Novel applications of triarylfluoroboranes in organic synthesis

The work described in this thesis is concerned with the utilisation of bulky boron-based Lewis acids as catalysts in organic synthesis. Chapter one provides a general introduction towards the bulky boron-based Lewis acids and diazo compounds, wherein their general synthesis, reactivity, and utilisation in the organic synthesis will be discussed in more detail. Furthermore, the introductory part of this thesis will cover frustrated Lewis pairs, including how they can activate small molecules via two different pathways and how such divergent activation was later utilised in the organic synthesis. Chapter two and three will cover the use of tris(pentafluorophenyl)borane as a catalyst for alkenylation and cyclopropenation of diazo esters, wherein the optimisation reactions, substrate scope, and reaction mechanism will be discussed. Lastly, frustrated Lewis pairs mediated cross-coupling reactions of arylacetylenes and aryl esters will be covered together with conducted studies for a determination of the reaction mechanism

Borane catalysed cyclopropenation of arylacetylenes

Triarylboranes have gained substantial attention as catalysts for C–C bond forming reactions due to their remarkable catalytic activities. Herein, we report B(C6F5)3 catalysed cyclopropenation of a wide variety of arylacetylenes using donor–acceptor diazoesters. A mild reaction protocol has been developed for the synthesis of functionalised cyclopropenes (33 examples) in good to excellent yields

Tris(pentafluorphenyl)boran-katalysierte Erzeugung von Carbenium-Ionen und autokatalytische Pyrazol-Synthese – eine theoretische und experimentelle Studie

In den letzten Jahren hat sich die metallfreie organische Synthese unter Verwendung von Triarylboranen als Katalysatoren zu einem weit verbreiteten Forschungsgebiet entwickelt. Hier berichten wir über eine umfassende theoretische und experimentelle Studie für die hochselektive Synthese von N-substituierten Pyrazolen durch die Erzeugung von Carbenium-Ionen aus der Reaktion zwischen Arylestern und Vinyldiazoacetaten in Gegenwart von katalytischem Tris(pentafluorphenyl)boran [B(C6F5)3]. DFT-Studien zum Reaktionsmechanismus zeigen, dass die In-situ-Generierung einer Carbenium-Spezies als Autokatalysator fungiert, welcher die regiospezifische Bildung von N-substituierten Pyrazolen in guter bis hervorragender Ausbeute (bis zu 81 %) auslöst

Tris(pentafluorophenyl)borane-catalyzed carbenium ion generation and autocatalytic pyrazole synthesis-a computational and experimental study

In recent years, metal-free organic synthesis using triarylboranes as catalysts has become a prevalent research area. Herein we report a comprehensive computational and experimental study for the highly selective synthesis of N-substituted pyrazoles through the generation of carbenium species from the reaction between aryl esters and vinyl diazoacetates in the presence of catalytic tris(pentafluorophenyl)borane [B(C6F5)3]. DFT studies were undertaken to illuminate the reaction mechanism revealing that the in situ generation of a carbenium species acts as an autocatalyst to prompt the regiospecific formation of N-substituted pyrazoles in good to excellent yields (up to 81 %)

Frustrated Lewis pairs in catalysis

Since frustrated Lewis pairs were first defined in 2006, they have become one of the most exciting areas of main group chemistry. In just 15 years, these systems have gone from chemical curiosities to commonplace catalysts, and many research groups are now exploring their ability to supersede conventional transition metal catalysts. This article aims to discuss the catalytic applicability of frustrated Lewis pairs, with a discussion on hydrogenation catalysis, 1,2-hydrofunctionalization catalysis, cyclization catalysis, and CO2 reduction catalysis

Borocations in catalysis

Even in its infancy, the catalytic application of cationic boron species has proven itself a powerful tool for main group catalyzed element bond activation. Though these highly Lewis acidic species have been known for almost half a century, their inherent reactivity made them difficult to work with in any fashion beyond chemical curiosity. This article will cover recent advances in the chemistry and catalytic applications of borocation species that are advancing main group chemistry

Frustrated Lewis pairs in organic synthesis

In 2006, Stephan defined frustrated Lewis pairs and, over the last 15 years, these systems have developed from chemical novelties to increasingly commonplace catalysts, with many research groups developing their capabilities to surpass those of transition metal catalysts. This book article aims to discuss the catalytic uses of frustrated Lewis pairs focusing on their application within organic synthesis, with a discussion on hydrogenation catalysis, air and moisture stable FLPs, enantioselective FLPs, 1,2-hydrofunctionalization catalysis, cyclization catalysis, and a brief look at the recent use of FLPs within radical chemistry

B(c6f5)3-catalyzed diastereoselective and divergent reactions of vinyldiazo esters with nitrones: synthesis of highly functionalized diazo compounds

Herein we report a mild, transition-metal-free, highly diastereoselective Lewis acid catalyzed methodology toward the synthesis of isoxazolidine-based diazo compounds from the reaction between vinyldiazo esters and nitrones. Interestingly, the isoxazolidine products were identified to have contrasting diastereoselectivity to previously reported metal-catalyzed reactions. Furthermore, the same catalyst can be used with enol diazo esters, prompting the formation of Mukaiyama–Mannich products. These diazo products can then be further functionalized to afford benzo[b]azepine and pyrrolidinone derivatives

Radical Reactivity of Frustrated Lewis Pairs with Diaryl Esters

Advances in the chemistry of metal-free systems known as frustrated Lewis pairs (FLPs) has exposed new reactivity of the p-block elements, particularly in small-molecule activation and catalysis. Typically, the mode of activation by FLPs has been predicated on a heterolytic two-electron process, although recently, select FLPs have been shown to participate in single-electron processes. Here, we report the reaction of diaryl substituted esters with FLPs. This results in divergent pathways, one whereby the diaryl moiety is stabilized by the Lewis basic phosphine, and the alternative pathway, wherein a single-electron transfer process occurs, generating the [Mes3P]+⋅/[C(H)Ar2]⋅ radical ion pair. The latter species undergoes a homocoupling reaction to yield tetraphenylethane derivatives. In the presence of olefins, this reactivity can be harnessed through an sp2-sp3 C–C heterocoupling reaction to generate α,β-substituted olefins. Notably, this work showcases an FLP approach to metal-free radical C–H bond activation with subsequent C–C bond formation, which also displays complementary reactivity to other approaches

Test of light-lepton universality in τ\tau decays with the Belle II experiment

International audienceWe present a measurement of the ratio $R_\mu = \mathcal{B}(\tau^-\to \mu^-\bar\nu_\mu\nu_\tau) / \mathcal{B}(\tau^-\to e^-\bar\nu_e\nu_\tau)$ of branching fractions $\mathcal{B}$ of the $\tau$ lepton decaying to muons or electrons using data collected with the Belle II detector at the SuperKEKB $e^+e^-$ collider. The sample has an integrated luminosity of 362 fb$^{-1}$ at a centre-of-mass energy of 10.58 GeV. Using an optimised event selection, a binned maximum likelihood fit is performed using the momentum spectra of the electron and muon candidates. The result, $R_\mu = 0.9675 \pm 0.0007 \pm 0.0036$, where the first uncertainty is statistical and the second is systematic, is the most precise to date. It provides a stringent test of the light-lepton universality, translating to a ratio of the couplings of the muon and electron to the $W$ boson in $\tau$ decays of $0.9974 \pm 0.0019$, in agreement with the standard model expectation of unity