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One- and Two-Electron Oxidations of Pagodanes and Bissecododecahedradienes Unusually Persistent Radical Cations.

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One- and Two-Electron Oxidations of
Pagodanes and Bissecododecahedradienes:
Unusually Persistent Radical Cations**
a)
By Horsr Prinzhach.* Bulusu A . R . C. Murty,
Wolf-Dieter Fessner. John Mortensen. Jiirgen Heinze. *
Georg Gescheidt. and Fabian Gerson*
[ I . 1. I . IIPagodane 1 (DZ,?jand the valence-isomeric diene
3 (La)
undergo oxidation in super acid solution to give
the same, novel 2n-aromatic dication 11 ( D z i C )Its
. surprising stability is closely connected with its special structure;
the dication 12, generated from [2.2.l.l]pagodane 2, is significantly less stable.l'] The preparatively important cleavage of the cyclobutane a bonds in 1 is made selectively
possible by the use of radical cation salts.'21 Further insight
into the course of this process should be provided by cyclovoltammetric and ESR spectroscopic
LO
L5
0.5
E [ vs. Ag/AgCl]/
6.0
V
b)
.--
,.
,..----
<.
;
, .
15
3 14
l/2
- I eo
-1e
k.
+.
.~
(.
:
718
5/6
I
0:
,
-le@
+
11/12
1,3,5,7,9,11, X=CH, ; 2,4,6,8,10,12, X=ICH,),
The oxidation of pagodane 1 and the diene 3 was investigated voltammetrically in CHZC12and S0,'4' in the presence of tetra-n-butylammonium hexafluorophosphate
(TBAPF6j as supporting electrolyte. Because of their low
nucleophilicity, both solvents are ideal for the generation
of reactive cations.''' In the case of 1 (Fig. la), an irrevers['I
Prof. Dr H. Prinzbach, Dr. Bulusu A. R. C. Murty, Dr. W.-D. Fessner
Chemisches Laboratortum der Universitat,
lnstitut f u r Organische Chemie und Biochemie
Albertstrasse 2 I , D-7800 Freiburg (FRG)
Priv.-Doz. Dr. J. Heinze, Dr. J. Mortensen
lnstitut fur Physikalische Chemie der Universitat
Albertstrasse 21, D-7800 Freiburg (FRG)
[**I
Prof Dr. F. Gerson. Dip1:Chem. G. Gescheidt
Institut f u r Physikalische Chemie der Universitat
Klingelbergstrasse 80, CH-4056 Base1 (Switzerland)
This work was supported by the Deutsche Forschungsgemeinschaft, the
Fonds der Chemischen Industrie, BASF AG, and the Schweizerischer
Nationalfonds zur Forderung der wissenschaftlichen Forschung. E . A .
R C M. thanks the Alexander von Humboldt Foundation for a fellowship
Angen.. Clwm In?. Ed Engl. 26 11987) No. 8
05
0.0
V
Fig. I . a) Cyclic voltammogram of 1 (SO:/U.I M TBAPF,,, -20 c', L ' =100
mV/s); b) cyclic voltammogram of 3 (S0JO.I M TBAPF,, -20°C.
u = 50 mV/s).
ax
X'
LO
E [ vs. Ag/AgC1]/
X'
ible wave is observed at + 1.2 V vs. Ag/AgCl in both solvents, which indicates that a rapid chemical reaction (cycloreversion 5 7 ) occurs following formation of the radical cation. The shape and height of this wave support the
assumption that a further charge transfer ( 7 I 1 j occurs
subsequently to this chemical reaction and is possibly followed by another chemical step. An ECE process is further
supported by the observation of a reversible wave at
f 0 . 6 5 V during the reverse scan, which is due to the reduction of the radical cation 7 to 3. The presumed oxidation to a dication ( l l ? ) around + 1.2 V cannot be confirmed under the experimental conditions owing to rapid
reaction with nucleophilic partners (residual H 2 0 in SO,;
CIo in CH2CIZ).The diene 3 is reversibly oxidized to 7 in
CH2C12 and SO, at +0.66 V vs. Ag/AgCI (Fig. Ib). The
radical cation 7 is very stable in SO, but undergoes slow
subsequent reactions in CH,C12. The oxidation to 1 1 occurs at
1.2 V. The dication 11 is appreciably less stable
than the radical cation 7 ; it reacts with CI" in CH2Clzand
with residual H 2 0 in SO2 so rapidly to give the known
quenching products (cf. 13b,c in Ref. [ I , 21) that its reduction can no longer be observed by cyclic voltammetry.
From measurements with scan rates between 10 and
50 V/s it is possible to estimate a lifetime of ca. 0.05 s.
For ESR studies, 1 was oxidized in CHzClzboth electrolytically on a helical Au anode['] (TBACIO, as the supporting salt) and chemically with AICI, or tris(p-bromopheny1)ammoniumyl hexachloroantimonate. The deeply colored radical cation
=610 nrn) thus generated is very
persistent, having in vacuo a lifetime of ca. ten and two
days at -70 and + 20°C, respectively. Its temperature-independent ESR spectrum (g=2.003 1 +-O.OOOl), shown in
Figure 2, extends over 13 mT; the nine groups of lines,
each group separated by 1.544 mT, are indicative of a hyperfine interaction with eight equivalent protons. The
splittings within the groups arise from three sets of four
equivalent protons each, with coupling constants of 0.1 17,
0.065, and 0.051 mT (+O.OOl mT), as well as from a second-order interaction associated with the large coupling
-
+
+
0 VCH Verlogsgeseil.vchofr mbH. 0-6940 Weinheim. 1987
0570-0833/87/0505-0457 $ 02.50/0
457
constant of 1.544 mT. By the use of the general TRIPLE
resonance technique,”’ the values of 1.544 and 0.051 mT
are found to have the same sign, which is opposite to that
of 0.1 17 and 0.065 mT.
*
stants, 1.76 0.0 i and 0.96 k 0.0 I mT, each due to a set of
four equivalent p protons. The hyperfine splittings of the
sixteen y protons belonging to six different sets are not resolved in the ESR spectrum; they d o not exceed 0. I mT, as
indicated by the ENDOR spectrum.’“’]
The persistence of the radical cations 718 is closely connected with their polycyclic skeletons. Under a variety of
oxidation conditions, it was not possible, by ESR spectroscopy, to detect radical cations generated from the “molecular moieties” of 1/3 (8/9 in Ref. [2]). The opening of the
four-membered ring, which occurs upon formation of the
dications 11/12 from the pagodanes 112, presumably also
occurs in 516 (ECE) but not in 9/10 (EEC). Owing to the
small exothermicity of the opening 1 - 3 (AAH;‘= -2.4
kcal/mol),”’l and presumably also of 5 - 7 , and owing to
the rigidity of the [I.l.l.l]pagodane 1 , there is a good
chance that 5 might be identified directly at low temperature1’21and that 5 and 7 as well as 9 and 11 might be differentiated in the gas-phase oxidation (“charge stripping”),[131
Received. January 19, 1987 [ Z 2055 IE]
German version: Angew. Chem. 99 (1987) 488
G. K. S . Prdkash, V. V. Krishnamurthy, R. Herges, R. Bau, H. Yuan, G.
A. Olah, W.-D. Fessner, H Prinzbach, J . Am. Chem So<. 108 (1986)
836.
W:D.
Fessner. Bulusu A. R. C . Murty, H. Prinzbach, Anyens. Chem. 99
(1987) 482; Angew Chem. In! Ed. Engl. 26 (1987) 451.
T h e He(la) PE spectrum of 1 (recording temperature ca. 100°C) exhibits a n initial. broad band with a maximum at I’;’=8.2 to 8.3 eV, fol-
Fig 2. ESR spectrum of the radical cation 7 generated from 1 o r 3 (CH2C12,
-40°C). T o p : complete spectrum. Bottom: part of the spectrum including
the three central groups o f lines at a n expanded magnetic field scale.
An identical ESR spectrum is observed for the diene 3
subjected to the same treatment; in line with the lower oxidation potential, the appearance voltage in the electrolysis
is substantially lower than for 1 . The results of the ESR
studies are thus fully consistent with the postulated cycloreversion 5 - 7 , so that the structure of the radical cation
should be properly described by formula 7.
Assignment of the large coupling constant (1.544 mT) to
the eight equivalent p protons, i.e., to those that are separated by one C-C bond from the n system of 7, is straightforward, considering the D2/,symmetry. The coupling constants, aH”,of protons arise from hyperconjugation and
obey the relationship‘’] u H ” = B.p”-cos’B, where p“ is the
spin population at the adjacent n center and 6’ stands for
the dihedral angle between the 2p, axis at this center and
the C-HI’ bond. Since the total rr-spin population in 7 is
evenly distributed among the four n centers, p” amounts to
0.25. Taking + 6 mT as the proportionality factor B appropriate for radical cations and setting B= 10” o n the basis of
structural data for 3,“’l one obtains uH11= + 1.5 mT. According to the evidence from TRIPLE, the positive sign
thus predicted for 1.544 mT also holds for 0.051 mT,
whence 0.1 17 and 0.065 mT must be negative. Assignments
of the three last-mentioned coupling constants to the three
sets of four equivalent y protons in specific positions of 7
are hardly feasible without further information.
=610 n m ) generated unThe colored radical cation (A,,,,,
der similar conditions from [2.2. I . Ilpagodane 2 is also
rather persistent; however, it decays faster than 7 . By analogy to 7, its structure should be described by formula 8.
The ESR spectrum (g = 2.0040 -t 0.000 1) reflects the lower
C,, symmetry of 8 by exhibiting two large coupling con-
s
458
0 V C H Verlaysgecellrcha/r mhH. 0-6940 Weinhetm. 1987
lowed by a band system with little structure. T h e point at which the first
band emerges above the noise level is at 7.7 to 7.8 eV, which allows a n
upper limit for the adiabatic ionization energy I.; to be determined (E.
Heilbronner, J Lecoultre, private communication).
J Heinze, Anyew. Chem. 96 (1984) X23: Angen.. Chem I n ! . Ed Enyl. 23
(1984) 83 I
M. Dietrich, J. Mortensen, J. Heinze, Angun,. Chum Y7 (1985) 502; A n yew Cliem. In!. Ed Engl. 24 (1985) 508.
H. Ohya-Nishiguchi, Bull. Chem. Soc. Jpn. 52 (1979) 2064.
Review: H. Kurreck, B. Kirste, W. Lubitz, Angen.. Chem. 96 (1984) 171;
Angew. Cheni. I n ! . Ed. Engl. 23 (1984) 173.
C. Heller, H. M. McConnell, J . Chem. P/i).c. 32 (1960) 1535.
W.-D. Fessner, H. Prinzbach, G. Rihs, Terruhedron Lerr. 24 (1983)
5857.
According to the relative intensities o f the E N D O R signals. a n appreciable part of the 16 y protons must have a coupling constant close to
0.025 mT.
P. R. Spurr, Bulusu A. R. C. Murty. W.-D. Fessner, H Fritz, H. Prinzbach. Angen,. Chem 99 (1987) 486, Angen’ Chem I n / . Ed Engl. 26
(1987) 455.
Cf. E. Haselbach, T. Bally, Z. Lanyiova, P. Baertschi, Heh;. Chim. A a a
62 (1979) 583; H. D. Roth, M. L. M. Schilling, T. Mukai, T. Miyashi,
Telrahedron L ~ w .24 (1983) 5815.
W. Koch, F. Maquin, D. Stahl, H. Schwarz, Chimra 39 (1985) 376
Enzyme-Catalyzed Cyanohydrin Synthesis in
Organic Solvents
By Frunz Effenberger.* Thomas Ziegier, and
Siegfried Forster
Pfeii et al. have described the enantioselective addition
of hydrogen cyanide 2 to benzaldehyde and numerous
other aldehydes 1 in the presence of the enzyme mandelonitrile lyase (“(R)-oxynitrilase”) to give optically active
(R)-cyanohydrins 3.“l However, in the aqueous and aqueous alcoholic systems exclusively used so far, the chemical
reaction, leading to the formation of racemates, occurs in
[*] Prof. Dr. F Effenberger, Dr. T. Ziegler, Dr. S . Forster
lnstitut fur Organische Chemie d e r Universitat
Pfaffenwaldring 55, 0-7000 Stuttgart 80 ( F R G )
0570-0833/87/0505-0458 S 02.50/0
Angew Chem. Inr Ed. Engl. 26 (1987) No. 5
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persistence, two, oxidation, unusually, one, pagodanes, electro, radical, cation, bissecododecahedradienes
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