Carbocations cyclopropylcarbinyl

Figure 1 Calculated structure of the cyclopropylcarbinyl carbocation over zeolite Ysurface, at B3LYP/6-31++G(d,p) MNDO.

A synthesis of 4-alkyl-2-methoxy-1-phenylthio-1,3-butadienes by a simple -elimination of thiophenol from a thioacetal is not possible owing to skeletal rearrangement that is fostered by stabilization of a cyclopropylcarbinyl carbocation intermediate by the alkyl substituent (eq 51). Interconversion of an initial a-phenylthio carbocation to a more stable a-methoxy carbocation intermediate leads to the generation of a 4-alkyl-l-methoxy-2-phenylthio- 1,3-butadiene instead. 

The rearrangement of the intermediate alkyl cation by hydrogen or methyl shift and the cyclization to a cyclopropane by a CH-insertion has been studied by deuterium labelling [298]. The electrolysis of cyclopropylacetic acid, allylacetic acid or cyclo-butanecarboxylic acid leads to mixtures of cyclopropylcarbinyl-, cyclobutyl- and butenylacetamides [299]. The results are interpreted in terms of a rapid isomerization of the carbocation as long as it is adsorbed at the electrode, whilst isomerization is inhibited by desorption, which is followed by fast solvolysis. 

This is less common than rearrangement of carbocations, but it does occur (though not when R = alkyl or hydrogen see Chapter 18). Perhaps the best-known rearrangement is that of cyclopropylcarbinyl radicals to a butenyl radical. The rate constant for this rapid ring opening has been measured in... 

Carbocations on Surfaces Formation of Bicyclobutonium Cation via Ionization of Cyclopropylcarbinyl Chloride over NaY Zeolite... 

The l3C NMR spectrum of the C4H7+ cation in superacid solution shows a single peak for the three methylene carbon atoms (72) This equivalence can be explained by a nonclassical single symmetric (three-fold) structure. However, studies on the solvolysis of labeled cyclopropylcarbinyl derivatives suggest a degenerate equilibrium among carbocations with lower symmetry, instead of the three-fold symmetrical species (13). A small temperature dependence of the l3C chemical shifts indicated the presence of two carbocations, one of them in small amounts but still in equilibrium with the major species (13). This conclusion was supported by isotope perturbation experiments performed by Saunders and Siehl (14). The classical cyclopropylcarbinyl cation and the nonclassical bicyclobutonium cation were considered as the most likely species participating in this equilibrium. 

There are no reported studies of this rearrangement on the zeolite surface and we argued that it could give some clues to the alkyl-aluminumsilyl oxonium ion/carbocation equilibrium. In this work we show experimental and theoretical results on the rearrangement of the cyclopropylcarbinyl chloride over NaY zeolite, aiming at demonstrating the equilibrium between the carbocation and the alkyl-aluminumsilyl oxonium ion. 

The three alkyl-aluminumsilyl oxonium ions are more stable than the carbocations, with the allylcarbinyl aluminumsilyl oxonium ion lying 4.5 and 4.7 kcal.mor1 lower in energy than cyclobutyl. and cyclopropylcarbinyl aluminumsilyl oxonium ions, respectively. This result is in agreement with the thermodynamic stability of the respective chlorides. 

Solvolytic studies provided the first structural indication for almost every carbocation-ic intermediate and the C4H,+ ion is no exception. Roberts observed that the solvolysis of cyclopropylcarbinyl or cyclobutyl systems and the diazotative deamination reactions of cyclopropylcarbinyl amine or cyclobutyl amine gave similar product mixtures consisting of cyclopropylcarbinyl, cyclobutyl and allylcarbinyl derivatives in essentially the same ratio1,2. A common cationic intermediate of C3v structure, the tricyclobutonium ion 1, was... 

That maximum acceleration occurs when the vacant p orbital is parallel to the plane of the cyclopropyl ring can be seen from the solvolysis of spiro[cyclo-propane-l,2 -adamantyl] chloride (71). The carbocation formed by departure of Cl is unable to adopt the geometry of the bisected cyclopropylcarbinyl cation, but can orient its empty p orbital properly to form the bicyclobutonium ion. This compound solvolyzes 103 times more slowly than 1-adamantyl chloride.82 On the other hand, 72 solvolyzes 10s times faster than 73. The cation from 72 does have its p orbital parallel to the plane of the ring as in the bisected cyclopropylcarbinyl cation.83... 

Long-lived cyclopropylcarbinyl cation chemistry, including spiro cations and dications, has been reviewed,7 and some of the more interesting newer carbocations, such as (1), are the subject of a short survey.8 The use of secondary deuterium isotope effects in the study of carbocation-forming reactions has been revisited,9 and the... 

Although radicals are not nearly so prone to rearrangement as are, for example, carbocations, there are a few such rearrangements which have become identified as characteristic of carbon radicals. These include radical cyclizations, particularly the 5-hexenyl radical cyclization, and radical C-C bond cleavages, particularly the cyclopropylcarbinyl to allyl carbinyl radical rearrangement. In hydrocarbon systems, as organic synthetic chemists have learned how to control rapid chain processes, such rearrangements have become important synthetic tools [176-179]. 

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