Camphene hydrochloride, rearrangement

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. 

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. 

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-... 

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. 

In the conversion of a-pinene (2.8) into bornyl chloride (2.9) endo isomer), the rearrangement to a 2° carbocation is favoured by relief of small-ring strain (Scheme 2.9). In a similar manner the conversion of camphene hydrochloride (2.10) into isobornyl chloride (2.11) involves rearrangement known as the Wagner-Meerwein rearrangement (Scheme 2.10). 

The rearrangement of carbonium ions was first postulated, by Meerwein (p. 160) in 1922, to account for the conversion of camphene hydrochloride into isobornyl chloride. Oddly enough, this chemical landmark is the most poorly... 

We can see, in a general way, how this particular rearrangement could take place. Camphene hydrochloride loses chloride ion to form cation I, which rearranges by a 1,2-alkyl shift to form cation II. Using models, and keeping careful... 

ProUem 28.19 (a) Show how a nonclassical ion intermediate could account for both the stereospecificity and the unusually fast rate (if it is unusually fast) of rearrangement of camphene hydrochloride into isobomyl chloride, (b) How do you account for the fact that optically active product is formed here, in contrast to what is obtained from solvolysis of norbomyl compounds ... 

Since the initial studies by Wagner at the turn of the century, the Wagner-Meerwein rearrangement has been the subject of almost continuous investigation. The reaction was initially examined in terms of the rearrangement of a-pinene (1) to bomyl chloride (2 Scheme 1), and of camphene (3) via camphene hydrochloride (4) to isobomyl chloride (5). 

It was Meerwein, who, in 1922, first recognized that the facile rearrangement of camphene hydrochloride (/) into isobomyl chloride (2) (1) involves a prior conversion into the corresponding ions or ion-pairs1 (2). This appears to be the first... 

In 1939, in the course of a discussion of the camphene hydrochloride-isobornyl chloride rearrangement, it was suggested by C. L. Wilson that such a rapidly equilibrating pair of cations (3 4) might exist instead as the mesomeric species 52. Previously 5 would have been considered to be the transition state separating 3 and 4. In effect, the proposal was that this transition state might be sufficiently stable so... 

An efficient route to 7-oxocamphene involved as its key step the solvolytic rearrangement of 3,3-ethylenedioxyisobomyl tosylate (derived from camphor-quinone).621 Brief (3 min 0 °C) treatment (PCl5-CaC03) of borneol gave excellent yields of camphene hydrochloride longer reaction times gave bornyl chlorides.522... 

It was Meerwein and van Emster who, in 1922, while studying the kinetics of the rearrangement of camphene hydrochloride (1) to isobomyl chloride (2) [Eq. (5.1)], noticed that the reaction rate increased in a general way with the dielectric constant of the solvent. Further, they found that metalhc chlorides such as SbCh, SnCb, FeCb, AlCb, and SbCf (but not BCh or SiCh), as well as dry HCl (all of which promote ionization of triphenylmethyl chloride by the formation of ionized complexes), also considerably accelerate the rearrangement of camphene hydrochloride. Meerwein concluded that the conversion of camphene hydrochloride to isobornyl chloride actually does not proceed by way of migration of the chlorine atom, but by a rearrangement of a cationic intermediate. Thus, the modern concept of carbocation intermediates was born. 

Wagner-Meerwein rearrangements occur extremely frequently among branched-chain aliphatic and alkylaryl compounds, and are particularly important in the terpene and camphor series. An example of rearrangement of type a) is that of camphene hydrochloride (1) into isobornyl chloride (2), for which Meerwein and van Emster141 give the following directions ... 

A specialized cation rearrangement first observed with bornyl systems is the acid catalyzed rearrangement of camphene hydrochloride (73) to isobornyl chloride (76).32 The reaction, which proceeds by a 1,2-alkyl shift called the Wagner-Meerwein rearrangement, initially generates bornyl cation 74, but rearrangement to... 

In 1939 Hevell, Salas and Wilson assumed an intermediate, bridge ion 2 to be formed when camphene hydrochloride 1 is rearranged into isobomyl chloride 3. This happened 17 years after Meerwein first postulated the intermediate formation of carbonium ions in chemical reactions. 

Depiction of Meerwein s interpretation, in 1922, of the rearrangement of camphene hydrochloride to isonorhomyl chloride, involving the intermediacy of camphene cation, which rearranges to isonorhomyl cation... 

There is evidence that a bonds can also participate in the formation of nonclassical carbocation intermediates in solvolysis reactions. In the Wag-ner-Meerwein rearrangement, camphene hydrochloride (45) rearranges to isobornyl chloride (46). It is possible to write a mechanism in which... 

---