Cleavage of bonds

C. Cleavage of Bonds Attaching Atoms or Groups to the a-Carbon of Ketones... 

Photochemical autooxidation of ketones, 316 Photochemical a-cleavage, 296 Photochemical cleavage of bonds attaching atoms to the a-carbon of ketones, 307 Photochemical cleavage of exocyclic. S-bonds, 323... 

The Mass Spectra of O-Isopropylidene Ketals 9 (Figure 6), 10, (Figure 7), and 11 (Figure 8). Three fragmentations characteristic of O-isopropylidene ketals are loss of a methyl radical from the ketal ring, cleavage of bonds adjacent to the ketal ring, and loss of acetone (12). 

Fragmentation Cleavage of bonds that are adjacent to the carbonyl group gives rise to R+, [RC O]and [OCH3]+. An intense peak in the mass spectra of C6-C26 methyl esters results from the McLafferty rearrangement ... 

Further nitrolysis (with cleavage of bond A) forms a hypothetical compound (compound II) as follows ... 

Compound II now undergoes further nitrolysis (with cleavage of bond, B) to give another hypothetical material, Compound III, and a known material, Compound IV ... 

A free-radical process consists of at least two steps. The first step involves the formation of free radicals, usually by homolytic cleavage of bond, that is, a cleavage in which each fragment retains one electron ... 

Cleaved derivatives cleavage of bond 2 and the regulatory function of the N-terminus... 

Castro CE, WC Kray (1963) The cleavage of bonds by low-valent transition metal ions. The homogeneous reduction of alkyl halides by chromous sulfate. J Am Chem Soc 85 2768-2773. 

Two examples from ketone photochemistry that has been recently analyzed within the context of solid-to-solid transformations are the Norrish type and Nor-rish-Yang type Ip44,i45 tactions. In general terms, the type I reaction consists of a homolytic cleavage of bond a-to the carbonyl to generate an acyl-alkyl radical pair (RP-A) or an acyl-alkyl biradical (BR-A) when the ketone is cyclic (Scheme 7.15). 

This pathway would involve simultaneous cleavage of bond 2-3 and formation of bonds 2-4 and 3-5. This picture is misleading in that, unlike the more refined pictures, it does not indicate inversion at C-3. 

The very efficient di-rr-methane rearrangement from the triplet state in this case is probably due to two factors (a) the energetics is favorable for cleavage of bond 3-4 and formation of bond 2-4 [see Eq. (8.26)] and (b) rotation about bond 1-2, cis-trans isomerization, should be restricted because of steric interactions. 

These enzymes catalyse the non-hydrolytic cleavage of bonds in a substrate to remove specific functional groups. Examples include decarboxylases, which remove carboxylic acid groups as carbon dioxide, dehydrases, which remove water, and aldolases. The decarboxylation of pyruvic acid (10.60) to form acetaldehyde (10.61) takes place in the presence of pyruvic decarboxylase (Scheme 10.13), which requires the presence of thiamine pyrophosphate and magnesium ions for activity. 

Usually, the goal of domino reactions is an increase of complexity of the products in comparison to the substrates. However, a domino-type cleavage of bonds might also be advantageous if the starting material is easily accessible and the product difficult to obtain by other methods. 

The cleavage of the cyclopropane ring takes place with siteselectivity at the less hindered bond a as shown in Scheme 5.14, and no product arising from cleavage of bond b is obtained. 

Examples taken either from his own laboratory or from the recent chemical literature allow Curran to show how retrosynthetic analysis can generate ideas for new methods and reagents, as well as for new synthetic strategies in which homolytic cleavage of bonds are taken into account. 

As in the heterolytic cleavage of bonds, the resulting synthons must be elaborated to give the reagents or synthetic intermediates that should be used in the laboratory. In our example, the dissonant 1,4-carbonyl system can be synthesised in the laboratory following either route A or B. 

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