<p>Since their discovery by Burg and Schlesinger in 1937, amine-boranes have enjoyed a rich preparative history and have experienced reinvigorated interest as valuable reagents for organic syntheses. Previously, the Herbert C. Brown Center for Borane Research has reported their synthesis from NaBH₄ and amines via the intermediacy of (NH₄)₂SO4 or NaHCO₃. Described herein is a CO₂-mediated amine-borane synthesis that accommodates all classes of amines, particularly long-chain trialkyl- and pyridine-like heteroarylamines.</p>

This thesis details the synthesis of novel Rh(I) and Rh(III) bis-phosphine fragments, and their use, along with other known rhodium species, to investigate the reactivity of amine-boranes, with a particular focus on the dehydrocoupling of the secondary amine-borane H3B.NMe2H (DMAB). Chapter 2 utilises the new mixed phosphine, PtBuiBu2, to investigate the role of the phosphine with regard to the corresponding low-coordinate organometallic species isolated. Their coordination and reactivity with amine-boranes is studied, leading to the development of a mechanism for an alkene hydroboration catalyst that employs H3B.NMe3 (TMAB). The final section of the chapter studies several fluxional processes pertinent to rhodium and iridium complexes of the model amine-borane TMAB using H/D exchange and low temperature NMR experiments. In Chapter 3, the mechanism of dehydrocoupling of DMAB is investigated in detail, employing catalysts based on the cationic bis¬-phosphine Rh fragment, {Rh(PCy3)2Ln}+. A series of stoichiometric and catalytic reactions are probed using NMR spectroscopy and mass spectrometry, revealing a complex mechanistic landscape. Subtleties include: the product of dehydrocoupling, [H2BNMe2]2, acting in an autocatalytic role; and parallel dehydrogenation of DMAB by a neutral catalyst present in a low but constant concentration. The mechanism was additionally interrogated through kinetic simulations conducted by Prof. Guy C. Lloyd-Jones (University of Bristol). From this, a generic mechanistic scheme has been suggested, aspects of which can be applied to transition metal and main group systems reported to catalyse the dehydrocoupling of DMAB. The final chapter moves on from cationic rhodium fragments to investigate the reactivity of the neutral rhodium species, Rh(H)2(PCy3)2Cl and [Rh(PCy3)2Cl]2, with amine-boranes. The mechanism by which Rh(H)2(PCy3)2Cl catalyses the dehydrogenation of DMAB has been investigated through initial rate and H/D exchange experiments, leading to the proposal of a reaction scheme. Additionally, the formation and characterisation of a base-stabilised boryl species has been reported resulting from the reactivity of an amino-borane with [Rh(PCy3)2Cl]2.

A series of cationic Rh(I) diphosphine and phosphine-alkene complexes have been isolated and fully characterised. The reactivity of these species towards hydrogenation, dehydrogenation and dehydrocoupling reactions has been investigated. The use of potentially hemilabile ligands DPEphos and XANTphos in the intramolecular dehydrogenation chemistry of tricyclopentylphosphine is reported. The comparison in reactivity of these isolated diphosphine phosphine-alkene complexes towards hydrogenation and with acetonitrile is discussed along with their ability to dehydrocouple secondary silane, Ph₂SiH₂, and amine-borane H₃B·NMe₂H. The acceptorless dehydrogenation of a tethered cyclopentane with cationic Rh(I) diphosphine complexes has also been extended to include thioethers. Isolated cationic Rh(I) phosphine-alkene complexes with labile fluorobenzene ligands are found to act as a source of the reactive 12-electron [Rh{PR₂(ƞ²-C₅H₇)}]+ (R = cyclopentyl (Cyp)/ iPr) fragment in solution and can coordinate two amine-borane ligands (either H₃B·NMe₃, H₃B·NMe₂H or H₃B·NMeH₂) in a novel and unique bis-σ-binding mode. The catalytic activity of some of these isolated complexes in the dehydrocoupling of H₃B·NMe₂H and H₃B·NMeH₂ has been determined. With a view to further understanding the mechanism of catalytic transition metal assisted amine-borane dehydrogenation and dehydrocoupling, known B-N intermediates H₃B·NMe₂BH₂·NMe₂H and [H₂B·NMeH]₃ were also coordinated to the [Rh{PCyp₂(ƞ²-C₅H₇)}]+ fragment and investigated with regard to their role in the catalytic cycle. Structure activity relationships determined from stoichiometric reactions of cationic Rh(I) diphosphine fluorobenzene complexes with amine-boranes enabled the design of a highly efficient homogeneous catalyst capable of dehydrogenating H₃B·NMe₂H to [H₂BNMe₂]₂ at 0.2 mol% loading in 30 minutes at 298 K. Rapid dehydrogenation and dehydrocoupling of H₃B·NMeH₂ to form high molecular weight poly(N-methylaminoborane) with a low PDI has also been achieved. Investigations using model aminoborane H₂B=NiPr₂ and intermediate B-N species H₃B·NMe₂BH₂·NMe₂H and [H₂B·NMeH]₃ has helped establish an overall mechanistic rationale for this process.

We present the synthesis and characterisation of novel single organic molecules known as phenoxylene borazines and borazatruxenes. Using temperature-dependant and concentration-dependant 1H NMR, we probe the supramolecular aggregation of these molecules in solution. Finally, we synthesise 2D hybrid material comprised of electron delocalised benzene rings and electron localised borazine rings. Using a combination of solid-state 11B and 13C NMR techniques, Raman spectroscopy and XPS, we confirm the presence of benzene and borazine regions in these novel materials.

Reported herein is a brief summary of the history, properties, and applications of amine-boranes. The past methods devised for their preparation are described and the routes used to produce the compounds used in the work presented here are detailed. Building on prior synthetic approaches to amine-boranes, a new carbon dioxide mediated synthesis is presented. Proceeding through a monoacyloxyborane intermediate, the borane complexes of ammonia, primary, secondary, tertiary, and heteroaromatic amine are provided in 53-99% yields. Utilizing the amine-boranes obtained from the methods described, two divergent methods for direct amidation are introduced. The first uses amine-boranes as dual-purpose reagents, where the carboxylic acid is first activated by the borane moiety to form a triacyloxyborane-amine complex. This allows the delivery of the coordinated amine to form the amide products. A series of primary, secondary, and tertiary amides were prepared in 55-99% yields using this protocol, which displays a broad functional group tolerance. Extended from this dual-purpose methodology, a catalytic amidation is described. Utilizing ammonia-borane as a substoichiometric (10%) catalyst, a series of secondary and tertiary amide are prepared directly from carboxylic acids and amines in 59-99% yields, including amines containing typically borane reactive functionalities including alcohols, thiols, and alkenes. Amine-boranes are additionally used in two borylation methodologies. By reaction with <i>n</i>-butyl lithium, the amine-boranes are converted to the corresponding lithium aminoborohydrides, which upon reaction with a terminal alkyne provides the alkynyl borane-amine complexes in 65-98% yields. This process is compatible with both alkenes and internal alkynes, as well as a range of aprotic functionalities. A new strategy for aminoborane synthesis is also described and applied to the borylation of haloarenes. Activation of a series of amine-boranes with iodine produces the iodinated amine-borane, which undergoes dehydrohalogenation with an appropriate base to produce either monomeric or dimeric aminoboranes. Several aminoboranes were synthesized exclusively as the monomeric species, which due to their greater reactivity, were used directly in the synthesis of a series of aryl boronates in 65-99% yields.

Les dérivés cyclopropanoïques sont des composés importants dans plusieurs domaines tels que la synthèse organique, la chimie médicinale et la science des matériaux. La synthèse asymétrique des dérivés cyclopropanoïques s'est de plus en plus concentrée sur la synthèse stéréocontrolée de cyclopropanes polysubstitutés qui arborent toute une gamme de substituants distincts. Ces méthodes permettent d’accéder à des synthèses divergentes pour préparer des composés pharmaceutiques comportant cette sous-unité. De plus, l'ouverture facile de ce cycle très tendu en fait une bonne cible pour étudier l'activation de la liaison C‒C. C’est pourquoi les cyclopropanes sont parmi les composés les plus attrayants et les plus diversifiés en synthèse organique. La synthèse divergente de dérivés cyclopropanoïques repose sur l'utilisation de précurseurs stables mais réactifs. L'une des réactions pour former des liaisons C‒C les plus couramment utilisées dans la fonctionnalisation à un stade avancé, est la réaction de couplage croisé de Suzuki-Miyaura. C'est l'une des raisons pour lesquelles les borocyclopropanes sont devenus des précurseurs synthétiques attrayants pour la fonctionnalisation et diversification des molécules complexes. L’accès à de telles molécules faciliterait la préparation de molécules cyclopropanoïques de structures diversifiées. Il est difficile de préparer des borocyclopropanes de manière énantiosélective. Dans cette thèse, une cyclopropanation énantiosélective d'acides boroniques protégés dérivés d'alcools allyliques a été réalisée via la réaction de cyclopropanation asymétrique en présence du ligand chiral de type dioxaborolane. Le développement de cette méthodologie a nécessité une modification de la décomplexation oxydative existante du dioxaborolane via son complexe dérivé de la diéthanolamine. Le protocole est maintenant applicable aux dérivés boronates qui incluent des groupements fonctionnels qui sont incompatibles avec les bases. Les borocyclopropanes tétracoordonnés obtenus permettent également la formation de liaisons C‒C et ont démontré une stabilité améliorée par rapport à leurs dérivés tricoordonnés. Une étude plus approfondie sur des complexes cyclopropylméthylamine-boranes (CAB) a démontré que ces derniers pouvaient conduire aux amine-boranes spirocycliques (SCAB). Ces SCAB ont été obtenus grâce à une cascade d'activation des CABs en utilisant le bis(trifluorométhanesulfonimide) (Tf2NH) comme initiateur. L'ouverture du cycle des CAB représente la première conversion des cyclopropanes en spirocycles contenant à la fois un N-spirocentre et un spiro amine-borane. Les amine-boranes ont démontré une activité pharmacologique telle que des propriétés anticancéreuses, anti-inflammatoires et anti-ostéoporotiques. L'incorporation de spirocycles dans un motif augmente le caractère sp3 et la chiralité inhérente. Les SCAB rendent alors des candidats attrayants pour la conception de médicaments. La réaction de SCAB avec de Tf2NH en quantités stoechiométriques a donné un complexe SCAB•NTf2 qui est capable de réduire les fonctions cétone, aldéhyde, imine, nitrobenzène, nitrosobenzène, anthracène, indole et aryl méthyl éther. Le complexe SCAB•NTf2 est également capable de réduire le diphénylacétylène de manière Z-sélective en cis-stilbène. Des études spectroscopiques approfondies ont donné plus d'informations sur la structure de SCAB•NTf2 et nous ont permis de proposer un mécanisme de réduction des groupements fonctionnels ci-dessus. Les études spectroscopiques (RMN, IR et Raman) ont également révélé l'implication d'une liaison α-C‒H au bore dans une liaison hydrogène hypsochromique « improper hydrogen bond » avec [Tf2N]-. L'hyperconjugaison avec l’atome de bore, un acide de Lewis, est proposée, ce qui rend la liaison C‒H acide et donc suffisamment polarisée pour agir comme un donneur de pont hydrogène. / Cyclopropane derivatives are incredibly versatile building blocks used in organic synthesis, medicinal chemistry, and materials science. The asymmetric synthesis of cyclopropane derivatives has increasingly focused on achieving polysubstituted cyclopropanes with a range of distinct substituents and their use in divergent syntheses to access pharmaceutical compounds. Moreover, the ring-opening potential of the cyclopropane ring, due to its inherent strain, makes it a facile target for C‒C bond activation and one of the most attractive and diverse cycloalkanes in organic synthesis. Divergent synthesis of cyclopropanes relies on stable pre-installed handles on cyclopropanes that can be activated readily. One of the most common C‒C bond formation approaches used in late-stage functionalization is the Suzuki-Miyaura cross-coupling reaction. As a result, borocyclopropanes have become attractive synthetic building blocks for their use in late-stage functionalization. Methods for the enantioselective synthesis of borocyclopropanes are scarce. In this thesis, the first enantioselective cyclopropanation of an allylic alcohol bearing a tetracoordinate boronate has been achieved via the Charette dioxaborolane-mediated enantioselective cyclopropanation reaction. The development of our method required modification of the existing oxidative decomplexation of dioxaborolane via diethanolamine. The protocol has now been expanded to include boronates and base-sensitive functionalities. The tetracoordinate borocyclopropane obtained was also shown to undergo C‒C bond formation and demonstrated enhanced stability compared to its tricoordinate boronate derivative. Further investigation of boron tethered cyclopropanes led to the discovery of the unique transformation of cyclopropane amine-boranes (CABs) to spirocyclic amine-boranes (SCABs). SCABs were obtained through a cascade activation of CAB via bis(trifluoromethane)sulfonimide (Tf2NH). The ring-opening of CABs represents the first conversion of cyclopropanes to spirocycles containing an N-spirocenter and furthermore an amine-borane spirocore. Amine-boranes have shown pharmacological activity such as anti-cancer, anti-inflammatory, and anti-osteoporotic properties. Incorporating spirocycles into a motif increases sp3 character and inherent chirality, rendering SCABs as attractive candidates for drug design. The reaction of SCAB with stoichiometric amounts of Tf2NH resulted in a SCAB•NTf2 complex that was found to be able to reduce ketone, aldehyde, imine, nitrobenzene, nitrosobenzene, anthracene, and indole functionalities as well as demethylate aryl methyl ethers. The SCAB•NTf2 complex was also capable of reducing diphenylacetylene in a Z-selective manner to cis-stilbene. In-depth spectroscopic studies revealed the structure of SCAB•NTf2 and a mechanism for the reduction of the above functionalities is proposed. The spectroscopic studies (NMR, IR and Raman) revealed the involvement of an α-C‒H bond to boron in improper hydrogen bonding with [Tf2N]-. Hyperconjugation to the Lewis acidic boron is proposed to make the C‒H bond acidic and therefore polarized enough to act as a hydrogen bond donor.

Among various nanomaterials, metal nanoparticles are the widely studied ones because of their pronounced distinct properties arising in the nanometer size regime, which can be tailored easily by tuning predominantly their size and shape. During the past few decades, scientists are engaged in developing new synthetic methodologies for the synthesis of metal nanoparticles which can be divided into two broad categories: i) top-down approach, utilizing physical methods and ii) bottom-up approach, employing chemical methods. As the chemical methods offer better control over particle size, numerous chemical methods have been developed to obtain metal nanoparticles with narrow size distribution. However, these two approaches have their own merits and demerits; they are not complementary to each other and also not sustainable for real time applications. Recent focus on the synthesis of metal nanoparticles is towards the development of green and sustainable synthetic methodologies. A solid state route is an exciting prospect in this direction because it eliminates usage of organic solvents thus, makes the overall process green and at the same time leads to the realization of large quantity of the materials, which is required for many applications. However, the major obstacle associated with the development of a solid state synthetic route is the lack of fundamental understanding regarding the formation mechanism of the nanoparticles in the solid state. Additionally, due to the heterogeneity present in the solid mixture, it is very difficult to ensure the proximity between the capping agent and nuclei which plays the most decisive role in the growth process. Recently, employment of amine–borane compounds as reducing agents emerged as a better prospect towards the development of sustainable synthetic routes for metal nanoparticles because they offer a variety of advantages over the traditional borohydrides. Being soluble in organic medium, amine– borane allows the reaction to be carried out in a single phase and due to its mild reducing ability a much better control over the nucleation and growth processes is realized. However, the most exciting feature of these compounds is that their reducing ability is not only limited to the solution state, they can also bring out the reduction of metal ions in the solid state. With the availability of a variety of amine–boranes of varying reducing ability, it opens up a possibility to modulate the nanostructure in both solid and solution states by a judicious choice of reducing agent. Although our current understanding regarding the growth behavior of nanoparticles has advanced remarkably, however, most often it is some classical model which is invoked to understand these processes. With the recent developments in in situ transmission electron microscopy techniques, it is now possible to unravel more complex growth trajectories of nanoparticles. These studies not only expand the scope of the present knowledge but also opens up possibilities for many future developments. Objectives • To develop an atom economy solid state synthetic methodology for the synthesis of metal nanoparticles employing amine–boranes as reducing agents. • To gain a mechanistic insight into the formation mechanisms of nanoparticles in the solid state by using amine–boranes with differing reducing ability. • Synthesis of bimetallic nanoparticles as well as supported metal nanoparticles in the solid state using ammonia borane as the reducing agent. • To develop a new in situ seeding growth methodology for the synthesis of core@shell nanoparticles composed of noble metals by employing a very weak reducing agent, trimethylamine borane and their transformation to their thermodynamically stable alloy counterparts. • Synthesis of highly monodisperse ultra-small colloidal calcium nanoparticles with different capping agents such as hexadecylamine, octadecylamine, poly(vinylpyrrolidone) and a combination of hexadecylamine/poly(vinylpyrrolidone) using the solvated metal atom dispersion (SMAD) method. To study the coalescence behavior of a pair of calcium nanoparticles under an electron beam by employing in situ TEM technique. Significant results An atom economy solid state synthetic route has been developed for the synthesis of metal nanoparticles from simple metal salts using amine–boranes as reducing agents. Amine–borane plays a dual role here: acts as a reducing agent thus brings out the reduction of metal ions and decomposes simultaneously to generate B-N based compounds which acts as a capping agent to stabilize the particles in the nanosize regime. This essentially minimizes the number of reagents used and hence simplifying and eliminating the purification procedures and thus, brings out an atom economy to the overall process. Additionally, as the reactions were carried out in the solid state, it eliminates use of organic solvents which have many adverse effects on the environment, thus makes the synthetic route, green. The particle size and the size distribution were tuned by employing amine–boranes with differing reducing abilities. Three different amine–boranes have been employed: ammonia borane (AB), dimethylamine borane (DMAB), and trimethylamine borane (TMAB) whose reducing ability varies as AB > DMAB >> TMAB. It was found that in case of AB, it is the polyborazylene or BNHx polymer whereas, in case of DMAB and TMAB, the complexing amines act as the stabilizing agents. Several controlled studies also showed that the rate of addition of metal salt to AB is the crucial step and has a profound effect on the particle size as well as the size distribution. It was also found that an optimum ratio of amine–borane to metal salt is important to realize the smallest possible size with narrowest size distribution. Whereas, use of AB and TMAB resulted in the smallest sized particles with best size distribution, usage of DMAB provided larger particles that are also polydisperse in nature. Based on several experiments along with available data, the formation mechanism of metal nanoparticles in the solid state has been proposed. Highly monodisperse Cu, Ag, Au, Pd, and Ir nanoparticles were realized using the solid state route described herein. The solid state route was extended to the synthesis of bimetallic nanoparticles as well as supported metal nanoparticles. Employment of metal nitrate as the metal precursor and ammonia borane as the reducing agent resulted in highly exothermic reaction. The heat evolved in this reaction was exploited successfully towards mixing of the constituent elements thus allowing the alloy formation to occur at much lower temperature (60 oC) compared to the traditional solid state metallurgical methods (temperature used in these cases are > 1000 oC). Synthesis of highly monodisperse 2-3 nm Cu/Au and 5-8 nm Cu/Ag nanoparticles were demonstrated herein. Alumina and silica supported Pt and Pd nanoparticles have also been prepared. Use of ammonia borane as the reducing agent in the solid state brought out the reduction of metal ions to metal nanoparticles and the simultaneous generation of BNHx polymer which encapsulates the metal (Pt and Pd) nanoparticles supported on support materials. Treatment of these materials with methanol resulted in the solvolysis of BNHx polymer and its complete removal to finally provide metal nanoparticles on the support materials. An in situ seeding growth methodology for the synthesis of bimetallic nanoparticles with core@shell architecture composed of noble metals has been developed using trimethylamine borane (TMAB) as the reducing agent. The key idea of this synthetic procedure is that, TMAB being a weak reducing agent is able to differentiate the smallest possible window of reduction potential and hence reduces the metal ions sequentially. A dramatic solvent effect was noted in the preparation of Ag nanoparticles: Ag nanoparticles were obtained at room temperature when dry THF was used as the solvent whereas, reflux condition was required to realize the same using wet THF as the solvent. However, no such behavior was noted in the preparation of Au and Pd nanoparticles wherein Au and Pd nanoparticles were obtained at room temperature and reflux conditions, respectively. This difference in reduction behavior was successfully exploited to synthesize Au@Ag, Ag@Au, and Ag@Pd nanoparticles. All these core@shell nanoparticles were further transformed to their alloy counterparts under very mild conditions reported to date. Highly monodisperse, ultrasmall, colloidal Ca nanoparticles with a size regime of 2-4 nm were synthesized using solvated metal atom dispersion (SMAD) method and digestive ripening technique. Hexadecylamine (HDA) was used as the stabilizing agent in this case. Employment of capping agent with a longer chain length, octadecylamine afforded even smaller sized particles. However, when poly(vinylpyrrolidone) (PVP), a branched chain polymer was used as the capping agent, agglomerated particles were realized together with small particles of 3-6 nm. Use of a combination of PVP and HDA resulted in spherical particles of 2-3 nm size with narrow size distribution. Growth of Ca nanoparticles via colaesence mechanism was observed under an electron beam. Employing in situ transmission electron microscopy technique, real time coalescence between a pair of Ca nanoparticles were detected and details of coalescence steps were analyzed.