Cyclopropanes neighboring group

Cyclopropyl as a Neighboring Group On page 180, we saw that the properties of a cyclopropane ring are in some ways similar to those of a double bond. Therefore it is not surprising that a suitably placed cyclopropyl ring can... 

Keywords SN2 and SN2 reactions Baldwin mles Oxirane opening Activation of a cyclopropane 1,2-rearrangement Ring contraction Ring expansion Solvolysis Neighboring group participation Oxirane rearrangement Classical and nonclassical carbocations 5-exo- vis-a-vis 5-endo cyclization Addition and elimination reactions... 

The last subject to be discussed in this section is one found with Group IV elements, and probably also with many Group II elements (cf. mercury, Section IV, E)—neighboring group effects and participation. Hawthorne (192) has pointed out that y-halopropylboranes, produced by hydroboration of allyl halides, undergo basic hydrolysis to yield cyclopropanes. 

At low temperature in superacid, ionization with neighboring group participation of the cyclopropane o bond gives a delocalized cation that must show three signals in its NMR spectrum (Fig. 21.63). 

Certain homoallylic tosylates react with KOAc to provide the corresponding acetylated cyclopropane derivatives and/or the expected homoallylic acetates (eq 10). Formation of the cyclopropane derivative is suggested to occur via neighboring group participation of the alkene, and is subject to steric and conformational constraints of the substrate. 

The cyclopropanation is sensitive to steric effects, adding from the less-hindered face. Having a neighboring hydroxyl group will generally accelerate the reaction, and will direct the cyclopropanation syn to the hydroxyl group even into sterically congested alkenes. 

Isomerization of cyclopropane was studied by Hall and co-workers (84, 103), who reported catalyst activity to increase with vacancy concentration. They postulated that the active site consisted of a vacancy and neighboring Al—OH group. Hydrogenation of ethylene at room temperature was found to increase with increase in catalyst reduction and irreversible hydrogen was apparently not involved in the reaction (104). 

Two of these carbon sp2 hybrids are used to form CH bonds, so one sp2 and one pure p orbital remain to bind to the neighboring carbons. The sp2 and p orbitals are orthogonal so, to a first approximation, the six MOs of the cyclopropane skeleton can de divided into two groups of linear combinations of the sp2 and p orbitals, respectively (Figure 7.4). 

The intramolecular reaction of the carbene from diazo ester 280, which contains a 1,3-diene moiety in the ester group and a double bond adjacent to the carbene center, leads to the formation of a substituted 1,2-divinyl-cyclopropane, whose CIS isomer then undergoes a Cope rearrangement to give substituted cycloheptadiene. In such a way, bicyclic 281 and tricyclic 282 y-lactones with a neighboring seven-membered carbocycle have been obtained (89JOC930). 

The selective oxidation of hydrocarbons by dry ozonation is not restricted to reaction at tertiary C atoms the CH2 groups adjacent to a cyclopropane ring are smoothly converted into carbonyl groups. This cyclopropyl activation can be explained by the well-known ability of the cyclopropyl group to effectively stabilize a neighboring R3C+ center. Although in the oxidation of alkanes with ozone the first intermediate, the hydrotrioxide, [11] does not arise from carbenium ions, the effect of polarity upon attack of the electronegative ozone molecule, either as a radical or by 1,3-dipolar... 

Where the ring generated is a cyclopropane ring, the latter process produces the same net result as a 1,2-shift of a neighboring vinyl group. If the carbon-carbon unsaturation should be a phenyl group, rearrange-... 

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