Regio- and Stereoselective Dimerization of Arylalkynes Mediated by a Transient Zinc Radical.

Abstract

Vinyl radicals are crucial intermediates in synthetic chemistry that readily undergo a range of well-established transformations, such as hydrogen atom transfer and cyclization. Herein, we report a distinct reaction mode in which vinyl radicals, generated from the attack of a transient zinc radical on an alkyne, are diverted toward selective dimerization to form a diene. Reaction of the dizinc compound (DIPPNacnacZn)2 (DIPPNacnac = [(DIPPNCMe)2CH]-; DIPP = 2,6-iPr2C6H3) with 1-phenylpropyne under UV irradiation affords a (Z,E)-2,5-dizincated-2,4-hexadiene derivative with high regio- and stereoselectivity. This transformation is also applicable to a variety of substituted 1-arylpropynes, yielding the corresponding diene products, although replacement of the methyl group with bulkier substituents (e.g., tert-butyl and trimethylsilyl) directs the reaction pathway toward 1,2-addition of the Zn-Zn unit across the alkyne. Mechanistic investigations involving radical trapping and radical clock experiments support a pathway initiated by the addition of a zinc radical to the alkyne, generating a β-zincated vinyl radical intermediate that subsequently undergoes selective dimerization. Complementary density functional theory calculations further elucidate the mechanistic details and the origins of selectivity in this reaction.

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