Hydrazines aldehyde scavenging

The aldehyde side product can react with the freshly released amine, thus limiting the chemical yield. To avoid this reaction one can add an aldehyde scavenger, such as semicarbazide hydrochloride, hydrazine or an aldehyde scavenging resin (e.g., supported hydrazines). An alternative is to alter the structure of the photolabile protecting group. 

Baxendale IR, Ley SV Sneddon H (2002a) A clean conversion of aldehydes to nitriles using a solid-supported hydrazine. Synlett 775-777 Baxendale IR, Lee A-L, Ley SV (2002b) A concise synthesis of carpanone using solid-supported reagents and scavengers. J Chem Soc Perkin Trans 1 1850-1867... 

The electron donor/hole scavenger to be used in actinide valence control must have two properties. The first is that it is capable of undergoing an irreversible oxidation, either by valence holes formed by the photo-excitation of the semiconductor photocatalyst or by means of the hydroxide radicals generated by the oxidation of water by the same valence band holes. Suitable materials include organic acids, alcohols, aldehydes, amino acids, and hydrazine and its oxidation products, such as hydroxylamine and dihydrox-ylamine. 

In the dehydration of aldehydes to form nitriles, Ley and colleagues used a polymer-supported hydrazine (Scheme 6.3). The immobilized hydrazine was reacted with aldehydes to give a hydrazone. Subsequent oxidation with mCPBA (meta-chloroperoxybenzoic acid) rendered the corresponding N-oxide, which spontaneously eliminated to form the desired nitrile [10]. Polyvinylpyridine was used as a scavenger for excess mCPBA. 

Kirschning and coworkers have recently reported the synthesis of a resin-bound diketone for the sequestration of amines and hydrazines [14]. The resin-bound diketone was synthesized via oxidation of a chloromethyl resin, followed by reductive coupling with 2,4-pentadione, to provide the requisite scavenger resin. This resin was found to be more efficient than supported-aldehydes for amine and hydrazine scavenging and, furthermore, found to be selective for the scavenging of primary amines over secondary amines (Scheme 8.10). 

In 2007, Aube reported the application of the synthesis of oxazolines and dihydrooxazines libraries using resin-bound scavengers via two methods [60]. In the first method, a polymer-bound PPh3 scavenger was used to scavenge excess hydroxyalkyl azide. Alternatively, a polymer-bound tosyl hydrazine was used to scavenge excess aldehyde (Scheme 8.45). 

Eschenmoser a,p-epoxy ketone cleavage (2,419-422 3,293). In the synthesis of a prostanoid, Corey and Sachdev encountered a case where the Eschenmoser cleavage reaction with p-toluenesulfonylhydrazine gave a very complex mixture from which none of the desired acetylenic aldehyde could be isolated. They then found that cleavage could be effected with 2,4-dinitrobenzenesulfonyl-hydrazine in CH2CI2 or THF at 0-25°. Pyridine, sodium carbonate, or sodium bicarbonate can be used as catalysts. The yield can sometimes be improved by addition of ethyl isocyanate to scavenge the sulfinic acid formed in the fragmentation. This modified procedure was shown to be effective in five cases. 

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