Communications
tions (e.g., higher amounts of catalyst, more electron-rich
perspective, shows that nickel is indeed impregnated within
the sheets. In conjunction with our ICP-atomic emission
spectrometric (AES) data indicating that only approximately
0.25% nickel relative to substrate is in solution, these data
suggest that reactions of Ni/Cg primarily take place on the
surface of the graphite layers. Thus, as is the case with nickel-
in-charcoal (Ni/C),[15] the hydrocarbon appears to support,
retain, and maintain the metal in a catalytically active state.
In summary, a readily available, very inexpensive catalyst
in the form of nickel-on-graphite has been prepared and
shown to afford chemoselective reductions of aryl tosylates
and mesylates. This heterogeneous material is amenable to
use under either conventional heating conditions, or micro-
wave irradiation which greatly enhances reaction rates.
Catalyst reisolation and recycling can be effected without
loss of activity. New additional chemistry associated with Ni/
Cg will be reported in due course.
ligands, microwave heating, rigorously dried solvent, etc.), are
illustrated in Scheme 6. For carbazole 5, N-deprotection was
observed, whether the nitrogen atom was masked as the
acetyl derivative or otherwise (e.g., pivaloyl, etc.). Both the
Scheme 6. Substrates that were not susceptible to reduction.
benzthiazole 6 and dicyanostyrene 7 cases led to net
hydrolysis of the tosylate (with or without the presence of
drying agents, molecular sieves, etc.), and the vitamin E
derivative 8 appears to be too hindered and/or deactivated to
react even at elevated temperatures (up to 2108C under
microwave irradiation).
To gauge the level of bleed of nickel from graphite under
conventional heating conditions, inductively coupled plasma
(ICP) analyses[12] were conducted on a sample reaction
mixture. Thus, following the conversion of tosylate 9 (cf. Ta-
ble 1, entry 5) into the corresponding reduced aromatic
compound,[13] hot filtration of the crude mixture was followed
by solvent removal and digestion in refluxing aqua regia. The
data indicated that 4.99% of the 5% Ni/Cg (or 0.155 mg
nickel for the amount of catalyst used) in the reaction mixture
could be determined to be in solution. Thus, only
0.0025 equivalents of nickel relative to substrate were
detected, an amount insufficient to effect the observed
chemistry.
Transmission electron microscopy (TEM) was used to
examine the physical nature of Ni-on-graphite.[14] There
appears to be no visible sign of extensive clustering of
nickel atoms, presumably due to the use of ultrasonication
during the preparation of the catalyst. From Figure 1a, scaled
at 100 nm, two graphite sheets can be distinguished that
indicate independent nickel clusters. A cross section of the
catalyst at 200 nm (Figure 1b), taken from a different
Experimental Section
Preparation of Niii/Cg: Graphite powder (3.75 g, 1–2 mm) was added
to a 100-mL round-bottomed flaskcontaining a stirring bar. A
solution of Ni(NO3)2·6H2O (727 mg, 2.30 mmol; Aldrich, 24,407-4, Ni
content determined by ICP analysis: 92%;) in deionized H2O
(35 mL) was added to the graphite, and deionized H2O (40 mL) was
added to wash down the sides of the flask. The flask was purged under
argon and stirred vigorously for 1 min. The flaskwas submerged in an
ultrasonic bath under a positive argon flow for 60 min. The flaskwas
attached to an argon-purged distillation setup and placed in a sand
bath preheated at 175–1808C with a stirring plate. As the distillation
ended, the flasktemperature rises automatically, but should be kept
below 2108C for an additional 15 min. Upon cooling to room
temperature, the blacksolid was washed with H 2O (2 50 mL)
under argon into an in vacuo predried 150-mL course-fritted funnel.
The H2O used to wash the Ni/C was removed with a rotatory
evaporator and analyzed for remaining nickel. The fritted funnel was
inverted under vacuum and allowed to stand for 3 h until the Ni/Cg
fell from the frit into a collection flask. Any Ni/Cg that remained on
the fritted funnel was scraped off with a spatula and collected. The
collection flaskwas then dried in vacuo at 100 8C for 18 h. Using these
specific amounts, all of the nickel, which corresponded to 0.552 mmol
Niii gÀ1 catalyst or 3.2% Ni/catalyst by weight, was mounted on the
support.
Representative procedure for Ni/Cg-catalyzed reductions of aryl
tosylates: Ni/Cg (96 mg, 0.05 mmol), PPh3 (70 mg, 0.27 mmol), K2CO3
(150 mg, 1.10 mmol), and borane–dimethylamine complex (68 mg,
1.10 mmol) were added to an argon-purged, flame-dried long-necked
10-mL round-bottomed flaskin a glove box. Dry DMF (2 mL) was
added to the reaction mixture and allowed to stir at room temper-
ature for 2 h. The tosylate (1.00 mmol) was added to the reaction
mixture and washed down the sides of the flaskwith additional DMF
(1 mL). The flaskwas then equipped with a reflux condenser and
placed into a preheated oil bath at 1208C. The reaction was followed
by TLC until complete disappearance of the starting material was
observed. The flaskwas then allowed to cool to room temperature
and filtered to remove Ni/Cg. The flaskwas washed with methanol
(3 10 mL) and CuCl (27 mg, 0.27 mmol) was added to sequester
PPh3.[16] The solvent was evaporated under reduced pressure, and the
reduced material was afforded by column chromatography on silica
gel of the residue.
Figure 1. TEM imagesof Ni/C g. a) Nickel clusters on graphite sheets;
Received: August 14, 2005
b) cross section showing exposed nickel atoms on the support.
Published online: December 21, 2005
802
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 800 –803
Lipshutz, Bruce H.
