Article
Organometallics, Vol. 29, No. 3, 2010 551
complexes, each rare earth metal ion is coordinated by one
indenyl ligand, two -CH2SiMe3 groups, and one THF
molecule. The geometry at the metal centers can be described
as distorted tetrahedral when the indenyl five-membered ring
is regarded as occupying a single polyhedral vertex. The
Table 2. Hydroamination of 2,2-Dimethylpent-4-enylamine Cat-
alyzed by 1-3 and (1,3-(SiMe3)2C9H5)Sc(CH2SiMe3)2(THF)a
˚
parameters of the slip distortion (ΔLn-C) (0.021 A in 1,
˚
˚
0.050 A in 2, and 0.029 A in 3) and the fold angle (Ω) (3.80° in
1, 2.70° in 2, and 3.74° in 3) indicate η5 hapticity of the
indenyl ligands in all of the complexes.13 The characteriza-
tion of the solid-state structures of complexes 1-3 and the
[cat.]/[sub.]
(%)
entry
catalyst
1(Y)
2(Lu)
3(Dy)
Sc complex
time (h) yield (%)b TOF (h-1 c
)
1
2
3
4
1
1
1
1
0.5
0.8
0.3
98
98
98
67
196
123
327
˚
previously reported Sc complex (ΔSc-C = 0.085 A, Ω =
3.08°), which differ only in the metal ions, provides the direct
crystallographic information of the metal size’s effect on the
indenyl hapticity. The structural data indicate that the slip
distortion value of the complex slightly decreases with an
24
2.8
a 60 °C, C6D6 as the solvent (0.50 mL), 0.50 mmol of substrate.
b NMR yield determined relative to ferrocene internal standard. c Turn-
over frequency.
increasing of the metal ion radii,14 from 0.085 A in the Sc
˚
˚
˚
complex to 0.050 A in the Lu complex, 0.021 A in the Y
sandwich indenyl complexes for the carbon atoms shared
by the five- and six-membered rings, which are opposite
the -SiMe3 substitutents, are longer than those for the
adjacent carbon atoms unique to the five-membered ring,
due to the indenyl ligand’s unique properties.
˚
complex, and 0.029 A in the Dy complex. On the other hand,
the change in the fold angle is small. Two distinct differences
between the structures of these half-sandwich indenyl
complexes and their Cp0 analogue 1,3-(SiMe3)2C5H3Sc-
(CH2SiMe3)2(THF)15 were observed. First, the distance
from metal ion to indenyl ligand is longer than that to Cp0
ligand when the difference in metal ion radii is counted. The
average Ln-C(five-membered ring) bond lengths in 1-3
and (1,3-(SiMe3)2C9H5)Sc(CH2SiMe3)2(THF) are 2.70, 2.66,
Catalytic Behaviors for Intramolecular Hydroamination of
Aminoalkenes. Intramolecular hydroamination of aminoalk-
enes offers an efficient and atom-economical method to
construct nitrogen heterocycles that are important for fine
chemicals and pharmaceuticals. Various metal complexes,
including those of alkali metals, early transition metals, and
late transition metals, have been investigated for this trans-
formation; the rare earth metal complexes are among the
most promising.16 The catalytic behaviors of half-sandwich
indenyl rare earth metal dialkyl complexes for intramolecular
hydroamination of aminoalkenes were initially tested by
employing 2,2-dimethylpent-4-enylamine (A) as the substrate
(Table 2). A solution of the metal complex, substrate, and
standard ferrocene in C6D6 was loaded into a NMR tube,
0
˚
2.72, and 2.56 A, respectively, while that in the Cp complex
˚
1,3-(SiMe3)2C5H3Sc(CH2SiMe3)2(THF) it is 2.51 A. Second,
the Sc ion of the Cp0 complex is closer to the carbon atoms
˚
opposite the -SiMe3 substituents (average 2.49 A) than to
other carbon atoms on the ring (average 2.52 A) due to
˚
the steric effect. In contrast, Ln-C bond lengths in the half-
(13) The slip distortion (ΔM-C) is defined as the difference between
the average distance from the metal center to the carbon atoms shared by
the five- and six-membered rings and the adjacent carbon atoms unique
to the five-membered ring. The fold angle (Ω) is defined as the angle
between the plane formed by the carbons unique to the five-membered
ring (C(2)-C(3)-C(4)) and the plane with C(1)-C(5)-C(2)-C(4).
1
and the reaction process was monitored by H NMR spec-
1
troscopy. The H NMR spectrum of the mixture showed a
˚
Typically, a slip distortion (ΔM-C) less than 0.30 A and a fold angle
rapid protonolysis of the rare earth metal alkyl by amine with
a release of SiMe4, and the indenyl ligand remains coordi-
nated to the metal ion. A clean formation of 2,4,4-trimethyl-
pyrrolidine, the Markovnikov-selective product, was ob-
served in several minutes, and no traces of other heterocyclic
regioisomers were detected through the proceeding in all
cases. Catalytic activity of the metal complex dramatically
increased with increasing ionic radii. The indenyl Sc complex
gave 67% yield over 24 h with 1.0 mol % catalyst loading.
When the Lu complex (2) was used, the reaction was much
faster and was complete within 0.8 h. For the Y complex (1)
and Dy complex (3), the time for completing the reaction was
only 0.5 and 0.3 h, respectively. The activities of 1 and 3 for
intramolecular hydroamination are comparable to the most
active rare earth metal complexes reported.
1 and 3 were subsequently applied for the intramolecular
hydroamination of a series of aminoalkenes (B-E) (Table 3).
All substrates were converted to the cyclic products in high
yield under mild reaction conditions (60 °C) with rather
low catalyst loading (1.0-3.0 mol %) and short reaction
time (2.5-9 h), and the Dy complex (3) showed higher
activities than the Y complex (1). According to Baldwin’s
rule for ring closure that the formation of a six-membered
ring is less favorable than that of a five-membered ring,17
(Ω) less than 8° indicate a η5 coordination mode of the indenyl ligand;
ꢀ
see: Bradley, C. A.; Flores-Torres, S.; Lobkovsky, E.; Abruna, H. D.;
Chirik, P. J. Organometallics 2004, 23, 5332.
3þ
3þ
(14) Six-coordinate ionic radii: Sc3þ (0.745 A), Lu (0.861 A), Y
˚
˚
3þ
˚
(0.900 A), Dy (0.912 A). Shannon, R. D. Acta Crystallogr., Sect. A
˚
1976, A32, 751.
(15) Li, X.; Baldamus, J.; Hou, Z. Angew. Chem., Int. Ed. 2005, 44,
962.
(16) For representative samples of intramolecular hydroamination
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ꢁ
Acc. Chem. Res. 2004, 37, 673. (b) Gagne, M. R.; Marks, T. J. J. Am. Chem.
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