Salicylidene

Fueled by the success of the Mn (salen) catalysts, new forays have been launched into the realm of hybrid catalyst systems. For example, the Mn-picolinamide-salicylidene complexes (i.e., 13) represent novel oxidation-resistant catalysts which exhibit higher turnover rates than the corresponding Jacobsen-type catalysts. These hybrids are particularly well-suited to the low-cost-but relatively aggressive-oxidant systems, such as bleach. In fact, the epoxidation of trans-P-methylstyrene (14) in the presence of 5 mol% of catalyst 13 and an excess of sodium hypochlorite proceeds with an ee of 53%. Understanding of the mechanistic aspects of these catalysts is complicated by their lack of C2 symmetry. For example, it is not yet clear whether the 5-membered or 6-membered metallocycle plays the decisive role in enantioselectivity however, in any event, the active form is believed to be a manganese 0x0 complex <96TL2725>. 

These catalysts were first tested as resin-bound derivatives via HTS, first with metals and then without. Three libraries of chiral molecules, based on three different enantiomerically pure diamines, bulky salicylidene moities and optically active ii-amino acids were used for structure optimisation (Scheme 37 TBSCN = fBuMe2SiCN) [152]. 

The reaction was first tested with these substances as ligands but the organic molecule, in the absence of any added metal ion, proved to be the most enantioselective catalyst (library 1 19% ee vs. less than 13% ee for the best metal catalyst). The effects of selective variations of the amino acid nature and of the salicylidene moiety on the diamine structure were investigated for urea and thiourea derivatives via HTS (library 2 48 urea compounds and... 

CoSANSE = [bis(salicylidene-N-(methyl-3>hydroxypropionate))] Co Punniyamurthy, T. Iqbal. J. Tetrahedron Lett., 1994, 35, 4007... 

A copper(II) complexes of 5-phenylazo-3-methoxy salicylidene thiosemicarbazone has been shown to have promising growth inhibition activity against P388 lymphocytic leukemia cells sensitive and resistant to adriamycin [196], The complex involves ON coordination of two deprotonated ligands and v(CS) is reported to be unaltered in intensity and position in the complex from its position in the spectrum of the ligand. Inhibition studies with the uncomplexed thiosemicarbazone indicating an important role for the copper(II). 

Ogunwumi and Bein synthesized the (R,R)-N,N -bis(3-tert-butyl-5-methyl-salicylidene)-l,2-cyclohexanediaminato Mn(III) chloride (5 in Fig. 7.6) (with... 

Formation of the adduct with dirhodium complex [6] has significantly changed the chemical shift of the compounds in which proton transfer occurred, for example, for the adduct of iV-(5-nitrosalicylidene)-2-ami nobutane, the chemical shift was —198.2, which was shielded by of 110 ppm in comparison to that in the starting Schiff base.12 The large signal shift was due to the shift of the proton transfer equilibrium towards NH tautomer. For the adduct of N-(salicylidene)-2-aminobutane, existing in the OH-form, the 15N chemical shift has changed from —87.5 to — 84.6 ppm. 

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... 

Neomycin is insufficiently volatile for direct mass spectrometric analysis. To overcome this problem Inouye- - prepared the volatile N-salicylidene Schiff s base, the M.S. of which, however, did not exhibit a peak for the molecular ion. To observe the molecular ion it was necessary to use the o-trimethylsilyl ether of the N-salicylidene Schiff s base. The spectrum of N-salicylidene neomycin was found to be dependant on the ion-chamber temperature indicating that thermal decomposition plays a significant part in the fragmentation process. 

It is interesting to note that the molecular ion corresponding to neamine(a major product of neomycin hydrolysis) was not observed in the mass spectrum of either the N-salicylidene or the T.M.S. derivative. 

Several Ru(III) salen complexes of the type Ruin(salen)(X)(NO) (X=C1-, ONO-, H20 salen = N,AP-bis(salicylidene)-ethylenediamine dianion) have been examined as possible photochemical NO precursors (19). Photo-excitation of the Rum(salen)(NO)(X) complex labilizes NO to form the respective solvento species Ruin(salen)(X)(Sol). The kinetics of the subsequent back reactions to reform the nitrosyl complexes (e.g. Eq. (8)) were studied as a function of the nature of the solvent (Sol) and reaction conditions. The reaction rates are dramatically dependent on the identity of Sol, with values of kNO (298 K, X = C1-) varying from 5 x 10-4 M-1 s-1 in acetonitrile to 4 x 107 M-1 s-1 in toluene, a much weaker electron donor. In this case, Rum Sol bond breaking clearly... 

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