Cycloheptane, isomerization

Trialkylsilanes are generally more effective than dialkyl- or monoalkylsilanes in minimizing isomerizations. The reduction of 2-bromododecane to dodecane proceeds under aluminum chloride catalysis in 82% yield using n-butylsilane and in 87% yield with tri-n-butylsilane.186 However, similar treatment of bromo-cycloheptane with triethylsilane yields a mixture of 39% cycloheptane and 26% methylcyclohexane. The same substrate yields 65% methylcyclohexane and less than 1% cycloheptane when n -butylsilane is the reducing agent.186... 

Cyclopropanes and cyclobutanes are not produced by isomerization of cyclopentane. Cycloheptane is irreversibly isomerized to methylcyclohexane by promoted aluminum chloride14 or bromide.15... 

The skeletal isomerization of cycloalkanes with more than six carbon atoms, that is cycloheptane, cyclooctane, cyclodecane, and cyclododecane, was performed over S04/Zr02 in the liquid phase at 50 °C. 

Scheme 17.3 Intermediates for the isomerizations of cycloheptane into methylcyclohexane and of cyclooctane into ethyl cyclohexane.

Cycloheptanes.—One synthesis of karahanaenone (231) depends upon thermal rearrangement of a 2-methylene-5-vinyltetrahydrofuran, and the conditions for this type of reaction have been examined on a simpler model (232), which arises from the dihydrofuran (233) at 140—200 °C. The reaction to the cycloheptenone (234) occurs rapidly at active sites on a glass surface, but is arrested in tubes coated with sodium hydroxide. Higher temperatures and lower pressures give two other compounds (235) and (236). All these reactions involve the biradical (237), as does the conversion of the cyclopropane (238) into the cycloheptenone (234). Karahanaenone (231) has also been made by isomerization of terpinolene epoxide (239) with boron trifluoride etherate. Eucarvone (240 R = H) should not be... 

Thermal homo [1,5] hydrogen shifts have been used as a ring-expansion route to cycloheptane derivatives e.g. heating the bicyclic ketone (89) at 200°C for 2h caused isomerization to the cycloheptyl ketone (90). This route was used to prepare a functionalized hydroazulene. ° ... 

Bicyclo[3,2,0]heptan-6-ones and bicyclo[3,2,0]hept-2-en-7-ones are cleanly iso-merized to cyclohept-2-enones and cyclohepta-2,4-dienones, respectively, in strong acids such as FSO3H and 96% H2SO4, e.g. (174) -> (175), and a useful method of converting phenols into cycloheptane derivatives of type (176) is by flash vacuum pyrolysis of their propiolate esters. Mixtures of isomeric cycloheptadienols are produced when the epoxide resulting from addition of dimethylsulphonium methyl-ide to acylvinyIcyclopropane is heated at 180 °C for 12 h. ... 

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