chapter 2 palladium catalysts in suzuki cross- coupling reaction

Table 5.3. The effect of phenylboronic acid reagent on Pd-NaY catalyzed Suzuki
reaction of 4-chloroacetophenone a
Entry PhB(OH)2 Conversion% b Yield% b
1 Aldrich 95 82
2 Fluka 87 75
3 Merck 100 90
4 c Merck 82 64
a NaOC2H5 was used as base. b GC yield. c In the absence of Bu4NBr.
We tried to lower Pd concentration under the optimized reaction condition (i.e.,
2 equiv. NaOEt, 1.2 equiv. PhB(OH)2 (Merck), 0.5 equiv. Bu4NBr, 160°C, in N2). It
was found that reduction of the catalyst loading from 0.68 to 0.1mol % Pd (0.018 mmol
Pd/g NaY zeolite) lowered the yield of the Suzuki product (Table 5.4, entry 1).
Interestingly, changing atmosphere condition from N2 to ambient atmosphere improved
the coupling yield (Table 5.4, entry 2).
It was reported recently that a Pd-NaY catalyst displayed higher activity in
Mizoroki-Heck reaction under O2 atmosphere. The promotive effect of O2 was ascribed
to the reduced propensity of Pd to agglomerate inactive Pd clusters by keeping most of
its fraction in Pd(II) state. This may also account for the promotive effect of air. In our
case, however, O2 atmosphere turned out to be highly detrimental for the reaction.
Since the addition of tetraalkylammonium salts enhanced the rate of the coupling
reaction, tetrabutylammonium salts of chloride and iodide were also screened under
defined conditions (Table 5.4, entries 3-5). Among salts tested, the best result was
obtained by using bromide salt (entry 4). Increasing the amount of Bu4NBr and
PhB(OH)2 to 1 and 2 molar equivalents, respectively, gave the yield of desired product
at 0.1% Pd concentration (Table 5.4, entries 4, 6). It is worth noting that only 5 minutes
of reaction period was sufficient to achieve 90 % of coupling product (Table 5.4, entry
7).
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