Toward Total Spontaneous Resolution ofsec-Butylzinc Complexes

Abstract

Di-sec-butylzinc has been used to synthesize [Zn(s-Bu) 2 L 2 ] complexes, where L denotes amine ligands: e.g., pyridine (py), 2-picoline (2-pic), and 3-picoline (3-pic). The crystal structures of [Zn(s-Bu) 2 (py) 2 ] (1), [Zn(s-Bu) 2 (2-pic) 2 ] (2), and [Zn(s-Bu) 2 (3-pic) 2 ] (3) were determined. Reaction of di-sec-butylzinc with the bidentate amine ligands N,N,N 0 ,N'-tetramethylethylenediamine (TMEDA) and N,N,N 0 ,N'-tetraethylethylenediamine (TEEDA) afforded the chiral complexes [Zn(s-Bu) 2 -(tmeda)] ( 4) and [Zn(s-Bu) 2 (teeda)] (5); their crystal structures were determined. Since they coordinate two s-Bu groups each, complexes 1-5 may exist both as chiral and meso molecules in solution, but the crystalline complexes were found to be exclusively chiral. The enantiomerization rate of 1-5 in benzene was found to be slow on the NMR time scale. Chiral crystalline complexes containing the Zn(s-Bu) 2 moiety are of particular interest, since they display reactive chirogenic R-carbon atoms. The identification of a Zn(s-Bu) 2 complex such as 1-5, which crystallized as a conglomerate of chiral crystals, would afford a simple and cheap way to optical resolution of a chiral organometallic reagent, without the need to use enantiopure ligands. Using preferential crystallization, it should be possible to perform total spontaneous resolution, which may be regarded as a form of absolute asymmetric synthesis. However, all complexes isolated so far, including the dinuclear alkylzinc amide [Zn(s-Bu)(Me 2 N(CH 2 ) 2 NH)] 2 (6), crystallize in centrosymmetric space groups. As a remedy, recrystallization of Zn(s-Bu) 2 complexes from different solvents was studied: in at least two cases, viz. [Zn(s-Bu) 2 (3-pic) 2 ] 3 C 7 H 8 ( 7) and [Zn(s-Bu) 2 (3,5-lut) 2 ] 3 C 7 H 8 (8), it was found that recrystallization from toluene gave rise to clathrates.

Analysis of Polymeryl Chain Transfer Between Group 10 Metals and Main Group Alkyls during Ethylene Polymerization

“…ZnMe 2 , ZnEt 2 , ZnPh 2 , AlEt 3 , GaMe 3 , and InMe 3 were purchased from Strem and used as received. Nickel and palladium α-diimines, ,− salicylaldimines, − [H­(OEt 2 ) 2 ] + [(3,5-(CF 3 ) 2 C 6 H 3 )) 4 B] − (HBArF), , Zn n Bu 2 , and Zn s Bu 2 were prepared according to literature procedures.…”

Zinc, cadmium and mercury

“…13 The ability to resolve sec-butylzinc compounds was investigated by formation of a series of [Zn(s-Bu) 2 (L) 2 ] (L = py, 2-picoline, 3-picoline) compounds. 14 Success will have important consequences in the area of asymmetric synthesis, but all compounds examined crystallized as chiral molecules but not in chiral crystals (i.e. centrosymmetric space groups).…”

Absolute Asymmetric Synthesis of Enantiopure Organozinc Reagents, Followed by Highly Enantioselective Chlorination

“…When such a reagent reacts with an electrophile, the chirogenic center in the organometallic reagent may remain intact, thus eliminating the need to transform helical chirality to central chirality. Our first attempts involved substituted benzyllithium complexes19 and sec ‐butylzinc complexes 32. To overcome difficulties encountered in handling extremely air‐sensitive solid organolithium reagents, and the low rate of enantiomerization of dialkylzinc reagents, we turned to diindenylzinc complexes.…”

“…Our first attempts involved substituted benzyllithium complexes [19] and sec-butylzinc complexes. [32] To overcome difficulties encountered in handling extremely air-sensitive solid organolithium reagents, and the low rate of enantiomerization of dialkylzinc reagents, we turned to diindenylzinc complexes. These reagents may enantiomerize through an h 3 intermediate or a rapid [1,3]-Zn shift in solution, whereas in the solid state zinc is prone to be s-bonded to the indenyl ligand in a monohapto fashion giving rise to a chirogenic a-carbon atom.…”