155135-24-3Relevant articles and documents
Mechanistic investigations of cooperative catalysis in the enantioselective fluorination of epoxides
Kalow, Julia A.,Doyle, Abigail G.
, p. 16001 - 16012 (2011/11/13)
This report describes mechanistic studies of the (salen)Co- and amine-cocatalyzed enantioselective ring opening of epoxides by fluoride. The kinetics of the reaction, as determined by in situ 19F NMR analysis, are characterized by apparent first-order dependence on (salen)Co. Substituent effects, nonlinear effects, and reactivity with a linked (salen)Co catalyst provide evidence for a rate-limiting, bimetallic ring-opening step. To account for these divergent data, we propose a mechanism wherein the active nucleophilic fluorine species is a cobalt fluoride that forms a resting-state dimer. Axial ligation of the amine cocatalyst to (salen)Co facilitates dimer dissociation and is the origin of the observed cooperativity. On the basis of these studies, we show that significant improvements in the rates, turnover numbers, and substrate scope of the fluoride ring-opening reactions can be realized through the use of a linked salen framework. Application of this catalyst system to a rapid (5 min) fluorination to generate the unlabeled analog of a known PET tracer, F-MISO, is reported.
X-Ray Structural Studies of Highly Enantioselective Mn(salen) Epoxidation Catalysts
Pospisil, Paul J.,Carsten, Douglas H.,Jacobsen, Eric N.
, p. 974 - 980 (2007/10/03)
The relationship between catalyst structure and enantioselectivity in the asymmetric epoxidation of unfunctionalized olefins by a series of chiral Mn(salen) complexes (1-10) was examined.The X-ray structures of 5-coordinate complexes 5, 8, of 6-coordinate 9 (+ClO4-), and 10 (6,6' = -tBu; 4,4' = -Br) were determined.Catalysts 1-9 were derived from (R,R)-1,2-diaminocyclohexane and catalysts 10 from (S,S)-1,2-diphenylethylenediamine.Catalysts 1-9 differ in the stereoelectronic substitution of the ortho (6,6') and para (4,4') positions of the salicylidene moiety.A comparison between structures 5, 8, and 9 reveals that the ligand geometry around the metal cnter and the chiral diimine backbone remains remarkably constant in both five- and six-coordinate cyclohexanediamine-derived complexes; in contrast, the salicylidene regions of the complexes display a wide range of conformations.The asymmetric epoxidation of indene and 6-cyano-2,2-dimethylchromene with NaOCl catalyzed by complexes 1-10 was effected.Systematically increasing the steric bulk on the ortho and then the para position in the order 1 (6,6' = -H; 4,4' = -H), 2 (6,6' = -CH3; 4,4' = -CH3), 3 (6,6' = -tBu; 4,4' = -H), 4 (6,6' = -tBu; 4,4' = -CH3), 5 (6,6' = -tBu; 4,4' = -tBu), and 6 (6,6' = -tBu; 4,4' = -trityl), and electronically modifying the para substituents in 7 (6,6' = -tBu; 4,4' = -OMe) and 8 (6,6' = -tBu; 4,4' = -OTIPS) resulted in enhanced enantioselectivities of the desired epoxides.The conformational variations observed in the solid state are likely to reflect accessible solution conformations and may help explain the high levels of stereoinduction obtained with these catalysts in the asymmetric epoxidation of unfunctionalized olefins. - Keywords: asymmetric epoxidations; catalysis; manganese complexes; structure elucidation
