Cyclopropyl anion

While thermodynamically, the direct metalation of cyclopropane can be envisioned from a synthetic point of view, this approach has been rarely used. A major obstacle appears to be kinetics which can be overcome by incorporation of a hydroxyl group (see Eq. 16)17). In special cases, such as bicyclo [1.1.0] butane and methylenecyclopropane (Eq. 17)18) the enhanced thermodynamic acidity is accom- 

A more general approach utilizes metal-halogen exchanges because of the ready availability of monobromocyclopropanes by either 

A slight modification of the cyclopropyl conjunctive reagent transforms a cyclopentannulation into a cycloheptannulation. Thus, the 2-vinylcyclopropyllithium reagent 5, converted to its cuprate 4, generates a 1,2-divinylcyclopropane. Heating to only 180 °C leads to smooth Cope type rearrangement, driven by the release of the cyclopropyl strain, to create a perhydroazulene ring systerh of many sesquiterpenoids (Eq. 19) 20). 

To direct a solvolytic ring opening, 2-methoxycyclopropyllithium (5) was developed as a chain extension conjunctive reagent. The failure of p-elimination to occur in 6 presumably derives from the high strain of cyclopropene and poor orbital overlap for elimination. The aldehyde adducts smoothly solvolyze to give p,y-unsaturated aldehydes (Eq. 22)23) which are best initially isolated as their hemithioacetals. 

The cuprate derived from the cis isomer of this reagent undergoes conjugate addition with cyclohexenone with unusually high diastereoselectivity (5 1, Eq. 23)24 In this case, electrophilically initiated ring opening with mercuric acetate chemo-selectively attacks the sterically least hindered cyclopropyl bond to give a branched product 7. Reductive work-up produces 8 in which the stereochemistry of the 

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The relative stability of the anions derived from cyclopropene and cyclopentadiene by deprotonation is just the reverse of the situation for the cations. Cyclopentadiene is one of the most acidic hydrocarbons known, with a of 16.0. The plCs of triphenylcyclo-propene and trimethylcyclopropene have been estimated as 50 and 62, respectively, from electrochemical cycles. The unsubstituted compound would be expected to fall somewhere in between and thus must be about 40 powers of 10 less acidic than cyclopentadiene. MP2/6-31(d,p) and B3LYP calculations indicate a small destabilization, relative to the cyclopropyl anion. Thus, the six-7c-electron cyclopentadienide ion is enormously stabilized relative to the four-7c-electron cyclopropenide ion, in agreement with the Hixckel rule. 

For a review of ring opening of cyclopropyl anions and related reactions, see Boche, G. Top. Curr. Chem., 1988, 146, 1. 

Table 1. Synthetic Constructions with Unstabilized Cyclopropyl Anions...

The isopropenyl side chain may derive by elimination of a tertiary alcohol or ether as in 202. Such a masking of the olefin avoids a possible competing vinylcyclopropane rearrangement. The correspondence of the cyclopentene of 202 with the vinylcyclopropane in 203 now becomes obvious. The presence of the dimethylcarbinol side chain now also offers the opportunity for its introduction by addition of a cyclopropyl anion to acetone. The feasibility of creating such an anion by fluoride initiated desilylation... 

Very high level ab initio [CCSD(T)//MCSF] calculations have been applied to singlet and triplet cyclopropenyl anion and cyclopropenyl radical. The anion ground state, a singlet with Cg symmetry, is destabilized relative to cyclopropyl anion as expected for an antiaromatic structure it is stabilized, with respect to its conjugate acid and the corresponding radical, by electron-withdrawing substituents such that 1,2,3-tricyanopropene has a predicted pK of 10-15. ... 

Despite our earlier enunciated electronegativity and bond polarity logic, we must forego nearly all comparison with the free (uncomplexed) carbanions. Unlike the rather stable cyclopropyl anion, the cyclobutyl and cyclopentyl ions are unbound with regard to loss of their extra electron. That is, the gas phase ionization process to form the radical from the carbanion, Ru R" -E e, is energetically favorable. 

The former reduction is thought to occur by two single electron transfers (SET) from the metal surface to the cyclopropane derivative providing a halide and cyclopropyl anion initially. The latter is protonated by the solvent thus leading to the monohalogen derivative which can undergo the reduction process for a second time. 

In the cases of cyclopropyl anions, destabilization by the d-d pairing and stabilization in the open forms for the most reactive substrates, and the opposite scenario for the least reactive substances are evident. 

The stereochemistry of the reaction has been examined both theoretically92 and experimentally.93 It has been found to be stereospecific, with disrotatory motion of the methylene groups. Cyclopropyl anions also undergo thermal rearrangements to allyl anions.94... 

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]. 

Also, reactions at vinylic carbons proceed with retention,101 indicating that the intermediate 14 has sp2 hybridization and not the sp hybridization that would be expected in the analogous carbocation. A cyclopropyl anion can also hold its configuration.102... 

Walborsky Motes 3. Am. Chem. Soc. 1970, 92, 2445 Moles Walborsky /. Am. Chem. Soc. 1970, 92, 3697 Bochc Harms Marsch /. Am. Chem. Soc. 1988, //0, 6925. For a monograph on cyclopropyl anions, cations, and radicals, sec Boche Walborsky Cyclopropane Derived Reactive Intermediates Wiley New York, 1990. For a review, sec Bochc Walborsky, in Rappoport The Chemistry of the Cyclopropyl Group, pt. 1 Wiley New York, 1987, pp. 701-808 (the monograph includes and updates the review). 

It appears that the combined effects of high s character and transannular interaction produce a basicity somewhat higher than that of the vinyl anion. The gas-phase proton affinity of the parent bridgehead anion (16) is 411 3.5 kcalmoT173. This value is close to those of the cyclopropyl anion (408 5 kcal moT1) and the vinyl anion (407 3 kcalmol ), in agreement with MP3/6-31 + G calculations43. 

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