Nickel salen Complexes

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... 

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. 

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. 

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. 

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. 

Since both nickel(II) and copper(II)(salen) complexes have been found to form asymmetric phase-transfer catalysts, the use of other metal(salen) complexes was investigated. Cobalt(salen) complexes 42a-d provided an opportunity to probe the influence of the oxidation state of the metal on the catalytic activity of the complex [42]. Hence, each of these complexes was prepared and tested as a catalyst for the benzylation of substrate 16a, according to the conditions specified in Scheme 8.18. 

In contrast to the behavior of the foregoing nickel(I) complexes as catalysts, the catalytic reactions of alkyl halides with cobalt(I) species such as vitamin Bi2s, cobaloximes(I), and cobalt(I) salen exhibit a significant difference. Cobalt(I) species, acting as potent nucleophiles in Sn2 reactions with alkyl halides, give stable alkylcobalt(III) intermediates. Lexa and coworkers [318] have discussed this mechanistic scheme for the catalytic reduction of l-bromobutane by the electrogenerated cobalt(I) tetraphenylpor-phin complex, where TPP denotes the ligand. Reversible one-electron reduction of the parent cobalt(II) species... 

Finally here it is worth noting that substitution by a fluorine atom does not always produce beneficial effects. For example, Chipperfield has synthesized the symmetric salen complexes shown in the Figure 32 containing copper(II), nickel(II) and oxovanadium(IV) [63]. Fluorination reduces the clearing point, as expected, in all but the oxovanadium complex with n = 6 where it remains constant. However, the mesophases are severely destabilized either disappearing completely or becoming monotropic. 

Related epoxidations of olefins with PhIO in the presence of Salen and related complexes of chromium(III), manganese(III) and cobalt(III) have been reported by Kochi and coworkers [58]. The use of nickel(II) Salen in conjunction with NaOCl was also described [59]. More recently, these systems formed the basis for the development, by Jacobsen and coworkers [60], of chiral manganese(III) Salen complexes for the enantioselective epoxidation of prochiral olefins by ArlO or NaOCl. Similarly, asymmetric epoxidations with moderate to good... 

Table II. Bond Lengths (pm) in Tetrahydrosalen, Dihydrosalen and Salen Complexes of Nickel(II).

It has to be pointed out that the salen complexes Ni (X )L and the dihydrosalen complex Ni [H2](Me)L do not add pyridine. The Lewis acidity of the nickel center is thus considerably increased by hydrogenation of the two C=N bonds in salen ligands. 

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