Enolates Hiickel calculations

The molybdenum-acyl enolate 4 has been characterized spectroscopically by NMR and has been reported to exist as a single (observable) isomer88. Extended Hiickel calculations on model complexes suggest that a conformation similar to A is most favorable for enolates such as 488. The deprotonated di-hapto acyl ligand may also be described as a 2-(C,0)-ketene ligand both the ketene and enolate terminology appear in the literature. 

Whereas the allyl anion, with a plane of symmetry through the central atom, has a node at that atom in j/ j, amides, esters, enamines, enol ethers and enolate ions do not have a node precisely on the central atom. Taking planar A V-dimethylviny-lamine and the enolate of acetaldehyde as examples, simple Hiickel calculations give the n orbitals in Fig. 2.15, which includes the allyl anion for comparison. 

Fig. 2.15 n Orbital energies and coefficients from simple Hiickel calculations of the allyl anion, enamine and enolate ion (orbital energies in kJ moP1 relative to a)... 

However, if we look at the LUMO, we find that it has the form 4.65, namely that of ift4 of benzene, but polarised by the nitrogen atom. This polarisation has reduced the coefficient at C-3, and the coefficient at C-4 is larger than that at C-2, as can be seen from the simple Hiickel calculation for pyridine itself in Fig. 4.11, which gives LUMO coefficients of 0.454 and —0.383, respectively, and an energy of 0.56/3 (compare benzene with 1/3 for this orbital). Thus, soft nucleophiles should attack at C-4, where the frontier orbital term is largest. Again this is the case cyanide ion, bisulfite, enolate ions, and hydride delivered from the carbon atom of the Hantsch ester 4.67 react faster at this site. 

Extended Hiickel calculations of the (hydroxy)quinolone 145 (X = NH R = Me) showed that while the tautomer 145a is more stable in the ground state, the tautomer 145b becomes the favored one in the excited state (80IJC(B)989). A similar keto-enol tautomerism was observed in fluorescence emission spectra of (hydroxy)quinolone 146 (98TAL1065). 

The program is reported to carry out simple Hiickel molecular orbital calculations to determine the relative sensitivity of aromatic carbon atoms to oxidation and the relative stability of keto and enol tautomers. Klopman et al. (1999) have reported that for polycyclic aromatic hydrocarbons, adequate reactivity is an essential but not sufficient condition for enzyme catalyzed reaction. The accessibility of the reactive site (i.e., the absence of steric hindrance) was also found to be important. Genetic algorithms have been used to optimize the performance of the biotransformation dictionary by treating the initial priority scores set by expert assessment as adjustable parameters (Klopman et al., 1997). 

An investigation into 3-substituted-2,4-pentadiones by H NMR and IR spectroscopy concluded that substituents that could conjugate mesomerically with the enol ring s n delocalization had most effect3). The electron withdrawal via this mechanism (D) was supported by Hiickel MO calculations. The effect of the SMe group was explained by invoking C-S pjt-djr bonding. 

Carbon atoms of the two f-butyl groups. " ) Presumable in ortho position. HMO calculation of spin densities. - By reduction of benzoyl acetone enolate a broad-lined ESR septet with On = b.45 mT was obtained. Hiickel and McLachlan calculations of spin densities. - Identical spectrum obtained by reduction of 3-deutero-dibenzoylmethide. ... 

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