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. 2016 Jan 1;7(1):628-641.
doi: 10.1039/c5sc03114a. Epub 2015 Oct 20.

Magnesium-catalysed nitrile hydroboration

Catherine Weetman  1 , Mathew D Anker  1 , Merle Arrowsmith  1 , Michael S Hill  1 , Gabriele Kociok-Köhn  1 , David J Liptrot  1 , Mary F Mahon  1
Affiliations

Magnesium-catalysed nitrile hydroboration

Catherine Weetman et al. Chem Sci. .

Abstract

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.

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Figures

Scheme 1
Scheme 1
Scheme 2
Scheme 2
Scheme 3
Scheme 3. Prototype mechanism for dihydroboration of nitriles.
Fig. 1
Fig. 1. ORTEP representation of the structure of compound 1 (30% probability ellipsoids). Hydrogen atoms, except for those attached to the C1 methylene carbon centre, are omitted for clarity.
Fig. 2
Fig. 2. ORTEP representations of (a) compound 2 and (b) compound 3 (30% probability ellipsoids). Hydrogen atoms, except for those attached to the C(30) and, for 3, the C(38) methine carbon centres, and iso-propyl groups are omitted for clarity. Symmetry transformations used to generate equivalent (′) atoms in (b) –x + 1, –y + 1, –z + 1.
Fig. 3
Fig. 3. ORTEP representation of compound 4 (30% probability ellipsoids). Hydrogen atoms apart from those attached to B1 and C30 and iso-propyl groups are omitted for clarity.
Fig. 4
Fig. 4. ORTEP representation of compound 5 (30% probability ellipsoids). Iso-propyl groups and hydrogen atoms other than those attached to B1 and C80 have been removed for clarity.
Fig. 5
Fig. 5. (a) Pseudo-zero order kinetics of propionitrile dihydroboration with varying [V]; (b) propionitrile dihydroboration reaction rate as a function of [V].
Fig. 6
Fig. 6. Hammett plot for dihydroboration of variously substituted aryl nitriles catalysed by V.
Scheme 4
Scheme 4
Scheme 5
Scheme 5
Scheme 6
Scheme 6
Scheme 7
Scheme 7
Scheme 8
Scheme 8
See this image and copyright information in PMC

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