Allenes and Derivatives in Gold(I)- and Platinum(II)-Catalyzed Formal Cycloadditions

Conspectus Cycloaddition reactions, by involving the formation of at least two bonds and one cycle in a single operation, represent one of the more practical ways to assemble carbo- and heterocyclic structures from simple acyclic precursors. Especially appealing are formal cycloadditions promoted by transition metals, owing to the ability of these reagents to open mechanisms that are not accessible using classical chemistry. Therefore, along the years, a great variety of annulations based on first-, and particularly second-row transition metals have been discovered. Most of these reactions involve inner sphere mechanisms, with the metal participating via standard oxidative addition or reductive elimination processes. Curiously, metals of the third row like platinum and, especially, gold remained largely unexplored, likely because of the belief that they were inert and expensive. However, from the beginning of this century, many groups realized that these metals can open very interesting mechanistic scenarios and promote novel types of transformations. In particular, the π-acidic, carbophilic behavior of gold(I) complexes, together with the possibility of tuning their reactivity using designed ligands, has triggered important activity in the field. Many gold-catalyzed transformations involved addition or cycloisomerization processes, but during recent years, there have been also important advances in the development of formal cycloaddition reactions. While many of these reactions rely on the activation of alkynes, there has been an increasing number of reports that exploit the peculiar reactivities of allenes and derivatives. In this Account, we present recent efforts on the development of platinum- and gold-catalyzed formal cycloadditions of allenes. For the sake of simplicity, we only include annulations initiated by a direct metal-promoted activation of the allene moiety. Thus, alternative Pt- or Au-catalyzed reactions wherein the allene does not interact with the metal catalyst are not covered. Upon activation by the metals, allenes generate allyl-cation alkenylmetal species that can behave as 1,2- or 1,3-carbon dipoles in cycloaddition processes. Especially relevant is the reactivity of allenamides. The presence of the amide substituent provides for the generation of gold intermediates with a good balance of reactivity and stability, which can therefore react with the corresponding partners in a controlled manner. Moreover, despite the difficulties associated with the transfer of stereochemical information from chiral linear gold(I) complexes, a variety of enantioselective gold-catalyzed annulations have been discovered. This Account is organized considering the number of atoms engaged in the annulation process, and when possible, we present the results in a chronological orderFinancial support from the Spanish MINECO (SAF2016-76689-R, CTQ2017-84767-P, FPU fellowship to I.V), the Xunta de Galicia (ED431C 2017/19, 2015-CP082, Centro Singular de Investigación de Galicia accreditation 20162019 ED431G/09), the ERDF, and ERC (Adv. Grant No. 340055) is acknowledgedS

Gold(I)–catalyzed enantioselective annulations between allenes and alkene-tethered oxime ethers: A straight entry to highly substituted piperidines and aza-bridged medium-sized carbocycles

NOTICE: This is the peer reviewed version of the following article: Marcote, D. C., Varela, I., Fernández-Casado, J., Mascareñas, J.L., López*, F. (2019), Gold(I)–catalyzed enantioselective annulations between allenes and alkene-tethered oxime ethers: A straight entry to highly substituted piperidines and aza-bridged medium-sized carbocycles. J. Am. Chem. Soc., 140, 16821-16833 [doi: 10.1021/jacs.8b10388]. This article may be used for non-commercial purposes in accordance with ACS Terms and Conditions for self-archivingPiperidine scaffolds are present in a wide range of bioactive natural products and are therefore considered as highly valuable, privileged synthetic targets. In this manuscript, we describe a gold-catalyzed annulation strategy that allows a straightforward assembly of piperidines and piperidine-containing aza-bridged products from readily available alkene-tethered oxime ethers (or esters) and N-allenamides. Importantly, we demonstrate the advantages of using oxime derivatives over imines, something pertinent to the whole area of gold catalysis, and provide relevant mechanistic experiments that shed light into the factors affecting the annulation processes. Moreover, we also describe preliminary experiments demonstrating the viability of enantioselective versions of the above reactionsThis work received financial support from the Spanish MINECO (SAF2016-76689-R, CTQ2017-84767-P, FPU fellowship to I.V. and J.F.-C.), the Xunta de Galicia (ED431C 2017/19, 2015-CP082, Centro Singular de Investigacion de Galicia accreditation 2016-2019 ED431G/09 and ́ predoctoral fellowship to D.C.M.), the ERDF, ERC (Adv. Grant No. 340055), and the Orfeo-Cinqa network (CTQ2016- 81797-REDC). Dr. Rebeca Garcí a-Fandiño is acknowledged for her contribution to the DFT studiesS

Gold(I)-Catalyzed Enantioselective [2+2+2] Cycloadditions: An Expedient Entry to Enantioenriched Tetrahydropyran Scaffolds

This is the peer reviewed version of the following article: Iván Varela, Hélio Faustino, Elena Díez, Javier Iglesias-Sigüenza, Francisca Grande-Carmona, Rosario Fernández, José M. Lassaletta, José L. Mascareñas* and Fernando López*, Gold (I)-Catalyzed Enantioselective [2+2+2] Cycloadditions: An Expedient Entry to Enantioenriched Tetrahydropyran Scaffolds. ACS Catal. 2017, 7, 2397-2402 [DOI: 10.1021/acscatal.6b03651]. This article may be used for non-commercial purposes in accordance with American Chemical Society Terms and Conditions for self-archivingA straightforward and atom-economical enantioselective approach to highly substituted tetrahydropyrans is reported. The process, which consists of an intermolecular gold-catalyzed [2+2+2] cycloaddition between allenamides, alkenes, and aldehydes, is efficiently catalyzed by both phosphoramidite- and chiral N-heterocyclic carbene-gold catalysts, occurs with complete chemoselectivity and regioselectivity, moderate diastereoselectivity, and moderate to very good enantioselectivitiesThis work was supported by the Spanish MINECO (grant:SAF2013-41943-R, CTQ2013-48164-C2-1-P and -2-P), the Consellería de Cultura, Educación e Ordenación Universitaria (GRC2013-041, 2015-CP082 and Centro Singular de Investigación de Galicia accreditation 2016-2019, ED431G/09) the European Regional Development Fund (ERDF), the European Research Council (Adv. Grant No. 340055), and the Junta de Andalucía (Grant 2012/FQM 1078). We also thank the Orfeo-Cinqa networkS