Catalyst-Free Enantiospecific Olefination with In Situ Generated Organocerium Species

Homoleptic Organolanthanum-Catalyzed Carbohalogenation

Metal-halogen exchange reactions are fundamental processes in chemistry that transform organic halides into organometallic reagents. However, using these reactions to build intricate structures in a cascade manner, especially in a catalytic mode, has been a challenge. In this study, we introduce a homoleptic organolanthanum catalyst to initiate lanthanum-halogen exchange and intramolecular carbohalogenation. The catalytic pathway can be achieved through metal-halogen exchange and carbometalation, followed by the extraction of halogen atoms from starting materials. Our approach offers a flexible and sustainable way to create a variety of useful compounds, showcasing its potential in chemical synthesis.

Recent Advances in Halogen-Metal Exchange Reactions

ConspectusThe halogen-metal exchange reaction is a very powerful method for preparing functionalized organometallic reagents in the fields of organic and organometallic chemistry. Since its inception, significant interest has been directed toward the on-demand development of new halogen-metal exchange reactions, primarily through the upgrading of exchange reagents. The enduring quest for optimal reactivity, superior functional group compatibility, and innovative synthetic applications of exchange reagents remains a fundamental objective. In the past several years, the emergence of some significant discoveries in halogen-metal exchange reactions has proclaimed a renaissance to this field. This Account outlines the latest advances within the domain contributed by the Knochel group, including the main points as follows.The stereoretentive I/Li exchange on stereodefined secondary alkyl iodides was developed for the synthesis of nonstabilized chiral secondary alkyllithium reagents. This provided a straightforward method to access chiral organolithium reagents, which can be trapped by various electrophiles or transmetalated with other metals such as copper, zinc, and magnesium, thus enabling the stereoselective synthesis of a series of functionalized compounds and natural products.Faster halogen-magnesium and halogen-zinc exchanges in toluene were realized using a novel kind of exchange reagent complexed with lithium alkoxide. These highly efficient exchange reactions are much faster than traditional ones and performed in an industrially friendly solvent. These advantages are of great value in practical synthesis, paving the way for new developments in this evolving area.Halogen-lanthanide exchanges and their novel applications in organic synthesis were established. These new exchanges introduced the lanthanide metals into halogen-metal exchange reactions for the first time, thereby opening new avenues in synthetic chemistry. Building on these achievements, a comparative analysis of the exchange reaction rates by kinetic study has quantified the relationship between the electronegativity of metals and the rates of halogen-metal exchanges.Br/Na exchange in continuous flow was achieved using a hexane-soluble exchange reagent, 2-ethylhexylsodium. This approach effectively circumvented the poor solubility of the organosodium reagent, which has proven to be of significant practical value and greatly enhanced the synthetic utility of the organosodium reagent in organic synthesis.These remarkable breakthroughs as mentioned above are fueled mainly by upgrading the exchange reagents, resulting in the development of new halogen-metal exchange reactions and innovative applications in organic synthesis. Given the importance of halogen-metal exchanges in synthetic chemistry, the pursuit of other types of exchange reactions, particularly those involving new metals, will be in continuous demand. This Account provides a timely summary of recent progress and will undoubtedly inspire further advances to drive this research field forward.

Stereoselective polar radical crossover for the functionalization of strained-ring systems

Radical-polar crossover of organoborates is a poweful tool that enables the creation of two C-C bonds simultaneously. Small ring systems have become essential motifs in drug discovery and medicinal chemistry. However, step-economic methods for their selective functionalization remains scarce. Here we present a one-pot strategy that merges a simple preparation of strained organoboron species with the recently popularized polar radical crossover of borate derivatives to stereoselectively access tri-substituted azetidines, cyclobutanes and five-membered carbo- and heterocycles.

Total synthesis of (-)-securingine G

In this study, we present the first total synthesis of (-)-securingine G. Diverging from the proposed biosynthetic pathway, our approach involves the addition of nucleophilic pyridine anion species to the electrophilic menisdaurilide congener. Crucially, incorporating a weakly basic yet nucleophilic tri(2-pyridinyl)lanthanum complex proved essential to circumvent undesired base-mediated pathways during the key coupling reaction. Notably, we introduce n-Bu3La·5LiCl as a new exchange reagent, facilitating efficient halide/lanthanum exchange of (hetero)aryl halides.

Transition Metal Catalyst Free Synthesis of Olefins from Organoboron Derivatives

Stereoselective preparation of highly substituted olefins is still a severe challenge that requires well defined elimination precursors. Organoboron chemistry is particularly suited for the preparation of molecules with adjacent stereocenters. As organo boron substrates with leaving groups in β-position can undergo stereospecific syn- or anti-elimination, this chemistry harbors great potential for the synthesis of complex olefins. In recent years three main strategies emerged, which differ in their approach to the β-functionalized organoboron elimination precursor. (i) Stereoselective preparation of such elimination precursor can be achieved by addition of a boron-stabilized anion (d1 ) to an aldehyde or ketone (a1 ) or diastereoselective 1,3-rearrangement of suitable boron-ate-complexes. Stereospecific methods rely either on (ii) diastereospecific 1,2-metalate rearrangement of boron-ate-complexes that involve opening of appropriate heterocycles or (iii) addition of chiral carbenoids (d1 *) to chiral boronates (a1 *) with a leaving group in α-position.

Transition Metal Catalyst-Free, Base-Promoted 1,2-Additions of Polyfluorophenylboronates to Aldehydes and Ketones

A novel protocol for the transition metal-free 1,2-addition of polyfluoroaryl boronate esters to aldehydes and ketones is reported, which provides secondary alcohols, tertiary alcohols, and ketones. Control experiments and DFT calculations indicate that both the ortho-F substituents on the polyfluorophenyl boronates and the counterion K+ in the carbonate base are critical. The distinguishing features of this procedure include the employment of commercially available starting materials and the broad scope of the reaction with a wide variety of carbonyl compounds giving moderate to excellent yields. Intriguing structural features involving O-H⋅⋅⋅O and O-H⋅⋅⋅N hydrogen bonding, as well as arene-perfluoroarene interactions, in this series of racemic polyfluoroaryl carbinols have also been addressed.

CeCl3 /n-BuLi: Unraveling Imamoto's Organocerium Reagent

CeCl₃(thf) reacts at low temperatures with MeLi, t‐BuLi, and n‐BuLi to isolable organocerium complexes. Solvent‐dependent extensive n‐BuLi dissociation is revealed by ⁷Li NMR spectroscopy, suggesting “Ce(n‐Bu)₃(thf)_x” or solvent‐separated ion pairs like “[Li(thf)₄][Ce(n‐Bu)₄(thf)_y]” as the dominant species of the Imamoto reagent. The stability of complexes Li₃Ln(n‐Bu)₆(thf)₄ increases markedly with decreasing Ln^III size. Closer inspection of the solution behavior of crystalline Li₃Lu(n‐Bu)₆(thf)₄ and mixtures of LuCl₃(thf)₂/n‐BuLi in THF indicates occurring n‐BuLi dissociation only at molar ratios of <1:3. n‐BuLi‐depleted complex LiLu(n‐Bu)₃Cl(tmeda)₂ was obtained by treatment of Li₂Lu(n‐Bu)₅(tmeda)₂ with ClSiMe₃, at the expense of LiCl incorporation. Imamoto's ketone/tertiary alcohol transformation was examined with 1,3‐diphenylpropan‐2‐one, affording 99 % of alcohol.

Uncommon Four-Membered Building Blocks - Cyclobutenes, Azetines and Thietes

Strained ring systems have gained considerable importance over the last few years for their implication in natural product syntheses or in drug discovery programs. We present herein a recollection of our work on the construction and functionalization of unsaturated four-membered carbo- and heterocycles in the context of the literature, as well as their applications in further reactions.

Tandem Mn-I Exchange and Homocoupling Processes Mediated by a Synergistically Operative Lithium Manganate

Pairing lithium and manganese(II) to form lithium manganate [Li₂ Mn(CH₂ SiMe₃ )₄ ] enables the efficient direct Mn-I exchange of aryliodides, affording transient (aryl)lithium manganate intermediates which in turn undergo spontaneous C-C homocoupling at room temperature to furnish symmetrical (bis)aryls in good yields under mild reaction conditions. The combination of EPR with X-ray crystallographic studies has revealed the mixed Li/Mn constitution of the organometallic intermediates involved in these reactions, including the homocoupling step which had previously been thought to occur via a single-metal Mn aryl species. These studies show Li and Mn working together in a synergistic manner to facilitate both the Mn-I exchange and the C-C bond-forming steps. Both steps are carefully synchronized, with the concomitant generation of the alkyliodide ICH₂ SiMe₃ during the Mn-I exchange being essential to the aryl homocoupling process, wherein it serves as an in situ generated oxidant.

Electro-Olefination-A Catalyst Free Stereoconvergent Strategy for the Functionalization of Alkenes

Conventional methods carrying out C(sp2 )-C(sp2 ) bond formations are typically mediated by transition-metal-based catalysts. Herein, we conceptualize a complementary avenue to access such bonds by exploiting the potential of electrochemistry in combination with organoboron chemistry. We demonstrate a transition metal catalyst-free electrocoupling between (hetero)aryls and alkenes through readily available alkenyl-tri(hetero)aryl borate salts (ATBs) in a stereoconvergent fashion. This unprecedented transformation was investigated theoretically and experimentally and led to a library of functionalized alkenes. The concept was then carried further and applied to the synthesis of the natural product pinosylvin and the derivatization of the steroidal dehydroepiandrosterone (DHEA) scaffold.

Single-Pot Access to Bisorganoborinates: Applications in Zweifel Olefination

Zweifel olefination is a catalyst-free reaction that serves alkene functionalization. While most methods employ commercially available boron pinacol esters, we have assembled a sequence in which the two partners of the formal coupling reaction are installed successively, starting from inexpensive boron alkoxides. The in situ formation of bisorganoborinates was accomplished by consecutive reaction of two different organometallic species. This single-pot procedure represents a great advancement in the generation of organoborinates and their involvement in C-C bond formation.