Marco Cavazzini

Copper-catalyzed aerobic oxidation of alcohols under fluorous biphasic conditions

A catalytic amount of perfluoroalkyl substituted bipyridine 1 (2 mol%), CuBr·Me2S (2 mol%) and TEMPO (3.5 mol%) allow the oxidation of various alcohols to aldehydes and ketones in a fluorous biphasic system of chlorobenzene and perfluorooctane directly with oxygen. The catalyst can be used for several reaction runs without a loss of reactivity.

PERFLUOROCARBON-SOLUBLE CATALYSTS AND REAGENTS AND THE APPLICATION OF FBS (FLUOROUS BIPHASE SYSTEM) TO ORGANIC SYNTHESIS

Abstract The new phase-separation and immobilization technique known as FBS (fluorous biphase system) is becoming more and more popular among researchers in both industry and academia. The immiscibility of perfluorocarbons with most organic solvents and water is of great advantage, in that it allows the easy recovery and recycling of the catalysts and reagents. The major results so far obtained in this field are discussed.

Metal Complexes of a Tetraazacyclotetradecane Bearing Highly Fluorinated Tails: New Catalysts for the Oxidation of Hydrocarbons under Fluorous Biphasic Conditions

Abstract The commercially available macrocycle tetraazacyclotetradecane (cyclam) has been converted into the fluorocarbon soluble ligand 1 by N-functionalization with RFCH2OCH2CH2OTs 3, wherein RF is a (per)fluorooxyalkylenic chain. Upon complexation with 1, transition metal salts can be selectively solubilized in fluorocarbons thus providing new catalysts for the oxidative functionalization of hydrocarbons under fluorous biphasic (FB) conditions.

Hydrolytic kinetic resolution of terminal epoxides catalyzed by fluorous chiral Co(salen) complexes

Abstract Cobalt complexes of fluorous chiral salen ligands have been synthesized and tested as catalysts in the hydrolytic kinetic resolution of terminal epoxides. Whereas the activity of heavily fluorinated complexes was found to be rather low, a ‘light fluorous’ complex was shown to be an efficient and highly selective catalyst for this asymmetric ring-opening reaction. Several strategies for the isolation of reaction products and the recovery of the fluorous catalyst are also discussed.

Synthesis of perfluoroalkylated bipyridines — New ligands for oxidation reactions under fluorous triphasic conditions

Abstract Fluorous soluble bipyridines bearing two perfluoroalkylated side chains in the 6,6′- or 4,4′-positions have been prepared in good yields via etherification of 6,6′-bis(chloromethyl)-2,2′-bipyridine or C -alkylation of 6,6′-dimethyl-2,2′-bipyridine. The new ligands L exhibit amphiphilic behaviour with respect to certain fluorous-organic biphasic systems. Nevertheless, their ruthenium complexes (RuL n )X generated in situ are efficient catalysts for the epoxidation of trans -stilbene in a fluorous triphasic system CH 2 Cl 2 /H 2 O/C 8 F 18 in the presence of NaIO 4 . The fluorous phase, where (RuL n )X is trapped, can be used up to four times without major loss of catalytic activity.

Synthesis, characterization, absorption spectra, and luminescence properties of multinuclear species made of Ru(II) and Ir(III) chromophores.

A series of new mixed-metal Ru(II)-Ir(III) trinuclear complexes have been prepared and characterized, together with their mononuclear parents and a series of closely related dinuclear and trinuclear homometallic Ru(II) and Ir(III) species, and their absorption spectra and luminescence properties (both at 77 K in rigid matrix and at room temperature in fluid solution) have been studied. The absorption spectra and luminescence properties of the Ru(II) species and subunits are dominated by metal-to-ligand charge-transfer (MLCT) transitions and excited states, whereas ligand centered (LC) transitions and excited states govern the spectroscopic and photophysical properties of most of the Ir(III) species here studied, with MLCT states playing an important role when cyclometalated Ir(III) subunits are present. Each metal-based subunit retains in the multinuclear arrays its own spectroscopic properties, but in the case of the mixed Ru-Ir species an efficient, additional decay channel is opened for the excited states involving the Ir-centered subunits, that is, photoinduced energy transfer to the lower-lying MLCT state(s) involving the Ru centers.

Asymmetric Epoxidation of Alkenes in Fluorinated Media, Catalyzed by Second-Generation Fluorous Chiral (Salen)manganese Complexes

The synthesis of sterically hindered chiral (salen)manganese complexes bearing perfluoroalkyl ponytails and their use in asymmetric epoxidation reactions are described. For better understanding of the relative influences of steric and electronic effects on the enantioselectivity of the fluorous catalysts, the epoxidation of 1,2-dihydronaphthalene and benzosuberene was first studied under homogeneous conditions. It was shown that the presence of sterically demanding tert-butyl groups and, to a lesser degree, the displacement of the electron-withdrawing perfluoroalkyl substituents from the ligand core provide ees higher than those attainable with first generation fluorous chiral (salen)manganese complexes featuring perfluoroalkyl substituents in the key positions (3,3′ and 5,5′) in the ligand. Second generation catalysts (Mn-6)C7F15COO and (Mn-7)C7F15COO were successfully employed in the fluorous biphase epoxidation of alkenes with PhIO as the oxidant and pyridine N-oxide as an additive. Epoxide yields (68−98%) and ees (50−92%) were similar to those obtained with the same oxidizing system and standard (salen)manganese complexes under homogeneous conditions. When the reaction was complete, the fluorous layer in which the catalyst was immobilized was easily recoverable by simple phase separation at room temperature and could be used up to three times before significant decline in yield and enantioselectivity was observed.

Chiral fluorous phosphorus ligands based on the binaphthyl skeleton: synthesis and applications in asymmetric catalysis

Two enantiopure fluorous phosphines have been conveniently synthesized by combining palladium-catalyzed coupling reactions of easily available binaphthyl building-blocks with the introduction of fluorous ponytails onto aromatic compounds via ether bond formation. These new fluorous chiral phosphines have been tested as ligands in metal-catalyzed asymmetric transformations, the best results being obtained in the palladium-catalyzed asymmetric allylic substitution of 1,3-diphenyl-2-propenyl acetate affording the products of up to 87% e.e.

Fluorous biphasic oxidation of sulfides catalysed by (salen)manganese(III) complexes

Abstract Quadridentate Schiff base ligands derived from 1,2-diamines and fluorous derivatives of salicylaldehyde were prepared and the corresponding manganese(III) complexes were tested as catalysts in the selective oxidation of alkyl aryl sulfides with PhIO. Complexes bearing two fluorinated ponytails were soluble in standard organic solvents and were used under classical homogeneous conditions, whereas heavily fluorinated complexes could be used in a CH3CN/perfluorooctane biphasic system. In both cases, sulfoxides were obtained as the main products, together with variable amounts of sulfones (≤10%), depending on the nature of the substrate and the catalyst. When reactions were carried out under fluorous biphasic (FB) conditions, the selectivity for sulfoxides was improved and the catalyst could be easily recovered by simple phase separation and reused up to four times. Despite their good chemoselectivity, catalytic efficiency and recyclability, chiral fluorous (salen)manganese(III) complexes showed low enantioselectivities in preliminary experiments run under fluorous biphasic conditions.

Highly homogeneous, transparent and luminescent SiO2 glassy layers containing a covalently bound tetraazacyclododecane–triacetic acid–Eu(III)–acetophenone complex

The preparation of sol–gel glasses is described wherein highly stable Eu(III) centres are anchored to the silica glass upon acidic hydrolysis and by using tetraethylorthosilicate (TEOS) as a silica source. The complex employed is Eu·1 and is obtained from bipartite ligand 1, which features a DO3A macrocycle as a hosting unit for the Eu(III) centre, a methoxy-acetophenone unit as an antenna chromophore, and an alkyl chain bearing one primary hydroxy group (DO3A is 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid). Eu·1 is linked to the forming silica glass through the –OH groups and a uniform distribution of the luminescent centres is obtained, as indicated by results from secondary ion mass spectrometry in-depth profiling. The luminescent efficiency of the Eu(III) centres in the solid matrix is evaluated to be 10%, which is comparable to that of Eu·1 in water solution and of a convenient reference complex, Eu·2 (2 is a ligand featuring the same hosting and antenna functionalities of 1, wherein the –OH residue is replaced by an unreactive group, a benzoylmethyl residue). A discussion is given of the photophysical properties exhibited by the complexes in the film and in solution.