308
M. Benaglia
a large scale; immobilization on a solid support may offer a solution to
this problem.
As mentioned before a PEG-supported TEMPO proved to be very
efficient in the oxidation of 1-octanol to octanal not only with sodium
hypochlorite, but also in combination with different terminal oxidants
such as bis(acetoxy)iodobenzene and trichloroisocyanuric acid. This reaction could be extended to acyclic and cyclic primary and secondary
alcohols with excellent results. It is remarkable that the PEG-supported
TEMPO maintained the good selectivity for primary vs secondary benzylic alcohol oxidation typical of non-supported TEMPO.
Catalyst recovery exploited the well known different solubility of
PEG in solvents of different polarity. In this case, where a PEG of relatively high M W was used, addition of diethylether to the reaction mixture induced the precipitation of compound 11. Subsequent filtration allowed recovery of the supported reagent with less than 10% weight loss
for each recovery. Catalyst recycling was shown to be possible for seven
reaction cycles in the oxidation of 1-octanol, that occurred in undiminished conversion and selectivity under the same reaction conditions.
The beneficial effect of the spacer was also demonstrated in a subsequent work, where spacer-containing and spacer-lacking pre-catalysts
(1–5 mol%) were used for the oxidation of primary and secondary alcohols using oxygen as the terminal oxidant in the presence of 2 mol% of
Co(NO 3 ) 2 hydrate and Mn(NO 3 ) 2 hydrate as co-catalyst (Pozzi 2005).
For instance, under these conditions oxidation of 1-octanol and cyclooctanol occurred in quantitative yield with 1 and in only about 65% yield
with 12 (Scheme 3). Recycling of pre-catalyst 1 was demonstrated for
the oxidation of 4-bromobenzyl alcohol for six reaction cycles occurring in slowly decreasing yields (>99% first cycle and 74% sixth cycle)
(equation b, Scheme 3).
3.2
Phase-Transfer Catalysts
The multifaceted applications of phase-transfer catalysts (PTC) in organic synthesis contributed decisively to the establishment of organic
catalysts as useful preparative tools. Polymer-supported PTC was examined extensively but it was noted that the catalytic activity of the
insoluble polystyrene-supported catalysts was strongly reduced in com-
M. Benaglia
a large scale; immobilization on a solid support may offer a solution to
this problem.
As mentioned before a PEG-supported TEMPO proved to be very
efficient in the oxidation of 1-octanol to octanal not only with sodium
hypochlorite, but also in combination with different terminal oxidants
such as bis(acetoxy)iodobenzene and trichloroisocyanuric acid. This reaction could be extended to acyclic and cyclic primary and secondary
alcohols with excellent results. It is remarkable that the PEG-supported
TEMPO maintained the good selectivity for primary vs secondary benzylic alcohol oxidation typical of non-supported TEMPO.
Catalyst recovery exploited the well known different solubility of
PEG in solvents of different polarity. In this case, where a PEG of relatively high M W was used, addition of diethylether to the reaction mixture induced the precipitation of compound 11. Subsequent filtration allowed recovery of the supported reagent with less than 10% weight loss
for each recovery. Catalyst recycling was shown to be possible for seven
reaction cycles in the oxidation of 1-octanol, that occurred in undiminished conversion and selectivity under the same reaction conditions.
The beneficial effect of the spacer was also demonstrated in a subsequent work, where spacer-containing and spacer-lacking pre-catalysts
(1–5 mol%) were used for the oxidation of primary and secondary alcohols using oxygen as the terminal oxidant in the presence of 2 mol% of
Co(NO 3 ) 2 hydrate and Mn(NO 3 ) 2 hydrate as co-catalyst (Pozzi 2005).
For instance, under these conditions oxidation of 1-octanol and cyclooctanol occurred in quantitative yield with 1 and in only about 65% yield
with 12 (Scheme 3). Recycling of pre-catalyst 1 was demonstrated for
the oxidation of 4-bromobenzyl alcohol for six reaction cycles occurring in slowly decreasing yields (>99% first cycle and 74% sixth cycle)
(equation b, Scheme 3).
3.2
Phase-Transfer Catalysts
The multifaceted applications of phase-transfer catalysts (PTC) in organic synthesis contributed decisively to the establishment of organic
catalysts as useful preparative tools. Polymer-supported PTC was examined extensively but it was noted that the catalytic activity of the
insoluble polystyrene-supported catalysts was strongly reduced in com-