Push-pull-type compounds

Figure 6.20 Push-pull type of compounds bearing heptafulvene moieties.

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... 

The ultraviolet absorption spectra of a number of untwisted and twisted compounds of the types 5, 6 and 7 have been studied43,45 and found to be compatible with the chromophore in the acceptor part. The same was found for the photoelectron spectra of a series of more or less twisted simple push-pull ethylenes56. 

The insertion of a photosensitive group or of a redox active unit into the push-pull system 1 yields switchable molecular wires and push-pull molecules that contain a photo-switch or a redox switch S, as represented in 4. Compounds of such type containing for instance electroactive ferrocene groups and photosensitive metal complexes, have been synthesized. Some of them are shown in series 5 (Marczinke, B. Przibilla, K.J. Lehn, J.-M., unpublished data). 

Heptafulvenes assume a push-pull character when the exocyclic carbon atom (C8) is substituted by acceptor groups90. Daub and coworkers91 have studied this type of compound with dimethylamino groups and other donor groups on C8. NMR spectra of the bis(dimethylamino) compound 41a show that the compound is boat-shaped with alternating single and double bonds, and a barrier to rotation about the exocyclic C=C bond >27 kcal mol-1 is reported for the 8-dimethylamino-8-triethylsilyloxy compound 41b. No low-temperature spectra are reported, but the C—N barrier is likely to be low. 

Structures of type 15 to 20 can also be considered aza analogues of ( )-stilbene compounds 17 and 18 are an unusual type of push-pull ( )-stilbene that should show a spectrum of properties ranging between those of ( )-stilbenes and betaines. The 19 types of compounds selected, 10-28, and their precursors, 29-47, outlined in Scheme 3 and Table I, have been ordered by (a) the nature of the interannular linkage (-Q-) (b) the nature of the two atoms linking the 7r-deficient nucleus and the interannular group (C—N bond type and C—C bond type) (c) the substitution pattern between the 7r-deficient nucleus and the interannular group. 

At present, the accessible physico-chemical properties have been studied mainly in liquid solution and the overall results provide evidence of their intrinsic high dipolar character. Moreover, compounds of type 1 may be ideal substrates for the study of their photophysical and other physical properties, especially for unconventional extended rr-systems 15-20, which are push-pull aza analogues of (E)-stilbene. Their capacity for specific physical behavior merits further exploration. 

The other type of stable cyclobutadiene has two electron-donating and two electron-withdrawing groups,and is stable in the absence of water. An example is 58. The stability of these compounds is generally attributed to the resonance shown, a type of resonance stabilization called the push-pull or captodative... 

Synthesis of amides and peptides. The simplest compound of this type, dimethylaminopropynal (CH3)aNC=CCHO, is known, but it polymerizes within minutes at room temperature. The thermal stability is improved by introduction of bulky constituents. A whole series of these acetylenes have been prepared of these 1 and a few related compounds have been found useful for synthesis of CO—NH bonds by a push-pull mechanism shown in equation (I). The reaction of 1 with a carboxylic acid proceeds by Michael addition to give a, which rearranges by a cyclic intermediate (b) to the enol ester (2). Reaction of 2 with an amine yields an amide (3) and the water adduct (4) of 1. Yields of amides are in the range 85-95y . This sequence cannot be used for esterification of carboxylic acids. 

As a further extension of push-pull dipole cycloaddition chemistry, the Rh (I I)-catalyzed cycHzation/cycloaddition cascade was applied toward the hexacyclic framework of the kopsifoline alkaloids. The kopsifolines 14 are structurally intriguing compounds, related to and possibly derived from an aspidosperma-type alkaloid precursor 12. A possible biogenetic pathway to the kopsifolines from 12 could involve an intramolecular epoxide-ring opening followed by loss of H2O as shown in Scheme 4. The interesting biological activity of these compounds, combined with their... 

The types of DPP molecule we have discussed are broadly classified into two groups mono-DPP where one DPP unit serves as the molecular core, and bis-DPP where two DPP molecules are incorporated as the end-cap units. The initial success of the former was replicated by the latter and has been appropriately extended to a tris-DPP molecular system, effectively increasing the n-conjugation of the system and enhancing the push-pull characteristics of the molecule. Compounds 19 and 20 (Figure 5.10), incorporate three DPP units, which impart a low-lying HOMO energy level of -5.4 and -5.2 eV... 

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