Metal-Schiff base

The intervention of a metal ion in the stoichiometry of a reaction has been illustrated several times previously. Reaction is forced to completion in ester hydrolysis since the carboxylate grouping forms a more stable complex than the ester moiety does. A similar driving force underlies the formation of macrocycles and the completion of transamination by formation of the metal-Schiff base complex. The latter is particularly relevant in dilute solution and at low pH. For example, the extent of aldimine formation between pyridoxal and alanine is undetectable at the physiological pH but occurs to the extent of = 10% in the presence of zinc... 

In contrast to the extensive coverage of transition metal Schiff base complexes, there is a somewhat desultory presentation of alkali and alkaline earth metal complexes of these ligands. 

In this chapter, the oxidations with hydroperoxides mediated or catalyzed by chiral titanium alcoholates will be discussed first. Then, results obtained with various chiral metal-Schiff bases... 

OLEDs) and achieved white EL with Pt(II) Schiff base complexes in 2004.29 Metal Schiff base complexes are potential candidates for the development of high performance PLEDs. 

Sigman and Jacobsen reported the first example of a metal-catalyzed enantioselective Strecker-type reaction using a chiral Alnl-salen complex (salen = N,N -bis(salicyhdene)-ethylenediamine dianion) [4]. A variety of N-allylimines 4 were evaluated in the reaction catalyzed by complex 5 to give products 6, which were isolated as trifluoroacetamides in good yields and moderate-to-excellent enantioselectivities (Scheme 3). Substituted arylimines 4 were the best substrates, while alkyl-substituted imines afforded products with considerably lower ee values. Jacobsen and co-workers also reported that non-metal Schiff base catalysts 8 and 9 proved to be effective in the Strecker reaction of imines 7 with hydrogen cyanide to afford trifluoroacetamides 10 after reaction with trifluoroacetic anhydride, since the free amines were not stable to chromatography (Scheme 4) [5]. 

Figure 4 Introduction of non-native metal Schiff base and substituted heme cofactors by noncovalent attachment in myoglobin. (Adapted from...

The assymetric Strecker reaction of diverse imines, including aldimines as well as ketoimines, with HCN or TMSCN provides a direct access to various unnatural and natural amino acids in high enantiomeric excesses, using soluble or resin-linked non-metal Schiff bases the corresponding chiral catalysts are obtained and optimized by parallel combinatorial library synthesis [93]. A rather general asymmetric Strecker-type synthesis of various imines and a, 9-unsaturated derivatives is catalyzed by chiral bifunctional Lewis acid-Lewis base aluminum-containing complexes [94]. When chiral (salen)Al(III) complexes are employed for the hydrocyanation of aromatic substituted imines, excellent yields and enatio-selectivities are obtained [94]. 

As already shown in Scheme 8, carpanone (44) has been synthesized by an electrochemical method. More efficient synthesis of 44 was effected with O2 catalyzed by metal-Schiff base complexes. Of the four complexes, Co(II)(salpr), Co(ll)(salen), Fe(II)(salen) and Mn(ll)(salen), Co(ll)(salen) provides the best results its solution with 4,5-methylenedioxy-... 

Evidently a fraction of the benzophenone is reduced to benzhydrol in a redox reaction in which an equivalent amount of Schiff base II is formed. After metallation of II by the remaining lithiumdiethylamide, the metallated Schiff base VI reacts with the reduced benzophenone to yield the aldimine adduct VII. Surprisingly, the entire process is completed within one minute at 0 °C. 

At first this new technique of adding metallated Schiff bases appeared problematical for the possible synthesis of natural products, because during its application to methylketones, such as jS-ionone, it could be expected that the proton active -COCH3 system would undergo a transmetallation or transanionization with the metallated Schiff base. Fortunately, this was not the case as was shown by model experiments with acetone, acetophenone, and cyclohexanone. 

Electropolymerized Metalloporphyrin, Metallophthalocyanine and Metal Schiff Base Complex Films Applications to Biomimetic Eiectrocataiysis and Bioeiectroanaiysis... 

Finally, innovative bimetallic complexes as dissymmetric ligands can easily be built, giving birth to an unprecedented class of metal Schiff base salen complexes that can be successfully electropolymerized concomitantly with an overoxidation process. The electrooxidation of 8-(3-acetylimino-6-methyl-2,... 

In other work, Wittig postulated the dehydrobenzene intermediate and proved its existence through trapping reactions of the Diels-Alder type. He discovered sodium tetraphenylborate, which is now used in the analytical determination of potassium and ammonium ions. His later work involved the chemistry of metalated Schiff bases. This work subsequently led to the development of the concept of directed aldol condensations. 

A heterodinuclear rare earth metal SchifF base catalyzes enantioselective desymmetrization of cyclic and acyclic weyo-aziridines with malonates (Scheme 13). The combined use of two rare earth metal sources with different properties, Bronsted basic rare earth metal alkoxide and Lewis acidic rare earth metal triflate, was important to promote the desired ring-opening reactions. The La(0 Pr)3/Yb(OTf)3/SchifFbase complex gave chiral cyclic and acyclic y-amino esters in high yields and high enantiomeric excesses. 

To 500(xL of collected chloioform, add 3 mL total of dithizone solution in three aUquots. Observe the different solution colors for extracted metal Schiff-base complexes ranging from blue to green to orange and pink according to the concentration of metal ion extracted. 

Solutions containing known amounts of metal bis-Schiff-base complex were analyzed in the dithizone assay. The expected decrease in free dithizone absorbance was observed, confirming that there was no interference from the metal Schiff-base complexes for this assay. 

Hrawitz CP, Murray RW (1988) Oxidative electropolymerization of metal schiff base complexes. Molec Cryst liquid Cryst 160 389—404... 

Polymerization of a dicarbonyl compound, a diamine, and a metal salt, is the most direct route to polymeric metal-Schiff bases (VII-12). One of the earliest uses of this reaction was as a colorimetric analytical method for metal ions (16). This reaction has been carried out in aqueous media, dimethyl-formamide (50), and mixed solvents incorporating benzene (50, 75). As with most chelate polymers, insolubility hampers obtention of high molecular weight. Molecular weights of 2000-11,000 were reported for the products obtained from some halogen-substituted bis(salicy aldehyde)-diamine-uranyl complexes (75). 

Homobimetallic Transition Metal Schiff Base Catalysts... 

---