Prevention decarbonylation

All these data could be obtained by means of two techniques, namely n.m.r. spectroscopy and the use of superacid solvent systems (such as HF—BF3, HF—SbFj, FHSO3—SbFs, SbFs—SOj). As will become evident in this article, this is equally true for the data of the carbonyl-ation and decarbonylation reactions (3). With less acidic systems the overall kinetics can, of course, be obtained but lack of knowledge concerning the concentrations of the intermediate ions prevents the determination of the rate constants of the individual steps. ... 

This chloride, purified by distillation at 58-60°C/l bar usually ignites spontaneously in air owing to presence of chloroacetylene (formed by decarbonylation), but vacuum distillation at cryogenic temperatures prevents formation of the impurity. 

The Stille coupling may be combined with carbonylation in two ways. Acid chlorides may be used as substrates for the reaction with vinyl or aryl stannanes. However, an atmosphere of carbon monoxide is frequently required to prevent decarbonylation after the oxidative addition step. 

The bright yellow color of the solution changes to light green after saturation with carbon monoxide. The presence of carbon monoxide prevents decarbonylation of the acylpalladium complex and thus the formation of ethyl p-nitrocinnamate. 

Acylstannanes will couple with acyl halides to give mixtures of mono- and diketones, but decarbonylation can be prevented by an atmosphere of CO, when the diketone is the predominant product.48... 

This alternative to the Friedel-Crafts reaction, extensively developed by Stille and coworkers, is particularly important, since the reaction conditions are essentially neutral, and so provides a method for acylation of compounds containing an acid-sensitive functionality which would preclude the use of the Friedel-Crafts reaction. Reaction temperatures are often below 100 C, and high (1000-fold) turnovers of the catalyst have been achieved. Solvents employed include chloroform, toluene, and, on occasions, HMPA. Some reactions have been carried out under an atmosphere of carbon monoxide to prevent excessive decarbonylation of the acyl palladium intermediate. Indeed, carbonylative coupling of alkenylstannanes with allyl halides in the presence of carbon monoxide ca. 3 atm or greater 1 atm =101 kPa) offers an alternative to the Friedel-Crafts acylation, ketones being formed by the reaction of the stannane with the acyl species formed by carbon monoxide insertion into the allyl palladium intermediate. ... 

The intramolecular hydroacylation of aldehydic olefins is catalyzed by cationic Rh complexes of chelating phosphines in polar nonprotic solvents. The rate decreases with increasing substrate Rh ratio since the substrate complex with the catalyst inhibits the reaction. However, since this complexation also prevents the decarbonylation of the aldehyde, catalyst deactivation decreases, leading to a higher turnover. The only effective catalytic C—H activations are carried out with [RhCl(CO)(PMe3)2] under photolysis which removes CO. Alkanes are converted to alkenes in the order cyclooctane > cyclohexane > n-decane n-hexane. Up to 200 turnovers cyclooctane per hour are observed. In order to generate terminal... 

Various Pd-catalyzed carbonylation reactions have often been referred to as carbonyla-tive cross-coupling reactions (Scheme 4). However, these reactions involving the formation of two C—C bonds with incorporation of CO clearly display a pattern of chemical transformation that is different from Scheme 1. So, these reactions are discussed in Parts VI and VIII. On the other hand, Pd-catalyzed acylation with acyl halides and related derivatives are examples of the reaction represented by Scheme 1, where is acyl, and they are therefore discussed in this Part (Sect. III.2.12.1), even if CO may be used to prevent decarbonylation. 

Another approach to the same natural product, pyrenophorin 2.189, is to carry out a dimerization by the Stille coupling of a stannane containing an acid chloride 2.188 (Scheme 2.65). The fact that these two functional groups can be in the same molecule illustrates further the functional group tolerance of the Stille reaction. Once again, decarbonylation is prevented by the use of a CO atmosphere. 

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