Silver complexes hydroxy

The use of Lewis acid drastically changes the regioselectivity. The highly enantioselective and O-selective nitroso aldol reactions of tin enolates with nitrosobenzene have been developed with the use of (i )-BINAP-silver complexes as catalysts. AgOTf and AgCICL complexes are optimal in the O-selective nitroso aldol reaction in both asymmetric induction (up to 97% ee) and regioselection (0/N= > 99/1), affording amino-oxy ketone. The product can be transformed to a-hydroxy ketone without any loss of enantioselectivity (Equation (71)).224... 

The ligand group can be introduced either on the meso or on the /5-pyrrole position of the porphyrin ring, but the synthesis of the meso-functionalized derivatives is easier and has been more widely exploited. Balch (50-53) reported that the insertion of trivalent ions such as Fe(III) (32) and Mn(III) (33) into octaethyl porphyrins functionalized at one meso position with a hydroxy group (oxophlorins) leads to the formation of a dimeric head-to-tail complex in solution (Fig. 11a) (50,51). An X-ray crystal structure was obtained for the analogous In(III) complex (34), and this confirmed the head-to-tail geometry that the authors inferred for the other dimers in solution (53) (Fig. lib). The dimers are stable in chloroform but open on addition of protic acids or pyridine (52). The Fe(III) octaethyloxophlorin dimer (52) is easily oxidized by silver salts. The one-electron oxidation is more favorable than for the corresponding monomer or p-oxo dimer, presumably because of the close interaction of the 7r-systems in the self-assembled dimer. 

Limited structural information is available for silver(I) carboxylates, despite their extensive use as catalysts in the manufacture of urethane polymers. This is in part due to their frequent insoluble and light-sensitive nature making chemical characterization of the complexes difficult. Dimeric structures have been reported for the perfluorobutyrate249 and trifluoroacetate complexes.250 In each case two-fold symmetry was crystallographically imposed. The Ag—O bond lengths were 223-224 pm, and in the more accurate determination of the trifluoroacetate, the Ag—Ag separation was found to be 297 pm. A dimeric structure was also found for the silver(I) complex of 3-hydroxy-4-phenyl-2,2,3-trimethylhexane carboxylate.251 In the asymmetric crystal unit the Ag---Ag separations were 277.8 and 283.4 pm. 

There are only scant reports of the reaction of silver ions with aliphatic or aromatic hydroxy acids and only a few stability constants have been reported.264 In general only weak complexes were formed and typical examples include reactions with ascorbic or tartaric acids. 

Methanolysis of penta-0-benzoyl-5-bromo-/ -D-glucopyranose, using silver oxide and methanol, gives a complex set of products, but hydrolysis in the presence of this solid affords the 5-hydroxy analog 128 which, in aqueous media, equilibrates with the 5-ulose 129 this loses benzoic acid, and the resulting aldehyde recyclizes, to afford 2,3,4,6-tetra-0-benzoyl-5-hydroxy-/ -D-glucose (130) (see Scheme 21).26... 

The aqueous layers containing the silver(I) complex of methyl (lR,5R)-5-hydroxy-2-cyclopentene-l-acetate are then treated with an excess of saturated brine to precipitate silver chloride and free the desired product. After precipitation is complete, the water layer is decanted from the solid silver chloride. The solids are washed with ether (4 x 100 mL) and each ether layer is used to extract the water layer (Note 20). The combined ether layers are washed with 50 mL of brine and dried over anhydrous magnesium sulfate. Filtration and removal of solvent under reduced pressure yield 16-19 g of crude product. The product is distilled through a 4"-Vigreux column at 0.1 mm pressure to yield 12.8-14.7 g (27-31 ) of methyl (lR,5R)-5-hydroxy-2-cyclopentene-l-acetate, bp 74-78°C at 0,1 mm, [a]p5 -132° (CHjOH, a 1.06) (Notes 21, 22). 

Similar cts-bis-carbene chelate complexes of palladium(Il) [327,330,331], but without the hydroxy functional groups on the wingtips, were used by the same research group for the copolymerisation of ethylene and CO. Once again, chelating bisphosphane complexes inspired the synthesis and application of their NHC counterparts [332,333]. The actual, defined catalyst precursors were the cationic complexes formed after haUde abstraction with silver salts in acetonitrile as donor solvent. 

Figure 4.4 Syntheses of a hydroxy functionalised bis-imidazolium salt and its silver(l) and copper I) complexes.

The simpler architecture is the 1,1 -biphenyl scaffold, likewise introduced by Hoveyda and coworkers [19]. The synthesis of the imidazolium salt starts with a chiral diamine and a substituted, achiral biphenyl [82-84], Subsequent introduction of a Mes substituent on the remaining primary amino end and ring closure reaction yields the chiral saturated imidazolium salt after hydrolysation of the methoxy group to liberate the phenolic hydroxy group (see Figure 4.22). Reaction with silver(I) oxide and carbene transfer to a Grubbs (Hoveyda) catalyst sets up the ruthenium catalyst complex. 

---