Cyclodecyl cation

Our interest in the forgoing dehydrocyclization mechanisms stems from some superacid-based studies we carried out some years ago, in which an observable cyclodecyl cation 1 loses H2 concomitant with C-C bond formation (shown in bold) to give the 9-decalyl cation 2, and the possible analogy with a 2-octyl cation 3 (as 3 ) giving H2 and 1,2-dimethyl cyclohexane 4 is shown in Scheme 1. The background to this work is reviewed in the next section. 

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. 

Theoretical Modeling of the Cyclodecyl Cation l- 9-Decalyl Cation 2 + H2 Reaction... 

Figure L Optimized 1,5-p-H-bridged secondary cyclodecyl cation 10, showing bond distance (A) and bond angle (degree) at the B3LYP/6-31G level... 

These calculations act as a backdrop for the next part of this Chapter, in which we describe calculations of ground-states, transition-states and products, for the loss of H2 from simple secondary alkyl carbocations, using the cyclodecyl cations as our model. 

Table IV. Geometry comparison between the p-H-bridged cyclodecyl cation 1 and the 2-octyl cations 24 and 25...

Rather large kH/kD isotope ratios (29) have been reported in model studies of dehydrocyclization using deuterated vs. normal alkanes, particularly when one considers the high temperatures being used, but the origin of these effects is difficult to sort out. In contrast to catalytic dehydrocyclization reactions, the dehydrocyclization reactions of the observable p-H-bridged cyclodecyl cations are much more amenable to mechanistic studies, albeit difficult because of the low temperatures involved. Examination of the dehydrocyclization transition... 

Cyclodecyl cation hydride bridge, 147 1,4-Cycloheptadiene, 170, 171 Cycloheptatriene, 281 Cycloheptatrienes rearrangements, 290 Cycloheptatrienylidene, 275 interaction diagram, 276... 

Figure 6.19 The cyclodecyl cation. From J. D. Dunitz and V. Prelog, Angew. Chem., 72, 896 (1960). Reproduced by permission of Verlag Chemie, GMBH.

Higher-order shifts are facile in medium-sized rings. The geometry of the ring forces some of the transannular hydrogens to be within it, close to the lobe of the empty p orbital, which also protrudes into it. For example, the cyclodecyl cation has approximately the structure shown in Figure 6.19.133 The 5-intra-annular hydrogen need hardly move to become bonded to the 1-carbon. 

Olah and coworkers have carried out reactions of alkanes in superacid solution (SbFs-HF or SbFs-FSOsH), leading to observable (by NMR) tertiary alkyl carbocations (17, 18, 19). This reaction is pictured as a protonation of the alkane to give a pentacoordinated intermediate which then loses H2. Thus, a plausible mechanism for the cyclodecyl -> 9-decalyl cation reaction could involve loss of the bridging hydrogen (as H ) to form decalin, and then reprotonation of the decalin to eventually form H2 + 9-decalyl cation. However, when the cyclodecyl cation 1 was prepared in SbFs-FSOsD solution, there was no evidence of H-D formation in the hydrogen gas that was produced. It was concluded therefore that the H2 gas must come directly from the carbocation hydrogens without any involvement by the superacid system. 

---