Allyl cations ring-opening

H. Kaga, Precision synthesis of (1 —> 6)-o -D-glucopyranan by cationic ring-opening polymerization of l,6-anhydro-tri-0-allyl-/8-D-glucopyranose, Macromol. Symposia, 181 (2002) 101-106 (b) A. Kusuno, M. Mori, T. Satoh, M. Miura, H. Kaga, and T. Kakuchi, Enantioseparation properties of (1 - 6)- -i)-glucopyranan and (1 - 6)-a-D-mannopyranan tris(phenylcarbamate)s as chiral stationary phases in HPLC, Chirality, 14 (2002) 498-502. 

Fig. 8 Synthesis of dextran via a cationic ring-opening polymerisation of 1,6-anhydro-2,3,4-tri-0-allyl-/J-D-glucopyranose (path A) and l)6-anhydro-2,3,4-tri-0-benzyl-/J-D-glucopyranose path B) [84,85]...

Fonnation of allylic products is characteristic of solvolytic reactions of other cyclopropyl halides and sulfonates. Similarly, diazotization of cyclopropylamine in aqueous solution gives allyl alcohol. The ring opening of a cyclopropyl cation is an electrocyclic process of the 4 + 2 type, where n equals zero. It should therefore be a disrotatory process. There is another facet to the stereochemistry in substituted cyclopropyl systems. Note that for a cri-2,3-dimethylcyclopropyl cation, for example, two different disrotatory modes are possible, leading to conformationally distinct allyl cations ... 

The disrotatory mode, in which the methyl groups move away from each other, would be more favorable for steric reasons. If the ring opening occurs through a discrete cyclopropyl cation, the W-shaped allylic cation should be formed in preference to the sterically less favorable U-shaped cation. This point was investigated by comparing the rates of solvolysis of the cyclopropyl tosylates 6-8 ... 

When the size of the fused ring permits ring opening to a fran -allylic cation, as in the case of compound 11, solvolysis proceeds at a reasonable rate ... 

Scheme 10.1 gives some representative examples of laboratory syntheses involving polyene cyclization. The cyclization in Entry 1 is done in anhydrous formic acid and involves the formation of a symmetric tertiary allylic carbocation. The cyclization forms a six-membered ring by attack at the terminal carbon of the vinyl group. The bicyclic cation is captured as the formate ester. Entry 2 also involves initiation by a symmetric allylic cation. In this case, the triene unit cyclizes to a tricyclic ring system. Entry 3 results in the formation of the steroidal skeleton with termination by capture of the alkynyl group and formation of a ketone. The cyclization in Entry 4 is initiated by epoxide opening. 

The change of cyclopropyl cation to allyl cation is an irreversible reaction because only the ring opening is observed. It is irreversible cyclopropyl cation, is thermodynamically unstable with respect to allyl cation and also because the ring is under strain. 

Other factors which affect the case of electrocyclic ring opening include the nature of substituents which can stabilize or destabilize the development of possible charge and the release of strain in small cyclic systems. Thus different stereochemistries have been observed in the ring opening of cyclopropyl derivatives. All cis derivatives generate an all-cis allyl cation but the anti derivatives will form the all trans cation. 

Dicyclopentadiene forms a radical cation (20 ) in which one of the bonds linking the monomer units is cleaved. The species contains two allyl moieties attached to a C4 spacer . Structure 20 + rests on an unmistakable CIDNP pattem " and is supported by an analysis of the electronic absorption spectmm. The large energy gap in the OS of this ion (AE = 1.67 eV) is incompatible with the photoelectron spectrum of the parent molecule (AE = 0.15 eV), but it fits the ring-opened structure 20 +. 

One of the problems associated with thermal cyclodimerization of alkenes is the elevated temperatures required which often cause the strained cyclobutane derivatives formed to undergo ring opening, resulting in the formation of secondary thermolysis products. This deficiency can be overcome by the use of catalysts (metals Lewis or Bronsted acids) which convert less reactive alkenes to reactive intermediates (metalated alkenes, cations, radical cations) which undergo cycloaddilion more efficiently. Nevertheless, a number of these catalysts can also cause the decomposition of the cyclobutanes formed in the initial reaction. Such catalyzed alkene cycloadditions are limited specifically to allyl cations, strained alkenes such as methylenccyclo-propane and donor-acceptor-substituted alkenes. The milder reaction conditions of the catalyzed process permit the extension of the scope of [2 + 2] cycloadditions to include alkene combinations which would not otherwise react. 

The electrocyclic reactions of 3-membered rings, cyclopropyl cation and cyclopropyl anion, may be treated as special cases of the general reaction. Thus the cyclopropyl cation opens to the allyl cation in a disrotatory manner (i.e., allyl cation, n = 0), and the cyclopropyl anion opens thermally to the allyl anion in a conrotatory manner (i.e., allyl anion, m = 1). Heterocyclic systems isoelectronic to cyclopropyl anion, namely oxiranes, thiiranes, and aziridines, have also been shown experimentally and theoretically to open in a conrotatory manner [300]. 

The ring opening of cyclopropyl cations (pp. 345, 1076) is an electrocyclic reaction and is governed by the orbital symmetry rules.389 For this case we invoke the rule that the o bond opens in such a way that the resulting/ orbitals have the symmetry of the highest occupied orbital of the product, in this case, an allylic cation. We may recall that an allylic system has three molecular orbitals (p. 32). For the cation, with only two electrons, the highest occupied orbital is the one of the lowest energy (A). Thus, the cyclopropyl cation must... 

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