Aldehydes base-catalysed decomposition

Because of the difficulties of purification and handling, many of the results for diazoalkanes have been obtained from reactions in which the diazo compound is not the starting material but in which it may reasonably be supposed to be an intermediate. Most popular in this respect has been the base-catalysed decomposition of arylsulphonyl-hydrazones of aliphatic aldehydes and ketones, a reaction which was first investigated by Bamford and Stevens (1952). Because the reaction... 

You can see why hemiacetals are unstable they are essentially tetrahedral intermediates containing a leaving group and, just as acid or base catalyses the formation of hemiacetals, acid or base also catalyses their decomposition back to starting aldehyde or ketone and alcohol. That s why the title of this section indicated that acid or base catalysts increase the rate of equilibration of hemiacetals with their aldehyde and alcohol components—the catalysts do not change the position of that equilibrium ... 

How can the Z selectivity in Wittig reactions of unstabilized ylids be explained We have a more complex situation in this reaction than we had for the other eliminations we considered, because we have two separate processes to consider formation of the oxaphosphetane and decomposition of the oxaphosphetane to the alkene. The elimination step is the easier one to explain—it is stereospecific, with the oxygen and phosphorus departing in a syn-periplanar transition state (as in the base-catalysed Peterson reaction). Addition of the ylid to the aldehyde can, in principle, produce two diastere-omers of the intermediate oxaphosphetane. Provided that this step is irreversible, then the stereospecificity of the elimination step means that the ratio of the final alkene geometrical isomers will reflect the stereoselectivity of this addition step. This is almost certainly the case when R is not conjugating or anion-stabilizing the syn diastereoisomer of the oxaphosphetane is formed preferentially, and the predominantly Z-alkene that results reflects this. The Z selective Wittig reaction therefore consists of a kinetically controlled stereoselective first step followed by a stereospecific elimination from this intermediate. 

The method used by Coates and Robinson" involved the copper-catalysed decomposition of trans,trans-farnesyl diazoacetate (4) to the cyclopropyl-lactone (5) having the stereochemistry shown. This was transformed into the cis-aldehyde-ester (6) by standard methods. Base epimerization gave the more stable transcompound (7). A Wittig reaction between the trans-aldehyde-ester (7) and the phosphorane (8), followed by lithium aluminium hydride reduction, yielded presqualene alcohol (1) as the major product accompanied by the minor isomer (9). 

The first general synthesis, which has been useful for the preparation of enamines derived from aldehydes, was discovered by Mannich and Davidsen. According to this method aldehydes and secondary amines react in the cold in the presence of potassium carbonate to give aminals 1 (equation 1). In many cases the aminal becomes the major product by the use of two equivalents of the amine. Destructive distillation of the aminal then leads to the enamine. Aminals are thermally stable up to temperatures of about 170 °C in the presence of base, and their decomposition to enamines at lower temperatures is acid-catalysed. ... 

Using the aldehyde in excess led to optimal conversions (5 equivalents). Due to the precedence of imidazole to catalyse the syn-anti isomerisation of aldol and Mannich products, it was selected as a base, which led to an increase in both reaction rate and yield (with 1 equivalent), however, excess imidazole led to byproduct formation and decomposition of the product (5 equivalents). 

From this study it seems that oxygenated aromatic compounds are less reactive towards acidic CO formylation in comparison to alkylated ones and this effect can be explained by the relative acid-base properties of the substrate and carbon monoxide. More than stoichiometric amounts of acid are required for acceptable rates and yields. Various acids apart from the HF/BF3 system were investigated without success in an attempt to facilitate formylation of anisole. Deuterium labelling experiments as a tool to probe the possible reaction mechanism provided interesting but inconclusive results in support of the intra-complex mechanism. Secondary product formation was shown to result mainly from acid catalysed reactions between the substrate and the aldehyde product. Preliminary economic evaluations indicated boron trifluoride to be the major cost driver in these reactions, necessitating its quantitative recovery. A successful FIF/BF3 retrieval process is described by Fujiyama et and involves the decomposition of the p-... 

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