Intramolecular hydroamination of aminoalkenes with group 2 precatalysts: mechanistic insights and ligand design

Long relegated to the background by the pre-eminence of magnesium-based, stoichiometric Grignard reagents, a distinct chemistry of the heavier alkaline earth metals, calcium, strontium and barium, is only now starting to emerge. As similarities have been drawn between the large, electropositive, redox-inert and d0 alkaline earth Ae2+ dications and the Ln3+ cations of the lanthanide series, a growing group 2-mediated catalytic chemistry has developed over the last decade, including polymerisation reactions, heterofunctionalisation reactions of multiple bonds and some rare examples of dehydrocoupling reactions. Among these catalytic reactions the magnesium- and calcium-catalysed intramolecular hydroamination of aminoalkenes has attracted particular interest. Mechanistic studies have demonstrated many parallels with the lanthanide-mediated catalytic cycle based upon successive σ-bond metathesis and insertion steps. In the first part of this thesis, further investigations into the hydroamination/cyclisation reaction have demonstrated the prominent role of the charge density of the catalytic group 2 cation (M = Mg, Ca, Sr, Ba), the beneficial influence of stabilising spectator ligands, and the importance of the choice of the reactive co-ligand for efficient catalyst initiation. Kinetic analyses of reactions monitored by NMR spectroscopy have given new insight into activation energies, entropic effects, substrate and product inhibition, and kinetic isotope effects, leading to a review of the previously suggested lanthanide-mimetic mechanism. In a second part, this study seeks to address two of the main challenges posed by the intramolecular hydroamination reaction in particular, and heavier alkaline earth-catalysed reactions in general: (i) The need to design new monoanionic spectator ligands capable of stabilising heteroleptic heavier alkaline earth complexes and preventing deleterious Schlenk-type ligand redistribution processes in solution; (ii) The stabilisation of highly reactive heteroleptic group 2 alkyl functionalities for fast, irreversible catalyst initiation and novel reactivity.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Direct access to a cAAC-supported dihydrodiborene and its dianion

The two-fold reduction of (cAAC)BHX2 (cAAC = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene); X = Cl, Br) provides a facile, high-yielding route to the dihydrodiborene (cAAC)2B2H2. The (chloro)hydroboryl anion reduction intermediate was successfully isolated using a crown ether. Overreduction of the diborene to its dianion [(cAAC)2B2H2]2– causes a decrease in the B-B bond order whereas the B-C bond orders increase

Facile synthesis of a stable dihydroboryl {BH2}– anion

While the one-electron reduction of (CAACMe)BH2Br (CAACMe = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) yields a hydride-shift isomer of the corresponding tetrahydrodiborane, a further reversible reduction leads to the first stable parent boryl anion, [(CAACMe)BH2]–, which acts as a powerful boron nucleophile

Magnesium-catalysed nitrile hydroboration

A β-diketiminato n-butylmagnesium complex is presented as a selective precatalyst for the reductive hydroboration of organic nitriles with pinacolborane (HBpin). Stoichiometric reactivity studies indicate that catalytic turnover ensues through the generation of magnesium aldimido, aldimidoborate and borylamido intermediates, which are formed in a sequence of intramolecular nitrile insertion and inter- and intramolecular B–H metathesis events. Kinetic studies highlight variations in mechanism for the catalytic dihydroboration of alkyl nitriles, aryl nitriles bearing electron withdrawing (Ar(EWG)CN) and aryl nitriles bearing electron donating (Ar(EDG)CN) substitution patterns. Kinetic isotope effects (KIEs) for catalysis performed with DBpin indicate that B–H bond breaking and C–H bond forming reactions are involved in the rate determining processes during the dihydroboration of alkyl nitriles and Ar(EDG)CN substrates, which display divergent first and second order rate dependences on [HBpin] respectively. In contrast, the hydroboration of Ar(EWG)CN substrates provides no KIE and HBpin is not implicated in the rate determining process during catalysis. Irrespective of these differences, a common mechanism is proposed in which the rate determining steps are deduced to vary through the establishment of several pre-equilibria, the relative positions of which are determined by the respective stabilities of the dimeric and monomeric magnesium aldimide and magnesium aldimidoborate intermediates as a result of adjustments to the basicity of the nitrile substrate. More generally, these observations indicate that homogeneous processes performed under heavier alkaline earth catalysis are likely to demonstrate previously unappreciated mechanistic diversity

Trans‐selective insertional dihydroboration of a cis‐diborene: synthesis of linear sp3‐sp2‐sp3‐triboranes and subsequent cationization

The reaction of aryl‐ and amino(dihydro)boranes with dibora[2]ferrocenophane 1 leads to the formation 1,3‐trans‐dihydrotriboranes by formal hydrogenation and insertion of a borylene unit into the B=B bond. The aryltriborane derivatives undergo reversible photoisomerization to the cis‐1,2-μ‐H‐3hydrotriboranes, while hydride abstraction affords cationic triboranes, which represent the first doubly base‐stabilized B3H4+ analogues

Alkaline earth catalysis for the 100% atom-efficient three component assembly of imidazolidin-2-ones

A variety of functionalised imidazolidin-2-ones may be synthesised under very mild reaction conditions using non-toxic and cost-effective alkaline earth bis(amide) pre-catalysts in a 100% atom-efficient, intermolecular one-pot assembly from inexpensive alkyne and cumulene reagents.</p

Bottleable neutral analogues of [B2H5]- as versatile and strongly binding eta2 donor ligands

Herein we report the discovery that two bottleable, neutral, base-stabilized diborane(5) compounds are able to bind strongly to a number of copper(I) complexes exclusively through their B-B bond. The resulting complexes represent the first known complexes containing unsupported, neutral σB-B diborane ligands. Single-crystal X-ray analyses of these complexes show that the X-Cu moiety (X = Cl, OTf, C6F5) lies opposite the bridging hydrogen of the diborane and is near perpendicular to the B-B bond, interacting almost equally with both boron atoms and causing a B-B bond elongation. DFT studies show that σ donation from and π backdonation to the pseudo-π-like B-B bond account for their formation. Astoundingly, these copper σB-B-complexes are inert to ligand exchange with pyridine under either heating or photoirradiation