Ruthenium complexes atom transfer radical

Ruthenium(II)-NHC systems ean be used for atom transfer radical polymerization (ATRP). Generally, similar results as for the analogous phosphine complexes are obtained. For the ATRP of styrene and methyl methacrylate (MMA) [(NHC)2peBr2] was found to rival copper(I)-based systems and to yield poly (MMA) with low polydispersities. Polymerizations based on olefin metathesis that are catalyzed by ruthenium-NHC complexes are discussed separately vide supra). 

The ruthenium indenylidene Schiff base complexes XXVIIIa and XXVIIId, synthesized by Verpoort, were evaluated in atom-transfer radical polymerization of methyl methacrylate. The polymerization was initiated by ethyl 2-bromo-2-methyl-... 

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). 

Representative ruthenium complexes active for Kharasch addition and atom transfer radical polymerization (ATRP). 

After having observed that the most active ruthenium-based catalyst systems for olefin metathesis also displayed a high efficiency in atom transfer radical polymerisation, we then became interested in comparing the role of the catalyst in those two different reaction pathways. Ruthenium alkylidene complexes 4-6 are unsaturated 16-electron species which formally allow carbon-halogen bond activation to form a 17-electron ruthenium(III) intermediate. Our preliminary results indicate that polymerisations occur through a pathway in which both tricyclohexylphosphine and/or imidazolin-2-ylidene ligands remain bound to the metal centre. 

Haas, M., Solari, E., Nguyen, Q.T., Gautier, S., Seopelliti, R., Severin, K, A bimetallic ruthenium complex as a catalyst precursor for the atom transfer radical polymerization of methacrylates at ambient temperature, Adv. Synth. Catal. 2006, 348 439-442. 

Simal, F., Demonceau, A., Noels, A.F., Kharasch addition and controlled atom transfer radical polymerisation (ATRP) of vinyl monomers catalysed by Grubbs ruthenium-carbene complexes. Tetrahedron Lett. 1999, 40 5689-5693. 

De Clerq, B., Verpoort, F., A new class of ruthenium complexes containing Schiff base ligands as promising catalysts for atom transfer radical polymerization and ring opening metathesis polymerization, J. Mol. Catal. A Chem. 2002,180 67-76. 

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. 

Atom-transfer radical cyclization (ATRC) is an atom-economical method for the formation of cyclic compounds, which proceeds under mild conditions and exhibits broad functional group tolerance. Okamura and Onitsuka described a planar-chiral Cp-Ru complex 124-catalyzed asymmetric auto-tandem allylic amidation/ATRC reaction in 2013. This protocol proceeds highly regio, diastereo, and enantioselec-tively to construct optically active y-lactams from readily available substrates in a one-pot manner (Scheme 2.32). In this process, a characteristic redox property of ruthenium complexes would work expediently in different types of catalyzes involving mechanistically distinct allylic substitutions (Ru /Ru ) and atom-transfer radical cyclizations (Ru /Ru ), thus leading to the present asymmetric auto-tandem reaction [48]. 

Kanbayashi, N., Takenaka, K., Okamura, T., Onitsuka, K. (2013). Asymmetric auto-tandem catalysis with a planar-chiral ruthenium complex sequential allyhc amida-tion and atom-transfer radical cychzation. Angewandte Chemie International Edition, 52, 4897-4901. 

This researeh was extended to include non-symmetrical amidinate complexes of the same type with mixed Cp and Cp ligands, such as the cationic species [Cp Ru 2-PrN=C(Me)NPr )RuCp(L)] (138 L = none, NCMe, CNBu , PMc3 BF4 salts). For 138 (L = CNBu ), there is NMR evidence that the isonitrile ligand switches between the ruthenium centers. Coordinatively unsaturated intermediates play a key role in catalytic processes, and thus complexes 138 (L = none PFe salt) and 135 (PFe salt) were found to catalyze the atom-transfer radical cyclization of A7-allyl trichloroacetamides. It was discovered that the greater protection offered by the steric bulk of the Cp" ligand led to the superior performance of 135 (PF salt). ... 

The iron species [Fe(X)2 CN(PP)CH(Me) = CH(Me)N(Pr ) ] (X = Cl, Br), containing highly donating imidazolyidene ligands, have been found to be extremely active and efficient catalysts for the atom transfer radical polymerisation of styrene and methylmethacrylate. A variety of indenyl ruthenium complexes containing either phenylacetylide (C = CPh) or vinyl (CH = CHPh) ligands have been found to catalyse the dimerisation of phenylacetylene to ( )-and (Z)-l,4-diphenyl-l-en-3-yne with the activity of the catalyst dependent upon the nature of the phosphine co-ligand bound to ruthenium. The vinylidene-ruthenium(II) complexes [Ru(Cl)(L)2(C = CHR)] (R = Bu, ferrocenyl L =... 

Ruthenium complexes are capable of catalyzing halogen atom transfer reactions to olefins. This has been illustrated in the enantioselective atom transfer reactions of alkane and arene-sulfonyl chlorides and bro-motrichloromethanes to olefins using chiral ruthenium complexes. Moderate ee s up to 40% can be achieved for these transformations [74-77]. These specific reactions are believed to follow a radical redox transfer chain process. 

The initial ruthenium(II) catalyst 66 abstracts a halogen (either chlorine or bromine) from the substrate forming a ruthenium(III) species 67. This is followed by pi complexation (68), radical addition (69) and halogen atom transfer to form the desired product (70). Starting from 65a, enantioselectivities of the resulting product 70a ranged from 20 to 40% ee with excellent chemical yields [28]. Reactions with a slightly different substrate bromotrichloromethane (65b) provided 70b in 32% ee, and a poor yield of 26% [29]. 

---