Main group species for catalytic hydroboration

Modern synthetic chemistry is unimaginable without transition metal catalysis. Yet the often high cost, toxicity and scarcity of many transition metals is driving attempts to find sustainable alternatives. Thus, the development of catalytic processes using main-group catalysts is now of broad interest. This thesis reports the development of a facile protocol for the aluminium-catalysed hydroboration of alkynes, alkenes and polar bonds using commercially-available catalysts. The catalytic hydroboration is proposed to occur by hydroalumination followed by product release through σ-bond metathesis with pinacol borane. An alternative route to alkenyl boranes is the 1,1-carboboration of alkynes using stoichiometric B(C6F5)3. A zwitterionic intermediate in the Piers’ borane-catalysed hydroboration and 1,1-carboboration of alkynes with B(C6F5)3 has been characterised and its divergent reactivity identified. This has led to the development of a B(C6F5)3 - catalysed hydroboration of alkynes using HBpin

Aluminum-Catalyzed Hydroboration of Alkenes

The aluminum-catalyzed hydroboration of alkenes with HBpin is reported using simple commercially available aluminum hydride precatalysts [LiAlH₄ or sodium bis­(2-methoxyethoxy)aluminum hydride (Red-Al)]. Good substrate scope and functional group tolerance is demonstrated for alkene hydroboration, and the protocol was also applied to the hydroboration of ketone, ester, and nitrile functional groups, showing the potential for wider application. The aluminum-catalyzed hydroboration is proposed to proceed by alkene hydroalumination, which generates an alkyl aluminum species that undergoes σ-bond metathesis with HBpin to drive turnover of the catalytic cycle

Characterization of the zwitterionic intermediate in 1,1‐carboboration of alkynes

The reaction of a Lewis acidic borane with an alkyne is a key step in a diverse range of main group transformations. Alkyne 1,1‐carboboration, the Wrackmeyer reaction, is an archetypal transformation of this kind. 1,1‐Carboboration has been proposed to proceed through a zwitterionic intermediate. We report the isolation and spectroscopic, structural and computational characterization of the zwitterionic intermediates generated by reaction of B(C6F5)3 with alkynes. The stepwise reactivity of the zwitterion provides new mechanistic insight for 1,1‐carboboration and wider B(C6F5)3 catalysis. Making use of intramolecular stabilization by a ferrocene substituent, we have characterized the zwitterionic intermediate in the solid state and diverted reactivity towards alkyne cyclotrimerization

Borane-Catalysed Hydroboration of Alkynes and Alkenes

Simple, commercially available borane adducts, H 3 B·THF and H 3 B·SMe 2, have been used to catalyse the hydroboration of alkynes and alkenes with pinacolborane to give the alkenyl and alkyl boronic esters, respectively. Alkynes and terminal alkenes underwent highly regioselective hydroboration to give the linear boronic ester products. Good functional group tolerance was observed for substrates bearing ester, amine, ether and halide substituents. This catalytic process shows comparable reactivity to transition-metal-catalysed hydroboration protocols

Iridium-Catalyzed Hydrochlorination and Hydrobromination of Alkynes via Shuttle Catalysis

Described herein are two different methods for the synthesis of vinyl halides by a shuttle catalysis based iridium‐catalyzed transfer hydrohalogenation of unactivated alkynes. The use of 4‐chlorobutan‐2‐one or tert‐butyl halide as donors of hydrogen halides allows this transformation in the absence of corrosive reagents, such as hydrogen halides or acid chlorides, thus largely improving the functional‐group tolerance and safety profile of these reactions compared to the state‐of‐the‐art. This method has granted access to alkenyl halide compounds containing acid‐sensitive groups, such as tertiary alcohols, silyl ethers, and acetals. The synthetic value of those methodologies has been demonstrated by gram‐scale synthesis where low catalyst loading was achieved.ISSN:1433-7851ISSN:1521-3773ISSN:0570-083

Palladium‐catalyzed Chlorocarbonylation of Aryl (pseudo)Halides through in situ Generation of Carbon Monoxide

An efficient palladium‐catalyzed chlorocarbonylation of aryl ( pseudo )halides to access a wide range of carboxylic acid derivatives has been developed. The use of butyryl chloride as a combined CO and Cl source eludes the need for toxic, gaseous carbon monoxide, thus facilitating the synthesis of high‐value products from readily available aryl ( pseudo )halides. The combination of palladium(0), Xantphos, and an amine base is essential to promote this broadly applicable catalytic reaction. Overall, this reaction provides access to a great variety of carbonyl‐containing products through in situ transformation of the generated aroyl chloride. Combined experimental and computational studies support a reaction mechanism involving in situ generation of CO.(© 2020 Wiley).ISSN:1433-7851ISSN:1521-3773ISSN:0570-083

Replacing the BO in BODIPY: Unlocking the Path to SBIDIPY and BIDIPY Chromophores

Boron-based dipyrrin chromophores (BODIPY) have found widespread application over the last twenty years in fields as diverse as medicine and materials. Thus, several efforts have been placed to exchange boron with other elements, with the aim of developing materials with complementary luminescence properties. However, despite the attempts, the incorporation of other main-group elements in dipyrrin scaffolds remains still rare. We have successfully synthesized and characterized novel chromophores based on heavy pnictogens antimony and bismuth, SBIDIPY and BIDIPY. Solution stabilities have been investigated by VT-UV/Vis spectroscopy and the fluorescence emission studied and supported by computational analysis. We were also able to isolate for the first direct analog of BODIPY containing fluoride handles, which featured a strong intensity of fluorescence emission

Nickel-Catalyzed Inter- and Intramolecular Carbon-Sulfur Bond Metathesis by Reversible Arylation

A nickel-catalyzed carbon-sulfur bond metathesis has been developed to access high-value thioethers. 1,2-bis(dicyclohexylphosphino)ethane (dcype) is essential to promote this highly functional group tolerant reaction. Further, synthetically challenging macrocycles could be obtained in good yield in an unusual example of ring-closing metathesis which does not involve alkene bonds. In-depth organometallic studies support a reversible Ni(0)-Ni(II) pathway to product formation. Overall, this work does not only disclose a more sustainable and more functional group tolerant alternative to previous catalytic systems based on Pd, but also presents new applications and mechanistic information which are highly relevant to the further development and application of unusual single bond metathesis reactions