Section 1.2 Cationic rearrangement

In Chapter 4 the decision to ionize or not to ionize the leaving group was part of your ability to predict whether the first step in an El or Sjsjl would proceed. Several other similar decisions are collected in this chapter. The most difficult choice is between a species serving as a nucleophile or as a base (Section 9.5). Other decisions are basically a choice of regiochemistry (Sections 9.3, 9.4, 9.6). One (Section 9.2) is a question of extent of reaction stop or keep going. Another (Section 9.7) is the competition between internal reactions and external ones. The last (Section 9.7) is the competition between nucleophilic trapping and cation rearrangement. 

This section covers only cationic rearrangements of halocyclopropanes. Rearrangements induced by exocyclic negative charge are treated in Section 2.4.1.3.2. 

In Chapter 4 you learned that carbocations could be captured by halide anions to give alkyl halides In the present chapter a second type of carbocation reaction has been introduced—a carbocation can lose a proton to form an alkene In the next section a third aspect of carbocation behavior will be described the rearrangement of one carbo cation to another... 

Dehydration of alcohols (Sections 5 9-5 13) Dehydra tion requires an acid catalyst the order of reactivity of alcohols IS tertiary > secondary > primary Elimi nation is regioselective and proceeds in the direction that produces the most highly substituted double bond When stereoisomeric alkenes are possible the more stable one is formed in greater amounts An El (elimination unimolecular) mechanism via a carbo cation intermediate is followed with secondary and tertiary alcohols Primary alcohols react by an E2 (elimination bimolecular) mechanism Sometimes elimination is accompanied by rearrangement... 

Figure 27.14 MECHANISM Mechanism of the conversion of 2,3-oxidosquaJene to lanosterol. Four cationic cyclizations are followed by four rearrangements and a final loss of H+ from C9. The steroid numbering system is used for referring to specific positions in the intermediates (Section 27.6). Individual steps are explained in the text.

We mention Williams work briefly here because it may also explain Blangey s observations strongly basic primary amines unequivocally form 7V-nitrosoanilinium ions in strongly acidic media. In contrast to the rate-limiting deprotonations of the less basic aromatic and heteroaromatic nitrosoamine cations discussed in this section, the TV-nitroso cation of a strongly basic amine deprotonates extremely slowly. Therefore, the nitroso rearrangement, the Fischer-Hepp reaction, competes effectively with the 7V-deprotonation. 

Their precursors must be the tricarbonyl o-allenyls with the uncoordinated C=C bonds. Neither an allylic rearrangement nor cis-trans isomerization has been observed in the reaction of CpMo(CO)3(cw-CH2CH=CHMe) with PPhj, the product being CpMo(CO)2(PPh3)(cw-COCH2CH=CHMe) (81). The interesting reaction leading to the formation of cationic carbene compounds was mentioned earlier [Eq. (17) and Section V] (78). 

Moreover, because of the involvement of cationic intermediates, rearrangements can occur in systems in which a more stable cation can result by aryl, alkyl, or hydrogen migration. Oxymercuration-reduction, a much milder and more general procedure for alkene hydration, is discussed in the next section. 

The subjects of this section are two reactions that do not actually involve carbo-cation intermediates. They do, however, result in carbon to carbon rearrangements that are structurally similar to the pinacol rearrangement. In both reactions cyclic intermediates are formed, at least under some circumstances. In the Favorskii rearrangement, an a-halo ketone rearranges to a carboxylic acid or ester. In the Ramberg-Backlund reaction, an a-halo sulfone gives an alkene. 

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