Exotherm cyclopentadiene dimerization

Sulphines may react as dienophiles with 1,3-dienes with the formation of cyclic sulphoxides. Unstable 2,2-dichloro-5,6-dihydro-2ff-thiin-l-oxide 191 was formed in an exothermic reaction between 173aandcyclopentadieneat — 40 (equation 101). The simplest, parent sulphine, CH2 = S = O, prepared in situ by treatment of a-trimethylsilylmethanesulphinyl chloride with cesium fluoride, reacts with cyclopentadiene to give bicyclic, unsaturated sulphoxide 192 as a mixture of two diastereoisomers in a 9 1 ratio (equation 102). On the other hand, a,j8-unsaturated sulphine 193 (generated by thermolysis of 2-benzylidene-l-thiotetralone dimer S-oxide) in boiling toluene behaves as a 1,3-diene and was trapped by norborene forming sulphoxide 194 in 78% yield ° (equation 103). 

The instability of cyclopentadiene also required detailed study. Under adiabatic conditions, cyclopentadiene can spontaneously dimerize exothermically and thus should be stored cold for only short duration, especially on kilogram scales (Scheme 3). The impact of container size, volume, temperature, and solvent dilution on storage conditions was carefully studied to assess the potential of a runaway reaction. The published results of this analysis clearly show that hazards increase significantly as the volume of stored cyclopentadiene increases.35 These generation and storage issues of a simple yet key building block threatened the earliest step in the benzazepine sequence. 

Cyclopentadiene is a product of petroleum cracking. It dimerizes exothermically in a Diels-Alder reaction to dicyclopentadiene, which is a convenient form for storage and transport. Dicyclopentadiene plus cyclopentadiene demand in the United States amounted to 270 million lb in 1998. 

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