Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis : Novel ...

214 NITRONES: NOVEL STRATEGIES IN SYNTHESIS
Enantioselective hydroxylation of the double bond in C=N nitrones with
diphenylsilane, using Ru2Cl4-[(S)-(−)-p-tolbinap]2(NEt3) [p-tolbinap = 2, 2’bis(di-p-tolylphosphino)-1,1’-binaphthyl]
as a catalyst at 0 ◦ C, gives the corresponding
optically active N,N -disubstituted hydroxylamines (482).
Deoxygenation of nitrones seems to be a useful transformation in organic
synthesis. It can be carried out in many ways, for example, with low valency
titanium (483), phosphorus- (484), sulfur- (485), and tellurium-derivatives (486),
tin tributyl hydride (487) and Pd/C (488). Many of the methods are limited
by side reactions, lack of chemoselectivity, low yields, or tedious procedures.
Deoxygenation of nitrones with triphenylphosphine, catalyzed by dichlorodioxomolybdenum
(VI), is achieved chemoselectively and under mild conditions (489).
The use of benzyltriethylammonium tetrathiomolybdate (PhCH2NEt3)MoS4 in
CH3CN at 25 ◦ C affords the corresponding imines in high yields (490). Deoxygenation
of nitrones with Mo(CO)6 proceeds under mild conditions and with
good yields (491).
Deoxygenation of nitrones occurs in photocatalyzed reduction with TiO2 in
acetonitrile, in a nitrogen atmosphere. 20 Selective nitrone deoxygenation is also
achieved with Zn-AlCl3.6H2O in THF (492), Zn(OTf)2, and Cu(OTf)2. (493) On
treatment with trimethylsilyl lithium, nitrones are converted into the corresponding
imines in high yields (494). A simple way to deoxygenate nitrones is to treat
their acetonitrile solutions with InCl3 (495). Upon treatment with indium in alcohol,
in the presence of ammonium chloride, and depending on the reaction conditions,
one observes either formation of the corresponding imines or reductive
coupling with formation of vicinal diamines (496). An efÞcient and easy method
of nitrone deoxygenation seems to be treatment with RuCl3xH2O (497). For
nitrone reduction, the use of a NiCl2xH2O-Li-arene catalytic reduction system has
been reported. Depending on reaction conditions it leads to de-oxygenation products
in high yield or secondary amines (498, 499). AlI3/MeCN (500) and AlCl3
x6H2O/KI/CH3CN/H2O (501) are effective deoxygenation systems. Deoxygenation
of nitrones proceeds mildly and selectively in a (CF3CO)2O/NaI system (502).
Except for special cases, neither hydrazine, nor amines reduce the nitrone
group. Even on reducing α-nitroaryl nitrones with hydrazine hydrate and Ni
Raney, only the nitro group is reduced (503). Deoxygenation of the nitrone group
with hydrazine is observed in paramagnetic 3-imidazoline-3-oxide derivatives
(504). Reduction of the nitrone group with hydrazine groups and with secondary
and tertiary amino groups takes place only intramolecularly in α-hydrazinoalkyl
and α-aminoalkyl nitrones (see Section 2.6.6.2).
2-Phenyl-3-phenylimino-3-H -indole N -oxide (an indolic nitrone) reacts with
triethyl and triisopropyl phosphite in reßuxing xylene and tert-butylbenzene to
give 2-phenylimino-3H -indole (indolenine) in very good yield (505).
2.6.3. Nitrone Oxidation
2.6.3.1. Nitrone Oxidation in Non-Nucleophilic Medium Treatment of
nitrones with strong oxidants such as PbO2, MnO2 (506) and ClO2 (507), in