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
1 (a) Allenes in Organic Synthesis, ed. H. F. Schuster, G. M. Copolla,
Wiley, New York, 1984; (b) Modern Allene Chemistry, ed. N. Krause,
A. S. K. Hashmi, Wiley-VCH, Weinheim, 2004.
2 Reviews: (a) R. Zimmer, Synthesis, 1993, 165; (b) T. G. Back,
Tetrahedron, 2001, 57, 5263.
3 Review: R. Zimmer, C. U. Dinesh, E. Nandanan and F. A. Khan,
Chem. Rev., 2000, 100, 3067.
4 Reviews: (a) R. W. Bates and V. Satchavoen, Chem. Soc. Rev., 2002, 31,
12; (b) N. Krause, A. Hoffman-Ro¨der and J. Canisius, Synthesis, 2002,
1759.
5 Y. Yamamoto, R. Fijikawa, A. Yamada and N. Miyaura, Chem. Lett.,
1999, 1069.
Fig. 1 Elimination of stannylated allyl alcohols.
502 | Chem. Commun., 2005, 501–503
This journal is ß The Royal Society of Chemistry 2005