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Is the Unpaired Electron in Radical Cations of Sterically Hindered Alkenes Localized or Delocalized.

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Vacuum Dynamic Gas Phase/Solid-Phase Reactions:
N-Chlorination of Primary Amines and
a-Elimination of the Resulting Chloramines;
Synthesis of Reactive (0and (a-Aldimines
By Jean-Claude Guillemin and Jean-Marc Denis*
The (E)- and (2)-aldimines 3 and 4 are intermediates in
chemical and enzymatic oxidations of primary amines, as
well as in the dehydration of aldehyde-ammonia adducts.
Several of them have been detected in the pyrolysis decomposition products of alkylamines, cyclic aldimine trimers, and alkylazides, but information concerning their stereochemistry is scarce and conflicting. Attempts to isolate
and characterize both the (E)- and (2)-isomers of this class
of compounds as stable free bases have so far failed.
Reactive species have been recently synthesized by vacuum dynamic gadsolid phase reactions1']. We report here
the direct synthesis of the reactive aldimines 3a-3f and
4a-4f by a "one-pot'' multistep sequence carried out in a
single vacuum line: N-chlorination of the primary amines
1 on solid N-chlorosuccinimide (NCS) is followed by an
a-elimination reaction of the resulting N-chloramines 2"l
over solid potassium tert-b~toxide"~]
(Scheme 1).
RH&\
H/ N-H
-
A/ i Y - C l L
1
a,
R
= CH,;
R)==N'
c, R
+
= C,H,;
4
d, R = CH=CH 2:
e , R = CH2-CH=CHz; f, R = =(CH&-CH=CH2
Scheme 1. Synthesis of 3a-3f and 4a-4f. a ; solid NCS, RT, l o - ' torr: b:
solid KOtBu, 55 "C, 10- ' torr. I n 3e and 4e R=CH=CH-CH,,
the C C
double bond is (Q-configurated.
Unsaturated N-chloramines 2d-2f are unstable and
cannot be revaporized. For this reason, vacuum chlorination of amines 1 on solid NCS and elimination of the resulting chloramines 2 on solid potassium t e r t - b ~ t o x i d e " ~ ~
were carried out successively in a single vacuum line, leading in good yields to a mixture of the reactive
and (3aldimines 3 and 4, which were characterized by their low
temperature IR ( - 196 "C), 'H-, and I3C-NMR spectra
( - 100 "C) (Table 2). N o (E)/(z)-isomerization occurs in
solution at low temperature. Stereoselectivity was ob-
(a-
Table 2. (E)- and (a-aldimines 3 and 4 from 1 [a].
a
b
89
85
c
55
d
30 [el
53
52
e [fl
I
2.3/1
2.6/1
3.0/1
2.7/1
2.3/1
2.3/1
15.9
16.1
15.4
15.9
15.5
15.6
25.3
23.0
24.4
25.1
27.0
24.4
CD
n8
9
10
found and, specifically, no intramolecular cyclization via
C-H
insertion was observed. 1-Azabicyclo[ 1.1.0lbutane
8'18]
was the main product in the elimination of 2d, but n o
bicyclic compounds 9 or 10 were detected in the elimination of 2e and 2f, respectively.
Received: April 6, 1982 [Z 5/6 IE]
revised: July 28, 1982
German version: Angew. Chem. 94 (1982) 7 I5
The complete manuscript of this communication appears in:
Angew. Chem. Suppl. 1982, I5 15- I524
hEID
R
3
2
b, R = C,H,;
HE HC
HA
RH2C\
served, the main product in all cases being the (E)-isomer
(72%), even though-as was found in the case of 3a[I4]-it
is the less thermodynamically stable isomer. Proof of the
constitution of the imines is provided by the low-temperature formation of s-triazines. In absence of solvent, the imines polymerize at ca. - 120 "C.
Furthermore, we examined the mechanism of the elimination b y isotopic labeling. Although a-elimination was
the main (if not the only) pathway for the transformation
of 2 to 3/4, no evidence of a typical nitrene process was
1642
1640
1640
1638
1639
1640
[a] For methanimine the following values are known: 'H-NMR: 'J,,,= 17.0
Hz, 3J,,0,,,=25.0Hz; IR: C(C=N)= 1639 c m - ' [lc]. [b] Yields calculated by
' H - N M R using tBuOH as internal standard. [c] 10% solution in
CD2C12+CFCI, at - 100 "C with TMS as internal standard. [d] NaCl window cooled with liquid nitrogen. [el Together with 45% of 8. [fl Only the I propenyl derivatives are formed by base-catalyzed isomerization.
[I] a) J. M. Denis, R. Niamayoua, M. Vata, A. Lablache-Combier, Terrahedron Lett. 21 (1980) 515: h) J. C. Guillemin, J.-M. Denis, A. LablacheCombier, J. Am. Chem. SOC.103 (1981) 468; c) C. Braillon, M. C. Lasne,
J. L. Ripoll, J. M. Denis, Nouu. J . Chim. 6 (1982) 122.
[7] N-Chloramines 2 were characterized by their low temperature 'HNMR, room temperature-IR (CCI,), and mass spectra.
[ 131 In a previous paper [ la], silica- or alumina-supported KOtBu was used to
minimize sublimation of the base (temperature > SO'C). When HCIelimination of primary chloramines occurs at lower temperature
( Q 55 "C), solid KOtBu can he used directly.
1141 1. Stolkin, T. K. Ha, H. Giinthard, Chem. Plqs. 21 (1977) 327.
[IS] W. Funke, Angew. Chem. 81 (1969) 35; Angew. Chem. Int. Ed. Engl. 8
(1969) 70.
Is the Unpaired Electron in
Radical Cations of Sterically Hindered Alkenes
Localized or Delocalized?**
By Horst Eierdanz and Armin Berndt*
An electron exchange between the localized structures A
and B was recently postulated['] for the sterically hindered
radical cations 1-3; it was suggested that the frequency
of this process strongly decreases with increasing steric
hindrance, and that 3 behaves like an alkyl radical ESR
spectroscopically.
By labeling an sp2 C atom of 3 with I3C we have now
been able to demonstrate that the unpaired electron in 3 and, hence, also in the less hindered radical cations 1 and
2 -is delocalized.
1 , m = n = 1;
2, m = I , n = 2.
3,m=n=2
[*I Dr. J. M. Denis, J. C. Guillemin
Laboratoire de Chimie Organique Physique
Associe li 1'E.N.S.C L.. E.R.A. C.N.R.S. n.827
UniverwC des Sciences et Techniques d e Lille
F-59655 Villeneuve d'Ascq Cedex (France)
690
0 Verlag Chemie GmbH. 6940 Weinheim.
[*] Prof. Dr. A. Berndt, H. Eierdanz
Fachbereich Chemie der Universitat
Hans-Meerwein-Strasse, D-3550 Marburg (Germany)
[**I This work was supported by the Fonds der Chernischen Industrie
1097
0570-0833/82/0909-0690 $02.50/0
Angew. Chem. Inr. Ed. Engl. 21 (1982) No. 9
The localized structure A or B for 3 was.derived from the
result that only 12 of the total 24 methyl protons and only
four of the total eight methylene protons lead to a splitting
of the ESR spectrum. From this is was concluded that the
unpaired electron couples with the protons "in only one cyclopentylidene-half'['l. The result is, however, also consistent with a delocalized structure A - B , if one assumes that
the cyclopentylidene rings occur in conformation C"'],
which is fixed o n the ESR time scale, with a total of four
axial and four equatorial methyl groups and methylene
protons, of which only one of each type couple with the
unpaired electron.
This alternative appears at first to be less attractive,
since it must be combined with a high inversion barrier[6h1
and infinitesimally small coupling constants for half of the
proton#"]. We obtained evidence, however, that this
could be the case by studying the radical cations 4 and 5
(a;,,,=
1.43 and 1.38 mT, respectively), which at -90 "C
exhibit ESR coupling constants of 0.23 and 0.25 mT, respectively ( 3 : 0.203 mT"') for only six of the twelve y-methy1 protons.
[I] F. Gerson, J. Lopez, A. Krebs, W. Kriiger, Angew. Chem. 93 (1981) 106;
Angew. Chem. I n [ . Ed. Engl. 20 (1981) 95.
[Z] U. Burkert, Tetrahedron 37 (1981) 333.
[31 H. Klusik, A. Berndt, Angew. Chem. 93 (1981) 903: Angew. Chem. fnr. Ed.
Engl.. and unpublished results.
[4] Following: J. von Braun, W. Sobecki, Chem. Ber. 44 (1911) 1918.
[5] A. A. Millard, M. W. Rathke, J. Org. Chem. 43(1978) 1834; A. Krebs, W.
Ruger, Tetrahedron Lerr. 1979, 1305.
[6] Furthermore, in 3 and 3" we observed a ' ) C coupling constant for four C
atoms in natural abundance with a'=0.98 mT, which we assign to the C
atoms of the four axial methyl groups. b) The barrier must be higher than
ra. 8 kcal/mol, since even at + 3 0 T the width of the hyperline lines is
only 0.007 mT, and hence no indication of an inversion process is obvious
(F. Gerson, personal communication). c) This is apparent both from the
narrow ESR line width and from the fact that no signals near the frequency of the free proton are observed in the ENDOR spectrum (F. Gerson, personal communication).
C,H,(CO)2Mn(m-Toluidinyl),
An Isolable Aminyl Complex**
By Dieter Sellmum*, Jiirgen Miiller, and Peter Hofmann
4
C
5
The alternatives should be clearly distinguishable from
one another in the I3C labeled radical 3": two coupling
constants having values similar to those of a tertiary alkyl
radical (ca. 4.5 mT for the radical center and ca. 1.2 mT for
the neighboring sp2-C atom with the positive charge) are
cted for the localized structure. In contrast, for the delocalized structure only one coupling constant is expected
which-as
in the delocalized radical cations 6*I3l
(a c--0.85 mT) and 7* (at -40°C: a"=1.43 mT (6H),
a H= 0.065 mT (18 H), ac = 0.86 mT, g = 2.0027)-should be
ca. 0.9 mT.
We obtained cyclopentanone with a I3C labeled carbonyl group in 38% yield by allowing the dimagnesium compound of 1,4-dibromob~tane[~]
to react with I3CO2(24%
I3C) in tetrahydrofuran; 3* was then prepared as described['].
The ESR spectrum of 3* can be simulated using the proton coupling constants mentioned in ['] and one I3C coupling constant of 0.88 mT l6l; this proves the delocalized
electronic structure A + B for 3. The less hindered radicals
1 and 2 should have the same electronic structure. The
temperature-dependent line-width effects observed for 2['l
may not arise from electron exchange but from ring inversion of the cyclopentyl ring, which because of the smaller
steric hindrance takes place with a higher frequency than
in 3.
Received: April 23, 1982 [Z 17 IE]
revised: May II. 1982
German version: Angew. Chem. 94 (1982) 716
Angew. Chem. Inr. Ed. En&. 21 (1982) No. 9
Aminyl radicals, 'NHR, usually cannot be isolated because of their high reactivity; they are assumed to be intermediates in oxidation reactions catalyzed by metal enzymes (e. g. peroxidase) and in the photolysis o r radiolysis
of arnines"'. During our investigation of redox reactions of
metal complexes with nitrogen ligands we succeeded in
trapping and stabilizing the toluidinyl radical, which is not
isolable as such, in the paramagnetic manganese complex
2, R = C H 3 ; to our knowledge this compound represents
the first aminyl complex.
OC."jMn
C
0
H
oc
I
1
If the m-toluidine complex 1 (vco: 1922, 1847 c m - ' , obtained from C5H5(C0)2Mn(THF)and m-CH3C6H4NH2by
standard procedured2"I) is oxidized with air o r HzOz, the
red-brown toluene solution immediately turns deep blue
and two new vco-bands can be observed IR-spectroscopically at 1968 and 1920 cm-'. After concentration of the solution and addition of hexane the blue-black m-toluidinyl
complex 2 crystallizes at -78 "C as analytically pure microcrystaIdZhl.
In the solid state 2 is stable at room temperature for several hours; it is characterized by one vNH-and two vcobands at 3340 and 1959/1902 c m - ' (KBr), by its mass[*] Prof. Dr. D. Sellmann, J. Muller
lnstitut fur Anorganische Chemie der Universitat Erlangen-Nurnberg
Egerlandstrasse I, D-8520 Erlangen (Germany)
Prof. Dr. P. Hofmann
lnstitut fur Organische Chemie der Universitat Erlangen-Niirnberg
Henkestrasse 42, D-8520 Erlangen (Germany)
[*'I Reactions of Complexed Ligands, Part 34. This work was supported by
the Fonds der Chemischen lndustrie and the Dr. Otto Rohm-Gedachtnisstiftung. We are indebted to Dr. G. Grampp for recording the ESR
spectra.-Part 33: D. Sellmann, H. E. Jonk, H. R. Pfeil, G. Huttner, J.
von Seyerl, J. Organomer. Chem. 191 (1980) 171.
0 Verlag Chentie GmbH. 6940 Weinheim. 1982
0570-0833/82/0909-0691 $02.50/0
69 1
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