Cyclopropylcarbinyl cation ring-openings

On the other hand, there is now a good deal of evidence that the solvolysis of most cyclobutyl derivatives does lead directly to the cyclopropylcarbinyl cation. For example, orbital symmetry considerations (Section 11.3) indicate that the conversion of cyclobutyl cations into cyclopropylcarbinyl cations should occur by disrotatory ring opening as shown in Figure 6.11 but any steric factors that would hinder such a process decelerate most cyclobutyl solvolyses. Thus 86 See note 84(b). 

The mechanism for bromination of semibullvalene has been proposed, based on quantum chemical calculations. The reaction pathway involves concerted bromine addition and cyclopropane ring opening to form an allylic cation, without the intermediacy of a bromonium or a cyclopropylcarbinyl cation.18... 

Cationic cyclization and its reverse have particular relevance, since ring closure and ring opening have been invoked to account for a large number of biosynthetic pathways.The gaseous cyclopropylcarbinyl-cyclobutyl-homoallyl system represents an extreme instance of a rapid, reversible isomerization of this sort, which has been studied over a period of 20 years both by /1-decay of tritiated cyclobutane and by radiolytic methods. This interconversion is so fast that the three structures... 

The data is consistent with the initial formation of a cyclopropylcarbinyl cation which then undergoes rapid ring-opening to the allylcarbinyl cation from which the products are derived. 

If the S configuration tentatively assigned to the carbinol carbon in natural artemisia alcohol (18) (55) is sustained, it would indicate that in the actual biosynthesis the nucleophilic substitution occurs with net retention of configuration. Since a direct substitution upon a cyclopropylcarbinyl ion should occur with inversion rather than retention 74—75), ring opening to the free allylic cation followed by stereospecific capture is proposed (55). 

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