Regioselective hydrostannations catalyzed by molybdenum isonitrile complexes

Article abstract of DOI:10.1016/S0022-328X(01)01309-2

Hydrostannations using Mo(CO)₃(CN^tBu)₃ give rise to α-stannylated alcohols and ethers in a highly regioselective fashion. The yields and selectivities obtained with this catalyst are significantly higher in comparison to t

Full text of DOI:10.1016/S0022-328X(01)01309-2

Journal of Organometallic Chemistry 641 (2002) 26–29
www.elsevier.com/locate/jorganchem
Regioselective hydrostannations catalyzed by molybdenum
isonitrile complexes
Sascha Braune, Uli Kazmaier *
Organisch-chemisches Institut der Uni6ersita¨t Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
Received 30 April 2001; accepted 10 August 2001
Abstract
Hydrostannations using Mo(CO)3(CNꢀtBu)3 give rise to a-stannylated alcohols and ethers in a highly regioselective fashion.
The yields and selectivities obtained with this catalyst are significantly higher in comparison to the results obtained with the
commonly used palladium and rhodium catalysts. The influence of the isonitrile incorporated into the molybdenum complex is
investigated. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Alkynes; Hydrostannation; Isonitriles; Molybdenum
used radical hydrostannation is only of limited value,
because the radical pathway results in a mixture of
regio- and stereoisomers [1], which are difficult to sepa-
rate. Better results are obtained in transition metal
catalyzed reactions. In palladium- [8] and rhodium-cat-
alyzed [9] reactions the addition of the tin hydride
occurs in a highly stereoselective fashion as a clean syn
addition, as a result of the reaction mechanism [2].
Very recently, we reported a new hydrostannation
catalyst based on molybdenum [10]. Herein we describe
the influence of isonitrile ligands on the regioselectivity
of the hydrostannation step.
1. Introduction
Metal catalyzed reactions are becoming more and
more important for organic synthesis, especially be-
cause of the mild reaction conditions under which
many of these reactions can be carried out [1]. Most of
them also tolerate a wide range of functional groups
and are, therefore, powerful tools for the synthesis of
complex molecules. Palladium plays a dominant role
under the metals used. Allylic alkylations, cyclizations
and cross coupling reactions are working horses for the
synthetic chemist [2]. During the last years, other metals
such as tungsten [3] and molybdenum [4] gained an
increased interest. For example, Mo(CO2)(CNꢀtBu)4
was shown to be also a good catalyst for allylic alkyla-
tions [5]. Hydrometallation reactions, such as hy-
drostannations are versatile tools for the synthesis of
interesting organometallic compounds. Vinylstannanes
obtained by hydrostannations of alkynes [6] are impor-
tant substrates for palladium-catalyzed cross coupling
reactions (Stille coupling) [7].
2. Results and discussion
For our early investigations we chose Mo(CO)6 and
Mo(CO)3(CNꢀtBu)3 (MoBI3) as catalysts. We found
that Mo(CO)6 in principle can be used for hydrostanna-
tions, but the yields and selectivities obtained were
moderate. Replacing three CO ligands by isoelectronic
isonitrile ligands [11] resulted in a significant increase in
Therefore, several approaches have been developed
for the synthesis of these stannanes. The commonly
t
the yield and the selectivity as well. Butylisonitrile was
chosen for steric reasons, with the expectation that the
sterically demanding butyl groups may have an influ-
ence on the regioselective outcome of the reaction.
Isonitriles also have the advantage to stay in solution
after dissociation from the metal, resulting in a pro-
* Corresponding author. Present address: Fachrichtung 8.12 Or-
ganische Chemie, Universita¨t des Saarlandes, Im Stadtwald, 66041
Saarbru¨cken, Germany. Tel.: +49-681-302-3409; fax: +49-681-302-
2409.
t
E-mail address: u.kazmaier@mx.uni-saarland.de (U. Kazmaier).
0022-328X/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)01309-2

S. Braune, U. Kazmaier / Journal of Organometallic Chemistry 641 (2002) 26–29
27
longed lifetime of the catalyst. Because of a weaker
p-back donation from the metal to the isonitrile, in
comparison to CO, the isonitriles are weaker bound in
the complex. Therefore, they can dissociate easily,
opening up free coordination sites on the catalytically
active metal. Based on these reasons one might expect a
strong influence of the number as well as of the nature
of the isonitrile ligands. To investigate this influence, we
chose THP-protected propargylic alcohol (Eq. (1)) as a
substrate, because the vinylstannanes obtained are sta-
ble and can easily be purified without side reactions
such as protodestannations [10].
Hydroquinone was added to the reaction mixture to
suppress the radical pathway which can also occur at a
raised temperature. The radical reaction should deterio-
rate the regioselectivity and should result in the addi-
tional formation of the (Z)-b-product. This
(Z)-product was not found in any of our hydrostanna-
tion reactions. Therefore, we can be sure that the
radical pathway is suppressed under our conditions,
and the regioselectivities observed are caused from the
catalyzed process.
The results obtained with several isonitrile complexes
are summarized in Table 1.
(1)
Subsequent substitution of CO ligands by the isoni-
trile (Table 1, entries 1–5) resulted in a dramatic in-
crease in the yield and the selectivity as well. The best
result was obtained with Mo(CO)3(CNꢀtBu)3 (MoBI3),
an additional isonitrile ligand had no positive effect on
the reaction. In contrast, the yield was somewhat lower,
although this decrease is not significant. The lower yield
might result from the lower stability of Mo(CO)2-
(CNꢀtBu)4 (MoBI4) in comparison to MoBI3. In con-
trast to the latter complex, which is perfectly stable at
room temperature even under air, MoBI4 decomposes
during storage even under argon at −18 °C. Obvi-
ously, three isonitrile ligands are the optimum, not only
for the yield and selectivity but also for the stability of
the catalyst.
Table 1
Catalytic hydrostannations of the THP-propargylether
To investigate the sterical and electronical influence
of the isonitrile, we synthesized Mo(CO3)(CNPh)3
(MoPI3) and the more electron rich Mo(CO3)-
(CNꢀpPhꢀOBu)3 (MoBPI3) according to a procedure
described by Albers et al. [11]. The isonitrile required
for the latter complex was obtained in three steps
starting from p-hydroxyaniline (Scheme 1).
Entry
Catalyst
Yield (%)
Selectivity a:b
1
2
Mo(CO)6
35
64
28:72
62:38
Mo(CO)5(CNꢀtBu)
(MoBI1)
3
4
5
Mo(CO)4(CNꢀtBu)2
(MoBI2)
89
91
85
87:13
Mo(CO)3(CNꢀtBu)3
(MoBI3)
\95:B5
\95:B5
The influence of these three complexes on the hy-
drostannation of various propargylic ethers was investi-
gated and the results are collected in Table 2. The
Mo(CO)2(CNꢀtBu)4
(MoBI4)
t
sterical bulk of the butyl groups in MoBI3 obviously
has a significant effect on the regioselectivity. The re-
lated phenyl derivative gave comparable yields, al-
though the selectivities are lower in all cases.
Introduction of the electron donating alkoxy groups on
the aromatic ring system significantly increased the
regioselectivity and one can assume, that not only steric
reasons are responsible for the selectivities obtained,
but also electronic factors. Further investigations are in
progress.
3. Experimental
3.1. Synthesis of Mo(CO3)(CNꢀpPhꢀOBu)3 (MoBPI3)
p-Butoxyphenylisonitrile (210 mg, 1.20 mmol) in tol-
uene (1 ml) was added slowly to a suspension of
CoCl2·2H2O (11 mg, 66 mmol) and Mo(CO)6 (98 mg,
0.37 mmol) in toluene (2 ml) at 100 °C. The mixture
Scheme 1. Synthesis of the catalyst Mo(CO)3(CNꢀpPhꢀOBu)3
(MoBPI3): (i) HC(O)OC(O)CH3, CH2Cl2, 90%; (ii) KOtBu, nBuBr,
DMF, 65%; (iii) POCl3, NEt3, CH2Cl2, 98%; (iv) Mo(CO)6,
CoCl2·2H2O, Toluol, 69%.

28
S. Braune, U. Kazmaier / Journal of Organometallic Chemistry 641 (2002) 26–29
Table 2
Hydrostannations of substituted alkynes
meinschaft (SFB 247) and the Fonds der Chemischen
Industrie is gratefully acknowledged.
was refluxed for 1 h before the resulting black suspen-
sion was evaporated in vacuo. The residue obtained
was purified by flash chromatography using CH2Cl2 as
an eluent. Yield: 180 mg (0.255 mmol, 69%) of a green
1
3
oil. H-NMR (300 MHz, CDCl3): l=0.96 (t, J=7.4
References
3
Hz, CH3), 1.47 (sext, J=7.4 Hz, CH2ꢀCH3), 1.76 (q,
3J=6.6 Hz, OꢀCH2ꢀCH2), 3.95 (t, 3J=6.5 Hz,
OꢀCH2), 6.85 (d, 3J=8.9 Hz, CHꢀCOBu), 7.27 (d,
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Financial support by the Deutsche Forschungsge-
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