Alkylation Reactions of dialkylzinc Compounds with 1,4-Diaza-1,3-butadienes : Cationic and Radical Anionic Organozinc intermediates. Molecular Structure of the Cationic Organozinc Species [MeZn(t-BuN=CHCH=N-t-Bu)]O3SCF3 and Me2Zn(bpy)(bpy = 2,2-Bipyridine)

1,4-Disubstituted 1,4-diaza-1,3-butadienes, R’N=CHCH=NR’ (R’DAB), are regioselectively alkylated by R2Zn compounds. It is suggested that the mechanism of these alkylation reactions involves prior formation of the 1:l RzZn(R’DAB) complex (R = alkyl) followed by intramolecular ligand-to-ligand electron transfer, generating, via homolytic R-Zn bond cleavage, a [RZn(R’DAB)] ‘/Re radical pair. Collapse of this latter pair results in the formation of N- or C-alkylation products. In the present study one aspect of a possible polar mechanism is probed, for which organozinc radical anions [R2Zn(R’DAB)I*- and organozinc cations [RZn(R’DAB)]+ have been suggested as the key intermediates. Representative examples of such organozinc cations, Le. the title compound [MeZn(t-BuDAB)]OTf (9) and [(2,6-xylyl)Zn(t-BuDAB)]OTf (lo), were separately prepared from their corresponding 1:l coordination complexes MezZn(t-BuDAB) and (2,6-xylyl)2Zn(t-BuDAB), respectively, by their reaction with 1 equiv of trifluoromethanesulfonic acid. The X-ray crystal structure of 9 shows a four-coordinate zinc atom with adistortedtetrahedral geometry, wherein the triflate is bonded almost perpendicular to the MeZn(t-BuDAB) plane. Structural parameters in the MeZn(t-BuDAB) plane are in good agreement with ab initio results for the MeZn(H-DAB) cation. The X-ray crystal structure determination of Me2Zn(bpy) (IC; bpy = 2,2’-bipyridine) has been carried out to compare the structural features of two neutral complexes, e.g. MezZn(bpy) and MezZn(t-BuDAB), with cationic species such as 9. 19F NMR spectra of 9 and 10 in benzene and tetrahydrofuran indicate that in solution the triflate-zinc bond dissociates to give the solvated cationic species [RZn(t-BuDAB)]+ (R = Me, 2,6-xylyl). The title compound 9 reacts with l-(trimethylsi1oxy)-l-ethoxycyclopropane via