文档介绍：R1
Pd +
R2
Cyclopropanation and Other
Reactions of Palladium-Carbene
(and Carbyne) Complexes
OLIVER REISER
A. INTRODUCTION
The cyclopropanation of alkenes via carbene transfer from pounds can be
achieved by a variety of transition metals. The most popular catalysts for this transforma-
tion are based on copper(I) and rhodium(II) complexes, however, palladium(II) has been
shown, as demonstrated first by Kirmse and Kapps,[1],[2] to be effective and even superior
for certain substrates and reagents.[3],[4]
The general mechanistic picture for these carbene transfer reactions starts with the initial
position of the pound 3 upon the influence of the metal (Scheme 1).
Although not conclusively proven, there is strong evidence that metal plexes 1
are formed, most prominently with copper and rhodium. These intermediates are attacked at
the electrophilic carbene center by the alkene 4 to form 2, which subsequently collapses to
the cyclopropane 5. However, there are distinctive differences in the range of reagents and
substrates being employed in copper and rhodium catalyzed carbene transfer reactions
compared to palladium. Due to the strong coordinating power of palladium(II) towards
alkenes, electrophilic addition of the metal plex 6 onto diazomethane to 7
followed by extrusion of nitrogen via metalocyclobutanes 8 or direct cyclopropane forma-
tion might be a plausible alternative.[5] Moreover, it was suggested that the reaction might
take place by a 1,3-dipolar cycloaddition with palladium(II) acting as a Lewis acid followed
by nitrogen extrusion from the pyrazoline 9. Indeed, 1,3-dipolar cycloaddition readily takes
[6]
place with alkenes bearing strong electron withdrawing substituents (CN, CO2Et), how-
ever, the overall mechanistic proposal for cyclopropane formation via this pathway has been
distinctively ruled out at least for some cases.[7]
X R
Y
NN
9
Formula 1
Handbook anopalladium Chemistry anic Synthesis, Edited by Ei-