Structure, Bonding, and Reactivity of Room-Temperature-Stable Lithium Chloride Carbenoids

Alkali-Metal Mediation: Diversity of Applications in Main-Group Organometallic Chemistry

Organolithium compounds have been at the forefront of synthetic chemistry for over a century, as they mediate the synthesis of myriads of compounds that are utilised worldwide in academic and industrial settings. For that reason, lithium has always been the most important alkali metal in organometallic chemistry. Today, that importance is being seriously challenged by sodium and potassium, as the alkali-metal mediation of organic reactions in general has started branching off in several new directions. Recent examples covering main-group homogeneous catalysis, stoichiometric organic synthesis, low-valent main-group metal chemistry, polymerization, and green chemistry are showcased in this Review. Since alkali-metal compounds are often not the end products of these applications, their roles are rarely given top billing. Thus, this Review has been written to alert the community to this rising unifying phenomenon of "alkali-metal mediation".

Taking advantage of lithium monohalocarbenoid intrinsic α-elimination in 2-MeTHF: controlled epoxide ring-opening en route to halohydrins

The intrinsic degradative α-elimination of Li carbenoids somehow complicates their use in synthesis as C1-synthons. Nevertheless, we herein report how boosting such an α-elimination is a straightforward strategy for accomplishing controlled ring-opening of epoxides to furnish the corresponding β-halohydrins. Crucial for the development of the method is the use of the eco-friendly solvent 2-MeTHF, which forces the degradation of the incipient monohalolithium, due to the very limited stabilizing effect of this solvent on the chemical integrity of the carbenoid. With this approach, high yields of the targeted compounds are consistently obtained under very high regiocontrol and, despite the basic nature of the reagents, no racemization of enantiopure materials is observed.

A practical guide for using lithium halocarbenoids in homologation reactions

ABSTRACT

Lithium halocarbenoids are versatile reagents for accomplishing homologation processes. The fast α-elimination they suffer has been considered an important limitation for their extensive use. Herein, we present a series of practical considerations for an effective employment in the homologation of selected carbon electrophiles.

GRAPHICAL ABSTRACT

Cooperative bond activation reactions with carbene complexes

This review highlights the recent advances in the application of carbene complexes in bond activation reactions via metal–ligand cooperation.

Alkali Metal Chlorine and Bromine Carbenoids: Their Thermal Stability and Structural Properties

The synthesis and structures of a series of M/X carbenoids of the type [Ph₂ P(S)]₂ CMX with M=Li, Na, and K and X=Cl and Br are reported, amongst the first isolated Na/Br and K/Br carbenoids. NMR spectroscopic as well as crystallographic studies showed distinct differences between the lithium carbenoids and their heavier congeners. In the solid state, all carbenoids showed no direct metal-carbon interaction, but an interaction between the metal and the halogen atom. This contact is only very weak in the case of the Li/Br carbenoid, but much more pronounced in the corresponding potassium and sodium compounds. Nevertheless, these interactions did not significantly influence the stability of the carbenoids by weakening the C-X bond and facilitating the MX elimination. As such all compounds were found to be stable up to approximately 60 °C in solution. Hence, M-X interactions-albeit being an essential feature for the structure formation of carbenoids-are not the only criterion determining the stability of such compounds. In the present systems, the stabilization by the thiophosphinoyl moieties is more important than the metal/halogen combination.

Contribution of microreactor technology and flow chemistry to the development of green and sustainable synthesis

Microreactor technology and flow chemistry could play an important role in the development of green and sustainable synthetic processes. In this review, some recent relevant examples in the field of flash chemistry, catalysis, hazardous chemistry and continuous flow processing are described. Selected examples highlight the role that flow chemistry could play in the near future for a sustainable development.

Stability and reactivity control of carbenoids: recent advances and perspectives

Metal carbenoids such as lithium or Simmons-Smith-type reagents are widely used in organic synthesis, particularly in cyclopropanation and homologation reactions. These reagents are often highly reactive and thermally labile, thus limiting their isolation and hampering the development of new synthetic applications. Recent years however, have shown that by means of systematic stabilization a control of reactivity and the development of new applications is possible. This feature article documents recent developments in the control of carbenoid reactivity and stability and highlights structural and electronic properties as well as applications in main group element and transition metal chemistry.

Lithium Halomethylcarbenoids: Preparation and Use in the Homologation of Carbon Electrophiles

α-Halomethyllithium carbenoids are useful homologating reagents which - reacting under proper reaction conditions as carbanions - enable the installation via nucleophilic addition of a reactive halomethyl fragment onto a preformed carbon-heteroatom bond. The pronounced thermolability represented - since seminal studies by Köbrich - the Achilles' heel of these reagents: the use of Barbier-type methodologies (i.e., the electrophile should be present in the reaction mixture prior to the formation of the carbenoid) was pivotal in order to suppress decomposition through α-elimination processes. Nowadays, the use of low temperatures (-78 °C) guarantees reliable procedures and, significantly, the employment of microreactor technologies allows external trapping to be performed even at higher temperatures as reported by Luisi. We will discuss the α-halomethyllithium-mediated homologations of a series of carbon electrophiles such as carbonyl compounds, imines, esters, Weinreb amides, and isocyanates.

Bromomethyllithium-mediated chemoselective homologation of disulfides to dithioacetals

An efficient, chemoselective homologation of disulfides and diselenides to the corresponding dithio- and diselenoacetals has been developed via the addition of bromomethyllithium. Chemoselectivity is fully preserved in the presence of concomitant electrophilic sites decorating the substrates. The synthetic potential of selected dithioacetals has been evaluated in Feringa-Fañanas-Mastral-type Pd-catalyzed coupling with an organolithium and in the unusual 1,4-addition to a Weinreb amide.

Formation of a zwitterionic boronium species from the reaction of a stable carbenoid with borane: CO2 reduction

The treatment of Li2C(PPh2NMes)2 (1, Mes = 2,4,6-Me3C6H2) with hexachloroethane yielded the corresponding carbenoid 2 in good yields. The reactivity of 2 was studied with BH3·SMe2 to give a zwitterionic boronium species 4, also a stable carbenoid. Both carbenoid species were found to be excellent catalysts for the CO2 reduction by BH3·SMe2.

Selective dehydrocoupling of phosphines by lithium chloride carbenoids

The development of a simple, transition-metal-free approach for the formation of phosphorus-phosphorus bonds through dehydrocoupling of phosphines is presented. The reaction is mediated by electronically stabilized lithium chloride carbenoids and affords a variety of different diphosphines under mild reaction conditions. The developed protocol is simple and highly efficient and allows the isolation of novel functionalized diphosphines in high yields.

Synthesis and stability of Li/Cl carbenoids based on bis(iminophosphoryl)methanes

Bis(iminophosphoryl) substituted Li/Cl carbenoids – accessable via different preparation methods – show high thermal stabilities, which however depend on the N-substituent.