Of cyclopropylmethyl

In contrast to the stability of cyclopropylmethyl cations (p. 222), the cyclopropyl group exerts only a weak stabilizing effect on an adjacent carbanionic carbon. ... 

Bicyclobutonium ions, are bridged carbocations with a pentacoordinated y-carbon which were first discussed as short lived intermediates in the solvolysis reaction of cyclopropylmethyl and cyclobutyl compounds (56, 57, 58, 59, 60, 61). 

Shoppee, C. W. Steroids and the Walden Inversion. Part II. Derivatives of d5-Cholestene and zl5-Androstene. J. chem. Soc. [London] 1946, 1147. 203- Siegel, S., and C. G. Bergstrom The Effect of a Cyclopropyl Group on a Displacement Reaction at an Adjacent Saturated Carbon Atom. I. The Ethanolysis of Cyclopropylmethyl Benzenesulfonate. J. Amer. chem. Soc. 74, 145 (1952). 

In 1951 Roberts and Mazur observed that the free radical chlorination of methylcyclopropane gave a mixture of cyclopropylmethyl chloride and 4-chloro-l-butene (equation 74). This reaction was studied furtherand in 1969 Kochi, Krusic, and Eaton observed the cyclopropylmethyl radical 46 by ESR and also monitored its rearrangement. 

Another important rearrangement is that of cyclopropylmethyl radicals to the corresponding homoallyl radicals. This is an exceptionally fast reaction (t1/2 10 8) and has been used as a radical clock to determine the rates of other free-radical reactions.95 Cyclopropylcarbene also undergoes rearrangement, leading to cyclobutene.96... 

This section deals with preparations of cyclobutanes, starting from but-3-enyl (homoallyl) compounds, independent of the occurrence of three-membered ring intermediates. Use of cyclopropylmethyl substrates for the preparation of cyclobutanes is described in Section 1.2.1. 

Over the past twenty years, the ring enlargement of cyclopropylmethyl cationic compounds has reached maturity and is now a major route for the construction of four-membered rings. 

The entire field of cyclobutane synthesis via ring enlargement of cyclopropylmethyl compounds is in continuous growth, and the development of asymmetric syntheses will undoubtly gain pace. 

Other examples of cyclopropylmethyl to cyclobutyl rearrangements24 38 are collected in Table 3. 

The cyclobutyl to homoallyl rearrangement was studied above all for mechanistic reasons, especially to differentiate between different kinds of cationic intermediates.7 In general, mixtures of cyclopropylmethyl, cyclobutyl, and homoallyl derivatives are formed, depending on the type of substitution in the substrate and stability of the precursor ions. 

For a summary of additional evidence for ihc symmetrical nature of cyclopropylmethyl cations, see Wibcrg Hess Ashe, Ref. 158. pp. 1300-1303. 

SYNTHESIS To a solution of 2.8 g of KOH pellets in 25 mL hot MeOH, there was added a mixture of 5.9 g 2,5-dimethoxythiophenol (see under 2C-T-2 for its preparation) and 5.0 g of cyclopropylmethyl bromide. There was an immediate exothermic reaction with spontaneous boiling and the formation of white crystals. This was heated on the steam bath for 4 h, and then added to 400 mL of H,0. After extraction with 3x75 mL CH2C12, the pooled extracts were washed first with dilute NaOH, then with saturated brine, then the solvent was removed under vacuum. The residue, 8.45 g of crude 2,5-dimethoxyphenyl cyclopropyl methyl sulfide, was distilled at 120-140 °C at 0.3 mm/Hg to give a white oil weighing 7.5 g. 

Scheme9.15. Reactivity of cyclopropylmethyl carbanions, radicals, and carbocations [55-59],...

Rate constants for ring-opening of cyclopropylmethyl radicals... 

A mixture of cyclopropylmethyl bromide derivative (0.4 mmol), allyltributyltin (0.8 mmol), and AIBN (5 mol%) in dry benzene (3 ml) was degassed by bubbling argon gas for 30 min. The mixture was refluxed for 12 h. After removal of the solvent, aq. KF solution (5 ml) and ether (10 ml) were added to the residue and stirred for one hour. After filtration and separation, the organic layer was dried over Na2S04. After removal of the solvent, the residue was chromatographed on silica gel using ethyl acetate/hexane to afford a diallyl product [95]. 

One case where C—C bonds are exceptionally effective in hyperconjugation is in the stabilisation provided by a cyclopropyl substituent to an empty p orbital. The cyclopropylmethyl cation is actually better stabilised than an allyl cation, as judged by the more rapid ionisation of cyclopropylmethyl chloride 2.19 than of crotyl chloride 2.20. In this case, hyperconjugation appears, unusually, to be better than n conjugation. 

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