Oxygen-substituted carbocations

These values have been discussed in some detail to indicate that care is required to take account of the differences in solvents for measurements. They illustrate, nevertheless that a good framework of stabilities of benzyl-related carbocations exists. Other (oxygen-substituted) benzylic cations for which pAR measurements have been reported are discussed below (p. 57-63). 

This concludes the discussion of the stabilities of carbocations with hydrocarbon-based structures and also of different methods for deriving equilibrium constants to express these stabilities. The remainder of the chapter will be concerned mainly with measurements of stabilities for oxygen-substituted and metal ion-coordinated carbocations. Consideration of carbocations as conjugate acids of carbenes and derivations of stabilities based on equilibria for the ionization of alkyl halides and azides will conclude the major part of the chapter and introduce a discussion of recent studies of reactivities. 

Effects of oxygen substitutents in an aromatic ring upon an exocyclic rather than endocyclic carbocation charge center have also been measured. The possibility of comparing HO, MeO, and O substituent effects for the benzylic cations is provided by recent studies of quinone methides, including the unsubstituted / -quinone methide 23, which may be considered as a resonance-stabilized benzylic cation with a /xoxyanion substituent. 

The lower reactivity of the o-thioquinone methide 81 (7.0 x 104 M-1 s 1) compared with the o-l (8.4 x 105 M-1 s 1) in acidic solution contrasts with the much higher reactivity of 81 at neutral pH (Table 1). This may represent the balance between the smaller concentration of the protonated thioquinone methide compared with protoned o-l due to the weaker basicity of o-S (pATa < —3 Table 1) compared with o-S (pKa < —1.7) and the presumably greater intrinsic reactivity of H-81 +. 58 However, the observed effects of sulfur for oxygen substitution on carbocation reactivity have in the past proven very difficult to rationize,130,160 and in the present case are probably not fully understood. 

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... 

The results of studies of the acid-catalyzed hydration of oxygen-, sulfur-, seleno-and nitrogen-substituted alkenes and the relevance of this work to partitioning of the corresponding carbocation intermediates (Chart 1) between deprotonation and nucleophile addition was reviewed in 1986.70. We present here a brief summary of this earlier review, along with additional discussion of recent literature. 

We suggest that the substitution of an a-oxygen by a a-sulfur or a-selenium at an oxocarbenium ion has several effects on the reactivity of the carbocation, each of which contributes to the large decrease in value of k%jkp for partitioning of the carbocation ... 

The substitution of an a-oxygen by an a-sulfur or an a-selenium results in a decrease in ks/kp for partitioning of oxocarbenium ions between addition of solvent and deprotonation. The largest effect is observed for the a-selenium substitution. These changes are probably the result of several different effects on carbocation reactivity, each of which contributes to the observed substituent effect on kjkp. 

Treated with trifluoroacetic anhydride, sulfoxides 218 undergo conversion to triazapentalenes 219 with high yields. The process must involve acylation of the sulfoxide oxygen atom and generation of a carbocation that attacks the N-2 atom of benzotriazole. Hydrogenation over Raney nickel cleaves the C-S and one of the N-N bonds to generate >rtfe -substituted anilines 220 (Scheme 27) <2002EJ0493>. 

We have examined the competing isomerization and solvolysis reactions of 1-4-(methylphenyl)ethyl pentafluorobenzoate with two goals in mind (1) We wanted to use the increased sensitivity of modern analytical methods to extend oxygen-18 scrambling studies to mostly aqueous solutions, where we have obtained extensive data for nucleophilic substitution reactions of 1-phenylethyl derivatives. (2) We were interested in comparing the first-order rate constant for internal return of a carbocation-carboxylate anion pair with the corresponding second-order rate constant for the bimolecular combination of the same carbocation with a carboxylate anion, in order to examine the effect of aqueous solvation of free carboxylate anions on their reactivity toward addition to carbocations. 

A new method for the synthesis of 2-substituted, as well as 2,4- and 2,5-disubstituted, cyclopentanones in 53-93% yield has been reported.81 For example, the Lewis acid catalyzed transformation of l-propanoyl-l-(4-tolylsulfanyl)cyclobutane gave 2-ethyl-2-(4-tolylsulf-anyl)cyclopentanone (1) in 93 % yield. The formation of the cyclopentanone is best explained by a mechanism which involves initial coordination of aluminum trichloride to the carbonyl oxygen, followed by ring expansion to form the sulfur-stabilized carbocation. Finally, migration of the ethyl group to the carbocation center regenerates concomitantly the carbonyl function.81... 

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