Carbon labilizing effect

Studies on the 1 1 complexing of NCS for H2O on aqua MosSj indicate an —10 labilizing effect as compared to MosO ". A cyclic voltammogram of [Mo3S4(ida)3] at a glassy carbon electrode shows two reversible one-electron waves indicating the existence of Mo and Moiii iii iv oxidation states. This is the first time the state has been identified. 

Alcoholic potassium hydroxide or sodium hydroxide are normally used to convert the halohydrins to oxiranes. Other bases have also been employed to effect ring closure in the presence of labile functional groups such as a-ketols, e.g., potassium acetate in ethanol, potassium acetate in acetone or potassium carbonate in methanol.However, weaker bases can lead to solvolytic side reactions. Ring closure under neutral conditions employing potassiunT fluoride in dimethyl sulfoxide, dimethylformamide or A-methyl-pyrrolidone has been reported in the patent literature. 

The greater lability of complex 146.C (compared to 145.c), as evinced by the much shorter reaction time, is typical of those that bear a carbomethoxy or acetyl substituent at the central carbon of an i73-allylic ligand. The temperature required for complete decarbonylation of complexes of type 146 and 148 increases with the size of the R-substituent, which suggests a mechanism involving hydride transfer.111 This would also explain the observed activating effect of the centrally located carbomethoxy group in 146.C, which would clearly labilize the methyl proton shown explicitly in 146. 

Alkaline hydrolysis with barium, sodium, or lithium hydroxides (0.2-4 M) at 110°C for 18-70 h126-291 requires special reaction vessels and handling. Reaction mixtures are neutralized after hydrolysis and barium ions have to be removed by precipitation as their carbonate or sulfate salts prior to analysis which leads to loss of hydrolysate. Correspondingly, peptide contents are difficult to perform by this procedure. Preferred conditions for alkaline hydrolysis are 4M LiOH at 145 °C for 4-8 h where >95% of tryptophan is recovered 291 An additional inconvenience of the alkaline hydrolysis procedure is the dilution effect in the neutralization step and thus the difficult application to the analyzer if micro-scale analysis is to be performed. The main advantage is the good recovery of tryptophan and of acid-labile amino acid derivatives such as tyrosine-0-sulfate1261 (Section 6.6) as well as partial recovery of phosphoamino acids, particularly of threonine- and tyrosine-O-phosphate (Section 6.5). 

The synthesis of chromones by the Kostanecki-Robinson method frequently yields a 3-acetylchromone. This acetyl group forms a 1,3-diketone with the pyran carbonyl group and is therefore labile in an alkaline medium. Treatment with aqueous carbonate or other base removes such groups but has no effect on other acyl substituents, for example the 6-acetyl of 3,6-diacetyl-2-methylnaphtho[l,2- ]pyran-4-one (516). 

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