Small molecule functionalization by metal-mediated borylene transfer chemistry

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Reactions of saturated and unsaturated organic molecules with boron-containing reagents represent a very powerful and versatile range of methodologies for the introduction of functionality into organic substrates, which have been widely exploited, e.g. in the syntheses of pharmaceuticals, natural products and functional materials. The resulting boron-derivatized products can be exploited directly for further synthesis (e.g. aryl boronate esters in Suzuki-Miyaura C-C coupling), or converted to a range of more useful functional groups (e.g. alcohols, amines, etc.) using established protocols. Landmark reactions in which borane reagents have been used to introduce functionality into organic substrates include (i) hydro- and diboration of multiple bonds; (ii) palladium-catalysed conversion of aryl halides to aryl boronates; and (iii) direct C-H bond functionalization of alkanes and arenes catalysed by rhodium or iridium complexes. For each of these methodologies, control of the B-C bond-forming step can be achieved within the coordination sphere of a transition metal, in some cases opening up the possibility for enantio-/diastereoselective syntheses via the use of chiral ligand sets. Transition metal boryl complexes, LnM-BX2, are widely implicated in the delivery of the BX2 fragment to organic substrates (e.g. in both hydro-/diboration and C-H activation processes) and as such have been the subject of intense recent investigation.By contrast, the chemistry of the borylene fragment (:BX) has not been exploited to any great extent in the formation of C-B bonds, despite obvious parallels with carbene (:CR2) and silylene (:SiR2) fragments, and the wide range of useful reactivity in which these subvalent group 14 systems have been implicated. Thus, for example, the applications of carbenes in metathesis, cyclopropanation and insertion chemistries, and of silylenes in C-C bond formation, have been widely investigated. In part, the lack of analogous boron chemistry reflects the extremely labile nature of 'free' borylene and its indiscriminate reactivity, e.g. towards C-H/C-C bonds. A strategy which has been successfully adopted to modulate reactivity in the cases of carbenes and silylenes is to confine the reactive fragment to the coordination sphere of a transition metal. We intend to exploit this approach to develop controlled bond-forming chemistry based on metal complexes containing the borylene fragment. In part, this will build on previous work within the Aldridge group which has opened up synthetic routes to borylene complexes analogous to Fischer carbenes/vinylidenes, and uncovered aspects of their fundamental chemistry. Crucially, these studies have identified cationic complexes containing the aminoborylene fragment as achieving the balance between being sufficiently robust for ease of synthesis/handling, but displaying clean room temperature reactivity towards archetypal organic and inorganic bonds.We intend to build on this preliminary work to develop new methodologies for introducing boron-containing fragments in to organic molecules, primarily concentrating (in the first instance) on developing insertion and cycloaddition protocols which are not possible (or are very difficult to achieve) using existing borylation methodologies. Initially we will explore issues of mechanism and selectivity by examining processes which are stoichiometric in the metal borylene complex, with the ultimate aim of incorporating these fundamental steps into catalytic processes. This second goal will require parallel developments leading, in particular, to the development of direct synthetic approaches utilizing readily-available borane precursors. Thus, these two goals: side-by-side development of synthetic methodology and exploitation of reactivity patterns will form the basis for the proposed work.