Amine catalysis in the vinylic substitution of α-methylthio-α-arylmethylene Meldrum's acids and its absence in the substitution of methyl β-iodo-α-nitrocinnamate by amines

Substitution of the iodine of (E)- and (Z)-methyl beta -iodo-alpha -nitrocinnamates (5) by amines-gives identical (Z)-enamines with aniline (Ani) and piperidine (Pip). No amine catalysis was observed with Pip, Ani, morpholine (Mor), or p-MeOC6H4NHMe (MMA) in MeCN nor with Pip or Mor in EtOH: k(pip)/k(Mor) = 115-138 (MeCN), 3:3-6.9 (EtOH); k(MeCN)/k(EtOH) = 25.5 +/- 2.2 (Pip), 0.79-1.16 (Mor); k((Z).5)/k((E).5) = 1.3-2.9 (13.5 with MMA in MeCN). Replacement of the MeS group in six alpha -methylthio-alpha -arylmethylene Meldrum's acid (6-X) by Pip resulted in amine catalysis in MeCN and EtOH: In EtOH, the p-anisyl derivative (6-MeO) and in MeCN 6-MeO, 6-Me and 6-H displayed second order catalysis in Pip. Other 6-X compounds show orders between one and two in Pip with amine catalyzed (k(3B))/non-catalyzed (k(2)) rate coefficient ratios of 281-731 (EtOH) and 504-635 (MeCN) at 30 degreesC. k(MeCN)/k(EtOH) 3.0-4.9. In MeCN DeltaH double dagger = -0.8 to -5.9 kcal mol(-1) and DeltaS double dagger == -50 to -72 e.u. An intermediate zwitterion, 3a, is formed in all cases: For system 5 the rate of I- expulsion from 3a exceeds its deprotonation rate, and the observed rate coefficient is composite: k(obs) = k(1)k(2)/k(-1) in MeCN (k(1) = rate coefficient of nucleophilic attack) but k(obs) = k(1) in EtOH. In MeCN the deprotonation is faster than the expulsion rate of MeS-, and more so for 6-X with X = p-Br, p-CF3, m,m'-(CF3)(2). Different electrophilicities of 6-X, different extents. of hydrogen bonding, steric and electronic effects account for the kinetic differences.