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

654 NITRONATES
E +
+
N OSi
N OSi +
OSi
E OSi
Si – trialkylsilyl; E + A
– cationic intermediate
Scheme 3.222
Another approach to the generation of bis-N,N-oxyiminium cations is shown
in Scheme 3.222.
This approach will be considered in more detail in Section 3.5.4.2.1. In these
processes, it should be noted, the only aim is to decompose the cations A as
rapidly as possible by facilitating elimination of Si + . Hence, the use of this
approach to the synthesis of intermediates A in C,C-coupling reactions is not
promising.
3.5.3. Elimination Reactions with Silyl Nitronates and Ene Nitroso Acetals
Silylation of AN and cyclic nitronates affords SENA and BENA or cyclic N -
siloxy-ene nitroso acetals as the major primary products (see Sections 3.2.1.3 and
3.5.1). All these relatively unstable derivatives can undergo various elimination
reactions, which will be considered in separately.
3.5.3.1. Elimination Reactions with Silyl Nitronates Most elimination reactions
of SENA involve cleavage of the weak N–O bond or cleavage of the O–Si
bond. In the latter case, the reactions could occur with the participation of hypervalent
silicon in the transition state, that requires the presence of an external
nucleophile.
3.5.3.1.1. 1,2-C,N Elimination These processes are typical of SENA containing
an electron-withdrawing substituent at the α-carbon atom and occur at high
temperatures (176, 203) (Scheme 3.223).
If such SENA contains the proton at the α-C atom (203), trimethylsilanol is
eliminated, which causes partial hydrolysis of the nitronate.
Trimethylsilyl ether of trinitromethane decomposes at ambient temperature
(176) to give very unstable trimethylsilyl nitrite. Another decomposition product,
nitronitrile oxide, can be trapped in low yield as the adduct with triphenylphosphine.
3.5.3.1.2. 1,3-C,N Elimination Standard SENAs are quite stable at ambient conditions.
However, if an activating electron-withdrawing substituent is attached at
the β-carbon atom, spontaneous elimination of trialkylsilanol occurs to form the
corresponding α-nitrosoalkene C (491, 496–498) (Scheme 3.224).
−Si +
E
N
OSi
O