Gilman-type versus Lipshutz-type reagents: Competition in lithiocuprate chemistry

Article abstract of DOI:10.1021/om801101u

CuCN reacts with RLi and TMPLi (TMP-2,2,6,6-tetramethylpiperidide) to give Gilman-type cuprates R(TMP)CuLi?nL (R = Ph,n = 3,L = THF 2; R = Me,n = 1,L = TMEDA 3). 3 and 3. LiCN have been tested in directed ortho cupration with data suggesting enhanced effi

Full text of DOI:10.1021/om801101u

38
Organometallics 2009, 28, 38–41
Gilman-Type versus Lipshutz-Type Reagents: Competition in
Lithiocuprate Chemistry
Joanna Haywood, James V. Morey, and Andrew E. H. Wheatley*
Department of Chemistry, UniVersity of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.
Ching-Yuan Liu
AdVanced Elements Chemistry Laboratory, The Institute of Physical and Chemical Research,
RIKEN, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan, and Organization for Frontier Research in
Pharmaceutical Sciences, Hokuriku UniVersity, Kanagawa-machi, Kanazawa 920-1181, Japan
Shuji Yasuike and Jyoji Kurita
Faculty of Pharmaceutical Sciences, Hokuriku UniVersity, Kanagawa-machi, Kanazawa 920-1181, Japan
Masanobu Uchiyama*
AdVanced Elements Chemistry Laboratory, The Institute of Physical and Chemical Research,
RIKEN, 2-1 Hirosawa, Wako-shi, Saitama 351-0198, Japan
Paul R. Raithby
Department of Chemistry, UniVersity of Bath, ClaVerton Down, Bath BA2 7AY, U.K.
ReceiVed NoVember 19, 2008
(Bn ) benzyl) confirming theoretical expectations.7 In spite of
Summary: CuCN reacts with RLi and TMPLi (TMP ) 2,2,6,6-
tetramethylpiperidide)togiVeGilman-typecupratesR(TMP)CuLi·nL
(R ) Ph, n ) 3, L ) THF 2; R ) Me, n ) 1, L ) TMEDA 3).
3 and 3·LiCN haVe been tested in directed ortho cupration with
data suggesting enhanced efficiency for Lipshutz-type 3·LiCN;
competition between Lipshutz- and Gilman-type formulations
is rationalized by DFT methods.
the observation of a complex Schlenk equilibrium, the dimeric
MesCu(NBn2)Li system showed negligible signs of deaggre-
gation in solution. While a deaggregated organo(amido)cuprate
has been established in solutionsa monomeric π-complex
between n-butyl{(R)-N-methyl-1-phenyl-2-(1-pyrrolidinyl)etha-
namido}cuprate and cyclohexanone being invoked in the Cu-
mediated 1,4-alkylation of R,ꢀ-unsaturated ketones8ssuch a
species has not hitherto been isolated.
It has recently been established that amido ligand components
of homoleptic cuprates can act as bases in the directed ortho
cupration (DoC) of functionalized arenes.1 The nontransferable
and potentially chiral nature of amido ligands in heteroleptic
cuprates is already theoretically established,2 and it is this which
has formed the basis of their deployment in synthetic chemistry.3
Whereas homoleptic bis(alkyl)cuprates have been the focus of
both synthetic applications and structural investigations,4 struc-
ture studies on heteroleptic organo(amido)cuprates are harder
to come by.5 It is only lately that Davies et al. reported the first
isolation and solid-state characterization of an organo(amido)-
cuprate,6 the head-to-tail dimeric structure of MesCu(NBn2)Li
In seeking to extend the field of directed aromatic deproto-
nation using heterobimetallic reagents,9 we have probed the
utility of both Gilman- and Lipshutz-type10 organo(amido)cu-
prates in DoC reactions and found that the latter type represent
substantially more effective substrates.1 During the course of
this work, we established that the treatment of CuCN with 2
equiv of TMPLi afforded the isolable Lipshutz-type complex
(TMP)2Cu(CN)Li2 · THF (1), which revealed a dimeric formula-
tion in the solid state, fundamental to the integrity of which
was (i) the ability of [TMP]- to act as an intermetal bridge
between Cu and Li and (ii) the retention of [CN]- in the dimer
core (Scheme 1). We next sought to investigate whether the
retention of cyanide (Lipshutz characteristics) occurred in
heteroleptic organo(amido)cuprates since metallo intermediates,
* To whom correspondence should be addressed. Fax: Int +44-1223-
336362 (A.E.H.W.); Int +81-48-467-2879 (M.U.). E-mail: aehw2@cam.ac.uk
(A.E.H.W.); uchi_yama@riken.jp.
(1) Usui, S.; Hashimoto, Y.; Morey, J. V.; Wheatley, A. E. H.;
Uchiyama, M. J. Am. Chem. Soc. 2007, 129, 15102.
(7) (a) Dieter, R. K.; Tokles, M. J. Am. Chem. Soc. 1987, 109, 2040.
(b) Dieter, R. K.; Hanks, T. W. Organometallics 1992, 11, 3549. (c) Rossiter,
B. E.; Eguchi, M.; Miao, G.; Swingle, N. M.; Hernandez, A. E.; Vickers,
D.; Fluckiger, E.; Patterson, R. G.; Reddy, K. V. Tetrahedron 1993, 49,
965.
(8) Eriksson, J.; Davidsson, O. Organometallics 2001, 20, 4763.
(9) Mulvey, R. E.; Mongin, F.; Uchiyama, M.; Kondo, Y. Angew. Chem.,
Int. Ed. 2007, 46, 3802, and references cited therein.
(2) Yamanaka, M.; Nakamura, E. J. Am. Chem. Soc. 2005, 127, 4697.
(3) (a) Lipshutz, B. H.; Sengupta, S. Org. React. 1992, 41, 135. (b)
Modern Organocopper Chemistry; Krause, N., Ed.; Wiley-VCH: Weinheim,
Germany, 2002. (c) Compounds of Groups 12 and 11 (Zn, Cd, Hg, Cu, Ag,
Au); O’Neil, I., Ed.; Georg Thieme: Stuttgart, Germany, 2004; Science of
Synthesis, Vol. 3.
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P. Chem. Eur. J. 1999, 5, 2680. (b) Henze, W.; Vyater, A.; Krause, N.;
Gschwind, R. M. J. Am. Chem. Soc. 2005, 127, 17335.
(5) Gschwind, R. M. Chem. ReV. 2008, 108, 3029.
(6) Davies, R. P.; Hornauer, S.; Hitchcock, P. B. Angew. Chem., Int.
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Tetrahedron 1984, 40, 5005. (c) Lipshutz, B. H.; Kozlowski, J. A.; Wilhelm,
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10.1021/om801101u CCC: $40.75
2009 American Chemical Society
Publication on Web 12/11/2008

Communications
Organometallics, Vol. 28, No. 1, 2009 39
Scheme 1
putatively of the type R(TMP)Cu(CN)Li2, have recently been
shown to be proficient DoC reagents.1 Notably, recent theoretical
work has suggested the importance of the seven-membered
X2Cu(CN)Li2 metallacycle in the chemistry of cyanide-contain-
ing higher order cuprates.11 Interestingly, however, whereas
several reports of cyanide-containing lower order cuprates
exist,12 the only fully characterized higher order cuprates to
incorporate cyanide are homoleptic. Whereas metallacycle
formation was noted in 1,1 the poor bridging ability of tBu
groups combined with the presence of strong Lewis base incurs
ion separation in [(tBu)2Cu]-[(CN){Li( · THF) · PMDETA}2]+,13
while in (2-Me2NCH2C6H4)2Cu(CN){Li( · 2THF)}2 the bridging
ligands favor polymerization instead.14 Herein we present the
preliminary structural results obtained when organo(cyano)cu-
prates are treated with a lithium amide. Data suggest that, in
contrast to the 2:1 reaction of lithium amide with CuCN, the
1:1 reaction of RCu(CN)Li (R ) alkyl, aryl) with a lithium
amide results in the expulsion of LiCN, that Gilman-type
products compete with the corresponding Lipshutz-type species,
and that solvent interactions prevent aggregation.
Figure 1. Structure of 2 at the 40% probability level with H atoms
and disorder omitted. Selected bond lengths (Å): C10-Cu1 )
1.903(2), N1-Cu1 ) 1.9116(18), N1-Li1 ) 2.188(4).
Scheme 2
1:1:3 ratio and therefore pointing to a Gilman-type formulation.
This was confirmed crystallographically, with 2 depositing in
j
the triclinic crystal system P1, and the crystal structure (Figure
1) revealing the monomeric complex PhCu(µ-TMP)Li · 3THF
in which the copper center adopts a near-linear geometry
(N1-Cu1-C10 ) 176.36(8)°) and the amide acts as an
intermetal bridge (Cu1-N1-Li1 ) 87.04(12)°). While such
behavior has been seen in the metallacyclic M(µ-R)(µ-N)Li
cores of related aluminates16 and zincates,9,17 and equivocally
in manganates,18 the motif seen here, in which essentially linear
geometry at Cu19 prevents the formation of a four-membered
metallacycle, bears close comparison to homoleptic 1 (Cu-N-Li
) 91.12(13), 94.92(14)°)1 and (Ph2N)2Cu(Ph2N)Li2 · 2OEt2
(Cu-N-Li ) 88.3(2)° in a six-membered ring),20 heteroleptic
MesNHCu(PhNH)Li · DME (Cu-N-Li ) 105.9(4), 107.2(5)°
in an eight-membered ring),20 and the aryl(amido)cuprate
MesCu(NBn2)Li (Cu-N-Li ) 89.96°).6 The observation of a
terminal Cu-Ph interaction (1.903(2) Å) is highly unusual.
While this interaction is known from other fields of organo-
copper chemistry,21 the only precedents in lithiocuprate chem-
The recently reported synthesis and isolation of 1 resulted
from the treatment of CuCN with excess TMPLi, after which
the solvent was removed and replaced by toluene for recrys-
tallization.1 The homoleptic product was shown to effect DoC
of the representative substrate N,N-diisopropylbenzamide with
the TMP ligand acting as a base, enabling homocoupling to
give a 2,2-biaryl through oxidation by PhNO2. Conversely, the
synthesis of heterocoupled 2-RC6H4C(O)NiPr2 (R ) Me, Ph)
was enabled by the use of the corresponding putative heteroleptic
cuprates R(TMP)Cu(CN)Li2. In seeking to probe the identity
of the aryl cuprate (R ) Ph), we sequentially treated a slurry
of CuCN in THF with THF stock solutions of PhLi and TMPLi
(1 equiv each). The resulting solution was concentrated prior
to storage at -30 °C, after which colorless plates of 2 were
obtained (Scheme 2). However, importantly, whereas the
spectroscopic analysis of bulk 1 supplied evidence for the
inclusion of cyanide (clearly seen at δ 167.1 by 13C NMR
spectroscopy and 2104.1 cm-1 by IR spectroscopysreplaced
by a strong signal at 2129.2 cm-1 upon hydrolysis),15 compa-
rable analysis of bulk 2 demonstrated the complete absence of
both the 13C NMR resonance and the infrared stretching modes
attributable to cyanide. Instead, 1H NMR spectroscopy yielded
signals suggesting the presence of phenyl, TMP, and THF in a
(16) (a) Naka, H.; Uchiyama, M.; Matsumoto, Y.; Wheatley, A. E. H.;
McPartlin, M.; Morey, J. V.; Kondo, Y. J. Am. Chem. Soc. 2007, 129, 1921.
´
(b) Garc´ıa-Alvarez, J.; Hevia, E.; Kennedy, A. R.; Klett, J.; Mulvey, R. E.
Chem. Commun. 2007, 2402.
(17) (a) Clegg, W.; Dale, S. H.; Hevia, E.; Honeyman, G. W.; Mulvey,
R. E. Angew. Chem., Int. Ed. 2006, 45, 2370. (b) Uchiyama, M.; Matsumoto,
Y.; Nobuto, D.; Furuyama, T.; Yamaguchi, K.; Morokuma, K. J. Am. Chem.
Soc. 2006, 128, 8748. (c) Kondo, Y.; Morey, J. V.; Morgan, J. M.; Raithby,
P. R.; Nobuto, D.; Uchiyama, M.; Wheatley, A. E. H. J. Am. Chem. Soc.
´
´
(11) (a) Nakamura, E.; Mori, S. Angew. Chem., Int. Ed. 2000, 39, 3750.
(b) Mori, S.; Nakamura, E.; Morokuma, K. J. Am. Chem. Soc. 2000, 122,
7294.
(12) (a) Hwang, C.-S.; Power, P. P. J. Am. Chem. Soc. 1998, 120, 6409.
(b) Hwang, C.-S.; Power, P. P. J. Am. Chem. Soc. 1999, 18, 697. (c) Eaborn,
C.; Hill, M. S.; Hitchcock, P. B.; Smith, J. D. Organometallics 2000, 19,
5780. (d) Bosold, F.; Marsch, M.; Harms, K.; Boche, G. Z. Kristallogr.
New Cryst. Struct. 2001, 216, 143. (e) Eaborn, C.; El-Hamruni, S. M.; Hill,
M. S.; Hitchcock, P. B. Dalton Trans. 2002, n/a, 3975. (f) Davies, R. P.;
Hornauer, S. Eur. J. Inorg. Chem. 2005, 51.
2007, 129, 12734. (d) Clegg, W.; Garcia-Alvarez, J.; Garcia-Alvarez, P.;
Graham, D. V.; Harrington, R. W.; Hevia, E.; Kennedy, A. R.; Mulvey,
R. E.; Russo, L. Organometallics 2008, 27, 2654.
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(18) Garcia-Alvarez, J.; Kennedy, A. R.; Klett, J.; Mulvey, R. E. Angew.
Chem., Int. Ed. 2007, 46, 1105.
(19) Power, P. P.; Ruhlandt-Senge, K.; Shoner, S. C. Inorg. Chem. 1991,
30, 5013.
(20) Reiss, P.; Fenske, D. Z. Anorg. Allg. Chem. 2000, 626, 1317.
(21) (a) Gambarotta, S.; Strologo, S.; Floriani, C; Chiesi-Villa, A.
Organometallics 1984, 3, 1444. (b) Dattelbaum, A. M.; Martin, J. D.
Polyhedron 2006, 25, 349. (c) Fischer, R.; Gorls, H.; Westerhausen, M.
Organometallics 2007, 26, 3269.
(13) Boche, G.; Bosold, F.; Marsch, M.; Harms, K. Angew. Chem., Int.
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(15) See the Supporting Information.
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J. Am. Chem. Soc. 1985, 107, 4337. (b) Davies, R. P.; Hornauer, S. Eur.
J. Inorg. Chem. 2005, 51.

40 Organometallics, Vol. 28, No. 1, 2009
Communications
Scheme 3
istry are the longer 1.925(10) and 1.916 Å (mean) bonds noted
in the ion separates [Ph2Cu]-[Li · 2(12-crown-4)]+ and
22
{[Ph2Cu]-}2[(Cu( · 2PPh3){(CN)Li}2 · 5THF)2]2+
,
while the
remaining reported terminal Cu-Ar bonds in this area feature
kinetically protective Mes,23 C6H3-2,6-Mes2,24 or C6H3-2,6-Trip2
(Trip ) C6H2-2,4,6-iPr3).12a,b Consistent with the high level of
external solvation of the alkali metal (O-Li1 range 2.041(4)-
2.067(4) Å), N1-Li1 is, at 2.188(4) Å, relatively long (viz.
1.982(5)and1.995(4)Åin1and1.969(10)ÅinMesCu(NBn2)Li).1,6
The ability of the putative cuprate Me(TMP)Cu(CN)Li2 to
achieve the heterocoupled product 2-MeC6H4C(O)NiPr2 led us
to probe the identity of the heterometallic species isolable from
a MeLi/CuCN/TMPLi mixture. Accordingly, a slurry of CuCN
in THF was sequentially treated with etherate solutions of MeLi
and TMPLi (1 equiv each). Concentration of the resulting
solution gave an oil, the storage of which at -30 °C failed to
yield any isolable material. However, a surprising modification
of the protocol led to greater success. Thus, a stock solution of
TMPLi (1 equiv) in dry toluene was added to a slurry containing
equimolar MeLi and CuCN in dry TMEDA at -78 °C. The
resulting cream-colored slurry changed color from medium
yellow to pale yellow and was left to reach 0 °C, whereupon
the introduction of THF gave a solution. The storage of this
mixture at -30 °C for 24 h produced colorless needles (Scheme
3). As for 2, 13C NMR and infrared spectroscopy again clearly
showed the complete absence of cyanide. 1H NMR spectroscopy
in C6D6 showed signals attributable to only Me, TMP, and
TMEDA, with the respective integrations (1:1:1) suggesting a
Gilman-type cuprate akin to 2, namely MeCu(µ-TMP)-
Li · TMEDA (3). The most notable spectroscopic feature of this
formulation is the cuprated methyl group, and this was plainly
Figure 2. Structure of 3 at the 40% probability level with TMEDA
disorder and lattice THF and H atoms omitted. Selected bond
lengths (Å) and angles (deg): C1-Cu1 ) 1.927(5), N1-Cu1 )
1.942(4),N1-Li1)1.992(9);Cu1-N1-Li1)92.1(3),N1-Li1-N2
) 139.2(5), N1-Li1-N3 ) 133.6(5).
dimethylcuprate ions have been noted in [Me2Cu]-[Li · nL]+ (n
) 2, L ) 12-crown-4, C-Cu ) 1.935(8) Å; n ) 3, L ) DME,
mean C-Cu ) 1.933 Å)22,26 and, of the alkyl(phosphido)cu-
prates RCu{µ-P(tBu)2}Li · 3THF (R ) Me, nBu, sBu, tBu), the
methyl homologue has been fully characterized (C-Cu )
1.940(4) Å).27
We have previously shown that Gilman-type cuprates rep-
resent inferior DoC substrates when they are compared to their
Lipshutz-type analogues.1 Accordingly, we conducted experi-
ments that proved the importance of LiCN inclusion in
amidocuprate chemistry by revealing the ortho iodination of
benzonitrile in 74% yield employing (TMP)2CuLi · LiCN but
complete failure of the reaction if (TMP)2CuLi · LiI is used
instead. To further test this idea, here we have treated N,N-
diisopropylbenzamide both with preisolated 3 and solutions
containing 3 and LiCN and, thereafter, with I2.15 Data reveal
that iodination proceeds in both cases but affords only 37%
2-iodo-N,N-diisopropylbenzamide when employing preisolated
3. This improved to 89% using a solution from which 3 had
deposited at -30 °C (and which logically therefore contained
LiCN) prior to its redissolution at ambient temperature. Having
noted this discrepancy, we sought to probe the nature of the
relationship between Lipshutz- and Gilman-type species theo-
retically (B3LYP method, SVP basis set for Cu and 6-31+G*
basis set for other atoms).15 The calculation of a negative ∆E
value of -3.5 kcal/mol suggests that in the presence of an excess
of etherate solvent (Me2O) the modeled heteroleptic (methyl/
dimethylamido) Lipshutz-type cuprate Me(Me2N)Cu(CN)-
Li2 · 2OMe2 (viz. 1) exists in facile equilibrium with its Gilman-
type analogue MeCu(NMe2)Li · 3OMe2 (viz. the tris(THF)
solvate 2, Scheme 4a). Of particular interest is the theoretical
observation that the corresponding homoleptic bis(amido)cuprate
(Me2N)2Cu(CN)Li2 · 2OMe2 dominates the modeled equilibrium
between itself and the Gilman-type species Me2NCu-
(NMe2)Li · 3OMe2 (∆E ) +9.1 kcal/mol, Scheme 4b). Not only
are these data consistent with our previous isolation and
characterization of 1,1 but the computed ability of
Me(Me2N)Cu(CN)Li2 · 2OMe2 to dissociate accounts for the
tandem observations that a solution containing 3 and LiCN gives
a high synthetic yield of 2-iodo-N,N-diisopropylbenzamide yet
deposits Gilman-type 3.
1
observable by both H and 13C NMR methods at δ -0.07 and
-11.0, respectively.
X-ray crystallography confirmed the proposed formulation
of 3, with the crystals depositing in the orthorhombic system
Pmcn. For each molecule of MeCu(µ-TMP)Li · TMEDA there
resides one unit of lattice THF, though analysis suggests that
this is largely removed in vacuo during isolation.15 The structure
demonstrates a monomer in spite of a reduction in the
coordination state of the alkali metal from distorted tetrahedral
in 2 to essentially trigonal planar in 3 (sum of angles at Li1
359.9°; Figure 2). The TMEDA-Li interactions are 2.120(11)
and 2.081(9) Å. However, in response to the decreased group
1 metal coordination state, the lithium-amide distance is, at
1.992(9) Å, rather short for a N-Li bond25 and significantly
less than the analogous interaction in 2. The angles at both the
intermetal amide bridge (Cu1-N1-Li1 ) 92.1(3)°) and the
copper atom (178.2(2)°) are comparable to those in 2. However,
both bonds to copper are extended in 3 relative to those in 2
(N1-Cu1 ) 1.942(4) Å and C1-Cu1 ) 1.927(5) Å). As with
that of the Cu-Ph bond in 2, the characterization of a Cu-Me
interaction in the solid state is, in itself, unusual. Homoleptic
(26) John, M.; Auel, C.; Behrens, C.; Marsch, M.; Harms, K.; Bosold,
F.; Gschwind, R. M.; Rajamohanan, P. R.; Boche, G. Chem. Eur. J. 2000,
6, 3060.
(27) Martin, S. F.; Fishpaugh, J. R.; Power, J. M.; Giolando, D. M.;
Jones, R. A.; Nunn, C. M.; Cowley, A. H. J. Am. Chem. Soc. 1988, 110,
7226.
(23) Davies, R. P.; Hornauer, S. Chem. Commun. 2007, 304.
(24) (a) Niemeyer, M. Organometallics 1998, 17, 4649. (b) Hwang, C.-
S.; Power, P. P. Bull. Korean Chem. Soc. 2003, 24, 605.
(25) Gregory, K.; Schleyer, P. v. R.; Snaith, R. AdV. Inorg. Chem. 1991,
37, 47.

Communications
Organometallics, Vol. 28, No. 1, 2009 41
Scheme 4
In summary, the isolation of 2 and 3 allows the first direct
observation of organo(amido)cuprate monomers. In line with
previous work on organo(amido)cuprates,6 the structures suggest
a predilection for crystallization of the Gilman-type species
RCu(amido)Li with the concomitant exclusion of LiCN. These
data are in contrast with the recent isolation and characterization
of a homoleptic Lipshutz-type bis(amido)cuprate and offer an
explanation for cyanide retention therein and the previous
observation of a seven-membered N2Cu(CN)Li2 metallacycle.
Hence, the crystal structures of 2 and 3 suggest that the ability
of a cuprate structure to exhibit Li(µ-CN)Li bonding must be
combined with a high affinity for intermetal bridging in both
of the Cu-bonded ligands in order to achieve the theoretically
expected X2Cu(CN)Li2 Lipshutz-type metallacycle.11a DFT
studies rationalize these data and, moreover, suggest the facile
interconversion of Lipshutz- and Gilman-type structures in
etherate solvent of heteroleptic alkyl(amido)cuprates. This
behavior explains the crystallographic observation of 2 and 3.
Moreover, calculations point to the contrasting tendency for
homoleptic bis(amido)cuprates to favor LiCN inclusion, as
observed previously by ourselves.1 Finally, conscious that
Gilman-type complexes are inefficient DoC substrates but that
the putative cuprate Me(TMP)Cu(CN)Li2 is an effective DoC
reagent,1 we have tested both preisolated 3 and solutions
containing 3 and LiCN and have noted the enhanced reactivity
of the Lipshutz-type species in the DoC of a representative
benzamide.
Acknowledgment. This work was supported by the U.K.
EPSRC (J.V.M., J.H.) and Hoansha and KAKENHI (Young
Scientist (A), Houga, and Priority Area No. 452 and 459)
(M.U.). The calculations were performed on the RIKEN
Super Combined Cluster (RSCC).
Supporting Information Available: Text, figures, and tables
giving synthetic and spectroscopic data and crystallographic data
for 1, 2, and 3 and a CIF file giving crystal data for 3. This material
is available free of charge via the Internet at http://pubs.acs.org.
OM801101U