+
X-
Cp2Mo
O
H
X-
+
H
-HX, -H2O
X-
Cp2Mo
Cp2Mo
Cp2Mo
X
X
H2O+
3
5
Scheme 1.

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M. Minato et al. / Journal of Organometallic Chemistry 689 (2004) 1025–1028
purification. All solvents were dried by standard meth-
ods and were stored under argon.
were carried out under essentially the same conditions.
The results are summarized in Table 1.
An equimolar amount of Cp2MoH2 (1) (0.186 g,
0.815 mmol) and unhydrated TsOH (0.139 g, 0.807
mmol) were loaded in a Schlenk tube. An excess of
homoallyl alcohol (2 ml) was vacuum-transferred into
the tube. The reaction mixture was cooled to )78 °C and
was degassed. The resultant solution was kept at 50 °C
with stirring for 6 h. As the reaction proceeded, the
color of the solution changed from yellow to dark-red.
After removal of the volatile components under vac-
uum, the residue was extracted with methanol. The ex-
tract was filtered, then concentrated in vacuo. The
resulting residue was dissolved in acetone. The red-
brown precipitates were formed on cooling the solution
in a refrigerator. The present precipitates (0.341 g)
contained the cyclic a-methyl- c-hydroxypropyl molyb-
denum complex (2) and the cationic g3-crotyl molyb-
denum complex (3) (see Table 1). Recrystallization from
THF/diethyl ether affords 2 as dark-red crystals. Isola-
tion of 3 was carried out following a procedure. After
removal of the volatile components from the reaction
mixture, the residue was washed with several portions of
ethanol and hexane to remove 2. High vacuum drying
afforded 3 as dark-red powders (30%).
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ꢀ
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3
ꢀ
ꢀ
ꢀ
b ¼ 7:760(5) A, c ¼ 13:77(2) A, V ¼ 2046(7) A , Z ¼ 4;
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ꢀ
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ꢀ
€
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