Carbocations cyclopropyl, ring opening

In the first step, Ag+ promotes the departure of Cl- to give a cyclopropyl carbocation. This undergoes two-electron disrotatory electrocyclic ring opening to give the chloroallylic cation, in which the empty orbital is localized on Cl and C3. Then 09 can add to C3 desilylation then gives the product. 

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. 

Carbalumination of 271b with tri-isopropylaluminium affords acyclic products, the formation of which can be attributed to rearrangement of the initially formed cyclopropyl carbocation (equation 84). Ring-opened products also result from reactions with trialkylboranes, but only cyclopropanes, from addition to the n bond, are seen in reactions with organomagnesium reagents . 

The opening of a cyclopropyl carbocation to form an allyl carbocation is an electrocyclic reaction involving two electrons. Because it involves an odd number of electron pairs, disrotatory opening is thermally allowed. The allyl carbocation is much more stable than the cyclopropyl carbocation because of resonance stabilization and relief of ring strain, so the allyl carbocation is favored. The thermal ring opening of the c/s-dimethylcyclopropyl carbocation occurs with a disrotatory motion. 

The ester Kulinkovich reaction was used to prepare a key allylic bromide 5333 in a synthesis of neolauh-malide and isolaulimalide (Scheme 5.88). The cyclopropanol 5.332 produced by the Kulinkovich reaction of ester 5.331, as its mesylate, was subjected to ring opening with magnesium bromide. This is likely to involve electrocyclic ring opening of the cyclopropyl carbocation, followed by bromide trapping. 

The largest l, -enyne that has been cyclized is a 1,9-enyne, which forms a 10-membered ring in the presence of a large amount of gold and silver complex (Scheme 1.26) [200]. The cycloisomerization presumably occurs via cyclopropyl gold carbene XXXVI, which can open to carbocation XXXVII or give directly the 10-membered ring. 

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