Twisted push-pull ethylenes

Low torsional barriers in combination with strong steric interactions between donor and acceptor groups in push-pull ethylenes have in several cases been demonstrated to cause permanently twisted double bonds, in which a planar arrangement of substituents at the double bond may represent an energy maximum. 

It is also clear that the delineation of these three cases is based on the lower limit to measurability of torsional barriers. In practice, it is difficult to go below 7 kcal/mol with push-pull ethylenes, since these rather polar compounds tend to aggregate at low temperatures and give very broad bands below -120 to - 130°C. As will be discussed later, several compounds that show a Case 1 type of NMR spectrum in solution are shown by X-ray crystallography to be twisted... 

Resolution of two chiral twisted push-pull ethylenes, 144 and 145, has been performed by chromatography on triacetylcellulose (209). The barriers obtained by thermal racemization in ethanol agree well with those found by NMR band-shape technique, taking the positive AS and the difference in solvent into account (Tables 17 and 22). 

The second chapter, by Jan Sandstrom, deals with stereochemical features of push-pull ethylenes. The focus is on rotational barriers, which span a large range of values. The ease of twisting is partly a matter of electron delocalization and partly a matter of steric and solvent effects. Electronic structure and such related items as dipole moments and photoelectron spectra for these systems are discussed. The chapter also deals with the structure and chiroptical properties of twisted ethylenes that do not have push-pull effects, such as frans-cyclooctene. 

The barriers to passage of the 90° twisted state in case 1 type push-pull ethylenes have been shown to correlate roughly with Ectr of the acceptors46 and the nitro group should be a better acceptor than acyl and thioacyl groups. Consequently, push-pull systems with two nitro groups as acceptors, like 11 and 12, show practically perpendicular acceptor parts with 0 = 89.0 and 86.9°, respectively47. 

Fig. 13a-c. Push-pull cyclobutadienes as biradicaloid species [20]. a The two degenerate frontier orbitals localize upon introduction of donor and acceptor substituents as shown and energetically split by the energy gap b. b Ground and S, states can therefore be assigned hole-pair (hp - one doubly occupied, one unoccupied frontier orbital) and dot-dot character (dd - two singly occupied frontier orbitals) similarly as in the case of twisted ethylene (Fig. 7). The energetic order is determined by the interplay of electron repulsion and the orbital energy gap h which depends on the substituents, c In-plane relaxational deformations in Si can lead to an approach of S and S, and thus to fluorescence red shifts or even to photochemical funnels...

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