Molybdenum-catalyzed hydrostannations of allenylcarbinols

Article abstract of DOI:10.1039/b413025a

Allenylcarbinols undergo regioselective hydrostannation in the presence of MoBl₃, a catalyst which was developed for the hydrostannation of propargyl alcohols and derivatives; allylstannanes are formed preferentially, which can easily be conver

Full text of DOI:10.1039/b413025a

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Molybdenum-catalyzed hydrostannations of allenylcarbinols{
Uli Kazmaier* and Manuela Klein
Received (in Cambridge, UK) 24th August 2004, Accepted 21st October 2004
First published as an Advance Article on the web 2nd December 2004
DOI: 10.1039/b413025a
Allenylcarbinols undergo regioselective hydrostannation in the
presence of MoBl3, a catalyst which was developed for the
hydrostannation of propargyl alcohols and derivatives; allyl-
stannanes are formed preferentially, which can easily be
Scheme 2 Molybdenum-catalyzed hydrostannation of alkynes.
converted into allyl iodides.
Herein, we now report on our investigations of MoBl3-catalyzed
During the last few years functionalized allenes have become more
hydrostannations of allenes. We chose allenylcarbinols 2 as
and more interesting not only as targets in natural product
substrates which could easily be obtained from the corresponding
synthesis, but also as valuable synthetic precursors for the synthesis
propargylic alcohols 1 according to Burgess and Jennings14
of complex molecules.1 In this context, carbon–carbon and
(Scheme 3).
carbon–heteroatom bond formations are of major interest,
Those allenylcarbinols were subjected to the reaction conditions
triggering the development of preparatively useful methods such
optimized for the hydrostannation of alkynes (THF, D) and the
as additions2 or palladium-catalyzed transformations.3 Besides
process was monitored by TLC. In general, the reactions were
cyclizations4 especially transition metal-catalyzed hydrometalla-
completed after 4 h, and therefore allenylcarbinols 2 are
tions such as hydroborations,5 hydrozirconations6 or hydrostan-
significantly more reactive than the corresponding propargyl
nations7 are synthetically useful transformations. Vinyl or allyl
derivatives 1. The product ratio was determined by NMR.
organometallics are obtained depending on the regioselectivity of
Overall, four different products were obtained, but products of
the hydrometallation step. In principle (Scheme 1), four different
type B and D were never observed. The results obtained are
products (A–D) are possible, and the product distribution depends
summarised in Table 1.
on the reaction conditions used.
First we investigated the hydrostannation of the phenylethyl-
While in Lewis acid-catalyzed reactions of terminal allenes
substituted allenylcarbinol 2a. Addition occurred exclusively at the
attack occurs at the internal double bond providing vinylstannanes
terminal double bond and only to give the allylstannanes 3a, which
B,8 in the presence of most palladium complexes the terminal
were obtained as an E/Z mixture in the ratio 1 : 2. To determine if
double bond reacts preferentially giving rise to allylstannanes C as
an E/Z mixture.9 Interestingly, the regioselectivity for the
hydrostannation of the terminal double bond can be converted
towards vinylstannanes D by switching from Pd(PPh3)4 to
Pd(OH)2/C.10 Hydrostannations of allenes bearing suitable func-
tional groups such as aryl halides11 or further double bonds12
Scheme 3 Preparation of allenylcarbinols.
allow subsequent cyclizations.
Recently, we developed a new molybdenum catalyst for
Table 1 Hydrostannation of allenylcarbinols 2
hydrostannations of alkynes. MoBl3 (Mo(CO)3(CNtBu)3) was
found to be an excellent catalyst for regioselective hydrostanna-
tions towards the internal vinylstannanes, being superior to the
generally used palladium catalysts (Scheme 2).13
Temp Yield
[%]
Allene
R
[uC]
Ratio (E)-3 : (Z)-3 : 4 : 5
2a
2a
2b
2c
2d
2e
2e
a
PhCH2CH2
PhCH2CH2
2-NO2Ph
4-ClPh
2,6-Cl2Ph
55
rt
68
74
76
68
74
86
23a
1 : 2 : 0 : 0
1 : 2 : 0 : 0
12 : 12 : 1 : 0
1 : 1 : 0 : 0
1 : 2 : 0 : 0
1 : 0 : 1 : 2
10 : 0 : 6 : 1
55
55
55
55
rt
Scheme 1 Hydrostannation of allenes.
2,4,6-(MeO)3Ph
2,4,6-(MeO)3Ph
{ Dedicated to Prof. Dr M. Veith on the occasion of his 60th birthday.
*u.kazmaier@mx.uni-saarland.de
Only 1% MoBl3, 2 equiv. Bu3SnH used.
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this ratio depends on the reaction conditions, e.g. on the
temperature, we carried out the reaction also at room temperature.
Under these milder conditions the yield was higher, but no
influence on the product ratio was observed.
To figure out if electronic factors in the substituent R might
have an influence on the product formation, we investigated
several aromatic allenylcarbinols bearing electron-withdrawing
and -donating groups. In the hydrostannation of o-nitrophenyl
derivative 2b the allylstannanes 3b were by far the major products,
the regioisomer 4b was formed in trace amounts. Similar results
were obtained in the reaction of p-chlorophenyl- and 2,6-
dichlorophenyl-substituted substrates (2c, 2d), but the situation
changed dramatically with electron-rich aromatic systems such as
the 2,4,6-trimethoxyphenyl derivative 2e. In this case the elimina-
tion product 5e was obtained preferentially, besides allylstannane
(E)-3e and vinylstannane 4e. Interestingly no (Z)-configured
product was obtained in this case, and we assume that this isomer
undergoes elimination according to Fig. 1.
Obviously the strong electron-donating groups facilitate the
cleavage of the OH group, probably via stabilization of the
carbenium ion intermediate. This would explain the high ratio of
elimination product obtained under the standard conditions. To
figure out if this proposal is reasonable, we investigated the
reaction also under milder conditions. And indeed, the amount of
elimination product could be reduced significantly.
Scheme 4 Proposed mechanism for the hydrostannation of allenes.
The high selectivities towards the allyl and vinyl stannanes 3 and
4 can be explained by the following mechanistic rationale
(Scheme 4). Probably in the first step some of the isonitrile ligands
dissociate from the molybdenum opening free coordination sides
for the oxidative addition of the tin hydride and coordination of
the allene. Depending on the substrate used, only the terminal or
both double bonds coordinate to the molybdenum giving rise to
intermediates A and B. Subsequent hydrometallation should
provide intermediates A9 and B9 with the metal fragment added
to the sterically least hindered position. The products 3 and 4 were
formed from these intermediates via reductive elimination. The
E/Z isomers were formed by coordination to the two diaster-
eotopic faces of the terminal double bond.
Scheme 5 Metal–halogen exchange of allylstannane 3.
most stable product is formed, which is an interesting substrate for
further synthetic applications.
In conclusion we have shown that molybdenum-catalyzed
hydrostannation of allenes proceeds regioselectively towards the
sterically least hindered stannylated product. Further research
concerning the selectivity and the reaction mechanism is currently
in progress.
Probably this mechanism is slightly different from the one
proposed for the hydrostannation of alkynes which seems to start
with a transfer of the stannyl group and a subsequent hydrogen
transfer in the reductive elimination step. But both possible
mechanisms were also discussed for palladium-catalyzed hydro-
stannations. Therefore, it is reasonable that the situation with the
molybdenum complexes is similar.
Financial support by the Deutsche Forschungsgemeinschaft as
well as the Fonds der Chemischen Industrie is gratefully
acknowledged.
Uli Kazmaier* and Manuela Klein
Universita¨t des Saarlandes, Institut fu¨r Organische Chemie, D-66123,
Saarbru¨cken, Germany. E-mail: u.kazmaier@mx.uni-saarland.de;
Fax: 49 681 3022409; Tel: 49 681 3023409
To improve the synthetic potential of this protocol we subjected
the mixture of (E/Z)-3b to a metal–halogen exchange (Scheme 5).
Although the isomeric mixture was used, the corresponding allyl
iodide (E)-6 was obtained as a single isomer in high yield.
Obviously under these conditions only the thermodynamically
Notes and references
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Fig. 1 Elimination of stannylated allyl alcohols.
502 | Chem. Commun., 2005, 501–503
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