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Magnesium-catalyzed hydrosilylation of ?,?-unsaturated esters.

ABSTRACT: To^MMgHB(C₆F₅)₃ (1, To^M = tris(4,4-dimethyl-2-oxazolinyl)phenylborate) catalyzes the 1,4-hydrosilylation of ?,?-unsaturated esters. This magnesium hydridoborate compound is synthesized by the reaction of To^MMgMe, PhSiH₃, and B(C₆F₅)₃. Unlike the transient To^MMgH formed from the reaction of To^MMgMe and PhSiH₃, the borate adduct 1 persists in solution and in the solid state. Crystallographic characterization reveals tripodal coordination of the HB(C₆F₅)₃ moiety to the six-coordinate magnesium center with a ?Mg-H-B of 141(3)°. The pathway for formation of 1 is proposed to involve the reaction of To^MMgMe and a PhSiH₃/B(C₆F₅)₃ adduct because the other possible intermediates, To^MMgH and To^MMgMeB(C₆F₅)₃, react to give an intractable black solid and To^MMgC₆F₅, respectively. Under catalytic conditions, silyl ketene acetals are isolated in high yield from the addition of hydrosilanes to ?,?-unsaturated esters with 1 as the catalyst.

SUBMITTER: Lampland NL

PROVIDER: S-EPMC5947512 | biostudies-literature | 2015 Dec

REPOSITORIES: biostudies-literature

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Magnesium-catalyzed hydrosilylation of α,β-unsaturated esters.

Lampland Nicole L NL Pindwal Aradhana A Neal Steven R SR Schlauderaff Shealyn S Ellern Arkady A Sadow Aaron D AD

Chemical science 20150826 12

ToMMgHB(C6F5)3 (1, ToM = tris(4,4-dimethyl-2-oxazolinyl)phenylborate) catalyzes the 1,4-hydrosilylation of α,β-unsaturated esters. This magnesium hydridoborate compound is synthesized by the reaction of ToMMgMe, PhSiH3, and B(C6F5)3. Unlike the transient ToMMgH formed from the reaction of ToMMgMe and PhSiH3, the borate adduct 1 persists in s ...[more]

PMID: 29861929

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Project description:The dimeric ?-diketiminato magnesium hydride, [(BDI)MgH]2, reacts at 80 °C with the terminal alkenes, 1-hexene, 1-octene, 3-phenyl-1-propene and 3,3-dimethyl-butene to provide the respective n-hexyl, n-octyl, 3-phenylpropyl and 3,3-dimethyl-butyl magnesium organometallics. The facility for and the regiodiscrimination of these reactions are profoundly affected by the steric demands of the alkene reagent. Reactions with the phenyl-substituted alkenes, styrene and 1,1-diphenylethene, require a more elevated temperature of 100 °C with styrene providing a mixture of the 2-phenylethyl and 1-phenylethyl products over 7 days. Although the reaction with 1,1-diphenylethene yields the magnesium 1,1-diphenylethyl derivative as the sole reaction product, only 64% conversion was achieved over a 21 day timeframe. Reactions with the ?,?-dienes, 1,5-hexadiene and 1,7-octadiene, provided divergent results. The initial 5-alkenyl magnesium reaction product of the shorter chain diene undergoes 5-exo-trig cyclisation via intramolecular carbomagnesiation to provide a cyclopentylmethyl derivative, which was shown by X-ray diffraction analysis to exist as a three-coordinate monomer. In contrast, 1,7-octadiene provided a mixture of two compounds, a magnesium oct-7-en-1-yl derivative and a dimagnesium-octane-1,4-diide, as a result of single or two-fold activation of the terminal C[double bond, length as m-dash]C double bonds. The magnesium hydride was unreactive towards internal alkenes apart from the strained bicycle, norbornene, allowing the characterisation of the resultant three-coordinate magnesium norbornyl derivative by X-ray diffraction analysis. Computational analysis of the reaction between [(BDI)MgH]2 and 1-hexene using density functional theory (DFT) indicated that the initial Mg-H/C[double bond, length as m-dash]C insertion process is rate determining and takes place at the intact magnesium hydride dimer. This exothermic reaction (?H = -14.1 kcal mol-1) traverses a barrier of 18.9 kcal mol-1 and results in the rupture of the dinuclear structure into magnesium alkyl and hydride species. Although the latter three-coordinate hydride derivative may be prone to redimerisation, it can also provide a further pathway to magnesium alkyl species through its direct reaction with a further equivalent of 1-hexene, which occurs via a lower barrier of 15.1 kcal mol-1. This Mg-H/C[double bond, length as m-dash]C insertion reactivity provides the basis for the catalytic hydrosilylation of terminal alkenes with PhSiH3, which proceeds with a preference for the formation of the anti-Markovnikov organosilane product. Further DFT calculations reveal that the catalytic reaction is predicated on a sequence of Mg-H/C[double bond, length as m-dash]C insertion and classical Si-H/Mg-C ?-bond metathesis reactions, the latter of which, with a barrier height of 24.9 kcal mol-1, is found to be rate determining.

Dataset Information

Magnesium-catalyzed hydrosilylation of ?,?-unsaturated esters.

Publications

Magnesium-catalyzed hydrosilylation of α,β-unsaturated esters.

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