A Convenient Approach to 2-Arylindenes via Suzuki Coupling Reaction of 2-Indenylboronate with Aryl Bromides

Abstract

Indenyl derivatives have been extensively employed in organometallic chemistry as ligands. Group 4 metallocene catalysts containing such ligands have shown activities in olefin polymerization as well as in various organic reactions of small molecules. Recently, unbridged bis(2-arylindenyl)zirconium dichlorides were reported to produce elastomeric polypropylene with a stereoblock microstructure when activated by MAO (methylaluminoxane). Waymouth and coworkers investigated several zirconocene catalysts containing a different aryl substituent in the 2-position of the indenyl ligand for their polymerization behavior, and reported that the bis(2-arylindenyl) framework was important for the production of elastomeric polyproplylene. Although the importance of 2-arylindenes as ligands for olefin polymerization catalysts is increasing, only a couple of synthetic methods to 2-arylindenes have been reported. These reported procedures, however, have a serious limitation, as strongly basic reagents are required. The reaction of an organolithium reagent or arylmagnesium bromide with 2indanone is troublesome due to the susceptibility of 2indanone to enolization. This method is particularly ineffective for most organolithium nucleophiles as well as sterically demanding nucleophiles. The reaction utilizing 1,2-di(magnesiomethyl)benzene dichloride and the corresponding methyl ester of the substituent moiety was reported. In this method, the starting methyl esters, if not commercially available, needed to be prepared separately and use of the di-Grignard reagent limited the functional group compatibility of this procedure. Therefore, a more convenient and general way to make a variety of 2-arylindenes is necessary. We now report a synthetic route to a family of 2-arylindenes via Suzuki coupling reaction of 2-indenylboronate with various aryl bromides. To develop an efficient protocol, we initially focused on the preparation of an indenyl boron derivative as the coupling component. Since various aryl halides are commercially available, this approach seems more attractive than the coupling between bromoindene and arylboronic acids. We found that Masuda’s borylation procedure could be employed to prepare the indenylboron derivative. The original reaction conditions was modified by changing the catalyst from PdCl2(dppf)/AsPh3 to PdCl2/ PPh3 and applied to bromoindene. The desired indenylboronate 1 was obtained in 80-88% yield in either THF or dioxane at 80 °C (Scheme 1). In this reaction, using an excess of toxic AsPh3 ligand with regard to the precatalyst was not necessary, and carrying out the coupling with a mixture of PdCl2 and PPh3 was enough to give the desired product in high yield. Next, reaction conditions for the Suzuki coupling reaction of indenylboronate 1 and bromobenzene were optimized by using Pd(OAc)2 as the precatalyst and K3PO4 as the base (Table 1). THF or dioxane was more effective than toluene as a solvent, and PPh3 was a better ligand than the other bidentate ligands that were screened. The coupling product, 2-phenylindene was isolated in 92% yield under the optimized conditions. Other aryl halides were tested in the coupling reaction and the results are shown in Table 2. Sterically hindered aryl halides with ortho substituents were effectively coupled to afford the corresponding 2-aryl substituted indenes in good yields (Table 2, entries 2-4). Ortho substituents such as methyl, phenyl, and methoxy groups were accommodated. It Scheme 1. Synthesis of 2-indenylboronate.