of ca. 1 nm in size were supported or stabilized by polyaminothiophenol (PTAP)
and were easily prepared by mixing the appropriate precursors. Reactions could
be performed in water using only 500 ppm of gold catalyst, albeit at 80
C. Although
trace amounts of gold were leached into the solution leading to debate regarding
the homo- vs. heterogeneity of the reaction, the catalyst was recycled six times
without noticeable differences in chemical yield. The chemistry can be done
with electron-poor (3) or electron-rich (4) chlorides. The normal reactivity trend
of I > Br > Cl was also followed to arrive at product 5. A bulky, di-ortho-halosubstituted boronic acid also gave product (6), although in this case the yield
was modest. The size of gold NPs has considerable effect on the reactivity, where
5 nm gold particles led to only trace amounts (ca. 10%) of product.
Preparation of the Polymer-Supported Au Catalyst 2-Aminothiophenol
(100.0 mg, 0.4 mmol) was dispersed in aqueous HCl solution (1.0 M, 20 mL)
with magnetic stirring at rt. for 1 h to obtain a uniform solution. After that, the
mixture was maintained at 20
C for 0.5 h before oxidative polymerization. Then,
a quantitative amount of an aqueous HAuCl 4 solution (0.1 M) was added to the
above mixture in one portion. The resulting solution was stirred for another 0.5 min
to ensure complete mixing, and then the reaction was allowed to proceed with
agitation for 24 h at 20
C. Finally, the product was washed with deionized water
until the filtrate became colorless and then dried under vacuum at 60
C for 24 h.
General Procedure for Suzuki-Miyaura Cross-Coupling The aryl halide
(225.0 mg, 2.0 mmol), phenylboronic acid (292.6 mg, 2.4 mmol), and NaOH
(320.0 mg, 8.0 mmol) were added to 40 mL of deionized water. The solution
was stirred at 80
C until the chemicals were completely dissolved. Then, an
aqueous solution of the Au catalyst (1.0 mM, 1.0 mL, 0.05 mol%) was added
to the stirred solution in one portion, and the reaction mixture was stirred for
another 4 h at this temperature. After the mixture had cooled to rt., the organic
product was extracted with Et 2 O (3 Â 20 mL). The organic layer was dried
with anhydrous Na 2 SO 4 . After filtration, all volatiles were removed under reduced
pressure to yield the final product.
Following this initial discovery, applications of Au NPs toward SM
couplings have gained attention from the synthetic community (Table 1). In
this context, Corma, Garcia, and co-workers [74] used Au-platelets grafted on
graphene (Au/fl-G) for SM couplings of aryl halides with phenylboronic acid in
water (entry 1). Interesting is the reactivity of aryl halides in the order of Cl > Br > I,
which is contrary to expectations based on C-X bond strength. DFT calculations
suggested that such reactivity could be observed due to a poisoning effect of
halide on gold, with iodide having a greater effect than bromide and much
higher than chloride. Thomas et al. [75] prepared a composite of gold NPs
with strontium, cross-linked alginate carboxymethyl cellulose, and graphene
oxide (Sr/Alg/CMC/GO/Au) and utilized 50 ppm of this catalyst for SM
couplings of chlorobenzene with phenylboronic acid in water at 80
C, leading
to a 76% yield of the desired product (entry 2). Nemygina et al. [76] have
Earth-Abundant and Precious Metal Nanoparticle Catalysis
105
and were easily prepared by mixing the appropriate precursors. Reactions could
be performed in water using only 500 ppm of gold catalyst, albeit at 80
C. Although
trace amounts of gold were leached into the solution leading to debate regarding
the homo- vs. heterogeneity of the reaction, the catalyst was recycled six times
without noticeable differences in chemical yield. The chemistry can be done
with electron-poor (3) or electron-rich (4) chlorides. The normal reactivity trend
of I > Br > Cl was also followed to arrive at product 5. A bulky, di-ortho-halosubstituted boronic acid also gave product (6), although in this case the yield
was modest. The size of gold NPs has considerable effect on the reactivity, where
5 nm gold particles led to only trace amounts (ca. 10%) of product.
Preparation of the Polymer-Supported Au Catalyst 2-Aminothiophenol
(100.0 mg, 0.4 mmol) was dispersed in aqueous HCl solution (1.0 M, 20 mL)
with magnetic stirring at rt. for 1 h to obtain a uniform solution. After that, the
mixture was maintained at 20
C for 0.5 h before oxidative polymerization. Then,
a quantitative amount of an aqueous HAuCl 4 solution (0.1 M) was added to the
above mixture in one portion. The resulting solution was stirred for another 0.5 min
to ensure complete mixing, and then the reaction was allowed to proceed with
agitation for 24 h at 20
C. Finally, the product was washed with deionized water
until the filtrate became colorless and then dried under vacuum at 60
C for 24 h.
General Procedure for Suzuki-Miyaura Cross-Coupling The aryl halide
(225.0 mg, 2.0 mmol), phenylboronic acid (292.6 mg, 2.4 mmol), and NaOH
(320.0 mg, 8.0 mmol) were added to 40 mL of deionized water. The solution
was stirred at 80
C until the chemicals were completely dissolved. Then, an
aqueous solution of the Au catalyst (1.0 mM, 1.0 mL, 0.05 mol%) was added
to the stirred solution in one portion, and the reaction mixture was stirred for
another 4 h at this temperature. After the mixture had cooled to rt., the organic
product was extracted with Et 2 O (3 Â 20 mL). The organic layer was dried
with anhydrous Na 2 SO 4 . After filtration, all volatiles were removed under reduced
pressure to yield the final product.
Following this initial discovery, applications of Au NPs toward SM
couplings have gained attention from the synthetic community (Table 1). In
this context, Corma, Garcia, and co-workers [74] used Au-platelets grafted on
graphene (Au/fl-G) for SM couplings of aryl halides with phenylboronic acid in
water (entry 1). Interesting is the reactivity of aryl halides in the order of Cl > Br > I,
which is contrary to expectations based on C-X bond strength. DFT calculations
suggested that such reactivity could be observed due to a poisoning effect of
halide on gold, with iodide having a greater effect than bromide and much
higher than chloride. Thomas et al. [75] prepared a composite of gold NPs
with strontium, cross-linked alginate carboxymethyl cellulose, and graphene
oxide (Sr/Alg/CMC/GO/Au) and utilized 50 ppm of this catalyst for SM
couplings of chlorobenzene with phenylboronic acid in water at 80
C, leading
to a 76% yield of the desired product (entry 2). Nemygina et al. [76] have
Earth-Abundant and Precious Metal Nanoparticle Catalysis
105