Carbene Transfer from Triazolylidene Gold Complexes as a Potent Strategy for Inducing High Catalytic Activity

A series
of gold­(I) complexes [AuCl­(trz)] were synthesized that
contain 1,2,3-triazolylidene (trz) ligands with variable wingtip groups.
In the presence of AgBF4, these complexes undergo ligand
redistribution to yield cationic complexes [Au­(trz)2]­BF4 in high yields as a result of efficient carbene transfer.
Identical reactivity patterns were detected for carbene gold complexes
comprised of Arduengo-type IMes ligands (IMes = N,N′-dimesityl-imidazol-2-ylidene). Reaction
of cationic complexes [Au­(trz)2]+ with [AuCl­(trz′)]
afforded the heteroleptic complex [Au­(trz)­(trz′)]+ and [AuCl­(trz)] (trz, trz′ = triazolylidene ligands with
different wingtip groups). Carbene transfer occurs spontaneously,
yet is markeldy rate-enhanced in the presence of Ag+. The
facile carbene transfer was exploited as a catalyst activation process
to form active gold species for the aldol condensation of isocyanides
and aldehydes to form oxazolines. The catalytic activity is strongly
dependent on the presence of Ag+ ions to initiate catalyst
activation. High turnovers (105) and turnover frequencies
(104 h–1) were accomplished. Structural
analysis at early stages of the reaction support the critical role
of triazolylidene dissociation to activate the precatalyst and dynamic
light scattering revealed the presence of nanoparticles (±100
nm diameter) as potential catalytically active species. Furthermore,
the triazolylidene scaffold had no impact on the diastereoselectivity
of the oxazoline formation, and chiral triazolylidenes did not induce
any asymmetry in the product. The facile dissociation of carbenes
from [AuCl­(carbene)] in the presence of Ag+ ions suggests
a less stable Au–Ccarbene interaction than often
assumed, with potential implications for gold-catalyzed reactions
that employ a silver salt as (putative) halide scavenger.

本研究合成了一系列一价金配合物[AuCl(trz)]，其搭载带有可变翼基的1,2,3-三唑卡宾（1,2,3-triazolylidene，简称trz）配体。在四氟硼酸银（AgBF4）存在的条件下，这类配合物会发生配体重分配反应，经由高效的卡宾转移过程，以高产率生成阳离子型配合物[Au(trz)₂]BF₄。对于搭载Arduengo型IMes配体（IMes = N,N′-二均三甲苯基咪唑-2-卡宾）的卡宾金配合物，也观测到了相同的反应活性模式。阳离子配合物[Au(trz)₂]⁺与[AuCl(trz′)]发生反应，可得到异配型配合物[Au(trz)(trz′)]⁺与[AuCl(trz)]（其中trz、trz′为带有不同翼基的三唑卡宾配体）。卡宾转移可自发进行，但在Ag⁺存在时会显著加速反应速率。研究将这种简便的卡宾转移策略用作催化剂活化过程，以制备活性金物种，用于异氰化物与醛的羟醛缩合反应来合成恶唑啉（oxazolines）。该催化活性强烈依赖于Ag⁺离子对催化剂活化的引发作用。研究实现了高达10⁵的周转数与10⁴ h⁻¹的周转频率。反应早期阶段的结构分析证实了三唑卡宾解离对于预催化剂活化的关键作用，而动态光散射实验表明体系中存在直径约±100 nm的纳米颗粒，其可作为潜在的催化活性物种。此外，三唑卡宾骨架对恶唑啉生成反应的非对映选择性无影响，手性三唑卡宾也不会在产物中诱导任何手性。在Ag⁺存在下，[AuCl(卡宾)]配合物中的卡宾可发生简便解离，这表明Au–C_carbene键的相互作用强度比以往普遍认为的更弱，这一发现对于以银盐作为（假定的）卤离子清除剂的金催化反应具有潜在的借鉴意义。