Angelo Clerici

Efficient acetalisation of aldehydes catalyzed by titanium tetrachloride in a basic medium

Abstract The acetalisation of aliphatic and aromatic aldehydes is achieved in a basic medium by using catalytic amount of Ti(IV) chloride in MeOH in the presence of NH3 or Et3N. The present protocol shows many advantages over the well known base or acid catalysis: in fact, in contrast to base-promoted acetalisation, aldehydes with electron-rich carbonyl groups react easily, enolizable aldehydes do not undergo aldol condensation and, in contrast to acid-catalysis, migration of the double bond does not occur in the preparation of α, β-unsaturated acetals. Download : Download full-size image

Mild acetalisation of mono and dicarbonyl compounds catalysed by titanium tetrachloride. Facile synthesis of β-keto enol ethers

Abstract The use of TiCl4, as a catalyst for the acetalisation, at room temperature, of carbonyl compounds is reported. Cyclic ketones and cyclic 1,4-diketones easily afford dimethyl acetals, but cyclic 1,3-diketones give β-keto enol ethers. Additionally, aryl ketones and acyclic ketones failed to react. β-keto aldehydes can be monoprotected either as β-keto enol ethers or β-keto dimethyl acetals depending on the reaction time and catalyst amount. Some mechanistic features are accounted for.

A highly dl-stereoselective pinacolization of aromatic aldehydes mediated by titanium trichloride in dichloromethane

Aromatic aldehydes are stereoselectively coupled to dl-hydrobenzoins on treatment with anhydrous TiCl3/CH2Cl2 solution at room temperature. The observed stereochemistry is briefly discussed in term of Ti(IV)-bridging control.

Pinacolic coupling of aromatic aldehydes and ketones promoted by aqueous titanium trichloride in basic media.

Aromatic aldehydes and ketones, which are not effected by aqueous titanium trichloride in acidic media, undergo rapid one-electron reduction to pinacols in basic media under very simple experimental conditions. The increase in the reducing power of the redox system with increasing the pH is discussed, and the stereoselectivity observed is shown in terms of Ti(IV) bridging control.

Arylative amination of aldehydes promoted by aqueous titanium trichloride

Free-radical decomposition of arene-diazonium salts 1, catalyzed by aqueous TiCl3, in the presence of aldehydes 2 and anilines 3 leads to secondary amines 4. A mechanism is proposed, according to which the aryl radical adds to the C-atom of the intermediate protonated imine.

Radical addition to the carbonyl carbon of ketones promoted by aqueos titanium trichloride in acidic medium, one step synthesis of pinacols and lactones

Abstract Carbon centered radical 1 , generated by reduction of methyl phenyl-glyoxalate with Ti(III) ion, adds to the carbonyl carbon of ketones 2 to afford pinacols 3 . Yields of 3 are remarkably influenced by the structure of 2 , in that the reaction is very sensitive to steric factors. Many other functional groups present in 2 do not interfere with carbonyl addition, but lactonization of 3 occurs when the additional groups are COOH, COOC2H5, and OH. Alkyl radical addition to ketones is a reversible process, but the rapid reduction of the resulting alkoxy radical 8 by Ti(III) ion makes the whole process stpractically irreversible and accounts for pinacol formation.

Reductive reactions of substituted pyridines by aqueous titanium trichloride

Abstract Aqueous titanium trichloride reductively removes cyano and halo groups from the correspondingly substituted pyridines by a two electron-transfer process and promotes reduction of pyridyl-ketones and aldehydes to glycols by one electron-transfer process under very simple experimental conditions.

A tricomponent reaction promoted by a titanium trichloride/pyridine system diastereoselective synthesis of β-amino-α-hydroxyesters

Abstract Methyl phenylglyoxylate, aniline and aromatic aldehydes, on treatment with TiCl 3 /pyridine in anhydrous THF at room temperature, undergo rapid condensation to produce syn -β-amino-α-hydroxyesters. The mechanism of the reaction is rationalized.

Nucleophilic character of alkyl radicals—IX : The benzyl radical

Abstract Homolytic benzylation of 4-substituted pyridines gives 1,2-diphenylethane and a benzyl-4-substituted pyridine in a ratio determined by the nature of the 4-substituents. Substituents in the benzyl radical affect its polar character, and a Hammett correlation is observed. The results are compared with those obtained for benzoyl radicals under the same conditions.

Facile Reduction of Aromatic Aldehydes, Ketones, Diketones and Oxo Aldehydes to Alcohols by an Aqueous TiCl3/NH3 System: Selectivity and Scope

A simple and rapid procedure for the almost quantitative reduction of aromatic aldehydes, ketones, diketones and oxo aldehydes to alcohols by use of TiCl3/NH3 in aqueous methanol solution is reported. The reducing system distinguishes between different classes of aldehydes and/or ketones, and many functionalities that usually do not survive under reducing conditions are tolerated well. The concept of reversal of chemoselectivity has also been developed. A mechanism based on two sequential one-electron transfers from TiIII to the carbonyl carbon atom is proposed, the second SET becoming operative only in the presence of ammonium ion (either added or formed in situ). (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)