Cyclopropyl cation transformations

This chapter begins by classifying the combinations of oxidation/reduction processes with subsequent cationic transformations, though to date the details of only two examples have been published. The first example comprises an asymmetric epoxidation/ring expansion domino process of aryl-substituted cyclopropyl-idenes (e. g., 7-1) to provide chiral cyclobutanones 7-3 via 7-2, which was first described by Fukumoto and coworkers (Scheme 7.1) [2]. 

It is interesting to mention the entirely different situation in the cyclopropyl-allyl cation system the ring-opening reaction is very fast as compared to the isomerization of the allyl cation. In agreement with this situation the disrotatory mode of the cyclopropyl-allyl cation transformation has been much easier to verify ... 

It was shown that the cyclopropyl cation is not an intermediate but that ring-opening occurs simultaneously with loss of tosylate. Prediction and experiments showed that for this electrocyclic transformation, susceptible to treatment by the Woodward-Hoffmann rules , the substituents cis to the leaving group rotate inwardly and substituents trans to the leaving group rotate outwardly (equation 2). 

Under the category of kn, k = 4q, electrons system, we can cite the ring closures of allyl anion to cyclopropyl anion, 1,3- and 1,4-pentadienyl cation to cyclopen-tenyl cation, and 1,3,5,7-octatetraene to 1,3,5-cyclooctatetraene. Under the category kn, k = 4q + 2, we can quote the ring closures of allyl cation to cyclopropyl cation and 1,3- and 1,4-pentadienyl anion to cyclopentenyl anion. The chemical equations for these transformations are given below. 

The Woodward-Hoffmann ru are not limited m application to the neutral systems that have been discussed up roTHis point. They also apply to charged systems. The conversion of cyclopropyl cation to allyl cation has been thoroughly studied and represents the simplest possible case of an electrocyclic transformation, since it involves only two tt-electrons. Because of the restrictions imposed on the internuc-lear angles in cyclopropyl rings, carbonium ions do not form readily, and cyclopropyl halides and arenesulfonates are quite unreactive under ordinary solvolytic conditions. For example, it was found that temperatures of 180°C were necessary for cyclopropyl tosylate to react in acetic acid, and the product was allyl acetate, rather than cyclopropyl acetate. A mechanism was considered in which cyclopropyl cation was formed in the rate-determining step, followed by rapid conversion to allyl... 

The usual notion of the coordinate of electrocyclic reactions is associated with the rotation of the end groups about double bonds. The conrotatory motion is thermally allowed for the reaction XXI-XXIII. Semiempirical [2,36-42] and ab initio [43-45] calculations of the critical regions of the PES of this reaction and of the still simpler cyclization of the allyl cation to the cyclopropyl cation have greatly refined the overall picture of the intrinsic mechanism and revealed some important distinguishing features common to all electrocyclic transformations. 

As far as the carbon atoms are concerned, there are fairly drastic charge migrations involved in this transformation. Thus, Ci which carries a substantial positive charge in cyclopropyl cation becomes C2 with a substantial negative charge in allyl cation. As a result, the Cis orbital energy changes from -11.7122 a.u. to -11.5613 a.u. 

As discussed earlier, Ila, Junjappa and coworkers used cyclopropyl units as cation-provider in cationic domino processes. Within their interesting approach, the indole derivatives 1-170 could be converted into the unexpected carbazoles 1-171 with 54-69% yield in a five-step transformation using SnCl4 as reagent (Scheme 1.41) [48],... 

Solvolysis of cyclopropyl derivatives leads directly to the allyl cation the ring opening is disrotatory as predicted. The most direct demonstration is the transformation of the 2,3-dimethyl-1-chlorocyclopropanes at — 100°C in strong acid... 

Initial interest in the solvolyses of cyclopropyl derivatives stemmed from the observation that they underwent solvolysis with concerted ring-opening , and that the reaction was strongly dependent on the nature and stereochemistry of substituents on the ring. This was explained by Woodward and Hoffmann who predicted from orbital symmetry considerations that the electrocyclic transformation in which a cyclopropyl carbocation is converted to an allyl cation should occur in a disrotatory fashion. Also, the particular disrotatory path a given system will take should be dependent on the stereochemistry of the leaving group. This is illustrated as follows. 

This section covers the formation of cyclopropanes via cyclization of reactive allylic intermediates (cations, anions, radicals). Included are those transformations of allylic functional derivatives (e.g. allylic halides, alcohols, aldehydes, ketones, acids, esters, boronates, Grignard reagents) to cyclopropyl derivatives that do not actually proceed via allylic reactive intermediates, but which are not covered by other sections of this volume. Additionally, this section will cover methods for the formation of cyclopropanes by pericyclic reactions. 

Consequently the formation of both these compounds can be visualized by the pathways shown on the previous page (with R changing from methoxycarbonyl to the carboxy functionalities in the final products). Direct attack of the intermediate cyclopropylmethyl cation C by water and loss of a proton leads to the nonisolated intermediate E which subsequently lactonizes as a consequence of the cis relationship of the cyclopropyl substituents. Similarly, the indirect attack of the nucleophile leads to the nonisolated hydroxy acid corresponding to F which on workup is transformed into a tetrahydrofuran derivative. 

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