Ruthenium-catalyzed allenyl carbamate formation from propargyl alcohols and isocyanates

Article abstract of DOI:10.1002/ejoc.200701067

Ruthenium complexes of redox-coupled cyclopentadienone ligands catalyze the formation of allenyl carbamates from propargyl alcohols and isocyanates. This efficient and atom-economical process represents the first catalytic access to allenyl carbamates, co

Full text of DOI:10.1002/ejoc.200701067

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200701067
Ruthenium-Catalyzed Allenyl Carbamate Formation from Propargyl Alcohols
and Isocyanates
Edgar Haak*[a]
Keywords: Allenes / Homogenous catalysis / Propargyl alcohols / Ruthenium / Vinylidene complexes
Ruthenium complexes of redox-coupled cyclopentadienone
ligands catalyze the formation of allenyl carbamates from
propargyl alcohols and isocyanates. This efficient and atom-
economical process represents the first catalytic access to al-
lenyl carbamates, compounds of high synthetic potential.
The reaction needs an acidic co-catalyst and can be per-
formed at room temperature. In addition, new (cyclopen-
tadienone)iron and -ruthenium complexes were synthesized,
and mechanistic aspects regarding catalytic transformations
of propargyl systems with ruthenium catalysts are obtained.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
Introduction
Results and Discussion
The development of new flexible reactions to obtain use-
In extension of previous work,[4b] several donor- and ac-
ful products with added value from simple and cheap mate- ceptor-substituted carbonyl(cyclopentadienone)ruthenium
rials is of high interest. The homogeneous transition metal and -iron complexes were synthesized, and their catalytic
catalysis delivers various processes and a broad range of activities towards propargyl alcohols – focused on amin-
new mechanisms to this field.[1] Our aim is the development ation reactions – were investigated. Four major groups of
of transition metal catalyzed processes using complexes of catalysts differing in the ligand’s substitution pattern were
redox-coupled ligands, mainly (cyclopentadienone)ruthe- investigated. Series 1 is acceptor-substituted, while the com-
nium derivatives, to obtain relevant, especially nitrogen- plexes of series 2–4 carry nitrogen donors in the 3- and 4-
containing, compounds. This type of catalyst, which is al- position. The complexes of series 3 and 4 contain additional
ready known from hydrogen transfer reactions,[2] provides acceptors, whereas the compounds of series 4 are hydrogen-
unique features towards catalytic transformations of bifunc- bonded (Figure 1, Table 1).
tional substrates, like propargyl alcohols, due to the elec-
tronic coupling of the dienone ligand and its basic coordi-
nation site. According to this concept, the range of selective
catalytic transformations of propargyl alcohols with ruthe-
nium catalysts[3] could recently be extended. With (cyclo-
pentadienone)ruthenium catalysts α- or β-amino ketones,
enamino ketones, aldehydes, ketones, imines, enynes or al-
kenes are accessible from propargyl alcohols (Scheme 1).[4]
Figure 1. Substituted (cyclopentadienone)ruthenium and -iron
complexes.
It had previously been shown that the substituents of the
cyclopentadienone ligand determine the mode of activation
of propargyl alcohols by the ruthenium complexes leading
to different catalytic cycles. Acceptor-substituted catalysts
of series 1 form alkenyl complexes 5 with terminal or in-
ternal secondary propargyl alcohols, while tertiary propar-
gyl alcohols are not reactive. The diamino-substituted ana-
Scheme 1. Catalytic transformations of propargyl alcohols.
[a] Institut für Organische Chemie der Universität Braunschweig,
Hagenring 30, 38106 Braunschweig, Germany
Fax: +49-531-3915388
E-mail: e.haak@tu-bs.de
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
788
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 788–792

Ruthenium-Catalyzed Allenyl Carbamate Formation
Table 1. Substitution pattern of catalysts 1–4.
method to obtain allenyl carbamates previously published,
has impressively been demonstrated by Hoppe et al.[5]
The fact that aliphatic propargyl carbamates are not
transformed by catalysts 2–4 under neutral reaction condi-
tions led us to investigate the effect of basic or acidic addi-
tives on the latter reaction. In the presence of a catalytic
amount of an amine base like DBU or DABCO, the well-
known 5-exo-dig cyclization of the initially formed propar-
gyl carbamate leading to 5-methylene-1,3-oxazolidin-2-ones
8 was observed.[6] This cyclization does not need the metal
catalyst and occurs in some cases already at room tempera-
ture. In the presence of acidic additives like BF3·Et2O,
Sc(OTf)3, HBF4 or NH4PF6 allenyl carbamates 9 are
formed accompanied by symmetrical urea derivatives 10 as
by-products. The formation of symmetrical ureas from car-
bon dioxide and amines in the presence of ruthenium com-
plexes and terminal propargyl alcohols has already been de-
scribed by Dixneuf et al.[3k] The best results regarding the
formation of 9 were obtained with BF3·Et2O as additive,
while the use of NH4PF6 leads to Rupe rearrangement[7] as
an undesired side reaction (Scheme 3). Conversion of com-
pound 8 in the presence of the ruthenium catalyst and a
Lewis acid was not observed.
Catalyst
R1
R2
R3
R4
1a[4b]
1b[4b]
1c[4b]
Ph
Me
CO2Me
Ph
Ph
Ph
Ph
Me
Me
Me
Me
Ph
Ph
Ph
Ph
Me
Me
Me
Me
Ph
Me
CO2Me
Ph
Ph
Ph
Ph
Me
2a[4b] (M = Ru) Ph
2b[4b] (M = Ru) Ph
2b-Fe (M = Fe)
2c[4b] (M = Ru)
2d (M = Ru)
Ph
H
H
2e[4b] (M = Ru)
Ph
CH2CH2OH CH2CH2OH Ph
3a[4b] (M = Ru) Ph
Me
Me
Me
Me
Me
Me
Ph
Me
Me
3b (M = Ru)
3c (M = Ru)
Ph
Me
Me
Me
Ph
3d (M = Ru)
3d-Fe (M = Fe)
3e[4b] (M = Ru)
3f[4b] (M = Ru)
CH2CH2OAc CH2CH2OAc Me
CH2CH2OAc CH2CH2OAc Me
CH2CH2OBz CH2CH2OBz Ph
Me
Me
Me
Me
OEt
Me
Me
Me
Me
Me
H
CO2Et
Ph
Ph
Ph
3g[4b] (M = Ru) OEt
4a[4b]
4b[4b]
4c[4b]
Ph
naphthyl
cyclohexyl Me
logues of series 2–4 seem to generate γ-hydroxyvinylidene
species 6 from terminal propargyl alcohols, while internal
propargyl alcohols are not reactive (Scheme 2).[4b] The cor-
responding iron complexes 2b-Fe and 2d-Fe are catalytically
inactive under similar reaction conditions.
Scheme 3. Catalytic transformations of propargyl alcohols with iso-
cyanates.
Catalysts of series 2 and 3 give the highest yields, while
those of series 4 show lower activity. If the reaction is per-
formed without a metal catalyst or with catalysts of series
1, with the analogues iron complexes 2b-Fe and 2d-Fe or
without a Lewis acid the corresponding propargyl carba-
mate 11 was quantitatively recovered (Table 2, Entries 1, 4,
5, 6, 9, 13, 21); the use of Ru3(CO)12 or RuCl2(PPh3)3, how-
ever, leads to the corresponding urea derivatives in high
Scheme 2. Different modes of propargyl alcohol activation.
Due to these different intermediates several selective yields (Table 2, Entries 2, 3).
transformations of propargyl alcohols could be
Various propargyl alcohols and isocyanates are suitable
achieved.[4a,4b] As one example, complexes of series 2–4 cat- substrates for this reaction but secondary propargyl
alyze the formation of α,β-unsaturated imines 7 from aryl- alcohols react slowly (Table 3, Entries 1–13). Propargyl
substituted propargyl alcohols and isocyanates under neu- carbamates derived from aliphatic propargyl alcohols and
tral conditions.[4b] Propargyl carbamates that derived from isocyanates behave similarly, while aryl-substituted propar-
aryl-substituted propargyl alcohols and isocyanates behave gyl carbamates form symmetric urea derivatives in high
similarly, while aliphatic propargyl carbamates are not yields. In case of labile substrates, the reaction can be per-
transformed under neutral reaction conditions. Herein, new formed at room temperature with complexes of series 3 due
results obtained by further investigations of the latter to the stronger coordination of the Lewis acid and better
reaction and leading to the first catalytic access to allenyl solubility of the resulting adduct (Table 3, Entries 14–17).
carbamates are reported. The synthetic potential of this
A plausible mechanism starts with the initial formation
class of compounds, including the only stoichiometric of a vinylidene species that is attacked in α-position. By
Eur. J. Org. Chem. 2008, 788–792
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789

E. Haak
SHORT COMMUNICATION
Table 2. Allenyl carbamate formation from 3-methyl-1-pentyn-3-ol and phenyl isocyanate with various catalysts.[a]
Entry
Catalyst
Additive
Yield[b] 9e [%]
Yield[b] 10 [%]
Recovered[b] 11 [%]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
–
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
–
–
–
–
–
–
–
99
40
–
90
92
91
10
5
98
45
9
18
97
6
8
7
12
7
7
5
98
44
RuCl2(PPh3)3
55
91
–
–
–
15
16
–
10
19
9
Ru3(CO)12
1a
1b
1c
–
2a
57
65
–
31
55
39
–
59
57
55
52
58
58
60
–
2b
2b
2b
Sc(OTf)3
HBF4
2b
2b
NH4PF6
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
BF3·Et2O
2b-Fe
2c
–
16
14
14
12
15
17
17
–
2d
2e
3a
3b
3c
3d
3d-Fe
4a
29
12
[a] Reaction conditions: 0.5 mL of toluene, 3 mol-% of catalyst, 3 mol-% of additive, 1 mmol of 3-methyl-1-pentyn-3-ol, 1 mmol of phenyl
isocyanate, 100 °C, 3 h. [b] Isolated yield (based on isocyanate).
Table 3. Allenyl carbamate formation from various propargyl alcohols and isocyanates.
Entry
R
RЈ
RЈЈ
Cat.
Yield[d] 9 [%]
Yield[d] 10 [%]
Recovered[d] 11 [%]
1[a]
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Et
Et
Et
Ph
Bn
Cy
allyl
Ph
Bn
Cy
allyl
Ph
Bn
Cy
allyl
Ph
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2c
2b
3b
3c
3b
50 (9a)
43 (9b)
45 (9c)
43 (9d)
65 (9e)
74 (9f)
48 (9g)
69 (9h)
62 (9i)
68 (9j)
59 (9k)
71 (9l)
50 (9m)
15 (9e)
49 (9e)
45 (9e)
24 (9m)
20
17
18
18
16
15
13
19
12
13
12
18
26
10
11
10
14
8
14
12
15
5
2
21
2
2[a]
3[a]
4[a]
5[a]
6[a]
7[a]
8[a]
Et
9[a]
C5H10
C5H10
C5H10
C5H10
7
8
10[a]
11[a]
12[a]
13[b]
14[c]
15[c]
16[c]
17[c]
14
2
18
69
28
31
55
H
Me
Et
Et
Et
Me
Me
Me
Me
H
Ph
Ph
Ph
Ph
[a] Reaction conditions: 0.5 mL of toluene, 3 mol-% of catalyst, 3 mol-% of BF3·Et2O, 1 mmol of ropargyl alcohol, 1 mmol of isocyanate,
100 °C, 3 h. [b] Reaction conditions: 0.5 mL of toluene, 3 mol-% of catalyst, 3 mol-% of BF3·Et2O, 1 mmol of propargyl alcohol, 1 mmol
of isocyanate, 100 °C, 8 h. [c] Reaction conditions: 0.5 mL of toluene, 3 mol-% of catalyst, 3 mol-% of BF3·Et2O, 1 mmol of propargyl
alcohol, 1 mmol of isocyanate, r. t., 3 d. [d] Isolated yield (based on isocyanate).
using an isocyanate that forms a propargyl carbamate with
the propargyl alcohol this nucleophilic attack occurs intra-
Conclusions
molecularly. Under neutral conditions the nitrogen atom
The first catalytic access to allenyl carbamates has been
acts as nucleophile, while under acidic conditions the car- reported. The new procedure using redox-coupled (cyclo-
bonyl oxygen atom attacks. The unsaturated system is re- pentadienone)ruthenium complexes as catalysts represents
ductively eliminated after elimination of the leaving group a simple selective, efficient and atom-economical method
in β-position (Scheme 4).
starting from readily available propargyl alcohols and isocy-
790
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Eur. J. Org. Chem. 2008, 788–792

Ruthenium-Catalyzed Allenyl Carbamate Formation
Scheme 4. Postulated catalytic cycle.
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Experimental Section
General Procedure: 3 mol-% of catalyst 2–4 was dissolved in 0.5 mL
of toluene; 3 mol-% of BF3·Et2O diluted in toluene, 1 mmol of the
propargyl alcohol and 1 mmol of the isocyanate were subsequently
added. The mixture was stirred under argon at 100 °C for 3 h (for
9m: 8 h) or at room temperature for 3 d. Evaporation of the solvent
and chromatography of the residue on silica furnished the allenyl
carbamates 9.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures and full characterizations.
Acknowledgments
Constant support by Prof. Dr. Dr. h. c. H. Hopf and by the Fonds
der Chemischen Industrie is gratefully acknowledged.
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Received: November 9, 2007
Published Online: January 7, 2008
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