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Electrochemical Evidence for Through-Space Orbital Interactions in Spiromethanofullerenes.

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tially separate components. The role of ion-molecule complexes has long been postulated in mass ~pectrometry,['~]
but was
based. in general. on indirect evidence from the fragmentation
pattern of excited species from the ionization of molecules in
which the putative components of the complex were initially
covalently bound. In the specific case of alkylbenzenes.
irrefutable mass spectrometric evidence is lacking for complexes
such iis 2; results of molecular orbital ~ a l c u l a t i o n s ' ~do
" ~ not
appear inconsistent with ours.
The gmcous s:iinpIc\ fix the radiolytic experimenls were prepared in se;iled 135 mL
glass \cssels. iri-.idiatrd in a 220 Gainmacell (Nuclear Canada Ltd.). fitted with a
thernio\tat. The gaseh and the chemicals used were research-grade products from
coininti-cia1source\. The radiolytic products were analyzed h) GC;MS (50 m long.
0.2 inin i . d f u x d d i c a column, coated with a 0.5 pm methylsilicons tilni: VG
Micronia\?, T R I O quadrupolc mass spectrometer). The FT-ICR experiments were
pcrlorinrd with .I Brukcr Spectrospin Apex T M 47e spectrometer equipped with a n
e x t e r i i i ion miircc
Received: March 1. 1995 [Z77541E]
German venion Angeilc C h w . 1995. 107. 1719 1721
Keywords: alkylations aromatic substitutions
chemistry . kinetics * ICR spectroscopy
*
gas-phase
[ I ] C I:riedcl, J. M. Crafts, Bid/. S U C(%ini. Fr 1877. 27. 530: Fri&-Crcl/rs irnd
f + / o i d K ~ w r f o ~ (Ed.'
is
G. A. Olali). Wiley-Interscience, New York. 1963,
G A. 0l:ih. Fricd& Cr-rrfi\ <'/mmtrv. Wiley-Interscience. New York. 1973;
K M . Roherls. A. A. Khalah, Friefle/-Cra/rs A/ky/ur;un Cherin.srr~~.Dekker,
New Yoi-k. 1984: R . Taylor, Electruphilic Aronluric Sub.uifuiioii. Wiley. Nen
Y w k . 1990
[2] (i. A 0l;ih. Abstract of Papers 3P. 133th Meeting of the American Chemical
New York. 1960: see ref. [I]
cc. . l ( c . (%mi.
Ke.s. 1988. 21. 215.
(41 (',,H,, leaking from the external source is actually present in traces within the
cell. hut 11 cannot he a source ofdetectable C,H,,D+ because o f i t s low partial
presure (lc\\ than 1 W q Torr) and the [C,H,];[C,H,] ratio greater than 5 x 10".
151 J. L. Heauchamp, D Holz. S. D. Woodgate. S.1 . Patt, J A m . Clieni. Suc. 1972.
94 2718: .I ltollis. J. M. Tedder. G. S. Walker. J. Chwn. Soc. Perkiri T,.OIIS.2
~
1991. 11x7
[6] M Sper;in/,i. N . Pepe. K.Cipollini. J Clrnir SW. PerXin fiuru. 2 1979, 1179.
171 Depcnding oii the composition of the sqstem. the yields of C,D,CH,, exp r c s w i h! !he corresponding G-b, values (number of molecules formed per
I 0 0 cV ah\oi-hrd) range from 0.3 to 0 4.
[XI 1 E. S/ulclko. T U. McMahon. J Am. C/rt,i?i.SO<.1993. 115 . 7839.
[9]The \uhstitutioii pattern at 37.5 C in C,H, at 730Torr is u : n i : p = 43 31 : 3 h :
M . Attinit. 1- Cacace. G . Ciranni. P. Giacomcllo. J A m . C/im77.S I J ~1977.
..
YY,
261 1 .
[lo] Tlieproloiiaftinityof~~C,H,,,is
189 kca11nol~~:S.
G. L1as.J. E. Bar1mess.J F.
I ichiixin. J L . Holmes, R. D. Levin. W. G. Mallard. J. P / I L LClteiii. Kc,/ D i m
1988. 17. Suppl. I.
[ I l l M . At1iii;i. 1- Cncacc. A Di Marzio. J ,4111. C/iwi. Soc. 1989. I l l . 6004.
1171 K ('ipolliiii. G. Lilla. N.Pepe. M Speriinza, J. P/II..\. (%cI?I. 1978. 87. 1207.
[I31 I~I) Boiicii. A ( < . .C/JCIII.
RPJ.1991. 2 4 , 364: A G . Harrison. Can. J. C h ~ f l J .
1985.64. 1051: R . W. Ho1man.M. L GI-osa,J. Ant Chrrii..So~.1989. 111.3560:
I ) Kuck. ilcfu . S p w / r f i i ? i . R?i, 1990, Y. 586.
[I41 I) Bcrthoinicu, V. Brenner. G . Ohanessian. J. P Denher. P. Millie. H. E. Auciier. .I ,-JIII. ( ' / w i n . SW. 1993. 1 1 5 . 2505.
Electrochemical Evidence for Through-Space
Orbital Interactions in Spiromethanofullerenes**
Matthias Eiermann, Robert C. Haddon, Brian Knight,
Q. Chan Li, Michele Maggini, Nazario Martin,
Toshinobu Ohno, Maurizio Prato, Toshiyasu Suzuki,
and Fred Wudl*
While buckminsterfullerene Ch0[l1can be converted efficiently into a remarkable number of methanofullerenes and
fulleroids.[21gaining control over its electronic properties still is
an interesting and demanding goal[31which might open up a
broad variety of applications similar to achievements in classical
benzene chemistry.
The cyclic voltammetry (CV) data of diphenylmethanofullerenes 1 with electron-donating groups (EDGs) and electron-withdrawing groups (EWGs) in the p,p' positions of the
phenyl groups are independent of
the functional groups.[41 In conR
trast, spiroannelated analogues
show remarkable substituent effects which we suggested calling
"periconj~gation".~'~
Here we describe our experimental evidence
obtained from 1,I -(4-oxocyclohexa-2.5-dieno)- and 9.9-fluorenofullerenes and present the
conclusions derived from the observations.
1 (R = EWG, EDG)
The most direct method for
studying the influence of addends
on the electronic properties of fullerene C,,) is cyclic voltammetry. The CV measurements were carried out in o-dichlorobenzene. because the spiroannelated methanofullerenes are most
soluble in this
and four reduction waves of C,, can be
detected. Since various irreversible processes can be observed
in some of the cyclic voltammograms, peak potentials rather
than the averaged data are given in Table 1 and in the discussion. All compounds described were prepared by known methods,[2.71
Since the 5;6- and 6,'6-isomers of 1,1-(4-0xocyclohexano)fullerene. 2 a and 2 b, respectively, can be separated. both could
be characterized by CV individually. Their first reductions are
fully reversible and occur at - 1158 and - 1164 mV (peak potentials, vs. ferroceneiferrocenium), respectively. well within the
expected range for "standard" methanofullerenes.['.
In comparison, the reduction is facilitated by more than 70 mV (vs. 2 b)
[*IProf. Dr. F. Wudl. Dr
[**I
M. Eiernidnn. B. Knight. Q. C . I I. N Martin. T Ohno.
T. Sumki
Institute for Polyiners mid Organic Solids
Departments of Physics and Chemistry. University 0 1 California
Santa Barbara, CA 93106 (USA)
Telefax: Int code + (805)893-4755
e-inail: wudl:o physics.ucsb edu
R . C Haddon
A T h T Bell Laboratories, Murray Hill, N J (USA)
M. Maggini
Centro Meccanisini Reiizioni Organiche dcl C N R
Dipartiniento di Chimica Organica. P a d o w (Jtaly)
M. Prlito
Dipartiinento di Scien7e Farmaceutiche. Trieste ( I t a l ? )
Thia work was partially supported by the M R L Program and the National
Science Foundation (award DMR-91-3048 and grants LIMR-91-22536. DMR91 -11097, and CHE-89-08323) M. E. is indehtcd t o the Alexander-von-Humboldt-Stiftung. and to Prof. P. C . Ford, UCSB, for support (Feodor-Lynen
fellowship) M. P. and F. W. are indebted to NATO fbi- 'i travel grant. N . M i h
indebted to Universidad Coinplutense de Madrid for a " D c l Amo" fellowship.
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Table 1 . Peak potentials E l o r reductions (in mV vs. ferrocene:ferrocentum) for C,
compounds 1- 13 from their cyclic voltammograms
Cvd.
E,L.rrdi
El,,.,,,
c,,,
-11'23
I ( R = H ) -1151
2a
-1158
2b
-1164
3
-10x1 [h, f]
4
1097 [el
5
-1042[a]
6
-1143
7[12]
-1176
8
-1070[a]
9
-1175
I O [ I ~ ] -1249
I1
-1040[a]
12 [12]
-1150 [a]
13
-1073 Id]
-
-1455
-1531
-1550
-1554
-1559 [f]
- 1620 [d]
-1197(d]
-1501
-1541 [a.h]
-1383[a]
-1479
-1590 [a C]
-1352[d]
-1510 [a]
-1368 [a]
and
~ ~ ~ 2 , . c o l
-1913. -2383
-2031
-2010. -2379[d]
-2044. -2504
principle of fluorenofullerenes applicable to the conversion of a
mono- into a bisadduct, no other regiochemical control besides
steric repulsion seems to direct the location of the second addition relative to the first.['] Thus all bis(fluoren0)fullerenes were
obtained as mixtures of five (12: six) regioisomers,['O1 which
could be separated only partially by HPLC (Scheme 1 ) .
- 2037
-1602, -2016
-1703. -2073
-1915. -2070
-1516[d], -1942[d]. -2092[d], -2477[d]
-1615. -2235
-2045. -2311
-1523[d], -1933[d], -2045[d]. -2444[d]
-1627 [d], -1894 [d], -2063 Id]. -2293 [dl
-1573 [d]. -2010 [d], -2391 [d]
R * R
[a] Irre\ersible. [b] Peak position depends on scan rate. [c] Structured a t low scan rates.
[d] Reversihility could not he determined unambiguously. [el Two-electron process.
[f] Scan rate I 0 m V s - l .
in the 616 adduct 3, which has two double bonds in the cyclohexane ring. The di-rerr-butyl and dimethyl derivatives 4 and 5,
respectively, exhibit the same behavior (Table 1); however, the
reduction of the latter is irreversible.
&
0
0
I
-\
R
~
'/
Schemc 1. Possihlc positions for ;I second addend on the "south pole" ( u ) , on the
"equator" ( h l . h2). and in between ( c , three possihle positions)[lO]. For clarity.
symmetry-equivalent positions have been omitted.
The CV spectra of the spiroaiinelated methanofullerenes
clearly demonstrate the striking influence of the addends on the
C,, moiety. The positions (Table 1 ) of the first two reduction
waves correlate with the electronic demand of the fluorenyl
groups." ' I In the unsubstituted and the exclusively EDG-substiR
tuted[12Jfluorenofullerenes (6,7,9, lo), the first and the second
reduction waves are strictly electrochemically reversible. The
uptake o f a third electron is electrochemically irreversible,
exhibiting a scan rate dependent gap between reduction and
reoxidation (Fig. 1).
The spiroannelated methanofullerenes bearing EWGs (8,
11 13) give rise to CV curves that are more difficult to resolve.
-
2b
2a
3:
R=H
4: R = t B u
5: R = M e
All fluorenofullerenes 6 - 13 described in the following could
be isolated exclusively as adducts on 6/6 junctions; no 5/6 isomers were detectable. Assuming that this is a general structural
R R-
R - R
6: R = H
7: R = N B n Z
8: R = N 0 2
9: R = H , R = H
10: R = NBn2, R
= NBn2
11: R = N O z , R ' = N O z
12: R = NBn2, R' = NO2
13
+I:Od &:40
b:204.'8011.40
-2'.d 12.b
u / v (VS. FC/FC+)Fig. 1. Cyclic voltammograms of 10 (upper scan: 10mVsC': lower scan:
100 tnVs- I : potential YS. ferroceneiferrocenium (Fc:Fc')).
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t
,+'
I
+0.800
0.000
i
1
i
IIpA
MNDO calculations on the spirocyclopentadiene compound 14 were used
to model the interaction. In the fully
optimized structure, which gives a
transannular
cyclopropane bond
length of 1.602 A,[141the HOMO is
bonding between the cyclopentadiene
and C,, residues (Fig. 4). The LUMO
has u , symmetry and is followed in energy by the h, and h , orbitals. When
the length of the transannular cyclo14
propane bond is constrained to
Fig. 4. HOMO interac2.0 A, the energy is increased by
tioii in cyc1opent;idieno13 kcalmol-' and the HOMO is no
f~illerene14. The remainlonger bonding between the two moiing ~itomicorbitals coneties. The h, orbital then becomes the
tributing t o the molecular
HOMO are left out for
LUMO followed by the a , and h, orclar I t )
bitals. Since the h , orbital is antibonding with respect to the transannular cyclopropane bond, its population (e.g. by electrochemical
reduction) will weaken the bond. These frontier orbital interactions may account for the complex findings in the electrochemical studies of 6-13.
The ability to control the electron acceptor and donor
strength of rnethanofullerenes-a
fine-tuning of the electrochemical properties-opens up more possibilities for fullerene
modification. Current research is aimed at an improved theoretical understanding of the effects as well as application of the
results described here to the preparation of more electropositive
and electronegative methanofullerenes.
i
i
i
+1:00 lo.40 -0.'20
h,:So-1."
i : o d 12.60
u I v (VS. FC~FC+)pig 7 ~ y c ~ iwltammograma
c
of 12 (upper scan: 1 0 m V 5 . ' ; lower scan:
100 m V s I . potential \s. Fc'Fc')
~
The first. second, and third reduction peaks are electrochemically irreversible. with complicated multiple reoxidation waves
(Fig. 3 ) . This behavior is not yet fully understood and will he the
subject of further investigations; however it may be ascribed to
the opening of a cyclopropane ring, as shown in Scheme 2.
Receiwd Fehrual-) 8. I995 [Z 7701 IE]
German version: A n g e u . Chrm 1995. /(17. 1733-1735
-+
Schcmc 2
The crucial structural difference between 9,9-fluorenofullerenes 6- 13 and (dipheny1methano)fullerenes 1 is that in the latter
the phenyl rings are free to rotate but prefer to lie "parallel" to
thc fullerenc surface, whereas the aromatic rings in 6-13 are
held rigidly perpendicular to the surface of the spheroid. The
cyclohexadienone derivatives 3-5 have similar structures.
Models show that there is a likelihood of unusual periconjugation, different from s p i r o ~ o n j u g a t i o n . [between
'~~
the p; orbitals
of the addends and the "p:" orbitals of the fullerene carbon
atoms ad-jacent to the bridge atom. Even though a spiro atom
also scparatcs the 7c systems in 3- 13, the mode of interaction of
the corresponding p L orbitals is geometrically completely different from spiroconjugation. This can be easily seen by looking at
the relative orientations of the atomic orbitals involved (Fig. 3).
Fig. 3. Modes (11.0rhital interaction in spiroconjugatlon (left) and periconjugation
(ripht).
I
Keywords: electrochemistry . fullerenes . through-space
interactions
[I] W. Kritschmer. L. D. Lamb. K Fostiropoulos. D. R. Huffrnan.
.Vufim ( l o t i d o n ) 1990. 347. 354: R. E Haullei-. .I. Conceicao.
L. P. F Chihante, Y Chai. N . E. Byrne. S.Flanagan, M. M Haley,
S. C. O'Brien. C . Pan. Z. Xiao. W. E. Billup, \I. A Ciufolini, R. H
Hauge, J. L. Margrave, L. J. Wilson. R. F. ('url. R E. Smalley. J
Phjs. (%em. 1990. 94, 8634.
[2] T. Suruki. Q Li, K. C. Khemani, F. Wudl. 0. Almarswn. Screiicr 1991, 354,
1186. F. Wudl. A r c . Climi. Rer. 1992. 25. 157: M. Prato. V Lucchini. M.
Maggini. E. Stimpfl, G. Scorrano. M . Eiermann, T S u i u k i . F. Wudl. J .4m.
C h i . Sot. 1993. /is.8476. and references therein.: E Diedci-ich. L Isaacs. D.
Philp, C/wii. Soc R e , . 1994. 243.
[3] M.Keshavarr-K.. B. Knight. E Wudl. .I Org. C'/im.. submitted
[4] However. the waves are shifted by 30-90 mV to inore negatibc potentials
relative to those of C,,,
[ 5 ] F. Wudl. T.Suzukr, M . Prato. Ssnih. Mei. 1993. 59.297 Thc CV data given i n
this paper were measured in THF.
[6] CV conditions. 0.15 M (nBu),NBF, in o-dichlorobenrcnc. 25 C. Workingrlectrode: Pt disk (2 mm diameter). Counter electrode Pt wire Reference
electrode: aq. Ag..A$CI (Fc/Fc' iiiternal standard). Polentio~tat:BAS-IOOA.
scan rate 100 mVsC'. Other standard CV solvents (THF. acetonitrile) a s well
as toluene;acetonilrile (Q Xis. E. Perez-Cordero. L. Ec1icgo)en. J . h i . C/im.
So? 1992. 114. 3978) did not provide sufficient solubity for all compounds of
our study
(71 All compounds g.ive satisfactor) spectroscopic and .inol) tical data. As an
example, the datii of 10 and 13 are gi\cn. 10: C,4211,,8!i,:.21, = I
' H N M R (CS~;[D,,]c)clohexane 10.1): A = 4.30 4.85 (m.I 6 H . Ph
6.65 7 55 (m. 48H. phenyl-H. fluorene-H). 7.80 8 50 (10d. .I = 1 Hr. 4 H ,
fluorene-I-H. -8-ti):
N M R (CS,[D,l]c~clohexane ill. I ) : A = 55.0 56.0
( 5 peaks). 76.9-798 (9 pcaka). 110.2-11 1.6 (9 peaks). 113 1 - I13 7 ( 5 peaks).
I 19 5 119.8 ( 5 peaks), 176.8 127.8 (10 praks). 128.7 129.3 ( 4 peaks), 131.4
132.5 ( X peaks), 137.6--149.0 (77 peak\): FAB MS. ni :1837 1879. FT-IR
(KBr): i. =I610 (m), 1570 ( w ) , 1495 (m). 1485 (s), 1431 (m),1355 ( w ) . 1230
(w).960 (w).800 (w). 730 (w). 695 (w)~
530 (w)c n i ~I : LiV,Vis (cyclohexane):
i,,, = 221. 145. 324. 344 sh, 430 sh, 470. 569 ah. 623 sh. 697 nin Anal. calcd.
for C,,,H,,N,.
C 93.19. H 3.75. N 3.06. found: C 93 11. H 3.78. N 799.
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13. C-,H,N,; ,M, = 886.85; ' H N M R (CS,:CDCI,): d =7.48 (dd. J = 8.1.
gen or r-oxygen substituents (R' R'CXLi) have "carbenoid"
4.4Hz,2H),8.92(dd.J=4.4,0.9Hz.3H).9.08(dd,J=X.1,0.9Hz.2H):
character, with bridging lithium and elongated C-X
13C NMR (CS,:CD,CI,): 6 =123.0. 132.1, 139.2. 141.5. 141.6. 142.4. 143.4.
143.6. 144.1. 144.9. 145.0. 145.3. 145.6, 149 0. 150.7. (too insoluble for dctcction of the bridge and bridgehead carbons at 6 = 7 0 - 8 5 ) ; FAB MS: ni:: 887.
8x6; IR (KBr): i. = 2910 (w). 1565 (w). 1410 (5). 1165 (w). 1035 (w).X I 5 ( w ) ,
750 (s). 745 (vs). 530 (s) cn1-l; UV,Vis (cyclohexane): i,,
=l
200,
, 211. 256.
308, 327. 433 sh. 470. 695 nm. Anal. calcd. for C7,H,N,: C 96 16. H O.6H.
N 3 16: found: C 96.02. H 0.81, N 3.17.
[8] M. Eiermann. F. Wudl. M. Prato, M. Maggini, J. A m Chcvn. Snc 1994. 106.
8364.
[Y] A. Hirsch. I. Lamparth, H. R. Karfunkel. Ang<m. Cheni. 1994, 1fM. 453;
An,qt'w Clwni. Inr. E d Engl. 1994. 33. 437.
[lo] The number of isomers was deduced from the ' H N M R spectra. If one designates the location of the first addition as the "north pole" of the Fullerene. there
are four different types of 6:6 junctions on the "southern" hemisphere
including the "south pole". giving rise to four regioisomeric bisadducts. Two
further types of 6/6 junctions on the "equator" lead to two additional bisadducts only if the second substituent is different from the first ( a total of six
regioisomcrs). Otherwise only one additional rcgioisomer is formed ( a total of
five regioisomers).
[l I] I n the cyclic voltammograms of all isomeric mixtures of bisadducts 9-12 the
signal shapes are. in general, similar to those of thc corresponding monoadducts, indicating that relative positions of the substituents hiis a surprisingl)
small effect on the redox potentials.
1121 All compounds with dibenzylamino groups (7. 10. 12) can be oxidized rcversibly in two steps due to a charging of the amino-substituted fluorcnyl moieties.
Oxidation peak potentials 7 : + 114 mV. +463 mV: 10: + 147 niV. +473 mV
(two-electron processes); 12. + 129 mV. +464 m V (see Figs. 1 and 2)(vs ferrocene:ferrocenium). This behavior was also detected in 2,7-bis(dibenzylaniino)9-fluorenone.
[13] H. Durr. R Gleiter, Angcw. Chem. 1978. YO, 591; A ! 7 p i . . Chrni. 1/11.Ed. Engl.
1975. 17, 559. T. A. Albtight. J. K . Burdett. M.-H. Whangbo. Orbird / n t r r o . riom in C / i c w i . ~ r rWiley.
~,
New York. 1985. p. 226. Spiroconjugation is manifested not only by a bathochromic shift i n the UV,'Vis spectra but also by
substantial changes in the cyclic voltammograms: P. Maalak. M. P. Augustine,
J. D. Burkey, J. Am. Chew. Sor. 1990. 1 / 2 . 5359.
1141 The first full structure of a inethanofullerenc was recently published: 1. Osterodt. M. Nieger, F. Vogtle. J. Cheni. Soc. C/imi. Connil. 1994. 1607. More
recently a low-temperature X-ray structure analysis of a 6:6-methanofullcrene
with a cyclopropane base bond length of 1.574(3) A was reported: H. L. Anderson. C. Boudon, F. Diederich. J-P. Gisselbrecht. M. Gross. P. Seiler.
Afigew. C k i n . 1994. 106. 1691 -1694. A ~ ~ P I IClinii.
'.
/m. Ed. €ng/. 1994. 33.
1628.
bonds.[2 51
Crystals of 1 (synthesized by tinilithium exchange) were obtained from hexane in the absence of cosolvents. The structure
of 1 adopts a self-assembled, polymeric chain in the solid state
(Fig. I).['] The asymmetric unit contains six H,C=C(Li)OEt
molecules. Four of them are aggregated to form a distorted
cubic Li,C, tetramer. the remaining two generate a second tetramer by a C , axis (Fig. 1a).
0
a-Ethoxyvinyllithium: An Unexpected
Polymeric Structure-Tetrameric Subunits
Linked by Li-C n Interactions**
Klas Sorger, Walter Bauer, Paul v o n Rague Schleyer,*
and Dietmar Stalke
Dedicated to Projessor Heins Viehe
on the occasion qf his 6Stl1 birtlidq
Serving as an acyl anion equivalent, a-ethoxyvinyllithium (1)
is a widely used synthetic reagent."] We now report the complex
X-ray structure of unsolvated 1, as
H,
,0CZH5
well as NMR spectroscopic evidence
for its nature in T H E In general,
1
,c=c \ Li
H
organolithium systems with cr-halo-
[*I
Prof. Dr. P. von R. Schleyer. Dipl.-Chem. K . Sorger. Dr. W. Bauer
Institut fur Organische Chemie der Universitit Erlangen-Nurnberg
Henkestrasse 42. D-91054 Erlangen (Germany)
Telefax: Int. code (9131)85-9132
+
[**I
Priv.-Doz. Dr. D. Stalke,
lnstitut fur Anorganische Chemie der Universitit
Tammannstrasse 4, D-37077 Gottingen (Germany)
This work was supported by the Fonds der Chemiwhen Industrie, the Stiftung
Volkwagenwerk, the Convex Computer Corporation. and the Deutschc
Forschungsgemeinschaft. K. S thanks the Freistnat Bayern for a scholarship.
0
O
C
oLi
Fig. 1 a ) Crystal structure of polymeric 1 showing the monoclinic unit cell. The
methylene and methyl hydrogen atoms of the ethoxy groups have been omitted for
clarity (the labels with a letter identify atoms which are related by a C, axis). b)
Chain structure of polymeric 1 showing the tetrameric subunits linked by C-Li
rr-interactions. Selected bond lengths [pm] and angles [ J: Lil - 0 1 1133.6(9). Lil 0 3
189.8(9), LilbC2 216.8(11). LilGC6 245.9(10). Lil-C6a 222.1(10), Li2-C2
223.4(10), Li2LC2a 232.1 (10). Li2ZC6 227.9(11). Li2-C9 244.6(10). Li2- C10
253.l( l o ) , Li3-Cl0 225.1( 1 I ). Li3-Cl4 231.3(10), Li3 -C22 227.4( lo), Li3-C5
244.8(10). Li3- C6 251 8(10), Li4-CI0 236.6(10), Li4-CI4 223.0 (10). L i 4 - C l 8
227.2(11). Li4-CI3a 242.5(10). Li4-CI4a 251.5(10). Li5-04 186.0(10). Li5-05
188.0(10). Li5-CI4 234.1(11). Li5 CIS 230.1(12), Li5-C22 221.4(11). Li6-03
187.9(9). Li6-06 186.6(10). Li6-CI0 231.3(11), Lib-CI8 214.4(10). Lih-C22
228.3(11). Li bridged C = C (mean) 133.3(8), nonbridged C = C (mean) 131.9(8);
C = C 0 (mean) 117.7, Cl-C2-LiI 172.1(5). C5-C6-Lll 149.0(4). C9- C10-Li6
156.5(5), C13-C14-Li5 156.2(5). cl7-ClX-L15 163.X(6). C21-C22-Li6 158.0(5).
The Li/oxygen carbenoid character of 1 is shown by the elongated C,-0 bond (142.1(6)-143.6(6) pm, mean: 142.8(7)
pm ;( 3 , 5a - dl the CvinY,-Obond length in vinylethers is about
136 pm (Fig. 2 bottom[']). Similar elongations of 6.8 and
8.5 pm were observed in r - l i t h i o b e n z o f ~ r a n [ and
~ ~ ] 3-bromo-2I i t h i o b e n z o f ~ r a n .respectively.
[~~~
The C,-0 bond of monomeric a-methoxyvinyllithium (computed ab initio at the Becke3LYP
/6-311 + G**[']level) is even longer (13 pm. Fig. 2 top) which
can be attributed to the absence of other lithium ligands. Due to
the unfavorable lone pair repulsions in the s-cis form, the r methoxyvinyl anion favors the s-rruns conformation (Fig. 2
middle). Both the conformation of a-methoxyvinyllithium and
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