Direkte Aminierung, Cycloaddition und Cyclopropanierung : ein breites Spektrum an Transformationen zugänglich durch Calcium-Katalyse

Topic of this work was the development of calcium catalyzed reactions. Despite its unexplored catalytic potential, calcium is an inexpensive and nontoxic metal and thus it is highly desirable to use it as a catalyst. In the first part the direct amination of pi-activated alcohols with nitrogen nucleophiles was developed. A catalytic amount of Ca(NTf2)2 und Bu4NPF6 as an additive allowed for an efficient conversion of secondary and tertiary benzylic and allylic as well as tertiary propargylic alcohols with carbamates, sulfonamides, anilines and carboxamides under very mild reaction conditions. Based on this project an asymmetric version of this reaction catalyzed by chiral calcium salts based on BINOL derivatives was developed. 2-substituted indolines could be obtained with enantiomeric excesses up to 15% by intramolecular cyclization. Another aspect of this work focused on the [3+2]-cycloaddition of donor-acceptor substituted cyclopropanes with aldehydes. A new type of 2,2-disubstituted cyclopropanes, bearing an alkyne moiety as the sole donor entity were transformed to highly substituted tetrahydrofurans in the presence of a catalytic amount of Ca(NTf2)2 and Bu4NPF6. By taking advantage of the marginal steric extension of this new donor, the tetrahydrofurans could be obtained with excellent trans-diastereoselectivities for the first time. In addition to that, mechanistic investigations were performed regarding the calcium-catalyzed cycloisomerization of enynols that was developed in this reearch group. It could be shown that the high distereoselectivity depends on the configuration of the double bond of the precursor and with the help of DFT-calculations the concerted asynchronous nature of this carbocation cascade could be confirmed