Oxygen-stabilized carbocations

Oxygen stabilized carbocations of this type are far more stable than tertiary carbocations They are best represented by structures m which the positive charge is on oxygen because all the atoms have octets of electrons m such a structure Their stability permits them to be formed rapidly resulting m rates of electrophilic aromatic substitution that are much faster than that of benzene... 

When this sequence is applied to carbohydrates, the first step takes place intramoleai-larly to yield a cyclic hemiacetal. The second step is intermolecular, requires an alcohol R"OH as a reactant, and proceeds readily only in the presence of an acid catalyst. An oxygen-stabilized carbocation is an intermediate. 

The first example of the use of the catalytic one-pot procedure described above, in which the deselenenylation occurs with substitution, is represented by the conversion of vinyl halides into a-alkoxy acetals [116]. This is illustrated in Scheme 38 in the case of -bromostyrene 235. The regioselective methoxy-selenenylation affords the a-bromo selenide 236, which undergoes a rapid solvolysis, through a selenium-stabilized carbocation to produce the selenide 237. Oxidation of this alkyl phenyl selenide with ammonium persulfate produces an oxygen stabilized carbocation, which affords the final product 238, and, at the same time, regenerates the selenenylating agent. 

Reaction of a divinyl alkyne such as 137 with aqueous acid generates a conjugated ketone (138), via hydration of an intermediate vinyl cation. Subsequent treatment with a mixture of phosphoric and formic acids leads to a new, oxygen-stabilized carbocation (139), which reacts with the adjacent alkene (a cyclization reaction) to form 140. Elimination led to the conjugated ketone (141). 3 This cationic ring-closing reaction... 

Step 5 Loss of water from the protonated hemiacetal gives an oxygen-stabilized carbocation. Of the resonance structures shown, the more stable contributor satisfies the octet rule for both carbon and oxygen. 

Step 6 Nucleophilic attack of ethanol on the oxygen-stabilized carbocation... 

The oxygen-stabilized carbocation that is formed reacts with the glycosyl acceptor ROH to form an anomeric mixture of (9-glycosides. 

Using curved arrows, write a mechanism to show how each of the glycosyl donors produces the oxygen-stabilized carbocation. For part (a) consult Section 16.17 on sulfides and 8.12 on triflates for part (b) examine step 2 in Mechanism 19.8, and assume that silylation occurs at nitrogen in the glycosyl imidate ester. 

The protonated hythoxy group is lost as water. This results in the formation of an oxygen-stabilized carbocation. 

When the oxocarbenium is drawn as resonance contributor 52A, it is an oxonium salt, clearly related to oxonium salts derived from ethers or alcohols in the preceding sections. Oxonium ion 52 is actually more stable than 13 or 47 (see preceding discussion) because it is resonance stabilized, and the positive charge can be delocalized on carbon (see 52A and 52B). The resonance form that has a positive charge on carbon (52B) is considered to be an oxygen-stabilized carbocation (hence the term oxocarbenium ion see Chapter 16, Section 16.3) and it can react with nucleophiles. Oxocarbenium ions will be generated in several chemical reactions to be presented in later chapters, and their reactions with suitable nucleophiles will be examined at that time. Ketones behave similarly. 

For a discussion of whether this is a metal carbene or a metal-stabilized carbocation or both or something in between, see Fiirstner, A. Davies, P. W. Angew. Chem., Int. Ed. 2007, 46, 3410. Similarly, an oxonium ion, may also be viewed as an oxygen-stabilized carbocation. 

Oxygen-Stabilized carbocations of this type are far more stable than tertiary carbocations. They are best represented by structures in which the positive charge is on oxygen because... 

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