Stereospecific reactions carbonylation

Vinyl halides (example 17, Table VII) were first observed by Kroper to form acrylic esters by reaction with carbon monoxide under pressure and tetracarbonylnickel in methanol at 100°C. These reactions were later shown to occur under much milder conditions. Highly stereospecific reactions were observed c/s-vinyl halides gave cis-carbonylation products and trans-vinyl halides trans-carbonylation products (example 18, Table VII). Retention of configuration of alkyl substrates in carbonylation seems to be a general feature in carbon monoxide chemistry (193a). 

VI, obtained from stereospecific reactions of the corresponding optically active alcohol (11) with thionyl chloride, phosphorus pentachloride, etc., was not racemized under any of the reaction conditions. Monitoring the carbonyl absorption frequencies in the IR during these decarbonylations showed that the transformations II — III — IV took place during the conversion of V to racemic VI. Thus rearrangement or decomposition or both could be responsible for the racemization. 

The palladium-catalyzed carbonylation of isomeric vinylic halides shows the reaction to be reasonably stereospecific and proceed with retention of the original halide structure. The degree of specificity, however, depends somewhat on the reaction conditions. Low reaction temperatures and/or excess triarylphosphine favor the stereospecific reaction (11). 

Stereochemical relationships in the synthesis and metabolism of citrate. When oxaloacetate labeled with l3C in the carbonyl group /3 to the keto group ( ) was used as substrate, the researchers expected that half of the label would be found in succinate and half in C02. That prediction was based on the assumption that the two —CH2—COO- arms of citrate must be equivalent in every way. In fact, all of the label was found in C02. Thus, only the intermediates shown on the left were produced. This result shows that both the condensation of acetyl-CoA with oxaloacetate and the isomerization of citrate are stereospecific reactions. The carbon atoms supplied by acetyl-CoA are shown in red. Neither of those atoms is lost in the first turn of the cycle after their entry. 

Two examples of Pd(II)-catalyzed carbomethoxylations of vinyl(phenyl)iodonium salts have been reported (equations 251 and 252)125,126. The mild reaction conditions and stereospecificity of carbonylation recommend further applications of vinyliodonium compounds for the synthesis of a,/ -unsaturated carboxylate esters. By way of comparison, similar carbobutoxylations of vinyl halides (Br, I) typically require higher temperatures (60-100 °C) and longer reaction times, and they sometimes proceed with low stereospecificity151. 

The Brown asymmetric crotylation is a highly regioselective and stereospecific reaction. Many organoboranes are now commercial available. Reviews (a) Denmark, S. E. Alrnstcad, N. G. In Modem Carbonyl Chemistry, Otera, J, Ed. Wiley-VCH Weinheim, 2000 Chapter 10 Allylation of Carbonyls Methodology and Stereochemistry,... 

Since the original preparation of optically active germyllithium by Brook and Peddle (31), these reagents have been found to undergo various stereospecific reactions. Carbonation and reaction with carbonyl compounds (eq. [12]) lead to retention of configuration at germanium (31). 

Two of the remaining chiral centres (c and d in 21) are adjacent the third (b) is separated by a CH2 group. It will be easy therefore to control (c) and (d) by stereospecific reactions but (b) will be difficult. In addition (b) is next to a carbonyl group and has a proton (H in 21) so that enolisation would destroy any stereochemistry set up here. A good strategy will therefore be to disconnect (21) to the symmetrical ketone (22) and an intermediate (23) containing both the chiral centres we aim to control. So far the analysis is the same as that in Chapter 1. 

The [2+2] cycloaddition reaction of isocyanates proceeds better with olefins having electron donating groups attached to the double bond system. Examples include vinyl ethers, enamines, ketene acetals, tetraalkoxy- or tetraalkylaminooleflns. The more reactive sulfonyl and carbonyl isocyanates undergo cycloaddition reactions with vinyl ethers especially well. For example, the reaction of vinyl ethers with p-toluenesulfonyl isocyanate affords the [2+2] cycloadducts in a stereospecific reaction... 

Ozonation ofAlkenes. The most common ozone reaction involves the cleavage of olefinic carbon—carbon double bonds. Electrophilic attack by ozone on carbon—carbon double bonds is concerted and stereospecific (54). The modified three-step Criegee mechanism involves a 1,3-dipolar cycloaddition of ozone to an olefinic double bond via a transitory TT-complex (3) to form an initial unstable ozonide, a 1,2,3-trioxolane or molozonide (4), where R is hydrogen or alkyl. The molozonide rearranges via a 1,3-cycloreversion to a carbonyl fragment (5) and a peroxidic dipolar ion or zwitterion (6). 

The parallel ability of oxiranes to undergo ring opening to carbonyl ylides was first noted in the case of tetracyanooxirane (68T2551), but such reactions have not been widely exploited. The addition to alkenes, leading to formation of tetrahydrofurans, is stereospecific (Scheme... 

Electron deficient carbon-carbon double bonds are resistant to attack by the electrophilic reagents of Section 5.05.4.2.2(t), and are usually converted to oxiranes by nucleophilic oxidants. The most widely used of these is the hydroperoxide ion (Scheme 79). Since epoxidation by hydroperoxide ion proceeds through an intermediate ct-carbonyl anion, the reaction of acyclic alkenes is not necessarily stereospecific (Scheme 80) (unlike the case of epoxidation with electrophilic agents (Section 5.05.4.2.2(f)) the stereochemical aspects of this and other epoxidations are reviewed at length in (B-73MI50500)). 

Site-specificity of the reaction is established in the first step since enolate formation involves the carbonyl carbon and the former halide bearing carbon, while the stereospecificity of the incoming deuterium is determined during the second step. It appears that the ketonization in deuterioacetic acid yields mainly the kinetic product (axial attack) although deuteration is... 

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