Group 5 metal binaphtholate complexes for catalytic asymmetric hydroaminoalkylation and hydroamination/cyclization

Article abstract of DOI:10.1021/om1011006

3,3′-Silylated binaphtholate tantalum and niobium complexes were shown to be efficient catalysts for the asymmetric hydroaminoalkylation of N-methylaniline derivatives and N-benzylmethylamine with simple alkenes in enantioselectivities of up to 80% ee. No

Full text of DOI:10.1021/om1011006

Organometallics 2011, 30, 921–924 921
DOI: 10.1021/om1011006
Group 5 Metal Binaphtholate Complexes for Catalytic Asymmetric
Hydroaminoalkylation and Hydroamination/Cyclization
†
†
Alexander L. Reznichenko, Thomas J. Emge, Stephan Audorsch,†,‡ Eric G. Klauber,†
€
Kai C. Hultzsch,*,† and Bernd Schmidt‡
†Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey,
610 Taylor Road, Piscataway, New Jersey 08854-8087, United States , and ‡Institut fu€r Chemie,
Organische Synthesechemie, Universita€t Potsdam, Karl-Liebknecht-Strasse 24-25, D-14476 Golm, Germany
Received November 23, 2010
Summary: 3,30-Silylated binaphtholate tantalum and niobium
complexes were shown to be efficient catalysts for the asym-
metric hydroaminoalkylation of N-methylaniline derivatives
and N-benzylmethylamine with simple alkenes in enantioselec-
tivities of up to 80% ee. No hydroaminoalkylation was observed
with aminoalkenes; rather, exclusive asymmetric hydroamina-
tion/cyclization took place in up to 81% ee.
added across an unsaturated carbon-carbon bond. The
hydroamination reaction has been studied intensively, also
with respect to asymmetric reactions,3 and a variety of catalyst
systems based on early2b,4 and late5 transition metals as well
as main-group metals6 have been developed. However, reports
on group 5 metal based hydroamination catalysts have been
scarce7,8 and only the hydroamination of alkynes,7a,b allenes,7a,d
and activated alkenes7a,c has been studied. The group
5 metal-catalyzed hydroamination of aminoalkenes has not
been reported, to the best of our knowledge, and no niobium-
based hydroamination catalysts are known thus far.
Furthermore, the only chiral tantalum catalyst system for
the asymmetric hydroamination of aminoallenes in up to
34% ee was introduced only very recently.7d
Hydroamination1 and hydroaminoalkylation2 are two
atom-economical processes for the synthesis of valuable
industrial and pharmaceutical amines in which an amine
N-H bond and an R-amino C-H moiety, respectively, are
*To whom correspondence should be addressed. E-mail: hultzsch@
rci.rutgers.edu.
(1) (a) Doye, S. In Science of Synthesis; Enders, D., Ed.; Thieme:
Initial reports on hydroaminoalkylation9 surfaced 30 years
ago,10 but more detailed studies utilizing titanium-,11
zirconium-,11c,12 and tantalum-based13 catalysts have been
performed only recently. In several of these studies hydro-
aminoalkylation was first observed as a side reaction of
hydroamination.11c-e,12,14 While initial catalytic studies10
€
Stuttgart, Germany, 2009; Vol. 40a, p 241. (b) Muller, T. E.; Hultzsch,
K. C.; Yus, M.; Foubelo, F.; Tada, M. Chem. Rev. 2008, 108, 3795. (c)
Brunet, J. J.; Neibecker, D. In Catalytic Heterofunctionalization from
Hydroamination to Hydrozirconation; Togni, A., Gr€utzmacher, H., Eds.;
Wiley-VCH: Weinheim, Germany, 2001; p 91. (d) M€uller, T. E.; Beller, M.
Chem. Rev. 1998, 98, 675.
(2) (a) Roesky, P. W. Angew. Chem., Int. Ed. 2009, 48, 4892. (b)
Eisenberger, P.; Schafer, L. L. Pure Appl. Chem. 2010, 82, 1503.
(3) For reviews and accounts on asymmetric hydroamination see: (a)
Reznichenko, A. L.; Hultzsch, K. C. In Chiral Amine Synthesis: Meth-
ods, Developments and Applications; Nugent, T., Ed.; Wiley-VCH:
Weinheim, Germany, 2010; p 341. (b) Hannedouche, J.; Collin, J.; Trifonov,
A.; Schulz, E. J. Organomet. Chem. 2011, 696, 255. (c) Zi, G.
J. Organomet. Chem. 2011, 696, 68. (d) Zi, G. Dalton Trans. 2009,
9101. (e) Chemler, S. R. Org. Biomol. Chem. 2009, 7, 3009. (f) Aillaud,
I.; Collin, J.; Hannedouche, J.; Schulz, E. Dalton Trans. 2007, 5105. (g)
Hultzsch, K. C. Adv. Synth. Catal. 2005, 347, 367. (h) Hultzsch, K. C. Org.
Biomol. Chem. 2005, 3, 1819. (i) Hultzsch, K. C.; Gribkov, D. V.; Hampel, F.
J. Organomet. Chem. 2005, 690, 4441. (j) Roesky, P. W.; M€uller, T. E.
Angew. Chem., Int. Ed. 2003, 42, 2708.
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2519. See also: Anderson, L. L.; Arnold, J.; Bergman, R. G. Org. Lett. 2006,
8, 2445. (b) Lorber, C.; Choukroun, R.; Vendier, L. Organometallics 2004,
23, 1845. (c) Maurya, M. R.; Arya, A.; Kumar, U.; Kumar, A.; Avecilla, F.;
Pessoa, J. C. Dalton Trans. 2009, 9555. (d) Near, K. E.; Chapin, B. M.;
McAnnally-Linz, D. C.; Johnson, A. R. J. Organomet. Chem. 2011, 696, 81.
(8) For a discussion on possible mechanisms of the tantalum-
catalyzed hydroamination of alkynes with anilines see: Anderson,
L. L.; Schmidt, J. A. R.; Arnold, J.; Bergman, R. G. Organometallics
2006, 25, 3394.
(9) The hydroaminoalkylation, as discussed in this communication,
should be distinguished from the hydroaminomethylation reaction that
proceeds via a tandem hydroformylation/reductive amination sequence;
(4) (a) Andrea, T.; Eisen, M. S. Chem. Soc. Rev. 2008, 37, 550. (b) Lee,
A. V.; Schafer, L. L. Eur. J. Inorg. Chem. 2007, 2245. (c) Severin, R.; Doye,
S. Chem. Soc. Rev. 2007, 36, 1407. (d) Odom, A. L. Dalton Trans. 2005,
225. (e) Hong, S.; Marks, T. J. Acc. Chem. Res. 2004, 37, 673. (f) Doye, S.
Synlett 2004, 1653. (g) Bytschkov, I.; Doye, S. Eur. J. Org. Chem. 2003,
935. (h) Pohlki, F.; Doye, S. Chem. Soc. Rev. 2003, 32, 104.
€
see: (a) Eilbracht, P.; Barfacker, L.; Buss, C.; Hollmann, C.; Kitsos-
Rzychon, B. E.; Kranemann, C. L.; Rische, T.; Roggenbuck, R.;
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A.; Jiao, H. Angew. Chem., Int. Ed. 2004, 43, 3368.
(10) (a) Clerici, M. G.; Maspero, F. Synthesis 1980, 305. (b) Nugent,
W. A.; Ovenall, D. W.; Holmes, S. J. Organometallics 1983, 2, 161.
(11) (a) Kubiak, R.; Prochnow, I.; Doye, S. Angew. Chem., Int. Ed.
2010, 49, 2626. (b) Kubiak, R.; Prochnow, I.; Doye, S. Angew. Chem., Int.
Ed. 2009, 48, 1153. (c) Bexrud, J. A.; Eisenberger, P.; Leitch, D. C.; Payne,
P. R.; Schafer, L. L. J. Am. Chem. Soc. 2009, 131, 2116. (d) Prochnow, I.;
Kubiak, R.; Frey, O. N.; Beckhaus, R.; Doye, S. ChemCatChem 2009, 1,
162. (e) M€uller, C.; Saak, W.; Doye, S. Eur. J. Org. Chem. 2008, 2731.
(12) Bexrud, J. A.; Schafer, L. L. Dalton Trans. 2010, 39, 361.
(13) (a) Herzon, S. B.; Hartwig, J. F. J. Am. Chem. Soc. 2007, 129,
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Commun. 2010, 46, 6296.
€
(5) (a) Jenter, J.; Luhl, A.; Roesky, P. W.; Blechert, S. J. Organomet.
Chem. 2011, 696, 406. (b) Brunet, J. J.; Chu, N. C.; Rodriguez-Zubiri, M.
Eur. J. Inorg. Chem. 2007, 4711. (c) Widenhoefer, R. A.; Han, X. Eur. J.
Org. Chem. 2006, 4555. (d) Hartwig, J. F. Pure Appl. Chem. 2004, 76, 507.
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O. R.; Tillack, A.; Trauthwein, H. Synlett 2002, 1579.
(6) For a review of alkali-metal-based catalyst systems see: (a)
Seayad, J.; Tillack, A.; Hartung, C. G.; Beller, M. Adv. Synth. Catal.
2002, 344, 795. More recently also alkaline-earth metals and aluminum have
been utilized. See for example: (b) Datta, S.; Gamer, M. T.; Roesky, P. W.
Organometallics 2008, 27, 1207. (c) Crimmin, M. R.; Arrowsmith, M.;
Barrett, A. G. M.; Casely, I. J.; Hill, M. S.; Procopiou, P. A. J. Am. Chem.
Soc. 2009, 131, 9670. (d) Zhang, X.; Emge, T. J.; Hultzsch, K. C. Organo-
metallics 2010, 29, 5871. (e) Dunne, J. F.; Fulton, D. B.; Ellern, A.; Sadow,
A. D. J. Am. Chem. Soc. 2010, 132, 17680. (f) Koller, J.; Bergman, R. G.
Chem. Commun. 2010, 46, 4577.
(14) As a side reaction in alkali-metal-catalyzed hydroaminations:
´
Horrillo-Martınez, P.; Hultzsch, K. C.; Gil, A.; Branchadell, V. Eur. J.
Org. Chem. 2007, 3311.
r
2011 American Chemical Society
Published on Web 01/12/2011
pubs.acs.org/Organometallics

922 Organometallics, Vol. 30, No. 5, 2011
Reznichenko et al.
Scheme 1
suggested that niobium complexes should be viable catalysts,
more recent studies have been unsuccessful in confirming this
result.13d Only two reports on stereoselective hydroami-
noalkylation reactions have been reported utilizing chiral
tantalum bis(amidate) catalyst systems,13c,d and it seems
quite astonishing that enantioselectivies of up to 93% ee13d
could be achieved despite the drastic reaction conditions
(130-160 °C).
Figure 1. ORTEP diagram of the molecular structure of the
isostructural complexes 2d-M (M = Nb, Ta). Thermal ellipsoids
are shown at the 50% probability level. Hydrogen atoms as well
as one position of the disordered naphthyl ring and the dis-
ordered silane group are omitted for clarity. Selected bond
lengths (A) and angles (deg) for 2d-Nb: Nb-O1 = 2.0916(9),
Nb-O2=1.9594(9), Nb-N1=1.9528(13), Nb-N2=1.9351(12),
Nb-N3=2.0380(12); O1-Nb-O2=85.43(4), O1-Nb-N3=
172.32(5), O2-Nb-N1=125.79(5), O2-Nb-N2=114.08(5),
O2-Nb-N3=97.77(4), N1-Nb-N2=119.95(6). Selected bond
lengths (A) and angles (deg) for 2d-Ta: Ta-O1 = 2.0763(12),
Ta-O2=1.9478(11), Ta-N1=1.9591(15), Ta-N2=1.9354(15),
Ta-N3 = 2.0303(14); O1-Ta-O2 = 85.10(5), O1-Ta-N3 =
172.27(5), O2-Ta-N1 = 125.36(6), O2-Ta-N2 = 114.80(6),
O2-Ta-N3 = 97.46(5), N1-Ta-N2 = 119.61(6). Binaphthyl
dihedral angle (deg): 68.5(2) (2d-Nb), 68.1(2) (2d-Ta).
We envisioned that 3,30-silylated binaphtholate group 5
complexes15 may be suitable catalysts for efficient hydro-
aminoalkylation and/or hydroamination, on the basis of our
previous experience with analogous thermally robust16 and
highly enantioselective17 rare-earth-metal hydroamination
catalysts. In this communication we report that niobium
complexes are indeed efficient catalysts for asymmetric
hydroaminoalkylations and that chiral tantalum complexes
are chemoselective catalysts for asymmetric hydroaminoalk-
ylations and hydroamination/cyclizations, depending on the
substrates employed in the transformation.
Table 1. Catalytic Asymmetric Hydroaminoalkylation of 1-Oc-
tene with N-Methylaniline using Binaphtholate Tantalum and
Niobium Complexesa
A series of enantiomerically pure (R)-binaphtholate tantalum
and niobium amido complexes were readily prepared via
rapid amine elimination reactions between M(NMe2)5 (M=
Nb, Ta) and the diols 1a-e, containing 3,30-silyl substituents
of various degrees of steric bulk (Scheme 1).15 While the
reactions are quantitative according to NMR spectroscopic
analysis, the isolated yields of the niobium complexes are
often lower in comparison to those of their tantalum counter-
parts due to their slightly higher solubility. The complexes
generally retain 1 equiv of HNMe2, according to NMR
spectroscopy, and the coordinated base could not be re-
moved even after extended heating under vacuum. However,
the tert-butyldimethylsilyl-substituted complexes 2d-Nb and
2d-Ta crystallized in the base-free form and an X-ray crystal-
lographic analysis of the isostructural18 complexes confirmed a
slightly distorted trigonal bipyramidal geometry around the
metal (Figure 1).
entry
cat.
T, °C; t, h
yield, %b
ee, %c
1
2
3
4
5
6
7
8
2a-Ta
2a-Nb
2b-Ta
2b-Nb
2c-Ta
2c-Tad
2c-Tae
2c-Ta
2c-Nb
2c-Nb
2d-Ta
2d-Nb
2e-Ta
170; 48
170; 48
170; 48
170; 48
150; 14
150; 14
150; 18
140; 27
150; 7
trace
trace
trace
trace
88
91
90
85
85
88
79
72
87
nd
nd
nd
nd
72
72
71
73
72
73
49
61
34
9
10
11
12
13
140; 11
150; 12
150; 8
(15) The synthesis of the tantalumcomplexes 2a-Ta, 2c-Ta, and 2e-Ta
has been previously reported by Rothwell; see: (a) Thorn, M. G.; Moses,
J. E.; Fanwick, P. E.; Rothwell, I. P. Dalton Trans. 2000, 2659. (b) Son,
A. J. R.; Schweiger, S. W.; Thorn, M. G.; Moses, J. E.; Fanwick, P. E.;
Rothwell, I. P. Dalton Trans. 2003, 1620. See also: (c) Weinert, C. S.;
Fanwick, P. E.; Rothwell, I. P. Organometallics 2002, 21, 484. (d) Weinert,
C. S.; Fanwick, P. E.; Rothwell, I. P. Organometallics 2005, 24, 5759.
(16) Reznichenko, A. L.; Nguyen, H. N.; Hultzsch, K. C. Angew.
Chem., Int. Ed. 2010, 49, 8984.
(17) (a) Gribkov, D. V.; Hultzsch, K. C.; Hampel, F. J. Am. Chem.
Soc. 2006, 128, 3748. (b) Reznichenko, A. L.; Hampel, F.; Hultzsch, K. C.
Chem. Eur. J. 2009, 15, 12819. (c) Gribkov, D. V.; Hultzsch, K. C. Chem.
Commun. 2004, 730.
150; 5
a Conditions (unless otherwise noted): N-methylaniline (0.2 mmol),
1-octene (0.8 mmol), cat. (0.01 mmol, 5 mol %), C6D6 (0.3 mL), 150 °C,
Ar atmosphere. b Isolated yield of N-benzamide. c Determined by chiral
HPLC of N-benzamide. d 2c-Ta was prepared in situ and was directly
used for the catalytic experiment. e Conditions: N-methylaniline
(0.5 mmol), 1-octene (0.75 mmol), cat. (0.05 mmol, 10 mol %), C6D6
(0.2 mL). nd = not determined.
For our initial tests on catalytic hydroaminoalkylation we
chose 1-octene and N-methylaniline as model substrates
(Table 1). The catalytic activity of the catalysts significantly
decreased with increasing steric bulk of the silyl substituents,
(18) Niobium and tantalum both have the identical ionic radius of 64
pm in six-coordinate compounds; see: Shannon, R. D. Acta Crystallogr.
1976, A32, 751.

Communication
Organometallics, Vol. 30, No. 5, 2011 923
Table 2. Substrate Scope in the Intermolecular Asymmetric Hydroaminoalkylation Catalyzed by 2c-M (M = Ta, Nb)a
a Reaction conditions (unless noted otherwise): amine (0.2 mmol), alkene (0.8 mmol), 2c-M (0.01 mmol, 5 mol %), C6D6, 140 °C, Ar atmosphere.
b Determined by chiral HPLC of the corresponding N-benzamide. c Determined by 1H NMR spectroscopy of (S)-Mosher amide. d Reaction conditions:
180 °C, 10 mol % cat., 10:1 regioisomer ratio, yield of major regioisomer. e Determined by 19F NMR spectroscopy of Mosher amide after benzyl group
cleavage. f Reaction conditions: 150 °C, 7 mol % cat., 8:1 regioisomer ratio, yield of major regioisomer. PMP = p-methoxyphenyl.
with the most encumbered catalysts 2a,b producing only trace
amounts of product even at 170 °C (Table 1, entries 1-4).
The sterically less encumbered complexes 2c-e showed good
activity at 140-150 °C to form the branched hydroami-
noalkylation products exclusively. The sterically least de-
manding trimethylsilyl-substituted catalyst 2e-Ta displayed
the highest activity but also the lowest selectivity in this
series. The niobium complexes were generally more active
than their tantalum analogues, but the enantioselectivities
were quite comparable. The highest selectivity of 73% ee in
this initial screening was observed for the most demanding,
yet still active, methyldiphenylsilyl-substituted 2c-Nb and 2c-
Ta (Table 1, entries 8 and 10).
It should be noted that complexes prepared in situ showed
reactivity and selectivity identical with those of analytically
pure samples isolated after recrystallization (Table 1, entries
5 and 6). A 4-fold excess of alkene was used in most cases;
however, a reduced 1.5:1 alkene to amine ratio and increased
substrate and catalyst concentrations resulted in only slightly
longer reaction times and comparable selectivity (Table 1,
entries 5 and 7). We found that the use of a smaller amount of
catalyst and 4 equiv excess of the alkene was more conve-
nient. Therefore, further studies on the scope of the reaction
were conducted using these optimized conditions and the
most selective catalysts 2c-Ta and 2c-Nb.
Table 2 illustrates that the reaction is general for terminal
olefins and the reactivity and selectivity patterns are similar
for linear, homoallyl, and allyl-substituted alkenes, in remark-
able contrast with the case of intermolecular hydroamina-
tion, where the sterically more hindered vinylcyclohexane
exhibited significantly diminished reactivity.16 Internal un-
activated alkenes, such as cyclohexene, were found to be
unreactive. The reaction is unaffected by the presence of
electron-withdrawing or electron-donating substituents in
the aromatic ring of the N-methylaniline substrate (Table 2,
entries 5-10). The niobium complex 2c-Nb displayed
systematically higher reactivity in comparison to its
tantalum analogue 2c-Ta. The unsymmetric dialkylamine
N-benzylmethylamine (Table 2, entries 11 and 12) was less
reactive than arylalkylamines. Here the difference in reactiv-
ity of niobium and tantalum was even more pronounced, with
the niobium complex reacting at 150 °C, while tantalum
required a significantly higher reaction temperature of
180 °C. The reactions of N-methylaniline derivatives provide
enantioselectivities in a relatively narrow range of 67-80%
ee, while the reaction of N-benzylmethylamine was signifi-
cantly less selective. None of the reactions produced any
detectable amount of the linear addition product; however,
the reaction of N-benzylmethylamine produced the regioisomer
originating from activation of the benzylic position as a minor
byproduct (Table 2, entries 11 and 12). While mechanistic
details in this system are scarce, it can be assumed that the
reaction proceeds via alkene insertion into a metallaaziridine
intermediate, as proposed previously.10b,11c,13a-13c
Recent studies on titanium- and zirconium-catalyzed trans-
formations of primary aminoalkenes have shown that the
hydroaminoalkylation and hydroamination may occur con-
currently, favoring one or the other process depending on the
chain length of the aminoalkene.11c-e,12 The reactivity of
group 5 complexes in these reactions has not been reported
previously, and to our surprise we discovered that the
binaphtholate tantalum complexes readily catalyze the hy-
droamination/cyclization of various primary aminopentenes
and aminohexenes (Table 3), but no hydroaminoalkylation
products were detected. The cyclization of the gem-diphenyl-
activated aminopentene 3a proceeded with the tantalum
catalysts at 100-110 °C with 5 mol % catalyst loading.
The highest enantioselectivity for this substrate was 64%
ee using the sterically most hindered catalyst 2a-Ta. Less
encumbered complexes showed higher activity but lower
selectivity, and the niobium analogue was significantly less
reactive (Table 3, entry 2). Other gem-dialkyl activated

924 Organometallics, Vol. 30, No. 5, 2011
Reznichenko et al.
Table 3. Catalytic Asymmetric Hydroamination/Cyclization of
Aminoalkenes using Binaphtholate Tantalum and Niobium
Complexes
Attempts to cyclize aminoheptene 3e, 1,2-disubstituted
alkenylamine 3f, or the secondary aminoalkene 3g were
unsuccessful. With only a few exceptions,23 most neutral
group 4 metal based catalysts are unable to cyclize sec-
ondary aminoalkenes, which has been interpreted with a
[2 þ 2] cycloaddition mechanism analogous to group 4
metal catalyzed alkyne and allene hydroaminations,24
as the secondary amine does not allow formation of
the metal imido species required in this mechanism. How-
ever, previous mechanistic studies were unable to confirm
an analogous [2 þ 2] cycloaddition mechanism in the case
of the amine addition to alkynes catalyzed by cationic
tantalum imido complexes.7a,8 Examples of selective azepane
ring formation through group 4 metal-catalyzed hydroamina-
tion are also rare,11c,21b,23c and it is noteworthy that no
hydroaminoalkylation was observed for 3d,e using either
2a-Ta or 2a-Nb, in contrast to the case for group 4 metal
based systems,11c-e,12 which often produce mixtures of
hydroamination and hydroaminoalkylation products when
applied to aminohexenes or aminoheptenes.
In summary, we have presented herein the first niobium-
based chiral hydroaminoalkylation catalyst that achieves
enantioselectivities of up to 80% ee. We have also discovered
the first group 5 metal based hydroamination catalysts for
the asymmetric hydroamination of aminoalkenes with en-
antioselectivities of up to 81% ee. The hydroaminoalkyla-
tion reactivity of the chiral catalysts presented herein is of the
same order of magnitude as that of previously studied chiral
catalyst systems.13c,d The binaphtholate ligands provide a
stable coordination environment under these harsh reaction
conditions, and a large variety of 3,30-substituted binaphtho-
late ligands are readily available in a few synthetic steps
starting from inexpensive enantiopure BINOL, which will
allow facile catalyst tuning. We are currently performing
further catalytic and mechanistic studies8 toward the scope
and limitation of complexes 2 in order to improve the
enantioselectivity and reactivity of these new catalysts. In
particular, it can be expected that the replacement of amido
ligands by electron-withdrawing ligands, e.g. halides,13b
may lead to more active hydroaminoalkylation catalyst
systems.
entry
substrate
cat.
T, °C; t, hb
ee, % (confign)
1
2
3
4
5
6
7
8
3a
3a
3a
3a
3a
3a
3b
3c
3d
3e
3f
2a-Ta
2a-Nb
2b-Ta
2c-Ta
2d-Ta
2e-Ta
2a-Ta
2a-Ta
2a-Ta
2a-Md
2a-Ta
2a-Ta
100; 16
140; 18
110; 14
100; 12
100; 12
100; 11
120; 30
140; 34
120; 36
170; 36
170; 36
170; 36
64 (S)c
50 (S)
<5% (S)
10 (S)
<5 (S)
21 (S)
81(S)
41 (S)
17
9
e
10
11
12
e
e
3g
a Reaction conditions: 5 mol % cat., C6D6, Ar atmosphere. b >95%
conversion based on 1H NMR spectroscopy using ferrocene as internal
standard. c 80% isolated yield. d M = Ta, Nb. e No reaction.
substrates were cyclized with selectivities of up to 81% ee
(Table 3, entry 7). Similar to the case for many previous
catalyst systems,19 the cyclization of aminohexene deriva-
tives, such as 3d, proceeded with significantly diminished
selectivity, due to the higher flexibility of the cyclization
transition state relative to that of the cyclization of amino-
pentenes. While the catalytic activity of complexes 2 in the
hydroamination/cyclization of aminoalkenes is significantly
lower than that of rare earth,1b,4e,17 alkaline,20 and alkaline
earth metal6b-e based hydroamination catalysts, the reactiv-
ity is quite comparable to many group 4 metal based catalyst
systems.21 However, there are only a limited number of
catalyst systems that provide enantioselectivities exceeding
80% ee in these types of cyclizations.17,19b,19c,22
Acknowledgment. This work was supported by the
National Science Foundation through a NSF CAREER
Award (CHE 0956021). S.A. thanks the Deutscher Aka-
demischer Austauschdienst (DAAD) for an overseas
exchange award. E.G.K. thanks the U.S. Department
of Education for a GAANN fellowship.
(19) See for example: (a) Ayinla, R. O.; Gibson, T.; Schafer, L. L.
J. Organomet. Chem. 2011, 696, 50. (b) Shen, X.; Buchwald, S. L. Angew.
Chem., Int. Ed. 2010, 49, 564. (c) Wood, M. C.; Leitch, D. C.; Yeung, C. S.;
Kozak, J. A.; Schafer, L. L. Angew. Chem., Int. Ed. 2007, 46, 354. (d)
Supporting Information Available: Text, figures, and tables
giving experimental procedures and characterization data,
NMR spectra of all new ligands and complexes, Mosher amides,
and HPLC traces and CIF files giving crystal data for 2d-Nb and
2d-Ta. This material is available free of charge via the Internet at
http://pubs.acs.org.
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