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

O
R 1
N
51o-t
OSiMe2R 3
R 2
PRINCIPAL PHYSICOCHEMICAL DATA AND CHARACTERISTICS 489
+ R 4 2BX
X
B Si
O N O
71n: R1 = R2 = CO2Me, R3 = Me, R4 = X = F (80%);
71o; R1 = R2 = CO2Me, R3 = Me, R4 = H, X = Cl (80%);
71r: R1 = R2 = Me, R3 = Me, R4 = Et, X = Cl (95%);
71s: R1 = R2 = Me, R3 = Pri , R4 71p: R
= Et, X = Cl (59%).
1 = H, R2 = CMe2CO2Me, R3 = Me, R4 = Et, X = Cl (70%);
R 1
R 2
Scheme 3.71
3.3. PRINCIPAL PHYSICOCHEMICAL DATA
AND CHARACTERISTICS
R 1
R 2
O
N
O
BR 4 2
BR 4 2
+
R3 71n-s
SiMe2 X
O
N
O
Since covalent nitronates contain several reaction centers, these compounds have
versatile reactivity. On the one hand, high reactivity of these derivatives offers
considerable scope for their use in organic synthesis. However, on the other hand,
high reactivity is to a large extent responsible for the instability of nitronates. As
a result, these compounds require special handling and special conditions for their
storage. Hence, it is appropriate to begin this section with a discussion of stability
of nitronates, as their main property, which is to a larger extent responsible for
the possibility of practical use of these derivatives.
3.3.1. Stability of Nitronates
Since different types of nitronates decompose through different pathways, their
stability will be considered separately.
3.3.1.1. Stability of Acyclic Alkyl and Acyl Nitronates A weak point of acyclic
alkyl nitronates is their thermal instability because these compounds can be
involved in two electrocyclic reactions presented in Scheme 3.72.
According to the Þrst process, acyclic alkyl nitronates (73) afford corresponding
oximes and carbonyl compounds (3) (Eq.1). This process is similar to the
well-known Cope rearrangement (Eq.1 ′ ) (233).
However, if protons at the α-carbon atom of the O-alkyl fragment are absent,
another process can occur resulting in elimination of oleÞn and generation of the
respective aci-nitro compound (Eq.2). In particular, this “anomalous” decomposition
was found by Nenitzescu and Isacescu (234) for nitronate 74a (Eq. 3).
Spectrophotometric study of the decomposition kinetics of nitronate
(MeO2C)2C=N(O)OMe (73a) demonstrated that this compound decomposes at
25 ◦ CbyaÞrst-order reaction according to Equation 1 (Scheme 3.72) to give the
R 2
R 1