Zirconium-Catalyzed Kinetic Resolution of Pyrans

Ketoreductase Catalyzed (Dynamic) Kinetic Resolution for Biomanufacturing of Chiral Chemicals

Biocatalyzed asymmetric reduction of ketones is an environmentally friendly approach and one of the most cost-effective routes for producing chiral alcohols. In comparison with the well-studied reduction of prochiral ketones to generate chiral alcohols with one chiral center, resolution of racemates by ketoreductases (KREDs) to produce chiral compounds with at least two chiral centers is also an important strategy in asymmetric synthesis. The development of protein engineering and the combination with chemo-catalysts further enhanced the application of KREDs in the efficient production of chiral alcohols with high stereoselectivity. This review discusses the advances in the research area of KRED catalyzed asymmetric synthesis for biomanufacturing of chiral chemicals with at least two chiral centers through the kinetic resolution (KR) approach and the dynamic kinetic resolution (DKR) approach.

Copper-catalysed asymmetric allylic alkylation of alkylzirconocenes to racemic 3,6-dihydro-2H-pyrans

Asymmetric allylic alkylation is a powerful reaction that allows the enantioselective formation of C–C bonds. Here we describe the asymmetric alkylation of alkylzirconium species to racemic 3,6-dihydro-2H-pyrans. Two systems were examined: 3-chloro-3,6-dihydro-2H-pyran using linear optimization (45–93% ee, up to 33% yield, 5 examples) and 3,6-dihydro-2H-pyran-3-yl diethyl phosphate with the assistance of a design of experiments statistical approach (83% ee, 12% yield). ¹H NMR spectroscopy was used to gain insight into the reaction mechanisms.

Evolution of catalytic stereoselective olefin metathesis: from ancillary transformation to purveyor of stereochemical identity

There have been numerous significant advances in catalytic olefin metathesis (OM) during the past two decades. Such progress has transformed this important set of reactions to strategically pivotal processes that generate stereochemical identity while delivering molecules that cannot be easily prepared by alternative routes. In this Perspective, an analysis of the origin of the inception of bidentate benzylidene ligands for Ru-based OM catalysts is first presented. This is followed by an overview of the intellectual basis that culminated in the development of Mo-based diolates and stereogenic-at-Ru complexes for enantioselective OM. The principles accrued from the study of the latter Ru carbenes and Mo alkylidenes and utilized in the design of stereogenic-at-Mo, -W, and -Ru species applicable to enantioselective and Z-selective OM are then discussed. The influence of the recently introduced catalytic OM protocols on the design of synthesis routes leading to complex organic molecules is probed. The impact of a better understanding of the mechanistic nuances of OM toward the discovery of stereoselective catalysts is reviewed as well.

Kinetic resolution of racemic alpha-olefins with ansa-zirconocene polymerization catalysts: Enantiomorphic site vs. chain end control

Copolymerization of racemic alpha-olefins with ethylene and propylene was carried out in the presence of enantiopure C1-symmetric ansa metallocene, {1,2-(SiMe2)2(eta5-C5H-3,5-(CHMe2)2)(eta5-C5H3)}ZrCl2 to probe the effect of the polymer chain end on enantioselection for the R- or S-alpha-olefin during the kinetic resolution by polymerization catalysis. Copolymerizations with ethylene revealed that the polymer chain end is an important factor in the enantioselection of the reaction and that for homopolymerization, chain end control generally works cooperatively with enantiomorphic site control. Results from propylene copolymerizations suggested that chain end control arising from a methyl group at the beta carbon along the main chain can drastically affect selectivity, but its importance as a stereo-directing element depends on the identity of the olefin.

Efficiency in nonenzymatic kinetic resolution

The Walden memorial at the Technical University in Riga is pictured in the frontispiece to mark the recent centennial of the Walden inversion. This is a rare public monument to key events from the first era of exploration in stereocontrolled synthesis, and may be the only such monument to use the language of organic chemistry expressed at the molecular level. The reaction of racemic substrates with chiral nucleophiles is one of many methods currently known to achieve kinetic resolution, a phenomenon that ranks as the oldest and most general approach for the synthesis of highly enantioenriched substances. The first nonenzymatic kinetic resolutions as well as the original forms of the Walden inversion were studied in the 1890s. All of these investigations were conducted within the first generation following the demonstration that carbon is tetrahedral, and provided abundant evidence that the principles and importance of enantiocontrolled syntheses were understood. However, a reliable, rapid technique to quantify results and guide the optimization process was still lacking. Many decades passed before this problem was solved by the advent of HPLC and GLPC assays on chiral supports, which stimulated explosive growth in the synthesis of nonracemic substances by kinetic resolution. The Walden monument is accessible to passers-by for hands-on inspection as well as for contemplation and learning. In a similar way, kinetic resolution is experimentally accessible and can be thought-provoking at several levels. We follow the story of kinetic resolution from the early discoveries through fascinating historical milestones and conceptual developments, and close with a focus on modern techniques that maximize efficiency.

Kinetic Resolution of Chiral α-Olefins Using Optically Active ansa-Zirconocene Polymerization Catalysts

A series of enantiopure C1-symmetric metallocenes, [(SiMe2)2[eta5-C5H(CHMe2)2][eta5-C5H2((S)-CHMeCMe3)]]ZrCl2, (S)-2, [(SiMe2)2[eta5-C5H(CHEt2)2][eta5-C5H2((S)-CHMeCMe3)]]ZrCl2, (S)-6, and [(SiMe2)2[eta5-C5HCy2][eta5-C5H2((S)-CHMeCMe3)]]ZrCl2, (S)-7 (Cy = cyclohexyl), zirconocene dichlorides that have an enantiopure methylneopentyl substituent on the "upper" cyclopentadienyl ligand, and diastereomerically pure precatalysts, [(SiMe2)2[eta5-C5H((S)-CHMeCy)(CHMe2)][eta5-C5H3]]ZrCl2, (S)-8a and (S)-8b, which have an enantiopure, 1-cyclohexylethyl substituent on the "lower" cyclopentadienyl ligand, has been synthesized for use in the polymerization of chiral alpha-olefins. When activated with methylaluminoxane, these metallocenes show unprecedented activity for the polymerization of bulky racemic monomers bearing substitution at the 3- and/or 4-positions. Due to the optically pure nature of these single site catalysts, they effect kinetic resolution of racemic monomers: the polymeric product is enriched with the faster reacting enantiomer, while recovered monomer is enriched with the slower reacting enantiomer. The two components are easily separated. For most olefins surveyed, a partial kinetic resolution was achieved (s = k(faster)/k(slower) approximately 2), but, in one case, the polymerization of 3,4-dimethyl-1-pentene, high levels of separation were obtained (s > 15). (13)C NMR spectroscopy of poly(3-methyl-1-pentene) produced with (S)-2 indicates that the polymers are highly isotactic materials. X-ray crystal structure determinations for (S)-2, [(SiMe2)2[eta5-C5H(CHMe2)2][eta5-C5H2((S)-CHMeCMe3)]]Zr(SC6H5)2, (S)-6, and (S)-7 have been used in combination with molecular mechanics calculations to examine the prevailing steric interactions expected in the diastereomeric transition states for propagation during polymerization. Precatalysts (S)-8a and (S)-8b are less selective polymerization catalysts for the kinetic resolution of 3-methyl-1-pentene than are (S)-2, (S)-6, and (S)-7.

General route from simple methyl, alkyl, and cycloalkyl arenes to polycyclic cyclopentenyl aryl derivatives. The CpFe(+) group as an activator and tag

[reaction: see text] The CpFe(+) group activates the perallylation of the benzylic groups of arenes using KOH and allylbromide under ambient conditions. This reaction can be followed by ruthenium-catalyzed RCM metathesis using Grubbs' catalyst at room temperature to give polycyclic aromatic derivatives in high yields, and these products are easily separated from the catalyst by extraction using ether. Alternatively, the RCM metathesis can be best carried out in ionic liquids at 80 degrees C, and extraction using ether is then facile.

Ring-opening reactions of oxabicyclic alkene compounds: enantioselective hydride and ethyl additions catalyzed by group 4 metals

Titanium and zirconium catalysts selectively catalyze either the ethyl or hydride addition to [2.2.1] 4, 5-bis(methoxymethyl)-7-oxabicycloheptene (6); the ring-opened products formed depend on catalyst, temperature, alkylaluminum reagent, and the concentration of alkylaluminum. Bis(neoisomenthylindenyl)zirconium dichloride catalyzes the ethyl addition ring-opening of 6 to produce (1R,2S,3S,6R)-2, 3-bis(methoxymethyl)-6-ethylcyclohex-4-enol (7) in 96% ee. Zirconium catalysts catalyze the ring-opening of [3.2.1] 2, 4-dimethyl-3-(benzyloxy)-8-oxabicyclo-6-octene (7) when ethylmagnesium bromide is used as a reagent. Both hydride and ethyl addition products are obtained at all conditions studied. Bis(neoisomenthylindenyl)zirconium dichloride catalyzes the ethyl addition ring-opening of 7 to produce (1S,2R,3S,4S,7S)-2, 4-dimethyl-3-(benzyloxy)-7-ethyl-5-cyclohexen-1-ol (8) in 48% ee.