Dehydrocyclization cation systems

Only the 1,6-p-H-bridged structures (24 and 25) are discussed in detail here since these lead to six-membered rings and their transition-states for dehydrocyclization are smaller than for the 1,5-p-H-bridged 23. However, since 23 is the global minimum for the octyl cation system, it will be used as an arbitraiy zero reference point for energy comparison between other isomers. 

Figure 9. B3LYP/6-3IG Energy profile (LE) for 6 ring (- - ) vs. 5 ring (—) dehydrocyclization in the 2-heptyl cation system. The arbitrary zero reference point is the cis 1-5-p-H-bridged cation 16 (see footnote (a) in Table II). The transition-state structures have been shown in Figure 6. The product ions are the I-2-dimethylcyclopentyl cation 20 and the I-methylcyclohexyl cation 21, respectively, and these have almost the same energy at B3LYP/6-3IG level.

The cyclodecyl system is unique however in undergoing a dehydrocyclization reaction to give the 9-decalyl cation, as was shown in Scheme 1. 

This introduction brings us back to the structural analogy shown earlier between the cation 1 dehydrocyclization reaction and the plausible connection between this reaction and that for a 2-octyl cation intermediate 3 (and which could include many other linear alkane carbocation systems with at least six contiguous carbons). Our initial aim therefore was to study computationally the dehydrocyclization of the cyclodecyl cation 1, to see if one could satisfactorily model this known reaction. 

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