A deprotonated intermediate in the amide methanolysis reaction of an N4O-ligated mononuclear zinc complex.
Treatment of [(ppbpa)Zn](ClO4)2 (1(ClO4)2, ppbpa = N-((6-(pivaloylamido)-2-pyridyl)methyl)-N,N-bis((2-pyridyl)methyl)amine) with 1 equiv of Me(4)NOH.5H(2)O in methanol-acetonitrile solution results within minutes in the stoichiometric formation of a complex having a deprotonated amide, [(ppbpa-)Zn]ClO4 (3). Complex 3 has been characterized by 1H and 13C NMR, FTIR, and elemental analysis. Notably, upfield shifts of specific 1H NMR resonances of the amide-appended pyridyl moiety in 3, versus those found for 1(ClO4)2, indicate delocalization of the anionic charge within the amide-appended pyridyl donor of this complex. Heating of analytically pure 3 in methanol-acetonitrile results in amide alcoholysis. Overall, this alcoholysis reaction is second-order, with a first-order dependence on both 3 and methanol. Analysis of the rate of decay of 3 as a function of temperature yielded activation parameters consistent with an intramolecular amide cleavage process (DeltaH++ = 15.0(3) kcal/mol, DeltaS++ = -33(1) eu). A possible reaction mechanism for amide alcoholysis is presented which involves reaction of the deprotonated amide intermediate 3 with methanol to produce a Lewis activated-type structure from which amide cleavage may be initiated. Additional support for this mechanistic pathway has been obtained through examination of the analogous ethanolysis reaction and via evaluation of the effect of varying steric hindrance near the amide carbonyl unit.
Szajna
Ewa
E
Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322-0300, USA.
Makowska-Grzyska
Magdalena M
MM
Wasden
Chad C
CC
Arif
Atta M
AM
Berreau
Lisa M
LM