Carbocations carbocationic complex

The authors succeeded in preparing (20-25) a series of long-lived carbocations bearing vinyl and related groups in the p-position to the carbocationic center (la-d, 3). Their structures were confirmed by and 13C NMR spectroscopy. These cations can be considered as a-complexes of C-centered electrophilic agents, namely vinyl and methyl-substituted vinyl cations derived from 9,10-dimethylphenanthrene and 1,2-dimethylacenaphthylene, respectively. Quite naturally, they had not been prepared by electrophilic vinyl-... 

Complexation of an alkyne to dicobalthexacarbonyl is a well-known way to stabilize carbocationic charges generated in the carbon a to the alkyne. These carbocations react with different nucleophiles. This process, the Nicholas reaction [112], has been used to generate enynes that undergo, in a domino fashion, a PKR. 

According to quantum chemical calculations, additions of carbocations to alkenes are expected to proceed via 7r-complexes [84, 85]. The high antistereoselectivity of carbocationic cyclizations had been explained by this hypothesis [86-88]. 

The investigations described in this chapter show that nature and concentration of the negative counterion has a great influence on the outcome of carbocationic telomerizations because of the different rates of ion-pair collapse and because of reionization of eventually produced 1 1 adducts from alkyl halides and alkenes. On the other hand, it was found that the rate of attack of a carbocation at an alkene is generally independent of the nature of the complex counterion and that free and paired carbocations react with equal rates. If this conclusion also holds for carbocationic polymerizations, a reinterpretation of many polymerization kinetics becomes necessary. 

Organozirconates. Zwitterionic zirconocene-ate see Ate Complexes) intermediates have been evoked to explain some reaction mechanisms (see Section 2.3.3). In fact, electron transfer from an organic ligand to 16-electron metal center permits the generation of a carbocationic center which can, then, undergoes isomerization and rearrangement characteristic of carbocations. It is now possible to isolate and characterize stable zwitterionic phosphonium zirconate complexes (eqnation 18). ... 

Stereospecific 2,3-epoxidation of squalene, followed by a nonconcerted carbocationic cyclization and a series of carbocationic rearrangements, forms lanosterol [79-65-0] (77) in the first steps dedicated solely toward steroid synthesis (109,110). Several biomimetic, cationic cydizations to form steroids or steroidlike nuclei have been observed in the laboratory (111), and the total synthesis of lanosterol has been accomplished by a carbocation—olefin cydization route (112). Through a complex series of enzyme-catalyzed reactions, lanosterol is converted to cholesterol (2). Cholesterol is the principal starting material for steroid hormone biosynthesis in animals. The cholesterol biosynthetic pathway is composed of at least 30 enzymatic reactions. Lanosterol and squalene appear to be normal constituents, in trace amounts, in tissues that are actively synthesizing cholesterol. 

It is evident that the interaction of a phosphorus(III) triester and the alkylating species RX can be pictured as an S 2 process (reaction 2) or, for those alkylating reagents capable of forming a carbocation, as an S l process (reaction 3). Several reactions testify to the importance of carbocationic carbon for the Michaelis-Arbuzov reaction in pursuance of its normal course they include the ease of reaction of cyclopropene dihalides, already encountered, and the ready formation of complexes with species having particularly weakly nucleophilic counter ions. Phosphonic acid formation also takes place with cyclic azonium salts and related ions. 9-Chloroacridine reacts with triethyl phosphite to afford a product thought to be the bisphosphonic acid ester 49 The related phosphonic esters 51 are obtainable when the onium salts 50 (X = NH, NR, O or S) are treated with trimethyl... 

The epoxidation of olefins does not appear at this time to involve radical intermediates in most systems, but epoxidation through two different spin states of the same complex has gained acceptance. " In some cases, side products imply that intermediates are present, but these intermediates have been concluded by Bruice to be carbocationic instead of radical. The carbocation would allow for Z/E isomerization and formation of oxidation products other than epoxides. " Studies on the epoxidations of an olefin attached to... 

Although the formation of the Zaitsev products is typical in El processes, it cannot be taken as evidence for the carbocationic mechanism Rearrangement of the intermediate carbocation and consecutive alkene isomerization (double bond migration) induced by the acidic catalyst may occur. These processes strongly affect product distribution and eventually give rise to the formation of a very complex mixture of isomeric alkenes. 

This one-step mechanism avoids the formation of a vinyhc carbocation but still invokes a transition state that exhibits some partial carbocationic character (notice the 6- - shown in the transition state). The development of a partial positive charge can effectively explain the observed regioselectivity, because the transition state will be lower in energy when this partial positive charge forms at the more substituted position. This more complex mechanism is supported in many cases by kinetic studies in which the rate expression is found to be overall third order ... 

The reason for this is simple. Since we are forming a complex with carbocationic character, it is possible for a carbocation rearrangement to occur. It is not possible for a methyl carbocation to rearrange. Similarly, an ethyl carbocation cannot rearrange to become any more stable. But a propyl carbocation CAN rearrange (via a hydride shift) ... 

---