Diethylzinc: A simple and efficient catalyst for the swift hydroamination at room temperature

Article abstract of DOI:10.1002/adsc.200900302

Diethylzinc and dimethylanilinium tetrakis(pentafluorophenyl)borate were found to catalyze hydroaminations at room temperature in high efficiency and very short reaction times. The reactivity of the proposed cationic zinc species, which is assumed to cata

Full text of DOI:10.1002/adsc.200900302

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DOI: 10.1002/adsc.200900302
Diethylzinc: A Simple and Efficient Catalyst for the Swift
Hydroamination at Room Temperature
Jens-Wolfgang Pissarek,a David Schlesiger,a Peter W. Roesky,b,*
and Siegfried Blecherta,*
a
Technische Universitꢀt Berlin, Institut fꢁr Chemie, Straße des 17. Juni 135, 10623 Berlin, Germany
Fax : (+49)-(0)30-3142-3619; phone: (+49)-(0)30-3142-2355; e-mail: blechert@chem.tu-berlin.de
Universitꢀt Karlsruhe (TH), Institut fꢁr Anorganische Chemie, Engesser Str. 15, 76313 Karlsruhe, Germany
b
Fax : (+49)-(0)721-608-4854, phone: (+49)-(0)721-608-6117; e-mail: roesky@chemie.uni-karlsruhe.de
Received: April 29, 2009; Published online: September 8, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900302.
Abstract: Diethylzinc and dimethylanilinium tetra-
kis(pentafluorophenyl)borate were found to cata-
lyze hydroaminations at room temperature in high
efficiency and very short reaction times. The reac-
tivity of the proposed cationic zinc species, which is
assumed to catalyze the reaction, strongly depends
on the coordinative abilities of the counterion.
single case for a olefin[16] but they needed elevated
temperatures.
We herein present the ZnEt2-catalyzed hydroami-
nation of aminoolefins and aminoalkynes which was
carried out at room temperature for most of the given
examples. We already reported the increased reactivi-
ty of Zn catalysts by adding [PhNHMe2][BACHTNURGTENUNG(C6F5)4]
(1) as activator.[17] This effect was even more dramatic
for ZnEt2, because it did not lead to any hydroamina-
tion at room temperature whereas the addition of 1
allows the reaction to proceed rapidly (Scheme 1).
Keywords: amines; catalysis; heterocycles; hydro-
ACHTUNGTRENNUNGamination; zinc
The importance of nitrogen-containing compounds
like amines, enamines and imines has caused many ef-
forts to develop efficient methods for their synthesis.
Hydroaminations of alkenes and alkynes represent
the most atom efficient processes in this respect. Con-
sequently the development of catalytic additions of
amines to C,C bonds has received much attention.[1–17]
Numerous metals were applied in the hydroamination
of alkenes and alkynes such as lithium,[1] group 4
metals,[2] the lanthanides,[3,4,5] group 3 metals,[6] the
platinum metals,[7] calcium,[8] copper,[9] silver[10] and
Scheme 1. Demonstration of the activation of ZnEt2 by 2.
gold.[11] The scope of catalytic hydroamination has Thus, substrate 2 was cyclized within 35 min (Table 2,
been reviewed recently.[12,13] Except for Zn these entry 1). In order to investigate the role of the activa-
metals are either expensive or lack tolerance towards tor we checked other Brønsted acids. Substrate 2 was
functional groups. The cheap metal Zn is somewhat chosen as a test system for it was shown to undergo
outstanding for it is non-toxic nature and the fact that hydroamination easily. We started our screening with
it tolerates many functional groups. Zn has already the non-fluorinated analogues of 1: [PhNHMe2]
been applied to the hydroamination of aminoolefins
ACHTUNGTRENNUG[BPh4] (Table 1, entry 4) and [Et3NH]ACHTUGNTERN[NUGN BPh4] (Table 1,
as the aminotroponiminate complex (ATIs).[14] The entry 3). In both cases, neither at room temperature
ATI-Zn catalysts turned out to be relatively robust to- nor at 808C, could 2 be cyclized. We also observed
wards moisture and air compared to dialkylzinc com- that [Et3NH]
pounds. Ligand-free Zn catalysts have also been re- tion whereas [PhNHMe2]
ported for the hydroamination of alkynes[15] and in a upon the addition of ZnEt2. Similar solubility behav-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
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Jens-Wolfgang Pissarek et al.
Conversion [%]/Time
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Table 1. Screening of different activators in the ZnEt2-catalyzed hydroamination of 2.
Entry
Activator[c]
Temperature [8C]
1
2
3
4
5
6
7
8
9
N
R
80
80
80
80
80
80
r.t.
r.t.
r.t.
0%/2 d
0%/2 d
0%/2 d
0%/2 d
30%/21 d
8%/21 d
25%/5 d
>99%/45 min
>99%/35 min
E
G
R
A
[a]
hFiP: hexafluoroisopropyl alcohol.
pFtB: perfluoro-tert-butyl alcohol.
Experiments were carried out with 2.5 mol% of each ZnEt2 and activator in 0.5 mL C6D6 in a flame-sealed NMR tube.
[b]
[c]
iour occurred for [NH4]
[PhNHMe2][BF4] (Table 1, entry 2). The ammonia catalyze the hydroamination of, for example, amino-
salt did not dissolve whereas the anilinium salt was
olefins[22] or norbornenes, we tested 1 without ZnEt2
A
As 1 displays a Brønsted acid which are known to
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
brought into solution spontaneously by adding ZnEt2. towards the hydroamination of 2. However, at room
Neither salt led to the desired hydroamination of 2. temperature the reaction proceeded very slowly so
Triflic acid and the corresponding anilinium salt that even with 10 mol% of 1 the conversion did not
showed good solubility and led to slow hydroamina- exceed 5% after 24 h (90% conversion after 8 days).
tion at 808C (Table 1, entries 5 and 6). The reactivity It is perfectly obvious that the combination of ZnEt2
of the Zn catalyst seemed to correlate with the coor- and 1 is much more reactive towards hydroamination
dination behaviour of the anion. In order to underline than the single components.
this suggestion we searched for a less coordinating
anion than the triflate anion.
Initial experiments showed successful hydroamina-
tion of primary and secondary amines. In order to test
Krossing et al. designed different WCAs (weakly functional groups (heterocycles) we focused on secon-
coordinating anions) based on aluminium showing dary amines. The results of the hydroamination of dif-
high thermal and chemical stability.[18] We synthesized ferent aminoolefins are shown in Table 2. The short
an analogue to 1 based on Krossingꢃs synthesis of reaction times reflect the high reactivity of our cata-
[(Et2O)2H][AlACHTUNGTRENNUNG ACHTUNTGERN(NUGN pFtB)4] (3) lytic system that nevertheless tolerates different func-
(pFtB)4].[19] [PhNHMe2][Al
showed similar reactivity as 1 towards hydroamina- tional groups such as furans (Table 2, entry 4), thio-
tion. Both 1 and 3 are claimed to carry the least coor- phenes (Table 2, entry 5), sulfonamides (Table 2,
dinating anions in our series.[20]] Therefore it is obvi- entry 6) and silyl ethers (Table 2, entries 7 and 8). A
ous that the reactivity of the Zn catalyst increases as catalyst loading of 2.5 mol% was sufficient and the
the coordination strength of the activatorꢃs counterion temperature could be kept at room temperature in
decreases.
most of the cases. Hydrazine-olefins did not lead to
The commercially available activator 1 showed the hydroamination at room temperature but cyclized
best results so far and so further experiments were swiftly at 808C within 3 h (Table 2, entry 10). To the
carried out with this compound. We also tested if an best of our knowledge, this is the first Zn catalyst that
excess of 1 could improve the reaction times but even cyclizes an aminoolefin that does not carry a digemi-
two equivalents related to the employed ZnEt2 did nal substitution (Thorpe–Ingold effect)[23] although
not show higher conversion rates. Use of only a “sub- high temperatures and long reaction times are neces-
stoichiometric” amount of activator led to prolonged sary (Table 2, entry 11). This also reflects the high-
reaction times. We observed the release of ethane. temperature and long-term stability of our catalyst.
The disappearance of the ethyl group can be observed For the isolation of the products the absence of li-
1
by H NMR spectroscopy during the reaction. There- gands which might be difficult to separate is a big ad-
fore we assume the formation of a cationic zinc spe- vantage. Thus only the filtration over a small pad of
cies. This hydroamination can proceed through two silica and removal of the solvent and volatiles was
mechanistic pathways: 1) Lewis-acid catalysis with ac- necessary to work up the reaction.
tivation of the C,C multiple bond or 2) activation of
the amine to a metalloamide which then undergoes cyclized with the new catalytic system. 5-Hexynyl-
olefin insertion.[13b,21]
amine (4) was subjected to 2.5 mol% of each ZnEt2
and 1. Within 30 h at 808C 4 was converted to an en-
We were also interested if aminoalkynes could be
AHCTUNGTRENNUNG
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Diethylzinc: A Simple and Efficient Catalyst for the Swift Hydroamination
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Table 2. Results for the hydroamination of different aminoolefins.
Entry[a]
Substrate
Product
Temperature [8C]
Time[b]
35 min
Yield [%][c]
95
1
r.t.
2
3
r.t.
r.t.
80 min
5.5 h
94
89
4
5
6
7
r.t.
r.t.
r.t.
r.t.
11 h
93
98
99
98
27 h
18 h
30 min
8
r.t.
r.t.
40 min
2 h
94
98
9
10
80
3 h
99
90
11
180
21 d
[a]
Experiments were carried out with 2.5 mol% of each ZnEt2 and [PhNHMe2][BACHTUNGTRNEG(UN C6F5)4] and 0.43 mmol of substrate in
0.5 mL of C6D6 in a flame-sealed NMR tube.
Elapsed time until full conversion.
Isolated yield.
[b]
[c]
amine which readily tautomerized to an imine
(Scheme 2). To isolate the corresponding volatile
amine the imine was reduced and converted to the
ammonium salt by adding concentrated HCl. After
drying at high vacuum 5 could be isolated as a colour-
less solid in quantitive yield. Also secondary amino-
AHCTUNGTRENNUNG
808C and furnished the corresponding enamine within
Scheme 2. Hydroamination of a primary aminoalkyne and
subsequent reduction.
50 h (Scheme 3). The enamine was hydrolyzed in
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Jens-Wolfgang Pissarek et al.
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General Procedure for the Zinc-Catalyzed
Hydroamination
To a solution of substrate (0.43 mmol) in 0.5 mL C6D6 were
added 11 mL of a 1M solution of ZnEt2 in hexane (11 mmol,
2.5 mol%) and 9 mg of [PhNMe2H][BACHTUNGRTNEUNG(C6F5)4] (11 mmol,
2.5 mol%) under a nitrogen atmosphere. A dried NMR tube
was charged with the resulting solution and flame sealed
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Acknowledgements
Part of this work was supported by Deutsche Forschungsge-
meinschaft (DFG). D.S. acknowledge support from the Clus-
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