Cyclobutyl, cation, rearrangement

In accordance with the previous observations of Dauben and Schleyer the cyclopropylcarbinyl-cyclobutyl cation rearrangement proceeds with net inversion at the migration origin upon nucleophilic attack. 

In a similar manner, the corresponding cyclopentane annulated spirocyclobutanones 40 were obtained in high yield. Clearly, initial formation of a terminal oxonium ion which undergoes stereoselective addition to the vinyl bond induces the cyclopropylmethyl to cyclobutyl cationic 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. 

Artemisyl, Santolinyl, Lavandulyl, and Chrysanthemyl Derivatives.— The presence of (41) in lavender oil has been reported earlier. Poulter has published the full details of his work (Vol. 5, p. 14) on synthetic and stereochemical aspects of chrysanthemyl ester and alkoxypyridinium salt solvolyses (Vol. 3, pp. 20—22) and discussed its biosynthetic implications. Over 98% of the solvolysis products are now reported to be artemisyl derivatives which are formed from the primary cyclopropylcarbinyl ion (93) which results from predominant (86%) ionization of the antiperiplanar conformation of (21)-)V-methyl-4-pyridinium iodide the tail-to-tail product (96 0.01%) may then result from the suprafacial migration of the cyclopropane ring bond as shown stereochemically in Scheme 3. This is consistent with earlier work (Vol. 7, p. 20, ref, 214) reporting the efficient rearrangement of the cyclobutyl cation (94) to (96) and its allylic isomer, via the tertiary cyclopropylcarbinyl cation (95). ... 

However, the ring expansion of 1-substituted tertiary cyclopropanemethanols to tertiary fluo-rocyclobutanes is efficiently brought about with pyridinium poly(hydrogenfluoride) in the presence of diisopropylamine and potassium fluoride-hydrogen fluoride.19 2(1 Under the strictly anhydrous conditions employed, the intermediate cyclobutyl cation is efficiently trapped. Illustrative is the rearrangement of methanesulfonate 9, the key step in a synthesis of ( )-2-fluorograndisol.21 Other examples are collected in Table 3. 

The final step in an assemblage of the (+ )-isocomene skeleton also requires the cyclobutyl-methyl to cyclopentyl cation rearrangement. However, the desired cyclobutylmethyl cation was generated by protonation of an alkene. Thus, p-toluenesulfonic acid induced rearrangement converted 2,6,8-trimethyl-5-methylenetricyclo[6.3.0.016]undccane to isocomene (6).20-21... 

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). 

No reactions of t with protic solvents have been reported however, its cyclic analogue 1,2-diphenylcyclobutene (7) reacts with the protic solvents methanol, acetic acid, and water, to yield adducts 85 and 86 (eq. 28). The proposed mechanism for the formation of 85 and 86 involves the formation of singlet exciplex followed by proton transfer to yield a cyclobutyl cation 87. Stereoselective nucleophilic capture of 87 by solvent from its less hindered side yields 85, while skeletal rearrangement of 87 yields the cyclopropylmethyl cation 88, which reacts with solvent to yield 86 ... 

Cyclobutyl cations certainly do exist if they are especially stabilized. For example, 1-phenylcyclobutyl cation shows no tendency to rearrange in superacid solution.89... 

The parent secondary cyclobutyl cation 47 undergoes threefold degenerate rearrangement viao-bond delocalization involving nonclassical bicyclobutonium ion-like system171,172 (see Section 3.5.2.5). Similar behavior is also observed for the 1-methylcyclobutyl cation 48a.173 176 The 1-phenylcyclobutyl cation 48b, on the other... 

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>. 

The parent 3-homonortricyclyl cation [147] underwent three-fold degenerate rearrangements in superacids, as shown by its temperature dependent nmr spectra, but only at higher temperatures (—85°C to 20 C) than for the corresponding dehydrohomoadamantyl [144 R = H] and dehydroadamantyl cations [126 X = H], The lower rearrangement rate of [147] was explained by a less favourable formation of the puckered cyclobutyl cation intermediates [159] in this geometrically more constrained system. The assignment of a symmetrical cyclopropylcarbinyl cationic structure to [147] was confirmed by comparison of its and C-nmr spectra with the static counterparts [157]. 

Ion [160], prepared by ionization of bicyclo[4.1.0]hepyl-2-ol [163], underwent a degenerate two-fold rearrangement with A(7 = 8.5 0.5 kcal mol" at —85°C (105). A puckered cyclobutyl cation was considered as the most probable intermediate. The 1-methyl substituted alcohol [164] also gave a... 

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. 

The parent secondary cyclobutyl cation 38 undergoes immediate rearrangement via o-bond delocalization into the equilibrating non-classical bicyclobutonium ion like system94,95 (see subsequent discussion of non-classical ions). Similar behaviour is also observed for the 1-methylcyclobutyl cation94, 94 98) 39. The 1-phenylcyclo-butyl cation 40 on the other hand is a classical tertiary carbocation94). 

The cationic rearrangement of cyclobutyl compounds to cyclopropylmethyl derivatives is a well-studied reaction which, like the homoallylic rearrangement, has been mainly investigated in connection with the fate and nature of the cyclobutyl cation.Again the synthetic value of this rearrangement is limited because generally mixtures of cyclobutanes, cyclopropanes and alkenes are obtained. 

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