Exploration of donor/acceptor substituted diazo compounds in asymmetric cyclopropanation
Lee, Gene Heejin
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Recently, the donor/acceptor substituted rhodium carbenoids have been known to have exceptional chemoselectivity, regioselectivity and stereoselectivity in intermolecular cyclopropanation. The primary objective goal of this project was to explore the scope and limitations of the acceptor (electron withdrawing) part of the donor/acceptor substituted diazo compounds in Rh(II) catalyzed asymmetric intermolecular cyclopropanation. The first area of exploration was the re-evaluation of the effect of the bulky tri- iso -propyl substituted arylsulfonyl group of the prolinate ligands on the enantioselectivities of intermolecular cyclopropanation reactions. The first phase of studies were conducted with an array of prolinate catalysts include the Davies' first generation catalysts Rh 2 ( S -DOSP) 4 and Rh 2 ( S -TISP) 4 , the second generation catalyst Rh 2 ( S -biTISP) 2 , and new derivatives of Andersson's catalysts Rh 2 ( R -DOSA) 4 and Rh 2 ( S -TISA) 4 synthesized in the Davies laboratory. These catalysts were first tested in the intermolecular cyclopropanation of alkyl vinyldiazoacetates with ester groups of different size and phenyldiazonitrile. The second phase of the studies included the prolinate catalysts and the C 2 symmetric Hashimoto's catalyst Rh 2 ( S -PTTL) 4 and its new derivative Rh 2 ( S -PTAD) 4 recently developed in the Davies group. These seven dirhodium chiral catalysts were tested in intermolecular cyclopropanations of α-diazophosphonates, α-vinyldiazoketone and three different kinds of cyclic diazo compounds categorized by the functional group: lactone, lactam, and ketone. In all cases, the catalysts with the bulkier tri- iso -propyl substituted arylsulfonyl group were shown to be more effective than the catalysts with the para -dodecyl substituted arylsulfonyl group. Furthermore, the C 2 symmetry catalyst Rh 2 ( S -PTAD) 4 was shown to be superior in intermolecular cyclopropanations of α-diazophosphonates and α-diazo-β-tetralone. In the final chapter of the thesis, the asymmetric cyclopropenation catalyzed by Rh 2 ( S -DOSP) 4 is presented. This study showcases the first example of the highly enantioselective synthesis of cyclopropene compounds containing quaternary center. This work expands the range of readily available chiral cyclopropenes, which can be used as chiral building blocks for organic synthesis.