Ir-Catalyzed Enantioconvergent Synthesis of Diversely Protected Allenylic Amines Employing Ammonia Surrogates

Article abstract of DOI:10.1002/anie.202005599

The first iridium catalyzed, enantioconvergent amination of allenylic carbonates is reported. This process utilizes various commercially available carbamates and sulfonamides to generate allenylic amines including commonly employed protected groups (Boc, Fmoc, Cbz, Ts, Ns) in 62–82 % yield and 87–98 % ee. The products generated through this scalable procedure serve as effective linchpins for the rapid, enantiospecific synthesis of a wide range of complex structures.

Full text of DOI:10.1002/anie.202005599

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Accepted Article
Title: Ir-Catalyzed Enantioconvergent Synthesis of Diversely Protected
Allenylic Amines Employing Ammonia Surrogates
Authors: Erick Moran Carreira, Fabian Glatz, and David A. Petrone
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Ir-Catalyzed Enantioconvergent Synthesis of Diversely Protected
Allenylic Amines Employing Ammonia Surrogates
Fabian Glatz, David A. Petrone and Erick M. Carreira*
Abstract: The first iridium catalyzed, enantioconvergent amination of
allenylic carbonates is reported. This process utilizes various
commercially available carbamates and sulfonamides to generate
allenylic amines including commonly employed protected groups (Boc,
Fmoc, Cbz, Ts, Ns) in 62-82% yield and 87-98% ee. The products
generated via this scalable procedure serve as effective linchpins for
the rapid, enantiospecific synthesis of a wide range of complex
structures.
Allenes have long been versatile platforms for reaction discovery
and development.[1] They form the basis of a number of widely
Scheme 1. Enatioconvergent synthesis of allenylic amines and applications.
used
transformations
which
include:
cyclizations,[2a-c]
hydroheterofunc-
annulations,[2d]
photocycloadditions,[2e]
tionalizations,[2f-h] and rearrangements.[2i-k] Their ease of
preparation, combined with their broad applications in
stereoselective catalysis has greatly increased their utility as
building blocks in organic synthesis.[1] Additionally, a number of
allene natural products displaying biological activity have been
isolated, and pharmacologically active anthropogenic allenes
have been designed as suicide inhibitors of enzyme activity.[3]
A
particularly interesting class of compounds are optically active
allenylic amines 3, as they can be used as effective linchpins in
the synthesis of diverse heterocycles (Scheme 1).[4-6] However,
the asymmetric synthesis of these amines remains greatly
underdeveloped by comparison to their allylic analogs.[7] Herein,
we report the development of an Ir-catalyzed enantioconvergent
amination of allenylic carbonates 1 with ammonia equivalents to
give 3 (Scheme 1). A key feature is that end users can choose
from a list of common amine protecting groups on the products,
such as Boc, Cbz, Fmoc as well as Ts, and Ns.
Scheme 2. Catalytic enantioselective strategies to protected allenylic amines.
tolylsulfonamides as nucleophiles. As such it underscores the
need for further development of these reaction types, which have
otherwise been extensively studied in the allylic series, to include
a broader range of amine nucleophiles.[9-11]
Modern, catalytic approaches to optically active protected
allenylic amines and their derivatives may be classified according
to the starting materials employed. In the first of these, additions
to N-Boc- or N-phosphinoylimines by allene-boronate
nucleophiles furnish allenylic amides stereoselectively (Scheme
2A).[8a,b] The second is the subject of a single publication in which
Ma and co-workers have reported the use of Pd/BIPHEP as a
catalyst for the substitution of racemic alkyl substituted allenylic
phosphates to give N-alkyl p-toluenesulfonamides in 89-94% ee
(Scheme 2B).[8c] Although it is state-of-the art, the scope is
restricted to alkyl derived substrates and employs N-
Our group has recently examined and reported Ir-(P,olefin)
catalyzed alkylation of racemic allenylic carbonates with
organozinc reagents.[12] Since metal-catalyzed reactions of
allenylic electrophiles have not been studied extensively, we have
embarked on expanding the versatility of the catalyst system to
include access to other adducts, such as allenylic amines.[10d,e]
Because tert-butyl carbamate is among the most widely employed
nitrogen protecting group, we examined the use of BocNH2 as a
nucleophile in the initial screening process. The use of this
inexpensive, crystalline, and commercially available nitrogen
source would lead to N-Boc allenylic amines, which can be
conveniently deprotected under mild acidic conditions or
transformed into more complex scaffolds, as shown below.
Evaluation of a range of parameters led to the identification of
optimal outcomes for the reaction of interest (Table 1). A system
comprising [Ir(cod)Cl]2 (2 mol%), (P,olefin)-ligand (S)-L1 (8 mol%),
ZnCl2 (10 mol%) and BocNH2 (1.1 equiv) in PhMe/THF (4:1) at 23
⁰C for 13 h provided product (R)-3a in 76% isolated yield and 96%
ee.[13,14]
[*]
F. Glatz, Dr. D. A. Petrone, Prof. Dr. E. M. Carreira
Laboratorium für Organische Chemie, HCI H335
Eidgenössiche Technische Hochschule Zürich
Vladimir-Prelog-Weg 3, 8093 Zürich (Switzerland)
E-mail: carreira@org.chem.ethz.ch
Supporting information for this article is given via a link at the
end of the document.
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Table 1. Screening of reaction parameters for the Ir-catalyzed amination.[a]
Table 2. Scope of Ir-catalyzed enantioconvergent amination.[a]
Entry
Deviation from the “standard” conditions
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
9
None
Commercial ZnCl2 solution(0.5 M THF)
THF instead of THF/PhMe
no [Ir(cod)Cl]2
82 (76)[d]
96
96
94
-
80
76
0
no (S)-L1
4
-
no ZnCl2
0
-
L2 instead of L1
0
-
L3 instead of L1
27
0
90
0
[e]
[Pd]/L4
[a] Reactions were conducted on 0.2 mmol scale. [b] Determined by 1H NMR
analysis of the unpurified reaction mixture using 1,1,2,2-tetrachloroethane as an
internal standard. [c] Determined by HPLC analysis using a chiral stationary
phase. [d] Conducted on 0.5 mmol scale. Isolated yield in parentheses [e]
[Pd(allyl)Cl]2 (2.5 mol%), (R)-L4 (6 mol%), DBU (2 equiv), BocNH2 (2 equiv) o-
xylene, 0 °C, 80 h. 2-Np = 2-naphthyl.
We highlight in Table 1 additional observations that are
noteworthy. The replacement of a freshly prepared THF solution
of ZnCl2 with a commercially available 0.5 M solution of ZnCl2 in
THF led only to a slight decrease in yield, while the ee remained
unchanged (entry 2). Replacing the PhMe/THF solvent mixture
with a THF-only solvent system led to slight decrease in both yield
and ee (entry 3). Additionally, control experiments confirmed that
all three components ([Ir(cod)Cl]2, (S)-L1 and ZnCl2) are crucial to
the success of the reaction (entries 4-6). Structural permutations
of ligand (S)-L1 led to less satisfactory outcomes. For example,
no product was observed when employing ligand (S)-L2 which
[a] Reaction conditions: [Ir] = 4 mol%, (S)-L1 = 8 mol%, (±)-1 (0.5 mmol), BocNH2
(1.1 equiv), ZnCl2 = 10 mol%, PhMe/THF (4:1, v:v) ([(±)-1] = 0.2 M), unless
otherwise noted. Isolated yields provided. Enantiomeric excess values (ee)
were determined by SFC or HPLC analysis using chiral stationary phases. [b]
(R)-L1 was used.
possesses
a
saturated iminostilbene moiety (entry 7).
Furthermore, the more rigid SPINOL derived (P,olefin) ligand (R)-
L3 gave the product in significantly reduced yield and
enantioselectivity (entry 8). Finally, the reaction conditions
reported by Ma and co-workers for amination of allenylic
phosphates did not lead to product formation when BocNH2 was
used as the nucleophile with 3a (entry 9).[8c] With the optimized
conditions in hand, the scope of the newly developed allenylic
amination was explored (Table 2). Phenyl substituted (R)-3b was
obtained in similar yield (73%) and identical ee (96%) to 3a.
Furthermore, enantiomer (S)-3b was obtained in 71% yield and
96% ee when (R)-L1 was employed. Allenylic amines (R)-3c-3e
possessing 4-haloarenes were generated in 63-75% yield and
95-96% ee. Due to their lower reactivity in comparison to 3a,
these substrates required heating to 40 °C for full conversion.
Substrates possessing strongly electron-withdrawing groups in
the para position of the arene were tolerated, and the
corresponding products (R)-3f and (R)-3g were obtained in 62%
and 67% yield along with 92% and 95% ee, respectively.
Furthermore, a series of aryl ethers [(R)-3i-k] were obtained in 68-
78% yield and 95-97% ee. Thiophene 1h furnished product (R)-
3h in 64% yield and 95% ee. Boronate-substituted substrate 1l
gave product (R)-3l in 62% yield and 94% ee. Some limitations of
this transformation were also uncovered. Hence, 1,1-disubstituted
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allene (R)-3m was only obtained in 62% yield and 10% ee, while
indole (R)-3n was isolated in 43% yield and 67% ee. Our attempts
with carbonates of alkyl allenyl carbinols were unsuccessful.[15]
Our results with aryl substituted allenylic carbonates thus
complement those reported by Ma employing N-tosyl amides and
Pd-catalysis, in which only alkyl substituted allenylic substrates
were reported.
We investigated the scalability of this transformation by
conducting experiments on both 1.00 g (4.06 mmol) and 10.0 g
(40.6 mmol) scale with (±)-1b as substrate (Table 2 bottom). In
these instances, the results were in agreement with those
obtained when the reaction was conducted on 0.5 mmol scale,
and product was obtained in 74% and 77% isolated yields and
95% and 96% ee, respectively.
Table 3 Scope of protected ammonia equivalents in Ir-catalyzed amination.[a]
Scheme 3. Mechanistic studies to probe catalyst vs substrate control.
with those from experiments involving the cyclobutyl homolog,
which was expectedly unreactive.[15]
In order to ascertain if the enantioconvergent allenylic amination
displayed significant rate differences between enantiomeric
starting carbonates, three experiments were conducted (Scheme
3B).[19] Firstly, when the reaction of (±)-1b was quenched after 1.5
h at 51% conversion, recovered 1b (49% yield) was racemic and
product (R)-3b was isolated in 30% yield and 95% ee. This result
is consistent with both enantiomeric carbonates (±)-1b
undergoing consumption at the same rate. Secondly, in separate
experiments enantiopure carbonate (S)-1b was allowed to react
under the standard conditions with either (S)- or (R)- L1. For both,
products were obtained in similar yields (75%/76%) and equally
high, but opposite, enantioselectivity (96 and –97% ee). These
observations indicate that substrate-dependent kinetic resolution
is not operative and that product configuration is solely dictated
by the chirality of Ir-catalyst.
The reactivity of the allenes and amines can be leveraged
to synthesize useful heterocyclic building blocks (Scheme 4). For
example, treatment of Boc-carbamate (R)-3a[20] with 5 mol%
Pd(PPh3)4 and PhI (1.05 equiv) in the presence of base, furnished
oxazolidinone 5 in 77% yield as a single diastereomer and a
crystalline solid (see SI for X-ray structure).[21] To the best of our
knowledge, there is no precedence for such a transformation
involving allenes. By contrast, under similar conditions N-Ts (R)-
4c gave aziridine 6 in 82% yield as a single diastereomer and
without erosion in ee.[4a] Au-catalyzed cycloisomerization of (R)-
3a using Echavarran’s catalyst (10) in a 1,2-DCE/PhMe solvent
mixture afforded 2,5-dihydropyrrole 7 in 89% yield.[5a] There have
been recent reports of the formation of 2,3-dihydropyrroles from
protected racemic allenylic amines using a collection of iron
catalysts by Bäckvall and Rueping.[6] The use of Fe-catalyst 11
leads to the cycloisomerization of optically active (R)-4c to
isomeric 2,3-dihydropyrrole 9 in 93% yield (98% ee). Finally,
enantioenriched primary amine hydrochloride salt 8 was obtained
in 88% yield by cleavage of the Boc group (HCl, 1,4-dioxane).[22]
[a] Reaction conditions: [Ir] = 4 mol%, (S)-L1 = 8 mol%, (±)-1b (0.5 mmol), RNH2
(1.1 equiv), ZnCl2 = 10 mol%, PhMe/THF (4:1, v:v) ([(±)-1b] = 0.2 M), 23 °C
Isolated yields provided. Enantiomeric excess values (ee) were determined by
HPLC analysis using chiral stationary phases. [b] reaction conducted on 2 mmol
scale.
We then proceeded to examine other protected ammonia
equivalents in the context of the allenylic amination reaction.
Conveniently, the conditions determined to be optimal for BocNH2
extended well to these other nucleophiles, which led to a diverse
series of N-protected allenylic amides (Table 3). For example,
when benzyl carbamate (CbzNH2) or FmocNH2 were used,
products (R)-4a and (R)-4b were isolated in 77% yield/96% ee
and 62% yield/87% ee, respectively. Furthermore, p-
tolylsulfonamide (TsNH2) gave product (R)-4c in 82% yield and
98% ee while o-nitobenzenesulfonamide (o-NsNH2) afforded (R)-
4d in 77% yield and 96% ee.
Our previous work on Ir-catalyzed substitution of allenylic
electrophiles indicated that substrate ionization at the allenylic
carbon leads to a highly delocalized cation.[12,16] The trends
observed in this study are consistent with such an intermediate.
To further probe this, cyclopropyl (±)-1o was examined as
substrate, given the known stability of carbinyl cyclopropyl
cabocations (Scheme 3A).[17] Indeed, product (R)-3o was formed
in 68% yield and 84% ee.[18] This result stands in direct contrast
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Scheme 4. Synthetic application of amination products, synthesis of nitrogen heterocycles. Reagents and conditions: a) Pd(PPh3)4 (5 mol%), PhI (1.05 equiv),
K2CO3 (4 equiv), DMF 70 °C; b) Pd(PPh3)4 (10 mol%), PhI (4 equiv), K2CO3 (4 equiv), 1,4-dioxane, 110 °C; c) 10 (5 mol%), 1,2-DCE/PhMe, 23 °C; d) 11 (5 mol %),
TMANO (6 mol %), K2CO3 (1 equiv), PhMe, 70 °C; e) HCl, 1,4-dioxane, 23 °C. TMANO = trimethylamine N-oxide
[4]
For examples with Pd, see: a) H. Ohno, M. Anzai, A. Toda, S. Ohishi, N.
Fujii, T. Tanaka, Y. Takemoto, T. Ibuka, J. Org. Chem. 2001, 66, 4904;
b) Y. Kayaki, N. Mori, T. Ikariya, Tetrahedron Lett. 2009, 50, 6491 c) S.
Li; J. Ye, W. Yuan, S. Ma, Tetrahedron 2013, 69, 10450.
In
summary,
we
have
developed
a
highly
enantioconvergent protocol for the amination of racemic allenylic
carbonates using an Ir-(P,olefin) catalyst system for the first time
with various, convenient used ammonia equivalents (BocNH2,
FmocNH2, CbzNH2, TsNH2, NsNH2). This procedure features mild
reaction conditions, readily available catalysts along with racemic
starting materials, and is easily scalable. Furthermore, the
general product scaffolds can be used as effective linchpins for
the streamlined preparation of various building blocks. The
synthesis of allenylic amines considerably extends the chemistry
of Ir-catalyzed substitution reactions, which should find wide
applications.
[5]
For examples with Au or Ag, see: a) R. J. Detz, Z. Abiri, R. le Griel, H.
Hiemstra, J. H. van Maarseveen, Chem. Eur. J. 2011, 17, 5921; b) A. C.
Breman, J. Dijkink, J. H. van Maarseveen, S. S. Kinderman, H. Hiemstra,
J. Org. Chem. 2009, 74, 6327; c) M. Billet, A. Schoenfelder, P. Klotz, A.
Mann, Tetrahedron Lett. 2002, 43, 1453; d) B. Alcaide, P. Almendros, C.
Aragoncillo, G. Gómez-Campillos, M. T. Quirós, E. Soriano, J. Org.
Chem. 2016, 81, 7362; e) Y. Gao, M. Fan, Q. Geng, D. Ma, Chem. Eur.
J. 2018, 24, 6547.
[6]
[7]
For examples with Fe, see: a) O. El-Sepelgy, A. Brzozowska, J. Sklyarut,
Y. K. Jang, V. Zubar, M. Rueping, Org. Lett. 2018, 20, 696; b) A.
Guđmundsson, K. P. J. Gustafson, B. K. Mai, V. Hobiger, F. Himo, J.-E.
Bäckvall, ACS Catal. 2019, 9, 1733.
For examples of asymmetric allylic amination using ammonia surrogates,
see: a) M. J. Pouy, L. M. Stanley, J. F. Hartwig, J. Am. Chem. Soc. 2009,
131, 11312; b) D. J. Weix, D. Marković, M. Ueda, J. F. Hartwig, Org. Lett.
2009, 11, 2944; c) M. Gärtner, R. Weihofen, G. Helmchen, Chem. Eur,
J. 2011, 17, 7605; c) B.-H. Zheng, C.-H. Ding, X.-L. Hou, Synlett, 2011,
2262.
Acknowledgements
ETH Zürich is thanked for support. DAP thanks the National
Sciences and Engineering Research Council of Canada for a
postdoctoral fellowship. The authors thank M. Solar and Dr. N.
Trapp (ETH Zürich) for X-ray analysis.
[8]
[9]
a) Y.-Y. Huang, A. Chakrabarti, N. Morita, U. Schneider, S. Kobayashi
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136, 3780; c) Q. Li, C. Fu, S. Ma, Angew. Chem. Int. Ed. 2014, 53, 6511;
Angew. Chem. 2014, 126, 6629.
Keywords: allenes • amination• enantioconvergent • iridium •
heterocycles
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Ed. Engl. 1985, 24, 700; Angew. Chem. 1985, 97, 706.
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2019, 11, 177; b) L. M. Stanley, J. F. Hartwig, Angew. Chem. Int. Ed.
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L. M. Stanley, Acc. Chem. Res. 2010, 43, 1461; c) J. Qu, G. Helmchen,
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Am. Chem. Soc. 2018, 140, 4697.
[13] For optimization details, see SI.
[14] The absolute configuration of (R)-3a was determined by X-ray, see SI.
[15] For details on substrate limitations, see SI
[16] M. Isomura, D. A. Petrone, E. M. Carreira, J. Am. Chem. Soc. 2019, 141,
4738.
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[18] The ee was determined by GC using a chiral stationary phase.
[19] For examples of Ir-catalyzed kinetic resolutions, see: a) S. Breitler, E. M.
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S. L. Rössler, E. M. Carreira, Angew. Chem. Int. Ed. 2019, 58, 9537;
Angew. Chem. 2019, 131, 9637.
[20] The 96% ee of 3a (Table 1) was upgraded to >99% by recrystallization
from hexanes/ether.
[21] Pd-catalyzed conversion of an achiral allenyl amine to an oxazolidinone
in the presence of CO2 has been reported.[4c] The approach we describe
directly converts the optically active, Boc-protected allenyl amide.
[22] P. Casara, K. Jund, P. Bey, Tetrahedron Lett. 1984, 25, 1891.
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Fabian Glatz, David A. Petrone, Erick
M. Carreira
Page No. – Page No.
Ir-Catalyzed Enantioconvergent
Synthesis of Diversely Protected
Allenylic Amines Employing
Ammonia Surrogates
Pick and choose: Ir- catalysis enables the synthesis of diversely protected allenylic amines employing readily available ammonia
equivalents. This highly scalable method grants access to these building blocks in high enantiomeric excess. The products obtained
may be transformed in to a variety of heterocycles.
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