Metal alkyls programmed to generate metal alkylidenes by α-H abstraction: prognosis from NMR chemical shift

• Published in: Chemical science (Cambridge) 2018-02, Vol.9 (7), p.1912-1918
• Main Authors: Gordon, Christopher P, Yamamoto, Keishi, Searles, Keith, Shirase, Satoru, Andersen, Richard A, Eisenstein, Odile, Copéret, Christophe
• Format: Article
• Language: English
• Subjects: Alkanes; Alkenes; Alkyl compounds; Alkylidene; Chemical reactions; Chemical Sciences; Chemistry; Metathesis; Molecular orbitals; NMR; Nuclear magnetic resonance; Tensor analysis; Tensors
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/c7sc05039a

Metal alkylidenes, which are key organometallic intermediates in reactions such as olefination or alkene and alkane metathesis, are typically generated from metal dialkyl compounds [M](CH 2 R) 2 that show distinctively deshielded chemical shifts for their α-carbons. Experimental solid-state NMR measurements combined with DFT/ZORA calculations and a chemical shift tensor analysis reveal that this remarkable deshielding originates from an empty metal d-orbital oriented in the M-C α -C α′ plane, interacting with the C α p-orbital lying in the same plane. This π-type interaction inscribes some alkylidene character into C α that favors alkylidene generation via α-H abstraction. The extent of the deshielding and the anisotropy of the alkyl chemical shift tensors distinguishes [M](CH 2 R) 2 compounds that form alkylidenes from those that do not, relating the reactivity to molecular orbitals of the respective molecules. The α-carbon chemical shifts and tensor orientations thus predict the reactivity of metal alkyl compounds towards alkylidene generation. Chemical shift analysis predicts the ease of alkylidene formation from bis-alkyl d 0 complexes via α-H abstraction.