Angewandte
Chemie
Springer,Heidelberg, 1999,pp. 833 – 886; c) B. M. Trost,D. L.
Van Vranken, Chem. Rev. 1996, 96,395 – 422.
[2] C. Markert,A. Pfaltz, Angew. Chem. 2004, 116,2552 – 2554;
Angew. Chem. Int. Ed. 2004, 43,2498 – 2500.
[3] P. Chen, Angew. Chem. 2003, 115,2938 – 2954; Angew. Chem.
Int. Ed. 2003, 42,2832 – 2847.
reactions. Evidence for the formation of dinuclear complexes
of this type was previously obtained from in situ X-ray
absorption fine structure (XAFS) studies,but no detailed
structural assignments could be made.[4d] Trinuclear com-
plexes and higher aggregates were also detected in that study,
and it was proposed that the dinuclear complexes undergo
association with additional Pd0 complexes to form polynuc-
lear clusters and eventually metallic Pd,resulting in catalyst
deactivation. Under the conditions used in this study,the
observed dinuclear complexes proved to be remarkably stable
and did not show any tendency to form higher aggregates.
Thus,these complexes represent a reservoir,from which
catalytically active mononuclear complexes can be released
under the reaction conditions. In the presence of excess
[4] a) H. Werner, Adv. Organomet Chem. 1981, 19,155 – 182; b) H.
Kurosawa, J. Organomet. Chem. 2004, 689,4511 – 4520; c) T.
Murahashi,H. Kurosawa, Coord. Chem. Rev. 2002, 231,207 –
228,and references therein; d) M. Tromp,J. R. A. Sietsma,S. A.
van Bokhoven,G. P. F. van Strijdonck,R. J. van Haaren,A. M. J.
van der Eerden,P. W. N. M. van Leeuwen,D. C. Koningsberger,
Chem. Commun. 2003,128 – 129.
[5] The unsaturated Pd0 complex 3 is most likely stabilized by a
weakly coordinated olefin. The structure of complex 7 is still
unknown. Formula 7 is based on ESI-MS data and analogy to
complex 6.
II
substrate,mononuclear allyl–Pd complexes are the domi-
[6] Samples for ESI-MS analysis were diluted with CH2Cl2 and
injected into a mass spectrometer without further purification. A
second aliquot of the reaction mixture was filtered through silica
gel and analyzed by HPLC to determine the conversion.
nating species. However,when the concentration of substrate
diminishes toward the end of the reaction,addition of the Pd 0
catalyst to the allyl–PdII intermediate becomes a competing
process and,consequently,the concentration of the resulting
dinuclear complex increases. The nature of the unusual
reaction with CH2Cl2 and the structure of the resulting
dinuclear complex 7 are still unclear and deserve further
investigation.
[7] In the 31P NMR spectra,the two diastereotopic phosphorus
3
atoms couple with each other (6a: 3JPP = 150 Hz; 6b: JPP
=
129 Hz). 31P signals of 6b were previously observed by G.
Helmchen and co-workers. However,no unambiguous structural
assignment could be made at that time: H. Steinhagen,M.
Reggelin,G. Helmchen, Angew. Chem. 1997, 109,2199 – 2202;
Angew. Chem. Int. Ed. Engl. 1997, 36,2108 – 2110.
Received: March 21,2007
Published online: June 14,2007
[8] CCDC-634865 (6b) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
Keywords: allylic substitution · dinuclear complexes ·
.
homogeneous catalysis · mass spectrometry · palladium
[9] The syn notation refers to a cis relationship between the terminal
À
substituent and the central C H bond.
[10] Such a stepwise mechanism would resemble the allyl transfer
mechanism proposed by Bosnich and co-workers: P. B. Mack-
enzie,J. Whelan,B. Bosnich, J. Am. Chem. Soc. 1985, 107,2046 –
2054; see also references [4b,c].
[1] Reviews: a) B. M. Trost,C. Lee in Catalytic Asymmetric Syn-
thesis,2nd ed. (Ed.: I. Ojima),Wiley-VCH,New York,
2000,
pp. 593 – 649; b) A. Pfaltz,M. Lautens in Comprehensive Asym-
metric Catalysis (Eds.: E. N. Jacobsen,A. Pfaltz,H. Yamamoto),
Angew. Chem. Int. Ed. 2007, 46, 5892 –5895
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5895