Atom transfer radical catalyst

Block copolymers were synthesized by a combination of fipase-catalyzed polymerization and atom transfer radical polymerization (ATRE). " " At first, the polymerization of 10-hydroxydecanoic acid was carried out by using lipase CA as catalyst. The terminal hydroxy group was modified by the reaction with a-bromopropionyl bromide, followed by ATRP of styrene using CuCE2,2 -bipyridine as catalyst system to give the polyester-polystyrene block copolymer. Trichloromethyl-terminated poly(e-CL), which was synthesized by lipase CA-catalyzed polymerization with 2,2,2-trichloroethanol initiator, was used as initiator for ATRP of styrene. 

Abstract Over the past decade significant advances have been made in the fields of polymerisation, oligomerisation and telomerisation with metal-NHC catalysts. Complexes from across the transition series, as well as lanthanide examples, have been employed as catalysts for these reactions. Recent developments in the use of metal-NHC complexes in a-olefin polymerisation and oligomerisation, CO/olefm copolymerisation, atom-transfer radical polymerisation (ATRP) and diene telomerisation are discnssed in subsequent sections. 

There are a number of reports of NHC complexes of mid-late transition metals being used as catalysts for atom transfer radical polymerisation (ATRP) of acrylates and styrene. Grubbs reported Fe(II) complexes of a simple monodentate carbene. 

Novel catalytic systems, initially used for atom transfer radical additions in organic chemistry, have been employed in polymer science and referred to as atom transfer radical polymerization, ATRP [62-65]. Among the different systems developed, two have been widely used. The first involves the use of ruthenium catalysts [e.g. RuCl2(PPh3)2] in the presence of CC14 as the initiator and aluminum alkoxides as the activators. The second employs the catalytic system CuX/bpy (X = halogen) in the presence of alkyl halides as the initiators. Bpy is a 4,4/-dialkyl-substituted bipyridine, which acts as the catalyst s ligand. 

Acrylate monomers may also be polymerized by atom transfer radical polymerization (ATRP). The reader is referred to Section 9.1.3.3 for an overview of catalyst systems. 

Other catalytic reactions involving a transition-metal allenylidene complex, as catalyst precursor or intermediate, include (1) the dehydrogenative dimerization of tributyltin hydride [116], (2) the controlled atom-transfer radical polymerization of vinyl monomers [144], (3) the selective transetherification of linear and cyclic vinyl ethers under non acidic conditions [353], (4) the cycloisomerization of (V2V-dia-llyltosylamide into 3-methyl-4-methylene-(V-tosylpyrrolidine [354, 355], and (5) the reduction of protons from HBF4 into dihydrogen [238]. 

Zhang H, Abeln CH, Fijten MWM, Schubert US (2006) High-throughput experimentation applied to atom-transfer radical polymerization automated optimization of the copper catalysts removal from polymers. e-Polymers... 

Indenylidene compounds VIII, K, XXI, XXIII, XXVIIIa and XXVIIIb act as atom transfer radical polymerization catalysts for the polymerization of methyl methacrylate and styrene in high yields and with good control (Table 8.7). The catalytic activity can be dramatically improved by transforming the complexes into cationic species by treatment with AgBp4 [61]. 

Another application of ruthenium indenylidene complexes was the atom transfer radical addition of carbon tetrachloride to vinyl monomers reported by Verpoort [61]. This Kharasch reaction afforded good yields for all substrates tested, especially with the catalyst VIII (Equation 8.11, Table 8.8). 

In an extension of atom-transfer radical reactions to heterocyclic systems, Byers has introduced a novel methodology for the addition of electron-deficient radicals to unprotected pyrroles and indoles in a stannane-fi ee, non-oxidative process <99TL2677>. For exanqrle, photochemical reaction of pyrrole (33) with etl l iodoacetate (34) in presence of thiosulfiite as an iodine reductant, phase transfer catalyst and propylene oxide led to high yields of the 2-alkylated pyrrole 35 <99TL2677>. 

Atom transfer radical polymerization (ATRP) [52-55]. Active species are produced by a reversible redox reaction, catalyzed by a transition metal/ligand complex (Mtn-Y/Lx). This catalyst is oxidized via the halogen atom transfer from the dormant species (Pn-X) to form an active species (Pn ) and the complex at a higher oxidation state (X-Mtn+1-Y/Lx). 

Kleij, A.W., Gossage, R.A., Gebbink, R.J.M.K., Brinkmann, N., Reijerse, E.J., Kragl, U., Lutz, M., Spek, A.L. and van Koten, G. (2000) A dendritic effect in homogeneous catalysis with carbosilane-supported arylnickel(II) catalysts observation of active-site proximity effects in atom-transfer radical addition. J. Am. Chem. Soc., 122, 12, 112. 

Triphenylamine derivatives are known to be efficient hole transport materials and are widely used in organic light-emitting devices. Thelakkat et al. reported the synthesis of a 2,2-bipyridine ligand capped with polyfvinyl-triphenylamine) at both ends.97 The polymer chain was synthesized by the atom transfer radical polymerization of 4-bromostyrene using 4,4-bis (chloromethyl)bipyridine as the initiator (Scheme 18). The bromide groups were then replaced by diphenylamine in the presence of palladium catalyst. Polymer 33 was then obtained by the metalation reaction. 

Brace reported atom transfer radical addition/cyclization sequences of 1,6-dienes 50 with CC14 using 1 mol% FeCl3 as the catalyst and benzoin as the reducing... 

Nagashima and colleagues showed subsequently that 5-10 mol% of Pd(PPh3)4 catalyzed atom transfer radical cyclizations (ATRC) of /V-allyl-difluoroiodo-acetamides 158b in fluorescent lab light at ambient temperature [194], Under these conditions, 34—98% yield of cyclized compounds 159b were obtained. No reaction occurred in the dark, while it was considerably slower in the absence of the catalyst. The low yield of 34% in one example (R=Bn) is due to the unfavorable rotational barrier in the substrate, which cannot be influenced by the presence of the catalyst. 

Evidence based on product mixtures now suggests, at least in the cases of a-halocarbonyl and perhaloalkyl starting marterials, that these reactions are in fact atom transfer radical cyclizations (equation 166)324,325. In them, the palladium catalyst is proposed to have roles both as the radical initiator and as a trap for iodine, similar to the more commonly used hexabutylditin. Intramolecular allyl halide-alkyne cyclizations proceed with trans-addition to the triple bond this is evidence that a still different mechanism may be operating in these cases (equation 167)1,326. 

Louie and Grubbs prepared an iron-based catalyst for atom transfer radical polymerization (ATRP) [49]. By heating a solution of Iz Prim and FeX2 (X = Br, Cl), crystals of Fe(Iz Prim)2X2 were obtained. These complexes mediated the homogeneous ATRP of styrene and methyl methacrylate with... 

The general mechanism in atom transfer radical polymerisation is depicted in Scheme 8.11. The main difference to conventional radical polymerisation is in the presence of a metal complex. Free radicals are generated from reaction between the initiator (such as an organic halide) and the metal species which further controls the reaction by reversibly transforming the free radicals into a dormant species.1"6 However, it ought to be pointed out that in ATRP contrary to, for example, Ziegler-Natta-type catalysts, the polymerisation does not take place at the metal centre. 

Cu(l) and Fe(ll) complexes prepared in situ by reacting copper(l) or iron(ll) chloride with 1 equiv of ligand LI (tris(pyridin-2-ylmethyl)amine) or L2 are efficient catalysts for atom-transfer radical addition reactions. For instance, pent-4-enyl trichloroacetate was converted into 3,3,5-trichlorooxocan-2-one in 90% and 99% yield, respectively, when CuCl-Ll and CuCl-L2 were used as catalysts (Scheme 30) <2000J(P1)575>. 

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