Department of Chemistry and Centre for Catalysis Research & Innovation, University of Ottawa , Ottawa, Ontario K1N 6N5, Canada.
J Am Chem Soc. 2016 Nov 9;138(44):14668-14677. doi: 10.1021/jacs.6b08372. Epub 2016 Oct 26.
Sterically accessible Lewis donors are shown to accelerate decomposition during catalysis, for a broad range of Grubbs-class metathesis catalysts. These include benzylidene derivatives RuCl(NHC)(PCy)(═CHPh) (Ru-2: NHC = HIMes, a; IMes, b; HIPr, c; IPr, d; HITol, e) and indenylidene complexes RuCl(NHC)(PCy)(═CH) (NHC = HIMes, Ru-2f; IMes, Ru-2g). All of these precatalysts form methylidene complex RuCl(NHC)(═CH) Ru-3 as the active species in metathesis of terminal olefins, and generate RuCl(NHC)(PCy)(═CH) Ru-4 as the catalyst resting state. On treatment with a 10-fold excess of pyridine, Ru-4a and Ru-4b decomposed within minutes in solution at RT, eliminating [MePCy]Cl A by net loss of three ligands (PCy, methylidene, and one chloride), and a mesityl proton. In comparison, loss of A from Ru-4a in the absence of a donor requires up to 3 days at 55 °C. The σ-alkyl intermediate RuCl(CHPCy)(NHC) (py) resulting from nucleophilic attack of free PCy on the methylidene ligand was undetectable for the HIMes system, but was spectroscopically observable for the IMes system. The relevance of this pathway to decomposition of catalysts Ru-2a-g was demonstrated by assessing the impact of pyridine on the in situ-generated methylidene species. Slow initiation (as observed for the indenylidene catalysts) did not protect against methylidene abstraction. Importantly, studies with Ru-4a and Ru-4b indicated that weaker donors (THF, MeCN, DMSO, MeOH, and even HO) likewise promote this pathway, at rates that increase with donor concentration, and severely degrade catalyst productivity in RCM, even for a readily cyclized substrate. In all cases, A was the sole or major P-containing decomposition product. For DMSO, a first-order dependence of decomposition rates on DMSO concentration was established. This behavior sends a warning about the use of phosphine-stabilized metathesis catalysts in donor solvents, or with substrates bearing readily accessible donor sites. Addition of pyridine to RuCl(HIMes)(PCy)(═CHMe) did not result in ethylidene abstraction, indicating that this decomposition pathway can be inhibited by use of substrates in which the olefin bears a β-methyl group.
空间位阻较大的路易斯供体被证明能在催化过程中加速分解，这一现象在广泛的 Grubbs 类复分解催化剂中都存在。这些催化剂包括苯亚甲基衍生物 RuCl(NHC)(PCy)(═CHPh)（Ru-2：NHC = HIMes，a；IMes，b；HIPr，c；IPr，d；HITol，e）和茚基亚甲基配合物 RuCl(NHC)(PCy)(═CH)（NHC = HIMes，Ru-2f；IMes，Ru-2g）。所有这些前催化剂在末端烯烃的复分解中形成亚甲基配合物 RuCl(NHC)(═CH) Ru-3 作为活性物种，并生成 RuCl(NHC)(PCy)(═CH) Ru-4 作为催化剂的静止状态。用 10 倍过量的吡啶处理时，Ru-4a 和 Ru-4b 在室温下几分钟内就在溶液中分解，通过净损失三个配体（PCy、亚甲基和一个氯离子）和一个均三甲苯质子，消除了 A。相比之下，在没有供体的情况下，Ru-4a 中 A 的损失需要在 55°C 下长达 3 天。由游离 PCy 对亚甲基配体进行亲核攻击生成的σ-烷基中间体 RuCl(CHPCy)(NHC)(py) 在 HIMes 体系中无法检测到，但在 IMes 体系中可以通过光谱观察到。吡啶对原位生成的亚甲基物种的影响证明了这种途径与催化剂 Ru-2a-g 分解的相关性。缓慢的引发（如茚基亚甲基催化剂所观察到的）并不能防止亚甲基的抽取。重要的是，Ru-4a 和 Ru-4b 的研究表明，较弱的供体（THF、MeCN、DMSO、MeOH，甚至 HO）也以与供体浓度成正比的速率促进这条途径，并且严重降低了 RCM 中的催化剂产率，即使对于易于环化的底物也是如此。在所有情况下，A 都是唯一或主要的含磷分解产物。对于 DMSO，建立了分解速率对 DMSO 浓度的一级依赖关系。这种行为对在供体溶剂中使用膦稳定的复分解催化剂或使用带有易于获得供体位点的底物发出了警告。向 RuCl(HIMes)(PCy)(═CHMe)中添加吡啶不会导致亚乙基的抽取，这表明通过使用烯烃带有β-甲基的底物可以抑制这种分解途径。
