Gold carbenes, gold-stabilized carbocations, and cationic intermediates relevant to gold-catalysed enyne cycloaddition

Abstract

Cationic gold complexes in which gold is bound to a formally divalent carbon atom, typically formulated as gold carbenes or α-metallocarbenium ions, have been widely invoked in a range of gold-catalyzed transformations, most notably in the gold-catalyzed cycloisomerization of 1,n-enynes. Although the existence of gold carbene complexes as intermediates in gold-catalyzed transformations is supported by a wealth of indirect experimental data and by computation, until recently no examples of cationic gold carbenes/α-metallocarbenium ions had been synthesized nor had any cationic intermediates generated via gold-catalyzed enyne cycloaddition been directly observed. Largely for this reason, there has been considerable debate regarding the electronic structure of these cationic complexes, in particular the relative contributions of the carbene (LAu(+)[double bond, length as m-dash]CR2) and α-metallocarbenium (LAu-CR2(+)) forms, which is intimately related to the extent of d → p backbonding from gold to the C1 carbon atom. However, over the past ∼ seven years, a number of cationic gold carbene complexes have been synthesized in solution and generated in the gas phase and cationic intermediates have been directly observed in the gold-catalyzed cycloaddition of enynes. Together, these advances provide insight into the nature and electronic structure of gold carbene/α-metallocarbenium complexes and the cationic intermediates generated via gold-catalyzed enyne cycloaddition. Herein we review recent advances in this area.

Two Amphoteric Silver Carbene Clusters

“…In analogy to silver NHC complexes,w hich serve coordination chemistry well as carbene transfer agents, [31] complex 5 reacted with LAuNTf 2 at low temperature to give the corresponding terminal gold carbene complexes 12 a (L = IPr) and 12 b (L = XPhos). [33][34][35] Thec haracteristic quinoid distortion in 12 a with short C1ÀC2 (1.433 )and C5ÀN1 (1.359 )bonds shows that the electron-rich carbene backbone,r ather than the metal center,l argely accounts for the metastability of this complex. [33][34][35] Thec haracteristic quinoid distortion in 12 a with short C1ÀC2 (1.433 )and C5ÀN1 (1.359 )bonds shows that the electron-rich carbene backbone,r ather than the metal center,l argely accounts for the metastability of this complex.…”

“…Al arger experimental data set is available for silver complexes carrying N-heterocyclic carbene (NHC) or diamino-substituted cyclopropenylidene ligands,m any of which are air-a nd moisture-stable and thermally fairly robust. [33] With complex 2,w hich comprises as ubstituted diphenylcarbene backbone,our group had isolated and characterized the first reactive gold carbene: [34,35] As this species cyclopropanates styrene derivatives even at low temperature,i ts structure is relevant for ab etter understanding of gold catalysis. Silver NHC complexes can also be used as versatile carbene transfer agents.…”

Gold Sulfonium Benzylide Complexes Undergo Efficient Benzylidene Transfer to Alkenes

“…[1][2][3][4] Although carbene transfer to alkenes is certainly not unique to gold, [5][6][7][8][9][10] the particularp roperties of gold including high d-electron count and electronegativity, [11] poor d!p backb onding, [12] and the formation of stable p-complexes [13] likely render the mechanismso fg old to alkenec arbenet ransfer distinct from other transition metals. [1][2][3][4] Although carbene transfer to alkenes is certainly not unique to gold, [5][6][7][8][9][10] the particularp roperties of gold including high d-electron count and electronegativity, [11] poor d!p backb onding, [12] and the formation of stable p-complexes [13] likely render the mechanismso fg old to alkenec arbenet ransfer distinct from other transition metals.…”

“…[14][15][16][17][18] As such, many gaps remaini no ur understanding of the mechanismso ft he carbenet ransfer processes of gold including control of stereoselectivity,t he nature of CÀCb ond formation, and the involvement of carbocationic and/or metallacyclobutane intermediates. [2,3] Recent efforts in this area have led to the identification of as mall number of well-defined gold carbene complexes that undergo carbene transfer to alkenes (A-C). [2,3] Recent efforts in this area have led to the identification of as mall number of well-defined gold carbene complexes that undergo carbene transfer to alkenes (A-C).…”

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

“…Structurally characterized copper carbenes have been prepared only by either decomposition of the diazo precursors with various copper sources30313233 or carbene transfer from chromium complex34. On the other hand, several groups have successfully isolated a variety of structurally characterized gold carbenes3536373839404142434445, but there remains an ongoing debate on the extent of the electronic character of gold carbenes as either carbene-like with a strong gold-to-carbon π-backbonding or carbocation-like with a weak π-backbonding46474849505152535455. Therefore, structurally characterized copper and gold carbenes, especially those involved in catalytic transformations, are highly desired for a clear understanding of the nature of gold carbenes and their essential role in the catalytic cycle.…”