Structural Organozinc Chemistry

CHAPTER 2 Structural organozinc chemistry JOHANN T. B. H. JASTRZEBSKI, JAAP BOERSMA and GERARD VAN KOTEN Debye Institute, Organic Chemistry and Catalysis, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands Fax: +31 30 252 3615; e-mail: [email protected] I. INTRODUCTION ...................................... 32 II. ORGANOZINCATES .................................... 34 A. Introduction ........................................ 34 B. Tetraorganozincates [R4Zn]M2 ............................ 35 C. Homoleptic Triorganozincates [R3Zn]M ...................... 41 D. Heteroleptic Triorganozincates [R2R Zn]M .................... 47 III. DIALKYL- AND DIARYLZINC COMPOUNDS ................. 53 A. Donor-base-free, σ ,σ -Bonded Homoleptic R2Zn and Heteroleptic RR Zn Compounds ........................................ 53 B. Diorganozinc Compounds Containing Multi-hapto Bonded Groups .... 60 C. Diorganozinc Compounds Containing Intramolecularly Coordinating Substituents ........................................ 64 D. Donor–Acceptor Complexes of Diorganozinc Compounds ......... 70 IV. HETEROLEPTIC RZnY COMPOUNDS ....................... 82 A. Introduction ........................................ 82 B. Monoorganozinc Cations ............................... 83 C. Monoorganozinc Compounds RZnY with Y = Halogen ........... 85 D. Monoorganozinc Compounds RZnY with Y = OR ............... 94 E. Monoorganozinc Compounds RZnY with Y = NR2 .............. 107 F. Monoorganozinc Compounds RZnY with Y = Other Heteroatom Bonded Group ........................................... 118 V. ORGANOZINC COMPOUNDS CONTAINING A ZINC–TRANSITION METAL BOND ........................................ 123 VI. CONCLUSIONS ....................................... 126 VII. REFERENCES ........................................ 127 The chemistry of organozinc compounds Edited by Z. Rappoport and I. Marek 2006 John Wiley & Sons, Ltd 31 32 Johann T. B. H. Jastrzebski, Jaap Boersma and Gerard van Koten I. INTRODUCTION Organozinc compounds are the first organometallic compounds ever made. Already in 1852, Edward Frankland prepared diethylzinc by heating ethyl iodide with zinc metal in a Carius tube1. Diethylzinc appeared to be a volatile colourless liquid that inflames spon- taneously upon exposure to air. Until the invention of the Grignard reagents around 1900 organozinc compounds were used extensively as alkylating agents in organic synthesis. Until fairly recently, little was known of the structures and properties of the organozinc compounds occurring as intermediates in various reactions. Interestingly, the complex- forming ability of organozinc compounds had already been recognized very early. In 1858, Wanklyn reported the formation of the ionic sodium triethylzinc complex2.One year later, Frankland observed that the formation of dimethylzinc from methyl iodide and zinc was accelerated by the addition of dimethyl ether or diethyl ether. It appeared that separation of the dimethylzinc from the ether was impossible, but it lasted until 1962 when it was established that dimethylzinc and dimethyl ether form a 1:1 complex in solution, which is appreciably dissociated in the vapour phase3. Most of the structures of organozinc compounds have been determined by X-ray crystal- lography, but, in view of the fact that many of these compounds are liquids, also Gas-phase Electron Diffraction of some volatile organozinc compounds as well as solution studies have been used. The latter category involves molecular weight measurements, microwave titrations, dipole moment determinations and complexation reactions. 57Zn NMR spec- troscopy has not played an important role since only one naturally occurring isotope of zinc possesses a nuclear magnetic moment, i.e. 67Zn with a natural abundance of 4.11% and a nuclear spin of 5/2. Because of its low receptivity and its quadrupole moment, 67Zn has received very little attention4 and in only a few cases have resonances of zinc ions in aqueous solutions been observed. In the February 2005 version of the CSD database5, 349 structures containing one or more direct zinc–carbon interactions have been found (excluding structures containing the Zn−CN structural motif). Together with the gas-phase data, this means that at that date a total of 354 molecular structures had been determined. Of these structures a large majority (225) deals with compounds in which 4-coordinate zinc is present. With its electron configuration 3d104s2, its normal valency is 2+ and the symmetrical, full, low-lying 3d shell of the Zn2+ ion does not cause ligand–field effects in coordination complexes. Its coordination behaviour is therefore relatively simple in that it basically forms trigonal planar or tetrahedral complexes with electron-donating ligands. Some examples of organozinc compounds containing formally Zn+ (organozinc cations) and Zn− (organozincates) are known. In a typical organozinc compound of the type R2Zn, the zinc–carbon bonds occupy two equivalent sp-hybridized molecular orbitals, resulting in a linear coordination. The dipole moment of symmetric diorganozinc compounds in non-polar solvents is therefore zero, as 6 indeed was observed for Me2Zn in cyclohexane solution . In this situation, the valency shell of the zinc atom has only two pairs of bonding electrons, but four empty low- energy orbitals available for bonding. As a consequence, two further coordinate bonds with ligands containing non-bonding electrons may form. Zinc is fairly electropositive with a Pauling electronegativity of 1.6 and diorganozinc compounds therefore contain covalent but rather polar zinc–carbon bonds. This bond polarity is responsible for the ease with which such compounds form coordination complexes. In the absence of electron donors, diorganozinc compounds containing saturated alkyl or aryl groups, with a few exceptions, occur as monomers. Apparently, the zinc in these compounds is unable to attain coordination saturation through the formation of aggregates via alkyl or aryl bridges by electron-deficient multi-centre bonds, which is the common structural motif in, 2. Structural organozinc chemistry 33 Me N NMe Zn Zn 2 Li Zn Li 2 Me2N NMe2 (1) (2) M Zn Zn M (3a) M = Cu (3b) M = Au FIGURE 1. Some examples of observed (1 and 2) and proposed (3) structures of organozinc compounds containing a bridging aryl group e.g., structural organocopper7 and organolithium8 chemistry. The relatively small electron deficiency of zinc together with the relative large size of the zinc atom are thought to be responsible for this lack of additional coordination. In a few cases structures having bridging alkyl or aryl groups between two zinc atoms, e.g. 19, or zinc and a metal like lithium, e.g. 210, copper 3a or gold, e.g. 3b11, or aluminium have been observed, or have been proposed in a few cases (Figure 1). Moreover, the exchange of alkyl groups between various alkyl metal compounds and alkyl zinc compounds in solution has been attributed to the formation of transient species in which bridging alkyl groups are present12–14. If one of the organic groups in a diorganozinc compound is replaced by an electronegative monoanionic grouping like a halogen atom or an alkoxy-, aryloxy- or organoamido group, but also by an alkynyl group, both the acceptor character of the zinc and the donor character of the zinc-bound electronegative group are enhanced. As a consequence, such compounds always form aggregates via multi-centre bonding of the electronegative substituent with various zinc centres. The self-association and/or complex formation of organozinc compounds involves con- siderable rehybridization of the zinc valence orbitals. When only one coordinate bond is formed, the zinc atom becomes sp2-hybridized and the resulting complex is planar or ◦ nearly so with bond angles around the zinc of about 120 . The zinc centre then still has one unoccupied valence orbital and remains coordinatively unsaturated. Three-coordinate zinc, however, is relatively rare and only occurs when steric crowding around the zinc prevents the approach of a fourth ligand. In general, both vacant valence orbitals of the zinc centre are used and consequently the bonding situation of zinc becomes close to sp3-hybridization reflected by a tetrahedral coordination geometry of the zinc centre. 34 Johann T. B. H. Jastrzebski, Jaap Boersma and Gerard van Koten As the 3d shell of zinc is completely filled it cannot function as a dative bond acceptor (Lewis acid). Also, the high ionization potential of the 3d electrons makes it very unlikely that they can be used in dative π-bonding (Lewis base) with electron-accepting ligands. However, the latter possibility cannot be excluded completely in some complexes of diorganozinc compounds with planar N-heterocyclic ligands. Although coordination numbers five and six are well-known for both monoorganozinc and inorganic zinc compounds, in diorganozinc compounds this five- or six-coordination is seen only in a few special instances. From a structural point of view, three classes of organozinc compounds can be distin- guished according to the number of carbon atoms directly bound to zinc. These classes are: (i) ionic organozinc compounds in which the number of directly zinc-bound carbon atoms (three or four) exceeds the valence number of zinc, the so-called organozincates; (ii) the diorganozinc compounds and their coordination complexes, which can be divided into subclasses depending on various types of coordinating ligands; and (iii) heteroleptic RZnX compounds in which X is an electronegative

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