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Captodative Substituent Effects and the Chromophoric System of Indigo.

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zener3] and o-dime~itoylbenzene~~J
with alkali metals in
aprotic solvents. Subsequent rearrangement of (3), involving
a 1,2-phenyl migration leads to a new radical anion intermediate (6), which on reaction with atmospheric oxygen gives
the hydroperoxide intermediate (5). Reduction of (5) with
further potassium leads to the intermediate (4), whose hydrolysis affords the final products (9).
If a radical anion such as (6) is involved in the transformation of (1) to (9), then it would be reasonable to assume that
the lactone (10)which can generate the same intermediate
(6), by reaction with potassium, should also give rise to the
same product (9), under similar conditions. Reaction of (10)
with potassium in THF, under conditions similar to those
employed in the case of ( l ) ,did indeed give (9) in 30% yield,
thereby supporting the possible participation of (6).
The formation of benzoic acid (11) in the reaction of (1)
with potassium may be rationalized in terms of the dioxetane
intermediate (12),'formed through the reaction of (3a) with
oxygen; the formation of anthracene could be possible by retro-Diels-Alder fragmentation of one of the radical anions.
Preliminary investigations have shown that cis-1,2-dibenzoylstyrene, 2,3-dibenzoylbicyclo[2.2.l]hepta-2,5-diene,and
2,3-dibenzoylbicyclo[2.2.2]octa-2,5-diene undergo similar
transformations, on treatment with potassium.
Received: April 10, 1980 [Z 594 IE]
German version. Angew. Chem. 92. 939 (1980)
CAS Registry numbers:
( I ) , 13391-20-3; (8).75476-33-4; (9). 75476-34-5; (lo), 75476-35-6: (11). 65-85-0;
anthracene, 120-12-7
111 For a comprehensive review on the addition of alkali metals to unsaturated
systems. see. V. Kalyanaraman, M. V. George, J . Organometal. Chem. 47,225
[2] A. G. Brook, H . L. Cohen, G. F. Wright. J . Org. Chem. 18, 447 (1953); J. W.
B. Reesor, J. G. Smith, G. F. Wrighf,ibrd. 19, 940 (1954); B. J. Herold, Rev.
Fac. Cienc. Univ. Lisboa, B 7, 155 (1959-60); Chem Ahstr. 55, 19877
(1961). Rev. Port. Quim. 3, 101 (1961); Chem. Abstr. 60, 13204 (1964)
[3] B. J. Herold, Tetrahedron Lett. 1962. 75; J. A. Campbell, R. W. Koch, J. Y
Hay, M. A. Ogliamso, J. F. Wore, J . Org. Chem. 39, 146 (1974).
141 B. J. Herold, A. F N . Correia, J. S.Veiga. I. Am. Chem. SOC.87, 2661 (1965).
B. J. Herold, M. Celina, R. L. R Lamna, H. M. Novais, Tetrahedron 33. 517
fords one bonding and one antibonding ~r-MOof the central
CC double bond which is substituted by two donor as well as
by two acceptor groups. If the energetic splitting of these two
MOs is sufficiently small they form the HOMO and the
LUMO of the combined system, and the difference AE of
their orbital energies is responsible for the absorption of light
of longest wavelength.
c ../5y
Fig. 1. Construction of the chromophoric system of indigo from two captodative
stabilized radicals; interaction of the singly occupied T-MOS.
In the HMO approximation, according to the 1st order
perturbation the0ry'~1the following applies:
AE= 2 c: c? B,,.
Thus the orbital energy difference A&is smaller, the smaller
are the absolute values of the LCAO-MO coefficients c t and
cb of the singly occupied MOs +R and
of the subsystems
R and S at the coupling sites p and u. The coeficients can
also be estimated with the aid of perturbation theory as a
function of the HMO parameters hx and hu for the Coulomb
integrals ax= a + hxP an ay= a + hyP of the heteroatoms X
and Y W ;starting from the butadienide ion as isoselectronic
hydrocarbon we find
C; = - 0.372+
0.184hx - 0.304hy
Thus, c: is smaller the more electronegative X is, and the less
electronegative Y is, i. e. the more pronounced are the acceptor and donor properties of the substituents -C=X and -Y
at the radical center. These are exactly the conditions corresponding to the greatest possible delocalization of the unpaired electron.
Captodative Substituent Effects and the
Chromophoric System of Indigo
By Martin KIessinger[']
The principle of radical stabilization by captodative substitution recently discussed by Viehe et aZ.('lis not only of importance regarding syntheses with radicals and radicophiles
but proves to be a fundamental model for understanding
many properties of organic compounds. This is demonstrated
using as example the chromophoric system of indigo ( I ) ,
whose unusual color''] can readily be explained in terms of
the principle of captodative substitution. At the same time
this example shows that the effect of captodative substitution
can be very easily assessed with the simple HMO model.
The r-electron system of the chromophoric system of indigo can be formally constructed from two identical subsystems (Fig. l), whereby the coupling of two radical centers af['I
Prof. Dr. M. Klessinger
Organisch-chemisches lnstilut der Universitat
Orleans-Ring 23, D-4400 Miinster (Germany)
0 Verlag Chemre, GmbH, 6940 Weinheim. 1980
Fig. 2. Orbital energy correlation diagram for the captodative stabilization of a
radical center by the substituents -C=X and -Y (the data were obtained from
a HMO calculation with hx = 1 .O and hy =0.8).
From the orbital energy correlation diagram (Fig. 2) it is
clear that the intuitive condition of maximum delocalization
of the radical electron agrees with the energetic criteria for
an optimum captodative radical stabilization: As a 2nd order
perturbation effect, the interaction between the radical center substituted by the acceptor group -C=X and the donor
group -Y is all the greater the smaller is the difference of
the corresponding orbital
Fe. the clos r are the
and the
singly occupied n-MO +,aR of the radical X=Cdoubly occupied donor MO +D. Starting from the ally1 radical, 1st order perturbation theory yields the energy E~~ of the
singly occupied radical MO as E A R = 1/2hx@,while the energy E D of the donor orbital is given by E~ = h,P; thus optimum
Angew. Chem. Inl. Ed. Engl. 19 (1980) No. I t
droperoxides, peracids, or iodosylbenzenel''. Evidence has
been presented for involvement of a high valency iron-oxo
species as the active oxygen transfer
Very few
chemical systems are known to efficiently catalyze alkane
hydroxylation under such mild conditions""]. Very recently,
iron-porphyrins have been shown to catalyze iodosylarenesupported hydroxylations of nonactivated C-H b o n d ~ l ~ . ~ ' .
As a part of our effort to understand the mechanisms of the
reactions between hemoproteins or metalloporphyrins and
two-electron oxidizing agentsIS1,and to define good catalysts
for hydroxylation of alkanes by readily available single oxygen donors, we have compared, in the present study, the abilities of various metalloporphyrins to catalyze cyclohexane
hydroxylation by alkyl hydroperoxides@'.
Cumyl hydroperoxide, C6H5C(CH3),00H, is stable for
days in benzene-cyclohexane (1:l) at 20°C. Addition of a
catalytic amount of rneso-tetraphenylporphyrin-iron(rrr)
chloride, Fe(TPP)Cl, to this mixture leads to fast decomposition of the hydroperoxide (&,2= 1 min) with formation of a
nearly stoichiometric amount of cumenol, C6H5C(CH3)20H,
(95%) and acetophenone in minor quantities (= 2-4%).
Study of the reaction mixture by HPLC and GLC also shows
a concomitant formation of cyclohexanol and cyclohexanone
with final yields (reached after about 10 min) of 40 and 20%
based on starting hydroperoxide171. These products were also
identified by IR and 'H-NMR after distillation or silica gel
chromatography of the reaction mixture. Visible spectroscopy shows that Fe(TPP)Cl is unchanged at the end of the
reaction and therefore behaves as a true catalyst. Very similar yields of cyclohexanol and cyclohexanone are obtained
under anaerobic conditions (argon), the reaction being only
slightly faster than in the presence of 02.Taking into account that two moles of oxidant are necessary for cyclohexa-
stabilization is reached for hy = 1/2 hx. A glance at the usual
shows that this situation cannot be
reached with the the common heteroatoms (N, 0, S, etc.) but
is more nearly approached the greater hx and the smaller hy,
i. e. the more pronounced the donor and acceptor properties
of the substituents.
Figure 2 shows, furthermore, that the stabilization of the
system substituted by captodative groups is greater than the
sum of the substituent effects: on using the given parameter
values, the interaction of the radical center with the acceptor
group -C=X or with the donor group -Y results in a stabilization of the =-system by 0.81 6 or 0.68 6, respectively;
the overall captodative stabilization is 1.676, i. e. O . l S @
greater than the sum of the individual effects.
A quantitative determination of the optimum radical stabilization on the basis of the donor and acceptor properties
of the substituents taking into consideration electron-interaction effects is possible[71with the LCFO modeF".
Received: June 2, 1980 [Z 597 IE]
German version: Angew. Chem. 92, 937 (1980)
[l] H G' Viehe, R. Merenyi, L. Stella, 2. Janousek, Angew. Chem. 91, 982
(1979); Angew. Chem. Int. Ed. Engl. 18,917 (1979).-I thank Prof. Viehe for
a strmulating discussion of the principle of captodative substitution.
[2] M. Klessinger, W. Lultke, Tetrahedron 19, Suppl. 2, 315 (1963).
[3] M J. S. Dewar, R. C. Dougherty: The PMO Theory of Organic Chemistry.
Plenum Press, New York 1975.
14) E. Hezibronner. H. Bock: Das HMO-Modell und seine Anwendung. Vol. 1 .
Verlag Chemie, Weinheim 1968, p. 179. The HMO Model and its Application. Wiley, London, and Verlag Chemie, Weinheim 1976, p. 179.
[5] A . Slreitwieser. Molecular Orbital Theory for Organic Chemists. Wiley, New
York 1961
[6] M. Klessinger, Theor. Chim. Acta 49, 77 (1978).
171 M. Klessinger, W. Merker, unpublished; cf. W Merker, Diplomarbeit, Unlversitat Munster 1980.
Table 1 . Cyclohexane oxidation by cumyl hydroperoxide [a] with various catalysts (molar ratio 120:1:0.05) [b]
Yield (X;][cl
after 15 min
1,,2 of
o r FeC13
or FeC12
0 [el
M = Cu. Ni, Zn,
M = Ti, V
2 5%
3.5 h
or Mg
0 [el
(25 after 10 d)
(12% after 10 d)
0.3 min
[a] Commercial (Fluka) samples containing 30% cumene; same results obtained with pure hydroperoxide with Fe(TPP)CI. [b] Addition of 50 m M hydroperoxide to 6 ml of
a benzene/cyclohexane mixture (1 : 1) containing 2.5 mmol I - ' catalyst. FeC12 and FeCI, were first dissolved in the minimum amount of CH3CN. [cl Based on starting hydroperoxide. [d] The visible spectrum of the porphyrin (in benzene for Os(TPP)(CO)(py) and in pyridine for Co(TPP)) is greatly modified at the end of the reaction. [el 5%
cyclohexanol after 20 d. [fl No significant decomposition of the hydroperoxide after 2 d.
Metalloporphyrin-Catalyzed Hydroxylation of
Cyclohexane by Alkyl Hydroperoxides:
Pronounced Efficiency of Iron-Porphyrins'"'
By Daniel Mansuy, Jean-Frangois Bartoli,
Jean-Claude Chottard, and Marc Lange'']
Cytochromes P450 are able to catalyze the hydroxylation
of nonactivated alkanes, either by O2 in the presence of a reducing agent, or by two-electron oxidants such as alkyl hy-
Dr. D. Mansuy. Dr. J. F. Bartoli, Prof. Dr. J. C. Chottard, Dr. M. Lange
Laboratoire de Chimie de I'Ecole Normale Superieure,
Associe au CNRS
24. rue Lhomond, F-75231 Paris Cedex 05 (France)
We are indebted to Dr. P. Batlioni for supplying Os(TPP)(CO)(py) and
Angew. Chem. Int. Ed. Engi. 19 (1980) No. 11
none formation from cyclohexane, 80%of the oxidizing equivalents of the hydroperoxide are used for cyclohexane oxidation. Neither ferric or ferrous chloride nor the free porphyrin
(TPPH,) gives any cyclohexane oxidation in the reaction
conditions (Table 1) indicating that both the metal ion and
its porphyrin ligand are necessary for the catalysis to take
Table 1 compares various rnetalloporphyrins~*~
as catalysts
for cyclohexane oxidation.
From these results, the metalloporphyrins studied can be
divided into three classes. (i) The Cull-, Nili-, Zn"-, Mgii-,
V1"-, and Ti'"-porphyrins are completely inactive in our
conditions. (ii) Co(TPP) and Os(TPP)(CO)(py) are able to
catalyze cyclohexane oxidation, the former even giving a
slightly faster reaction and better yields than Fe(TPP)(Cl),
0 Verlag Chemie, GmbH, 6940 Weinheim. 1980
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effect, indigo, substituents, chromophore, system, captodative
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