Models for Understanding Divergent Reactivity in Lewis Acid-Catalyzed Transformations of Carbonyls and Olefins

Mechanism to model: a physical organic chemistry approach to reaction prediction

The application of mechanistic generalizations is at the core of chemical reaction development and application. These strategies are rooted in physical organic chemistry where mechanistic understandings can be derived from one reaction and applied to explain another. Over time these techniques have evolved from rationalizing observed outcomes to leading experimental design through reaction prediction. In parallel, significant progression in asymmetric organocatalysis has expanded the reach of chiral transfer to new reactions with increased efficiency. However, the complex and diverse catalyst structures applied in this arena have rendered the generalization of asymmetric catalytic processes to be exceptionally challenging. Recognizing this, a portion of our research has been focused on understanding the transferability of chemical observations between similar reactions and exploiting this phenomenon as a platform for prediction. Through these experiences, we have relied on a working knowledge of reaction mechanism to guide the development and application of our models which have been advanced from simple qualitative rules to large statistical models for quantitative predictions. In this feature article, we describe the models acquired to generalize organocatalytic reaction mechanisms and demonstrate their use as a powerful approach for accelerating enantioselective synthesis.

Insights into Synergistic Effects of Counterion and Ligand on Diastereoselectivity Switch in Gold-Catalyzed Post-Ugi Ipso-Cyclization

The concept of diastereoselectivity switch in gold catalysis is investigated, which primarily depends on the effects of ligand and counterion. The origins of gold-catalyzed post-Ugi ipso-cyclization for the diastereoselective synthesis of spirocyclic pyrrol-2-one-dienone have been explored with density functional theory calculations. The reported mechanism emphasized the importance of the cooperation of ligand and counterion in diastereoselectivity switch, leading to the stereocontrolling transition states. Furthermore, the nonbonding interactions primarily between the catalyst and the substrate play a significant role in the cooperation of ligand and counterion. This work would be useful to further understand the reaction mechanism of gold-catalyzed cyclization and the effects of ligand and counterion.

Controlling Catalyst Behavior in Lewis Acid-Catalyzed Carbonyl-Olefin Metathesis

Lewis acid-catalyzed carbonyl-olefin metathesis has introduced a new means for revealing the behavior of Lewis acids. In particular, this reaction has led to the observation of new solution behaviors for FeCl3 that may qualitatively change how we think of Lewis acid activation. For example, catalytic metathesis reactions operate in the presence of superstoichiometric amounts of carbonyl, resulting in the formation of highly ligated (octahedral) iron geometries. These structures display reduced activity, decreasing catalyst turnover. As a result, it is necessary to steer the Fe-center away from inhibiting pathways to improve the reaction efficiency and augment yields for recalcitrant substrates. Herein, we examine the impact of the addition of TMSCl to FeCl3-catalyzed carbonyl-olefin metathesis, specifically for substrates that are prone to byproduct inhibition. Through kinetic, spectroscopic, and colligative experiments, significant deviations from the baseline metathesis reactivity are observed, including mitigation of byproduct inhibition as well as an increase in the reaction rate. Quantum chemical simulations are used to explain how TMSCl induces a change in catalyst structure that leads to these kinetic differences. Collectively, these data are consistent with the formation of a silylium catalyst, which induces the reaction through carbonyl binding. The FeCl3 activation of Si-Cl bonds to give the silylium active species is expected to have significant utility in enacting carbonyl-based transformations.

Short Scalable Route to Apiaceae Sesquiterpene Scaffolds: Total Synthesis of 4-epi-Epiguaidiol A

The oxy-Cope/ene reaction cascade to form a locked elemane conformer allows the short scalable synthesis of versatile Apiaceae scaffolds. The divergent fate of the obtained macrocyclic germacrane is surveyed under cationic and dioxygen-induced Prins-type reaction conditions to allow the diastereoselective synthesis of oxidized Apiaceae guaiane congeners and the total synthesis of 4-epi-epiguaidiol A. Additionally, the unprecedented reduction of a hydrogen-bond-biased guaiane substrate permits the chemoselective synthesis of desoxo-jungiaguaiane.

Crystallographic evidence for a continuum and reversal of roles in primary-secondary interactions in antimony Lewis acids: applications in carbonyl activation

Primary and secondary interactions form the basis of substrate activation in Lewis-acid mediated catalysis, with most substrate activations occurring at the secondary binding site. We explore two series of antimony cations, [(NMe₂CH₂C₆H₄)(mesityl)Sb]⁺ (A) and [(NMe₂C₆H₄)(mesityl)Sb]⁺ (B), by coordinating ligands with varying nucleophilicity at the position trans to the N-donor. The decreased nucleophilicity of the incoming ligands leads to reversal from a primary bond to a secondary interaction in A, whereas a constrained N-coordination in B diminishes the border between primary and secondary bonding. Investigations on carbonyl olefin metathesis reactions and carbonyl reduction demonstrate increased reactivity of a Lewis acid when the substrate activation occurs at the primary binding site.

Toward Homogeneous Brønsted-Acid-Catalyzed Intramolecular Carbonyl-Olefin Metathesis Reactions

The carbonyl-olefin metathesis (COM) reaction is an attractive approach for the formation of a new carbon-carbon double bond from a carbonyl precursor. In principle, this reaction can be promoted by the activation of the carbonyl group with a Brønsted acid catalyst; however, it is often complicated as a result of unwanted side reactions under acidic conditions. Thus, there have been only a very few examples of Brønsted-acid-catalyzed COM reactions, all of which required specially designed setups. Herein, we report a new practical homogeneous Brønsted-acid-catalyzed protocol using nitromethane, a readily available solvent, to promote intramolecular ring-closing COM reactions.

Syntheses of Bufospirostenin A and Ophiopogonol A by a Conformation-Controlled Transannular Prins Cyclization

Controlling the conformation of medium-sized rings is challenging because of their flexibility and ring strain effects. Herein, we report non-Curtin-Hammett conditions for the precise control of the conformation of cyclodecenones to effect the first cis-selective transannular Prins cyclization, which enabled concise syntheses of the 5(10→1)abeo-steroids bufospirostenin A and ophiopogonol A in only seven steps from inexpensive starting materials. Computational results indicated that the key cyclization was kinetically controlled and proceeded via either a Prins pathway or a carbonyl-ene pathway, depending on the reaction conditions. Moreover, conformational isomerization played a critical role in determining the stereochemistry of the products.

Brønsted Acid-Catalyzed Carbonyl-Olefin Metathesis: Synthesis of Phenanthrenes via Phosphomolybdic Acid as a Catalyst

Compared with the impressive achievements of catalytic carbonyl-olefin metathesis (CCOM) mediated by Lewis acid catalysts, exploration of the CCOM through Brønsted acid-catalyzed approaches remains quite challenging. Herein, we disclose a synthetic protocol for the construction of a valuable polycycle scaffold through the CCOM with the inexpensive, nontoxic phosphomolybdic acid as a catalyst. The current annulations could realize carbonyl-olefin, carbonyl-alcohol, and acetal-alcohol in situ CCOM reactions and feature mild reaction conditions, simple manipulation, and scalability, making this strategy a promising alternative to the Lewis acid-catalyzed COM reaction.

Origin of enantioselectivity reversal in Lewis acid-catalysed Michael additions relying on the same chiral source

Enantiodivergence is an important concept in asymmetric catalysis that enables access to both enantiomers of a product relying on the same chiral source as reagent. This strategy is particularly appealing as an alternate approach when only one enantiomer of the required chiral ligand is readily accessible but both enantiomers of the product are desired. Despite the potential significance, general catalytic methods to effectively reverse enantioselectivity by changing an achiral reaction parameter remain underdeveloped. Herein we report our studies focused on elucidating the origin of metal-controlled enantioselectivity reversal in Lewis acid-catalysed Michael additions. Rigorous experimental and computational investigations reveal that specific Lewis and Brønsted acid interactions between the substrate and ligand change depending on the ionic radius of the metal catalyst, and are key factors responsible for the observed enantiodivergence. This holds potential to further our understanding of and facilitate the design of future enantiodivergent transformations.

Enantiodivergence is an important concept in asymmetric catalysis that enables access to both enantiomers of a product relying on the same chiral source as reagent.

Untargeted foodomics reveals molecular mechanism of magnetic field effect on Feng-flavor Baijiu ageing

Ageing is a time-consuming step in Baijiu manufacture, stimulating an urgent requirement of optimization. Variation of artificial aged Feng-flavor Baijiu by inhomogeneous alternating magnetic field was investigated through quantitative foodomics combined with confirmed ultra high performance liquid chromatography quadrupole-orbitaltrap high resolution mass spectrometry (UHPLC-Q-Orbitrap). A total of 153 substances were identified with significant variables (p < 0.05, VIP > 1) and 16 metabolic pathways related to Feng-flavor Baijiu functions were obtained. The method showed good accuracy with recovery values between 80.4% and 117.4% and precision lower than 9.8% for all characteristic substances. Limit of detection (LOD) was ranging between 1.6 and 10.0 μg/L with R² ≥ 0.99. Factor analysis demonstrated that ageing degree of magnetized samples increased with rise of magnetic field intensity and the maximum effect was equivalent to 12.81 years of natural ageing. The results of stoichiometric analysis revealed that regulation of magnetic field on proportion in Baijiu was mainly performed through entropy and the hydrogen bond strength of Baijiu molecules. Sensory evaluation illustrated that score of Baijiu samples reached the highest at 150 mT, demonstrating that magnetic field treatment can be considered as an optimized ageing means for Feng-flavor Baijiu.

Carbonyl-Olefin Metathesis

This Review describes the development of strategies for carbonyl-olefin metathesis reactions relying on stepwise, stoichiometric, or catalytic approaches. A comprehensive overview of currently available methods is provided starting with Paternò-Büchi cycloadditions between carbonyls and alkenes, followed by fragmentation of the resulting oxetanes, metal alkylidene-mediated strategies, [3 + 2]-cycloaddition approaches with strained hydrazines as organocatalysts, Lewis acid-mediated and Lewis acid-catalyzed strategies relying on the formation of intermediate oxetanes, and protocols based on initial carbon-carbon bond formation between carbonyls and alkenes and subsequent Grob-fragmentations. The Review concludes with an overview of applications of these currently available methods for carbonyl-olefin metathesis in complex molecule synthesis. Over the past eight years, the field of carbonyl-olefin metathesis has grown significantly and expanded from stoichiometric reaction protocols to efficient catalytic strategies for ring-closing, ring-opening, and cross carbonyl-olefin metathesis. The aim of this Review is to capture the status quo of the field and is expected to contribute to further advancements in carbonyl-olefin metathesis in the coming years.