Chemistry Reference
In-Depth Information
Jiang and Ragauskas (see Chapter 2, Figure 2.9a)
30
reported an oxidation
system in which the catalyst (a modified TEMPO with an NHAc group) sol-
uble in DMSO was recovered by simple extraction with n-pentane of the final
products and the catalytic phase was reused by addition of fresh starting
material. Another similar interesting example recovered the catalyst by
antisolvent precipitation (addition of hexanes to the reaction mixture and
decantation), as described in Section 10.7.4.
23
A different and unusual approach for catalyst recovery was developed by
Mizuno and co-workers, who showed that a ruthenium hydroxide species on
magnetite [Ru(OH)
x
/Fe
3
O
4
] performed very well and catalyst/product(s) sep-
aration was extremely simple. Indeed, after completion of the oxidation re-
action, a permanent magnet was attached to the outside wall of the glass
reactor to 'hold' the catalyst magnetically and the reaction solution in-
cluding the product(s) was separated by simple decantation (see also
Chapter 3, Figure 3.14).
31
d
n
4
r
4
n
g
|
6
10.9 Some Scaled-Up Procedures
Large-scale applications of aerobic alcohol oxidation are often prevented by
safety concerns associated with the combination of O
2
and organic solvents
and reagents and by the frequent use of halogenated solvents.
32
Moreover,
some oxidation methods have been optimized with non-standard solvents
such as fluorinated compounds or ionic liquids, and some of the catalysts
with the best reported activities are not commercially available or they have a
limited lifetime. All these factors limit the widespread adoption of aerobic
oxidation reactions in synthetic chemistry and in particular make the
development of practical and scalable procedures very dicult.
However, some ecient and scaled-up procedures have been reported.
The method of Stahl and co-workers employing (bpy)Cu(I) salts/TEMPO in
CH
3
CN as solvent with ambient air as the oxidant was applied on a 10 g scale
to eight different alcohols, with reaction times varying from 20 min to 24 h,
depending on the alcohol used and the purification methods, each taking
about 2 h.
33
The total time necessary for the complete protocol ranged from
3to26h.
The safety hazards typically associated with aerobic oxidations on a large
scale could also be overcome by using dilute air (8% O
2
in N
2
) as the oxidant
in a continuous flow reactor, which allowed a Pd(OAc)
2
/pyridine-catalyzed
aerobic oxidation reaction to be tested on a 1 kg scale in a 7 L flow reactor.
34
.
Representative experimental procedure for aerobic alcohol oxidation in a 7 L
flow reactor.
34
An oven used to regulate the reaction-zone temperature of
the flow reactor was set to 100 1C. The flow reactor was rinsed with dry
toluene and dried by passing nitrogen gas through the tubing at 100 1C.
The reactor was pressurized by applying a 500 psig nitrogen back-
pressure from a high-pressure nitrogen cylinder connected to the vapor-
liquid separator. The regulator for the diluted O
2
gas cylinder (8% O
2
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