| 1. |
- Gerdin, Martin, 1976-, et al.
(author)
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Enantioselective silicon-boron additions to cyclic 1,3-dienes catalyzed by the platinum group metal complexes
- 2008
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In: Journal of Organometallic Chemistry. - : Elsevier B. V.. - 0022-328X .- 1872-8561. ; 693:23, s. 3519-3526
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Journal article (peer-reviewed)abstract
- Silaborations of 1,3-cyclohexadiene and 1,3-cycloheptadiene were achieved using catalysts prepared from different combinations of phosphorus ligands and group 10 metal compounds. For the six-membered compound, 1,4-adducts with up to 82% ee were obtained employing Pt(0) and phosphoramidite ligands. For the seven-membered diene optimal conditions were found using catalysts based on Ni(0), but the highest selectivity observed was merely 22% ee. No improvement of the chiral induction was obtained using chiral silylboranes in combination with chiral phosphoramidite ligands in the additions to 1,3-cyclohexadiene. The adduct obtained from cyclohexadiene was used in allylborations of aldehydes under microwave irradiation to produce homoallylic alcohols with moderate to good diastereoselectivity.
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| 2. |
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| 3. |
- Vasse, Jean Luc, et al.
(author)
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Influence of steric symmetry and electronic dissymmetry on the enantioselectivity in palladium-catalyzed allylic substitutions
- 2003
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In: Journal of Organic Chemistry. - : American Chemical Society (ACS). - 0022-3263 .- 1520-6904. ; 68:8, s. 3258-3270
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Journal article (peer-reviewed)abstract
- Chiral P,N-ligands with pseudo-C2 and pseudo-Cs symmetry based on chiral pyrrolidine and phospholane rings or on dinaphthatodihydroazepino and dinaphthatodihydrophosphepino moieties were prepared and assessed in the palladium-catalyzed allylic substitutions of allylic acetates. Higher selectivity was achieved with pseudo-C2-symmetric ligands based on the binaphthyl skeleton than with the analogous C2-symmetric P,P- and N,N-analogues. Pseudo-C2-type ligands had properties superior to those of pseudo-meso-type ligands when 1,3-diphenyl-2-propenyl acetate was used as a substrate, whereas the reverse situation was found for 3-cyclohexenyl acetate. Chirally flexible ligands, prepared by substitution of one of the rigid binaphthyl skeletons for a flexible biphenyl system, were found to induce chirality to the same extent as a 1:1 mixture of the rigid ligands.
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| 4. |
- Zalubovskis, Raivis, 1975-
(author)
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Flexibility – a tool for chirality control in asymmetric catalysis
- 2006
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Doctoral thesis (other academic/artistic)abstract
- This thesis deals with the design and synthesis of ligands for asymmetric catalysis: palladium catalyzed allylic alkylations, and rho-dium and iridium catalyzed hydrogenations of olefins. Chirally flexible phosphepine ligands based on biphenyl were synthesized and their properties were studied. The rotation barrier for configurationally flexible phosphepines was determined by NMR spectroscopy. The ratio of the atropisomers was shown to depend on the group bound to phosphorus. Only complexes with two homochiral ligands bound to the metal center were observed upon complexation with Rh(I). It was shown that one diastereomer of the flexible ligand exhibits higher activity but lower selectivity than its diastereomer in the rhodium catalyzed hydrogenation of methyl alfa-acetamidocinnamate. These ligands were also tested in nickel catalyzed silabora-tions. Chiral P,N-ligands with pseudo-C2 and pseudo-CS symmetry based on pyrrolidines-phospholanes or azepines-phosphepines were synthesized and studied in palladium catalyzed allylic alkylations. Semi-flexible azepine-phosphepine based ligands were prepared and their ability to adopt pseudo-C2 or pseudo-CS symmetry depending on the substrate in allylic alkylations was studied. It was shown on model allyl systems with flexible N,N-ligands that the ligand prefers CS-symmetry in compexes with anti-anti as well as syn-syn allyl moieties, but that for the latter type of complexes, according to computations, the configuration of the ligand is R*,R* in the olefin complexes formed after addition of a nucleophile to the allylic group. A preliminary investigation of the possibilities to use a su-pramolecular approach for the preparation of P,N-ligands with pseudo-C2 and pseudo-S symmetry was made. An N,N-ligand with C2 symmetry was prepared and its activity in palladium catalyzed ally-lic alkylation was studied. Pyridine-based P,N-ligands were tested in iridium catalyzed hy-drogenations of unfunctionalized olefins with good activities and se-lectivities. In order to attempt to improve the selectivity, ligands with a chirally flexible phosphepine fragment were prepared and applied in catalysis with promising results.
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| 5. |
- Zalubovskis, Raivis, et al.
(author)
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Iridium-catalyzed asymmetric hydrogenation of olefins using pyridine-phosphinites derived from the chiral pool
- 2008
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In: ARKIVOC. - : ARKAT USA, Inc.. - 1424-6376 .- 1551-7012. ; , s. 58-66
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Journal article (peer-reviewed)abstract
- Cationic iridium complexes containing chiral pyridine-phosphinites, with their chirality derived from menthol or mandelic acid, were assessed in hydrogenations of trisubstituted non-functionalized olefins. Complexes with the new ligands showed high reactivity towards most of the olefins tested. The enantioselectivities varied with the structures of the substrate and the ligand. Best results were obtained in the hydrogenation of 3-hydroxy-2-methyl-1-phenylpropene using a ligand prepared from (-)-menthol, pyridyllithium and diphenylchlorophosphine (>99% conversion, 92% ee).
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| 6. |
- Zalubovskis, Raivis, et al.
(author)
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Self-adaptable Catalysts : Substrate-Dependent Ligand Configuration
- 2008
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In: Journal of the American Chemical Society. - Washington DC : American Chemical Society. - 0002-7863 .- 1520-5126. ; 130:6, s. 1845-1855
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Journal article (peer-reviewed)abstract
- Pd(II) allyl and Pd(0) olefin complexes containing the configurationally labile ligand 1,2-bis-[4,5dihydro-3H-dibenzo[c-e]azepino]ethane were studied as models for intermediates in Pd-catalyzed allylic alkylations. According to NMR and DFT studies, the ligand prefers C-s conformation in both eta(3)-1,3-diphenylpropenyl and eta(3)-cyclohexenyl Pd(II) complexes, whereas in Pd(0) olefin complexes it adopts different conformations in complexes derived from the two types of allyl systems in both solution and, as verified by X-ray crystallography, in the solid state. These results demonstrate that the Pd complex is capable of adapting its structure to the reacting substrate. The different structural preferences also provide an explanation for the behavior of 1,3-diphenyl-2-propenyl acetate and 2-cyclohexenyl acetate in Pd-catalyzed allylic alkylations using pseudo-C-2 and pseudo-C-s symmetric ligands.
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| 7. |
- Zalubovskis, Raivis, et al.
(author)
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Stereochemical Control of Chirally Flexible Phosphepines
- 2007
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In: European Journal of Organic Chemistry. - Weinheim : Wiley-VCH Verlag. - 1434-193X .- 1099-0690. ; 2007:1, s. 108-115
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Journal article (peer-reviewed)abstract
- The barriers to interconversion of the two enantiomeric atropisomers of 6-methoxy-6,7-dihydro-5H-dibenzo[c,e]phosphepine and that of the diastereomeric forms of 6-(-)menthoxy-6,7-dihydro-3H-dibenzo[c,e]phosphepine were determined by NMR spectroscopical methods to be 19.3 and 18.5 kcalmol(-1), respectively, at 298 K. The ratio of the atropisomers was shown to depend on the group bound to phosphorus. Only complexes with two homochiral ligands bound to the each metal center were obtained upon reaction with [Rh(COD)(2)](+) BF4-. The Rh complexes catalyzed the hydrogenation of alpha-acetamidocinnamate. The major isomer of 6-(-)-menthoxy-6,7-dihydro-5H-dibenzo[c,e]phosphepine was found to exhibit higher activity but to afford a product with lower ee than its diastereomer.
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