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An Intramolecular Base-Catalyzed Proton Transfer in 1 3-Bis(4-fluorophenyl)triazene.

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trindenyl unit a paramagnetic effect. In contrast, l b 3 - exhibits
pronounced diamagnetic ring currents along the periphery of
the molecule.
Received: September 19, 1995 [Z8412IE]
German version: Angew. Chem. 1996, 108. 456-457
Keywords: hydrocarbons
- redox reac-
[l] a) V. D.Parker. J. Am. Chem. Soc. 1976, 98, 98; b) K.Deuchert, S. Hunig,
Angeit.. Chem. 1978, 90, 927; Angex,. Chem. Int. Ed. Engl. 1978. 17, 875; c) K.
Nakasuji, K . Yoshida, 1. Murata. J. A m . Chem. Soc. 1982, 104. 1432, d) ibid.
1983. f05. 5136; e) Chcm. Leu. 1982. 969: f ) I. Murata, S. Sasaki, K. -U.
Klabunde. J. Toyoda. K. Nakasuji, Angebt.. Chem. 1991, 103, 198.: Angew.
Chrm. Itit. Ed. EngI. 1991, 30, 172; g) K.Takahashi, T. Suzuki, J. Am. Chem.
Soc. lY89, 1 I l . 5483.
[2] To our knowledge. only a few examples are known of compounds that exhibit
six-stage amphoteric redox behavior. See: J Heinze. Angew. Chem. 1981, 93,
186: Angeii, Chem. Int. Ed. Engl. 1981, 20. 202.
[3] The structure of 2 was confirmed unambigously by X-ray analysis. Yellow
needles of 2 suitable for X-ray structure analysis were grown by slow evaporation of a solution of 2 in CH,CI, and n-hexane. Details of the structure will be
reported in a separate paper.
(41 Decacyclene (Aldrich) was recrystallized from a xylene solution twice.
[5] Two regioisomeric tribromides could not be separated by column chromatography or recrystallization; however, this did not disrupt our synthesis, because
they were converted to a single symmetrical product in the final steps, 10 to Ib',
and 10 to lb3?
[6] Compounds 2 - 10 were fully Characterized spectroscopically ('H NMR, MS.
[7] Neutral monoradical species lb' and l c were stable in degassed toluene; their
ESR signals did not change for a week at room tempetrature.
[8] A. D. McLachlan. Mol. P h p . 1960, 3, 233.
191 Elemental analysis: found C 43.06, H 3.60%: calcd for C,,H,,(SbCI,),,
43.58. H 3.66Vo;m.p. >3OO"C; UV/Vis(H,SO,): imaX(&)
=775 (sh, 7760). 676
(sh. 26700), 601 (42500). 550 (sh, 37900), 494 (118000). 231 nm (81800).
[lo] UV;Vis ( T H F ) . am ax(^:) = 543 (56300)- 507 (52300), 421 (21000), 361 (37300).
333 (34300). 256 nm (39200).
[ i l l H. Spiesecke. W.G. Schneider. Tetrahedron Lett. 1986, 27, 937.
(121 In phenalenyl derivatives the positions 1, 3, 4, 6,7. and 9 are referred to as a
and 2, 5. and 8 as B. The shift changes (A&) are 51.8 at the a position and 4.9
at the [j position. I. Sethson, D. Johnels, U. Edlund, A. Sygula, J. Chem. Soc.
Perkin Tran,. 2 1990, 1339.
[I31 For super-charged mono- and polycyclic ions, see: K. Mullen, Cheni. Rev.
1984.84. 603: M.Rabinovitz. I. Willner, A. Minsky, Acc. Chem. Res. 1983. 16,
[I41 F. London. J P17~.Radium. 1937. 8. 397; R. McWeeny, Mol. PhJs. 1958, 1,
volving degenerate single proton transfer processes, which are
coupled to charge transfer and therefore to solvent reorientation, are currently available. Recently, in the conjugated porphyrin m ~ n o a n i o n [ ' an
~ ] intramolecular single proton transfer
that is characterized by a high activation barrier was observed.
In contrast, the barrier for proton transfer is very low in hydrogen-bonded complexes formed by acetic acid and pyridine at
low temperatures in organic solvents.['51 Here we introduce a
model system that lies between these two extremes : bis(4-fluorophenyl)[l,3-'5N,]triazene (1, Fig. 1) dissolved in an aprotic solvent together with a base such as trimethylamine (2). The NMR
experiments described below show that the intramolecular proton transfer in 1 occurs only in the presence of 2. Therefore,
2 acts as catalyst carrying the mobile proton of 1 from one
nitrogen site to the other, as illustrated in Figure l a .
- 1 11.1
An Intramolecular Base-Catalyzed Proton
Transfer in 1,3-Bis(4-fluorophenyl)triazene
Ferdinand Mannle and Hans-Heinrich Limbach*
Proton transfer processes are elementary steps in many acidand base-catalyzed organic and biochemical reactions." '] Because of their complexity, simple model compounds are required if the proton motion is to be followed experimentally, for
example, by NMR spectroscopy or other fast reaction techniques. Model systems are also important for the theoretical
treatment of proton transfers." l o ] Whereas several model systems for double proton transfer within or between neutral molecules have been
only a few model systems in-
[*] Prof. Dr. H.-H. Limbach, Dip].-Chem. Dr. F. Mlnnle
Institut fur Orgdnische Chemie der Freien Universitat
Takustrasse 3. D-14195 Berlin (Germany)
Fax. Int. code +(30)838-5310
e-mail: limbdch(a
This work was supported by the Deutsche Forschungsgemeinschdft and the
Fonds der Chemischen Industrie.
Angrn. Chem. In(. Ed. Engl. 1996, 35, No. 4
Fig. 1. Proton transfer pathways for 1,3-bis(4-fluorophenyl)[l,?-'
'N,]triazene (1):
a) intramolecular proton transfer, catalyzed by trimethylamine: b) hypothetical
intramolecular uncatalyzed proton transfer; c) intermolecular double proton transfer in hypothetical cyclic dimers of 1.
The 'HNMR spectra of sealed samples of 1 in deuterated
ethyl methyl ether that are carefully prepared with well-established vacuum techniques to exclude water and other acid or
basic impuritites do not show any sign of proton mobility. The
partial room-temperature 'H NMR spectrum of such a sample
is shown in Figure 2a. The signal of the mobile proton of 1 is
split into a doublet by scalar coupling with a single I5N nucleus
= 93.5 Hz), which indicates that the mobile proton
of 1
is localized at a single nitrogen site. More precisely, potential
VCH Verlagsgrsell~chuf/mhH, 0-69451 Weinheim, 1996
0570-0833i96i3S04-0441 5 15.00f .25 0
intramolecular proton jumps (Fig. 1 b), intermolecular double
proton transfer in a hypothetical dimer (Fig. Ic), and intermolecular proton exchange catalyzed by impurities as discussed
previously['6-'9] are slow on the N M R time scale. This also
applies if a small amount of trimethylamine (2) is added before
recording the spectrum
at 137 K (Fig. 2c). Due
to enhanced hydrogen
bond formation the proton signal shifts to low
field. However, if the
temperature is increased
to 248 K, a triplet can be
observed, indicating a
scalar coupling to both
atoms ('JN.H=
47 Hz). In the absence of
2 this doublet-triplet
transition does not occur. The observation of
this triplet verifies that a
11 0
intramolecular pro-6
ton transfer according to
Fig. 2. a) ' H NMR (500 MHz) signal of
Figure 1a is mediated by
the mobile proton o f a 0.1 M solution of 1 in
2. Intermolecular proton
[DJethyI methyl ether, at 298 K. b),c) Corexchange would lead to a
responding signals a t 248 and 137 K a t the
same concentration of 1. but with 0 . 0 2 ~
doublet-singlet transitrimethylamine (2) added.
t i ~ n . " ~Since
only one
signaI was observed for
the mobile proton of 1, the hydrogen-bond exchange between
solvated monomeric 1 and its 1 : 1 complex with 2 is fast on the
NMR time scale. The same proton therefore shuttles frequently
between the two nitrogen sites of 1, whereas a different base
molecule (2) catalyzes each jump.
During the proton carrier process the proton must be temporarily transferred from 1 to 2-this creates a contact ion pair.
The properties of the contact pair ensure that proton transfer to
the initial or second nitrogen site of 1 is faster than the dissociation of the contact ion pair that would lead to intermolecular
proton exchange. This finding is plausible, because the free energy of dissociation of the contact ion pair is enormous in a solvent with a low dielectric constant as a result of the Coulomb
interaction. By contrast, the dissociation energy of the molecular 1 : 1 complex between 1 and 2 is minimal, leading to the fast
hydrogen bond exchange mentioned above.
The uncatalyzed intramolecular proton transfer of 1 is understandably slow, since no intramolecular hydrogen bond is
present. Furthermore, 1 does not form cyclic dimers in which a
double proton transfer could take place according to Figure lc,
as is the case with the related diarylamidine~."~]This observation can be rationalized, because 1 is planar, and the associated
steric repulsion between the aromatic protons within a dimer
hinders the approach of the two molecules and the formation of
strong hydrogen bonds.
In conclusion, in an aprotic but not necessarily apolar solvent, bases such as trirnethylamine are capable of picking up
mobile protons at one molecular site and carrying them rapidly
to another site. The transfer occurs faster than the dissociation
of the intermediate contact ion pair. The system diaryltriazene/
trimethylamine therefore is an interesting model for further experimental and theoretical studies of proton carrier processes,
which could perhaps contribute to the understanding of the
mechanisms of enzyme reactions.
Received: November 17. 1995 [Z8561 IE]
German version: Angew. Chem. 1996. 108. 477-479
Q VCH Verlugsgesellschuf/ mbH, 0-69451 Weinheim. 1996
Keywords: catalysis
N M R spectroscopy
proton transfer
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Wehrle, J. Brdun. C. Hoelger, H. Benedict, G. Buntkowsky, W. P. Fehlhammer,
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[ l l ] M. Schlabach. H. Rumpel, H. H. Limbach, Angen. Chem 1989. 101. 84:
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1991. 113, 3550. F. Aguilar-Parrilla, G . Scherer. H . H. Lirnbach, M. C. FocesFoces. F. H. Cano, J. A S. Smith. C. Toiron, J. Elguero, ;bid. 1992, 114, 9657;
M. Schlabach, H H. Limbach, E.Bunnenherg, A. Shu. B. R. Tolf, C. Djerassi,
;hid 1993, 115. 4554, C. G. Hoelger, B. Wehrle. H. Benedict, H. H. Limbach,
J Phys. Chem. 1994. 98.843; J. Braun, M. Schlabach, B. Wehrle, M. Kocher,
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Regioselective Palladium-Catalyzed
Hydrostannylation of Unsymmetrical
Oxabicyclic Alkenes**
Mark Lautens* and Wolfgang Klute
Hydrometalation reactions are widely used for the generation
of stereochemically defined organometallic compounds. Transition metal catalysts have been shown to accelerate the hydroboration,['] hydrosilylation,[zfand hydroalumination of a l k e n e ~ . ~ ~ ]
Significant control of the regio-, stereo-, and enantioselectivity
has been achieved.
Prof. M. Lautens. Dr. W. Klute
Department of Chemistry. University of Toronto
Toronto, Ontario MSS 1Al (Canada)
Fax: Int. code +(416)978-6083
e-mail: mlautens(n
M. L. thanks NSERC (Canada) for support of this work in the form of an
E. W. R Steacie Fellowship. W. K. thanks the Deutsche Forschungsgemeinschaft for a postdoctoral fellowship.
0S70-0833~96/3504-0442$ 15.00+ .25/0
Angew. Chein. Inr. Ed. Engl. 1996, 35, No. 4
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base, fluorophenyl, intramolecular, triazene, transfer, bis, proto, catalyzed
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