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M. Benaglia
Given these excellent results, it is surprising that this approach has
not been used more extensively for chiral organic catalyst discovery.
The success of this methodology is even more significant if one considers that catalysts 10 are among the few chiral organic catalysts to be
currently used at the industrial level.
The preparation of supported catalysts for use in environmentally
friendly or green solvents, as part of the drive towards developing more
Green Chemistry, is also becoming more widespread. In this context
PEG-supported enantiomerically pure bisoxazolines have been prepared
and used in combination with Cu(II) salts as the catalyst for the Mukaiyama aldol condensation between the trimethylsilyl keteneacetal of methyl isobutyrate and various aldehydes carried out in water as the only
reaction solvent. Enantiomeric excesses similar to those obtained under
the same conditions with non-supported ligands were observed. The reaction proceeded at higher yields when electron-poor aldehydes were
used as the substrates. The high solubility of the ligand in water allowed a very convenient catalyst-recycling procedure involving simple removal of the reaction product by extraction in diethylether and
addition of fresh reagents to the catalyst-containing aqueous solution.
The chemical and stereochemical efficiency of the catalyst was only
marginally eroded over its use in three reaction cycles.
However, PEG supported metal-free catalysts have also been shown
to perform well in water. For example the synthesis of a PEG-supported
TEMPO (2,2,6,6-tetramethyl-piperidine-1-oxyl), and its use as a highly
efficient, recoverable and recyclable catalyst in oxidation reactions was
described (Pozzi et al. 2004).
TEMPO-catalysed oxidation of alcohols to carbonyl compounds with
buffered aqueous NaOCl has found broad application, even in largescale operations. Indeed, this selective methodology involves the use
of safe, inexpensive inorganic reagents under mild reaction conditions.
A new supported catalyst PEG–TEMPO 11, soluble in organic solvents
such as CH 2 Cl 2 and AcOH, but insoluble in ethers and hexanes, was
prepared and proved to be an effective catalyst for the selective oxidation of 1-octanol with various stoichiometric oxidants. When 11 was
used at 1 mol% as a catalyst in combination with KBr (10 mol%) a slight
excess of buffered bleach (pH = 8.6) as the terminal oxidant, partial
over-oxidation of 1-octanol to octanoic acid was observed (91% yield in
M. Benaglia
Given these excellent results, it is surprising that this approach has
not been used more extensively for chiral organic catalyst discovery.
The success of this methodology is even more significant if one considers that catalysts 10 are among the few chiral organic catalysts to be
currently used at the industrial level.
The preparation of supported catalysts for use in environmentally
friendly or green solvents, as part of the drive towards developing more
Green Chemistry, is also becoming more widespread. In this context
PEG-supported enantiomerically pure bisoxazolines have been prepared
and used in combination with Cu(II) salts as the catalyst for the Mukaiyama aldol condensation between the trimethylsilyl keteneacetal of methyl isobutyrate and various aldehydes carried out in water as the only
reaction solvent. Enantiomeric excesses similar to those obtained under
the same conditions with non-supported ligands were observed. The reaction proceeded at higher yields when electron-poor aldehydes were
used as the substrates. The high solubility of the ligand in water allowed a very convenient catalyst-recycling procedure involving simple removal of the reaction product by extraction in diethylether and
addition of fresh reagents to the catalyst-containing aqueous solution.
The chemical and stereochemical efficiency of the catalyst was only
marginally eroded over its use in three reaction cycles.
However, PEG supported metal-free catalysts have also been shown
to perform well in water. For example the synthesis of a PEG-supported
TEMPO (2,2,6,6-tetramethyl-piperidine-1-oxyl), and its use as a highly
efficient, recoverable and recyclable catalyst in oxidation reactions was
described (Pozzi et al. 2004).
TEMPO-catalysed oxidation of alcohols to carbonyl compounds with
buffered aqueous NaOCl has found broad application, even in largescale operations. Indeed, this selective methodology involves the use
of safe, inexpensive inorganic reagents under mild reaction conditions.
A new supported catalyst PEG–TEMPO 11, soluble in organic solvents
such as CH 2 Cl 2 and AcOH, but insoluble in ethers and hexanes, was
prepared and proved to be an effective catalyst for the selective oxidation of 1-octanol with various stoichiometric oxidants. When 11 was
used at 1 mol% as a catalyst in combination with KBr (10 mol%) a slight
excess of buffered bleach (pH = 8.6) as the terminal oxidant, partial
over-oxidation of 1-octanol to octanoic acid was observed (91% yield in