Study of Intermediates in Iridium-(Phosphoramidite,Olefin)-Catalyzed Enantioselective Allylic Substitution

Article abstract of DOI:10.1021/jacs.6b12421

Experimental mechanistic studies of iridium-catalyzed, enantioselective allylic substitution enabled by (phosphoramidite,olefin) ligands are reported. (η²-Allylic alcohol)iridium(I) and (η³-allyl)iridium(III) complexes were synthesiz

Full text of DOI:10.1021/jacs.6b12421

Communication
pubs.acs.org/JACS
Study of Intermediates in Iridium−(Phosphoramidite,Olefin)-
Catalyzed Enantioselective Allylic Substitution
Simon L. Rossler, Simon Krautwald, and Erick M. Carreira*
̈
̈
̈ ̈
Eidgenossische Technische Hochschule Zurich, Vladimir-Prelog-Weg 3, HCI H335, 8093 Zurich, Switzerland
*
S Supporting Information
Scheme 1. Ir−(P,olefin)-Catalyzed Allylic Substitutions
ABSTRACT: Experimental mechanistic studies of iridi-
um-catalyzed, enantioselective allylic substitution enabled
2
by (phosphoramidite,olefin) ligands are reported. (η -
3
Allylic alcohol)iridium(I) and (η -allyl)iridium(III) com-
plexes were synthesized and characterized by NMR
spectroscopy as well as X-ray crystallography. The
substrate complexes are catalytically and kinetically
competent to be intermediates in allylic substitutions of
branched, racemic allylic alcohols with various nucleo-
philes. In addition, we have identified an off-cycle pathway
involving reversible binding of molecular oxygen to
iridium, which contributes to the air tolerance of the
catalyst system.
ridium-catalyzed allylic substitutions have emerged as versatile
I
methods for the enantioselective formation of carbon−carbon
1
and carbon−heteroatom bonds. These processes are charac-
terized by preferential attack of nucleophiles at the more
substituted terminus of unsymmetrical allylic electrophiles,
complementing the regioselectivity generally observed with
Previous studies have established the mechanism of allylic
2
10−12
substitutions with a variety of palladium and iridium
2
3
4
palladium. Following Takeuchi’s and Helmchen’s seminal
catalysts. In light of this precedent, our own studies were guided
by the proposed catalytic cycle shown in Scheme 1b.
Accordingly, coordination of allylic alcohol to iridium gives η
reports on iridium-catalyzed allylic substitution in 1997, a variety
of chiral ligands have been shown to induce enantiocontrol. The
5
2
complex I, which undergoes acid-promoted oxidative addition to
best-studied catalyst system, however, relies on Feringa’s
3
5
f
furnish (η -allyl)iridium(III) species II. Nucleophilic attack on
phosphoramidites, the use of which in iridium-catalyzed allylic
6
the allyl fragment affords product complex III, and subsequent
displacement of the product by allylic alcohol completes the
catalytic cycle.
We first set out to examine whether adducts of allylic alcohols,
iridium, and ligand L, such as substrate complex I, could be
formed and spectroscopically observed. We found that
amination was first described by Hartwig. This catalyst system
typically employs linear allylic carbonates as substrates along with
7
8
iridacyclic catalysts and operates under alkaline conditions.
Our laboratory has been studying and developing iridium
catalysts derived from (phosphoramidite,olefin) ligand L, which
allows direct enantio- and regioselective substitution of easily
accessible branched, racemic allyl alcohols in the presence of a
combining 2 equiv of (R)-1, 0.5 equiv of [Ir(cod)Cl] , and 2
2
equiv of ligand (R)-L led to a well-defined species in solution as
9
variety of acid promoters (Scheme 1a). The iridium−L catalyst
31
1
determined by P{ H} NMR spectroscopy (Scheme 2). Its
characterization was facilitated by removal of the released 1,5-
cyclooctadiene and uncomplexed (R)-1. Precipitation with
system is remarkably tolerant of oxygen and water, enabling
operationally simple execution. While the mechanisms by which
the iridacyclic complexes of Hartwig and Helmchen operate have
31
1
pentane gave a powder whose P{ H} NMR spectrum displayed
the same signals as observed in the initial experiment. The
10−12
been studied in detail,
no such experimental studies have
2
been reported to date for the Ir−(P,olefin) system. However,
formation of (η -allylic alcohol)iridium(I) complex (R,R,R)-2
1
3
31
1
Sunoj has recently disclosed a detailed computational study.
was evidenced by two doublets in the P{ H} NMR spectrum at
2
2
Herein we report the isolation and characterization of (η -allylic
alcohol)iridium(I) and (η -allyl)iridium(III) complexes and
δ = 114.1 and 106.4 ppm ( J
= 27.1 Hz), indicating two
P−P
3
inequivalent phosphorus donors at the iridium center. In
1
particular, the upfield shifts of the vinyl protons in the H
demonstrate their competence as intermediates in a variety of
catalytic reactions, giving products in yields and selectivities that
match those observed for the in situ-generated catalyst originally
reported.
Received: December 2, 2016
©
XXXX American Chemical Society
A
DOI: 10.1021/jacs.6b12421
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Scheme 2. NMR Experiment Showing Coordination of Allyl
Alcohol (R)-1 to Iridium
main structural differences compared with its diastereomer are
the orientation of the azepine moiety and the spatial disposition
of the allylic alcohol C−O bond in relation to the substrate olefin
(
(
(
C1−C2−C3−O1, 3.4(5)° in (R,R,R)-2 vs −127.0(7)° in
R,R,S)-2). Notably, the solid-state structures of (R,R,R)-2 and
R,R,S)-2 closely resemble the lowest-energy structures of the
allylic alcohol complexes predicted by Sunoj’s computational
13
study.
We then examined the reactions of the allylic alcohol
complexes with acid on the premise that (η -allyl)iridium(III)
complexes might be formed and observed. Following treatment
3
NMR spectrum (δ = 3.53, 2.35, and 1.66 ppm) provide evidence
2
for η coordination of the terminal olefin in (R)-1 to iridium.
2
13
Consistent with the η hapticity for (R)-1, the C NMR
spectrum displayed upfield resonances for the olefinic carbons (δ
of (R,R,R)-2 in CDCl with triflic acid, two new species were
3
31
1
observed in a 1.6/1 ratio by P{ H} NMR spectroscopy (δ =
2
=
63.8 and 28.5 ppm).
The solid-state structure of (R,R,R)-2 confirmed the NMR
1
(
05.4 and 66.5 ppm ( J = 31.6 Hz); δ = 101.9 and 69.9 ppm
P−P
2
JP−P = 25.0 Hz)), and HRMS analysis showed a major peak at
+
assignment and revealed coordination of one phosphoramidite
m/z 1359.2788, corresponding to [M] of the allyl complex. We
suspected that the novel species were two diastereomeric endo-
2
ligand in bidentate (η -κP) fashion, whereas the other (P,olefin)
3
ligand is bound solely through phosphorus (Scheme 3).
and exo-(η -allyl)iridium(III) complexes. Remarkably, treatment
of (R,R,S)-2 with acid also led to the same two diastereomers in
2
Scheme 3. Synthesis and X-ray Structures of (η -Allylic
identical ratio. Thus, either enantiomer of the allylic alcohol leads
15
3
alcohol)iridium(I) Complexes (R,R,R)-2 and (R,R,S)-2
to the same mixture of (η -allyl)iridium(III) species. If oxidative
addition proceeds with inversion of configuration, our
observations are consistent with rapid isomerization of the two
16
3
diastereomeric allyl complexes. The (η -allyl)iridium(III)
complexes could also be synthesized directly without prior
2
isolation of (η -allylic alcohol)iridium(I) intermediates, as shown
3
in Scheme 4. This procedure enabled the synthesis of several (η -
allyl)iridium(III) complexes bearing various allyl groups.
3
Scheme 4. Synthesis of (η -Allyl)iridium(III) Complexes and
15
X-ray Structure of [3c]OTf
Complex (R,R,R)-2 adopts a distorted trigonal bipyramidal
geometry in which the equatorial loci are occupied by the
terminal olefin of (R)-1, P1A, and the olefin of the chelating
phosphoramidite ligand; P1B and chloride reside in the axial
positions. The bond of the equatorial phosphorus to iridium
(
Ir1−P1A, 2.3272(9) Å) is longer than that of the axial P (Ir1−
P1B, 2.1917(9) Å). The coordinated olefin of the ligand (C27B−
C28B, 1.425(5) Å) and that of the allyl alcohol (C1−C2,
The putative endo/exo ratio in these was dependent on the
electronic and steric properties of the arene substituents. Thus,
the o-nitro substrate formed complex [3c]OTf as a single
1
.425(6) Å) are elongated compared with those of the
31
1
uncoordinated olefin (C27A−C28A, 1.338(6) Å) and free
ligand (R)-L (C23−C32, 1.350(11) Å; Figure S14), indicating
significant back-bonding. Interestingly, the dibenzazepine nitro-
diastereomer (>20/1 dr) as indicated by P{ H} NMR analysis
2
13
(δ = 104.7 and 62.1 ppm ( J
= 32.2 Hz)). The C NMR
P−P
spectrum of [3c]OTf displayed the resonance belonging to the
2
gen atom undergoes a hybridization change from sp in the
terminal CH of the allyl group at δ = 33.0 ppm, providing
2
3
14
monodentate ligand to sp in the bidentate ligand. The
diastereomeric complex (R,R,S)-2 was prepared analogously
from allyl alcohol (S)-1. The spectroscopic features of (R,R,S)-2
are similar to those of complex (R,R,R)-2, and its solid-state
structure is also a distorted trigonal bipyramid (Scheme 3). The
evidence against a σ-bound allyl Ir−CH moiety since these
2
1
7
typically appear at lower chemical shifts. After several
unsuccessful attempts to crystallize enantiomerically pure
[3c]OTf, crystals suitable for X-ray diffraction were grown
from the racemate obtained by mixing [3c]OTf and its
B
DOI: 10.1021/jacs.6b12421
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
1
8
enantiomer in equimolar amounts. The solid-state structure of
3c]OTf is shown in Scheme 4 and allowed assignment of the
competence of (R,R,R)-2 and [3a]OTf to be intermediates in
[
the catalytic cycle (Figure S8).
3
1
1
31
1
major diastereomer as exo-[3c]OTf. P− H HOESY and
P− H COSY experiments confirmed the exo configuration as
When allylation experiments were monitored by P{ H}
NMR spectroscopy, a singlet resonance at δ = 141.1 ppm
indicated the presence of a minor species. The same species had
3
1
1
19
the major diastereomer in solution (Figure S5). It is important
to note that outer-sphere attack on this intermediate would lead
to the minor product enantiomer. Thus, the high enantiose-
lectivities observed would be explained either by isomerization
prior to nucleophilic attack or an inner-sphere mechanism.
Additional experiments will be required to determine the
detailed course of events. Complex [3c]OTf adopts a severely
distorted six-coordinate octahedral geometry with the allyl
group, the phosphorus atom of the nonchelating L, and the olefin
of the chelating L in one plane. The phosphorus atom of the
chelating ligand and the chloride lie apically with respect to this
plane. The bond from the benzylic carbon of the allyl fragment to
iridium is 0.35 Å longer than that from the terminal carbon (Ir1−
C1, 2.161(3) Å; Ir1−C3, 2.508(3) Å). Such a difference is
unusually large for it to be attributed solely to the trans influence
of the phosphoramidite and olefin ligands. Considering the
spatial environment about the iridium, the elongated bond may
also stem from an allyl plane deflection due to steric hindrance
between the allyl’s arene fragment and the ligand’s azepine
moiety. Although the terminus with the longer metal−carbon
bond has been proposed to be more electrophilic, Hartwig’s
recent studies have established that Ir−C bond lengths do not
govern regioselectivity in allylic substitutions involving triphe-
2
also been observed to slowly form upon dissolution of (η -allylic
alcohol)iridium(I) complexes in the absence of substrate to
function as an additional ligand. Indeed, the complex associated
with this resonance was synthesized independently from
[Ir(cod)Cl] and 2 equiv of (R)-L and was identified as 4, in
2
which both ligands bind in bidentate fashion (eq 1; the solid-state
22
structure of 4 is shown in Figure S12).
20
Importantly, treatment of complex 4 with an excess of allylic
alcohol 1 in CDCl did not lead to formation of the allylic alcohol
3
complexes 2 at room temperature (eq 1). This observation
indicates that the allylic alcohol substrate does not displace the
olefin of the iminostilbene portion of the ligand in 4. In our
9
21
published procedures, [Ir(cod)Cl] and ligand L are combined
2
in solvent and stirred for 15 min prior to addition of allylic
alcohol and nucleophile. An important corollary of the above
observation is that when the reactions are conducted with in situ-
generated catalyst, it is best to include substrate from the outset
10g
nylphosphite as a ligand. Additional data will therefore be
needed to clarify the influence of Ir−C bond lengths on the
regioselectivity of the (P,olefin) catalyst system.
To evaluate the catalytic competence of the isolated
complexes, (R,R,R)-2 and [3a]OTf were employed as catalysts
in a number of our published methods, as shown in Scheme 5.
(
Figure S8) in order to avoid slow formation of 4 (Figure S7).
The Ir/(R)-L catalyst system is remarkably tolerant of oxygen
and moisture. Indeed, some reactions can be carried out in mixed
9d
organic−aqueous media, and generally no special precautions
are needed to exclude air. In this regard, we were curious to
examine the effect of oxygen on the catalyst system. Stirring a
Scheme 5. Allylic Substitution Reactions Catalyzed by
a
solution of 0.5 equiv of [Ir(cod)Cl] and 2 equiv of (R)-L in
2
(R,R,R)-2 and [3a]OTf
CHCl saturated with oxygen followed by precipitation with
3
Et O gave a solid, which we characterized as oxygen adduct
2
complex 5 on the basis of analysis by NMR spectroscopy and X-
ray diffraction (Scheme 6). The solid-state structure of complex 5
15
Scheme 6. Synthesis and X-ray Structure of 5
a
(
R,R,R)-2 and [3a]OTf were employed as catalysts with the reported
catalyst loadings. See the Supporting Information for experimental
details.
revealed side-on binding of dioxygen in which the two oxygen
atoms are approximately equidistant to iridium (Ir1−O1,
2
.027(6) Å; Ir1−O2, 2.028(6) Å). The average O−O distance
The yields and selectivities observed with the isolated complexes
as catalysts match our previous observations, establishing that
of 1.465(8) Å, while longer than that in O (1.21 Å), is
comparable to that in the oxygen adduct of Vaska’s complex
(1.47 Å) and corresponds closely to that in O2 (1.49 Å).
2
9
23
2−
24
(R,R,R)-2 and [3a]OTf are catalytically competent in a broad
range of reactions. Furthermore, the allylation of 1 with
allyltrimethylsilane was monitored, establishing the kinetic
Accordingly, the iridium center in 5 can be described as six-
coordinate with a formal oxidation state of III.
C
DOI: 10.1021/jacs.6b12421
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Journal of the American Chemical Society
Communication
Reversible binding of molecular oxygen to low-valent iridium
complexes has been known since Vaska’s pioneering studies,
Angew. Chem., Int. Ed. 2003, 42, 2054. (e) Miyabe, H.; Matsumura, A.;
Moriyama, K.; Takemoto, Y. Org. Lett. 2004, 6, 4631. (f) Teichert, J. F.;
Feringa, B. L. Angew. Chem., Int. Ed. 2010, 49, 2486.
25
and treatment of 5 with allyl alcohol rac-1 (15 equiv) in CDCl3
indeed resulted in the formation of a 1:1 mixture of 2 and 5.
Furthermore, complex 5 also catalyzed the allylation of 1 with
allyltrimethylsilane at a decreased rate (Figure S8). This
experiment underscores the robustness of the catalyst system,
since iridium can re-enter the catalytic cycle upon oxygen
dissociation.
(
(
6) Ohmura, T.; Hartwig, J. F. J. Am. Chem. Soc. 2002, 124, 15164.
7) Kiener, C. A.; Shu, C.; Incarvito, C.; Hartwig, J. F. J. Am. Chem. Soc.
2
003, 125, 14272.
(8) For selected examples, see: (a) Bartels, B.; García-Yebra, C.;
Helmchen, G. Eur. J. Org. Chem. 2003, 2003, 1097. (b) Liu, W.-B.;
Zhang, X.; Dai, L.-X.; You, S.-L. Angew. Chem., Int. Ed. 2012, 51, 5183.
(c) Gartner, M.; Mader, S.; Seehafer, K.; Helmchen, G. J. Am. Chem. Soc.
̈
2011, 133, 2072. (d) Liu, W.-B.; Reeves, C. M.; Stoltz, B. M. J. Am.
Chem. Soc. 2013, 135, 17298. (e) Chen, W.; Hartwig, J. F. J. Am. Chem.
Soc. 2013, 135, 2068. (f) Chen, M.; Hartwig, J. F. Angew. Chem., Int. Ed.
2014, 53, 8691. (g) Alexakis, A.; Polet, D. Org. Lett. 2004, 6, 3529.
(h) Zhuo, C.-X.; Zheng, C.; You, S.-L. Acc. Chem. Res. 2014, 47, 2558.
In summary, we have isolated and characterized key complexes
relevant to the catalytic cycle of iridium-catalyzed allylic
substitution involving (P,olefin) ligand L, including substrate−
2
iridium complexes, namely, (η -allylic alcohol)iridium(I) and
3
(
η -allyl)iridium(III). Two practical consequences emanate from
(9) (a) Defieber, C.; Ariger, M. A.; Moriel, P.; Carreira, E. M. Angew.
this work: the catalyst is optimally prepared in situ in the
presence of allyl alcohol, and the robustness of the iridium
catalyst system stems in part from reversible dioxygen binding.
These results form the basis of in-depth studies currently
ongoing in our laboratory.
Chem., Int. Ed. 2007, 46, 3139. (b) Lafrance, M.; Roggen, M.; Carreira, E.
M. Angew. Chem., Int. Ed. 2012, 51, 3470. (c) Schafroth, M. A.; Sarlah,
D.; Krautwald, S.; Carreira, E. M. J. Am. Chem. Soc. 2012, 134, 20276.
(d) Hamilton, J. Y.; Sarlah, D.; Carreira, E. M. J. Am. Chem. Soc. 2013,
135, 994. (e) Hamilton, J. Y.; Sarlah, D.; Carreira, E. M. Angew. Chem.,
Int. Ed. 2013, 52, 7532. (f) Krautwald, S.; Sarlah, D.; Schafroth, M. A.;
Carreira, E. M. Science 2013, 340, 1065. (g) Hamilton, J. Y.; Sarlah, D.;
Carreira, E. M. J. Am. Chem. Soc. 2014, 136, 3006. (h) Krautwald, S.;
Schafroth, M. A.; Sarlah, D.; Carreira, E. M. J. Am. Chem. Soc. 2014, 136,
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.6b12421.
■
*
S
3
020. (i) Hamilton, J. Y.; Hauser, N.; Sarlah, D.; Carreira, E. M. Angew.
Chem., Int. Ed. 2014, 53, 10759.
Experimental procedures and characterization data (PDF)
Crystallographic data for (R,R,R)-2, (R,R,S)-2, [3c]OTf,
(10) Hartwig’s studies: (a) Leitner, A.; Shu, C.; Hartwig, J. F. Proc. Natl.
Acad. Sci. U. S. A. 2004, 101, 5830. (b) Leitner, A.; Shekhar, S.; Pouy, M.
J.; Hartwig, J. F. J. Am. Chem. Soc. 2005, 127, 15506. (c) Markovic, D.;
́
Hartwig, J. F. J. Am. Chem. Soc. 2007, 129, 11680. (d) Madrahimov, S. T.;
Markovic, D.; Hartwig, J. F. J. Am. Chem. Soc. 2009, 131, 7228.
(e) Hartwig, J. F.; Stanley, L. M. Acc. Chem. Res. 2010, 43, 1461.
(f) Madrahimov, S. T.; Hartwig, J. F. J. Am. Chem. Soc. 2012, 134, 8136.
(g) Madrahimov, S. T.; Li, Q.; Sharma, A.; Hartwig, J. F. J. Am. Chem.
Soc. 2015, 137, 14968.
4, 5, and (R)-L (CIF)
AUTHOR INFORMATION
■
Corresponding Author
carreira@org.chem.ethz.ch
ORCID
*
(11) Helmchen’s studies: (a) Bartels, B.; Helmchen, G. Chem.
̈
Simon L. Rossler: 0000-0002-5057-0576
Commun. 1999, 741. (b) Bartels, B.; García-Yebra, C.; Rominger, F.;
Helmchen, G. Eur. J. Inorg. Chem. 2002, 2002, 2569. (c) Spiess, S.;
Welter, C.; Franck, G.; Taquet, J.-P.; Helmchen, G. Angew. Chem., Int.
Erick M. Carreira: 0000-0003-1472-490X
Notes
Ed. 2008, 47, 7652. (d) Spiess, S.; Raskatov, J. A.; Gnamm, C.; Bro
K.; Helmchen, G. Chem. - Eur. J. 2009, 15, 11087. (e) Raskatov, J. A.;
Spiess, S.; Gnamm, C.; Brodner, K.; Rominger, F.; Helmchen, G. Chem. -
Eur. J. 2010, 16, 6601. (f) Raskatov, J. A.; Jakel, M.; Straub, B. F.;
̈
dner,
The authors declare no competing financial interest.
̈
ACKNOWLEDGMENTS
■
̈
We are grateful to ETH Zu
̈
rich and the Swiss National Science
Rominger, F.; Helmchen, G. Chem. - Eur. J. 2012, 18, 14314.
(12) You’s studies: Liu, W.-B.; Zheng, C.; Zhuo, C.-X.; Dai, L.-X.; You,
S.-L. J. Am. Chem. Soc. 2012, 134, 4812.
Foundation (200020_152898) for financial support. We thank
Dr. N. Trapp and M. Solar for X-ray crystallographic analysis and
Dr. M.-O. Ebert, R. Arnold, R. Frankenstein, and S. Burkhardt for
NMR measurements.
(13) Bhaskararao, B.; Sunoj, B. S. J. Am. Chem. Soc. 2015, 137, 15712.
(
2
(
14) Drinkel, E.; Bricen
010, 29, 2503.
15) Thermal ellipsoids are shown at the 50% probability level.
̃
o, A.; Dorta, R.; Dorta, R. Organometallics
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DOI: 10.1021/jacs.6b12421
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX