Other double-bond reactions

The best-known nitrogen-containing compounds (other than those associated with the carboxyl group discussed in Section 10.15) are the aziridines or epimino compounds and the amino compounds derived from these by ring-opening processes. These compounds have a formal resemblance to the epoxides and diols and to the corresponding sulphur compounds. 

Epithio compounds are obteuned from epoxides by reaction with sulphur-containing compounds such as thiourea, thiocyanogen, thiocyanates, potassium methyl xanthate and base, or 3-methylbenzothiazol-2-one and trifluoroacetic acid. 9,10-Epithiostearic acid melts at 58 °C (cis) and 64 °C (trans). Sulphur-containing compounds can also be made by radical addition (usually photolytic) of hydrogen 

Aziridines are formed from alkenes in a number of multistep processes involving initial reaction with iodine azide, iodine isocyanate or NN-dichlorourethane  

Some of the more important reactions of the aziridines cire set out in the following equations. The v/c-diamine can also be prepared from an epoxide by successive reaction with sodium azide, methane-sulphonyl chloride, sodium azide and hydrogen/ platinum. 

Nitrogen-containing compounds are also prepared by the Ritter reaction (see Section (d)). 

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Other Double-Bond Reactions. - Alkenoates react with dimethyl sulphide and iodine to give bisthioethers,the mass spectra of which are useful for locating the original position of the double bond. 

The results suggest that once hydroalumination has occurred at one end of the diene, the course of the reaction at the other double bond is significantly affected by the alkylaluminum group, possibly due to chelation in which a palladium-chloride-aluminum bond is thought to be important. 

In an interesting reaction, -santonin was reduced with lithium in liquid ammonia so that the lactone was hydrogenolyzed to an add and one of the double bonds conjugated with the carbonyl was reduced. The other double bond as well as the keto group did not undergo reduction [1091]. 

As has been exemplified in this chapter, fluorinations with fluorine-18 can be classified into two categories (1) the nucleophilic reactions, which usually involve no-carrier-added [ F]fluoride of high-specific radioactivity as its Kf FIF-K complex and include substitutions in the aliphatic and the /lomoaromatic series and (2) the electrophilic reactions, which mainly use moderately low-specific radioactivity molecular [ F]fluorine, or other reagents prepared from it, such as acetyl [ F] hypofluorite, and include addition across double bonds, reactions with carbanions and especially fluorodehydrogenation and fluorodemetallation reactions. 

Endocyclic allenes which contain other double bonds lead to rearranged products. An example is the reaction of dibromobicyclononatriene (17) with methyllithium to form initially a carbenoid (18) which... 

In the above discussion of stereoselectivity the mechanisms of various reactions have been used to rationalize why some are stereoselective and some are not. Thus the bromination of olefins proceeds via a bridged bromonium ion intermediate and gives only trails addition across the double bond [reactions (6.2) and (6.3)]. In contrast, the addition of HBr across a double bond gives a carboca-tion intermediate that does not maintain the facial integrity of the olefin and is thus much less stereoselective [reaction (6.1)]. In these examples the mechanism of the reaction is used to explain and understand the diastereoselectivity that is observed. There are many other examples (usually in textbooks) where the mechanism of a reaction is used to rationalize the stereoselectivity of the process. To do this requires that the mechanism be known with certainty. 

Squalene is then converted into squalene epoxide in a reaction that uses 02 and NADPH (Fig. 2b). The squalene epoxide cyclizes to form lanosterol, and finally cholesterol is formed from lanosterol by the removal of three methyl groups, the reduction of one double bond by NADPH, and the migration of the other double bond (Fig. 2b). 

There are nonetheless just a few isolated examples of three- or four-membered ring forming anionic cyclisations curiously it was in fact a three- and a four-membered ring forming reaction which provided the first two pieces of evidence that organolithiums do indeed cyclise onto unactivated double bonds. In 1960, Wittig107 showed that n-BuLi not only added to one of the double bonds of norbornadiene but that the major product of the reaction arose from organolithium cyclisation of the intermediate 201 onto the other double bond. This odd reaction is presumably made possible by the strain already inherent in the bicyclic system. After protonation, the major product was 202. 

Reactions of fluorine with other double bonds and aromatic compounds.678... 

With these reactions, a vitamin A synthesis based on acetylene was developed in the 50s, but this synthesis gained no industrial significance since the individual synthesis steps were not economically practicable. It was not possible to realise the concept aimed at, namely to link a C15 unit with a C5 building block, in the last stage. For this purpose, a reaction was required in which C—C linkage takes place with the formation of an olefmic double bond. The other double bonds are also possible linking points for the synthesis of the vitamin A molecule (2) ... 

The Regioselectivity ofHetero Diels-Alder Reactions. In a few cases, carbonyl, nitrosyl, cyano, and other double bonds with one or more electronegative heteroatoms have acted as dienophiles in Diels-Alder reactions. The carbonyl group has a HOMO and a LUMO as shown in Fig. 1.51. The energies of both orbitals are relatively low, and most of their Diels-Alder reactions will therefore be guided by the interaction between the HOMO of the diene and the LUMO of the carbonyl compound. This explains the regioselectivity in the cycloaddition of dimethylbutadiene 6.176 and formaldehyde, and between 1-substitituted butadienes 6.177 and nitrosobenzenes. 

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