Neutral organocopper(III) complexes

Article abstract of DOI:10.1039/b717290g

Neutral organocopper(III) complexes have been prepared from organocuprate(I) reagents and alkyl halides in the presence of certain strongly electron donating ligands. The Royal Society of Chemistry.

Full text of DOI:10.1039/b717290g

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Neutral organocopper(III) complexes
a
b
a
a
Erika R. Bartholomew, Steven H. Bertz,* Stephen Cope, Donna C. Dorton,
a
a
Michael Murphy and Craig A. Ogle*
Received (in Maryland, USA) 8th November 2007, Accepted 19th December 2007
First published as an Advance Article on the web 22nd January 2008
DOI: 10.1039/b717290g
3
1
Neutral organocopper(III) complexes have been prepared from
organocuprate(I) reagents and alkyl halides in the presence of
certain strongly electron donating ligands.
(6 : 1). Complex 2 was stable at ꢁ100 1C; however, the
NMR peak was broad, and we did not observe any J coupling.
Injection of EtI into Me under the same
CuLiꢀLiI/P(OMe)
conditions gave EtMe Cu[P(OMe) ] (3) and A (1 : 1). As in the
P
2
3
2
3
III
Cu Li, recently joined
Organocuprate(III) complexes,
R
4
case of 2, J coupling was not observed.
Injection of EtI into Me
CuLiꢀLiI/PPh
conditions gave EtMe Cu(PPh ) (4) and A (2 : 3) along with
I
Cu Li (‘Gilman reagents’), in
organocuprate(I) complexes, R
2
2
under the same
3
1
–3
the panoply of organocopper compounds.w
rapid injection nuclear magnetic resonance (RI-NMR) stu-
Before our
2
3
a small amount (o10%) of propane. A large trans-coupling
( J = 118.0 Hz) was observed between P and the methylene C
1
,2,4,5
III
such Cu complexes had been relegated to the
2
dies,
realm of ‘elusive’ intermediates, for example, in the reactions
atom; however, the peaks were too broad to resolve
cis-couplings. When the reaction mixture was warmed to
ꢁ80 1C, 4 was no longer present; it had decomposed to
6
7
of Gilman reagents with a-enones or alkyl halides.
The first such ‘copper(III) intermediate’ to be characterized
was prepared via the reaction of Me
CuLiꢀLiI with 2-cyclo-
hexenone and TMSCN, namely lithium cyanobis(methyl)-
3
propane and MeCuPPh before the first scan (ca. 0.1 h).
2
No complex was observed with tri(t-butyl)phosphine, pre-
sumably owing to steric hindrance. Furthermore, no complex
was observed with triphenylarsine (vide infra).
1
(
3-trimethylsiloxy-2-cyclohexenyl)cuprate(III). It was fol-
III
lowed by several more examples of anionic Cu species from
reactions of methyl cuprates with alkyl halides, for example,
2
The amine complexes in Chart 1 exhibited a wide range of
stabilities at ꢁ100 1C. In the presence of pyridine (py, 1 equiv.)
,3
EtMe
3 2
CuLi (A) and EtMe Cu(CN)Li (B).
III
However, the Cu analogs of the well-known organo-
as the neutral ligand, Me
2
CuLiꢀLiI and EtI gave EtMe
2
Cu(py)
I
6,7
copper(I) compounds, RCu (L) (L = neutral ligand), were
still unknown. Now, by using RI-NMR techniques, we have
been able to prepare the first examples of such neutral
0
III
organocopper(III) complexes, RR Cu (L) (Chart 1).
2
When EtI (1 equiv., 0.5 M in THF-d
THF-d solution of Me ,z spinning in the probe
CuLiꢀLiI/PBu
of an NMR spectrometer at –100 1C, EtMe Cu(PBu ) (1) was
formed almost exclusively with only a small amount (o5%) of
A. Above ꢁ80 1C, 1 decomposed to MeCuPBu and propane,
the expected products of reductive elimination.
The structure of 1 was assigned on the basis of two-bond
8
) was injected into a
8
2
3
2
3
3
2
I
couplings J across Cu, which were introduced for Cu com-
8
pounds, but have also proven invaluable for structure assignment
III
1,2
2
in Cu chemistry. The trans-coupling ( J = 130.4 Hz) between
1
3
13
P and the methylene C atom in CH CH ( CH ) Cu(PBu ) is
3
2
3 2
3
2
much larger than the cis-coupling ( J = 14.6 Hz) of P with the
methyl C atoms (see Fig. 1), which is also the general case for Pt
9
complexes. The small cis-coupling, previously observed between
1
,2
methylene and methyl C atoms, was not resolved in this case.
When Me was injected with EtI under the
2
CuLiꢀLiI/PMe
3
2 3
same conditions, the products were EtMe Cu(PMe ) (2) and A
a
Department of Chemistry, University of North Carolina–Charlotte,
Charlotte, NC 28223, USA. E-mail: cogle@uncc.edu;
Fax: +1-704-687-3151; Tel: +1-704-687-2524
Complexity Study Center, 88 East Main Street, Suite 220,
Mendham, NJ 07945, USA. E-mail:
b
sbertz@complexitystudycenter.org; Fax: +1-973-628-4007;
Tel: +1-973-644-0285
Chart 1 Organocopper(III) compounds prepared in this study with
13 1
chemical shifts (ppm) at ꢁ100 1C in THF-d
8
for C (red) and H (blue).
1
176 | Chem. Commun., 2008, 1176–1177
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and arsine complexes is correlated with the order of base
hardness in the series, PBu B PMe 4 PPh 44 AsPh
3
3
3
3
.
Chemical exchange is not unexpected in coordinatively
8
unsaturated, 16-electron, d transition metal species such as
3
–10, and it appears to be manifested in the P NMR spectra,
1
1
1
3
31
where the peaks are broad and the expected C– P couplings
are frequently absent.
Phosphine and phosphite complexes of copper(I) salts were
I
introduced as soluble sources of Cu for the preparation of
6
,7
organocopper reagents. Our results suggest a more signifi-
cant role for such ligands in reactions involving them. Bertz
et al. noted the beneficial effect of pyridine on the conjugate
addition reaction of organocuprates, and they proposed a
3
1
13
13
Fig. 1
P NMR spectrum of CH
3 2 3 2 3
CH ( CH ) Cu(PBu ) (ppm
scale) at ꢁ100 1C in THF-d
8
. See text for coupling constants.
1
4
pyridylcopper(III) intermediate to account for it. Complex
5 provides substantial support for this suggestion.
(
5) as a short-lived intermediate (0.5 h to max. concentration).
In addition to propane, which increased continuously, the final
1
Given the diversity of neutral ligands capable of coordinat-
ing to organocopper(III) and the resulting range of stabilities,
there appears to be an abundance of possibilities for fine-
tuning organocopper reactivity based upon them.
0
products were A and Me Cu Li.
3
2
With 4-dimethylaminopyridine (DMAP) under the same
conditions, EtMe Cu(DMAP) (6) was formed in high yield
ca. 90%). A small amount of propane (ca. 10%) was also
2
(
The authors thank D. Deadwyler for the fabrication and
maintenance of the RI-NMR apparatus and the USA
National Science Foundation for funding (grant 0718368).
3 2
produced. Most significantly, A and Me Cu Li were not
observed in this case.
When the cyano-Gilman reagent, Me
2
CuLiꢀLiCN, was
treated sequentially with py or DMAP (0.1 or 1 equiv.) and
then EtI, neither 5 nor 6, respectively, was observed. Instead,
Notes and references
the product in virtually quantitative yield was EtMe Cu(CN)-
2
w The terms organocopper and organocuprate refer to compounds
Li (B). In the reaction without amine, which we studied
2
previously, the yield of B (ca. 65%) was significantly lower,
I
with Cu–C bonds to alkyl or aryl groups R in RCu , R
I
Cu Li,
2
III
4
R Cu Li, etc. Following the usual convention, the superscript Ro-
owing to the competing formation of A and propane.
III
It appears that cyanide stabilizes the Cu center signifi-
man numerals are the formal oxidation numbers (see also ref. 11).
z The slash in formulas such as R CuLiꢀLiX/L indicates that we make
no assumption regarding interaction between R
CuLiꢀLiX (X =
anionic ligand) and L, a neutral ligand. Me (0.06 M)
CuLiꢀLiI/PBu
was prepared from 2 equiv. of MeLi (1 M in THF-d ) and 1 equiv. of
CuI(PBu
) in an unused NMR tube at ꢁ78 1C. Sonication at 0 1C for
.1 h was used to complete the reaction. Ligands that might be
sensitive to alkyllithiums were added in THF-d solution to the pre-
2
2
cantly more than a strongly electron donating amine such as
DMAP. While not incorporated into the product, py and
DMAP nevertheless play an important role in the elimination
of side-products. This route appears to be the best preparation
of a lithium cyanocuprate(III) complex to date.
2
3
8
3
0
8
formed cuprate at ꢁ78 1C. Otherwise, they were added along with the
1
-Methylimidazole (MI), 1-methylbenzimidazole (MBI) and
,5-diazabicyclo[4.3.0]non-5-ene (DBN) also formed stable
complexes, EtMe Cu(MI) (7), EtMe Cu(MBI) (8) and EtMe -
Cu salt to the NMR tube, and then THF-d and MeLi were added.
8
1
The trimethylphosphine was added as a commercial 1 M solution in
toluene. The chemical shift of tributylphosphine was set at ꢁ32.50
2
2
2
ppm versus 85% phosphoric acid.
Cu(DBN) (9), respectively. In contrast, quinuclidine and
triethylamine did not give complexes.
1
2
3
S. H. Bertz, S. Cope, M. Murphy, C. A. Ogle and B. J. Taylor, J.
Am. Chem. Soc., 2007, 129, 7208–7209.
S. H. Bertz, S. Cope, D. Dorton, M. Murphy and C. A. Ogle,
Angew. Chem., Int. Ed., 2007, 46, 7082–7085.
T. Gartner, W. Henze and R. M. Gschwind, J. Am. Chem. Soc.,
¨
Under our RI-NMR conditions, t-butyl isocyanide gave
t
Cu(CNBu ) (10) as a transient complex (0.1 h to max.
EtMe
2
concentration). The final products in essentially quantitative
t
III
I
3
yield were EtMe Cu Li (A) and MeCu (CNBu ), while the
2007, 129, 11362–11363.
4 S. H. Bertz, C. M. Carlin, D. A. Deadwyler, M. D. Murphy, C. A.
Ogle and P. H. Seagle, J. Am. Chem. Soc., 2002, 124, 13650–13651.
formation of propane (o5%) was effectively suppressed. This
route appears to be the best preparation of a lithium tetra-
alkylcuprate(III) complex to date.
5
M. D. Murphy, C. A. Ogle and S. H. Bertz, Chem. Commun., 2005,
54–856.
6 G. H. Posner, Org. React., 1972, 19, 1–113.
8
The most stable of these new organocopper(III)w compounds
have powerfully electron donating ligands. As summarized in
Chart 1, the chemical shifts for these complexes are similar to
7
8
9
G. H. Posner, Org. React., 1975, 22, 253–400.
S. H. Bertz, J. Am. Chem. Soc., 1991, 113, 5470–5471.
31
13
P. S. Pregosin and R. W. Kunz, P and C NMR of Transition
Metal Phosphine Complexes, Springer-Verlag, Berlin, 1979.
III
those reported previously for the anionic Cu ate com-
1
,2
plexes, which suggests that the charge on Cu is approxi-
10 E. C. Ashby and J. J. Watkins, J. Am. Chem. Soc., 1977, 99,
312–5317, see also ref. 5.
5
1 J. P. Snyder, J. Am. Chem. Soc., 2007, 129, 7210–7211, and
mately the same. Snyder has calculated that the atomic charge
1
on Cu in the ate complexes is ca. +1, consistent with the
1
1
references therein.
12 L. Pauling, The Nature of the Chemical Bond, Cornell University
Press, Ithaca, NY, 3rd edn, 1960.
13 R. G. Pearson, Chemical Hardness—Applications from Molecules
to Solids, Wiley-VCH, Weinheim, 1997.
1
2
Pauling Electroneutrality Principle.
1
According to HSAB theory, the most stable complexes are
3
I
formed between acids and bases of similar hardness. While Cu
III
is a soft acid, Cu is much harder. Consistent with this simple
III
theory, the qualitative order of stability of the Cu phosphine
1
4 S. H. Bertz, G. Miao and M. Eriksson, Chem. Commun., 1996,
815–816.
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Chem. Commun., 2008, 1176–1177 | 1177