Halogens nucleofuge

These results suggest an unprecedented halogen nucleofugality (Br>I) for a halogen-metal exchange (HME) or ET reaction. However, competition experiments of 1-iodo and l-bromobicyclo[2.2.2]octanes established that the iodine is ca twice as reactive as the bromine derivative131. 

Apart from a,fl-unsaturated carbonyl compounds having 0-, N- and S-leaving groups at the p-position, compounds 706 with halogen nucleofuges (i.c. Cl, F) were also used in the synthesis of 4-fluoroalkylpyrimidines. In particular, reaction of aldehyde 775 [481], ketone 776 [482], or ester 777 [483] with amidines or guanidines gave pyrimidines 778-780 in 39-98 % yields (Scheme 154). 

Reactions of P-halogen-NHPs with Lewis acids leading to halide abstraction, or metathetic replacement of the halide by a nucleofugic, noncoordinating anion form by far the most important routes to access phosphenium ions in general [51, 52] and also 1,3,2-diazaphospholenium cations in particular. As Lewis acid assisted P-X... 

Other substrates, in which one of the two nucleofuges is not a halogen, have been studied. Haloarene-diazo sulfides on treatment with arenethiolate ions give substantial formation of bis(arylthio)arenes except that the monofluorophenyl sulfide is produced when the halogen is fluorine,64 and p-iodophenyl-trimethylammonium ion, on treatment with benzenethiolate ion in liquid ammonia, gives p-bis(phenyl-thio)benzene in 95% yield.90... 

In these reactions the nucleophilic carbanion is produced either by deprotonation or by metal reduction of a C-halogen bond. It is interesting to note that, as in other cyclopropane formation reactions , even a sluggish nucleofuge such as EtO" can be displaced in these intramolecular nucleophilic reactions (equation 5). 

The sulfur species is thus activated by S-halogenation into a nucleofugic leaving group, which is substituted by fluoride. The fluorodesulfuration of thiocarbonyl compounds is supposed to follow a similar principal pathway. 

Alcohols with a nucleofuge leaving group in the r-position can be cyclized to give oxetanes. Thus the cyclodehydrohalogenation of halo alcohols occurs in an analogous way to the oxirane synthesis from y -halogenated alcohols (see p 20). Oxetanes can be prepared from 1,3-diols via monoarene sulfonates ... 

From the very mechanism it stems that L in the carbanions should be a nucleofugal group, which is able to be eliminated from the o adducts as HL. Besides halogen atoms (Cl, Br), alkoxy, aryloxy, alkylthio, and arylthio groups and many other substituents can be eliminated in this way. There are practically no limitations concerning substituents R and groups Y, stabilizing the carbanions. 

Replacement of the halogens of the above precursors with different nucleofugic groups generates another set of aryne sources (32g-321) which once again probably react via the o-substituted anions (34). 

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