Metal-Catalyzed Di-tert-butylsilylene Transfer: Synthesis and Reactivity of Silacyclopropanes

Three-Membered Aluminacycles

Three-membered-ring scaffolds of carbocycles, namely, cyclopropanes and cyclopropenes, are ubiquitous in natural products and pharmaceutical molecules. These molecules exhibit a peculiar reactivity, and their applications as synthetic intermediates and versatile building blocks in organic synthesis have been extensively studied over the past century. The incorporation of heteroatoms into three-membered cyclic structures has attracted significant attention, reflecting fundamental differences in their electronic/geometric structures and reactivities compared to their carbon congeners and their associated potential for exploitation in applications. Recently, the chemistry of low-valent aluminum species, alumylenes, dialumenes, and aluminyl anions, has dramatically developed, which has allowed access to hitherto unprecedented aluminacycles. This Perspective focuses upon advances in the chemistry of three-membered aluminacycles, including their synthetic protocols, spectroscopic and structural properties, and reactivity toward various substrates and small molecules.

Diastereoselective silylene transfer reactions to chiral enantiopure alkenes: effects of ligand size and substrate bias

Silylenes are useful reactive intermediates for the stereoselective construction of compounds containing carbon-silicon bonds. Despite their synthetic utility, the development of either an enantioselective or diastereoselective metal-catalyzed silylene transfer reaction, in which ligands on the metal catalyst control stereoselectivity, has not been achieved. In this article, we report that the structure of the alkene is the most important for controlling stereoselectivity in these reactions. The stereochemical course of kinetically controlled silacyclopropanation reactions was not affected by the nature or chirality of the ligands on the metal. When silylene transfer reactions were reversible, however, products can be formed with a high degree of diastereoselectivity (90 : 10 d.r.).

Shuttle Catalysis-New Strategies in Organic Synthesis

Shuttle catalysis has recently emerged as a powerful new concept that provides a platform for performing both functionalization and defunctionalization reactions. In this concept article, applications of shuttle catalysis as a novel strategy in organic synthesis are discussed. This includes using forward shuttle catalysis reactions for challenging bond-forming processes that avoid the use of hazardous chemicals. Shuttle catalysis also facilitates the transfer of reactive functionality as a route to procure a broad range of compounds using one simple procedure. Reverse shuttle catalysis reactions are also discussed as a method for the valorization of biomass and waste materials. Another area of interest, shuttle-catalysis-assisted reactions, wherein the transfer of a small molecule is utilized in a catalytic cycle, is also described. Possible future directions in this exciting new field are also suggested.

A Green Recyclable Poly(4-Vinylpyridine)-Supported Copper Iodide Nanoparticle Catalyst for the Chemoselective Synthesis of Pentaerythritol Diacetals from Aromatic Aldehydes

Poly(4-vinylpyridine)-supported copper(I) iodide nanoparticles have been used as an efficient, green recyclable catalyst for the chemoselective synthesis of pentaerythritol diacetals from aromatic aldehydes and pentaerythritol under solvent-free conditions. This catalyst was reused six times without any loss of its activity.

Silylene transfer to allylic sulfides: formation of substituted silacyclobutanes

Silylene transfer to allylic sulfides results in a formal 1,2-sulfide migration. The rearrangement yields substituted silacyclobutanes, not the expected silacyclopropanes. The silacyclobutanes were elaborated by insertions of carbonyl compounds selectively into one carbon-silicon bond. A mechanism for the 1,2-sulfide migration is proposed involving an episulfonium ion intermediate.

Strained organosilacyclic compounds: synthesis of anti-Bredt olefins and trans-dioxasilacyclooctenes

Insertions of silylenes into the allylic carbon-oxygen bond of vinyl epoxides was shown to generate 1,2-silaoxetanes. These intermediates undergo highly diastereoselective additions to aldehydes to form anti-Bredt olefins and trans-dioxasilacyclooctenes. Additions of electrophiles could be performed selectively on the outside face of these strained trans-cycloalkenes to provide valuable functionalized compounds.

Catalytic Addition of Simple Alkenes to Carbonyl Compounds Using Group 10 Metals

Recent advances using nickel complexes in the activation of unactivated monosubstituted olefins for catalytic intermolecular carbon-carbon bond-forming reactions with carbonyl compounds, such as simple aldehydes, isocyanates, and conjugated aldehydes and ketones, are discussed. In these reactions, the olefins function as vinyl- and allylmetal equivalents, providing a new strategy for organic synthesis. Current limitations and the outlook for this new strategy are also discussed.

Alkylidenesilacyclopropanes derived from allenes: applications to the selective synthesis of triols and homoallylic alcohols

Several alkylidenesilacyclopropanes were prepared by silver-mediated silylene transfer to allenes. Oxasilacyclopentanes derived from allenes were prepared with high regio- and diastereoselectivity by a two-step, one-flask silacyclopropanation/carbonyl insertion reaction. Triols and homoallylic alcohols were formed diastereoselectively by functionalizing the oxasilacyclopentanes. An optically active allene (>98% ee) was utilized to synthesize an enantiopure homoallylic alcohol in 96% ee.

Intermolecular silacarbonyl ylide cycloadditions: a direct pathway to oxasilacyclopentenes

Silacarbonyl ylides, generated by metal-catalyzed silylene transfer to carbonyls, participate in formal intermolecular 1,3-dipolar cycloaddition reactions with carbonyl compounds and alkynes to form dioxasilacyclopentane acetals and oxasilacyclopentenes in an efficient, one-step process.

Nickel-catalyzed coupling of alkenes, aldehydes, and silyl triflates

A full account of two recently developed nickel-catalyzed coupling reactions of alkenes, aldehydes, and silyl triflates is presented. These reactions provide either allylic alcohol or homoallylic alcohol derivatives selectively, depending on the ligand employed. These processes are believed to be mechanistically distinct from Lewis acid-catalyzed carbonyl-ene reactions, and several lines of evidence supporting this hypothesis are discussed.

Synthesis of (±)-epi-Stegobinone Utilizing Silacyclopropanes as Synthetic Intermediates

The synthesis of (+/-)-1'-epi-stegobinone has been accomplished in ten steps and 17% overall yield from a recently reported silacyclopropane-derived diol. All stereocenters of the final product were established relative to the stereochemistry of the initial silacyclopropane. This synthesis represents the first time silacyclopropane reactivity has been employed in a target-directed synthesis.

Formation and Reactivity of Silacyclopropenes Derived from Siloxyalkynes: Stereoselective Formation of 1,2,4-Triols

Silver phosphate-catalyzed silylene transfer to siloxyalkynes provided silacyclopropenes possessing a silyl enol ether functional group. Copper-catalyzed insertions of carbonyl compounds afforded the corresponding oxasilacyclopentenes. The embedded silyl enol ether functionality was treated with various aldehydes and a catalytic amount of Sc(OTf)3 to provide dioxasilacycloheptanones, which resulted from an aldol addition/rearrangement. Stereoselective reduction or allylation of the cyclic ketone, followed by n-Bu4NF deprotection, provided high yields of 1,2,4-triols possessing four contiguous stereocenters.

Silylenes, Silenes, and Disilenes: Novel Silicon-Based Reagents for Organic Synthesis?

Silylenes, silenes, and disilenes are silicon analogues of carbenes and alkenes. Since the first detection and isolation of these species a few decades ago, focus has been given to their fundamental structure and reactivity properties. Recent developments show that the time is set to exploit their unique chemistry in applied areas. Emerging applications in catalysis and stereoselective synthesis point to a new field within synthetic organosilicon chemistry.

Metal-Catalyzed Rearrangement of Homoallylic Ethers to Silylmethyl Allylic Silanes in the Presence of a Di-tert-butylsilylene Source

[reaction: see text] In examining the scope of the di-tert-butylsilylene transfer to gem-disubstituted alkenes to form silacyclopropanes, we discovered an unprecedented reaction of homoallylic ethers. When silylene transfer was performed at room temperature or above, two di-tert-butylsilylene units were incorporated into the molecule, and complete rearrangement of the carbon backbone occurred. This report describes the scope of this unique reaction as well as the mechanistic studies conducted that led to a proposed mechanism.

Silylene Transfer to Carbonyl Compounds and Subsequent Ireland−Claisen Rearrangements to Control Formation of Quaternary Carbon Stereocenters

In this communication, we demonstrate that silylene transfer to carbonyl compounds provides a new method for regio- and stereoselective enolate formation. Silylene transfer to a range of alpha,beta-unsaturated esters under silver-catalyzed conditions proved to be a general method for the formation of oxasilacyclopentenes containing a cyclic silyl ketene acetal functionality. These oxasilacyclopentenes are useful synthetic intermediates that can undergo facile and selective Ireland-Claisen rearrangements and aldol addition reactions to provide products with multiple contiguous stereocenters and quaternary carbon centers.