Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 424

OMe CN

P d(OAc) 2 (10 mol% )
O 2, HPMo 11 V, acacH
EtCOOH, 90°C, 5 h

+

CN

MeO

63% (o:m:p = 13:23:64)

Scheme 9.104 Oxidative arylation by simple arenes in the presence of substoichio-
metric amounts of Pd salts.

Further development of this reaction applied to simple arenes has been
rather slow, as the reaction fails to compete with the regular Mizoroki-Heck
reaction. A few improvements have been targeted on making the reaction
more economical through the use of catalytic amounts of Pd salt. Thus,
Obora et al. showed that oxygen can be used as a terminal oxidant if the
reoxidation is promoted by molybdovanadophosphoric heteropolyacid
(Scheme 9.104). 313,314
The original Fujiwara-Moritani reaction is applied to electron-rich arenes.
This limitation seems natural as the palladation seemed to be a regular
electrophilic substitution reaction obeying typical trends common for text-
book reactions of this type, the closest analogues being mercuration, thal-
lation, etc. However, organometallic reactions involving transition metals
such as Pd often follow concerted pathways not involving carbenium ions,
carbanions and other discreet ionic species common in non-transition metal
chemistry. The palladation of arenes most probably is no different - the
rupture of a C-H bond and the formation of a Pd-C bond are most probably
concerted, taking place within the coordination shell of palladium with one
of the ligands probably lending assistance for deprotonation, 315-318 hence
the process is an inner-sphere reorganization of bonds. As a result, the effect
of substituents is mitigated (in common electrophilic substitution the dif-
ference in reactivity between PhH, PhMe, PhOMe, PhNMe 2 is extremely
large, exceeding 10-15 orders of magnitude along the series, whereas in
palladation chemistry the difference is only modest). On the other hand, by
varying the transition state it is possible to reach the ''carbanion-like''
transition state (rupture of the C-H bond is slightly ahead of Pd-C bond
formation) and to observe the reaction with electron-deficient substrates
(this can be considered as a switch-over of distinct mechanisms, 319 but in
common physical organic chemistry the response of the transition state
intimate structure to variations of substituent effects, substrate structure
and environment effects which are ubiquitous in classical mechanistic
paradigms, such as S N 2, E2, etc., are conveniently discussed as a mechanistic
continuum of a variable transition state within a single mechanism).
Such abuse of the electrophilic palladation paradigm often happens in
directed palladation (see below), but in non-assisted palladation it seems not
to have been observed until recently. The reason why non-directed substi-
tution with electron-deficient substrates is rare, whereas in directed sub-
stitution the difference between electron-rich and electron-deficient
substrates is less manifested, is probably very simple - the attack of the Pd