Structure of a Reactive Gold Carbenoid

Abstract

A formal gold‐for‐chromium transmetalation allowed the gold carbenoid species [Cy3PAuCAr2]NTf2 (11) (Ar=pMeOC6H4‐) to be obtained in crystalline form. The structure in the solid state suggests that there is only little back donation of electron density from gold to the carbene center of 11 and hence very modest AuC double‐bond character; rather, it is the organic ligand framework that is responsible for stabilizing this species by resonance delocalization of the accumulated positive charge. Because 11 is capable of cyclopropanating p‐methoxystyrene even at low temperature, the discussion of its structure is deemed relevant for a better understanding of the mechanisms of π‐acid catalysis in general.

Synthesis, Structure, and Reactivity of a Gold Carbenoid Complex That Lacks Heteroatom Stabilization

“…[1] Nevertheless, there remains considerable debate regarding the nature of these complexes, [2][3][4] which is in large part due to the lack of direct experimental information regarding the structure and behavior of cationic gold carbenoid complexes. [1][2][3][4][5] In 2008, Hammond reported the spectroscopic detection of a gold carbenoid/oxonium complex, [6] and since then, a number of cationic gold carbenoid complexes have been isolated, [7][8][9][10] including Fürstners recent disclosure of a reactive dianisylcarbenoid complex ( Figure 1). [1][2][3][4][5] In 2008, Hammond reported the spectroscopic detection of a gold carbenoid/oxonium complex, [6] and since then, a number of cationic gold carbenoid complexes have been isolated, [7][8][9][10] including Fürstners recent disclosure of a reactive dianisylcarbenoid complex ( Figure 1).…”

“…[5] Rather, information regarding the nature of cationic gold carbenoid complexes was for many years restricted to indirect experimental evidence and computational studies. [9] However, in all cases, these complexes rely on the conjugation of heteroatoms to the electron-deficient C1 carbon atom for stabilization. [9] However, in all cases, these complexes rely on the conjugation of heteroatoms to the electron-deficient C1 carbon atom for stabilization.…”

“…[8] In an effort to impart greater stability in a heteroatomfree gold carbenoid complex, we sought to exploit the potential aromatic stabilization of the cycloheptatrienylidene ligand. [6][7][8][9] [*] R. 4 ), which are presumably formed through the attack of unreacted 3 on 4·B(C 6 F 5 ) 4 . Attempted synthesis of gold diphenylcarbenoid complex 1.…”

Isolierung eines nicht‐Heteroatom‐stabilisierten Goldcarbens

“…Ohne p-Rückbindung sollte die Au-CMes 2 -Bindung also länger sein. [6] In der Kristallstruktur sind die ortho-Methylgruppen der Mesitylsubstituenten paarweise diastereotop. Diese Interpretation deckt sich mit Brooners und Widenhoefers eleganter strukturellen Beobachtung einer schwachen Gold-zu-Carben-Rückbindung, die jene einer Methyl-oder Phenylgruppe übertrifft (Abbildung 1, Mitte rechts).…”

Katalyse und Totalsynthese: eine persönliche Zwischenbilanz

“…Hingegen ist die Elektronenrückbindung vom Goldzentrum in das an sich freie Orbital des Singulettcarben-Kohlenstoffatoms offenbar gering (obschon nicht null); [38] jedenfalls ist die Au-C-Bindungsordnung auch in dieser Spezies nahe an eins. [37] Dieser experimentelle Befund erhärtete unsere früh geäußerte Vermutung, dass p-Säure-Katalyse auf "carbenoiden" Intermediaten mit erheblicher Ladungsdichte am Liganden beruht. [14,15] Man mag es als semantisches Problem ansehen, ob man solche Spezies als "Goldcarbenoide", "Goldcarbene" oder "goldstabilisierte Kationen" bezeichnet.…”