Stereodivergent Construction of 1,3-Chiral Centers via Tandem Asymmetric Conjugate Addition and Allylic Substitution Reaction

Herein, we report a synthesis of cyclohexanones bearing multi-continuous stereocenters by combining copper-catalyzed asymmetric conjugate addition of dialkylzinc reagents to cyclic enones with iridium-catalyzed asymmetric allylic substitution reaction. Good to excellent yields, diastereoselectivity and enantioselectivity can be obtained. Unlike the stereodivergent construction of adjacent stereocenters (1,2-position) reported in the literature, the current reaction can achieve the stereodivergent construction of nonadjacent stereocenters (1,3-position) by a proper combination of two chiral catalysts with different enantiomers.

An Enolate-Structure-Enabled Anionic Cascade Cyclization Reaction: Easy Access to Complex Scaffolds with Contiguous Six-, Five-, and Four-Membered Rings

Catalyst-free addition of ketone enolate to non-activated multiple C-C bonds involves non-complementary reaction partners and typically requires super-basic conditions. On the other hand, highly aggregated or solvated enolates are not reactive enough to undergo direct addition to alkenes or alkynes. Herein, we report a new anionic cascade reaction for one-step assembly of intriguing molecular scaffolds possessing contiguous six-, five-, and four-membered rings, representing a formal [2+2] enol-allene cycloaddition. Reaction proceeds under very mild conditions and with excellent diastereoselectivity. Deeper mechanistic and computational studies revealed unusually slow proton transfer phenomenon in cyclic ketone intermediate and explained peculiar stereochemical outcome.

Facile Synthesis of Polysubstituted Indolizines via One-Pot Reaction of 1-Acetylaryl 2-Formylpyrroles and Enals

An efficient method for the synthesis of polysubstituted indolizines has been developed based on formal [4+2] annulation of 1-acetylaryl 2-formylpyrroles with enals, followed by oxidative aromatization. Pyridine-type six-membered rings were constructed in this transformation. This transition metal-free reaction features mild reaction conditions, a broad substrate scope, and excellent functional group tolerance. Notably, the formyl group is well tolerated under reaction conditions.

Recent Advances in the Chemistry of Heavier Group 14 Enolates

Recently heavier Group 14 enolates showed their importance and applicability in a broad range of chemical transformations. They were found to be key intermediates during the synthesis of photoinitiators, as well as during the formation of complex silicon frameworks. This Minireview presents general strategies towards the synthesis of heavier Group 14 enolates (HG 14 enolates). Structural properties, as well as their spectroscopic behavior are outlined. This study may aid future development in this research area.

Synthesis of Structurally Complex Silicon Frameworks through the First Sila-Aldol Reaction

Herein, we report on the first sila-aldol reaction, which emphasizes the tight connection between silicon and carbon chemistry. This novel synthetic method provides straightforward access to 2-oxahexasilabicyclo[3.2.1]octan-8-ide, a structurally complex silicon framework, in quantitative yield. Its structure was confirmed by NMR spectroscopy and X-ray crystallography, and it displays a distinctive charge-transfer transition. The complete mechanism of this highly selective rearrangement cascade is outlined and supported by density functional theory (DFT) calculations, which highlight the thermodynamic driving force and the low activation barriers of this powerful silicon-carbon bond-forming strategy.

Preparation and Application of Solid, Salt-Stabilized Zinc Amide Enolates with Enhanced Air and Moisture Stability

The treatment of various N-morpholino amides with TMPZnCl⋅LiCl (TMP=2,2,6,6-tetramethylpiperidyl) and Mg(OPiv)₂ in THF at 25 °C provides solid zinc enolates with enhanced air and moisture stability (t_1/2 in air: 1-3 h) after solvent evaporation. These enolates undergo Pd- and Cu-catalyzed cross-couplings with (hetero)aryl bromides as well as allylic and benzylic halides. The arylated N-morpholino amides were converted into various ketones by LaCl₃ ⋅2 LiCl mediated acylation with Grignard reagents. The new, solid enolates were used to prepare a potent anti-breast-cancer drug candidate in six steps and 23 % overall yield.

Conformational Polymorphism of Lithium Pinacolone Enolate

A metastable, polymorphic hexameric crystal structure of lithium pinacolone enolate (LiOPin) is reported along with three preparation methods. NMR-based structural characterization implies that the lithium pinacolate hexamer deaggregates to a tetramer in toluene but retains mainly the hexameric structure in nonaromatic hydrocarbon solvents such as cyclohexane. Moreover, the presence of a small amount of lithium aldolate (LiOA) dramatically influences the aggregation state of LiOPin by forming a mixed aggregate with a 3:1 ratio (LiOPin₃·LiOA).

Mixed Aggregates of the Dilithiated Koga Tetraamine: NMR Spectroscopic and Computational Studies

A combination of (1) H, (6) Li, (13) C, and (15) N NMR spectroscopies and density functional theory computations explores the formation of mixed aggregates by a dilithium salt of a C2 -symmetric chiral tetraamine (Koga's base). Lithium halides, acetylides, alkoxides, and monoalkylamides form isostructural trilithiated mixed aggregates with few exceptions. (6) Li-(13) C and (6) Li-(15) N couplings reveal heretofore undetected transannular contacts (laddering) with lithium acetylides and lithium monoalkylamides. Marked temperature-dependent (15) N chemical shifts seem to be associated with this laddering. Computational studies shed light on the general structures of the aggregates, their penchant for laddering, and the stereochemical consequences of aggregation.

Non-stabilized enolates – versatile nucleophiles in transition metal-catalysed allylic alkylations

This review covers new developments in the transition metal-catalyzed allylic alkylations of non-stabilized enolates, preferentially generated from ketone, esters or amides.

Gold-acetonyl complexes: from side-products to valuable synthons

A new synthetic strategy was devised leading to the formation of complexes, such as [Au(IPr)(CH2 COCH3)]. The approach capitalizes on the formation of a decomposition product observed in the course of the synthesis of [Au(IPr)(Cl)]. A library of gold acetonyl complexes containing the most common N-heterocyclic carbene (NHC) ligands has been synthesized. These acetonyl complexes are good synthons for the preparation of numerous organogold complexes. Moreover, they have proven to be precatalysts in common gold(I)-catalyzed reactions.

Mixed aggregates of 1-methoxyallenyllithium with lithium chloride

A combined computational and ¹³C NMR study was used to investigate the formation of mixed aggregates of 1-methoxyallenyllithium and lithium chloride in tetrahydrofuran (THF) solution. The observed and calculated chemical shifts, as well as the calculated free energies of mixed aggregate formation (MP2/6-31+G(d)), are consistent with the formation of a mixed dimer as the major species in solution. Free energies of mixed dimer, trimer, and tetramer formation were calculated by using the B3LYP and MP2 methods and the 6-31+G(d) basis set. The two methods generated different predictions of which mixed aggregates will be formed, with B3LYP/6-31+G(d) favoring mixed trimers and tetramers in THF solution, and MP2/6-31+G(d) favoring mixed dimers. Formation of the sterically unhindered mixed dimers is also consistent with the enhanced reactivity of these compounds in the presence of lithium chloride. The spectra are also consistent with some residual 1-methoxyallenyllithium tetramer, as well as small amounts of higher mixed aggregates. Although neither computational method is perfect, for this particular system, the calculated free energies derived using the MP2 method are in better agreement with experimental data than those derived using the B3LYP method.

Aggregation and cooperative effects in the aldol reactions of lithium enolates

Density functional theory and Car-Parrinello molecular dynamics simulations have been carried out for model aldol reactions involving aggregates of lithium enolates derived from acetaldehyde and acetone. Formaldehyde and acetone have been used as electrophiles. It is found that the geometries of the enolate aggregates are in general determined by the most favorable arrangements of the point charges within the respective Lin On clusters. The reactivity of the enolates follows the sequence monomer≫dimer>tetramer. In lithium aggregates, the initially formed aldol adducts must rearrange to form more stable structures in which the enolate and alkoxide oxygen atoms are within the respective Lin On clusters. Positive cooperative effects, similar to allosteric effects found in several proteins, are found for the successive aldol reactions in aggregates. The corresponding transition structures show in general sofa geometries.

Highly stereoselective modifications of peptides via Pd-catalyzed allylic alkylation of internal peptide amide enolates

Pd-catalyzed allylations are excellent tools for stereoselective peptide modifications, showing several advantages compared to normal alkylations. Reactions of internal peptide amide enolates with Pd-allyl complexes proceed not only with high yields of up to 86%, they show also high regio- and diastereoselectivities (88-99%), giving rise to the trans-configured products. Therefore, this protocol is a powerful synthetic tool for the synthesis of natural product and drug molecules.

Enantioselective and diastereoselective Tsuji-Trost allylic alkylation of lactones: an experimental and computational study

The Tsuji-Trost protocol has been successfully employed for the allylic alkylation of preformed lactone enolates. It has been demonstrated that this Pd-catalyzed reaction can be carried out in an enantio- and diastereoselective manner. The use of additives, such as LiCl, was found to be crucial for reaching high levels of product selectivity. For one particular pair of reactants, density functional theory was used to investigate the mechanism of the nucleophilic addition. Among the five pathways considered, the reaction between an (allyl)Pd(BINAP) complex and a LiCl-lithium enolate adduct is predicted to be the most likely route for C-C bond formation. LiCl plays a key role as the connecting link between the noble metal and the enolate in the kinetically favored transition state. The computed diastereoselectivity ratio is in good agreement with the experimentally observed value.

On the nature of the oxidative heterocoupling of lithium enolates

The coupling of enolates through single-electron oxidation is one of the most direct routes for generating 1,4-dicarbonyls. Recent work on the intermolecular heterocoupling of equimolar amounts of two different enolates through single-electron oxidation has shown that synthetically useful yields beyond those predicted by statistics can be obtained. To determine the underlying basis for the selective formation of heterocoupled products, kinetic, (7)Li NMR, and synthetic studies were performed. The collection of data obtained from these experiments shows that the selective formation of heterocoupled products is a consequence of heteroaggregation of lithium enolates.