A pentagonal pyramidal zirconium imido complex for catalytic hydroamination of unactivated alkenes

Article abstract of DOI:10.1021/om060545n

The first, isolable group 4 imido complexes capable of promoting intramolecular olefin hydroamination for the preparation of N-containing heterocycles are presented. The structurally characterized 6-coordinate bis(amidate)-supported zirconium imido complex is a rare example of distorted-pentagonal-pyramidal geometry.

Full text of DOI:10.1021/om060545n

Organometallics 2006, 25, 4069-4071
4069
A Pentagonal Pyramidal Zirconium Imido Complex for Catalytic
Hydroamination of Unactivated Alkenes
Robert K. Thomson, Jason A. Bexrud, and Laurel L. Schafer*
Department of Chemistry, UniVersity of British Columbia, 6174 UniVersity BouleVard,
VancouVer, British Columbia, Canada V6T 1Z1
ReceiVed June 22, 2006
Summary: The first, isolable group 4 imido complexes capable
of promoting intramolecular olefin hydroamination for the
preparation of N-containing heterocycles are presented. The
structurally characterized 6-coordinate bis(amidate)-supported
zirconium imido complex is a rare example of distorted-
pentagonal-pyramidal geometry.
decades of research in the field of zirconium imido chemistry,
there have been no previous reports of characterized group 4
imido complexes that mediate alkene hydroamination.7 Here we
present the first bis(amidate)-supported group 4 imido complex,
which is also the first isolable imido complex that is a competent
precatalyst for alkene hydroamination.
Our research on the development of a new class of hydroami-
nation catalysts has shown that bis(amidate)-bis(amido) com-
plexes of Ti are efficient, modular systems for intra- and inter-
molecular alkyne and allene hydroamination.8 Here we show
that, with elevated temperatures, these complexes are also active
for catalytic cyclohydroamination of alkenes. Complexes 1 and
2 were tested with a commonly used aminoalkene substrate (eq
1),1a and it was observed that the Zr complex 2 was much more
Selective formation of C-N bonds has many potential
applications in the pharmaceutical and fine chemical industries.
Although there are many synthetic routes to C-N bonds, the
most efficient methodology is catalytic hydroamination: the
formal addition of N-H to C-C unsaturation.1 Many transition-
metal complexes have been developed to promote this reaction,
and in particular, group 4 metals have been successfully
exploited for the catalytic hydroamination of alkynes.2 This well-
studied reaction is understood to involve an imido complex,
which undergoes a cycloaddition reaction to form an azamet-
allacyclobutene intermediate.2c,d,f,3 The analogous reaction with
alkenes has met with less success using group 4 metals;
however, cationic complexes have been shown to mediate this
transformation with secondary aminoalkene substrates.4 These
amido complexes likely employ a σ-bond insertion mechanism,
analogous to the isoelectronic lanthanide catalysts pioneered by
Marks and co-workers.1a,5 Interestingly, these cationic precata-
lysts cannot be used with primary amine substrates, which has
been postulated to be due to the in situ formation of catalytically
inactiVe imido complexes.4a More recently, select neutral group
4 amido complexes have been shown to mediate the catalytic
cyclohydroamination of primary aminoalkenes.6a,b Substrate
scope investigations with these systems suggest that imido
complexes are the catalytically active species.6 However, despite
reactive than the analogous Ti complex 1.9 While reported
computational3 and substrate scope investigations support the
intermediacy of an imido species, these reports do not obviate
a σ-bond insertion mechanism.6a,b To probe the reactivity of
related imido species, the novel complexes 3 and 4 were
synthesized from the bis(amido) precursors 1 and 2 (eq 2).
The synthesis of complexes 3 and 4 was accomplished in
high yield in a single step. Isolation of 5-coordinate imido
complexes was not successful, due to the electropositive nature
of these compounds. Trapping of crystalline 16-electron,
* To whom correspondence should be addressed. E-mail: schafer@
chem.ubc.ca.
(1) (a) Hong, S.; Marks, T. J. Acc. Chem. Res. 2004, 37, 673. (b) Muller,
T. E.; Beller, M. Chem. ReV. 1998, 98, 675. (c) Pohlki, F.; Doye, S. Chem.
Soc. ReV. 2003, 32, 104. (d) Roesky, P. W.; Muller, T. E. Angew. Chem.,
Int. Ed. 2003, 42, 2708. (e) Gribkov, D. V.; Hultzsch, K. C.; Hampel, F. J.
Am. Chem. Soc. 2006, 128, 3748. (f) Michael, F. E.; Cochran, B. M. J.
Am. Chem. Soc. 2006, 128, 4246 and references therein.
(6) (a) Bexrud, J. A.; Beard, J. D.; Leitch, D. C.; Schafer, L. L. Org.
Lett. 2005, 7, 1959. (b) Kim, H.; Lee, P. H.; Livinghouse, T. Chem.
Commun. 2005, 5205. (c) For the proposed catalytic cycle see Figure 4 in
the Supporting Information.
(7) (a) Walsh, P. J.; Hollander, F. J.; Bergman, R. G. J. Am. Chem. Soc.
1988, 110, 8729. (b) Hoyt, H. M.; Michael, F. E.; Bergman, R. G. J. Am.
Chem. Soc. 2004, 126, 1018. (c) Cummins, C. C.; Baxter, S. M.; Wolczanski,
P. T. J. Am. Chem. Soc. 1988, 110, 8731. (d) Cummins, C. C.; Schaller, C.
P.; Duyne, G. D. V.; Wolczanski, P. T.; Chan, A. W. E.; Hoffmann, R. J.
Am. Chem. Soc. 1991, 113, 2985. (e) Li, Y.; Shi, Y.; Odom, A. L. J. Am.
Chem. Soc. 2004, 126, 1794. (f) Basuli, F.; Bailey, B. C.; Watson, L. A.;
Tomaszewski, J.; Huffman, J. C.; Mindiola, D. J. Organometallics 2005,
24, 1886. (g) Mountford, P. Chem. Commun. 1997, 2127. (h) Hazari, N.;
Mountford, P. Acc. Chem. Res. 2005, 38, 839. (i) Kissounko, D. A.; Guzei,
I. A.; Gellman, S. H.; Stahl, S. S. Organometallics 2005, 24, 5208. (j) Basuli,
F.; Aneetha, H.; Huffman, J. C.; Mindiola, D. J. J. Am. Chem. Soc. 2005,
127, 17992. (k) Kissounko, D. A.; Epshteyn, A.; Fettinger, J. C.; Sita, L.
R. Organometallics 2006, 25, 1076 and references therein.
(2) (a) McGrane, P. L.; Jensen, M.; Livinghouse, T. J. Am. Chem. Soc.
1992, 114, 5459. (b) Baranger, A. M.; Walsh, P. J.; Bergman, R. G. J. Am.
Chem. Soc. 1993, 115, 2753. (c) Walsh, P. J.; Baranger, A. M.; Bergman,
R. G. J. Am. Chem. Soc. 1992, 114, 1708. (d) Johnson, J. S.; Bergman, R.
G. J. Am. Chem. Soc. 2001, 123, 2923. (e) Ackermann, L.; Bergman, R. G.
Org. Lett. 2002, 4, 1475. (f) Pohlki, F.; Doye, S. Angew. Chem., Int. Ed.
2001, 40, 2305. (g) Heutling, A.; Doye, S. J. Org. Chem. 2002, 67, 1961.
(h) Tillack, A.; Castro, I. G.; Hartung, C. G.; Beller, M. Angew. Chem.,
Int. Ed. 2002, 41, 2541. (i) Shi, Y.; Ciszewski, J. T.; Odom, A. L.
Organometallics 2001, 20, 3967. (j) Ong, T.-G.; Yap, G. P. A.; Richeson,
D. S. Organometallics 2002, 21, 2839. (k) Ackermann, L.; Bergman, R.
G.; Loy, R. N. J. Am. Chem. Soc. 2003, 125, 11956 and references therein.
(3) Straub, B. F.; Bergman, R. G. Angew. Chem., Int. Ed. 2001, 40, 4632.
(4) (a) Gribkov, D. V.; Hultzsch, K. C. Angew. Chem., Int. Ed. 2004,
43, 5542. (b) Knight, P. D.; Munslow, I.; O’Shaughnessy, P. N.; Scott, P.
Chem. Commun. 2004, 894.
(8) (a) Li, C.; Thomson, R. K.; Gillon, B.; Patrick, B. O.; Schafer, L. L.
Chem. Commun. 2003, 2462. (b) Zhang, Z.; Schafer, L. L. Org. Lett. 2003,
5, 4733. (c) Ayinla, R. O.; Schafer, L. L. Inorg. Chim. Acta 2006, 359,
3097.
(5) (a) Gagne, M. R.; Marks, T. J. J. Am. Chem. Soc. 1989, 111, 4108.
(b) See Figure 5 in the Supporting Information for the catalytic cycle.
(9) This trend has been seen previously with cationic group 4 catalysts.
10.1021/om060545n CCC: $33.50 © 2006 American Chemical Society
Publication on Web 07/21/2006

4070 Organometallics, Vol. 25, No. 17, 2006
Communications
6-coordinate imido complexes was possible upon addition of
the electron donor triphenylphosphine oxide (TPPO). These
complexes were characterized by 1H, 13C, and 31P NMR
spectroscopy as well as mass spectrometry and elemental
analysis.10a Complex 4 was also characterized by X-ray crystal-
lography and found to be of distorted-pentagonal-pyramidal
geometry, with the amidate and TPPO ligands bound pseu-
doequatorially and the imido moiety located in the axial position
with a short MdN bond length of 1.853 Å (Figure 1).10b These
structural features are consistent with a strongly donating imido
moiety and the TPPO donor binding to an orthogonal orbital in
the pseudoequatorial plane, along with the ionic amidate
ligands.11
Figure 1. Solid-state structure of 4 with 6-coordinate, distorted-
pentagonal-pyramidal geometry. Non ipso phenyl carbons have been
removed for clarity. Selected bond distances (Å) and angles (deg):
Zr(1)-N(3) ) 1.8531(18), Zr(1)-N(1) ) 2.3124(18), Zr(1)-O(1)
) 2.2034(15), Zr(1)-O(3) ) 2.2086(14); C(57)-N(3)-Zr(1) )
173.54(16), N(3)-Zr(1)-O(3) ) 100.07(7), O(1)-Zr(1)-O(3) )
78.14(6), N(1)-Zr(1)-N(2) ) 121.90(7).
Group 4 imido complexes have been widely studied over the
past 20 years, resulting in stoichiometric reactions including
C-H activation,7a-d epoxide ring opening reactions,12 and [2
+ 2] cycloadditions to form azametallacyclic compounds.2c,7h
Catalytic advances include imine metathesis and hydroamination
of alkynes2a,c,d,j,7e and allenes,2k,8c but interestingly, no group 4
imido complexes have been shown to promote the hydroami-
nation of alkenes. Here, complexes 1 through 4 are all active
for the cyclohydroamination reaction (eq 1), with the Zr imido
complex once again being more reactive than the Ti analogue.
Recent studies have suggested that proton also promotes these
reactions in certain systems.13 In the presence of a bulky base
(2,6-di-tert-butyl-4-methylpyridine) the catalytic activity of
complexes 1-4 is unaffected, confirming metal-mediated
reactivity.
Figure 2. Plot of first-order substrate consumption by 4 and
comparison between Zr amido (2) and imido (4) complexes (inset).
Some amido systems that have been developed for alkyne
hydroamination show a significant induction period, during
which the catalytically active imido species is postulated to
form.2d However, preliminary kinetic investigations directly
comparing complexes 2 and 4, with constant catalyst concentra-
tions, confirmed that the reaction is first order in substrate
(Figure 2), consistent with a protonolysis event being rate
limiting. This is in contrast to reports of hydroamination catalysts
using group 3 and lanthanide metal complexes, which are known
to proceed through a σ-bond insertion mechanism. For those
systems careful kinetic investigations have shown a zero-order
dependence upon substrate, consistent with the intramolecular
olefin insertion step being rate-limiting.1a Furthermore, in our
case there is no observable induction period for either precatalyst
(Figure 2).10c
A comparison of the half-lives for the bis(amido) and imido
complexes (2 vs 4) showed no appreciable difference between
the two complexes for two separate runs at 5% and 10% catalyst
loading, implying that both precatalysts share a common
catalytically active species. The reaction was determined to be
Figure 3. Plot of first-order catalyst dependence for cyclohy-
droamination with complex 4.
first order in catalyst, consistent with a single-site catalyst
(Figure 3).10d
Finally, if an imido complex is the catalytically active species,
then no reaction would be expected for the secondary ami-
noalkene substrate (R ) Me in eq 1), as the requisite imido
species cannot form. It should be noted that cationic group 4
complexes, which have been proposed to promote this reaction
by a σ-bond insertion mechanism, are known to mediate the
cyclohydroamination of such secondary amine substrates.4 In
our case, no reaction was observed for the secondary amine
substrate shown in eq 1 (R ) Me). Neither complex 2 nor
(10) (a) See the Supporting Information for details. (b) For full
crystallographic details, see the Supporting Information. (c) See the
Supporting Information for kinetic details. (d) For a kinetic plot see Figure
22 in the Supporting Information.
(11) Thomson, R. K.; Zahariev, F. E.; Zhang, Z.; Patrick, B. O.; Wang,
Y. A.; Schafer, L. L. Inorg. Chem. 2005, 44, 8680.
(12) Blum, S. A.; Walsh, P. J.; Bergman, R. G. J. Am. Chem. Soc. 2003,
125, 14276.
(13) Anderson, L. L.; Arnold, J.; Bergman, R. G. J. Am. Chem. Soc.
2005, 127, 14542.

Communications
Table 1. Catalytic Cyclohydroamination Resultse
Organometallics, Vol. 25, No. 17, 2006 4071
was capable of cyclizing a substrate with an internal alkene,
albeit slowly (entry 3). The more volatile substrate in entry 4
demonstrates the importance of size on the gem-dialkyl effect.
Finally, good diastereoselectivity was observed for the cyclo-
hydroamination in entry 5 with precatalyst 4. This selectivity
is consistent with a chairlike transition state in which there is a
preference for the R-methyl substituent being in the equatorial
position.
While the aforementioned lack of reactivity for the secondary
amine substrate may be considered to be due to steric bulk,
comparison to the bulky primary amine substrate in entry 5
(which also lacks reactivity-enhancing geminal substituents)
suggests that the Zr precatalyst 4 can accommodate steric bulk
at the reactive site. Furthermore, it should be noted that
substantially more congested cationic Ti complexes are known
to mediate the cyclohydroamination of secondary aminoalkenes.4a
In conclusion, we have presented the first amidate-supported
Zr imido complex (4) that exhibits a unique pentagonal-
pyramidal geometry in the solid state. This is also the first
neutral group 4 imido complex that is a precatalyst for
intramolecular alkene hydroamination. Preliminary kinetic
experiments and substrate scope investigations support an imido-
based cycloaddition mechanism for this reactivity, and current
efforts are focused on the full elucidation of the mechanism for
this reaction. This mechanistic insight will serve to guide the
rational design of new complexes displaying enhanced reactivity
and selectivity.
a Isolated yield. b Yield determined by 1H NMR (internal standard 1,3,5-
trimethoxybenzene). c Ratio determined by 1H NMR. d 10% catalyst. e Un-
less otherwise stated, 5 mol % of catalyst was utilized and reaction times
were unoptimized.
complex 4, even with prolonged reaction times (up to 4 days)
and elevated reaction temperatures (up to 140 °C), showed any
trace of product formation. Thus, both kinetic investigations and
substrate scope experiments support the intermediacy of an
imido complex.14
Preliminary substrate scope investigations were performed
(Table 1) to directly compare the bis(amido) complex 1 with
the imido complex 4 as precatalysts for heterocycle formation.
All results demonstrate the improved reactivity obtained with
Zr catalysts vs Ti catalysts, in contrast to alkyne hydroamination.7a
In addition to pyrrolidine products (entry 1), substituted pip-
eridine compounds can also be isolated (entry 2). Complex 4
Acknowledgment. We thank the UBC, the CFI, and the
NSERC for funding. L.L.S. thanks Boehringer-Ingleheim Canada
Inc., R.K.T. thanks the NSERC for a scholarship, and J.A.B.
thanks the UBC for a fellowship. L.L.S. thanks M. D. Fryzuk,
B. R. James, and J. A. Love for fruitful discussions.
Supporting Information Available: Text, figures, and tables
giving experimental procedures and characterization data for new
compounds and a CIF file giving X-ray crystallographic data for
4. This material is available free of charge via the Internet at
http://pubs.acs.org.
(14) We cannot rule out the conversion of an imido species to a
catalytically active bis(amido) species that can subsequently catalyze
hydroamination via a σ-bond insertion process.
OM060545N