Copper-mediated 1,4-diamination of 1,3-butadienes

Article abstract of DOI:10.1002/ejoc.201301805

A copper(II)-mediated diamination of 1,3-butadienes is reported. The reaction employs an inorganic base, copper dibromide as a reagent, and saccharin as the nitrogen source. The diamination proceeds for a series of 1,3-butadienes under relatively mild conditions with complete selectivity in favor of the 1,4-oxidation products. Copyright

Full text of DOI:10.1002/ejoc.201301805

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201301805
Copper-Mediated 1,4-Diamination of 1,3-Butadienes
Claudio Martínez,[a] Luis Martínez,[a] Jonathan Kirsch,[a] Eduardo C. Escudero-Adán,[a]
Eddy Martin,[a] and Kilian Muñiz*[a,b]
Keywords: Amination / Copper / Dienes / Oxidation / Regioselectivity
A copper(II)-mediated diamination of 1,3-butadienes is re-
ported. The reaction employs an inorganic base, copper di-
bromide as a reagent, and saccharin as the nitrogen source.
The diamination proceeds for a series of 1,3-butadienes un-
der relatively mild conditions with complete selectivity in
favor of the 1,4-oxidation products.
more difficult to accomplish than the corresponding vicinal
diaminations. A pioneering example was provided by
Bäckvall, who investigated the palladium-mediated diamin-
ation of butadiene and cyclohexadiene with dimethylamine
as the nitrogen source. This work demonstrated that selec-
tive 1,4-diamination reactions between free nitrogen sources
and 1,3-butadienes by using metal promoters could indeed
be realized.[9]
Introduction
The diamination of unfunctionalized alkenes has recently
emerged as a suitable route to establish vicinal diamines
within a single, operationally simple oxidation reaction.[1]
This work comprises both metal-mediated[2] and metal-cat-
alyzed reactions.[3] In contrast to isolated alkenes, 1,3-buta-
dienes have received significantly less attention, as, gen-
erally, conditions for the oxidative diamination of alkenes
are not readily transferable to the substrate class of conju-
gated dienes. Shi developed a series of important 1,2-di-
amination reactions of 1,3-dienes under palladium and cop-
per catalysis.[4] Our laboratory recently contributed an iod-
ine(III)-mediated reaction that uses a combination of
PhI(OAc)2 and bistosylimide (HNTs2) to transform conju-
gated dienes into diamines.[5] Although this metal-free
method has its advantages, the reaction shows enhanced Figure 1. 1,4-Diamines as important building blocks.
substrate dependence in the formation of 1,2- and 1,4-di-
amination products. Thus, a more general method for selec-
tive 1,4-diamination is still highly desirable.
Results and Discussion
1,4-Diamines are interesting compounds in their own
right. For example 1,4-diaminobutane (putrescine, Fig-
ure 1) was recognized as an important natural polyamine,
and together with its derivatives cadaverine, spermine, and
spermidine became part of a group of substances deeply
involved in cell proliferation, in vivo protein synthesis, pro-
grammed cell death, tumor prevention, and antiviral activi-
ties.[6,7] It has also been shown that the putrescine derivative
(E)-1,4-diaminobut-2-ene (A) and its derivatives are of bio-
logical interest and have, for example, important fungicidal
activity.[6,8] However, 1,4-diamination reactions are usually
In view of this important structural motif, we became
interested in exploring alternative metal-based routes
towards the 1,4-diamination of alkenes. A series of condi-
tions was tested with 2-methyl-1,3-butadiene (1a) as the
substrate (Table 1).
An initial attempt to employ palladium catalysis in the
presence of copper(II) bromide (3 equiv.) as the terminal
oxidant resulted in the isolation of desired diamine 2a in
32% yield (Table 1, entry 1). However, a control experiment
revealed that the same outcome was achieved in the absence
of the palladium compound (Table 1, entry 2). The reaction
was improved by using dry potassium phosphate as the base
(Table 1, entry 3). In general, the reaction outcome was
found to depend crucially on the exclusion of moisture.
Upon successive increases in the temperature, the reaction
went to full conversion to provide the product in 87% yield
after column chromatography (Table 1, entries 4 and 5).
Solvents other than DMF (Table 1, entries 6–8) and bases
[a] ICIQ – Institute of Chemical Research of Catalonia,
Av. Països Catalans 16, 43007 Tarragona, Spain
E-mail: kmuniz@iciq.es
http://www.iciq.es/portal/Lang!en-US/924/
DesktopDefault.aspx
[b] ICREA – Catalan Institution for Research and Advanced
Studies,
Pg. Lluís Companys 23, 08010 Barcelona, Spain
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201301805.
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K. Muñiz et al.
SHORT COMMUNICATION
Table 1. Optimization of the copper-mediated 1,4-diamination of (1c) and 1,3-pentadien-5-yl benzyl ether (1e; Table 2, en-
1,3-butadienes (HNSacc = saccharin).
tries 4,5). These compounds bearing aliphatic substituents
had also generated 1,4-diamination products in previous
iodine(III)-mediated reactions.[5a] The terpene myrcene (1f)
underwent clean diamination of the 1,3-butadiene function-
ality to yield corresponding diamines 2f/2fЈ as a 3:1 dia-
stereomeric mixture. It is an important observation that the
double bond remained untouched under the present condi-
tions. In addition to the selective oxidation, the absence of
any possible cyclization products was also noted. We were
then pleased to observe that 1,3-butadienylarenes 1g–n
(Table 2, entries 6–12) and their derivatives (Table 2, en-
tries 7–10) gave excellent yields of the intermolecular 1,4-
addition products. For product 2g, the 1,4-constitution was
unambiguously assured by X-ray structure analysis (Fig-
ure 2).[10] This regioselectivity outcome is an exciting con-
trast to the case of iodine(III) chemistry, which usually fa-
vors the exclusive 1,2-addition onto the terminal position.[5]
Under the present conditions, 1,4-diamination was ob-
tained in all cases. For para-substituted arenes, good to ex-
cellent yields were obtained. We noted the formation of
minor amounts of aminobromination products as side
products in these cases.[11] These were formed as N-(1-
bromo-4-arylbut-3-enyl) saccharins 3g–n,[12] and their con-
stitution was assigned through X-ray structure analysis of
p-methoxy derivative 3i.[10]
[a] Determined by analysis of the crude reaction mixture by
1H NMR spectroscopy. [b] Yield of isolated product after column
chromatography. [c] Reaction was performed with Pd(OAc)2
(10 mol-%). [d] n.d.: not determined. [e] With CuBr2 (2.0 equiv.).
[f] With CuBr2 (30 mol-%) under an atmosphere of molecular di-
oxygen. [g] With Cu(OAc)2 (2.0 equiv.) instead of CuBr2. [h] With
phthalimide (2.0 equiv.) instead of saccharin (a complex product
mixture was obtained).
other than K3PO4 (Table 1, entries 9–11) were found to be
less effective. Decreasing the amount of copper(II) bromide
resulted in a significant decrease in the chemical yield, as
Figure 2. X-ray structure of 2f.[10]
did an attempt to work under aerobic conditions with a
Given that we had experienced some problems with the
substoichiometric amount of copper (Table 1, entries 12
removal of the saccharin moiety in the past, we undertook
and 13). Notably, the reaction depended on copper(II)
some preliminary investigations into its deprotection for the
bromide as the promoter. An attempt to replace this reagent
present case (Scheme 1).
by copper(II) acetate, which was introduced by Chemler for
intramolecular diamination reactions,[2n,2o] did not result in
any observable oxidation of 1a (Table 1, entry 14). Attempts
to employ phthalimide as an alternative nitrogen source un-
der otherwise unchanged conditions did not lead to any
observable diamination product (Table 1, entry 15).
As a result, it was decided to explore the scope of the
1,4-diamination reaction for different 1,3-butadienes
(Table 2). 1,3-Butadiene (1b) itself was condensed into the
reaction solution at low temperature. The sealed vessel was
then warmed to room temperature over several hours. For
this particular case, the reported yield of 41% is based on
saccharin (Table 2, entry 2). As in the case of 1a, exclusive
1,4-diamination was obtained. The same regioselectivity
was observed for the diamination reactions of 1,3-octadiene Scheme 1. Selective cleavage reactions of saccharin in 2a,c.
2018
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Copper-Mediated 1,4-Diamination of 1,3-Butadienes
Table 2. Scope of the copper-mediated 1,4-diamination of 1,3-butadienes.
[a] Yield of isolated product after column chromatography. [b] In condensed matter as a cosolvent, yield is based on saccharin. [c] 1,3-
Butadienes 1g–n were used as 1:1 mixtures of (E)/(Z) isomers. [d] Yield in parentheses refers to the corresponding aminobromination
products 3g–n (see text).
The synthetic utility of 1,4-diamination products 2 as tive yield. This method allowed the saccharin ring to be
suitable precursors to 1,4-diamine building blocks was in- selectively opened at the sulfamide side to afford amide 4.
vestigated within some representative reactions. First, 2a Derivatives of 4 are recognized for their biologic properties
was treated with 4-(dimethylamino)pyridine (DMAP) and such as their role in probiotic formulations[13] and for their
pyridine in water at 80 °C to afford product 4 in quantita- proadhesin and nonadhesin properties.[14] Treatment of 2c
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K. Muñiz et al.
SHORT COMMUNICATION
with LiAlH4 led to the opening of the saccharin ring with to BЈ). Given that this process gives an allylic bromide, a
concomitant reduction of the carbonyl group to the second nucleophilic amination is feasible to provide ob-
alcohol, which resulted in product 5, the structure of which served diamination products 2. This mechanistic proposal
was confirmed by X-ray structure analysis.[10] Derivatives is also supported by the increased formation of amino-
of 5 are known for their biological properties, as, for exam- bromination products for substrates 1l–n. In these cases, the
ple, their cytotoxic activity against human tumor cell li- enhanced volume of the aromatic substituent should render
nes.[7c] The full deprotection of the saccharin to the amine the amination in the benzylic position less feasible, which
was previously reported, the first report of which was under would result in the observed decrease in the formation of
acidic conditions with the use of HCl and another approach the diamination product. The assumption of allylic copper
was through the use of CsF in DMF.[3f]
intermediate BЈЈ would also explain the observation that
Although full mechanistic studies have to await future both (E)- and (Z)-1n lead to single (E)-configured product
investigation, we did perform some control experiments to 2n.[12] The nature of the copper in allylic intermediates B/
clarify major aspects. First, the formation of diamination BЈ/BЈЈ is unclear at present. Owing to the observation that
products 2 and aminobromination products 3 is assumed to 3 equivalents of the copper(II) promoter are required for
proceed through identical initial pathways.[15] We postulate complete conversion, the involvement of higher copper ag-
an initial regioselective bromocupration of butadiene 1 with gregates or the formation of a copper(III) species may be
copper(II) bromide. This step should apparently not pro- concluded.[2o,17]
ceed through radical C–Br bond formation, as in the exam-
ple of myrcene oxidation (Table 2, entry 6) no cyclization
product was observed. The intermediate product should be
1,2- or 1,4-cuprate B or BЈ, respectively, which could be
Conclusions
In summary, we have developed a new copper-promoted
formulated as allyl cuprate BЈЈ (Scheme 2). An alternative
1,4-selective diamination of 1,3-butadienes. The reaction
mechanism may be considered on the basis of the known
proceeds under mild conditions by using copper dibromide
decomposition of CuBr2 into CuBr and elemental brom-
ine.[16] However, subsequent formation of a dibromide
species such as 6 as an intermediate prior to product forma-
tion through two nucleophilic allylic substitution reactions
appears less probable. A control experiment revealed that 6
undergoes the expected formation of 2b in the presence of
saccharin and base; however, the same was observed for
phthalimide, which in contrast to saccharin does not pro-
mote the diamination reaction under the copper-mediated
conditions (Table 1, entry 19). Instead, direct nucleophilic
displacement from B/BЈ/BЈЈ is expected. This can either pro-
mote the formation of aminobromination product 3 upon
nucleophilic substitution in B or promote the SN2Ј addition
to BЈ. Diamination product 2 would then arise from the
alternative pathways (SN2 addition to B and SN2Ј addition
and saccharin as reagents and gives rise to a series of 1,4-
diaminobut-2-enes. The reported reactivity should be of
interest for the development of additional 1,4-difunctionali-
zation reactions with the use of copper(II) promoters.
Experimental Section
General Procedure: A flame-dried Schlenk tube was charged with
CuBr2 (268 mg, 1.2 mmol, 3.0 equiv.), K3PO4 (170 mg, 0.8 mmol,
2.0 equiv.), saccharin (147 mg, 0.8 mmol, 2.0 equiv.), and dry DMF
(2.0 mL) under a nitrogen atmosphere. The diene (0.4 mmol,
1.0 equiv.) was then added. After stirring for 12 h at 50 °C, the
reaction mixture was cooled to room temperature and water
(10 mL) was added. The copper salt was removed by filtration
through Celite. The aqueous layer was extracted with dichloro-
methane (5ϫ 7 mL), and the combined organic layer was dried
with Na2SO4 and evaporated under reduced pressure. The crude
product was purified by column chromatography (silica gel; n-hex-
ane/ethyl acetate, 4:1 to 1:1) to give the pure diamination product.
Supporting Information (see footnote on the first page of this arti-
cle): Detailed experimental description; characterization data for
all new compounds; details of the X-ray analyses; and copies of
1
the H NMR, 13C NMR, and 19F NMR spectra.
Acknowledgments
The authors thank Institute of Chemical Research of Catalonia
(ICIQ) Foundation and the Spanish Ministry for Economy for gen-
erous financial support (grant number CTQ2011-25027).
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2020
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Copper-Mediated 1,4-Diamination of 1,3-Butadienes
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Received: December 4, 2013
Published Online: February 24, 2014
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