Heck reactions in water

Microwave-heated Heck reactions in water have also been performed using phase-transfer catalysis. A number of phase-transfer systems were investigated with TBAB, once more giving the best results51 (Scheme 2.18). 

Scheme 2.18 Heck reaction in water employing phase-transfer catalysts.

Microwave-heated Heck reactions in water using ultralow palladium catalyst concentrations have been performed by Arvela and Leadbeater [46]. Different catalyst concentrations were investigated using a commercially available 1000 ppm palladium solution as the catalyst source. Impressively, useful Heck arylations were performed with palladium concentrations as low as 500 ppb (Scheme 8). 

A later report demonstrated similar chemistry under milder conditions. The apparently reduced effectiveness of the PTA in the previous work was noted, as was a further report where Pd/MjCOj/PTA had been demonstrated to catalyze the Heck reaction in water in excellent yield under mild conditions. This chemistry was therefore adapted to the solid phase. After tethering 4-iodobenzoic acid to TentaGel resin, the reaction with ethyl acrylate was examined and found to be successful with the conditions shown in Scheme 2. Initial attempts to run the reaction in neat water failed to convert starting material to product in much more than about 50% yield, but introduction of a DMF-water solvent mixture solved this problem. The chemistry was adapted for the coupling of a number of olefins (generally those with attached electron-withdrawing groups). In contrast to the previous report, where these reactions were shown with reversal of polarity (i.e., the reaction of solution-phase iodides and bromides with resin-bound 4-vinylbenzoic acid), no products were obtained in these reversed cases. 

Microwave-promoted Heck reactions in water using ultra-low concentrations of palladium catalyst have also been performed. Different catalyst concentrations... 

Polymer-supported carbine-palladium complexes were used for the Mizoroki-Heck reaction in water [152]. The product could be separated by extraction with diethyl ether. However, the catalytic activity decreased after reuse of the catalyst. The authors claim that... 

Hagiwara, H., Sugawara, Y, Hoshi, T. and Suzuki, T. (2005) Sustainable Mizoroki-Heck reaction in water remarkably high activity of Pd(OAc)2 immobilized on reversed phase silica gel with the aid of an ionic liquid. Chem. Commun., 23, 2942-4. 

Botella, L. and Najera, C. (2004) Controlled mono and double Heck reactions in water catalyzed by an oxime-derived palladacycle. Tetrahedron Lett., 45, 1833-6. 

Uozumi, Y. and Kimura, T. (2002) Heck reaction in water with amphiphihc resin-supported palladium-phosphine complexes. Synlett, 2045-8. 

The Heck reaction (also called the Mizoroki-Heck reaction) is the chemical reaction of an unsaturated halide with an alkene in the presence of a base and a palladium catalyst (or palladium nanomateiial-based catalyst) to form a substituted alkene. An efficient and simple protocol for phosphine-free Heck reactions in water in the presence of a Pd(L-proline)2 complex as the catalyst under controlled micro-wave irradiation conditions is versatile and provides excellent yields of products in short reaction times (Scheme 8.17) [20], The reaction system minimizes costs, operational hazards, and envirorunental pollution. 

Imidazolium-styrene copolymers were prepared by copolymerization of 1-vi-nyl-3-butylimidazolinm-based ionic liquids ([VBImJX, X=C1 , BF ", and PFg) with styrene, which were used as polymeric supports to immobilize Pd(OAc)2 using a method of alcohol reduction [22]. It was demonstrated that Pd existed in the form of Pd nanoparticles (NPs) on these imidazolium-styrene copolymers. Using the [VBIm]Cl-styiene copolymer as a support, Pd NPs of less than 6 nm were formed, which was particularly interesting, as usually only a Pd carbene complex was formed when Pd(OAc)2 was treated with 1,3-dialkyimidazolium ionic liquids containing a halide anion. The copolymer-supported Pd catalysts were found to be efficient and reusable catalysts for the Heck reaction in water in the absence of a phosphine ligand and phase-transfer catalyst. 

Allam BK, Singh KN (2011) An elEcient phosphine-free heck reaction in water using Pd(L-Proline)2 as the catalyst under microwave irradiation. Synthesis 2011(7) 1125-1131... 

QiaoK, SugimuraR, BaoQ, TomidaD, YokoyamaC(2008)An efficient Heck reaction in water catalyzed by palladium nanoparticles immobilized on imidazolium-styrene copolymers. Catal Commun 9(15) 2470-2474... 

Gaikwad, D. S., and Pore, D. M. (2012). Palladium-nanoparticle-catalyzed Matsuda-heck reaction in water. Synlett, 23, 2631-2634. 

Other polymeric catalysts have also been investigated. Examples are seen in the supported oxime-based Hgands 54 [151d] and 55 [151e]. Both are active for the Heck reaction in water. The latter was shown to be efficient for the coupling of heterocychc aryl bromides with tert-butyl acrylate or styrene under thermal or MW heating [15 le]. 

A fluorous nano-paUadium catalyst based on pyrrolidine imide 22 was developed for the Suzuki coupling reaction (Scheme 7.12) [21]. The catalyst 22 promoted the coupling of aryl boronic acid and aryl halide with high yield, and the catalyst was recovered by fluorous liquid-liquid extraction and reused for three times. This fluorous nano-paUadium catalyst 22 was also employed in the Heck reaction in water [22]. The catalyst was recovered by fluorous liquid-liquid phase separation and reused for four times with Utde loss of catalytic activity. 

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