A novel iron-catalyzed borylation of propargylic acetates leading to allenylboronates has been developed. The method allows the preparation of a variety of di-, tri- and tetrasubstituted allenylboronates at room temperature with good functional group compatibility. Stereochemical studies show that an anti-S_N2’ displacement of acetate by boron occurs; this also allows transfer of chirality to yield enantiomerically enriched allenylboronates. The synthetic utility of this protocol was further substantiated by transformations of the obtained allenylboronates including oxidation and propargylation. 

Herein, we describe an efficient nanocopper-catalyzed Alder-ene reaction of allenynamides. The copper nanoparticles were immobilized on amino-functionalized microcrystalline cellulose. A solvent-controlled chemoselectivity of the reaction was observed, leading to the chemodivergent synthesis of pyrrolines (2,5-dihydropyrroles) and pyrroles. The heterogeneous copper catalyst exhibits high efficiency and good recyclability in the Alder-ene reaction, constituting a highly attractive catalytic system from an economical and environmental point of view. 

One of the main challenges of modern chemical industry is the inevitable transition to greener, more sustainable manufacturing processes that utilize raw materials more effectively, minimize waste streams, and avoid the use of toxic and hazardous materials. The development of new iron-based catalytic systems can eliminate the need for the currently widely used, costly and potentially toxic heavy metal catalysts in industrially relevant chemical reactions. Prominent examples of such processes include transition metal catalyzed cross-coupling reactions for formation of carbon-carbon (C -C) bonds. This minireview outlines recent developments in the area of iron-catalyzed cross-coupling chemistry during the last couple of decades, with special emphasis on the use of propargylic substrates and Grignard reagents as coupling partners.

Herein we report on the development and application of chemoenzymatic Dynamic Kinetic Asymmetric Transformation (DYKAT) of α-substituted β-hydroxyketones (β-HKs), using Candida antartica lipase B (CALB) as transesterification catalyst and a ruthenium complex as epimerization catalyst. An operationally simple protocol allows for an efficient preparation of highly enantiomerically enriched α-substituted β-oxoacetates. The products were obtained in yields up to 95 % with good diastereomeric ratios. 

Herein we disclose an iron-catalyzed cross-coupling reaction of propargyl ethers with Grignard reagents. The reaction was demonstrated to be stereospecific and allows for a facile preparation of optically active allenes via efficient chirality transfer. Various tri- and tetrasubstituted fluoroalkyl allenes can be obtained in good to excellent yields. In addition, an iron-catalyzed cross-coupling of Grignard reagents with α-alkynyl oxetanes and tetrahydrofurans is disclosed herein, which constitutes a straightforward approach towards fully substituted β- or γ-allenols, respectively.

Palladium(ΙΙ)-catalyzed carbon-carbon and carbon-heteroatom bond formation via selective C-H bond oxidation constitutes a step-economical and versatile approach towards complex target molecules. This thesis has been focused on the development of new selective palladium(ΙΙ)-catalyzed transformations of allenes under oxidative conditions catalyzed by either homogenous or heterogeneous palladium(ΙΙ) catalysts.

The first part of this thesis describes carbocyclization-borylation of bisallenes, alkynylation of enallenes and preparation of [3]dendralenes from readily available allene starting materials. These homogenous palladium(ΙΙ)-catalyzed transformations involve selective allenic C-H cleavage under oxidative conditions. The aforementioned reactions can be carried out under aerobic biomimetic conditions by using electron transfer mediators (ETMs) and oxygen gas as the stoichiometric oxidant.

The second part deals with highly diastereoselective palladium-catalyzed oxidative carbocyclization reactions of enallenes assisted by weakly coordinating functional groups, such as hydroxyl or sulfonamide groups. It was demonstrated, that this weak coordination is sufficient to trigger the allene attack on the metal center enabling formation of (vinyl)Pd(II) intermediates, previously achievable by the directing effect of unsaturated hydrocarbon groups. A broad range of borylated cyclohexanol derivatives, as well as cis-fused [5,5] bicyclic γ-lactones and γ-lactams were obtained with superb diastereoselectivity via palladium(ΙΙ)-catalyzed carbocyclization-functionalization reactions of enallenols.

The final part of the thesis describes applications of the heterogenous catalyst Pd-AmP-MCF in oxidation reactions of allenes. Chemodivergent oxidative cascade reactions of enallenols were realized by switching between heterogeneous (Pd-AmP-MCF) and homogeneous (Pd(OAc)₂) palladium catalysts. In separate cases, the heterogeneous catalyst Pd-AmP-MCF showed improved performance over its homogeneous counterpart, accompanied by high recyclability and no detectable metal leaching. Synthetically important cyclic skeletons such as cyclobutenols, furanes, oxaboroles, and fused γ-lactones/lactams were obtained via borylative or carbonylative carbocyclization reactions of enallenols in high yields and with excellent stereoselectivity.

A heterogeneous palladium-catalyzed oxidative cyclization of enallenols has been developed for the construction of highly substituted furan and oxaborole derivatives. The heterogeneous catalyst (Pd-AmP-MCF) exhibits high activity, high site- and stereoselectivity, and efficient palladium recyclability in the transformations.

A palladium-catalyzed oxidative cascade carbonylative carbocyclization of enallenols was developed. Under mild reaction conditions, a range of cis-fused [5,5] bicyclic gamma-lactones and gamma-lactams with a 1,3-diene motif were obtained in good yields with high diastereoselectivity. The obtained lactone/lactam products are viable substrates for a stereoselective Diels-Alder reaction with N-phenylmaleimide, providing polycyclic compounds with increased molecular complexity.

A highly selective and efficient oxidative carbocyclization/borylation of enallenols catalyzed by palladium immobilized on amino-functionalized siliceous mesocellular foam (Pd-AmP-MCF) was developed for diastereoselective cyclobutenol synthesis. The heterogeneous palladium catalyst can be recovered and recycled without any observed loss of activity or selectivity. The high diastereoselectivity of the reaction is proposed to originate from a directing effect of the enallenol hydroxyl group. Optically pure cyclobutenol synthesis was achieved by the heterogeneous strategy by using chiral enallenol obtained from kinetic resolution.

A palladium-catalyzed oxidative tandem process of enallenols was accomplished within a homogeneous/heterogeneous catalysis manifold, setting the stage for the highly chemodivergent and diaster-eoselective synthesis of gamma-lactones and gamma-lactams under mild conditions.