Atom transfer radical additions copper

Electron transfer from copper or copper salts to alkyl halides has been used to initiate atom transfer radical additions. One modification of this process involves catalytic amounts of copper powder and fluorinated alkyl iodides the radicals so generated may react in either inter- or intramolecular fashion with alkenes (equation 13)19. Alternatively, a-chloroesters with remote alkene functions undergo cyclization in the presence of cat-... 

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>. 

Copper-homoscorpionate complexes as active catalysts for atom transfer radical addition to olefins... 

Greening of Copper Catalyzed Atom Transfer Radical Addition (ATRA) and Cyclization (ATRC) Reactions... 

A parallel development was initiated by the first publications from Sawamoto and Matyjaszweski. They reported independently on the transition-metal-catalyzed polymerization of various vinyl monomers (14,15). The technique, which was termed atom transfer radical polymerization (ATRP), uses an activated alkyl halide as initiator, and a transition-metal complex in its lower oxidation state as the catalyst. Similar to the nitroxide-mediated polymerization, ATRP is based on the reversible termination of growing radicals. ATRP was developed as an extension of atom transfer radical addition (ATRA), the so-called Kharasch reaction (16). ATRP turned out to be a versatile technique for the controlled polymerization of styrene derivatives, acrylates, methacrylates, etc. Because of the use of activated alkyl halides as initiators, the introduction of functional endgroups in the polymer chain turned out to be easy (17-22). Although many different transition metals have been used in ATRP, by far the most frequently used metal is copper. Nitrogen-based ligands, eg substituted bipyridines (14), alkyl pyridinimine (Schiff s base) (23), and multidentate tertiary alkyl amines (24), are used to solubilize the metal salt and to adjust its redox potential in order to match the requirements for an ATRP catalyst. In conjunction with copper, the most powerful ligand at present is probably tris[2-(dimethylamino)ethyl)]amine (Mee-TREN) (25). 

Munoz-MoHna JM, Belderrain TR, Perez PJ. An efficient, selective, and reducing agent-free copper catalyst for the atom-transfer radical addition of halo compounds to activated olefins. Inorg Chem. 2010 49 642-645. 

ATRP is analogous to atom transfer radical addition reactions which are well known in the field of organic chemistry as Kharasch addition reactions [83]. These methods often utilize a transition metal complex based on copper, iron, ruthenium, and nickel to abstract a halogen and produce a carbon-based radical [84, 85]. Since the first reports in 1995 of living radical polymerizations based on copper(I) for styrene and methyl methacrylate [86] and ruthenium(II) for methyl methacrylate [87], this technique has become widely utilized in polymer science. 

Eckenhoff WT, Pintauer T. Copper catalyzed atom transfer radical addition (ATRA) and cyclization (ATRC) reactions in the presence of reducing agents. Catal. Rev. 2010 52 (l) l-59. 

Abstract During the past decade, atom transfer radical polymerization (ATRP) has had a tremendous impact on the synthesis of macromolecules with well-defined compositions, architectures, and functionalities. Structural features of copper and copper(II) complexes with bidentate, tridentate, tetradentate, and multidentate nitrogen-based ligands commonly utilized in ATRP are reviewed and discussed. Additionally, recent advances in mechanistic understanding of copper-mediated ATRP are outlined. 

The need to better control surface-initiated polymerization recently led to the development of controlled radical polymerization techniques. The trick is to keep the concentration of free radicals low in order to decrease the number of side reactions. This is achieved by introducing a dormant species in equilibrium with the active free radical. Important reactions are the living radical polymerization with 2,2,4,4-methylpiperidine N-oxide (TEMPO) [439], reversible addition fragment chain transfer (RAFT) which utilizes so-called iniferters (a word formed from initiator, chain transfer and terminator) [440], and atom transfer radical polymerization (ATRP) [441-443]. The latter forms radicals by added metal complexes as copper halogenides which exhibit reversible reduction-oxidation processes. 

Controlled/ Living radical polymerization (CRP) of vinyl acetate (VAc) via nitroxide-mediated polymerization (NMP), organocobalt-mediated polymerization, iodine degenerative transfer polymerization (DT), reversible radical addition-fragmentation chain transfer polymerization (RAFT), and atom transfer radical polymerization (ATRP) is summarized and compared with the ATRP of VAc catalyzed by copper halide/2,2 6 ,2 -terpyridine. The new copper catalyst provides the first example of ATRP of VAc with clear mechanism and the facile synthesis of poly(vinyl acetate) and its block copolymers. 

VAc has been successfully polymerized via controlled/ living radical polymerization techniques including nitroxide-mediated polymerization, organometallic-mediated polymerization, iodine-degenerative transfer polymerization, reversible radical addition-fragmentation chain transfer polymerization, and atom transfer radical polymerization. These methods can be used to prepare well-defined various polymer architectures based on PVAc and poly(vinyl alcohol). The copper halide/t is an active ATRP catalyst for VAc, providing a facile synthesis of PVAc and its block copolymers. Further developments of this catalyst will be the improvements of catalytic efficiency and polymerization control. 

In addition, Cui-N-propyl-2-pyridylmethanimine mediated "living" radical polymerization of vinyl monomers by use of l-butyl-3-methylimidazolium hexafluorophosphate as solvent has been reported (Carmichael et al., 2000). It has been pointed out that the rate of polymerization was enhanced in comparison to other polar/coordinating solvents. Moreover, the polymerization product was made copper free by a simple solvent wash, which avoids the contamination of the polymer product by the catalyst. Other atom transfer radical polymerizations in ionic liquids have recently been reported (Sarbu Matyjaszewski, 2001 Biendron Kubisa, 2001). 

Whenever vinyl monomers are electropolymerized, two situations must be distinguished depending on whether the vinyl monomer is the precursor of the initiating species or not. Either the polymerization is directly initiated by the activated monomer or the initiation is indirect whenever the active species (radical, anion, or cation) is generated by a compound other than the monomer (conducting salt, solvent, or properly selected additives). Besides these electroinitiated polymerization processes, a very recent report also demonstrated that electrochemistry is a valuable tool to mediate atom transfer radical polymerization (ATRP). Indeed, an externally applied electrochemical potential can activate the copper catalyst by a one-electron reduction of an irutially added air-stable cupric species (Cu /ligand) allowing... 

Arylation of activated double bonds with diazonium salts in the presence of copper catalysts is known as the Meerwin reaction. The reaction is postulated to either proceed through an organocopper intermediate or through a chlorine atom transfer from chiral CuCl complex to the a-acyl radical intermediate. Brunner and Doyle carried out the addition of mesityldiazonium tetrafluoroborate with methyl acrylate using catalytic amounts of a Cu(I)-bisoxazoline ligand complex and were able to obtain 19.5% ee for the product (data not shown) [79]. Since the mechanism of the Meerwin reaction is unclear, it is difficult to rationalize the low ee s obtained and to plan for further modifications. 

The Meerwein arylation is at least formally related to the atom transfer method because a net introduction of an aromatic ring and a chlorine across a double bond is accomplished (Scheme 62). Facile elimination of HC1 provides an efficient route to the kinds of substituted styrenes that are frequently prepared by Heck arylations. Standard protocol calls for the generation of an arene diazonium chloride in situ, followed by addition of an alkene (often electron deficient because aryl radicals are nucleophilic) and a catalytic quantity of copper(II) chloride. It is usually suggested that the copper salt operates in a catalytic redox cycle, reducing the diazonium salt to the aryl radical as Cu1 and trapping the adduct radical as Cu11. 

An example of conjugate free-radical addition to methyl acrylate mediated by a copper Lewis acid has been reported (Sch. 30) [65]. In this example the Lewis acid 127 activates the substrate for conjugate addition by the aryl radical which is followed by an enantioselective chlorine atom-transfer step. Chemical and optical yield for the transformation are both low. 

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