Isolation of catalytic intermediates in hydroamination reactions: Insertion of internal alkynes into a zirconium-amido bond

Article abstract of DOI:10.1002/anie.201001927

Insert alkyne to continue: Reaction of a bis(ureate) zirconium dimethylamido complex with electron-rich alkynes leads to vinylamine insertion products (see scheme). The insertion of C=C unsaturated bonds into M=N bonds is proposed as a key step in lanthan

Full text of DOI:10.1002/anie.201001927

Communications
DOI: 10.1002/anie.201001927
Catalytic Intermediates
Isolation of Catalytic Intermediates in Hydroamination Reactions:
Insertion of Internal Alkynes into a Zirconium–Amido Bond**
David C. Leitch, Courtney S. Turner, and Laurel L. Schafer*
Metal-catalyzed hydroamination, a direct and atom-economic
azametallacycles, resulting from stoichiometric [2+2] cyclo-
À
À
approach for the formation of C N bonds, is becoming an
additions between Group 4 metal–imido complexes and C C
increasingly powerful method in organic synthesis.[1] This
unsaturated bonds, have been isolated and structurally
characterized.[3c,7] As a consequence of this imido-mediated
catalytic cycle, nearly all Group 4 metal catalysts can use only
primary amines as substrates.[1,3,8]
À
À
catalytic addition of N H bonds across C C unsaturated
bonds allows for the preparation of complex nitrogen-
containing compounds using readily available and inexpen-
sive starting materials. Furthermore, these reactions proceed
without the need for stoichiometric amounts of reagents, or
the generation of waste by-products. There have been a
staggering number of catalytic systems developed for this
reaction, exhibiting a corresponding high degree of mecha-
nistic diversity.[1–8] In particular, the hydroamination of
alkynes, allenes, and alkenes using Group 4 metal catalysts
has been proposed to occur through a [2+2] cycloaddition
between the unsaturated substrate and a metal–imido bond.[3]
More recently, proposals of a s-bond insertion of the
unsaturated substrate into a metal–amido bond have been
asserted for select cationic[4] and neutral[5] Group 4 metal
catalysts, akin to that established for Group 3 and lanthanide
metals[6] (Scheme 1). The vast majority of previous kinetic,
A small number of Group 4 metal systems have been
reported that exhibit contrasting reactivity.[4,5] Cationic zirco-
nium species, which are isovalent with Group 3 and lantha-
nide metal complexes, can successfully cyclize secondary
aminoalkene substrates;[4] however, these same systems are
ineffective with primary aminoalkenes. Notably, there are a
limited number of neutral, sterically accessible zirconium
catalysts that are able to catalyze hydroamination reactions
using both primary and secondary amines.[5] A detailed
synthetic and kinetic study of “constrained-geometry” cata-
lysts by Stubbert and Marks provides compelling evidence
that a s-bond insertion mechanism is operative,[5c] which is in
contrast to nearly every other neutral Group 4 metal system
reported to date.[1,3,8]
While stoichiometric insertions of polar, electrophilic
carbon–heteroatom multiple bonds into titanium- and zirco-
nium–amido bonds are well established,[9] those of nonpolar
À
C C unsaturated bonds are exceedingly rare. There is a single
characterized example of alkyne insertion into a Group 4
metal–nitrogen bond, involving the highly strained [Cp2Zr-
(N2Ph2)] metallacycle (Cp = cyclopentadienyl), reported by
Bergman and co-workers in 1990.[10] Apart from this, there is
only one characterized example of electron-rich alkyne
insertion into an unstrained amido bond of any metal,
which was reported by Odom and co-workers for a d2-
molybdenum(IV) center.[11] The rarity of these electron-rich,
Scheme 1. Two pathways for zirconium-catalyzed alkyne hydroamina-
tion.
À
C C multiple bond insertions into metal–nitrogen bonds can
be partly explained by the electronic nature of the d0-metal–
synthetic, and computational studies to determine the mech-
anism for hydroamination reactions using Group 4 metals
supports the [2+2] mechanism.[1,3,7,8] A compelling piece of
evidence for this proposal is that catalytically relevant
amido linkage. Coulombic repulsion between the formal
À
metal–nitrogen double bond and the incoming C C multiple
bond results in a substantial kinetic barrier to insertion.[12]
Thermodynamically, these insertion reactions are estimated
to be thermoneutral or only slightly exothermic,[1,13] owing to
[14]
À
the strength of the Zr N bond.
[*] D. C. Leitch, C. S. Turner, Prof. L. L. Schafer
Department of Chemistry, University of British Columbia
2036 Main Mall, Vancouver, B.C., V6T 1Z1 (Canada)
Fax: (+1)604-822-2847
During the course of our own research on hydroamination
reactions, we have identified a ureate-supported zirconium
precatalyst (1) that is highly effective for the hydroamination
of alkynes and alkenes with both primary and secondary
amines.[5a] Herein, we report the isolation and character-
ization of vinylamine complexes (2a–c) resulting from
E-mail: schafer@chem.ubc.ca
[**] We gratefully acknowledge Boehringer Ingelheim (Canada) Ltd. and
the NSERC for financially supporting this work. D.C.L. thanks
NSERC for a CGSD scholarship. We thank N. Yonson for assistance
with X-ray crystallography.
À
electron-rich, nonpolar alkyne insertion into a Zr N bond
of 1 [Eq. (1)]. These represent the first characterized exam-
À
ples of stoichiometric C C multiple bond insertion into an
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001927.
unstrained, d0-metal–amido bond. Not only are 2a–c derived
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Chemie
cis to the axial site accessible for neutral ligand coordination.
Our hypothesis is that during the hydroamination catalysis,
coordination of the alkyne substrate occurs in the axial
position on the electrophilic, sterically accessible zirconium
metal center. This coordinated species would then facilitate
the intramolecular nucleophilic attack of the alkyne substrate
by the electron-rich equatorial amido nitrogen atom, thus
À
resulting in C N bond formation and formal s-bond inser-
tion.[11a]
This hypothesis is supported by comparing the use of aryl/
alkyl and dialkyl amines in hydroamination reactions cata-
lyzed by 1 [Eq. (2)]. N-methylaniline is ineffective as a
substrate for alkyne hydroamination, and gives none of the
desired product under catalytic conditions. Here, we suggest
that delocalization of the nitrogen lone pair of electrons into
the aromatic system disfavors nucleophilic attack, thereby
preventing the insertion. In contrast, dialkyl-substituted
amines are transformed efficiently into the anticipated
enamine products.[5a] Furthermore, to the best of our knowl-
edge, no catalytic intermolecular hydroamination system that
is proposed to operate via s-bond insertion is able to use N-
aryl amines as substrates.[1]
from an active hydroamination precatalyst (1), they are also
themselves catalytically competent.
As part of our efforts to determine the source of the
unique catalytic activity of 1 relative to other Group 4 metal
systems, we undertook a structural analysis of the precatalyst
(Figure 1). The seven-coordinate complex exhibits distorted
Figure 1. ORTEP representation of the molecular structure of 1 (ellip-
soids drawn at 50% probability, H atoms except that on N7 are
omitted). Selected bond lengths (ꢀ) and angles (8): Zr–N1 2.280(1),
Zr–N5 2.092(2), Zr–N6 2.135(1), Zr–N7 2.503(2), Zr–O1 2.240(1), N1-
Zr-O1 57.91(5).
To test our mechanistic model, we have examined the
À
feasibility of stoichiometric alkyne insertion into the Zr
NMe2 bonds of complex 1. Treatment of 1 with an excess
amount of 1-phenyl-1-propyne, diphenylacetylene, or 3-
hexyne at 1008C resulted in the formation of vinylamine
complexes 2a–c in moderate yields after recrystallization
[Eq. (1)].[17] The compounds are crystalline solids, and are
therefore easily isolated, purified, and characterized. The
NMR spectroscopic features of 2a–c are very similar to one
another, and are consistent with the molecular formulations
in Equation (1). Upon conversion from 1, the protons of the
gem-dimethyl moiety of the ureate ligand become inequiva-
lent, as do the adjacent methylene protons, which resonate as
a diagnostic AB quartet in all three cases. Two signals are
observed for the methyl protons on the NMe2 groups, one for
the axial dimethylamido ligand, and one for the new
alkenyl(dimethylamino) ligand. Complex 2a is formed as a
single isomer, with no indication of the presence of the other
insertion regioisomer. The 13C NMR spectra of 2a–c are
devoid of any alkynyl carbon signals; in each case, a new
resonance near d = 180 ppm is observed, which corresponds
to the carbon atom directly attached to the zirconium center,
as has been previously characterized for other metallacyclic
pentagonal bipyramidal geometry, with the bis(ureate) ligand
located in the equatorial plane. The dimethylamido ligands
are oriented cis to the zirconium center, and the seventh
coordination site is occupied by a neutrally bound dimethyl-
amine group that is oriented trans to the axial amido ligand.
An examination of the geometrical parameters reveals a
À
remarkably longer Zr NMe2 bond of 2.135(1) ꢀ for the
À
equatorial amido ligand, while the axial Zr NMe2 bond is
2.092(2) ꢀ. Structurally analogous seven-coordinate amidate
À
and sulfonamidate complexes have Zr N bond lengths that
range between 2.06–2.07 ꢀ for both axial and equatorial
amido ligands.[8b,h]
The length of this equatorial amido bond cannot be
rationalized in steric terms,[15] nor can it be explained by
electron delocalization with an adjacent unsaturated organic
group.[16] Instead, we propose that this elongation is a result of
À
the reduced p-bonding character in the equatorial Zr NMe2
2
[3c]
À
linkage, which is in the same plane as the other four donors of
the ureate ligand. In contrast, the axial amido ligand is
oriented to maximize p-orbital overlap. We suggest that the
decreased metal–ligand p bonding of the equatorial amido
ligand in this case results in a more nucleophilic nitrogen atom
sp -hybridized C Zr bonds.
Electron impact mass spec-
trometry enables the observation of signals corresponding to
molecular ions and diagnostic fragments, and finally combus-
tion analyses are consistent with the empirical formulae (see
the Supporting Information).
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6383

Communications
Single crystals of 2a and 2b were subject to X-ray
crystallographic analysis to confirm the structural assign-
ment.[18] The molecular structure of 2a is shown in Figure 2.
All of the above experimental observations can be
rationalized by the proposed mechanism shown in
Scheme 2.[20] Dissociation of dimethylamine from 1 would
Figure 2. ORTEP representation of the molecular structure of 2a
(ellipsoids drawn at 50% probability, H atoms and methyl groups
from isopropyl groups are omitted). Selected lengths (ꢀ) and angles
(8): Zr–N1 2.208(1), Zr–N5 2.443(1), Zr–N6 2.087(2), Zr–C20 2.324(2),
Zr–O1 2.200(1), C20–C27 1.326(2), N1-Zr-O1 59.28(5), N5-Zr-C20
58.25(6).
Both complexes have analogous features, and have under-
gone only minimal geometric reorganization from the parent
zirconium species. The structure of 2a reveals which regio-
isomer is formed, and is consistent with the previously
determined regioselectivity of the hydroamination between
1-phenyl-1-propyne and morpholine catalyzed by 1.[5a] The
observed regioselectivity is also consistent with the results
previously observed for other hydroamination systems that
employ a formal s-bond insertion mechanism.[2f,6b] The
observed geometry of 2a and 2b suggests that the alkyne
À
substrate inserts preferentially into the weaker equatorial Zr
N bond, as proposed.[19] The C20 C27 bond length of
À
Scheme 2. Proposed mechanistic pathway leading to the formation of
isolated intermediates 2a–c and observed enamines 3a–c.[20]
À
1.326(2) ꢀ is consistent with a C C double bond, while the
C27 N5 bond length of 1.477(3) ꢀ indicates a C N single
bond. The long Zr N5 bond length of 2.443(1) ꢀ confirms
that the N donor of the vinylamine group is bound to the
zirconium center in a neutral fashion.
À
À
À
give a coordinatively unsaturated intermediate 4, which could
accommodate alkyne coordination in the axial position to
regenerate a seven-coordinate intermediate. This coordina-
tion would then activate the alkyne toward insertion by
lowering the energy of the p* LUMO, and positioning it in
close proximity to the reactive equatorial amido ligand.
Nucleophilic attack by the lone pair of electrons on the
nitrogen atom of the equatorial amido ligand onto the
coordinated alkyne could then form the vinylamine com-
plexes 2a–c through a formal s-bond insertion. In the
presence of the dimethylamine unit liberated to form 4,
1
In situ monitoring of the insertion reactions by H NMR
spectroscopy reveals not only the nearly quantitative forma-
tion of the organometallic products, but also 0.3 to 0.6 equiv-
alents of free N,N-dimethylenamines 3a–c. To confirm that
enamine products can be formed through aminolysis of the
À
Zr C bond of 2a, and to confirm that the isolated vinylamine
complexes are catalytically viable for alkyne hydroamination,
complex 2a was treated with a large excess of morpholine at
1008C [Eq. (3)]. Complete conversion into the enamine 3a
was observed in less than 16 hours. Finally, complex 2a is itself
a viable hydroamination precatalyst. A catalyst loading of
10 mol% of 2a resulted in the hydroamination of phenyl-
acetylene with morpholine in 16 hours at 1008C; which
corresponds to the same reactivity previously reported using
precatalyst 1.[5a]
À
rapid aminolysis of the Zr C bond would give the observed
enamines 3a–c, and regenerate 4. As the soluble HNMe2 is
consumed in the stoichiometric reactions, the cycle would
become arrested, thus enabling the isolation of complexes
2a–c. Alternatively, in the presence of an excess amount of a
dialkylamine substrate, such as morpholine, efficient turnover
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Angewandte
Chemie
46.3, 57.9 (CH2), 152.4 (C), 168.9 (C), 184.2 ppm (C); MS(EI) m/z 614
is observed and results in the catalytic synthesis of the
enamine.
In summary, we have identified, isolated, and character-
ized key catalytic intermediates in the intermolecular hydro-
amination reaction of alkynes with a highly active zirconium
catalyst system. The vinylamine complexes 2a–c have been
(M+), 570 ([LZr–C(Et) = C(Et)(NMe2)]+), 488 ([LZrNMe2]+); Ele-
mental analysis calcd for C29H60N6O2Zr: C 56.54, H 9.82, N 13.64;
found: C 56.20, H 10.14, N 13.85.
Received: March 31, 2010
Revised: June 8, 2010
Published online: July 21, 2010
À
isolated and are the first examples of formal nonpolar C C
multiple bond insertion into unstrained d0-metal–nitrogen
bonds. Furthermore, these vinylamine complexes are them-
selves catalytically competent. According to our mechanistic
hypothesis the unique bonding environment supported by the
electron-rich, tethered ureate ligand is able to reduce the
multiple bond character of the equatorial amido ligand,
consequently the nitrogen lone pair of electrons is capable of
nucleophilic attack on the coordinated alkyne substrate.[20]
Kinetic, synthetic, and computational studies are currently
underway to provide valuable insight into the details of this
process, with the goal of effectively designing the next
generation of Group 4 metal hydroamination catalysts.
Keywords: alkynes · hydroamination · insertion · ureate ·
zirconium
.
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Experimental Section
Synthesis of complexes: 1 (100.0 mg, 0.173 mmol) and the alkyne
(0.346 mmol for 2a and 2b, 1.73 mmol for 2c) were dissolved in
benzene (1 mL). The solution was heated to 1008C for 16 h (for 2a
and 2b) or 24 h (for 2c) prior to solvent removal under vacuum. The
residue was recrystallized from pentane at À358C to give 78.0 mg
(69%) of 2a, 70.0 mg (57%) of 2b, 59.3 mg (56%) of 2c.
Characterization of 2a: 1H NMR (C7D8, 400 mhz) d = 0.77 (3H, s,
CH3), 1.09 (3H, s, CH3), 1.22 (12H, d, J = 8.0 Hz, 2xCH(CH3)2), 1.24
(12H, d, J = 8.0 Hz, 2xCH(CH3)2), 1.66 (3H, s, [Zr]–C(Ph) = C-
=
(CH3)(NMe2)), 2.81 (6H, s, C C(CH3)(N(CH3)2)), 3.07 (4H, AB q,
J = 10.0 Hz, 2xMe2C(CH2)N), 3.29 (6H, s, [Zr]–N(CH3)2), 3.57 (4H,
sept, J = 8.0 Hz, 4xCH(CH3)2), 6.93 (3H, m, 3xPh-H), 7.24 ppm (2H,
m, 2xPh-H); 13C NMR (C7D8, 100 mhz, CH2 and C determined from
DEPT experiments) d = 6.5, 21.8, 22.1, 22.2, 27.0, 36.3 (C), 43.6, 45.4,
46.3, 57.3 (CH2), 121.4, 124.8, 127.3, 145.6 (C), 149.8 (C), 168.5 (C),
182.2 ppm (C); MS(EI) m/z 648 (M+), 604 ([LZr–C(Ph) = C(Me)-
(NMe2)]+), 488 ([LZrNMe2]+); Elemental analysis calcd for
C32H58N6O2Zr: C 59.12, H 8.99, N, 12.93; found: C 59.31, H 8.91, N
12.76.
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1
Characterization of 2b: H NMR (C7D8, 400 mhz) d = 0.83 (3H,
s, CH3), 1.16 (3H, s, CH3), 1.25 (12H, d, J = 6.7 Hz, 2xCH(CH3)2), 1.28
=
(12H, d, J = 6.7 Hz, 2xCH(CH3)2), 2.92 (6H, s, C C(CH3)(N(CH3)2)),
3.17 (4H, AB q, J = 11.2 Hz, 2xMe2C(CH2)N), 3.32 (6H, s, [Zr]–
N(CH3)2), 3.61 (4H, sept, J = 6.7 Hz, 4xCH(CH3)2), 6.75 (1H, t, J =
7.3 Hz, 1xPh-H), 6.90 (2H, d, J = 7.9 Hz, 2xPh-H), 6.96 (1H, t, J =
7.3 Hz, 1xPh-H), 7.02–7.09 (4H, m, 4xPh-H), 7.26 ppm (2H, d, J =
6.8 Hz, 2xPh-H); 13C NMR (C7D8, 100 mhz, CH2 and C determined
from DEPTexperiments) d = 21.8, 21.9, 22.3, 27.2, 36.4 (C), 44.9, 45.3,
46.3, 57.5 (CH2), 121.7, 125.7, 126.3, 127.0, 127.4, 130.8, 135.2 (C),
148.3 (C), 149.4 (C), 168.5 (C), 183.4 ppm (br, C); MS(EI) m/z 710
(M+), 666 ([LZr–C(Ph) = C(Ph)(NMe2)]+), 488 ([LZrNMe2]+); Ele-
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found: C 62.76, H 8.52, N 11.55.
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1.35 (3H, s, CH3), 1.40 (3H, t, J = 7.6 Hz, CH2CH3), 2.24 (2H, q, J =
=
7.6 Hz, CH2CH3), 2.55 (2H, q, J = 7.6 Hz, CH2CH3), 2.87 (6H, s, C
C(CH3)(N(CH3)2)), 3.27 (2H, d, J = 11.2 Hz, Me2C(CH2)N), 3.42
(2H, d, J = 11.2 Hz, Me2C(CH2)N), 3.51 (6H, s, [Zr]–N(CH3)2),
3.67 ppm (4H, sept, J = 6.8 Hz, 4xCH(CH3)2); 13C NMR (C6D6,
100 mhz, CH2 and C determined from DEPT experiments) d = 15.2
(CH2), 16.3, 16.8, 21.9, 22.1, 22.6, 26.7, 27.6,(CH2), 36.7 (C), 44.6, 45.8,
Angew. Chem. Int. Ed. 2010, 49, 6382 –6386
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Communications
À
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À
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À
neutral complex: b) S. Tobisch, Chem. Eur. J. 2008, 14, 8590;
DG° values of about 18–20 kcalmolÀ1 were calculated for alkene
insertion with an analogous cationic complex: c) S. Tobisch,
Dalton Trans. 2006, 4277; compare with DG° values of about 8–
13 kcalmolÀ1 for insertion of C C unsaturated bonds into Ln N
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À
À
[13] T. J. Marks, M. R. Gagnꢅ, S. P. Nolan, L. E. Schock, A. M.
Seyam, D. Stern, Pure Appl. Chem. 1989, 61, 1665, and
references therein.
À
[14] The mean Zr N bond dissociation enthalpy in Zr(NMe2)4 is
84 kcalmolÀ1, considerably higher than the Zr C bond dissoci-
À
ation values for either Zr(CH2Ph)4 (60 kcalmolÀ1
) or Zr-
(CH2CMe3)4 (54 kcalmolÀ1): a) L. E. Schock, T. J. Marks, J.
Am. Chem. Soc. 1988, 110, 7701; b) J. A. Conner, Top. Curr.
Chem. 1977, 71, 71.
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Soc. 2000, 122, 751.
[17] Attempted alkene insertions have thus far been unsuccessful.
[18] CCDC 769163 (2a), 769164 (2b), and 769165 (2c) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
[19] Alternatively, this ligand arrangement could result from isomer-
[10] a) P. J. Walsh, F. J. Hollander, R. G. Bergman, J. Organomet.
Chem. 1992, 428, 13; b) P. J. Walsh, F. J. Hollander, R. G.
Bergman, J. Am. Chem. Soc. 1990, 112, 894.
ization to
a
thermodynamic product via
a
six-coordinate
[11] a) E. Katayev, Y. Li, A. L. Odom, Chem. Commun. 2002, 838;
alkyne insertion into other metal–nitrogen bonds: b) J. A.
Cabeza, I. del Rio, F. Grepioni, M. Moreno, V. Riera, M.
Suꢉrez, Organometallics 2001, 20, 4190; c) C. Krꢁger, H. Kisch, J.
Chem. Soc. Chem. Commun. 1975, 65; alkene insertion into a
intermediate.
À
[20] A reviewer has pointed out that the reduced Zr N p bonding
proposed for complex 1 may not be common to intermediates 4
and 5. An alternate electronic rationale for the observed
reactivity, such as a formal [2+2] cycloaddition between the
À
À
Pd amide bond: d) P. S. Hanley, D. Markovic, J. F. Hartwig, J.
alkyne and the p contribution to the Zr N bond, cannot
therefore be ruled out. Computational work is currently under-
way to address these questions.
Am. Chem. Soc. 2010, 132, 6302; J. D. Neukom, N. S. Perch, J. P.
Wolfe, J. Am. Chem. Soc. 2010, 132, 6276; norbornene insertion
6386 www.angewandte.org
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