Magnesium-catalysed nitrile hydroboration.

Chemical Science View Article Online Open Access Article. Published on 20 October 2015. Downloaded on 15/05/2018 14:26:15. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. EDGE ARTICLE View Journal | View Issue Magnesium-catalysed nitrile hydroboration† Cite this: Chem. Sci., 2016, 7, 628 Catherine Weetman, Mathew D. Anker, Merle Arrowsmith, Michael S. Hill,* Gabriele Kociok-Köhn, David J. Liptrot and Mary F. Mahon A b-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 interand 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 Received 21st August 2015 Accepted 19th October 2015 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 DOI: 10.1039/c5sc03114a homogeneous processes performed under heavier alkaline earth catalysis are likely to demonstrate www.rsc.org/chemicalscience previously unappreciated mechanistic diversity. Introduction The reduction of organic nitriles to primary amines is an essential component of many industrial processes (e.g. the production of dyes, polyesters, agrochemicals and as precursors for pharmaceutical compounds).1 Catalytic nitrile hydrogenation may be achieved under heterogeneous conditions. These latter processes, however, are typically poorly selective and also result in the unwanted formation of imine and secondary amine side products at the high temperatures required.2 While their reliance on the use of poorly abundant and/or toxic precious metals is a further indicator of the unsustainability of these heterogeneous systems, it is notable that the development of well-dened solution-phase catalysis is limited to a handful of reports which deploy similarly expensive species derived from heavy precious metals.3,4 Although the reduction of nitriles may also be achieved through the use of stoichiometric quantities of Department of Chemistry, University of Bath, Claverton Down, Bath, BA2 7AY, UK. E-mail: † Electronic supplementary information (ESI) available: Experimental procedures and full characterisation data, details of the X-ray analyses of compounds 1–5, protocols and data associated with the kinetic analyses and NMR spectra. CCDC 1018705–1018709. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5sc03114a 628 | Chem. Sci., 2016, 7, 628–641 main group reducing agents such as LiAlH4 and NaBH4,5 the ammable nature of these reagents, coupled with large amounts of inorganic waste by-products, again renders them unattractive. More recent reports have shown that reduction may be accomplished using amine borane reagents,6 whilst other novel hydrogenation methods have exploited the use of catalytic amounts of ‘frustrated’ Lewis pairs to provide the rst metal-free systems to reduce nitriles, albeit under rather energetic (120
C) reaction conditions.7 Whilst nitrile hydrogenation provides a direct route to the desired amine product, reductive hydrosilylation or hydroboration can be advantageous in their provision of further functionality to the resultant amine.8 Although the catalytic hydroboration of a wide range of multiply-bonded substrates has been achieved, only a handful of nitrile hydroboration reactions have been devised and all but one of these previous reports described non-catalysed H–B addition and required the use of more activated and less discriminating borane reagents.9 A unique case of a catalysed addition, therefore, has been provided by Nikonov's report of the catalytic hydroboration of nitriles using 5 mol% of the Mo(IV) imido–hydrido complex (I) with catecholborane (HBcat).10 With this system acetonitrile and benzonitrile were reduced to the 1,1-bis(boryl)amine products, which were themselves shown to undergo This journal is © The Royal Society of Chemistry 2016 View Article Online Edge Article Chemical Science Open Access Article. Published on 20 October 2015. Downloaded on 15/05/2018 14:26:15. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. chemoselective coupling with aldehydes, R0 C(O)H, to afford imines RCH2N]C(H)R0 . Our own research has focussed on the development of a homogeneous catalytic chemistry for complexes, LAeX (Ae ¼ Mg, Ca, Sr and Ba; L ¼ unreactive spectator ligand; X ¼ reactive substituent), derived from the heavier alkaline-earth elements.11 The negligible toxicity and high natural abundance of calcium and magnesium (the fourth and sixth most abundant lithospheric elements respectively) in particular designate species of this type as environmentally benign and sustainable. Although these are primary motivating factors for the development of this chemistry, an additional major concern is a deconvolution of basic reactivity patterns for this relatively understudied family of elements. Based upon an immutable +2 oxidation state and effectively ionic ligand and substrate binding under catalytic conditions, a level of ‘lanthanide mimetic’ behaviour was initially assumed. Consequently, a wide variety of heterofunctionalisation catalyses, predicated on sequences of regioselective and polarisation-dependent sigmabond metathesis and insertion events have now been described (Schemes 1 and 2).11 A majority of detailed studies have focussed on the intramolecular hydroamination/cyclisation of aminoalkenes as an appropriate baseline reaction that is wellprecedented in homogeneous 4f-element centred catalysis.12,13 Within this one reaction type alone, distinct variations which occur with changing group 2 atomic weight have been rationalised as a consequ (...truncated)