Cyclopropyl-cyclobutyl rearrangement

This rearrangement, which accounts for the scrambling, is completely stereospecific.The rearrangements probably take place through a nonplanar cyclobutyl cation intermediate or transition state. The formation of cyclobutyl and homoallylic products from a cyclopropyl-methyl cation is also completely stereospecific. These products may arise by direct attack of the nucleophile on 58 or on the cyclobutyl cation intermediate. A planar cyclobutyl cation is ruled out in both cases because it would be symmetrical and the stereospecificity would be lost. 

It is also evident from the data of Bly et al. (95), Jacobs and Macomber (91), and Garry and Vessiere (99) that neopentyl-type homoallenic systems do not yield cyclopropyl derivatives upon solvolysis, in contrast to the unsubstituted parent system. If they have no substituents at Cj or C3, neopentyl homoallenic substrates yield rearranged acyclic olefins and rearranged solvent-incorporated products exclusively. If they carry an alkyl substituent at Ci, they give both rearranged and unrearranged acyclic products. If a substituent is present on C3 besides the acyclic derivatives, cyclobutyl products also are formed. 

The comparable strain between a cyclobutyl and cyclopropyl ring suggests the feasibility of a vinylcyclobutane to cyclohexane rearrangement. Nevertheless, such a process appears to proceed much less facilely and is complicated by [2 + 2] cycloreversion. Thermal rearrangements of vinylcyclobutanones have been reported (Eq. 92)118). However, the synthetically most useful reactions appear to be the base... 

They have postulated that the solvolysis of 3-substituted cyclobutyl tosylates 12 (X = Alkyl, Ar, Cl, OEt, SiR 3) proceeds through the initial formation of bicyclobutonium ion in the rate determining step, which rearranges stereospecifically to the cyclopropyl-... 

From solvolytic studies of iso topically labeled substrates it was shown that cyclopropyl-carbinyl-cyclobutyl interconversion is stereospecific51 52. The stereospecific interconversion of cyclobutyl cations to the corresponding cyclopropylcarbinyl cation was also cleanly observed in superacid medium, and was used to prepare otherwise unstable cis-(a-methylcyclopropyl)carbinyl cation 1753. Thus ionization of d.s-2-chloro- or cw-3-chloro-l-methylcyclobutane in SbF5-S02ClF at -135 °C yielded the ris-isomer which rapidly rearranged irreversibly into the trans-isomer 18 at about -100 °C. The trows-isomer 18 is the only cation obtained when the preparation was carried out at -80 °C, or when prepared from the cyclopropylmethyl carbinol20b 38 50ac (equation 24). 

This would make it appear that cyclopropyl and cyclobutyl derivatives each solvolyze to give ions that are similar in structure to the starting material. Solvent capture may occur at this stage. If it does not, the first-formed ion rearranges. 

Enyne 190 undergoes platinum(n)-catalyzed ring closures to afford the cyclopropyl-fused 3,4-dihydropyran 191. The reaction is thought to proceed via formation and rearrangement of the intermediate cyclobutyl cation 192 (Scheme 59) <2000JA6785, 2001JA11863>. 

This may all seem far-fetched, but it happens in laboratory reactions too Treatment of the simplest cyclopropyl alcohol with HBr gives cyclobutyl bromide by a similar rearrangement. 

The investigations carried out in this area were done primarily to determine the magnitudes of steric and electronic effects on the solvolytic rates and products of reaction in the cyclopropylcarbinyl cation system. The goal of most of these studies was to learn more about the nature of the charge delocalization in the cyclopropylcarbinyl system and of the stereochemistry of the cyclopropylcarbinyl-cyclobutyl and cyclopropyl-carbinyl-allylcarbinyl cation rearrangements. Key papers in these studies were those in 1966 by Schleyer and Van Dine, in 1971 by Majerski and Schleyer and in 1974 by Poulter and Spillner which demonstrated that in the simple cyclopropylcarbinyl system... 

Paquette and coworkers have carried out a solvolytic study of a tricyclic system in which a 3,4-ethano group is attached to the 2-bicyclo[3.1.0]hexyl system. This was done to assess the importance of cyclopropyl versus cyclobutyl neighboring group involvement. Some relative rate data are given in Scheme 10 for 3,5-dinitrobenzoate hydrolyses in 80 % aqueous acetone at 115 °C. Product results are also shown. Deuterium labeling revealed the lack of any cyclopropylcarbinyl-cyclopropylcarbinyl cation rearrangements. Also, the results showed that cyclopropyl participation predominates over initial involvement of the cyclobutyl group. 

First, we will take up cyclopropyl group formation by the rearrangement of carbon skeletons via cationic intermediates encountered in various mono- and sesquiterpenes, and also examine the illudin biosynthesis where contraction of a cyclobutyl cation to a cyclopropane has been invoked. We will then discuss the head-to-head condensation of isoprenoid alcohols at the C15 or C20 level to generate the cyclopropyl intermediates, presqualene pyrophosphate and prephytoene pyrophosphate, on the way to the C30 and C40 polyene hydrocarbons, squalene and phytoene respectively. Conversion of 2,3-oxidosqualene via common intermediate protosterol cation to cycloartenol or lanosterol represents an important pathway in which the angular methyl group participates in the three-membered ring formation. The cyclopropanation outcome of this process has been carefully studied. 

Bicyclic cyclopropyl compounds are anodically dimethoxylated to give stereoisomeric cis and trans) cyclopropane ring-opened products, as in Eq. (48) [322]. Shono and coworkers [323] reported a different type of ring-opening reaction of a cyclobutyl compound through anodic methoxylation. They also found that anodic oxidation of borneol and isoborneol in methanol resulted in a rearrangement to provide methoxylated stereoisomeric products with the same endo-exo ratio, as in Eq. (49) [324] ... 

Side products are observed in the case of both carbenes for example, 1-chlorocyclobutene was formed from chloro(cyclopropyl)carbene via 1,2-carbon shift (kinetic and activation parameters °° of this rearrangement have been studied), and products of 1,2-carbon and hydride shifts (kinetics of these reactions have been investigated ) for chloro(cyclo-butyl)carbene. The chloro(cyclobutyl)carbene is particularly prone to these rearrangements. 

On the basis of literature data, dealing with the stereochemistry of cyclobutyl-cyclopropyl-methyl cation rearrangements of related secondary cyclobutyl systems, the correlation leads also to absolute configurational assignments of the cyclopropane derivatives by a process retaining the configuration at Cl and inverting it at C3. 

It has been suggested that this rearrangement involves oxidative addition on the metal to give a cyclobutyl cation which, via a metallacyclopentane, rearranges to a new stable cyclopropyl-methyl cation. Then, subsequent reductive elimination of the metal ion would yield the... 

The interesting formal parallel that exists between the rearrangements of the chrysanthemyl cation and the conversion of presqualene alcohol into squalene (and now of prephytoene alcohol into phytoene) has been further explored. Solvolyses ° of the cyclopropyl (65) and cyclobutyl (63) esters both afford head-to-head coupled Cio chains analogous to squalene. A versatile new method provides access to 9-substituted p-menthanes. This starts with natural limonene and proceeds via the anion (135) which retains chirality and leads to chiral products (see below). Skeletal rearrangements in the bicycloheptane series, an historic field in the study of organic reaction mechanisms, has received a fresh impetus from the extended work of Kirmse and his colleagues, - which is of preparative and mechanistic significance. 

The l-(trimethylsilyl)cyclobutyl cation is obtained by reaction of [l -(trimethylsilyl)cyclopropyl]-methanol with SbFs at -130°C. It has a hypercoordinated puckered l-(trimethylsilyl)-bicyclobutonium structure 5. Cation 5 undergoes a fast threefold degenerate methylene rearrangement (compare Fig. 2 with R = SiMe3). Averaged NMR signals are observed for the... 

Extensive studies have been devoted to these radicals in connection with their structure (classical or nonclassical) and their reactivity (homoallylic rearrangement and 1,2-vinyl migration) and the main results have been reviewed.For instance, the classical nature of the allylcarbinyl, cyclopropyl carbinyl, and cyclobutyl radicals now seems well established. 

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