Cyclopropane-propane rearrangement

Methoxycyclopropanemcthanols, as potential precursors of cyclobutanones. are also obtained by addition of 1-methoxy-l-vinyllithium to carbonyl compounds followed by cyclo-propanation of the resulting allylic alcohol. Starting with cyclohexanone the final product was spiro[3.5]nonan-l-one (3).154 Cyclopropanation and rearrangement of an isomeric allylic alcohol 4 yielded spiro[3.5]nonan-2-one (5).15S... 

A different result was obtained in the cycloaddition to methylenecyclo-propanes 216-218 tearing alkoxycarbonyl substituents on the cyclopropyl ring. In this instance, 1,2,3-triazoles 220 isomeric with the triazolines 219 were formed in the reaction [57]. The formation of triazoles 220 is rationalised by the intermediate formation of triazolines 219, which are unstable under the reaction conditions and undergo a rearrangement to the aromatic triazoles via a hydrogen transfer that probably occurs with the assistance of the proximal ester carbonyl (Scheme 35). The formation of triazoles 220 also confirms the regio-chemistry of the cycloaddition for the methylene unsubstituted methylene-cyclopropanes, still leaving some doubt for the substituted ones 156 and 157. 

Since the addition of methylene to an olefin should be exothermic, with the evolution of about 90 kcal/mole, isomerisations of the initially formed cyclo-propanes are very likely, since they only need about 64 kcal/mole. RRKM-studies demonstrate that this isomerisation should be faster than the rearrangement of cyclopropanes 32, 33 to the pentenes 34, 35 Numerous studies of the photochemical generation in the gas phase provided conclusive evidence in favour of these findings uo.iii.iis). 

Pillai and Pines (84) found that neopentyl alcohol, mixed with 10% by weight of piperidine and passed over alumina prepared from aluminum isopropoxide, yielded 2-methyl-l-butene and 2-methyl-2-butene, in a maximum ratio of 3, and small amounts of 1,1-dimethylcyclo-propane. However, lert-pentyl alcohol yielded these two olefins in a maximum ratio of only 1.4, and none of the cyclopropane was produced (Table VI). Because of these facts a carbonium ion mechanism which is applicable to ferf-pentyl alcohol is not adequate to explain the rearrangement taking place during the dehydration of neopentyl alcohol,... 

The thermolysis of cyclopropane, cyclobutane and their derivatives has received considerable attention. The thermal rearrangement of cylcopropane to propene is a clean, first-order process.79 Information concerning the course of the reaction was provided by a study of the thermal isomerization of cis- and mmr-1,2-dideuteriocyclopropane (18).80 The process occurs significantly faster than conversion to propene, suggesting a propane-1,3-diyl 19 as an intermediate. 

Diphenylvinylidenecyclopropanes 12 were selectively epoxidized in the methylenecyclo-propane part and rearranged to give diphenylmethylenecyclobutanones.52 If there was a choice, both regioisomers were formed. It is interesting to note that the stereochemistry of the substituents in the cyclopropane was preserved. 

The increasing tendency toward thermal extrusion of CF2 with increasing number of fluorine substituents on the cyclopropane ring has been demonstrated in studies of 1,1-difluorocyclo-propane, 1,1,2-trifluorocyclopropane, 1,1.2,2-tetrafluorocyclopropane,19 and perfluorocyclo-propane.20 Studies of the thermolysis of 1,1,2,2-tetrafluorospiropentane 8 21 and 1,1.2,2-tetra-fluoro-3-methylenecyclopropane (25),22 however, demonstrated that the CF2-CF2 bond in a tetrafluorocyclopropane is substantially weakened. In the case of 25, rearrangement to 2-(di-fluoromethylene)-l,l-difluorocyclopropane (26) occurs irreversibly at 150°C at a rate that was calculated to be 7850 times faster than that of the gem-difluoro analog 21. 

Photochemical procedures for the preparation of electrophilic cyclopropanes are relatively important. Appropriately substituted electron-deficient olefins on photolysis undergo a di-7r-methane rearrangement to give the corresponding cyclopropane derivatives. For example, l,l-dicyano-2-methyl-3-arylpropenes (455) provide the dicyanocyclo-propanes (456), presumably via the mechanism of equation 154 . ... 

A limited number of rearrangements of electrophilic cyclopropanes are known in the literature, but synthetic applications have not been encountered except for the vinylcyclo-propane (528)- yclopentene rearrangement . This phenomenon is due to the rather... 

When l-diazo-3-(l-methylcyclopenta-2,4-dienyl)propan-2-one (14) was photolyzed in hexane, Wolff rearrangement occurred on direct irradiation to give 15 and intramolecular cyclopropanation on triplet-sensitized photolysis to give 16. ... 

A further elaboration of this method involves alkylation of a sulfanyl- or selanyl(vinyl)cyclo-propane (see Section 5.2.2.7.3.) and reaction with base, which gives a rearranged alkylidenecy-clopropane. Repetition of the sequence leads to elimination of the selenide or sulfide and formation of a diene, as illustrated by the synthesis of (2-methyleneoctylidene)cyclopropane (15). ... 

In the case of 1-siloxy-l-vinylcyclopropanes, treatment with base produces the corresponding ethyl ketones. For example, l-[l-(trimethylsiloxy)cyclopropyl]cyclopentene (24) can be effectively converted to 1 -(cyclopent-l-enyl)propan-l -one (25). In contrast, reaction of cyclopropane 24 with acid results in rearrangement to give cyclobutanone 26 in good yield. " However, thermolysis of 1-vinylcyclopropanol at 100 C for 10 minutes gives a mixture of 2-methylcyclo-butanone (85%) and pent-l-en-3-one (15%). °... 

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