Current Organic Chemistry - Volume 13, Issue 9, 2009

Palladacycles containing tridentate ligands have become a very promising family of complexes due to an excellent balance between stability and reactivity that enables them to catalyze a number of organic reactions. Moreover, special chemical properties of such interesting compounds have been considered as keys to determine the mechanisms of many palladium-catalyzed reactions. Although the most commonly employed are those with two identical donor groups, that is, symmetrical pincers, the interest in the preparation and synthetic applications of unsymmetrical ones has considerably increased because the presence of two different donor groups provides them unique properties and reactivity. In fact, high efficiency in terms of outstanding turnover numbers and frequencies has been achieved by using this emerging class of palladacycles. In this article it is projected to compile the advances achieved in the synthesis and catalytic applications of unsymmetrical palladium pincer complexes with a special focus on cross-coupling reactions, the processes in which the catalytic activity of such specially designed catalysts has been mainly evaluated.

A comprehensive review of the reports in the literature concerning conjugation of polyunsaturated acid and their esters (vegetable oils such as: linseed oil, sunflower oils, rape oil, soybean oil; linoleic acid, methyl linoleate etc.) catalyzed by supported transition metals and their complexes is presented. The following issues are analyzed: chemo-, regio-, and stereoselectivity of the double bond migration in polyenes, mechanism of conjugation and accompanying isomerization process, hydrogenation and E-Z isomerization, stability of homo- and heterogenous catalysts, removal of catalysts from the post-reaction mixture, and possibility of catalysts' recycling.

A broad range of fundamental free radical reactions, such as hydrogen and halogen atom transfer, radical deoxygenations, radical cyclization reactions, radical additions to C=C and C=N double bonds, in β-cyclodextrin based molecular reactors at room temperature are investigated. The reactions proceeded smoothly in moderate to excellent yield at ambient temperature.

Intramolecular haloetherification and transannular hydroxycyclization of alkenes is a synthetic methodology to obtain polycyclic ethers and amines in a regio- and stereoselective manner. The stereoelectronic effects controlling this selectivity are discussed. Also, the methodology based on [4+3]-cycloaddition followed by intramolecular haloetherification or transannular hydroxycyclization is described as a very useful tool to prepare the dioxa- and oxazatricyclic cores of several bioactive natural products, whose molecular scaffold is quite difficult to reach by other synthetic approaches.

The main methods reported in literature to obtain stereoselective photochemical reaction is reviewed. The most important approached attempted to obtain stereoselective photochemical reaction are: 1.) the use of diastereoselective photochemical reactions, 2.) the use of molecules with prevented mobility (this approach can be obtained 1. introducing chiral auxiliary, 2. introducing chiral molecules in the reaction mixture able to give complexes with a reduced mobility, 3. performing the reaction in organized media such as zeolites or cyclodextrins, 4. performing the reaction on single crystal, 5. performing the reaction on single crystals of molecules able to give crystal in chiral space group, and, 6. finally performing the reaction in solid phase on inclusion complexes of the substrate with chiral molecules); 3.) using chiral solvents and chiral light, and 4.) using chiral photosensitizers. The case of the diastereoselective Paternò-Büchi reaction on furan derivatives is also discussed.