First X-ray crystal structure and internal reference diffusion-ordered NMR spectroscopy study of the prototypical posner reagent, MeCu(SPh)Li(THF) 3

Article abstract of DOI:10.1002/chem.201301415

Grow slow: The usual direct treatment of MeLi and CuSPh did not yield X-ray quality crystals of MeCu(SPh)Li. An indirect method starting from Me ₂CuLi×LiSPh and chalcone afforded the desired crystals by the slow reaction of the intermediate π-complex (see scheme). This strategy produced the first X-ray crystal structure of a Posner cuprate. A complementary NMR study showed that the contact ion pair was also the main species in solution. Copyright

Full text of DOI:10.1002/chem.201301415

DOI: 10.1002/chem.201301415
First X-Ray Crystal Structure and Internal Reference Diffusion-Ordered
NMR Spectroscopy Study of the Prototypical Posner Reagent,
MeCu(SPh)Li(THF)3
Steven H. Bertz,* Richard A. Hardin, Thomas J. Heavey, Daniel S. Jones,
T. Blake Monroe, Michael D. Murphy, Craig A. Ogle,* and Tara N. Whaley[a]
Dedicated to Professor Gary Posner, an organocopper pioneer, a gentleman and a scholar, on the occasion of his 70th birthday
Heterocuprates, RACTHNUTRGNEUNG(Het)CuLi, are a successful solution to
the efficiency problem of homocuprates, R2CuLi (“Gilman
reagents”), which only transfer one equivalent of R in most
applications.[1,2] The non-transferred ligand, Het, in these
mixed cuprates[3] is bonded to copper via a heteroatom (e.g.,
Het=SPh, NCy2, PPh2). The first synthetically useful
heteroACHTUNGTRENNUNGcuprates were the phenylthiocuprates, introduced by
Posner et al.[4] They were followed by more stable and more
reactive amidocuprates[5] and phosphidocuprates;[5–7] never-
theless, thiocuprates are still very useful, as the usual precur-
sor (CuSPh) is stable and commercially available; moreover,
they are key intermediates in some interesting reactions of
homocuprates.[2] By using a novel preparative procedure
(1; Scheme 1), we have been able to obtain the first X-ray
Figure 1. ORTEP plot (50% ellipsoids) for 1. Selected bond distances
crystal structure of a Posner cuprate, MeCuACTHNUTRGENNUG(SPh)LiACHTUGNTREN(NUNG THF)3
(1; Figure 1), and by using diffusion-ordered NMR measure-
ments, we have found that this structure persists in solution.
À
À
À
[ꢀ] and angles [8]: Cu C, 1.914(3); Cu S, 2.1797(7); Li S, 2.431(4);
À
À
À
À
À
Li O1, 1.920(5); Li O2, 1.914(5); Li O3, 1.914(4); C Cu S, 176.15(12);
À À
Cu S Li, 98.94(10).
Crystals of 1, grown from solutions that had been pre-
pared by adding MeLi (1 equiv) to CuSPh as usual,[4] were
not suitable for X-ray crystallography, as they grew too
quickly. Therefore, we set out to prepare MeCuACTHNUTRGENUG(N SPh)Li by
an alternative procedure, which involved the treatment of
MeCu with PhSLi, both formed in situ. To generate MeCu
slowly, we developed a modification of the method that
Bertz et al.[8] used to prepare halide-free MeCu, which was
based on the observation by House et al.[9] that the 1,4-addi-
tion of Me2CuLi to a-enones yielded MeCu and a lithium
enolate. Instead of 2-cyclohexenone, we used chalcone for
the current application, as its conjugate addition reaction
with organocuprates is much slower.[10]
Scheme 1. Slow synthesis of 1 by an indirect method.
Accordingly, chalcone was added to
Me2CuLi·LiSPh, prepared from MeLi (2 equiv) and CuSPh,
a solution of
[a] Prof. Dr. S. H. Bertz, R. A. Hardin, T. J. Heavey, Prof. Dr. D. S. Jones,
T. B. Monroe, Dr. M. D. Murphy, Prof. Dr. C. A. Ogle, T. N. Whaley
Department of Chemistry
and the resulting solution, which contained cuprate–olefin
p-complex 2[10] and PhSLi
ACTHNUTRGNEN(UG THF)4 (3), was stored at approxi-
University of North Carolina–Charlotte
Charlotte, NC 28223 (USA)
Fax : (+1)704-687-0960
mately À508C. The ensuing “slow synthesis” proceeded via
the usual 1,4-addition reaction to give soluble enolate 4 and
MeCu, which was intercepted efficiently, as no amorphous
yellow precipitate was observed. Instead, we obtained the
desired product 1 as exquisite, X-ray quality crystals, which
allowed us to determine its structure.[11]
E-mail: sbertz1@uncc.edu
cogle@uncc.edu
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301415; including lists of
bond lengths and angles for 1 and details of MW determinations.
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Chem. Eur. J. 2013, 19, 10138 – 10141

COMMUNICATION
Table 1. Molecular weight data for selected organocopper compounds.[a]
Reagent[b]
Solution species
MeCu(SPh)Li(THF)3 (1)
76% CIPM
[MeCuSPh]À[Li(THF)4]+
[MeCu(SPh)Li(THF)2]2
Mcalcd
Mexptl
[CIPM]
426/430
[SSIP]
[CIPD]
MeCu(SPh)Li
G
R
G
411
428
483
648
ACHTUNGTRENNUNG
R
G
ACHTUNGTRENNUNG
E
G
N
ACHTUNGTRENNUNG
Scheme 2. CIPD–CIPM–SSIP equilibria for MeCuACHTNUTRGNEUNG(SPh)Li in THF (Note
that 1 equiv of CIPD yields 2 equiv of CIPM or SSIP).
Me2CuLi·LiSPh
Me2CuLi
65% CIPM
[Me2Cu]À[Li
[Me2CuLi(THF)2]2
G
317
342
389
490
ACHTUNGTRENNUNG
G
E
ACHTUNGTRENNUNG
R
U
ACHTUNGTRENNUNG
À
The Cu C bond length in 1 (1.91 ꢀ) is slightly shorter
PhSLi
PhSLi
14% CIPM
[PhS]À[Li(THF)4]+ (3)
G
332
395
405
ACHTUNGTRENNUNG
than it is in homocuprate [Me2Cu]À[Li(12-crown-4)2]+ (5;
1.93 ꢀ)[12] and phosphidocuprate [MeCu[P
A
N
G
ACHTUNGTRENNUNG
(6; 1.94 ꢀ).[7] The C-Cu-S angle in 1 is 1768 versus 1808 for
the C-Cu-C angle in 5 and 1798 for the C-Cu-P angle in 6.
[a] Conditions: À808C, 60 mm in [D8]THF–[D6]benzene (9:1). [b] Canoni-
cal representation. [c] The estimated uncertainty (95% confidence level)
in the mean for these three values is 340Æ7 (SE=1.67, t=4.30).
[d] Me2CuLi from CuI. [e] Me2CuLi from CuCN.
À
The Cu S bond in 1 (2.18 ꢀ) is marginally shorter than
À
À
the Cu P bond in 6 (2.22 ꢀ), and the Li S bond in 1
À
(2.43 ꢀ) is significantly shorter than the Li P bond in 6
(2.54 ꢀ)[7] (see the Supporting Information for lists of bond
lengths and angles). It is interesting to note that the struc-
tures of these heterocupates are very similar, with a lone
pair of electrons from the heteroatom coordinated to lithi-
um, along with those from three THF ligands.
The solid state structures of copper reagents are impor-
tant for theoretical reasons;[13] nevertheless, when it comes
to practical applications, the solution structures are of para-
mount importance.[14,15] Solvent-separated ion pairs (SSIP)
are present in THF, and contact ion pairs (CIP) are favored
in diethyl ether, where the CIP is a dimer.[14] The X-ray
structure of 1 (Figure 1) shows that in the solid state, the
CIP is a monomer, which suggests that it might also be
mono
pair dimer (CIPD), contact ion pair monomer (CIPM) and
SSIP for MeCu(SPh)Li are shown in Scheme 2.
We have previously studied MeCu(SPh)Li and some of its
ACHTUNGTRENmNGNU eric in THF solution. Equilibria among contact ion
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
p-complexes in [D8]THF solution,[16] but did not address the
issue of solvation. To explore the solution structure of se-
lected organocuprates, apparent molecular weights were de-
termined by using the internal reference diffusion-ordered
NMR method of Williard and co-workers.[17] Figure 2 and
Figure 3 show typical diffusion attenuation curves, and the
results are summarized in Table 1 (see the Experimental
Section for reference compounds).
Figure 2. Diffusion attenuation curves for
CuSPh) and internal references with molecular weight (MW) values
shown in the key.
1 (prepared from MeLi+
The experimental values for the molecular weight of 1,
M
exptl =426 and Mexptl =430, fall between the calculated
values, Mcalcd, for CIPM and SSIP. The average, Mexptl =428,
is consistent with 76% CIPM and 24% SSIP in THF solu-
tion. We round this to a 3:1 ratio of CIPM/SSIP, considering
the experimental uncertainty (Table 1, footnote [c]). We
cannot rule out a small amount of CIPD for this heterocup-
rate; nevertheless, Gschwind et al. have shown that for the
homocuprate very little CIPD is present at equilibrium in
THF.[14]
The apparent molecular weight for the corresponding ho-
mocuprate, Me2CuLiACTHNUTRGNEUNG(THF)3 (7), was measured starting from
CuSPh, CuI, and CuCN, and all three determinations were
very close to each other (<2% difference between values).
The measured molecular weight of 7 (from CuSPh) suggests
that THF solutions contain 65:35 CIPM/SSIP under our
conditions.
Figure 3. Diffusion attenuation curves for 7 (prepared from 2MeLi+
CuSPh) and internal references with molecular weight (MW) values
shown in the key.
Chem. Eur. J. 2013, 19, 10138 – 10141
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10139

S. H. Bertz, C. A. Ogle et al.
The fact that all three measured values for the homocup-
rate are the same within experimental uncertainty strongly
suggests that the lithium salts (LiSPh, LiI or LiCN, respec-
tively) from the metathesis reactions used to prepare the
cuprate solutions are not intimately associated with the cup-
rate cluster. Therefore, we conclude that the corresponding
anions (PhSÀ, IÀ or CNÀ, respectively) cannot be bonded to
the copper atoms in these cuprates (i.e., they are not
“higher order” cuprates).[18]
Acknowledgements
This work was supported by National Science Foundation (USA) grants
1012493 and 0321056.
Keywords: aggregation · cuprates · NMR spectroscopy ·
S ligands · X-ray diffraction
In contrast to 1 and 7, which are mainly CIPM in solution,
3 is predominately a SSIP (ca. 86%). Interestingly, X-ray
crystal structures of the 2,4,6-tri-tert-butyl[19] and 2,4,6-tri-
phenyl[20] derivatives of 3 are CIPM with three THF ligands
per lithium, as for 1 and 6.
In conclusion, we have obtained the first single crystal
X-ray structure of a Posner cuprate, MeCuACHTNUTRGENNUG(SPh)LiACHTUNTGERN(NGUN THF)3
(1), and shown that its solution structure in THF is largely
the same (ca. 3:1 CIPM/SSIP). Finally, the solution structure
of this heterocuprate is very similar to that of the Gilman
[1] a) G. H. Posner, Org. React. 1972, 19, 1–113; b) G. H. Posner, Org.
React. 1975, 22, 253–400; c) G. H. Posner, An Introduction to Syn-
thesis Using Organocopper Reagents, Wiley, New York, 1980.
[2] Both R groups are transferred in special cases: a) G. H. Posner, D. J.
Brunelle, Chem. Commun. 1973, 907–908; b) S. H. Bertz, G. Dab-
bagh, L. M. Williams, J. Org. Chem. 1985, 50, 4414–4415; c) S. H.
Bertz, Y. Moazami, M. D. Murphy, C. A. Ogle, J. D. Richter, A. A.
Thomas, J. Am. Chem. Soc. 2010, 132, 9549–9551.
[3] Mixed cuprates RR’CuLi, where R’ is attached through a hetero-
AHCTUNGTREGaNNNU tom or a carbon atom, have two different groups bonded to Cu.
For examples of the former, see refs. [4]–[7]. For examples of the
latter, see ref. [16], and references cited therein.
reagent, Me2CuLiACHTUNGTRENNUNG(THF)3 (7; ca. 2:1 CIPM/SSIP), which is
independent of the cuprate precursor.
[4] a) G. H. Posner, C. E. Whitten, J. J. Sterling, J. Am. Chem. Soc.
1973, 95, 7788–7800; b) G. H. Posner, D. J. Brunelle, L. Sinoway,
Synthesis 1974, 662–663; c) G. H. Posner, C. E. Whitten, Org. Synth.
1976, 55, 122–127. For some applications, see: d) D. M. Knotter,
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b) S. H. Bertz, G. Dabbagh, G. M. Villacorta, J. Am. Chem. Soc.
1982, 104, 5824–5826.
Experimental Section
Crystals of 1 were prepared by adding methyllithium in diethyl ether
(6.30 mL, 1.60m, 10.1 mmol, Sigma–Aldrich) to copper(I) thiophenylate
(860 mg, 4.98 mmol, Sigma–Aldrich), suspended in dry THF (25 mL,
freshly distilled from Na/benzophenone) under nitrogen in a dry Schlenk
flask (100 mL), cooled to À788C in a dry ice/acetone bath. The suspen-
sion was swirled and sonicated in an ultrasonic bath at 08C until a homo-
genous solution was obtained (ca. 0.1 h). The colorless solution was
cooled to À788C, and a solution of chalcone (1.00 g, 4.80 mmol) in dry
THF (10 mL) was added with magnetic stirring. The resulting solution
was stored at À478C in a low temperature refrigerator for 7 days, during
which time the color lightened from orange to pale yellow with the con-
comitant formation of pale yellow plates. A small sample of crystals was
transferred under nitrogen with a spatula to a drop of Paratone-N oil on
a cold glass slide (À408C). An oil-coated crystal was picked up with a
Cryoloop (0.4–0.5 mm) and transferred using Cryotongs (80 K) to the
precooled goniometer head, which was placed in the cold stream (100 K
nitrogen) of the diffractometer.[11]
[6] S. H. Bertz, G. Dabbagh, J. Org. Chem. 1984, 49, 1119–1122.
[7] S. F. Martin, J. R. Fishpaugh, J. M. Power, D. M. Giolando, R. A.
Jones, C. M. Nunn, A. H. Cowley, J. Am. Chem. Soc. 1988, 110,
7226–7228.
[8] S. H. Bertz, A. S. Vellekoop, R. A. J. Smith, J. P. Snyder, Organo-
AHCTUNGTERGmNNUN etallics 1995, 14, 1213–1220.
[9] a) H. O. House, J. M. Wilkins, J. Org. Chem. 1976, 41, 4031–4033;
b) H. O. House, W. L. Respess, G. M. Whitesides, J. Org. Chem.
1966, 31, 3128–3141.
[10] a) S. H. Bertz, R. A. Hardin, M. D. Murphy, C. A. Ogle, J. D. Richt-
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7838.
[11] a) Data were collected on an Agilent Xcalibur Atlas Gemini Ultra
diffractometer with CrysAlisPro. Refined formula, C19H32CuLiO3S;
Mr =410.99; crystal dimensions, 0.43ꢂ0.33ꢂ0.10 mm; crystal system,
orthorhombic; space group, Pna21; unit cell, 18.91434(15)ꢂ
8.61215(6)ꢂ12.91449(8) ꢀ [2103.68(3) ꢀ3]; Z=4; calculated density,
1.298 g cm–3; linear absorption coefficient, 2.485 mm–1; radiation,
Preparation of NMR samples has been described.[10,16] For details of the
molecular weight measurements, see the Supporting Information. Refer-
ence compounds (internal standards)[21] for these diffusion-ordered NMR
studies are shown below:
CuKa 1.54184 ꢀ; measurement temperature, 99.95(10) K; 2qmax
,
133.938; total reflections, 98793; independent reflections, 3769; R-
A
;
deepest hole, À0.242
ꢀ–3. Structure solved and refined with
ShelX-97 in Olex2: b) O. V. Dolomanov, L. J. Bourhis, R. J. Gildea,
J. A. K. Howard, H. Puschmann, J. Appl. Crystallogr. 2009, 42, 339–
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3060–3068.
CCDC-928158 (1) contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_re-
quest/cif.
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Chem. Eur. J. 2013, 19, 10138 – 10141

Prototypical Posner Reagent, MeCuACHTNUGTRNENUG(SPh)LiHCATUNGTERN(NUNG THF)3
COMMUNICATION
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Received: April 15, 2013
Published online: June 18, 2013
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10141