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

NITRONE COMPLEXES 203
radiolysis in squalane (439, 440). Analysis of HFI constants and quantumchemical
calculations indicate that the RA of a Þve-member cyclic nitrone has
a non-planar geometry with a pyramidal N + -O − group and that the spin density
in the RA is localized in the C=N + -O − fragment.
Other experiments suggest that nitrone deoxygenation proceeds through intermediate
formation of nitrone RC, rather than the product of its isomerization, a
oxaziridine (441).
2.5. NITRONE COMPLEXES
The electron-donor N -oxide oxygen atom of a nitrone makes it suitable for complexation
and protonation. Such properties of nitrones have been widely used to
inßuence their reactivity, using Lewis acids and protonation in nucleophilic addition
reactions (see Section 2.6.6). In this chapter, the chemistry of nitrones with
various metal ions [Zn (II), Cu(II), Mn (II), Ni (II), Fe (II), Fe (III), Ru (II), Os
(II), Rh (I), UO2 2− ] (375, 382, 442–445), and diarylboron chelates is described
(234–237, 446). Accurate descriptions of the structures of all complexes have
been established by X-ray analysis.
DMPO has been used in the synthesis of the Þrst metalloporphyrin nitrone
complex (443). On the basis of nitrone ligands (L) (Scheme 2.81) the synthesis
of rhodium (I) carbonyl complexes of the type [Rh(CO)2ClL] was carried out.
These complexes are used as effective catalysts of methanol carbonylation into
acetic acid and its ester (444).
OC
Rh
OC
Cl
Cl
CO
Rh
CO
+
L
stirred at. r.t. in CH2Cl2
2 h
OC
Rh
OC
L = a, N-diphenylnitrone (a); α-styryl-N-phenylnitrone (b); N,N ' -diphenyldinitrone
(c); a-(2-furyl)-N-phenylnitrone (d)
Me
O
Ph
Me Ph
+
B O
N
−
Ph
Scheme 2.81
ΔT
B O
Me Ph
Me
N
O
Ph
Ph
238 239
Scheme 2.82
Cl
L