Nickel-salen

Significant synthetic applications of the nickel-salen catalysts are the formation of cycloalkanes by reduction of <>, -a-dihaloalkanes255,256 and unsaturated halides,257,258 the conversion of benzal chloride (C6H5CHC12) into a variety of dimeric products 259 the synthesis of 1,4-butanediol from 2-bromo- and 2-iodoethanol260 or the reduction of acylhalides to aldehydes261 and carboxylic acids.262... 

The low level of asymmetric induction obtained with nickel(salen) complexes was not synthetically usefiil, but did prove that the concept of using metal(salen) complexes as asymmetric phase-transfer catalysts was feasible. Fortunately, changing... 

At even higher pH values, metal catalysis is required for chlorination to proceed. Thus, at pH =11, nickel(salen) catalyzes the chlorination of adamantane, cyclohexane and toluene785 and manganese porphyrin promotes the chlorination of cyclohexane786. Selective side-chain chlorination by sodium hypochlorite under PTCs at pH = 8.5 has been used for benzylic chlorinations787,788 and for the functionalization of poly(4-methyl-styrene)789,790. Similarly, with calcium hypochlorite in acetic acid, ring chlorination is reported for toluene, xylenes, anisole and other activated aromatics791. 

Other Metal Complexes Apart from metal complexes derived from BINOL, other metal complexes, such as the lithium-aluminum amiuo diol complex, " aluminum and nickel salen complex, ruthenium diamine complex, and ruthenium phosphinite diamine complex were also found applicable for the asymmetric Michael addition of 1,3-dicarbonyl compound to cyclic enone. All these metal complexes afforded about 90% of asymmetric induction in the Michael reaction of 2-cyclohexen-l-one and malonate. 

Martin CS, Dadamos TRL, Teixeira MFS (2012) Development of an electrochemical sensor for determination of dissolved oxygen by nickel-salen polymeric film modified electrode. Sensors Actuators B Chem 175 111-117. doi 10.1016/j.snb.2011.12.098... 

Dadamos TR, Martin CS, Teixeira MFS (2011) Development of nanostmctured electrochemical sensor based on polymer film nickel-salen for determination of dissolved oxygen. Procedia Eng 25 1057-1060. doi 10.1016/j.proeng.2011.12.260... 

The Kumada-Corriu reaction is characterized by mild conditions and clean conversions [2]. A disadvantage of previous Kumada-Corriu reactions was due to the use of homogeneous catalysts, with more difficult product separation. Recently, an unsymmetrical salen-type nickel(II) complex was synthesized with a phenol functionality that allows this compound to be linked to Merrifield resin polymer beads (see original citation in [2]). By this means, heterogeneously catalyzed Kumada-Corriu reactions have become possible. 

Electrogenerated nickel(I)251 and cobalt(I)252 complexes of Salen (Salen = bis(salicylidene)ethane-1,2-diamine) have displayed good catalytic properties in the cleavage of carbon-halogen bonds in a variety of organic compounds. Recent research in this field has been reviewed.253... 

It was first suggested that the reaction of an alkyl halide with a nickel(I) Schiff base complex yields an alkylnickel(III) intermediate (Equation (56)). Homolytic cleavage of RBr to give an alkyl radical R and a nickel(II) complex (Equation (57)) or, alternatively, one-electron dissociative reduction leading to R (Equation (58)) are possible pathways.254 A mechanism based on the formation of R via dissociative electron transfer of Ni -salen to RX (Equation (59)) has also been proposed.255... 

The electrochemistry of cobalt-salen complexes in the presence of alkyl halides has been studied thoroughly.252,263-266 The reaction mechanism is similar to that for the nickel complexes, with the intermediate formation of an alkylcobalt(III) complex. Co -salen reacts with 1,8-diiodo-octane to afford an alkyl-bridged bis[Co" (salen)] complex.267 Electrosynthetic applications of the cobalt-salen catalyst are homo- and heterocoupling reactions with mixtures of alkylchlorides and bromides,268 conversion of benzal chloride to stilbene with the intermediate formation of l,2-dichloro-l,2-diphenylethane,269 reductive coupling of bromoalkanes with an activated alkenes,270 or carboxylation of benzylic and allylic chlorides by C02.271,272 Efficient electroreduc-tive dimerization of benzyl bromide to bibenzyl is catalyzed by the dicobalt complex (15).273 The proposed mechanism involves an intermediate bis[alkylcobalt(III)] complex. 

Extensive work has been carried out on microsensors built from electropolymerized nickel porphyrin films.328,329 Films of Prussian blue (Fe4[Fe(CN)6]3) 345 metal-salen complexes (M = Co, Fe, Cu, Mn)346 or the ferrocene-containing Nin-tetraaza[14] annulene (24),347 also exhibit interesting activity for NO electrooxidation and sensing. 

Although salen complexes of chromium, nickel, iron, ruthenium, cobalt, and manganese ions are known to serve as catalysts for epoxidation of simple olefins, the cationic Mn-salen complex is the most efficient. 

In order to give the usual overview of nickel complexes at increasing coordination numbers we begin with the usual square planar complexes of the Schiff bases salen and saloph.149,150 As an example, Figure 98 shows the molecular structure of [Nin(salen)]. 

Electrogenerated nickel(I) salen has been employed catalytically for the reduction of benzal chloride [152] and for the reductive coupling of 2-bromo- and 2-iodoethanol to prepare 1,4-butanediol [153]. Electrogenerated nickel(I) cyclams have been used as catalysts for the reductive intramolecular cyclizations of o-haloaryl... 

The second group includes nickel salts ligated to a tetradentate ligand like cyclam and related compounds (CR, tet a), or salen. The electroreduction at around -1.6 V vs SCE of these compounds gives the corresponding nickel(I) intermediate which is only further reduced at quite negative potential. We will see that the two classes of nickel compounds display different behavior towards organic halides. 

Ni-salen is a good alternative to nickel-cyclam to generate Ni intermediates. Indeed the cyclisation of 6-phenyl-l-halo-pent-5-yne occurs very efficiently in the presence of 10% of catalyst, as compared to the direct electroreduction of the reagent (Table 7, entry 8) [78]. 

While this manuscript was under preparation, a considerable number of examples of sohd-phase-attached catalysts appeared in the literature which is a clear indication for the dynamic character of this field. These include catalysts based on palladium [131, 132], nickel [133] and rhodium [134] as well applications in hydrogenations including transfer hydrogenations [135, 136] and oxidations [137]. In addition various articles have appeared that are dedicated to immobilized chiral h-gands for asymmetric synthesis such as chiral binol [138], salen [139], and bisoxa-zoline [140] cinchona alkaloid derived [141] complexes. 

Stretching frequencies used to assign the structures. Ni(N03)2 reacts with [Ni(R-salen)2] (R = Et or Pr ) to give trinuclear complexes (165) which contain two pseudo-tetrahedral and one octahedral nickel atoms. ... 

Steric constraints dictate that reactions of organohalides catalysed by square planar nickel complexes cannot involve a cw-dialkyl or diaryl Ni(iii) intermediate. The mechanistic aspects of these reactions have been studied using a macrocyclic tetraaza-ligand [209] while quantitative studies on primary alkyl halides used Ni(n)(salen) as catalyst source [210]. One-electron reduction affords Ni(l)(salen) which is involved in the catalytic cycle. Nickel(l) interacts with alkyl halides by an outer sphere single electron transfer process to give alkyl radicals and Ni(ii). The radicals take part in bimolecular reactions of dimerization and disproportionation, react with added species or react with Ni(t) to form the alkylnickel(n)(salen). Alkanes are also fonned by protolysis of the alkylNi(ii). 

This is the case for secondary and tertiary alkyl bromides. If the stability is high, however, as, for example, with primary alkyl bromides, the organo nickel(III) complex is further reduced to an alkyl nickel(II) complex which loses the alkyl group in form of the alkyl anion. An electroinactive Ni(II) species remains. The number of regenerative cycles is consequently low. The structure of the ligand also influences the lifetime of the alkyl nickel(ni) complex thus, a less stable complex is formed in the case of [A,A -ethylene-bis(salicylidene-irainato)]nickel(II) ([Ni(salen)]) as compared with (5,5,7,12,12,14-hexamethyl-l,4,8,ll-tetraazacyclo-tetradecane)nickel(II) ([Ni(teta)] ), and hence the former complex favors the radical pathway even with primary alkyl halides. 

With primary halides, dimers (R—R) are formed predominantly, while with tertiary halides, the disproportionation products (RH, R(—H)) prevail. Both alkyl nickel(III) complexes, formed by electrochemical reduction of the nickel(II) complex in presence of alkyl halides, are able to undergo insertion reactions with added activated olefins. Thus, Michael adducts are the final products. The Ni(salen)-complex yields the Michael products via the radical pathway regenerating the original Ni(II)-complex and hence the reaction is catalytic. In contrast to that, the Ni(III)-complex formed after insertion of the activated olefin into the alkyl-nickel bond of the [RNi" X(teta)] -complex is relatively stable. Thus, further reduction leads to the Michael products and an electroinactive Ni"(teta)-species. 

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