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Ligand-Mediated Coupling of Organometallic Reaction Centers.

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Instead of treating D with water, D was allylated in situ (81 % overall yield) by
addition of CuBr.Me,S (1.5 equiv) and then ally1 bromide (3 equiv). The mixtrure
was stirred for 3 h stirring and then subjected t o aqueous workup as above (Table 1,
entry 3). The hydrazone product was smoothly hydrolyzed (85% yield) according
t o a literature procedure [Sd] by stirring a mixture of the hydrazone (1 mmol).
CuCI, (1 mmol), 0.05~
phosphate buffer (3 mL), water (5 mL), and T H F (15 mL)
at 25-35 ‘C for 9 h.
Received: April 8, 1997
Revised version: July 14, 1997 [Z10326IE]
German version: Angew. Chem. 1997, 109,2581 -2583
Keywords: alkenes
- hydrazones . ketones
[l] A. T. Nielsen. W. J. Houlihan, Org. React. 1968, 16, 1-438; T. Mukaiyama,
ibrd. 1982. 2X. 203-331.
[2] The difficulty of this reaction is seen in the endothermicity of the reaction
(26.5 kcalmol-’. as calculated for free acetone enolate +ethylene, HF/321 G ) .
[3] To improve the unfavorable thermodynamics a) the ring strain of a cyclic
olefin was used (E. Nakamura, K. Kubota, J. Org. Chem. 1997,62,792-793)
and b) an intramolecular reaction was carried out with an enolate bearing a
BrZn” cation (P. Karoyan, G. Chassaing, Tetrahedron Letr. 1997,38, 85-88;
E. Lorthiois. I Marek, J. F. Normant, ibid. 1997, 38, 89-92).
[4] Another recent (formally related) solution to the problem is the use of vinylmagnesium bromide as an “activated form” of ethylene (E. Nakamura, K
Kubota. G. Sakata. J Am. Chem. Soc. 1997, 119, 5457-5458). This reaction,
which takes place by a metalla-Claisen rearrangement, may be mechanistically
unrelated to the present reaction.
[5] Utility of metalated hydrazones in the synthesis of carbonyl compounds is well
documented a ) G. Stork, J. Benaim, 1 Am. Chem. Soc. 1971,93,5938-5939;
b) E. J. Corey. D. Enders. Chem. Ber. 1978, f f / , 1337-1361 and 1978, ill,
1362- 1383; c) D E. Bergbreiter, M. Momongan. Comprehensive Organic Synfhesu. Vol. 2 (Eds.: B. M. Trost. I. Fleming), Pergamon, Oxford, 1991,
pp. 503-526: d ) E J. Corey, S. Knapp, Tetrahedron Lett. 1976, 41. 36673668.
[6] The structure of the zinc hydrazone is presently unknown. For the structures
of lithium hydrazones, see D. B. Collum. D. Kahne, S. A. Gut, R. T. DePue, F.
Mohamadi. R A. Wanat, J. Clardy, G. V. Duyne. J. Am. Chem SOC.1984.106.
4865-4869; R A. Wanat, D. B. Collum, ibid. 1985, 107, 2078-2082; D. Enders, G . Bachstadter, K. A. M. Kremer, M. Marsch, K. Harms, G. Boche,
Angea.. Chein. 198%.100, 1580-1581; Angew Chem. Int. Ed. Engl 1988, 27,
1522- 1524.
[7] For a review of the related chemistry of homoenolates, see I. Kuwajima, E.
Nakamura, Comprehensive Orgunic Synrhesis Vol. 2 (Eds.: B M. Trost, I.
Fleming), Pergamon, Oxford, 1991. pp. 441 -454.
[8] It is notable that Ihe azaenolate moiety in C, and not the seemingly more
reactive butyl group, reacts preferentially with the olefin. This lacking reactivity of the alkyl-zinc bond agrees with the fact that the product E does not react
further with the starting olefin.
[9] The MeZn and /err-BuZn analogues of C are less reactive than C.
[lo] See also K. Sekiya, E Nakamura, Tetrahedron Left. 1988, 29, 5155-5156;
M. Arai, T Kawasuji. E. Nakamura, J Org. Chem. 1993, 58, 5121-5129,
and references therein.
I l l ] D. Enders in A.synzmefric Synthesis, Vol. 3 (Ed.: J. M. Morrison), Academic
Press, 1984. pp 275-339.
[12] The configuration of the new chiral center was assigned in anlogy to the reactions with SAMP hydrazones [ll].
1131 Theory suggests that electrophilic activation of the olefin by the metal cation
is important for the carbometalation of olefins: E. Nakamura, Y. Miyachi, N.
Am. Chem. SOC.1992,114,6686-6692; M. NakamuKoga. K. Morokuma, .
Chem. SOC.Foruduj Trans. 1994,
ra, E Nakamura. N. Koga, K. Morokuma, .
2Y, 1789-1798, E. Nakamura, Pure Appl. Chem. 1996.68, 123-130.
[14] Competition experiments indicated that 1-octene and the allylstannane react at
roughly the same rate.
[15] The question still remains as to why the zinc hydrazone reacts much faster with
olefins than with the polarized C = N bond.
[16] M. Nakamura. A. Htrai, E. Nakamura, J. Am. Chem. Soc. 1996, 118, 84898490: M. Nakamura, M. Arai. E. Nakamura, &id. 1995, 1 / 7 , 1179-1180.
Angeu‘.Cheni. Inr Ed. Engl. 1997, 36, No. 22
Ligand-Mediated Coupling of Organometallic
Reaction Centers**
Wolfgang Kaim,* Ralf Reinhardt, and Jan Fiedler
Dedicated to Professor WoEfgang Beck
on the occasion of his 65th birthday
The electronic coupling of reversible single-electron transfer
at individual redox centers as mediated by bridging molecules
has played a pivotal role for the understanding of intra- and
intermolecular electron transfer reactivity.[’] The mechanistic
concepts of ligand-mediated “communication” between two or
more electron-transfer-active
metal sites have developed
from simple systems, marked
by the mixed-valent CreutzTaube ion 1 as a molecular
model for degenerate electron
transfer between two separated metal ions,[1b3C’
21 to encompass a high degree ofexperimental
and theoretical sophistication, and now extend from small molecules to large biological systems.[lal
One common way[lclto measure the metal-metal interaction
in species like 1 is to determine the comproportionation constant K c , which is defined in Equations (1) and (2) (Int: mixed-
K , = [Int]z/[Ox] x [Red] = 10AE’59mV
[Ox]+[Red] 6 2[Int]
valent intermediate). However, for chemical purposes directed
at synthetic transformations and eventual multielectron catalysis it would be more relevant to couple “redox reactivities”
instead of mere electron transfer. Among the redox reactions
beyond pure electron transfer processes, the EC-type “composite”
represent the best understood examples:
electron transfer (E) is followed by an elementary chemical step
(C) such as a dissociation, and the thus created new species may
eventually revert back to the starting compound through another set of EC steps. Composite processes such as EC or ECE (a
two-electron variant) are crucial in organometallic chemistry
and catalysis, for example, within reductive elimination/oxidative addition ~equences.1~~
A well established reversible ECE processr4’ involving Rh”’
and coordinatively unsaturated Rh’ species has been used for
some years for the catalysis of hydride transfer, for example, to
H’, NAD’, or f ~ r m a t e . [A~ single
~ . ~ ~{Cp*CIRhf} fragment
(Cp* = C,Me,) bound to an a-diimine ligand such as 2,2’bipyridine or a 1,4-dia~a-l,3-butadiene[~~,
undergoes an ECE
process in which the initial electron uptake is followed by a rapid
dissociation of chloride as the chemical step and by a second
electron acquisition. This reaction produces deeply colored neutral “Rh’” compounds that are reoxidized at a significantly less
Prof. Dr. W. Kaim, Dr. R. Reinhardt
lnstitut fur Anorganische Chemie der Universitit
Pfaffenwdldring 55, D-70550 Stuttgart (Germany)
Fax: Int code +(711)685 4165
e-mail: kaim&
Dr. J. Fiedler
J. Heyrovsky Institute for Physical Chemistry and Electrochemistry
Dolejskova 3, CZ-18223 Prague 8 (Czech Republic)
e-mail: jan.fiedler&;
[**I This work was supported through grants from the Deutsche Forschungsgemeinschaft (SFB 270. German-Czech Exchange Program). the Volkswagenstiftung. and the Fonds der Chemischen Industrie. Donations of RhCI, by
Degussa AG are also gratefully acknowledged.
6 WILEY-VCH Verlag GmbH, D-69451 Weinheim,
0570-083319713622-2493S 17.50+.5(1/0
negative potential and then bind to the additional ligand.C4I In
the catalytic cycle the active form Cp*Rh(cc-diimine)can oxidatively add a proton to form the essential, reactive hydride intermediate [Cp*RhH(a-diimine)]+
b, 51 The less reactive iridium
analogues may be isolated and characterized.[6]
In a first systematic attempt to extend the coupling ofelectron
transfer sites to the coupling of redox reaction centers through
conjugated bridging ligands we have synthesized[71the symmetrically dinuclear compounds [Cp*CIRh(p-L)RhClCp*](PF,),
L = bpipr8"](2) and bptz[sbl(3) and studied their extended but
-0.5 -1.0 -1.5
E/V vs. Fcffc'
Figure 1. Cyclic voltammogram of [Cp*ClRh(p-bptz)RhClCp*](PF,), in acetonitriIe/O.l~Bu,NPF6 at 100 mVs-' (solid line) and 50 mVs-' (broken line).
Dotted line: response from rotating disk electrode (glassy carbon) at 20 mVs-I,
w = 2000 rpm. Fc/Fc+ = ferrocene/ferrocenium.
- 1 I
\. .\
1000 1200 1400 1600 1800 2000
llnm -+
Figure 2. Spectroelectrochemical reduction of [Cp*Rh(p-bpip)RhClCp*]'
[Cp*Rh(p-bpip)RhCp*]' in acetonitrile/O.l M Bu,NPF,.
Table 1. Electrochemical potentials [a], absorption maxima [b], and EPR data of dinuclear complexes with L = bpip (bptz).
= bpip : 2
= bptz : 3
fully reversible electrochemistry by EPR and UV/Vis spectroscopic techniques. Whereas bpip resembles a bis-chelate modification of the familiar pyrazine ligand (see l), the bptz molecule
was chosen because of its proven ability to promote very strong
coupling between transition metal centers; for example,
K, = lo1' for the bisftetraammineruthenium)
The results of the cyclic voltammetry of the dinuclear rhodium compounds reveal fully reversible cycles, including two separated halide dissociation/association processes and up to six
electron transfer steps (Figure 1). Diagnostic criteria for the
oxidation state formulation of the intermediates are the UV/
Vis/NIR absorption spectra from OTTLE spectroelectrochemistry (Figure 2),[91the release of chloride as studied by Hg polar~graphy,[~']
and-in the case of paramagnetic compounds-the
EPR signals. From this information we can derive the sequence
summarized in Table I .
In both cases (L = bpip and bptz), the first reduction is no
longer an ECE process but an electrochemically reversible oneelectron reduction. Absorption and EPR spectra clearly indicate
L-centered spin (a radical anion ligand['OI). Chloride dissociation occurs only after the second one-electron reduction, indicating a decoupling of the ECE process["] into an E and EC
step. In other words, the ligand L acts as a remote electron
storage or "reservoir"[3. 121 until the second electron triggers a
0 WILEY-VCH Verfag GmbH, D-69451 Weinhelm, 1997
-0.43 (0.04)
-0.50 (-0.03)
-0.43 (- 0.46)
-0.76 (-1.06)
-1.03 (-1.07)
- 1.33[c]
- 1.39[c]
-2.37 (-1.37)
-2.45 (-1.44)
690[e] (420,sh)[fl
[Cp*Rh(L)RhCp*]'+ [c]
(866)[d. 81
(- 1.94)[d]
( - 2.02)[d]
(691) [dl
[a] From cyclic voltammetry at 100 mVsC' in acetonitrile/O.l M Bu,NPF6, anodic and
cathodic peak potentials En. and E, in V vs. Fc'/Fc". [b] Long-wavelength absorption
maxima 1,, in nm from spectroelectrochemistry. [c] Only for L = bpip. [d] Only
for L = bptz. [el g,,, = 1.9934 (270 K). [f] g , = 2.0019, g,, =1.9914 (3.4K).
[g] g, = 2.151, g 2 = 2.035, g , ~ 1 . 9 6 (3.4K).
combined metal-centered two-electron reduction of Rh"' to
yield chloride-free Rh'. The appearance of the {Cp*Rh') moiety
bound to a x-acceptor ligand is evident from intense long-wavelength
Interaction between the two organometallic reaction centers[' * I is evident from the fact that the loss of chloride occurs
stepwise, separated by significant potential differences of more
than 300 mV. The class I['3' mixed-valent species [Cp*ClRh-
0570-083319713622-2494$17.50+ .50/0
Angen.. Chem. Int. Ed. Engl. 1997.36, No. 22
(p-L)RhCp*]' formed in the first chloride-releasing process is
reduced for L = bptz in an ECE step to neutral [Cp*Rh(p-bptz)RhCp*] at more negative potential; there are two more
reversible one-electron steps with a paramagnetic intermediate
that may be f ~ r m u l a t e d ~according
to [Cp*Rh'(p-bptz-')Rh'Cp*]'- c, [Cp*Rh'(p-bptz-")Rh"Cp*]'- based on the sig~ ]comparison to that of [Cp*ClRh"'nificant g a n i s o t r ~ p y [ 'in
(p-b"z-')Rh'''ClCp*]'+ (Table 1).
For L = bpip the second ECE step is once again decoupled
but-unlike the first-in an EC process followed by an electron
transfer E step. This sequence produces an unusual mixedvalent Rh"/Rh' intermediate with a comproportionation conand a series of fairly intense long-wavestant K, of about
length absorption bands in the near-infrared (NIR) region (Figure 2 ) . This intermediate differs from the much investigated
Creutz-Taube ion 1[ I c . 21 by being a d7/d8 [l 51 instead of a d5/d6
mixed-valent dimer. The higher reduced states of the bpip complex were too unstable for further characterization. In summary, the following general points can be made:
The ECE processes observed for the mononuclear systems['. can be split in various ways (E + EC, EC + E).
Acceptor ligands may act as passive electron storage sites
until the necessary number of electrons is collected and then
transferred to the potentially reactive site at the right potential for chemical reactivity.
There is clearly a "communication" between the organometallic reaction sites in the dinuclear rhodium complexes
bridged by bpip or bptz; the halide-releasing steps are separated by about 0.3 (L = bptz) and 0.5 V (L = bpip).
At this point the differences between the two systems are not
immediately apparent, this first systematic study thus produces
further questions related to the eventual construction of multielectron redox systems coupled with potentially catalytic chemical reactivity.
Received: April 24, 1997 [Z103441E]
German version: Angew. Chem. 1997, f09, 2600-2602
Keywords: cyclic voltammetry
transfer * reaction mechanisms
- electrochemistry - rhodium
[I] a) Ekctron Transfer in Inorganic, Organic and Biological Systems (Eds.: J. R.
Bolton, N. Mataga, G. McLendon), Adv. Chem. Ser. No. 228, ACS, Washington, 1991; see also chapter I in ref.131; b) H. Taube, Angew. Chem. 1984, 96,
315: A n g e k . Chem. Inr. Ed. Engl. 1984, 23, 329; c) C. Creutz, Prog. Inorg.
Cheni. 1983. 30. 1.
[2] C. Creutz. H. Taube, J Am. Chem. Soc. 1973, 95, 1086.
[3] D. Astruc, Electron Transfer and Radical Processes in Transition-Meral Chemi.ytrj, VCH, New k'ork, 1995.
[4] a) U. Kolle. M. Gratzel, Angew. Chem. 1987, 99, 572; Angew. Chem. In!. Ed.
Engl. 1987,26.568, b) U. Kolle, B:S. Kang, P. Infelta, P. Compte, M. Gratzel,
Chem. Bur 1989. 122, 1869; c) M. Ladwig, W. Kaim, J. Organomer. Chem.
1991, 419. 233; d) R. Reinhardt, W Kaim, 2. Anorg. Ali'g. Chem. 1993, 619,
1998; e) W Kaim, R. Reinhardt, E. Waldhor, J. Fiedler, J Organome!. Chem.
1996. 524. 195; f) W. Kaim, R. Reinhardt, M . Sieger, Inorg. Chem. 1994, 33,
4453: g) R. Reinhardt. J. Fees, A. Klein, M. Sieger, W. Kaim in Wasserstoffals
Energielrigu. VDI-Verlag, Diisseldorf, 1994, p. 133.
[5] a) D. Westerhausen, S. Herrmann, W. Hummel, E. Steckhan, Angew. Chem.
1992, 104. 1496; Angew Chem. Int Ed. Engl. 1992, 31, 1529; b) E. Steckhan,
S. Herrmann. R. Ruppert, J. Thommes, C. Wandrey, ibid. 1990, 102, 445 and
1990, 29, 388, c) C. Caix, S. Chardon-Noblat, A. Deronzier, R. Ziessel, J
Elecrrounul Chem. 1993,362,301. d) R. Ziessel, Angew. Chem. 1991,103, 863;
Angrpr. Chem. Int. Ed. Engl. 1991,30,844, e) C. Caix, S. Chardon-Noblat, A.
Deronzier, R Ziessel. J: Elec!roanai. Chem. 1996, 403, 189.
[6] a) S. Greulich. W. Kaim, A. Stange, H. Stoll, J Fiedler, S. Zalis, Inorg. Chem.
1996.35. 3998; b) R. Ziessel, J: Am. Chem. Soc. 1993,1/5,118;
c) M.Ladwig,
W. Kaim. J Organomer. Chem. 1992,439,79.
[7] a) General procedure: [{Cp*CI,Rh},] (250 mg, 0.4045 mmol) [4]was suspended In acetone (50 mL) and treated with AgPFs (204.5 mg, 0.809 mmol) and
stirred for 30 min. The resulting AgCl was filtered off and Bu,NPF, (620 mg,
1 6 mmol) was added to the filtrate. A solution of the bridging ligand (L = bpip
or bptz. 0.32 mmol) in acetone (ca. 20 mL) was then added. After the mixture
Angrn Chem Inr Ed Engl 1997,36, No 22
had been stirred for 6 h at room temperature the volume was reduced to about
20 mL and the light brown (L = bpip) or dark red (L = bptz) product was
completely precipitated by addition of pentane. Filtration and washing with
diethyl ether yielded about 70% of the product. Correct elemental analyses
(C,H,N). b) Thecomplexes are formed asmixtures ofcis and irunsisomers with
respect to CI and Cp* ligands. For L = bptz (ratio 4: 1) the NMR signals of the
minor component could not be completely identified due to partial overlap:
'H NMR (250 MHz, CD,NO,, 298 K): 6 = 1.98 (s, 30H; Cp*. major isomer),
1 99 (s, 30H; Cp*, minor isomer), 8.36(ddd, 2 H ; H5.5'). 8 71) (td, 2H; H4,4),
9.26 (m, 2H; H3,3'). 9.28 (m. 2 H ; H6,6); J(H3,H4) 7 8. J(H4,HS) 7.8,
J(HS,H6) 5 5, J(H3,H5) 1.2, J(H4,H6) 1.4 Hz. c) For L = bpip the cis and trans
isomers are formed in a 1:1 ratio that can be shifted to 1 . 1 by fractional
crystallization. As expected, cyclic voltammetry and spectroelectrochemistry
are not perceptibly affected by the changing isomer ratio. 'H NMR (250 MHz,
CD,NO,, 298 K): 6 = 1 65 (s, 30H; Cp*. isomer I), 1 63 (s. 30H, Cp*, isomer
11). 2.94 (s, 6 H ; CH,. isomer I), 2.98 (s, 6 H ; CH,, isomer 11). 7.65 (m. 4 H ;
"-H,"'-H, two isomers), 7.69 (m, 2 H ; P-H,P'-H, two isomers), 7.83 (m, 4 H ;
"-H." -H, two isomers), 9.64 (s, 2H, Pz-H;isomer I), 9.61 (s. 2H. Pz-H;isomer
[S] a) T. Stahl, V. Kasack, W. Kaim, J Chem. SOC.Perkin Dons. 2 1995, 2127;
b) J. Poppe, M. Moscherosch, W. Kaim, Inorg. Chem. 1993. 32, 2640.
[9] OTTLE cell (optically transparent thin-layer electrolytic cell): M. Krejcik. M.
Danek, F. Hart], J. Electroanai. Chem. 1991, 317, 179.
[lo] W. Kaim, Coord. Chem. Rev. 1987, 76, 187.
[l 11 The mononuclear complex [Cp*RhCl(bptz)](PF,) exhibits electrochemically
irreversible reduction at Epc= - 0.59 V vs. Cp,Fe+!Fco.
[12] a) D. Astruc, Chem. Rev. 1988, 88, 1189; b) Arc. Chem. R i x 1986, 19, 377.
[13] M. B. Robin, P. Day, Adv. Inorg. Chem. Radiochem. 1967. 10, 247.
[14]For diruthenium analogues see V. Kasack, W. Kaim. H. Binder, J. Jordanov,
E. Roth. Inorg. Chem. 1995, 34, 1924.
[15]A . Klein, W. Kaim, J. Fiedler, S. Zalis. Inorg. Chim. Actu. i n press.
Isolation and X-ray Structural Determination
of Both Folded and Twisted Conformers
of Bis(4H,SH-4-(dicyanomethylene)benzo[1,2-~:4,5-~']bis[1,2,5]thiadiazol-8-ylidene},
an Overcrowded Ethylene with High Electron
Takanori Suzuki,* Takanori Fukushima,
Tsutomu Miyashi, and Takashi Tsuji
Recently much attention has been focused on optically responsive molecules, which are interesting as molecular devices
or switches.[" Overcrowded ethylenes are one of the promising
candidates for this purpose,[" because they exhibit a characteristic color change associated with their conformational isomerism. The leading compound in this area is bianthrone (1),[31
which adopts the doubly folded conformation 1 A in the ground
state to reduce the steric repulsion in the fjord region.I4]Triggered by heating, photoexcitation, or pressure, this yellow conformer partly transforms into a green metastable
is believed to be the twisted conformation 1B.16' Because the
['I Prof. T. Suzuki, T. Fukushima,'*' Prof. T. Tsuji
Division of Chemistry, Graduate School of Science
Hokkaido University
Sapporo 060 (Japan)
Fax: Int. code +(11)746-2557
[ Permanent address: Tohoku University, Graduate School of Chemistry
Sendai (Japan)
Prof T. Miyashi
Department of Chemistry, Tohoku University, Sendai (Japan)
[**I We thank Prof. Tamotsu Inabe (Hokkaido University) for the use of the X-ray
structure analysis system and FT-IR microscope. T. F. thanks the Ministry of
Education, Science, and Culture, Japan for a research grant (No. 0067) and
JSPS Research Fellowships for Young Scientists.
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organometallic, reaction, couplings, center, ligand, mediated
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