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Me
Me
Me
Me
Br
O
Benzene, r
t
Br
Pd
2
dba
3
Ph
2
P
Pd
NC
O
PPh
2
PPh
2
PPh
2
NC
trans
-(Xantphos)Pd(Ar)Br
Figure 2.33
Synthesis of trans-(Xantphos)Pd(Ar)Br.
Related observations of trans-chelating bisphosphines in cationic Pd com-
plexes have been made by Sato et al.
124
and Hartwig and co-workers
28
with
(dppf)Pd(PPh
3
)(BF
4
)
2
and (dtbpf)Pd(Ar)(BF
4
), respectively, both of which
feature a Pd-Fe interaction. It was proposed that a weak interaction between
Pd and the oxygen atom (2.697 Å) could be a contributing factor to the ex-
tremely large bite angle.
119
The trans-(Xantphos)Pd(Ar)Br complex also
proved to be catalytically competent in aryl amidation.
121
Like BINAP, the
use of Xantphos as a ligand has widespread applications in many C-N bond-
forming processes such as the synthesis of alkyldiarylamines,
125
arylation of
heteroarylamines,
126
regioselective aminations of polyhalopyridines,
127
amidation of enol triflates
128
and several others.
129
Another prominent ligand from the family of chelating bisphosphines
which was first synthesized by van Leeuwen's group is (oxydi-2,1-phenyle-
ne)bis(diphenylphosphine) (DPEPhos).
118
DPEPhos
130
is structurally similar
to Xantphos, but lacks only the bridge junction. They then studied the
Kumada-Tamao-Corriu coupling of sec-butylmagnesium chloride and bro-
mobenzene (the same reaction studied earlier by Hayashi et al. when they
observed the beneficial effect of dppf with respect to activity and selectivity
over b-hydride elimination
7
) and observed that the analogous reaction using
DPEPhos was significantly more active and selective (Figure 2.34).
131
A ratio
of 98 : 1 : 1 of branched coupling product (A):linear product (B):homo-
coupling (C) was observed with DPEPhos, whereas the Pd/dppf-catalyzed
reaction gave 95 : 2 : 3. The increased activity using DPEPhos was attributed
to its slightly larger bite angle relative to dppf (102.91 versus 99.11), hence it
was able to promote reductive elimination better. However, the corres-
ponding Pd/Xantphos-catalyzed reaction gave poor selectivity with only 24%
conversion, implying that a bite angle of
1001 is optimal and ligands with
smaller or larger bite angles gave diminished reactivity/selectivity.
The first use of DPEPhos in aryl amination was reported by Buchwald and
co-workers in 1998 in a study to identify a more cost-ecient alternative to
rac-BINAP, which was ''moderately expensive'' at that time.
132
The DPEPhos-
based catalyst system was demonstrated to be at least as active as the BINAP
systems, but significantly more active than Pd/dppf in the amination of aryl
bromides with anilines. Preliminary studies indicated that this system was
B
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