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PhSnBu
3
Pd(OAc)
2
(1 0 mol%)
Cu( OAc )
2,
LiOA c
DMF, 100°C, 24 h
O
O
Ph
64%
Scheme 9.156 Example of destannylative arylation.
SnB u
3
Pd(IPr)(OTs)
2
(6 mol%)
Cu(OTf)
2,
O
2,
MS 3A
DMA, r.t., 72 h
+
X
OTBS
X
+
OTBS
OTBS
X
X
X
OMe
Cl
13%
36%
82%
46%
Scheme 9.157 Destannylative arylation versus 1,1-diarylation.
co-workers,
428
which led to discovery of interesting trends seeming highly
relevant in the more common context of Heck chemistry.
The reaction, performed with a cationic palladium complex with a het-
erocyclic carbene ligand, led, in addition to the expected Heck product, to
1,1-diarylation (Scheme 9.157).
428
The distribution of products depended on
the substituent in the organotin compound - electronegative substituents
favoured the Heck pathway, whereas electron-rich organotin compounds
gave mostly 1,1-diarylation, which in these cases can be regarded as a se-
lective reaction.
The results are extremely intriguing and reveal important aspects of the
internal functioning of Heck pathways. First, the formation of diarylation
products is an evident outcome of the Heck-Stille cascade, with reactive
organotins being able to trap carbopalladation intermediates. In debor-
ylative reactions, such cascades do not occur because the activation of
organoboron compounds to transmetallation requires the presence of base,
which usually is not added to reaction mixtures in oxidative deborylative
reactions.
In Sigman and co-workers' process, the primary carbopalladation adduct
is not trapped to a significant extent, but instead undergoes PdH elimination
giving either the monoarylation product or the isomeric adduct, which is
what is captured by the second molecule of the organotin compound
(Scheme 9.158).
Such ''strange'' behaviour is likely to be accounted for by the kinetics of
the stages. Transmetallation with organotins should be slow, particularly
with electron-deficient or sterically hindered compounds. This gives a
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