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R
R
NN
H
H
R
NN
Ar
Ar
R=iPr ,
89a
R=Me,
89b
R=H,
89c
Cl
Pd Br
N
Pd
Cl
Ar = 2,4,6-trimethylphenyl
88a
Ar = 2,6-diisopropylphenyl
88b
Cl
Figure 4.39
Structures of some precatalysts active in the amination reaction.
promote the coupling. The throwaway ligand also played a crucial role, as
the system needs to be rapidly activated.
Other groups also developed new NHC ligands for the amination reaction.
For example, Dastgir et al. prepared two [Pd(BIAN-SIAr)(allyl)Cl] complexes
(SIAr = SIMes and SIPr, 88, Figure 4.39).
178
Both complexes were active in the
reaction using reasonably low catalyst loadings (1 mol%) at room tempera-
ture, for the coupling of various deactivated and activated aryl chlorides and
bromides. Notably, sterically hindered bromomesitylene was found to be a
more dicult substrate. Tu and co-workers reported the synthesis of ben-
zimidazolylidene PEPPSI-type complexes (89, Figure 4.39).
179
Among three
new congeners, 89a was the most active in amination. In these structure, the
metal centre is less congested and the s-donation is modified by the ben-
zimidazole ring. The catalytic activity of 89a was found to be good but not
exceptional.
4.3.6.1.2 Development of New Well-Defined Precatalysts Bearing New
Throwaway Ligands. Jin et al. reported the preparation of well-defined
air- and-moisture stable [Pd(NHC)(sal)Cl] (sal = salicylaldimine) com-
plexes.
180
The influence of the steric and electronic parameters of the sali-
cylaldimine ligands was studied in the reaction between 4-chlorotoluene
and morpholine, using 1 mol% of the precatalyst and NaOtBu as the base
in DME at 80 1C. Sterically encumbered aryl groups on the salicylaldimine
nitrogen or a methyl group in the ortho-position of the salicylaldimine oxy-
gen were not found to be beneficial in the reaction. From an electronic
point of view, electron-rich N-aryl groups were found to have a negative ef-
fect on the catalytic activity, whereas electron-deficient groups exhibited a
greater effect. Consequently, 90 was identified as the most ecient pre-
catalyst to couple a range of deactivated, activated and sterically hindered
aryl chlorides with various aromatic and aliphatic primary and secondary
amines (Scheme 4.65).
Interestingly,
the reaction could be conducted
under aerobic conditions.
Nolan and co-workers investigated the modification of the structure of the
acac ligand in complexes of formula [Pd(IPr)(R-acac)Cl] (91-94,
Figure 4.40),
181
previously developed by the same group.
160a
This work was
carried out in analogy with the [Pd(NHC)(R-allyl)Cl] complexes, where they
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