Rearrangements of cyclopropylmethyl

This example is related to the rearrangement of cyclopropylmethyl radicals 32 to give homoallylic systems 33 (Scheme 8). The parent reaction is very fast =... 

The rearrangement of cyclopropylmethyl carbocations is an attractive route to cyclobutane derivatives, and further illustrations of the method have been published this year (Scheme... 

Scheme 3 Rearrangement of [C4H6R] + cations, for R = H a threefold degenerate interconversion of bicyclobutonium ion (54) and cyclopropylmethyl cations (55).

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... 

To allow fast orientation. Table 1 summarizes the most important methods for the synthesis and rearrangement of the cyclopropylmethyl compounds available today. The first column indicates the basic structure(s) to be rearranged, the second details the substituent(s), and the third gives information on the most general methods for the synthesis of the substrates. The fourth column specifies whether cyclobutanes, cyclobutenes and/or cyclobutanones may be obtained, and in the last column the section is given where the corresponding transformation is discussed. Asymmetric versions are indicated with an asterisk ( ). 

Rearrangements of Monofunctional Cyclopropylmethyl Compounds 3.2.1.1. Rearrangements of Hydrocarbons and Solvolysis of Vinyl Halides... 

Rearrangements of 1 -Donor-Substituted Cyclopropylmethyl Compounds with the Exception of Ethers, Sulfides and Selenides... 

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. 

No reactions of t with protic solvents have been reported however, its cyclic analogue 1,2-diphenylcyclobutene (7) reacts with the protic solvents methanol, acetic acid, and water, to yield adducts 85 and 86 (eq. 28). The proposed mechanism for the formation of 85 and 86 involves the formation of singlet exciplex followed by proton transfer to yield a cyclobutyl cation 87. Stereoselective nucleophilic capture of 87 by solvent from its less hindered side yields 85, while skeletal rearrangement of 87 yields the cyclopropylmethyl cation 88, which reacts with solvent to yield 86 ... 

Evidently, Pi is obtained from BRi by a straightforward combination of the two radical centres, but an extensive skeleton rearrangement must occur on the route to the product P2. Because it is natural to assume that no carbon-hydrogen bonds are broken in that process, the polarization of each proton serves as a label of the carbon atom it is attached to. The mechanism displayed in Chart 9.4 sums up the result, and identifies the structural changes as a cyclopropylmethyl-homoallyl rearrangement of the quadricylcane-derived moiety. 

Radical clock rearrangements can be used to provide evidence for radical intermediates these include the ring opening of cyclopropylmethyl radical and the ring closing of the hexenyl radical (equation 21). 

This rearrangement, which accounts for the scrambling, is completely stereospecific.The rearrangements probably take place through a nonplanar cyclobutyl cation intermediate or transition state. The formation of cyclobutyl and homoallylic products from a cyclopropylmethyl cation is also completely stereospecific. These products may arise by direct attack of the nucleophile on 65 or on the cyclobutyl cation intermediate. A planar cyclobutyl cation is ruled out in both cases because it would be symmetrical and the stereospecificity would be lost, iv. The rate enhancement in the solvolysis of secondary cyclobutyl substrates is probably caused by participation by a bond leading directly to 65, which accounts for the fact that solvolysis of cyclobutyl and of cyclopropylmethyl... 

The cationic rearrangement of homoallylic compounds to cyclopropylmethyl derivatives is a well-known reaction which has received considerable attention, mainly due to studies of the homoallylic methyl ion intermediate. ... 

Using an alternative method, homoallylic iodides are efficiently transformed into cyclopropylmethyl acetates using silver acetate in anhydrous benzene (Table 1). y-Disubstituted or conjugated homoallylic iodides are particularly reactive and rearrange quantitatively. y-Monosub-stituted iodides afford mixtures of cyclopropylmethyl and homoallyl acetates in a 1 1 ratio, whereas the absence of a y-alkyl or aryl group in the homoallylic iodides leads to elimination, only. 

The cationic rearrangement of cyclobutyl compounds to cyclopropylmethyl derivatives is a well-studied reaction which, like the homoallylic rearrangement, has been mainly investigated in connection with the fate and nature of the cyclobutyl cation.Again the synthetic value of this rearrangement is limited because generally mixtures of cyclobutanes, cyclopropanes and alkenes are obtained. 

The cyclobutyl-cyclopropylmethyl rearrangement of 2-oxabicyclo[4.2.0]octan-3-ones 7 is strongly dependent on their substitution pattern at the bridgehead positions.When unsubstituted or monomethyl-substituted -lactones are treated with acetic acid or 4-toluenesulfonic acid under reflux, no rearrangement occurs. However, under similar conditions dialkyl-sub-stituted lactones lead to an almost quantitative yield (at 20-40% conversion) of y-lactones 8. 

Additionally, regioisomeric differentiation can result at some stage of the actual codimerization reaction, e.g. the initial C —C bond-formation step of the distal opening (A) or in the course of the cyclopropylmethyl to but-3-enyl rearrangement of the proximal opening (B). The occurrence of formal [2-I-2]-cycloaddition products of MCP and alkene can also be rationalized from this mechanistic scheme. Thus, these products could arise from the spirometallaheptane by reductive elimination instead of the cyclopropylmethyl to but-3-enyl rearrangement. 

The thermolytically induced cyclopropyl to allyl rearrangement can be combined with a cyclopropylmethyl to but-l-enyl rearrangement (see Section 1.2.4.1,5.). °° For example, rearrangement of 19 to 20. °°... 

Cyclobutanol was obtained by deamination of (cyclopropylmethyl)amine in 40% yield. Cyclopropylmethanol was a major byproduct in this reaction. A more efficient synthesis of cyclobutanol is the acid-catalyzed rearrangement of cyclopropylmethanol with aqueous hydrochloric acid " cyclobutanol was obtained in 72% yield containing a trace of the allyl alcohol. This transformation forms the basis of an efficient cyclobutene synthesis. ... 

An interesting application of this cyclopropylmethyl to cyclobutyl rearrangement is the rearrangement of 3. A cascade of such rearrangements led to exotic cyclohexane and cyclopentane derivatives polyannulated with cyclobutanes, 4 and 5, respectively, in 63% yield. ... 

Rearrangements of a-substituted cyclopropylmethyl radicals afford mixtures of the (E)-and (Z)-butenyl radicals usually with a preponderance of the Z-isomer. This stereoselectivity is probably a consequence of the higher proportion of the a /-conformer compared to the j n-conformer, the former being lower in energy for steric reasons i.e. nonbonded interactions between the substituent and ring hydrogens are less important. The -alkene is also thermodynamically more stable than the Z-alkene. ... 

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