Modern Heck Reactions - New Trends in Cross-Coupling: Theory and Applications - page 398

increase in catalytic activity. A cross-comparison of parameters may give
such clues, if desired.
Salicylaldehyde thiosemicarbazones as ONS-tridentate ligands, such as 17,
were used for the preparation of a series of heteroleptic Pd complexes. These
complexes were used as precatalysts for the Mizoroki-Heck reaction of aryl
iodides and bromides with acrylates showing a modest level of activity
(system: 0.1-1 mol% Pd, Na 2 CO 3 , DMF, 130-145 1C, 24-36 h). The least
reactive substrate studied was p-bromotoluene, giving a 78% yield of methyl
p-methylcinnamate with a 0.1 mol% loading after 36 h at 145 1C. 225
In addition to preformed complexes, new ligands are also appearing. The
conditions of the catalytic processes, however, show that no apparent ligand-
acceleration effect is exhibited and the ligands just bind Pd, forming an
SRPC in situ. Thus, a bisphosphite derivative of pentaerythritol (18)
(Scheme 9.62) was tested as a ligand for the Mizoroki-Heck reaction of aryl
iodides and bromides with standard olefins. The least reactive substrates
used were o- and p-bromoanisole and p-bromo-N,N-dimethylaniline, giving
only modest yields (35-65%) of the respective cinnamates [system: Pd(OAc) 2
(1 mol%), L, Na 2 CO 3 , DMF, 140 1C, 12 h]. 226
Another interesting example was reported by Buckley and Neary, 227
namely the sterically hindered thiadiazolidine S-oxide 19, a structural
analogue of the most useful carbene ligands. This ligand was used in a
typical catalytic system containing NaOAc and Bu 4 NBr for reactions of aryl
iodides and bromides with standard olefins to give high yields of the re-
spective products with high TON values. Even the least reactive substrate
used, p-bromoanisole, was processed with an impressive TON of 740 000
(Scheme 9.63). The reactions were run at high temperatures, achieved by
microwave heating.
A tetraselenide forms two different complexes - a bis-pincer (20) and a
regular chelate (21) involving seven-membered rings (Scheme 9.64). Both
served as precatalysts for the Mizoroki-Heck reaction of a series of aryl
bromides with acrylate and styrene (0.001 mol% Pd as 20 or 21, n-BuNH 2 ,
DMA, 100 1C). 228 In spite of the great difference in structure - the pincer Se-
C-Se complex should be much more stable than the seven-membered simple

O R

O

N NH 2

N N

N

Pd S

O

N Pd N

PPh 3

OMe

(16) (17)

O O

t Bu t Bu

O P P O

O O

t Bu t Bu

(18)

Scheme 9.62 Precatalysts 16-18.