Addition, 1,4-, to dienes

Later m this chapter we 11 see how allylic carbocations are involved m elec trophihc addition to dienes and how the principles developed m this section apply there as well... 

Addition of HCN to unsaturated compounds is often the easiest and most economical method of making organonitnles. An early synthesis of acrylonitrile involved the addition of HCN to acetylene. The addition of HCN to aldehydes and ketones is readily accompHshed with simple base catalysis, as is the addition of HCN to activated olefins (Michael addition). However, the addition of HCN to unactivated olefins and the regioselective addition to dienes is best accompHshed with a transition-metal catalyst, as illustrated by DuPont s adiponitrile process (6—9). 

Diester (40) is a 1,6-dicarbonyl compound and reconnection gives (50) which might be made by CH addition to diene (51). k. o-Zysis 2... 

Pyrolysis of the parent thiirane oxide 16a monitored by microwave spectroscopy led to the conclusion that the sulfur monoxide is generated in its triplet ground state, although the singlet state ( A) cannot be excluded completely (equation 8). A later study presented evidence, based on the stereoselective addition to dienes of sulfur monoxide generated from thiirane oxide as well as thermochemical data, that the ground state S is formed exclusively ° . ... 

Iron(II) salts, usually in conjunction with catalytic amounts of copper(II) compounds, have also been used to mediate radical additions to dienes91,92. Radicals are initially generated in these cases by reductive cleavage of peroxyesters of hydroperoxides to yield, after rearrangement, alkyl radicals. Addition to dienes is then followed by oxidation of the allyl radical and trapping by solvent. Hydroperoxide 67, for example, is reduced by ferrous sulfate to acyclic radical 68, which adds to butadiene to form adduct radical 69. Oxidation of 69 by copper(H) and reaction of the resulting allyl cation 70 with methanol yield product 71 in 61% yield (equation 29). 

The concentration of copper(II) has a pronounced effect on the course of the reaction. In the presence of very low copper(II) concentrations, oxidation of allyl radical 69 is slow and major amounts of allyl radical dimer are formed. In the presence of very high concentrations of copper(II), radical 68 is oxidized rapidly before addition to diene can take place. An optimum yield of product 71 can therefore only be achieved at certain copper(II) concentrations. The metal-ion-promoted addition of chloramines to butadiene appears to follow the same mechanism93. 

Finally, another interesting use of singlet oxygen in the oxidation of dienes concerns the reactivity of allenes. Besides the formation of endoperoxides by addition to dienes and hydroperoxide formation via the ene reaction, singlet oxygen reacts with electron-rich... 

Pyramidalization is thought not to be relevant to the different face selectivity of 4 + 2 cycloaddition reactions of diene systems exocyclic to the bicyclo[2,2,l]heptane structure, because selectivity persists for additions to dienes which are not significantly distorted (Gallucci et al., 1985). 

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