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We have earlier postulated the intermediacy of organozinc radical species in the regioselective alkylation reactions of 1,4-di-tert-butyl-1,4-diaza-1,3-butadiene (t-BuNCHCHN-t-Bu) with diorganozinc compounds (ZnR2). To verify these postulates, we have prepared and studied the neutral organozinc radical complex MeO(CH2)3Zn(t-BuNCHCHN-t-Bu) (A) and two diorganozinc radical-anionic complexes, K[R2Zn(t-BuNCHCHN-t-Bu)] (R = Me (8a), Et (8b)). A was prepared in situ by the reaction of t-BuNCHCHN-t-Bu with bis(methoxypropyl)zinc. Complexes 8 were prepared by reducing the 1:1 coordination complex ZnMe2(t-BuNCHCHN-t-Bu) (1a) with potassium or by the nucleophilic addition of K(t-BuNCHCHN-t-Bu) to ZnR2 (R = Me, Et). The resulting radical-anionic complexes are thermally unstable and readily undergo an intermolecular single-electron transfer, giving a mixture of the metallacyclic heteroleptic zincate complexes K[ZnR(t-BuNCHCHN-t-Bu)] (9) and K[ZnR(t-BuNC(R)CHN-t-Bu)] (10). The ratio in which the latter complexes are formed depends on the R group and on the concentration and the temperature. Alternatively, 9 has been prepared separately from the reduction of [ZnR(t-BuNCHCHN-t-Bu)]2 (4) with 2 equiv of potassium. The crystal structures of two complexes of 9, i.e. {9a(THF)}n and {9c(Et2O)1/2}n, have been determined. Both complexes form linear coordination polymers containing alternating potassium cations and zinc−diazabutadiene anions.

A new enantiomerically pure S,N-chelated zinc bis(aminoarenethiolate), (R,R)-Zn(SC6H4C(Me)HNMe2-2)2 ((R,R)-3b), has been synthesized by the reaction of the (R)-trimethylsilyl aminoarenethiolate species (R)-2b with ZnCl2 in a 2:1 molar ratio. (R,R)-3b is an efficient catalyst for the addition of dialkylzinc compounds to aliphatic and aromatic aldehydes to give the corresponding secondary alcohols in nearly quantitative yields with optical purities of 69−99% ee under mild reaction conditions. Although excellent selectivities were obtained with this simple ligand containing only one stereogenic (carbon) center, further modifications of the amino substituents were studied. Cyclic N(CH2)4 or N(CH2)5 amino-substituted aminoarenethiolate ligands considerably enhanced the reaction rates, resulting in shorter reaction times and higher ee's. The mechanism of these 1,2-addition reactions has the general characteristics as reported by Noyori et al. This conclusion is based on the synthesis, isolation, and characterization (X-ray, ¹H and ¹³C NMR) of the enantiopure zinc bis(aminoarenethiolate) and organozinc aminoarenethiolate intermediates and on monitoring of the reaction process. We present evidence for an interpretation of the binding in the product-forming key intermediate complex in terms of an organozinc cation/anion pair. The possibility that the very efficient transfer of chiral information in this compact complex may be due to a combination of the shortness of the Zn−N coordinate bonds with concomitant η² bonding of the aldehyde substrate is discussed. The solid-state structure of the zinc bis(aminoarenethiolate) (R,R)-Zn(SC6H4C(Me)HNMe2-2)2 is reported.

Homoleptic lithium zincates with intramolecular Li−N coordination, i.e., Li(thf)nZn(C6H4CH2NMe2-2)3 (n = 0 (1); n = 1 (1(thf))) and Li2Zn(C6H4CH2NMe2-2)4 (2) have been prepared by reacting [Li(C6H4CH2NMe2-2)]4 with Zn(C6H4CH2NMe2-2)2. In the solid state 1(thf) is a monomer containing a distorted tetrahedral zinc atom. The zinc is surrounded by three monoanionic C6H4CH2NMe2-2 (dmba) ligands, which all show a different bonding mode:  η¹-C bonding to zinc and nitrogen coordination to lithium; η¹,μ²-C bridge bonding to zinc and lithium with Li−N coordination; and C,N-chelate bonded to zinc. One thf coordinates to lithium. In the solid state, 2 is a monomeric dilithium tetraarylzincate complex containing a tetrahedral zinc atom. The four dmba ligands are all similarly η¹,μ²-C bridge-bonded to zinc and lithium, and each lithium is four-coordinate by two additional Li−N bonds. The reaction of LiCH2SiMe3 and Zn(C6H4CH2NMe2-2)2 did not give a mixed zincate complex but the respective homoleptic zincate complexes instead. Both homoleptic and in situ prepared mixed zincate complexes react readily with 2-cyclohexen-1-one, but the product composition shows that the in situ prepared zincate disproportionates into the corresponding homoleptic zincates prior to reaction.

The synthesis of perfluoroalkyl-functionalized arene trimethylsilyl ethers and their conversion to ethylzinc thiolates is described. These compounds have been successfully applied as catalysts in the enantioselective addition of diethylzinc to benzaldehyde. This is the first example of a two-phase organic/perfluorous enantioselective catalytic system.