Improved catalytic procedures for the copper(I)-promoted reactions of β-amido zinc reagents

Article abstract of DOI:10.1039/b102446a

Copper-catalysed cross coupling of the β-aminoalkylzinc reagents 1a and 2 with unsaturated alkyl halides gives β-unsaturated ethylamines in 47-78% yield (4 examples) and enantiomerically pure β-unsaturated propylamines in 55-69% yield (3 examples). This m

Full text of DOI:10.1039/b102446a

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Improved catalytic procedures for the copper(I)-promoted reactions
of ꢀ-amido zinc reagents
Christopher Hunter,a Richard F. W. Jackson *†a and Harshad K. Ramib
1
a Department of Chemistry, Bedson Building, The University of Newcastle,
Newcastle upon Tyne, UK NE1 7RU
b Discovery Chemistry, GlaxoSmithKline, New Frontiers Science Park North, Third Avenue,
Harlow, Essex, UK CM19 5AW
Received (in Cambridge, UK) 15th March 2001, Accepted 25th April 2001
First published as an Advance Article on the web 16th May 2001
Copper-catalysed cross coupling of the β-aminoalkylzinc reagents 1a and 2 with unsaturated alkyl halides gives
β-unsaturated ethylamines in 47–78% yield (4 examples) and enantiomerically pure β-unsaturated propylamines in
55–69% yield (3 examples). This method is more efficient for simple β-aminoalkylzinc reagents than that using
stoichiometric copper.
Zinc organometallics are important nucleophilic reagents
having considerable synthetic potential.1,2 The improved stabil-
ity of β- and γ-amino acid-derived organozinc reagents in
DMF,3 and the fact that Knochel and co-workers have prepared
the β-benzamido organozinc reagent 1b, and the corresponding
zinc–copper reagent, in a DMSO–THF solvent mixture,4 led us
to develop the β-amido organozinc reagent 1a and similarly the
enantiomerically pure analogues 3 and 4.4
ing zinc–copper reagents in THF. It was clearly important
to define conditions in which analogues with more easily
deprotected N-protecting groups could be employed. We now
report conditions that allow the use of the simple zinc–copper
reagents 8 and 9 in an effective and reliable manner.
Results and discussion
The necessary alkyl iodide precursors 10 and 11 were prepared
in two simple steps from 2-aminoethanol and (S)-alaninol using
the general methods previously described.5
Subsequent reaction of these reagents with substituted aryl
iodides under palladium catalysis gave the corresponding
arylated products in moderate to good yields.5 We wished to
explore further the potential of the simple β-amido zinc reagent
1a and the previously unreported organozinc reagent 2 in the
context of copper-mediated cross coupling processes. The
extensive work by Knochel and co-workers in the area of zinc–
copper reagents has led us to develop the β-amido zinc–copper
reagents 5, 6 and 7, derived from serine, aspartic acid and
glutamic acid, respectively. Reaction of these reagents with a
range of electrophiles allowed the synthesis of a variety of
enantiomerically pure unsaturated α-, β- and γ-amino acids, not
available from the corresponding zinc reagents.6,7
Reactivity of zinc–copper reagents 8 and 9
The organozinc reagent 1a was generated from the iodide
10 using activated zinc dust in DMF. An equimolar amount of
a DMF solution of CuCNؒ2LiCl8 was added to a solution
of 1a at Ϫ55 ЊC. The reaction was then allowed to warm
briefly to 0 ЊC to ensure formation of the cuprate, where-
upon it was re-cooled to Ϫ55 ЊC and allyl bromide added
(Scheme 1).
Standard work-up and purification via flash chromatography
gave the allylated compound 12 in moderate yield (33%). The
mass balance was accounted for by protonated zinc reagent and
BocNH2, a degradation product formed via β-elimination of
the carbamate protecting group. Analogous treatment of iodide
11 gave the compound 16 in similar yield (36%). A represen-
tative selection of unsaturated alkyl halide electrophiles was
employed and moderate yields of coupled products were
obtained in most cases (Table 1). While these results were a
significant improvement on our previous work, the instability
of the zinc–copper reagents 8 and 9 was clearly having a dele-
terious effect on the efficiency of the process. In an attempt to
address this issue, we therefore turned to the catalytic use of
copper.
Our previous efforts to prepare simple analogues of the zinc–
copper reagents 5–7 lacking the ester function, using THF as
solvent, were frustrated by low yields. These poor yields
appeared to be a reflection of the instability of the correspond-
Copper-catalysed cross coupling of zinc reagents 1a and 2
Hiemstra and co-workers recently reported that it is possible to
† Present address: Dept. of Chemistry, Dainton Building, The
University of Sheffield, Sheffield, UK S3 7HF.
couple
a pyroglutamic acid-derived β-amido organozinc
DOI: 10.1039/b102446a
J. Chem. Soc., Perkin Trans. 1, 2001, 1349–1352
This journal is © The Royal Society of Chemistry 2001
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Table 1 Preparation of compounds 12–18 using stoichiometric or catalytic copper
Yield (%)a
Stoichiometric
CuCNؒ2LiCl
Zn
reagent
Catalytic
CuBrؒDMS
Electrophile
Product
1a
Allyl bromide
33
51
63
1a
1a
Propargyl chloride
30
Ethyl 2-bromomethylacrylate
60
38
78
47
1a
3-Iodocyclohex-2-enone
2
2
Allyl bromide
36
29
55
60
Propargyl chloride
2
Ethyl 2-bromomethylacrylate
59
69
a Based on starting iodide 10 or 11.
Scheme 2 Reagents and conditions: i, allyl bromide and CuBrؒSMe2
(5 mol%) in DMF, Ϫ10 ЊC to room temperature, 14 h.
pound was isolated in a much improved 51% yield, which was
obtained consistently on a 0.75 mmol scale. Reaction of zinc
reagents 1a and 2 with the other electrophiles in the presence of
catalytic CuBrؒSMe2 brought about increased yields in all cases
compared to the use of stoichiometric CuCNؒ2LiCl (Table 1).
The work-up for reactions using catalytic amounts of copper is
substantially more straightforward, especially given the need
for special precautions for the disposal of aqueous cyanide
waste.
The main decomposition pathway for reagents such as 8 and
9 involves a β-elimination process, which leads to ethene and
propene respectively. It is likely that the higher yields using
catalytic amounts of copper simply reflect the greater stability
of the organozinc reagents 1a and 2, compared with the zinc–
copper reagents 8 and 9.
Scheme 1 Reagents and conditions: i, Zn* (prepared from Zn dust
using Me3SiCl, in DMF), 5–15 min, 0 ЊC; ii, CuCNؒ2LiCl in DMF, 5
min, Ϫ55 to 0 ЊC; iii, allyl bromide (1.33 equiv.), Ϫ55 ЊC to room
temperature, 14 h.
reagent with propargylic‡ tosylates in good yield provided a
catalytic quantity of CuBrؒSMe2 was used in the reaction,9 a
process we have recently applied to amino acid synthesis.10 In
the latter case, the use of catalytic copper gave good results,
albeit slightly inferior to those obtained using stoichiometric
CuCNؒ2LiCl.
The organozinc reagent 1a was generated as described above.
The excess zinc dust was allowed to settle and the supernatant
was then removed by syringe and added to a pre-mixed DMF
solution of CuBrؒSMe2 (5 mol%) and allyl bromide at Ϫ10 ЊC
(Scheme 2). After subsequent purification the allylated com-
The enantiomeric excess of the representative product 18 was
established as greater than 98% by preparation of a racemic
sample, followed by chiral phase HPLC analysis. This estab-
lished that no significant racemisation had occurred during the
reaction of zinc reagent 2 with ethyl 2-bromomethylacrylate.
Conclusions
We have shown that the use of a catalytic quantity of
‡ The IUPAC name for propargyl is prop-2-ynyl.
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CuBrؒSMe2 instead of a stoichiometric amount of CuCNؒ
2LiCl is beneficial in promoting the reactions of the β-amido
zinc reagents 1a and 2 with unsaturated electrophiles. In addi-
tion, we have provided evidence that for the zinc reagent 2,
complete retention of stereochemical integrity occurs during
the coupling process.
aqueous ammonium chloride (20 mL) and then filtered. The
organic layer was washed with water (20 mL) and brine (20 mL)
and then dried, and the solvent removed at reduced pressure.
Flash column chromatography on silica gel, eluting with an
appropriate petroleum ether–ethyl acetate gradient yielded the
unsaturated products 12–15 and 16–18.
Method (ii): preparation of ꢀ-unsaturated ethylamines and
propylamines via CuBrؒSMe2 catalysis
Experimental
Dry DMF was distilled from calcium hydride and stored over 4
Å molecular sieves. Dry dichloromethane was distilled from
calcium hydride. Dry THF was distilled from potassium–
benzophenone ketyl. Petroleum ether refers to the fraction with
a boiling point between 40 and 60 ЊC. Specific rotations were
measured at 20 ЊC, unless otherwise stated and values are given
The organozinc reagent (1a or 2) was prepared as described
above. The excess zinc dust was allowed to settle for 5 minutes
at 0 ЊC then the supernatant was transferred under nitrogen by
syringe to a pre-mixed solution of CuBrؒSMe2 (0.010 g,
5 mol%), and the electrophile (1.00 mmol, 1.3 equiv.) in DMF
(0.5 mL) at Ϫ10 ЊC (ice–salt). The solution was then allowed to
warm slowly to room temperature and stirred for a further 14 h.
The reaction mixture was diluted with ethyl acetate (30 mL) and
washed successively with water (20 mL) and brine (20 mL),
dried and evaporated to dryness. Flash column chromato-
graphy over silica with an appropriate petroleum ether–ethyl
acetate gradient furnished the products 12–15 and 16–18.
1
in 10Ϫ1 deg cm2 gϪ1. H NMR spectra were recorded in CDCl3
solvent at 500 MHz, referenced to TMS. 13C NMR spectra were
recorded in CDCl3 at 125 MHz and referenced to TMS. Chem-
ical shifts are given in ppm. Coupling constants are given in
hertz. Organic extracts were dried over MgSO4 and the solvent
removed on a rotary evaporator. The preparation of iodide 10
has been described,5 and iodide 11 was prepared by the same
method.
N-Pent-4-enylcarbamic acid tert-butyl ester 12. Treatment
with allyl bromide (1.0 mmol) yielded pent-4-enylcarbamic acid
tert-butyl ester 12 (0.071 g, 51%) isolated as a colourless oil
(Found Mϩ 185.1410; C10H19NO2 requires 185.1416); IR (KBr
disc)/cmϪ1 3350, 2978, 1691, and 1641; NMR δH 1.44 (9H, s,
C(CH3)3), 1.55–1.61 (2H, m, C(2)H2), 2.06–2.11 (2H, m,
C(3)H2), 3.13 (2H, q, J 7, C(1)H2), 4.54–4.58 (1H, br s, NH),
4.96–5.05 (2H, m, C(5)H and C(5)HЈ), 5.75–5.84 (1H, m,
C(4)H); δC 28.43 (CH3), 29.24 (CH2), 30.44 (CH2), 40.10 (CH2),
79.09 (quat.), 115.09 (C(5)H2), 137.86 (C(4)H2), and 155.96
(CO); m/z (EI) 183 (Mϩ, 64%), 110 (88), 57 (100), and 53 (12).
(2S)-N-tert-Butoxycarbonyl-2-amino-1-iodopropane 1111
Isolated as a white solid, (2S)-N-tert-butoxycarbonyl-2-amino-
1-iodopropane 11 was recrystallised from petroleum ether–
ethyl acetate (9.21 g, 60%). Mp 60–62 ЊC (lit.11 58–59 ЊC)
(Found Mϩ 285.0211; C8H16NO2I requires 285.0226); [α]1D4
Ϫ15.3 (c 1.00 in CH2Cl2) (Found: C, 33.8; H, 5.6; N, 4.8%;
C8H16NO2I requires C, 33.7; H, 5.7; N, 4.9%); IR (KBr disc)/
cmϪ1 3285, 2976, 1678, 1533, and 1172; NMR δH 1.20 (3H, d,
J 7, C(3)H3), 1.45 (9H, s, C(CH3)3), 3.30 (1H, dd, J 10, 4,
C(1)H), 3.38–3.43 (1H, m, C(1)HЈ), 3.50–3.56 (1H, m, C(2)H),
4.60–4.66 (1H, br s, NH); δC 16.02 (CH3), 21.12 (CH2), 28.52
(CH3), 45.89 (CH), 79.67 (quat.), and 154.79 (CO); m/z (EI) 285
(Mϩ, 11%), 144 (61), 102 (32), 88 (15), and 57 (33).
N-(tert-Butoxycarbonyl)penta-3,4-dienylamine 13. Treatment
with propargyl chloride (1.0 mmol) yielded N-(tert-butoxy-
carbonyl)penta-3,4-dienylamine 13 (0.086 g, 63%) isolated as a
colourless oil (Found Mϩ 183.1260; C10H17NO2 requires
183.1259); IR (KBr disc)/cmϪ1 3347, 2954, and 1705; NMR
δH 1.44 (9H, s, C(CH3)3), 2.15–2.22 (2H, m, C(2)H2), 3.22 (2H,
d, J 6, C(1)H2), 4.64–4.69 (1H, br s, NH), 4.70–4.74 (2H, m,
C(5)H and C(5)HЈ), 5.05–5.11 (1H, m, C(4)H); δC 28.38 (CH3),
28.80 (CH2), 39.81 (CH2), 75.34 (C(5)), 79.17 (quat.), 87.09
(C(3)), 155.84 (CO), and 208.90 (C(4)); m/z (EI) 183 (Mϩ, 22%),
110 (88), 71 (42), and 57 (100).
A racemic sample rac-11 of the above material was prepared
in an identical manner. Compound rac-11 exhibited identical
spectroscopic data to the enantiomerically pure sample 11 but
had melting point of 53–54 ЊC.
Generation of zinc reagents 1a and 2. General procedure
Zinc dust (325 mesh, 0.147 g, 2.25 mmol, 3.0 equivalents) was
weighed into a 50 mL round-bottom flask with side arm, which
was repeatedly evacuated (with heating using a hot air gun) and
flushed with nitrogen. Dry DMF (0.5 mL) and trimethylsilyl
chloride (6 µL, 0.046 mmol) were added, and the resultant mix-
ture was stirred for 30 min at room temperature. The iodide 1a
or 2 (0.75 mmol) was dissolved in dry DMF (0.5 mL) under
nitrogen. The iodide solution was transferred by syringe to the
zinc suspension and stirred at 0 ЊC. TLC analysis (petroleum
ether–ethyl acetate, 2 : 1) showed complete consumption of the
iodide within 5–15 min.
Ethyl (5-N-tert-butoxycarbonylamino)-2-methylenepentano-
ate 14. Treatment with ethyl 2-bromomethylacrylate (1.0 mmol)
yielded ethyl (5-N-tert-butoxycarbonylamino)-2-methylene-
pentanoate 14 (0.151 g, 78%), isolated as a colourless oil
(Found MHϩ 258.1712; C13H23NO4 requires 258.1705); IR
(KBr disc)/cmϪ1 3376, 2978, 1731, and 1631; NMR δH 1.31 (3H,
t, J 7, CO2CH2CH3), 1.44 (9H, s, C(CH3)3), 1.64–1.70 (2H, m,
C(4)H2), 2.33 (2H, t, J 7.5, C(3)H2), 3.15 (2H, q, J 6, C(5)H2),
4.21 (2H, q, J 7, CO2CH2CH3), 4.65–4.70 (1H, br s, NH), 5.56
(1H, d, J 1, C methylene H), 6.17 (1H, d, J 1, C methylene HЈ);
δC 14.21 (CH3), 28.44 (CH3), 28.96 (CH2), 29.06 (CH2), 39.96
(CH2), 60.70 (CH2), 79.04 (quat.), 125.09 (C methylene), 140.08
(C(2)H2), 156.01 (carbamate), and 167.15 (ester); m/z (EI) 258
(MHϩ, 26%), 202 (51), 184 (5), 112 (9), 85 (6), and 57 (100).
Method (i): preparation of ꢀ-unsaturated ethylamines and
propylamines via zinc–copper reagents 8 and 9
The pre-formed zinc reagent (1a or 2) was cooled to Ϫ55 ЊC
(cryostat temperature). A solution of CuCNؒ2LiCl, prepared
by dissolving copper() cyanide (0.067 g, 0.75 mmol) and
vacuum-dried (at 180 ЊC, for 4 h) lithium chloride (0.064 g, 1.50
mmol) in dry DMF (0.5 mL), was transferred via syringe to the
reaction mixture, which was then allowed to warm to 0 ЊC for
5 min. After re-cooling to Ϫ55 ЊC, the electrophile (1.00 mmol)
was introduced, and then the mixture was stirred at this tem-
perature for 4 h, then allowed to warm slowly to room temper-
ature and stirred for a further 10 h. The reaction mixture
was partitioned between ethyl acetate (30 mL) and saturated
3-(2Ј-N-tert-Butoxycarbonylaminoethyl)cyclohex-2-en-1-one
15. Treatment with 3-iodocyclohex-2-enone (1.0 mmol) yielded
3-(2Ј-N-tert-butoxycarbonylaminoethyl)cyclohex-2-en-1-one
15 (0.084, 47%), isolated as a colourless oil. IR (KBr disc)/cmϪ1
3344, 2933, 1694, and 1667; NMR δH 1.42 (9H, s, C(CH3)3),
1.98–2.03 (2H, m, C(5)H2), 2.32–2.41 (6H, m, C(1)H2, C(4Ј)H2,
C(6Ј)H2), 3.33 (2H, q, J 6, C(2)H2), 4.56–4.62 (1H, br s, NH),
5.87 (1H, s, C(3Ј)H ); δC 22.72 (CH2), 28.43 (CH3), 29.44, 37.34,
J. Chem. Soc., Perkin Trans. 1, 2001, 1349–1352
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37.95, 38.81, 79.64 (quat.), 127.24, 155.56 (CO), 162.96, and
199.69; m/z (EI) 74 (13%), and 57 (100).
46.19 (CH2), 60.65 (CH), 79.00 (quat.), 124.87, 140.29, 155.39
(carbamate), 167.13 (ester); m/z (EI) 272 (MHϩ, 9%), 216 (17),
198 (7), 172 (47), 155 (13), 144 (4), 110 (7), 88 (7), 81 (9), and 57
(100).
A racemic sample rac-18 of the above material was prepared
in an identical manner using rac-11 as the starting material.
Compound rac-18 exhibited identical spectroscopic data to the
enantiomerically pure sample 18. The racemic sample was
analysed by chiral phase HPLC (Chiralpak AD, eluent 98 : 2
hexane–ethanol, flow rate 1 ml minϪ1, detection at 215 nm),
which gave baseline enantiomer separation. Analysis of 18
indicated an enantiomeric excess of 98%.
(2S)-N-(Hex-5-en-2-yl)carbamic acid tert-butyl ester 16.12
Treatment with allyl bromide (1.0 mmol) yielded (2S)-N-(hex-5-
en-2-yl)carbamic acid tert-butyl ester 16 (0.082 g, 51%), isolated
as a colourless oil (Found Mϩ Ϫ CH3 184.1388; C10H18NO2
requires 184.1337); [α]2D3 1.8 (c 2.65 in CH2Cl2) (Found: C, 66.1;
H, 11.0; N, 7.2%; C11H21NO2 requires C, 66.3; H, 10.6; N,
7.0%); IR (KBr disc)/cmϪ1 3339, 2977, 1686, and 1641; NMR
δH 1.13 (3H, d, J 7, C(1)H3), 1.45 (9H, s, C(CH3)3), 1.46–1.52
(2H, m, C(3)H2), 2.05–2.11 (2H, m, C(4)H2), 3.62–3.71 (1H, m,
C(2)H), 4.25–4.32 (1H, br s, NH), 4.93–5.05 (2H, m, C(6)H and
C(6)HЈ), 5.76–5.83 (1H, m, C(5)H); δC 21.47 (CH3), 28.68
(CH3), 30.57 (CH2), 36.76 (CH2), 46.39 (CH), 79.26 (quat.),
115.02, 138.36, and 155.19 (CO); m/z (EI) 157 (Mϩ Ϫ C3H6,
32%), 144 (87), 142 (67), 126 (5), and 57 (100).
Acknowledgements
We thank the EPSRC for a CASE studentship (C.H.), and
GlaxoSmithKline for support. We also thank Dr M. N. S. Hill,
Mrs L. Cook, Mr D. Dunbar and Mr S. Addison for obtain-
ing spectroscopic and other data, Dr G. Jonas (Discovery
Chemistry, GlaxoSmithKline) for chiral phase HPLC analysis,
and Mr E. Hart for technical assistance.
(2S)-N-(tert-Butoxycarbonyl)hexa-4,5-dien-2-ylamine
17.
Treatment with propargyl chloride yielded (2S)-N-(tert-butoxy-
carbonyl)hexa-4,5-dien-2-ylamine 17 (0.089 g, 60%), isolated as
a colourless oil (Found MHϩ 198.1489; C11H20NO2 requires
198.1494); [α]2D5 7.2 (c 0.970 in CH2Cl2); IR (KBr disc)/cmϪ1
3364, 2977, and 1689; NMR δH 1.15 (3H, d, J 7, C(1)H3), 1.45
(9H, s, C(CH3)3), 2.12–2.19 (2H, m, C(3)H2), 3.70–3.77 (1H, br
s, C(2)H), 4.41–4.48 (1H, br s, NH), 4.49–4.70 (2H, m, C(6)H2),
and 5.02–5.08 (1H, m, C(4)H2); δC 20.42 (CH3), 28.39 (CH3),
35.96 (CH2), 46.30 (CH2), 79.06 (quat.), 85.93, 86.70, 155.23
(CO), and 209.56 (C(5)); m/z (CI, methane) 198 (Mϩ, 8%), 142
(27), 98 (100), and 57 (13).
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Ethyl
(5S)-5-(N-tert-butoxycarbonylamino)-2-methylene-
hexanoate 18. Treatment with ethyl 2-bromomethylacrylate (1
mmol) yielded ethyl (5S)-5-(N-tert-butoxycarbonylamino)-2-
methylenehexanoate 18 (0.140 g, 69%) isolated as a colourless
oil (Found MHϩ 272.2577; C14H26NO4 requires 272.1861); [α]2D6
Ϫ5.5 (c 2.1 in CHCl3); IR (KBr disc)/cmϪ1 3368, 2977, 1716,
and 1631; NMR δH 1.14 (3H, d, J 7, C(6)H3), 1.31 (3H, t, J 7,
CO2CH2CH3), 1.45 (9H, s, C(CH3)3), 1.55–1.60 (2H, m,
C(4)H2), 2.27–2.41 (2H, m, C(3)H2), 3.64–3.72 (1H, m, C(5)H),
4.20 (2H, q, J 7, CO2CH2CH3), 4.37–4.42 (1H, br s, NH), 5.56
(1H, s, C methylene H), 6.15 (1H, s, C methylene HЈ); δC 14.21
(CH3), 21.33 (CH3), 28.43 (CH3), 28.58 (CH2), 36.05 (CH2),
1352
J. Chem. Soc., Perkin Trans. 1, 2001, 1349–1352