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Influence of H-Donor and Temperature on the Stereoselectivity of Radical Reactions.

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Influence of H-Donor and Temperature on the
Stereoselectivity of Radical Reactions**
. ,,
. ,
.. ,,
1 1 (D2)
1121 After 2 h reaction time the ratio of the unreacted diastereomeric complexes is 27:73 in favor o f the compound with the carbonyl signal at
[I31 We have also been able to monitor the reaction described here by ' H NMR spectroscopy: M. Hiillmann. Dissertation. Universitit Marburg
1 Oa
~ 5 ~ 2'
1 2 (D3)
1 3 (D5)
ligand exchange reactions between 6a/6b and 10a/10b are
conceivable, which would simulate an intermolecular CC
bond formation.
To check this ambiguity, kinetic experiments were performed. In the case of a purely intramolecular CC coupling step 6a/6b-7 a first order reaction with respect to
the complex is expected, whereas in case of an intermolecular reaction, second order kinetics should pertain.
Experimentally, we observed the latter (k=3.29 x
L m0L-l s - ' at - 15°C). Hence an intramolecular methyl
transfer can be ruled out.
Our investigations thus show"'] that Cram's chelate
model has a real basis and that a seemingly simple and
stereochemically clean reaction can proceed in a complex
manner (multistep).
By Bernd Giese,* Juan Antonio Gonzaiez-Gomez,
Stephen Lachhein, and Jurgen 0 . Metzger*
Radical reactions are finding increasing use in organic
synthesis, whereby a knowledge of the reactivities and selectivities is of decisive importance. Stereoselectivity plays
a crucial role, for it is dependent not so much on the product stability but rather on the shielding of the radical center because of the early transition states of rapid radical
reactions. This is important in the case of H-abstraction
reactions because the transfer of the small H-atoms from
the less shielded side leads to the thermodynamically less
stable product. Thus the addition of alkyl radicals to methylmaleic anhydride 1 and to phenylacetylene 3 in the
subsequent H-abstraction step by cyclohexylmercury hydride preferentially affords the isomers ( 9 - 2 and (27-4,
respectively, whereby the selectivity increases with increasing size of the substituent R.121
20 oc
Received: January 9, 1987:
revised: February 5, 1987 [ Z 2043 IE]
German version: Angew. Chem. 99 (1987) 478
CAS Registry numbers:
5, 107798-64.1: MeTiCI,, 4015-75-2.
[I] D. J. Cram, J. D. Knight, J. Am. Chem. Soc. 81 (1959) 2748.
[2] W. C. Still, J. H. McDonald, Tetrahedron Lett. 21 (1980) 1031.
131 Review of chelation and non-chelation controlled additions to chiral alkoxy-ketones and -aldehydes: M. T. Reetz, Angew. Chem. 96 (1984) 542:
Angew. Chem. I n / . Ed. Engl. 23 (1984) 556.
141 M. T. Reetz, M. Hiillmann, J. Chem. SOC.Chem. Commun. 1986. 1600.
[5] Distilled CH,TiC12 was used. Review of organotitanium reagents in organic synthesis: M. T. Reetz: Organolitanrum Reagents in Organic Synrhesrs. Springer, Berlin 1986.
161 Racemic 5 was used; only one enantiomer is shown.-When CH,TiCI,
is mixed with 5 at -78°C virtually only the diastereomer with the carboriyl absorption at higher field is formed in a kinetically controlled
reaction. On warming to -45°C. isornerization to give a mixture of the
diastereomers takes place first, followed by C C coupling.
[7] Chirdl u- and b-alkoxycarbonyl compounds react with TiCI, to give
characteristic I : I complexes, which have been investigated ' H - and "CNMR spectroscopically: M. T. Reetz, K. Kesseler, S . Schmidtberger, B.
Wenderoth, R. Steinbach, Angew. Chem. 95 (1983) 1007: Angew Chem.
Inr Ed. Engl. 22 (1983) 989: Angew. Chem. Suppl 1983. 151 I ; G. E.
Keck, S. Castellino, J . Am. Chem. Soc. I08 (1986) 3847: K. Kesseler,
Ui\\ermrion, Universitat Marburg 1986: see also IS].
[8] R. J. H. Clark, A. J. McAlees, Inorg. Chem. I 1 (1972) 342.
191 J Dawoodi, M. L. H. Green, V. S. B. Mtetwa, K. Prout, J . Chem. Soc
('hem Commun. 1982. 1410.
[ 101 In contr%t, the position of the benzyl groups (arrangement around the
oxygen o f the ether function) is unclear; NOE experiments d o not allow
any reliable conclusions to be drawn.
[ I I ] Some additional but very weak signals are observed in the original spectrum; they probably arise from a product of ether cleavage. The titanium
alkoxide 7 is likely to have the chelate structure shown, since the " C NMR signals of the u,u'-C-atoms of the ether function are shifted
markedly downfield compared to the absorptions of the titanium-free
product ( M . Hullmann, Dissertation. Universitat Marburg 1986). Normal titanium alkoxides of the type ROTiCl, are monomeric compounds
which d o not display any additional coordination: R. L. Martin, G .
Winter. J . Chem. Soc. 1961. 2947.
Angea. Chem. In/ Ed. Engl. 26 (1987) No 5
(2)-2 : ( Q - 2
62 . 38
(274 : ( 0 - 4
c-CoH I I
8 9 : I1
74 : 26
94 : 6
97 : 3
In the case of the n-vinyl radical Sb,''] formed by addition of cyclohexyl radicals to phenylacetylene, we have
now observed that the ( Z ) / ( E )ratio can also be influenced,
and even reversed, by variation of the H-donor and the
reaction temperature. Measurements between - 20°C and
260°C show that ( Z ) - 4 b is formed with less activation enthalpy then (E)-4b (Table 1).
The approach of the H-donor from the anti side of the
vinyl radical 5b (away from the cyclohexyl group), for instance, requires less activation enthalpy than the attack
from the syn side. The energy difference AHf((E)4b) -AH' ((Z)-4b) thereby increases with decreasing
Prof. Dr. B. Giese, Dr. J. A. Gonzalez-Gomez, Dr. S. Lachhein
lnstitut fur Organische Chemie und Biochemie
der Technischen Hochschule
Petersenstrasse 22, D-6100 Darmstadt (FRG)
Priv.-Doz. Dr. J. 0. Metzger
Fachbereich Chemie der Universitat
Carl-von-Ossietzky-Strasse 9- 1 I , D-2900 Oldenburg (FRG)
This work was supported by the Deutsche Forschungsgerneinschaft and
the Fonds der Chernischen Industrie.
0 VCH Verlagsge.sellschafr mbH. 0-6940 Weinheim. 1987
$ 02.50/0
T,ihlr I . Activdtioii parameters for the stereoselectivity of H-transfer to the
iinyl radical 5b.
H - Ilonor
[Jmol ' K - ' I
4.6k 1.5
I 1 . 7 i 1.0
1.2 20.5
7 is
28 ? 1.3
- 20-80
0- 84
reactivity of the H-donor. It increases from 2.5 (cyclohexylmercury hydride) to 4.6 (tributyltin hydride) to 11.7 kJ/
mol (cyclohexane). At the same time, the rate of H-transfer
in this series decreases by about a factor of
(Z)- 4 b
( E ) -4b
Apparently, the differences in the steric shielding have
greater influence on the activation enthalpies the less reactive the H-donor is, because the distance between the reactants is smaller in the later transition states.''] Since the difference in the activation entropies in the same series also
increases from 1.2 to 7 to 28 J/rnol-' K - ' , the compensation of the activation enthalpies and activation entropies
leads to an isoselective temperature,f61which lies between
60 and 80°C (Fig. I). In this temperature range the Hdonors mentioned here react with the same selectivity. In
the case of cyclohexane the entropy effects are so large
that above 140°C the isomer (E)-4b is the major product.
Thus, at 0°C the ratio ( 9 - 4 b :(E)-4b is 78:22 with
Bu,SnH as donor, whereas at 260°C with cyclohexane as
H-donor the selectivity (29 : 7 1) is reversed.[']
These investigations on the vinyl radical 5b show how
the stereoselectivities of radical reactions can be steered by
varying the H-donors (radical trapping agents) and the
reaction temperature.
Received: January 13, 1987 [Z 2045 1 9
German version: Angew Chem. Y9 (1987) 478
[ I ] R. Giese, Anyew Chem. Y7 (1985) 5 8 5 : Angen,. Chem. In,. Ed. Eiigl. 24
(1985) 853: Radicab in Organic S.vnthecir. Pergamon Press, Oxford
121 R Giese. S. Lachhein, Angen, Chem. 94 (1982) 7 8 0 ; Angew. Clzem. Int.
Ed. Engl. 21 (1982) 768: B. Giese, G. Kretzschmar, Chem Ber. 117 (1984)
131 T h e phenyl suhstituent enforces s p hybridization at the radical center: R.
M. Kochik, J. A. Kampmeier, J . Am. Chem. Soc. YO (1968) 6733; J. E.
Rennett, J. A. Howard, C/iem. P h ~ tLett Y (1971) 460: L. Bonazzola, S.
Fenistein, R. Marx, Mol. Phys 22 (1971) 689.
I41 T h e rate coefficients for H-transfer to alkyl radicals for cyclohexylmercury hydride, tributyltin hydride. and cyclohexane have been reported to
he about lo', 10" a n d I L mol- ' s - I, respectively at 20°C: B. Giese, G.
Kretzschmar, Cliem. Ber. 117 (1984) 3160; C. Chatgilialoglu, K. U. Ingold, J. C. Scaiano, J Am. Clzem. Soc. If13 (1981) 7739: D. J. Roddy, E
W. R. Steacie, Can J Chem. 3Y (1961) 13.
[ S ] Later transition states are required by the Hammond postulate for slower
reactions: G. S. Hammond, J . Am. Chem. So< 77 (1955) 334
161 R. Giese, Anyen Chem. 89 (1977) 162; Angew. Clzem. Inf Ed. Engl. 16
(1977) 125; Acc. C h n . Re.s. 17 (1984) 438.
[7] For the experimental procedure with cyclohexane see J Hartmanns, K.
Klenke, J. 0. Metzger. Chem Ber. 119 (1986) 488. Control experiments'
\how that an isomerization (Z)-4b-(E)-4b is not apparent u p to 260°C.
Asymmetric Synthesis of
a-Amino-y-nitrocarboxylic Esters by the
Bislactim Ether Method**
By Ulrich SchollkopJ* Wulf'Kiihnle, Ernst Egert. * and
Michael Dyrbusck
Dedicated to Professor Hans Paulsen on the occasion of
his 65th birthday
Optically active y-nitro-a-amino acids are attracting attention lately because of their potential biological activity
and there usefulness as building blocks for modified oligopeptides. Moreover, they are suitable as starting compounds for the synthesis of further unusual amino acids,
since the nitro group can be reduced to an amino group'
or converted into a carbonyl group by the Nef reaction." 'I
So far, however, no method for the asymmetric synthesis
of this class of compounds has been reported in the literature. We now describe here an asymmetric synthesis for
the methyl a-amino-y-nitrocarboxylates 7 starting from
the titanium derivative 3 of the (commercially available[41)
bislactim ether 1 of cyclo(-L-Val-Gly) and the nitroolefins
4 . Intermediates of the synthesis are the adducts 5, which,
in the case of the (E)-nitroolefins 4a-c, are formed with
high asymmetric induction with respect to the two stereocenters (C-2 and C-1') (Table 1). Surprising is not only the
[*] Prof. Dr. U. Schollkopf, DiplLChem. W. Kuhnle
lnstitut fur Organische Chemie der Universitrit
Tammannstrasse 2, D-3400 Gottingen ( F R G )
10311 I K - ' ~
Dr. E. Egert, M. Dyrbusch
lnstitut fur Anorganische Chemie der Universitiit
Tammannstrasse 4, D-3400 Gottingen (FRG)
Fig. I . Temperature drpendrnce 01 the \iurcoxleciiiit) lor H-transfer to the
vinyl radical 5b by c-C,,H,?. Bu:SnH, a n d c-C,,H,,HgH as H-donors.
0 VCH V e d u g r ~ e ~ e l l d z anzhH.
0-6940 Weinheim. I987
Asymmetric Syntheses bia Heterocyclic Intermediates, Part 34.-Part
33: U. Schollkopf, J. Bardenhagen, Liehigs Ann. Chem. 1987. 393
0.~70-0833/87/0505-f~480S 02.50'0
Angen,. Chem. Int. Ed. Engl. 26
119871 No.
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stereoselective, reaction, temperature, donor, radical, influence
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