Formation of mixed aryl-, alkyl-lithium aggregates in the heteroatom assisted lithiation of α,α′-dialkyl substituted 1,3-bis[(dimethylamino)methyl]benzene

Article abstract of DOI:10.1016/S0022-328X(97)00554-8

The heteroatom assisted lithiation of 1,3-bis[1-(dimethylamino)ethyl]benzene with n-BuLi afforded 2,6-bis[1-(dimethylamino)ethyl]phenyllithium. An X-ray crystal structure determination revealed a dimeric aggregate in which the four benzylic chiral centers

Full text of DOI:10.1016/S0022-328X(97)00554-8

Ž
.
Journal of Organometallic Chemistry 463 1998 463–467
Short communication
Formation of mixed aryl–, alkyl–lithium aggregates in the heteroatom
assisted lithiation of a,aX-dialkyl substituted
1,2
wž
/
x
1,3-bis dimethylamino methyl benzene
a
a
a
a
Johannes G. Donkervoort , Jose L. Vicario , Evelien Rijnberg , Johann T.B.H. Jastrzebski ,
´
Huub Kooijman , Anthony L. Spek b,3, Gerard van Koten
Debye Institute, Department of Metal-Mediated Synthesis, Utrecht UniÕersity, Padualaan 8, Utrecht 3584 CH, The Netherlands
b
a,)
a
b
BijÕoet Centre for Biomolecular Research, Department of Crystal and Structural Chemistry, Utrecht UniÕersity, Padualaan 8, Utrecht 3584
CH, The Netherlands
Received 9 April 1997
Abstract
The heteroatom assisted lithiation of 1,3-bis 1- dimethylamino ethyl benzene with n-BuLi afforded 2,6-bis 1- dimethyl-
w
Ž
.
x
w Ž
.
x
amino ethyl phenyllithium. An X-ray crystal structure determination revealed a dimeric aggregate in which the four benzylic chiral
w
Ž
.
x
centers are identical, pointing to stereoselective crystallization. In contrast, reaction of 1,3-bis 1- dimethylamino propyl benzene with
n-BuLi afforded a dimeric aggregate comprising the parent lithiated compound and n-BuLi in a 1:1 molar ratio. The four Li atoms and
the four bridging carbon atoms are arranged in a unique ladder-type C–Li2–CX2–Li2–C framework. q 1998 Elsevier Science S.A.
Keywords: Heteroatom assisted lithiation; Stereoselective crystallization; Mixed organolithium aggregate
1. Introduction
properties which lead to the synthesis of organometallic
complexes in which the metal center can adopt various
oxidation states. In addition, this ligand enables the
control of the reactivity and stereochemistry of its
organometallic complexes. Both properties are result of
the electronic and geometric restrictions imposed on the
metal center by this ligand.
In order to fine-tune the metal environment in a way
that would create a chiral envelope around the metal
center, enantiopure derivatives of proline were used to
The use of diamine-, Fig. 1A, and diphosphine-, Fig.
1B, aryl ligands has in recent years led to
synthesis of many new types of complexes 1–8 , ii a
better understanding of reaction mechanisms 9–11 ,
Ž .
i
the
w
x
Ž .
w
x
Ž
.
and iii novel applications in the field of homogeneous
w
x
catalysis 12–14 .
In particular the potentially terdentate monoanionic
Ä
Ž
.
4
aryldiamine 2,6-bis dimethylamino methyl phenyl
Ž
. Ž
.
5NCN Fig. 1A, R5H has been the subject of
w
x
synthesize a C2-symmetric analogue of NCN 12 . The
chirality in this ligand is introduced at the nitrogen
atoms, which are part of the pyrrolidinyl ring systems,
by placing a methyl substituent on the 2- or 5-positions
of the latter ring. Unfortunately, the corresponding
organonickel compounds, although active, did not show
any stereochemical induction in the Kharasch addition
reaction of CCl4 to C5C bonds. A possible explanation
for this is that the chiral centers are too far away from
the reactive site to induce significant enantiomeric ex-
cesses.
w
x
intensive studies in our group 1–8 . We have found
that this ligand has some very specific and unique
)
Corresponding author.
1 These results were presented in part at the Symposium ‘Frontiers
in Organometallic Chemistry’ at the Spring 1997 American Chemical
Society meeting in San Francisco.
2 Dedicated to Prof. Ken Wade on the occasion of his 65th
birthday, thanking him for his ‘rules’, discussions and long friend-
ship.
3 Address correspondence pertaining to crystallographic studies to
this author.
One way to solve this problem is to introduce the
0022-328Xr98r$19.00 q 1998 Elsevier Science S.A. All rights reserved.

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464
J.G. DonkerÕoort et al.rJournal of Organometallic Chemistry 550 1998 463–467
the corresponding lithiated compound rac-1a in 75%
yield as a white crystalline solid.
Ž .
1
Fig. 1.
chiral information at the benzylic positions of the lig-
Ž
.
and, e.g., as in Fig. 1A R5Me , as this would place a
chiral center in close proximity to the metal atom even
in cases where metal–nitrogen bond dissociation occurs.
This C2-symmetric ligand system shows similarities
with a-methylbenzylamine, a ligand which has found
applications as a chiral auxiliary in enantioselective
That lithiation had occurred exclusively at the 2-posi-
tion of the aromatic ring became evident from the
observed 1H and 13C NMR spectra of the products
obtained after reaction of rac-1a with D2O.
Ž
Molecular weight determination by cryoscopy in
w
x
.
homogeneous catalysis 15–17 .
benzene of rac-1a showed that this compound exist in
Here we report our first results towards the synthesis
of both racemic and enantiopure a,aX-disubstituted
aryldiamines and their subsequent heteroatom assisted
lithiation to the corresponding chiral aryllithiums.
solution as a discrete dinuclear aggregate. Previously
w
Ž
.
x
we have shown that 2,6-bis dimethylamino methyl -
w
x
phenyllithium 19 , and more recently for a series of
w
x
4-substituted derivatives 20 , are discrete dinuclear ag-
gregates. From the latter compounds an X-ray crystal
structure determination of the 4-phenyl derivative
showed that this compound exist as a dimer also in the
2. Results and discussion
w
x
solid state 20 .
Racemic 1,3-bis 1- dimethylamino ethyl benzene,
Preliminary 1H and 6 Li NMR studies of rac-1a
revealed very complex temperature dependent spectra
that are not completely understood at this time. Of
course for rac-1a, complex spectra are expected be-
cause several mixed diastereoisomeric aggregates may
be formed in solution. These topics are currently under
investigation.
w
Ž
.
x
Ž
.
w
Ž
.
rac-1 dlrmeso 1:1 , and 1,3-bis 1- dimethylamino
x
Ž
.
propyl benzene, rac-2 dlrmeso 1:1 , were prepared via
Ž
.
a multistep synthesis Scheme 1a . The corresponding
X
Ž
.
enantiopure diamine R,R -2 was prepared via an
enantioselective addition of Et2 Zn over the carbonyls of
isophthaldehyde Scheme 1b . The e.e. of each addition
step of Et2 Zn is 86%, resulting in a RR : SS :meso
ratio of 74:2:24. The resulting enantiomerically en-
riched diols were converted to the corresponding
dimethylamino derivatives as outlined in the racemic
case. The details of the synthesis of rac-1, rac-2, and
R,R -2 will be published separately 18 .
Ž
.
Ž
.
Ž
. Ž
.
To get insight in the type of organolithium aggre-
gates formed in the lithiation of rac-1, attempts were
undertaken to obtain crystals of this compound that are
suitable for an X-ray crystal structure determination.
Crystals that were suitable for an X-ray crystal struc-
ture determination were obtained after cooling a satu-
rated solution of rac-1a for 3 days at y308C. The
crystal structure of 1a involves the packing of 1 dimeric
X
Ž
.
w x
Ž
Ž ..
Reaction of rac-1 with butyllithium see Eq. 1 in a
non-polar solvent like pentane, afforded after work up
Scheme 1.

(
)
J.G. DonkerÕoort et al.rJournal of Organometallic Chemistry 550 1998 463–467
465
.
molecule in the asymmetric unit. The stereochemistry of
mation, calculated projection shown in Fig. 3B it is
obvious that in the latter situation a considerable steric
interference occurs between the methyl groups and both
the four benzylic carbon atoms are identical. The struc-
X
Ž
.
ture of enantiopure R,R -1 thus obtained points to
˚
Ž
stereoselective crystallization.
lithium atoms calculated distances 3.0 and 3.1 A, re-
X
Ž
.
.
The molecular geometry of R,R -1a is depicted in
Fig. 2 together with relevant bond distances and angles.
Each of the monoanionic terdentate aryl groups bridges
via a two-electron three-center bond 2e–3c the two
lithium atoms, as is corroborated by the acute Li–C–Li
spectively .
A similar l-conformation for the five-membered
chelate ring in octahedral tungsten compounds contain-
ing the 2- 1- dimethylamino ethyl phenyl ligand is ob-
served. The alternative d-conformation would result in a
considerable steric interference between the benzylic
methyl group and one of the meridional substituents at
Ž
.
w
Ž
.
x
Ž
Ž . .
angles both 67.2 7 8 . Distorted tetrahedral coordina-
tion at each lithium atom is achieved by intramolecular
coordination of each of the NMe2 groups to one of the
lithium atoms. The almost perfect symmetric bridging
of the aryl units is reflected in only small differences in
w
x
tungsten 23 .
w
Ž
.
The reaction of rac-2, i.e., 1,3-bis 1- dimethylamino
propyl benzene, with n-butyllithium indicates that the
heteroatom assisted lithiation of this type of aryl di-
amines is largely influenced by the presence and nature
of the benzylic substituents. It should be noted that the
heteroatom assisted lithiation of a-substituted dimethyl-
benzylamines is largely influenced by the bulk of this
substituent 24 . It appeared that reaction of rac-2 with
butyllithium gives rise to the formation of an aggregated
species, rac-2a Eq. 2 , comprising the parent lithiated
compound and n-butyllithium in a 1:1 molar ratio. The
formation of such mixed aggregates is not unprece-
dented, e.g., a similar aggregate was obtained by the
x
Ž
.
C–Li bonding distances see Fig. 2 . The overall struc-
tural features relevant bond distances and angles of
R,R -1 are comparable to those of 2,6-bis dimethyl-
amino methyl 4-phenyl phenyllithium 20 and 2,6-
Ž
.
X
Ž
.
.
wŽ
Ž
x
Ž
.
w x
wŽ
.
x
w x
bis diethylamino methyl phenyllithium 21 .
X
Ž
.
w
x.
A notable feature of R,R -1a, however, is the fact
that the four five-membered chelate rings are puckered
in such a way that the benzylic methyl groups are
Ž
Ž ..
Ž
oriented almost co-planar with the aryl ring l-confor-
. Ž
.
mation see Fig. 3A . It has been well established that a
situation in which the substituents are oriented perpen-
Ž
.
w
Ž
.
x
dicular to the aryl ring d-conformation , and conse-
quently, the lithium and benzylic hydrogen atom are in
anti-periplanar position, is energetically more favourable
lithiation of 1,3-bis dimethylamino methyl -2,4,6-
w
x
trimethylbenzene with n-BuLi 25 .
w
x
22 . However, as a consequence of the bridging nature
X
Ž
.
Ž
.
2e–3c bond of the aryl ring in R,R -1a the two
lithium atoms are located below and above the plane of
Ž .
2
Ž
.
the aryl group see Fig. 3 . When the two possible
situations with respect to the puckering of the chelate
rings are compared l-conformation as found in the
Ž
X
Ž
.
crystal structure of R,R -1a, Fig. 3A vs. the d-confor-
A molecular weight determination of the mixed ag-
gregate rac-2a points to a dimeric compound. To eluci-
date the structure of rac-2a an X-ray crystal structure
determination was carried out.
The crystal structure of 2a involves the packing of
two discrete dimeric molecules each located on a crys-
tallographic inversion center. The two molecules are
chemically identical, but slightly different in the solid
state structure. The molecular geometry together with
relevant bond distances and angles of one of these
molecules is shown in Fig. 4.
It appeared that the 2:2 mixed aggregate comprises
of four lithium atoms and four bridging carbon atoms
arranged in a ladder-type C–Li2–CX2–Li2–C frame-
work. A similar type has recently been found for the
mixed aggregate obtained from the reaction of 1,3-
X
Ž
.
Fig. 2. Molecular geometry of R,R -1a. Relevant bond distances
˚
Ž .
Ž . Ž .
Ž .
Ž . Ž .
Ž .
Ž .
Ž .
Ž .
A and angles 8 : C 1 –Li 1 2.26 2 , C 1 –Li 2 2.21 2 , C 21 –
w
Ž
.
x
Ž
bis dimethylamino methyl -4,6-dimethyl-2- trimethyl-
Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .
Ž
.
Li 1 2.25 2 , C 21 –Li 2 2.23 2 , Li 1 –N 1 2.12 2 , Li 1 –N 21
.
w x
silyl methylbenzene with n-butyllithium 26 . In this
Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .
Ž
.
2.11 2 , Li 2 –N 2 2.15 2 , Li 2 –N 22 2.10 2 , Li 1 –C 1 –Li 2
Ž . Ž . Ž . Ž .
67.2 7 , Li 1 –C 21 –Li 2 67.2 7 .
Ž
.
respect, it should be noted that ladder type frame-works

(
)
466
J.G. DonkerÕoort et al.rJournal of Organometallic Chemistry 550 1998 463–467
X
Ž
.
Ž .
Ž .
Fig. 3. Puckering of the five-membered chelate rings in R,R -1a. A l-Conformation as observed in the solid state. B Calculated alternative
d-conformation.
˚
Ž
.
Ž . Ž .
Ž .
are a typical feature observed in lithium amides and
asymmetric manner C 17 to Li 1 2.13 2 A, to Li 2
˚
˚
Ž .
Ž
w
x
Ž .
Ž
.
lithium phosphides see Ref. 27 and references cited
therein , but only very recently the first examples of a
2.31 2 A and to Li 2a 2.25 2 A. As a consequence of
different bonding modes of the aryl two electron-three
. Ž .
center and n-butyl two electron-four center bridging
.
Ž
ladder-type frame-work have been reported in aggre-
gates containing multicenter bonded carbon atoms in
N–Li–C–Li or C–Li–O–Li structural units. In this case
the bridging carbon atom and the bridging heteroatom
Ž .
carbon atoms, the coordination geometry of Li 2 is a
Ž .
distorted tetrahedral and that of Li 1 is more or less
Ž
.
trigonal see Fig. 4 . It should be noted however, that
the b-carbon of the n-butyl group is in very close
proximity of this lithium atom 2.33 2 A as a likely
consequence of the coordinatively unsaturated character
of the three-coordinate lithium atom. Similar interac-
Ž
.
are part of the same molecule intramolecular chelation
˚
Ž . .
w
x
Ž
28,29 .
The obtained 2:2 aryl-, n-butyl–lithium aggregate
may be regarded as consisting of two Aryl Li2 cationic
Ž
.
Ž
.
units linked together by two n-butyl groups. The two
tions have been observed in the structures of n-BuLi
30 and c-C6 H11Li 31 and have been explained in
6
Ž
.
w
x Ž
. w x
6
cationic Aryl Li2 units are symmetry related by a
Ž
.
crystallographic center of symmetry. In the Aryl Li2
Ž .
terms of electrostatic repulsion.
unit the bridging carbon atom, C 1 , is asymmetrically
In the determined structure of rac-2a, the stereo-
chemistry of the chiral centers within one aryl diamine
unit are the same, but as a requirement of the space
group symmetry is inverted in the other aryldiamine
unit. Obviously, a diastereoselective aggregation has
occurred. Another interesting feature of rac-2a is that
the four five-membered chelate rings are now puckered
in such a way that the ethyl substituents are oriented
perpendicular to the plane of the aryl ring, i.e., the
d-conformation which is energetically more favourable
˚
˚
.
Ž . Ž
Ž .
.
Ž . Ž
Ž .
bonded to Li 1 2.16 2 A and Li 2 2.30 2 A while
the two nitrogen atoms of the terdentate, monoanionic
aryl anion are each coordinating to one lithium atom.
Ž
.
The two n-butyl anions connecting the two Aryl Li2
units are each bound to three lithium atoms in an
Ž
.
than the l-conformation vide supra . The fact that the
d-conformation is the most stable one in rac-2a while
X
Ž
.
in R,R -1a the l-conformation is found in the solid
state, is most likely a result of the asymmetric bonding
of the bridging aryl group in rac-2a. Also, one of the
lithium atoms is three-coordinate, resulting in a consid-
erable reduction in steric repulsion between the benzylic
substituents and the two lithium atoms.
For obvious reasons, rac-2a is not a suitable starting
material for other organometallic compounds via a
transmetallation reaction. It is expected that the forma-
tion of transient metallobutyl intermediates will give
rise to extensive decomposition, e.g., via b-elimination
reactions. However, it appears that Me3SiCl reacts se-
lectively with the n-butyl moieties in rac-2a to give a
novel n-butyl free organolithium species. This was
subsequently used in the synthesis of the corresponding
˚
Ž .
Fig. 4. Molecular geometry of rac-2a. Relevant bond distances A
Ž .
and angles 8 , values in square brackets refer to the other unique
Ž . Ž . Ž . w Ž .x Ž . Ž . Ž . w Ž .x
molecule: C 1 –Li 1 2.16 2 2.12 2 , C 1 –Li 2 2.30 2 2.35 2 ,
Ž . Ž . w Ž .x Ž . Ž . w Ž .x
Ž
Ž
.
.
Ž
.
Ž .
C 17 –Li 1 2.13 2 2.10 2 , C 17 –Li 2 2.31 2 2.35 2 , C 17 –
Ž . w Ž .x Ž . Ž . Ž . w Ž .x Ž . Ž .
Li 2a 2.25 2 2.22 2 , Li 1 –N 1 2.03 2 2.04 2 , Li 2 –N 2
Ž . w Ž .x Ž . Ž . Ž . Ž . w Ž .x
2.07 2 2.08 2 , Li 1 –C 1 –Li 2 65.3 7 65.2 6 .

(
)
J.G. DonkerÕoort et al.rJournal of Organometallic Chemistry 550 1998 463–467
467
w x
4
H.C.L. Abbenhuis, N. Feiken, H.F. Haarman, D.M. Grove, E.
Horn, H. Kooijman, A.L. Spek, G. van Koten, Angew. Chem.,
w
x
organoplatinum compound 32 . However, it may a
priori not be excluded that this novel n-butyl-free
organolithium species is in fact an aggregate of the
parent lithiated compound and LiCl. This topic and the
elucidation of the structures in solution by multinuclear
NMR studies will be the subject of a forthcoming paper.
Ž
.
Int. Ed. Engl. 30 1991 996.
w x
5
C. Chuit, J.P. Corriu, A. Mehdi, C. Reye, Angew. Chem., Int.
´
Ž
.
Ed. Engl. 32 1993 1311.
w x
6
M.R.P. Rietveld, D.M. Grove, G. van Koten, N.J. Chem. 21
Ž
.
1997 751.
w x
Ž
.
7
H. Rimml, L.M. Venanzi, J. Organomet. Chem. 259 1983 C6.
J.P. Sutter, S.L. James, P. Steenwinkel, T. Karlen, D.M. Grove,
N. Veldman, W.J.J. Smeets, A.L. Spek, G. van Koten,
w x
8
3. X-ray structure determinations
Ž
.
Organometallics 15 1996 941.
w x
9
J. Terheijden, G. van Koten, I.C. Vinke, A.L. Spek, J. Am.
Ž
.
Chem. Soc. 107 1985 2891.
Both structures were solved with direct methods and
refinement on F2 SHELXL-93 . Refinement included
w
x
10 J. Terheijden, G. van Koten, P. Mul, D.J. Stufkens, F. Muller,
Ž
.
Ž
.
C.H. Stam, Organometallics 5 1986 519.
anisotropic thermal parameters for non-H atoms; H-
atoms were included on fixed positions. Pertinent data
for R,R -1a: C28 H46 Li2 N4, colourless, monoclinic,
w
w
x
Ž
.
11 A.D. Ryabov, Chem. Rev. 90 1990 403.
x
12 L.A. van de Kuil, Y.S.J. Veldhuizen, D.M. Grove, J.W. Zwikker,
X
Ž
.
L.W. Jenneskens, W. Drenth, W.J.J. Smeets, A.L. Spek, G. van
Ž
.
˚
Koten, Recl. Trav. Chim. Pays-Bas 113 1994 267.
Ž .
Ž .
Ž .
P21, as10.041 4 , bs10.152 2 , cs14.075 7 A,
3
w
w
w
w
x
13 D.M. Grove, A.H.M. Verschuuren, G. van Koten, J. Mol. Catal.
˚
Ž .
Ž .
Ž
.
bs95.97 2 8, Vs1427.0 7 A , Zs2, m CuK a s
Ž
.
45 1988 169.
4.3 cmy1. 3500 reflections measured, 1889 indepen-
x
14 J.G. Donkervoort, J.L. Vicario, J.T.B.H. Jastrzebski, G. Cahiez,
Ž
˚
dent, Rint s0.076 Ts295 K, CuK a radiation, Ni fil-
Ž
.
G. van Koten, Recl. Trav. Chim. Pays-Bas 115 1996 547.
.
ter, ls1.54184 A . Final wR2 value for 307 parame-
x
15 E. Rijnberg, J.T.B.H. Jastrzebski, M.D. Janssen, J. Boersma, G.
2
w
2 Ž
.
F q
Ž
.
van Koten, Tetrahedron Lett. 35 1995 6521.
ters is 0.226, R1 s 0.076, w s 1r s
2
2
2
x
16 M. van Klaveren, E.S.M. Persson, D.M. Grove, J.-E. Backvall,
¨
Ž
.
x
w
Ž
.
x
0.1216P , Ps Max Fo ,0 q2Fc r3, y0.17-Dr
Ž
.
G. van Koten, Tetrahedron Lett. 35 1994 5931.
-3
˚
-0.27 eA . Pertinent data for rac-2a: C40 H72 Li4 N4,
Ž . Ž .
w
w
x
Ž
.
17 G. van Koten, Pure Appl. Chem. 66 1994 1455.
colourless, triclinic, P1, as12.537 3 , bs12.668 3 ,
x
18 J.G. Donkervoort, J.L. Vicario, E. Rijnberg, J.T.B.H. Jastrzeb-
˚
Ž .
Ž .
Ž .
cs13.355 2 A, as92.25 2 , bs91.65 2 , gs
ski, G. van Koten, manuscript in preparation.
3
w
w
w
w
w
x
˚
19 J.T.B.H. Jastrzebski, G. van Koten, M. Konijn, C.H. Stam, J.
Ž .
Ž .
Ž
.
101.04 2 8, Vs2078.7 8 A , Zs2, m MoK a s0.6
cmy1. 5048 reflections measured, 4792 independent,
Ž
.
Am. Chem. Soc. 104 1982 5490.
x
20 P. Steenwinkel, S.L. James, D.M. Grove, N. Veldman, A.L.
Ž
Rint s0.096 Ts150 K, MoK a radiation, graphite
Ž
.
Spek, G. van Koten, Chem. Eur. J. 2 1996 1440.
˚
.
monochromator, ls0.71073 A . Final wR2 value for
x
21 R. Schlengermann, J. Sieler, S. Jelonek, E. Hey-Hawkins, J.
2
w
2Ž
.
F
459 parameters is 0.226, R1s0.103, ws1r s
Ž
.
Chem. Soc., Chem. Commun. 1997 197.
2
2
2
x
Ž
.
x
w
Ž
.
x
22 J.T.B.H. Jastrzebski, J. Boersma, G. van Koten, J. Organomet.
q 0.0457P , Ps Max Fo ,0 q2Fc r3, y0.25-
Dr-0.22 eAy3. Full crystallographic details may be
Ž
.
Chem. 413 1991 43.
˚
x
23 P.A. van der Schaaf, J. Boersma, H. Kooijman, A.L. Spek, G.
Ž
.
obtained from one of the authors ALS .
Ž
.
van Koten, Organometallics 12 1993 4334.
w
w
x
Ž
.
24 G. van Koten, J.T.B.H. Jastrzebski, Tetrahedron 45 1989 569.
x
25 P. Wijkens, E.M. van Koten, M.D. Janssen, J.T.B.H. Jastrzeb-
ski, A.L. Spek, G. van Koten, Angew. Chem., Int. Ed. Engl. 34
Acknowledgements
Ž
.
1995 219.
w
w
w
w
w
w
w
x
26 P. Wijkens, J.T.B.H. Jastrzebski, N. Veldman, A.L. Spek, G.
Ž
.
This work was supported in part J.G.D., A.L.S by
the Netherlands Foundation for Chemical Research
SON with financial aid from the Netherlands Organi-
van Koten, Chem. Commun., 1997 in press.
x
27 F. Pauer, P.P. Power, Lithium Chemistry, in: A.-M. Sapse,
Ž
.
Ž
.
P.v.R. Schleyer Eds. , Wiley, New York, 1995, p. 295.
x
Ž
.
28 K. Sorger, P.v.R. Schleyer, D. Stalke, J. Chem. Soc., Chem.
zation for Scientific Research NWO .
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