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

REACTIONS OF DIPOLAR 1,3-CYCLOADDITION ([3 + 2] CYCLOADDITION) 379
[M]
N1 - N4
+
N C R
N =
HC O
[M]
N C
TS
R
N1: N2:
+
N
−
O
O
N3: N4:
N+
−
O
R = CH3; CF 3; [M] = no
[M]
HC
O
N
N C
O
R
N+
O
O
N+
O
R = CH 3; [M] = trans-[PtCln(NCCH 3)]; n = 2 (750) and 4 (751)
Scheme 2.309
Experimental and theoretical studies of 1,3-dipolar cycloadditions of nitrones
to organonitriles, RCN—both free (R = CH3, CF3), or ligated to Pt 2+ and Pt 4+
(in the complexes trans-[PtCl2(NCCH3)2 (750) andtrans-[PtCl4(NCCH3)2(751)]
have been performed. The effectiveness of two types of dipolarophile activation,
(by introducing a strong electron-acceptor group R and by coordinating to a
metal center) has been analyzed and compared (830, 831). The reaction proceeds
through the formation of a Þve-membered cyclic transition state (TS) (Scheme
2.309).
The reactivity of dipoles and dipolarophiles increases along the series
N4 < N1∼N3 < N2 and CH3CN < CF3CN < (750) < (751). The latter demonstrates
that the coordination of RCN to a Pt center provides an even higher activation
effect upon cycloaddition than the introduction of a strong electron-acceptor
group R, such as CF3. The higher reactivity of the cyclic dipole N1 than of
acyclic nitrones (e.g., CH3CH=N(CH3)O) is interpreted to be a result of its
existence in a more strained and hence more reactive E-conÞguration rather than
Z -conÞguration. Consideration of solvent effects shows an increase of the activation
barriers. Such enhancement is less pronounced for the nonpolar or low polar
solvents. On the basis of kinetic and thermodynamic considerations heterocyclic
nitrones with the oxygen atom at the sixth position of the ring, such as, N2) are
predicted to be even more reactive than N1 in reactions with nitriles. Thus, use
of cyclic nitrones of the N2-type on one hand, and of the metal-bonded nitriles
on the other hand, is the most efÞcient and promising way for the acceleration
of the nitrone-to-nitrile 1,3-cycloaddition reactions. However, the nitronate N4 is
found to be less reactive toward nitriles than the acyclic nitrones (830e).
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