Carbenium ions cyclopropylmethyl

On the other hand, cyclobutyl compounds are not obtained from cyclopropylmethyl derivatives with carbenium ion stabilizing substituents on the carbinyl carbon. For example, reactions of methylcyclopropyl methanol (101) with acidic reagents led to unrearranged or on further heating to 3-penten-l-yl derivatives (equation 70) . 

The ring contraction of monofunctionalized four-membered rings (i.e. halo- or tosyloxycyclobutanes), involving carbenium ion intermediates, usually affords mixtures of cyclobutyl, cyclopropylmethyl and but-3-enyl compounds, but are of limited synthetic applicability (Section 4.1.2.1.). On the other hand, the ring eontraction of vicinally difunctionalized cyclobutanes appears easier and does not necessarily require carbenium ion formation. 

Process (a) is characteristic of the photolysis of cholesta-3,5-diene (R = H), which gave bicyclobutane derivative in at least 75% yield. Subsequent treatment with water or alcohol provided a homoallylic carbenium ion, in which the protonation had occurred stereoselective-ly. Coordination with solvent then provided either cyclopropylmethyl ethers and alcohols (R = H, Et) or cyclopentylmethyl ethers and alcohols. 

Cyclopropylmethyl systems containing a carbenium ion stabilizing group at the 1-position of the cyclopropane ring lead to the exclusive formation of cyclobutanols.Thus, 1-methylcy-clobutanol (2 a) was obtained in 90% yield by hydrochloric acid catalyzed ring expansion of (l-methylcyclopropyl)methanol (la). Hydrolysis of 1-phenyl-l-(tosyloxymethyl)cyclo-propane (lb) in 90% acetone/water or dimethyl sulfoxide afforded a quantitative yield of 1-phenylcyclobutanol (2b). 

The first cyclopropylmethyl cation to be directly observed was the tricyclo-propylmethyl cation (100) and the subsequent study of a variety of cyclopropylmethyl cations " led to the conclusion that the tertiary cations are static and, in the absence of constraining skeletal rigidity, adopt a bisected geometry rather than an eclipsed one (making the ex substituents on the carbenium ion... 

Three types of products have been observed in intermolecular acylations of homoallylic silanes, the major one being cyclopropylmethyl ketones, along with minor amounts of 3-butenyl ketones and -chlo-ro ketones. It is likely that all derive from the carbenium ion formed by acylation of the double bond, which then undergoes cyclodesilylation or hydride transfer followed by 3-elimination (Scheme 14). The former leads to the cyclopropane, which can ring open to give the chloro products. The latter pathway gives the butenyl ketone, and is supported by location of substituent positions on methylated substrates. However, the direct acylation of the carbon-silicon bond should not necessarily be excluded in consideration of more general cases. Titanium tetrachloride seems the preferred catalyst in these cyclodesilyl-ations, and low temperatures minimize the formation of the chloro by-products. Intramolecular versions... 

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