Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 389

favour of the stable ancillary effect. The reaction, however, was highly
dependent of additives, typical for ''phosphine-free'' reactions (Scheme 9.50).
With aryl bromides, diarylation was also developed into a useful protocol
applicable to various aryl bromides. All these data suggest that the
palladacycle-carbene complex on heating releases active Pd(IMes) complex,
serving as catalyst. The high temperature required for aryl iodides, bromides
and triflates, that is disregarding the reactivity of the substrate, is thus ac-
counted for by precatalyst thermal activation via stripping off the pallada-
cycle ligand shell. In a more recent study, a similar complex of IPr carbene
with ferrocenylimine palladacycle showed a more typical performance lim-
ited to aryl and alkenyl bromides and iodides and standard olefins, but also
showing consistently high yields for about two dozen different substrates,
many of them electron-rich (system 0.2 mol% Pd, K 3 PO 4 ,Bu 4 NBr, DMF,
140 1C). 193
Whereas in the conventional Mizoroki-Heck reactions the heterocyclic
carbene ligands have so far failed to find a reasonable role, in the very exotic
Heck alkylation one of the few existing protocols does rely on a carbene
ligand. Intramolecular Heck alkylation was demonstrated by Firmansjah
and Fu to afford exomethylenecyclopentenes via ligand-controlled selective
carbopalladation using an alkyl bromide or chloride reactive centre (an ex-
ample is shown in Scheme 9.51). 194 Only the heterocyclic carbene SIMes was
found to be able to control the process by suppressing premature b-hydride
elimination.
Among new additions to the arsenal of ligands, an unprecedented phos-
phazane system was reported by Iranpoor et al., 195 which showed very un-
usual performance. The phosphazanes are structurally different from
common phosphorus-based ligands used in Pd catalysis, bearing a certain
similarity only to Verkade's triaminophosphine and proazaphosphatrane
ligands, which, however, have not been found particularly useful in Heck
chemistry. The new ligand can be used not only in common polar solvents
(DMF, DMSO) but also in the non-polar solvent toluene and even in neat
water in heterogeneous systems not using any solubilizing or phase-transfer

COO t Bu

COO t Bu OTf

Pd(IMes)(dmba)Cl (2 mol%)
K 2 CO 3, NMP, Bu 4 NBr, 4A MS
140°C, 18 h

79%

Scheme 9.50 Example of arylation by triflates in the presence of IMes complex.

Br

EtO 2 C EtO 2 C

Pd 2 (MeOdba) 3 (10 mol% Pd)
SIMes ( 40 mol% ), Cs 2 CO 3
MeCN, 65°C

EtO 2 C EtO 2 C

85%

Scheme 9.51 Example of an intramolecular Heck reaction involving an sp 3 -carbon
centre.