Acid generation from formation rearrangement product

Figure 5 Acid Generation from Formation of Rearrangement Products...

The reaction is especially suited to the generation of optically active diazonium ions with specifically oriented counter-ions. In this respect it has possibilities which are absent for the reaction of diazoalkanes with acids and the deamination of aliphatic amines. However, in carrying out stereochemical studies, great care must be exercised to avoid spurious results, since the transient formation of a diazoalkane, either by loss of a proton from the diazonium ion or by what is probably a concerted elimination reaction of the diazoester, can lead to racemisation of the alkyl function and loss of asymmetry in the anion. Moreover, the diazoester is liable to nucleophilic displacement, for example by an acid molecule formed from already rearranged nitrosoamide, and this can lead to inverted product. 

The ketocarbene 4 that is generated by loss of Na from the a-diazo ketone, and that has an electron-sextet, rearranges to the more stable ketene 2 by a nucleophilic 1,2-shift of substituent R. The ketene thus formed corresponds to the isocyanate product of the related Curtius reaction. The ketene can further react with nucleophilic agents, that add to the C=0-double bond. For example by reaction with water a carboxylic acid 3 is formed, while from reaction with an alcohol R -OH an ester 5 is obtained directly. The reaction with ammonia or an amine R -NHa leads to formation of a carboxylic amide 6 or 7 ... 

Many hydroxy compounds would not survive such harsh treatment therefore other methods must be used. Some alcohols were hydrogenolyzed with chloroalanes generated in situ from lithium aluminum hydride and aluminum chloride, but the reaction gave alkenes as by-products [605], Tertiary alcohols were converted to hydrocarbon on treatment at room temperature with triethyl- or triphenylsilane and trifluoroacetic acid in methylene chloride (yields 41-92%). Rearrangements due to carbonium ion formation occur [343]. 

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