Regiodivergent Ring-Opening Cross-Coupling of Vinyl Aziridines with Phosphorus Nucleophiles: Access to Phosphorus-Containing Amino Acid Derivatives

Article abstract of DOI:10.1021/acs.orglett.8b03309

Catalytic ring-opening phosphonation and phosphatation of vinyl aziridines have been developed in a regiodivergent fashion, giving linear and branched products. Generation of P-centered radicals enables S_N2′-type ring-opening reactions of vinyl aziridines to afford δ-amino alkylphosphorus products at room temperature. On the other hand, in situ generated phosphate anions via the Ag-catalyzed aerobic oxidation of phosphonyl reactants underwent S_N2 reaction to provide branched phosphorus-containing amine products. Furthermore, this divergent methodology serves as a powerful tool for the stereospecific synthesis of phosphorus-containing amino acid derivatives.

Full text of DOI:10.1021/acs.orglett.8b03309

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Regiodivergent Ring-Opening Cross-Coupling of Vinyl Aziridines
with Phosphorus Nucleophiles: Access to Phosphorus-Containing
Amino Acid Derivatives
Dahye Kang, Taehwan Kim, Hyunpyo Lee,†,‡ and Sungwoo Hong*,†,‡
†
,‡
†,‡
†
Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Korea
‡
Center for Catalytic Hydrocarbon Functionalizations, Institute for Basic Science (IBS), Daejeon 34141, Korea
*
S Supporting Information
ABSTRACT: Catalytic ring-opening phosphonation and phospha-
tation of vinyl aziridines have been developed in a regiodivergent
fashion, giving linear and branched products. Generation of P-
centered radicals enables S 2′-type ring-opening reactions of vinyl
N
aziridines to afford δ-amino alkylphosphorus products at room
temperature. On the other hand, in situ generated phosphate
anions via the Ag-catalyzed aerobic oxidation of phosphonyl
reactants underwent S 2 reaction to provide branched phosphorus-containing amine products. Furthermore, this divergent
N
methodology serves as a powerful tool for the stereospecific synthesis of phosphorus-containing amino acid derivatives.
rganophosphorus motifs are frequently used in a wide
As outlined in Scheme 1, we speculated the possibility of
1
O
range of applications in organic chemistry, medicinal
controlling the reaction pathways (S 2 or S 2′-type) by the
N
N
2
3
chemistry, and the materials industries. In particular,
compounds containing amino phosphonate or amino phos-
phatate units have attracted attention because they exhibit
Scheme 1. Regioselective and Divergent Catalytic Ring-
Opening Reactions of Vinyl Aziridines
4
intriguing biological activities. Among them, phosphorus-
containing chiral amino acids are an important class of
4a,b,5
biologically active scaffolds,
and (2R)-amino-5-phospho-
novaleric acid (AP-5) is a selective NMDA receptor
4
d,5f
antagonist.
Although tremendous efforts have been
6
directed toward the development of efficient phosphonation
and phosphatation reactions, a divergent and stereoselective
7
synthesis of enantioenriched phosphorus functionalized amino
acid derivatives remains unknown.
Aziridines serve as versatile building blocks in organic
8
9,10
synthesis. In particular, vinyl aziridines
possess multiple
features of the nucleophile source, which can differentiate and
amplify site-discriminating interactions with the vinyl azir-
idines. We discovered that the ring-opening phosphonation of
vinyl aziridines occurred via the cleavage of an allylic C−N
bond under Cu or Ag catalysts to give δ-amino alkylphospho-
rus derivatives. Notably, by tuning the reaction conditions
reactive sites and are prone to various ring opening reactions
that can be exploited for the construction of a wide range of
valuable β-amino-functionalized motifs.
8
,10
In general, the
reactions proceed via transition-metal-catalyzed C−N bond
1
1
cleavage or direct nucleophilic attack of the activated
8b
aziridines at the more substituted carbon. The cleavage of
an allylic C−N bond and direct displacement of the C−N
bond of the aziridine are the major competing pathways that
result in the formation of linear-type and branched-type
products. Consequently, much effort has been devoted toward
achieving high discrimination among the reactive sites of the
vinyl aziridines. Despite recent progress, site-selective
catalytic ring-opening reactions of vinyl aziridines by
phosphorus nucleophiles remain a challenging synthetic
problem. In this regard, predictable and controllable divergent
synthetic tools applicable to common vinyl aziridine starting
materials would be highly valuable.
under an O atmosphere at 60 °C, we observed that branched
2
products were generated with the incorporation of phosphates.
Our mechanistic studies revealed that the formation of
branched products likely occurs via in situ-generated
phosphate anions derived from the Ag-catalyzed oxidation of
phosphonyl reactants, followed by the addition of phosphate to
the vinyl aziridines. Herein, we describe our efforts toward
developing catalyst-controlled ring-opening cross-coupling
reactions of vinyl aziridines, which enable the divergent
1
2
Received: October 16, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b03309
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
synthesis of linear- or branched-type amino-functionalized
molecules from common versatile starting materials. Further-
more, we achieved the stereospecific synthesis of two types of
optically active phosphorus-containing unnatural amino acid
derivatives with the retention of the chiral information.
To corroborate this scenario, our investigation began with
the reaction of vinyl aziridine 1a and diphenylphosphine oxide
Scheme 2. Substrate Scope of S 2′ Phosphonation
N
(
2a) under different conditions, and selected data are listed in
Table 1. After evaluating different reaction conditions, vinyl
Table 1. Reaction Optimization for the S 2′
N
a
Phosphonation
b
entry
catalyst
AgNO3
solvent
time (h)
yield (%)
c
1
MeCN
MeCN
MeCN
toluene
DCE
MeCN
toluene
toluene
toluene
17
24
12
2
4
4
1
1
1
52
63
66
71
51
21
72
NR
trace
c
2
Cu O
2
c
3
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
c
4
5
6
7
8
a
Conditions: 1a (1.0 equiv), 2 (2.0 equiv), and Cu(OAc) (20 mol
2
%
) in toluene (0.2 M) at rt under air atmosphere. Yields of isolated
b
Cu(OAc)2
products. 2 (3.0 equiv) and DBU (1.5 equiv) were used. PMP = p-
methoxybenzene.
d
9
Cu(OAc)2
a
Conditions: 1a (1.0 equiv), 2a (2.0 equiv), catalyst (20 mol %) in
(2f−k) and diphenyl and dibenzyl phosphites (2k and 2l) all
reacted well to provide the desired products. In addition, ethyl
phenylphosphinate readily participated in the reaction and
provided desired product 3n. We subsequently assessed the
applicability of our method to cyclic phosphorus reactants,
such as cyclic phosphonate and cyclic oxaphosphine oxide,
which worked well and provided desired products 3m and 3o,
proving that the present reaction protocol is suitable for
various types of phosphorus reagents, including H-phosphine
oxides, H-phosphonates, and H-phosphinates.
b
1
solvent (0.1 M) at rt under air atmosphere. H NMR yields of
products. Under N atmosphere. TEMPO (2.0 equiv) was added.
DCE = 1,2-dichloroethane.
c
d
2
aziridine 1a was found to undergo an S 2′-type phosphonation
N
using either Cu or Ag as the catalyst to afford linear-type
product 3a. The structure of 3a was unambiguously confirmed
by X-ray crystallographic analysis. Among the solvents
screened, toluene was most efficient for this reaction, and
the desired product was obtained in 71% yield (entry 4). Of
the various catalysts tested, Cu(OAc)2 displayed an out-
standing performance for this transformation. Control experi-
ments confirmed the essential role of the Cu catalyst in this
reaction (entry 8). The inhibition of the reaction by the
addition of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)
We next conducted a series of experiments to investigate the
scope of the vinyl aziridine substrates. As shown in Scheme 3,
we observed that substrates with a variety of substitution
patterns were successfully converted to the desired products.
Aziridine substrates bearing nosyl and N-phthalimide groups
worked well to afford the desired products 3p and 3q.
Dimethyl-substituted vinyl aziridine 1d was well tolerated, and
the coupled product was isolated in 74% yield. Further
exploration demonstrated that cyclohexene cores containing
both amino and phosphorus groups can be accessed under the
reaction conditions (3s). The utility of the present method was
further broadened by the reaction with vinyl aziridine
substrates possessing benzyl and isobutyl groups, which
afforded products 3t and 3u, respectively. Next, the importance
of phosphorus-containing amino acids in natural products and
biologically active molecules prompted us to establish a general
protocol to access these molecular scaffolds. To understand the
stereochemical fate of the stereogenic center, enantiopure vinyl
(
entry 9) supports a radical process being active. Through
systematic screening of the reaction conditions, a stirred
solution of 1a, phosphine oxide 2a, and Cu(OAc) (20 mol %)
2
in toluene at rt for 1 h afforded 3a exclusively in 72% yield.
With the optimal conditions in hand, we next set out to
determine the substrate scope with respect to phosphine
oxides to demonstrate the utility and generality of this method
as summarized in Scheme 2. A series of diphenylphosphine
oxides bearing various substituents were employed in the
present Cu-catalyzed reaction and were successfully converted
to the corresponding coupled products in moderate to good
yields (3a−d). In addition, reactions of di(naphthalen-2-
yl)phosphine oxide smoothly afforded phosphonated product
9b,14
aziridine esters 1h−k
were subjected to the reactions with
3
e in 84% yield. Expanding the scope from phosphine oxides
phosphine oxides. Our optimization studies revealed that the
Ag catalyst was more effective than Cu with this type of
substrate. To our delight, optically active chiral aziridine (R)-
1h with 92% ee was converted into chiral phosphonated amino
acid derivative 3v in 72% yield. Notably, chiral HPLC analysis
of 3v revealed that the stereochemistry was completely
retained in the reaction (92% ee, S). The structure and
absolute configuration of 3v was unambiguously confirmed by
to H-phosphite reactants was possible, leading to the formation
of corresponding products 3f−o. In the case of H-phosphite
reactants, the use of 1,8-diazabicyclo[5.4.0]undec-7-ene
13
(
DBU) was required to ensure the high reactivity. With
slightly modified reaction conditions, the Cu-catalyzed
phosphonation reactions were successfully conducted with a
variety of H-phosphites. For example, alkyl H-phosphinates
B
DOI: 10.1021/acs.orglett.8b03309
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a
Scheme 3. Substrate Scope of Various Vinyl Aziridines
Table 2. Reaction Optimization for S 2 Phosphatation
N
b
yield (%)
entry
catalyst
atm
temp (°C)
time (h)
3f
4a
1
2
3
4
5
6
7
8
8
AgNO3
N2
air
air
air
N2
air
O2
O2
O2
N2
O2
rt
rt
rt
12
12
1
15
12
15
5
5
9
12
12
NR
NR
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
c
65
24
58
60
60
60
40
60
60
60
60
38
69
44
69
77
9
NR
NR
d
1
0
AgNO3
a
Conditions: 1a (1.0 equiv), 2f (2.0 equiv), catalyst (20 mol %) in
b
1
c
solvent (0.1 M). H NMR yields of products. DBU (1.5 equiv) was
added. TEMPO (2.0 equiv) was added. NR = no reaction.
d
approaches to produce either linear product 3f or branched
product 4a as the dominant product. Intrigued by this
interesting transformation, we screened reaction media and
catalysts to optimize the reaction outcome. We reasoned that
the molecular oxygen is necessary as it oxidizes the phosphonyl
reactant. The best result was obtained when toluene was used
as the solvent. Next, we studied the influence of temperature
on the reaction outcomes, and the desired product was
produced in 44 and 69% yields when the reaction mixture was
heated at 40 and 60 °C, respectively (entries 7 and 8).
Mechanistically, it can be postulated that phosphate anions
generated in situ via the oxidation of the phosphonyl functional
groups by the Ag catalysts subsequently engage in an addition
to vinyl aziridine 1a in an S 2 ring-opening reaction through
N
3
the direct displacement of the C(sp )−N bond.
With a divergent strategy for accessing branched-type
products, we explored the substrate scope of the reaction. As
summarized in Scheme 4, a wide range of phosphonates
underwent the desired cross-coupling reaction to yield the
corresponding products (4b−f). In addition, the catalytic
system was applicable to di-p-tolylphosphine oxide, which
afforded desired product 4g. Next, we sought to demonstrate
the utility of this methodology by further exploring its practical
applicability in stereospecific synthesis of phosphorus-contain-
ing amino acid derivatives that are synthetically and bio-
logically important. Notably, the respective chiral trans-vinyl
aziridine esters could be subjected to these reaction conditions
and effectively converted to phosphate-containing chiral amino
acid products 4h, 4i, and 4j, and the formation of only a single
stereoisomer was ensured by chiral HPLC analysis of 4h
(>99% ee, see the SI for details).
To elucidate the reaction mechanisms of these two divergent
synthetic transformations, several control experiments were
conducted as shown in Scheme 5. To determine the possibility
of a radical pathway in the phosphonation, diene 5 was
subjected to the standard reaction conditions. Indeed, 5
underwent a cascade reaction under the optimized conditions
and gave cyclized product 6 by capturing the radical species
(Scheme 5a). Next, we evaluated our hypothesis that the use of
a
Conditions: 1 (1.0 equiv), 2a (2.0 equiv), and Cu(OAc) (20 mol
2
%
) in toluene (0.2 M) at rt under air atmosphere. Yields of isolated
b
products. Instead of Cu(OAc) , AgNO (20 mol %) was used in
2
3
MeCN (0.1 M) for 4 h.
single-crystal X-ray diffraction analysis (see the Supporting
15
Information (SI) for details). Similarly, chiral phosphonated
amino acid derivatives 3w and 3x were formed in a selective
manner. These results indicate that the current catalytic system
is stereospecific and operates with retention of stereochemical
information.
During these investigations, we discovered that branched
phosphate product 4a was formed as a major product in a site-
selective manner when an Ag catalyst was employed under an
O atmosphere at 60 °C (Table 2). We speculated that these
results might offer a unique opportunity to develop divergent
2
C
DOI: 10.1021/acs.orglett.8b03309
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Organic Letters
Letter
a
Scheme 4. Substrate Scope of S 2 Phosphatation
N
Figure 1. Plausible mechanism of divergent ring-opening of vinyl
aziridine.
aziridine nitrogen to the Cu or Ag catalyst provides complex A.
Subsequent regioselective addition of phosphinoyl radical B to
vinylaziridine complex A delivers linear product 3. In the
oxidative catalytic cycle, the phosphate anion is generated via
the Ag-catalyzed oxidation of the phosphonyl group, and the
subsequent aziridine ring-opening takes place at the most
electrophilic carbon in an S 2 fashion to afford phosphated
N
branched-type product 4.
In summary, we have developed a product-switchable
catalytic ring-opening cross-coupling of vinyl aziridines and
phosphorus reactants to access valuable phosphorus-containing
amino compounds. Importantly, the regioselective divergency
is controlled by the choice of reaction atmosphere (O vs N )
a
Conditions: 1 (0.1 equiv), 2 (2.0 equiv), and AgNO (20 mol %) in
3
toluene (0.1 M) at 60 °C under O atmosphere for 11−17 h. Yields of
isolated products. AgNO (30 mol %) and 2 (3.0 equiv) were used.
2
b
3
2
2
Scheme 5. Control Experiments and NBO Charge Analysis
of Vinyl Aziridines
and temperature. This divergent strategy offers a convenient
and powerful synthetic tool for accessing optically active
phosphorus-containing amino acid derivatives with potential
applications in the construction of focused chemical libraries of
medicinally relevant compounds.
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b03309.
Experimental procedures and characterization of new
1
13
31
compounds ( H, C, and P NMR spectra) (PDF)
Accession Codes
CCDC 1872730, 1872732, 1872880, 1872886−1872887, and
873505 contain the supplementary crystallographic data for
1
this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
the AgNO catalyst promotes the formation of phosphate
3
anions from phosphonyl reactants and investigated the
proposed two-step sequence: (1) the generation of phosphate
anions and (2) the addition of the resulting phosphate anions
to vinyl aziridine. Indeed, phosphate anions were detected
under the optimized conditions in the absence of a vinyl
aziridine substrate. In addition, we observed the formation of
branched product 4a when H-phosphonate 2g was used in the
presence or absence of the Ag catalyst (Scheme 5c). Based on
the analysis of NBO charges of vinyl aziridines, in which the
C3 position is notably positive, we suggest that the
regioselectivity of ring opening of the vinyl aziridines could
be rationalized by the electronic properties of the vinyl
AUTHOR INFORMATION
Corresponding Author
■
*
E-mail: hongorg@kaist.ac.kr.
ORCID
Sungwoo Hong: 0000-0001-9371-1730
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
12
aziridine and the incoming nucleophile (Scheme 5d).
On the basis of the above experimental results, a plausible
mechanism is proposed in Figure 1. Coordination of the
This research was supported financially by the Institute for
Basic Science (IBS-R010-G1). We thank Dr. Dongwook Kim
(IBS) for XRD analysis.
D
DOI: 10.1021/acs.orglett.8b03309
Org. Lett. XXXX, XXX, XXX−XXX

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