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Towards the Definition of the Maximum Allowable Tightness of an Electron Transfer Transition State in the Reactions of Radical Anions and Alkyl Halides.

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[I 11 a ) 0 . Mitsunobu. Sxnlhesis 1981. 1--28 (review); b) 0.Mitsunobu, K . Kato. J.
titions and mechanistic crossovers in the reactions between radKimura. J. Am. Chrm. Soc. 1969. 91. 6510-6511; J. Kimura, Y. Fujisawa, T.
ical anions and alkyl halidesk2,31 The present communication
Yoshrzawa. K. Fukuda, 0 Mitsunobu. Bull. Chem. Sor. Jpn. 1979, 52, 1191 addresses
the fundamental questions by using the ET and SUB
1196.
reactions for combinations of formyl radical anions and methyl
[12] Yield of isolated product based on 4. The yields by N M R analysis were 37. 19,
and 35% for 3,4, and 5, respectively. Only one of two possible diastereomers
halides. We have found that within a distinct range of interwas obtained. which was determined to have the same configuration as that of
molecular distances the TS is transformed from an ET-TS into
the 1,2is-oxaphosphetane [lb] by NOE difference experiments. 5 : M.p. 58 a SUB-TS.
61 C (decomp); ' H N M R (500.1 MHz, CDCI,, 27'C. TMS): 6 = 3.41 (s,
Following our recent studies described in ref. [4], we investi3 H), 5.54 (d. 'J(H.P) = 18.9 Hz, 1 H). 6.30 (d, 'J(H.H) = 8.1 Hz. 2H). 6.54 (t.
'J(H.H) =7.2 Hz,1 H).h.71-6 74(m.ZH).7.13-7.18(m,4H).7.29-7.37(rn. gated a group of reactions between the substituted formyl radi4H),7.54-7.58(m,1H).7.75-785(m.3H).7.94(d.3J(H.H)=8.3Hz,lH),
cal anions and the methyl halides (systems I in Scheme 1). These
8.19 (dd. 'J(H.P) =11.7. 3J(H.H) = 8.0 Hz. 1 H): "P NMR (109 MHz,
CDCI,. 27'C. 85% H,PO,): 6 = - 33.0.
[13] The downfield shift was first reported by 3. C. Martin et 21.: 1. Granoth. J. C.
Martin. J A m . Chem. Sor. 1979, 101. 4618-4622.
[14] C,,H,,F,NOP.
M , = 621.56. crystal size 0.25 x 0.25 x 0.05 mm, monoclinic,
CHs + X space group C2/c, (I = 31.969(6). h = 8.857(7). L' = 22.270(4) A,
/l
= 1 11 41(2)". V = 587014) A3, Z = 8. [)rrlrd = 1.406 gcm 3. R = 0.056
( R , = 0.060). Crystallographic data (excluding structure factors) for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-179-22. Copies
of the data can be obtained free of charge on application to The Director,
CCDC, 12 Union Road. Cambridge CB2 1EZ. UK (fax: int code +(1223)
336-033; e-mail: techedcn chemcrys.cam.ac.uk).
[I 51 For tetracoordinate 1.2-azaphosphetidienes. see: a ) E. S. Gubnitskaya. 2. T.
Semashko, A. V. Kirsanov, Zh. Ohshrh Khim. 1978.48.2624; b) E. S. Gubnitskaya, V. S . Parkhomenko, Z. T. Semashko, L. 1 Samaray. Phosphorus Sulfur
1983. 15. 257--258: c) M.-J. Menu, Y. Dartiguenave, M. Dartiguenave. A.
Baceiredo, G. Bertrand, Phosphorus Sulfur Silimn 1990. 47. 327 -334; d ) K.
Afarinkia, J. 1 . G Cadogan, C. W. Rees. J. Chem Soc. Chmi. Cuniniun. 1992,
285-287. For pentacoordinate 1.2.3-diazaphosphetidines. see e) A. Schmidpeter. J. Luber, D. Schomburg, W. S. Sheldrick, Chem. Ber. 1976, 109. 3581 3587; f) H. W. Roesky, K. Ambrosius. W. S. Sheldrick, ;bid 1979. 112, 13651371; g) H. W. Roesky, K Ambrosius, M. Banek. W. S. Sheldrick. ;bid. 1980,
113, 1847-1854: h) 1. M. Abd-ellah. E. H. M. Ibrahim. A. N. El-khazandar.
Phosphorus Sulfur 1987. 29. 239-247.
[16] E. Vedejs, C. F. Marth, J Am. Chem. Soc. 1989. 111, 1519-1520.
[17] R Holler. H. Lischka, J. A m Chem. Soc. 1980, 102, 4632-4635: F. Volatron,
0 . Eisenstein. ihid. 1987. 109, 1-14; T. Naito, S. Nagase, H. Yamatka, ihid.
1994, 116, 10080-10088. Whether this reaction follows a C - P heterolysis
mechanism or not is open to further investigation For this mechanism, see:
H. J. Bestmann, Pure Appl. Cliem. 1980. 52, 771-788: H J. Bestmann. 0.
Vostrowsky, Top. Curr. Chern. 1983, / 0 9 , 85.
Towards the Definition of the Maximum
Allowable Tightness of an Electron Transfer
Transition State in the Reactions of Radical
Anions and Alkyl Halides""
G. Narahari Sastry, David Danovich, and
Sason Shaik*
How tightly bonded can the transition state (TS) of an electron transfer (ET) reaction be before it collapses to a classical TS
for a bond-forming reaction? In other words: is it possible to
define the minimum allowed separation o r the maximum allowed bonding between the reactants of an ET process? These
fundamental questions are related to the long-standing interest
in the mechanistic dichotomy between ET and nucleophilic substitution (SUB) processes,"' and to renewed interest in the
problem through the recent demonstrations of ETjSUB compe[*] Prof. Dr. S. Shaik. Dr. G N. Sastry, Dr. D. Danovich
Department of Organic Chemistry
and
[**I
The Fritz Haber Centre of Molecular Dynamics
The Hebrew Univerisity. Jerusalem 91904 (Israel)
Fax: Int. code +(2)658-5345
This research was sponsored by the Volkswagen-Stiftung.
i-
H
4+
i
1
CH,X
Y
c-sus,
1
a
b
Y
2
0'
\
X-
c
3
C
N
g
h
i
Scheme 1. Reactions studied.
systems undergo three types of reactions: 0-alkylation, C-oriented electron transfer (C-ET)14"] to give 2, and C-alkylation
(C-SUB) to give 3. While the 0-alkylation has generally a distinct TS, which has been discussed before,[41the C-ET and CSUB reactions are mutually exclusive and exhibit a mechanistic
crossover, C-ET + C-SUB, when the substituents on the formyl
radical anion and also the halides are varied. This communication focuses on the latter two processes and addresses precisely
the fundamental question posed at the outset.
All the calculations were performed with the Gaussian 92
program package.[51Geometries were gradient optimized and
characterized by frequency analysis. Mechanisms were ascertained by following the reaction path, by using the intrinsic
reaction coordinate (IRC) technique with internal (non-massweighted) coordinates, IRC(interna1) .I6, 'I Since spin contamination becomes at times significant along the UHF and UMP2
paths,[6b1the study was carried out with R O H F theory,[6a1
which is free of spin Contamination and gives geometric and
energetic results compatible with those of previous
The 6-31G* basis set was used for C, 0, H, and N atoms, while
for the halides we used the Hay and Wadt effective core potential (ECP)['] with the LANLIDZ valence basis set augmented
with two sets of uncontracted d-type polarization functions (CI:
ad = 0.150 and 0.375; Br: ad = 0.232 and 0.600; I : a,,= 0.270
and 0.730). A comparative all-electron study, which was carried
out for X = CI, using the 6-31G* basis set gave virtually identical results with those of the ECP basis set.
An ion-dipole cluster C, 4, precedes the transition state 5,
which proceeds either to the ET products 2 o r to the C-SUB
products 3. For each pair of reactants in Scheme 1 the C-attack
gave only one TS structure (5)either of the C-ET o r C-SUB
variety, but never both. The critical distances for the various C,
and transition state structures are given in Table 1 , In the transition from C, to TS the R,, distance decreases while R,, increases. For case f the TS is early and its C-I bond is shorter
than that in the cluster but clearly longer than that in the free
(IS7fi-0833~9~/3510-109X
$ /5.00+ 25/0
A n p w . Chern. In[. Ed. EnRI. 1996, 35. No. 10
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radical anion. were found to have transition states that correspond to the C-SUB variety, resulting in C-alkylation. Further
increase of the acceptor ability of the alkyl halide to X = I in
system i led to an ET-TS structure even with Y = CN.["] Apparently,
a mutually exclusive competition exists between the
Y
ET
and
SUB
mechanisms along the same C - C - X trajectory of
4. CR
5 . TS
attack. and the entire group of reactions define therefore a
mechanistic spectrum with an ET/SUB changeover.
Table 2 also gives the computed energies, entropies of activaTable I . Important structural parameters [A] of TS 5 and C, 4 for the reactions in
Scheme I
tion, and the s(-C"/C'~kinetic isotope effects. The activation
barriers AE* depend on X and Y with the following orders:
System Y
X
&<>
RCX
Rcc
Cl > Br > I for X. and H < Me cc CN for Y. Ail the TS strucC,
TS
C,
TS
C,
TS
tures contain C-C bonding, as reflected in large values for the
1.2X7
1.262
a
H Cl[,i]
1.818
2.101
3 289
2.529
entropy of activation for all the reactions and in normal x (1.849) (2.072)
(I ,287) ( I .265)
(3.316) (2.572)
ClZiCl3isotope effects.['*] Even in the "loosest" TS structure in
1.288
1.269
3.291
2.679
2.031
2.169
b
k1
Br
the group, 5c, exhibits significant bonding, as implied by the
2.200
2.285
1 2Y0
1.277
3.336
2.846
C
H
I
normal S ( - C ' ~ / Cisotope
'~
effect.
1.262
1.815
2.108
3.343
2.524
d
Me CI [.I] 1.2XY
(1.822) (2.081)
(1.2Y0) (1.265)
(3.360) (2.565)
The mechanistic selection follows from a recent valence bond
2.028
2.179
1.290
1.268
3.333
2.667
C
Me Br
configuration mixing (VBCM) treatment" of the problem in
1.292
1.276
2.311
f
Me 1
2 296
3.236
2.820
terms of three principal VB configurations, which for brevity
12.1 51J [b]
may be described by the occupancy of the nco and n:o fragment
3.693
2.250
1.269
1.261
1.816
2.326
g
CNCl['l]
(1.269) (1.259)
(3.680) (2.314)
(1.824) (2.297)
orbitals of the formyl group, and ocxand o& orbitals of the
1269
1.256
1.992
2.424
3.664
2.370
h
C" Br
alkyl halide moiety. These are: the reactant configuration
1270
1.252
3.630
2.481
2.204
2.567
I
CN I
(&,
ofx)the charge transfer configuration (rtf, ofxo,*i)
[a] Value.; in the parentheses were obtained with the all-electron 6-31C* basis set.
and the substitution configuration (&
ofx 0:;).
The sub[b] The \iilue in square bracketscorresponds to the length o f t h e C - I bond in CHJ.
stitution configuration involves triplet excitation of the formyl
moiety from n,, to R : ~ in addition to an electron transfer to the
o& orbital. It is the pairing of the two 7t electrons of the triplet
reactant. CH31. The decrease in R,, promotes the C-ET process
formyl moiety with the electron in the orxorbital that is eventu(acting to eject the electron from the formyl radical anion), while
ally responsible for formation of the new C-C bond in the
the increase in & is common to both C-ET and C-SUB processsubstitution product. At the geometry of the reactants, the eneres. The IRC(interna1) analysis for the C-ET process shows initialgy of the CT state lies above that of the reactant state by the
ly a C--C approach followed by C-C recoil. In turn, the C-C
vertical electron transfer energy gap GET.The vertical energy
recoil is synchronized with C - X bond cleavage; hence this is a
gap of the substitution state, G,,,, is obtained by adding the
dissociative and bonded C-ET process. In contrast. the IRC(intertriplet 3 7 c ~ * excitation energy of the formyl moiety to GE.r.This
nal) analysis for the C-SUB process shows a continuous C-C
triplet 371n* excitation energy for formaldehyde and methyl
bond formation synchronized with C-X bond cleavage; hence
formaldehyde is much larger than that for cyanoformaldehyde,
this is a substitution mechanism. Both types ofTSs are connected
and therefore the CN(H)C=O'-/CH,X reactant pairs are typito cluster C, in the reverse direction of IRC. Thus, the IRC results
fied both by a higher lying charge transfer configuration and a
for the two processes are distinct and characteristic.
more closely lying substitution Configuration than the configuAs specified in Table 2, the TS structures for systems a-f
rations of the corresponding pairs, H,C=O'-/CH,X
and
correspond to ET-TS structures. In contrast, systems g and h,
CH,(H)C=O' -/CH,X.
which involve the poorest electron donor, cyanoformaldehyde
These three configurations cross and mix with each other
along the C-C-X trajectory." I h l Consider first the crossing of
Table 2. Thcrinochemical quantities [a], entropies of activation [b. c], and x-Ci2/C" kinetic the reactant and CT configurations in terms of two key quantiisotope effect, [c. d]
ties shown in Table 2, the reaction exothermicity AEETand the
vertical electron transfer energy
-. gap
- . GpT.
._.A large
- G,,
... and weakSystem Y
X
Process AE* BEFT[el A&,, [el G,,[f.h]
A S * s-C'"C''
ly exothermic A& will lead to a tighter crossing point with a
a
H
CI [g] [GET
6.4
-39.1
-61.3
80.0
-24.9
1.056
small intermolecular distance R,.__ In turn, the smaller the C-C
( 5 5 ) (-48.0) (-70.2)
(72.4) (-25.1) (1.059)
distance at the crossing point is, the greater the contribution of
b
H
Br
C'-ET
2.8
-50.8
-73.0
45.7
-24.8
1.052
the
substitution configuration will be, until the point at which its
E
H
i
C-ET
0.6
-66.7
-88.9
21.5
-24.9
1.028
d
Me cI [,,I c.ET
7,9
-42.4
-57.x
78.1
-25,7
1.057
mixing with the reactant configuration prevails and the corre( 5 8 ) (-51.3) (-66.7)
(70.5) ( - 2 5 8 ) (1.058)
sponding structure collapses accordingly to a SUB-TS. This
e
Me Br
C-ET
3.4
-54.1
-69.6
43.9
-25.5
1.054
effect of the substitution configuration will become significant
f
Mc I
C-ET
1.4
-70.0
-85.5
19.6
-25.6
1.036
for the CN(H)C=O'-/CH,Cl and CN(H)C=O'- 'CH,Br pairs;
7.5
-15.8
112.6
-27.7
1.057
g
CN C'I [g] ('-SUB 21.7
(18.7) (-1.4)
(-24.7)
(105.0) (-26.9) (1.060)
the mixing of the substitution configuration will be enhanced by
h
CN Br
('-SUB 14.3
-4.3
-27.5
78.3
-261
1.056
both the large GET gaps and the low triplet 3nx* excitation
1
CN I
('-ET
9.4
-20.2
-43.4
54 1
-24.6
1.052
energy of cyanoformaldehyde,
Since G,,, depends on GET,a combination of GETand AEET
[a] In kcalniol-'. [b] The standard state is 1 Mat 298.15K. [c] The parameters correspond
scaled bv the recommended factor0.8929 to obtain variations can Serve as the minimum necessary predictor of both
J. ~ d Frequenciesare
i
to the process I
the thermochemical v.d& [el AEquantities refer to the process from separated reactants the C - c distance at the TS as well as in the mechanistic changeI 10 separxted product.r 2 or 3. [f] GrT corresponds to the energy difference between the Over C-ET + C-SUB , The factor
= 0.5G,/(GET P A E E T ) '
ground state Y H C = O ' - CH,X and the charge transfer state YHC=O;CH,X'- at the
derived from the Marcus expression,"'] gives effectively the
reactants geometry. [g] Values in parentheses were obtained with the all-electron 6-31C*
b,'isisset.
,.
[h] G,,,,, = C,, +AE,,;(YHC=O). Note that AE,,:(YHC=O) ismuch smaller for appropriate 'Ombination
Of GET and A E t 3 that can gauge the
earliness of the TS in terms of its R,, distance. Figure 1 is a plot
Y = CN than for Y = H. CH3.
@A
-.
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OO0t
i lT
SUB
0 100
0200
0400
0300
0500
0600
+
aFig. 1. The variation of C C distances at the TS as a function of x. Filled circles:
results of calculations with the ECP basis set; asterisks: results ofcalculations with
the 6-31G* basis set (systems a', d',and g'), gray band: area of mechanistic changeover.
of the R,, distances, calculated for the transition states of the
reactions of systems I , against the index a. It is seen that the
C - C distances in the transition states decrease linearly as a
increases; in the ET-TS structures, RC,>2.4S1
while in the
SUB-TS structures. R,, 2 = 2.370 A. These two critical values,
2.370 and 2.481 A, define the mechanistic changeover zone from
the C-ET to C-SUB, over a narrow range of approximately
0.1 A. It follows therefore that the ETjSUB dichotomy characterized by Figure 1 corresponds to two mutually exclusive mechanisms that exhibit a continuous mechanistic spectrum with a
narrow changeover zone from C-ET to C-SUB. Thus, while TS
earliness is gauged by the Marcus parameter of the ET process,
the C-ET + C-SUB mechanistic changeover itself originates in
the multiconfigurational nature of the problem, which is chardcterized by the VB mixing of at least three VB configurations.["]
We have described here the first structural foundations for the
long-sought['". b, 31 ETjSUB dichotomy in a set of reactions of
radical anions (A*-) and alkyl halides (RX). There seems to be
a continuous spectrum of distances between the reactants in the
ET-TS, and at a certain minimum distance there is a mechanistic
changeover to the SUB-TS. Thus, the ET series up to the
changeover zone exhibits an ET-TS with variable C-C bonding
ranging from weak bonding in cases c and f, to significant bonding in cases a, d, and i. Of course the choice of the computational
method may shift the changeover zone, but nevertheless, the
sharpness of the zone indicates that ET-TS structures can indeed
be strongly bonded and structurally adjacent to their SUB
analogs. The critical zone may of course depend on the nature
of the A'-/RX family; each family may exhibit i t s own characteristic changeover zone, and its own point of maximum bonding for the ET-TS.
A,
Received: Noveinber 17, 1995 [Z8564IE]
German version: Angew. Chem 1996. 108, 1208-1211
Keywords: a b initio calculations
anions * reaction mechanisms
-
electron transfer
. radical
[ l ] a) L. Eberson, Elutron Tuunsfer Rcuctions in Orgunic ChrrnDtry, Springer,
Heidelberg, 1987; b) A. Pross, Ace. C'hem. R e . 1985, 18, 212; c) M. Chanon,
BUN. So<.Chin?. A. 1982, Part II, p. 197.
[2] a) N . Kimura, S. Takamuku, Bull. Chem. Soc. Jpn. 1992.65,1668; b) ihid. 1993,
66,3613; c) .I An?. Chen7. Sue. 1994, 116, 4087.
1100
5.'
VCll Verlug.sgesrilrchoffrmhH, 0-69451 Wonheim, 1996
[3] a) K . Daasberg, T. B. Christensen, Acla Chem. Scund. 1995. 49, 128; b) K.
Daasberg, S. U. Pedersen, H. Lund. ihrd 1991.45,424; c) H. Lund, K. Daasberg, T. Lund, S. U. Pedersen, Acc. Chem. Ray. 1995, 28, 313, and references
Org. Cheni 1990. 26, 1
therein; d) JLM. Savkant, A d v . P/ZJ,S.
[4] a) G . N. Sastry, S. Shaik, J. Am. C'hem. Soc. 1995, 117, 3290; b) G. N. Sastry.
A. C. Reddy, S. Shaik. AIZ,QC~IV.
Chem. 1995, 107, 1619; Aizjini, Chenz. In,. Ed.
Engl. 1995.34, 1495.
[S] M. J. Frisch, G. W. Trucks, M. Head-Gordon, P M W. Gill, M. W. Wong. J. B.
Foresman. B. G. Johnson, H. B. Schlegel, M. A. Robb, E. S. Replogle, R.
Gomperts, J. L. Andres, K . Raghavachari. J. S. Binkley, C. Gonzalez, R. L.
Martin, D. J. Fox, D. J. Defrees, J. Baker, J. J. P. Stewart, J. A. Pople, Gaussian 92, Rcvision C3, Gaussian, Inc., Pittsburgh, PA, 1992.
[6] The term IRC(interna1) is used as in ref. IS]. The reaction pathways were
examined also by means of IRC with mass-weighted (MW) coordinates [a],
IRC(MW). It was found that the energy along the IRCfinternal) path is consistently lower than that along the IRC(MW) path in all the cases. Hence the
IRC(interna1) path qualifies as the path of steepest descent and was therefore
used throughout to designate the mechanisms ofthe reactions. a) At the R O H F
level both IRC(interna1) and IRC(MW) gave identical mechanistic conclusions
for the combinations a - e , g, and h with both the 6-310* and the ECP basis sets
G*, 6-31G**,
(reaction a was tested with many other basis sets-6-31
6-311G*, and 6-311G**-all of which led to the same conclusion, an ET
mechanism). b) In U H F (for a, c, g) and UMP2/6-31G* (for a, g), the IRC(internal) path has a rather small spin contamination throughout and leads to the
same mechanistic conclusions as the R O H F path In contrast, the IRC(MW)
path passes through highly spin-contaminated regions (for example
( S 2 ) = 0.875 for reaction a) and produces nonsmooth energy profiles tcrminating at C-SUB products. For this reason the R O H P level was preferred over the
U H F and UMP2 level in the present study. c) For the reaction in system i
(Y = CN, X = I) the IRC(MW) at the R O H F level gave a nonsmooth profile
resulting in C-SUB, while IRC(interna1) produced a consistently steeper path
and a smooth energy profile leading to ET. As the path of steepest descent
starting from 5i Icads to ET products, 5i was designated as an ET-TS.
[7] C. Gonzalez, H . B Schlegel, J Chem. P h p . 1989, 90, 2154.
[8] C. Gonzalez, H. B. Schlegel, J Phvs. Chem. 1990. 94, 5523.
(91 a) P. J. Hay. W. R Wadt, J Chern. PIzjs. 1985.82.270; b) W. R. Wadt, P. J. Hay,
ihirl 1985,82, 284; c) P. J. Hay. W R. Wadt, h i . 1985,X2, 299.
[lo] B. S. Axelsson, B. LBngstrom, 0 Matsson, J. Am. t h m z . Suc. 1987, 109.7233.
[ I l l a) L. Eberson, S. S. Shaik, J. Am. Chern. So?. 1990, 112, 4484. b) Based on
Figure 4 in ref. [l la]. The C T state and reactant configurations mix by optimization of then&, -a& overlap, while thc substitution and reactant configurations mix by optimization of the riro a& overlap. Hence all three configurations mix along the C C X trajectory
[12] The Marcus expression is given by T = 0.5(1 +AE,,/i.). where 1 =
Gr, -A&.
Substituting for 2 and rearranging leads to c( = 0.5G,/
(GET-A&).
Synthesis, Structure, Photophysics, and
Excited-State Redox Properties
of the Novel Luminescent Tetranuclear
Acetylidocopper(1) Complex
ICu4(~-dPPm)4(~4-r1,r2-C~C-)l
(BF4)2**
Vivian Wing-Wah Yarn,* Wendy Kit-Mai Fung, and
Kung-Kai Cheung
There has been a growing interest in the synthesis of alkynyl
metal complexes because of their potential applications as nonlinear optical materials and molecular wires."] Moreover, the
ability of acetylido ligands to bond to transition metals in a
number of different bonding modes to form a variety of mono[*] Dr. V W-W. Yam, W. K.-M. Fung, Dr. K.-K. Cheung
Department of Chemistry
The University of Hong Kong
Pokfulam Road, Hong Kong (Hong Kong)
Fax: Int. code +(2)857-I586
e-mail: wwyam(n,hkucc.hku.hk
[**I V. W.-W. Y. acknowledges financial support from the Research Grants Council,
the Croucher Foundation, and The University of Hong Kong. W. K.-M. F.
acknowledges the receipt of a postgraduate studentship, administered by The
University of Hong Kong. dppm = bis(dipheny1phosphano)methane.
0570-0833196~3510-1100
$ 15.00f .25/0
Angew. Chem 1/11. Ed. Engl. 1996, 35, N o . 10
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