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Electron Transfer-Catalyzed DielsЦAlder Reactions with 2-Vinylindoles.

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of A6 = 0.374 with two equivalents. This host-guest complexation ratio of 1:2 was also reported by others and is
attributed to dimerization of the H,PO, ion by the formation of intermolecular hydrogen bonds.['] The observed
chemical shifts d o not arise from proton transfer from
H,PO, and HSO, to the ligands, since no shifts in the
position of the CH,N signal were observed in the 'H N M R
spectra of the complexes. Moreover, when ligands 1-8 were
protonated by addition of p-toluenesulfonic acid or picric
acid, a distinct downfield shift of the CH,N signal of
A 6 = 1 .O-1.1 was observed. The FAB- mass spectra of the
complexes of all the host molecules H with Bu,NiAexhibit strong signals corresponding to [HA-]- and
[HBu,N+A-]-. The association constants (Table 1) for the
complexation of ligands 1-8 with anions A - (Bu,Nf as the
counterion) were determined in acetonitrile by conductometry.[*I
Table 1. Association constants K [M-'] for the reaction of 1- 8 with the anions
H,PO,. HSO,. and C1- determined by conductometry [a].
H 2P0,
3 500
[a] Reactions in acetonitrile. The counterion in all cases was Bu,N+. Measurements were conducted at constant ionic strength with starting concentrations of
1-8 of 1-3 mM and of the salt of 0.9-1.1 mM. The concentrations of the salt
and ligand were varied by dilution of the sample solution with a stock solution
of the free salt. Initial Concentrations were prepared by adding the appropriate
amount of free ligdnd to 10mL of the stock solution. The K values were
determined with curve-fitting methods. The error is 5% for K > lo2 M - ' and
10% for K < l o 2 M - ' .
From this data it is clear that all ligands bind phosphate
ions preferentially (H,PO, > C1- > HSO,). The reference
compound chloroacetamide was not complexed in these conductometric titrations. Ligand 8 shows highest affinity towards the phosphate ion, probably because of the increased
electrophilicity of the sulfonamide NH group and preorganization of the binding sites by n-stacking interactions of the
The chloroacetamide derivatives 1 and 3
have a larger binding affinity than the caproamides 2 and 4,
which may be attributed to the higher polarization of the
amide moieties in 1 and 3 caused by the inductive electronwithdrawing effect of the a-chloro substituents.
In conclusion, host molecules 1-8 show selective binding
affinity towards anions exclusively by hydrogen-bond formation. and are thereby a unique mimic for anion-binding
proteins. Although the association constants are not as high
as those of the natural proteins, the simple structure of the
models offers the possibility for synthetic manipulation. To
achieve higher association constants and selectivities we are
currently examining calixarenes and crown ether templates
for organizing H-bond donor and acceptor binding sites for
Esper iment al Procedure
" C N M R : 6 ~136.5.
134.8,132.1. 129.7.,128.5.,
122.5 (Arc), 54.4(CH,NH), 40.9(CH,N); FABMS (NBA matrix): m/z 717.1
(A4 +I);correct C,H.N analysis.
Received: December 12. 1992 [Z 5751 IE]
German version: Angeii,. Chem. 1993. 105, 942
[l]a) J.-M. Lehn, Angrw. Chrm. 1990,102. 1347;Angew. Chem. l n t Ed. Engl.
1990.29,1304;b) H. J. Schneider, ibid. 1991, 103,1419 and 1991.30,1417:
c) F. P. Schmidtchen, A. Gleich, A. Schummer, Purr Appl. Chem. 1989.61,
[2]a) B. Dietrich, J. Guilhem, J.-M. Lehn, C. Pascard, E. Sonveaux. Helb,.
Chim.Acla 1984,67,91;
b) F.P. Schmidtchen, J. Org. Chem. 1986,.51,5161:
c) M. W.Hosseini. A. J. Blacker, J.-M. Lehn. J. A m . Chrm. Soc. 1990. 112,
3896:d) P. D. Beer, J. W. Wheeler. A. Grieve, C.Moore, T. Wear. J. Chcm.
Soc. Chem. Cummun. 1992. 1225;e) G.Deslongchdmps. A. Galin, J. de
Mendoza, J. Rebek, Jr., Angew. Chrm. 1992,104,58:Angew. Chcvn. h t . Ed.
Engl. 1992,31, 61.
[3] a) M. T. Blanda, J. N . Homer, M. Newcomb. J. Org. Chem. 1989.54.4626.
and references therein; b) M. T. Reetz, C. M. Niemeyer, K. Harms. Angcmr.
Chem 1991,103, 1515; Angen. Chem. Int. Ed. Engl. 1991.30. 1472;c) X.
Yang, C. B. Knobler. M.F. Hawthorne, ibid. 1991,103. 1519 and 1991.30.
1507;d) V. B. Chur, I. A. Tikhonova, A. I. yanovskii, Yu. T. Struchkov.
P V Petrovskii, S. Yu. Panov, G. G. Furin. M. E. Vol'pin. J Orgunornet.
Chrm. 1991, 418,C29; e) P. D. Beer, J. Hodacova. S. E. Stokes. J. Chein.
Soc. Chem. Commun. 1992, 270; e) D. M. Rudkevich, W. P. R. V. Stauthamer. W. Verboom, J. F. J. Engbersen, S. Harkema, D. N. Reinhoudt. J.
Am. Chem. Sue. 1992,114,9671.
Pascal et al. synthesized a neutral host molecule which is claimed to bind a
fluoride ion in its cavity; however, no evidence was given: R. A. Pascal Jr.,
J. Spergel, D. V. Engen, Tetrahedron Lett. 1986,27,4099.
a) H. Luecke, F. A. Quiocho, Nuture 1990, 347. 402; b) J. W. Ptlugrdth,
F. A. Quiocho, 1 Moi. Biol. 1990,200,163: c) J. L. Jacobson. F. A. Quiocho, ihid. 1988,204,783.
The structure of our ligands is somewhat similar to that of the endo-tridentate podands developed by Potvin et al. However, these receptor molecules
were designed for complexing Z n 2 +ions mimicking the activity of carbonic
anhydrase: R. Jairam. P. G. Potvin. J. Org. Chem. 1992.57. 4136.
L.S.Flatt. V. Lynch, E. V. Anslyn, Tetrohedron Lett. 1992.33.1785.
The data obtained for the 'HN M R chemical shifts in chloroform did not
allow quantitative determination of the complexation constants. mainly
because of the high association of the host and guest species in this moderately polar solvent and the relatively high concentrations required to obtain
the N M R signals. In the conductometric experiments, however, complexation constants could be determined in the more polar solvent acetonitrile
with much lower concentrations of host and guest molecules. Under these
conditions the formation of only 1 : 1 host-guest complexes was observed.
[9]W. L. Jorgensen, D. L. Severance, J Am. Chen?. Suc. 1990,112. 4768
Electron Transfer-Catalyzed Diels-Alder
Reactions with 2-Vinylindoles**
By 0. Wiest and E. Steckhan*
Electron Transfer (ET) is one of the most important elementary processes in chemistry.[" During the last 15 years,
mechanistic-theoretical investigations of ET reactions have
made a great comeback, culminating in the award of the
Nobel Prize for Chemistry to R. A. Marcus in 1992. In certain cases, reactions which proceed too slowly or not at all
with neutral molecules can be effected by electron transfer.
Thus, the [4 + 21 cycloaddition between two electron-rich
The synthesis of 8 described is representative for the preparation of 1 -8: To a
solution of tris(aminomethy1)amine (1 .Og, 6.8mmol) and triethylamine
(3.5 mL) in CH,CI, (100mL) at 1O'C was added 2-naphthalenesulfonyl chloride (5.4g. 23.9mrnol). The reaction mixture was stirred for 3 h, allowed to
warm up to room temperature, stirred for another 3 h, and poured into water
(200mL). The aqueous layer was extracted with CH,CI, (2x 100 mL). The
A ~ ~ P I C'liein.
In1 Ed. Engl. 1993.32.Nu. 6
combined organic layers were concentrated, and the resulting solid was recrystallized from methanol to give 8 In 84% yield. M.p. 127-128°C.' H N M R
(250MHz, CDCI,, 25'C): b = 8.48 (s, 3H,ArH). 7.9-7.7(m, 12H. ArH),
$3 VCH
Prof. Dr. E.Steckhan. DipLChem. 0. Wiest
Institut fur Organische Chemie und Biochemie der UniversitHt
Gerhard Domagkstrasse I , D-W-5300Bonn 1 (FRG)
This work was supported by the Deutsche Forschungsgemeinschaft (Ste
the Fonds der Chemischen Industrie, and the BASF AG. We
thank Prof. Dr.G. P. Kreishman, University of Cincinatti, USA, for
recording the N M R spectra.
mbH. W-6940 Weinheim, 1993
$ 10.00+ ,2510
90 1
substrates can be initiated by a one-electron oxidation of one
of the components to its radical cation. After a cycloaddition
at the radical cation stage, a back-electron transfer terminates the reaction sequence. The reaction is thus catalyzed by
ET in the sense of a redox umpolung. The synthetic potential
of these reactions has not until recently been investigated
The importance of 2-vinylindoles as building blocks for
indole alkaloids has been pointed out several times.I3' Compounds of the type I with a wide variety of substituents are
easily accessible by using the method of Blechert et al. However, their use as 4nelectron components in Diels-Alder
reactions is limited to very electron-deficient dienophiles,
sometimes demanding drastic reaction c o n d i t i ~ n s . ~ ~ l
We now report for the first time electron transfer-catalyzed Diels-Alder reactions of I with electron-rich
dienophiles under mild reaction conditions and in good to
moderate yields.
Based on our studies on the photoinduced electron transfer(PETj-catalyzed cycloadditions of indole,"] we wanted to
establish whether compounds of the type 1 react as dienes or
dienophiles. Therefore, 1 a was allowed to react with 1,3-cyclohexadienes 2 under PET conditions. As PET sensitizers,
triarylpyrylium salts such as 3 were used, since they have
proven to be efficient electron transfer catalysts even in catalytic amounts.[2b.
The cycloadduct 4 a is smoothly
formed with 2a, that is 1 reacted as a diene. If the chiral
dienophile (5Rj-2b is used, (R)-configurated 4 b is isolated as
the only isomer. As a side reaction, the dimerization of the
Table 1. Results of the ET-catalyzed Diels-Alder reaction of 2-vinylindoles 1
with 1,3-cyclohexadienes2.
Yield [%]
211 [a]
1611 [d.b]
only endo
3 411 [d]
[a] Determined by gas chromatography in the crude product. [b] Thermal elec.
tron transfer with 10 mol% 5 (based on 1).
Under PET conditions, the primary adduct 7 is aromatized by double dehydr~genation[~'
to give the carbazole
derivative 8. If the reaction is performed under conditions
le, R'
- 4 f f
- 4 8
for thermal electron transfer, the mdolexo isomers of 7 are
obtained in good yields. It can be seen from the results
(Table 2) that the difference of the oxidation potentials determines the outcome of the reaction.
40: R' = Me,
RZ = R3 = H
4b: R' = R3 = Me,
RZ = iPr
R' = Ph,
R2 = R3= H
Table 2. Results of the ET-cavalyzed Diels-Alder reaction of 2-vinylindoles 1
with substituted styrenes 6 and dependence on the difference of the oxidation
potentials of the starting materials.
Dienophile R2
AEp(0x) [V] Product
69 la1
[a] Thermal electron transfer with 10 mol% 5 (based on 1). [b] Traces of the
product detected by GC/MS.
1,3-cyclohexadienes is observed, a reaction known1" to be
also catalyzed by PET. Alternatively, the reaction can be
initiated by thermal ET with tris(4-bromphenyl)aminium
hexachloroantimonate 5.['] However, lower yields are obtained in this case. The indole 1 b (Table I ) reacts in an
analogous manner.
From previous investigations,['] it was established that a
cycloaddition can only take place if the oxidation potentials
E, (Ox) of the starting materials d o not differ by more than
500 mV. Hence, styrenes 6 were chosen for further investigations because their oxidation potentials could be varied by
suitable substitution over a wide range with negligible steric
VCH V e r l u ~ ~ ~ e ~ e l l . rwhH,
\ ~ h u f W-6Y40
Weinlieftn, 1993
Non-conjugated dienophiles such as 1 -methoxycyclohexene d o not react despite their suitable E,(Ox) values. Therefore, we conclude that the intermediate stabilization of a
benzyl or ally1 radical plays a decisive role. Based on our
experiences with the indole system['.
and the preliminary
results of mechanistic investigations," the mechanism outlined in Scheme 1 for the reaction of the distonic radical
cation formed by ET appears probable.
For the rearomatization of the indole system, the most
likely mechanism is a thermal [1,,3,] H shift followed by
back-electron transfer. The [Is,3J H shift is symmetry forbidden for the neutral molecule, but not for the radical
cation.['21Shift reactions of this type have been well investigated by using spectroscopic[' 31 and theoretical['41methods.
0570-0833/93/0606-0902 $ /0.00+ ,2510
Angeu.. Cliem. I n ! . Ed. EngI. 1993. 32, No. 6
2) back-electron
Scheme 1. Proposed mechanism of the ET-catalyzed 14
tween 1 and 2.
+ 21 cycloaddition be-
If the [1,3] H shift is prevented by suitable substitution (9) of
the 2-~inylindole,['~]
the double bond remains in the 1,9aposition of the tetrahydrocarbazok intermediate after backelectron transfer. Under the reaction conditions, in situ dehydrogenation takes place to give the highly substituted
4,4a-dihydrocarbazole 10 as the only
[l] For reviews on this topic see for example a ) L. Eberson, Elecrron Truns/er
Reactions in Orgunic Chemistry, Springer, Berlin, 1987: b) J. Mattay,
Angew. Chem. 1987, 99. 849-870. Anger!:. Chem. Int. Ed. EngI. 1987, 26?
825-846; C)Pholoinduced Electron Transfer Part A-D (Eds.: M . Chanon,
M. A. Fox), Elsevier, Amsterdam, 1988.
121 a ) B. Harirchian. N. L. Bauld. J. Am. Chem. Soc. 1989, 111, 1826-1828,
b) I. Mattay. Synthesis 1989, 233-252; c) M. Schmittel. H. von Seggern,
Angew. Chem. 1991, 103,981 -983; Angew Chem. Int. Ed. En@. 1991,30,
999-1001; d) F. Muller. J. Mattay, ihrd. 1992, 104.207-208 and 1992.31,
209 - 210.
[3] a) S. Blechert, Liebigs Ann. Chem. 1985. 673-682; b) J. Wilkens. A.
Kiihling. S. Blechert, Tetrahedron 1987.43, 3237-3246; c) U. Pindur, H r t rrocycles 1988.27.1253-1268;d) U. Pindur, L. Pfeuffer, Chem. Zrg. 1986,
110.95 - 99.
141 a) H. Schroers, dissertation. UniversitAt Bonn, 1989; b) J. Levy. J. Sapi.
1.-Y Laronze. D. Royer. L. Toupet, Synktr 1992, 601 -602.
[5] a) A. Gieseler, E. Steckhan.0. Wiest, Synktt 1990, 275-277; b) A. Gieseler, E. Steckhan, 0. Wiest, F. Knoch, J. Org. Chem. 1991.56, 1405-1411
[b] a)J. Mlcoch, E. Steckhan, Angew. Chem. 1985. 97. 429-431: Angfw.
Chem. I n t . Ed. Engl. 1985.24.412-414; b) J. Mlcoch, E.Steckhan, Tftruhedron Lert. 1987, 28, 1081- 1084; c) M. Martiny, E. Steckhan, T. Esch.
Chem. Ber,. in press.
[7] D. J. Bellville, D. D. Wirth. N. L. Bauld. J. Am. Chem. Soc. 1981, 103.
[8] F. A. Bell, A. Ledwith. D. C. Sherrmgton, J. Chem. Sol. C 1969, 2719~2720.
[Y] Aromdtization by simple dehydrogenation is known M. J. Climent, H.
Garcia, S. Iborra. M. A. Miranda. J. Primo, Heterocycles 1989. 29. 115121.
[lo] 0. Wiest, E. Steckhan. F. Grein, J. Org. Chem. 1992, 57, 4034-4037.
[11] Further mechanistic studies are in progress and will be published elsewhere.
[12] This [1.3] H shift was also observed on other systems: a) J. Delaunay, A.
Orliac-Le Moing, J. Simonet, L. Toupet, Tetrahedron Lerr. 1986.27,62056208, b) Ref. [6a].
[13] Review: H. Schwarz. Top. Curr. Chem. 1981, 97, 1-31.
[14] For a recent review seea) K. N. Houk, Y Li, 1. D. Evansek, Angen. Chem.
1992, 104, 711-739; Angen'. Chem. Int. Ed. Engl. 1992. 31. 682-708,
b) T. Clark, J. Am. Chem. Soc. 1987,109,6838-6840; c) Y. Hoppiliard, G.
Bouchoux, Org. Mass Specrrom. 1982, 17, 534-536.
[15] T. Wirth. S. Blechert, Synletr, in press. We thank Dr. T. Wirth for a sample
of 9.
[16] The [4 21 cycloaddition of 9 and 6e can be induced alternatively by
potentiostatic electrolysis (undivided cell, working electrode: graphite,
auxilaryelectrode: Pt, CH,CN/CH,C& 1: 1.0.1 M LiCIO,. workingpotential670 mV YS. Ag/AgNO,). Thus, it is shown that the key intermediate of
the reaction is the radical cation of 9.
The reactions presented here open a new pathway for
Did-Alder reactions with 2-vinylindoles. The exact mechanism and the application of the reactions to the synthesis of
indole alkaloids are currently under investigation.
Experimental Procedure
a ) General procedure for PET-catalyzed reactions: Vinylindole 1 (0.5 mmol),
dienophile 2 (0.7 mmol) or 6, and 3 (0.02 mmol) were dissolved in anhydrous
CH,Cl, (50 mL). and the solution was degassed with argon in a Schlenk tube.
The solution was irradiated in a water bath (15 "C) with a 450 W xenon lamp
( h > 345 nm). The pyrylium salt 3 was removed after the reaction with a short
filtration column.
b) General procedure for the thermal electron transfer: Vinylindole 1
(0.5 mmol) and dienophile 2 (0.7 mmol) or 6 were dissolved in anhydrous
CH,CI, (50 mL) and degassed with argon. The solution was cooled to 0°C and
a suspension of 5 (0.05 mmol) in CH,Cl, (10 mL) was added dropwise.
For workup, in both cases the solvent was removed, and the residue was purified by chromatography. The endo/exo isomers were separated by HPLC
(LiChrospher RP18-5, waterpacetronitrile mixtures). All new compounds were
identified spectroscopically. Regio- and stereochemistry were proven by twodimensional NMR spectroscopy (COSY, HETCOR, NOESY). for example for
4 b : ' H N M R (400 MHz, CDCI,): 6 = 8.04 (br. 1 H, H-7a), 7.65 (1 H, d,
C-H Bond Addition to a V=NR Bond:
Hydrocarbon Activation by a Sterically Crowded
Vanadium System
By Jan de With and Andrew D. Horton*
Electrophilic complexes whose central atom is a metal
from Group 3 or 4, or an f-block element catalyze alkene
polymerization['1 and activate C-H bonds;['"* the related
vanadium chemistry has been hindered by the perceived instability of Vv complexes with cT-bound hydrocarbon liga n d ~ . [ ~Recent
reports of the highly reactive nature of
M =NR bonds in coordinatively unsaturated imido comJ=7.8H~.H-11),7.33(1H,d,J=8Hz,H-8),7.18(lH,t,J=7.8H~,H-Y),
7.09 (1H. t. J = 8 H z . H-lo), 5.71 ( l H , d. J=10.8Hz, H-4), 5.66 ( l H , d,
p l e ~ e s , [together
with our development of a series of robust
J =10.8 Hz.H-3).3.93(1 H , d . J = 4.9 Hz,H-6),2.99(1H.m,H-llc)2.36(1 H.
Vv complexes with bulky nitrogen ligands,IS1led us to atm, H-1). 2.08 ( I H. qd, J = 6.8 and 4.9 Hz, H-5), 1.94 (1 H. ddd, J = 12.6. 9.7
und2.9Hz.H-l),1.55(2H.m,H-5.H-12),1.36(3H,d,J=6.8Hz,H-16),1.18tempt to synthesize three-coordinate imidovanadium complexes. We postulated that generation of 1 on thermolytic
(3H.s. H-15). 0.86(3H.d, J = 6.1 Hz. H-12),0.85(3H,d.J = 6.1 Hz, H-14);
"CNMR (100.6 MHz, CDCI,): 6 =136.7 (C-7a). 132.8 (C-3). 129.9 (C-4),
expulsion of R'-H from [RN=V(NHR),R'], would be facil126.5(C-6a),125.6(C-lla). 122.4(C-9), 119.6(C-lO.C-l1), 118.4(C-l7), 113.8
itated by the small covalent radius and relatively labile na(C-llb). 111.2 (C-8). 39.5 (C-5). 38.6 (C-llc), 38.15 (C-2), 38.1 (C-4a), 31.5
(C-13). 30.9 (C-6). 26.8 (C-16). 26.03 (C-1). 19.9 (C-12). 19.5 (C-14). 14.2
(C-15). [XI:' = 94.5 ( C = 0.49 in CH,CI,).
Received: January 18, 1993 [ Z 5815 IE]
German version: Angew. Chem. 1993. 105, 932
Angew. C h m . I n l . Ed. EngI. 1993, 32. No. 6
[*] Dr. A. D. Horton, J. de With
Amsterdam (Shell Research B.V.)
P.O. Box 3003, NL-1003 AA Amsterdam (The Netherlands)
Telefax: Int. code + (20) 630-8025
mhH, W-6940 Weinheim, 1993
S 10.00+ .25/0
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dielsцalder, reaction, transfer, electro, vinylindoles, catalyzed
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