Isomerisation about the C–N and CN bonds of E- and Z-amidines

Abstract

The unstable 2-formamidines are 'frozen' in the perpendicular configuration (6), ready isomerisation [vza the conjugate acid (4)] occurs to the E-form (2)WE have recently reported' that 1 1-addition of secondary amines to isocyanides at low temperature in the presence of silver chloride leads stereospecifically to isolable Z-formamidines, such as (12) These 2-formamidines are thermo-

Solution Chemistry of N,N’‐Disubstituted Amidines: Identification of Isomers and Evidence for Linear Dimer Formation

“…The main reasonsf or this are their rich acid-base chemistry (several protonation/deprotonation steps may occur depending on the substitution pattern), andt he fact that configurational isomerism (E/Z)w ith respect to theC =Nd ouble bond as wella sr otational isomerism (syn/anti)w ith respectt ot he CÀNs ingle bond are possible. [6][7][8][9][10][11] For example, the Z isomer is favored as the steric bulk of the substituent R 1 increases. The isomerization mechanisms comprise C=Ni somerization (a), CÀNr otation (b), andt automerization (c).…”

“…[7] In simple N,N'-disubstitutedamidines the E isomer seems energetically favored over the Z isomer;h owever,t he typea nd number of substituentsm ay change this very general trend by steric or electronic effects. [6][7][8][9][10][11] For example, the Z isomer is favored as the steric bulk of the substituent R 1 increases. [13,14] Similarly,t he presence of rotational isomersa lso depends on the substituents, but solvent effects have to be considered as well.…”

“…In simple N,N’ ‐disubstituted amidines the E isomer seems energetically favored over the Z isomer; however, the type and number of substituents may change this very general trend by steric or electronic effects . For example, the Z isomer is favored as the steric bulk of the substituent R 1 increases .…”

The Reactivity of Hexa(N‐methylideneimine)CoIII Complexes towards Nucleophiles: Structure and Mechanism

“…A type I form is also in agreement with the structure observed for the organic analogue in which the Co centre and the taci subunit are replaced by two methyl groups 27. Our calculations further revealed that the type I isomer with an E orientation for Co and NH‐phenyl and a syn configuration of the amidine moiety and the methyl group is of lowest energy 28. For this particular geometry, the calculations predicted Co III –N bond lengths in the range of 1.95–1.99 Å (mean value 1.98 Å) with the 1.95 Å and the 1.99 Å distances corresponding to the bonds to the imine and methylamine nitrogen donor, respectively.…”

D2O‐catalyzed proton transfer selectivity of 4 different NH protons in the same molecule

“…The other iminium groups will yield lower pKa. For instance, in Figure 3, the H E pKa (down figure) is lower than the H z top one because, in the former, the amidine has free single bond rotation [16] that releases the MeCNR E interaction. The 2 iminium groups of 3 ( Figure 3) with R = benzyl are even more acids due to the differences in pKa of benzylamine (9.34 [17] ) and methyl amine (10.64 [17] ).…”