Current Organic Chemistry - Volume 6, Issue 10, 2002

Pd-catalyzed furan ring formation processes involving acyclic units are reviewed. A number of methods, which allow the construction of wide variety of functionalised furan rings from acetylenic building blocks such asalkynones, alkynols, epoxyalkynes, and o-alkynyl phenols have been described. Alkynes containing proximate oxygen nucleophile in the presence of palladium complexes undergo cyclization to form furans via formation of new oxygen-carbon bond. Tandem cyclization reactions have been utilized for the construction of a wide variety of functionalised furans rings and providing important new dimensions in the design of synthetic strategies.

In this review we introduced many kinds of bifunctional N-substituted amides, including phosphinamides and phosphoramides, squaramides, sulfonamides and so on which were applied to the asymmetric reactions. More emphasis were given to the asymmetric borane reduction of prochiral ketones, the reaction conditions and mechanisms were discussed systematically. At the same time, other reactions, including asymmetric addition, Diels-Alder reaction and Aldol reactions catalyzed by chiral N-substituted amides were also reviewed.

The Diels-Alder reaction of 1,2-dihydrophosphinine oxides with dienophiles, such as dimethyl acetylenedicarboxylate (DM AD) and maleic acid derivatives affords phosphabicyclo [2. 2.2] oct adien es or bicyclooctenes, respectively, that are precursors of low-coordinated P-fragments, methylenephosphine oxides useful in the phosphorylation of alcohols, phenols and amines. An alternative mechanism involving an intermediate with a pentavalent, pentacoordinated phosphorus atom was also substantiated and proved. Reaction of the P-2, 4, 6-trialkylphenyl dihydrophosphinine oxides with DMAD followed an unexpected protocol to give the corresponding stabilised phosphonium ylides. The intermediates are oxaphosphetes formed by the [2+2] cycloaddition of the P=O group of the P-cycle and the triple bond of DMAD. This kind of reaction, that is the consequence of the presence of the electronreleasing trialkylphenyl group, has never been observed earlier and proved to be general to take place with a variety of cyclic phosphine oxides.

The scope of hydrosilylation of carbonyl and imine compounds in synthesis has greatly expanded because of the wide array of catalyst systems that are now available for these reactions. Beside rhodium and titanium complexes that have retained most attention in the past decades, new catalyst systems based on less traditional transition metals and on main group elements have recently emerged, that enable outstanding achievements in chemo- and / or enantioselective transformations. Also, investigators have focused on the development of friendly, cost-effective processes utilization of safe and inexpensive polyhydrosiloxanes such as PMHS (polymethylhydrosiloxane) instead of the usual molecular hydrosilanes and direct reduction of carbonyl compounds in protic solvents have attained high synthetic efficacy. This review will cover the development and application of these new hydrosilylation catalyst systems.