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Mechanistic Origins of Chemo- and Regioselectivity of Ru(II)-Catalyzed Reactions Involving ortho-Alkenylarylacetylene, Alkyne, and Methanol: The Crucial Role of a Chameleon-like Intermediate
journal contribution
posted on 2014-10-03, 00:00 authored by Yanfeng Dang, Shuanglin Qu, Yuan Tao, Chunyu Song, Zhi-Xiang WangM06-DFT
computations have been applied to understand four catalytic systems
which involved [Ru(Cp*)(MeCN)3]PF6 or [Ru(Tp)(PPh3)(MeCN)2]PF6 as mediator and ortho-alkenylarylacetylene, terminal alkyne, and methanol
as reactants. Potentially, the products of these systems could be
dihydrobiphenylenes, 1,3-dienyl ether, and naphthalene. Remarkably,
each system afforded product selectively. Our computed mechanisms
successfully account for the chemo- and regioselectivities of these
systems. Furthermore, the study demonstrates that the chameleon-like
mono(carbene) intermediates formed via the intermolecular alkyne–alkyne
oxidative coupling play a crucial role to complete the reactions.
According to their geometric and electronic structures, three resonance
structures were introduced to characterize their reactivity properties,
which address the features of the classical alkyne–alkyne oxidative
coupling intermediates, mono(carbene) species, and electrophilicity
of the intermediates, respectively. The reactivity properties lead
to three channels isomerizing the intermediates to three isomers.
Surprisingly, the bis(carbene) isomers, which are similar to the bis(carbene)
intermediates generally considered to be crucial in the neutral RuCp*Cl-catalyzed
systems, are accessible but not reactive enough to continue the subsequent
reaction steps partially due to aromaticity. The other two isomers
continue subsequent reaction steps. These findings may help not only
to understand the four specific catalytic reactions but also to advance
the [2 + 2 + 2] synthetic methodology.