Structural effects in lithiocuprate chemistry: The elucidation of reactive pentametallic complexes

Article abstract of DOI:10.1002/chem.201304824

TMPLi (TMP=2,2,6,6-tetramethylpiperidide) reacts with Cu^I salts in the presence of Et₂O to give the dimers [{(TMP) ₂Cu(X)Li₂(OEt₂)}₂] (X=CN, halide). In contrast, the use of DMPLi (DMP=cis-

Full text of DOI:10.1002/chem.201304824

DOI: 10.1002/chem.201304824
Communication
&
Amidocuprates
Structural Effects in Lithiocuprate Chemistry: The Elucidation of
Reactive Pentametallic Complexes
Philip J. Harford,[a] Andrew J. Peel,[a] Joseph P. Taylor,[a] Shinsuke Komagawa,[b]
Paul R. Raithby,[c] Thomas P. Robinson,[c] Masanobu Uchiyama,*[b] and
Andrew E. H. Wheatley*[a]
theory)[7] heteroleptic monomers[8,9] and dimers (Figure 1a, b,
d).[10] Conversely, cyanide-containing Lipshutz cuprates[11] with
Abstract: TMPLi (TMP=2,2,6,6-tetramethylpiperidide)
reacts with CuI salts in the presence of Et2O to give the
heteroaggregate structures have now been elucidated (Fig-
ure 1e),[3] with very recent work proving that the replacement
dimers [{(TMP)2Cu(X)Li2(OEt2)}2] (X=CN, halide). In contrast,
the use of DMPLi (DMP=cis-2,6-dimethylpiperidide) gives
an unprecedented structural motif; [{(DMP)2CuLi-
(OEt2)}2LiX] (X=halide). This formulation suggests a hither-
to unexplored route to the in situ formation of Gilman-
type bases that are of proven reactivity in directed ortho
cupration.
of X=CN by X=halide affords structurally analogous com-
plexes,[4c,d,6] and suggesting use of the term Lipshutz-type to
Organocuprate(I) complexes in general,[1] and amidocuprates
in particular, have proved to be tremendously important in ef-
fecting stereoselective organic transformations,[2] with recent
work yielding lithiocuprates of the type RR’CuXLi2 (R, R’=or-
ganyl, TMP (TMP=2,2,6,6-tetramethylpiperidide); X=CN,
halide) and the new field of directed ortho cupration (DoC). It
has been noted that the TMP group could react to achieve
chemoselective DoC transformations and the subsequent trap-
ping of electrophiles or oxidative ligand coupling showed the
significant potential of these reagents in CÀC and CÀO bond
formation.[3,4]
Figure 1. Established lithium amidocuprate structure types; a) AM=N-
(CH2Ph)Et, R=Mes, n=3, S=THF;[9] AM=TMP, R=Ph, n=3, S=THF;[8]
AM=TMP, R=Me, n=1, S=TMEDA;[8] b) AM=N(CH2Ph)2, R=Mes;[10]
c) AM=X=NPh2, n=1, S=OEt2;[12] d) AM=NHMes, AM’=NHPh, n=1,
S=DME;[12] e) AM=TMP, X=CN, Cl, Br, I, n=1, S=THF.[3,4b–d,6]
Structural organocuprate(I) chemistry was recently the sub-
ject of review.[5] Recent advances have revealed so-called
Gilman-type monomers and dimers[6] and (in line with
describe this wider group of comparable cuprates. Interesting-
ly, although the reactivity of Lipshutz-type cuprates has been
considered to often exceed that of their Gilman-type counter-
parts,[4b,8,13] it was recently suggested that a Lipshutz-type re-
agent could be used to generate a more reactive Gilman-type
intermediate in situ.[6]
[a] P. J. Harford, A. J. Peel, J. P. Taylor, Dr. A. E. H. Wheatley
Department of Chemistry, University of Cambridge
Lensfield Road, Cambridge, CB2 1EW (UK)
E-mail: aehw2@cam.ac.uk
[b] Dr. S. Komagawa, Prof. Dr. M. Uchiyama
Presently we report the investigation of ligand effects
through varying the amido component of new lithiocuprate
bases. Data reveal a hitherto unrecognised class of cuprate
heteroaggregate.
Advanced Elements Chemistry Research Team
RIKEN Center for Sustainable Resource Science (CSRS)
and Elements Chemistry Laboratory, RIKEN
2-1 Hirosawa, Wako-shi, Saitama 351-0198 (Japan)
Graduate School of Pharmaceutical Sciences, The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
E-mail: uchiyama@mol.f.u-tokyo.ac.jp
We have recently sought to develop our understanding of
ligand and solvent influences on lithiocuprate structures by
modifying our previous syntheses of [{(TMP)2Cu(X)Li2(thf)}2]
(X=CN,[3] halide[6]). To this end, TMPLi was added to CuCN
before introducing Et2O. Storage of the resulting solution af-
forded crystalline 1, the X-ray diffraction of which showed Lip-
shutz cuprate [{(TMP)2Cu(CN)Li2(OEt2)}2]. Though the quality of
the data were poor (Rint >10%), the connectivity was unambig-
uous and the dimer was plainly analogous to that seen with
THF.[3] Comparable reactions using CuHal gave [{(TMP)2Cu-
[c] Prof. P. R. Raithby, T. P. Robinson
Department of Chemistry, University of Bath
Claverton Down, Bath, BA2 7AY (UK)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304824.
ꢀ 2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons At-
tribution License, which permits use, distribution and reproduction in any
medium, provided the original work is properly cited.
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Communication
(Hal)Li2(OEt2)}2] (Hal=Cl 2, Br 3, I 4), establishing the general
isolation of the structure-type seen for 1 (Scheme 1). In each
case superior crystal data were obtained, with structural pa-
rameters found to be closely related to those seen in
[{(TMP)2Cu(Hal)Li2(thf)}2] (Figure 2).[4c,d,6] The monomeric Lip-
shutz-type building blocks in 2–4 revealed symmetrical 6-
Scheme 2. Synthesis of 5 (M=Cu, X=Cl, n=bulk), 6 (M=Cu, X=Br;
n=bulk or 2 equiv with respect to Cu) and 7 (M=Cu, X=I; n=1 equiv with
respect to Cu).
Scheme 1. Synthesis of 1–4.
Figure 3. Adduct [{(DMP)2CuLi(OEt2)}2LiCl] (5). H atoms and minor Et2O disor-
der are omitted and the adduct core is viewed along the Li2ÀCl1 axis in (b).
Selected bond lengths [ꢁ] and angles [8]: Li1-Cl1 2.354(6), Li2-Cl1 2.412(7),
Li3-Cl1 2.301(7), Li1-N1 1.986(8), Li2-N2 2.035(7), Li2-N3 2.054(6), Li3-N4
1.970(9), Li1-N1-Cu1 87.6(2), Li2-N2-Cu1 94.1(2), Li2-N3-Cu2 94.2(2), Li3-N4-
Cu2 90.1(2), N2-Li2-N3 129.2(4), Li1-Cl1-Li3 139.5(2).
observable presence of an NH resonance (at d=0.85 ppm) was
inconsistent with the spectral data obtained for 1–4. Crystallo-
graphic analysis revealed a unique triangulated structure
based on a lithium halide core and having the formulation
[{(DMP)2CuLi(OEt2)}2LiCl] (5). This identification suggests that
the (reproducible) observation of DMPH in solution is attribut-
able to extreme moisture sensitivity in spite of the storage of
deuterated solvents over a fresh Na mirror. The solid-state
structure of 5 can be viewed as representing the first full char-
acterisation of an adduct between a Lipshutz- and a Gilman-
type cuprate.
Figure 2. [{(TMP)2Cu(Cl)Li2(OEt2)}2] (2). H atoms are omitted. Selected bond
lengths [ꢁ] and angles [8]: Li1-Cl1 2.344(6), Li2-Cl1 2.332(7), Li1-N1 2.024(6),
Li2-N2 1.953(7), Li1-N1-Cu1 90.4(2), Li2-N2-Cu1 91.8(2), Cl1-Li1-N1 125.2(3),
Cl1-Li2-N2 127.3(3).
membered rings with each amide acting as an intermetal
linker through the construction of uniform Cu-N-Li bridges
(Cu-N-Li 90.4(2)-92.26(12)8, Cu-N-Li(OEt2) 89.1(3)-91.8(2)8).
Recent studies have suggested the importance of steric ef-
fects in controlling amidocuprate reactivity.[3] To further probe
this issue we have investigated the effect of replacing TMP
with less bulky DMP (cis-2,6-dimethylpiperidide). Notably,
DMPH also retails at a fraction of the cost of TMPH.[14] In the
present case, DMPLi was added to CuX in the presence of
Lewis base (bulk or 1 or 2 equiv Et2O with respect to Cu). At-
tempts to isolate a product when X=CN are yet to bear fruit.
However, for X=Hal a remarkable series of structurally analo-
gous complexes was obtained (Scheme 2 and illustrated for
X=Cl in Figure 3).
The formation of this new class of cuprate adduct was next
replicated using CuBr in the presence of Et2O in order to pre-
pare [{(DMP)2CuLi(OEt2)}2LiBr] (6). Initial attempts used nBuLi
(1 equiv with respect to amine) in the preparation of DMPLi. In
the case of bulk Et2O this led to 6 (Scheme 2).[15] In contrast,
the use of 2 equiv Et2O afforded [{(DMP)2CuLi(OEt2)}1.45
-
{(DMP)2CuLi(DMPH)}0.55LiBr] (6’). This problem could be over-
come, and the reproducible formation of 6 was obtained, by
using 1.1 equiv nBuLi.[15]
Whereas the preparations of 5 and 6 used solvent condi-
tions of bulk Et2O (for Cl) or of either bulk or limited Et2O (for
Br), attempts to prepare the iodide analogue required that
strictly limited quantities of Et2O be present. The use of bulk
donor failed to afford isolable material, whereas the presence
For the use of X=Cl, the presence of bulk Et2O (Scheme 2)
1
allowed the isolation of crystals that H NMR spectroscopy sug-
gested contained Et2O and DMP in a 1:2 ratio. However, the
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of 2 equiv Et2O with respect to Cu afforded only LiI(OEt2).[16]
However, the documented solubility of lithium iodide in Et2O
led us to suspect that this was causing the salt to largely
remain in solution during filtration of the reaction mixture and
to be subsequently crystallising. The amount of donor solvent
was therefore further restricted to promote lithium iodide pre-
cipitation and removal. Storage of the resulting filtrate at
À278C yielded crystalline blocks that X-ray crystallography
confirmed to be [{(DMP)2CuLi(OEt2)}2LiI] (7; Scheme 2).[15] As
with 5 and 6, the spectroscopic observation of NH was inter-
preted in terms of extreme moisture sensitivity.
The structures of 6 and 7 are highly analogous to that of 5
and all exhibit approximate C2 symmetry about a central lithi-
um halide axis, as shown representatively in Figure 3. In each
case the halide shows triangulated coordination and two types
of bond to Li+, with Li2ÀX being relatively extended (Li2ÀX
2.412(7), 2.592(7) and 2.971(16) ꢁ in 5, 6 and 7, respectively).
The Li1/3ÀX bonds are somewhat inequivalent: 2.354(6)/
2.301(7), 2.474(8)/2.515(8) and 2.720(13)/2.667(14) ꢁ in 5, 6 and
7, respectively. As would be expected, the metalÀhalide bonds
extend as Group VII is descended. However, this extension is
not consistent and the ratio between Li2ÀX and the mean of
Li1/3ÀX is greater for iodide (1.10) than for chloride or bromide
(1.04 in either case). This suggests that, rather than simply at-
tributing this bond extension to the ionic radius of the halide,
competition between metal stabilisation by hard and soft
donors must also be considered. Thus, in the presence of soft
iodide, Li2 is more inclined to be stabilised by the N2/3-based
DMP ligands. This is reflected also in the Li2-N-Cu angles,
which increase in response to the higher halide: Li2-N2/3-Cu1/
2 94.1(2)/94.2(2)8 (5), 97.3(2)/97.5(3)8 (6), 100.1(5)/101.2(5)8 (7).
A similar trend is seen for the remaining two (N1/4-based) li-
gands, though, consistent with the shorter Li1/3ÀX bonds, the
angles are smaller: Li1/3-N1/4-Cu1/2 87.6(2)/90.1(2)8 (5),
92.5(3)/89.0(3)8 (6), 96.1(5)/96.8(5)8 (7). The asymmetry in these
angles at nitrogen contrasts with the more symmetrical rings
in 2–4, in which the difference between Cu-N-Li and Cu-N-
Li(OEt2) was never more than 38.
Figure 4. Adduct [{(DMP)2CuLi(thf)2}2LiBr] (8). H atoms are omitted. Selected
bond lengths [ꢁ] and angles [8]: Li1-Br1 2.609(11), Li2-Br1 2.677(11), Li3-Br1
2.602(12), Li1-N1 2.067(13), Li2-N2 2.045(11), Li2-N3 2.029(11), Li3-N4
2.094(15), Li1-N1-Cu1 94.3(4), Li2-N2-Cu1 93.9(4), Li2-N3-Cu2 94.2(4), Li3-N4-
Cu2 92.0(4), N2-Li2-N3 132.2(6), Li1-Br1-Li3 147.3(4).
solid-state structures. In all cases a low-field signal (at d=2.16–
2.18 ppm) matches the dominant signal (at d=2.20 ppm) in
a DMPLi reference spectrum. For each of 5, 6 and 7 in
[D6]benzene the dominant signals are seen at d=1.83–1.84
and 1.41–1.50 ppm in a 1:2 ratio, consistent with the crystallo-
graphic data. In the case of 6’, the spectrum is more complicat-
ed, yet still consistent with crystallography. The presence of
Li(DMPH) now introduces a signal at d=1.66 ppm. However,
the proximity of this to the d=1.48 ppm signal attributable to
Li(OEt2) prevents their separate integration. Lastly, for 8 in
[D6]benzene a single environment is observed by 7Li NMR
methods, and we attribute this to the four THF molecules pres-
ent in 8, which create a more polar medium than the two
Et2O/DMPH molecules in 5–7.
Subsequent investigation focused on the reasons for the
transition in structure type from dimers 1–4 to adducts 5–8.
The possibility that solvent identity or quantity was a determin-
ing factor having been removed, competition was presumed
to be dictated by the amide. This can be seen from the chlo-
ride species shown in Figures 2 and 3. The two TMP ligands as-
sociated with any given Cu atom (see N1, N2 in Figure 2) proj-
ect away from one another so as to lie endo,endo with respect
to the structure core (Figure 5, left). In contrast, the presence
Lastly, treatment of DMPLi with CuBr in dry toluene followed
by recrystallisation in the presence of THF at À278C yielded
[{(DMP)2CuLi(thf)2}2LiBr] (8) and established that adduct forma-
tion is not limited to the deployment of Et2O (Figure 4). The
two THF-solvated Li+ ions are now pseudotetrahedral. Al-
though the crystal structure of 8 is largely analogous to that of
6, the effect of using a stronger Lewis base can be noted.
Whereas Li2ÀN2/3 bonds are essentially unaffected, both Li1À
N1 and Li3ÀN4 are extended in 8. Similarly, Li1/3ÀBr1 increases
significantly from 2.474(8)/2.515(8) ꢁ in
6 to 2.609(11)/
2.602(12) ꢁ in 8. The asymmetry in the Li-N-Cu bond angles
noted in 6 is also now absent; the four angles in 8 being es-
sentially identical. Lastly, evidence for additional stability con-
ferred by the four THF molecules in 8 comes from the observa-
tion of a substantially smaller NH resonance in the 1H NMR
spectrum (c.f. 5–7).[15]
Although 1H NMR spectroscopy on adducts 5–7 suggests
some sensitivity towards trace moisture, Li NMR spectroscopic
Figure 5. Structures of the Lipshutz-type monomers incorporated in TMP-
based 2 (left) and DMP-based 5 (right) highlighting the endo,endo and
exo,exo amide orientations.
7
analysis is consistent with a significant level of retention of the
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of DMP much reduces steric interaction between the methyl
groups in the pair of amide ligands, allowing the piperidide
rings to reside face-on to each other in a way that would be
sterically precluded for TMP. The consequence of the face-on
motif adopted by the DMP ligands is that they project away
from the aggregate core in exo,exo fashion (Figure 5, right).
This configuration of the DMP ligands in 5–8 also avoids steric
congestion between the two amides that are bonded to the
single unsolvated Li+ centre in the structure.
We already know that Gilman-type cuprates show inferior
DoC activity when compared to their Lipshutz-type ana-
logues.[3] However, we have also established that DoC reactivity
actually requires monomeric Gilman-type reagents accessed
from a Lipshutz-type precursor,[6] and we here reinforce the im-
portance of LiHal inclusion in amidocuprate chemistry
(Scheme 3). In THF, N,N-diisopropylbenzamide reacted to give
Scheme 4. The interconversion of Lipshutz- and Gilman-type dimers via
adduct [(Me2N)2CuLi(OMe2)]2LiCl (LD-LMGM-GD) at B3LYP/SVP level.[15,17] DE
1
[DG] are in kcalmolÀ1. “LiCl” is /4 [LiCl(OMe2)]4ÀOMe2.
compares favourably with that recently calculated using a Lip-
shutz monomer as the starting point,[6] and reinforces the view
that LMGM adducts, such as 6, represent viable DoC reagents.
To conclude, a previously unexplored class of triangulated
lithium amidocuprate best viewed as a 1:1 adduct between
Gilman- and Lipshutz-type monomers, is reported. The forma-
tion of 5–8 can be viewed as resulting from the abstraction of
lithium halide from a Lipshutz-type dimer and the relative ori-
entations of the amide ligands in both dimers 2–4 and adducts
5–8 can be rationalised sterically. Derivatisations of an aromatic
tertiary amide undertaken with: 1) 2:1 DMPLi/CuBr, and 2) 6 re-
inforce the importance of LiX-containing systems in amidocup-
rate reactivity.[6] In both cases, high conversion is achieved
using bases made with DMPH, suggesting major cost bene-
fits.[14] To improve our mechanistic understanding, we have
now initiated a detailed study of the solution behaviour of
these adducts.[18] We are also seeking to use various amines to
probe the relationship between ligand bulk and structure type.
CCDC-964430–964439 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Scheme 3. Directed ortho iodination using DMPLi, CuBr and N,N-diisopropyl-
benzamide in a 4:2:1 ratio (top) or 6 and N,N-diisopropylbenzamide in a 1:1
ratio (bottom).
2-iodo-N,N-diisopropylbenzamide (9) in 80% yield using
DMPLi, CuBr and the benzamide in a 4:2:1 ratio (i.e., 2 equiv
Lipshutz-type Cu per arene) prior to I2 work-up. Meanwhile,
dissolution of pre-isolated 6 and N,N-diisopropylbenzamide in
a 1:1 ratio (i.e., 2 equiv Cu per arene) gave 9 in an essentially
identical yield of 82%. These data show that an adduct such
as 6 can, like a Lipshutz-type dimer,[6] be viewed as an efficient
source of reactive Gilman-type monomers.
We sought to probe the relationship between Lipshutz- and
Gilman-type dimers using DFT methods (Scheme 4).[17] Results
obtained with the simplified (Me2N)2CuLi(OMe2)+LiCl
system[3,6,8] suggest that Lipshutz-type dimer LD exhibits an en-
thaplic preference (DE=À13.7 kcalmolÀ1) for eliminating a lithi-
um halide solvate and forming a Gilman-type dimer, GD.[15]
Meanwhile, a small entropy decrease, consistent with solvation
of the eliminated halide, explains a slight increase (+4.5 kcal
molÀ1) in DG. Though the adduct between a Lipshutz- and
a Gilman-type monomer (LMGM) is not the preferred cuprate of
the three, the energetic balance between species is a fine one.
Lastly, the ability of LMGM to associate with a reagent, such as
N,N-dimethylbenzamide, prior to effecting a DoC reaction has
been investigated.[15,17] Results indicate that the conversion of
LMGM to a complex between GM (Me2N)2CuLi(OMe2) and N,N-di-
methylbenzamide along with 0.5LD is accompanied by
Acknowledgements
We thank the UK EPSRC (EP/J500380/1), the GB Sasakawa
Foundation (A.W.), JSPS KAKENHI (S) (No. 24229001), the
Takeda Science, Asahi Glass and Sumimoto Foundations, the
Foundation NAGASE Science Technology development (M.U.),
and the JSPS for a Research Fellowship for Young Scientists
(S.K.). We thank Dr. Joanna Haywood for help with crystallogra-
phy. Calculations were performed using the RICC facility at
RIKEN.
Keywords: copper · density functional calculations · directed
metalation · lithium · solid-state structures
a change in DG of only +6.1 kcalmolÀ1 [15]
suggests a route to a N,N-dimethylbenzamide–GM adduct that
.
This energy profile
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Communication
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Received: December 10, 2013
Published online on February 19, 2014
Chem. Eur. J. 2014, 20, 3908 – 3912
www.chemeurj.org
3912ꢀ 2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim