9-Decalyl 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. 

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

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

A transition state (TS) for H2 loss was easily located in both 1,6-p-H-structures, and an intrinsic reaction coordinate (IRC) following, shows that each TS is connected to the respective cis- or trans- 9-decalyl cation + H2 product. The cis- 1,5-bridged structure 10 also has a transition-state for H2 loss, but this is higher in energy than those for the 1,6-analogs since the product tertiary cation... 

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. 

Gream has studied the generation of the 9-decalyl cation (363) from pre-... 

In such a process, the water molecule fonned in the elimination step is captured primarily fiom the fixmt side, leading to net retention of configuration for the alcohol. For the ester, the extent of retention and inversion is more balanced, although it vari among individual systems. It is clear om die data in Table 5.18 that the two pairs of stereoisomeric amines do not form the same intermediate, even though a simple mechanistic interpretation would sugg that both would fmm the 2-decalyl cation. The coUap of the ions to product is pvidoitly so rapid that diere is not time for relaxation of the initially formed intermediates to reach a common stnicture. 

Class (3) reactions include proton-transfer reactions of solvent holes in cyclohexane and methylcyclohexane [71,74,75]. The corresponding rate constants are 10-30% of the fastest class (1) reactions. Class (4) reactions include proton-transfer reactions in trans-decalin and cis-trans decalin mixtures [77]. Proton transfer from the decalin hole to aliphatic alcohol results in the formation of a C-centered decalyl radical. The proton affinity of this radical is comparable to that of a single alcohol molecule. However, it is less than the proton affinity of an alcohol dimer. Consequently, a complex of the radical cation and alcohol monomer is relatively stable toward proton transfer when such a complex encounters a second alcohol molecule, the radical cation rapidly deprotonates. Metastable complexes with natural lifetimes between 24 nsec (2-propanol) and 90 nsec (tert-butanol) were observed in liquid cis- and tra 5-decalins at 25°C [77]. The rate of the complexation is one-half of that for class (1) reactions the overall decay rate is limited by slow proton transfer in the 1 1 complex. The rate constant of unimolecular decay is (5-10) x 10 sec for primary alcohols, bimolecular decay via proton transfer to the alcohol dimer prevails. Only for secondary and ternary alcohols is the equilibrium reached sufficiently slowly that it can be observed at 25 °C on a time scale of > 10 nsec. There is a striking similarity between the formation of alcohol complexes with the solvent holes (in decalins) and solvent anions (in sc CO2). 

Many more cyclic and polycyclic equilibrating carbocations have been reported. Some representative examples, namely, the bisadamantyl (499),859 2-norbornyl (500),40 7-perhydropentalenyl (501),188 9-decalyl (502),188 and pentacylopropylethyl (503)860 cations, are given in Scheme 3.19. All these systems again involve hypercoordinate high-lying intermediates or transition states. 

The water molecule formed in the elimination step is evidently captured primarily from the front side, leading to retained configuration for the alcohol. The ester product can be formed by solvent collapse from the front or back side or by capture of the acetate ion. It is clear that the two stereoisomeric amines do not form the same intermediate, even though a simple mechanistic picture would show the 2-decalyl ion as a common intermediate. The product composition is very significantly different for the two starting materials. Similar results have been found for the cis-and 4-t-butylcyclohexylamines. Some of the data are summarized in Table 5.16. The general picture which arises from these and other diazotization studies then is one of a very rapid collapse of the cationic intermediate with the product ratio and stereochemistry determined by the immediate solvation environment. 

Our studies on the adamantyl (10) and decalyl (1) systems are onlytwo of several examples of the successful application of the ionic chiral auxiliary approach to asymmetric induction in the Yang photocyclization reaction. Space does not permit a full exposition of our results in this area. The interested reader may wish to consult references 17, 25, 27, 31, 35, 38 and 44 for additional examples and further details. As a note of interest, reference 44 describes the use of anionic chiral auxiliaries in the Yang photocyclization of a cationic counterion in the crystalline state aU of our other studies have been of the reverse type. 

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