Julia Rehbein

Do we fully understand what controls chemical selectivity

Reaction rates and product selectivity of kinetically controlled reactions are not always sufficiently described by standard RRKM or TST theory. Reactions taking place on potential energy surfaces featuring a valley ridge inflection point belong to this class of reactions. Though various research groups could show that reaction path bifurcations are far from being an exception in organic reactions the underlying principles that govern product distributions of those bifurcating reaction pathways are yet not fully understood. This Perspective has the intention to provide an overview of how far our understanding and the development of the theoretical foundation have progressed.

Facile Oxidative Rearrangements Using Hypervalent Iodine Reagents

Aromatic substituents migrate in a novel oxidative cyclization mediated by iodine(III) reagents. 4-Arylbut-3-enoic acids are cyclized and rearranged to 4-arylfuran-2(5H)-ones by hypervalent iodine compounds in good to excellent yields under mild reaction conditions. Other ring sizes are also accessible. The mechanism of the reaction is described in detail, and calculations highlight the cationic nature of the intermediates in the rearrangement. The fast access to heavily substituted furanones is used for the synthesis of biologically active derivatives.

Total Synthesis of Natural and Non-Natural Δ5,6Δ12,13-Jatrophane Diterpenes and Their Evaluation as MDR Modulators

We report the details of the total synthesis of natural and non-natural jatropha-5,12-dienes. The successful tactic for the assembly of the strained trans-bicyclo[10.3.0]pentadecane scaffold employed a B-alkyl Suzuki-Miyaura cross-coupling for the formation of the C5/C6 double bond and a ring-closing metathesis for the construction of the C12/C13 double bond. The key step of the synthesis of the cyclopentane fragment, an uncatalyzed intramolecular carbonyl-ene reaction, was studied computationally by DFT calculations. The members of the ensemble of synthetic natural and non-natural jatrophanes were subsequently examined as modulators for the ABCB1, ABCG2, and ABCC1 efflux proteins, which are associated with multidrug resistance in cancer chemotherapy.

Gosteli−Claisen Rearrangement of Propargyl Vinyl Ethers: Cascading Molecular Rearrangements

The ambivalent reactivity of 2-alkoxycarbonyl-substituted propargyl vinyl ethers has been explored. Depending on the conditions, the catalyzed and uncatalyzed Gosteli-Claisen rearrangement triggers downstream transformations that cascade from initially formed γ-allenyl α-keto esters to highly substituted furanes and cyclopentenes. In support of a mechanistic hypothesis, the results of a DFT study using the B1B95 and B3LYP functionals are revealed.

Gosteli−Claisen Rearrangement: Substrate Synthesis, Simple Diastereoselectivity, and Kinetic Studies

The results of kinetic studies on the uncatalyzed [3,3]-sigmatropic rearrangement of 2-alkoxycarbonyl-substituted allyl vinyl ethers are reported. Apparently first reported by Gosteli in 1972, this variation of a Claisen rearrangement enjoyed a shadowy existence for three decades until its potential for the development of a catalytic asymmetric Claisen rearrangement was discovered. Inspired by this development, we have studied substituent and solvent rate effects, and we provide evidence that a chairlike transition state is highly favorable for the uncatalyzed Gosteli-Claisen rearrangement.

Gosteli-Claisen Rearrangement: DFT Study of Substituent-Rate Effects

The uncatalyzed Gosteli-Claisen rearrangement of four double bond isomeric allyl vinyl ethers has been studied at the B3LYP/6-31G* and B3LYP/6-31G*+PCM levels of theory. The experimentally determined structure-reactivity relationship was successfully reproduced; the relative reactivity of the (E,E)-, (E,Z)-, (Z,E)-, and (Z,Z)-configured allyl vinyl ethers can be attributed to unfavorable interactions caused by pseudoaxial substituents within the chairlike transition-state structures. As expected, the isolated assessment of the calculated ground-state or transition-state stabilities is not suitable to explain the experimentally observed structure-reactivity relationship.

Enantioselective Oxidative Rearrangements with Chiral Hypervalent Iodine Reagents

Abstract A stereoselective hypervalent iodine‐promoted oxidative rearrangement of 1,1‐disubstituted alkenes has been developed. This practically simple protocol provides access to enantioenriched α‐arylated ketones without the use of transition metals from readily accessible alkenes.

Elucidation of the regio- and chemoselectivity of enzymatic allylic oxidations with Pleurotus sapidus – conversion of selected spirocyclic terpenoids and computational analysis

Summary Allylic oxidations of olefins to enones allow the efficient synthesis of value-added products from simple olefinic precursors like terpenes or terpenoids. Biocatalytic variants have a large potential for industrial applications, particularly in the pharmaceutical and food industry. Herein we report efficient biocatalytic allylic oxidations of spirocyclic terpenoids by a lyophilisate of the edible fungus Pleurotus sapidus. This ‘’mushroom catalysis’’ is operationally simple and allows the conversion of various unsaturated spirocyclic terpenoids. A number of new spirocyclic enones have thus been obtained with good regio- and chemoselectivity and chiral separation protocols for enantiomeric mixtures have been developed. The oxidations follow a radical mechanism and the regioselectivity of the reaction is mainly determined by bond-dissociation energies of the available allylic CH-bonds and steric accessibility of the oxidation site.

Visible-Light-Accelerated Copper(II)-Catalyzed Regio- and Chemoselective Oxo-Azidation of Vinyl Arenes

The visible-light-accelerated oxo-azidation of vinyl arenes with trimethylsilylazide and molecular oxygen as stoichiometric oxidant was achieved. In contrast to photocatalysts based on iridium, ruthenium, or organic dyes, [Cu(dap)2 ]Cl or [Cu(dap)Cl2 ] were found to be unique for this transformation, which is attributed to their ability to interact with the substrates through ligand exchange and rebound mechanisms. CuII is proposed as the catalytically active species, which upon coordinating azide will undergo light-accelerated homolysis to form CuI and azide radicals. This activation principle (CuII -X→CuI +X. ) opens up new avenues for copper-based photocatalysis.

Synthesis of aryl sulfides via radical–radical cross coupling of electron-rich arenes using visible light photoredox catalysis

Electron-rich arenes react with aryl and alkyl disulfides in the presence of catalytic amounts of [Ir(dF(CF3)ppy)2(dtbpy)]PF6 and (NH4)2S2O8 under blue light irradiation to yield arylthiols. The reaction proceeds at room temperature and avoids the use of prefunctionalized arenes. Experimental evidence suggests a radical–radical cross coupling mechanism.