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Hindered Rotation of tert-Butyl Groups in Radical Cations of Highly Substituted Alkenes.

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[6]
[7]
[8]
[9]
[lo]
(C,H,), 122.2 (CF3, Jr1.=254 Hz), 127.0 (CF,, Jcy =261 Hz), 127.4 (CF2,
Jc.,.=260 Hz), 221.8, 222.0, 222.3, 236.3 (m),247.8.
R. D. Adams, D. A. Katahira, L.-W. Yang, Oryanomeiallics 1 (1982)
231.
Monoclinic, P2,/m, a = 1083.6(3), h= 1378.6(4), c=681.6(2) pm,
8=92.73(5)", Z = 2 ; p,.,l,=2.03
g cm-';
R=0.056, R,=0.029
(MoK,=71.06 pm, 2264 reflections, 1575 with 1>2a(l)), absorption correction @ = 12.3 cm-I), anisotropic temperature factors, no H-atom positions, 2 18 refined parameters). Further details of the crystal structure
investigation are obtainable on request from the Fachinformationszentrum Energie Physik Mathematik, D-7514 Eggenstein-Leopoldshafen 2,
o n quoting the depository number CSD 50861, the names of the authors, and the journal citation.
a) M. D. Curtis, L. Messerle, N. A. Fotinos, R. F. Gerlach in M. H. Chisholm: Reacfiuifyof Metal-Metal Bonds. A m . Chem. SOC.Symp. Ser. 155
(1981) 221: b) L. K . Bell, W. A. Herrmann, M. L. Ziegler, H. Pfisterer,
Organomeiallics I (1982) 1673.
H. Oberhammer, D. Lentz, unpublished.
F. A. Cotton, W. J. Roth, J . A m . Chem. SOC.105 (1983) 3734.
Hindered Rotation of tevt-Butyl Groups in
Radical Cations of Highly Substituted Alkenes""
By Horst Eierdanz, Siegfried Potthoff; Rudolf Bolze,
and Armin Berndt*
Hindered rotation of tert-butyl groups bound to sp2 Catoms has hitherto only been observed in l&di-tert-butylnaphthalenes"] and has been predicted for Z-1,2-di-tertbutylethene 1 ( R = H ) on the basis of force field calculat i o n ~ [ ~Herein
. ~ ] . we describe with the radical cations 1"
and 3" the first examples of hindered rotation of sp2
C-bonded Z-vicinal and geminal tert-butyl groups.
a , R = M e ; b, R
Table I . ESR coupling constants of the radicals 1°a-300.
Radical
Oxidation
method [a]
T
["C]
a:'Mci
- 85
- YO
1.7 (6H)
1.6 (6H)
- I10
- 40
to
0.65 (18H) [b]
61
0.65 (1 8 H)
- YO
- 4s
2.5 (6H)
0.87 (18H)
2.5 (6H)
-
- 110
-
a: [GI
[GI
xx
14.2 (6 H)
23.0 (2 H)
1.6 (2H)
14.8 (6 H) [b]
15.9 (2H) [c]
9.3 (2 H) [c]
12.5 (6H)
12.5 (6H)
13.2 (2H) [c]
9.5 (2 H) [c]
2.9 (6 H )
[a] A: electrochemically, B: with AIC'I,. [b] In [lo], these data have been erroneously assigned to 3'@. I n the meantime we have found that 3a isomerizes to Za in the presence of AICI,. [c] Line-broadening effects by hindered
rotation of the ethyl groups.
dered rotation of the tert-hutyl groups and not of the methyl
groups as in the neopentyl radical"21.The ESR spectrum of
the deuterated radical shows a splitting at -90" by six tertbutyl protons, while in the case of hindered rotation of the
methyl groups a splitting by only five tert-butyl protons is
to be e~pected"~].
-
55O
= Et
Me
a, 11 = M e ; b, 12 = ~t
K, R = CMe,CH,CMe,
Me
4@@,n = 2 , 3
t
9.7x 10%'
C,
The radicals 1 '@-3 were generated by electrochemical oxidation of the corresponding alkenesk7]on a gold spiral as anode (platinum wire as counter electrode) or by oxidation with AIC13 in dichloromethane at - 80 to - 100°C.
The ESR coupling constants are collected in Table 1.
In the radicals l o @
and 3'@ only six of the eighteen
tert-butyl protons lead to a splitting in the ESR spectrum
at low temperatures. Only 3a0@is stable enough to be investigated in the temperature range of the intermediate
and fast exchange of the six with the remaining twelve y
protons. The analysis of the ESR spectra of 3aoQ (Fig. 1)
gives a rotation barrier of EA= 4.7 kcal/mol["]. By replacing one of the six methyl groups of the tert-butyl groups in
3
by a trideuteriomethyl group we were able to prove
that the phenomenon under investigation involves a hin@
k
-75
7.1 x 10%'
I
@
[*I Prof.
Dr. A. Berndt, Dr. H. Eierdanz, S. Potthoff, Dr. R. Bolze
Fachbereich Chemie der Universitat
Hans-Meerwein-Strasse, D-3550 Marburg (FRG)
[**I This work was supported by the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie.
526
0 Verlag Chemie GmhH. 0-6940 Weinheim, 1984
Fig. 1. Downfield halves of the ESR spectra of 3a" at various temperatures
(left) and simulations of the central multiplet with the pertinent rate constants of the exchange (right).
0570-0833/84/0707-0526 $ 02.50/0
Angew. Chem. I n f . Ed. Engl. 23 (1984) No. 7
The hindered rotation of the tert-butyl groups in 3'@ is
consistent with the relatively high barriers of ring inversion
which we have recently demonstrated for the radicals
4 0 @[lo, 141
The free rotation down to -110°C of the tert-butyl
groups of the radicals 2 " of E-l,2-di-tert-butylalkenes,
isomeric to l a @shows
,
that hindered rotation of sp2
C-bonded tert-butyl groups occurs only when two tert-butyl groups are in close neighborhood (1,l-, 2-1,2-, or
"endo"-l,3-position in 3' @,1 '@,
and l&di-tert-butylnaphthalenes). This finding is consistent with the assumption of a correlated rotation[*] of the two tert-butyl
groups.
Received: March 15, 1984;
revised: May 24, 1984 [Z 760 IE]
German version: Angew. Chem. 96 (1984) 513
W. L. Mandella, J . Am. Chem. SOC.94
(1972) 4608.
[21 R. B. Nachbar, Jr., C. A. Johnson, K. Mislow, J. Org. Chem. 4 7 ( 1 9 8 2 )
4829.
131 Hindered rotation of other sp2 C-bonded alkyl groups, e.g. in alkenes or
their radical cations: (CH2)2CCH3[4a], CH(CH& [4b, 51, CH,C(CH,),
[4c, d], CH2Si(CH,), [4e], CH2CH, 161, and Table 1.
I41 a) T. Loerzer, R. Gerke, W. Liittke, Tetrahedron Lett. 24 (1983) 5861; b )
D. S . Bomse, T. H. Morton, ibid. 1975, 781; 0. Ermer, Angew. Chem. 95
(1983) 1010; Angew. Chem. Int. Ed. Engl. 22 (1983) 998; Angew. Chem.
Suppl. 1983, 1353; C) G. A. Olah, G. K. S . Prakash, J . Org. Chem. 42
(1977) 580; d) H. Klusik, A. Berndt, Angew. Chem. 93 (1981) 903; Angew. Chem. lnt. Ed. Engl. 20 (1981) 870; e) H. Bock, W. Kaim, J . Am.
Chem. SOC.102 (1980) 4429.
[5] Hindered rotation of the CH(CH,)> groups in the radical cation of tetraisopropylethene, which we have generated electrochemically at
-8O"C, follows from the coupling constants of the p-protons:
a z = 2 . 2 8 G (2H), 0.4G (2H); aF=0.57G (24H).
[6] Hindered rotations of the ethyl groups in the ethenes l b , Zb, and 3b follow from line-broadening effects in the 'H-NMR spectra. syn-anti-Isomers are detectable for l b and Zb below -50 and - 1 1 5 T , respectively.
[7] l a and 2a were prepared as described by Rice et al. IS], l b , 2b and 3ac according to the method of Barton et al. [9]. The Z - , E-isomers were
separated by gas chromatography.
[8] J. E. Rice, Y. Okamoto, J . Org. Chem. 47 (1982) 4189.
[9] T. G. Back, D. H. R. Barton, M. R. Britten-Kelly, F. S . Guziec, Jr., J.
Cfiem. SOC.Perkin Trans. I 1976, 2079, and references cited therein.
[lo] H. Eierdanz, A. Berndt, Angew. Chem. 94 (1982) 716; Angew. Chem. Irzt.
Ed. Engl. 21 (1982) 690; see also footnote [b] of Table I.
[11] Hindered rotation of the tert-butyl groups in the ethene 3a follows from
the marked broadening of the corresponding "C-NMR signal at
- 155°C. Since neither the temperature range of the coalescence nor
that of slow exchange was accessible, only an upper limit can be estimated for the barrier. Assuming A ~ = 1 0 0 - 4 0 0 Hz, AC' must be less
than 5 kcal/mol.
(121 K. U. Ingold, J. C. Walton, J . Am. Chem. SOC.104 (1982) 616.
[I31 In the case of hindered rotation of the tert-butyl groups, only the six
protons of the two "axial" methyl groups of the conformations A and
A'lead to splitting into a 1:6:15:20:15:6:1 septet with a H = 2 . 5 G .
The additional ESR spectrum of the conformation B is not resolved,
since each line of the 1 :3 :3 : 1 quartet (2.5 C) of the axial CH, group is
s p l i t i n t o a 1 : 3 : 6 : 7 : 6 : 3 : 1 septet witha"=2.5/=6.5=O0.4G,whilethe
line width is 1.7 G.
Phosgene as a Reagent for the Enantiomeric
Resolution of 1,2- and 1,3-Diols, a-Amino Alcohols,
a-Hydroxy Acids, and N-Methyl-a-Amino Acids
by Gas Chromatography**
By Wirfried A . Konig*, Eberhard Steinbach, and
Karin Ernst
Isocyanates have proved to be versatile reagents for the
preparation of carbamate, urea, and amide derivatives of
chiral hydroxy, amino, and carboxy compounds which can
be resolved by gas chromatography[']. We now report on
the use of phosgene as a reagent for enantiomeric resolution. Phosgene-containing solutions react with a-amino alcohols (such as l ) to give the oxazolidin-2-one 2[*],with
1,2- and 1,3-diols to give the cyclic carbonates 3 and 4['11,
respectively, with a-hydroxy acids to give the 1,3-dioxolane-2,4-diones 5I4l, and with N-methyl-a-amino acids to
give the N-methyloxazolidine-2,5-diones615].
[ I ] J. E. Anderson, R. W. Franck,
A
On hindered rotation of the methyl groups only five protons would lead
to splitting, since one deuterium of the CD, group always adopts a position which would lead to coupling.
[I41 The similarity of 3'' and 4Beis also reflected in the size of the ESR
coupling constants of the y-protons (2.3 and 2.5 G in the case of 4'",
2.5-2.9 G in the case of 3O').
Angew. Chem. l n t . Ed. Engl. 23 (1984) No. 7
2
rt -0
L
K
3
4
5
6
IL = Alkyl
With 30-50 pL of phosgene solution (20% in toluene)
and between 0.1 and 1 mg of starting material in dichloromethane or ether, the reactions afford good yields at room
temperature in 30-60 min. When ammonium salts are
used, addition of a small amount of 0.5 M NaOH is advantageous. a-Hydroxy acids react particularly well in dioxane
in the presence of a trace of pyridine. The derivatives, obtained from racemic compounds, can be readily resolved
into their enantiomers by gas chromatography (glass or
fused silica capillary columns, stationary phase: polysiloxane XE-60-~-valine-(R)-a-phenylethylamide~~~)
(Table 1).
The derivatization proceeds without racemization.
Aliphatic 1,2- and 1,3-diols (Fig. 1)-frequently used
chiral building blocks for the synthesis of natural produ c t ~ " and
~ for which various enantioselective synthetic
methods have been developed[xl-could previously hardly
be resolved into enantiomers by gas chromatography; this
could only be achieved with aryl-substituted 1,2-diols via
perfluoroacylated derivatives'"'. Diols and a-amino alcohols with additional functional groups can also be resolved
as cyclic carbonates and oxazolidin-2-ones, respectively.
By this means, racemic tartaric acid and N-methylthreonine were resolved after esterification of the carboxy groups
with isopropanol and subsequent reaction with phosgene.
[*] Prof. Dr. W. A. Konig, E. Steinbach, K. Ernst
Institut fur Organische Chemie der Universitat
Martin-Luther-King-Platz 6 , D-2000 Hamburg 13 (FRG)
[**I This work was supported by the Deutsche Forschungsgemeinschaft and
by the Fonds der Chemischen Induscrie.
0 Verlag Chernie GrnbH, 0-6940 Weinheim, 1984
0570-0833/84/0707-0527$ 02.50/0
527
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