Minimization of organocuprate complexity through self-organization: Remarkable orientation effect in mixed cuprate π complexes

Article abstract of DOI:10.1002/anie.201107060

They "know" where to go: A powerful orientation effect has been observed in complexes of mixed organocuprates [R_TR_NTCuLi] and substrates with C-C, C-N, and C-S double bonds (see scheme; Th=thienyl). The preferred geometry of the intermediate complex sets up the facile addition of R_T to the double bond, rather than addition of the "dummy ligand", R_NT. Copyright

Full text of DOI:10.1002/anie.201107060

Angewandte
Chemie
DOI: 10.1002/anie.201107060
Organocuprates
Minimization of Organocuprate Complexity through
Self-Organization: Remarkable Orientation Effect in Mixed Cuprate
p Complexes**
Steven H. Bertz,* Richard A. Hardin, Michael D. Murphy, Craig A. Ogle,* Joshua D. Richter,
and Andy A. Thomas
A great deal of effort has gone into the research, development
and application of mixed organocuprate(I) reagents
[RTRNTCuLi].[1–7] These “mixed cuprates” contain one group
RT that is transferred to a substrate and a second, non-
transferred group RNT (sometimes called a “dummy ligand”)
that is assumed to be inert under the reaction conditions. They
were introduced in order to solve the efficiency problem of
homocuprates R2CuLi (Gilman reagents), which only transfer
one equivalent of the R-group in most applications. Out-
standing examples include thienyl (Th),[2] cyano,[3] alkynyl,[4]
thiolato,[5] trimethylsilylmethyl,[6] and phosphido[7] groups.
We have found that upon addition of typical a,b-
unsaturated ketones, esters, nitriles, or sulfones, these mixed
cuprates give p complexes that exhibit a remarkable orienta-
tion effect in which RT is cis to the site of the usual addition
reaction, for example, the b-carbon in a,b-unsaturated
plexity of mixed organocuprate reagents, and it is essential for
a complete understanding of their reaction mechanisms.
The observation of this heretofore “hidden” phenomenon
was made possible by our rapid injection NMR method, in
which a solution is injected from a glass capillary into an
NMR tube that is spinning in the probe of an NMR
spectrometer at a controlled temperature under a nitrogen
atmosphere. For example, when 2-cyclohexenone (1a) was
injected into a solution of [Me(Th)CuLi] in [D8]THF at
À1008C, p complex 2a was observed in equilibrium with the
starting materials (Keq = 11). The structure was tentatively
1
assigned as 2a, and not iso-2a, on the basis of the H NMR
shift (À0.09 ppm) for the methyl group on copper, MeCu,
which was close to the downfield peak for the corresponding
homocuprate complex 3a (À0.10 ppm, Table 1).[8] This struc-
ture was confirmed by NOESY.
1
À
carbonyl compounds 1 (Scheme 1). Complexes with C N
There were no new peaks in the H NMR region at ca.
À
and C S double bonds also display this effect. The self-
À1 ppm, where MeCu for iso-2a would be expected to appear.
However, in the vicinity of the MeCu peak for 2a, we observed
small (< 10%) side-product peaks, which may be due to
aggregates, as in the case of 3a, which was in equilibrium with
a minor amount of 3a·LiI.[8]
organization we report here substantially reduces the com-
À
À
À
A number of diverse substrates with C C, C N and C S
double bonds were investigated by treating them with
Table 1: 1H and 13C NMR chemical shifts for the MeCu groups in
complexes of [Me(Th)CuLi] (and [Me2CuLi]) with selected substrates.
Substrate Product 1H NMR, ppm
([Me2CuLi])[a]
13C NMR, ppm
([Me2CuLi])[a]
1a
1b
2a
2b
À0.09 (À0.10, À1.12)[b] À1.30 (À0.57, À5.02)[b]
(À0.24, À1.16)[c]
(À1.85, À5.56)[c]
À0.13 (À0.36, À1.09)[b,d] À4.74 (À4.74, À7.93)[b,d]
Scheme 1. Stable h2 p complexes from diverse mixed cuprates and
a,b-unsaturated carbonyl compounds.
(À0.14, À1.00)[c,d]
(À3.05, À6.44)[c,d]
1c
1d
2c
2d
À0.11 (À0.43, À1.06)
8.33 ( 4.07, À3.44)
À0.28 (À0.54, À1.12)[b] À7.23 (À8.22, À8.55)[b]
(À0.48, À1.16)[c]
À0.29 (À0.48, À0.88)
À0.10 (À0.42, À0.69)
0.09 (À0.26, À1.07)
0.79 ( 0.43, À0.79)
0.16 (À0.27, À0.52)
0.01
(À6.71, À7.90)[c]
À7.98 (À7.86, À8.37)
À5.79 (À6.26, À8.18)
11.26 ( 5.02, À1.05)
14.29 ( 10.81, À9.09)
13.03 ( 9.18, À9.18)
À5.82
4a
5a
6a
7a
8a
4b
5b
6b
7b[e]
8b
8c
9
[*] Prof. Dr. S. H. Bertz, R. A. Hardin, Dr. M. D. Murphy,
Prof. Dr. C. A. Ogle, J. D. Richter, A. A. Thomas
Department of Chemistry, University of North Carolina-Charlotte
Charlotte, NC 28223 (USA)
E-mail: sbertz1@uncc.edu
cogle@uncc.edu
CS2
0.76 ( 0.37, À0.76)
24.06 ( 20.10, À10.54)
[**] Rapid Injection NMR in Mechanistic Organocopper Chemistry, part
10. For previous papers in this series, see references [8–15]. We are
grateful to the National Science Foundation (USA) for their support
of our work through grants 0353061 and 0321056.
[a] Me2CuLi complexes were prepared from the substrates and their
spectra were measured under the same conditions as the mixed cuprate
complexes. [b] Major complex. [c] Minor complex (20–30%). [d] 2D NMR
À
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201107060.
connects the downfield 1H and upfield 13C peak in this case. [e] C S
double-bond complex.
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[Me(Th)CuLi] at À1008C. A single mixed cuprate complex
was formed in every case except for 8a, which gave two
complexes, 8b and 8c (Scheme 2). Initially, they were present
in a 1:1 ratio; however, the latter slowly isomerized to the
former at À1008C, or rapidly upon warming the probe to
À808C. Upon cooling it back to À1008C, the sole product
remained 8b.
[Me2CuLi].[15] In the case of the less electron-rich cuprate,
À
[Me(Th)CuLi], no intermediate C N complex was observed,
À
and only C S complex 7b appeared upon injection of 7a.
To further elucidate the scope of the orientation effect, we
investigated the reactions of a number of mixed cuprates with
the prototypical a-enone, methyl vinyl ketone (MVK; 1b). In
addition to Nilssonꢀs thienylcuprate [CH3(C4H3S)CuLi],[2] we
examined Levisallesꢀ cyanocuprate [CH3(CN)CuLi],[3]
[4]
ꢀ
Coreyꢀs alkynylcuprate [CH3{CH3O(CH3)2CC C}CuLi],
Posnerꢀs phenylthiocuprate [CH3(C6H5S)CuLi],[5] Bertzꢀs
(partially deuterated) trimethylsilylmethylcuprate [CH3-
{(CD3)3SiCH2}CuLi],[6] and his diphenylphosphidocuprate
[CH3{(C6H5)2P}CuLi],[7] all under the usual conditions
([D8]THF, À1008C). Complexes from MVK and dithioester
8a are summarized in Scheme 3 (see Scheme 2 for 2b).
Scheme 2. Complexes from the treatment of [Me(Th)CuLi] with diverse
À
À
À
substrates containing C C, C N and C S double bonds.
With 13C-labeled thienyl cuprate, [13CH3(C4H3S)CuLi],
the more stable complex 8b had a small cis two-bond coupling
(2JC-C = 1.4 Hz) between MeCu (13.03 ppm) and the thiocar-
bonyl carbon (92.80 ppm). Less stable isomer 8c had a larger
trans coupling (2JC-C = 17.7 Hz) between them (À5.82,
85.77 ppm, Figure 1). The use of scalar coupling across Cu
to establish stereochemistry has been well-established.[9–15]
Thus, the two products from 8a result from different
orientations and not different aggregation states.
Scheme 3. Complexes from mixed cuprates and methyl vinyl ketone
(1b) or methyl dithio-4-(trifluoromethyl)benzoate (8a).
À
The results for C N double bonds deserve special com-
ment. Complex 6b from [Me(Th)CuLi] and 2-azachalcone 6a,
as well as the corresponding [Me2CuLi] complex (Table 1),
À
are rare examples of stable cuprate p complexes with C N
À
double bonds. An unstable C N complex, which rearranged
Upon injection of MVK, [CH3(C4H3S)CuLi] gave p com-
plex 2b. The orientations of MeCu and Th were confirmed by
NOESY, which also proved that the a-enone moiety adopted
the s-cis conformation in the p complex. Likewise, both major
and minor complexes 3b and 3b’ from MVK and [Me2CuLi]
had the s-cis conformation. Therefore, we tentatively assign
3b’ to an aggregate of 3b with LiI. Complex 2b was stable
over the period of ca. 6 h, during which NMR spectra were
collected.
À
to a stable C S complex, was observed with 7a and
Injection of (CD3)2(C6H5)SiCl into the solution at À1008C
gave a quantitative conversion of 2b to a 9:1 Z/E mixture of
silyl enol ethers from the 1,4-addition of methyl. The Z isomer
corresponds to trapping the observed s-cis conformation of
the p complex in what appears to be a “least motion” process.
The minor amount of lithium enolate (20%, ca. 1:1
Z/E) that appeared immediately upon injection of the
substrate was not silylated under the reaction conditions.
We believe its formation was the result of local heating during
Figure 1. Thiocarbonyl region of the 13C NMR spectra for complexes
8b and 8c (top) and 16 and 16’ (bottom). See text for d and J values.
2
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the injection, since the amount did not increase significantly
thereafter.
When MVK was injected into
a
solution of
[CH3(CN)CuLi], a p complex was not observed; nevertheless,
the presence of a small equilibrium concentration of one such
complex (e.g., 10) was inferred from the relative broadening
of the cuprate methyl peak and enone vinyl peaks in the
1H NMR spectrum. Reversible p-complex formation between
[tBu(CN)CuLi] and methyl phenylpropiolate was reported by
Nilsson et al.[16]
In contrast, [CH3(CN)CuLi] formed a stable complex 11
with dithioester 8a, a more powerful electron acceptor than
MVK.
Figure 2. 1H-31P HMQC for 16 and 16’ from [Me(Ph2P)CuLi] and 8a.
ꢀ
Upon injection of MVK, [CH3{CH3O(CH3)2CC C}CuLi]
and [CH3(C6H5S)CuLi] each gave the expected p complex, 12
and 13, respectively, with the usual orientation of MeCu.
Unexpectedly, they underwent metathesis to homocuprate
complex 3b during the course of several hours.
and the other pair (À0.91 ppm, 3JH-P = 2.0 Hz; À0.94 ppm,
3JH-P = 4.9 Hz) had them oriented toward the a-carbon (17b
and 17b’). NOE data was not conclusive, as ROESY indicated
rapid exchange of MeCu groups.
The downfield peak in each pair (À0.39, À0.91 ppm) was
correlated with a 31P NMR peak at À0.47 ppm, and the
upfield peak in each pair (À0.51, À0.94 ppm) with a 31P peak
at À1.36 ppm. Thus, it appears that there were two orienta-
tions of the methyl group, as shown in 17a and 17b (in
analogy to 8b and 8c), and we propose an aggregated version
of each, 17a’ and 17b’ (in analogy to 16 and 16’), for a total of
four complexes.
Injection of MVK into
a
solution of [CH3-
{(CD3)3SiCH2}CuLi], which had deuterated methyl groups
1
on silicon in order to simplify the H NMR spectral window
from 1 to À1 ppm, did not result in the expected p complex
14. Instead, a quantitative conversion of starting cuprate to
enolate (ca. 1:1 Z/E) from 1,4-addition of Me was observed in
the first 1H NMR spectrum of the rapid-injection experiment,
which was obtained within 5 s at À1008C. (We note that 30%
of
a
p complex was formed between MVK and
[{(CD3)3SiCH2}2CuLi].)
[Me2CuLi(MVK)] complex 3b was stable under the same
conditions, which confirmed the dramatic activating effect of
b-silicon on copper reagents, predicted by Bertz and
Snyder.[17] Yamanaka and Nakamura further elucidated the
retention of the trimethylsilylmethyl group on copper.[18]
Treatment of [CH3{(CD3)3SiCH2}CuLi] with 8a gave
complex 15 with the usual MeCu orientation. Complexes 11
and 15 highlight the remarkable stability of cuprate–thiocar-
bonyl complexes.
Injection of (CD3)2(C6H5)SiCl into the mixture of p com-
plexes at À1008C converted them to the silyl enol ethers from
1,4-addition of both Me (40%, 9:1 Z/E) and Ph2P (10%, 3:2
Z/E). The initial lithium enolate (50%, 1:1 Z/E) did not react
under these conditions. The product distribution appears to
be determined by the relative rates of reductive elimination of
Me versus PPh2.
We chose thienylcuprates for additional study because our
group had previously prepared scalemic thienyl ligands for
use as chiral auxiliaries in asymmetric induction reactions
with a-enones.[19] Upon rapid injection of chalcone 1c into
a solution of scalemic cuprate 18 (Scheme 4, prepared from
the corresponding lithium reagent), [Me2CuLi(chalcone)]
complex 3c grew monotonically as cuprate 18 disappeared
at À1008C.
Treatment of [CH3{(C6H5)2P}CuLi] with 8a afforded two
stable products (ca. 1:1), designated 16 and 16’. Their
3
1H NMR (À0.34 ppm, 3JH-P = 6.2 Hz; À0.40 ppm, JH-P
=
2
5.9 Hz) and 13C NMR shifts (14.91, 13.88 ppm, JC-P = 8.5 Hz
each) for MeCu were very similar. The thiocarbonyl 13C shifts
2
(108.71, 106.29 ppm, JC-P = 40.5 Hz each, Figure 1) and 31P
shifts (9.47, 7.26 ppm, Figure 2) were also similar. Since they
both had the same large trans two-bond 13C-31P coupling
across Cu, we concluded that they both had the usual
orientation, MeCu cis to thiocarbonyl carbon (cf. 16).
This conclusion was further supported by using [13CH3-
{(C6H5)2P}CuLi] to prepare the complexes, which both had
small cis splittings (2JC-C = 2 Hz) in the thiocarbonyl carbon
peaks in addition to the large trans splittings (2JC-P = 40 Hz).
We tentatively view 16’ as an aggregate of 16.
When MVK was injected into a solution of [CH3-
{(C6H5)2P}CuLi], approximately equal amounts of four
p complexes were observed in addition to a substantial
amount of enolate from 1,4-addition of Me (50%, ca. 1:1
Z/E). The 1H NMR data indicated that one pair of complexes
(À0.39 ppm, 3JH-P = 5.2 Hz; À0.51 ppm, 3JH-P = 5.3 Hz) had
MeCu groups oriented toward the b-carbon (17a and 17a’),
Scheme 4. Reaction of chalcone 1c with substituted thienylcuprate 18,
which has a side chain capable of chelating lithium ions.
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The simple thienylcuprate–chalcone complex 2c (Table 1)
was stable under the reaction conditions. Apparently, the
presence of a moiety capable of chelating Li ions destabilized
the mixed cuprate p complex and promoted the metathesis
reaction. This experiment highlights the importance of Li ions
in the structures of mixed cuprate p complexes, as predicted
by Yamanaka and Nakamura.[18]
The second major factor that influences the structures of
mixed cuprates with dummy ligands such as cyano, alkynyl,
and thiolato was identified as a structural trans effect (“trans
influence”).[18] Erdik and ꢁzkan have presented experimental
Experimental Section
Mixed cuprates were prepared by adding MeLi·THF[28] in
[D6]benzene (1m, 1 equiv) to a suspension of CuI (30 mmol, Aldrich
“Ultrapure”) in [D8]THF (420 mL, freshly distilled from Na/K) to give
a suspension of MeCu in a new NMR tube, dried in an oven (1508C),
sealed under argon with a septum, and cooled to À808C in a dry ice/
acetone bath. Addition of the dummy ligand in its lithiated form (ca.
ꢀ
0.9 equiv, e.g., ThLi, RC CLi, TMSCH2Li, Ph2PLi) to the NMR tube
at À808C gave the mixed cuprate. The sample was checked by
1H NMR at À1008C, and additional dummy ligand was added when
needed, based on integration of the spectrum. In the case of
[Me(PhS)CuLi], the precursor was [PhSCu] (Aldrich) and in the
case of [Me(Th)CuLi], CuCN was also used.[2c] In all cases, the
suspension of copper salt (CuI, CuCN, or PhSCu) was sonicated for
1 min at 0–208C before addition of the lithium reagents. When
a satisfactory sample was obtained, the septum was removed in
a stream of dry nitrogen, and the tube immediately lowered into the
probe, which was filled with dry nitrogen (used to spin the sample).
The substrate was then injected and single-pulse 1H NMR spectra
were obtained periodically (2–20 s, depending on the rate of
À
evidence that the strength of the Cu RNT bond plays an
important role in mixed cuprate chemistry.[20]
The observation of small cis and large trans two-bond
coupling across Cu in the NMR spectra of cuprate–thiocar-
bonyl complexes suggests a pseudo square planar arrange-
ment of ligands,[15,21] which in turn implies a significant degree
of CuIII character. We believe there is a small but significant
amount of CuIII character in the a-enone complexes, as well.
An intermolecular SN2-like mechanism for ligand exchange
reactions in square-planar CuIII complexes has been proposed
by Nakamura, Gschwind et al.[22]
1
reaction). H NMR (13C NMR) spectra were referenced to benzene
at 7.340 (128.59) ppm, and 31P NMR spectra were referenced to
MePPh2 at À27.00 ppm (external std method).
Received: October 5, 2011
The only case where both orientations of the ligands
appear to have essentially the same stability comprises the
p complexes from MVK and [Me(Ph2P)CuLi]. The trans
effects of ÀCH3 and ÀPPh2 are expected to be comparable,
given that phosphines PR3 are only slightly below methyl
anion on the trans-effect scale.[23]
With the exception of complexes involving [Me-
(Ph2P)CuLi], we do not see evidence for substantial aggre-
gation in complexes of mixed cuprates from CuI in THF.
Aggregates of cuprates and lithium halides are the major
species in ether, and THF has been used to break them
down.[24] Thus, by working in [D8]THF at À1008C, we can
study the simple p complexes that are the foundation upon
which the aggregates in ether are built, as elucidated by
Gschwind et al.[25]
Revised: November 23, 2011
Published online: February 1, 2012
Keywords: 1,4-addition · copper · isotopic labeling ·
.
NMR spectroscopy · rapid injection
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In summary, mixed organocuprate(I) reagents are not as
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