Asymmetrische Cycloisomerisierung von 1,6-Dienen
Within this thesis an highly selective and active catalyst system for the cycloisomerisation of 1,6-dienes towards exo-methylenecyclopentanes could be developed. The combination of [{Ni(allyl)Br}2], Wilke´s azaphospholene and Na[BARF] led to a catalyst, which transformed diethyl diallylmalonate to 3-methyl-4-methylenecyclopentan-1,1-dicarboxylic diethylester in 91% regioselectivity and 79% enantiomeric excess. Furthermore the use of halide free precursors of the type [Ni(allyl)(cod)][Y] with weakly coordinating counterions Y led to a strong increase in activity without loss in regio- and only with small loss in enantioselectivity. With [Ni(allyl)(cod)][BARF] as precursor, a TOF of 144 h-1 and an enantiomeric excess of 72% could be reached in the cyclisation of diethyl diallylmalonate. Concerning activity and regioselectivity our catalyst system is comparable with the most successful achiral protocols. Apart from diethyl diallylmalonate, we were able to transform further 1,6-dienes into the desired exo-methylenecyclopentanes. The highest enantioselectivity was observed with 4,4-bis-(hydroxymehtyl)-1,6-heptadiene as substrate. A value of 91% ee was reached, being currently by far the highest ee known for the diene cycloisomerisation. Additionally to the already mentioned substrates leading to carbocycles, also N,N-diallyl tosylamide could be transformed into the corresponding exo-methylenepyrrolidine. With this substrate ee values up to 54%, TOFs up to 128h 1 accompanied with perfect regioselectivities (up to >99%) were achieved. These values are currently the highest reported for this class of substrates. Furthermore, we were successful in the cycloisomerisation of unsymmetric 1,6-dienes. trans-Diethyl-2-cinnamyl-2-allylmalonate and other 1-arylated 1,6-heptadienes could be converted into the corresponding (E)-exo-benzylidenecyclopentane, which arose in very high selectivity. As counterion effects are often very pronounced in olefin dimerisation, this aspect was assessed also for the new cycloisomerisation system. We observed that the catalyst activity increases strongly by decreasing the nucleophilic character of the counterion, whereas regio- and enantioselectivity are mostly independent from the catalyst anion. Apart from Wilke´s Azaphospholen, also monodentate phosphoramidites were tested as chiral ligands in the Ni-catalysed cycloisomerisation of 1,6-dienes. Here we observed that MonoPhos type ligands only show moderate activity and regioselectivity combined with no chiral induction. The use of MonoPhos derivatives containing the 3,3’-substituted binaphthol moiety resulted in an increase of the catalyst performance in terms of activity, regio- as well as enantioselectivity. The most promising results however were obtained with BINAPHOSQUIN-type ligands. With this ligand class TOF values of 40h-1, regioselectivities up to 97% and ee values up to 46% were reached. The variation of the substituent in 2-position of the dihydro-quinoline backbone does not have a major effect on the catalysis outcome.
info:eu-repo/classification/ddc/540, Chemie, Homogene Katalyse, Asymmetrische Synthese, Nickel-Komplexe, Olefin-Dimerisierung, Homogeneous Catalysis, Asymmetric Synthesis, Nickel Complexes
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