Complex-Induced Proximity Effects: Stereoselective Carbon-Carbon Bond Formation in Chiral Auxiliary Mediated beta-Lithiation-Substitution Sequences of beta-Substituted Secondary Carboxamides

Nickel-catalyzed reductive coupling of homoenolates and their higher homologues with unactivated alkyl bromides

The catalytic generation of homoenolates and their higher homologues has been a long-standing challenge. Like the generation of transition metal enolates, which have been used to great affect in synthesis and medicinal chemistries, homoenolates and their higher homologues have much potential, albeit largely unrealized. Herein, a nickel-catalyzed generation of homoenolates, and their higher homologues, via decarbonylation of readily available cyclic anhydrides has been developed. The utility of nickel-bound homoenolates and their higher homologues is demonstrated by cross-coupling with unactivated alkyl bromides, generating a diverse array of aliphatic acids. A broad range of functional groups is tolerated. Preliminary mechanistic studies demonstrate that: (1) oxidative addition of anhydrides by the catalyst is faster than oxidative addition of alkyl bromides; (2) nickel bound metallocycles are involved in this transformation and (3) the catalyst undergoes a single electron transfer (SET) process with the alkyl bromide.

Preparation of Thioanisole Biscarbanion and C-H Lithiation/Annulation Reactions for the Access of Five-Membered Heterocycles

The synthesis, isolation, and X-ray structure of a thioanisole-based trilithium complex are reported. On the basis of the double-lithiation strategy, two novel synthetic methodologies have been developed under mild reaction conditions (room temperature): (1) reactions of lithiated thioanisoles with nitriles give benzoisothiazoles via a [3 + 2]-type of approach with two new bond formations and (2) formation of benzothiophenes from thioanisoles and amides through a [4 + 1] pattern forming 4 new chemical bonds.

Trapping of carbolithiation-derived tertiary benzylic α-lithio piperidines with carbon electrophiles: Controlling the formation of α-amino quaternary and vicinal stereocenters

The interception of carbolithiation-derived tertiary benzylic α-lithio piperidines with carbon electrophiles has led to the diastereoselective synthesis of vicinally functionalized piperidines bearing α-amino quaternary stereocenters.

Simple, versatile, and chemoselective reduction of secondary amides and lactams to amines with the Tf2O–NaBH4 or Cp2ZrHCl–NaBH4 system

A convenient method for the direct reduction of secondary amides and lactams to amines is reported.

Rh-catalyzed addition of β-carbonyl pinacol alkylboronates to aldehydes: asymmetric synthesis of γ-butyrolactones

The rhodium-catalyzed 1,2-addition of chiral benzylic secondary alkylboronic esters with a coordinating carbonyl group to aldehydes was demonstrated with high levels of enantiospecificity. Pinacol boronic ester derivatives can be employed directly for the addition in the presence of KHF2 without the use of corresponding trifluoroborate salts where retention of the configuration was observed. Enantiomerically enriched β,γ-diaryl-substituted γ-butyrolactones were synthesized in good yields.

Synthetic applications and inversion dynamics of configurationally stable 2-lithio-2-arylpyrrolidines and -piperidines

In diethyl ether, N-Boc-2-lithio-2-arylpiperidines have been found to be configurationally stable at -80 °C, whereas N-Boc-2-lithio-2-arylpyrrolidines are configurationally stable at -60 °C. Several tertiary benzylic carbanions derived from enantioenriched 2-aryl heterocycles have been successfully alkylated or acylated with little to no loss of enantiopurity. The scope of the reactions has been explored. The enantiomerization dynamics of N-Boc-2-lithio-2-phenylpyrrolidine and N-Boc-2-lithio-2-phenylpiperidine have been studied in the presence of different solvents and achiral ligands.

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.

Remote Stereocontrol by Sulfinyl Groups: Reduction of δ-Ketosulfoxides

The reduction of delta-ketosulfoxides constitutes the first evidence of the efficiency of the sulfinyl group to control the stereoselectivity of 1,5-asymmetric induction processes. The use of DIBAL/Yb(OTf)3 or L-Selectride as the reducing agents provides delta-hydroxysulfoxides with the opposite configuration at the hydroxylic carbon in a highly stereoselective way.

Enantioselective synthesis of 2-substituted 2-phenylethylamines by lithiation-substitution sequences: synthetic development and mechanistic pathway

The lithiation and asymmetric substitution of N-(2-phenylethyl)isobutyramide (2) with selected electrophiles, under the influence of (-)-sparteine, provides benzylically substituted products in 58-90% yields with enantiomeric ratios (ers) from 72:28 to 91:9. Syntheses of enantioenriched dihydroisoquinolines (S)-18 and (S)-19 and a tetrahydroisoquinoline (4S)-20 provide examples of synthetic applications. Mechanistic investigations suggest the enantiodetermining step at -78 degrees C is a dynamic thermodynamic resolution.

Synthesis of alpha-substituted beta-amidophosphines by diastereoselective alkylation. A new access to chiral ligands for asymmetric catalysis

Chiral beta-amidophosphine boranes 7a-f can be diastereoselectively alkylated, using O-protected amino-alcohols as chiral inducers, to furnish alpha-substituted beta-amidophosphine boranes 8a-f and 9-12 with up to 72% diastereoisomeric excess. Selective deprotection afforded optically pure carboxylic derivative 13 which is a key intermediate for the synthesis of various potential chiral ligands for asymmetric catalysis.

Stereoselective synthesis of naturally occurring alpha-methylenebis-gamma-butyrolactones: an application of novel oxiranyl "remote" anions

Stereospecific deprotonation of the epoxy proton at the beta-position of the alpha,beta-epoxy esters 5 and 6 yielded oxiranyl "remote" anions 7 and 8, which could then be used for alkylation. The anions 7 and 8 underwent a consecutive aldol lactonization to give, respectively, epoxy lactones 11 and 13 with high stereoselectivity. Generation of the remote anions as well as their stereoselective reactions served as a new synthetic route to the naturally occurring alpha-methylenebis-gamma-butyrolactones, 1.

Tandem addition beta-lithiation-alkylation sequence on alpha,beta-unsaturated aldehydes

A tandem reaction between (E)-cinnamaldehyde, 1a, and phenyllithium affording beta-substituted dihydrochalcones was recently reported. NMR spectroscopic studies on the reaction mixture, as well as isotopic exchange reactions and trapping of two intermediates, provide clues on the several mechanistic steps of this new reaction. Extended studies revealed that beta-alkyl-substituted alpha,beta-unsaturated aldehydes and aliphatic lithium reagents did not afford good yields of the tandem reaction products, while aromatic lithium reagents gave good results. The aggregation features of the aryllithium reagents and the extended charged delocalization effects are considered to promote beta-selectivity. This approach provides a convenient route for the synthesis of a wide variety of beta-alkyl-substituted dihydrochalcones.