Synthesis of chiral N-phosphoryl aziridines through enantioselective aziridination of alkenes with phosphoryl azide via Co(II)-based metalloradical catalysis

Article abstract of DOI:10.3762/bjoc.10.129

The Co(II) complex of a new D₂-symmetric chiral porphyrin 3,5-DiMes-QingPhyrin, [Co(P6)], can catalyze asymmetric aziridination of alkenes with bis(2,2,2-trichloroethyl)phosphoryl azide (TcepN₃) as a nitrene source. This new Co(II)-based metalloradical aziridination is suitable for different aromatic olefins, producing the corresponding N-phosphorylaziridines in good to excellent yields (up to 99%) with moderate to high enantioselectivities (up to 85% ee). In addition to mild reaction conditions and generation of N₂ as the only byproduct, this new metalloradical catalytic system is highlighted with a practical protocol that operates under neutral and non-oxidative conditions.

Full text of DOI:10.3762/bjoc.10.129

Synthesis of chiral N-phosphoryl aziridines through
enantioselective aziridination of alkenes
with phosphoryl azide via Co(II)-based
metalloradical catalysis
Jingran Tao, Li-Mei Jin and X. Peter Zhang*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2014, 10, 1282–1289.
Department of Chemistry, University of South Florida, Tampa, Florida
33620, USA
doi:10.3762/bjoc.10.129
Received: 29 January 2014
Accepted: 24 April 2014
Published: 04 June 2014
Email:
X. Peter Zhang* - xpzhang@usf.edu
* Corresponding author
This article is part of the Thematic Series "Organophosphorus chemistry".
Guest Editor: P. R. Hanson
Keywords:
asymmetric aziridination; aziridine; chiral porphyrin; cobalt complex;
metalloradical catalysis; organophosphorus; phosphoryl azide
© 2014 Tao et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
The Co(II) complex of a new D2-symmetric chiral porphyrin 3,5-DiMes-QingPhyrin, [Co(P6)], can catalyze asymmetric aziridina-
tion of alkenes with bis(2,2,2-trichloroethyl)phosphoryl azide (TcepN3) as a nitrene source. This new Co(II)-based metalloradical
aziridination is suitable for different aromatic olefins, producing the corresponding N-phosphorylaziridines in good to excellent
yields (up to 99%) with moderate to high enantioselectivities (up to 85% ee). In addition to mild reaction conditions and generation
of N2 as the only byproduct, this new metalloradical catalytic system is highlighted with a practical protocol that operates under
neutral and non-oxidative conditions.
Introduction
Aziridines, the smallest three-membered nitrogen-containing the abundance of both alkenes and nitrene sources [9-12]. The
heterocycles, are highly valuable heterocyclic compounds that enantioselective olefin aziridination is of particular significance
are widely used in organic synthesis and pharmaceuticals [1,2]. due to the streamlined approach for the installation of chiral
As a result, tremendous efforts have been made for the aziridines, which are versatile intermediates in organic syn-
construction of this class of nitrogen-containing three- thesis. To date, several different types of transition metal-based
membered ring compounds [3-8]. Among synthetic methodolo- chiral catalysts, such as Mn, Fe, Cu, Rh, Ru and Co complexes,
gies, catalytic aziridination of alkenes with nitrene sources via have been demonstrated as effective catalysts in asymmetric
“C2 + N1” addition has received the most attention because of olefin aziridination with various nitrene sources, including the
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Beilstein J. Org. Chem. 2014, 10, 1282–1289.
widely used iminoiodanes and their in situ variants, chloramine- new Co(II)-based metalloradical catalytic system by employing
T, bromamine-T, tosyloxycarbamates and organic azides [9-15]. D2-symmetric chiral amidoporphyrins as the supporting ligands
Among them, the organic azides have recently emerged as [28]. It was shown that the chiral metalloradical catalyst
attractive alternative nitrene sources for metal-catalyzed aziridi- [Co(P1)] (P1 = 3,5-DitBu-ChenPhyrin) could catalyze the for-
nation because of many advantages such as ease of preparation, mation of optically enriched phosphoryl aziridine through direct
structural diversity, and N2 gas as the only byproduct [13-15]. aziridination of alkenes with DPPA (Scheme 2) [21]. While this
While sulfonyl and aryl azides have been effectively employed [Co(P1)]/DPPA catalytic system represented the first asym-
for metal-catalyzed asymmetric aziridination [16-19], the metric version of olefin aziridination with phosphoryl azide,
catalytic system based on other types of azides, such as phos- both the yields and enantioselectivities were moderate even
phoryl azides, remains underdeveloped.
using 10 mol % catalyst loading. It would be desirable if a more
effective Co(II)-based metalloradical system could be devel-
Phosphoryl azides, a family of common organic azides that can oped for asymmetric aziridination of alkenes with phosphoryl
be directly synthesized from commercially available phos- azides with both improved reactivity and enantioselectivity.
phoryl chlorides, have been recently explored as nitrene sources
for transition metal-catalyzed nitrene transfer reactions [20-23]. The stable 15e-metalloradicals Co(II) complexes of
Their use in a catalytic asymmetric aziridination would provide D2-symmetric chiral amidoporphyrins ([Co(D2-Por*)] repre-
an attractive approach for the synthesis of valuable chiral phos- sent a new type of chiral catalysts that have been demonstrated
phorous-containing aziridines, producing nitrogen gas as the to be effective for asymmetric olefin aziridination using
only and also environmentally friendly byproduct. Chiral phos- different types of nitrene sources, particularly with sulfonyl and
phorylated aziridines and their derivatives have been demon- aryl azides [16,18]. Computational and experimental studies
strated with pharmaceutical and other important synthetic appli- have provided increasing evidences to suggest a stepwise
cations. In addition to the fundamental and practical signifi- radical mechanism for the Co(II)-catalyzed metalloradical
cance of the phosphorous-containing aziridines, the easy depro- aziridination that involves an unprecedented Co(III)–nitrene
tection of phosphoryl groups makes them even more syntheti- radical intermediate [29-34]. It is worthy to note the impor-
cally useful [24-27]. However very few catalytic systems are tance of dual functions of the chiral amide units of the
available for the direct asymmetric olefin aziridination with D2-symmetric chiral amidoporphyrin ligands played in the
phosphoryl azides. In this regard, our group initially reported in Co(II)-based metalloradical catalysis (MRC): the rigid amide
2006 a racemic olefin aziridination system with diphenylphos- spacers do not only support and orient the chiral environments
phoryl azide (DPPA) using Co(II) complexes of common por- toward the cobalt metalloradical center, but also function as
phyrin ligands as catalysts, including [Co(TPP)] (Scheme 1) potential donors to engage in hydrogen bonding with acceptors
[20]. Despite the first demonstration of DPPA as a new nitrene located at the nitrene moiety in the Co(III)–nitrene radical inter-
source, this Co(II)-based catalytic transformation, however, mediate [18,35,36]. These secondary hydrogen bonding interac-
suffered from low-yielding formation of the desired aziridine tions are expected to lower the energy barrier of the transition
products. To improve the efficiency and control the enantio- state and thus lead to acceleration of the reaction rate as well as
selectivity of the nitrene transfer process, we then developed a improvement of the stereoselectivity [18,29]. Given that the
Scheme 1: [Co(TPP)]-catalyzed olefin aziridination with DPPA.
Scheme 2: [Co(P1)]-catalyzed asymmetric olefin aziridination with DPPA.
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P=O group can serve as a potential hydrogen bond acceptor, we With this assumption in mind, we have carried out a systematic
hypothesized that the resulting Co(III)–nitrene radical inter- study to identify more effective phosphoryl azides and to
mediate from activation of phosphoryl azides would benefit employ Co(II) complexes of suitable D2-symmetrical chiral
from a similar hydrogen bonding interaction (Figure 1).
porphyrin ligands ([Co(Por*)]) (Figure 2) for the development
of Co(II)-based asymmetric aziridination via MRC to improve
Figure 1: (A) Potential H-bonding interaction in postulated nitrene radical complex of [Co(D2-Por*)]. R* represents a chiral unit. (B) Geometry corres-
ponding to the minimum energy from simplified computer modeling by molecular mechanics with Spartan 10. The P=O…H–N distance of (1.87 Å)
suggests possible existence of significant hydrogen bonding interaction. However, besides computer modeling there is no direct experimental evi-
dence for such interactions. For clarity, the other two meso-groups of the porphyrin are omitted in both (A) and (B). In addition, the other two amido
units below the porphyrin ring are also omitted in (B).
Figure 2: Structures of D2-symmetric chiral cobalt(II) porphyrins.
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Beilstein J. Org. Chem. 2014, 10, 1282–1289.
the reactivity and selectivity. As the result of this study, herein 5,10,15,20-tetraphenylporphyrin) as catalyst to search for a
we wish to report an effective catalytic system for asymmetric more effective phosphoryl azide (Table 1). In the presence of
olefin aziridination based on the use of bis(2,2,2-tri- 10 mol % of [Co(TPP)], the phosphoryl azides 2a–c were found
chloroethyl)phosphoryl azide (TcepN3) as nitrene source and to be ineffective nitrene sources for the catalytic reaction, with
the employment of new generation of chiral Co(II) metallorad- no detectable aziridine product but remaining of the starting
ical catalysts. The aziridination via Co(II)-based MRC is applic- azides (Table 1, entries 1–3). It should be noted that azide 2c
able for a broad range of aromatic olefins, producing the corres- was previously shown to be a productive nitrene source for the
ponding N-phosphorylated aziridines in good to excellent yields catalytic aziridination reaction only at a high temperature of
with moderate to high enantioselectivities. In addition to gener- 80 °C [20]. Afterwards, we were pleased to find that the phos-
ating N2 as the only byproduct, the new metalloradical aziridi- phoryl azide bis(2,2,2-trichloroethyl)phosphoryl azide (TcepN3,
nation process is highlighted by a practical protocol that oper- 2d) was an effective nitrene source even at low temperature.
ates under neutral and non-oxidative reaction conditions For instance, at 40 °C, styrene could be aziridinated with the
without the need of any additives.
phosphoryl azide TcepN3 in low but significant yield when
using [Co(TPP)] as the catalyst (Table 1, entry 4). Subsequent
experiments showed that Co(II) complexes of D2-amidopor-
Results and Discussion
Our initial study was focused on the aziridination reaction of phyrin ligands (Figure 2) were more effective catalysts to acti-
styrene (1a) as a model reaction and [Co(TPP)] (TPP = vate TcepN3 for the aziridination reaction. For example, under
Table 1: Optimization of catalytic aziridination of styrene with phosphoryl azides by Co(II)-based metalloradical catalysts.a
entry
1d
R–N3
catalyst
solvent
PhCl
yield (%)b
0
ee (%)c
–
[Co(TPP)]
2a
2d
[Co(TPP)]
PhCl
0
–
2b
2c
3d
4d
[Co(TPP)]
[Co(TPP)]
PhCl
PhCl
0
–
–
11
2d
2d
2d
2d
2d
2d
2d
2d
2d
2d
5
[Co(P1)]
[Co(P2)]
[Co(P3)]
[Co(P4)]
[Co(P5)]
[Co(P6)]
[Co(P6)]
[Co(P6)]
[Co(P6)]
PhCl
PhCl
PhCl
PhCl
PhCl
PhCl
PhCF3
C6H6
C6H6
77
<5
99
7
53
nd
40
65
81
75
77
81
82
6
7
8
9
23
98
75
98
99
10
11
12
13e
aReaction conditions: 2 mol % of catalyst; olefin:azide = 5:1; [azide] = 0.1 M. bIsolated yield. cDetermined by chiral HPLC. d10 mol % of catalyst. eat
35 °C in 36 h.
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Beilstein J. Org. Chem. 2014, 10, 1282–1289.
the similar conditions, the reaction catalyzed by [Co(P1)] (P1 = Accordingly, we designed and synthesized a new D2-symmetric
3,5-DitBu-ChenPhyrin) [28], gave the desired aziridine in 77% amidoporphyrin 3,5-DiMes-QingPhyrin (P6), whose Co(II)
yield and 53% ee even using only 2 mol % of catalyst loading complex [Co(P6)] was shown to be the optimal catalyst for this
(Table 1, entry 5). The dramatic difference observed in the reaction, producing the desired aziridine in 98% yield and 75%
catalytic performance between [Co(P1)] and [Co(TPP)] is in ee using only 2 mol % of catalyst loading (Table 1, entry 10).
accordance with N-H…O=P hydrogen bonding which we After screening various solvents, it was found that benzene was
assume to play an important role in activating the phosphoryl the solvent of choice for the catalytic process, giving the desired
azide (Figure 1). Further studies showed that the Co(II) com- product with high enantioselectivity (81% ee) while main-
plex of the more sterically hindered amidoporphyrin ligand taining the excellent yield (98%) (Table 1, entries 10–12). Some
[Co(P2)] (P2 = 2,6-DiMeO-ChenPhyrin) gave almost no reac- reduction in reaction temperature (from 40 °C to 35 °C) and
tion (Table 1, entry 6), signifying the steric demand of the time (from 48 h to 36 h) was shown to have no obvious effect
catalytic process. Gratifyingly, [Co(P3)], in which the 3,5-posi- on both product yield and enantioselectivity (Table 1, entry 13).
tions of the meso-phenyl rings of the porphyrin were installed However, the catalytic reaction became significantly slower as
with mesityl groups, was found extremely effective in the temperature further decreased.
catalyzing this olefin aziridination reaction with TcepN3,
producing the desired aziridine product in almost quantitative Under the optimized reaction conditions, we then investigated
yield although with lower enantioselectivity (Table 1, entry 7). the scope and limitation of the [Co(P6)]/TcepN3-based catalytic
Further improvement in enantioselectivity was achieved when system for asymmetric olefin aziridination. The Co(II)-
[Co(P4)] (P4 = 3,5-DitBu-Xu(2’-Naph)Phyrin), a second-gener- catalyzed asymmetric aziridination was shown to be effective
ation MRC catalyst that was previously shown to be optimal for for a variety of styrene derivatives with varied electronic and
asymmetric olefin aziridination with aryl azides [18], was used steric properties (Table 2). Similar to styrene, the styrene
as a catalyst, reaching 65% ee but in a poor yield (Table 1, entry derivatives with electron-donating groups, such as the para-
8). To our delight, the use of catalyst [Co(P5)] (P5 = 3,5-DitBu- methylated styrene 1b could be effectively aziridinated to afford
QingPhyrin), which was shown to be effective in asymmetric the corresponding N-phosphoryl aziridine in a high yield with
intramolecular olefin cyclopropanation [37], led to significant good enantioselectivity (Table 2, entries 1 and 2). In addition to
further improvement in enantioselectivity to 81% ee although the electron-rich aromatic olefins, styrenes with electron-defi-
the yield for the aziridination reaction with TcepN3 remained to cient substituents at various positions were found to be suitable
be low (Table 1, entry 9). These studies on the relationship substrates as well for the Co(II)-based asymmetric aziridination.
between catalytic reactivity and the porphyrin ligand structure For instance, the meta-nitro-substituted styrene 1c could be
indicate the importance of both the chiral amido units and the aziridinated in a moderate yield and good enantioselectivity
non-chiral substituents of the porphyrin ligand in influencing (Table 2, entry 3). Interestingly, when the nitro group is located
the catalytic performance of the Co(II) metalloradical center. at the para-position as in the case of olefin 1d, the corres-
Table 2: Enantioselective aziridination of olefins with TcepN3 catalyzed by [Co(P6)].a
entry
olefin
1a
aziridine
yield (%)b
99
ee (%)c
82
1
3a
3b
2d
86
66
76
66
1b
3
1c
3c
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Table 2: Enantioselective aziridination of olefins with TcepN3 catalyzed by [Co(P6)].a (continued)
4
90
23
–
1d
3d
5e
98
64
1e
3e
6
48
1f
3f
7
98
74
98
85
74
72
1g
3g
8d
1h
3h
9
1i
3i
10d
85
99
66
85
1j
3j
11
1k
3k
aReaction conditions: 2 mol % of catalyst in the presence of 4 Å MS; olefin:azide = 5:1; [azide] = 0.1 M, benzene as solvent; 35 °C in 36 h. bIsolated
yield. cDetermined by chiral HPLC. dAt 40 °C in 48 h. eThe enantiomers could not be resolved.
ponding aziridine was produced in an excellent yield but in low with 85% ee (Table 2, entry 7). Like p-fluorostyrene, the
enantioselectivity (Table 2, entry 4). An excellent yield was p-chlorostyrene (1h) and p-bromostyrene (1i) were also effec-
also achieved for the catalytic aziridination reaction of the steri- tive substrates for the metalloradical aziridination system,
cally hindered substrate o-trifluoromethylstyrene (1e) (Table 2, forming the corresponding chiral aziridines in good yields and
entry 5). When p-trifluoromethylstyrene (1f) was used as the enantioselectivities (Table 2, entries 8 and 9). In addition to
substrate, however, a decrease in reaction yield was observed p-bromostyrene, both m-bromostyrene (1j) and o-bromostyrene
(Table 2, entry 6). Furthermore, [Co(P6)] could effectively (1k) could also be productively aziridinated (Table 2, entries 10
catalyze the aziridination reactions of various halogenated and 11). Similar to the case of o-CF3-subtituted styrene 1e
styrenes. For example, under similar conditions, p-fluoro- (Table 2, entry 5), the catalytic reaction of the sterically
styrene (1g) could be aziridinated with TcepN3 in 98% yield demanding o-Br-substituted olefin 1k gave the desired aziri-
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In summary, we have shown that the Co(II) complex of the new
D2-symmetric chiral porphyrin 3,5-DiMes-QingPhyrin,
[Co(P6)], is an effective metalloradical catalyst for asymmetric
olefin aziridination with bis(2,2,2-trichloroethyl)phosphoryl
azide (TcepN3) as a new nitrene source. This [Co(P6)]/TcepN3-
based new aziridination system, which can be operated under
neutral and non-oxidative conditions without the need of any
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enantioenriched N-phosphorylaziridines may find potential
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Supporting Information File 1
Experimental procedures and characterization data. Copies
of 1H, 13C, and 31P NMR spectra and HPLC data for all
new compounds.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-10-129-S1.pdf]
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