Cyclopropylcarbinyl anions

The cyclopropylcarbinyl anion is stabilized by phenyl groups attached to the carbinyl carbon toward ring opening, provided that potassium or sodium is the counterion. However, replacement of the potassium by lithium or magnesium leads to immediate... 

Finally, the allylcarbinyl/cyclopropylcarbinyl anion rearrangement (42a) (42b) should be mentioned as a striking example of a solvent-dependent valence isomerization [207]. 

The cyclopropylcarbinyl radical (4), the cyclopropylcarbinyl cation (5) and the cyclopropylcarbinyl anion (6) all dominate the expected reactivity of vinylcyclopropanes since one of these forms will be expected to be a major contributor to either a radical or a polar transition state. It is important to consider the various reactive subunits in some detail in order to understand the big picture reactivity of a vinylcyclopropane system, especially as perturbed by additional substituents. 

The dominant contributor to the reactivity of vinylcyclopropanes in any radical reaction is the form (4a), the cyclopropylcarbinyl radical system. The opening of a cyclopropylcarbinyl radical to a butenyl radical is among the fastest radical processes known, with a rate constant of 1.3 x 10 sec". - The various stereoelectronic effects of this rearrangement have been reviewed. The structure of (4a), deduced from its ESR spectrum - and in agreement with calculations (STO-36 basis set), is in the bisected conformation shown, predicted to be 1.4 kcal mol more stable than its perpendicularly oriented counterpart. Above -KX) T only the butenyl radical (4b) can be detected. Substituent efiects do not seem to operate here when the substituents are on the cyclopropane (i.e. product stabilization). The cy-clopropylcaibinyl cation and anion have structures similar to (4a), bisect conformations (5) and (6), respectively. A concise summary of solvolytic and mechanistic data for system (5) has recently appeai Reviews of cyclopropylcarbinyl anions and carbenes are also available. - ... 

A simple HMO calculation for the cyclization of the allylcarbinyl to the cyclopropylcarbinyl anion (161) has appeared (equation 33) and, based on proton affinities, 161 was calculated to be 10 kcal mol" more stable than ethyl anion (SCF double-zeta basis) and 14.3 kcal mol" more stable than i-PrCH (STO-3G). The bisected geometry was predicted to be most stable, with electron donation from the filled p orbital at C(4) to the unoccupied Walsh 4ai MO (162) at C(l). Inductive electron withdrawal by cyclopropyl would also stabilize this anion. 

The reaction follows the stoichiometry which requires 3 equivalents of the anion for each tris(halogenomethyl)ethane, furnishing one equivalent of the cyclopropylcarbinyl complex and one dinuclear complex [CpM(CO) ]2. One possible mechanism is shown in Scheme 2, in which the key step proposed for ring formation is a metathesis involving the metal, the halogen and two carbon atoms. 

Analogously, reaction of group 7 metal carbonyl anions M(CO)5 (M = Mn, Re) with MeC(CFFI), gave cyclopropylcarbinyl complexes. Flowever, whereas rhenium produces the normal (/-metal bonded cyclopropylcarbinyl complex, the manganese complex furnished (cyclopropylacetyl)Mn(CO)5, where carbonyl insertion into the manganese-alkyl bond occurred (equation 56)114. 

TABLE 4. Effect of substituents on the rate of ring opening of cyclopropylcarbinyl radicals and related radical anions... 

Mattay et al. used triethylamine as electron donor in tandem fragmentation/cyclization reactions of a-cyclopropylketones (49) [48]. The initial electron transfer to the ketone moiety is followed by subsequent cyclopropylcarbinyl-homoallyl rearrangement yielding a distonic radical anion (50). With an appropriate unsaturated side chain attached to the molecule both annelated and spirocyclic ring systems are accessible in moderate yields. Scheme 26 shows some representative examples. 

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