Bicyclo heptyl cations

One of the major contemporary chemical controversies in which I was inadvertently involved developed in the 1950s, surprisingly over the structnre of a deceptively simple seven carbon-containing bicyclic carbocation, the 2-norbornyl (bicyclo[2.2.1]heptyl) cation. The in-... 

X-r crystal structure determinations have been completed on two salts containing bicyclo[2.2.1]heptyl cations (Fig. 5.12). Both are more stable than the 2-norbomyl cation itself 18 is tertiary whereas 19 contains a stabilizing methoxy group. The crystal structure of 18 shows an extremely long (1.74 A) C—C bond between C-1 and C-6. The C(1)—C(2) bond is shortened to 1.44 A. The distance between C-2 and C-6 is shortened from 2.5 A in norbomane to 2.09 AThese structural changes can be depicted as a partially bridged structure. 

A fi-silyl group stabilizes (/i-silyl effect) the bicyclo[2.2.1]heptyl cation. For example, endo-2-(4-bromophenylsulfonyloxy)-enc/o-6-trimethylsilylbicyclo[2.2.1]heptane on solvolysis in trifluo-roethanol, in the presence of 2,6-dimethylpyridine, gave predominantly bicyclo[3.1.1]hept-2-ene (norpinene, 29 86% by GC 83% overall yield of solvolysis products).75... 

In a subsequent calculational study [HF and MP2 levels of theory with 6-31G(d) basis set and MP4(SDQ)], Szabo and Cremer849 explored the he C7Hn+ potential energy surface. Cation 487 (tricyclo[4.1,0.01,3]heptyl cation), the protonated ethano-bridged derivative of spirocyclopentyl cation, was considered to be the missing link between the bicyclo[3.2.0]hept-3-yl cation 488 and the 7-norbomyl cation 489. It is a kinetically stabilized species separated from cations 488 and 489 by 18.9 and 15.9 kcal mol-1, respectively. 

The intermediate bicyclo[2,2,l]heptyl cation has been written in Fig. 1 in its unbridged form by analogy with conclusions reached from studies of the Bamford-Stevens reaction of 18 stereospecifically deuteriated in either the 6-exo- or 6-endo positions (Nickon and Werstiuk, 1966). Under aprotic conditions (diglyme/sodium methoxide), the product is entirely norticyclene (19), formed without loss of deuterium in keeping with carbene formation followed by intramolecular insertion. Under standard protic conditions, 19 still constitutes more than 90% of the reaction product, but 19% of the label is lost from ea o-deuteriated starting material and 52% from the endo-deuteriated compound,... 

Carrupt, P. A., Vogel, P. Ab initio MO calculations on the rearrangements of 7-oxa-2-bicyclo[2,2.1]heptyl cations. The facile migration of acyl groups in Wagner-Meerwein rearrangements. J. Phys. Org. Chem. 1988, 1, 287-298. 

X-ray crystal stmcture determinations have been completed on two salts containing bicyclo[2.2.1]heptyl cations (Fig. 5.9). Both are more stable than would be the 2-norbomyl cation itself 18 is tertiary whereas 19 contains a stabilizing methoxy... 

The behavior of members of the bicyclo[2.2.1]heptene family is also different from that of other common 1,2-disubstituted alkenes.230 The parent bicy-clo[2.2.1]heptene gives bicyclo[2.2.1]heptane in only 3.5% yield when it is treated with Et3SiH/TFA. The major product is reported to be a 2-bicyclo[2.2.1]heptyl trifluoroacetate of unspecified configuration (Eq. 70).230 The carbocation intermediate is presumably the 2-norbornyl cation. Addition of small amounts of boron trifluoride etherate to the reaction mixture causes the yield of hydrocarbon product to rise to 22% after a reaction time of 24 hours at room temperature. Further... 

The nature of the intermediate cationic species, whether it is a classical methyl cation or a nonclassical bridged cation, has also been determined. The deamination of optically active e c/o-2-aminobicyclo[2.2.1]hcplane-exo-2-carbonitrile gave 2-hydroxybicyclo[3.1. l]-heptane-1-carbonitrile (19) in 9% yield with 92% retention of the optical activity. Similarly, deamination of e.vo-2-trifluoromethylbicyclo[2.2.1]heptyl-e Jo-2-amine gave 1-trifluoromethyl-bicyclo[3.1.1]heptan-2-ol (20) in 11 % yield with 88% retention of the optical activity. These results are interpreted in terms of intermediate bridged cations.87... 

Deuterium-labelling studies revealed that no more than a few percent cyclo-propylcarbinyl-cyclopropylcarbinyl cation rearrangement takes place in the parent 2-bicyclo[4.1.0]heptyl system. However, acid-catalyzed alcohol rearrangement and... 

Consequently, reaction (3) should depend on the stability of the cation corresponding to the radical R. This is clearly the case, as shown in Scheme 8 by the good correlation obtained between the stability of the cation (measured by the SN1 rate constant) and k0 [21]. This interpretation explains why radicals having a a configuration (pyramidalized radicals) such as the cyclopropyl or the bicyclo[2.2.1]heptyl-l [34], less nucleophilic than n radicals (quasi planar radicals having their unpaired electron in a p orbital), give no or only small quantities of... 

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