Unexpected stereoselective synthesis of (Z)-β-alkenyl substituted β-amino phosphonates through β,γ-dihydrogen shift reaction catalyzed by a copper(I) complex and iodine [Cu(MeCN)4]PF 6/I2

Article abstract of DOI:10.1002/adsc.201300838

A series of dialkyl a-diazophosphonates has been prepared from natural amino acids. The diazo decomposition of these diazophosphonate compounds with tetrakis(acetonitrile)copper(I) hexafluorophosphate/iodine, [Cu(MeCN) ₄]PF₆/I₂, as catalyst has been investigated. It was found that the diazo decomposition of dialkyl a-diazophosphonates gave a mixture of β,γ-dihydrogen shift and 1,2-hydride migration products and afforded β-alk- enyl-substituted β-amino phosphonates with the Z configuration. The mechanism of this novel diazo decomposition process was discussed.

Full text of DOI:10.1002/adsc.201300838

UPDATES
DOI: 10.1002/adsc.201300838
Unexpected Stereoselective Synthesis of (Z)-b-Alkenyl
Substituted b-Amino Phosphonates through b,g-Dihydrogen
Shift Reaction Catalyzed by a Copper(I) Complex and Iodine
[Cu(MeCN)4]PF6/I2
G
Yan Cai,a Hairong Lyu,a Chengbin Yu,a and Zhiwei Miaoa,*
a
State Key Laboratory and Institute of Elemento-Organic Chemistry, Collaborative Innovation Center of Chemical
Science and Engineering (Tianjin), Nankai University, Weijin Road 94, Tianjin 300071, Peopleꢀs Republic of China
Fax : (+86)-22-2350-2351; e-mail: miaozhiwei@nankai.edu.cn
Received: September 15, 2013; Revised: October 14, 2013; Published online: January 28, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300838.
Abstract: A series of dialkyl a-diazophosphonates
has been prepared from natural amino acids. The
diazo decomposition of these diazophosphonate
compounds with tetrakis(acetonitrile)copper(I)
tomerization.[8] Furthermore, 1,2-hydride migration
promoted by Rh2ACTHNUTRGNEUNG(O2CCF3)4 provides an efficient
route to (Z)-a,b-unsaturated carbonyl compounds[9]
(Scheme 1).
hexa
fluorophosphate/iodine, [Cu
E
In contrast to that found in a-diazo carbonyl com-
pounds, a-diazophosphonyl compounds have not been
studied systematically in metal carbene reactions.[10]
Recently, we reported that tertiary b-alkoxy-substitut-
ed b-amino phosphonate derivatives could be synthe-
sized by treatment of a-diazo phosphonyl compounds,
which are derived from natural amino acids, with al-
cohols catalyzed by tetrakis(acetonitrile)copper(I)
catalyst has been investigated. It was found that the
diazo decomposition of dialkyl a-diazophospho-
nates gave a mixture of b,g-dihydrogen shift and
1,2-hydride migration products and afforded b-alk-
ACHTUNGTRENNUNGenyl-substituted b-amino phosphonates with the Z
configuration. The mechanism of this novel diazo
decomposition process was discussed.
hexafluorophosphate/iodine, [CuACHTNUTRGNEUNG(MeCN)4]PF6/I2, at
room temperature (Scheme 1).[11] The 1,2-hydride mi-
gration product – (Z)-diethyl [2-(1,3-dioxoisoindolin-
2-yl)prop-1-en-1-yl]-phosphonate – was obtained as
a side product. As a natural extension of the 1,2-hy-
dride migration reaction of diazo compounds, we con-
ceived that a-diazo phosphonyl compounds may un-
dergo a similar migration. However, when running
a control experiment in the absence of alcohol, we
Keywords: copper(I); [CuACHTNUGTRNEUNG(MeCN)4]PF6; diazo com-
pounds; a-diazophosphonates; b,g-dihydrogen shift;
1,2-hydride migration; iodine; tetrakis(acetonitrile)-
copper(I) hexafluorophosphate
Diazo compounds are remarkably versatile metal car- unexpectedly observed the b,g-dihydrogen shift prod-
bene precursors which participate in metal-catalyzed uct – a (Z)-b-alkenyl-substituted b-amino phospho-
rearrangements,[1] cycloadditions,[2] X H (X=C, N, O, nate. As a continuation of this investigation, we
À
Si, S, etc.) bond insertions,[3] and 1,2-migration reac- report our detailed study on this unprecedented reac-
tions.[4] Among the 1,2-migration reactions, the 1,2-hy- tion.
dride migration is generally predominant.[5] This mi-
To systematically study the possible 1,2-hydride mi-
gratory preference has also been known to be affect- gration and b,g-dihydrogen shift reaction, (S)-diethyl
ed by steric and conformational factors.[6] Because of [1-diazo-2-(1,3-dioxoisoindolin-2-yl)propyl]phosphon-
their unique properties and extensive applications, the
ACHTUNGTRENaNGNU te 6a was synthesized through the condensation of l-
interest in the chemistry of diazo compounds has alanine 1 and phathalic anhydride. Upon successful
been long standing. b-Substitution of a-diazo carbonyl chlorination with SOCl2 in toluene, 2a afforded acyl
compounds has been found to have a marked influ- chloride 3a. After an Arbuzov reaction of 3a, a-keto
ence on the reaction pathway of diazo decomposi- phosphonate 4a was converted to tosylhydrazone
tion.[7] Exposure of b-substituted diazo compounds to phosphonate 5a and the desired a-diazo phosphonate
Rh
2ACHTUNGTRENNUNG(OAc)4 led to formation of b-keto esters as 6a could be readily obtained by elimination from 5a
a result of 1,2-hydride migration presumably via tau- with Et3N in CH2Cl2 (Scheme 2).[11]
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UPDATES
Unexpected Stereoselective Synthesis of (Z)-b-Alkenyl Substituted
Scheme 1. 1,2-Hydride migration and b,g-dihydrogen shift.
Scheme 2. Synthesis of a-diazo phosphonate 6a.
With the diazo phosphonyl compounds 6 in hand,
From Table 1, it is clear that the diazo decomposi-
we then proceeded to study their reaction with differ- tion could be effected under various reaction condi-
ent kind of the catalysts. a-Diazophosphonyl com- tions and that the ratio of the three products depends
pound 6a was the first substrate studied to examine on the catalysts employed (Table 1, entries 1–8). In
the effect of the catalysts on the reaction, and the re- the presence of TfOH (20 mol%) or BF·Et2O
sults are summarized in Table 1. The results revealed (20 mol%), the diethyl a-diazo phosphonate 6a can
that with iodine (20 mol%) as co-catalyst participate in this reaction in good yield (Table 1, en-
Cu
CuI, TCCu, AgOTf and Rh
mote this reaction. Three products have been isolated three products range from 28 to 65%. The ratio of 7a/ACHTUNG-
A
ACHTUNGRTEN(NGNU OTf)2, tries 9 and 10). It is worthwhile noting that the com-
CTHUNGTRENNUNG
1
or identified in the H NMR spectra, the b,g-dihydro-
TRENN(UG 8a+9a) reflects the migratory aptitude of the b,g-di-
gen shift product diethyl 2-(1,3-dioxoisoindolin-2- hydrogen shift vs. the 1,2-hydride migration, and
yl)allylphosphonate 7a, the 1,2-hydride migration shows a noticeable dependence on the kind of cata-
product (E)-diethyl [2-(1,3-dioxoisoindolin-2-yl)prop- lysts. With Rh
2ACHTUNGTNER(NUNG OAc)4 as the catalyst, b,g-dihydrogen
1-en-1-yl]phosphonate 8a, and its Z-isomer 9a shift was predominant (50:50, Table 1, entry 8). Stron-
(Scheme 1).
ger electron-withdrawing ligands in [Cu
ACHTUNGRTEN(NUNG MeCN)4]PF6
resulted in better selectivity between b,g-dihydrogen
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UPDATES
Yan Cai et al.
Table 1. Optimization of the reaction conditions.[a]
Entry
Catalysts
Cu(MeCN)4BF4
CuOTf
CuOTf·1/2C6H6
Solvent
Product ratio (7a:8a:9a)[b]
Overall yield [%][c]
1
2
3
4
5
6
7
8
G
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
PhCH3
THF
50:35:15
28:44:28
50:30:20
25:50:25
28:50:22
29:29:42
29:43:28
50:33:17
46:27:27
32:37:31
55:33:12
–
51
30
50
28
28
33
30
50
65
53
55
–
CuACTHNGURETNNU(G OTf)2
CuI
TCCu[d]
AgOTf
Rh2ACTHUNGTRENNU(G OAc)4
TfOH
BF3·Et2O
9[e]
10[e]
11
12
13
14
15
16[f]
Cu
Cu
Cu
Cu
Cu
Cu
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
T
E
G
AHCTUNGTRENNUNG
–
–
–
–
40
38
Et2O
ClCH2CH2Cl
CH2Cl2
40:44:16
0:93:7
[a]
Unless otherwise noted, all reactions were carried out using a-diazo phosphonate 6a (0.28 mmol, 1 equiv.) in 2 mL solvent
with 5 mol% of [Cu(MeCN)4]PF6 and 20 mol% of I2 at 258C for 4 h (before addition 1 h, after addition 3 h).
The product ratio was determined by H NMR of the crude product.
Overall yield of the mixture of 7a, 8a and 9a after silica gel chromatograph.
TCCu=copper(I) thiophene-2-carboxylate.
AHCTUNGTRENNUNG
[b]
[c]
[d]
[e]
[f]
1
20 mol% catalyst was used.
No I2 was added as the co-catalyst.
shift and 1,2-hydride migration (55:45, Table 1, position of dialkyl a-diazophosphonates 6 which are
entry 11).
derived from different natural amino acids was evalu-
With the best catalyst [CuAHCTUNGETRG(NUNN MeCN)4]PF6 being iden- ated. The tested a-diazophosphonates 6a and 6b with
tified, we next carried out the b,g-dihydrogen shift re- different substituents on the b-position, such as
action in different solvents to determine the best sol- methyl and isobutyl groups afforded good yields of
vent for this reaction. Among the various solvent b,g-dihydrogen shift products 7a and 7b, the 1,2-hy-
tested, toluene, tetrahydrofuran, and diethyl ether dride migration products 8a and 8b and Z isomers 9a
could not afford the expected products (Table 1, en- and 9b (Table 2, entries 1 and 2). In cases where the
tries 12–14). The reaction only gave a complex mix- substituent groups on the b-position of dialkyl a-diaz-
ture in these solvents. On the other hand, the Cu(I)- ophosphonates 6 are changed to benzyl and p-tolyl
catalyzed reaction of 6a occurred efficiently in 1,2-di- acetate groups, a significant amount of 7c and 7d was
chloroethane with 40% combined yield (Table 1, formed, this was isolated and the yields were deter-
entry 15). The most suitable solvent was found to be mined after column chromatographic purification
dichloromethane. With further optimization of the re- (Table 2, entries 3 and 4). It is astonishing to note that
action conditions, we found that in the absence of diethyl a-diazophosphonates 6e–6g which are derived
iodine the 1,2-hydride migration products 8a and 9a from valine, isoleucine, and methionine could not un-
were formed in significant amounts (Table 1, dergo this reaction to give the desired products. The
entry 16). Thus, the optimal reaction conditions for starting materials 6e–6g were decomposed under the
this transformation were determined to be 0.28 mmol reaction conditions (Table 2, entries 5–7).
a-diazo phosphonate 6a, 5 mol% of [Cu
as catalyst and 20 mol% of I2 as co-catalyst in 2 mL tivity, we set out to study reactions of a series of
CH2Cl2 as solvent at room temperature. dialkyl a-diazophosphonates 6h–6m under
Based on the above optimization efforts, the sub- [Cu(MeCN)4]PF6/I2 catalytic conditions. The migrato-
A
To assess the effect of substrates on product selec-
AHCTUNGTRENNUNG
strate scope of this reaction was investigated ry product aptitude was dependent upon the size of
(Table 2). The impact of substituent groups at the b- the R3 group. When the bulk of R3 group was in-
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UPDATES
Unexpected Stereoselective Synthesis of (Z)-b-Alkenyl Substituted
Table 2. Scope of the reaction.[a]
Entry
Product
R1
R2
R3
Product ratio (7:8:9)[b]
Overall yield (%)[c]
1
2
3
4
5
6
7
8
7a
7b
7c
7d
7e
7f
7g
7h
7i
7j
7k
7l
7m
7n
H
CH
Ph
p-AcOC6H4
CH3
CH3
CH2SCH3
H
H
H
H
H
H
CH3
Et
H
H
H
H
H
H
H
Et
Et
Et
Et
Et
Et
Et
Me
i-Pr
n-Bu
Me
i-Pr
n-Bu
Et
55:33:12
62:27:11
76:0:24
76:0:24
–
55
67
52
51
–
(CH3)2
–
–
–
–
54:34:12
70:26:4
60:31:9
67:0:33
84:0:16
75:0:25
trace:55:45
68
72
74
60
70
80
33
9
10
11
12
13
14
H
Ph
Ph
Ph
(CH2)3NPhth
H
[a]
Reaction conditions: a-diazo phosphonate 6 (0.28 mmol) in 2 mL of CH2Cl2 at 258C in the presence of 5 mol% of
[Cu(MeCN)4]PF6 and 20 mol% of I2 for 4 h (before addition 1 h, after addition 3 h).
ACHTUNGTRENNUNG
[b]
[c]
1
The product ratio was determined by H NMR of the crude product and the configuration of 7 was assigned as Z from
1
the H-1H NOESY spectrum.
Overall yield of the mixture of 7, 8 and 9 after silica gel chromatograph.
creased from methyl to butyl, the main products (Z)-
b-alkenyl-substituted b-amino phosphonates 7 result-
ed (Table 2, entries 8–13). Interestingly, when dialkyl
a-diazo phosphonates 6k–6m which are derived from
phenylalanine were treated with 5 mol% [Cu-
ACHTUNGTRENNUNG(MeCN)4]PF6 and 20 mol% iodine in CH2Cl2 at room
temperature, the diazo compounds disappeared with
30 min to give a mixture of 7k–7m and 9k–9m. 1,2-
Hydride migration E isomers 8k–8m could not be de-
1
tected in the H NMR spectra of the crude reaction
mixtures in these cases (Table 2, entries 11–13). We
suspect that the increased steric congestion between
benzyl and phosphonate groups reduces the ratio of 8
(Scheme 3). When (S)-diethyl [1-diazo-2,6-bis(1,3-di-
oxoisoindolin-2-yl)hexyl]phosphonate 6n was em-
ployed in this reaction, only 1,2-hydride migration
products 8n and 9n were observed (Table 2, entry 14). Figure 1. X-ray crystal structure of 9a.
The structure of 9a was confirmed by single crystal X-
ray diffraction (Figure 1).[12]
The possible mechanism for the reaction is shown
in Scheme 4. The a-diazo phosphate 6 initially formed
a metal carbene complex A with [Cu(MeCN)4]PF6
AHCTUNGTRENNUNG
and iodine. It has been suggested that in the metal
carbene intermediate A, the carbon attached to the
metal has a partial positive charge (intermediate
B).[13] The iodine may play a role in stabilizing the
carbocation intermediates A and B.[11] Then the g-hy-
drogen (H2) on the phosphonate migrated to the car-
bene center through a resonance complex B to form
Scheme 3. The steric congestion between benzyl and phos-
phonate groups.
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UPDATES
Yan Cai et al.
Scheme 4. Plausible reaction mechanism.
a tertiary carbocation intermediate C. For the migra-
The substituent effect on migratory aptitude ob-
tion to occur, the migrating bond needs to be parallel served in this study can be simply rationalized by as-
to the p orbital of the carbene carbon.[14] Further- suming that the 1,2-migration can stabilize the posi-
more, the proton H1 was picked up by the copper cat- tive charge in the transition state. The migratory apti-
alyst and followed by the extrusion of Cu(I) catalyst tude is largely governed by the stability of the prod-
to give the final product 7 (pathway a). The b-hydro- ucts. The steric bulkier group in the b,g-dihydrogen
gen on the phosphonate could also migrate to the car- shift product 7 will obviously favor a large group,
bene center to form a carbocation intermediate D. such as the benzyl group, because the steric hindrance
Following the 1,2-hydride migration and the losing of can thus be relieved.
[Cu
G
In order to explore the mechanism of this reaction,
transformed to by-products 8 and 9 through pathway CD2Cl2 was used for the deuterium labelling experi-
b.
ment to investigate the proton-transfer process
(Scheme 5). Compound 6a was treated under the
1
Scheme 5. Deuterium labeling experiments processed with CDCl3, determined by H NMR.
600
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Adv. Synth. Catal. 2014, 356, 596 – 602

UPDATES
Unexpected Stereoselective Synthesis of (Z)-b-Alkenyl Substituted
mento-Organic Chemistry in Nankai University for financial
support.
standard reaction conditions with CD2Cl2 as solvent.
After the reaction was complete, the mixture was ex-
tinguished with D2O. The H NMR results show that
1
no deuterium was detected in the product. The results
clearly indicate that the b,g-dihydrogen shift was
really an intramolecular process and ruled out the
possibility that the hydrogen arouse from the work-up
process after this reaction. Furthermore, with the use
of aqueous dichloromethane solutions, the reaction
proceeded efficiently and only afforded 1,2-hydride
migration product 9a with 52% yield.
In conclusion, we have reported an unexpected ste-
reoselective synthesis of (Z)-b-alkenyl-substituted b-
amino phosphonates through a b,g-dihydrogen shift
reaction of a-diazo phosphonates catalyzed by tetra-
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ACHTUNGTRENNUNG
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Supporting Information.
General Procedure for [Cu
ACHTUNGTREN(GNUN MeCN)4]PF6/I2-Catalyzed
Reaction of Dialkyl a-Diazo Phosphonates 6
[CuACHTUNGTRENNUNG(MeCN)4]PF6 (0.014 mmol) and I2 (0.056 mmol) in an
oven-dried Schlenk tube were dissolved in 2 mL of freshly
distilled CH2Cl2 under nitrogen. Dialkyl a-diazo phosphon-
ACHTUNGTRENNUNGate 6 (0.28 mmol) was diluted with 2 mL of CH2Cl2 and was
drawn into a gastight syringe. It was then added to the reac-
tion mixture dropwise over a period of 1 h with the help of
a syringe pump. After the addition was complete, the reac-
tion mixture was stirred for another 3 hour at 258C. The sol-
vent was then removed under reduced pressure and the
crude residue was purified by silica gel chromatography
with the eluent [CH2Cl2/EtOAc, 15:1 (v:v)] to give the cor-
responding products 7, 8 and 9.
À
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We thank the National Natural Science Foundation of China
(21072102), the Committee of Science and Technology of
Tianjin (11JCYBJC04200) and State Key Laboratory of Ele-
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