On Ni catalysts for catalytic, asymmetric Ni/Cr-mediated coupling reactions

Article abstract of DOI:10.1021/ja302177z

The importance of the Ni catalyst in achieving catalytic asymmetric Ni/Cr-mediated coupling reactions effectively is demonstrated. Six phenanthroline-NiCl₂ complexes 1a-c and 2a-c and five types of alkenyl iodides A-E were chosen for the study, thereby demonstrating that these Ni catalysts display a wide range of overall reactivity profiles in terms of the degree of asymmetric induction, geometrical isomerization, and coupling rate. For three types of alkenyl iodides A-C, a satisfactory Ni catalyst(s) was found within 1a-c and 2a-c. For disubstituted (Z)-alkenyl iodide D, 2c was identified as an acceptable Ni catalyst in terms of the absence of Z → E isomerization and the degree of asymmetric induction but not in terms of the coupling rate. Two phosphine-based Ni catalysts, [(Me)₃P]₂· NiCl₂ and [(cy)₃P]₂·NiCl₂, were found to meet all three criteria for D. The bond-forming reaction at the C16-C17 position of palytoxin was used to demonstrate the usefulness of the Ni catalysts thus identified.

Full text of DOI:10.1021/ja302177z

Communication
pubs.acs.org/JACS
On Ni Catalysts for Catalytic, Asymmetric Ni/Cr-Mediated Coupling
Reactions
Xiang Liu, Xiaoyong Li, Yu Chen, Yimin Hu, and Yoshito Kishi*
Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, United States
S
* Supporting Information
under catalytic conditions.4 Several chiral ligands5 and a ligand-
optimization strategy6 were introduced. Thus, the Ni/Cr-
ABSTRACT: The importance of the Ni catalyst in
achieving catalytic asymmetric Ni/Cr-mediated coupling
reactions effectively is demonstrated. Six phenanthroline−
NiCl2 complexes 1a−c and 2a−c and five types of alkenyl
iodides A−E were chosen for the study, thereby
demonstrating that these Ni catalysts display a wide
range of overall reactivity profiles in terms of the degree of
asymmetric induction, geometrical isomerization, and
coupling rate. For three types of alkenyl iodides A−C, a
satisfactory Ni catalyst(s) was found within 1a−c and 2a−
c. For disubstituted (Z)-alkenyl iodide D, 2c was identified
as an acceptable Ni catalyst in terms of the absence of Z →
E isomerization and the degree of asymmetric induction
but not in terms of the coupling rate. Two phosphine-
based Ni catalysts, [(Me)3P]2·NiCl2 and [(cy)3P]2·NiCl2,
were found to meet all three criteria for D. The bond-
forming reaction at the C16−C17 position of palytoxin
was used to demonstrate the usefulness of the Ni catalysts
thus identified.
mediated coupling reactions give chiral allylic alcohols at a
synthetically useful level.7
Overall, this bond-forming process is a Grignard-type
carbonyl addition reaction. However, it is noteworthy that
this reaction displays a remarkable selectivity toward aldehydes
over other carbonyl compounds. Activation of halides/triflates
in the presence of aldehydes provides not only an experimental
convenience but also an opportunity to achieve chemical
transformations in an unconventional manner (e.g., cycliza-
tion). Undoubtedly, the most valuable feature of this coupling
is its exceptional compatibility with a wide range of functional
groups. This unique potential is appreciated most when applied
to polyfunctional molecules. There are numerous examples in
which this bond-forming process has been used at a late stage in
a multistep synthesis.8
In an earlier study of the catalytic asymmetric version of Ni/
Cr-mediated coupling, we focused on the chromium chemistry,
as the C−C bond formation takes place through the
alkenylchromium(III) species.9 However, we gradually realized
that the Ni catalyst plays an equally important role.10 In this
communication, we report three areas in which Ni catalysts play
important roles in achieving catalytic asymmetric Ni/Cr-
mediated couplings effectively: (1) geometrical isomerization
of (Z)-alkenyl halides, (2) the degree of asymmetric induction,
and (3) the coupling rate.
oupling of an alkenyl halide/triflate with an aldehyde
C
mediated by CrCl2 was reported by Takai, Hiyama,
Nozaki and co-workers1 in 1983 (eq 1 in Scheme 1).
Scheme 1. Ni/Cr-Mediated Coupling Reaction and Probable
Reactive Intermediates
The first issue concerns geometrical isomerization during the
Ni/Cr-mediated couplings. In particular, Ni/Cr-mediated
couplings with a (Z)-alkenyl iodide give the expected coupling
products, but they are formed as a mixture of geometrical
isomers. The observed partial Z → E isomerization apparently
takes place via alkenylnickel species, which are known to be
prone to geometrical isomerization.11 Two mechanisms were
proposed for the isomerization,12, one via a and the other via b
in Scheme 2, thereby suggesting the possibility that the
Scheme 2. Two Possible Intermediates Suggested for
Geometrical Isomerization of Alkenylnickel Species
Subsequently, it was revealed that the coupling is initiated by
a catalytic amount of NiCl2.2 It is now generally accepted that
this coupling involves (1) oxidative addition of Ni(0), formed
from NiCl2 via reduction with CrCl2 in situ, to an alkenyl
halide/triflate to form an alkenylnickel(II) species; (2)
transmetalation of the resultant Ni(II) species to CrCl2 to
form alkenylchromium(III) and nickel(I) halides; and (3)
carbonyl addition of the resultant Cr(III) species to an
aldehyde to form the chromium(III) alkoxide product (Scheme
1).3 The chemistry was developed to achieve this coupling
electronic nature of the Ni ligand might affect the degree of Z
→ E isomerization.
Received: March 5, 2012
Published: March 23, 2012
© 2012 American Chemical Society
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Communication
The second issue is the degree of asymmetric induction. One
would assume that the Ni catalyst has no effect on the
asymmetric induction, as the C−C bond formation takes place
through the Cr complex in the Ni/Cr-mediated couplings.
However, the degree of asymmetric induction under catalytic
conditions is often lower than that under stoichiometric
conditions.10 For example, 2,9-dimethylphenanthroline−
NiCl2, abbreviated as (H)2Phen(Me)2·NiCl2, is an excellent
Ni catalyst for translating the asymmetric induction achieved
under stoichiometric conditions to catalytic conditions, as least
for exo-type alkenyl iodides (e.g., A in Table 1). To the
contrary, catalytic asymmetric couplings with some Ni catalysts,
represented by 3,3′-dimethyl-2,2′-dipyridyl−NiCl2, progressed
well, but the degree of asymmetric induction was noticeably
lower than that observed under stoichiometric conditions. We
have suggested that the observed reduction in the asymmetric
induction is due to a background reaction, with the C−C bond
formation taking place through a Cr complex formed with an
achiral ligand scrambled from the Ni catalyst.13
Table 1. Catalytic Asymmetric Ni/Cr-Mediated Couplings
To Assess the Performance of Phenanthroline−NiCl2
a
Complexes
The third issue is the overall coupling rate. The Ni/Cr-
mediated coupling reaction is initiated by oxidative addition of
Ni(0) to an alkenyl halide/triflate. Thus, the Ni catalyst has an
effect on the overall coupling rate even though it is not directly
involved in the C−C bond formation step.
Although (H)2Phen(Me)2·NiCl2 (1b) serves well for many
cases, we felt that it was important to conduct a thorough study
of Ni catalysts for effective use of catalytic asymmetric Ni/Cr-
mediated couplings. For this study, we selected six phenanthro-
line−NiCl2 complexes 1a−c and 2a−c14 (Scheme 3) and
a
Couplings were done with 10 mol % Cr catalyst prepared from
sulfonamide 3a and 2 mol % Ni catalyst at rt in MeCN (c = 0.4 M) in
the presence of Mn metal (2 equiv), Zr(cp)2Cl2 (1.2 equiv), and LiCl
(2 equiv); E/Z and er were estimated from 1H NMR spectra of crude
products and Mosher esters of coupling products, respectively. 1.5
equiv of A was used, as homodimerization was more significant for A
than for the other alkenyl iodides. Conversion was ∼70%. Couplings
were very sluggish with 10 mol % Cr catalyst and 2 mol % Ni catalyst;
the conversion was ∼20, 55, 60, 70, 65, and 60% at the indicated times.
b
c
d
Scheme 3. Phenanthroline−NiCl2 Complexes and Chiral
Sulfonamide Ligands [Best = 3,5-Di(10H-phenothazine-10-
carbonyl)benzene]
Through this study, a satisfactory Ni catalyst(s) was
identified for four types of alkenyl iodides. First, 1a,b and
2a,b are satisfactory Ni catalysts for alkenyl iodide A,
representing 2-iodoolefins. Second, 2b is a satisfactory Ni
catalyst for alkenyl iodides B and C, representing di- and
trisubstituted (E)-1-iodoolefins, respectively; no E → Z
isomerization was detected with 1c and 2b,c, whereas E → Z
isomerization was observed with 1a,b and 2a. However, 2b is
far superior to 1c and 2c in terms of (1) the degree of
asymmetric induction and (2) the time for coupling
completion. Third, 2c is the only Ni catalyst to avoid Z → E
isomerization of alkenyl iodide D, representing disubstituted
(Z)-1-iodoolefins. Fourth, no Ni catalyst was found completely
to suppress Z → E isomerization of alkenyl iodide E,
representing trisubstituted (Z)-1-iodoolefins.15 It is noteworthy
that under the original stoichiometric coupling conditions, the
olefin geometry is retained for disubstituted (E)- and (Z)-vinyl
iodides and trisubstituted (E)-vinyl iodides but is lost for
trisubstituted (Z)-vinyl iodides.1
As mentioned, the geometrical isomerization takes place
through the alkenylnickel species, thereby suggesting the
possibility that the geometrical isomerization should be
suppressed by lowering the Ni/Cr ratio. This notion was
tested by conducting the coupling with two different of Ni/Cr
catalyst ratios; the coupling of D with 1 mol % Ni catalyst 2b
and 10 mol % Cr catalyst gave only the Z product with er =
3.8:1, whereas the coupling with 2 mol % Ni catalyst 2b and 10
mol % Cr catalyst gave a 11:1 Z/E product mixture (er =
3.9:1).15 Overall, this approach did help in maintaining the Z
configuration but significantly slowed the coupling (i.e., the
studied their behavior in catalytic asymmetric Ni/Cr-mediated
coupling reactions in the presence of the Cr catalyst prepared
from CrCl2 and chiral sulfonamide ligand 3a.
For this study, we used the coupling of dihydrocinnamalde-
hyde with five types of alkenyl iodides A−E and focused on (1)
the degree of geometrical isomerization, if relevant; (2) the
degree of asymmetric induction; and (3) the time for
completion of the coupling (Table 1). This experiment
revealed that the six Ni catalysts exhibited a wide range of
reactivity profiles. Nonetheless, we recognized three general
trends: (1) substitution of 2,9-hydrogens with methyl or ethyl
groups resulted in an enhancement of the asymmetric induction
and coupling rate; (2) substitution of 2,9-hydrogens with
methyl or ethyl groups resulted in an enhancement of Z → E
isomerization; and (3) substitution of 4,7-hydrogens with
methoxy groups resulted in a suppression of the Z → E
isomerization.
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Communication
a
times for coupling completion were 10 h and <1 h with 1 and 2
mol % 2b, respectively).
Scheme 4. Palytoxin C16−C17 Bond-Forming Examples
For disubstituted (Z)-alkenyl iodides, (MeO)2Phen-
(H)2·NiCl2 (2c) was found to be a satisfactory Ni catalyst in
maintaining the olefin geometry, but we wished to enhance the
coupling rate. To achieve this goal, we expanded the search area
of Ni catalysts and eventually found that some phosphine-based
Ni catalysts exhibit an appealing profile (Table 2). For
Table 2. Catalytic Asymmetric Ni/Cr-Mediated Coupling To
a
Assess Performance of Phosphine−NiCl2 Complexes
a
For experimental details, see the footnotes of Table 1.
disubstituted (Z)-vinyl olefins, represented by D, both
[(Me)3P]2·NiCl2 (4a) and [(cy)3P]2·NiCl2 (4b) meet all the
three criteria, i.e., no Z → E isomerization, a high degree of
asymmetric induction, and a high coupling rate. However, for
trisubstituted (Z)-vinyl olefins, represented by E, these Ni
catalysts still gave a mixture of Z and E products.15 We should
also note that these phosphine-based Ni catalysts are not as
effective as the phenanthroline-based NiCl2 catalysts for the
vinyl iodides represented by A, B, and C in terms of the
coupling rate.
Lastly, it is noteworthy that the effective Ni catalysts 2b, 2c,
4a, and 4b gave only insignificant amounts of homodimers.
This observation probably implies that no significant amounts
of alkenylnickel(II) species accumulated in these cases.16
Naturally, we were interested in testing the performance of
these Ni catalysts for catalytic asymmetric Ni/Cr-mediated
coupling of polyfunctional substrates. As a demonstration, we
chose the C16−C17 bond-forming reaction of palytoxin for
three reasons. First, this is the coupling reaction that led us to
the discovery of the Ni/Cr-mediated coupling reaction.2a
Second, aldehyde 5 is labile and does not agree well with
standard organometallic reagents.17 Third, the original non-
asymmetric stoichiometric protocol gave only very modest
diastereoselectivity.2a
On the basis of the results outlined in Tables 1 and 2, we
chose Ni catalysts 2b and 4a18 for (E)- and (Z)-vinyl iodides 6
and 7, respectively. A brief ligand optimization via the toolbox
approach6 allowed us to find a suitable chiral sulfonamide for
each case. Thus, four allylic alcohols, namely, 16α- and 16β-8
and 16α- and 16β-9 in both the (19R,20R) and (19S,20S)
series, were obtained at a synthetically useful level of
stereoselectivity without geometrical isomerization (Scheme
4).19
a
Couplings were done with 20 mol % Cr catalyst prepared from
sulfonamide 3a and 4 mol % Ni catalyst at rt in MeCN (c = 0.4 M), in
the presence of Mn metal (2 equiv), Zr(cp)2Cl2 (1.2 equiv), and LiCl
(2 equiv); E/Z and er were estimated from HPLC or 1H NMR
analysis, respectively.
For disubstituted (Z)-alkenyl iodide D, 2b was identified as an
acceptable Ni catalyst in terms of the absence of Z → E
isomerization and the degree of asymmetric induction but not
in terms of the coupling rate. Two phosphine-based Ni
catalysts, [(Me)3P]2·NiCl2 and [(cy)3P]2·NiCl2, were found to
meet all three criteria for D. The C16−C17 bond-forming
reaction of palytoxin was used to demonstrate the usefulness of
the Ni catalysts thus identified.
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details and 1H and 13C NMR spectra. This
material is available free of charge via the Internet at http://
pubs.acs.org.
In conclusion, we have shown the importance of the Ni
catalyst in achieving catalytic asymmetric Ni/Cr-mediated
coupling reactions effectively. Six phenanthroline−NiCl2
complexes 1a−c and 2a−c and five types of alkenyl iodides
A−E were chosen for the study. These Ni catalysts were found
to display a wide range of overall reactivity profiles in terms of
the degree of asymmetric induction, geometrical isomerization,
and coupling rate. For three types of alkenyl iodides A−C, a
satisfactory Ni catalyst(s) was found within 1a−c and 2a−c.
AUTHOR INFORMATION
■
Corresponding Author
kishi@chemistry.harvard.edu
Notes
The authors declare no competing financial interest.
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Communication
this observation is interesting because alkenylpalladium species are
known to be configurationally more stable. In spite of extensive efforts,
we were unable to activate CrCl2 by doping with Pd salts. We now
speculate that the Pd(OAc)2 used in the original work might have
been contaminated with some metal salt.
ACKNOWLEDGMENTS
■
We are grateful to the Eisai USA Foundation for generous
financial support.
(12) For examples, see: (a) Huggins, J. M.; Bergman, R. G. J. Am.
Chem. Soc. 1981, 103, 3002. (b) Hirata, Y.; Tanaka, M.; Yada, A.;
Nakao, Y.; Hiyama, T. Tetrahedron 2009, 65, 5037.
(13) In the presence of some Ni complexes, the coupling reaction
slowly progresses in MeCN with only free CrCl2, roughly
corresponding to the background coupling.
(14) The colors of the NiCl2 complexes are violet (1a, 2a, and 2b),
yellow (1b), green (1c), and blue (2c). The yellow and violet
complexes are known to be the α- and β-forms of 1b. See: Preston, H.
S.; Kennard, C. H. L. J. Chem. Soc. A 1969, 2682.
(15) With a lower Ni/Cr-ratio, the degree of Z → E isomerization
can be suppressed even for the trisubstituted (Z)-olefin E: in the
presence of 20 mol % Cr catalyst and (2 + 2) mol % Ni catalyst (the
second 2 mol % was added at 4 h), the coupling of trisubstituted (Z)-
vinyl olefin E was complete within 16 h, giving the product with Z/E =
3.2/1.
(16) An alkenylnickel(I) species was suggested to play a key role in
homodimerization. See: Tsou, T. T.; Kochi, J. K. J. Am. Chem. Soc.
1979, 101, 7547. Also see: Semmelhack, M. F.; Helquist, P. M.;
Gorzynski, J. D. J. Am. Chem. Soc. 1972, 94, 9234. Colon, I.; Kelsey, D.
R. J. Org. Chem. 1986, 51, 2627. Amatore, C.; Jutand, A.
Organometallics 1988, 7, 2203.
(17) Aldehyde 5 readily eliminates benzyl alcohol even under weakly
basic conditions, yielding the corresponding α,β-unsaturated aldehyde.
This instability limited the choices and reaction conditions.
Experimentally, it was found that only simple organocuprates gave
the desired adduct in satisfactory yields.
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