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Isoselective Relationship for the Stereoselectivity of the Transfer of Hydrogen Atoms to Cyclic Alkyl Radicals.

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triangle are comparably involved in a mutual spin polarization which would tend to give an S = 0 ground state.
Received: January 2, 1991 [Z 4365 IE]
German version: Angew. Chem. 103 (1991) 592
[I] a)S. J. Lippard, Chem. Br. 22 (1986) 222-227; b)S. J. Lippard, Angew.
Chem. 100(1988)353-371; Angew. Chem.Int.Ed. Engl.27(1988) 344-361;
c) L. Que, Jr., R. C. Scarrow, Metal Clusters in Proleins (ACS Symposium
Ser. 372 (1988) 152-178); d) P. C. Wilkins, R. G. Wilkins, Coord. Chem.
Rev. 79 (1987) 195-214; e)B.-M. Sjoberg, A. Graslund, Adv. Inorg.
Biochem. 5 (1983) 87-110; f) E. C. Thiel, Meral Clusters in Proteins (ACS
Symposium Ser. 372 ( 1 988) 179 - 195).
[2] Fe,-complexes see a) S. J. Lippard, S. M. Gorun, J. Am. Chem. Soc. 107
(1985) 4568-4570; b) S . M. Gorun, G. C. Papaefthymiou, R. B. Frankel,
S. J. Lippard, ibid. / 0 9 (1987) 4244-4255; Fed-complexes see: c) S. M.
Gorun, S. J. Lippard, Inorg. Chem. 27 (1988) 149-156, and references
therein; d) S . Driieke, K. Wieghardt, B. Nuber, J. Weiss, E. L. Bominaar, A.
Sawaryn, H. Winkler, A. X. Trautwein, ibid. 28 (1989) 4477-4483, and
references therein; e) J. L. Sessler, J. W. Sibert, V. Lynch, ibid. 29 (1990)
4143-4146; Fe, complex: f) A. S. Batsanov, Yu. T. Struchkov, G. A.
Timko, Koord. Khim. 14 (1988) 266-270; g) W. Micklitz. S. J. Lippard,
Inorg. Chem. 27 (1988) 3067-3069; h) W. Micklitz, S . G. Bott, J. G. Bensten, S. J. Lippard, J. Am. Chem. SOC. I// (1989) 372-374; i) K.
Hegetschweiler, H. Schmalle, H. M. Streit, W. Schneider, Inorg. Chem. 29
(1990) 3625-3627; for an example of an Fe, complex see: j ) K. Wieghardt,
K. Pohl, I. Jibril, G. Huttner, Angew. Chem. 96 (1984) 66-67; Angew.
Chem. lnt. Ed. Engl. 23 (1984) 77- 78; For examples of Fe,, complexes see:
k) S . M. Gorun, G. C. Papaefthymiou, R. B. Frankel, S. J. Lippard, J. Am.
Chem. SOC.109 (1987) 3337-3348, and references therein.
[3] X-ray structure data: triclinic, spacegroup Pi, (I = 12.167(2) A, 6 =
12.921(4)A, c = 15.394(4) .&, a = 314.41(2)".
= 97.641(15)", y =
102,17(2)", V = 2087.4 A,, Z = 1, data collected 4.0' 5 28 5 45.0". 4179
unique reflections with F > 6.0u(F), R = 0.046. R, = 0.059. Further details
of the crystal structure investigations are available on request from the
Director of the Cambridge Crystallographic Data Center, University
Chemical Laboratory, Lensfield Road, GB-Cambridge CB2 IEW (U. K.),
on quoting the complete journal citation.
Satisfactory elemental analysis for Fe,CI,OZ,N,C,,H,,(3~CH~Cl,).
Mossbauer hyperfine parameters for two doublets in area ratio of 1 :2
(values relative to iron foil): 300 K: 6 [0.406(6), 0.383(3) mm/s]; AEQ
[1.056(11), 0.729(5) mmls]. 200 K : 6 [0.510(6), 0.350(14) mm/s]; AEQ
[I .061(12), 0.78(3) mm/s].
K. Kambe, J. Phys. Soc., Jpn. 5 (1950) 48.
S . Ghose, A. W. Hewat, M. Pinkney, SolidState Communications 74 (1990)
413, and references therein.
a) J. K. McCusker, J. B. Vincent, E. A. Schmitt, M. L. Mino, K. Shin, D. K.
Coggin, P. M. Hagen, J. C. Huffman, G. Christou and D. N. Hendrickson,
J. Am. Chem. Soc. /13(1991) 3012-3021; b) J. K. McCusker, E. A. Schmitt
and D. N. Hendrickson in D. Gatteschi, (Ed.): Magnetic Molecular Materials, Kluwer Academic Publishers, in press.
and selectivities is of decisive importance.['] Especially important is an exact knowledge of the stereoselectivity of hydrogen transfer and the possibility of controlling it, since this
reaction is frequently the decisive product-forming step in
radical reactions.[*'
Using the a-alkenyl radical 1 as example we have shown
that the stereoselectivity of hydrogen transfer to give (23-2
and (a-2can be steered by both variation of the H donor
RH as well as by the temperature, and that it obeys the
isoselective relationship (a) with an isoselective temperature
Ti,of 60 -80 0C.[31At this isoselective temperature, variation
of the H donor has no influence on the selectivity.[4a1
&(AH: - AH:)
= &(AS: - AS,')
Such isoselective realtionships are of practical importance,
but also enable us to assess the selectivity of a series of
reactions over a large temperature range and possibly to
anticipate reversal of selectivity as a function of the tempera t ~ r e .In
' ~ addition,
the occurrence of an isoselective relationship points to a common reaction mechanism. Hitherto
a series of reactions in which isoselectivity occur, were experimentally measured with variation of only one reaction partner.14"]We have now for the first time, by H trapping with the
radical 4, measured a series of reactions, which upon variation of both reaction partners-radical 4 and H donor RHleads to a single isoselective relationship (Scheme 1).
I +R'
Lsoselective Relationship for the Stereoselectivity
of the Transfer of Hydrogen Atoms
to Cyclic Alkyl Radicals**
By Bernd Giese,* Hassan Farshchi, Jorg Hartmanns,
and Jiirgen 0. Metzger *
With the increasing use of radical reactions in organic
synthesis a more detailed understanding of the reactivities
I'] Priv.-Doz. Dr. J. 0. Metzger, Dr. J. Hartmanns
Fachbereich Chemie der Universitat
Carl-von-Ossietzky-Strasse 9-1 1, W-2900 Oldenburg (FRG)
Prof. Dr. B. Giese
Institut fur Organische Chemie der Universitat
St.-Johanns-Ring 19, CH-4056 Basel (Switzerland)
Dr. H. Farshchi
Institut fur Organische Chemie der Technischen Hochschule Darmstadt
Petersenstr. 22
W-6800 Darmstadt (FRG)
[**I This work was supported by the Deutsche Forschungsgemeinschaft.
Verlagsgesellschafr mbH, W-6940 Weinheim. 1991
Scheme 1. X = 0, PhN; RH = Bu,SnH, PhMe, PhEt, c-C,H,,; R : M e (a),
n-C6H,3 (b), PhCH, (c), c-C,H,, (a), PhCH(Me) (e).
The radicals 4 were obtained by addition of a radical R"
to methylmaleic anhydride or N-phenyl(methy1)maleimide
3. Transfer of H from H donors RH to the planar n-alkyl
radical 4 leads to formation of (2)-5and (E)-5,whose ratio
was determined in the temperature range 0-400 "C. The
stereoselectivity of the H transfer is significantly greater with
the alkyl radical 4 than with the alkenyl radical 1 as acceptor.
It is strongly influenced by the P-substituent R , by the
H donor and by the temperature, but is largely independent
of whether 3 is employed as the N-phenylimide or the anhydride (Table 1).
Since the approach of the H donor RH upon 4 from the
anti-side with respect to R is less hindered than from the
syn-side, the activation enthalpy for the formation of (Z)-5
is 3.5-10.9 kmol-' lower than for the formation of (E)-5.
$3.50+ ,2510
Angew. Chem. Inf. Ed. Engl. 30 (1991) No. 5
Table 1. Activation parameters for the stereoselectivity of the transfer of H to
the alkyl radicals 4 and ratio of the products [(Z)-S]:[(E)-S].
AAH* [el
[kJ mol - '1
5.1 f 0.3
5.4 f 0.3
5.7 k 0.7
8.2 f 0.5
9.4 1.2
dIbl C-Ce.Hi2 9. 7f0 . 3
d[bl C-Ce.Hi2 10.9 f 0.6
3.5 f 0.2
c[a] PhMe
4.8 k 0.4
e[a] PhEt
[a] X = PhN. [b] X
= 0.
AAS* [el
[J mol-'K-']
8.6 f 0.8
9.0 f 1.2
11.3 f 2.1
14.3 f 1.5
16.6 f 3.7
17.6 k 1.2
7.0 f 0.5
8.7 f 0.8
3.0: 1 [c]
3.5:1 [c]
5.6: 1 [c]
1.04: 1[d]
1.18:l [d]
0- 140[7]
0-80 [7]
0- 140[7]
0- 110[7]
200- 260[8]
180- 260 [8]
180-260 [S]
[c] 20°C. [d] 200°C. [el In each case the value for
(E)-5- the value for (Z)-5.
Thereby, the energy difference AH*((E)-5) - AH*((Z)-5)
with the same H donor tributyltin hydride increases with
increasing size of the P-substituent R from 5-6 (methyl, nhexy1,benzyl) to 8.2 kJ mol- (cyclohexyl), and accordingly
the stereoselectivity increases. Since also the differences in
the activation entropies increase in the same direction from
9 (methyl, n-hexyl) through 11.3 (benzyl) to 14.3 J mol-'
K - (cyclohexyl), the compensation of the activation enthalpies and activation entropies leads to an isoselective temp e r a t ~ r e ~of~ ~750
l K with [(Z)-5]/[(E)-5] = 0.72 0.07
(Fig. 1). At this temperature, which lies outside the measurement range, the radicals 4s-4d (X = PhN) react with the
H donor tributyltin hydride (Experiments 1 - 4 ) with the
same stereoselectivity.
the less reactive the H donor
Our result-a further
deviation from the reactivity-selectivity principlel6l-shows
that the steric effect of tributyltin hydride on 4c (X = PhN)
is greater than that of toluene in the transition state of the
H transfer, so that the above effect is overcompensated. In
contrast, in the case of the radical 4 d (Experimental pairs 4
and 7 as well as 5 and 6), the expected behavior is observed
even when the differences are smaller than in the case of the
alkenyl radical l.[31
The temperature dependence of the stereoselectivity with
incorporation of the isoselective point is shown in Figure 1 .
The selectivity lg{[(Z)-5]/[(E)-5]} decreases as expected with
increasing temperature, and between 475 K (Experiment 8)
and 638 K (Experiment 6) is equal to zero. Above these
temperatures the selectivity increases again, and entropically
favored preferentially (E)-5 is formed. This reversal of selectivity lies within the range of measurement in the case of
Experiments 6-9, and could therefore be measured directly.
Our investigations on the alkyl radicals 4 show that the
stereoselectivity of radical hydrogen transfer reactions can
be controlled by variation of the B-substituent, the H donor,
and the reaction temperature. These results are important
for the planning of syntheses in which the subsequent H
transfer takes place at a prochiral center.
Received: November 16, 1990 [Z 4283 IE]
German version: Angew. Chem. 103 (1991) 619
0 .o
2 .o
lo3 T-'
Fig. 1 . Temperature dependence of the stereoselectivityof the H transfer to the
alkyl radicals 4 by Bu,SnH, c-C,H,,, PhMe and PhEt. The numbers on the
Arrhenius curves correspond to the experiment numbers in Table 1. The unbroken portions of the curves demarcate the ranges of measurement (cf. Table 1).
The H transfer from toluene to 4c (X = PhN), from ethylbenzene to 4e (X = PhN), and from cyclohexane to 4d
(X = 0)(Experiments 8,9, and 6) obey the same isoselective
relationship as the H transfer from tributyltin hydride to the
radical 4 (Fig. 1). This is a remarkable result, since it shows
that the reactions of five sterically differently shielded radicals 4 with four differently reactive H donors RH151 obey a
common isoselective relationship. It can therefore be concluded that the stereoselectivity of the H transfer is determined by the steric interaction between H donor and B-substituent in radical 4. The finding that 4c (X = PhN) reacts
more selectively with the H donor tributyltin hydride (Experiment 3) than with the much less reactive toluene, is surprising, since the difference in the steric shielding by R should
have all the more stronger effect on the activation energies
Angew. Chem. l n t . Ed. Engl. 30 (1991) No. 5
CAS Registry numbers:
3 (X = 0),616-02-4; 3 (X = PhN), 3120-04-5; 4 a (X = PhN), 132750-69-7; 4b
(X = PhN), 132750-70-0; 4 c (X = PhN), 132750-71-1;4d (X = PhN), 13275072-2; 4d [b] (X = 0), 65149-70-4; 4 e (X = PhN), 132750-73-3; (E)-5a
(X = PhN), 35393-95-4; (Z)-5a (X = PhN), 6144-74-7; (E)-5b (X = PhN),
132750-74-4;(Z)-Sb (X = PhN), 132750-78-8; ( E ) - ~ c(X = PhN), 13275075-5;(Z)-Sc(X = PhN), 132750-79-9;(E)-Sd(X = PhN), 132750-76-6;(2)5d (X = PhN), 132750-80-2;(E)-5d (X = 0),65424-96-6; (Z)-5d (X = O),
65424-93-3; 5 e (X = PhN), 132750-77-7.
[l] M. Regitz, B. Giese (Eds.): C-Radikale (Methoden Org. Chem. (HoubenWeyl) 4th Ed., Vol. E19a (1989)); B. Giese: Radicals in Organic Synthesis:
Formation of Carbon-Carbon Bonds, Pergamon, Oxford 1986; further reviews: D. J. Hart. Science (Washington D. C.) 223(1984) 883; M. Ramaiah,
Tefrahedron 43 (1987) 3541; W P. Neumann, Synrhesis 1987, 665; D. P.
Curran, ibid. 1988, 417, 489.
[2] B. Giese, Angew. Chem. 101 (1989) 993; Angew. Chem. I n f . Ed. Engl. 28
(1989) 969, and references cited therein.
[3] a) B. Giese, J. A. Gonzalez-Gomez, S. Lachhein, J. 0. Metzger, Angew.
Chem. 99 (1987)475; Angew. Chem. Inf. Ed. Engl. 26 (1987) 479; b) see also
L. A. Singer Sel. Org. Transform. 2 (1972) 239; 0. Simamura, Top. Stereochem. 4 (1969) 1.
[4] a) B. Giese, Arc. Chem. Res. 17 (1984) 438; b) see also H. Buschmann,
H.-D. Scharf, N. Hoffmann, P. Esser, Angew. Chem. 103 (1991) 480;
Angew. Chem. Inr. Ed. Engl. 30 (1991), 477.
[5] The rate constants for the transfer of H to primary alkyl radicals were
determined for tributyltin hydride, ethylbenzene, toluene, and cyclohexane
at 100°C to be 8.3 x lo6, 1.3 x lo3, 4.2 x lo2, and 16 x lo2 M-'s-', respectively: C. Chatgihaloglu, K. U. Ingold, J. C. Scaiano, 1 Am. Chem. SOC.103
(1981) 219; V. E. Agabekov, N. L. Budeiko, E.T. Denisov, N. 1. Mitskovich, React. Kinef. Cafal. Left. 7 (1972) 437; P. J. Boddy, E. W. R. Steacie, Can. J. Chem. 39 (1961) 13.
[6] B. Giese, Angew. Chem. 89 (1977) 162; Angew. Chem. I n f . Ed. Engl. 16
(1977) 125.
[7] A mixture of one equivalent of alkyl iodide or alkyl bromide R Y , two
equivalents of tributyltin hydride, two equivalents of alkene 3, and 0.1
equivalent of azobisisobutyronitrile in toluene was irradiated for 5 30 min or heated, and the ratio of the products [(Z)-S]:[(E)-S] determined
gas chromatographically.
[S] 3 and the H-donor, which concomitantly supplies R , (molar ratio 1: 1000)
contained in pyrex ampoules below the critical temperature of the H donor(91, in a high-pressure/high-temperature flow reactor above the critical
temperature[9] were allowed to react in the absence of air. The reaction time
was so chosen that the conversion sufficed for determination of the sterisomeric ratio even though no isomerization was detectable. The ratio [(Z)51:[(E)-51was determined by capillary gas chromatography with on-column
[9] J. Hartmanns, K. Klenke, J. 0. Metzger, Chem. Ber. 119 (1986) 488.
0 VCH Verlagsgesellschafr mbH.
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hydrogen, alkyl, stereoselective, cyclic, isoselective, atom, transfer, radical, relationships
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