Highly active and diastereoselective N,O- and N,N-yttrium complexes for intramolecular hydroamination

Article abstract of DOI:10.1002/adsc.201000892

The intramolecular hydroamination of aminoalkynes and unactivated aminoalkenes catalyzed by yttrium N,O- and N,N-complexes has been investigated. The N,N-yttrium complexes are highly active, catalyzing the conversion of a wide range of terminal aminoalkenes at room temperature, and internal aminoalkenes at elevated temperature, to yield pyrrolidine and piperidine products in high yields. A high diastereoselectivity of up to 23:1 is observed at 0°C with 1-methyl-4-pentenylamine as substrate.

Full text of DOI:10.1002/adsc.201000892

FULL PAPERS
DOI: 10.1002/adsc.201000892
Highly Active and Diastereoselective N,O- and N,N-Yttrium
Complexes for Intramolecular Hydroamination
Frank Lauterwasser,a,b Paul G. Hayes,c Warren E. Piers,c Laurel L. Schafer,b,*
and Stefan Brꢀsea,*
a
Institute for Organic Chemistry, Karlsruhe Institute of Technology (KIT), Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany
Fax : (+49)-721-6084-8581; e-mail: braese@kit.edu
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, V6T 1Z1,
b
Canada
E-mail: schafer@chem.ubc.ca
Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary Alberta, T2N 1N4, Canada
c
Received: November 28, 2010; Revised: March 27, 2011; Published online: May 17, 2011
Abstract: The intramolecular hydroamination of yield pyrrolidine and piperidine products in high
aminoalkynes and unactivated aminoalkenes cata- yields. A high diastereoselectivity of up to 23:1 is ob-
lyzed by yttrium N,O- and N,N-complexes has been served at 08C with 1-methyl-4-pentenylamine as sub-
investigated. The N,N-yttrium complexes are highly strate.
active, catalyzing the conversion of a wide range of
terminal aminoalkenes at room temperature, and in- Keywords: cyclohydroamination; hydroamination;
ternal aminoalkenes at elevated temperature, to rare earth metals; yttrium
Introduction
tion, reaction temperatures of 65 to 1108C are typical-
ly required.
À
The hydroamination reaction (addition of N H across
a carbon-carbon multiple bond) is a very powerful rare earth complexes has been investigated by Marks
Indeed the impact of ionic radius upon reactivity in
[1]
À
and efficient method for C N bond formation. In for a series of related Cp* (C5Me5) ligated complex-
particular, intramolecular hydroamination is a very es.[8f,11] These investigations showed that increased
selective and atom economic reaction to synthesize ionic radius results in higher turnover frequencies.
nitrogen heterocycles, which play an important role in Furthermore, modification of the ligand environment
material and life sciences.[2] Pioneering work in the to include constrained geometry organolanthanide
development of catalytic inter- and intramolecular hy- catalysts
droamination reactions was conducted by Marks and
{CGC,
AHCTUNGRTEN(NGUN SiMe3)} revealed that less sterically demanding
Me2SiACTHNGUTRENNUG ACTHUNGTREN(NUNG t-BuN)]LnN-
[h5-C5Me4)
co-workers[3] using rare earth elements, while Berg- ligand sets also result in improved reactivity.[12] Thus,
man achieved this transformation using group 4 as non-Cp derived systems have emerged as easily
metals.[4] Further research in the field by Living- tunable catalyst systems,[1] the variable steric bulk of
house,[5] Odom,[6] ourselves[7] and others[8] has focused different ligand sets can be used to advantage to pro-
on non-cyclopentadienyl (non-Cp) ligands in combi- mote desirable reactivity and selectivity trends.
nation with a broad range of metals. Our research
Compared to scandium, yttrium has a larger ionic
group has shown that commercially available Ti- radius and thus the coordination sphere is more ac-
[9]
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
have also disclosed one of the first applications of the coordination sphere can be further tuned. Here
scandium as a reactive metal with N,O- and N,N-che- we present highly active and diastereoselective N,O-
lating ligands for alkene hydroamination.[10] In this and N,N-chelated yttrium complexes for the room
work we have observed that improved access to the temperature intramolecular hydroamination of a wide
metal coordination sphere improves reactivity. Thus, range of terminally substituted aminoalkynes and
when the smaller scandium complexes or TiACTHNUTRGNEUNG
(NMe2)4 aminoalkenes.[13] This series of complexes further il-
are used as catalysts for intramolecular hydroamina- lustrates the influence of steric effects upon reactivity
trends in rare earth cyclohydroamination catalysis.
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Adv. Synth. Catal. 2011, 353, 1384 – 1390

Highly Active and Diastereoselective N,O- and N,N-Yttrium Complexes
Table 1. Intramolecular alkyne hydroamination.
Results and Discussion
Metal Complexes
Preferred ligand sets for non-Cp rare earth hydroami-
nation precatalyst synthesis include a variety of N
and/or O bound ligands such as amides,[5a,14] arylox-
ides,[15] guanidinates,[14a] amidates,[16] and b-diketimi-
nate derived systems.[8f,10,17] Very reactive and selec-
tive N,S-chelating complexes have also been used to
advantage for selective hydroamination catalysis.[5c,e]
While high reactivities are typically observed for ami-
noalkyne and aminoalkene intramolecular hydroami-
nation, comparisons of various yttrium complexes
provide valuable insight for ligand design.
Entry
Catalyst
Time [min]
Conversion [%][a]
1
2
3
1
2a
2b
25
25
25
>95
>95
>95
[a]
1
Conversions determined by H NMR spectroscopy
Table 2. Intramolecular hydroamination of aminoalkene 5.
This investigation focuses on N,O-chelating bis(sali-
cylaldiminato) complex 1 and the N,N-chelating anili-
do-imine complexes 2a and 2b (Figure 1). The synthe-
ses of these complexes have been previously reported,
and their application in hydroamination catalysis is
disclosed here.[18,19]
Entry Catalyst Temp. [8C] Time [min] Yield [%][a]
1
2
3
1
2a
2b
65
25
25
180
40
40
>95
>95
>95
Hydroamination Catalysis
[a]
1
Yield determined by H NMR spectroscopy using 1,3,5-
trimethoxybenzene as an internal standard.
For the initial experiments, 5-phenyl-4-pentynyl-1-
amine (3) (Table 1) has been employed as the intra-
molecular hydroamination substrate for catalyst
screening purposes. Product formation of the pyrroli- we cannot differentiate between the bis-ligated and
1
dine 4 can be monitored by H NMR spectroscopy the mono-ligated complexes. This rapid reactivity
using an NMR tube loaded with the appropriate cata- prompted further investigation of these catalyst sys-
lyst, substrate 3, and C6D6 under an inert atmosphere. tems with the more challenging unactivated aminoal-
The conversion of substrate to product is observed kenes.
with the appearance of the benzylic singlet at d=3.55,
To this end, the hydroamination of 2,2-diphenyl-4-
indicating product formation as well as the disappear- pentenylamine (5) (Table 2) catalyzed by 1 and 2, has
ꢀ
ance of the diagnostic triplet at d=2.45 (C CCH2), been performed. Reaction progress is monitored by
characteristic of the starting material.
the disappearance of the characteristic alkene proton
As shown in Table 1 all three yttrium complexes signals at d=5.03 and 5.40 ppm, and the appearance
catalyze the reaction at room temperature in 25 min of the diagnostic multiplet at d=3.18.
with a catalyst loading of 5 mol%. These results are
As observed with the alkynes, all three yttrium
consistent with many other rare earth catalysts.[1] complexes are able to promote the reaction in excel-
Here, due to the rapid reactivity using aminoalkyne 3, lent yields (>95% with 5 mol% catalyst loading).
However, disparate reactivity between the yttrium
complexes could be observed in the conversion of
aminoalkene 5. While the bis(salicylaldiminato) com-
plex 1 requires a reaction temperature of 658C to
reach yields greater than 95% within 3 h, the monoli-
gated complexes 2a and 2b can promote the reaction
at room temperature in 40 min. Temperatures of
>508C are often reported for yttrium-catalyzed ami-
noalkene hydroamination,[5e,14c,17,20] however, room
temperature hydroamination of disubstituted pyrroli-
dine has been reported for complexes with geometri-
cally constrained tethered ligand sets, where accessi-
bility to the metal center is enhanced.[13,15,16b,21] Thus,
the increased accessibility of the metal center in the
Figure 1. N,O- and N,N-chelating yttrium complexes for hy-
droamination catalysis.
monoligated complexes 2a and 2b results in higher ac-
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Table 3. Scope of reactivity with monoligated complex 2a as catalyst.
[a]
Yield determined by p-xylene as internal standards. All reactions were performed in tolu-
ene-d8 with 5% catalyst 2a.
tivity compared to the bisligated complex. As expect- substituted piperidines, such as 14, at room tempera-
ed, there is no difference between the reactivity of ture in 24 h in yields greater than 95%. The longer re-
catalysts 2a and 2b, consistent with the mechanistic in- action time compared to the pyrrolidine product 6
terpretation that these catalysts, upon treatment with (40 min) may be due to the seven-membered transi-
excess aminoalkene substrate, undergo protonolysis tion state required for the piperidine substrate. Most
À
of the Y C bonds to yield the catalytically active yt- importantly, the more hindered a-branched aminoal-
trium amido complexes. Therefore, only complex 2a kene 1-methyl-4-pentenylamine (15) can also be con-
has been employed to determine the scope of reactivi- verted to the desired product 16 in >95% yield at
ty with a variety of aminoalkene hydroamination sub- room temperature with extended reaction times
strates.
As shown in Table 3, all substrate scope reactions
(36 h) (see also Table 4).
Encouraged by the efficient catalysis of the chal-
have been carried out with 5 mol% catalyst loading at lenging a-branched aminoalkenes, an unactivated in-
room temperature with excellent yields (>92%). The ternal aminoalkene, 2,2-diphenyl-4-hexenylamine (17)
first three entries in Table 3 indicate that disubstitu- (Scheme 1), has been evaluated. Initial experiments
tion at the b-position (diphenyl, methyl/phenyl or di- with up to 10 mol% catalyst at room temperature pro-
methyl substitution) promotes reactivity, as reaction vide no conversion, however increasing the reaction
times are under one hour. This is consistent with the temperature to 658C results in product formation.
Thorpe–Ingold effect being significant for this yttrium Indeed a change of solvent from benzene-d6 to tolu-
complex. For example, when there is only one methyl ene-d8, and subsequent heating to 1008C provides the
group in the ß-position, thereby significantly reducing product in quantitative yield in 3 h with 5 mol% cata-
the steric bulk, the reaction time increases to 18 h at lyst loading.
room temperature (entry 4, Table 3, dr not recorded).
Even with these optimized conditions, secondary a-
As shown in entry 5, it is also possible to synthesize branched aminoalkenes, such as butyl-(1-methyl-4-
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Highly Active and Diastereoselective N,O- and N,N-Yttrium Complexes
Table 4. Diastereoselectivity of intramolecular alkene hydro-
amination.
perature is increased to 658C with catalyst 2b in an
attempt to reduce the reaction time, yields of >95%
were obtained within 90 min, however, the trans/cis
ratio dropped significantly to 11:1. Following this
trend, an improved diastereoselectivity of 23:1 was
achieved with a reaction temperature of 08C. While
some high diastereoselectivities have been previously
reported for yttrium complexes, these are for dianion-
ic, tethered ligand sets which typically result in the
Entry Catalyst Temp. Time [h] Yield [%][a] trans/cis[a]
1
2
3
4
2a
2b
2b
2b
258C 36
258C 36
658C 1.5
>95
>95
>95
>80
17:1
17:1
formation
of
sterically
demanding
reactive
sites.[5a,e,8g,24] Thus the anilido-imine ligand reported
here provides desirable stereochemical control and
good reactivity with a simple monoanionic ligand
framework. Indeed the hydroamination reaction at
08C is a testament to the reactive nature of the steri-
cally accessible N,N-chelating yttrium anilido-imine
complexes. In fact, yields of >90% are obtained by
longer reaction times (up to 4 days) without sacrific-
ing diastereoselectivity.
11:1
08C
48
23:1[b]
[a]
Yield and diastereoselectivity determined by 1H NMR
spectroscopy, p-xylene is used as an internal standard.
Toluene-d8 was used as solvent.
[b]
Persuaded by the results with commercially avail-
able TiACHTUNGTRNEUNG
(NMe2)4,[9] and the noted impact of the auxili-
ary ligand on slowing reactivity due to reduced steric
accessibility of metal center, we also tried a compari-
son with the known hydroamination catalyst
Y[NACHTNUTGRNEUNG
(SiMe3)2]3, which has no auxiliary ligands.[25]
Indeed this complex shows good reactivity with select
substrates. Pyrrolidine 6 (Scheme 3) could be synthe-
sized in >95% yield with 5 mol% of this commercial-
Scheme 1. An internal aminoalkene as substrate for the in-
tramolecular hydroamination with complex 2a.
pentenyl)amine (19) (Scheme 2) are unsuitable sub- ly available precatalyst at room temperature in 2.5 h.
strates and no N-substituted pyrrolidine product for- It is also possible to catalyze the conversion of sub-
mation can be observed regardless of catalyst loading strate 13 to the piperidine product 14 with 5 mol%
and reaction temperature.[22]
catalyst loading to obtain yields >95%, although here
Similarly, catalysis of the intermolecular hydroami- a reaction temperature of 658C is required. Notably,
nation of 1-hexyne and tert-butylamine did not prove the monoligated 2a can promote this reaction at room
possible with our preferred yttrium complex. Indeed, temperature. The fact that improved substrate scope
intermolecular hydroamination with rare earth com- and reactivity can be promoted through judicious se-
plexes is much less common than cyclohydroamina- lection of auxiliary ligands with tunable steric bulk
tion.[23]
can be seen when comparing entry 4 Table 3 (>90%
With the substrate scope of the yttrium complexes yield in 18 h at room temperature) with a previous
established, further experiments were conducted to report using Y[NACHTNUTRGNEG(UN SiMe3)2]3 (>95% yield in 10 days at
determine the stereoselectivity of the reaction. The 908C).[25a] Furthermore, only modest diastereoselec-
intramolecular hydroamination of a-branched amino- tivity (trans:cis, 7:1)[25a] was reported for the cyclohy-
alkene 1-methyl-4-pentenylamine (15) (Table 4) in the
presence of 5 mol% of 2a or 2b proceeds in quantita-
tive yield and with a 17:1 diastereomeric ratio, of the
trans-pyrrolidine as the major product. When the tem-
Scheme 2. Secondary a-branched aminoalkenes are not suit-
able substrates
Scheme 3. Y[N
hydroamination.
ACHTUGNRTNE(NUNG SiMe3)2]3 as a precatalyst for aminoalkene
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Frank Lauterwasser et al.
FULL PAPERS
over molecular sieves. p-Xylene as was distilled from
sodium under an atmosphere of argon and stored over mo-
lecular sieves.
droamination of 15 with Y[NACHTUNGRTNEUNG(SiMe3)2]3 as precatalyst,
which further illustrates the dramatic impact this one
monoanionic ligand has upon reactivity and selectivity
in hydroamination.
All aminoalkenes were distilled from CaH2 under reduced
pressure or under an argon atmosphere and stored in the
glove box, for details see ref.[10] Aminoalkenes 2,2-diphenyl-
4-pentenyl-1-amine (5),[26] 2-methyl-2-phenyl-4-pentenyl-1-
amine (7),[7c] 2,2-diphenyl-5-hexenyl-1-amine (13),[27] 1-meth-
ylpent-4-enylamine (15),[28] 2,2-dimethyl-4-pentenyl-1-amine
(9) and 2-methyl-4-pentenyl-1-amine (11)[29] were prepared
as described in the literature. The spectral data for the cis-
and trans-isomers of 2,5-dimethylpyrrolidine (cis-16 and
trans-16)[28] were entirely consistent with those described in
the literature. The diastereoselectivity for the cyclization of
Conclusions
We have presented bisligated N,O- and monoligated
N,N-yttrium complexes for intramolecular hydroami-
nation. The monoligated N,N-yttrium complexes 2a
and 2b show the highest reactivity over a wide range
of substrates with yields >92% at room temperature.
Only the internal aminoalkene required a reaction
temperature of up to 1008C. While steric accessibility
1
15 to trans-16 and cis-7f was determined based on H NMR
integration; trans-16/cis-16=3.14 (septet)/2.93 (m) for NCH
peaks.
promotes reactivity,
sphere promoted by an auxiliary ligand is preferred,
as Y[N(SiMe3)2]3 only shows high reactivity with
a well-defined coordination
ACHTUNGTRENNUNG
select substrates.[25] The catalysts 2a and 2b are suffi-
ciently reactive that reaction temperatures as low as
08C can be used, resulting in high diastereoselectivity
(up to 23:1). Even with the increased steric accessibil-
ity of 2, the sterically bulky secondary aminoalkene
19 could not undergo cyclohydroamination, even at
elevated temperatures. In conclusion, these results il-
lustrate how subtle changes in steric bulk on either
the substrate or the rare earth catalyst can dramatical-
ly perturb reactivity trends. Furthermore, we have
shown that a monosubstituted yttrium complex, with
a simple, monoanionic and sterically demanding che-
lating ligand can provide an excellent balance of de-
sirable reactivity and selectivity trends. On-going ef-
forts address the development of improved catalysts
with broad substrate scope, with the aim of realizing
intermolecular hydroamination catalysis at modest
temperatures.
General Procedure for Intramolecular Aminoalkene
Hydroamination Reactions
All reactions were carried out in an N2-filled glovebox on
an NMR-tube scale. The NMR tubes were equipped with a
Teflon screw cap. In the NMR tubes the aminoalkene, the
internal standard (1,3,5-trimethoxybenzene or p-xylene), the
complexes and benzene-d6 or toluene-d8 were combined.
The reactions were either heated in an oil bath to 658C,
cooled to 08C in a cryostat apparatus or allowed to stand at
room temperature.
2-Methyl-5,5-diphenyl-piperidine (14): Colourless liquid:
1H NMR (300 MHz, CDCl3): d=1.02 (d, J=6.4 Hz, 3H,
CH3), 1.17 (m, J=3.1, 10.9 Hz, 1H, CHH-CHCH3), 1.64
(dd, J=3.1, 13.3 Hz, 1H, CHH-CHCH3), 2.10–2.30 (m, 2H,
N-H, CPh2CHHCH2), 2.67 (dq, J=3.1, 13.6 Hz, 1H,
CPh2CHHCH2) 2.75–2.90 (m, 1H, CHCH3), 3.10 (d, J=13.7,
1H, CHHNH), 3.92 (dd, J=2.9, 13.6 Hz, 1H, CHHNH)
7.05–7.43 (m, 10H, Harom.); 13C NMR (75 MHz, CDCl3): d=
22.07 (CH3), 32.60 [CH2CH
ACHTNUGTRENN(UGN CH3)], 36.71 [CH2CH2CH-
ACHTNUTGRE(GUNNN CH3)], 46.62 (CH2CPh2CH2), 53.77 (CHCH3), 56.88
(CH2NH), 127.32, 127.84, 129.59, 129.67, 130.13, 145.86,
149.99 (Carom.); EI-MS (50 eV): m/z (%)=252 (2) [M+H+],
251 (7) [M+], 236 (2), 221 (2), 205 (2), 180 (20), 165 (18), 91
(8), 71 (6), 58 (100); HR-EI-MS: m/z=251.1673, calcd.:
251.1674.
Experimental Section
General
1H and 13C NMR spectra were recorded on either a Bruker
300 MHz or 400 MHz Avance spectrometer at ambient tem-
perature. Chemical shifts are given relative to residual sol-
vent. IR samples were prepared as nujol mulls on NaCl
disks or KBr pellets and recorded on a BOMEM Michelson
Series MB-100 FT-IR spectrophotometer. Mass spectra were
recorded on a Kratos MS-50 spectrometer using an electron
impact (70 eV) source. HR-MS determinations were per-
formed at the Department of Chemistry, University of Brit-
ish Columbia.
All reactions were carried out using standard Schlenk line
and glovebox techniques, under an atmosphere of nitrogen,
unless stated otherwise. Tetrahydrofuran and diethyl ether
were distilled from sodium benzophenone ketyl under an
inert gas atmosphere. Hexanes and toluene were purified by
passage through a column of activated alumina and de-
gassed with nitrogen. Benzene-d6 was degassed and dried
2-Ethyl-4,4-diphenyl-pyrrolidine (18): Colourless liquid;
1
Rf =0.24 (CH2Cl2/5% MeOH); H NMR (300 MHz, CDCl3):
d=0.95 (t, J=7.41 Hz, 3H, CH3), 1.40–1.67 (m, J=6.87 Hz,
2H,
CH2CH3),
2.05
[dd,
J=11.67 Hz,
1H,
CHHCH(NH)CH2CH3], 2.80 [dd, J=11.33 Hz, 1H,
CHHCH(NH)CH2CH3], 2.86–3.05 (bs, 1H, NH), 3.06–3.26
[m, 1H, CH(NH)CH2CH3], 3.43 (d, J=9.90 Hz, 1H,
CHHNH), 3.74 (d, J=9.36 Hz, 1H, CHHNH), 7.03–7.37 (m,
10H, Harom.); 13C NMR (75 MHz, CDCl3): d=11.49 (CH3),
29.63 (CH2CH3), 44.75 [CH2CH(NH)], 56.46 (Cquart.), 57.08
(CH2NH), 59.55 (CH), 126.05 (Carom.), 126.87 (Carom.), 126.95
(Carom.), 128.27 (Carom.), 128.36 (Carom.), 146.49 (Carom., quart.),
147.32 (Carom. quart.); EI-MS (50 eV): m/z (%)=251 (10)
[M+], 223 (26), 222 (100), 178 (65), 165 (67), 115 (61), 91
(51), 71 (98), 70 (89), 56 (90); HR-EI-MS: m/z=251.1673,
calcd. 251.1674.
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Adv. Synth. Catal. 2011, 353, 1384 – 1390

Highly Active and Diastereoselective N,O- and N,N-Yttrium Complexes
Casley, M. S. Hill, J. Am. Chem. Soc. 2005, 127, 2042;
Acknowledgements
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This work was supported in part by the Fonds der Chemi-
schen Industrie and the Deutsche Forschungsgemeinschaft
(DFG) (SFB 380) and also by the Natural Science and Engi-
neering Research Council of Canada (NSERC), Boehringer
Ingelheim Canada Ltd., and the University of British Colum-
bia.
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