Carbonyl group, reactions involving

Three-dimensional potential energy diagrams of the type discussed in connection with the variable E2 transition state theory for elimination reactions can be used to consider structural effects on the reactivity of carbonyl compounds and the tetrahedral intermediates involved in carbonyl-group reactions. Many of these reactions involve the formation or breaking of two separate bonds. This is the case in the first stage of acetal hydrolysis, which involves both a proton transfer and breaking of a C—O bond. The overall reaction might take place in several ways. There are two mechanistic extremes ... 

Two of the four general carbonyl-group reactions—carbonyl condensations and basic conditions and involve enolate-ion intermediates. Because the experimental conditions for the two reactions... 

You are what you eat. Food molecules are metabolized by pathways that involve the four major carbonyl-group reactions. 

Biochemistry is carbonyl chemistiy. Almost all metabolic pathways used by living organisms involve one or more of the four fundamental carbonvl-group reactions we ve seen in Chapters 19 through 23. The digestion and metabolic breakdown of all the major classes of food molecules—fats, carbohydrates, and proteins—take place by nucleophilic addition reactions, nucleophilic acyl substitutions, a substitutions, and carbonyl condensations. Similarly, hormones and other crucial biological molecules are built up from smaller precursors by these same carbonyl-group reactions. 

Although single-electron-transfer (SET) processes would be expected to be important in reactions that use metals as reagents, this type of process has also been recognized in the reduction of carbonyl groups that involve 1,4-dihydronicotinamide derivatives . Recent work by Oae and coworkers" has shown that an SET process is operative in the reduction of dibenzothiophene S-oxide by l-benzyl-l,4-dihydronicotinamide when the reaction is catalyzed by metalloporphins. The reaction is outlined in equation (18), but the study gave results of much more mechanistic than synthetic value. This type of study is relevant to understanding biochemical mechanisms since it is known that methionine sulphoxide is reduced to methionine by NADPH when the reaction is catalyzed by an enzyme isolated from certain yeasts . 

We should include the somewhat similar cyclisation onto a phosphine oxide here, even though no carbonyl group is involved. In this strange reaction, deprotonation of 88 a to phosphorus protects this position while halogen-metal exchange generates an aryllithium which displaces PhLi from phosphorus to give 89.50... 

Under acidic conditions, reactions at positions a to carbonyl groups often involve the enol tautomer acting as a nucleophile. 

A proton has to be transferred from one oxygen atom to the other we have shown ethanol doing this job, with one molecule being protonated and one deprotonated. There is no overall consumption of ethanol in the pro to nation/deprotonation steps, and the order in which these steps happen is not important. In fact, you could reasonably write them in one step as shown in the margin, without involving the alcohol, and we do this in the next hemiacetal-forming reaction below. As with all these carbonyl group reactions, what is really important is the addition step, not what happens to the protons. 

Dimanganese Decacarbonyl. This reaction should be described first, since no replacement of carbonyl groups is involved. When solutions of dimanganese decacarbonyl and of dinitrogen tetroxide in light petroleum at 0° C. are mixed, the solution becomes deep orange. Within 30 minutes precipitation begins, and... 

Two of the four general carbonyl-group reactions—carbonyl condensations and a substitutions—take place under basic conditions and involve enolate ion intermediates. Since the experimental conditions for the two reactions are so similar, how can we predict which will occur in a given case When we generate an enolate ion with the intention of carrying out an < alkylation, how can we be sure that a carbonyl condensation reaction won t occur instead ... 

Examples of carbonyl insertion reactions involving compounds of Group VIII metals are more common. The reaction of iron pentacarbonyl with the cyclopropylalkene (41) involves both carbonyl insertion and cyclopropane ring opening. Reduction of cobalt(n) to cobalt(o) in aqueous ammonia is thought to involve a carbonyl insertion stage, with... 

Manganese(II), iron(II), zinc(II) and mercury(II) are not effective as template centres for the assembly of L778 and L779. Products of unidentifiable composition containing carbonyl groups not involved in the reaction are formed in the presence of these ions. 

Carbonyl-ene reactions involving vinylsilanes promoted by a Lewis acid provide an avenue for regio- and stereocontrolled introduction of a vinyl functionality. The selectivity changes dramatically by placement of a MesSi group into the substrates as shown in equations 56 and 57 °. Similarly, upon reaction with aldehydes in the presence of MeaAlCl, the (r-BujMeaSi group in optically pure 138 assists the chirality transfer to give adducts 139 in 80-99 %ee (equation 58) . ... 

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