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Exposing the Hidden Complexity of Stoichiometric and Catalytic Metathesis Reactions by Elucidation of Mg-Zn Hybrids

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Exposing the Hidden Complexity of Stoichiometric and Catalytic Metathesis Reactions by Elucidation of Mg-Zn Hybrids Exposing the hidden complexity of stoichiometric and catalytic metathesis reactions by elucidation of Mg-Zn hybrids Eva Hevia1, Jonathan Z. Chua, Pablo García-Álvarez, Alan R. Kennedy, and Matthew D. McCall WestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow, UK, G1 1XL Edited by Jack Halpern, University of Chicago, Chicago, IL, and approved January 14, 2010 (received for review November 17, 2009) Studying seemingly simple metathesis reactions between ZnCl2 role in these vital, new synthetic methodologies. In fact, the num- and t BuMgCl has, surprisingly, revealed a much more complex ber of organometallic compounds combining magnesium and chemistry involving mixed magnesium-zinc compounds that could zinc within the same molecule is scarce (7, 8), contrasting with be regarded as Mg-Zn hybrids. Thus, the reaction of equimolar the numerous reports on the synthesis of mixed-metal com- t amounts of ZnCl2 and BuMgCl reveals the formation of the unpre- pounds (ates) containing an alkali-metal with either magnesium μ t cedented mixed Mg-Zn complex [ðTHFÞ4Mgð -ClÞ2Znð BuÞðClÞ] (1), (magnesiates), or zinc (zincates), that show enhanced reactivity as a result of the co-complexation of the two anticipated exchange beyond the confines of the monometallic reagents from which products of the metathesis. This magnesium zincate adopts a con- they are derived (9–11). tacted ion-pair structure, closely related to Knochel’s pioneering In situ metathetical approaches are by far the most common “Turbo” Grignard reagents. Furthermore, a second coproduct iden- vehicles of choice to prepare either these mixed magnesium-zinc tified in this reaction is the solvent-separated mixed magnesium- putative intermediates or neutral organozinc reagents. Herein, 2þ 2− zinc chloride complex [fMgðTHFÞ6g fZn2Cl6g ] (3) that critically we shed light on this fundamental synthetic tool, by isolating diminishes the amount of ZnCl2 available for the intended metath- and characterising (structurally and spectroscopically) mixed-me- esis reaction to take place. In another surprising result, when the tal magnesium-zinc species (Mg-Zn hybrids) resulting from the t reaction is carried out by using an excess of 3 M equivalents of the reaction of ZnCl2 with BuMgCl, which reveal that metathesis re- Grignard reagent (closer to the catalytic conditions employed by actions and the constitutions of the products formed can be de- synthetic chemists), solvent-separated magnesium trialkyl zincate cidedly more complex than their simple reaction equations would þ t − [fMg2Cl3ðTHFÞ6g fZnð BuÞ3g ] (4) is obtained that can be viewed as suggest. Furthermore, we disclose their application to efficiently a model for the active species involved in the increasingly impor- prepare tris(aryl) magnesium zincates via direct metal-halogen tant organic transformations of Grignard reagents catalysed by exchange reactions, one of the most important and commonly ZnCl2. Furthermore, preliminary reactivity studies reveal that com- used methodologies in synthesis (12). plex 4 can be used as an effective new reagent for direct Zn-I exchange reactions that allow the preparation and structural iden- Results and Discussion þ tification of the magnesium tris(aryl) zincate [fMg2Cl3ðTHFÞ6g Stoichiometric Metathesis. Firstly following the standard protocol p − fZnð -TolÞ3g ] (5) that represents the first example of complete used in the synthetic arena we reacted equimolar amounts of 3-fold activation of a zincate in a Zn-I exchange reaction which, ZnCl2 dissolved in THF with a commercial solution of the in turn, can efficiently be used as a precursor for Negishi cross- Grignard reagent tBuMgCl in THF, from which a white solid coupling reactions. was obtained almost instantaneously. Initially, we logically but mistakenly attributed this to the formation of MgCl2 (in fact it rganozinc compounds (together with organolithium and appears to be largely the hybrid magnesium-zinc chloride 2þ 2− 3 Oorganomagnesium compounds) are amongst the most com- [fMgðTHFÞ6g fZn2Cl6g ](), vide infra) that was removed monly used and vitally important reagents in synthesis, playing a under filtration. Addition of hexane to the remaining solution key role in many fundamental organic transformations. Their allowed the crystallisation of a second product, namely the unex- main advantage is their unrivalled compatibility with a rich vari- pected mixed magnesium-zinc mixed alkyl-chloride complex t ety of organic functionalities that stems from the greater stability [ðTHFÞ4Mgðμ-ClÞ2Znð BuÞðClÞ](1), as revealed by multinuclear 1 13 and more covalent character of Zn-C bonds (in comparison with NMR ( H and C) spectroscopy and X-ray crystallography, in- Li-C or Mg-C bonds) (1). Together with the oxidative insertion of stead of the originally anticipated homometallic zinc compound t Zn metal into C-X bonds, metathesis reactions of ZnCl2 with BuZnCl (Scheme 1). The isolated yield of 1 was a modest 10%. more polar, more reactive organometallic reagents (typically The molecular structure of 1 established by X-ray crystallo- organolithiums, RLi, or Grignard reagents, RMgX) are one graphic studies (Fig. 1) confirms its heterobimetallic constitution, of the most important synthetic tools to prepare organozinc re- where both metals magnesium and zinc are intimately contacted agents (2). through the presence of two chlorine bridges giving rise to a Related to this stoichiometric metathesis, a flurry of recent planar [Mgðμ-ClÞ2Zn] four-membered ring. The zinc center reports have established that the use of catalytic amounts of en- vironmentally benign ZnCl2 with Grignard reagents greatly en- Author contributions: E.H. designed research; J.Z.C. and M.D.M. performed research; hances the reactivity and selectivity of the latter in numerous P.G.-A. and A.R.K. contributed new reagents/analytic tools; E.H. and M.D.M. analyzed data; cornerstone methodologies in organic synthesis such as alkyla- and E.H. wrote the paper. tions (3, 4), nucleophilic substitutions (5), or cross-coupling ap- The authors declare no conflict of interest. R plications (6). Mixed magnesium-zinc species 3ZnMgCl, This article is a PNAS Direct Submission. “ ” resulting from the in situ metathesis reaction of the relevant Data deposition: The atomic coordinates have been deposited in the Cambridge Structural Grignard reagent with the catalyst ZnCl2, seem to be implicated Database (CCDC). in these important transformations. However, their presence (as 1To whom correspondence should be addressed. E-mail: [email protected]. transient intermediates or defined compounds) is only assumed This article contains supporting information online at www.pnas.org/cgi/content/full/ and there is no tangible spectroscopic or structural proof of their 0913307107/DCSupplemental. 5294–5299 ∣ PNAS ∣ March 23, 2010 ∣ vol. 107 ∣ no. 12 www.pnas.org/cgi/doi/10.1073/pnas.0913307107 Downloaded by guest on October 3, 2021 Scheme 1. completes its distorted tetrahedral coordination sphere by bond- ing to a terminal chlorine and a terminal tert-butyl group that was originally bound to magnesium. Magnesium achieves its larger distorted octahedral geometry by also bonding to four molecules of THF as well as to the Cl bridges. Tetrahedral Zn forms a strong sigma bond [Zn1-C1 2.000(3) Å] with the quaternary carbon of the formally anionic tert-butyl ligand that is marginally elongated Fig. 1. Molecular structure of 1 with hydrogen atoms omitted for clarity. t to that found in discrete, linear Bu2Zn [1.977(4) Å] (13). Com- 1 t pound exhibits a unique contacted ion-pair organometallic a Bu∶THF 1∶4 ratio identical to that found in the crystal struc- “ 13 structure combining Mg with Zn and it can be envisaged as a mo- ture. In addition, the C NMR spectrum of 1 displayed two re- ” lecular salt structure where the ionic MgCl2 fragment is trapped sonances at 21.9 and 33.6 ppm for the quaternary carbon and the within a molecular framework before it can aggregate to build a methyl groups of the tBu ligand, respectively. These 13C chemi- lattice.* Thus, the two expected products of the metathesis reac- t t cals shifts are similar to those found for the related BuZnCl spe- tion, BuZnCl and MgCl2 have formed an unprecedented type of cies in the same deuterated solvent (21.0, 33.2 ppm). However, 2þ 0 2− magnesium-zincate [Mg ðZnR3R Þ ] that can be regarded as a they differ significantly to those found in related organomagne- hybrid Mg-Zn Grignard reagent (14). The mixed-metal nature of 1 t t sium compounds [ Bu2Mg, 15.8, 36.0 ppm; BuMgCl, 15.6, ’ 13 1 t allows its comparison with Knochel s Turbo Grignard reagents 35.8 ppm (the Cf HÞ NMR spectrum of BuMgCl in deuterated that combine a conventional Grignard reagent RMgX with LiCl THF is highly complex showing in addition three minor species – t (15 17), from which a myriad of successful regioselective functio- that also contain Bu groups that is consistent with the compli- nalisations of a wide range of substituted aromatic and heterocyc- cated structure that Grignard reagents in general can exhibit lic molecules have been reported. Furthermore, recent reports by in solution )] (22) suggesting that mixed-metal species 1 retains the same group show that the addition of ZnCl2 to these bime- most of its “zinc character.” In addition, no changes in the 1H CHEMISTRY tallic reagents to generate trimetallic systems can enhance their spectrum of 1 were observed when low temperature NMR studies – reactivity even further (18 20). In addition, Mulvey et al. have were carried out at −40 °C, suggesting that, even at subambient recently unveiled the structure of the related Turbo Hauser base 1 μ 2 temperatures, retains its bimetallic constitution. [ðTHFÞ2Lið -ClÞ2MgðTMPÞðTHFÞ]() (Fig. 2) that exhibits a Because using this metathetical approach compound 1 is ob- 1 structural motif related to that of where both metals are con- tained only in poor yield, we next turned our attention to inves- nected by two chlorine bridges with the remaining anionic ligand, tigate in detail the composition of the white solid formed when in this case the amide TMP, is bonded to magnesium.
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