Alcohols, cyclopropylcarbinyl

This method is recommended for conversion of cyclopropylcarbinyl alcohols into the corresponding halides without formation of homoallylic halides via ring cleavage.-Example ... 

The bromination method mentioned above that uses triphenylphosphine and either CBr4 or elemental bromine is a particularly mild method for this transformation. Cyclopropylcarbinyl alcohols are acid sensitive substrates, for example, usually giving ring-opening products or cyclobutane derivatives on treatment with acid (see above). S,158 pjjg reaction of 147 with PPh3 and bromine gave a 79% yield of 148. S9.160 3 35... 

Acid treatment of the cyclopropylcarbinyl alcohol (144) results in a two-carbon ring expansion to the benzocyclodecadienol (145) which, after conversion into the siloxy-Cope precursor (146), readily undergoes a further two-carbon ring expansion leading to benzocyclododecadienone (147) (Scheme 32). ... 

Furthei-more, the cyclization of the iododiene 225 affords the si.x-membered product 228. In this case too, complete inversion of the alkene stereochemistry is observed. The (Z)-allylic alcohol 229 is not the product. Therefore, the cyclization cannot be explained by a simple endo mode cyclization to form 229. This cyclization is explained by a sequence of (i) e.vo-mode carbopallada-tion to form the intermediate 226, (ii) cydopropanation to form 227. and (iii) cyclopropylcarbinyl to homoallyl rearrangement to afford the (F3-allylic alcohol 228[166]. (For further examples of cydopropanation and endo versus e o cyclization. see Section 1.1.2.2.)... 

In solvolytic reactions it was observed that cyclopropylcarbinyl cations are formed as transient intermediates from the homoallylic alcohols 4 (equation 5). 

In suitable cases, allylic alcohols can be converted to the cyclopropylcarbinyl cations by reaction with superacids. The reaction involves the rearrangement of the initially formed ally] cation to the homoallyl cation by a 1,2-hydride transfer followed by its cyclization to the cyclopropylcarbinyl cation19 (equation 9). 

Cyclopropylcarbinyl radicalsGeneration of a radical adjacent to a cyclopropyl group is attended by cleavage of the cyclopropane ring. The precursors are available by cyclopropanation of a cyclic allylic alcohol followed by replacement of... 

Later Julia and coworkers developed another important mode of ring breaking addition of Grignard compounds or reduction converts the ester 22 to the corresponding alcohols which are startingpoints for a cyclopropylcarbinyl/homoallyl cation rearrangement. After acid treatment p/y-unsaturated carbonyl compounds (e.g. 23) can be isolated, which sometimes isomerize to the oc,(3-unsaturated systems 10). 

The reduction of methyl 2-siloxycyclopropanecarboxylates can also be started at the ester function when lithium aluminum hydride in ether is the reagent. The resulting alcohols undergo the wellknown cyclopropylcarbinyl/homoallyl rearrangement upon treatment with acid to provide P/y-unsaturated carbonyl compounds 117. These are synthesized isomerically pure and in good yields in a number of cases, if the two-phase-system 2N hydrochloric acid/pentane is employed 78). Otherwise the very easy isomerization to the conjugated a,p-unsaturated compounds 118 occurs to some extend, which can intentionally be completed by base catalysis. 

CH2, Jqh - 189 Hz). Thus all three methylene carbons are identical, and the protons attached to each of them are nonequivalent. Equilibrating classical cyclopropylcarbinyl cations can be ruled out not only based on substantially shielded averaged methylene carbon NMR shift, but also due to the presence of nonequivalent geminal protons on each of the methylene carbons. A pentacoordinated nonclassical bicyclobutonium ion can account for the observed results. The proton NMR absorptions at 3 4.64 and 4.21 are assigned to the endo- and exo-methylene hydrogens, respectively, based on the NMR spectra of the stereospeciflcally deuteriated cyclopropylcarbinyl cations The endo-deuteriated cyclopropylcarbinyl cation was prepared from alcohol 5a, whereas a 1 1 mixture of endo- and exo-deuteriated cations were prepared from alcohol 5b (equations 10 and 11). 

Olah and coworkers NMR study of the 8,9-dehydro-2-adamantyl cation, obtained from the corresponding alcohol in superacidic medium, showed the equivalence of C2, C8 and C9 carbons ((5 C 157.0) in agreement with the conclusions of solvolytic studies Even at -120 °C the structure of the cation could not be frozen out to a static cation, showing the extremely fast equilibration of the threefold degenerate cyclopropylcarbinyl cation 74. An identical NMR spectrum was obtained from the ionization of the 2,5-dehydro-4-protoadamantanol, which prompted the suggestion of the intermediacy of the 2,5-dehydro-4-protoadamantyl cation 75. The ion rearranges to an ally lie cation 76 at -78 (equation 45). 

Cyclobutyl- or cyclopropylcarbinyl chloride or the corresponding alcohols react with SbFj to give stable ion solutions with identical H- and C-nmr spectra. These spectra indicate either an ion with three-fold symmetry, the tricyclobutonium ion [32], or a set of rapidly equilibrating, less symmetrical ions with the same effective averaged symmetry. These include cyclopropylcarbinyl cation [33], bicyclobutonium ion [34] or bent cyclobutyl cations [35]. Theoretical calculations predict similar energies for [33]-[35], but indicate that [32] is less likely (Hehre and Hiberty, 1974 Hehre, 1975). Isotopic perturbation studies by Saunders and Siehl (1980) indicate that the... 

Olah et al. (1976a) continued the studies of potentially degenerate secondary cyclopropylcarbinyl cations incorporated into polycyclic carbon skeletons with the preparation of 2,4-dehydro-5-homoadamantyl cation [144 R = H]. This ion and two 5-substituted tertiary derivatives were prepared from the corresponding 5-R-2,4-dehydrohomoadamantyl alcohols [145 R = H, CH3... 

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