Homo-3,3-shift

The same mixture of compounds was obtained from pyrolysis of trans-, 9-dihydroindene. Since the major products were not obtained upon pyrolysis of bicyclo[6.1.0]nonatriene, it is clear that this material and bicyclo[5.2.0]nonatriene and all c/ -cyclononatetraene or its c,t,c,c or t,c,c,c isomer are not involved in these thermal isomerizations (see Section 2.1). It would appear reasonable that 1,5-hydrogen shifts could convert the rra/i5 -8,9-dihydroindene to the major product and ultimately to the other products however, not clear is how the tetracyclononene is converted to rra/i5 -8,9-dihydroindene. This latter material could undergo a concerted or stepwise homo-3,3-shift to tricyclo[4.3.0.0 ]nona-4,7-diene, but progress from there to fra/i5 -8,9-dihydroindene is not obvious (Scheme 10.25). 

Treatment of dibromocarbene adduct (43) (Rji, = O) with aqueous methanol containing silver nitrate or perchlorate gives A-homo-estra-1 (10), 2,4a-triene-4,17-dione (45) in 21 % overall yield from the enol ether (42). The exact pathway is not known, but the first step may be formation of a bromo-homo-dienone facilitated by the methoxyl group, which then undergoes further loss of hydrogen bromide involving shift of a double bond by enolization. ... 

The relative graduation of ECT and x(HOMO) is not influenced during variation of X by introduction of a methyl group at a- and p-position. However, ECT as well as x(HOMO) are shifted in the direction of increased donor ability of the monomer by the methyl group. This shift is much more visible at the p-position than the a-position, for instance, the CT energy is decreased by an a-methyl group by 9 kJ mol-1 and by P-methyl group by about 24 kJ mol-1 (see Table 12). 

This rearrangement, which has been called an enolene rearrangement, a homodienyl [1,5] sigmatropic hydrogen shift (see 18-29), and a [1,5] homo sigmatropic... 

Strong donor-acceptor interaction shifts the reaction from the pseudoexcitation band to the transfer band. Electrons delocalize from the HOMO of propene to the LUMO of X=Y too much to form a bond between the double bonds. One electron transfers and a radical ion pair forms. The negatively charged X=Y... 

How can we determine the carbon-carbon connectivities in a molecule through a combination of homo- and heteronculear shift[Pg.259]

NMR provides one of the most powerful techniques for identification of unknown compounds based on high-resolution proton spectra (chemical shift type integration relative numbers) or 13C information (number of nonequivalent carbon atoms types of carbon number of protons at each C atom). Structural information may be obtained in subsequent steps from chemical shifts in single-pulse NMR experiments, homo- and heteronuclear spin-spin connectivities and corresponding coupling constants, from relaxation data such as NOEs, 7) s 7is, or from even more sophisticated 2D techniques. In most cases the presence of a NOE enhancement is all that is required to establish the stereochemistry at a particular centre [167]. For a proper description of the microstructure of a macromolecule NMR spectroscopy has now overtaken IR spectroscopy as the analytical tool in general use. 

The terminal lobes of the HOMO will be of the same phase in a nonatetraenyl radical also, i.e. for (36, x = 3), and 1,9-shifts (in a lOe system overall) should thus be allowed, and suprafacial. Formation of the required 10-membered T.S. could present some geometrical difficulty, however, and it is somewhat doubtful whether any such concerted 1,9-shifts have actually been observed. Suprafacial thermal shifts have not been observed in other allowed , i.e. (4n + 2)e overall —(36, x = 3,5. ..), systems either. 

As can be deduced from the electronic transition in the visible spectra, the HOMO and LUMO of flavonoids can shift strongly on changing the substituents, pH and solvents. For acidic pH values, the molecule is positively charged, whereas for basic pH values the species is negatively charged (see Scheme 9.1). 

The adsorption spectra in the UV-visible range of the extracts exhibit different broad bands depending on the component of the mixtures photocurrent measurements show larger bands in comparison with those derived from adsorbed dyes. In particular, in the case of Mix, a bathochromic shift of about 40 nm was observed the proposed rationale is a band interruption between density states of Ti02 and the HOMO-LUMO in the dye. Moreover, the photocurrent response derives predominantly from the superposition of those of Morus nigra and carminic acid. 

---