Ligands hydroxamic acid

A combination first coordination shell-second coordination shell based recognition BLM transport system was devised, including active transport (200). This is based on a labile dihydroxamic acid system, including alcaligin, and a free lysine hydroxamic acid ligand capable of ternary complex formation to... 

The development of transition metal mediated asymmetric epoxidation started from the dioxomolybdcnum-/V-cthylcphcdrinc complex,4 progressed to a peroxomolybdenum complex,5 then vanadium complexes substituted with various hydroxamic acid ligands,6 and the most successful procedure may now prove to be the tetroisopropoxyltitanium-tartrate-mediated asymmetric epoxidation of allylic alcohols. 

SCHEME 60. Optically active hydroxamic acid ligands for the vanadium-catalyzed asymmetric epoxidation of allylic alcohols,... 

The asymmetric epoxidation of allylic alcohols with cumene hydroperoxide or rerf-butyl hydroperoxide (TBHP) was first examined by using chiral amino alcohol-Mo complexes 45) and V complexes with chiral hydroxamic acid ligands (Scheme 20) 46). The highest optical yields were 33% with geraniol and 50% with 2-phenylcinnamyl alcohol. Combined use of VO(acac)2 and a hydroxamic acid derived from proline led to 80% optical yield with 2-phenylcinnamyl alcohol 47). 

Figure 4.4.8 The hydroxamic acid ligands used for the synthesis of the organotins in Ref 22...

Many of the iron transport compounds have hydroxamic acid ligands which bind the ferric ion and so synthetic iron (III) hydroxamic acid compounds have been prepared. 

Again, these ferric complexes are very stable (/>K 28) as are the natural iron chelating compounds and they bind ferrous iron relatively weakly (50). In an acid medium (pH 2—6) the hydroxamic acid forms a deep purple 1 1 complex with ferric iron while in neutral or basic solution a brown-red 3 1 complex is formed in which the hydroxamates act as three bidentate ligands which occupy the six octahedral positions about the iron. The 1 1 purple complex has an absorption maximum at 5000—5200 A depending on the nature of the hydroxamic acid ligand. The 3 1 red-brown complex (it becomes yellow-orange on dilution) has an absorption maximum usually between 4250—4400 A with many of the same spectral features as the naturally occurring complexes of ferric iron. 

Enterobactin (MW 669) is derived from certain enteric bacteria such as Escherichia coli and is classified as a siderochrome by Neilands (1) in which the iron is bound by three bidentate catechol groups instead of the hydroxamic acid ligands described above. There are three charged oxygens which bind the iron either in a facial or meridional configuration. 

Enantiomerically pure manganese complexes using ligands other than the salen structure have been reported, but so far with lower enantioselectiv-ities. Better results have been achieved using molybdenum complexes bearing hydroxamic acid ligands and TBHP or cumylhydroperoxide as oxidant. This system has been used to effect the epoxidation of a range of olefins with up to 96% ee. 

Scheme 5.9 Pd-catalyzed enantioselective C(sp )—H bond functionalization using a chiral hydroxamic acid ligand reported by Yu.

Wilfong, E.M., Du, Y and Toone, E.J. (2012) An enthalpic basis of additivity in biphenyl hydroxamic acid ligands for stromelysin-1. Bioorganic and Medicinal Chemistry Letters, 22, 6521-6524. 

Hydroxylamine hydrochloride (34.8 g, 500 mmol) is dissolved with rapid stirring in 185 mL of hot methanol. Potassium hydroxide (42.0 g, 750 mmol) is dissolved in 115 mL of room-temperature methanol. After cooling both solutions to ambient temperature the potassium hydroxide solution is added with stirring to the hydroxylamine hydrochloride (an exothermic process ). The resulting solution and precipitated potassium chloride are cooled to — 20°C in a methanol ice bath before vacuum filtration. The clear colorless supernatant hydroxylamine KOH solution is immediately used in the preparation of the hydroxamic acid ligands. 

Yamamoto took a similar approach for the epoxidation of allyUc and homoallyhc alcohols (Schane 4.20). This process employed chiral hydroxamic acid ligand 101 to induce stereocontrol and VO(OtPr)3 to serve as the Lewis acid [49]. ( )-HomoaIlyhc alcohol 102 was converted to epoxide 103... 

SCHEME 34.15. Asymmetric epoxidation of allyl alcohols 54 leading to enantioenriched epoxides 56 mediated by vanadium(V) catalysts with a chiral hydroxamic acid ligand 55 and t-butyl hydroperoxide as a terminal oxidant. 

FIGURE 35.4. Chiral hydroxamic acid ligands for vanadium-catalyzed epoxidation. 

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