Iron Catalyzed Double Bond Isomerization: Evidence for an FeI /FeIII Catalytic Cycle

Stereodivergent, Kinetically Controlled Isomerization of Terminal Alkenes via Nickel Catalysis

Because internal alkenes are more challenging synthetic targets than terminal alkenes, metal-catalyzed olefin mono-transposition (i.e., positional isomerization) approaches have emerged to afford valuable E- or Z- internal alkenes from their complementary terminal alkene feedstocks. However, the applicability of these methods has been hampered by lack of generality, commercial availability of precatalysts, and scalability. Here, we report a nickel-catalyzed platform for the stereodivergent E/Z-selective synthesis of internal alkenes at room temperature. Commercial reagents enable this one-carbon transposition of terminal alkenes to valuable E- or Z-internal alkenes via a Ni-H-mediated insertion/elimination mechanism. Though the mechanistic regime is the same in both systems, the underlying pathways that lead to each of the active catalysts are distinct, with the Z-selective catalyst forming from comproportionation of an oxidative addition complex followed by oxidative addition with substrate and the E-selective catalyst forming from protonation of the metal by the trialkylphosphonium salt additive. In each case, ligand sterics and denticity control stereochemistry and prevent over-isomerization.

Germacrene B - a central intermediate in sesquiterpene biosynthesis

Germacranes are important intermediates in the biosynthesis of eudesmane and guaiane sesquiterpenes. After their initial formation from farnesyl diphosphate, these neutral intermediates can become reprotonated for a second cyclisation to reach the bicyclic eudesmane and guaiane skeletons. This review summarises the accumulated knowledge on eudesmane and guaiane sesquiterpene hydrocarbons and alcohols that potentially arise from the achiral sesquiterpene hydrocarbon germacrene B. Not only compounds isolated from natural sources, but also synthetic compounds are dicussed, with the aim to give a rationale for the structural assignment for each compound. A total number of 64 compounds is presented, with 131 cited references.

Cycloisomerization of Alkyne-Tethered N-Acyloxycarbamates to 2-(3H)Oxazolones through Nitrenoid-Mediated Carboxyamidation

The cycloisomerization of alkyne-tethered N-benzoyloxycarbamates to 2-(3H)oxazolones is described. Two catalytic systems are tailored for intramolecular 5-exo-alkyne carboxyamidation and concomitant alkene isomerization. PtCl₂ /CO (5 mol%, toluene, 100 °C) promotes both carboxyamidation and alkene isomerization but has a limited substrate scope. On the other hand, FeCl₃ (5 mol%, CH₃ CN, 100 °C) promotes carboxyamidation effectively but a cocatalyst is required for the exocyclic alkene isomerization. Thus, a two-step one-pot protocol has been developed for a broader reaction scope, which involves FeCl₃ -catalyzed carboxyamidation and base-induced alkene isomerization. Crossover experiments suggest that these reactions proceed mainly through a mechanism involving acylnitrenoid intermediates rather than carbenoid intermediates.

Parts-per-million of ruthenium catalyze the selective chain-walking reaction of terminal alkenes

The chain–walking of terminal alkenes (also called migration or isomerization reaction) is currently carried out in industry with unselective and relatively costly processes, to give mixtures of alkenes with significant amounts of oligomerized, branched and reduced by–products. Here, it is shown that part–per–million amounts of a variety of commercially available and in–house made ruthenium compounds, supported or not, transform into an extremely active catalyst for the regioselective migration of terminal alkenes to internal positions, with yields and selectivity up to >99% and without any solvent, ligand, additive or protecting atmosphere required, but only heating at temperatures >150 °C. The resulting internal alkene can be prepared in kilogram quantities, ready to be used in nine different organic reactions without any further treatment.

The chain-walking of terminal alkenes is an industrially relevant reaction. Here, the authors show that part-per-million amounts of a variety of ruthenium compounds catalyze the reaction in yields and selectivity up to >99%, without any solvent or additive.

Synthesis of mono- and polyfunctional organosilicon derivatives of polyhedral carboranes for the preparation of hybrid polymer materials

A series of mono- and polyfunctional carborane organosilicon derivatives were prepared with good yields based on the hydrosilylation reactions of allylcarboranes with hydride-containing organosilicon compounds such as tetramethyldisiloxane, decamethylpentasiloxane and triethoxysilane in the presence of Karstedt's catalyst.

Highly Regioselective Copper-Catalyzed Transfer Hydrodeuteration of Unactivated Terminal Alkenes

Catalytic transfer hydrodeuteration of unactivated alkenes is challenging because of the requirement that chemically similar hydrogen and deuterium undergo selective insertion across a π-bond. We now report a highly regioselective catalytic transfer hydrodeuteration of unactivated terminal alkenes across a variety of heteroatom- or heterocycle-containing substrates. The base-metal-catalyzed reaction is also demonstrated on two complex natural products. Reaction studies indicate modular conditions that can also be extended to perform either an alkene transfer hydrogenation or transfer deuteration.

Polymerization of Allenes by Using an Iron(II) β-Diketiminate Pre-Catalyst to Generate High Mn Polymers

Herein, we report an iron(II)-catalyzed polymerization of arylallenes. This reaction proceeds rapidly at room temperature in the presence of a hydride co-catalyst to generate polymers of weight up to Mn =189 000 Da. We have determined the polymer structure and chain length for a range of monomers through a combination of NMR, differential scanning calorimetry (DSC) and gel permeation chromatography (GPC) analysis. Mechanistically, we postulate that the co-catalyst does not react to form an iron(II) hydride in situ, but instead the chain growth is proceeding via a reactive Fe(III) species. We have also performed kinetic and isotopic experiments to further our understanding. The formation of a highly unusual 1,3-substituted cyclobutane side-product is also investigated.

Iron Catalyzed Double Bond Isomerization: Evidence for an FeI /FeIII Catalytic Cycle

Iron-catalyzed isomerization of alkenes is reported using an iron(II) β-diketiminate pre-catalyst. The reaction proceeds with a catalytic amount of a hydride source, such as pinacol borane (HBpin) or ammonia borane (H3 N⋅BH3 ). Reactivity with both allyl arenes and aliphatic alkenes has been studied. The catalytic mechanism was investigated by a variety of means, including deuteration studies, Density Functional Theory (DFT) and Electron Paramagnetic Resonance (EPR) spectroscopy. The data obtained support a pre-catalyst activation step that gives access to an η2 -coordinated alkene FeI complex, followed by oxidative addition of the alkene to give an FeIII intermediate, which then undergoes reductive elimination to allow release of the isomerization product.