Journal - The Institution of Engineers, Malaysia (Vol. 66, No. 1, March 2005) 53 HIGHLY REACTIVE ELECTROGENERATED ZINC: PREPARATION AND ITS USE IN CROSS-COUPLING REACTIONS Aishah Abdul Jalil 1 , Sugeng Triwahyono 2 , Kurono Nobuhito 3 , Takasugi Shingo 3 , and Tokuda Masao 3 1 Department of Chemical Engineering, Faculty of Chemical and Natural Resources Engineering, 2 Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor 3 Division of Molecular Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan ABSTRACT Highly reactive zinc metal was prepared by electrolysis of a N,N-dimethylformamide (DMF) solution containing naphthalene and a supporting electrolyte in a one-compartment cell fitted with a platinum cathode and a zinc anode. This highly reactive electrogenerated zinc (EGZn/Naph) was used for transformation of bromoalkanes into the corresponding organozinc bromides, which can not be achieved by the use of usual zinc metals. Reaction of the organozinc compounds were thus prepared with various aryl iodides in the presence of 5 mol% of palladium catalyst to give the corresponding cross-coupling products in high yields. Arylzinc iodides were also prepared by the use of this highly reactive zinc, and they were reacted with other iodides to give the corresponding cross-coupled biaryls in good yields. Keywords : Highly Reactive Zinc, Preparation, Cross-Coupling, Palladium Catalyst 1.0 INTRODUCTION Organozinc compounds are very useful organometallic compounds for the forming reaction of carbon-carbon bonds [1]. Organozinc halides can usually be prepared by direct insertion of zinc metal into organic halides, but commercially available zinc metal is generally poorly reactive [2]. Therefore, activation of the metal is necessary for preparation of organozinc halides. Various method of zinc activation, such as the reduction of zinc halide with alkaline metal or alkali metal naphthalenide, have been reported [3]-[5]. These methods, however, require high temperatures and long reaction times, or vigorous stirring during the reaction. A new method for preparation of reactive zinc by electrolysis and its use in facile isoprenylation [6] and allylation of aldehydes and ketones was reported elsewhere [7],[8]. It was shown that this electrogenerated reactive zinc (EGZn) was an aggregation of very fine crystalline zinc particles with a large surface area [8]. A facile preparation of organic compounds from functionalized alkyl iodides by using EGZn and their cross-couplings with aryl halides (Scheme 1), was also reported [9]. However, no organozinc compounds have been obtained from alkyl bromides or were only obtained in very low yields from the corresponding organic bromides, even by the use of reactive EGZn. Recently, the authors have developed a new electrochemical method for preparation of more highly reactive zinc by using naphthalene as a mediator. Such methods for preparing reactive zinc are very convenient and useful in organic synthesis. In this paper, the authors report a new method for preparing highly reactive zinc by electrolysis and its use in cross-coupling reaction of bromoalkanes with aryl iodides as well as in aryl-aryl cross-coupling reaction. 2.0 EXPERIMENTAL A. General methods IR spectra were recorded on a JASCO IR-810 infrared spectrometer (neat between NaCl plates). 1 H NMR spectra were recorded on a JEOL JNM-EX270 (270 MHz) and JNM- LA400FT NMR (400 MHz). 1 H chemical shift is reported in ppm (δ) using tetramethylsilane as an internal standard. High and low resolution mass spectra were determined with a JEOL JMS-AX500 or JEOL JMS-SX102A spectrometer. Products were isolated by kugel rohr distillation or thin layer chromatograph and column chromatograph (Merck Kieselgel 60 PF254). B. Solvent and reagents Commercially available anhydrous N,N-dimethylformamide (DMF) and tetrahydrofuran (THF) packed under a nitrogen atmosphere (Kanto Chemical) were used without further purification. Tetraethylammonium perchlorate was prepared according to the previous reported method [9]. The zinc plate (Nilaco) is commercially available in more than 99.9% Pt cathode – Zn anode “Zn” (EGZn) electrolysis 0.1M Et4NCIO4 – DMF I n CO2Et IZn n CO2Et Ar n CO2Et EGZn 0ºC, 10 min Ar-X Pd(II) cat. Scheme 1 053-060•highly reactive 9/20/05 12:38 PM Page 53