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

REACTIONS OF NITRILE OXIDES 69
A phosphorylnitrile oxide (Me2CHO)2P(O)CNO reacts with tetracyanoethylene
to give the bisadduct 330 (296). Reaction of cyanodinitrochloromethane with
3-nitrobenzonitrile oxide gives 5-chlorodinitromethyl-3-(3-nitrophenyl)-1,2,4oxadiazole
331 (394).
O
(Me2CHO)2P
N
O2N
O
N
CN
C C
NC
330
N
O
N
O
N O
N C(NO2)2Cl 331
P(OCHMe) 2
A signiÞcant acceleration of 1,3-dipolar cycloaddition of nitriles with nitrile
oxides is shown, in the absence of solvent, at microwave irradiation (395). The
reactions are Þnished within 2 to 10 min, to give 1,2,4-oxadiazoles in good yields.
A new route to 1,2,4-oxadiazoles and their complexes via Pt- and Pd-mediated
1,3-dipolar cycloaddition of nitrile oxides to organonitriles, has been reported.
The sequence of the metal-mediated [2 + 3] cycloaddition offers an alternative
route for the preparation of oxadiazoles.
SigniÞcant activation of the CN group in organonitriles upon their coordination
to a Pt(IV) center has been found in the reaction of [PtCl4(RCN)2] (R= Me, Et,
CH2Ph) with stable aromatic nitrile oxides, ArCNO (Ar = 2,4,6-R ′ 3C6H2; R ′ =
Me, OMe), to give the (1,2,4-oxadiazole)platinum(IV) complexes, PtCl4.HetH
(HetH = 3-Ar-5-R-1,2,4-oxadiazole; Ar and R see above). The [2 + 3] cycloaddition
has been performed under mild conditions even starting from complexed
acetonitrile and propionitrile, which exhibit low reactivity in the free state. The
reaction between complexes PtCl4.HetH and one equivalent of Ph3P:CHCO2Me
in CH2Cl2 leads to the appropriate Pt(II) complexes PtCl2.HetH; the reduction
fails only in the case of PtCl4.HetH (Ar = mesityl, R = Me) because it is
insoluble in most common organic solvents. The oxadiazoles formed in the
metal-mediated reaction are liberated, almost quantitatively, from their Pt(IV)
complexes by reaction of the latter with an excess of pyridine in CHCl3, giving
free 1,2,4-oxadiazoles and trans-[PtCl4(pyridine)2] (396).
The reactions between stable 2,4,6-trisubstituted benzonitrile oxides ArCNO
and trans-[PdCl2(RCN)2] complexes, or RCN (R = Me, Et, CH2CN, NMe2,
Ph), in the presence of PdCl2, proceed under mild conditions and give the
1,2,4-oxadiazole trans-complexes of the type 332 in 40% to 85% yields. In
CH2Cl2, the reaction between the nitrile oxides and [PdCl2(MeCN)2] furnishes