Carbonyl electron-accepting

Photochemical reactions of carbonyl compounds with alkenes give the oxetanes (Scheme 30). The stereochanical course depends on the substituents of the alkenes [16]. The reactions proceed with the retention of the configuration of the alkenes for the electron accepting substituent, e.g., CN. The stereochemical integrity is lost for the donating group, e.g., OCH. ... 

Scheme 30a,b [2 + 2] Cycloaddition reactions of excited carbonyl compounds with the alkenes substituted by electron-accepting (a) and -donating (b) groups... 

The [2h-2] cycloaddition could occur thermally in the pseudoexcitation band. In fact, an alkyne with electron-donating group, ethoxyacetylene, and electron accepting carbonyl compound, perfluoroacetone, form the oxetene at low temperature (-78 °C) without light irradiation (pseudoexcitation band in Scheme 6) [26, 27],... 

Thermal [2h-2] cycloaddition reactions of carbonyl compounds were catalyzed by a Lewis acid. The catalyst forms complexes with the carbonyl compounds and enhances the electron-accepting power. The reaction shifts from the delocalization band to the pseudoexcitation band. Catalyzed [2h-2] cycloaddition reactions were observed with acetylenic compounds [28] and ketenes [29-31]. 

Electron-donating substituents in conjugation with the carbonyl group have the effect of stabilising the (7r,7r ) triplet state but destabilise the (n,7t ) triplet state. On the other hand, the (n,7r ) triplet state is stabilised relative to the (n,n ) triplet state by the presence of electron-accepting substituents. 

Amides possess planar or almost planar structures (1 and 2). Their rotational ground state is stabilized because the amino group is a strongly electron-donating group and the carbonyl function is strongly electron accepting. Excellent reviews on this topic have been published (28,29,30), and should be consulted by readers interested in amide rotation. 

Silane o-complexes [M(p -HSiR3)(CO)5] are interesting in that here the electron density of metal is effectively delocalized over five electron-accepting carbonyl ligands so that a situation of almost negligible backdonation is modeled well. Therefore, the M" H "Si interaction in this system is very close to the 3c-2e limit thus, it is not surprising that these complexes are highly unstable. 

Figure 14.7. Aromatic dianhydrides arranged in order of calculated electron affinity (EAjai) and C-NMR chemical shift of carbonyl carbon ( ( ). Larger EAd and smaller 6(. correspond to the strong electron-accepting property of a dianhydride.

---