Micellar promoted alkenes isomerization in water mediated by a cationic half-sandwich Ru(II) complex

Highlights

• A Ru complex catalyzes the double bond migration in substituted allylbenzenes.

• High selectivity towards the E isomer is observed.

• The reaction is carried out in water under mild conditions with an ordinary catalyst.

• Surfactants are used to solubilize reactants, products and catalyst.

Abstract

Micellar media in water provide a simple and efficient environment to favor the double bond isomerization of terminal alkenes catalyzed by the cationic half-sandwich complex 1 at 95 °C. The micellar medium favors both catalyst dissolution in water by means of ion-pairing with the preferred anionic surfactants as well as substrate dissolution thus favoring its conversion into products.

Introduction

Recently the stringent constraints required by the environmental legislation in many countries are imposing to industry new paradigms and methods pushing towards drastic modifications in the way in which fine chemicals and pharmaceuticals are produced. We are currently living in a new era where industrial synthesis must comply with the implementation of sustainability criteria [1] based on the almost 20 year old twelve principles of green chemistry [2]. Among others, the use of catalytic rather than stoichiometric methods and the avoidance of organic solvents or their replacement with less hazardous media are often the first steps to convert an old synthetic method into a more sustainable one. In an industrial chemical production the choice of the solvent is not trivial [3] because about half of the total amount of waste is constituted by the solvent [4], [5] and, quite commonly, the simplest way to dispose it is eventually incineration to recover heat. Chlorinated, aromatic and highly polar solvents like DMF and DMSO are usually considered for use only if no alternatives are possible. Water, as the most abundant liquid on earth, is the only one that, together with low cost and high safety, provides almost negligible impact for the environment. The latter aspect is clearly demonstrated by the fact that its E-factor value is considered equal to zero [4]. In all recent solvent selection guides [6] developed by different chemical and pharmaceutical companies, water is always considered as highly desirable [7]. Recent studies emphasized also that reactions in water can benefit from extra features like enhanced selectivity (chemo-, regio-, stereo- and enantio-) if compared to the use of organic media for the same process [8]. Because of its many advantages [9], water is witnessing a sort of renaissance as an alternative reaction medium [10]. The peculiarity of water is often related to the hydrophobic effect that is responsible for most of the increased selectivities performed by reactants both when dissolved in water and under “on water” conditions [11]. On the other hand, the same properties cause also some challenging drawbacks, in primis the generally low solubility of organic substrates and of most metal complex catalysts that in some cases are also inactivated/decomposed by this medium. In order to overcome such limitations, traditional catalysts are modified making use of water soluble ligands bearing polar and/or charged tags [12] to increase their solubility.

A simpler and more straightforward approach consists in the use of surfactants whose role is to cope with reagents and catalysts. The addition of surfactants to water leads to the spontaneous formation (self-assembly) of micelles as supramolecular aggregates that, thanks to their apolar core and polar surface, are able to dissolve apolar substrates and catalysts in their core but also ionic catalysts close to the surface. Micellar catalysis [8](e), [13], [14] has been an alternative approach to traditional catalysis in organic media for years, but because of the recent, more stringent environmental concerns, it is becoming a real alternative gaining pace also in drug productions [15]. Several are in fact the examples of reactions that benefit from the use of micellar media vs. organic media on grounds such as reaction rate, selectivity at all levels, ease of product isolation and recyclability [16].

Micellar catalysis is particularly suited for reactions where soft Lewis acidic metal catalysts interact with soft Lewis basic substrates, in particular reactions involving late transition metals and unsaturated substrates are usually unaffected by the presence of water or by the nature of the surfactant. In this field our research group has obtained very good results using micellar catalysis in water in e.g. alkyne hydration [17] or alkene hydroformylation [18] and epoxidation [14b], or sulfoxidation [14c], mediated by cationic Pt(II) catalysts using economic, traditional, commercially available anionic surfactants like e.g. sodium dodecyl sulfate, observing in some cases possible catalyst recycling.

Half-sandwich complexes of ruthenium bearing aryl or cyclopentadienyl ligands turned out to be interesting catalysts for a series of reactions. Examples span from the allylic substitution reaction [19], to the Diels-Alder cycloaddition reaction with high selectivity towards the exo product [20], to the [2 + 2 + 2] cyclotrimerization of alkynes leading to aromatic compounds [21] and to the cyclopropanation of alkenes with diazoacetate carbene precursors with good selectivity for the formation of the cis isomer [22], just to name a few. As long as intramolecular reactions are concerned, the Ru-catalyzed isomerization of alkenes has been known for decades [23] including some recent examples based on monocationic Ru(II) species bearing 1-phenyl-indenyl and chloride anionic ligands [24].

Recently we successfully investigated the nitrile hydration reaction to amides in micellar media catalyzed by neutral half-sandwich Ru(II) complexes bearing cymene, phosphite and chlorine ligands [25]. Spurred by these positive results related to a good solubilization of both catalyst and substrate in water, we became interested with the double bond migration in terminal alkenes to give the corresponding internal isomers. The reaction has some interesting industrial applications like e.g. the estragole, eugenol, and safrole isomerization into the corresponding internal alkenes [26] used as fragrances, and can be carried out with both homogeneous and heterogeneous catalysts [27].

Herein we report an alternative supramolecular approach to this reaction utilizing readily available and cheap surfactants to promote the terminal alkene isomerization reaction in the presence of 1 (Scheme 1). To the best of our knowledge the use of micellar environments has never been investigated for this reaction.