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be presented, with focus on stereoselective transformations. Also here, asymmetric C- H
functionalizations have not been developed to great extent, but the synthetic value of
the reactions will be highlighted with achiral examples.
4.2. ASYMMETRIC C - H ACTIVATION VIA METAL
CARBENOID INSERTION
4.2.1. Generation, Classifi cation, and Reactions of Metal Carbenoids
Metal carbenes are typically generated by metal-catalyzed nitrogen extrusion from diazo
compounds [49]. Alternative carbene precursors include iodonium, phosphonium, sul-
fonium, sulfoxonium, and thiophenium ylides, but their synthetic use is less developed
[70-78]. According to the currently accepted mechanism [79,80,81] for the generation
of carbenes via dirhodium(II) complexes (Scheme 4.1), in the presence of the appropri-
ate metal complex M, the diazo compound will coordinate reversibly to an open active
site and undergo rate-limiting extrusion of nitrogen to form the highly reactive interme-
diate metal carbenoid complex [49,80-83]. The high reactivity of the electrophilic car-
benoid complex will lead to immediate reaction with a suitable trapping agent present
in the reaction mixture or embedded in the diazoacetate structure (k
3
is large) [49,80].
Common reactions of metal carbenoids include cyclopropanation/propenation reactions
[6,7,45,48,49,55,84,85] , [3 + 2] cycloadditions [86 - 88] , C - H insertions [3,6,7,45,48,55] ,
and ylide formation [45,49,55]. C-H insertion processes will be discussed at length in
this chapter.
R
H
R'
N
2
k
1
k
-1
M
R
R'
k
3
N
2
M
H
R
R'
k
2
M
N
2
(g)
R
R'
Scheme 4.1.
Mechanism of carbenoid C - H insertion [79,80,81] .
The transient metal carbenoid intermediate has a large number of reaction pathways
available due to its reactive nature [6,7,46,48,49,52,54,56,61,85,89 - 92] . The development
of practical methods for C-H activation via such high-energy intermediates has there-
fore centered on achieving control over the broad spectrum of reactions that can take
place [3]. The intrinsic reactivity of the metal carbenoid intermediate is modulated by
