INVERSION OF KINETIC AND THERMODYNAMIC PREFERENCES IN MEISENHEIMER COMPLEX-FORMATION - REGIOSELECTIVITY IN THE REACTION OF 2,4,6-TRIMETHYLPHENOXIDE ION WITH 2,4,6-TRINITROANISOLE AND THE IMPORTANCE OF STEREOELECTRONIC FACTORS

To probe regioselectivity in Meisenheimer complexation, the reaction o f 2,4,6-trimethylphenoxide ion (MesO-) with 2,4,6-trinitroanisole (TNA ) was followed by H-1 and C-13 NMR spectroscopy in the new low-tempera ture solvent acetonitrile-glyme (1:1, v/v) at -40-degrees-C to ambient temperatures. The key results include finding a kinetic preference fo r C-1 attachment and that the sigma-adduct that results from C-3 attac k is thermodynamically more stable (K1T3 behavior); this behavior is i n direct contrast to the ''normal'' C-3 to C-1 isomerization pathway d isplayed by alkoxides and hydroxide with TNA (K3T1 behavior) as well a s different from the regioselectivity shown by the phenoxide ion as an O-nucleophile (K1T1 behavior). Stereoelectronic stabilization of C-1 O-adducts through n-sigma interaction of antiperiplanar lone pairs wi th the requisite C-O acceptor bonds is recognized as a major factor in the regioselectivity of C-1 versus that of C-3 attack. Such an antipe riplanar conformer is found to be energetically inaccessible for the C -1 TNA.OMes- adduct 3, unlike the C-1 TNA.OMe- adduct 4. Estimation of the kinetic barriers and thermodynamics of adduct formation demonstra tes the instability of 3 relative to that of 4 and thereby provides a basis for the slide or metamorphosis of K3T1 to K1T3 behavior.