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Coupling of Two Ethyne Molecules at a Nickel Center to Form a Nickelacyclopentadiene Complex.

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Coupling to the dimer takes place via two Li-0-Li-0
four-membered rings. Each Lie (e.g. Li2) is bound to an
ortho C atom (C6), to two 0 atoms of two different sulfonyl groups ( 0 1 and 02’) and to one diethyl ether 0 atom
(04). The sp2 orbital at C6 is oriented almost centrally between Li2 and Lil‘ (the torsion angles S-CLC6-Li2 and
S-C 1-C6-Li 1‘ are 37.8(5) and - 47.1(4) respectively). The
two Lie (e.g. Li2 and Lil’) which are coupled to an ortho C
atom (C6) are complexed by the neighboring sulfone 0
atoms 01 and 0 2 such that five-membered rings are
formed (e.g. C6-Li 1’-02-S-CI).r’01The ease of ortho-lithiation of arenes with an RS02 groupf4]is thus understandable. The structure computed for Li,CHS02CH3 by Streitwieser et al.L”lis very similar to that of [3.(OEt2),], in one
respect: the two Lie are bound to the two sulfone 0 atoms
and an “anionic” C (a-C).
The structural characteristics of the a-sulfonyl anion
moiety of [3-(OEt,),], are not influenced to any great extent but, nevertheless, noticably so by the dianion formation: the C7-S bond is shortened from 174.7(5) pm in the
sulfone la“*] to 163.7(5) pm, and the S - 0 bonds are
lengthened from 144.3(3) pm to 149.4(3) pm; hence, the SO bonds here are ca. 3 pm longer than in the known asulfonyl carb bani on^".^^^ This could be due to the fact that
each 0 atom in the “dianion” [ 3 .(OEt2)2]2is coordinated
to two Li5, but in the monoanions to only one Lie. The
dihedral angles OI-S-C7-Sil ( - 50.8(4)”)and 02-S-C7-Si2
(13.3(4)”) would indicate that the lone pair on C7 assumes
a gauche conformation to the sulfone 0 atoms, which is
rotated through ca. 18.5” from the ideal position; the same
situation is found in m ~ n o a n i o n s . [ ~
C7
~ Iis almost planar
coordinated: the sum of the torsion angles Cl-S-C7-Sil
and CI-S-C7-Si2 is 169.7’. Further, the eight-membered
ring (e.g. S-O1-Lil-O2’-S’-O1 ’-Lil’-O2), which characterizes a-sulfonyl “carbanion” dimers, is also still present.
The X-ray structure analysis of [3 -(OEt2)2]2thus confirms the structural characteristics found in a-sulfonyl
“carbanions” ; further, the five-membered ring complexation of Lie by the 0 atoms of an RS02 group in an aromatic ortho anion is recognizable which contributes to its
ease of formation.
O,
Received: August 17, 1987 [Z 2401 IE]
German version: Angew. Chem. 99 (1987) 1279
[I] D. Seebach, R. Hassig, J. Gabriel, Helu. Chrm. Acta 66 (1983) 308.
121 The significance of these selective reactions is covered in a series of new
reviews: a) P. Beak, D. B. Reitz, Chem. Reu. 78 (1978) 275; H. W.
Gschwend, H. R. Rodriguez, Org. React. 26 (1979) I ; c) P. Beak, V.
Snieckus, Acc. Chem. Res. 15 (1982) 306; d) P. Beak, A. 1. Meyers, Acc.
Chem. Res. 19 (1986) 356; e) G. W. Kiumpp, J. R. Neth. Chem. Sac. I05
(1986) 1.
131 X-ray structure data have recently become available for some a-sulfonyl
“carbanions”: a) [(PhCHSO2Ph)Li(tmeda)l2:G. Boche, M. Marsch, K.
Harms, G. M. Sheldrick, Angew. Chem. 97 (1985) 577; Angew. Chem.
Inr. Ed. Engl. 24 (1985) 573; b) [(CH,SO,Ph)Li(tmeda)],: H.-J. Gais, H.
J. Lindner, J. Vollhardt, ibid. 97 (1985) 865 and 24 (1985) 859; c)
[(CH2=CHCHSO2Ph)Li(diglyrne)l2:H.-J. Gais, H. J. Lindner, J. Vollhardt, ibrd. 98 (1986) 916 and 25 (1986) 939; d) the structure of
[(MeiSiCHSOzPh)Li(tmeda)]2 corresponds in many respects to the structures of the compounds mentioned under (a-c): G. Boche, W. Hollstein,
M. Marsch, K. Harms, unpublished.
[4]The strong directing effect of the RSOl group (R=aryl, tert-butyl, NR;,
NLiR‘) is known: a ) W. E. Truce, M. F. Amos, J. Am. Chem. Sac. 73
(1951) 3013; b) H. Gilman, S. H. Eidt, ibid. 78 (1956) 2633; c) H. Watanabe, C. R. Hauser, J . Org. Chem. 33 (1968) 4278; d ) H. Watanabe, R. A.
Schwarz, C . R. Hauser, J . Lewis, D. W. Slocum, Can. J. Chem. 47(1969)
1543; e) F. M. Stoyanovich, R. G. Karpenko, Y. L. Gol‘dfarb, Tetrahedron 27 (1971) 433; f) J. G. Lombardino, J. Org. Chem. 36 (1971) 1843;
g) D. Hellwinkel, M. Supp, Chem. Eer. 109 (1976) 3749; h) D. W. Slocum, D. I. Sugarman, Adu. Chem. Ser. 130 (1974) 222; i) S. J. Shafer, w.
D. Closson, J. Org. Chem. 40 (1975) 889; j) D. W. Slocum, C . A. Jen-
1288
0 VCH Verlagsgesellscha/r mbH, 0-6940 Weinheim, 1987
nings, ibid. 41 (1976) 3653; k) y-deprotonation of a,P-unsaturated sulfones: E. Block, V. Eswarakrishnan, K. Gebreyes, Tetrahedron Leu. 25
(1984) 5468.
orrho-Li compounds which are complexed intramolecularly by RO- or
R,N-groups have been investigated X-ray crystallographically: a) J. T.
B. H. Jastrzebski, G. van Koten, M. Konijn, C. H. Stam, J. Am. Chem.
Sac. 104 (1982) 5490; b) I. R. Butler, W. R. Cullen, J. Reglinski, S . J.
Rettig, J. Organomet Chem. 249 (1983) 183: c) J. T. B. H. Jastrzebski, G.
van Koten, K. Goubitz, C. Arlen, M. Pfeffer, ibid. 246 (1983) C 7 5 ; d) H.
Dietrich, D. Rewicki, ibrd. 205 (1981) 281; e) see also: W. Neugebauer,
A. J. Kos, P. von R. Schleyer, rbid. 228 (1982) 101.
a) In the reaction of ally1 phenyl sulfone with two molar equivalents of
nBuLi in T H F at low temperatures the initially formed a-sulfonyl “carbanion” reacts further to give the I,o-dianion corresponding to 3. Transprotonation to the I,l-dianion takes place at 50°C [7]; see J. Vollhardt,
H.-J. Gais, K. L. Lukas, Angew. Chem. 97 (1985) 607; Angew. Chem. Int.
Ed. Engl. 24 (1985) 608; b) the second lithiation of (trimethylsily1)methy1 phenyl sulfone proceeds exclusively to the 1,l-dianion; see J. Vollhardt, H.-J. Gais, K. L. Lukas, ibid. 97 (1985) 695 and 24 (1985) 696; c)
the a-sulfonyl “carbanions” mentioned under [3a-d] are formed from
the sulfone and one molar equivalent of nBuLi; nothing has been reported about a further reaction product. 2,o-Dilithio compounds are
also known: J. E. Mulvaney, S. Groen, L. J. Carr, Z. G. Garlung, J. Am.
Chem. Sac. 91 (1969) 388; H. M. Walborsky, P. Ronman, J. Org. Chem.
43 (1978) 731.
1,l-Dilithiosulfones have been known for some time: a) E. M. Kaiser, L.
E. Solter, R. A. Schwarz, R. D. Beard, C. R. Hauser, J. Am. Chem. Sac.
93 (1971) 4237; b) J. B. Evans, G. Marr, J. Chem Sac. Perkin Trans. I
1972, 2502; c) K. Kondo, D. Tunemoto, Tetrahedron 1975, 1397; d) A.
Roggero, T. Salvatori, A. Proni, A. Mazzei, J . Organomel. Chem. I77
(1979) 313; e) S. P. J. M. van Nispen, C. Mensink, A. M. van Leusen,
Tetrahedron Lett. 21 (1980) 3723; f) M. C. Mussatto, D. Savoia, C.
Trombini, A. Umani Ronchi, J. Org. Chem. 45 (1980) 4002; g) J. J. Eisch,
S. K. Dua, M. Behrooz, ibid. 50 (1985) 3674.
Procedure for [3-(OEt2),],: A solution of la (90 mg, 0.30 mmol) in diethy1 ether (1 mL) was treated at room temperature with 0.33 mmol of
nBuLi in hexane. After 10min the solution was evaporated down to
about 0.5 mL in an oil-pump vacuum. Single crystals suitable enough for
an X-ray analysis could be isolated from the solution after I hour.
Yield: 44 mg (0.05 mmol), 30°/o.
The exclusive formation of 28 from l b additionally shows that the deprotonation of 1 does not initially lead to an ortho anion that is transprotonated in a second step to the a-anion 2. Otherwise 2b and, after
protonation, l e would have to be formed; deuterium determinations
were carried out mass spectrometrically. A corresponding experiment
was carried out for the first time by Gais et al., see [6b]. A related problem has recently been investigated by other authors: A. I. Meyers, K. B.
Kunnen, W. C . Still, J. Am. Chem. Sac. 109 (1987) 4405. For the mechanism of the second lithiation in some other compounds see: W. Bauer,
T. Clark, P. von R. Schleyer, J. Am. Chem. Sac. 109 (1987) 970; W.
Bauer, G. Miiller, R. Pi, P. von R. Schleyer, Angew. Chem. 98 (1986)
1130; Angew. Chem. I n t . Ed. Engl. 25 (1986) 1103.
I-Magnesio-2-pivaloyl-l,2,3,4-tetrahydroisoquinolineis characterized
by a structurally related five-membered ring formation by intramolecular complexation of the carbonyl 0 atom with the C-bonded Mg:
D. Seebach, J. Hansen, P. Seiler, J. M. Gromek, J. Organomet. Chem.
285 (1985) 1.
D. A. Bors, A. Streitwieser, Jr., J. Am. Chem. Sac. 108 (1986) 1397.
Structure of l a : W. Hollstein, M. Marsch, K. Harms, G. Boche, unpublished.
Coupling of Two Ethyne Molecules at a Nickel
Center to Form a Nickelacyclopentadiene Complex
By Klaus-Richard Porschke*
The nickel catalyzed cyclooligomerization reactions of
acetylene discovered by Reppe presumably proceed stepwise by a mechanism involving a nickelacyclopentadiene
Compounds of this type are already known
for other
whereas with nickel so far only substituted derivatives have been obtained.L31Continuing our investigations on ethyne complexes of nickel(^)[^^ we have
now observed for the first time a coupling of two ethyne
molecules on nickel(o) at -78°C (!), leading to formation
[*I Dr. K.-R. Porschke
Max-Planck-lnstitut fur Kohlenforschung
Postfach 10 13 53, D-4330Mulheim a. d. Ruhr 1 (FRG)
0570-0833/87/1212-1288 $ 02.50/0
Angew. Chem. Int. Ed. Engl. 26 (1987) No. I 2
of a nickelacyclopentadiene complex with bis(diisopropy1phosphino)ethane as stabilizing ligand.
When tris(ethene)nickel(~)[~~
is treated with stoichiometric amounts of iPrzPCzH4PiPrzin pentane the crystalline
ethene complexes [(y-iPr2PCzH4PiPrz){Ni(C2H4)z}~]
(1)
and (iPr2PC2H4PiPrz)Ni(CzH4)
(2) can be isolated. 1 reacts
with ethyne in T H F at - 78°C to yield the mixed ethene/
ethyne
dinuclear
complex
[(p-iPrzPC2H4PiPrz){Ni(C2H4)(CzHz))z]
(3)'61as a colorless crystalline and
rather sensitive compound. A similar reaction proceeds in
ether at -30°C to afford yellow crystals of [(piPr,PCrH4PiPrz)(Ni(CzH4)}2(y-CZH2)]
(4);'' in which only
one ethyne ligand is present, bridging the nickel atoms.
From 2 and ethyne the complex [(iPrzPCzH,PiPr~)Ni(CzH2)]
( 5 ; CZH2: v = 1598 cm-I,
&=7.29,
&= 123.8,
'J(CH) = 202 Hz) is obtained, which, particularly in solution, is comparatively stable. 5 reacts with the ethene complex 2 at 0°C with liberation of ethene to afford
[((iPrzPCzH4PiPrz)Ni)z(p-CzHz)](6; p-C2HZ: v = 1315
cm-', SH=5.52, 6,=86.3, 'J(CH)=188 Hz), in which the
ethyne ligand bridges two bis(phosphane)nickel(o) moieties. 6 is cleaved by ethyne at 0°C into two molecules of 5 .
Single crystal X-ray structure analyses have been carried
out on 5 (d(C=C) 1.287(7)A) and 6 (d(C=C)
1.335(7)
A).'*]
the nickelacyclopentadiene complex 7 separate out in the
course of 48 h. Apparently, 7 is formed via 3: presumably
the bridging iPrzC2H4PiPrzligand of 3 transforms under
the influence of the added iPrZPCzH4PiPrzinto the chelating binding mode and in the course of this reorientation
two of the ethyne molecules combine to form the nickelacyclopentadiene group, while a third ethyne molecule
forms a bridging ligand between the formal nickel(I1) ion
and the nickel atom.
iPr
7
7 has been characterized by its 'H-, I3C-, 3'P-NMR, and
IR spectra[']. The C = C stretching vibration of the bridging
ethyne ligand of 7 occurs at the same wave number
v = 1315 cm-' as that of 6 . The 'H- and I3C-NMR data of
the ethyne ligand of 7 (6, ~ 5 . 9 6 &=98.8,
,
'J(CH)= 190
Hz) are similar to those of 6 and also indicate a bridging
ethyne ligand (cf. values of 5 ) . For the p-C4H4ligand the
chemical shifts and coupling constants of
= 5.09, 4.85
and 6,=115.6 ('J(CH)=151 Hz, NiCH=CH-), 110.8
('J(CH)= 143 Hz, Ni-CH=) are in the range expected for
a metal alkenyl group. In the "P-NMR spectrum two singlets for the iPr2PC2H4PiPrzligands show that the P
atoms of each ligand do not couple with those of the
neighboring ligand. We are currently studying further reactions of 7.
Received: September 4, 1987;
revised: October 8, 1987 [ Z 2422 IE]
German version: Angew. Chem. 99 (1987) 1321
CAS Registry numbers:
1, 111497-32-6; 2, 111497-33-7; 3, 111497-34-8; 4, 111524-69-7; 5 , 11149735-9; 6, I 1 1556-66-2; 7, I 1 1524.70-0; iPr2PCH2CH2PzPr2,87532-69-2; tris(ethene)nickel(O) 50696-82-7.
4
Ill P. W. Jolly in G. Wilkinson, F. C . A. Stone, E. W. Abel (Eds.): Compre-
i Pr
"
When iPrzPCzH4PiPrzand excess ethyne are added to
a pentane solution of 1 at -78"C, tine yellow crystals of
Angew. Chem. lnt. Ed. Engl. 26 (1987) No. 12
hensive Organometallic Chemistry. Vol. 8. Pergamon Press, Oxford 1982,
p. 653f.
[2] Examples: a) Ti: H. G. Alt, H. E. Engelhardt, M. D. Rausch, L. B. Kool,
J. Organornet. Chem. 329 (1987) 61; b) Mo: M. H. Chisholm, K. Folting,
J. C. Huffman, I. P. Rothwell, J. Am. Chem. Soc. 104 (1982) 4389; c) W:
M. H. Chisholm, K. Folting, D. M. Hoffman, J. C. Huffman, J. Leonelli,
J. Chem. SOC.Chem. Commun. 1983, 589; M. H. Chisholm, D. M. Hoffman, J. C. Huffman, J . Am. Chem. Soe. 106 (1984) 6806; d) Fe: G. Dettlaf, E. Weiss, J. Organomet. Chem. 108 (1976) 213; e) Co: H. Yamazaki,
Y. Wakatsuki, J . Organornet. Chem. 272 (1984) 251.
[3] H. Hoberg, W. Richter, J. Organomet. Chem. 195 (1980) 355.
[4] a) K. R. Porschke, R. Mynott, K. Angermund, C. Kriiger, Z. Naturforsch.
8 4 0 (1985) 199; b) K. R. Porschke, R. Mynott, ibid. 4 2 (1987) 421; c) K.
R. Porschke, Y.-H. Tsay, C. Kriiger, Angew. Chem. 97 (1985) 334; Angew.
Chem. l n f . Ed. Engl. 24 (1985) 323; d) K. R. Porschke, Xllth I n i . ConJ
Organornet. Chem., September 8-13, 1985, Abstracts No. 207.
[5] K. Fischer, K. Jonas, G. Wilke, Angew. Chem. 85 (1973) 620; Angew.
Chem. l n t . Ed. Engl. I2 (1973) 565.
[6J 3: 'H-NMR (200 MHz, [D8]THF, -3OOC): 6=7.10,6.34 (m. 4H, C2H2),
2.72 (8H. C2H4), 2.25 (m, 4H, PCH), 1.93 (4H, PCH2), 1.08 (m, 24H,
LCH,); "P-NMR (32.4 MHz, [DslTHF, -30°C): 6=51.6.
[71 4 : 'H-NMR (200 MHz, [D8]THF, -3OOC): 6=6.08 (1 H, C2H2),4.86 (t,
J(PH)= 17.2 Hz, 1 H, CIHZ),2.49,2.27 (m, 8 H , C2H4),2.25 (m,4H, PCH),
2.1 (m, 4 H , PCH2), 1.1 (m, 24H, PCH,); "P-NMR (32.4 MHz, [D,]THF,
-30°C): 6=34.7.
I81 K. R. Porschke, W. Bonrath, Y.-H. Tsay, C. Kruger, Z . Naturforsch. B , in
press.
0 VCH Verlagsgesellschaft mbH. 0-6940 Weinheim. 1987
0570-0833/87/1212-1289 $ 02.50/0
1289
191 7: IR (KBr, -30°C): v=3010 c m - ' ( d - H or =C-H), v=1315 c m - '
(CsC), y=756 c m - ' (C-H); 'H-NMR (400 MHz, [D,]THF, rel. TMS,
-30°C): 6=5.96 (m, 2H, C2H2),5.09 (m, 2H, Ni-CH=), 4.85 (m, 2H,
=CH-), 2.20, 2.16 (m, 4H, PCH), 1.94, 1.91 (m, 4H, PCH), 1.5-0.9 (8
multiplets, 48H, PCH,); "C-NMR (75.5 MHz, [D,ITHF, rel. TMS,
-30°C): 6=115.6 (m. 2C, 'J(CH)=ISI Hz, =CH-), 110.8 (m, 2C,
'J(CH)= 143 Hz, Ni-CH=), 98.8 (m, 'J(CH)= 190 Hz, H G C H ) ; "PNMR (32.4 MHz, [D,]THF, rel. 85% aqueous H,P04, -30°C): 6=83.6,
71.4, intensity ratio 1 : I .
zene to formation of the previously unknown spiro[4.4]nona-2,7-diene 6 as sole product (Table 1).
7
The Dependence of Intramolecular Electron Transfer
on Structure in a Spiro Compound Containing Two
Cyclooctatetraene Moieties**
By Giinter Krummel, Walter Huber, and Klaus Miillen*
Dedicated to Professor Emanuel Vogel on the occasion of
his 60rh birthday
1
The previously unknown compound 1, 10,10'-spirobi(bicyclo[6.3.0]undeca-2,4,6,8-tetraene), offers the following
advantages over already investigated model systems: 1) the
subunits are fixed orthogonally to each other by the spiro
c o ~ p l i n g ; ' 2)
~ ,both
~ ~ a direct ~1,nconj~gation''-~]as well as
a through-space conjugation161 between the subunits are
ruled out; 3) because of the planarization of the cyclooctatetraene (COT) necessary for ion formation,"'] the reorganization energy required for an intramolecular electron
transfer between the subunits should be increased.
We describe here the synthesis of 1 and structurally related compounds, the generation of lee, lz@,
and 140
and discuss the possibility of electron transfer between
COT subunits of these anions. The method of choice for
the preparation of 1 (Scheme 1) is that previously used by
us on several occasions: cycloannelation by reaction of
dianions with bifunctional electrophiles.l' 'I After alkylation of the cyclooctatetraene dianion Li2-2 (2 equiv.) with
tetrabromoneopentane 3b and aqueous work-up the polycycle 5 is isolated in 56% yield. Apparently a twofold cycloannelation takes place, but the primary product undergoes a valence isomerization under the reaction conditions. The I3C-NMR spectrum of 5 (Table 1) shows that
only one species of several conceivable configurational
isomers exists. Photolysis of 5 leads via cleavage of ben-
[**I
Prof. Dr. K. Miillen, Dip].-Chem. G. Krummel
Institut fur Organische Chemie der Universitat
J.-J.-Becher-Weg 18-20, 6500 Maim I (FRG)
Priv.-Doz. Dr. W. Huber
Institut fur Physikalische Chemie der Universitat
Klingelbergstrasse 80, CH-4056 Basel (Switzerland)
Reductive Transformations, Part 9. This work was supported by the
Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie.-Part 8: G. Neumann, K. Miillen, J. Am. Chem. SOC. 108 (1986)
4105.
1290
0 VCH Veriagsqeseiischaji mbH. 0-6940 Weinheim. 1987
, L G L
ie
L
4
5
bl
1
The rate constants of the intramolecular electron transfer between separate redox units"' are not only dependent
on the substrate and ion-pair structure[21but also on the
length and conformation of the bridging g r o ~ p . ' ~ We
.~]
have generated extended redox sequences of several "biselectrophoric" systems and have been able to alter the energy profiles of intramolecular electron
[*I
8
Scheme 1. a) NHI, -33°C; HzO, 20°C. b) Pentane, 3 x lo-'
c) NH,, -60°C; d) NH3, -6O"C, KNH2. e) CdC12.
M,
Pyrex lamp.
The course of the cycloannelation reaction depends on
the structure of the ion pair of the nucleophile and on the
leaving group of the electrophile 3 (Scheme 1). Alkylation
of Liz-2 with the chloride 3a instead of with the bromide
3b leads to formation of only the monoadduct 7 (Table 1).
O n the other hand, if the potassium salt Kz-2 is allowed to
react with 3b only the polycycle 8 is obtained after dehydrogenation (cf. 4+ 14@+
I ) (Table l), i.e., here a reductively induced three-membered ring formation follows the
cycloannelation.
The conversion of 4 into 5 can be partially suppressed
by working at lower temperatures (Scheme l), and the target compound 1 is obtained via the tetraanion 148.1121
Table 1. Melting points ( 5 , 7) and "C-NMR data [a] of 5 - 8 .
5 : 83°C; 6 = 127.6, 127.5, 120.6 (C3-C6, C3'-C6'), 57.6 (CIO), 52.0, 51.1 (C2,
C7, C2', C77, 47.4, 46.3 (C1, C8, CI', CS?, 37.0, 36.8 (C9, C11, C9', CII')
6 : 8= 130.1 (C2, C3, C7, CS), 48.7 (C5), 47.5 (Cl, C4, C6, C9)
7:84-85°C; 6= 134.8, 128.2, 127.7 (C2-C7), 51.5, 50.6 (C12, C13), 46.3 (ClO),
43.0, 41.8 (Cl, C8, C9, C11)
8: 6 = 140.2 (C1, CS), 132.5, 131.7, 131.2 (C2-C7), 47.3 (C9, C1 I), 19.0 (CIO),
14.7 (C12, C13)
[a] CDCll; 5 , 6 100 MHz; 7, 8 50 MHz.
The spectroscopic characterization of 1 at room temperature (Table 2) indicates a structure with effective Dzd symmetry. Cooling leads to a line-broadening of the singlet
signal of the methylene protons and finally at -60°C to
the appearance of the signals of two AB systems. This dynamic behavior confirms that both rings are present in a
tub conformation.
When a solution of 1 in [D8]tetrahydrofuran is brought
into contact with lithium the tetraanion 14@
is formed
again. The number and position of the NMR signals (Table 2) indicate a structure with &d symmetry which contains two planar COTZe units. If one monitors the reduction of 1 N M R spectroscopically it is possible to detect the
dianion 1'' as intermediate. The number of the 'H- and
I3C-NMR signals (Table 2) shows beyond doubt that the
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Angew. Chem. hi.Ed. Engl. 26 (1987) No. 12
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