Allylcarbinyl derivatives

Scheme 3 - Product distribution from solvolysis of cyclopropylcarbinyl cyclobutyl and allylcarbinyl derivates.

The Nature of the Intermediate in Carbonium-Ion Type Interconversion Reactions of Cyclobutyl, Cyclopropylcarbinyl and Allylcarbinyl Derivatives. J. Amer. chem. Soc. 73, 3542 (1951). 

Solvolytic studies provided the first structural indication for almost every carbocation-ic intermediate and the C4H,+ ion is no exception. Roberts observed that the solvolysis of cyclopropylcarbinyl or cyclobutyl systems and the diazotative deamination reactions of cyclopropylcarbinyl amine or cyclobutyl amine gave similar product mixtures consisting of cyclopropylcarbinyl, cyclobutyl and allylcarbinyl derivatives in essentially the same ratio1,2. A common cationic intermediate of C3v structure, the tricyclobutonium ion 1, was... 

Note that if the bicyclobutonium ion were formed directly upoii ionization of an allylcarbinyl derivative, it would be a case of homoallylic participation in an acyclic system. In fact, the bicyclobutonium ion is similar to the carbonium ion proposed by Winstein for homoallylic participation in the 7-norbornenyl system— cf. Figures 6.7 and 6.8 and Structures 65 and 66. The difference between them is that 66 is more symmetrical. 

There have been a very large number of investigations of carbocationic reactions of cyclobutyl, cyclopropylcarbinyl and allylcarbinyl derivatives under so-called stable-ion as well as solvolytic conditions. Bartlett (1965) has stated Among nonclassical ions the ratio of conceptual difficulty to molecular weight reaches a maximum with the cyclopropylcarbinyl-cyclobutyl system . The term nonclassical was first used by Roberts and Mazur (1951) to describe the nature of tricyclobutonium ion [32] suggested to be involved in reactions of cyclopropylcarbinyl derivatives. Later Roberts and coworkers (Mazur et al., 1959) favoured a set of rapidly equilibrating nonclassical bicyclo-butonium ions [34] instead of a single non-classical species. Essentially all experimental evidence on indicates that the species is a nonclassical... 

The reaction of bicyclobutanes with acids can result in the formation of three major products. Thus, when bicyclobutane itself is allowed to react with HCIO4. in MeOH (equation 51), cyclobutyl, cyclopropylcarbinyl and allylcarbinyl derivatives are obtained In most cases, the allylcarbinyl products are obtained only in minute amounts... 

The solvolysis of both cyclopropylcarbinyl and cyclobutyl derivatives proceeds with rearrangement to give mixtures of cyclopropylcarbinyl (515), cyclobutyl (516), and allylcarbinyl derivatives (517)401,402. Under certain conditions the product distributions are very similar, starting from either (57. ) or (514) (Table 16). These data... 

In 1951 Roberts observed that most cationic reactions of cyclopropylcarbinyl and of cyclobutyl derivatives give the same products in nearly the same ratio.73 For example, cyclopropylcarbinyl and cyclobutyl amines on deamination form the products shown in Equation 6.31.74 Moreover, when allylcarbinyl tosylate (63)... 

Cyclopropylmethanol derivatives (750 X = COPh, CO Me, CONH2, NOj, or NO) have been synthesized by the silver-ion-assisted acetolysis of a-substituted bromocyclobutanes (749). In the case of (749 X = COPh), the allylcarbinyl acetate (751) was also formed, along with (750). 

Numerous routine synthetic applications have appeared which make use of the stereoselective ring-opening of cyclopropylcarbinyl derivatives to E-olefins. The allylcarbinyl-cyclopropylcarbinyl interconversion is frequently used for the protection of double bonds in steroids during synthetic procedures for the preparation of homosteroids and may be involved in the fluorination of A -3-tri-methylsiloxy-steroids. ... 

The data is consistent with the initial formation of a cyclopropylcarbinyl cation which then undergoes rapid ring-opening to the allylcarbinyl cation from which the products are derived. 

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