Late transition metal catalyzed intramolecular hydroamination: The effect of ligand and substrate structure

Article abstract of DOI:10.1021/om700428q

A series of cationic rhodium(I) and iridium(I) complexes of the type [M(N-N)(CO)₂][BPh₄ containing the imidazole-based bidentate nitrogen donor ligands (N-N), including bis(N-methylimidazol-2-yl) methane (bim) (1, a M = Rh, b M = Ir), bis(N-methylimidazol-2-yl)ketone (bik) (5, a M = Rh, b M = Ir), 1,1-bis(N-methylimidazol-2-yl)ethane (bie) (3, M = Rh), 2,2-bis(N-methylimidazol-2-yl)propane (bip) (4, M = Rh), and bis(N-methylbenzimidazol-2-yl)methane (bbnzim) (6a M = Rh, 6b M = Ir), were synthesized and characterized. Complexes incorporating the pyrazole analogue bis(1-pyrazolyl)methane (bpm) [M(bpm)(CO)₂] [BPh₄] (2, a M = Rh, b M = Ir) were also prepared. The efficiency of each of the complexes as catalysts for the cyclization of 4-pentyn-1-amine to 2-methyl-1-pyrroline is reported. The influence of structural variations of the nitrogen donor ligand on the catalytic efficiency of cationic complexes of the type [M(N-N)(CO) ₂][BPh₄] for the hydroamination reaction was found to be much less than the influence of the nature of the counterion. The scope of the substrates for the intramolecular hydroamination of alkynamines was also investigated using Rh and Ir catalysts with the bim and bpm ligands, using 3-butyn-1-amine (7a), 3-pentyn-1-amine (7b), 4-phenyl-3-butyn-1-amine (7c), 4-pentyn-1-amine (8a), 4-hexyn-1-amine (8b), 5-phenyl-4-pentyn-l-amine (8c), 5-hexyn-1-amine (9), and 6-heptyn-1-amine (10) as substrates. The rhodium and iridium catalysts under investigation preferentially catalyze the formation of five-membered rings and do not catalyze the formation of four- or seven-membered rings. The effect of the substituents on the alkyne on the efficiency of hydroamination differentiates Rh(I) and Ir(I) and suggests the nature of the reactive metal-bound alkynyl intermediate.

Full text of DOI:10.1021/om700428q

Organometallics 2007, 26, 4335-4343
4335
Late Transition Metal Catalyzed Intramolecular Hydroamination:
The Effect of Ligand and Substrate Structure
Suzanne Burling,† Leslie D. Field,*,‡ Barbara A. Messerle,*,‡ and Sarah L. Rumble‡
School of Chemistry, UniVersity of Sydney, Sydney, NSW 2006, Australia, and School of Chemistry,
UniVersity of New South Wales, Sydney, NSW 2052, Australia
ReceiVed May 2, 2007
A series of cationic rhodium(I) and iridium(I) complexes of the type [M(N-N)(CO)2][BPh4] containing
the imidazole-based bidentate nitrogen donor ligands (N-N), including bis(N-methylimidazol-2-yl)methane
(bim) (1, a M ) Rh, b M ) Ir), bis(N-methylimidazol-2-yl)ketone (bik) (5, a M ) Rh, b M ) Ir),
1,1-bis(N-methylimidazol-2-yl)ethane (bie) (3, M ) Rh), 2,2-bis(N-methylimidazol-2-yl)propane (bip)
(4, M ) Rh), and bis(N-methylbenzimidazol-2-yl)methane (bbnzim) (6a M ) Rh, 6b M ) Ir), were
synthesized and characterized. Complexes incorporating the pyrazole analogue bis(1-pyrazolyl)methane
(bpm) [M(bpm)(CO)2][BPh4] (2, a M ) Rh, b M ) Ir) were also prepared. The efficiency of each of the
complexes as catalysts for the cyclization of 4-pentyn-1-amine to 2-methyl-1-pyrroline is reported. The
influence of structural variations of the nitrogen donor ligand on the catalytic efficiency of cationic
complexes of the type [M(N-N)(CO)2][BPh4] for the hydroamination reaction was found to be much
less than the influence of the nature of the counterion. The scope of the substrates for the intramolecular
hydroamination of alkynamines was also investigated using Rh and Ir catalysts with the bim and bpm
ligands, using 3-butyn-1-amine (7a), 3-pentyn-1-amine (7b), 4-phenyl-3-butyn-1-amine (7c), 4-pentyn-
1-amine (8a), 4-hexyn-1-amine (8b), 5-phenyl-4-pentyn-1-amine (8c), 5-hexyn-1-amine (9), and 6-heptyn-
1-amine (10) as substrates. The rhodium and iridium catalysts under investigation preferentially catalyze
the formation of five-membered rings and do not catalyze the formation of four- or seven-membered
rings. The effect of the substituents on the alkyne on the efficiency of hydroamination differentiates
Rh(I) and Ir(I) and suggests the nature of the reactive metal-bound alkynyl intermediate.
been achieved using lanthanide,9,10 actinide,11 and late transition
metal complexes.12 Catalytic systems employed for the intramo-
lecular hydroamination reaction include calcium,13 early transi-
tion metals,14 rare earth metals,15 and lanthanides,9,16-18 as well
as late transition metals,6,19-23 especially palladium.24-27 In
contrast to both early transition metal and lanthanide complexes,
late transition metal complexes have the advantage of a low
Introduction
The hydroamination reaction involves the addition of N-H
bonds across carbon-carbon double and triple bonds and offers
an atom-efficient pathway to produce new primary, secondary,
and tertiary amines as well as imines and enamines.1,2 While
the hydroamination reaction is thermodynamically reasonable,
there is a high activation barrier under normal conditions, and
the reaction is only practical when it is appropriately catalyzed.3
Many catalytic systems are known to promote hydroamination,
including acids,4 bases,5 and alkali earth metals,1 but the most
successful approaches have been the use of transition metal
complexes and lanthanide complexes.1,6,7,10 Recent significant
developments have been the use of active group IV metal
complexes including alkyltitanocenes and titanium amido
complexes.8 Intermolecular hydroamination reactions have also
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* Corresponding author. E-mail: b.messerle@unsw.edu.au.
† University of Sydney.
‡ University of New South Wales.
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10.1021/om700428q CCC: $37.00 © 2007 American Chemical Society
Publication on Web 07/21/2007

4336 Organometallics, Vol. 26, No. 17, 2007
Burling et al.
oxophilicity and better functional group tolerance. There are
few rhodium or iridium complexes that have been reported to
catalyze intramolecular hydroamination.22,28-30
The generality of the catalyzed hydroamination reaction is
important in developing synthetic approaches to amines and
nitrogen-containing heterocycles. The late transition metal
catalysts preferentially form five-membered rings.20,27 Late
transition metal complexes have also been shown to efficiently
promote the synthesis of six-membered heterocycles from
appropriate alkynamine starting materials.22,26 Where the major-
ity of catalysts reported to date for hydroamination are effective
for the addition of amines to terminal alkynes, the efficiency
of the catalysis is also significantly affected by the use of
substituted alkynes.18,20,25
We have previously shown that the cationic Rh(I) and Ir(I)
complexes [M(bim)(CO)2][BPh4] (1, a M ) Rh, b M ) Ir) and
[M(bpm)(CO)2][BPh4] (2, a M ) Rh, b M ) Ir), containing
the bidentate imidazole ligands bis(N-methylimidazol-2-yl)-
methane (bim) and bis(1-pyrazolyl)methane (bpm), are effective
catalysts for the intramolecular hydroamination of 4-pentyn-1-
amine.28,30 With the aim of establishing the key structural
features of the ligand system as well as making improvements
to the catalyst performance, a series of analogous complexes
of the type [M(N-N)(CO)2][BPh4] (3-6) were prepared
with modifications to the bidentate nitrogen donor ligands (N-
N), including bis(N-methylimidazol-2-yl)ketone (bik) (5, a M
) Rh, b M ) Ir), 1,1-bis(N-methylimidazol-2-yl)ethane (bie)
(3), 2,2-bis(N-methylimidazol-2-yl)propane (bip) (4), and bis-
(N-methylbenzimidazol-2-yl)methane (bbnzim) (6, a M ) Rh,
b M ) Ir).
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Using the most active catalysts, the dependence of the
efficiency of the hydroamination reaction on substrate chain
length was investigated using the substrates 3-butyn-1-amine
(7a), 3-pentyn-1-amine (7b), 4-phenyl-3-butyn-1-amine (7c),
4-pentyn-1-amine (8a), 4-hexyn-1-amine (8b), 5-phenyl-4-
pentyn-1-amine (8c), 5-hexyn-1-amine (9), and 6-heptyn-1-
amine (10).
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Results and Discussion
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(a) Rhodium-Catalyzed Cyclization of 4-Pentyn-1-Amine
(8a) to 2-Methyl-1-pyrroline (11) with Modifications of
N-Donor Ligands. The efficiency of each of the rhodium
complexes [Rh(bim)(CO)2][BPh4] (1a), [Rh(bie)(CO)2][BPh4]
(3), [Rh(bip)(CO)2][BPh4] (4), [Rh(bik)(CO)2][BPh4] (5a), [Rh-
(bbnzim)(CO)2][BPh4] (6a), and [Rh(bpm)(CO)2][BPh4] (2a) for
promoting the hydroamination reaction was tested using 4-pen-
tyn-1-amine (8a) as a substrate (Scheme 1, Table 1). Similarly,
the efficiency of each of the iridium complexes [Ir(bim)(CO)2]-
[BPh4] (1b), [Ir(bik)(CO)2][BPh4] (5b), [Ir(bbnzim)(CO)2][BPh4]
(6b), and [Ir(bpm)(CO)2][BPh4] (2b) for promoting the hydroami-
nation reaction was tested (Table 1). All metal complexes tested
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Late Transition Metal Catalyzed Intramolecular Hydroamination
Organometallics, Vol. 26, No. 17, 2007 4337
Table 1. Yield of 2-Methyl-1-pyrroline (11) Obtained from
the Rhodium(I)- and Iridium(I)-Catalyzed Cyclization of
4-Pentyn-1-amine (8a) in Tetrahydrofuran-d8 at 60 °C, with
1.5 mol % Catalyst
alkyl groups, at the bridging carbon having only a minor effect
on reactivity. Substitution with very bulky alkyl groups was
not considered here, but may well have a more significant effect
on the catalytic efficiency.
complex
time (h) % conversion TOF (h-1)a
(b) Rhodium- and Iridium-Catalyzed Intramolecular
Cyclization of a Series of Alkyn-1-amines. The catalyzed
intramolecular hydroamination of a series of alkynamine
substrates of varying chain length and with both terminal and
internal acetylene bonds (7a-c, 8a-c, 9, 10) was investigated
to assess the selectivity and limitations of the intramolecular
hydroamination reaction using cationic Rh(I) and Ir(I) catalysts.
In order to compare the metal centers rhodium and iridium, and
the two nitrogen donor ligands bis(N-methylimidazol-2-yl)-
methane and bis(1-pyrazolyl)methane, the efficiency of the four
catalysts [Rh(bim)(CO)2][BPh4] (1a), [Rh(bpm)(CO)2][BPh4]
(2a), [Ir(bim)(CO)2][BPh4] (1b), and [Ir(bpm)(CO)2][BPh4] (2b)
in promoting the cyclization was established. The same reaction
procedure as was used above for the metal-catalyzed cyclization
of 4-pentyn-1-amine (8a) was applied to the catalyzed cycliza-
tion of all alkynamine substrates (Table 2).
[Ir(bpm)(CO)2][BPh4], 2b
[Ir(bim)(CO)2][BPh4], 1b
[Ir(bik)(CO)2][BPh4], 5b
[Ir(bbnzim)(CO)2][BPh4], 6b
[Rh(bim)(CO)2][BPh4], 1a
[Rh(bie)(CO)2][BPh4],b 3
1.5
24
12
12
14
12
24
12
24
12
24
12
24
12
100
92
98
93
100
71
87
78
84
79
89
5
7
10
17
6
[Rh(bip)(CO)2][BPh4],c 4
[Rh(bik)(CO)2][BPh4],c 5a
[Rh(bbnzim)(CO)2][BPh4], 6a
[Rh(bpm)(CO)2][BPh4], 2a
8
10
11
20
94
86
100
90
a Turnover frequency ) moles of product produced per mole of catalyst
used per hour; typically calculated at time of 50% conversion. b 1.6 mol %
catalyst. c 1.3 mol % catalyst.
Scheme 1. Catalyzed Cyclization of 4-Pentynamine
(i) Cyclization of 3-Butyn-1-amine (7a) to 1-Pyrroline (12).
Intramolecular cyclization of 3-butyn-1-amine (7a) via hy-
droamination may give two possible isomers resulting from
either 4-exo or 5-endo ring closure. Heating 3-butyn-1-amine
(7a) for 24 h with [Rh(bim)(CO)2][BPh4] (1a) at reflux led to
a low level of conversion (20%) to 1-pyrroline (12) resulting
from 5-endo ring closure. The other complexes did not promote
the cyclization of this substrate.
catalytically cyclized the substrate 8a regioselectively to
2-methyl-1-pyrroline (11), with varying degrees of efficiency.
No six-membered heterocyclic products resulting from the
alternative endo-cyclization pathway were observed.
(ii) Cyclization of 3-Pentyn-1-amine (7b) to 2-Methyl-1-
pyrroline (11). Reaction of 3-pentyn-1-amine (7b) using
complexes 1a, 1b, 2a, and 2b as catalysts, at 60 °C, afforded
2-methyl-1-pyrroline (11) as the only product, which represents
the successful cyclization of a substrate with an internal
acetylene functionality (Table 2). The rhodium complexes [Rh-
(bim)(CO)2][BPh4] (1a) and [Rh(bpm)(CO)2][BPh4] (1b) both
reached >90% conversion to 2-methyl-1-pyrroline (11) after 2
After 24 h, all Rh catalysts had promoted conversion of
between 84 and 100%, with small variation in the reaction curve
profiles. For the Ir(I) catalysts, greater variation was observed,
with the [Ir(bim)(CO)2][BPh4] (1b) catalyst being significantly
less effective in 12 h than the other Ir(I) catalysts. It can be
concluded that the exact nature of the substituents on the bis-
(imidazolyl) ligand is not a critically important feature of the
catalyst, with substitution by electron-withdrawing CO, or by
Table 2. Summary of the Rhodium- and Iridium-Catalyzed Intramolecular Hydroamination of Alkynamines Using Catalysts
[Rh(bim)(CO)2][BPh4] (1a), [Rh(bpm)(CO)2][BPh4] (2a), [Ir(bim)(CO)2][BPh4] (1b), and [Ir(bpm)(CO)2][BPh4] (2b) with between
1.4 and 1.6 mol % Catalyst Loading, in Tetrahydrofuran-d8
a At reflux. b At 60 °C.

4338 Organometallics, Vol. 26, No. 17, 2007
Burling et al.
Table 3. Yield of 2-Phenyl-1-pyrroline (16) from the
Catalyzed Cyclization of 4-Phenyl-3-butyn-1-amine (7c)
(thf-d8 at 60 °C) and Yield of 2-Benzyl-1-pyrroline (17) from
the Catalyzed Cyclization of 5-Phenyl-4-pentyn-1-amine (8c)
(dioxane-d8 at 95 °C)
days. The iridium analogues were far less active catalysts, with
the reaction catalyzed by [Ir(bpm)(CO)2][BPh4] (2b) producing
only 50% conversion after 2 days. Only minor conversion was
achieved using [Ir(bim)(CO)2][BPh4] (1b) as the catalyst.
For all of the complexes, 5-endo cyclization is a considerably
less facile process than the 5-exo process, which is utilized in
the cyclization of 4-pentyn-1-amine (8a) to afford the same
product, with >90% conversion. The preference for 5 exo
cyclization is most pronounced for the iridium catalyst [Ir(bpm)-
(CO)2][BPh4] (2b).
(iii) Cyclization of 4-Hexyn-1-amine (8b) to 2-Ethyl-1-
pyrroline (13). The reaction of 4-hexyn-1-amine (8b) was
catalyzed by each of the four complexes 1a,b and 2a,b in
tetrahydrofuran-d8 at both 60 °C and reflux. The major product
in all cases was 2-ethyl-1-pyrroline (13), resulting from exo
addition of the N-H to the acetylene. A second minor product
was also formed and was identified as 2-methyl-3,4,5,6-
tetrahydropyridine (14), the six-membered heterocycle resulting
from the alternative 6-endo cyclization.
Although all four complexes 1a, 1b, 2a, and 2b achieved
approximately 80% conversion of 4-hexyn-1-amine (8b) in 6 h
at high temperature, at lower temperatures it was clear that the
best conversions to 2-ethyl-1-pyrroline (13) were achieved using
the rhodium complexes (Table 2). The formation of 2-ethyl-1-
pyrroline (13) occurs via a 5-exo ring closure in the same fashion
as the cyclization of 4-pentyn-1-amine (8a) to 2-methyl-1-
pyrroline (11), although at a considerably slower rate.
(iv) Cyclization of 5-Hexyn-1-amine (9) to 2-Methyl-
3,4,5,6-tetrahydropyridine (14). The reaction of 5-hexyn-1-
amine (9) catalyzed by each of the four complexes 1a,b and
2a,b at reflux gave 2-methyl-3,4,5,6-tetrahydropyridine (12),31
resulting from 6-exo cyclization of the substrate. Almost
quantitative conversion of 9 to 2-methyl-3,4,5,6-tetrahydropy-
ridine (14) was achieved using the Ir catalyst, 2a, after 24 h,
whereas after the same time the equivalent Rh catalyst, 1a,
catalyzed the reaction with only moderate conversion (32%).
Low conversions (12%) were also achieved after 24 h using
both of the rhodium or iridium complexes containing bis(1-
pyrazolyl)methane ligands (Table 2). No conversion was
observed for any of the catalysts at 60 °C. Mu¨ller and
co-workers have studied the same cyclization reaction exten-
sively, catalyzed by a series of transition metals.21,22 Superior
results were achieved using 1 mol % of [Cu(CH3CN)4][PF6] in
acetonitrile at reflux, with complete conversion in under 9 h.20
(v) Attempted Cyclization of 6-Heptyn-1-amine (10). Li
and Marks reported the production of a seven-membered
heterocycle (92%) via the intramolecular hydroamination of
7-phenyl-6-heptyn-1-amine, although with a very low turnover
rate (0.11 h-1).18 In this work, the catalyzed cyclization of
6-heptyn-1-amine (10) was attempted, although the reaction as
catalyzed by each of the Rh and Ir complexes 1a,b and 2a,b
failed to cleanly produce the seven-membered heterocycle,
3,4,5,6-tetrahydro-7-methyl-azepine (15), which would result
from 7-exo cyclization (Table 2).
complexes 1a, 1b, 2a, and 2b required more forcing conditions
to be efficient and was therefore conducted in dioxane-d8 at
95 °C (Table 3). The rhodium complexes 1a and 2a were
significantly better catalysts for both of these cyclizations than
their iridium analogues, confirming that the rhodium complexes
are more reactive toward internal alkynes than the iridium
complexes.
For the cyclization of 5-phenyl-4-pentyn-1-amine (8c), the
rhodium complexes 1a and 2a promote a rapid initial reaction
rate, with a high turnover rate at 45% of 580 h-1 for [Rh(bpm)-
(CO)2][BPh4] (2a) and lower initial turnover rate of 189 h-1
for [Rh(bim)(CO)2][BPh4] (1a). [Rh(bpm)(CO)2][BPh4] (2a)
appears to be more prone to catalyst deactivation as the reaction
rate slows considerably after the first 10 min, reaching only
87% after 4.5 h.
In the case of the cyclization of 4-phenyl-3-butyn-1-amine
(7c), the best catalyst was [Rh(bpm)(CO)2][BPh4] (2a), with
quantitative conversion being achieved in just 2.5 h. However,
[Rh(bim)(CO)2][BPh4] (1a) had a faster initial rate of reaction,
with a turnover rate at 50% conversion of 75 h-1 for [Rh(bim)-
(CO)2][BPh4] (1a), and for [Rh(bpm)(CO)2][BPh4] (2a) a lower
initial rate of 35 h-1. This indicates that [Rh(bim)(CO)2][BPh4]
(2a) is more susceptible to catalyst deactivation with
non-terminal alkynes as substrates. Previous work has shown
that the iridium complexes are more robust and con-
tinue to act as effective catalysts over several reaction cycles
for the cyclization of 8a, while the performance of the rho-
dium analogues was reduced on performing addition of new
substrate.30
The catalytic activity of [Rh(bim)(CO)2][BPh4] (1a) for the
cyclization of 5-phenyl-4-pentyn-1-amine is the highest of all
late transition metal complexes reported to date for the cycliza-
tion of this substrate (Table 4).20,23
Rhodium(I). [Rh(bim)(CO)2][BPh4] (1a) was the most active
catalyst over the range of substrates studied, so to compare the
effect of the nature of the substituent on the alkyne moiety on
the efficiency of catalyzed cyclization, the cyclization of the
terminal alkyne as well as methyl- and phenyl-substituted
alkynes were compared using [Rh(bim)(CO)2][BPh4] (1a) as
catalyst.
The efficiency of cyclization by 1a of the substituted
4-alkynamines 5-phenyl-4-pentyn-1-amine (8c), 4-hexyn-1-
amine (8b), and 4-pentyn-1-amine (8a) is shown in Table 5.
The rate of cyclization decreases with substrate reactivity in
the order H > Me > Ph. Mu¨eller observed a similar rate
comparison for the late transition metal (Pd, Zn, and Cu
complexes) catalyzed cyclization of 4-alkynamines and sug-
gested that 4-pentyn-1-amine (8a) is cyclized more rapidly than
the 5-phenyl-4-pentynamine (8c) due to steric hindrance in the
transition state in the cyclization of 8c.20 Marks et al. observed
(b) Alkynyl Substituent Effects: Comparing Rh(I) and
Ir(I). The cyclization of two alkynamines with methyl substit-
uents on the alkyne (7b and 8b) has been demonstrated above.
The catalyzed reaction of 4-phenyl-3-butyn-1-amine (7c) with
all four complexes 1a, 1b, 2a, and 2b in tetrahydrofuran-d8 at
60 °C led to the formation of 2-phenyl-1-pyrroline (16) as the
sole product (Table 3). The cyclization of 5-phenyl-4-pentyn-
1-amine (8c) to 2-benzyl-1-pyrroline (17) catalyzed by all four
(31) Hua, D. H.; Miao, S. W.; Bharathi, S. N.; Katsuhira, T.; Bravo, A.
A. J. Org. Chem. 1990, 55, 3682-3684.

Late Transition Metal Catalyzed Intramolecular Hydroamination
Organometallics, Vol. 26, No. 17, 2007 4339
Table 4. Comparison of Yields and Turnover Frequencies for the Cyclization of 5-Phenyl-4-pentyn-1-amine (8c) to
2-Benzyl-1-pyrroline (17) Catalyzed by Late Transition Metal Complexes
% conversion
(time in h)
entry
catalyst (mol %)
solvent and temp
TOF (h-1)
1
[Rh(bim)(CO)2][BPh4] (S7) (4.8mol%)
[Cu(CH3CN)4]PF6 (1 mol %)
Zn(CF3SO3)2 (1 mol %)
[Pd(triphos)](CF3SO3)2 (1 mol %)
Ru3(CO)12 (1.3 mol %)
dioxane-d8, 95 °C
CH3CN, 82 °C
toluene, 111 °C
toluene, 111 °C
diglyme, 110 °C
96 (2.3)
100 (16)
100 (8.3)
100 (9)
189
29
55
220
320
420
523
51
>99 (4)
Table 5. Hydroamination of 3- and 4-Alkynamines 8a-c
and 7a-c Catalyzed by [Rh(bim)(CO)2][BPh4], 1a, in thf-d8
at 60 °Ca
In the case of the Rh(I) catalyst 1a, the influence of the
substituent on the alkyne moiety on the cyclization rate has the
opposite effect for 4-alkynamines (H > Me > Ph) than for the
3-alkynamines (Ph > Me > H). Steric effects in the transition
state are therefore unlikely to provide the explanation for the
order of substrate reactivity as both the 4-alkynamines and the
3-alkynamines should have similar steric constraints with regard
to the substituent on the alkyne moiety. The mechanistic
pathway for hydroamination includes an activation of the alkyne
moiety by metal binding. The major difference between the
cyclization of the two classes of substrates here (4-alkynyl- and
3-akynyl-) is the ring-closing pathway, where the cyclization
of the 4-alkynamines proceeds through an exo-cyclization
pathway and the cyclization of the 3-alkynamines proceeds
through an endo-cyclization pathway (Scheme 2). Postulated
structures for key transition states are shown in Figure 1.
The binding of the alkyne moiety to the metal center activates
the triple bond, and the amino group is then able to attack one
of the carbon atoms. In the endo-cyclization pathway, attack
occurs on the external carbon. Carbocations are stabilized by
increasing substitution on the cationic carbon and by aryl
substituents, so that when R ) H the carbocation will be least
stabilized. Therefore the order of stability of the endo-derived
transition state would be Ph > Me > H, which correlates with
the order of reactivity observed for the 3-alkynamine substrates.
In the case of the exo-cyclization pathway the internal carbon
must have greater carbocation character (Figure 1). In this case,
the carbon bearing the R substituent would have some carban-
ionic character, and this would have the opposite dependence
on R for the order of stability of that of the transition state for
the endo cyclization, as observed (Table 5).
a 7% conversion to the 6-endo product 2-methyl-3,4,5,6-tetrahydropy-
ridine also observed.
Figure 1. Postulated transition states for the rhodium-catalyzed
cyclization of alkynamines proceeding through endo- and exo-
cyclization pathways.
Scheme 2. Cyclization Routes for 3-Alkynamines (n ) 1)
and 4-Alkynamines (n ) 2)
Conclusions
Each of the rhodium and iridium complexes investigated were
catalytically active for the cyclization of 4-pentyn-1-amine (8a)
to 2-methyl-1-pyrroline (11). On comparison of the activity of
complexes 3, 4, 5, and 6 to that of [Rh(bim)(CO)2][BPh4] (1a),
it was clear that the modifications to the bis(imidazolyl) ligand
in complexes 3, 4, 5, and 6 did not produce a significant effect
on the rate of the reaction. Introduction of the alternate bidentate
nitrogen donor bis(1-pyrazolyl)methane in complex 2a also
produced a catalyst comparable to 1a. These results indicate
that small structural and electronic variations to the bidentate
sp2 nitrogen donor ligand did not significantly affect the
reactivity of the catalyst.
The catalyzed formation of a series of N-containing hetero-
cycles demonstrated that these catalysts have a strong preference
for the formation of five-membered heterocycles over four- or
six-membered rings. 3-Butyn-1-amine (7a) and 3-pentyn-1-
amine (7b) afford pyrroline products by 5-endo cyclization,
although not as readily as the 5-exo cyclization of 4-pentyn-1-
amine (8a), and 4-hexyn-1-amine (8b) afforded pyrroline
products. Intramolecular hydroamination to afford the six-
membered heterocycle 2-methyl-3,4,5,6-tetrahydropyridine (14)
the same order of reactivity using lanthanide complex catalyzed
hydroamination reactions, with H > Me > Ph, and also cited
steric effects as the primary reason for the relative activities,
although the differences were not as pronounced as those in
Table 5.18
The efficiency of hydroamination catalyzed by 1a for the
substituted 3-alkynamines 4-phenyl-3-butyn-1-amine (7c), 3-pen-
tyn-1-amine (7b), and 3-butyn-1-amine (7a) is presented in
Table 5. In this case, the rate of cyclization is affected by the
nature of the substituents on the alkyne with a decrease in
substrate reactivity in the order Ph > Me > H. Utimoto et al.
have investigated the Pd(II)-catalyzed cyclization of a similar
series of 3-alkynamines and found that the rate of cyclization
was affected by the nature of the substituents on the alkyne
with Ph ≈ n-C4H9 . H and postulated that the terminal alkyne
substrate would form a palladium acetylide, which could not
be transformed into a nitrogen heterocycle.25

4340 Organometallics, Vol. 26, No. 17, 2007
Burling et al.
degassed using three consecutive freeze-pump-thaw cycles and
vacuum distilled from suitable drying agents immediately
prior to use.
was less facile but still occurred, both as a minor product via
6-endo cyclization from 8b and as the sole product of the 6-exo
cyclization of 5-hexyn-1-amine (27). Attempts to produce a
seven-membered ring via hydroamination were unsuccessful.
Bulk compressed gases nitrogen (>99.5%) and carbon monoxide
(>99.5%) were obtained from British Oxygen Company (BOC
Gases) and used as supplied.
Comparison of the catalysis results for each of the four
complexes [Rh(bim)(CO)2][BPh4] (1a), [Rh(bpm)(CO)2][BPh4]
(2a), [Ir(bim)(CO)2][BPh4] (1b), and [Ir(bpm)(CO)2][BPh4] (2b)
over the range of substrates places the rhodium complex 1a as
the most versatile catalyst examined. [Rh(bim)(CO)2][BPh4] (1a)
successfully catalyzed more reactions and usually with higher
rates of conversion than the three remaining analogues 1b, 2a,
and 2b. There were, however, two anomalous results produced
by the iridium catalysts: the superior rate of cyclization achieved
by [Ir(bim)(CO)2][BPh4] (1b) when reacted with 5-hexyn-1-
amine (9) and the unexpectedly rapid rate with which [Ir(bpm)-
(CO)2][BPh4] (2b) cyclized 4-pentyn-1-amine (8a).
Infrared spectra were recorded on a Perkin-Elmer 1600 Series
FTIR spectrometer. Microanalyses were carried out at the Campbell
Microanalytical Laboratory, University of Otago, New Zealand.
Mass spectra of organic compounds were recorded on a Finnigan
Polaris Q mass spectrometer using a direct exposure probe (DEP).
Electrospray mass spectra of organometallic complexes were
recorded on a Finnigan LCQ mass spectrometer by direct infusion
of a tetrahydrofuran solution of the complex into the source. Melting
points were determined using a Gallenkamp melting point apparatus
and are uncorrected.
1H and 13C NMR spectra were recorded on a Bruker DRX400
spectrometer at 400.13 and 100.62 MHz, respectively, or a Bruker
DPX300 operating at 300 and 75 MHz, respectively. Spectra were
recorded at 300 K for characterization purposes unless otherwise
The Rh(I) catalysts were significantly more efficient catalysts
for the cyclization of alkynamines with phenyl- and methyl-
substituted alkynyls than the Ir(I) catalysts. Differences were
observed between the Rh(I)-catalyzed cyclization rates of
butynyl and pentynyl alkynamines with substituents on the
alkyne. The dependence of reaction efficiency on the nature of
the substituents was consistent with a mechanism where the
substituents could stabilize or destabilize the developing charge
in the metal-bound intermediate.
stated, as determined using a variable-temperature unit. 1H and 13
C
NMR chemical shifts (δ) were referenced to internal solvent
references.
(a) Synthesis of 1,1-(Bis(N-methylimidazol-2-yl))ethane (bie).
A solution of n-butyllithium (1.3 mL, 2 mmol) in hexanes was
added dropwise to a suspension of bis(N-methylimidazol-2-yl)-
methane (330 mg, 1.9 mmol) in tetrahydrofuran (20 mL) at
-78 °C under nitrogen. After stirring for 30 min at -78 °C,
iodomethane (0.12 mL, 1.9 mmol) in tetrahydrofuran (5 mL) was
added. The mixture was immediately warmed to -40 °C, at which
temperature a saturated ammonium chloride solution (10 mL) was
added to quench the reaction. The mixture was allowed to warm
to room temperature and water (5 mL) was added. Following
acidification with 3 M hydrochloric acid, the organic layer was
separated and extracted with 3 M hydrochloric acid (3 × 5 mL).
The combined aqueous layers were neutralized with sodium
carbonate, filtered, and extracted with dichloromethane (3 × 10
mL). The organic extracts were dried over magnesium sulfate and
filtered, and the solvent was removed under reduced pressure. 1,1-
(Bis(N-methylimidazol-2-yl))ethane remained as a cream solid (134
mg, 37%), mp 82-85 °C (lit.16 82-83 °C) and was used in
Experimental Section
All manipulations of metal complexes and air-sensitive reagents
were carried out using standard Schlenk techniques or in a Vacuum
Atmospheres nitrogen-filled glovebox. Organic starting materials
were obtained from Aldrich Chemical Co. Inc. The metal halide
salts RhCl3‚xH2O and IrCl3‚xH2O were purchased from Strem Inc.
and used as received.
Di-µ-chlorotetracarbonyldirhodium,32 di-µ-chlorobis(1,5-cyclooc-
tadiene)diiridium,33 bis(N-methylimidazol-2-yl)ketone (bik),28 bis-
(N-methylimidazol-2-yl)methane (bim),28 bis(pyrazol-1-yl)methane
(bpm),34 bis(N-methylbenzimidazol-2-yl)methane (bbnzim),35 and
the complexes [Rh(bim)(CO)2][BPh4],28 [Rh(bik)(CO)2][BPh4],35
[Rh(bbnzim)(CO)2][BPh4],35 and [Ir(bpm)(CO)2][BPh4]36 were pre-
pared by literature methods.
The substrates 4-pentyn-1-amine (8a)37 and 4-hexyn-1-amine
(8b)18 were prepared by literature methods and gave identical
spectroscopic detail to the published data. The alkynamines 3-butyn-
1-amine (7a), 3-pentyn-1-amine (7b), 5-hexyn-1-amine (9), 6-hep-
tyn-1-amine (10), 4-phenyl-3-butyn-1-amine (7c), and 5-phenyl-
4-pentyn-1-amine (8c) were prepared following literature procedures.38
1
subsequent reactions without further purification. H NMR (400
MHz, acetone-d6): δ 6.91 (d, 2H, 3JH4-H5 ) 1.0 Hz, H4), 6.78 (d,
2H, H5), 4.52 (q, 1H, 3JH-CH3 ) 7.4 Hz, C-H), 3.44 (s, 6H, N-CH3),
1.68 (d, 3H, C-CH3) ppm. 13C{1H} NMR (100 MHz, acetone-
d6): δ 148.3 (C2), 127.1 (C4), 122.3 (C5), 33.0 (N-CH3), 32.6 (C-
H), 17.5 (C-CH3) ppm. MS m/z (%): 190 (M+, 60), 175 (64), 134
(25), 96 (73), 81 (45), 42 (100).
(b) Synthesis of 2,2-(Bis(N-methylimidazol-2-yl))propane
(bip). A solution of n-butyllithium (1.25 mL, 2 mmol) in hexanes
was added dropwise to a suspension of 1,1-(bis(N-methylimidazol-
2-yl))ethane (370 mg, 1.95 mmol) in tetrahydrofuran (20 mL) at
-78 °C under nitrogen. After stirring for 30 min at -78 °C,
iodomethane (0.13 mL, 2.1 mmol) in tetrahydrofuran (5 mL) was
added. The mixture was stirred for 1 h, followed by warming to
-40 °C, at which temperature saturated ammonium chloride
solution (20 mL) was added to quench the reaction. The mixture
was allowed to warm to room temperature, and water (10 mL) was
added. Following acidification with 3 M hydrochloric acid, the
organic layer was separated and extracted with 3 M hydrochloric
acid (3 × 5 mL). The combined aqueous layers were neutralized
with sodium carbonate, filtered, and extracted with chloroform (3
× 10 mL). The organic extracts were dried over magnesium sulfate
and filtered, and the solvent was removed under reduced pressure.
2,2-(Bis(N-methylimidazol-2-yl))propane remained as small cream
crystals (158 mg, 40%), mp 110-112 °C, and was used in
subsequent reactions without further purification. High-resolution
For the purposes of air-sensitive manipulations and in the
preparation of metal complexes tetrahydrofuran and hexane were
predried over sodium wire and distilled from benzophenone ketyl
immediately prior to use. Methanol was dried over and distilled
from magnesium methoxide under nitrogen. Deuterated solvents
for NMR purposes were obtained from either Merck or Cambridge
Isotopes. Chloroform-d was used as supplied. Acetone-d6 and
tetrahydrofuran-d8, for use with air-sensitive compounds, were
(32) McCleverty, J. A.; Wilkinson, G. Inorg. Synth. 1991, 28, 84-86.
(33) Herde, J. L.; Lambert, J. C.; Senoff, C. V. Inorg. Synth. 1974, 15,
18-19.
(34) Elguero, J.; Julia, S.; Mazo, J. M. d.; Avila, L. Org. Prep. Proc.
Int. 1984, 16, 299-307.
(35) Elgafi, S.; Field, L. D.; Messerle, B. A.; Turner, P.; Hambley, T.
W. J. Organomet. Chem. 1999, 588, 69-77.
(36) Soler, L. P. Ph.D. Thesis; University of Sydney, 1999.
(37) Dumont, J.-L.; Chodkiewicz, W.; Cadiot, P. Bull. Soc. Chim. Fr.
1967, 2, 588-596.
(38) Morgan, J.; Field, L. D. Manuscript in preparation.

Late Transition Metal Catalyzed Intramolecular Hydroamination
Organometallics, Vol. 26, No. 17, 2007 4341
mass spectrum: calculated for C11H16N4 (M+) 204.1375; found
204.1377. 1H NMR (400 MHz, chloroform-d): δ 7.02 (d, 2H,
3JH4-H5 ) 1.0 Hz, H4), 6.81 (d, 2H, H5), 3.10 (s, 6H, N-CH3),
1.86 (s, 6H, C-CH3) ppm. 13C{1H} NMR (100 MHz, chloroform-
d): δ 150.7 (C2), 126.5 (C4), 123.5 (C5), 38.2 (C-(CH3)2) 33.8
(N-CH3), 27.5 (C-(CH3)2) ppm. MS m/z (%): 204 (M+, 83), 189
(100), 123 (99), 109 (96), 96 (99) 83 (49), 42 (36).
125.8 (m-C), 122.0 (p-C), 39.0 (N-CH3) ppm. ν (KBr): 2082 (Rh-
CO), 2015 (Rh-CO) cm-1. MS m/z: (ES+) 439 (100%,
[Ir(bik)(CO)2]+).
(f) Synthesis of [Ir(bbnzim)(CO)2][BPh4]. Bis(N-methylbenz-
imidazol-2-yl)methane (45 mg, 0.16 mmol) in methanol (5 mL)
was added to a solution of [Ir(COD)Cl]2 (50 mg, 75 µmol) in
methanol (10 mL) and hexane (3 mL) at room temperature. The
mixture was stirred for 30 min before adding a solution of sodium
tetraphenylborate (70 mg, 0.2 mmol) in methanol (5 mL). A bright
yellow precipitate formed. The mixture was degassed and placed
under an atmosphere of carbon monoxide gas and stirred for 2 days.
The yellow solid was isolated by filtration and washed with hexane
and methanol to give (bis(N-methylbenzimidazol-2-yl)methane)-
dicarbonyliridium(I) tetraphenylborate (109 mg, 86%), mp 194-
196 °C (dec), which was used without further purification. Anal.
Found: C, 61.1; H, 4.4; N, 6.8. C43H36BIrN4O2 requires: C, 61.2;
H, 4.3; N, 6.6. 1H NMR (400 MHz, acetone-d6): δ 8.05-8.02 (m,
2H, H5 or H4), 7.85-7.83 (m, 2H, H7), 7.63-7.60 (m, 4H, H6
and H5 or H4), 7.34 (m, 8H, o-H), 6.89 (m, 8H, m-H), 6.73 (m,
4H, p-H), 5.05 (s, 2H, CH2), 4.18 (s, 6H, N-CH3) ppm. 13C{1H}
NMR (100 MHz, acetone-d6): δ 173.3 (s, Ir-CO), 166.0-164.1
(m, B-C), 150.9 (C2), 140.7 (C8 or C9), 136.8 (o-C), 135.3 (C8 or
C9), 126.6 and 126.1 (C4 or C5 and C6), 126.0 (m-C), 122.2
(p-C), 117.9 (C4 or C5), 113.1 (C7), 32.1 (N-CH3), 25.8 (CH2)
ppm. ν (KBr): 2074 (Ir-CO), 2005 (Ir-CO) cm-1. MS m/z: (ES+)
525 (100%, [Ir(bbnzim)(CO)2]+).
General Procedures for Catalytic Reactions. Rhodium(I)- and
iridium(I)-catalyzed reactions were performed on a small scale under
nitrogen, in NMR tubes fitted with a concentric Teflon valve. The
substrate was typically added to the catalyst dissolved in deuterated
solvent, in a NMR tube at room temperature. The catalytic reactions
were performed at elevated temperature by heating in an oil bath
at the desired temperature or heating within the NMR spectrometer,
in the case of time course experiments. The temperature within
the magnet was calibrated using ethylene glycol.39 Products were
confirmed by comparison with spectral NMR data from the
literature and by comparison with authentic samples.
The conversion of starting material to product was determined
by integration of the product resonances relative to the substrate
resonances in the 1H NMR spectrum. 100% conversion was taken
to be the time where no remaining substrate peaks were evident.
The turnover rate (Nt h-1) was calculated as the number of moles
of product/mol of catalyst/hour and was usually calculated at the
point of 50% conversion of substrate to product.
Cyclization of 4-Pentyn-1-amine (8a) to 2-Methyl-1-pyrroline
(11). The rhodium(I)- and iridium(I)-catalyzed cyclization of
4-pentyn-1-amine (8a) led to 2-methyl-1-pyrroline (11) as the single
product. A typical reaction was performed as follows:
4-Pentyn-1-amine (8a) (56 mg, 0.67 mmol) was added to [Rh-
(bim)(CO)2][BPh4] (1a) (6 mg, 9.2 µmol) in tetrahydrofuran-d8 (0.5
mL) in a NMR tube under an atmosphere of nitrogen. The mixture
was heated at 60 °C, and 1H spectra were recorded at regular
intervals. The product, 2-methyl-1-pyrroline (11), was formed
quantitatively from starting material after 14 h. Assignment of the
product was based on comparison of the 1H and 13C NMR spectra
of 11 with the 1H and 13C NMR spectra of an authentic sample of
2-methyl-1-pyrroline. 1H NMR (tetrahydrofuran-d8): δ 3.67
(m, 2H, CH2CdN), 2.39 (t, 2H, 3JCH2-CH2 ) 7.9 Hz, CH2N), 1.91
(s, 3H, CH3), 1.78 (m, 2H, CH2) ppm. 13C{1H} NMR (tetrahydro-
furan-d8): δ 173.6 (CdN), 61.6 (CH2N), 39.0 (CH2CdN), 23.7
(CH2CH2N), 19.3 (CH3) ppm.
(c) Synthesis of [Rh(bie)(CO)2][BPh4]. 1,1-(Bis(N-methylimi-
dazol-2-yl))ethane (43 mg, 0.23 mmol) in methanol (5 mL) was
added to a solution of [Rh(CO)2Cl]2 (44 mg, 0.11 mmol) in
methanol (3 mL) at room temperature. The mixture was stirred for
30 min before adding excess sodium tetraphenylborate (50 mg) in
methanol (2 mL). A white-yellow gelatinous precipitate formed
immediately and this was isolated by filtration and washed with
methanol. The precipitate was recrystallized from acetone, affording
(1,1-(bis(N-methylimidazol-2-yl)ethane))dicarbonylrhodium(I) tet-
raphenylborate as a fine yellow-green crystalline solid (40 mg,
27%), mp 211-213 °C (dec). Anal. Found: C, 64.8; H, 5.2; N,
1
8.4. C36H34BN4O2Rh requires: C, 64.7; H, 5.1; N, 8.4. H NMR
(400 MHz, tetrahydrofuran-d8): δ 7.30 (m, 8H, o-H), 7.19 (d, 2H,
3JH4-H5 ) 1.5 Hz, H4), 7.04 (d, 2H, H5), 6.81 (m, 8H, m-H), 6.65
3
(m, 4H, p-H), 4.30 (q, 1H, JH-CH3 ) 7.2 Hz, C-H), 3.35 (s, 6H,
N-CH3), 1.48 (d, 3H, C-CH3) ppm. 13C{1H} NMR (100 MHz,
tetrahydrofuran-d8): δ 185.2 (d, 1JRh-CO ) 68 Hz, Rh-CO), 165.7-
164.3 (m, B-C), 147.6 (C2), 137.0 (o-C), 130.5 (C4), 125.6 (m-C)
124.1 (C5), 121.8 (p-C), 34.0 (N-CH3), 29.6 (C-H), 22.6 (C-CH3)
ppm. ν (KBr): 2090 (Rh-CO), 2028 (Rh-CO) cm-1. MS m/z:
(ES+) 349 (100%, [Rh(bie)(CO)2]+).
(d) Synthesis of [Rh(bip)(CO)2][BPh4]. 2,2-(Bis(N-methylimi-
dazol-2-yl))propane (40 mg, 0.2 mmol) in methanol (3 mL) was
added to a solution of [Rh(CO)2Cl]2 (36 mg, 93 µmol) in methanol
(5 mL) at room temperature. The mixture was stirred for 1 h before
adding excess sodium tetraphenylborate (30 mg) in methanol (3
mL). The precipitate that formed immediately was isolated by
filtration and washed with methanol, affording 2,2-(bis(N-meth-
ylimidazol-2-yl)propane)dicarbonylrhodium(I) tetraphenylborate as
a silver-black solid (82 mg, 61%), mp 172-176 °C (dec). Anal.
Found: C, 65.0; H, 5.4; N, 8.3. C37H36BN4O2Rh requires: C, 65.1;
1
H, 5.3; N, 8.2. H NMR (400 MHz, acetone-d6): δ 7.34 (m, 8H,
3
o-H), 7.31 (d, 2H, JH4-H5 ) 1.6 Hz, H4), 7.19 (d, 2H, H5), 6.91
(m, 8H, m-H), 6.77 (m, 4H, p-H), 4.07 (s, 6H, N-CH3), 2.44 (s,
6H, C-(CH3)2) ppm. 13C{1H} NMR (100 MHz, acetone-d6): δ 186.0
(d, 1JRh-CO ) 68.2 Hz, Rh-CO), 165.6-164.6 (m, B-C), 151.2 (C2),
137.4 (o-C), 130.5 (C4), 126.8 (C5), 126.4 (m-C), 122.6 (p-C), 43.7
(C-(CH3)2), 39.4 (N-CH3), 32.3 (C-(CH3)2) ppm. ν (KBr): 2089
(Rh-CO), 2027 (Rh-CO) cm-1. MS m/z: (ES+) 363 (100%, [Rh-
(bip)(CO)2]+). m/z (%): 434 (M+, 77), 406 (47), 378 (100), 349
(11), 276 (29), 149 (31). ν (KBr): 2054 (Rh-CO), 1993
(Rh-CO) cm-1
.
(e) Synthesis of [Ir(bik)(CO)2][BPh4]. Solutions of bis(N-
methylimidazol-2-yl)ketone (60 mg, 0.32 mmol) in methanol (3
mL) and sodium tetraphenylborate (120 mg, 0.35 mmol) in
methanol (3 mL) were added to a solution of [Ir(COD)Cl]2 (90
mg, 0.13 mmol) in methanol (10 mL) and hexane (3 mL) at room
temperature. A dark red precipitate formed immediately. The
mixture was stirred for 1 h, degassed, and placed under an
atmosphere of carbon monoxide gas. The red solid became yellow
within 5 min. The mixture was left to stir at room temperature for
24 h. The yellow solid was isolated by filtration and washed with
hexane and methanol, affording (bis(N-methylimidazol-2-yl)ke-
tone)dicarbonyliridium(I) tetraphenylborate (143 mg, 73%), mp 160
°C (darkens), 201-202 °C (dec), which was used without further
purification. 1H NMR (tetrahydrofuran-d8, 290 K): δ 7.52 (s, 2H,
H4), 7.29 (m, 8H, o-H), 7.24 (s, 2H, H5), 6.82 (m, 8H, m-H), 6.67
(m, 4H, p-H), 3.79 (s, 6H, N-CH3) ppm. 13C{1H} NMR
(tetrahydrofuran-d8): δ 172.3 (Ir-CO), 166.3 (CdO), 165.8-164.3
(m, B-C), 139.4 (C2), 137.1 (o-C), 135.0 (C4), 131.5 (C5),
Cyclization of 3-Butyn-1-amine (7a) to 1-Pyrroline (12). The
catalyzed cyclization of 3-butyn-1-amine (7a) was attempted using
complexes 1a, 2a, and 2b in tetrahydrofuran-d8 at 60 °C and at
reflux. No new products were found in reactions with complexes
(39) Amman, C.; Meier, P.; Merbach, A. E. J. Magn. Reson. 1982, 46,
319-321.

4342 Organometallics, Vol. 26, No. 17, 2007
Burling et al.
2a and 2b. Catalysis with complex 1a resulted in the formation of
1-pyrroline (12). Assignment of the product was based on com-
Cyclization of 5-Hexyn-1-amine (9) to 2-Methyl-3,4,5,6-
tetrahydropyridine (14). The cyclization of 5-hexyn-1-amine (9)
to 2-methyl-3,4,5,6-tetrahydropyridine (14) was catalyzed by
complexes 1a, 1b, 2a, and 2b in tetrahydrofuran-d8 at reflux.
Assignment of the product was based on comparison of the 1H and
13C NMR spectra with literature data.31 A typical reaction was
performed as follows:
1
parison of the H NMR spectra with literature data.40
3-Butyn-1-amine (7a) (20 mg, 0.29 mmol) was added to [Rh-
(bim)(CO)2][BPh4] (1a) (3 mg, 4.6 µmol) in tetrahydrofuran-d8 (0.5
mL) in a NMR tube under an atmosphere of nitrogen. The mixture
was heated at reflux in an oil bath for 24 h. 1-Pyrroline (12) was
formed in 20% conversion from starting material. 1H NMR
(tetrahydrofuran-d8): δ 7.44 (s, 1H, CH), 3.73 (m, 2H, CH2CdN),
5-Hexyn-1-amine (9) (44 mg, 0.45 mmol) was added to [Ir(bim)-
(CO)2][BPh4] (1b) (5 mg, 6.7 µmol) in tetrahydrofuran-d8
(0.5 mL) in a NMR tube under an atmosphere of nitrogen. The
3
2.44 (t, 2H, JH5-H4 ) 8.0 Hz, CH2N), 1.73 (m, 2H, CH2) ppm.
1
13C{1H} NMR (tetrahydrofuran-d8): δ 165.7 (CdN), 62.0 (CH2N),
37.2 (CH2CdN), 21.4 (CH2) ppm.
mixture was heated at reflux in an oil bath, and H spectra were
recorded at regular intervals. 2-Methyl-3,4,5,6-tetrahydropyridine
(14) was formed in 95% conversion from starting material
after 24 h.
Cyclization of 3-Pentyn-1-amine (7b) to 2-Methyl-1-pyrroline
(11). The cyclization of 3-pentyn-1-amine (7b) to 2-methyl-1-
pyrroline (11) was catalyzed by complexes 1a, 1b, 2a, and 2b in
tetrahydrofuran-d8 at 60 °C. Assignment of the product was based
Attempted Cyclization of 6-Heptyn-1-amine (10). The cata-
lyzed cyclization of 6-heptyn-1-amine (10) was attempted with
complexes 1a, 1b, 2a, and 2b. No new cyclized products were
detected. A typical reaction was performed as follows:
6-Heptyn-1-amine (10) (40 mg, 0.36 mmol) was added to [Rh-
(bim)(CO)2][BPh4] (1a) (5.5 mg, 8.4 µmol) in tetrahydrofuran-d8
(0.5 mL) in a NMR tube under an atmosphere of nitrogen. The
1
1
on comparison of the H and 13C NMR spectra of 11 with the H
and 13C NMR spectra of an authentic sample of 2-methyl-1-
pyrroline. A typical reaction was performed as follows:
3-Pentyn-1-amine (7b) (34 mg, 0.41 mmol) was added to [Rh-
(bim)(CO)2][BPh4] (1a) (4 mg, 6.1 µmol) in tetrahydrofuran-d8 (0.5
mL) in a NMR tube under an atmosphere of nitrogen. The mixture
1
mixture was heated at 60 °C in the spectrometer magnet, and H
1
spectra were recorded every 10 min using an automated time
was heated at 60 °C in the spectrometer magnet, and H spectra
1
course program. The H NMR spectrum showed no evidence of
were recorded every 10 min using an automated time course
program. 2-Methyl-1-pyrroline (11) was formed with 63% conver-
sion from starting material after 12 h.
Cyclization of 4-Hexyn-1-amine (8b) in Tetrahydrofuran-d8
at 60 °C. The cyclization of 4-hexyn-1-amine (8b) in tetrahydro-
furan-d8 at 60 °C was catalyzed by complexes 1a, 1b, 2a, and 2b.
A typical reaction was performed as follows:
the desired cyclic product after 16 h. The tube was placed in an oil
bath at reflux for 48 h, after which time no new products had
formed.
Cyclization of 4-Phenyl-3-butyn-1-amine (7c) to 2-Phenyl-1-
pyrroline (16). The cyclization of 4-phenyl-3-butyn-1-amine (7c)
in tetrahydrofuran-d8 at 60 °C was catalyzed by the complexes 1a,
1b, 2a, and 2b. A typical reaction was performed as follows:
[Rh(bpm)(CO)2][BPh4] (2a) (5.0 mg, 8.0 µmol) was weighed
into an NMR tube and dissolved in freshly distilled, dry, and
degassed tetrahydrofuran-d8 (0.6 mL). 4-Phenyl-3-butyn-1-amine
(7c) (63 mg, 0.43 mmol) was injected directly into the NMR tube.
4-Hexyn-1-amine (8b) (20 mg, 0.2 mmol) was added to [Rh-
(bim)(CO)2][BPh4] (1a) (2 mg, 3 µmol) in tetrahydrofuran-d8 (0.5
mL) in a NMR tube. The mixture was heated at 60 °C in the NMR
1
spectrometer magnet, and H spectra were recorded every 10 min
using an automated time course program. The major product,
2-ethyl-1-pyrroline (13), was formed with 68% conversion from
starting material after 16 h. The minor product 2-methyl-3,4,5,6-
tetrahydropyridine (14) was formed in 7% conversion. Complete
conversion was achieved within 48 h. Assignment of the products
1
The reaction was monitored by H NMR while being heated at
60 °C. 2-Phenyl-1-pyrroline (16) was formed in quantitative
conversion from the starting material after 2.5 h. The product was
identified by comparison of the 1H and 13C NMR data with literature
data.25
1
was based on comparison of the H and 13C NMR spectra of the
1H NMR (300 MHz, tetrahydrofuran-d8): δ 7.85 (m, 2H, ArH),
product with literature spectral data.
3
4
2-Ethyl-1-pyrroline (13).18 1H NMR (tetrahydrofuran-d8): δ
7.33 (m, 3H, ArH) 3.95 (tt, 2H, JH5-H4 ) 7.2 Hz, JH5-H3 ) 1.9
3
4
3
Hz, H5), 2.86 (tt, 2H, JH3-H4 ) 7.9 Hz, JH3-H5 ) 1.9 Hz, H3),
3.68 (m, 2H, CH2CdN), 2.39 (t, 2H, JH5-H4 ) 8.1 Hz, CH2N),
3
3
1.93 (p, 2H, JH4-H5 ) 7.2 Hz, JH4-H3 ) 7.9 Hz, H4) ppm. 13C
NMR (75 MHz, tetrahydrofuran-d8): δ 172.3 (CdN), 135.9 (ArC),
130.4 (ArCH), 128.7 (ArCH), 128.2 (ArCH), 62.1 (C5), 35.0 (C3),
23.3 (C4) ppm.
3
2.26 (q, 2H, JCH2-CH3 ) 7.5 Hz, CH2CH3), 1.78 (m, 2H, CH2),
1.09 (t, 3H, CH3) ppm. 13C{1H} NMR (tetrahydrofuran-d8): δ 177.4
(CdN), 61.5 (CH2N), 37.4 (CH2CdN), 27.2 (CH2CH2N), 23.4
(CH2CH3), 10.9 (CH3) ppm.
2-Methyl-3,4,5,6-tetrahydropyridine (14).31 1H NMR (tetrahy-
Cyclization of 5-Phenyl-4-pentyn-1-amine (8c) to 2-Benzyl-
1-pyrroline (17). The cyclization of 5-phenyl-4-pentyn-1-
amine (8c) in dioxane-d8 at 95 °C was catalyzed by the complexes
1a, 1b, 2a, and 2b. A typical reaction was performed as
follows:
3
drofuran-d8): δ 3.42 (m, 2H, CH2CdN), 2.05 (t, 2H, JH6-H5
)
6.4 Hz, CH2N), 2.05 (s, 3H, CH3), 1.56 (m, 2H, CH2), 1.46
(m, 2H, CH2) ppm. 13C{1H} NMR (tetrahydrofuran-d8): δ 166.1
(CdN), 49.8 (CH2N), 30.5 (CH2CdN), 27.4 (CH3), 22.8 (CH2),
20.7 (CH2) ppm.
Cyclization of 4-Hexyn-1-amine (8b) in Tetrahydrofuran-d8
at Reflux. The cyclization of 4-hexyn-1-amine (8b) in tetrahydro-
furan-d8 at reflux was catalyzed by complexes 1a, 1b, 2a, and 2b.
A typical reaction was performed as follows:
[Rh(bim)(CO)2][BPh4] (1a) (6.0 mg, 9.2 µmol) was weighed into
an NMR tube and suspended in dioxane-d8 (0.6 mL). 5-Phenyl-4-
pentyn-1-amine (8c) (31 mg, 1.9 mmol) was injected directly into
1
the NMR tube. The mixture was heated at 95 °C, and H NMR
spectra were recorded at regular intervals. 2-Benzyl-1-pyrroline (17)
was formed with 96% conversion from starting material after 2.3
4-Hexyn-1-amine (8b) (45 mg, 0.47 mmol) was added to [Rh-
(bim)(CO)2][BPh4] (1a) (5 mg, 7.6 µmol) in tetrahydrofuran-d8 (0.5
mL) in a NMR tube. The mixture was heated at reflux in an oil
1
h. The product was identified by comparison of the H and 13C
NMR data with literature data.18
1H NMR (300 MHz, dioxane-d8): δ 7.27-7.15 (m, 5H, ArH),
1
bath, and H spectra were recorded at regular intervals. 2-Ethyl-
3.73 (tt, 2H, 3JH5-H4 ) 7.2 Hz, 4JH5-H3 ) 1.7 Hz, H5), 3.61 (s, 2H,
1-pyrroline (13) was formed in 85% conversion from starting
material after 6.5 h. The minor product 2-methyl-3,4,5,6-tetrahy-
dropyridine (14) was formed in 9% conversion.
3
4
CH2Ph), 2.32 (t, 2H, JH3-H4 ) 7.9 Hz, JH3-H5 ) 1.7 Hz, H3),
1.74 (p, JH4-H5 ) 7.2 Hz, JH4-H3 ) 7.9 Hz, 2H, H4) ppm. 13C
NMR (75 MHz, dioxane-d8): δ 175.4 (CdN), 138.3 (ArC), 129.5
(ArCH), 128.9 (ArCH), 126.8 (ArCH), 61.2 (C5), 40.6 (CH2Ph),
36.6 (C3) 23.0 (C4) ppm.
3
3
(40) Cantrell, G. K.; Geib, S. J.; Meyer, T. Y. Organometallics 2000,
19, 3562-3568.

Late Transition Metal Catalyzed Intramolecular Hydroamination
Organometallics, Vol. 26, No. 17, 2007 4343
Acknowledgment. We gratefully acknowledge the
Australian Government for Australian Postgraduate Scho-
larships (S.B. and S.L.R.) and the Australian Research
Council for funding. We would also like to thank Ms.
Jackie Morgan for her invaluable assistance with
compound preparation. We would like to thank Prof.
Todd Marder and Dr. Michael Whittlesey for helpful
discussions.
OM700428Q