Dynamic resolution of N-alkyl-2-lithiopyrrolidines with the chiral ligand (−)-sparteine

Nonclassical Routes for Amide Bond Formation

The present review offers an overview of nonclassical (e.g., with no pre- or in situ activation of a carboxylic acid partner) approaches for the construction of amide bonds. The review aims to comprehensively discuss relevant work, which was mainly done in the field in the last 20 years. Organization of the data follows a subdivision according to substrate classes: catalytic direct formation of amides from carboxylic and amines ( section 2 ); the use of carboxylic acid surrogates ( section 3 ); and the use of amine surrogates ( section 4 ). The ligation strategies (NCL, Staudinger, KAHA, KATs, etc.) that could involve both carboxylic acid and amine surrogates are treated separately in section 5 .

Dynamics of catalytic resolution of 2-lithio-N-Boc-piperidine by ligand exchange

The dynamics of the racemization and catalytic and stoichiometric dynamic resolution of 2-lithio-N-Boc-piperidine (7) have been investigated. The kinetic order in tetramethylethylenediamine (TMEDA) for both racemization and resolution of the title compound and the kinetic orders in two resolving ligands have been determined. The catalytic dynamic resolution is second order in TMEDA and 0.5 and 0.265 order in chiral ligands 8 and 10, respectively. The X-ray crystal structure of ligand 10 shows it to be an octamer. Dynamic NMR studies of the resolution process were carried out. Some of the requirements for a successful catalytic dynamic resolution by ligand exchange have been identified.

Asymmetric substitutions of 2-lithiated N-boc-piperidine and N-Boc-azepine by dynamic resolution

Proton abstraction of N-tert-butoxycarbonyl-piperidine (N-Boc-piperidine) with sBuLi and TMEDA provides a racemic organolithium that can be resolved using a chiral ligand. The enantiomeric organolithiums can interconvert so that a dynamic resolution occurs. Two mechanisms for promoting enantioselectivity in the products are possible. Slow addition of an electrophile such as trimethylsilyl chloride allows dynamic resolution under kinetic control (DKR). This process occurs with high enantioselectivity and is successful by catalysis with substoichiometric chiral ligand (catalytic dynamic kinetic resolution). Alternatively, the two enantiomers of this organolithium can be resolved under thermodynamic control with good enantioselectivity (dynamic thermodynamic resolution, DTR). The best ligands found are based on chiral diamino-alkoxides. Using DTR, a variety of electrophiles can be used to provide an asymmetric synthesis of enantiomerically enriched 2-substituted piperidines, including (after Boc deprotection) the alkaloid (+)-beta-conhydrine. The chemistry was extended, albeit with lower yields, to the corresponding 2-substituted seven-membered azepine ring derivatives.

The barrier to enantiomerization of N-Boc-2-lithiopyrrolidine: the effect of chiral and achiral diamines

(-)-Sparteine and TMEDA dramatically lower both enthalpy and entropy of activation for the barrier to enantiomerization of N-Boc-2-lithiopyrrolidine in diethyl ether, whereas N,N'-diisopropylbispidine has little effect; the entropy of activation for enantiomerization is zero in the presence of TMEDA and slightly negative in the presence of sparteine; these data suggest a subtle change in mechanism of enantiomerization in the presence of TMEDA and sparteine.

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.

Dynamic kinetic resolution of N-Boc-2-lithiopyrrolidine

Asymmetric substitution of the organolithium derived either from N-Boc-2-tributylstannylpyrrolidine by tin-lithium exchange or from N-Boc-pyrrolidine by deprotonation occurs in the presence of a commercially available chiral diamine ligand with high levels of enantioselectivity by a dynamic kinetic resolution pathway.