Exchange reactions displacement

The rather unreactive chlorine of vinyl chloride can be displaced with nucleophiles by the catalytic action of PdCb. The conversion of vinyl chloride to vinyl acetate (797) has been studied extensively from an industrial standpoint[665 671]. DMF is a good solvent. 1,2-Diacetoxyethylene (798) is obtained from dichloroethylene[672]. The exchange reaction suffers steric hindrance. The alkenyl chloride 799 is displaced with an acetoxy group whereas 800 and 801 cannot be displaccd[673,674]. Similarly, exchange reactions of vinyl chloride with alcohols and amines have been carried out[668]. 

Maltose phosphorylase cannot carry out a similar reaction. The P exchange reaction of sucrose phosphorylase is accounted for by a double-displacement mechanism where E = E-glucose ... 

Maltose phosphorylase proceeds via a single-displacement reaction that necessarily requires the formation of a ternary maltose E Pi (or glucose E glucose-l-phosphate) complex for any reaction to occur. Exchange reactions are a characteristic of enzymes that obey double-displacement mechanisms at some point in their catalysis. 

At present most-perchlorate salts are prepd from Na perchlorate or perchloric ac by exchange or displacement reactions (Ref 13, p4). 

For the halogen-metal exchange reaction of bulkier halopyrimidines, steric hindrance retards the nucleophilic attack at the azomethine bond. As a consequence, halogen-metal exchange of 5-bromo-2,4-di-r-butoxypyrimidine (43) with n-BuLi could be carried out at -75 °C [20]. The resulting lithiated pyrimidine was then treated with n-butylborate followed by basic hydrolysis and acidification to provide 2,4-di-f-butoxy-5-pyrimidineboronic acid (44). 5-Bromopyrimidine 43 was prepared from 5-bromouracil in two steps consisting of a dehydroxy-halogenation with phosphorus oxychloride and an SnAt displacement with sodium r-butoxide. 

Copper iodide acts as an efficient reagent for the nucleophilic displacement of 1-haloalkynes. It transforms 1-bromoalkynes (72) into 1-iodoalkynes (73) which, on further treatment with copper(II) bis(arenesulfinate), are converted into the corresponding alkynyl aryl sulfones (74). An electron transfer between 1-haloalkynes and copper(I) salts is believed to take place for the copper-assisted halogen-exchange reaction at the acetylenic carbon atom. 

Aromatic fluorine for halogen (F-X) exchange reactions (DMSO, 160°C, 20 min) in an [ F]fluoride-cryptand-oxalate system using 4 -halo-acetophe-nones (F, Cl, Br and I) has also been studied. The relative efficacy of the exchange is the following one F-F > F-Cl > F-Br > F-I, the radiochemical yield for the exchange F-F being similar to that of the commonly employed NO2 or +NMej displacements [113]. 

However, the hemiaminal 17 is unstable as a free base and readily undergoes exchange reactions. Since the hydroxy moiety of 17 is more easily displaced than the amine moiety, a highly reactive cyclopropyliminium salt 18 is formed, which then reacts with weak nucleophiles such as ethanol, to give e. g., 19. Otherwise in water solution 17 can also probably eliminate ammonia to form the highly reactive cyclopropanone 20, which is in equilibrium with its hydrate 21 and hemi-acetal 22, Eq. (8) [20]. It has been reported that hydrate 21 is also a potent inhibitor of ALDH [20,21]. 

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