On the Trapping of Vinylgold Intermediates

Abstract

An internal aryl-substituted ortho-alkyn-A C H T U N G T R E N N U N G ylphenol and a similar aniline with stoichiometric amounts of N,N'-bis(2,6-diisopropylphenyl)imidazol-2-ylidene-gold tosylate [(IPr)AuOTs] and tri-A C H T U N G T R E N N U N G ethylamine gave the aurated heterocycles as stable intermediates of the corresponding gold(I)-catalysed hydrooxylation and hydroamination reactions. X-ray crystal structure analyses of both products could be obtained. A similar internal alkyl-substituted ortho-alkynylphenol gave only the cycloisomerised product, no aurated intermediate could be detected.Keywords: alcohols; alkynes; amines; gold; hetarenes; mechanism Most reactions in the very active field of homogeneous gold catalysis [1] are based on the attack of a nucleophile Nu À H to either an allene-gold complex 1 or an alkyne-gold complex 2, both elementary reactions of central importance for gold catalysis. The activation of the carbon-carbon multiple bond by the gold complex is crucial, without this activation usually the nucleophilic addition is not observed. In both cases vinylgold species are formed; allenes deliver 3 with the nucleophile in an allylic position, alkynes lead to 4 with the nucleophile in a vinyl position (Scheme 1).For a long time, the vinylgold intermediates eluded detection and isolation, but in the last year three groups managed to isolate such species and unambiguously characterised them by X-ray crystal structure analyses. The first examples were provided by Hammond et al., who observed an only slow cyclisation of allenic esters with gold(I) catalysts and finally could show that in stoichiometric rather than catalytic reactions they can conveniently isolate stable vinylgold(I) phosphane complexes 5 (Figure 1), [2] the proto-deauration of 5 needs higher temperatures and strong acids. Next GagnØ et al. managed to isolate another allene-derived vinylgold(I) complex, by an intramolecular hydroarylation the phosphane complex 6 was obtained.[3] Then our group could show that with internal alkynes not only the regioselectivity of the ring closure of N-propargylic carboxamides would switch from a 5-exo-dig cyclisation to a 6-endo-dig cyclisation, also the vinylgold species could be detected when using phosphane ligands on gold and could even be isolated when using the gold(I) complex of the common NHC ligand N,N'-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr). [4] This was a quite general aproach, several representatives of the products 7 could be prepared in stoichiometric reactions in good yields and the spectroscopic properties as well the reactivity of this species could be investigated.

One‐Pot Gold‐Catalyzed Synthesis of Azepino[1,2‐a]indoles

“…At this stage, the reaction sequence could proceed through a protodeauration of the 3-[Au]-indolyl species D and subsequent dehydration (Scheme 2, step 2). [18,19] Next, the condensation of 2-vinyl indole 3 a on the carbonyl unit takes place, resulting in an annulated intermediate with the formation of a seven-membered ring. A ring-closing process promoted by a Lewis acid (LA) or a Brønsted acid (Prins-type condensation) does not adequately account for the observed reactivity, as 1) treating 3 a with a catalytic amount of TfOH (5 mol % in refluxing toluene, or 15 mol % at RT) produced 4 a in only trace amounts up to 25 % yield, and 2) a [Au I ]/LA-type assisted mechanism does not properly explain the unique reactivity observed with Au-NHC species when compared to more electrophilic phosphine-based gold complexes.…”

“…[16] After optimizing the reaction conditions, the generality of the method was next explored by subjecting a series of alkynyl anilines 1 b-p to the cascade ring-closing procedure. The results are summarized in Table 2.…”

Synthesis and structures of gold and copper carbene intermediates in catalytic amination of alkynes

“…Moreover, Hashmi et al 6566. also reported that addition of a base can slow down the protodemetallation to conserve organometal species.…”

Direct Evidence for the Origin of Bis‐Gold Intermediates: Probing Gold Catalysis with Mass Spectrometry

“…Compared with earlier discovered/proposed gold-containing intermediates, such as gold-alkyne π-complexes [3] and vinyl gold derivatives, [4] the dinuclear gold complexes (bis-gold) have received sustained attention only recently. Based on NMR analysis, Gagn8 and co-workers identified the existence of dinuclear gold intermediates in intramolecular hydroarylation reactions.…”