Copper(II) Triflate as a Source of Triflic Acid: Effective, Green Catalysis of Hydroalkoxylation Reactions

Abstract

Abstract:The hydroalkoxylation of dicyclopentadiene (DCPD) and norbornene (NB) with 2-hy-A C H T U N G T R E N N U N G droxyethyl methacrylate (HEMA) for the synthesis of industrially relevant monomers has been investigated with various metal-based Lewis acids and strong Brønsted acids. In the absence of other additives, copper(II) triflate is the most efficient catalyst system. Kinetics, electron spin resonance (ESR), catalyst poisoning and cross experiments indicate that triflic acid (TfOH) is the true active catalyst in these reactions. This in situ generation of TfOH occurs via reduction of CuA C H T U N G T R E N N U N G (OTf) 2 by the olefin reagent (DCPD, NB). The copper ions present in the reaction mixture act as radical polymerization retardants, preventing polymerization of HEMA (which is observed with most other metal salts and strong Brønsted acids investigated), thus improving the selectivity and yield (up to 95%) for the desired products. These observations have led to the development of a highly effective green process, using bulk reagents (no solvent) and a cheap, metal-free catalyst system, based on TfOH and a phenolic radical inhibitor (2,5-di-tert-butylhydroxytoluene, BHT).

Experimental and Theoretical Studies on the Bismuth‐Triflate‐Catalysed Cycloisomerisation of 1,6,10‐Trienes and Aryl Polyenes

“…In particular, the role of an active proton in the presence of metal triflates in hydroalkoxylation reactions was proposed, in the specific case where the substrate bears a hydroxyl functional group 22. A striking example of TfOH generation from Cu(OTf) 2 was given, but involved a redox process of copper(II) with unavoidable liberation of the acid 23…”

Copper Salts as Additives in Gold(I)‐Catalyzed Reactions

“…We have seen that with the Au/Ag system, the quantitative instant production of catalytically active [Ph 3 PAu][OTf] is rapidly followed by its deactivation, which is caused by the organic substrate in the form of [(Ph 3 P) 2 Au][OTf] and (Au) n . As TfOH could also be formed in appreciable amount under these experimental conditions, [29] it was tested as a catalyst (10 mol %), but only 14 % of the alkyne was converted in this case. With the Au/Cu catalytic mixture, we assume that the concentration of [Ph 3 PAu][OTf] remains very low, actually too low to allow this complex to be detected (Figure 1).…”

Discovery and Comparison of Homogeneous Catalysts in a Standardized HOT‐CAT Screen with Microwave‐Heating and qNMR Analysis: Exploring Catalytic Hydration of Alkynes

“…The system is active in aqueous methanol at low acidity and is thus different from recently published Cu(OTf) 2 ‐based systems, which work in acidic media with a preference for aryl‐alkynes . An earlier Cu(II)‐system of Meier and Marsella in alcoholic solution may well have contained a similar active component formed by reaction of Cu(II) with alcohol …”

“…Some catalyst systems used Cu(OTf) 2 in “dry” media (1–2 equiv. of H 2 O), i. e., under conditions that favor hidden Brønsted acid catalysis and which may not be specific to copper . A less acidic copper catalyst (Cu(OAc) 2 ) used trifluoroacetic acid as reaction medium and was limited to haloalkyne substrates .…”