Sustainable Micellar Gold Catalysis – Poly(2‐oxazolines) as Versatile Amphiphiles

Abstract

The application of five polymer amphiphiles in the gold‐catalyzed allene cycloisomerizations under aqueous micellar conditions is described. The polymers were prepared by ring‐opening cationic polymerization based on poly(2‐methyl‐2‐oxazoline) as hydrophilic segment and different hydrocarbon‐ or fluorocarbon‐based hydrophobic segments. The catalytic activity in the gold‐catalyzed allene cyclization is strongly dependent on the type of gold precursor, the salt concentration in the bulk aqueous medium, and the concentration of the polymeric amphiphile. Best results were obtained with 2 mol% of gold(III) bromide, 1 mM of amphiphile and 5 M sodium chloride, affording over 80% yield for different heterocyclic products. The catalyst system is also suitable for the dehydrative cyclization of acetylenic diols to furans. Moreover, successful catalyst recycling was demonstrated in three consecutive runs when using optimized extraction conditions.magnified image

The Hydrophobic Effect Applied to Organic Synthesis: Recent Synthetic Chemistry “in Water”

“…New fluorinated and non-fluorinated amphiphilic poly(2-oxazolines) (Scheme 34) have been studied asm edia for gold catalysis in water.T he Krause and Weberskirch groups in Dortmund have teamedu pt od evise these surfactants that enable both allenic alcohols and tosylated amines to undergo facile intramolecular cycloisomerizations, while acetylenic diols can be smoothly converted to furans via cyclodehydrations. [65] Critical micellec oncentrations (CMCs) for the fivep olymers fashioned were in the 1-2 10 À6 m range. Best resultsw ere obtained using the poly(2-oxazolines), with x = 20, y = 7, together with alterationo ft he ionic strength of the water by inclusion of NaCl (5 m) [58,66] which led to greatly increased reaction rates.…”

“…New fluorinated and non‐fluorinated amphiphilic poly(2‐oxazolines) (Scheme ) have been studied as media for gold catalysis in water. The Krause and Weberskirch groups in Dortmund have teamed up to devise these surfactants that enable both allenic alcohols and tosylated amines to undergo facile intramolecular cycloisomerizations, while acetylenic diols can be smoothly converted to furans via cyclodehydrations . Critical micelle concentrations (CMCs) for the five polymers fashioned were in the 1–2×10 −6 m range.…”

“…Interesting observations have been notedi ns ome cases of surfactants bearingc harge(s), such as greatly enhanced reaction rates betweenp olar substrates, where, in the presence of cationic or anionic surfactants, the normale xchange process between micellar arrays through water by an eductm ay be slowed duet oe lectrostatic attractions, keeping them in closer proximity to the micelles in which their reactions take place. [34,64,65,69,82] Another interesting observation that wasn ot scriptedi ncludes the lack of solubility of dissolved oxygen within ap articular surfactant-derived nanomicelle, that has perhaps clear implicationsf or the lifetime of an oxygen-sensitive phosphine ligand-chelated metal catalyst. [24] On the other hand, as urfactant hasb een specifically designed to increase molecular oxygen solvation within the core of the micelles for oxidation reactions.…”

Shielding Effect of Micelle for Highly Effective and Selective Monofluorination of Indoles in Water

“…[12] So far,u sage of water as ar eaction medium for organic synthesis and catalysis has not gained as much attention as it deserves, especially in academia. [13] The inherited insolubility and instability of reagents and catalysts in aqueous media is also responsible for as low and sporadicp rogress of aqueous organic or organometallic chemistry.The false perception of generation of more aqueous waste in aqueous chemistry is also responsible.N onetheless,m ore aqueous waste is generated from workup of reactions conducted in organic solvents. [12] Here, we disclose the results of our strategy:ageneral, highly efficient, scalable, mild, greener,a nd sustainable methodf or monofluorination of indoles that can also be applied to monofluorination of arenes with further fine-tuning of arene solubilityw ith optimal temperature.…”

Micelle‐Enabled One‐Pot Guanidine Synthesis in Water Directly from Isothiocyanate using Hypervalent Iodine(III) Reagents under Mild Conditions

“…[1][2][3] Since then many elegant reactions have been reported. [4][5][6][7][8][9][10][11][12] Although on-water reactions offer much benefits, wide-spread adoption has been limited by inability of on-water reaction to be adapted to all traditional chemical transformation reactions due to heterogeneous behaviour.I nr ecent years, new developments in surfactant technology created by Lipshutz [13][14][15][16][17][18] and others [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] have significantly made an improvement for aqueous reactions allowing reaction to occur "in-water" via the formation of micelles,t hereby providingh igh reproducibility and reactivity.…”