Enantioselective ruthenium-catalyzed carbonyl allylation via alkyne-alcohol C-C bond-forming transfer hydrogenation: Allene hydrometalation vs oxidative coupling

Article abstract of DOI:10.1021/jacs.5b00747

Chiral ruthenium(II) complexes modified by Josiphos ligands catalyze the reaction of alkynes with primary alcohols to form homoallylic alcohols with excellent control of regio-, diastereo-, and enantioselectivity. These processes represent the first examp

Full text of DOI:10.1021/jacs.5b00747

Communication
pubs.acs.org/JACS
Enantioselective Ruthenium-Catalyzed Carbonyl Allylation via
Alkyne−Alcohol C−C Bond-Forming Transfer Hydrogenation: Allene
Hydrometalation vs Oxidative Coupling
Tao Liang, Khoa D. Nguyen, Wandi Zhang, and Michael J. Krische*
Department of Chemistry, University of Texas at Austin, Austin, Texas 78712, United States
S
* Supporting Information
Scheme 1. Strategies for Enantioselective Carbonyl
Allylation
ABSTRACT: Chiral ruthenium(II) complexes modified
by Josiphos ligands catalyze the reaction of alkynes with
primary alcohols to form homoallylic alcohols with
excellent control of regio-, diastereo-, and enantio-
selectivity. These processes represent the first examples
of enantioselective carbonyl allylation using alkynes as
allylmetal equivalents.
nantioselective carbonyl allylation is of broad use in
1
Echemical synthesis. As exemplified in the seminal work of
Hoffmann in 1978,2a preformed allylmetal reagents modified by
chiral auxiliaries are effective in enantioselective carbonyl
allylation (Scheme 1, eq 1).2 Alternatively, as first demonstrated
by Yamamoto (1991), achiral allylmetal reagents can used in
combination with chiral catalysts (Scheme 1, eq 1).3 Finally,
enantioselective carbonyl allylation can be achieved through the
reductive coupling of allylic halides, as in Nozaki−Hiyama−
Kishi type allylations,4 and umpoled reactions of allylic
carboxylates under the conditions of metal catalysis.5 The
latter reaction type typically requires stoichiometric metallic or
metal-based reductants. Exploiting hydrogen embedded in
alcohol reactants, we have developed redox-neutral enantio-
selective carbonyl allylations,6 wherein alcohol oxidation is
coupled to reductive cleavage of allylic carboxylates7 or the
hydrometalation of dienes,8 allenes,9 or enynes10 to generate
aldehyde−organometal pairs (Scheme 1, eq 2).
In the course of our studies, Obora and Ishii reported the
iridium-catalyzed reaction of 1-aryl-1-propynes with alcohols to
form racemic products of carbonyl allylation.11 Having
encountered such products in allene-alcohol C−C couplings
catalyzed by iridium9 and ruthenium,12 alkyne-to-allene isomer-
ization is likely operative in this process. The possibility of
promoting such transformations using a ruthenium catalyst
appeared tenuous, as established alkyne-alcohol C−C couplings
are known to form allylic alcohols.13,14 Despite this precedent,
we recently found that ruthenium catalysts generated upon the
acid−base reaction of H2Ru(CO)(PPh3)3 and 2,4,6-(2-
Pr)3PhSO3H convert 2-alkynes and primary alcohols to (Z)-
homoallylic alcohols through pathways involving alkyne-to-
allene isomerization with subsequent allene−carbonyl oxidative
coupling (Scheme 1, eq 3).15 In this Communication, we
disclose key alterations to the reaction conditions that suppress
allene-carbonyl oxidative coupling of the transient allene to
favor allene hydrometalation,16 resulting in the first examples of
catalytic enantioselective carbonyl allylation using alkynes as
chiral allylmetal equivalents (Scheme 1, eq 4).17
Under established conditions for the ruthenium-catalyzed
coupling of 4-methyl-2-pentyne 1a with p-bromobenzyl alcohol
2a, the linear (Z)-homoallylic alcohol 4a is generated in 70%
isolated yield as a single alkene stereoisomer (Table 1, entry 1).
It was found that introduction of Bu4NI (10 mol%) inhibited
conversion to the (Z)-homoallylic alcohol 4a (Table 1, entry
2). Using the chelating phosphine ligand dppf in the absence of
Bu4NI, roughly equimolar quantities of the branched allylation
product 3a and the (Z)-homoallylic alcohol 4a were obtained
Received: January 22, 2015
Published: March 3, 2015
© 2015 American Chemical Society
3161
DOI: 10.1021/jacs.5b00747
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Communication
Table 1. Selected Experiments Illustrating the Partitioning of
Hydrometalative and Oxidative Coupling Pathways in the
Reaction of 1a and 2a to Form Isomers 3a and 4a
Table 2. Regio-, anti-Diastereo-, and Enantioselective C−C
Coupling of Alkyne 1a and Alcohols 2a−2l to Form
a
a
Branched Products of Carbonyl Allylation 3a−3l
a
Yields are of material isolated by silica gel chromatography. See
b
Supporting Information for further experimental details. 2,4,6-(2-
Pr)3PhSO3H (14 mol%). 2,4,6-(2-Pr)3PhSO3H (5 mol%). THF (1
M).
c
d
in modest yield (Table 1, entry 3). To our delight, the
combination of Bu4NI and dppf led to an 83% yield of the
branched allylation product 3a as a 15:1 (anti:syn) diastereo-
meric mixture (Table 1, entry 4). Using dippf as ligand, the
branched allylation product 3a was formed as a single anti-
diastereomer in 57% yield (Table 1, entry 5). In general, P-
alkyl-substituted chelating ligands provided superior levels of
diastereoselectivity compared to their P-phenyl congeners
(Table 1, entry 4 vs 5; entry 6 vs 7). At this stage, diverse
chiral chelating phosphine ligands were evaluated. Promising
levels of asymmetric induction were observed using the
Josiphos ligands SL-J009-1 and SL-J002-1 (Table 1, entries 8
and 9). Lowering the loading of 2,4,6-tri(2-propyl)-
phenylsulfonic acid (5 mol%),18,19 the branched allylation
product 3a could be obtained in excellent yield and 94%
enantiomeric excess (ee) using either SL-J009-1 or SL-J002-1 as
ligand (Table 1, entries 10 and 11). Omission of 2,4,6-tri(2-
propyl)phenylsulfonic acid led to over-oxidation of 3a to form
ketones, that is, products of allene hydroacylation.14 Omission
of 2-PrOH led to a roughly 20% reduction in yield of 3a and
accumulation of unreacted aldehyde, dehydro-2a.
With these optimized conditions in hand, diverse alcohols
2a−2l were surveyed for their ability to participate in this new
protocol for asymmetric allylation (Table 2). Benzylic alcohols
2a−2f were converted to adducts 3a−3f, respectively, in good
yield with complete levels of anti-diastereoselectivity and
uniformly high levels of enantiomeric enrichment (94−96%
ee). Allylic alcohols 2g−2i could potentially engage in internal
redox isomerization to form the corresponding aldehydes,20 but
such products are not observed. Rather, adducts 3g−3i are
formed in a highly regio- and stereoselective manner. Similarly
high levels of selectivity were observed in the conversion of
aliphatic alcohols 2j−2l to adducts 3j−3l. Under these first-
generation conditions, branched primary alcohols such as
a
Yields are of material isolated by silica gel chromatography.
1
Diastereoselectivities were determined by H NMR analysis of crude
reaction mixtures. See Supporting Information for further exper-
imental details. 2-PrOH was omitted. 125 °C. 48 h.
b
c
d
isobutyl alcohol provided adducts in the range of 30−40%
yield. Under these conditions, secondary alcohols oxidize to
form ketone products and do not engage in C−C coupling.
To further explore the scope of this method for catalytic
enantioselective allylation, a series of substituted propynes 1b-
1d possessing tert-butyl (1b), 4-(N-Boc-piperidinyl) (1c), and
phenyl (1d) moieties were explored in couplings to benzylic
alcohol 2a and aliphatic alcohol 2j (Table 3). In the case of the
tert-butyl-substituted alkyne 1b, adducts 3m and 3p were
generated in good yield with complete levels of anti-
diastereoselectivity and excellent levels of enantioselectivity,
94% ee and 90% ee, respectively. Similarly, for 4-(N-Boc-
piperidinyl)-substituted alkyne 1c, adducts 3n and 3q were
formed with good control of relative and absolute stereo-
chemistry, despite potential cleavage of the acid-sensitive Boc-
protecting group. Finally, the phenyl-substituted alkyne 1d
coupled to alcohols 2a and 2j to provide adducts 3o and 3r,
respectively, in good yield as single diastereomers and good to
excellent control of enantioselectivity, 86% ee and 90% ee,
respectively. Attempted use of 2-pentyne resulted in a complex
mixture of products. The relative and absolute stereochemical
assignment of adducts 3a-3r is made in analogy adduct 3m,
which was determined by single-crystal X-ray diffraction
analysis.
A general catalytic mechanism was proposed (Scheme 2).
Allene hydrometalation forms a nucleophilic allylruthenium
complex. The stoichiometric reaction of HXRu(CO)(PR3)3 (X
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Communication
as precatalyst in the absence of arylsulfonic acid under
otherwise standard conditions, alcohol 2a is converted to the
branched allylation product 3a in 68% yield, >20:1 dr, and 81%
ee, corroborating intervention of iodide as a counterion for
ruthenium. The strong σ-donicity of iodide may destabilize
ruthenium(0), suppressing competitive oxidative coupling
pathways. The indicated deuterium labeling experiments are
consistent with this interpretation of the mechanism, and
further reveal that the reaction products are inert with respect
to alcohol dehydrogenation, which prevents any erosion in
kinetic stereoselectivity under the reaction conditions. It should
be noted that protons from the reactant alcohol or adventitious
water may contribute to diminished levels of deuterium
incorporation.22
Table 3. Regio-, anti-Diastereo-, and Enantioselective C−C
Coupling of Alkynes 1b−1d and Alcohols 2a and 2j to Form
a
Branched Products of Carbonyl Allylation 3m−3r
In summary, we report a ruthenium-catalyzed enantio-
selective carbonyl allylation wherein alkynes and primary
alcohols serve as redox pairs. A notable feature of these
transformations is the use of alkynes as a reservoir for allenes,
which are kinetically more reactive and which form in situ
through a second, parallel ruthenium-catalyzed isomerization
process. Of further note are the subtle changes in reaction
conditions that enable partitioning of two competing catalytic
pathways, allene−carbonyl oxidative coupling vs allene hydro-
metalation, while the parallel ruthenium-catalyzed alkyne
isomerization pathway is maintained. More broadly, this work
contributes to a growing body of catalytic processes developed
in our laboratory wherein the native reducing ability of alcohols
is used to generate transient organometallics from π-
unsaturated precursors, enabling carbonyl addition from the
alcohol oxidation level in the absence of stoichiometric
organometallic reagents.
a
Yields are of material isolated by silica gel chromatography.
1
Diastereoselectivities were determined by H NMR analysis of crude
reaction mixtures. See Supporting Information for further exper-
imental details. 72 h. 75 °C. 2-PrOH was omitted. 7.5 mol% of
H2Ru(CO)(PPh3)3, ArSO3H, SL-J009-1, and 15 mol% Bu4NI. 48 h.
b
c
d
e
f
= Cl, Br) with allenes to form π-allylruthenium complexes is
known.21 We have found that such π-allylruthenium species are
highly fluxional in nature, undergoing rapid geometrical
isomerization by way of the σ-bound haptomers.12 Coordina-
tion of aldehyde to the pentacoordinate (E)-σ-allylruthenium
species leads to stereospecific addition through a closed
transition structure to form the homoallylic ruthenium alkoxide.
Protonolytic cleavage of the alkoxide by the arylsulfonic acid
releases the product and provides the indicated ruthenium
sulfonate, which is displaced by a primary alcohol. Subsequent
β-hydride elimination generates aldehyde along with the
ruthenium hydride to close the catalytic cycle. Notably, in the
absence of acid, β-hydride elimination occurs at the stage of the
homoallylic ruthenium alkoxide to furnish the β,γ-enone.14
Hence, substoichiometric arylsulfonic acid appears to catalyze
alkoxide exchange at the metal center. Using HIRu(CO)(PR3)3
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures and spectral data; HPLC traces
corresponding to racemic and enantiomerically enriched
samples; single-crystal X-ray diffraction data for compound
3m. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*mkrische@mail.utexas.edu
a
Scheme 2. Deuterium Labeling Studies and General Catalytic Mechanism
a
2
1
2
The extent of H incorporation was determined using H and H NMR. For the deuterium labeling experiments, reactions were conducted using
dippf as ligand. See Supporting Information for further experimental details, including equations accounting for the regioselectivity and extent of
deuterium incorporation at positions Ha−He.
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(6) For a recent review, see: Ketcham, J. M.; Shin, I.; Montgomery,
T. P.; Krische, M. J. Angew. Chem., Int. Ed. 2014, 53, 9142.
(7) For selected examples of catalytic enantioselective redox-triggered
carbonyl allylations employing allylic carboxylates as pronucleophiles,
see: (a) Kim, I. S.; Ngai, M.-Y.; Krische, M. J. J. Am. Chem. Soc. 2008,
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2008, 130, 14891. See also literature cited in ref 6.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The Robert A. Welch Foundation (F-0038) and the NIH-
NIGMS (RO1-GM069445) are acknowledged for partial
support of this research.
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