Ruthenabenzene: A Robust Precatalyst

Metallaaromatics constitute a unique
class of aromatic compounds
where one or more transition metal elements are incorporated into
the aromatic system, the parent of which is metallabenzene. One of
the main concerns about metallabenzenes generally deals with the structural
characterization related to their relative aromaticity compared to
the carbon archetype. Transition metal-containing metallabenzenes
are also implicated in certain catalytic processes such as alkyne
metathesis polymerization; however, these transition metal-based metallaaromatic
compounds have not been developed as a catalyst. Herein, we describe
an effective strategy to generate diverse arrays of ruthenabenzenes
and demonstrated them as an aromatic equivalent of the Grubbs-type
ruthenium alkylidene catalysts. These ruthenabenzenes can be prepared
via an enyne metathesis and metallotropic [1,3]-shift cascade process
to form alkyne-chelated ruthenium alkylidene intermediates followed
by spontaneous cycloaromatization. The aromatic nature of these complexes
was confirmed by spectroscopic and X-ray crystallographic data, and
the mechanistic pathways for the cycloaromatization process were studied
by DFT calculations. These ruthenabenzenes display robust catalytic
activity for metathesis and other transformations, which illustrates
that metallabenzenes are not only compounds of structural and theoretical
interests but also are a novel platform for new catalyst development.