1779
Carbonium Ion Rearrangements in the Fenchyl Series Sir : The dehydration of a-fenchol by various acidic catalysts has been a n extensively studied reaction and presently five different fenchenes (CY-, and cyclofenchene have been characterized. Although the carbonium ion rearrangements relating these fenchenes are well known,2 little is known concerning the relative thermodynamic stability of the various carbonium ions or the kinetics of the rearrangements relating the ions. Addition of a-fenchene (7,7-diniethyl-2-methylenenorbornane), /3-fenchene (5,5-dimethyl-2-niethylenenorbornane), or cyclofenchene, but not a-fenchol, t o 4 : 1 SOsCIF-FS03H at -130" yields a solution which contains, by nmr,4 an equilibrium mixture of the cations I and I1 (Scheme I). The equilibrium constant is close to one and the value of kl (Scheme I) was estimated5 to be 1 sec-' at -78" (AF* ca. 11.2 kcal/ mol). On warming t o -92", I a I1 disappear completely forming 111, with a rate constant k2 of 5.1 + 0.2 X sec-l (AF* = 13.1 kcal/mol). Like the 1,2-dimethyl-2-norbornyl cation,'j it has not been possible to observe the individual isomers IIIa and IIIb in the nmr spectra, since the Wagner-Meerwein rearrangements interconverting the two are still very rapid at - 130". Ion 111 further exists in two geometric forms, the equilibrium favoring the endo-anti pair IIIa-IIIb ( K = 6.3 i 0.3). At -15.0", ion I11 re-
r)
In this work, recovery experiments (i.e,, quenching the ion solution in aqueous base) have been performed on ions 111 and IV yielding the results shown in Scheme I. The known products of the acid-catalyzed dehydration of a-fenchol stop at ion 111 derived compounds and it is apparent that ka is too slow to be of significance in the reactions previously studied. The two new "fenchenes" derived from IV have been isolated in a pure form and characterized by physical and chemical means. Scheme I Related fenchenes . - - - *i.i.dimethyl-2-methyIeiienor~rnnne ( Q ) Z,i,i-trimethyl."norbornene ( f ) _ r
H H
CH, I
-- - - - - - +
1,5,5-trimethyl.2-norbornene ( 6 )
HvH
I1
"1 m
1,2,3-trimethyl.2.norbornene
(e)
+
minor othersa lk3
A
ck,
y CH3
CH2
kH,
3
1
rv
t
b
minor others"
V
arranges to the "ultimate rearrangement" product of this bicyclo[2.2.l]heptyl series, ion IV, with a rate constant k3 of 1.1 X sec-l (AF" = 18.5 kcal/mol). The W M shift in this cation is also rapid on the nmr time scale at - 130". The final product is the allylic cation V, formed at 25.0' with a rate constant k i of 3.2 0.2 X sec-l (AF* = 22.2 kcal/mol). The formation of this cation from a-fenchol in sulfuric acid solvent was first reported by Den0 and Houser.'
Exocyclic compounds derived from IIIa-d. Only one of these compounds could be isolated pure. A small amount of IV-derived in products were also present. Pure 1,2,3-trimethyl-2-norbornene acid gives only the IIIa-1Ilb ions initially. * At least two further fenchenes are formed in small amounts and their structures is under investigation. All these compounds can be used to regenerate IV. Q
*
( I ) Reviewed i n "Rodd's Chemistry of Carbon Compounds," 2nd ed, S. Cofey, Ed., Elscvicr, Amsterdam, 1969, part 11, pp 243-249. (2) W. Hiickel and D. Volkmann, An17, Chew., 664, 31 (1963).
(3) This endo alcohol and others are simply protonated at this tenipcrature and heterolysis of the C-0 bond does not occur rapidly ~ i i i t i lthe temperature reaches about -40". (4) The peaks for the two ions are so similar in chemical shift that onc is unablc to assign the peaks in detail. The gross features are, howevcr, coiisistent with these structures, ( 5 ) See part I1 of this series: E. Huang, I
