Isopropyllithium diamine adducts: From a non symmetric aggregate to monomeric i-PrLi·(1R,2R)-N,N,N′,N′-tetraethylcyclohexane-1,2- diamine

Article abstract of DOI:10.1039/b804855j

By means of isopropyllithium crystal structures, the transition from dimeric (i-PrLi·TMEDA)₂ (4) to monomeric i-PrLi·(R,R)- TECDA (6) via the non-symmetric aggregate [(i-PrLi)₃·(TEEDA) ₂] (5) is shown, depending on the steric demand of the ligand. The Royal Society of Chemistry.

Full text of DOI:10.1039/b804855j

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Isopropyllithium diamine adducts: from a non symmetric aggregate to
monomeric i-PrLiꢀ(1R,2R)-N,N,N0,N0-tetraethylcyclohexane-
1,2-diaminew
Carsten Strohmann,* Viktoria H. Gessner and A. Damme
Received (in Cambridge, UK) 20th March 2008, Accepted 22nd April 2008
First published as an Advance Article on the web 20th May 2008
DOI: 10.1039/b804855j
By means of isopropyllithium crystal structures, the transition
from dimeric (i-PrLiꢀTMEDA)2 (4) to monomeric i-PrLiꢀ(R,R)-
TECDA (6) via the non-symmetric aggregate [(i-PrLi)3ꢀ(TEE-
DA)2] (5) is shown, depending on the steric demand of the ligand.
Organolithium compounds are amongst the most often used
reagents in synthetic chemistry. Therefore, the addition of
Fig. 1 Diamine ligands for alkyllithium compounds.
Lewis bases, such as nitrogen ligands, plays an important role
in controlling and increasing the reactivity of these com-
pounds.1 Especially chiral ligands mainly in combination with
sec-butyllithium have become of great interest because of their
application in asymmetric synthesis.2,3 Due to the struc-
ture–reactivity relationship of lithium organics the under-
standing of the process of these reactions is strongly
connected to the understanding of the structure formation
patterns. However, to date there is only little structural
information known about Lewis-base coordinated s-BuLi
adducts.4 In the case of chiral ligands, this lack of knowledge
is probably due to the formation of diastereomers between the
ligand and the lithium alkyl. These diastereomers lead to a
hindered crystallisation of the adduct. Nevertheless, the eluci-
dation of these compounds is still endeavoured for a better
understanding of the reaction mechanism. An approach to this
problem is the use of isopropyllithium instead of s-BuLi as
organolithium compound. Both lithium bases possess almost
the same steric demand and basicity. However, i-PrLi is more
suitable for crystallisation due to the absence of diastereomeric
mixtures.
Out of a mixture of isopropyllithium and an equivalent
amount of TMEDA in n-pentane, (i-PrLiꢀTMEDA)2 (4) crys-
tallises as colorless needles in the orthorhombic crystal system,
space group Fdd2 (one molecule was detected in the asym-
metric unit, Fig. 2). The central motif of the C2 symmetric
molecule is the central Li–C–Li–C four-membered ring, which
shows a slight deformation from planarity (sum of angles
346.61). The Li–C distances of the ring amount to 2.255(8)
and 2.236(8) A, the Li–N 2.234(8) and 2.258(8) A and are thus
comparable with other known dimeric lithium alkyl struc-
tures.1,5 4 is the second known dimeric structure of i-PrLi,
which has also been detected with the chiral diamine (1R,2R)-
N,N,N0,N0-tetramethylcyclohexane-1,2-diamine
[(R,R)-
TMCDA].4 However, in this adduct the Li–C bond lengths
[2.206(6)–2.332(6) A] are considerably longer than in (i-PrLiꢀ
TMEDA)2 (4) and other known dimeric alkyllithium com-
pounds.4,5
We present herein the structure elucidation of three isopro-
pyllithium adducts with the commonly used diamine ligands,
N,N,N0,N0-tetramethylethylenediame (TMEDA, 1), N,N,N0,N0-
tetraethylethylenediamine (TEEDA, 2) and the chiral (1R,2R)-
tetraethylcyclohexane-1,2-diamine [(R,R)-TECDA, 3] (Fig. 1).
These structures show a transition from the dimeric compound
(i-PrLiꢀTMEDA)2 (4) to the monomer, i-PrLiꢀ(R,R)-TECDA
(6), via the non-symmetrical structure [(i-PrLi)3ꢀ(TEEDA)2] (5),
depending on the steric demand of the ligand. Therefore, the
presented molecules impressively demonstrate the influence of
the ligand on structure formation patterns.
Institut fur Anorganische Chemie, Universitat Wurzburg, Am
¨
¨
¨
Hubland, 97074 Wurzburg, Germany. E-mail: mail@carsten-
¨
strohmann.de; Fax: (+49)931-888-4605; Tel: (+49)931-888-4613
w Electronic supplementary information (ESI) available: Experimen-
tal, crystallographic and quantum chemical data, including the co-
ordinates of all calculated species. CCDC 681928–681930. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b804855j
Fig. 2 Molecular structure of (i-PrLiꢀTMEDA)2 (4) in the crystal.
Selected bond lengths (A) and angles (1) [symmetry operation for (0): 1/
2 ꢁ x, 1/2 ꢁ y, z]: C(1)–Li(1) 2.255(8), C(1)–Li(10) 2.236(8), Li–N(1)
2.234(8), Li–N(2) 2.258(8); N(1)–Li(1)–N(2) 81.9(3), C(1)–Li1–C(10)
108.2(3), Li(1)–C(1)–Li(10) 65.1(3).
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Fig. 4 Drawing of the molecular structures of (i-PrLiꢀTMEDA)2 (4)
and [(i-PrLi)3ꢀ(TEEDA)2] (5).
Fig. 3 Molecular structure of [(i-PrLi)3ꢀ(TEEDA)2] (5) in the crystal.
Selected bond lengths (A) and angles (1) (hydrogens of the ligand are
omitted for clarity): C(11)–Li(2) 2.175(5), C(11)–Li(1) 2.264(5), C(14)–
Li(2) 2.231(5), C(14)–Li(1) 2.282(5), C(14)–Li(3) 3.402(5), C(17)–Li(3)
2.095(4), C(17)–Li(2) 2.229(4), Li(1)–N(1) 2.227(5), Li(1)–N(2)
2.279(4), Li(3)–N(3) 2.078(4), Li(3)–N(4) 2.111(4); Li(2)–C(11)–Li(1)
66.04(16), Li(2)–C(14)–Li(1) 64.85(14), C(11)–Li(1)–C(14) 107.25(18),
C(11)–Li(2)–C(14) 112.35(19), Li(3)–C(17)–Li(2) 82.20(16), Li(3)–
C(14)–Li(2) 83.24(15), C(14)–Li(2)–C(17) 129.85(19), C(11)–
Li(2)–C(17) 117.56(19).
Furthermore, contrary to other known structures with such
an uneven number, the isopropyllithium molecules are asym-
metrically arranged and coordinated in the molecule (Fig. 4).6
The formation of [(i-PrLi)3ꢀ(TEEDA)2] (5) can be under-
stood by computational studies at the B3LYP/6-31+G(d)
level.7 These calculations show an energetic preference by 19
kJ molꢁ1 of compound 5 over 1/2 (i-PrLi)6 and two molecules
TEEDA and an preference of 17 kJ molꢁ1 over 1/6 (i-PrLi)6
and a hypothetic dimeric (i-PrLiꢀTEEDA)2. The formation of
monomeric i-PrLiꢀTEEDA is even more disfavoured by 26 kJ
molꢁ1 in comparison to [(i-PrLi)3ꢀ(TEEDA)2] (5) and 1/6
(i-PrLi)6. The disfavoured formation of the dimer can be
explained by the steric demand of the isopropyllithium mole-
cules. Most recently, we isolated dimeric [i-PrLiꢀ(R,R)-
TMCDA]2 with the chiral ligand (R,R)-TMCDA, showing
strongly elongated Li–C distances.4 The more demanding
TEEDA finally prevents such an expanded dimer, so that only
the formation of the non-symmetrical adduct 5 is possible.
Replacing the ethylene bridge of TEEDA with a cyclohexyl
ring results in the diamine (R,R)-TECDA (3). Structure eluci-
dation with this ligand yields again a change in the degree of
aggregation. Monomeric i-PrLiꢀ(R,R)-TECDA crystallises
from an equivalent amount of the amine in n-pentane in the
orthorhombic crystal system, space group P212121 (one mole-
cule was detected in the asymmetric unit, Fig. 5). The lithium
center possesses three contacts, two to the nitrogen atoms of
the ligand and one to the carbanionic center of the isopropyl
group. In comparison to oligomeric alkyllithium compounds,
6 shows shortened Li–N [2.073(5) and 2.115(5) A] and Li–C
distances [2.099(5) A], which are typical for monomeric alkyl-
lithiums.4,8–10 Contrary to t-BuLiꢀ(ꢁ)-sparteine, the i-PrLi
molecule is almost vertically coordinated to the ligand (sum
of angles: 359.51). However, both sides of the molecule are
unequally free due to the isopropyl group. This is depicted by
the Connolly surface and the electrostatic potential, indicating
a free, positively charged side for coordination or an electro-
philic attack (Fig. 5).11 Besides t-BuLiꢀ(ꢁ)-sparteine8 and the
adducts of t-BuLi and s-BuLi with (R,R)-TMCDA,4,9 i-PrLiꢀ
(R,R)-TECDA (6) is one of the rare monomeric saturated
alkyllithium compounds.4,8–10 Additionally it is the first
monomeric isopropyllithium adduct isolated so far.
A totally different type of structure is formed with the
higher homologous of TMEDA, TEEDA (2). [(i-PrLi)3ꢀ
(TEEDA)2] (5) crystallises out of n-pentane in the monoclinic
crystal system, space group P21/n (one molecule was detected
in the asymmetric unit, Fig. 3). Compound 5 consists of three
isopropyllithium units coordinated by two TEEDA mole-
cules.6 The central structural motif is formed by a Li–C–Li–C
four-membered ring which is linked with another isopropyl-
lithium unit. However, the contact between C14 and Li3 is
considerably elongated in comparison to other known Li–C
distances. While the lithium atoms, Li1 and Li2, show Li–C
distances typical for dimeric compounds [2.175(5) to 2.282(5)
A], Li3 possesses a very short contact in the range of mono-
meric organolithiums [Li3–C17: 2.095(4) A] and an uncom-
monly long one [Li3–C14: 3.402(5) A], indicating no bond.
Thus, the lithium atoms Li2 and Li3 possess three contacts
each, whereas Li1 shows four contacts, two to the nitrogen
atoms of the ligand and two to the carbanionic centers.
Interestingly, between C19 and Li3 a very short Li–Cb contact
of only 2.377(5) A is found, which is in the range of the
elongated Li–C distances of the dimeric [i-PrLiꢀ(R,R)-
TMCDA]2. This short non-bonding contact is due to steric
reasons forcing the isopropyl group into this specific arrange-
ment. The Li–N distances show analogous tendencies as the
Li–C distances, with those to Li1 in range of dimers and those
of Li3 in the range of monomers. Altogether, [(i-PrLi)3ꢀ
(TEEDA)2] can formally be considered as an isopropyllithium
dimer, into which an additional i-PrLi molecule is inserted.
Consequently, compound 5 represents a transition from the
dimeric adduct to the monomer.
[(i-PrLi)3ꢀ(TEEDA)2] is a remarkable type of alkyllithium
structure. While these compounds generally tend to form
symmetric structures as dimers, tetramers or hexamers, com-
pound 5 possesses an uneven number of lithium units.
On first glance, TEEDA (2) and (R,R)-TECDA (3) possess
almost the same steric demand. However, both diamines form
different i-PrLi structures in the crystal. Hence, the question
ꢂc
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the first monomeric isopropyllithium structure, i-PrLiꢀ(R,R)-
TECDA (6), via the interesting, non-symmetrical aggregate
[(i-PrLi)3ꢀ(TEEDA)2] (5). These three adducts indicate the
influence of the ligand on the structure formation patterns
and thus of the reactivity of organolithium compounds.
Notes and references
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Fig. 5 (left) Molecular structure of i-PrLiꢀ(R,R)-TECDA (6) in the
crystal. Selected bond lengths (A) and angles (1): C(15)–Li 2.099(5),
Li–N(1) 2.073(5), Li–N(2) 2.115(5); N(1)–Li–C(15) 134.0(2),
C(15)–Li–N(2) 138.7(2), N(1)–Li–N(2) 86.8(2); (right) Connolly sur-
(f) E.-U. Wurthwein, K. Behrens and D. Hoppe, Chem.–Eur. J.,
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=
ꢁ0.126] of both sides of compound 6 [B3LYP/6-31+G(d) probe
4. C. Strohmann and V. H. Gessner, J. Am. Chem. Soc., 2007, 129,
8952.
radius, 1.2 A].
5. Examples
of
dimeric
organolithium
compounds:
(a) M. A. Nichols and P. G. Williard, J. Am. Chem. Soc., 1993,
115, 1568; (b) C. Strohmann, K. Strohfeldt and D. Schildbach,
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(a) B. Walfort, R. Bertermann and D. Stalke, Chem.–Eur. J.,
Fig. 6 Drawing of monomeric i-PrLiꢀ(R,R)-TECDA and illustration
of the repulsion between the cyclohexane ring and the ethyl groups in a
hypothetical dimer or aggregate.
2001, 7, 1424; (b) A. Furstner, H. Krause and C. W. Lehmann,
¨
¨
Angew. Chem., 2006, 118, 454; A. Furstner, H. Krause and
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A. Otero, S. Prashar, A. M. Rodriguez and E. Villasenor, Orga-
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arises why (R,R)-TECDA forms a monomer and TEEDA an
aggregate with i-PrLi. This can be explained by both crystal
structures. In [(i-PrLi)3ꢀ(TEEDA)2] (5), one ethyl group of
each nitrogen atom is arranged in the direction of the iso-
propyllithium molecules and one towards the ligand. By
contrast, in the monomeric i-PrLiꢀ(R,R)-TECDA no ethyl
group is directed towards the cyclohexane ring of (R,R)-
TECDA. Such an arrangement would lead to strong repulsion
between the ethyl groups and the axial hydrogen atoms of the
ring. Therefore, all ethyl groups are arranged towards the
i-PrLi molecule (Fig. 6), thus preventing the formation of a
hypothetic dimer [i-PrLiꢀ(R,R)-TECDA]2 or an analogous
structure to 5. This is confirmed by DFT studies [B3LYP/
6-31+G(d)] showing no stationary point for the dimer and an
opening of an analogous structure to 5. Consequently, for
steric reasons (R,R)-TECDA and i-PrLi can only form a
monomeric structure.
7. M. J. Frisch et al., GAUSSIAN 03 (Revision D.01), see ESIw.
8. C. Strohmann, T. Seibel and K. Strohfeldt, Angew. Chem., 2003,
115, 4669; C. Strohmann, T. Seibel and K. Strohfeldt, Angew.
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C. Strohmann and V. H. Gessner, Angew. Chem., Int. Ed., 2007,
46, 8281.
10. Examples of further monomeric organolithium compounds:
(a) U. Schumann, J. Kopf and E. Weiss, Angew. Chem., Int. Ed.
¨
Engl., 1985, 24, 215; (b) J. Arnold, V. Knapp, J. A. R. Schmidt and
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M. Marsch, K. Harms and G. Boche, Chem. Ber., 1989, 122, 2303;
(d) C. Strohmann, T. Seibel and D. Schildbach, J. Am. Chem.
Soc., 2004, 126, 9876.
11. (a) S. T. Chadwick, R. A. Rennels, J. L. Rutherford and
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(b) S. T. Chadwick, A. Ramirez, L. Gupta and D. B. Collum,
J. Am. Chem. Soc., 2007, 129, 2259; (c) P. G. Williard and
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In summary, we have presented molecular structures of
isopropyllithium with three nitrogen ligands. The different
steric demand of these Lewis bases leads to a change in the
degree of aggregation from the dimeric TMEDA adduct (4) to
ꢂc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 3381–3383 | 3383