Methyl iodide ions, decomposition

The first observation of the instability of carbohydrate orthoesters toward alkali came from Haworth, Hirst and Miller in connection with their experiments on the simultaneous deacetylation and methylation of L-rhamnose methyl 1,2-orthoacetate. These authors noticed that methylation by methyl iodide and silver oxide in the presence of solid sodium hydroxide resulted in the formation of crystalline methyl tri-methyl-/3-L-rhamnopyranoside. A similar result was obtained by Bott, Haworth and Hirst on the simultaneous deacetylation and methylation of triacetyl-D-mannose methyl 1,2-orthoacetate by the use of excessive quantities of dimethyl sulfate and alkali. The reaction produced a mixture of a. and /3 forms of methyl tetramethyl-D-mannopyranoside but the yield was only 40%. When the acetylated orthoester was submitted to methylation with silver oxide and methyl iodide in the presence of sodium hydroxide, the product was mainly trimethyl-rhamnose methyl 1,2-orthoacetate. This result indicates that for the alkaline hydrolysis of orthoesters, hydroxyl ions are necessary. Such ions are present in the dimethyl sulfate-alkali process, but are absent in the methyl iodide treatment except when the reaction mixture contains a little water either by accident or from the decomposition of the sugar molecule. Haworth, Hirst and Samuels examined the behavior of dimethyl-L-rhamnose methyl 1,2-orthoacetate in alkaline solution. When the substance was heated under various conditions with 0.1 A alkali at 70 there was no appreciable hydrolysis at the end of ninety minutes, whereas at 80 for... 

The reaction of perchloro-p-xylene with iodide ion in DMSO gives cis-4//,4 //-decachlorostilbene (Diaz-Alzamora, 1968). The formation of this dealkylated condensation product is interpreted in (48) as a nucleophilic attack on a-chlorine, giving the carbanion [63], the relevant transition state being stabilized by the electron-attracting effect of the second trichloro-methyl substituent. The decomposition of this carbanion would take place as indicated on p. 295. These results could hardly be consistent with the dealkylation occurring after the reductive condensation. 

The dimercurial differed markedly from its methyl analogue in that it was a volatile white crystalline solid, which was soluble both in organic solvents and in water. It could, indeed, be recovered from water without any evidence of decomposition. The aqueous solution also had a small electrical conductivity, though no evidence has as yet been obtained to hhow what ions are present. Analogous products may be prepared from pentafluoroiodoethane and higher fluoralkyl iodides of this series. 

Use of trimethylsilylmethyl triflate enables the effective formation of intermediate iminium salts in the reaction mixture because the counteranion, triflate ion, is nonnucleophilic both to carbon and silicon atoms. N-Silyl-methylation can also be performed with other alkylating agents, such as silymethyl chloride, bromide, and iodide. However, the resulting iminium salts desilylate immediately after they are formed by the attack of the halide counteranions, leading to a serious decomposition of the requisite iminium intermediates. The final step of desilylation generating azomethine ylides is effected by a fluoride anion which is selectively nucleophilic to a silicon atom. 

The mechanism and kinetics of a novel autocatalytic degradation of A-methylmor-pholine A-oxide (70) into morpholine and formaldehyde, induced by carbenium-iminium ions (Mannich intermediates) has been explored. " The study was prompted by the observation that NMMO as an oxidant is often consumed far beyond the stoichiometric ratio, with generation of morpholine and some formaldehyde, and at a rate faster than its reaction with reductant. Decomposition of NMMO was apparently promoted by the combination of products mentioned and could also be induced by dimethyl(methylene)iminium iodide. The proposed mechanism is in Scheme 8. Addition of small amounts of base favour the abstraction of the proton from the A-methyl group but do not prevent carbenium-iminium ion formation however, larger concentrations of base terminate the reaction. The presence of only trace amounts of water is prerequisite for the protonation-deprotonation steps to proceed. 

---