Camphene rearrangement

Keywords camphene, rearrangement, stereospecific, waste-free, gas-solid reaction, isobornyl halide... 

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. 

Both acetolyses were considered to proceed by way of a rate-determining formation of a carbocation. The rate of ionization of the ewdo-brosylate was considered normal, because its reactivity was comparable to that of cyclohexyl brosylate. Elaborating on a suggestion made earlier concerning rearrangement of camphene Itydrochloride, Winstein proposed that ionization of the ero-brosylate was assisted by the C(l)—C(6) bonding electrons and led directly to the formation of a nonclassical ion as an intermediate. 

Carbocation rearrangements occur in the reactions of some secondary alco hols with DAST, thus isobutyl alcohol gives a mixture of isobutyl fluoride and tert-hxAy] fluonde [95] (Table 6), and both bomeol and isoborneol rearrange to the same 3-fluoro-2 2,3-tnraethylbicyclo[2 2 IJheptane (72-74%) accompanied by camphene [95]... 

Of synthetic importance is the Wagner-Meerwein rearrangement especially in the chemistry of terpenes and related compounds." For example isoborneol 5 can be dehydrated and rearranged under acidic conditions to yield camphene 6 ... 

It is possible, however, that camphene hydrochloride is not a uniform body, but that some of the terpene suffers some rearrangement in the molecule by the action of hydrochloric acid, and that the hydrochloride consists of a mixture of a-camphene hydrochloride and /8-camphene hydrochloride there is, however, no evidence to suggest that camphene itwlf is a mixture of two terpenes, so that the two camphenes are not known to exist. Aschan obtained an alcohol, camphene hydrate, by acting on camphene hydrochloride with milk of lime, a reagent which does not produce molecular rearrangement in the terpene nucleus. 

Isoborneol (Problem 27.37) is converted into camphene on treatment with dilute sulfuric acid. Propose a mechanism for the reaction, which involves a carbocation rearrangement. 

Reactions. Part. I. The Mechanism of the Wagner-Meerwein Rearrangement. Exchange of Radioactive Chlorine and of Deuterium between Camphene Hydrochloride and Hydrogen Chloride. J. chem. Soc. [London] 1939, 1188. 

The retention of optical activity of camphene rules out methyl migration (Nametkin rearrangement) (71) or a symmetrical intermediate. On the acidic alumina at low contact time the retention of optical activity is high, about 80%. At longer contact time, however, there is essentially complete racemization. Hence, the dehydration mechanism seems to be the same on the acidic and on the base-modified alumina. The acidic alumina, however, causes the readsorption of the dehydration product leading to isomerization and equilibration. 

Various unsubstituted solid alkenes are able to quantitatively add gaseous halogenohydrides. Prominent examples are the cholesterol esters 128 that give stereospecifically the bromides 129 at -30 °C [75,75al and camphene (130) that gives stereospecifically the rearranged bromide 131 or the elusive camphene hydrochloride (132) with 100% yield [11] (Scheme 15). The quantitative solid-... 

Bomeol is oxidized to camphor with chromic or nitric acid dehydration with dilute acids yields camphene. Bomeol is readily esterified with acids, but on an industrial scale bornyl esters are prepared by other routes. For example, levorotatory bomeol is synthesized industrially from levorotatory pinenes by Wagner Meerwein rearrangement with dilute acid, followed by hydrolysis of the resulting esters [86]. 

Commercially camphene is obtained by acid-catalyzed rearrangement of pinene, recognized many years prior to the isolation from natural sources. 

The rearrangement of camphene hydrochloride is called the Nametkin Rearrangement... 

WAGNER-MEERWIN REARRANGEMENT. Carbon-to-carbon migration of alkyl, aryl, or hydnde ions. The original example is the acid-catalyzed rearrangement of camphene hydrochloride lo isobornyl chloride. 

The oxidation of camphene is somewhat more difficult. Using deuterated chloroform as the solvent and H2O2 as the oxidant, a low conversion is obtained after 3 h (27%), and only cleavage and rearrangement products are formed. When switching to UHP as the oxidant, a very good conversion of 92% is obtained after 30 h with complete selectivity toward the epoxide [37]. 

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