Development of Chemo- and Enantioselective Palladium-Catalyzed Decarboxylative Asymmetric Allylic Alkylation of α-Nitroesters

Article abstract of DOI:10.1002/anie.201904034

We describe the development of a Pd-catalyzed decarboxylative asymmetric allylic alkylation of α-nitro allyl esters to afford acyclic tetrasubstituted nitroalkanes. Optimization of the reaction parameters revealed unique ligand and solvent combinations crucial for achieving chemo- and enantioselective C-alkylation of electronically challenging benzylic nitronates and sterically encumbered 2-allyl esters. Substrates were efficiently accessed in a combinatorial fashion by a cross-Claisen/ α-arylation sequence. The method provides functional group orthogonality that complements nucleophilic imine allylation strategies for α-tertiary amine synthesis.

Full text of DOI:10.1002/anie.201904034

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Accepted Article
Title: Development of a Chemo- and Enantioselective Pd-Catalyzed
Decarboxylative Asymmetric Allylic Alkylation of α-Nitroesters
Authors: Barry M. Trost, Johnathan E. Schultz, and Yu Bai
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10.1002/anie.201904034
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Development of Chemo- and Enantioselective Palladium-
Catalyzed Decarboxylative Asymmetric Allylic Alkylation of α-
Nitroesters
Barry M. Trost,[a] Johnathan E. Schultz, and Yu Bai
transfer mechanisms.[5b] Within only the last decade, racemic
Abstract: We describe the development of
a Pd-catalyzed
nitroalkane alkylation has been made general through metal-
catalyzed radical processes.[6] Watson and coworkers have
shown that copper-catalysis facilitates the alkylation of radical-
stabilized electrophiles,[6a-c] and more recently, they have
disclosed a nickel-catalyzed alkylation of unactivated alkyl
iodides.[6d]. This reaction has been rendered asymmetric using
chiral nickel-catalysis with primary nitroalkanes and α-
bromoamides.[6e] However, these approaches have not shown
primary or secondary benzylic nitroalkanes to be competent
nucleophilic partners.
decarboxylative asymmetric allylic alkylation of α-nitro allyl esters to
afford acyclic tetrasubstituted nitroalkanes. Optimization of the
reaction parameters revealed unique ligand and solvent
combinations crucial for achieving chemo- and enantioselective C-
alkylation of electronically challenging benzylic nitronates and
sterically encumbered 2-allyl esters. Substrates were efficiently
accessed in a combinatorial fashion by a cross-Claisen / α-arylation
sequence. The method provides functional group orthogonality that
complements nucleophilic imine allylation strategies for α-tertiary
amine synthesis.
Nitroalkanes have a rich history as synthetic intermediates in
organic chemistry, with a variety of post-synthetic modifications
of the nitro group affording amines and alkanes by reduction and
carbonyl compounds by Nef oxidation.[1] Nitroaromatic
compounds are more commonplace in natural products and
pharmaceutical agents;[2a] however, a surprisingly large number
of biologically active, naturally occurring nitroalkanes have been
discovered (Figure 1).[2b] More importantly, nitroalkanes can
serve as synthetic precursors to biologically active amines,
including synthetically challenging α-tertiary amines.[3]
Figure 1. Select naturally occurring nitroalkanes and biologically active α-
tertiary benzylic amines
In the area of C-C bond construction, the asymmetric
Michael,[4a] aldol (Henry reaction),[4b] and Mannich (aza-Henry)[4c]
reactions are fairly well-developed with nitronates as the
nucleophilic partner. However, for ionic alkylation of nitroalkanes
using alkyl halides, the issue of C- vs O-alkylation has been long
recognized, with early approaches toward solving this problem
utilizing alkylation of doubly deprotonated nitroalkanes[5a] or
reactions with specific electrophiles through single electron-
Scheme 1. Pd-AAA of nitroalkanes
For allyl electrophiles under Pd-catalyzed asymmetric allylic
alkylation (Pd-AAA), C-alkylation can be achieved with
asymmetric induction on the electrophilic partner[7a] and with
simultaneous
diastereoselection
when
using
primary
nitroalkanes[7b-d] For the alkylation of secondary nitroalkanes,
Ooi and coworkers demonstrated that ester-stabilized nitronates
undergo alkylation when using 3-aryl-substituted allyl
electrophiles (Scheme 1a).[7e] This work was important for
demonstrating nitroalkanes as pronucleophiles in the synthesis
of acyclic, fully-substituted nitrogen stereocenters; however, O-
[a]
[b]
Prof. Dr. B. M. Trost, J.E. Schultz, Y. Bai
Department of Chemistry, Stanford University
Stanford, CA 94305-5080 (USA)
E-mail: bmtrost@stanford.edu
Supporting information for this article is given via a link at the end of
the document.
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alkylation was unlikely a problem due to the extra stabilization of
the ester substituent. Surprisingly, the asymmetric C-allylation of
secondary benzylic nitronates remains an unsolved problem.
The absence of asymmetric allylation of these species is likely
due to the inherent propensity of the nucleophilic nitronate to
undergo O-allylation. For instance, our group has demonstrated
enantioselectivity on the electrophilic partner by O-alkylation of a
hindered benzylic potassium nitronate salt (Scheme 1b).[7f] In
addition to the chemoselectivity problem, achieving asymmetric
induction employing secondary benzylic nitroalkanes should
prove challenging, due to a small degree of differentiation of the
carbon substituents compared to primary nitroalkanes. This
problem is compounded by the fact that setting stereochemistry
solely on the nucleophilic partner remains the most difficult
Scheme 3. A Pd-catalyzed DAAA strategy for the synthesis of acyclic α-
tertiary nitroalkanes
mode of asymmetric induction in Pd-AAA. This is
a
Metal-catalyzed decarboxylative methods have been shown to
be powerful reactions for efficient C-C bond-formation,[9a] For
asymmetric decarboxylative methods, Pd-DAAA has recently
been exploited in setting fully-substituted acyclic carbon
consequence of the outer-sphere mechanism of attack of
stabilized nucleophiles in Pd-AAA.[8] For substrates without an
ester for α-stabilization, a single example of Pd-AAA with a
secondary di-alkyl nitroalkane with modest enantioselectivity has
been described by Shibasaki.[7c] In addition, setting fully-
substituted acyclic stereocenters by Pd-AAA presents unique
challenges.[8d] With this in mind, we were particularly intrigued
with the prospect of developing a method for the chemoselective
allylic alkylation of racemic secondary nitroalkanes to afford
enantiopure acyclic tertiary nitroalkanes (Scheme 1c) using the
diphenylphosphino benzamide ligands L1-L4 (Fig, 2).
atoms,[10] a longstanding challenge in organic synthesis.[11]
A
decarboxylative alkylation of nitroalkanes has been described by
both Tsuji and Tunge.[12] In Tsuji’s first report,[12a] O-alkylation
was observed as a major side reaction. Tunge explored the
scope of this transformation and discovered conditions for rapid
C-alkylation using tetrakis(triphenylphosphine)palladium(0).[12b]
This method was limited to cases where the nitronate
substitutents were both alkyl groups, save for one example of
benzylic alkylation with additional fluoro-stabilization. In this
process, the stereochemistry of the starting nitroester would be
ablated in the process,[13] furnishing
a planar nitronate
counterion-paired with the π-allyl palladium(II) complex. We
postulated that if one of the nitronate substitutents were an
aromatic group, enantio-induction could be observed due to
facial differentiation of the α-substituents; however, C-alkylation
would prove more difficult due to the conjugation of the nitronate
to the aromatic system.[7f]
Table 1. Substrate dependence on C-alkylation
Figure 2. Chiral ligands employed in these studies
Our initial approach toward this problem involved
intermolecular Pd-AAA of model benzylic nitroalkane 1 in the
presence of chiral palladium catalyst and allyl methyl carbonate
2 (Scheme 2). A ligand and solvent screen revealed C-allylation
proceeded smoothly but with modest enantioselectivity using
bis(dibenzylidenediacetone)palladium(0) chloroform adduct in
dimethoxyethane. The ability to exploit lower temperatures was
limited by long reaction times. Furthermore, 2-substituted
carbonates failed to provide the product efficiently. For this
reason, we looked to exploit the enhanced kinetics of
decarboxylative asymmetric allylic alkylations (Scheme 3).[9b-d]
conditi substra
entry
onsa
A
te
T (˚C)
-70
yield (%)
90
e.r.b
90:10
1
2
4a
4b
4b
4b
4c
4c
RT
A
28
83:17
RT
3
B
42
93:7
-78 to 4
-78 to 4
-78 to 4
4
B
76
95:5
5
6 c
B
29
95.5:4.5
77.5:22.5
B
31
a
b
Conditons A: L2, dimethoxyethane (DME), Conditions B: L4, toluene.
Enantiomeric ratio determined by chiral HPLC. cReaction performed at 1M
concentration.
An initial optimization for substrates 4 revealed conditions A
and B for the Pd-DAAA reaction of allyl 4a and 2-substituted allyl
substrates 4b,c (for full screening, see SI). However, these
conditions led to capricious results, and changing the methyl
substituent to ethyl (Table 1, entry 5) resulted in high
enantioselectivity but poor yield due to significant formation of
the oxime by-product of O-alkylation. For the re-optimization of
the reaction (Table 2), we chose 4c as the model substrate
since both an ethyl side chain and a β-methyallyl ester were
detrimental to chemoselective C-alkylation (Table 1). It was
postulated that these difficulties would potentially reveal more
differential catalyst conditions and better aid in solving the
inherent chemoselectivity problem. The absolute configuration of
product 3a was readily determined by reduction of the nitro
Scheme 2. Intermolecular Pd-AAA of an α-secondary benzylic nitroalkane
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10.1002/anie.201904034
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group and comparison of the optical rotation with the known
compound.[14] The configuration of all other products are
assigned by analogy (see SI for details.)
advent of methods for the robust and scalable synthesis of
symmetrical aryl iodonium salts[19a,b] as well as mixed salts
bearing
non-transferable
electron-rich
substituents.[19c]
Fortunately, no sigmatropic rearrangement was observed in the
arylation reaction.
Table 2. Solvent and Ligand Effects in Pd-DAAA Reaction
Table 3. Scope of Pd-DAAA Reactiona
entry
1
ligand
L1
Solvent
THF
T (˚C)
RT
Yield (%)
57
e.r.a
88.5:11.5
86:14
RT
2
L2
THF
30
RT
3
L3
THF
42
87.5:12.5
87:13
RT
4
L4
THF
42
4
5
L1
THF
29
89:11
RT
86:14
6
L1
DME
36
RT
7
L1
EtOAc
MTBE
EtOAc
EtOAc
EtOAc
85
90:10
88:12
RT
8
L1
69
-78 to 4
-78 to 4
-78 to 4
9
L1
87
93.5:6.5
93.5:6.5
91.5:8.5
10
11 b
L4
31
L1
65
a
Enantiomeric ratio determined by chiral HPLC bPalladium pre-catalyst was
changed to cyclopentadienyl cinnamyl palladium. THF=tetrahydrofuran,
DME=dimethoxyethane, EtOAc=ethyl acetate, MTBE=methyl tert-butyl ether.
The ligand and solvent screen for the DAAA of 4c revealed
that our initial conditions were far from optimal. All four of the
standard ligands (Figure 2) afforded the product with similar
levels of enantioselectivity (Table 2, entries 1-4). However, L1
proved differential for C-alkylation compared to the other ligands
in THF (Table 2, entry 4). The isolated yield of C-alkylation
product was strongly dependent on solvent, with the highest
yield obtained in ethyl acetate (Table 2, entry 7). The high rate of
the DAAA pathway allowed for lowering the temperature of the
reaction; beginning the reaction at -78⁰ and warming to 4⁰C
increased the enantioselectivity to 93.5:6.5 e.r. in 87% yield
(Table 2, entry 9).
Scheme 4. A Combinatorial Approach to Chiral α-Tertiary Nitroalkanes by Pd-
DAAA
The development of an efficient synthesis of α-aryl nitro esters
4 was required for the preparation of Pd-DAAA substrates.
Lithium nitronate salts 6 and allyl acyl imidazoles 7 were
combined to form nitroesters 8 which in turn were further
elaborated using aryl iodonium salts[15] in an α-arylation reaction
(Scheme 4).[16]
A procedure described by Mosher and
coworkers[17a] for the cross-Claisen condensation was found to
be slow due to poor solubility of the nitronate salts; however,
gradual heating of the reaction suspension until it became
homogeneous was found to produce nitroesters 8 rapidly.
Utilizing an α-arylation of nitroalkanes procedure developed by
Olofsson and coworkers,[18a] DAAA substrates 4 were produced
efficiently using both electron-rich and electron-poor aryl
iodonium salts. We found this route highly practical due to the
a.) Reactions were performed on 0.1 mmol scale. b.) Product was de-silylated
for determination of e.r. c.) Performed using conditions A, Table 1. d.)
Performed using conditions B, Table 1. e.) Performed using conditions A,
Table 1, -78 ⁰C to 4 ⁰C.
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With the modified conditions in hand, we were able to achieve
high levels of chemoselectivity for more sterically demanding
substrates with both electron-withdrawing and electron-donating
aromatic substituents (Table 3). In addition, both α-ethyl
nitroesters and more bulky β-siloxymethallyl esters (tert-
and monomeric species with differing levels of activity and
enantiodiscrimination, as demonstrated by Lloyd-Jones and
Norrby.[20] Their model (Figure 3) suggests that the monomeric
form of catalyst places the amide hydrogen atoms in proximity
for both reaction acceleration and substrate recognition. This
could potentially explain suppression of the O-alkylation pathway
in our nitroalkane allylation. The dependence of ligand backbone
and C-alkylation is unsurprising considering small changes in
the backbone result in changes in the placement of hydrogen-
bond donors. Additionally, the dependence of the
enantioselectivity on concentration is in agreement with a model
where the degree of oligomerization affects reaction selectivity.
It is important to note that this application of the Lloyd-Jones /
Norrby model does not involve catalyst recognition for
differentiation of enantiotopic sites on the electrophile, but rather
accounts for facial differentiation of a prochiral nucleophile.
Superimposed upon the above effects are the chiral recognition
elements of projecting phosphino aromatic substituents, as
explained by the ‘wall and flap’ model.[8c]
butyldiphenylsilyl=TBDPS)
were
alkylated
in
high
enantioselectivity and yield. Gratifyingly, aromatic groups of
various electronic patterns are all tolerated, most notably a
heterocyclic pyridyl system (3o). Yields were only slightly
diminished for electron-rich aromatic substrates, due to greater
formation of oxime by-product from O-alkylation. Substitution at
the 3-position of the benzene ring was demonstrated by smooth
formation of 2-naphthyl product 3n in 83% yield and 91:9 e.r.
The high enantioselectivities for 2-siloxymethylene substrates
are noteworthy, as these demonstrate the use of more
elaborated systems and allow for installation of a versatile
functional group handle at the 2-allyl position. The reaction is not
without its limitations: although high yields were obtained for
unsubstituted allyl esters, the enantioselectivities were
diminished (3s-3v). Additionally, 2-bromo aryl substitution
resulted in little conversion of the starting material (3w).
Highly elaborated product 3q was formed on gram scale with
half the standard catalyst loading (Scheme 5). Simple functional
group interconversions were demonstrated with chemoselectivity
for the different functional group handles and the alkyl side
chain. Conversion of the allyl functional group to ketone 10 was
achieved in the presence of the alkyne by careful introduction of
ozone. The alkyne was chemoselectively reacted in the
presence of Lindlar’s catalyst without reduction of the nitro- or
olefinic functional groups. Over-reduction of styrenyl 11 to the
alkane was suppressed by the addition of 30 mol% quinoline.
Reduction of the potentially acid sensitive tertiary benzylic
nitroalkane 3q by zinc powder in acetic acid, afforded after
neutralization, basic amine 12.
Figure 3. Stereochemical rationale based on Lloyd-Jones / Norrby model
In summary, we have developed a Pd-DAAA reaction of allyl
nitroesters that furnishes the alkylation products of benzylic
nitronate alkylation of sterically hindered π-allyl species. The
ability to overcome the inherent O-alkylation pathway was
enabled by L1 in ethyl acetate, a particular solvent and ligand
combination favoring the C-alkylation pathway. Further, the
ready introduction of the aromatic functionality by arylation of
nitroesters and the ease of reduction of the nitro group
empowers the ability to have synthetic access to α-tertiary
benzylic primary amines asymmetrically. This potentially
provides a highly useful umpolung version of nucleophilic
ketimine additions,[21] as well as modularity in synthetic
sequences toward chiral α-tertiary amines.
Scheme 5. Gram-Scale Synthesis and Chemoselective Transformations of
elaborated product
Acknowledgements
Funding was provided in part by the Tamaki Foundation.
Keywords: asymmetric catalysis • allylic alkylation • palladium
catalysis • chemoselectivity • enantioselectivity
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Table of Contents Artwork:
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