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Nomenclature for Intramolecular Exchange Processes.

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I
I
LO06
3982
I
EB,&V1
[4] R. Grashey, M . Baumann, and K-D. Lubos, Tetrahedron Lett. 1968,
5881.
[5] K. T. Potts and C . Sapino, Chem. Commun. 1968,672.
[6] The methiodides are conveniently obtained from the mesoionic
compounds by means of methyl iodide in boiling acetonitrile.
[7] C . J . Thoman and D. J . Voaden, Org. Synth. 45,96 (1965).
[8] F. C. Hengebaert and J . F. Willems, Tetrahedron 22,913 (1966).
[9] R . K Hill and L. E . Sutton, J. Chem. Phys. 46, 244 (1949).
[lo] G . J . Leigh, J . N . Murrell, K Bremser, and G. G. Proctor, Chem.
Commun. 1970,1661.
[ I l l L. D. Hulett and T. A. Carlson, Clin. Chem. 16, 677 (1970).
I
1637 1611
Fig. 1. X-ray photoelectron spectra of ( I ) .
Nomenclature for Intramolecular Exchange
Processes
By Gerhard Binsch, Ernest L. Eliel, and Horst Kessler“’
4007 3994
EBlndleVl
164 5
Fig. 2. X-ray photoelectron spectra of (3)
they can be correlated satisfactorily with the units of
charge obtained from the Pading electronegativity relations“’]. Thus a difference of 2.3 eV for the binding energy
of the N-Is electrons in ( I ) and (2) corresponds to a
charge difference of 0.46 charge unit. Carlson found a
comparable value for 7-methylinosine, a typical betaine,
We wish to propose a system of nomenclature for describing
the interchange of identical ligands between distinguishable
chemical and/or magnetic environments. Such interchanges
correspond to ‘‘null reactions”, involving no change in
chemical structure, but they can frequently be studied
quantitatively by dynamic NMR spectroscopy. Names
have been suggested for the process of exchange, notably
“degenerate isomerization”, “isodynamic change”“], and
“automerization”f21.We should like to point out that a
useful, alternative classification can be based on the
recently developed nomenclat~re[~
in which identical
ligands in stereochemically different environments are
designated as “enantiotopic” or “diastere~topic”[~~,
the
overall term for the two cases being “heterotopi~”[~]
or
Table 1. X-ray photoelectron spectra and dipole moments of compounds ( I ) to ( 6 ) [a].
(1)
(2)
(3)
(4)
(5)
(61
398.2;
398.3;
399.4;
399.1;
399.7;
399.4;
400.6
400.5
400.7
400.1
400.6
401.4
AEdeV)
Dipole
moment V(D)
9.28 i- 0.1
8.13 i0.02
2.4
2.2
1.3
1.o
0.9
161.1 ;
160.7;
163.7
163.2
164.5
163.5
163.7
2.6
2.5
0
0
2.1
4.32 ?0.06 [9]
2.0
0 , l s : 530.3; 532.7
2.4
6.48 & 0.02 [9]
161.6;
[a] The spectra of compounds ( 3 ) to (6) were resolved by a DuPont curve analyzer model 310.
with 2.6 eV = 0.52 charge unit“ ‘I. The difference of 2.5 eV
for the S-2p electrons in (I) and (2) represents, by a similar
calculation, a charge difference of 0.49 charge unit. These
results confirm Katritzky’s view that a betaine structure
predominates in the ground state of mesoionic 1,3,4thiadiazolethiones and similar compounds, with charge
distribution as shown in the formulations ( I ) , ( 2 ) ,and (6).
Received: April 19,1971 [Z 444 IE]
German version: Angew, Chem, 83, 588 (1971)
[l] a) K . Siegbahn et a/.: ESCA, Atomic, Molecular and Solid State
Structure Studies by Means of Electron Spectroscopy. Almquist & Wiksells, Uppsala 1967; b) J . M . Hollander and K L. Jolly, Accounts Chem.
Res. 3, 193 (1970); c) D. M . Hercules, Anal. Chem. 42, 20A (1970).
[2] A. R. McCarthy and K D. Ollis et al., J. Chem. SOC.B 1969, 1185.
[3] The spectra were measured on a Varian IEE-I5 instrument. The
AI-Ka, line (1486 eV) provided the activation energy. The instrument
was adjusted so that the Au-4f,,, line fell at 83.0 eV. The electronbinding energies given were standardized on the C-1s signal from Tesa
film (= 284 eV).
570
“stereoheterotopic”~sl;in contrast, ligands in identical
chemical surroundings are called “ h o r n ~ t o p i c ” ~ We
~-~~.
therefore propose that the process leading to exchange of
the positions of identical ligands be called “topomerization”; in this context, it is sometimes convenient to have a
name for the indistinguishable species involved in the
exchange and we suggest they be called “topomers”.
It may sometimes be desirable (and often, though not
always, possible) to employ sub-classifications, similar to
those used for isomerization. Thus bullvalene ( I ) ‘’I
represents a case of multiple valence bond or constitutional
topomerization, whereby one and the same ligand (carbon,
hydrogen) is placed in four chemically different positions
[*] Prof. Dr. G. Binsch and Prof. Dr. E. L. Eliel
Department of Chemistry, University of Notre Dame
Notre Dame, Ind. 46556 (USA)
Prof. Dr. H. Kessler
Institut fur Organische Chemie der Universitat Frankfurt/Main
Robert-Mayer-Strasse 7-9 (Germany)
Angew. Chem. internat. Edit. Vol. 10 (1971)1 N o . 8
(allylic, two kinds of vinylic, cyclopropanoid). Examples of
cis-trans topomerization or diastereotopomerization of the
type (2) (e.g.: X=CN, Y=C,H,, A=N(CH,),) have
detectable by dynamic NMR spectroscopy. The internal
rotation in l,l,l-trifluoroethane (7) provides an example
of such a “homotopomerization”. Here the three hydrogen
atoms (and also the three fluorine atoms) are structurally
equivalent in each of the three conformers, but the expected
inequality of the gauche and anti hydrogen-fluorine
coupling constants should lead to spectra of the [AX],
type[’5i when rotation is slow on the NMR time scale and
of the [A,X,] type in the fast-exchange limit. The bondshift process in cyclooctatetraene (8) represents another
intriguing case of (constitutional) homotopomerization,
recently been studied‘“’ by dynamic NMR spectroscopy.
Another well-known case of diastereotopomerization is
that of the cyclohexane ring inversion, e.g. (3)[‘11. Enantiotopomerization is represented by ( 4 ) ; the enantiotopic groups AR and As might, in principle, be distinguishable in a chiral solvent when inversion is slow. The
detectable in principle from a study of [per-’ 3C]cyclooctatetraene, where the spectrum pertaining to the averaged
planar form would be of the [[AX],I4 or [AXIS type
depending on the rate of the bond-shift process[’8!
Finally we wish to point out that a detailed classification of
topomerization processes and/or of topomers along the
lines suggested in this note may not always be possible.
(4)
enantiotopomerization ( 5 ) has in fact been studied
experimentally“Z1in achiral solvents.This becomes possible
because the enantiotopic nuclei are ani~ogamous‘’~~
; the
methylene proton NMR spectrum of ( 5 ) changes from an
AA’BB’ type (or
type in the notation of H ~ i g h “ ~ ] )
to an A,B, type on increase of the exchange rate. Case
( 6 ) ,like case (31,may be described as one ofconformational
topomerism[“Z 7’.
’
(9)
The in-plane nitrogen inversion process (9)[’*], which
simultaneously interchanges the diastereotopic ligands X
and the enantiotopic ligands Y, can be characterized
uniquely (as enantiotopomerization or diastereotopomerization) with reference to specific ligands, but the classification of the topomers involved would necessarily have to
be ambiguous. Similarly, the internal rotation (10)
corresponds to a homotopomerization with respect to the
fluorine ligand, but to a diastereotopomerization with
respect to the three hydrogen atoms. Finally case ( l l ) ,a
homotopomerization with respect to either fluorine atom,
is a “mixed” topomerization with regard to the three hydro-
In the examples (i) - (6) the exchanging ligands differ in
“topicity” and hence we are dealing with “heterotopo-
@F@@
HO
merizations”, more specifically, in cases (2) - ( 6 ) , with
“stereoheterotopomerizations”. But even if the exchanging
ligands are homotopic, the exchange process may still be
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71
Angew. Chem. internat. Edit.
Vol. 10 (1971) j No. 8
gen atoms. (“Mixed” because positions labeled 2 and 3 in
the first conformer on the left are enantiotopic and 2 and
I-or 3 and I-are diastereotopic.) Both cases (10) and
(11) represent heterotopomerization with respect to the
hydrogen atoms. The even more complicated situation in
57 1
the stereochemically nonrigid six-coordinate molecule
FeH,[P(OC,H,),],
has recently been analyzed quantitativeIy[211.
We believe that a detailed nomenclature capable of
coping with the most general case would become so
unwieldy as to lose its appeal for the majority of situations
commonly encountered in practice. For a rigorous
description of very complex topomerizations it will clearly
be necessary to resort to a formal permutational approach.
The electronic destabilization confers high reactivity on
these systems, which thus become interesting intermediates.
Planar systems, as small as possible, with bond angles of
ca. 120°[21are particularly advantageous models for antiaromatic behavior. Such systems should be readily obtainable in the heterocyclic series if one starts from six-membered dihydro heterocycles ( I ) and (Z)[31. If these are
converted into the corresponding anions, substantially
planar 8.rr electron systems result.
Received: May 24, 1971 [Z 445 IE]
German version: Angew. Chem. 83,618 (1971)
[I] J . F. M . Oth, paper presented at the International Symposium on
Conformational Analysis, Brussels, Belgium, September 9, 1969.
[2] A . T Balaban and D. Farcasiu, J. Amer. Chem. SOC.89,1958 (1967).
[3] K . Mislow and M . Raban, Top. Stereochem. 1,l (1967).
[4] D.Arigoni and E. L. Eliel, Top. Stereochem. 4, 127 (1969)
[5] K . R. Hanson and H . Hirschmann, personal communication; see
also E. L. Eliel, J . Chem. Educ. 48, 163 (1971).
[6] For prochiral enantiotopic and diastereotopic ligands [3] we use
the designations L, and Lsdefined elsewhere [4,7]; heterotopic ligands
in an olefin are denoted by L, and L, [S].
[7] K . R. Hanson, J. Amer. Chem. SOC.88,2731 (1966).
[El J . E. Blackwood, C . L. Gladys, K . L. Loening, A . E. Petrarca, and
J . E. Rush, J. Amer. Chem. SOC.90,509 (1968).
[9] M . Saunders, Tetrahedron Lett. 1963,1699.
[lo] a) H.Kessler, Angew. Chem. 82,237 (1970); Angew. Chem. internat. Edit. 9, 219 (1970); b) H . Kessler, Chem. Ber. 103, 973 (1970).
1111 S . L. Spassou, D. L. Grf$th, E. S. Glazer, K . Nagarajan, and J . D .
Roberts, J. Amer. Chem. SOC.89, 88 (1967).
[I23 G . M . Whitesides, M . Wiranowski, and J . D. Roberts, J. Amer.
Chem. SOC.87,2854 (1965).
[I31 This term, suggested by Laszlo 1141, is self-explanatory and
descriptive, whereas the ambiguous “magnetically nonequivalent” and
even the more specific “spin-coupling nonequivalent” [3] are misleading
unless explicitly curtailed by restrictive definitions [3].
[14] P. Laszlo, personal communication.
[15] C . W Haigh, J. Chem. SOC.A 1970,1682.
[16] For cases of this type, see W E. Heyd and C. A . Cupas, J. Amer.
Chem. SOC.91,1559 (1969)and reference [loa].
[17] Topomerization by rotation about the carbon-carbon bond of
(6) is confined to cyclic permutations (123*231$312); the remaining
permutational possibilities (213, 132, 321) are inaccessible by rotation.
The situation could be characterized by an extension of the CahnIngold-Prelog rules, where the labeling determines the priority sequence and the “configurational” symbol is enclosed in square brackets.
In the topomers of ( 6 ) the methyl group then belongs to the [ R ] family.
[lS] This experiment has not been carried out, but we known about
the bond-shift process from an NMR study of the closely related
heterotopomerization in [mono-13C]cyclooctatetraene [19].
[19] F. A . L. Aner, J. Amer. Chem. SOC.84, 671 (1962).
[20] H . Kessler and D. Leibfritz, Chem. Ber. 104, 2143 (1971).
1211 P. Meakin, L. J . Guggenberger, J . P. Jesson, D. H . Gerlach, F. N .
Tebbe, W G. Peer, and E. L. Muetterries, paper presented at the Symposium on Dynamic NMR Spectroscopy, Los Angeles, Calif., March
30, 1971.
Synthesis of a 6-0xa-2-azabicyclo[3.l.O]hex-3-ene:
Heterocyclic8n-SystemsasReactiveIntermediates“’
By Richard R. Schmidt[*]
Cyclic conjugated planar 4n-71 systems are destabilized by
resonance and according to R. Bredow[” are antiaromatic.
[*] Doz. Dr. Richard R. Schmidt
Institut fur Organische Chemie der Universitat
7 Stuttgart 1, Azenbergstr. 14 (Germany)
p*]This work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie.
572
R2
(31
4H-1,3-Oxazine ( 3 ) was used as an example. When it is
treated with butyllithium in anhydrous tetrahydrofuran
(THF) at -78°C a deep blue intermediate (h:E=570;
612 nm)[“l is formed, to which we ascribe the structure ( 4 ) .
This 871 system undergoes rapid further reaction, yielding
a new, colorless product. For an intramolecular reaction
the conversion into especially the valence-tautomeric
anions ( 5 ) , (6), and (7) must be taken into account. The
anion (7) is thermodynamically more stable than ( 5 ) or
(6), and indeed careful working up of the reaction mixture
permits isolation of a compound whose analytical and
spectroscopic data and subsequent reactions accord only
with its formulation as the triphenyl derivative (8) of the
previously unknown 6-oxa-2-azabicyclo[3.1.0]hex-3-ene[51.
In protic solvents compound (8) rearranges rapidly, even
at room temperature, into the open-chain iminobutenone
(9) (in ethanol, 5:p2”‘=2 h), presumably in a pericyclic
[of + of + n3.1 reaction. On thermolysis, the pyrrolinone
is obtained from (8) by way of (9). By variation
of the substituents R’, R2, and R3 it can be proved that
in this reaction R’ migrates to the C atom to which R3 is
bonded”]. On treatment of (8) with triethyl phosphite the
pyrrole (11) is formed in 50% yield.
The equilibrium between ( 4 ) and (7) lies much to the
side of the latter; it can also be set up from (8)”’. The
Angew. Chem. internat. Edit. / Vol. I0 (1971) 1 No. 8
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