nanoparticles are best stored under argon in a refrigerator; otherwise, the color
may change indicative of a drop in reactivity.
General Procedure for Reductions Iron-based nanomaterial (6 mg) was added
to an oven-dried 10 mL round-bottomed flask (RBF) containing a PTFE-coated
magnetic stir bar. An aqueous solution of 2 wt% TPGS-750-M (0.5 mL) was added
via syringe, and NaBH 4 (28.5–59.0 mg, 0.75–1.50 mmol) was added to the reaction
mixture. (Caution: NaBH 4 should be added slowly, especially for large-scale
reactions, i.e., >1 mmol.) During addition of NaBH 4 , the reaction mixture
turned black with evolution of hydrogen gas. The reaction flask was covered
with a rubber septum, and the mixture was stirred for 2 min at rt. The nitro
group-containing substrate (0.5 mmol, pre-dissolved or dispersed in mixture of
0.5 mL aqueous TPGS-750-M and 0.1 mL THF in advance) was then added
to the catalyst suspension via syringe (substrates which are not soluble in aqueous
TPGS solution were first dissolved in a minimum amount of THF (160 μL for
0.5 mmol of educt)). The RBF was filled with argon and covered again with a
rubber septum. Finally, the reaction mixture was vigorously stirred at rt. Progress
of the reaction was monitored by TLC or GCMS. After complete consumption of
starting material as monitored by TLC, the septum was removed, and argon was
bubbled through the mixture. Minimal amounts of an organic solvent (EtOAc,
i-PrOAc, Et 2 O, MTBE, etc.) were added, and the mixture was stirred gently for
2 min. Stirring was stopped, and the organic layer was then allowed to separate,
after which it was removed via pipette. The same extraction procedure was repeated,
and the combined organic extracts were dried over anhydrous Na 2 SO 4 . Volatiles
were evaporated under reduced pressure, and semi-pure product was purified
by flash chromatography over silica gel. Caution: Never use acetone for TLC
monitoring or column chromatography. Occasionally during the progress of the
reaction, the reaction vial requires gentle shaking to avoid adherence of reaction
material to the glass. Always use fresh and good quality NaBH 4 .
Further optimization of these NPs led to an improved catalyst exhibiting
synergistic effects between Pd (80 ppm) and Ni (1,600 ppm) [19]. While
electron-rich nitroaromatics on occasion showed sluggish behavior toward the
initially reported catalyst, the second-generation Fe/Pd/Ni NPs are, in general,
far more reactive. Ether-, thioether-, and aniline-containing nitroaromatics were
reduced in good-to-excellent yields with remarkable chemoselectivity (Fig. 5).
For example, despite the presence of super-stoichiometric amounts of borohydride,
reduction of aryl hydrazones to their corresponding hydrazines was not observed.
EXAFS analysis of the first-generation NPs revealed high shell scattering indicative
of tight Pd-Pd interactions. The nickel-containing particles contained no such
feature. It is postulated that the presence of nickel dilutes the palladium on
the surface of the NPs inhibiting clustering (i.e., Pd-Pd interactions) and therefore
increasing the availability of highly reactive single atoms of Pd.
In 2018, Ming Bao and co-workers reported that allylboronates can be used
as a carbon-based ligand for in situ generation of Pd NPs [20]. These have been
shown to be useful in carboxylative Suzuki-Miyaura coupling reactions of benzylic
chlorides with allylpinacolborate (Fig. 6). The reaction conditions were relatively
Earth-Abundant and Precious Metal Nanoparticle Catalysis
83
may change indicative of a drop in reactivity.
General Procedure for Reductions Iron-based nanomaterial (6 mg) was added
to an oven-dried 10 mL round-bottomed flask (RBF) containing a PTFE-coated
magnetic stir bar. An aqueous solution of 2 wt% TPGS-750-M (0.5 mL) was added
via syringe, and NaBH 4 (28.5–59.0 mg, 0.75–1.50 mmol) was added to the reaction
mixture. (Caution: NaBH 4 should be added slowly, especially for large-scale
reactions, i.e., >1 mmol.) During addition of NaBH 4 , the reaction mixture
turned black with evolution of hydrogen gas. The reaction flask was covered
with a rubber septum, and the mixture was stirred for 2 min at rt. The nitro
group-containing substrate (0.5 mmol, pre-dissolved or dispersed in mixture of
0.5 mL aqueous TPGS-750-M and 0.1 mL THF in advance) was then added
to the catalyst suspension via syringe (substrates which are not soluble in aqueous
TPGS solution were first dissolved in a minimum amount of THF (160 μL for
0.5 mmol of educt)). The RBF was filled with argon and covered again with a
rubber septum. Finally, the reaction mixture was vigorously stirred at rt. Progress
of the reaction was monitored by TLC or GCMS. After complete consumption of
starting material as monitored by TLC, the septum was removed, and argon was
bubbled through the mixture. Minimal amounts of an organic solvent (EtOAc,
i-PrOAc, Et 2 O, MTBE, etc.) were added, and the mixture was stirred gently for
2 min. Stirring was stopped, and the organic layer was then allowed to separate,
after which it was removed via pipette. The same extraction procedure was repeated,
and the combined organic extracts were dried over anhydrous Na 2 SO 4 . Volatiles
were evaporated under reduced pressure, and semi-pure product was purified
by flash chromatography over silica gel. Caution: Never use acetone for TLC
monitoring or column chromatography. Occasionally during the progress of the
reaction, the reaction vial requires gentle shaking to avoid adherence of reaction
material to the glass. Always use fresh and good quality NaBH 4 .
Further optimization of these NPs led to an improved catalyst exhibiting
synergistic effects between Pd (80 ppm) and Ni (1,600 ppm) [19]. While
electron-rich nitroaromatics on occasion showed sluggish behavior toward the
initially reported catalyst, the second-generation Fe/Pd/Ni NPs are, in general,
far more reactive. Ether-, thioether-, and aniline-containing nitroaromatics were
reduced in good-to-excellent yields with remarkable chemoselectivity (Fig. 5).
For example, despite the presence of super-stoichiometric amounts of borohydride,
reduction of aryl hydrazones to their corresponding hydrazines was not observed.
EXAFS analysis of the first-generation NPs revealed high shell scattering indicative
of tight Pd-Pd interactions. The nickel-containing particles contained no such
feature. It is postulated that the presence of nickel dilutes the palladium on
the surface of the NPs inhibiting clustering (i.e., Pd-Pd interactions) and therefore
increasing the availability of highly reactive single atoms of Pd.
In 2018, Ming Bao and co-workers reported that allylboronates can be used
as a carbon-based ligand for in situ generation of Pd NPs [20]. These have been
shown to be useful in carboxylative Suzuki-Miyaura coupling reactions of benzylic
chlorides with allylpinacolborate (Fig. 6). The reaction conditions were relatively
Earth-Abundant and Precious Metal Nanoparticle Catalysis
83