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Expensive transition metal complexes derived from ruthenium and irid-
ium were the most ecient catalysts for these transformations, which do not
require the addition of molecular oxygen as an oxidant. Searching for a more
practical catalytic system, Beller and co-workers investigated copper com-
plexes for the alkylation of sulfonamides with alcohols. They found that
copper(
II
) acetate catalyzed such a reaction and produced the alkylated sul-
fonamide in excellent yields (Scheme 7.21).
48
Importantly, the presence of an air atmosphere was beneficial whereas an
inert or a higher oxygen content atmosphere inhibited the reaction. Based
on mechanistic studies, the authors proposed the mechanism shown in
Scheme 7.22, similar to that previously described (see Scheme 7.20). HRMS
analysis revealed in situ formation of bissulfonylated amidine 76, which
played the role of a stabilizing ligand for the copper catalyst (Figure 7.2). The
interaction between ligand 76 and copper was observed by UV-visible
measurements. Labeling experiments with benzyl-d
7
alcohol showed that the
dehydrogenation-imide formation-hydrogenation sequence was a reversible
process. Furthermore, the C-H bond cleavage (i.e. dehydrogenation step),
facilitated by molecular oxygen, is the rate-determining step.
O
O
S
R
1
OH
R
1
R
2
H
[Cu]
O
2
[Cu-H]
O
O
R
1
O
S
R
1
R
2
O
O
N
S
R
2
NH
2
Scheme 7.22 Proposed mechanism of the Cu-catalyzed sulfonamide alkylation of
alcohols.
O
O
S
S
N
H
O
O
bissulfonylated amidine
76
Figure 7.2 Copper-stabilizing ligand in catalytic alkylation of sulfonamides.
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