A DFT computational study on electrophilic substitutions upon alpha-oxy-substituted benzylorganolithium compounds: lithium catalysis is the hidden piece of the puzzle

From bifunctional to trifunctional (tricomponent nucleophile-transition metal-lewis acid) catalysis: the catalytic, enantioselective α-fluorination of acid chlorides

We report in full detail our studies on the catalytic, asymmetric α-fluorination of acid chlorides, a practical method that produces an array of α-fluorocarboxylic acid derivatives in which improved yield and virtually complete enantioselectivity are controlled through electrophilic fluorination of a ketene enolate intermediate. We discovered, for the first time, that a third catalyst, a Lewis acidic lithium salt, could be introduced into a dually activated system to amplify yields of aliphatic products, primarily through activation of the fluorinating agent. Through our mechanistic studies (based on kinetic data, isotopic labeling, spectroscopic measurements, and theoretical calculations) we were able to utilize our understanding of this "trifunctional" reaction to optimize the conditions and obtain new products in good yield and excellent enantioselectivity.

A reductive cyclization approach to attenol A

A reductive cyclization strategy was applied to the synthesis of attenol A. This nontraditional approach to the spiroacetal structure illustrated several advantages of the reductive cyclization methodology. The attenol A core was formed in a carbon-carbon bond coupling that gave rise to a previously inaccessible spiroacetal epimer, a new method to synthesize thioketene acetals from a phenyl sulfone was realized, and the configurational stability of a nonanomeric spiroacetal was evaluated. A minor byproduct in the reductive cyclization reaction was identified that for the first time allowed direct evaluation of the stereoselectivity in a reductive cyclization of a dialkyloxy alkyllithium reagent.

Dynamic Thermodynamic and Dynamic Kinetic Resolution of 2-Lithiopyrrolidines

Dynamic resolution has been studied as a method for the asymmetric synthesis of 2-substituted pyrrolidines. Highly enantioselective electrophilic substitutions of racemic 2-lithiopyrrolidines in the presence of a chiral ligand have been achieved. The organolithium compounds were prepared by tin-lithium exchange from the corresponding tributylstannanes and n-butyllithium or by deprotonation of N-(tert-butyloxycarbonyl)pyrrolidine with sec-butyllithium. A range of N-substituents and chiral ligands were investigated for the dynamic resolution. Electrophilic quench of the resolved diastereomeric 2-lithiopyrrolidine-chiral ligand complexes provided the enantiomerically enriched 2-substituted pyrrolidines. With N-alkyl derivatives, the resolution occurs conveniently at (or just below) room temperature and either enantiomer of the product can be formed by appropriate choice of the chiral ligand. The asymmetric induction occurs as a result of a thermodynamic preference for one of the diastereomeric complexes. The minor complex was found to have a faster rate of reaction with the electrophile. The use of N-allylic derivatives provides a means to prepare the N-unsubstituted pyrrolidine products. Best results were obtained with the N-2,3-dimethylbut-2-enyl derivative, and this N-substituent could be cleaved using 1-chloroethyl chloroformate. With N-Boc-2-lithiopyrrolidine, the enantioselectivity arises by a kinetic resolution and high levels of asymmetric induction in the presence of excess n-butyllithium can be obtained. Dynamic kinetic resolution of the N-Boc derivative is limited in the scope of electrophile that can be used.

Gas-Phase Reactions of Lithium Dimethylaminoborohydride and Related Species

[reaction: see text] Ab initio and density functional theory (DFT) calculations were used to examine the mechanisms of reduction and amination of chloromethane by lithium dimethylaminoborohydride (LAB) in the gas phase. For comparison, the amination of chloromethane by lithium dimethylamide and the reduction by borane, diborane, and borohydride ions were also examined. The reduction of chloromethane by LAB occurred most readily from a conformation that allowed coordination of the lithium atom to the chloride leaving group, and the most favorable amination pathway occurred by a backside attack of the nitrogen nucleophile on chloromethane.