Coordination-Induced Stereocontrol over Carbocations: Asymmetric Reductive Deoxygenation of Racemic Tertiary Alcohols

Article abstract of DOI:10.1021/jacs.9b00862

The inherent difficulty in eliciting facial control over carbocations has limited their utility as intermediates in asymmetric catalysis. We have now shown that a docking strategy involving the reversible coordination of a substrate to a chiral transition-metal catalyst can be used to enable highly stereoselective nucleophilic attack on intermediate tertiary carbocations. This approach has been implemented to achieve the first example of enantioselective reductive deoxygenation of tertiary alcohols. This reduction occurs with high enantio- (up to 96% ee) and regioselectivity (up to >50:1 rr) by applying a novel Hantzsch ester analogue as a convenient hydride source. In-depth mechanistic studies support the involvement of a tertiary carbocation that is coordinated to the iridium metal center via the key allene moiety.

Full text of DOI:10.1021/jacs.9b00862

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Article
Coordination-Induced Stereocontrol over Carbocations:
Asymmetric Reductive Deoxygenation of Racemic Tertiary Alcohols
Mayuko Isomura, David A. Petrone, and Erick M Carreira
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 20 Feb 2019
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Coordination-Induced Stereocontrol over Carbocations:
Asymmetric Reductive Deoxygenation of Racemic Tertiary Alcohols
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Mayuko Isomura,§ David A. Petrone,§ and Erick M. Carreira*
ETH Zürich, Vladimir-Prelog-Weg 3, HCI, 8093 Zürich, Switzerland.
ABSTRACT
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The inherent difficulty in eliciting facial control over carbocations has limited their utility as intermediates in asymmetric catalysis. We
have now shown that a docking strategy involving the reversible coordination of a substrate to a chiral transition metal catalyst can be
used to enable highly stereoselective nucleophilic attack on intermediate tertiary carbocations. This approach has been implemented to
achieve the first example of enantioselective reductive deoxygenation of tertiary alcohols. This reduction occurs with high enantio- (up
to 96% ee) and regioselectivity (up to >50:1 rr) by applying a novel Hantzsch ester analog as a convenient hydride source. In-depth
mechanistic studies support the involvement of a tertiary carbocation which is coordinated to the iridium metal center via the key allene
moiety.
INTRODUCTION
Carbocations have prompted the development of myriad
chemical reactions since their independent discovery by Norris
and by Kehrman and Wentzel in 1901.1 Despite their ubiquity in
organic chemistry, the role of carbocations as intermediates in
asymmetric synthesis has lagged behind considerably.2 This
disparity lies in the intrinsic difficulty in biasing nucleophilic
attack on one of the two prochiral faces of their planar structure
(Scheme 1A, left). Accordingly, the two main strategies for
stereocontrol over carbocations have involved diastereoselective
substrate-control (Scheme 1A, center)3 or the formation of ion
pairs between chiral anions (i.e. *X-) and achiral carbocations
(Scheme 1A, right).4-7 These approaches have been most
successfully applied to transformations proceeding through
secondary carbocations, whereas their use in achieving
tertiary α-allenyl alcohols to a chiral metal catalyst through the
remaining double bond,17 and, if so, whether a regio- and
enantioselective SN1-type reaction could be accomplished.
enantiocontrol over tertiary species has been less fruitful.8,9
A
major breakthrough in this area was recently made by Jacobsen
who exploited ion pair interactions involving chiral hydrogen
bond-donor-acceptor complexes to achieve enantiocontrol over
discrete tertiary carbocations.10 Therein, enantioenriched
quaternary carbons could be constructed via asymmetric
allylation, while circumventing the need for dynamic substrate
epimerization,8a or heteroatoms that assist in solvolysis and
carbocation stabilization.8b
We recently reported the asymmetric alkylation of
secondary allenyl carbonates (Scheme 1B).11 Accompanying
theoretical studies pointed to the involvement of an unusual
intermediate that contains an electrophilic η2-allene ligand with
a highly delocalized positive charge (i.e. A).12 The obvious
parallels between this overall transformation and the SN1
reaction manifold led us to hypothesize that this mode of
reactivity could also be used to command stereocontrol of
tertiary carbocations. In this approach, the chiral metal complex
emulates the effect of an existing stereocenter via coordination
to the substrate, while the ensuing ionization would generate a
tertiary carbocation which is confined within the chiral
coordination sphere (Scheme 1C).13
In 1982, Siehl and Mayr reported that cations of type B
could be generated via the ionization of tertiary α-allenyl
alcohols and spectroscopically characterized (Scheme 1D).14-16
These studies provided insight into the nature of the charged
intermediate and revealed that the positive charge was mostly
localized at Cα and Cγ (i.e. B1 or B2, respectively). On account
of the orthogonality of the p-orbitals constituting the 1,2-diene
motif, resonance stabilization is possible without loss of the Cγ‒
Cδ double bond. We have been interested in examining whether
cations of type B could be generated after coordination of
Scheme 1. A. (left) The innate challenge in stereocontrol with carbocations;
(center) using substrate control; (right) using chiral ion pairing to elicit
stereocontrol; B. Asymmetric Ir-catalyzed alkylation of allenyl carbonates
involving an electrophilic η2-bound butadienylium intermediate; C. Applying
an anchoring strategy involving metal‒substrate coordination as a means to
gain enantiocontrol over tertiary carbocations; D. Generation of stable α-
vinyl substituted vinyl cations. E. Coordination-induced stereocontrol of 3°
carbocations in asymmetric reductive deoxygenation.
Herein, we outline the implementation of this strategy for
stereocontrolled additions to tertiary carbocations as a design
element in the first catalytic, asymmetric, reductive,
deoxygenation reaction (Scheme 1E).18-22
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It is important to note that metal-catalyzed allenylic
Table 1. Effect of Reaction Parameters.a
substitutions have largely been reported for Pd and Ir complexes
with activated secondary alcohol derivatives (Scheme 2A).11,23,24
In general, asymmetric substitution reactions of tertiary allenylic
leaving groups are, to the best of our knowledge, unknown.25
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The use of bismuth(III) triflate26 and
a chiral Ir(I)-
bis(phosphoramidite,olefin) complex has now enabled the direct
and stereoselective reductive deoxygenation of racemic tertiary
α-allenyl alcohols. A collection of mechanistic studies has
provided experimental evidence for the involvement of a
coordinated tertiary carbocation as a key catalytic intermediate
in this highly enantioconvergent transformation.27
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entry
variation from the
“standard” conditions
None
HEH-2 instead of HEH-1
HEH-3 instead of HEH-1
3 instead of HEH-1
1 mol% of Bi(OTf)3
no Bi(OTf)3
(S)-2a
(%)b,c
91(81)e
91
18
<2
75
<2
16
53
rrc
2a:2a’
>20:1
11:1
>20:1
‒
ee
(%)d
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80
‒
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5
Scheme 2. Transition metal-catalyzed allenylic substitution.
14:1
86
6 f
7g
8h
9
‒
‒
4 instead of Bi(OTf)3
4 °C instead of rt
no [Ir]/(S)-L1
12:1
>20:1
7:1
94
92
0
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12i
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14j
15
no [Ir]
no (S)-L1
1:1 [Ir]:L1
(S)-L2 used
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6
23
6
<2
<2
6:1
2:1
15:1
3:1
‒
0
0
74
39
‒
(S,S,S)-L3 used
[Rh(cod)Cl]2 used
a Reactions run on 0.5 mmol scale. ≤ 3% of vinyl allene 5a was observed.
‒
‒
b Combined yield of regioisomers. c Determined by 1H NMR analysis of
the unpurified reaction mixture. Determined by GC using a chiral
d
e
f
stationary phase.
Isolated yield of (S)-2a. Starting material was
recovered quantitatively. g Reaction was run using 5 mol% of 4; 13% of
vinyl allene 5a was observed. h Reaction time = 50 h. [Ir] = 4.0 mol%
i
and (S)-L1 = 4.0 mol%. j Catalyst pre-formed using nPrNH2, see ref. 33. ‒
= not determined; CPME = cyclopentolates methyl ether; cod = 1,5-
cyclooctadiene; 2-Np = 2-naphthyl; OTf = trifluoromethanesulfonate.
RESULTS AND DISCUSSION
Reaction Optimization. Our studies were initiated using
racemic α-allenyl alcohol (±)-1a which could be prepared in two
steps from commercially available 2-acetonaphthone via a route
involving
Ma’s
modified
Cu(I)
catalyzed
Crabbé
homologation.28 Hantzsch pyridines were chosen as the hydride
source in this process due to their successful use in numerous
catalytic asymmetric transformations including allylic
reduction29 and transfer hydrogenation.30,31 Furthermore, their
ease of preparation and their high degree of modularity greatly
facilitates reaction optimization.
After evaluating a wide array of reaction parameters, a
system comprising [Ir(cod)Cl]2 (2 mol%) and chiral
phosphorous‒olefin ligand (S)-L1 (8 mol%) and Bi(OTf)3 (0.4
mol%) in combination with the novel bis-isopropyl Hantzsch
dihydropyridine analog HEH-1 (1.5 equiv) in CPME ([(±)-1a] =
0.2 M) at room temperature was found to be optimal. Under
these conditions, allene (S)-2a was obtained in 91% yield with
92% ee and >20:1 regioselectivity over the corresponding
achiral 1,3-diene (E)-2a’ (Table 1, entry 1). Moreover, the
formation of vinyl allene 5a could be almost completely
supressed under these conditions. The application of standard
Hantzsch dihydropyridine HEH-2 had no deleterious effect on
yield; however, marked decreases in both regio- and
enantioselectivity were observed (entry 2). Although larger
groups at the 2- and 6-positons of the HEH motif impacted the
overall selectivity profile of this reaction, larger tert-butyl esters
at the 3- and 5-positions resulted in lower yield and
enantioselectivity, albeit with no observed change in
regioselectivity (entry 3). Furthermore, commonly employed N-
benzyl dihydronicotinamide 3 was found to be an unsuitable
hydride donor in this transformation (entry 4). Of note was the
sensitivity of the enantioselectivity to the loading of the
Bi(OTf)3 co-catalyst.26 When the loading was increased from
0.4 mol% to 1.0 mol%, the yield, regio- and enantioselectivity
all decreased to 75%, 14:1 rr, and 86% ee, respectively (entry 5).
This suggests that a higher co-catalyst loading may increase the
rate of the non-Ir-catalyzed background reduction (vide infra).
Conversely, no products are observed in the absence of
Bi(OTf)3, and only starting material is recovered (entry 6). Our
group previously showed that sulfonamide 4 was an optimal
Brønsted acid co-catalyst for asymmetric allyl-alkene
couplings.32 The use of 5 mol% of 4 in place of Bi(OTf)3
resulted in decreased reaction efficiency (16% yield), albeit with
slightly increased enantioselectivity (entry 7). Decreasing the
reaction temperature to
4 °C in an effort to increase
enantioselectivity rendered the reaction sluggish and had no
positive effect on selectivity (entry 8).
The highly activated nature of (±)-1a presumably increases
its susceptibility to background reaction giving racemic product.
In order to determine the extent to which these reactions occur,
experiments were performed in the absence of the
[Ir(cod)Cl]2/(S)-L1 catalyst combination or its other individual
components (entries 9-11). Therein, products (S)-2a and (E)-2a’
could be observed, albeit with low yield (6-9%) and
regioselectivity (2:1 to 7:1). When the [Ir]:(S)-L1 ratio was
changed from 1:2 to 1:1, the yield, regio- and enantioselectivity
all significantly decreased to 23%, 15:1 rr, and 74% ee,
respectively (entry 12). In accord with our previously reported
asymmetric alkylation of allenylic carbonates, these findings
support the notion that a 1:2 Ir:(P,olefin) complex is operative
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(vide infra).11,18 Both structural and electronic permutations of
practical and can be performed using standard benchtop
techniques, without the need to strictly avoid air and moisture.
L1 led to less satisfactory reaction outcomes. The olefin motif of
the ligand was found to be crucial for overall reaction efficiency
and selectivity. When dihydro analog (S)-L2 was employed, the
product was obtained in only 6% yield with 3:1 rr and 39% ee
(entry 13). In our previously allenylic alkylation, we observed
that employing a catalyst system comprising (S,S,S)-L3 leads to
exclusive formation of the undesired 1,3-diene isomer.11,33
However, neither reduction regioisomer was observed with the
catalyst system employed as optimal in this study (entry 14).
Recently, You and co-workers have shown that the combination
of [Rh(cod)Cl]2 and L1 was an effective catalyst for the
asymmetric allylic alkylation of racemic allylic alcohols.34
When [Rh(cod)Cl]2 was used in place of [Ir(cod)Cl]2, no
reaction occurred, and the starting material was recovered
quantitatively (entry 15). Overall, this reaction is simple,
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Reaction Scope. Various racemic tertiary α-allenyl alcohols
were found to undergo the desired transformation to afford
products with high levels of enantiomeric excess (Table 2). 2-
Naphthyl analogs containing electron-donating (2b) and -
withdrawing (2c) substituents were tested, and the desired
products could be obtained in 84% and 67% yield with 90% and
94% ee, respectively. Less reactive non-naphthyl-based
substrates were also examined. Products containing a wide array
of aryl analogs with para electron-withdrawing groups including
halogens (2e-2g), a nitrile (2h), a methyl ester (2i), or an ‒OCF3
(2j) group were furnished in 55-72% yield with 90-94% ee. The
pinacolborane functionality was also retained under these
conditions, and the corresponding product (2k) was obtained in
74% yield with 96% ee. Substrates incorporating meta
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substituents
such
as
‒CF3
(2l)
Table 2. Scope of the asymmetric Ir-catalyzed reductive deoxygenation of racemic tertiary alcohols.a
a Reaction conditions: [Ir(cod)Cl]2 = 3.0 mol%, (S)-L1 = 12 mol%, (±)-1 (0.5 mmol), HEH-1 (1.5 equiv), CPME (c = 0.2 M), rt unless otherwise noted. Isolated
yields shown. Enantiomeric excess values (ee) were determined by GC analysis using a chrial stationary phase. Regiochemical ratios (rr) were determined by 1H
NMR analysis of the unpurified reaction mixtures. b Reaction was run using [Ir] = 4.0 mol% and (S)-L1 = 8 mol%. c 0.2 mol% of Bi(OTf)3 was used. d Reaction
was run in Et2O. 3.0 mol% of Bi(OTf)3 was used. f 2.5 mol% of Bi(OTf)3 was used. g 1.0 mol% of Bi(OTf)3 was used. h 1.5 mol% of Bi(OTf)3 was used. i 0.8
e
mol% of Bi(OTf)3 was used.
or ‒OMe (2m) were also tolerated, and the corresponding
products were obtained in 55% and 68% yield with 93% and
92% ee, respectively. Products containing electron-donating
groups (2n-2q) such as ‒Me (2n), ‒OMe (2o, 2q), and
cyclopropyl (2p) were also obtained in 61-88% yield with 71-
90% ee. Given the well-established stability of cyclopropyl
carbinyl cations,16,36 substrates containing cyclopropyl rings at
the stereogenic carbon center were also examined. Subjecting
the analogous cyclopropyl carbinols possessing 2-naphthyl
(2r), para-fluoro (2s) or para-chloro (2t) aromatic groups to
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the reaction conditions led to the formation of the desired
cyclopropane-containing products in 72-79% yield with 93-
96% ee.
The use of cyclopropyl-containing substrates led to several
interesting observations. The corresponding products were
obtained with the opposite absolute configuration with respect
to the corresponding Me analogs, and neither the achiral diene
regioisomer, nor products resulting from cyclopropane ring-
opening were observed. The exclusive regioselectivity
observed with these substrates is presumably a manifestation
the standard conditions (Scheme 3B). It was found to undergo
the desired transformation with nearly identical efficiency to
non-deuterated (±)-1a, and without erosion of isotopic purity
or scrambling of the label. Second, a cross-over experiment
involving methoxynaphthyl substrate (±)-1b and vinyl allene
5a led only to the formation of the asymmetric reductive
deoxygenation product (S)-2b (Scheme 3C). Together, these
results suggest that the tertiary alcohol substrate undergoes
direct conversion to product by way of a mechanism which
does not involve an initial dehydration step.37
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of the cyclopropanes ability to stabilize
a significant
proportion of the positive charge, generated upon ionization at
the benzylic carbon. The scalability of this transformation was
highlighted using substrate 1t where the corresponding
product could be obtained in 81% yield and with >50:1 rr and
93% ee when the experiment was conducted on a 1.2 gram
scale. Tetrahydropyridine derivative containing an N-CBz
group was also tolerated under the reaction conditions and the
desired product 2u was obtained in 57% yield with >20:1 rr
and 99% ee. Some limitations of the methodology were
encountered. For example, tosylated indole 2v was obtained in
72% yield and with >20:1 rr, albeit with only 38% ee.
Furthermore, use of a substrate containing an ‒Et group at the
stereocenter led to both an attenuation in yield and selectivity
and product 2w was obtained in 44% yield with 17:1 rr and
50% ee. Finally, 1,4-ynallene product 2x was obtained in 58%
yield with >20:1 rr and 55% ee. This reaction proved to be
intolerant of substrates containing highly Lewis basic
Scheme 3. Deuterium labeling studies and cross-over
experiments.
i
functionalities (i.e. 1y) or sterically large groups like Pr at the
stereocenter (i.e. 1z). These effects likely arise from the
poisoning of the Bi(OTf)3 co-catalyst and
encumbered approach of HEH-1 in the reduction step,
respectively.
a highly
The general trend observed across this series of substrates
is that electron-deficient ones typically lead to higher levels of
enantioselectivity and attenuated levels of regioselectivity
while the opposite is true for electron-rich substrates. The
variation in regioselectivity across electron-rich and -poor
substrates is in line with our proposed model of a coordinated
carbocation wherein positive charge is mostly localized at the
two carbons at which hydride transfer delivers the competing
regioisomers (i.e. Cα in B1 and Cγ in B2). The work of Mayr
and co-workers concerning allyl cations indicates that
electron-releasing substituents increase and decrease positive
charge at the proximal and distal carbons, respectively,
whereas the opposite is true for electron-accepting
substituents.36 In this regard, higher allene:diene
regioselectivities in substrates containing electron-rich aryl
groups or cyclopropyl substituents is likely the result of
increased positive charge population at Cα in the coordinated
carbocation.
Mechanistic Studies. We undertook in-depth experimental
investigations to examine the underlying catalytic mechanism.
A series of experiments were initially performed using
isotopically labeled reagents and substrates (Scheme 3). The
reductive deoxygenation was carried out using 4,4-d2-HEH-1
(>99% D) which led to the complete transfer of the deuterium
label to the benzylic carbon of the substrate (Scheme 3A).
This finding confirms the sole origin of the hydrogen at the
stereogenic benzylic carbon to be the 4-position of the
Hantzsch ester. Product (S)-2a-d1 was obtained in comparable
yield (84%) and selectivity (>20:1 rr and 92% ee) as to when
non-labeled HEH-1 was employed. Vinyl allene 5a, resulting
from dehydration of the tertiary alcohol substrate, was
observed as a side product in varying quantities during
reaction optimization. Two sets of experiments were carried
out to probe whether this species is an intermediate in the
The proposed reductive deoxygenation resembles an SN1-
type reaction that is likely to proceed via a mechanism
involving turnover-limiting ionization of the tertiary alcohol
motif.38 In line with this hypothesis, a series of kinetic isotope
effect (KIE) studies were first conducted to determine if
hydride transfer could be ruled out as the turnover-limiting
step. As such, intermolecular primary KIE experiments (same
vessel) were carried out on two different racemic tertiary
alcohol substrates (±)-1a or (±)-1g in the presence of a
mixture of HEH-1 and 4,4-d2-HEH-1. Therein, substrates
(±)-1a and (±)-1g led to calculated kH/kD values of 2.90 and
2.33, respectively. (Scheme 4A). Since findings of this nature
do not always provide sufficient evidence to conclude that C‒
H bond cleavage, here in the step of hydride attack, is
turnover-limiting, parallel KIE experiments (separate vessels)
were also conducted.39
During these parallel KIE studies, we determined that the
minute quantities of Bi(OTf)3 required under the standard
conditions (0.4 mol%) precluded its usefulness for obtaining a
high level of reproductivity between runs. This challenge was
compounded by its low solubility in organic solvents which
negated our ability to precisely administer this reagent from a
stock solution. We hypothesized that if a well-defined hydride
transfer pathway was operative, it would lead to the generation
‒
of a pyridinium species which possesses a counterion (i.e.
OTf) derived from the Lewis acid promoter.29b Due to the
acidity of this resulting pyridinium hydrotriflate, we
hypothesized that it could also serve as an effective Brønsted
reaction mechanism. Substrate (±)-1a-d3 containing
perdeuterated methyl group (>98% D) was first subjected to
a
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acid promoter for the reductive deoxygenation. Indeed, when
as little as 0.3 mol% of 6·HOTf was employed in place of
1
2
1.2
0.8
0.4
0.3
0
80
84
86
84
0
>20:1
>20:1
>20:1
>20:1
‒
84
87
91
92
‒
Bi(OTf)3,
nearly
identical
yield,
regioselectivity,
3e
4f
5
enantioselectivity and intermolecular KIE values were
obtained in comparison to the standard conditions (Table 3).
Fortunately, 6·HOTf is an easily-prepared, crystalline solid
which displays high solubility in organic solvents such as
methylene chloride and tetrahydrofuran. Given this data,
6·HOTf was used in place of Bi(OTf)3 during the parallel KIE
studies.
a Reaction run on a 0.5 mmol scale. b Determined by 1H NMR analysis of
the crude reaction mixture using 1,3,5,-trimethoxybenzene as an internal
standard. c Combined yield of regioisomers. d Determined by GC using a
e
chiral stationary phase. Intermolecular KIE using HEH-1 and 4,4-d2-
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HEH-1 remained unchanged. f Reaction was run for 18 hours. ‒ = not
determined.
Scheme 4. Kinetic isotope effect and reversibility
experiments.
The nature of the ionization step of the asymmetric
reductive deoxygenation was then examined. A linear free-
energy relationship (LFER) study was carried out, and a
strong linear correlation with a moderately negative slope (ρ =
-1.16, Scheme 5A) was obtained from the corresponding plot
of the Hammett substituent constants (σ)41 versus log(kX/kH)
for a variety of para-substituted analogs.
Scheme 5. Linear free-energy relationship and 2° β-
deuterium kinetic isotope effect studies.
Accordingly, the initial rates observed for the reactions
employing either HEH-1 or 4,4-d2-HEH-1 in separate
reaction vessels were carefully measured over triplicate runs
and led to negligible isotope effects with average calculated
kH/kD values of 1.13 ± 0.09 and 1.02 ± 0.12 for (±)-1a and (±)-
1g, respectively (Scheme 4B). We also noted that the
subjection of monodeuterated product (S)-2a-d1 to the
standard condition with HEH-1 led to no observable H/D
exchange (Scheme 4C) which suggests that the product-
forming step of hydride attack is irreversible. Together, the set
of data displayed in Scheme 3 is consistent with C‒H bond
cleavage of Hantzsch ester (product C‒H bond-forming) in a
step that is both irreversible and product-determining, but this
step is not turnover-limiting.39,40
The observation of a linear free-energy dependence in this
context provides strong evidence for positive charge
accumulation in the turnover-limiting transition state, which is
consistent with an SN1-type ionization step. In order to further
study the nature of the turnover-limiting step, a β secondary
isotope effect study was conducted (Scheme 5B). Rate
attenuation of carbonium ion-forming reactions in cases when
β-hydrogen atoms are substituted for deuterium atoms is a
well-established phenomenon when ionization/solvolysis is
rate-limiting. In reactions which exhibit a secondary β-KIE
effect, values of around 1.1-1.2 can typically be calulated.38,42
We calculated a normal secondary KIE (kH/kD) of 1.08 ± 0.01
from the intermolecular competition experiment between (±)-
1g and (±)-1g-d3 across a triplicate series of runs. Although
this experimentally obtained value is outside of the above-
stated range, it is in line with our previous observations, and
Table 3. Evaluating the competency of pyridinium triflate
salt 6·HOTf as a Brønsted acid promoter.a
entry
6·HOTf
(mol%)
(S)-2a (%)b,c
rr
ee
2a:2ab
(%)d
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suggests that the benzylic carbon may change hybridization
acts to erase the stereochemical information originating from
from sp3 to sp2 during the turnover-limiting transition state.
The enantioconvergent nature of this reaction, whereby
both starting material enantiomers are converted to a single
product enantiomer,27 led us to explore the nature of the
interaction between the chiral catalyst and the racemic
substrate. When the reaction involving (±)-1a was run to
the starting material. If attack of HEH-1 is assumed to occur
exclusively from the top face of the planar, cationic ligand (i.e.
from the side nearest the axial Cl- ligand),11 the observed
product enantiomeric ratio would reflect the selectivity of
attack on the syn versus the anti complexes. The observation
that scalemic substrate (‒)-1a (87% ee, i.e. 93.5:6.5 er) leads
partial
conversion,
1a
was
recovered
without
to
products
possessing
opposite,
yet
non-equal
enantioenrichment, whereas (S)-2a was obtained in 46% yield
with 91% ee and >20:1 rr (Scheme 6A). This data evokes two
significant conclusions: first, a kinetic resolution is not
operative, meaning the catalyst processes both enantiomers of
the racemic mixture with equivalent rates, and second, the
enantiomeric excess of the product largely remains constant
over the course of the reaction. To emphasize this point,
scalemic substrate (‒)-1a (87% ee) was subjected to the
standard conditions using the individual enantiomers of L1,
and the reactions were run to partial conversion (Scheme 6B).
In both cases, recovered (‒)-1a was observed to have slightly
eroded enantiomeric excess (79% for (S)-L1 and 80% ee for
(R)-L1 vs. 87% ee).
enantioselectivities (96% ee vs. -84% ee) in the presence of
either (S)- or (R)-L1 sheds light on this process. In the
presence of (S)-L1, stereoretention is observed for the
enantiomer of (‒)-1a in excess (major). By contrast, the
enantiomer in excess undergoes substitution with inversion of
configuration when (R)-L1 is employed. Therefore, since a
higher proportion of the ‘matched’ starting material
enantiomer is already present (and subsequently, the anti
complex) when (S)-L1 is employed, only a small quantity of
the syn complex is left to convert to the anti complex. This
leads to a higher product enantioselectivity compared to when
(±)-1a is employed (94% ee vs. 92% ee). Conversely, since a
higher proportion of the ‘mismatched’ starting material
enantiomer is present when (R)-L1 is employed, a large
quantity of the resulting syn complex must convert to the anti
complex which ultimately leads to the major enantiomer
observed upon hydride attack. This leads to a lower product
enantioselectivity compared to when (±)-1a is employed (-
84% ee vs. 92% ee). These results reflect a mechanism in
which ionization occurs after the substrate coordinates to the
chiral Ir catalyst. Moreover, the observations are consistent
with a rate of product-forming hydride attack which is
competitive with the rate of syn-to-anti interconversion. As
such, if ionization were to occur in the absence of Ir-
coordination (i.e. off metal), the separate experiments
employing (R)-L1 or (S)-L1 would be expected to give the
identical absolute-value for enantiomeric excess.43
Given, the large body of work concerning allylic
substitution with the present metal-ligand combination, it is
reasonable to assume that the analogous allylic reduction
would also proceed.44 However, subjection of racemic tertiary
allyl alcohol (±)-7 to the standard conditions led to no
observable quantities of the known 2-allyl naphthalene
derivative 8 or its regioisomer, and starting material was
recovered quantitatively (Scheme 6D). This finding indeed
stresses the overall importance of the allene moiety in this
reductive deoxygenation. As the sensitivity of this class Ir-
phosphoramidite catalyst to increased steric effects at allylic
stereocenters is well-established, we hypothesize that the three
carbon allene functionality (versus two for olefins) provides
an extra one carbon atom spacer which places the bulky Ir-
catalyst further away from the tetrasubstituted carbon
stereocenter upon η2 coordination. As a result of the decreased
substrate-catalyst steric clash in the case of tertiary α-allenyl
alcohols, substitution can proceed under mild conditions.
The geometry of the proposed mode of substrate binding
was supported experimentally. Complex (S,S)-Ir-1 bearing an
η2-tertiary α-allenyl alcohol ligand could be prepared in 41%
yield by combining [Ir(coe)Cl]2 (0.5 equiv), (±)-1r (2.5 equiv)
and (S)-L1 (1.7 equiv) in CDCl3 followed by trituration with
pentane (Scheme 7). The off-white powder obtained via
filtration was readily characterized by NMR spectroscopy (1D
and 2D experiments). Furthermore, X-ray quality crystals of a
single diastereomer could be obtained via vapor diffusion into
an equimolar mixture of (S,S)-Ir-1 and its enantiomer which
was prepared using the enantiomeric ligand (R)-L1.45,46 As in
our previous studies,11 this analysis unambiguously confirms
that binding of the allene occurs via the terminal double bond,
which results in the considerable bending of the allene (∠C1‒
C2‒C3 = 147.6°). Furthermore, the terminal carbon of the
10
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Scheme 6. Studies probing enantioconvergence and
catalyst control of enantioselectivity.
In separate experiments, product (S)-2a was obtained in 59%
yield with 96% ee using (S)-L1, while (R)-2a was obtained in
49% yield with opposite absolute stereochemistry (-84% ee)
when (R)-L1 was used. These results suggest that the reaction
is non-stereospecific, and enantioselectivity is under catalyst
control. This rules out the possibility of a dynamic kinetic
resolution wherein rapid substrate epimerization occurs while
one enantiomer is processed selectively by the chiral Ir
catalyst. Ionization of the two enantiomers of the η2-
coordinated α-allenyl alcohol would, in principle, lead to two
diastereomeric complexes. In one of these, the aryl moiety
finds itself in an anti relationship to the Ir center, while in the
other the corresponding relationship is syn (Scheme 6C). The
interconversion of these syn and anti diastereomers effectively
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allene is further from the metal center than the central carbon
(C1‒Ir = 2.167 Å vs. C2‒Ir = 2.095 Å).
Scheme 9. Proposed mechanism for the asymmetric Ir
catalyzed direct reductive deoxygenation (R = iPr).
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6
7
Scheme 7. Synthesis of an η2-allene Ir(I) complex bearing a
coordinated substrate molecule.
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Although the generation of HOTf through hydrolysis of metal
In order to examine the reversibility of allene binding which
acts to facilitate catalyst turnover, known complex (S,S)-Ir-2
bearing η2-coordinated unfunctionalized allene 9 was prepared
(Scheme 8A),47 and shown to act as an effective pre-catalyst
(4 mol%) for the asymmetric reductive deoxygenation of (±)-
1a (Scheme 8B). This enabled product (S)-2a to be obtained in
81% yield with 92% ee and >20:1 rr. This experiment also
provides additional support for a 1:2 Ir:(P,olefin) complex
being operative in this transformation (cf. Table 1, entry 12).
triflate salts is
a known phenomenon, base-quenching
experiments utilizing the hindered 2,6-di-tertbutyl-4-methyl
pyridine as a Brønsted acid scavenger suggest that Bi(OTf)3 is
indeed acting primarily as a Lewis acid instead of as a
precursor to HOTf (see Supporting Information for details).48
Complex 11 can partition itself between two pathways. The
first minor (<3%) pathway leads to the irreversible formation
of a vinyl allene (i.e. 5d) by loss of a proton. In the second
(major) path, enantiodetermining hydride transfer from the 4-
position of HEH-1 to the benzylic carbocation results in
product-bound Ir(I) complex 12. Finally, allene exchange
occurs to free the product and regenerate active Ir(I) complex
10.
Scheme 8. Preparation and use of an η2-allene complex as
a pre-catalyst for asymmetric reduction deoxygenation.
CONCLUSIONS
We have developed the first catalytic, enantioselective
direct reductive deoxygenation of tertiary alcohols by
employing a coordination-based strategy for stereocontrol
over tertiary carbocations. Transition metal Lewis acid co-
catalysis involving an Ir-bis(phosphoramidite,olefin) complex,
Bi(OTf)3 and a novel bis-isopropyl Hantzsch ester analog
enables the highly enantioconvergent reduction of racemic
tertiary α-allenyl alcohols. Using this method, allene-
containing products are obtained in high enantiomeric excess
(up to 96% ee) and with excellent regioselectivity (up to
>50:1) over the corresponding achiral 1,3-dienes. This method
also represents the first asymmetric substitution of tertiary
alcohols using Ir catalysis to the best of our knowledge. A
combination of Hammett analysis with crossover, isotope
labeling and KIE experiments have provided insight into the
mechanism of this transformation. The data obtained herein
supports a turnover-limiting SN1-type ionization event that
generates a discrete allenyl cation coordinated to the Ir center.
These studies also suggest that this intermediate undergoes
irreversible hydride attack where the facial, and thus,
enantioselectivity is controlled by the Ir center. This approach
is expected to inspire the development of other new
coordination-based methods for facial control of carbocations.
Proposed Mechanism. Based on the experimental data
described herein, in addition to previously described
theoretical evidence,11 we propose the tentative mechanism for
the present asymmetric reductive deoxygenation shown in
Scheme 9. The catalyst precursor [Ir(cod)Cl]2 coordinates to
(S)-L1 and the racemic α-allenyl alcohol substrate (i.e. (±)-1d)
to generate complex 10, wherein the substrate is bound to Ir
via the terminal C=C bond of the allene in an η2-fashion. In
the presence of Bi(OTf)3, the carbon‒oxygen bond of the
tertiary alcohol undergoes turnover-limiting, heterolytic
cleavage resulting in the formation of intermediate 11
containing a planar, η2-bound cationic ligand.
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Diastereoselective SN1-Type Reactions of Chiral Propargylic
Acetates. Angew. Chem. Int. Ed. 2008, 47, 10106; d) Chung, J.
Y.; Mancheno, D.; Dormer, P. G.; Variankaval, N.; Ball, R.
G.; Tsou, N. N. Diastereoselective Friedel-Crafts Alkylation
of Indoles with Chiral α-Phenyl Benzylic Cations.
Asymmetric Synthesis of Anti-1,1,2-Triarylalkanes. Org. Lett.
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K. T. Evidence for π-Stacking as a Source of Stereocontrol in
the Synthesis of the Core Pyranochromene Ring System
Common to Calyxin I, Calyxin J, and Epicalyxin J. J. Org.
Chem. 2009, 74, 7529; f) Devoust, M.; Kitching, J. A.;
Fleming, M. J.; Lautens, M. Diastereoselective Benzylic
Arylation of Tetralins. Chem. Eur. J. 2010, 16, 50; g) Li, G.-
X.; Qu, J. Friedel–Crafts Alkylation of Arenes with Epoxides
Promoted by Fluorinated Alcohols or Water. Chem. Commun.
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McDougal, N. T.; Schaus, S. E.; Porco, J. A., Jr. Skeletal
Diversity via Cationic Rearrangements of Substituted
Dihydropyrans. Org. Lett. 2010, 12, 3222; i) Medjahdi, M.;
Gonzalez-Gomez, J. C.; Foubelo, F.; Yus, M. Concise Route
to (–)‐ and (+)‐Aphanorphine. Eur. J. Org. Chem. 2011, 2230;
j) Liebert, C.; Brinks, M. K.; Capacci, A. G.; Fleming, M.J.;
Lautens, M. Diastereoselective Intramolecular Friedel–Crafts
Alkylation of Tetralins. Org. Lett. 2011, 13, 3000; k) Guin, J.;
Besnard, C.; Pattison, P.; Lacour, J. Highly Selective
Additions of Hydride and Organolithium Nucleophiles to
Helical Carbenium Ions. Chem. Sci. 2011, 2, 425.
ASSOCIATED CONTENT
The information is available free of charge via the
internet at http://pubs.acs.org.
Experimental details of synthetic procedures, X-ray
data, and computational details (PDF).
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AUTHOR INFORMATION
Corresponding Author
*erickm.carreira@org.chem.ethz.ch
ORCID
Mayuko Isomura: 0000-0002-6235-5937
David A. Petrone: 0000-0001-9867-9178
Erick M. Carreira: 0000-0003-1472-490X
Author Contributions
§ These authors contributed equally.
Notes
[4] a) Brak, K.; Jacobsen, E.N. Asymmetric Ion-Pairing
Catalysis. Angew. Chem. Int. Ed. 2012, 52, 534; b) Ungarean,
C.N.; Southgate, E.H.; Sarlah, D. Enantioselective Polyene
Cyclizations. Org. Biomol. Chem. 2016, 14, 5454.
[5] a) Guo, Q.-X.; Peng, Y.-G.; Zhang, J.-W.; Song, L.; Feng,
Z.; Gong, L.-Z. Highly Enantioselective Alkylation Reaction
of Enamides by Brønsted-Acid Catalysis. Org. Lett. 2009, 11,
4620; b) Wilcke, D.; Herdtweck, E.; Bach, T. Enantioselective
Brønsted Acid Catalysis in the Friedel-Crafts Reaction of
Indoles with Secondary ortho-Hydroxybenzylic Alcohols.
Synlett 2011, 1235; c) Rueping, M.; Uria, U.; Lin, M.;Y.
Atodiresei, I. Chiral Organic Contact Ion Pairs in Metal-Free
Catalytic Asymmetric Allylic Substitutions. J. Am. Chem. Soc.
2011, 133, 3732; d) Wang, S.-G.; Han, L.; Zeng, M.; Sun, F.-
L.; Zhang, W.; You, S.- L. Enantioselective Synthesis of
Fluorene Derivatives by Chiral N-Triflyl Phosphoramide
Catalyzed Double Friedel–Crafts Alkylation Reaction. Org.
Biomol. Chem. 2012, 10, 3202; e) Tsuji, N.; Kennemur, J.L.;
Buyck, T.; Lee, S.; Prévost, S.; Kaib, P.S.J.; Bykov, D.; Farès,
C.; List, B. Activation of Olefins via Asymmetric Brønsted
Acid Catalysis. Science 2018, 359, 1501.
The authors declare no competing financial interests.
ACKNOWLEDGEMENTS
ETH Zürich and the Swiss National Science Foundation
(200020_152898) are gratefully acknowledged for financial
support. MI thanks the Funai Foundation for a Funai Overseas
Scholarship. DAP thanks the National Sciences and
Engineering Research Council of Canada for a postdoctoral
fellowship. The authors thank Michael Solar and Dr. N. Trapp
(ETH Zürich) for X-ray analysis, Dr. M.-O. Ebert, R. Arnold,
R. Frankenstein, and S. Burkhardt for NMR measurements,
and Oswald L. Greter for HRMS measurements and analysis.
Dr. Ivan Franzoni (NuChem Therapeutics) and Simon L.
Rössler (ETH Zürich) are thanked for helpful discussions and
Greta Vastakaite is thanked for assistance with SFC and
HPLC separations. This manuscript is dedicated to Professor
Mark Lautens on the occasion of his 60th birthday.
[6] Surendra, K.; Corey, E. J. Highly Enantioselective Proton-
Initiated Polycyclization of Polyenes. J. Am. Chem. Soc. 2012,
134, 11992.
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