Carbon-heteroatom bonds, cleavage

The isomerization of vinyl- or ethynyl-oxiranes provides a frequently exploited source of dihydrofurans or furans, but analogous conversions of vinylaziridines have not been applied so often. While most of the examples in Scheme 87 entail cleavage of the carbon-heteroatom bond of the original heterocycle, the last two cases exemplify a growing number of such rearrangements in which initial carbon-carbon bond cleavage occurs. 

Hydrogenolysis of the C-C bond takes place on different metal catalysts but the rupture of the nonpolarized C-C bond is more difficult than cleavage of the more polarized carbon-heteroatom bonds.482-486... 

The five-membered ring heteroaromatics (furan, thiophene, and pyrrole) show very similar ring cleavage patterns. The first step in each case is cleavage of the carbon-heteroatom bond. 

Finally, cytochromes P-450 catalyze the oxidation of heteroatom-con-taining molecules, leading to products that derive either from the transfer of an oxygen atom to the heteroatom (N-or S-oxygenation) or from the oxidative cleavage of a carbon-heteroatom bond (N- or S-dealkylation) [46],... 

Retrosynthetic cleavage of a carbon-heteroatom bond in 2.13 gives enol 2.14 which is equivalent to ketone 2.15. Repeating the process gives us a 1,4-dicarbonyl compound and the heteroatom-containing fragment such as a primary amine or hydrogen sulphide. 

Our retrosynthetic analysis of generalised pyridine 5.4 commences with an adjustment of the oxidation level to produce dihydropyridine 5.5. This molecule can now be disconnected very readily. Cleavage of the carbon-heteroatom bonds in the usual way leaves dienol 5.6 which exists as diketone 5.7. The 1,5-dicarbonyl relationship can be derived from a Michael reaction of ketone 5.8 and enone 5.9, which in turn can arise from condensation of aldehyde 5.10 and ketone 5.11. 

Other reactions involving the cleavage of a carbon heteroatom bond include a promising method for the deprotection of benzyl ethers by irradiation in the presence of acceptors (Scheme 46) [240-241] and the liberation of alkyl radicals (capable of initiating a polymerization) from alkyltriarylborate salts [242-243], The PET induced decomposition of phenyldiazomethane leads to cis-stilbene the reaction however appears to involve addition of the radical cation to a neutral molecule prior to nitrogen loss [244]. The detachment of a halogen after intramolecular electron transfer from the a C-X bond to an electron-rich... 

Vinylaziridines such as (162) have been rearranged theimally to 3-pynolines (equation 51),presumably through one of the species (164). Substituents play an important role in the likelihood of these inteim ates. The energy of activation for carbon-nitrogen bond cleavage has been estimated to be 12-14 kcal mol" (compared with 27 kcal mol for cyclopropane). Vinylaziridines of type (165 R = alkyl) furnish 1-pyirolines (166) or, if X is another heteroatom, the corresponding heterocyclopentenes (equation 52 Section 8.1.7). - ... 

Electrochemical reductive cleavage of a-amino ketones becomes easier as the acidity of the medium is increased, indicating that they are reduced as their conjugate acids. As with a-ketols (Section 4.8.3.1.3), reductive cleavage of the carbon-heteroatom bond is frequently accompanied by reduction of the carbonyl group (equation 28). ... 

More recently, a radical-mediated variation of this addition-fragmentation has been explored. The reaction, summarized in Scheme 77 for a one-carbon expansion, involves the generation of a radical at the terminus of a chain by homolytic cleavage of a carbon-heteroatom bond. Addition of the radical to the carbonyl produces a bicyclic intermediate, which on cleavage of the alternate bond regenerates the ketone carbonyl group with formation of a new radical. The sequence is terminated by the reduction of the radical with the tributyltin hydride reagent. The near neutral conditions of the reaction avoid the reclo-... 

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