Simone Manzini

The Activation Mechanism of Ru-Indenylidene Complexes in Olefin Metathesis

Olefin metathesis is a powerful tool for the formation of carbon-carbon double bonds. Several families of well-defined ruthenium (Ru) catalysts have been developed during the past 20 years; however, the reaction mechanism for all such complexes was assumed to be the same. In the present study, the initiation mechanism of Ru-indenylidene complexes was examined and compared with that of benzylidene counterparts. It was discovered that not all indenylidene complexes followed the same mechanism, highlighting the importance of steric and electronic properties of so-called spectator ligands, and that there is no single mechanism for the Ru-based olefin metathesis reaction. The experimental findings are supported quantitatively by DFT calculations.

Simple synthetic routes to ruthenium–indenylidene olefin metathesis catalysts

An efficient synthetic protocol involving reactions between the free carbene and [RuCl(2)(PPh(3))(2)(Ind)] followed by addition of pyridine leads to the isolation of olefin metathesis active [RuCl(2)(L)(Py)(Ind)] (L = SIMes and SIPr) complexes. This novel approach circumvents the use of costly tricyclohexylphosphine.

Facile and efficient KOH-catalysed reduction of esters and tertiary amides

Esters and tertiary amides were efficiently reduced to their corresponding alcohols and amines in high yields under mild and environmentally friendly conditions. The presented KOH-catalysed system involves a simple hydrosilylation procedure that is carried out under solvent-free conditions and does not require the use of inert conditions.

How phenyl makes a difference: mechanistic insights into the ruthenium(ii)-catalysed isomerisation of allylic alcohols

[RuCl(η5-3-phenylindenyl)(PPh3)2] (1) has been shown to be a highly active catalyst for the isomerisation of allylic alcohols to the corresponding ketones. A variety of substrates undergo the transformation, typically with 0.25–0.5 mol% of catalyst at room temperature, outperforming commonly-used complexes such as [RuCl(Cp)(PPh3)2] and [RuCl(η5-indenyl)(PPh3)2]. Mechanistic experiments and density functional theory have been employed to investigate the mechanism and understand the effect of catalyst structure on reactivity. These investigations suggest a oxo-π-allyl mechanism is in operation, avoiding intermediate ruthenium hydride complexes and leading to a characteristic 1,3-deuterium shift. Important mechanistic insights from DFT and experiments also allowed for the design of a protocol that expands the scope of the transformation to include primary allylic alcohols.

The use of the sterically demanding IPr* and related ligands in catalysis

This account highlights the synthesis and applications of one of the very bulky NHC ligands, IPr* (1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazo-2-ylidene). This ligand and some of its derivatives have been found very effective in several catalytic applications and have enabled the isolation of highly reactive organometallic complexes. More specifically, applications of this ligand in Pd and Ni chemistry have permitted challenging transformations under mild reaction conditions and low catalyst loadings. We report the successes as well as the limitations encountered using transition-metal systems bearing this ligand-type. This report will hopefully serve as a guide to synthetic chemists, providing insights as to when the very sterically demanding IPr* ligand (and its congeners) and in a broader context, very bulky NHC ligands, should be used.

How does the addition of steric hindrance to a typical N-heterocyclic carbene ligand affect catalytic activity in olefin metathesis?

Density functional theory (DFT) calculations were used to predict and rationalize the effect of the modification of the structure of the prototype 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) N-heterocyclic carbene (NHC) ligand. The modification consists in the substitution of the methyl groups of ortho isopropyl substituent with phenyl groups, and here we plan to describe how such significant changes affect the metal environment and therefore the related catalytic behaviour. Bearing in mind that there is a significant structural difference between both ligands in different olefin metathesis reactions, here by means of DFT we characterize where the NHC ligand plays a more active role and where it is a simple spectator, or at least its modification does not significantly change its catalytic role/performance.

Ruthenium catalysed C–H bond borylation

An easily prepared series of phenylindenyldihydridosilyl ruthenium complexes () was obtained by reaction of tertiary silanes with the commercially-available [RuCl(3-phenylindenyl)(PPh3)2] (). The [RuH2(3-phenylindenyl)(SiEt3)] () complex was shown to be highly efficient (1.5 mol%) in the ortho-selective borylation of pyridyl substrates, with yields of up to 90%. A novel ruthenium(iv)-catalysed C-H activation borylation/functionalization reaction using a remarkably low catalyst loadings is described.

Insights into the Decomposition of Olefin Metathesis Precatalysts

The decomposition of a series of benzylidene, methylidene, and 3-phenylindenylidene complexes has been probed in alcohol solution in the presence of base. Tricyclohexylphosphane-containing precatalysts are shown to yield [RuCl(H)(H2)(PCy3)2] in isopropyl alcohol solutions, while 3-phenylindenylidene complexes lead to η(5)-(3-phenyl)indenyl products. The potential-energy surfaces for the formation of the latter species have been probed using density functional theory studies.

From a Decomposition Product to an Efficient and Versatile Catalyst: The [Ru(η5-indenyl)(PPh3)2Cl] Story

Conspectus One of the most important challenges in catalyst design is the synthesis of stable promoters without compromising their activity. For this reason, it is important to understand the factors leading to decomposition of such catalysts, especially if side-products negatively affect the activity and selectivity of the starting complex. In this context, the understanding of termination and decomposition processes in olefin metathesis is receiving significant attention from the scientific community. For example, the decomposition of ruthenium olefin metathesis precatalysts in alcohol solutions can occur during either the catalyst synthesis or the metathesis process, and such decomposition has been found to be common for Grubbs-type precatalysts. These decomposition products are usually hydridocarbonyl complexes, which are well-known to be active in several transformations such as hydrogenation, terminal alkene isomerization, and C–H activation chemistry. The reactivity of these side products can be unwanted, and it is therefore important to understand how to avoid them and maybe also important to keep an open mind and think of ways to use these in other catalytic reactions. A showcase of these decomposition studies is reported in this Account. These reports analyze the stability of ruthenium phenylindenylidene complexes, highly active olefin metathesis precatalysts, in basic alcohol solutions. Several different decomposition processes can occur under these conditions depending on the starting complex and the alcohol used. These indenylidene-bearing metathesis complexes display a completely different behavior compared with that of other metathesis precatalysts and show an alternative competitive alcoholysis pathway, where rather than forming the expected hydrido carbonyl complexes, the indenylidene fragment is transformed into a η1-indenyl, which then rearranges to its η5-indenyl form. In particular, [RuCl(η5-(3-phenylindenylidene)(PPh3)2] has been found to be extremely active in numerous transformations (at least 20) as well as compatible with a broad range of reaction conditions, rendering it a versatile catalytic tool. It should be stated that the η5-phenyl indenyl ligand shows enhanced catalytic activity over related half-sandwich ruthenium complexes. The analogous half-sandwich (cyclopentadienyl and indenyl) ruthenium complexes show lower activity in transfer hydrogenation and allylic alcohol isomerization reactions. In addition, this catalyst allows access to new phenylindenyl ruthenium complexes, which can be achieved in a very straightforward manner and have been successfully used in catalysis. This Account provides an overview of how mechanistic insights into decomposition and stability of a well-known family of ruthenium metathesis precatalysts has resulted in a series of novel and versatile ruthenium complexes with unexpected reactivity.

A Highly Active Cationic Ruthenium Complex for Alkene Isomerisation: A Catalyst for the Synthesis of High Value Molecules

A novel cationic ruthenium complex is shown to be efficient in the isomerization of important feedstocks derived from essential oils, which can then be functionalized through olefin metathesis.