Iridium-Catalyzed Diastereoselective and Enantioselective Allylic Substitutions with Acyclic α-Alkoxy Ketones

Article abstract of DOI:10.1002/anie.201600235

The asymmetric alkylation of acyclic ketones is a longstanding challenge in organic synthesis. Reported herein are diastereoselective and enantioselective allylic substitutions with acyclic α-alkoxy ketones catalyzed by a metallacyclic iridium complex to form products with contiguous stereogenic centers derived from the nucleophile and electrophile. These reactions occur between allyl methyl carbonates and unstabilized copper(I) enolates generated in situ from acyclic α-alkoxy ketones. The resulting products can be readily converted into enantioenriched tertiary alcohols and tetrahydrofuran derivatives without erosion of enantiomeric purity.

Full text of DOI:10.1002/anie.201600235

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International Edition: DOI: 10.1002/anie.201600235
German Edition: DOI: 10.1002/ange.201600235
Asymmetric Catalysis
Iridium-Catalyzed Diastereoselective and Enantioselective Allylic
Substitutions with Acyclic a-Alkoxy Ketones
Xingyu Jiang, Wenyong Chen, and John F. Hartwig*
Abstract: The asymmetric alkylation of acyclic ketones is
a longstanding challenge in organic synthesis. Reported herein
are diastereoselective and enantioselective allylic substitutions
with acyclic a-alkoxy ketones catalyzed by a metallacyclic
iridium complex to form products with contiguous stereogenic
centers derived from the nucleophile and electrophile. These
reactions occur between allyl methyl carbonates and unstabi-
lized copper(I) enolates generated in situ from acyclic a-
alkoxy ketones. The resulting products can be readily con-
verted into enantioenriched tertiary alcohols and tetrahydro-
furan derivatives without erosion of enantiomeric purity.
In the presence of suitable metal cations, acyclic carbonyl
compounds bearing a-heteroatoms form enolates with
a defined geometry created by chelation. This structure has
been exploited for the allylation of glycine derivatives.[6]
However, these reactions occurred with low diastereoselec-
tivity when forming products containing adjacent tetrasub-
stituted and tertiary stereocenters.[6a] With the same strategy,
Evans and co-workers achieved diastereoselective allylations
of a-hydroxy, as well as a-alkoxy or a-siloxy acetophenone
catalyzed by an achiral rhodium catalyst.[7] However, only
products containing adjacent tertiary and trisubstituted
stereocenters, bearing an oxygen atom, were formed, and no
enantioselective transformation was reported. We envisioned
that this strategy could be used to achieve the enantioselec-
tive allylation of unstabilized ketones with cyclometallated
iridium catalysts we developed.[8]
Herein, we report diastereo- and enantioselective allylic
alkylations with unstabilized enolates of acyclic a-alkoxy
ketones catalyzed by the iridium complex 3 (Scheme 1). The
geometry of the enolates is controlled through chelation in
the presence of a copper(I) cation. These reactions form, with
high diastereo- and enantioselectivity, products containing
vicinal oxygen-bearing tetrasubstituted and tertiary stereo-
centers. Products containing a MOM (methoxymethyl) group
on the tertiary alcohol were formed in good yield with high
d.r. and ee values, and these products can be readily converted
into tertiary alcohols and tetrahydrofuran (THF) derivatives
without erosion of enantiomeric purity.
T
ransition metal catalyzed asymmetric allylation of enolates
serves as an efficient and reliable method to construct
carbon–carbon bonds with high levels of asymmetric induc-
tion.[1] The majority of these reactions form products con-
taining a single stereocenter from either a prochiral enolate as
the nucleophile or a prochiral allylic compound as the
electrophile. If both nucleophile and electrophile are prochi-
ral, synthetically valuable dyads containing contiguous ste-
reocenters could be assembled in a catalytic and stereoselec-
tive fashion.[2] However, this transformation is challenging
because a new bond needs to be formed between two
sterically hindered prochiral carbon atoms with control of
both absolute and relative configurations.
Metallacyclic iridium complexes catalyze allylic substitu-
tions with a variety of carbon and heteroatom nucleophiles
regio- and enantioselectively.[1b, 3] Although reactions have
been reported between prochiral enolates and prochiral
electrophiles to afford products containing vicinal tetrasub-
stituted and trisubstituted stereocenters with excellent dia-
stereo- and enantioselectivity, reactions with unstabilized,
acyclic, prochiral ketones have not been reported.[4] The main
challenge facing this transformation results from the lack of
control of the geometry of the unstabilized enolate of an a-
branched acyclic ketone. In contrast to cyclic enolates, the
backbone of the nucleophile does not dictate the geometry.
Also, because a-branched, acyclic ketones do not readily form
enamines, the use of amine auxiliaries has not been effective
to control the geometry.[5]
Scheme 1. Iridium-catalyzed diastereo- and enantioselective allylations
with unstabilized copper(I) enolates of acyclic a-alkoxy ketones.
[*] X. Jiang, W. Chen, Prof. J. F. Hartwig
Department of Chemistry, University of California
Berkeley, CA 94720 (USA)
and
To assess the potential of developing an iridium-catalyzed
allylation of an acyclic ketone enolate, we conducted the
reactions between O-methyl benzoin (1a) and methyl cin-
namyl carbonate (2a; Table 1). Treatment of 1a and 2a with 3
in the presence of LHMDS at 58C for 12 hours furnished the
Division of Chemical Sciences
Lawrence Berkeley National Laboratory, Berkeley, CA 94720 (USA)
E-mail: jhartwig@berkeley.edu
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201600235.
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Table 1: Evaluation of reaction conditions for the iridium-catalyzed
a significantly lower d.r. of 2.7:1 (entry 10). Reactions
conducted with one equivalent of copper in place of two
equivalents led to excellent reactivity and afforded the
product in identical d.r. and ee values (entry 11), but reactions
allylation.[a]
with 0.5 equivalents occurred with
a lower d.r. value
(although slightly higher ee), as shown in entry 12.[10] In all
cases, the branched product was obtained exclusively. Reac-
tions run with CuBr2 as an additive gave no product, thus
indicating the critical role of the copper(I) cation in this
reaction, rather than a copper(II) cation which might be
formed by disproportionation or oxidation of a copper(I) salt
(entry 13).
The scope with respect to the allylic electrophiles which
underwent the iridium-catalyzed allylic substitution with
acyclic a-alkoxy ketones is summarized in Table 2. Various
para-substituted cinnamyl carbonates are suitable electro-
philes. Electron-neutral (4ab), electron-donating (4ac) and
electron-withdrawing (4ah, 4ai) functional groups on the
cinnamyl aryl ring were all tolerated in this reaction, and the
corresponding products were formed in excellent yield (ꢀ
94%) with high d.r. (ꢀ 10:1) and ee values (ꢀ 90%). Cin-
namyl carbonates bearing halogens at either the para- or
meta-position reacted cleanly, thus furnishing 4ad–ag in
greater than or equal to 92% yield, 12:1 d.r., and 91%
ee.[11] The absolute configuration of 4ag was established by
single-crystal X-ray diffraction.
The reaction also occurred with allylic carbonates con-
taining heteroaryl, alkenyl, and alkyl substituents. The
reaction of the thienyl carbonate 2k afforded 4ak in high
yield with excellent diastereo- and enantioselectivity (> 99%,
> 20:1 d.r., 92% ee). Methyl sorbyl carbonate (2l) reacted to
form the product 4al in 75% yield with greater than 20:1 d.r.
and 94% ee. Even the simple crotyl carbonate (2m) reacted
to form product 4am in good yield, although the d.r. and
ee values were slightly lower than those with aryl-substituted
allylic carbonates.
The scope of the acyclic a-alkoxy ketones that underwent
the iridium-catalyzed allylation is summarized in Table 3.[12,13]
MOM- (1b), MEM- (methoxyethoxymethyl; 1c), and PMB-
protected (para-methoxybenzyl; 1d) benzoins underwent
allylation in high yield with excellent diastereo- and enantio-
selectivity. The reaction between 1b and 2a required a higher
catalyst loading of 4 mol% to reach full conversion within
12 hours. Several acyclic O-Me benzoin derivatives bearing
identical substituents at both aryl rings, such as 1e and 1 f, as
well as their O-MOM analogues 1i and 1j were suitable for
this transformation (4ea, 4 fa, 4ic, and 4jk). The reactions
with nucleophiles derived from nonsymmetrical benzoins
were also examined. The benzoin 1g, bearing a thienyl group,
underwent allylation in quantitative yield with an excellent
d.r. value of 15:1 and 96% ee (4ga). The benzoin analogue
1h, containing an isobutyl group, reacted with 2a in a low
yield of 38% and low branched/linear selectivity of 4:1 (5:1
d.r. for the branched product).[14] However, the identical
reaction with the less reactive cinnamyl acetate as the
electrophile afforded branched product 4ha exclusively in
high yield (74%, isolated yield of the major diastereomer)
with acceptable diastereoselectivity and excellent enantiose-
lectivity.
Entry
Additive
Yield [%][b]
d.r.[c]
ee [%][d]
1
2
3
4
5
6
7
8
–
LiCl
ZnCl2
CuI
CuCl
CuBr
CuCN
CuOAc
CuSCN
CuBr
CuBr
CuBr
CuBr2
95
>99
72
97
2.0:1
1:1.1
16:1
5.7:1
10:1
14:1
2.5:1
1.5:1
2.3:1
2.7:1
14:1
12:1
–
n.d.
n.d.
n.d.
n.d.
n.d.
92
n.d.
n.d.
n.d.
n.d.
93
58
>99 (>99)
34
52
94
42
>99
>99
0
9
10[e]
11[f]
12[g]
13
95
n.d.
[a] The molar ratio of 1a/2a/3/LHMDS/additive=2:1:0.02:2:2. The
absolute configuration of 4aa was assigned by analogy. [b] Combined
yield of two diastereoisomers. Determined by 1H NMR analysis with
mesitylene as an internal standard. The yield within parentheses is that
of the two diastereoisomers isolated. [c] Determined by 1H NMR analysis
of the crude reaction mixtures. [d] Determined by chiral-phase SFC
analysis of the major isomer. [e] KHMDS was used as the base instead of
LHMDS. [f] 1 equiv of CuBr was used. [g] 0.5 equiv of CuBr was used.
LHMDS=lithium hexamethyldisilazide, n.d.=not determined,
THF=tetrahydrofuran.
branched product 4aa in 95% yield (combined yield of two
diastereoisomers), but with a low d.r. of 2.0:1 (entry 1). The
reaction conducted after addition of LiCl[4o] to the lithium
enolate gave the product with a lower d.r. of 1:1.1, slightly
favoring the formation of the other diastereoisomer (entry 2).
The reaction with added ZnCl2[6] afforded 4aa with excellent
diastereoselectivity (16:1 d.r., entry 3), albeit in a lower yield
of 72%. In contrast, the reaction conducted with added CuI
occurred with a higher diastereomeric ratio of 5.7:1 while
maintaining excellent conversion into 4aa in a 97% yield
(entry 4). Similarly, Evans and co-workers observed higher
diastereoselectivity with the copper(I) enolate of a-hydroxy
acetophenone derivatives than with the corresponding lith-
ium enolate.[7a] Because the anion of the copper(I) salt could
influence the transmetalation, we further evaluated a series of
copper(I) salts. Reactions conducted with added CuBr
occurred with a higher diastereomeric ratio of 14:1 with an
excellent ee value of 92% (Table1, entry 6). The major
diastereoisomer was isolated in 93% yield (see the Support-
ing Informaiton for details). Reactions with other copper(I)
additives, such as CuCl, CuCN, CuOAc or CuSCN, occurred
in significantly lower yield (58%, entry 5) or with lower
d.r. values (entries 7–9).[9] The identity of the cation of the
anionic base was crucial to obtaining high yields and
diastereoselectivities. Reactions conducted with KHMDS
instead of LHMDS afforded only 42% yield of 4aa with
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Table 2: Iridium-catalyzed allylations of acyclic a-alkoxy ketone enolates:
Table 3: Iridium-catalyzed allylation of acyclic a-alkoxy ketone enolates:
Scope of ketones.[a]
Scope of the allylic carbonates.[a]
[a] The molar ratio of 1/2/3/LHMDS/CuBr=2:1:0.02:2:2. The absolute
configurations were assigned by analogy. The yields were reported as the
combined yields of two diastereoisomers. The diastereomeric ratios were
1
determined by H NMR analysis of crude reaction mixtures. The
enantiomeric excesses were determined by chiral-phase SFC analysis of
major isomers. [b] 4 mol% of 3 was used. [c] 1 equiv of CuBr was used.
[d] 0.5 equiv of CuBr was used. [e] Cinnamyl acetate was used instead of
cinnamyl carbonate, and the reaction time was extended to 36 h. The
yield reported is that of the isolated major diastereomer.
[a] The molar ratio of 1a/2/3/LHMDS/CuBr=2:1:0.02:2:2. The absolute
configurations were assigned by analogy. The structure of 4ag was
determined by X-ray analysis. The yields were reported as the combined
yields of two diastereoisomers. The diastereomeric ratios were deter-
mined by 1H NMR analysis of crude reaction mixtures. The enantiomeric
excesses were determined by chiral-phase SFC analysis of major isomers.
[b] 0.5 equiv of CuBr was used.
(step b), yielded the terminal alcohol, which was subsequently
converted into the corresponding tosylate (step c). Removal
of the MOM protecting group (step a) afforded the free
tertiary alcohol, which underwent a 5-exo-tet cyclization
in situ to furnish the THF derivative 6ba in 45% yield over
three steps.
These synthetic sequences were also applied to the
products of allylation bearing different substituents, R1, R2,
and R3. In all cases, the substrates 4ic (R1 = R2 = 4-tol, R3 =
4-anisyl), 4jk (R1 = R2 = 4-anisyl, R3 = 2-thienyl), and 4km
(R1 = Ph, R2 = 2-thienyl and R3 = Me) afforded the corre-
sponding tertiary alcohols 5ic, 5jk, and 5km in high yield
without erosion of enantiomeric purity. Similarly, the corre-
sponding THF derivatives 6ic, 6jk, and 6km were obtained in
high enantiomeric purity.
Allylation products containing an O-MOM group on the
tertiary alcohol were readily transformed into enantio-
enriched tertiary alcohols (5) containing adjacent tertiary
stereogenic centers (Table 4). Deprotection of 4ba by reac-
tion with acidic Dowex-50W-X8 resin[15] (step a) afforded the
corresponding alcohols 5ba in quantitative yield without any
erosion of enantiomeric purity. The synthetic value of these
allylated benzoin derivatives was further demonstrated by
their transformation into highly substituted THF derivatives.
Hydroboration of 4ba with 9-BBN, followed by oxidation
The olefin moiety is a useful precursor to many functional
groups. For example, ozonolysis, hydrogenation and the
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Table 4: Synthesis of enantioenriched tertiary alcohols (5) and tetra-
hydrofuran derivatives (6).[a]
taining adjacent tertiary stereogenic centers. Studies to gain
insight into the origin of diastereoselectivity in this reaction
are ongoing in our laboratories.
Acknowledgements
Financial support provided by the Director, Office of Science,
of the U.S. Department of Energy under contract no. DE-
AC02- 05CH11231 and the NIH-NIGMS (GM-55382).
Keywords: alkylation · asymmetric catalysis ·
diastereoselectivity · iridium · ketones
How to cite: Angew. Chem. Int. Ed. 2016, 55, 5819–5823
Angew. Chem. 2016, 128, 5913–5917
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combination of hydroboration and oxidation of the products
of allylation 4aa afforded the aldehyde 7aa, ketone 8aa, and
primary alcohol 9aa, respectively, in high yields without
erosion of enantiomeric purity (Scheme 2).
In summary, we have developed iridium-catalyzed dia-
stereo- and enantioselective allylic substitutions with unsta-
bilized copper(I) enolates of acyclic a-alkoxy ketones. By
employing the metallacyclic complex 3 as the catalyst,
LHMDS as the base, and CuBr as the additive, the allylation
reactions give products containing vicinal tetrasubstituted
and tertiary stereocenters in high yield with excellent d.r. and
ee values. The geometry of the enolates is controlled by
chelation in the presence of a copper(I) cation. The synthetic
utility of this method was demonstrated by the synthesis of
enantioenriched THF derivatives and tertiary alcohols con-
[5] R. Mahrwald, Modern Aldol Reactions, Wiley-VCH, Weinheim,
2004.
Scheme 2. Derivatizations of 4aa.
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[6] a) T. Kanayama, K. Yoshida, H. Miyabe, T. Kimachi, Y. Take-
the presence of 2 equiv of CuBr gave the product 4ag in a similar
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91% yield, higher 15:1 d.r., but slightly lower 89% ee than the
reaction with 0.5 equiv of CuBr (see the Supporting Information
for details), whereas the same reaction with 0.5 equiv of CuBr
resulted in similar yield (92%), slightly lower d.r. (12:1), but
excellent ee (96%).
[7] a) P. A. Evans, M. J. Lawler, J. Am. Chem. Soc. 2004, 126, 8642;
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[12] The reaction of 2-methoxyacetophenone with cinnamyl methyl
carbonate afforded the product quantitatively with a d.r. value of
1.9:1. It is possible that the initially formed product underwent
epimerization under the reaction conditions to cause the
d.r. value of the final product to be low.
[9] We also investigated the reactions with a metallacyclic catalyst
generated in situ, with the preformed catalysts bearing different
ligands, and at different temperatures. The reaction catalyzed by
the preformed complex 3 at 58C or 08C afforded the product in
the highest yield and d.r. value. See Table S1 in the Supporting
Infomation for details.
[10] The solution of copper enolate was heterogeneous and was
transferred into the vial containing the allyl carbonate 2 and
catalyst 3. The loss of CuBr was inevitable and not measurable.
Considering the low price of CuBr and that an inadequate
amount of CuBr would result in lower diastereoselectivity in this
reaction (see Table 1, entry 11 versus entry 12), we decided to
use excess CuBr (2 equiv) as the additive for further study.
[11] As observed for the test substrate 2a (see Table 1, entry 6 vs.
entry 12), the reaction of 3,4-dichlorocinnamyl carbonate (2g) in
[13] In several cases (4ca, 4ea–ga, 4ic, 4km), viscous solutions of
copper enolates were observed if 2 equiv of CuBr were added,
and the consumption of limiting allylic carbonates was not
completed within 12 hours. However, if less CuBr (1 or
0.5 equiv; see the Supporting Information for details) was
added, full conversion of the allylic carbonates was obtained
within 12 hours.
[14] Approximately 30% of allylic carbonate remained, and about
30% of the material was a mixture of uncharacterized products.
[15] H. Seto, L. N. Mander, Synth. Commun. 1992, 22, 2823.
Received: January 8, 2016
Published online: April 1, 2016
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