Gold(I) Carbenoids: On‐Demand Access to Gold(I) Carbenes in Solution

Abstract

Chloromethylgold(I) complexes of phosphine, phosphite,a nd N-heterocyclic carbene ligands are easily synthesized by reaction of trimethylsilyldiazomethane with the corresponding gold chloride precursors.Activation of these gold(I) carbenoids with av ariety of chloride scavengers promotes reactivity typical of metallocarbenes in solution, namely homocoupling to ethylene,o lefin cyclopropanation, and Buchner ring expansion of benzene.Carbene complexes of transition metals are reactive intermediates [1] frequently invoked in awide variety of C À Cbondforming processes that range from industrial scale olefin metathesis [2] to natural product synthesis. [3] Despite their central role in gold catalysis, [4] few non-heteroatom-stabilized gold(I) carbene complexes have been structurally characterized.[5] Ther educed steric shielding provided by commonly used ligands and the intrinsically high electrophilicity [6] exhibited by simple gold(I) carbenes preclude their isolation in condensed phase.Such species are however of high interest as demonstrated by gas-phase studies by Chen, [7] Schwarz, [8] and others. [9] Theisolation of this type of compounds,ortheir functional equivalents,i st herefore of great importance for the fundamental understanding of the reactivity of electrophilic gold carbenes.T ransition-metal complexes of the type [M(CHXR)],formally defined as carbenoids, [10] show similar reactivity to their carbene counterparts [11] and offer an attractive alternative to otherwise non-isolable species.T o date,h owever, very little is known about gold(I) carbenoids and their reactivity.[12] Herein we report the synthesis of easily accessible gold carbenoids bearing bulky ligands as gold carbene equivalents in solution.Previous synthesis of chloromethylgold(I) complexes used either toxic and potentially explosive diazomethane [12a] or low-temperature in situ formation of Mg(CH 2 Cl)Cl.[12d] We have developed amore convenient approach that utilizes the methanol promoted decomposition of trimethylsilyldiazomethane [13,14] at room temperature (Scheme 1). Tr eatment of phosphine,phosphite,and NHC gold chloride complexes with trimethylsilyldiazomethane in benzene solution in presence of methanol afforded gold carbenoids 1a-d within minutes. Complexes 1a-c could be easily purified by column chromatography and have been fully characterized. Remarkably, these gold(I) carbenoids can be stored indefinitely when protected from air,a nd only decompose slowly when left under ambient conditions. Measured AuÀCd istances (Table 1) are close to 2.088-(9) observed for the previously reported [(PPh 3 )AuCH 2 Cl] complex (2).[12d] On the other hand, CÀCl distances (1.828-1.830 )are markedly longer than found in 2 (1.68(1) ), and Scheme 1. Synthesis of chloromethylgold(I) complexes 1a-d and structures of complexesa )1a,b)1b,c)and 1c.[38] ORTEP plots with ellipsoids set at 50 %p robability;h ydrogen atoms and solvent molecules are omitted for clarity.

Cyclic (Alkyl)- and (Aryl)-(amino)carbene Coinage Metal Complexes and Their Applications

“…To probe the a-hydride abstraction, the gold(I)-neopentyl complex 19 was reacted with Ph3C + BF4 -, and the two diastereomeric gold(I)-(2-methylbut-2-ene) complexes 21 were isolated, although the reaction was drastically slower than in the case of 17. All attempts to characterize the putative carbene intermediate 20 failed, which is not surprising since, with rare exceptions, 44,45,46,47,48,49,50 8 heteroatom substituted carbene gold complexes are very unstable. However, when triphenylcarbenium tetrafluoroborate was reacted with the gold(I) dithianyl complex 22, the expected cationic bis(carbene) gold(I) complex 23 was obtained, demonstrating the a-hydride abstraction process.…”

Gold-Catalyzed Synthesis of Small Rings

“…These carbenoids, upon activation with TMSOTf or AgOTf, display the typical reactivity patterns of metal carbenes: cyclopropanation of alkenes, dimerization, and Buchner reaction ( Scheme 111 A). 637 …”

Migratory Insertion of Carbenes into Au(III)–C Bonds

“…We were particularly intrigued at the outset of this venture, by the complete absence of precedent for migratory insertion of carbenes into Au–C bonds, despite numerous reports of spectroscopically observable or even isolable gold alkylidenes, − insertions of electrophilic species such as SO 2 into Au–C bonds, ,, formal carbene insertions into Au–Cl bonds, and Au-catalyzed generation of carbenes from diazoalkanes and subsequent carbene transfer reactions. , Concurrently with this work, an independent investigation in our laboratory into alkyl–CF 3 reductive elimination serendipitously uncovered an example that was proposed to involve migratory insertion of difluorocarbene into Au–C bonds . With only this example known to date, migratory insertion of carbenes into Au–C bonds remains virtually unexplored.…”