Studies of polarised ethylenes—IV: Barriers to rotation around formal double bonds and formal single bonds in 1, 1‐bis‐dimethylaminoethylenes

Abstract

1 .l-Bis-dimethylaminoethylenes with electron attracting substituents on C , show rotations around the C-=C and C-N bonds, the rates of which are in most cases measurable by the N M R line shape method. The temperature-dependent four-and eight-site N M R spectra have been analysed and the barricrs to rotation around the different bonds have been evaluated by a complete line shape treatment based on the Bloch formalism. Assignment of sites to methyl groups has been made with the aid of aromatic solvent induced shifts and anisotropy effects.

Combined use of complete lineshape analysis of 1D spectra subjected to reference deconvolution and linear prediction, 2D-EXSY spectra and a double fitting method for the study of chemical exchange. Application to an eight-site exchange system

“…The steric effects are important in cases where the steric repulsion obstructs the formation of a planar -electron system in the ground state; this effect would raise its energy and lower the C C barriers. 11 In the framework of the above general discussion, our results could be explained in the following way. As mentioned above, the minor rotamer has a higher ground state energy compared with the ground state energy of the major rotamer, owing to steric repulsion between Ph and N-CH 3 groups.…”

“…Neither H 6 D nor S 6 D values nor error analysis were reported in Ref. 11. The two sets of data were obtained in different ways: rate constant calculations, solvents.…”

“…The assignment of the N-CH 3 signals was done by taking into consideration the magnetic anisotropy effect of the neighbouring groups and also the aromatic solvent induced shifts (ASIS) 11 and was confirmed by 2D-EXSY spectra. Figure 3 represents a schematic spectrum of the compound at low temperature.…”

“…This compound is an example of exchange between eight possible sites. It has already been examined by Wennerbeck and Sandström 11 and some exchange data were available.…”

Synthesis of 1,2,3,4‐Tetrahydro‐4‐oxothieno[3,2‐d]pyrimidine and Perhydropyrimidine Derivates from Alkyl Dicyanoacetates

“…The C=C bond in 3-10 and 12-19 is highly polarized by the pushpull effects of the CN and COOR groups (electron withdrawing) as well as the NH and NH, or NHCO groups (electron donating). One of the characteristics of such pushpull systems [17][18][19][20] is the very low I3C-NMR chemical shift of the carbon C-atom on the side of the electron-withdrawing groups of the C=C bond, since the electron densities on this C-atom are higher than normal. This was indeed observed in our investigations: The I3C-NMR chemical shifts of C(a) of 3-10 are ca.…”