Carbocyclizations catalyst control

A method for highly efficient asymmetric cyclopropanation with control of both relative and absolute stereochemistry uses vinyldiazomethanes and inexpensive a-hydroxy esters as chiral auxiliaries263. This method was also applied for stereoselective preparation of dihydroazulenes. A further improvement of this approach involves an enantioselective construction of seven-membered carbocycles (540) by incorporating an initial asymmetric cyclopropanation step into the tandem cyclopropanation-Cope rearrangement process using rhodium(II)-(5 )-N-[p-(tert-butyl)phenylsulfonyl]prolinate [RhjtS — TBSP)4] 539 as a chiral catalyst (equation 212)264. 

Miscellaneous Iminium Catalyzed Transformations The enantioselective construction of three-membered hetero- or carbocyclic ring systems is an important objective for practitioners of chemical synthesis in academic and industrial settings. To date, important advances have been made in the iminium activation realm, which enable asymmetric entry to a-formyl cyclopropanes and epoxides. In terms of cyclopropane synthesis, a new class of iminium catalyst has been introduced, providing the enantioselective stepwise [2 + 1] union of sulfonium ylides and ot,p-unsaturated aldehydes.As shown in Scheme 11.6a, the zwitterionic hydro-indoline-derived catalyst (19) enables both iminium geometry control and directed electrostatic activation of sulfonium ylides in proximity to the incipient iminium reaction partner. This combination of geometric and stereoelectronic effects has been proposed as being essential for enantio- and diastereocontrol in forming two of the three cyclopropyl bonds. 

The tremendous potential of cyclopropenes and methylene-cyclopropanes as multifunctional reagents in organic syntheses has only been recognized in the last decade. The use of transition metal catalysts allows an effective control of their transformations enabling highly selective syntheses ranging from three- to sevenmembered carbocycles. The activities in this field are summarized in this review for the first time with an emphasis on the preparative aspects of this work. 

Although hydrogenation of aromatic systems requires more energetic conditions than do those of olefinic double bonds, carbocyclic aromatics are readily hydrogenated to the fully saturated product. Controlling hydrogenolysis, selectivity and stereochemistry are major problems connected with arene hydrogenations. The substituents play an important role, and their effects are not the same for all catalysts. ... 

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