Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 446

explanation may involve the activation of boronic acid by the basic RNMe 2
group, although in this hypothesis no six-membered rings can be drawn in
the intermediates. An increase in the length of the pendant moiety or a
change to a bulkier NEt 2 group or to secondary NHMe ruins the diarylation.
Another interesting feature of these reactions was manifested in the case
of Michael acceptors used as olefins is the competition between the normal
Heck pathway and the conjugate addition pathway. This is particularly im-
portant for cyclic enones, the primary products of the carbopalladation of
which cannot eliminate PdH directly, but are engaged in metallotropic re-
arrangements leading to either of two products. This behaviour is in fact well
known in regular Mizoroki-Heck chemistry (for a discussion, see Ref. 87).
Lee and co-workers showed that in the arylation of cyclohexenones and
similar enones by arylboroxines, the outcome depends dramatically on the
conditions used, and pathway can be induced as needed by changing the
conditions (Scheme 9.150). 409
Stahl and co-workers developed this approach further by performing a
one-pot arylation of cyclohexenones with subsequent isomerization induced
by the addition of DMSO to the reaction mixtures to afford 3-arylated phe-
nols (Scheme 9.151). 410
This mechanistic dichotomy in the reactions of organoboron compounds
with enones and other Michael acceptors was observed also with catalysis by
Ni, Co 411 and Rh 412,413 complexes.
An interesting reaction formally looking like an oxidative deborylative
Heck reaction was described by Liwosz and Chemler. 414 Alkylation of al-
kenes by alkyltrifluoroborates was achieved in the presence of copper(II)
triflate (Scheme 9.152). Both the unusual substrate profile and mechanistic
studies, however, revealed a free-radical pathway without the involvement of
organometallic species.
Deborylative Heck reactions have been applied to a variety of interesting
methodologies, e.g. for the synthesis of 4-arylated coumarins, 415 selective

arylation of allylamines 416 and allylic ethers 417

and cascaded arylation-

oxypalladation to afford 2-aryltetrahydrofurans from homoallylic

alcohols. 418

9.3.3 Desilylative Heck Reaction
In cross-coupling chemistry, organosilicon compounds belong to the ''Big
Five'' major organometallics used as transmetallation agents (B, Sn, Zn, Mg,
Si). In oxidative Heck reactions, so far organoboron compounds are beyond
competition, but there are some data on Sn and Si compounds showing that
the potential exists, but is largely unexploited.
Organosilicon compounds are generally less reactive than organoboron
compounds and require more energetic activation, usually by fluoride-ions
(the Hiyama cross-coupling reaction), as compared with common bases
used in the Suzuki-Miyaura reaction. Concerning the oxidative Heck re-
action, where the transmetallation is believed to be effected by more reactive