Camphene, protonation

This involves the formation of a carbenium ion which is best described as a hybrid of the two structures shown. This then rearranges by migration of a bond, and in so doing forms a more stable tertiary carbenium ion. Elimination of a proton yields camphene. 

Sorensen and co-workers799-801 have studied the fate of observable camphene hydrocation 190 prepared from isoborneol 187, camphene 188, or tricyclene 189 in HSO3F acid medium (Scheme 5.74). The intermediate cycloalkenyl cation 191 can also be prepared by protonation of a-terpineol (192), sabine (193), and /3-pinene (194) (Scheme 5.75). 

The corner-protonated cyclopropane configuration 45 is analogous to the non-classical norbornyl ion at the center of controversy for the past three decades. The non-classical cation concept had its origin with Wilson and coworkers in 1939 who depicted structure 58 as a possible intermediate in the camphene hydrochloride-isobornyl chloride... 

This argument cannot be applied to our examples when large amounts of polymers are formed. Besides, Deno and co-workers 41) have detected in concentrated sulfuric acid solutions of camphene and related compounds (borneol, fenchol) high concentrations of a mono-cyclic cyclohexenic carbonium ion, produced also by proton addition to l-methyl-3-isopropylidene-l-cyclohexene, a clear case of ring breaking in acid media. More recently, Olah and co-workers reported that in highly acidic systems such as HF or HSO3F and SbFs, cyclohexane produces, besides the expected cyclohexyl- and methylcyclopentyl carbonium ions, hexyl and isohexyl carbonium ions 42). 

The esterification of camphene (2) with acetic acid at a reaction temperature of 80 °C showed a conversion of 88% with a selectivity of 87% with the catalyst SAC 13 and a selectivity of 88% with the Amberlyst A 15. The carboxylic acid was in four-fold molar excess with 10 wt% catalyst. Both, the SAC 13 and the A 15 show high activity (each 88% conversion) and high selectivity in this reaction. This olefinic compound seems to be so reactive as to render high conversion independently of the acid strength of the catalyst. This could be due to the terminal double bond and to the very stable carbocation formed by protonation. 

It is assumed that this reaction involves the protonation of exo double bond of camphene to generate a tertiary carbocation, followed by the carbon skeleton rearrangement to give a transient secondary carbocation, which is immediately trapped by acetic acid. Upon saponification via Sn2, the endo acetate is converted into exo alcohol as outlined in the mechanism shown here. 

Thus, in Scheme 6.59, l,7,7-trimethylbicyclo[2.2.1]hept-2-ene ( bornene ) is protonated to yield a secondary carbocation, capable of undergoing 1,2-enrfo-hydride migration and/or the migration of a bond in a Wagner-Meerwein rearrangement to a new bicyclic (but now tertiary) carbocation and thence, by proton loss, to 3,3-dimethyl-2-methylenebicyclo[2.2.1]heptane ( camphene ). 

Scheme 7.46. The formation of camphene hydrochloride (ex< -2-chloro-233-trimethylbicyclo[2.2.1]heptane and its rearrangement to isobomyl chloride (ex<7-2-chloro-l,7,7-trimethylbicyclo[2.2.1]heptane).The addition of a proton to the carbon-carbon double bond of camphene 3,3-dimethyl-2-methyienebicyclo[2.2.1]heptane is shown as accompanied by o-bond migration to produce a singie ion with partial bonding to two sites (called, variously, nonclassical or bridged ) or a pair of rapidly equilibrating ions. The classical ions are shown, leading to the observed products. Debate raged over a period of years about the nature of the ion or ions lying between the starting materials and products. Additional discussion is provided in Chapter 8.

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