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Ion Cyclotron Resonance Studies on tert-Butyl 2-Norbornyl and 1- and 2-Adamantyl Cations.

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cludes an intramolecular 1,2 hydride shift"]. However, only
(1) has hitherto been detected as a stable cation in a superacid medium and in the gas phase['I; all attempts to isolate
(2) or to identify it by spectroscopy
&:
Received: May 15. 1979;
supplemented: June 15, 1979 [Z 352a IE]
German version: Angew. Chem. 91, 1017 (1979)
Publication delayed at authors' request
&H
CAS Registry numbers:
(f), 2819-03-6; (4,21517-94-2; (3), 768-90-1; (4), 7314-85-4; (So), 281-23-2; (Sb),
19215-02-2 p c ) , 72152-35-3
(2)
(1)
R'
(31
deuterium is exclusively eliminated from the tetradeuterated adamantane (Sb). The trideuterated species (5c) loses
both H' and D'; after correcting for the statistical probability
of H/D elimination the experimental data afford a kinetic
isotope effect of k H / k D= 1.9.
(4)
We have generated the [M- Br] + ions from 1- and 2-bromoadamantane (3) and (4), respectively, by dissociative ionization in the gas phase and recorded their collision activation
(CA) spectraf4].In spite of certain common features, the CA
spectra of the [M- Br] ions of (3) and (4) differ to such a
degree (for example, see the section between m/e=65 and
m/e=81 shown in Fig. 1Is1) that we have to assign the ions
d@eerent structures (fingerprint criterion).
+
79
[I] a) P. u. R. Schleyer, Angew. Chem. 81, 539 (1969); Angew. Chem. Int. Ed.
Engl. 8, 529 (1969); b) P. v. R. Schleyer, L. K. M. Lam, D. J. Raher, J. L. Fry,
M. A . McKervey, J. R. Alford, 8. D. Cuddy, V. G. Keizer, H . W . Geluk, J. L.
M . A . Schlatmann, J. Am. Chem. SOC.92,5246 (1970); c) D. M . Brouwer. H .
Hogeveen, Rec. Trav. Chim. Pays-Bas 89, 21 1 (1970); d) M. L. Sinnott, M . C.
Whiting, J. Chem. SOC.Perkin Trans. I1 1975, 1446; e) P. Vogel. M . Saunders,
W . Thielecke, P. v. R. Schleyer, Tetrahedron Lett. 1971. 1429.
121 P. u. R. Schleyer, R. C. Fort, Jr., W E. Watts, M. B. Comisarow, G. A. Olah, J.
Am. Chem. SOC.86,4195 (1964); G. A. Olah, M. B. Comisarow, C. A . Cupas,
C. U . Pittman, Jr., ibid. 87, 2997 (1965); G. A. Olah, J. Lukas, ibid. 90, 933
(1968): R. H . Sta/e.v. R. D. Wieting, J. L. Beauchamp, ibid. 99, 5964 (1977).
131 P. I'. R. Schlryer, personal communication. We thank Prof. Schleyer, Erlangen, for suggestions concerning this work.
141 Concerning the method: K. Leusen, H. Schwarz, Angew. Chem. 8X. 589
(1976); Angew. Chem. Int. Ed. Engl. I S , 509 (1976); R. G. Cooks: Collision
Spectroscopy. Plenum Press, New York 1978; F. W. McLaferty in M. L.
Gross: High Performance Mass Spectrometry: Chemical Applications. ACS
Symp. Ser. 70, American Chemical SOC.,Washington, D. C., 1978; K. Leusen;
Fundamental Aspects of Organic Mass Spectrometry. Verlag Chemie. Weinheim 1978.
151 N o attempt has been made to reproduce the complete space-consuming spectra here.
[6] See: D. H . Williams. Acc. Chem. Res. 10, 280 (1977); D. H. Williams, B. J.
Slapleton, R. D. Bowen, Tetrahedron Lett. 1978, 2919; R. D. Bowen. D. H.
Williams, H. Schwarz, Angew. Chem. 91,484 (1979); Angew. Chem. Int. Ed.
Engl. 18,451 (1979).
Ion Cyclotron Resonance Studies on tert-Butyl,
2-Norbornyl, and 1- and 2-Adamantyl Cations'")
By Raymond Houriet and Helmut Schwarz[']
0)
b)
C)
Fig. 1. Section from the CA spectra of the ions C,,H :5: a) from (3). [ M - Br] +;
b) from (4). [M-Brj +;c) from (5a). [ M - H I +;the numbers are the m / e values
of the collision-induced fragments.
Exothermic isomerization of the CIoH ions before or
during elimination of Br' can be ruled out since the kinetic
energy liberated on unimolecular decomposition of (3) and
(4), i.e. TI/', and also the line shape of the transition sigBr] we obtain
nalsl6', are identical: for [(3)] +.+[A&
T"'=0.25 kcal mol-' and for the decomposition of [(4)] +.a
value of P I 2 = 0.26 kcal mol- '.
Since the decomposition of (3) can certainly be regarded
as a simple dissociation to form the stable I-adamantyl cation ( l ) , we deduce from the CA and T results that bromine is
eliminated from the radical cation of (4) in a mechanistically
analogous reaction (i. e. without rearrangement or a-neighboring group participation), thus giving the long-sought 2adamantyl cation (2).
Remarkably, the CA spectrum of the [M- H] ion of adamantane (Sa) and that of (1) are identical. This means that
either the tertiary C- H bond dissociates specifically or the
cation subsequently isomerizes to (1) following nonspecific
H elimination. The latter alternative can be ruled out because
+
+
Angi'n'. Chem. Ini. Ed. Engl. I N (1'17Y) No. 1 2
The relative stability of carbocations R@in the gas phase
can be determined by ion cyclotron resonance (ICR) spectroscopy[']; in this technique the direction of anion transfers
and the position of the equilibrium are established [eq. (a),
X = F, Br]. The heterolytic bond dissociation energy
D(R@--X"), which can be regarded as a measure of the relative stability of R012,31
and is defined by eq. (b), can be calculated from the equilibrium constant of reaction
R'X
RX + '
R
R @+ R"X
+ R'@
(a)
+ X o ; AH,=D(R'-Xe)
(b)
We have examined the bromide transfer between the
cations (1)-(6) (Fig. 114') and obtained the stability sequence
given in Scheme 1 from the position of the equilibria. These
agree both quantitatively and qualitatively with the sequence
given by Beauchamp et aZ.13b1
in the case of (1)-(4), while
qualitative agreement was found for (6). However, we found
[*] Prof. Dr. H. Schwatz
Institut fur Organische Chemie der Technischen Universitat Berlin
Strasse des 17. Juni 135, D-1OM) Berlin 12 (Germany)
Dr. R. Houriet
Physikalisch-chemisches Institut
der Eidgenossischen Technischen Hochschule
CH-1015 Lausanne (Switzerland)
1'7 This work was supported by the Fonds der Chemischen lndustrie and by
the Schweizer Nationalfonds zur Forderung der wissenschaftlicben Forschung.-~H. S. is grateful to the ETH Lausanne for a Visiting Professorship
(1979) and to Prof. T. Gaumann, Lausanne, for his hospitality.
0 Verlag Chemie, CmbH, 6940 Weinheim, 1979
#570-0833/79/1212-09SI
$ 02.50/0
951
D(RmB8:
161
151
149
147
(kcal.ma14)
V
A(AGI: 4 f l kcal.mol.'
Scheme 1. Stability sequence ofcations (1) to (6); D(R@ X"')values taken from
l3bl.
that the A(A@ values for the pair (3)/(6),and hence also the
differences in the bond dissociation energies, to be s 4 kcal
molFtf5I.This means that, with regard to its stability, (6)
must be regarded as a normal tertiary carbocation~'~,and not,
as frequently assumed, as a highly stabilized carbocation.
[4] In order to ensure internal consistency, a total of eleven pairs was examined
(some under double resonance conditions). Br- transfer for the pairs (3)/(4),
/4)/(S), and (4)/(6) are reproduced in Fig. 1. In cases of equal mass, as with
/S)//6), deuterated compounds were used.-The ICR spectrometer built at
the ETH Lausanne and used for the measurements contains a Varian V 7300
magnet: the experiments were performed in a flat four-section cell under trapped ion conditions [Z B. McMahon, J. L. Beauchamp, Rev. Sci. lnstrum. 43,
509 (1972)]. The dissociative ionization of the bromides was accomplished by
a 2.5-ms pulse of 20-eV electrons. All compounds were introduced via a dual
inlet system.
[5] The data reported in [3a] yield a difference of 10 kcal mol - ' for (3)/(6).
However, it is emphasized in that publication that the value for (6) is attended by considerable uncertainty. On using data from the literature, our meas1 5 9 5 3 kcal
urements yield the enthalpy of formation of (6) as
mol~
'. This value is in excellent agreement with the results obtained by other
methods, e.g. 160.7 [6a] or 160 kcal mol-' [6b].
16) a) J. Allison, D. P. Ridge, J. Am. Chem. Soc. 101, 4998 (1979); b) R. C. Fort.
Jr. in G. A. Olah. P. u. R. Schleyer: Carbonium Ions. Vol. IV. Wiley, New
York 1973
bi
CI
k,-h----1
'*i"-.i,
IO,1-/6al,m/e= 138
h
ILaJ.
0.2
04
f
02
06
04
181-
f
Br
( C H3)3C-Br
b
B
r
B~~
D&D
a stable ion in the gas phase[']. The stability difference of ca.
16 kcal molto be expected for classical sec/tert cations is
not present; (5) and (6) differ by 4 kcal mol-' at most. The
analogy with the contro~ersial[~l
system 2-norbornyl/2-methyl-2-norbornyl cation cannot be overlooked.
Received October 3, 1979 [Z 352b IE]
German version: Angew. Chem. 91, 1018 (1979)
CAS Registry numbers:
( I ) , 3889-74-5; (2). 3170-69-2; (3), 1605-73-8; (30). 509-19-7: 14). 30967-37-4;
(4a), 2534-77-2; (S)> 21517-94-2; (Sa), 7314-85-4; (6), 2819-03-6; [D3]-/6a).
72152-34-2
111 Selected literature: a) J. L. Beauchamp, Annu. Rev. Phys. Chem. 22, 527
(1971); b) T. A. Lehman, M. M. Bursey: Ion Cyclotron Resonance Spectrom-
etry. Wiley-Interscience, New York 1976.
[2] W. Kirmse, Top. Cum. Chem. 80. 125 (1979).
[3] a) R. J. Blinr, T. B. McMahon, J. L. Beauchamp, J. Am. Chem. Soc. 96. 1269
(1974): b) R. H. Staley, R. D. Wieting, J. L. Beauchamp, ibid. 99, 5964
(1977).
952
0 Verlag Chemie, GmbH, 6940 Weinheim, 197Y
10
08
[sl-
Fig. 1. Ion intensities as function of time in binary matures of a) exo-2-norbornyl bromide (la) (2.3 x 10 ' torr) and terr-hutyl bromide (3a) (8 x 10
adamantyl bromide (Sa) (1 : 1 mixture, total pressure: 5.5 x 10- ' torr), and c) (40) and [Dr]-/6a) (1 : I mixture, total pressure: 7.2 x 10 ' torr).
The situation is completely different in the 2-adamantyl cation (S), which has recently been detected for the first time as
m/e = 95
' torr). b) /4a) and 2-
(71 This view has already been voiced P. Vogel. M.Saunders, W. Thielicke, P. c.
R. Schleyer. Tetrahedron Lett. 1971, 1429.-Nole added in prouf(Nov. 19,
1979): Recently published STO-3G calculations ID. E. Sunko. S. Hiril-Slur&uit, S. K. Pollock, W. J. Hehre, J. Am. Chem. Soc. 101. 6163 (1979)] show (6)
to be only 4. I kcal mol ' more stable than (3). However, the stability difference ( 5 ) / ( 6 ) is predicted by these calculations [M. Kausch, P. u. R. Schleyer,
personal communication of Nov. 1 I . 19791 to be about 11.7 kcal mol
[8] C. Wesdemiofis, M. Schilling, H . Schwarz, Angew. Chem. 91. 1017 (1979);
Angew. Chem. Int. Ed. Engl. 18, 956 (1979).
[9] H. C. Brown (commentary by P. c. R. Schleyer): The Non-Classical Ion
Problem. Plenum Press, New York 1978.
~
Adamantene in the Gas Phase'"]
By Helmut Schwarz, Manfred T. Reetz, Wilhelm F. Maier,
Chrysostomos Wesdemiotis, loannis Chatziiosl$dis,
and Marita schilling^']
Adamantene (I) occupies a special place among the
bridgehead olefins'']. So far, however, the intermediacy of
(1) has only been established indirectly by trapping experiments [such as cycloaddition to give (2)]['l. If the reaction is
[*] Prof. Dr. H. Schwarz, Dipl.-Chem. C. Wesdemiotis, M. Schilling
Institut fur Organische Chemie der Techniscben Universitat
Strasse des 17. Juni 135, D-1000 Berlin 12 (Germany)
Prof. Dr. M.T. Reetz, Dr. W. F. Maier, Dipl.-Chem. I. Chatziiosifidis
Institut f i r Organische Chemie und Biochemie der Universitat
Gerhard-Domagk-Strasse I , D-5300 Bonn (Germany)
['"I
This work was supported by the Deutsche Forschungsgemeinschaft and the
Fonds der Chemischen Industrie. We are grateful to Prof. Dr. H. Vorbniggen,
Schering AG Berlin, for chemicals.
0570-0833/79/1212-0952
$ 02.50/0
Angeu. Chem. Inr. Ed. Eng/. 1 8 (1970) N o . I 2
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cyclotron, butyl, norbornyl, ion, tert, cation, resonance, adamantyl, studies
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