and characterized an unstable tris(pyrazolyl)borate analogue
by generating more reactive cationic intermediates.[3b,4c] This
salt metathesis results in a vacant coordination site on the Pd
center. In accordance with these studies, 3a was completely
transformed into 4 and 5 when treated with AgBF4 and
AgOTf, respectively, at 1208C for 6 hours (Scheme 1).
Product 6 was formed in trace amounts, but there was no
by 1H NMR spectroscopy at À408C.[13b] To our knowledge, we
have isolated and fully characterized the first PdIV complexes
[1,14,15]
À
containing a Pd SO2R bond.
We discovered that both aryl and alkyl sulfonyl chlorides
oxidize complex 1 to generate palladium(IV) sulfinate com-
plexes 3 (Table 1). Both electron-donating (methoxy) and
electron-withdrawing (fluoro) substituents on the aryl sulfo-
nyl chloride are tolerated (Table 1, entries 2 and 3). Oxidation
of 1 with benzylsulfonyl chloride (2d) results in the corre-
sponding palladium(IV) alkylsulfinate complex 3d, albeit in
low yield (Table 1, entry 4).[16] In contrast, the use of
trifluoromethanesulfonyl chloride (2e) provides complex 3e
in 97% yield in less than one minute at room temperature
(Table 1, entry 5).
Scheme 1. Reductive elimination of 3a upon addition of Ag salts.
Tf =trifluoromethanesulfonyl.
Next, we studied the transformation of complex 3a at
elevated temperatures in various solvents. We anticipated
that decomposition of complex 3a could result in various
products including sulfone 4, ligand dimer 5, chloride 6, and/
product 7. In each case, 4 was formed in more than 80% yield.
À
À
À
À
or biphenyl 7. These products result from C S, C C, C Cl,
This result suggests that the formation of the C SO2 bond
and/or desulfitative C C bond formation, respectively. Heat-
may occur from a PdIV species via a five-coordinate cationic
PdIV intermediate. Next, we examined the oxidation of
complex 9 bearing a benzoquinoline ligand with 3 equivalents
of 2a at room temperature (Scheme 2). Complex 10 was
À
ing complex 3a results in the formation of all four products
(Table 2). These results represent the first examples of
Table 2: Reductive elimination study of complex 3a.[a]
Scheme 2. Formation and reductive elimination of benzoquinoline-
ligated palladium(IV) sulfinate complex 10. Ar=4-MeC6H4.
Entry
Solvent
4 [%]
5 [%]
6 [%]
7 [%]
1
DCE
DCE
AcOH
pyridine
DMSO
52
71
58
64
58
19
14
7
17
30
8
9
2
10
5
1
1
0
<1
<1
2[b]
3
produced in 80% yield. Sulfone 11 was obtained in 71% yield
when complex 10 was treated with 1 equivalent of AgBF4.
Crossover products were observed in less than 5% yield
(based on gas chromatographic analysis) when complex 10
and 3b were heated together. These results further support
reductive elimination directly from PdIV as the major mech-
anistic pathway.
4
5
[a] Reductive elimination reactions were carried out at 1208C for 6 h.
Yields were determined by gas chromatography based on an average of
2–3 runs, using dodecane as an internal standard. [b] 18 h.
À
À
C SO2R bond formation and C C bond formation (with
While these studies were inspired by our original sulfo-
nylation reaction,[10] they are not direct mechanistic studies of
the catalytic process. Indeed, the use of complex 3a as a
loss of SO2) from a PdIV species. Our observations reveal that
À
the formation of the C SO2R bond is favored over the
À
À
À
formation of either C C or C Cl bonds. In 1,2-dichloro-
ethane (DCE), sulfone 4 was observed as the major product
(52%, 6 h; 71%, 18 h; Table 2, entries 1 and 2). In both acidic
solvent (AcOH) and basic media (pyridine) we observed a
catalyst for the ligand-assisted sulfonylation of C H bonds
results in less than two turnovers. Further kinetic and
mechanistic studies are necessary to establish relevance to
catalysis, but these results support the feasibility of palladium-
À
À
À
preference for C S bond formation, while desulfitative C C
bond formation was suppressed (Table 2, entries 3 and 4).
Complex 3a underwent reductive elimination efficiently in
DMSO to yield 4 as the major product (58%; Table 2,
entry 5). Complexes 3b and 3c undergo reductive elimination
to give comparable results to 3a, while complex 3e decom-
poses to give dimerization of the ligand.[17]
catalyzed sulfonylation and desulfitative C C cross-coupling
reactions through a PdII/PdIV catalytic cycle.
To conclude, sulfonyl chlorides can oxidize PdII to PdIV to
generate stable palladium(IV) sulfinate complexes. These
novel complexes undergo reductive elimination to give
products with C S, C C, and C Cl bonds. This fundamental
contribution to organopalladium(IV) chemistry will aid
future studies on carbon–sulfur bond formation and bond
activation.
À
À
À
Elsevier and Sanford have shown that the addition of Ag
salts (such as AgBF4 and AgOTf) aids reductive elimination
Angew. Chem. Int. Ed. 2011, 50, 932 –934
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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