Sept. 5 , 1960
FORMATION OF ADAXANTANE
4645
[CONTRIBUTION FROM FRICK CHEMICAL LABORATORY, PRINCETUX UMVERSITY,PRINCETON, h-.J.]
The Relative Stability of Bridged Hydrocarbons. 11. endo- and exo-Trimethylenenorbornane. The Formation of Adamantanel,' BY
PAUL VON R. SCHLEYER h S D ~Z'~ALCOLM hl.
DONLLLDSON~
RECEIVEDFEBRUARY 8, 1960 Strong sulfuric acid rearranged endo-trimeth!-lenenorbornane (endo-tetrahydrodicyclopentadiene) ( I ) t o t h e e x o isomer 111. T h e equilibrium mixture contained 99(,', ero and lye endo material. I t is shown t h a t t h e rearrangement-addition of protonic acids t o dicyclopentadiene also does not occur with complete endo to exo conversion. n'ith aluminum halides, more powerful Lewis acid catalysts, further isomerization of I and I11 t o adainantane (IX) occurred. This profound rearrangement, which makes adamantane readily accessible for the first time, is pictured as proceeding by a multi-stage ionic process. Conditions are described which make i t possible t o obtain a 10-20% yield of adamantane.
I n 1903, EijknianAd observed t h a t prolonged heating of endo-trimcthylenenorbornane (endotetrahydrodicyclopentadiene, I),4bm.p. T7", in the presence of concentrated sulfuric acid to which a small amount of fuming sulfuric acid had been added, gave an isomeric material, melting a t about 9". At that time the structure of the starting material had not been established, but Eijkman proposed t h a t the transformation might be some kind of cis-trans isomerization. The reaction does not appear t o have been repeated, but the subsequent elucidation of the correct structure (I)5 of the starting material has allowed speculation into the nature of the isomerization product. For example, the trans structure I1 has been proposed in the review literature.6 This suggestion, however, is unlikely, since models show that three methylene groups cannot bridge the %-ex0 and the 3-endo positions of the bicyclo[2 2.11 heptane nucleus without the introduction of a prohibitive amount of train.^
There are many rearrangements in the dicyclopentadiene series in which the five-membered ring is transformed from the cis-endo t o the cis-exo An example is the addition of protonic acids t o dicyclopentadiene, (IV) ; this M'agner-hleerwein rearrangement is considered t o proceed through a bridged-ion intermediate V.3 Consequently, it was felt t h a t exo-trimethylenenorbornane, 111, was a more likely structure than 11.
n v.?
IV
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I1
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This expectation was further strengthened by analogy with compounds VI1 and VIII, for which heat of combustion data are a~ailab1e.I~The endo isomer VI1 was found t o be 3.6 kcal. less stable than VIII. A similar difference would be predicted for compounds I and 111; equilibration between the two, if possible, should favor the C.YO isomer. On the other hand, I11 is a known
111 (1) P a p e r 111 of d berieb on Bridged Ring 5ysteins paper I1 I K a p l a n , H K w a r t and P von R Schleyer, T F I IJ~ O U R N A L , 82, 2341 (1960) (2) F o r preliminary accounts o f this work, see P. von R . Schleyer, ibid., 79, 3292 (1957); P . von R . Schleyer a n d M. M . Donaldson, Ab-
VI1
stracts, 133rd S a t l . h'Itg., Am. Chem. SOC., San Francisco, Cal., Apr., 1958, p . 95-N. (3) Armstrong Cork Fellow, 1956-1937. T h e material here presented was t a k e n , in p a r t , f r o m the P h . D . Thesis of 1 I . h I . U . . Princeton University, 1958. (4) (a) J. F. Eijkinan. (./zriiz. W e e k b l n d , 1, 7 i l Y O S ) ; 3 , (i83 ( I U O f i ) . ( b ) T h e nomenclature used here. which emphasizes t h e relationship between t h e dicyclopentadiene a n d the norbornane ring systems, will be t h a t suggested previously (P. von I
