Catalysts, acidic electron-release

The electron-releasing 4-substituents are considered to stabilize the 3,4 double bond toward hydrogenation. An electron-releasing 7-substituent (OMe, NH2, or OH) may also contribute to the stabilization of the 3,4 double bond. Thus the hydrogenation of 7-methoxy-4-methylquinazoline to the 3,4-dihydro derivative was successful only in aqueous hydrochloride or aqueous acetic acid, using palladium and platinum catalysts (eq. 12.84).158 No reduction took place at room temperature in nonaqueous solvents such as ethanol, ethanol-HCl, and acetic acid. 

Allylic alcohols can also allylate indole in the presence of triethylborane and a Pd catalyst [94]. This system is capable of allylating a number of nucleophiles and is believed to proceed through a 7t-allylic-Pd intermediate. The borane functions as a Lewis acid, activating the allylic alcohol towards oxidative addition [95]. With unsymmetrical alcohols, a mixture of the allylic regioisomers is observed. The reaction proceeds satisfactorily with both electron-releasing and electron-withdrawing C-ring substiments. With 3-methylindole the indolenine is isolated. 

Besides the use of dienes and dienophiles that have complementary electron-releasing and electron-donating properties, other factors found to enhance the rate of Diels-Alder reactions include high temperature and high pressure. Another widely used method is the use of Lewis acid catalysts. The following reaction is one of many examples where Diels-Alder adducts form readily at ambient temperature in the presence of a Lewis acid catalyst. (In Section 13.IOC we see how Lewis acids can be used with chiral ligands to induce asymmetry in the reaction products.)... 

Oxygen Nucleophiles. A reagent such as permanganate oxidizes toluene to benzoic acid, whereas benzylic oxidation by palladium acetate results in benzyl alcohol derivatives. The oxidation is favored by electron-releasing substituents in the phenyl ring. Catalytic amounts of palladium acetate and tin diacetate, in combination with air, effects an efficient palladium-catalyzed benzylic oxidation of toluene and xylenes. For the latter substrates, the Q, Q -diacetate is the main product.A mixed palladium diacetate-copper diacetate catalyst has also been found to selectively catalyze the benzylic acyloxylation of toluene (eq 64). ... 

This side reaction makes it difficult to control the anode reaction, and reduces the number of electrons released in BH4 electrooxidation. The reduction of hydrolysis on the anode catalyst is important for the improvement of DBFCs. Thus, the development of a new BH4 electrooxidation catalyst that does not stimulate BRt hydrolysis is required. A Rh porphyrin catalyst was investigated. Previously, it is demonstrated that carbon-supported Rh porphyrin catalysts have strong activity for the electrooxidation of several small molecules (CO, glucose, and oxalic acid) [33-36]. On the other hand, many hydride complexes of Rh have been reported [37]. Thus, it is expected that Rh porphyrin catalysts would exhibit BH4 electrooxidation activity. The catalytic activity of Rh porphyrin catalyst for BH4 electrooxidation was investigated. This new catalyst is different from other noble metal electrocatalysts such as Pt/C or PtRu/C in that the molecule is an active site. Hence, only a small amount of Rh is needed in Rh porphyrin catalysts for sufficient activity. 

Aldehydes that have available only one hydrogen in the alpha position, such as the a-alkyl aldehydes (—CHR—CHO), are less acidic than those containing a —CH2— group in the alpha position. This is due to the electron-release effect of the alkyl group which displaces the C—electron pair nearer to the proton and makes the proton removal by the basic catalyst more difficult. Aldolization of such aldehydes would, therefore, require strongly basic catalysts. 

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