Active aldehyde, electronic charges

In the case of Pt-Fe, the two metals were suggested389,421 to be in the metallic state forming an alloy. EXAFS studies indicated an electron transfer from Fe to Pt, which allowed the conclusion that the aldehyde double bond is more easily adsorbed on the induced sites . In the case of Pt-Sn and Pt-Ge, the authors suggested that Snn+ or Gen+ ions (Lewis acids) activate the carbonyl group by enhancing the positive charge of the carbonyl carbon (equation 32). 

Regioselectivity in hydroformylation is influenced by electronic and steric effects [4, 5]. Thus the formation of the C a-Rh bond is favored over that of the C P-Rh bond by the well known P-silicon effect (Fig. 3), which stabilizes a positive charge on the p-C atom. From the resulting intermediate la the /50-product should form predominantly. On the other hand, steric effects induced by bulky substituents on silicon or rhodium would favor the sterically less hindered normal alkyl rhodium complex with the C P Rh intermediate Ila as the precusor to the -aldehyde. The observed //so-ratios very close to 1 1 for the Rh-catalyzed hydroformylation of vinyltrimethylsilane indicate that the electronic P-effect obviously is canceled out by the steric demand of the MesSi-groups. Since addition of PPha will favor an active complex with a larger number of bulky phosphine ligands (L = PPhs in Fig. 2), the formation of the linear alkylrhodium complex intermediate Ila to lid is prefered [6]. 

Solutions of maleic anhydride in ether will initiate catiOTiic polymerizations of isobutyl vinyl ether or A -vinyl carbazole, if subjected to attacks be free radicals. The same is true if the solutions are irradiated with ultraviolet light or gamma rays [62, 63], Also, active species are generated from reactions of aldehydes or ketones with maleic anhydride when attacked by free radicals or irradiated by UV-light, or gamma rays from °Co [63]. These active species are presumed to be formed through charge-transfer reactions that occur between the electron acceptors, 7i-acids, or electron donors, n-bases, which form cations [64]. 

Glyceraldehyde phosphate dehydrogenase probably holds the distinction of being the classic thiol enzyme in the minds of most biochemists . The thiol is believed to be involved in the initial attachment of the aldehyde substrate as a thiohemiacetal. The em me-bound thiohemiacetal is then oxidized by NAD generating an enzyme-bound thioester. In more sophisticated proposals for this mechanism the nicotinamide cofactor interacts with the active centre thiol as a charge transfer type of complex. This facilitates the reaction of the thiol with the carbonyl of the substrate. The thiol addition and the electron transfer to nicotinamide occur... 

When electron-withdrawing substituents such as chlorine and fluorine are substituted on a silicon atom of silyl enol ethers and allylsilanes, the silicon center becomes Lewis acidic enough to activate carbonyls. Coordination of a Lewis base carbonyl to the silicon atom further enhances the Lewis acidity of the silicon center due to delocalization of a negative charge over ligands to allow diastereoselective allylation of aldehydes through a six-membered transition state involving a hexacoordinate silicate species (Scheme 3-112). " ... 

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