Chemoselective deprotonations of a cationic zirconium primary amido complex to either a neutral zirconium terminal imido or a noninterconverting tautomer

Article abstract of DOI:10.1021/om051007y

Chemoselective deprotonation of {Cp*Zr(NH^tBu)-[N( ⁱPr)C(Me)N(ⁱPr)]}[B(C₆F₅) ₄] (2a) cleanly provides either the neutral enolamide, Cp*Zr(NH^tBu)[N(ⁱPr)C(CH₂)N( ⁱPr)] (5), or the terminal imido, Cp*Zr(N^tBu) [N(ⁱPr)C(CH₃)N(ⁱPr)] (6). These two tautomers do not interconvert, and no evidence was obtained for deprotonation of 2a in the presence of an excess of the primary amine ^tBuNH₂.

Full text of DOI:10.1021/om051007y

1076
Organometallics 2006, 25, 1076-1078
Communications
Chemoselective Deprotonations of a Cationic Zirconium Primary
Amido Complex to Either a Neutral Zirconium Terminal Imido or a
Noninterconverting Tautomer
Denis A. Kissounko, Albert Epshteyn, James C. Fettinger, and Lawrence R. Sita*
Department of Chemistry and Biochemistry, UniVersity of Maryland, College Park, Maryland 20742
ReceiVed NoVember 22, 2005
Summary: ChemoselectiVe deprotonation of {Cp*Zr(NHtBu)-
[N(iPr)C(Me)N(iPr)]}[B(C6F5)4] (2a) cleanly proVides either
the neutral enolamide, Cp*Zr(NHtBu)[N(iPr)C(CH2)N(iPr)] (5),
or the terminal imido, Cp*Zr(NtBu)[N(iPr)C(CH3)N(iPr)] (6).
These two tautomers do not interconVert, and no eVidence was
obtained for deprotonation of 2a in the presence of an excess
Scheme 1
t
of the primary amine BuNH2.
Inter- and intramolecular metal-catalyzed hydroaminations of
carbon-carbon multiple bonds (i.e., alkenes, alkynes, and
allenes) represent highly desirable synthetic organic transforma-
tions.1 In this regard, Bergman and Doye pioneered the use of
zirconium and titaninum bis(amido) complexes as catalysts for
the intermolecular hydroamination of alkynes and allenes with
primary amines.2,3 For these transformations, support now exists
for a mechanism in which the bis(amido) metal complex
undergoes R-elimination of a primary amine to generate a highly
reactive transient metal terminal imido species according to path
a in Scheme 1. Unfortunately, in contrast to their reactivity with
alkynes, such group 4 metal imido complexes do not appear to
readily engage in [2+2] cycloadditions with alkenes,4 and
therefore, the development of well-defined group 4 metal
catalysts for the more thermodynamically difficult intermolecular
hydroamination of alkenes with primary amines remains a major
challenge. Recently, Scott and co-workers5 introduced a suc-
cessful strategy for the intramolecular hydroamination cycliza-
tion of aminoalkenes that is reasonably proposed to proceed
via alkene insertion into the Zr-N bond of a cationic metal
amido species that is initially generated in situ through ami-
nolysis of a zirconium alkyl cation precursor according to path
b in Scheme 1. The inactivity of ω-aminoalkenes containing
primary amine groups (i.e., R′ ) H in Scheme 1) to undergo
similar hydroaminations was tentatively rationalized as possibly
being due to facile deprotonation of the corresponding cationic
zirconium primary amido complex to generate a neutral
zirconium imido species according to path c in Scheme 1, which,
as previously stated, would then be unreactive toward alkenes.
Herein, we experimentally probe this deprotonation hypothesis
further using relevant, well-characterized model complexes in
order to better understand the relative importance of this possible
deactivation process that could represent a significant barrier
to the development of successful cationic group 4 metal amido
catalysts for the hydroamination of alkenes with primary amines.
Our interest in hydroamination stems from previous studies
establishing that cationic methyl monocyclopentadienylzirco-
nium acetamidinate complexes of the general formula {Cp*Zr-
(X)[N(R1)C(Me)N(R2)]}[B(C6F5)4] (Cp* ) η5-C5Me5, R1 and
R2 ) alkyl, X ) Me) (1) can function as highly active initiators
for the living Ziegler-Natta polymerization of R-olefins.6 Thus,
we became intrigued with the question of whether the corre-
sponding coordinatively unsaturated cationic zirconium primary
amido complexes 2 (X ) NHR′ in 1) could serve in the capacity
of catalysts for the hydroamination of alkenes by primary
amines. To systematically explore this concept, we desired a
synthesis that could generate these coordinatively unsaturated,
highly electron deficient, cationic metal complexes in the
absence of any strong σ-donors, such as amines.
* To whom correspondence should be addressed. E-mail: lsita@umd.edu.
(1) For recent reviews, see: (a) Hong, S.; Marks, T. J. Acc. Chem. Res.
2004, 37, 673-686. (b) Bytschkov, I.; Doye, S. Eur. J. Org. Chem. 2003,
935-946. (c) Hultzsch, K. C. AdV. Synth. Catal. 2005, 347, 367-391.
(2) See for instance: (a) Walsh, P. J.; Baranger, A. M.; Bergman, R. G.
J. Am. Chem. Soc. 1992, 114, 1708-1719. (b) Baranger, A. M.; Walsh, P.
J.; Bergman, R. G. J. Am. Chem. Soc. 1993, 115, 2753-2763.
(3) Doye, S. Synlett 2004, 1653-1672, and references therein.
(4) A few reports of the reversible [2+2] cycloaddition of ethene and
norbornene to group 4 and 5 imido complexes have appeared, see: (a) de
With, J.; Horton, A. D. Organometallics 1993, 12, 1493-1496. (b) Walsh,
P. J.; Hollander, F. J.; Bergman, R. G. Organometallics 1993, 12, 3705-
3723. (c) Bennett, J. L.; Wolczanski, P. T. J. Am. Chem. Soc. 1994, 116,
2179-2180.
To begin, protonolysis of the η2-styrene zirconium complex
37 with 2 equiv of tBuNH2 cleanly provided a high yield of the
bisamido species 4 according to Scheme 2.8 As the zirconacy-
clopropane ring of 3 has been shown to undergo ring-opening
protonolysis with terminal acetylenes,7 the coproduct of this
reaction is presumed to be ethylbenzene. Interestingly, all
(6) (a) Jayaratne, K. C.; Sita, L. R. J. Am. Chem. Soc. 2000, 122, 958-
959. (b) Kissounko, D. A.; Zhang, Y.; Harney, M. B.; Sita, L. R. AdV.
Synth. Catal. 2005, 347, 426-432, and references therein.
(7) Kissounko, D. A.; Epshteyn, A.; Fettinger, J. C.; Sita, L. R.
Organometallics 2006, 25, 531-535.
(5) Knight, P. D.; Munslow, I.; O’Shaughnessy, P. N. O.; Scott, P. J.
Chem. Soc., Chem. Commun. 2004, 894-895.
(8) Details are provided in the Supporting Information.
10.1021/om051007y CCC: $33.50 © 2006 American Chemical Society
Publication on Web 02/02/2006

Communications
Organometallics, Vol. 25, No. 5, 2006 1077
Figure 1. Molecular structures (30% thermal ellipsoids) of compounds (a) 2a, (b) 5, and (c) 6. The [B(C6F5)4] counterion of 2a and all
hydrogen atoms, except for some selected crystallographically located ones for 2a and 5, have been omitted for the sake of clarity.
Scheme 2
center, it is reasonable to conclude that these geometric features
might be associated with partial multiple-bond character between
zirconium and nitrogen. Finally, no insertion of ethene, propene,
or higher R-olefins into the Zr-N bond of 2a has been observed
under a variety of conditions.
With compound 2a in hand, efforts to form a zirconium imido
complex through deprotonation yielded quite unexpected results.
Thus, as Scheme 3 presents, the sterically hindered base NaN-
(SiMe3)2 provided only the “enolamido” complex 5 through
selective deprotonation of the amidinate ligand.8,10 Compound
5 was isolated in analytically pure form through recrystallization,
and Figure 1b provides the molecular structure of this complex
as determined by X-ray crystallography.8 Of particular note is
that the Zr(1)-N(3) bond length of 2.0533(16) Å is only slightly
contracted relative to the Zr(1)-N(1) and Zr(1)-N(2) bond
lengths of 2.0596(15) and 2.0631(15) Å, respectively. Impor-
tantly, both the short C(11)-C(12) bond length of 1.351(3) Å
and the trigonal coplanarity of C(12) support the depicted
enolamido structure [cf. the C(11)-C(12) bond length of 1.498-
(3) Å in 2a]. There are also no short distances observed in the
solid state between the fluorines of the borate anion and the
cationic metal center. Finally, in the 1H NMR (400 MHz,
benzene-d6, 25 °C) spectrum of 5, a distinct singlet resonance
for the two methylene protons of the enolamido structure is
observed at 3.39 ppm.
Scheme 3
Compound 5 was found to be surprisingly quite stable in
solution, resisting tautomerization to a zirconium terminal imido
up to temperatures of at least 100 °C. It was eventually
discovered, however, that among a wide range of bases (e.g.,
n-butyllithium, triethylamine, DBU, and LiSiPh310sall of which
provide a complex mixture of products) commercially available
EtNP(NMe2)2NP(NMe2)3 (abbreviated as P2Et)3,11 was excep-
tional in selectively deprotonating 2a to provide a high isolated
yield of the desired imido complex 6 as shown in Scheme 3.8
Compound 6 was again isolated as single crystals, and X-ray
analysis was used to determine the molecular structure shown
in Figure 1c. Relative to both 2a and 5, compound 6 possesses
a significantly shorter Zr(1)-N(3) bond of 1.839(2) Å, which
is similar in magnitude to other known zirconium imido
complexes and which supports the depicted multiple-bond
character [cf. Zr(1)-N(1) and Zr(1)-N(2): 2.232 (2) Å].7,12,13
The Zr(1)-N(3)-C(19) bond angle is also nearly collinear at
175.52(18)°.
attempts to prepare 4 through protonolysis of Cp*ZrMe2[N(iPr)-
C(Me)N(iPr)] under identical conditions failed to provide the
desired product in acceptable yield and purity. Next, as Scheme
3 reveals, direct amide abstraction of 4 using the borate [Ph3C]-
[B(C6F5)4] provided the desired cationic zirconium amido 2a
(R1 ) R2 ) iPr) in high yield. Although similar methide (-CH3)
abstraction using this borate is commonly performed to prepare
zirconium alkyl cations from neutral dimethyl precursors,6,9 to
the best that we can determine, this is the first example of
analogous amide abstraction chemistry, and it may represent a
general route to a variety of other cationic metal amido species.
Analytically pure 2a was obtained through recrystallization
from pentane, and single crystals were subjected to X-ray
analysis.8 Figure 1a presents the molecular structure of 2a, and
of particular interest are the Zr(1)-N(3) bond length of 2.021-
(5) Å involving the exocyclic amido group, which is shorter
than the Zr-N bond lengths involving the amidinate fragment
[cf. Zr(1)-N(1) and Zr(1)-N(2) of 2.1516(17) and 2.1549(18)
Å, respectively], and the nearly trigonal coplanar geometry of
the N(3) atom. Given the electrophilic nature of the d0 metal
(10) Zhang, Y.; Kissounko, D. A.; Fettinger, J. C.; Sita, L. R. Organo-
metallics 2003, 22, 21-23.
(11) Schwesinger, R.; Schlemper, H.; Hasenfratz, C.; Willaredt, J.;
Dambacher, T.; Breuer, T.; Ottaway, C.; Fletschinger, M.; Boele, J.; Fritz,
H.; Putzas, D.; Rotter, H. W.; Bordwell, F. G.; Satish, A. V.; Ji, G.-Z.;
Peters, E. M.; Peters, K.; von Schnering, H. G.; Walz, L. Liebigs Ann. 1996,
1055-1081.
(9) Chen, E. Y.; Marks, T. J. Chem. ReV. 2000, 100, 1391-1434.

1078 Organometallics, Vol. 25, No. 5, 2006
Communications
which was isolated in crystalline form.8 Interestingly, a 2D 1H
EXSY NMR spectrum established that, at 25 °C, the solution
structure of 7 is static with no evidence for either proton or
group exchange occurring between the two chemically distinct
exocyclic nitrogen-containing substituents on the NMR time
frame. Compound 7 also proved to be thermally stable in toluene
solutions up to at least 100 °C. Finally, in the presence of excess
tBuNH2 (10 equiv), solutions of 2a in chlorobenzene once again
yielded only 7, even at elevated temperatures, and with no
evidence for deprotonation.
Scheme 4
A preliminary survey of the stability and reactivity of 6
revealed that neither alkenes nor alkynes engage in formal [2+2]
cycloadditions, and no C-H additions of alkanes across the Zrd
N bond occur as previously observed for other zirconium imido
species.14 Addition of 1 equiv of tBuNH2 to a toluene solution
of 6 quantitatively provided the bisamido 4, and protonation
using 1 equiv of [PhNHMe2][B(C6F5)4] in chlorobenzene cleanly
regenerated 2a (Scheme 3). Finally, at elevated temperatures
up to 100 °C, 6 again does not tautomerize to 5 in solution as
determined by NMR spectroscopy.
Since pKa’s of cationic zirconium primary amido species can
reasonably be expected to vary according to overall formal
electron count of the complex, it remains to be seen whether
model complexes for the more electron-deficient intermediates
(vis-a`-vis 2a) that are involved in Scott’s catalytic process are
more susceptible to deprotonation by primary amines. In the
present case of catalysts based on 2, this process might not pose
as big of a threat to their success as simple irreversible
coordination of these primary amines to potential olefin binding
sites of the highly electrophilic metal center. Fortunately, steric
tuning of the ligand environment about the metal may serve to
help ameliorate the situation with respect to the latter deactiva-
tion process. Further hydroamination studies with several
structural derivatives of 2 are now in progress.
With the ability to spectroscopically establish formation of
3-5 by NMR, final attention was turned to determining whether
a stoichiometric or an excess amount of a primary amine could
t
deprotonate 2a. With 1 equiv of BuNH2, only complexation
occurred to provide the amine adduct 7 shown in Scheme 4,
(12) Compound 6 is structurally analogous to monocyclopentadienyl-
titanium imido amidinate complexes prepared by Mountford and co-workers,
see: (a) Guiducci, A. E.; Cowley, A. R.; Skinner, M. E. G.; Mountford, P.
J. Chem. Soc., Dalton Trans. 2001, 1392-1394. (b) Boyd, C. L.; Clot, E.;
Guiducci, A. E.; Mountford, P. Organometallics 2005, 24, 2347-2367.
(13) See for instance: (a) Walsh, P. J.; Hollander, F. J.; Bergman, R. G.
Organometallics 1993, 12, 3705-3723. (b) Ong, T. G.; Wood, D.; Yap,
G. P. A.; Richeson, D. S. Organometallics 2002, 21, 1-3.
Acknowledgment. Funding for this work was provided by
the NSF (CHE-0092493), for which we are grateful.
Supporting Information Available: Experimental details,
including crystallographic analyses of 2a, 5, and 6. This material
is available free of charge via the Internet at http://pubs.acs.org.
(14) See, for instance: (a) Cummins, C. C.; Baxter, S. M.; Wolczanski,
P. T. J. Am. Chem. Soc. 1988, 110, 8731-8733. (b) Hoyt, H. M.; Michael,
F. E.; Bergman, R. G. J. Am. Chem. Soc. 2004, 126, 1018-1019.
OM051007Y