Cyclohexene precautions

It is important to exclude air in all hydrazone-type reductions involving olefins (otherwise, over-reduction occurs due to diimide formation) in the above example, as an added precaution cyclohexene was used as a solvent. 

Catalysts that in themselves are completely safe may catalyze combustion of hydro n or of organic vapors or solvents. Compounds that are de-hydro nated readily, such as lower alcohols and cyclohexene, are particularly apt to ignite. Other solvents are ignited with much more difficulty and very rarely, but this should not be relied on, and in all cases due precaution should be taken. 

The intermediate in both reactions is a cation but the first (from cyclohexene) adds an anion while the second (from benzene) loses a proton so that the aromatic system can be restored. Notice also that neutral bromine reacts with the alkene but the cationic AICI3 complex is needed to get reaction with benzene. Bromine itself is a very reactive electrophile. It is indeed a dangerous compound and should be handled only with special precautions. Even so it does not react with benzene. It is difficult to get benzene to react with anything. 

It is reported that unless special precautions are taken for the vacuum distillation of crude -ionylideneacetaldehyde extensive rearrangement is observed and the tricyclic ketone (327) may be isolated in about 50% yield. " The formation of (327) can be rationalized by a scheme involving successive E-Z isomerization, [1, 5] shift of aldehyde hydrogen, and intramolecular cycloaddition of the unsaturated ketene so formed to the cyclohexene part of the molecule. A second product is formed in the course of the reaction and the compound (328) is a tentative suggestion for its structure since a second mode for the ketene-cyclohexene addition is possible. 

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