chapter 2 palladium catalysts in suzuki cross- coupling reaction

11). PhB(OH)2 of different sources was tested again under the improved reaction
conditions which were applied in the case of entry 7. The result from the Aldrich
product was comparable with that obtained with Merck’s whereas the reagent from
Fluka resulted about 10% lower yield (Table 5.4, entries 12, 13).
To understand the role of the zeolite itself in reactions (i.e., whether it plays an
active role), the reaction was performed with only ~60 mg of Pd-loaded zeolite in the
absence of extra amount of zeolite. Surprisingly, no product formation was observed
(Table 5.5, entry 1). Moreover, when the zeolite additive which was vacuum dried at
140 °C for 2 h prior to the reaction (Table 5.5, entry 2) was used, the catalyst displayed
no activity. As a result of these data, we concluded that the intrinsic water content of the
zeolite (which was measured to be around 23% based on the weight loss during vacuum
drying at 140 °C for 2 h) was crucial for the activity of the system. When 230 mg of
water (which is equal to the amount of water that would be introduced by the addition
of 1 g of NaY zeolite in as-received form) was added into reaction medium without the
zeolite additive, a moderate product formation (61%) was observed (Table 5.5, entry 3),
indicating that the reaction can operate only in moist medium and, suggesting the
existence of a synergism between water and zeolite combination.
Also, the effect of zeolite amount (in as-received form) was examined at 140 ºC
(Table 5.5, entries 4-7). The yield formation was modest (48%) when the reaction was
carried out with an overall zeolite amount of 1 g for 5 min (Table 5.5, entry 4). About 3
to 4 grams of the zeolite seemed to be required for the optimum product recovery
(compare entries 5-7) and the reaction proceeded smoothly even at a lower reaction
temperature of 120 ºC to give 88% coupling product after 15 min (Table 5.5, entry 8).
Four grams of NaY zeolite would introduce nearly 1 g of water into the reaction
medium. Therefore, a reaction was conducted in the presence of 1 g of zeolite along
with 1 g of added water in order to clarify if the yield improvement was solely due to
the increased water amount in the system. The yield was identical to that obtained in the
experiment performed with 1 g original zeolite alone (Table 5.5, compare entries 4, 9).
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