trans-Mutation at Gold(III): A Mechanistic Study of a Catalytic Acetylene Functionalization via a Double Insertion Pathway

The
Au­(III) complex Au­(tpy)­(OAcF)2 (OAcF = OCOCF3; tpy = 2-(p-tolyl)­pyridine)
catalyzes the anti addition of trifluoroacetic acid
(HOAcF) to acetylene to furnish vinyl trifluoroacetate.
The Au­(III) vinyl complex Au­(tpy)­(OAcF)­(CHCHOAcF) (vinyl group bonded trans to tpy-N) is formed during the early stage of the reaction. The
vinyl complex, which has been isolated and structurally characterized,
resists protolytic cleavage of the vinyl group, and therefore catalysis
does not proceed by a simple formal insertion (i.e., coordination-nucleophilic
attack-protolysis at the site trans to tpy-N) mechanism. Experimental evidence, including isotopic
labeling, rather suggests that a double-insertion process is operative.
The unobserved Au­(III) divinyl complex Au­(tpy)­(CHCHOAcF)2 is a crucial intermediate for which the true
catalytic activity, comprising a coordination-nucleophilic attack-protolysis
sequence, occurs at the site trans to tpy-C. The overall mechanism is in full agreement with DFT calculations
and is a result of the considerable differences in the kinetic and
thermodynamic trans effects of tpy-N versus tpy-C on each reaction step. The computational
data provide a rationale for the catalytic functionalization of acetylene trans to tpy-C, whereas ethylene (previously
reported) only undergoes a stoichiometric insertion, and then comes
to a full stop, trans to tpy-N.

Au(III)配合物Au(tpy)(OAcF)2（OAcF = OCOCF3；tpy = 2-(p-甲氧基苯基)吡啶）催化三氟乙酸（HOAcF）对乙炔的反式加成反应，生成乙烯基三氟乙酸酯。在反应的早期阶段，形成了Au(III)乙烯基配合物Au(tpy)(OAcF)(CH≡CHOAcF)（乙烯基与tpy-N以反式键合）。该乙烯基配合物已被分离并进行了结构表征，它能够抵抗乙烯基的亲核裂解，因此催化过程并非通过简单的形式插入（即，在tpy-N反式位置的配位亲核攻击-亲核裂解）机制进行。包括同位素标记在内的实验证据反而表明，双插入过程是起作用的。未观测到的Au(III)双乙烯基配合物Au(tpy)(CH≡CHOAcF)2是关键的中间体，真正的催化活性，包括配位亲核攻击-亲核裂解序列，发生在tpy-C反式位置。整体机制与DFT计算完全一致，这是由于tpy-N与tpy-C在每个反应步骤上动力学和热力学反效应的显著差异所致。计算数据为乙炔在tpy-C反式位置的催化官能团化提供了理论依据，而乙烯（此前已有报道）仅经历了一种化学计量学插入，然后反应在tpy-N反式位置完全停止。