Regiospecific and highly stereoselective synthesis of β-amino (Z)-enylphosphonates via β-hydrogen migration reaction of dialkyl α-diazophosphonates catalyzed by AgOTf

Article abstract of DOI:10.1039/c4ra02520b

A series of dialkyl α-diazophosphonates bearing different substituents have been prepared from natural amino acids in order to investigate the steric effect in 1,2-migration reaction of metal carbenes. The diazo decomposition of the diazophosphonates using the AgOTf/NaBAr_F complex resulted in β-hydrogen migration to give β-amino (Z)-enylphosphonates in good yields with high regio- and stereoselectivity. A possible reaction mechanism shows that the steric effect could dramatically influence the geometric isomerism aptitude. This new method for constructing (Z)-β-amino vinylphosphonates should be of general utility in organic synthesis. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra02520b

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Regiospecific and highly stereoselective synthesis
of b-amino (Z)-enylphosphonates via b-hydrogen
migration reaction of dialkyl a-diazophosphonates
catalyzed by AgOTf†
Cite this: RSC Adv., 2014, 4, 21492
Received 22nd March 2014
Accepted 2nd May 2014
*
Yan Cai, Haihong Ge, Chengbin Yu, Weize Sun, Junchen Zhan and Zhiwei Miao
DOI: 10.1039/c4ra02520b
www.rsc.org/advances
A series of dialkyl a-diazophosphonates bearing different substituents 1,2-hydride migration products as the major products,
have been prepared from natural amino acids in order to investigate depending on the reaction conditions.9
the steric effect in 1,2-migration reaction of metal carbenes. The diazo
In contrast to that found in a-diazo carbonyl compounds,
decomposition of the diazophosphonates using the AgOTf/NaBArF a-diazophosphonyl compounds have not been studied system-
complex resulted in b-hydrogen migration to give b-amino (Z)-enyl- atically in metal carbene reactions.10 Recently, we reported a
phosphonates in good yields with high regio- and stereoselectivity. A kind of novel a-diazophosphonyl compounds prepared from
possible reaction mechanism shows that the steric effect could natural amino acid, which could afford b-alkoxy substituted
dramatically influence the geometric isomerism aptitude. This new b-amino phosphonates derivatives through a combined C–H
method for constructing (Z)-b-amino vinylphosphonates should be of functionalization/O–H insertion process (Scheme 1, eqn (1)).11
general utility in organic synthesis.
As a natural extension of the diazo compounds 1,2-hydride
migration reaction, we developed a stereoselective [Cu(MeCN)4]
PF6/I2 catalyzed b,g-dihydrogen shi reaction for the synthesis
of (Z)-b-alkenyl substituted b-aminophosphonates (Scheme 1,
eqn (2)).12
b-Aminophosphonates are the phosphorus analogues of
b-amino acids, and therefore have widely used for biological
and pharmaceutical applications, such as enzyme inhibitors,
Diazo compounds are commonly used important precursors of
metal carbenes, which can subsequently undergo diverse
chemical transformation.1 Numerous efficient transformation
such as cyclopropanations,2 X–H (X ¼ C, Si, O, N, S, etc.)
insertions,3 cycloadditions4 and ylide transformations5 have
been developed, as well as studies on reactivities and mecha-
nisms of the involved transition-metal carbenoids.6 In the
catalytic reaction of a-diazo carbonyl compounds with both
metal carbene and free carbene as reactive intermediates, 1,2-
hydrogen migration is a frequently encountered reaction, which
referred to as b-hydride shi.7 In some cases, 1,2-hydrogen
migration can nd useful application in organic synthesis. For
example, Taber and co-workers have shown that a-diazo esters
undergo b-hydride elimination with rhodium(II) triuoroacetate
dimer produce (Z)-a,b-unsaturated esters in good yields.8 Wang
has found that the diazo decomposition of b-(N-tosyl)amino
diazo carbonyl compounds gives either 1,2-aryl migration or
State Key Laboratory of Elemento-Organic Chemistry, Institute of Elemento-Organic
Chemistry, Nankai University, Collaborative Innovation Center of Chemical Science
and Engineering (Tianjin), Nankai University, State Key Laboratory of Medicinal
Chemical Biology (Nankai University), Tianjin 300071, P. R. China. E-mail:
miaozhiwei@nankai.edu.cn; Fax: +86 22-2350-2351; Tel: +86 22-2350-4783
† Electronic supplementary information (ESI) available: Experimental section,
characterization of all compounds, and copies of 1H and 13C NMR spectra for
selected compounds. CCDC 893994. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c4ra02520b
Scheme 1 Previous and proposed work.
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agrochemicals, or antivirus activities.13 b-Amino vinyl- phosphonate 3a in low yield (Table 1, entry 1). CuOTf and
phosphonates can be regarded as isosters of b-amino- Rh2(OAc)4 could only decompose 1a with low yields and obtain
phosphonates. These compounds can be prepared via 2a and 3a with poor stereoselectivities (Table 1, entries 2 and 3).
formation of C–C-, C–N-, and C–P-bonds.14 For example, Pala- Hg(OTf)2 could not decompose 1a at all and the starting mate-
cios reported the preparation of uoroalkyl b-enamino- rial recovered (Table 1, entry 4). To our delight AgOTf could
phosphonates from alkylphosphonates and peruoroalkyl promote the reaction smoothly with higher yield and good
nitriles (Scheme 1, eqn (3)).14a Ionin also reported the addition stereoselectivity (Z/E ¼ 73 : 27) (Table 1, entry 5). When the
reaction of secondary amines to alkynylphosphonates catalyzed reaction proceeded in MTBE, the Z/E ratio will increase to
by Cu(I) salts to form (E)-b-enaminophosphonates (Scheme 1, 76 : 24 (Table 1, entry 6). To further improve the reactivity and
eqn (4)).14b In spite of these results up to now there is only one stereoselectivity, the effects of additive and solvent were inves-
example of the synthesis of b-enaminophosphonates from the tigated. In recent years the use of weakly or noncoordinating
corresponding a-diazoethylphosphonates underwent an exclu- anions as counter anions is of signicant interest in both
sive 1,2-aryl shi to form the enamine products (Scheme 1, eqn synthesis and catalysis.16 The lack of reactivity and non-nucle-
(5)).14c Continuing with our interest in the chemistry of ami- ophilic character of [BArF]ꢀ have led to the widespread use as
nophosphorus derivatives,15 here we report the rst example for noncoordinating.17 In our investigation, aer screening NaBArF,
converting dialkyl a-diazophosphonates into b-amino (Z)-enyl- it was found that PhCONH2, CH3CONH2, t-BuOH, DMF and
phosphonates in a regiospecic and highly stereoselective PhOH could not give superior results in terms of reactivity and
manner (Scheme 1, eqn (6)).
stereoselectivity (Table 1, entries 7–12).
Initially, the amino acid derived a-diazophosphonyl
With the best AgOTf catalyst combined with NaBArF as
compound 1a11 was the rst substrate studied to examine the additive (Table 1, entry 7), we next carried out the reaction in
effect of the catalysts on the reaction, and the results were different solvents to determine the best solvent for this reaction.
summarized in Table 1. The results revealed that Cu(MeCN)4PF6 Among the various solvents tested, diethyl ether, toluene, iso-
decomposed
1a
mainly
afforded
(E)-a,b-unsaturated propyl ether, anisole and acetone afforded lower yields of the
Table 1 Optimization of the reaction conditionsa
Entry
Catalysts
Solvent
Additive
Z/E ratiob (2a : 3a)
Overall yieldc (%)
1
2
3
4
5
6
7
8
Cu(MeCN)4PF4
CuOTf
Rh2(OAc)4
Hg(OTf)2
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
MTBEd
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
MTBE
Et2O
PhCH3
i-Pr2O
PhOMe
Acetone
MTBE
—
—
—
—
—
—
7 : 93
38
11
15
N.R.
64
65
77
63
71
75
65
70
73
51
61
60
51
45
75
73
77
63 : 37
23 : 77
—
73 : 27
76 : 24
63 : 37
64 : 36
61 : 39
60 : 40
69 : 31
62 : 38
78 : 22
86 : 14
78 : 22
80 : 20
80 : 20
73 : 27
80 : 20
75 : 25
80 : 20
e
NaBArF
PhCONH2
f
g
9
CH3CONH2
t-BuOHh
DMFi
10
11
12
13
14
15
16
17
18
19k
20l
21m
PhOHj
NaBArF
NaBArF
NaBArF
NaBArF
NaBArF
NaBArF
NaBArF
NaBArF
NaBArF
AgOTf
AgOTf
AgOTf
AgOTf
MTBE
MTBE
a
Unless otherwise noted, all reactions were carried out using a-diazophosphonate 1a (0.28 mmol, 1 equiv.) in 2 mL solvent with 5 mol% of catalyst
and 6 mol% additive in 2 mL solvent at 25 ꢁC for 7.5 h (before addition 1.5 h, aer addition 6 h). b The product ratio was determined by 31P NMR of
the crude product. c Overall yield of the mixture of 2a and 3a aer silica gel chromatograph. d MTBE ¼ methyl tert-butyl ether. e NaBArF ¼ sodium
f
g
h
i
tetrakis[3,5-bis(triuoromethyl)-phenyl]borate. 50 mol% PhCONH2 was used. 50 mol% CH3CONH2 lwas used. 5eq. t-BuOH was used. 5 eq.
j
k
DMF was used. 5 eq. PhOH was used. 4 mL MTBE was used to dissolve a-diazophosphonate 1a. 2 mol% AgOTf and 3 mol% NaBArF was
used. m 10 mol% AgOTf and 11 mol% NaBArF was used.
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expected products 2a and 3a with moderate stereoselectivities signicant amount of Z-isomers 2c–e was formed (Table 2,
(Table 1, entries 14–18). The most suitable solvent was found to entries 3–5).
be methyl tert-butyl ether (MTBE), (Z)-diethyl 2-(1,3-dioxoi-
It was found that the Z/E isomer selectivity of 2 and 3
soindolin-2-yl)prop-1-enylphosphonate 2a was obtained in good impacted by the size of the R1 and R2 groups of a-diazo-
yield and stereoselectivity (Table 1, entry 13). With further phosphonates 1. With the increasing bulk of R1 group, the Z/E
optimization of the reaction conditions, we found that ratio of 2 and 3 ranged from 80 : 20 to 98 : 2 with moderate to
increasing the solvent amount to 6 mL, the Z/E ratio could good yields (Table 2, entries 1–5). In cases where the substituent
increase to 80 : 20 with good yield (Table 1, entry 19). Further- groups on b-position of dialkyl a-diazophosphonates change to
more, a decrease in the catalyst loading to 2 mol% of AgOTf and isopropyl and isobutyl groups, no desired products 2f–g and 3f–
3 mol% of NaBArF led to a decrease in yield and stereoselectivity g obtained due to the poor solubility of substrate 1f–g in MTBE.
(Table 1, entry 20). Similar result was obtained when the reac- When use CH2Cl2 as solvent without the add of NaBArF, a
tion was performed in MTBE with increasing the catalyst signicant amount of Z-isomers 2f–g were formed with good
loading to 10 mol% of AgOTf and 11 mol% of NaBArF (Table 1, yields (Table 2, entries 6 and 7). It is worthwhile to note that
entry 21). Thus, the optimal reaction conditions for this trans- diethyl a-diazophosphonate 1h which derived from methionine
formation were determined to be 0.28 mmol a-diazo- can not proceed this reaction and get the desired product (Table
phosphonate 1a, 5 mol% of AgOTf as catalyst and 6 mol% of 2, entry 8).
NaBArF as additive in 6 mL MTBE as solvent at room tempera-
ture. The two stereoisomers 2a and 3a could be separated aer set out to study reactions of a series of dialkyl a-diazo-
purication. phosphonates 1i–n under AgOTf/NaBArF catalytic condition.
To access the effect of substrates on product selectivity, we
Based on the above optimization efforts, the substrate scope The migratory product aptitude was dependent upon the size of
of this reaction was investigated (Table 2). The impact of the R3 group. When the bulk of R3 group was increased from
substituent groups on b-position of dialkyl a-diazo- methyl to butyl, the Z/E isomer products 2 and 3 were obtained
phosphonates 1 which derived from different natural amino with lower yields. Interestingly, when the reaction proceeding in
acids was evaluated. The tested a-diazophosphonates 1a–b with CH2Cl2 without the add of NaBArF, the low yields could be
different substituents on b-position, such as methyl and iso- improved with some changes for the Z/E isomer selectivities
butyl groups afforded good level yields of b-amino-a,b-unsatu- (Table 2, entries 9–14). The structure of 2a was conrmed by
rated phosphonates 2a–b in favor of the Z-isomer (Table 2, single crystal X-ray diffraction (see the ESI† for details).18
entries 1 and 2). In cases where the substituent groups on
The Z/E stereoselectivity of b-amino-a,b-unsaturated phos-
b-position of dialkyl a-diazophosphonates 1 change to benzyl, phonates implies that conformational factors may play a role in
2-propylisoindoline-1,3-dione, and p-tolyl acetate groups, a the migration process. For the migration to occur, it is necessary
that the migrating bond needs to be parallel to the p orbital of
Table 2 Scope of the reactiona
Entry
Substrate
R1
R2
R3
Z/E ratiob (2 : 3)
Overall yieldc (%)
1
2
3
4
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
H
H
H
H
H
Et
Et
Et
Et
Et
Et
Et
Et
Me
i-Pr
n-Bu
Me
i-Pr
n-Bu
80 : 20
80 : 20
93 : 7
91 : 9
98 : 2
100 : 0
100 : 0
—
75
87
87
62
63
76
73
CH(CH3)2
Ph
(CH2)3NPhth
p-AcOPh
CH3
CH3
CH2SCH3
H
H
H
Ph
Ph
Ph
5
H
6d
7d
8
CH3
Et
H
H
H
H
H
H
H
N.R.
62 (71)d
71 (72)d
61 (89)d
79
9
90 : 10 (69 : 31)d
78 : 22 (72 : 28)d
72 : 28 (69 : 31)d
90 : 10
10
11
12
13
14
96 : 4 (88 : 12)d
92 : 8 (89 : 11)d
51 (79)d
28 (94)d
a
Reaction conditions: a-diazophosphonate 1 (0.28 mmol) in 6 mL of MTBE at 25 ꢁC in the presence of 5 mol% of AgOTf and 6 mol% of NaBArF for
7.5 h (before addition 1.5 h, aer addition 6 h). b The product ratio was determined by 31P NMR of the crude product and the conguration of 2 was
c
d
assigned as Z by the 1H–1H NOESY spectrum. Overall yield of the mixture of 2 and 3 aer silica gel chromatograph. CH2Cl2 was used as the
solvent without the addition of NaBArF.
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Fig. 1 Conformations leading to b-hydrogen migration.
the carbene carbon in the transition states.19 Of the two
conformations A and B, B is likely to be disfavored because of
steric hindrance between the phosphonate group and the R1
group. Thus, the b-hydrogen migration is proposed to occur via
transition state A which leads to the observed Z-isomer. The
effect is most likely due to the increased steric hindrance
between R1 and the phosphonate group, which affects the
populations and barriers to their interconversion, of the ster-
eoelectronically required conformations for migration. Besides
the steric hindrance inuence, Ag+ participates chelation effect
should not be ruled out. As shown in Fig. 1, Ag+ may coordinate
with phosphonate group and phthalimide group in transition
state A which lead to the formation of Z-isomer. In transition
state B, the chelation effect is lacked, therefore conformation A
is more favored than B.
In conclusion, we have developed a new and convenient
synthesis of b-amino (Z)-enylphosphonates from amino acid-
derived dialkylphosphonates with complete control over regio-
and stereochemistry. The inuence factors on the Z/E isomer
selectivity have been discussed. The investigation demonstrated
that, steric factors play the important role in affecting the
geometric isomerism aptitude in this carbene reaction. Further
researches for extension of this reaction are currently underway
in our laboratory.
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;
(p)
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Acknowledgements
We thank the National Natural Science Foundation of China
(21072102), the Committee of Science and Technology of
Tianjin (11JCYBJC04200) and State Key Laboratory of Elemento-
Organic Chemistry in Nankai University for nancial support.
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