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- Cheruku, Pradeep, et al.
(författare)
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Asymmetric Hydrogenation of Di and Trisubstituted Enol Phosphinates with N,P-Ligated Iridium Complexes
- 2008
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 130:16, s. 5595-5599
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Tidskriftsartikel (refereegranskat)abstract
- The iridium-catalyzed asymmetric hydrogenation of various di- and trisubstituted enol phosphinates has been studied. Excellent enantioselectivities (up to >99% ee) and full conversion were observed for a range of substrates with both aromatic and aliphatic side chains. Enol phosphinates are structural analogues of enol acetates, and the hydrogenated alkyl phosphinate products can easily be transformed into the corresponding alcohols with conservation of stereochemistry. We have also hydrogenated, in excellent ee, several purely alkyl-substituted enol phosphinates, producing chiral alcohols that are difficult to obtain highly enantioselectively from ketone hydrogenations.
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- Cheruku, Pradeep, et al.
(författare)
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Iridium-N,P-ligand-catalyzed enantioselective hydrogenation of diphenylvinylphosphine oxides and vinylphosphonates
- 2009
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 131:23, s. 8285-8289
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Tidskriftsartikel (refereegranskat)abstract
- Diphenylvinylphosphine oxides and di- and trisubstituted vinylphosphonates have been employed as substrates in iridium-catalyzed asymmetric hydrogenations. Complete conversions and excellent enantioselectivities (up to and above 99% ee) were observed for a range of substrates with both aromatic and aliphatic groups at the prochiral carbon. We have also hydrogenated electron-deficient carboxyethylvinylphosphonates with excellent stereoselectivity (up to and above 99% ee). The hydrogenated products of both classes of substrates are synthetically useful intermediates.
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- Diéguez, Montserrat, et al.
(författare)
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Chiral pyranoside phosphite-oxazolines : a new class of ligand for asymmetric catalytic hydrogenation of alkenes
- 2008
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 130:23, s. 7208-7209:130, s. 7208-7209
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Tidskriftsartikel (refereegranskat)abstract
- We have described the first successful application of a phosphite-oxazoline ligand library in the asymmetric Ir-catalyzed hydrogenation of several unfunctionalized olefins. The introduction of a bulky biaryl phosphite moiety in the ligand design is highly adventitious in the product outcome. By carefully selecting the ligand components, we obtained high activities (TOFs up to >1500 mol x (mol x h)(-1) at 1 bar of H2) and enantioselectivities (ee values up to >99%) and, at the same time, show a broad scope for different substrate types. So, this is an exceptional ligand class that competes favorably with a few other ligand series that also provide high ee values for tri- and disubstituted substrate types.
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- Engman, Mattias, et al.
(författare)
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Iridium-Catalyzed Asymmetric Hydrogenation of Fluorinated Olefins Using N,P-Ligands : A struggle with hydrogenolysis and selectivity
- 2007
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 129:15, s. 4536-4537
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Tidskriftsartikel (refereegranskat)abstract
- To broaden the substrate scope of asymmetric iridium-catalyzed hydrogenation, fluorine-functionalized olefins were synthesized and hydrogenated with iridium complexes. Preliminary results showed high levels of fluorine elimination together with low selectivity. The loss of vinylic fluorine at first seemed difficult to handle, but further studies revealed that a catalyst with an azanorbornyl scaffold in the ligand gave more promising results. With this in mind, a new ligand was developed. This gave among the best results published to date for fluorine asymmetric hydrogenation, yielding high conversion and very high ee's with very little fluorine elimination. Further increasing the selectivity, the trials also revealed that tetrasubstituted fluorine-containing olefins can be hydrogenated with high ee's, despite that this class of compounds has usually shown low reactivity in this reaction type.
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- Henriksen, Signe, et al.
(författare)
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Combined Experimental and Theoretical Study of the Mechanism and Enantioselectivity of Palladium-Catalyzed Intermolecular Heck Coupling
- 2008
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 130, s. 10141-10421
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Tidskriftsartikel (refereegranskat)abstract
- The asymmetric Heck reaction using P,N-ligands has been studied by a combination of theoretical and experimental methods. The reaction follows Halpern-style selectivity, that is, the major isomer is produced from the least favored form of the pre-insertion intermediate. The initially formed Ph-Pd(P,N) species prefers a geometry with the phenyl trans to N. However, the alternative form, with Ph trans to P, is much less stable but much more reactive. In the preferred transition state, the phenyl moiety is trans to P, but significant electron density has been transferred to the alkene carbon trans to N. The steric interactions in this transition state fully account for the enantioselectivity observed with the ligands studied. The calculations also predict relative reactivity and nonlinear mixing effects for the investigated ligands, these predictions are fully validated by experimental testing. Finally, the low conversion observed with some catalysts was found to be caused by inactivation due to weak binding of the ligand to Pd(0). Adding monodentate PPh3 alleviated the precipitation problem without deteriorating the enantioselectivity and led to one of the most effective catalytic systems to date.
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