Cyclopropanes, activated: ring opening

An elegant route to 11-deoxyprostaglandins uses a 1,5 attack of an enolate on such an activated cyclopropane. The ring-opening of activated cyclopropanes by intramolecul-arly situated nucleophiles has also been achieved An interesting case of intramolecular 1,7 attack on an activated vinylcyclopropane was observed upon treatment with dimsyl sodium-Me2SO at 90°C (equation 18) . 

Lithium benzeneselenolate reacted with acetylcyclopropane (5, R = H R = Me) and other mono-activated cyclopropanes to ring-opened products 6 with a benzeneselanyl substituent at the terminal carbon atom. ... 

Under acidic conditions, aryl-substituted cyclopropanes underwent ring-opening reactions and added benzenethiol and toluenethiol, respectively. Contrary to the carbonyl-activated cyclopropanes, the nucleophile attacked at the aryl-substituted carbon atom due to the preferred protonation of the cyclopropyl ring. 

We envisioned that an alkynyl substituent on a cyclopropane could be complexed as a cobalt hexacarbonyl derivative [8], which would enable activation of the cyclopropane toward ring opening in order to undergo subsequent cycloadditions. This is explained by the ring-opening of a cobalt complexed cyclopropanediester I under the influence of a Lewis acid to form the Nicholas carbocation 2 as illustrated in Scheme 10.3. 

While generation of a Mn(V)oxo salen intermediate 8 as the active chiral oxidant is widely accepted, how the subsequent C-C bond forming events occur is the subject of some debate. The observation of frans-epoxide products from cw-olefins, as well as the observation that conjugated olefins work best support a stepwise intermediate in which a conjugated radical or cation intermediate is generated. The radical intermediate 9 is most favored based on better Hammett correlations obtained with o vs. o . " In addition, it was recently demonstrated that ring opening of vinyl cyclopropane substrates produced products that can only be derived from radical intermediates and not cationic intermediates. ... 

One of the most efficient procedures for the synthesis of cyclopropanes is the reaction of alkenes with electrophilic carbene complexes. In this process up to three stereogenic centers can be generated in one step. Cyclopropanes are a key structural element encountered in many natural products with interesting biological activity. Further, by virtue of the ability of cyclopropanes to undergo ring-opening reactions these compounds can be valuable synthetic intermediates. 

Although this view is oversimplified and borderline metal carbene complexes have been isolated, this approach is convenient for discussing the activity of metal carbene species in the ring-opening metathesis polymerisation of cycloolefins. Calculations have predicted [81,82] and recent results have shown [83] that, in some systems, metal alkylidene reactivity is competitive with metal carbene reactivity, i.e. olefin metathesis is competitive with olefin cyclopropanation. 

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