A remarkable switch from a diamination to a hydrohydrazination catalyst and observation of an unprecedented catalyst resting state

Article abstract of DOI:10.1002/anie.201206249

A small change with a big effect: Reaction of the titanium complex 1 with terminal alkynes gave the unusual acetylide-vinylhydrazide(1-) compounds 2 (R=4-C₆H₄X (X=Me, CF₃, OMe), C ₆F₅, SiMe₃) as hydrohydrazination intermediates. In contrast, a complex similar to 1 with bulkier trimethylsilyl substituents in place of the isopropyl groups is known to catalyze the 1,2-diamination of alkynes with hydrazines. Copyright

Full text of DOI:10.1002/anie.201206249

.
Angewandte
Communications
DOI: 10.1002/anie.201206249
Catalytic Intermediates
A Remarkable Switch from a Diamination to a Hydrohydrazination
Catalyst and Observation of an Unprecedented Catalyst Resting
State**
Andrew D. Schwarz, Chee S. Onn, and Philip Mountford*
[1]
=
Group 4 imido complexes, (L)M NR, and, more recently,
mediates VI. The more widely studied hydroamination
reaction of alkynes, alkenes, and allenes with Group 4 met-
als[1c,e,4] has been explored in a series of elegant mechanistic
and computational[3b,5] studies. Three mechanisms have been
[2]
=
hydrazido complexes, (L)M NNR2, undergo a range of
addition or insertion reactions of their M N multiple bonds
with many unsaturated and saturated substrates. This reac-
tivity has led to a number of new bond-forming method-
ologies. In the case of hydrazides, reductive cleavage of the
=
established: the “imide route” (the most common) through
[3a,6]
=
a [2+2] cycloaddition reaction of an M NR bond,
the
ꢀ
Na Nb bond can also occur with formally oxidizable sub-
“amide route” through substrate migratory insertion into an
[7]
strates, such as CO,[2b] isocyanides,[2g,o] and alkynes,[2j,l,m] in the
presence of suitably activating supporting ligand sets, such as
bis(cyclopentadienyl) and diamide–amine types. We recently
found[2l,m] that the hydrazide complex I (Scheme 1) and
M NRR’ amide bond, and a proton-assisted C N bond-
ꢀ
ꢀ
forming mechanism via an amide intermediate.[8]
We have recently been developing further the chemistry
of compounds of the type I and its homologues.[2m,o,9] As part
of this program we prepared the closely related compound
[Ti(N2iPrN)(NNPh2)(py)] (1; N2iPrN = MeN(CH2CH2NiPr)2)
containing sterically less demanding isopropyl substituents
in the ligand periphery in place of the SiMe3 groups in I.
Compound 1 was prepared in 72% yield from Li2N2iPrN and
[Ti(NNPh2)Cl2(py)3]. The solid-state structure[10] (see the
Supporting Information) establishes the trigonal-bipyramidal
geometry shown in Scheme 2. This geometry is also predom-
Scheme 1. Structures of the alkyne-diamination catalyst I, intermedi-
ates II and III, and products IV, as well as the proposed conventional
intermediate in alkyne hydrohydrazination VI and the product hydra-
zone V. py=pyridine.
certain homologues react with terminal or internal alkynes to
form metallacycles of the type II. In the case of I, these
metallacycles cannot be isolated but proceed immediately at
temperatures below 08C to form exclusively vinyl imides III.
Furthermore, both I and III catalyze the 1,2-diamination of
terminal alkynes to form diaminoalkenes IV.[2l] This trans-
formation is a new reaction of hydrazines with alkynes, which
usually undergo hydrohydrazination, also via postulated
metallacycles of the type II.[1e,2c,d,3] These metallacycles
Scheme 2. Reaction of [Ti(N2iPrN)(NNPh2)(py)] (1) with terminal
alkynes.
inantly maintained in solution according to NOE experi-
ments. The hydrazide ligand in the solid-state structure of
1 occupies the electronically preferred[11] axial position trans
to the NMe donor, whereas in I it lies exclusively in the
equatorial position, which is the sterically preferred site.
ꢀ
ꢀ
undergo protonolysis of the Ti C and Ti N bonds to form
1
hydrazones V via transient mixed bis(hydrazido(1ꢀ)) inter-
Minor resonances in the H NMR spectrum of 1 are attrib-
uted to the isomer with NNPh2 in the equatorial position.
The treatment of 1 with HCCTol (1 equiv; Tol = 4-
C6H4Me) in C6D6 led to 50% conversion of 1 and complete
conversion of the alkyne into a new compound, 2a. A second
reaction with a 2:1 ratio of the alkyne to 1 gave quantitative
conversion into 2a, which was isolated in 91% yield when the
reaction was scaled up and carried out in Et2O. Compound 2a
is the unusual acetylide–vinylhydrazide(1ꢀ) compound [Ti-
[*] A. D. Schwarz, C. S. Onn, Prof. P. Mountford
Chemistry Research Laboratory, University of Oxford
Mansfield Road, Oxford, OX1 3TA (UK)
E-mail: Philip.Mountford@chem.ox.ac.uk
[**] We thank the UK EPSRC (grant EP/H01313X/1) and Ministry of
Education, Brunei (scholarship to C.S.O.) for support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201206249.
(N2iPrN)(CCTol){N(NPh2)C(H)C(H)Tol}]
illustrated
in
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ꢀ
Scheme 2. In solution it exists as two isomers with the
vinylhydrazide moiety cis or trans to the NMe donor of N2iPrN,
as established by NOE measurements (the ratio of cis-2a to
trans-2a is ca. 10:1).
The isomer cis-2a was structurally characterized by X-ray
crystallography (Figure 1).[10] The distances and angles asso-
ciated with the various ligand fragments and with the titanium
reaction of Ti C(R) would furnish cis- or trans-2, respectively.
In an attempt to identify an intermediate species, we followed
ꢀ
the reaction between 1 and HCC 4-C6H4OMe in [D8]toluene
from ꢀ708C to room temperature. No intermediates were
observed; however, interestingly, the first-formed product
was trans-2c, which was then converted predominantly into
cis-2c as room temperature was approached. This observation
may suggest that the kinetically preferred reaction proceeds
via trans-3 at the s-bond-metathesis step. An alternative
ꢀ
reaction sequence based on alkyne C H bond activation via
[Ti(N2iPrN)(CCR){NH(NPh2)}] (4) and then alkyne insertion
into the Ti N(H)NPh2 bond to form [Ti(N2iPrN)-
ꢀ
(CCR){C(R)C(H)N(H)NPh2}] (5), followed by the isomer-
ization of 5 to form 2 is considered unlikely. Although alkyne
ꢀ
C H addition to titanium–ligand multiple bonds (titanocene
imido, oxo, vinylidene bonds)[13] and alkyne insertion into
early-transition-metal–nitrogen single bonds[7e,14] have prece-
dent, these reactions are very rare.
No apparent reaction occurred when 2a was treated with
a further equivalent of HCCTol or DCCTol. The latter
experiment shows that the formation of 2a from the likely
metallacycle intermediate is irreversible. However, when
ꢀ
a solution of 2a was treated with HCC 4-C6H4CF3 (1 equiv),
Figure 1. Displacement ellipsoid plot (20% probability level) of [Ti-
(N2iPrN)(CCTol){N(NPh2)C(H)C(H)Tol}] (cis-2a). H atoms are omitted,
except for the vinylic atoms.
an equilibrium mixture of this alkyne, HCCTol, 2a, and
iPr
ꢀ
a
species tentatively assigned as [Ti(N2 N)(CC 4-
C6H4CF3){N(NPh2)C(H)C(H)Tol}] was formed, probably by
a s-bond-metathesis reaction as proposed for the formation of
compounds 2 from intermediate metallacycles 3.
A reaction of 1 with a mixture of HCCTol and DCCTol (1/
HCCTol/DCCTol = 1.0:1.5:1.5) was also carried out in an
NMR tube. The 1H and D NMR spectra of the organometallic
product mixture showed the presence of a mixture of four
isotopomers of the form [Ti(N2iPrN)(CCTol){N(NPh2)C24-
(HaDb)C25(HxDy)Tol}], in which (a+b) = (x+y) = 1 and C24
and C25 are labeled according to the numbering scheme in
center itself are within the usual ranges.[12] Isomer cis-2a may
be thermodynamically favored on steric grounds, as it
positions the bulkier N(NPh2)C(H)C(H)Tol group furthest
from the isopropyl substituents of N2iPrN. Single crystals of cis-
2a immediately reform the equilibrium mixture of cis and
trans isomers in solution. The vinylic hydrogen atoms of the
vinylhydrazide moiety in cis-2a (H(241) and H(251) in
Figure 1) appear as mutually coupled doublets at d = 7.99
1
1
and 5.79 ppm, respectively, in the H NMR spectrum. The
Figure 1. For C24, there was an equal amount of H and D
absence of these 1H resonances in the isotopomer [Ti(N2iPrN)-
(CCTol){N(NPh2)C(D)C(D)Tol}] ([D2]cis-2a) prepared from
1 and DCCTol (the expected resonances are present in the
D NMR spectrum) shows that both of these hydrogen atoms
are derived from the alkyne.
Compound 1 also reacted immediately and quantitatively
in C6D6 with HCCR (R = 4-C6H4CF3, 4-C6H4OMe, C6F5, or
SiMe3) to form the corresponding cis and trans analogues of
2a (Scheme 2). These compounds were isolated in good to
excellent yield when the reactions were scaled up. In contrast,
HCCtBu did not react with 1 at room temperature or on
heating, whereas HCCnBu, PhCCMe, and MeCCMe gave
unidentified mixtures. Compounds 2a–e are stable in solution
in C6D6 over a number of days at room temperature but
decompose on heating (e.g., the half-life for the decomposi-
tion of 2a at 708C is ca. 3 h).
Formation of the acetylide–vinylhydrazides 2a–e probably
proceeds by [2+2] cycloaddition reactions of 1 (or its
pyridine-free analogue) via intermediate metallacycles of
the type [Ti(N2iPrN){N(NPh2)C(H)C(R)}] (cis- or trans-3;
Scheme 3) as observed and/or isolated previously for II
(Scheme 1) or its homologues.[2m] The subsequent reaction of
cis- or trans-3 with HCCR through a s-bond-metathesis
attached on average (a/b = 1). This result implies, as expected,
that there is no significant kinetic isotope effect (KIE) for the
formation of the intermediate metallacycles 3a or [D]3a from
2a and either HCCTol or DCCTol, respectively. In contrast,
there was a significantly higher extent of H attachment to C25
as compared to D attachment (x/y = 2.4(1) on the basis of 1H
and D spectroscopic analysis); thus, a KIE value of 2.4(1) was
obtained for the s-bond-metathesis process leading from 3a
to 2a.
Although Group 4 imide- and hydrazide-derived aza-
metallacyclobutene species (L)M{N(R)C(R1)C(R2)} are now
quite well established, and unsaturated substrates can be
[1c,e,2e,f,o]
ꢀ
inserted into the M C bond,
a subsequent s-bond-
metathesis reaction has never been observed.[15] As men-
tioned, catalytic hydrohydrazination and hydroamination
reactions of alkynes by the “imide route” are proposed to
proceed via transient species of the type VI (Scheme 1)
containing hydrazido(1ꢀ) (or amido) and vinylhydrazido(1ꢀ)
ꢀ
(or vinylamido) ligands formed by R2N H protonolysis of the
1
ꢀ
M C(R ) bond of the precursor metallacycle. Compounds
2a–e can be viewed as the first arrested models of this type of
N-bound vinylhydrazido(1ꢀ) moiety. The research groups of
Odom[14a] and Schafer[7e] reported complementary C,N-bound
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aminovinyl complexes formed by alkyne insertion into a high-
ꢀ
ꢀ
energy Mo NMe2 or Zr NMe2 amide moiety, respectively.
ꢀ
Alkyne insertion into Zr NMe2 is relevant to alkyne hydro-
amination by the “amide route”.
Preliminary experiments have shown that 1 is a catalyst
for the hydrohydrazination of certain terminal alkynes with
Ph2NNH2 [Eq. (1) and Table 1]. This reactivity contrasts
Table 1: Hydrohydrazination of alkynes HCCR with Ph2NNH2 and [Ti-
(N2iPrN)(NNPh2)(py)] (1).[a]
Entry Alkyne R
group
Reaction kobs [hꢀ1 [b]
time [h]
]
Conversion
[%][c]
Yield
[%][d]
1
2
3
4
5
6
7
8
4-C6H4Me
4-C6H4CF3
4-C6H4OMe
4-C6H4CF3
C6F5
C6F5
SiMe3
tBu
48
68
47
47
116
45
6.8(3)ꢀ10ꢀ2 96
90
75
58
–
40
–
4.7(2)ꢀ10ꢀ2 94
4.2(1)ꢀ10ꢀ2 74
Figure 2. Plot of hydrazine conversion into the hydrazone (top, dotted
line drawn as an aid to the eye) and the corresponding first-order ln
plot of hydrazine consumption (bottom, linear regression fit,
–
91
0.66(5)ꢀ10ꢀ2 55
20
4.4(1)ꢀ10ꢀ2 95
k
obs =4.7(2)ꢀ10ꢀ2 hꢀ1). Data are for the hydrohydrazination of
–
HCC 4-C6H4CF3 with Ph2NNH2 and [Ti(N2iPrN)(NNPh2)(py)] (1; C6D6,
65
116
73
0
ꢀ
0
0
room temperature, 10 mol% catalyst loading).
[a] Reactions were carried out in C6D6 with a 10 mol% loading of 1.
[b] The rate constant kobs was determined on the basis of first-order rate
plots (R2 =0.995–0.996) for ꢁ90% substrate consumption, except for
the reaction in entry 3, which showed significant decay of the catalyst
after 27 h (67% conversion, R2 =0.997; no further reaction beyond 47 h),
and the reaction in entry 6 (116 h, 55% conversion, R2 =0.974).
[c] Conversion was determined by NMR spectroscopy by integration of
the hydrazine and/or alkyne, and hydrazone resonances. [d] Yield of the
isolated product after purification by column chromatography.
when the same catalytic reaction of HCCTol and Ph2NNH2
was carried out with 2a (10 mol%) as the added catalyst
rather than 1, the reaction proceeded at approximately the
same rate to form 6a, and once again 2a was the only titanium
species observed. Further experiments in NMR tubes showed
that the reaction of 2a with 1 equivalent of Ph2NNH2
liberated 1 equivalent of 6a and consumed 0.5 equivalents
of 2a in accordance with Scheme 3.[17] The corresponding
reaction with XylOH (Xyl = 2,6-C6H3Me2) immediately
formed HCCTol (no 6a was formed), which is consistent
remarkably with that of the SiMe3-substituted homologue I
(Scheme 1), which is a 1,2-diamination catalyst under these
conditions.[2l,16] The catalysis is totally specific for the
formation of the anti-Markovnikov hydrazones 6a–e, as
ꢀ
with preferential protonolysis of the Ti CCTol moiety of 2a
ꢀ
over [Ti N(NNPh2)C(H)C(H)Tol]. The metal product of this
ꢀ
confirmed for (E)-HC(NNPh2)CH2 4-C6H4CF3 (6b) by X-
reaction was not identified, but the kinetically favored
ray crystallography (see the Supporting Information).[10]
The catalysis is relatively slow at room temperature. The
best conversions were observed for 6a, 6b, and 6e: 91–95%
conversion after 47–68 h at room temperature with a catalyst
loading of 10 mol%. However, with the exception of a system
reported recently by Gade,[3d] these performances are not
untypical of titanium hydrohydrazination catalysts at room
temperature.[2c,d,3c] All systems showed first-order consump-
tion of the hydrazine (see Figure 2 for the reaction with
protonation of 2 at the Ti C(sp) bond is reminiscent of the
ꢀ
well-established protonation reactions of [2+2] Group 4
2
ꢀ
metallacycles at the M C(sp ) bond during hydroamination/
hydrohydrazination catalysis.
Overall, these results are consistent with the catalytic
cycle shown in Scheme 3. Alkyne [2+2] cycloaddition to base-
free 7 (previously established as an intermediate in 1,2-
diamination reactions catalyzed by I) forms transient 3,
which, in turn, rapidly forms 2. Alternatively, the cycle can be
ꢀ
ꢀ
HCC 4-C6H4CF3 and Figure S3 in the Supporting Informa-
entered directly from 2. Protonolysis of Ti CCR forms 8,
tion for HCCTol). For 6c, the catalyst system seemed to
degrade after approximately 1 day, whereas with HCCC6F5,
the catalysis was about an order of magnitude slower.
analogous to the intermediates (e.g. VI, Scheme 1) proposed
in direct reactions of metallacycles with hydrazines or amines.
Finally, elimination from 8 generates the hydrazone 6. Since
the rates of the reactions in Scheme 2 of 1 with HCCR to form
2a–e are effectively independent of R, whereas hydrohydra-
zination with R = C6F5 is approximately 10 times slower
(owing to the lower basicity of the acetylenic carbon atom in
We monitored the reactions in Equation (1) by NMR
spectroscopy and found that the acetylide–vinyl hydrazido
compounds 2a–e were the only titanium species observed and
were present throughout the catalytic process. Furthermore,
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k
obs = 6.8(3) ꢀ 10ꢀ2 hꢀ1) was repeated with [HCCTol]0/
[Ph2NNH2]0/[1]0 = 5:10:1 (i.e., a 50% decrease in the alkyne
loading) the experimental kobs value was identical (6.7(1) ꢀ
10ꢀ2 hꢀ1; see Figure S3 in the Supporting Information), which
indicates that the kobs value and the TLS are independent of
[alkyne]t. The use of monodeuterated tolyl acetylene with
[DCCTol]0/[Ph2NNH2]0/[1]0 = 10:10:1 gave kobs([d]alkyne)
=
6.2(3) ꢀ 10ꢀ2 hꢀ1 (the KIE value based on HCCTol and
DCCTol is 1.1(1); Figure 3, top): again no experimentally
significant effect of the deuterated alkyne on the overall
reaction rate was found (recall that we showed independently
above that the (fast) s-bond-metathesis step 3a!2a has
a KIE value of 2.4(1)).
Scheme 3. Proposed mechanism for the catalytic hydrohydrazination of
alkynes via [Ti(N2iPrN)(CCR){N(NPh2)C(H)C(H)R}] (2). Only the cis
isomers of 2, 3, and 6 are illustrated for clarity. Compound 5 may also
be formed by pyridine loss from 1 (this step is omitted for clarity).
2d), the turnover-limiting step (TLS) appears to be from 2 to
8. This conclusion is also consistent with the observation that
2 is the resting state.
These results alone do not rule out the direct conversion
of 3 into 8 as proposed in the conventional “imide”
mechanism for alkyne hydroamination, although such a trans-
formation seems unlikely from our observations that 1) 2 is an
active (pre)catalyst; 2) 2 is also the resting state of the
titanium in the system; 3) the formation of 3 from 2 does not
occur (according to labeling studies). To probe this possibility
further, we carried out a stoichiometric competition reaction
of 1 with HCCTol and Ph2NNH2 (1:1:1 molar ratio) in an
NMR tube. All of the HCCTol and half of 1 were immediately
converted into 2a. No hydrazone 6a was formed and no
hydrazine was consumed until 30 min after consumption of
the alkyne to form 2. Therefore, in the presence of equimolar
amounts of the alkyne and the hydrazine (the ratio used in the
catalytic reactions, Table 1), the formation of 2a from 3a is
much faster than a possible reaction of 3a with Ph2NNH2 to
form 8a. This result is consistent with the observations we
made when we monitored the catalytic reactions by NMR
spectroscopy: complete conversion of all of the starting 1 into
2 occurred well in advance of the appearance of any
hydrazone.
Figure 3. Top: First-order ln plot of hydrazine conversion and linear
regression fits for the hydrohydrazination of HCCTol (squares and
solid line, kobs =6.8(3)ꢀ10ꢀ2 hꢀ1) and DCCTol (diamonds and dotted
line, kobs =6.2(3)ꢀ10ꢀ2 hꢀ1) with Ph2NNH2. Bottom: Corresponding
first-order plots of hydrazine conversion and linear regression fits for
the hydrohydrazination of HCCTol with Ph2NNH2 (squares and solid
line, kobs =6.8(3)ꢀ10ꢀ2 hꢀ1) and Ph2NND2 (triangles and dotted line,
k
obs =1.9(1)ꢀ10ꢀ2 hꢀ1). All experiments were carried out with [Ti-
(N2iPrN)(NNPh2)(py)] (1; C6D6, room temperature, 10 mol% catalyst
loading).
In previous mechanistic and computational studies of
hydroamination catalysis by an imide mechanism, both the
[2+2] cycloaddition and subsequent protonation of the
metallacycle have been proposed to be the TLS (as well as
In contrast, use of the N-dideuterated hydrazine with
[HCCTol]0/[Ph2NND2]0/[1]0 = 10:10:1 gave kobs([d]hydrazine)
=
the generation of the free imido species (L)M NR from
1.9(1) ꢀ 10ꢀ2 hꢀ1, which corresponds to a significant KIE
value of 3.7(2) based on Ph2NNH2 and Ph2NND2 (Figure 3,
bottom). Taken together, these results imply a tentative rate
expression of d[hydrazone]/dt = kobs[Ph2NNH2]. Further stud-
ies are in progress to fully characterize the kobs term, but our
results unambiguously establish 2!8 as the TLS for the
hydrazines and alkynes studied to date with 1.
=
a bis(amide) or dimeric resting state), depending on the
system under study.[3a,b,5,6] In our system, it is apparent that the
reaction of 3 with the alkyne to form 2 is significantly faster
than that of 7 with the alkyne, and the protonolysis of 2 is
considerably slower than the conversion of 1 into 2.
We carried out additional experiments to investigate
further the contribution of steps 7!3, 3!2, and 2!8 to the
overall rate of hydrohydrazination. When the experiment
with [HCCTol]0/[Ph2NNH2]0/[1]0 = 10:10:1 (Table 1, entry 1;
The high selectivity of the catalytic process is controlled
=
by the sense of addition of the alkyne to the Ti NNPh2 bond
of 3 as found in related systems.[1c,3b,4d,18] We have explained
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the origin of this preference previously in the 1,2-diamination
reactions of I with alkynes.[2m] The lack of reaction of 1 with
HCCtBu is attributed to the unfavorable energy of formation
of the metallacycle 3 with the bulky, electron-releasing tert-
butyl substituent, which destabilizes the electron-rich
metallacycle.[2m]
Received: August 3, 2012
Revised: October 2, 2012
Published online: November 4, 2012
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Keywords: homogeneous catalysis · hydrazide complexes ·
.
hydrazones · hydrohydrazination · titanium
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[15] We make a formal distinction between the metathesis reaction
ꢀ
observed with C H bonds and protonolysis-type chemistry well-
ꢀ
known with N H bonds.
[16] J. D. Selby, DPhil thesis, University of Oxford 2009.
[17] Scheme 3 predicts that a stoichiometric reaction of Ph2NNH2
with 2a should initially form base-free 7 (1 equiv), 6a (1 equiv),
and HCCTol (1 equiv). However, the rapid reaction of 7 with the
liberated HCCTol (1:2 ratio) leads to the regeneration of 2a
(0.5 equiv) through the [2+2]/s-bond-metathesis reaction.
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