SHORT COMMUNICATION DOI: 10.1002/ejoc.201101527 Catalyst-Free Suzuki-Type Coupling of Allylic Bromides with Arylboronic Acids Alberto Scrivanti,* [a] Valentina Beghetto, [a] Matteo Bertoldini, [a] and Ugo Matteoli [a] Keywords: C–C coupling / Allylation / Boron / Cross-coupling The coupling of arylboronic acids with electron-rich allylic bromides is accomplished in the absence of any transition- metal catalyst through conventional heating. The reaction is Introduction There is great interest in developing practical and simple approaches to the synthesis of allylated arenes owing to their importance as natural products [1] and owing to the variety of chemical transformations that are feasible on aryl and allyl moieties. [2] In the last two decades, Suzuki–Miyaura coupling emerged as a versatile and efficient tool for the formation of carbon–carbon bonds. [3] Exploiting the Suzuki reaction, the synthesis of allylated arenes can be carried out accord- ing to two different approaches (Scheme 1): either an allyl- boronic acid is coupled with an aryl halide [Equation (1)] [4] or an arylboronic acid is coupled with an allylic halide [Equation (2)]; [5] both reactions require a transition-metal catalyst, usually a palladium complex. In Equation (2), the allylic halide may be replaced by an allylic alcohol or one of its derivatives. [6] Scheme 1. Synthesis of allylated arenes through a Suzuki–Miyaura reaction. During studies focused on employing Pd(OAc) 2 as the catalyst [7] in the coupling of (E)-cinnamyl bromide with phenylboronic acid (Scheme 2), we serendipitously ob- served that the reaction takes place at 90 °C even in the [a] Dipartimento di Scienze Molecolari e Nanosistemi, Università Cà Foscari di Venezia, Calle Larga S. Marta 2137-30123 Venezia, Italy Fax: +39-0412348967 E-mail: scrivanti@unive.it Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejoc.201101527. © 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Org. Chem. 2012, 264–268 264 completely regioselective, affording only the α-coupled prod- uct, and can be carried out under mild aerobic conditions in an organic solvent; the presence of a base is required. absence of any transition-metal catalyst. Delighted by this discovery, we decided to thoroughly investigate this cou- pling reaction and its potential applications. Scheme 2. Coupling of cinnamyl bromide with phenylboronic acid. Results and Discussion In the initial investigations, commercial arylboronic acids (Aldrich) were employed as received, but soon we observed that samples belonging to different production lots may dis- play significant differences in reactivity. Inspection of the 1 H NMR spectra of the less reactive arylboronic acids re- vealed that they invariably contained high amounts of the corresponding anhydrides (boroxines, see Scheme 3). [8] Scheme 3. The arylboronic acid/triarylboroxine equilibrium. In fact, reactivity studies supported by 1 H NMR investi- gations (see below) allowed us to establish that the actual reacting species is the acid, whereas the cyclic anhydride is by far less reactive. This finding is not surprising, as in the literature there are several examples of reactions in which a boronic acid and the corresponding boroxine display dif- ferent reactivities. [8b,9] To address the reproducibility difficulties due to the dif- ferent arylboronic acid/boroxine molar ratios in different lots of the commercial reagents, the arylboronic acids were recrystallized from hot water and suction dried in air as described by Ellman. [9a] This procedure affords “wet” bor-