Phosphoramidate tantalum complexes for room-temperature C-H functionalization: Hydroaminoalkylation catalysis

Article abstract of DOI:10.1002/anie.201304153

A cooled reaction: Phosphoramidate-ClTaMe₃ complexes promote the first example of room-temperature hydroaminoalkylation catalysis. This reaction can be realized under solvent-free conditions and with challenging substrates such as styrenes and dialkyl amines. When using a vinylsilane substrate, for the first time the linear regioisomer is obtained preferentially using a Group5 metal. TBS=tert-butyldimethylsilyl, TMS=trimethylsilyl. Copyright

Full text of DOI:10.1002/anie.201304153

Angewandte
Chemie
Communications
À
C H Activation
P. Garcia, Y. Y. Lau, M. R. Perry,
L. L. Schafer*
&&&& — &&&&
Phosphoramidate Tantalum Complexes
À
for Room-Temperature C H
Functionalization: Hydroaminoalkylation
Catalysis
A cooled reaction: Phosphoramidate–
ClTaMe3 complexes promote the first
example of room-temperature hydroami-
noalkylation catalysis. This reaction can
be realized under solvent-free conditions
and with challenging substrates such as
styrenes and dialkyl amines. When using
a vinylsilane substrate, for the first time
the linear regioisomer is obtained prefer-
entially using a Group 5 metal.
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
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DOI: 10.1002/anie.201304153
À
C H Activation
À
Phosphoramidate Tantalum Complexes for Room-Temperature C H
Functionalization: Hydroaminoalkylation Catalysis**
Pierre Garcia, Ying Yin Lau, Mitchell R. Perry, and Laurel L. Schafer*
Amines are critical functional groups that are incorporated
into many biologically active compounds and functional
materials of importance to the biomedical, agrochemical
and fine-chemical industries.[1] An idealized synthetic
approach for the preparation of this important class of
compounds would take advantage of the direct and by-
product free conversion of feedstock alkenes directly into
unprotected amines with good regio- and stereoselectivity
under mild reaction conditions. These goals could be realized
with early transition metal catalyzed hydroaminoalkylation
Ligand screening investigations by Herzon and Hartwig
have shown that electron-withdrawing chloride ligands
enhance reactivity, such that select substrate combinations
yield products at 908C.[5b] Notably, the more challenging
dialkylamine substrates required temperatures of 1508C and
thermally polymerizable styrene derivatives were not repor-
ted.[4b] Herein, we show that our phosphoramidate–ClTaMe3
precatalyst is easily synthesized and can achieve room-
temperature hydroaminoalkylation with a broad range of
substrates. Importantly this complex shows functional group
tolerance (OTBS, Cl, CF3, OMe, TMS) in both the amine and
alkene substrates, it is effective with thermally sensitive
styrene substrates, and even dialkylamines can be used at
room temperature. While unactivated alkenes react regio-
selectively to give branched products in high yield, remark-
ably this low temperature reactivity permits a substrate-
controlled shift in regioselectivity such that for the first time
with Group 5 complexes, linear amine products can be
obtained in good yield. Most importantly, this phosphorami-
date Ta complex can be used under solvent-free conditions to
promote the high yielding, room temperature, and atom-
economic synthesis of substituted amines directly from
alkenes.
À
(Scheme 1), a C H functionalization reaction a to nitrogen
À
Scheme 1. Hydroaminoalkylation: a by-product-free C H functionaliza-
tion reaction using simple amine and alkene feedstocks.
that results in selective C C bond formation.[2,3] However, to
À
date, all promising Group 4 and 5 metal complexes for this
transformation demand harsh reaction conditions.[4,5] The
identification of a system that can be used with mild reaction
conditions is desirable. Here we show that by using a sterically
demanding, N,O-chelating, electron-withdrawing phosphor-
amidate as an easily installed auxiliary ligand, room-temper-
ature alkene hydroaminoalkylation can be achieved for the
first time.
In an effort to identify catalyst systems that can realize
À
C H functionalization under mild reaction conditions,
a benchmark hydroaminoalkylation reaction of 1-octene
with N-methyl-p-methoxyaniline has been selected to com-
pare various catalyst systems (Scheme 2). N-methyl-p-
methoxyaniline is a commonly used substrate in hydro-
aminoalkylation catalyst development work,[4d,5a,c,f–j] as it has
been shown to be favorably reactive while affording the
opportunity for oxidative deprotection to access primary
amine products.[5i] Notably, previous reports using Ta pre-
Unlike late transition metal catalysts (Ir, Ru)[6,7] for this
reaction, early transition metal catalysts (Ti,[4] Zr,[4a] Ta,[5]
Nb,[5e–g]) do not require a removable directing group or
activated alkene substrates. Hydroaminoalkylation results in
unprotected amines ready for further functionalization. This
transformation gives selectively substituted amines in a single
and atom-economic catalytic reaction, using inexpensive
early transition metals of low toxicity. Thus, hydroamino-
alkylation is an excellent reaction to target for advances in
green chemistry.
[*] Dr. P. Garcia, Y. Y. Lau, M. R. Perry, Prof. Dr. L. L. Schafer
Department of Chemistry, University of British Columbia
2036 Main Mall, Vancouver, BC, V6T 1Z1 (Canada)
E-mail: schafer@chem.ubc.ca
[**] We thank Jacky C.-H. Yim for assistance with X-ray crystallography
and Boehringer Ingelheim (Canada) Ltd. and NSERC for financial
support of this work. This research was funded by the Canada
Research Chairs program.
Scheme 2. Developing a pecatalyst for room-temperature reactivity.[a]
[a] Reaction conditions: 1a (0.5 mmol), 2a (0.75 mmol), [D8]toluene
(0.6 mL), [Ta] (0.025 mmol), 20 h. Conversion determined by 1H NMR
spectroscopy. [b] From isolated complex. [c] From in situ generated
complexes.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201304153.
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catalysts for hydroaminoalkylation of unactivated alkenes
with N-methyl-p-methoxyaniline typically disclose reaction
temperatures of 130–1608C with a few exceptions reported at
90–1108C.[5b,g,i,j]
substituted phosphoramidate was not tolerated (entry 5). An
increase in catalyst loading to 10 mol% resulted in a more
favorable conversion within 20 h (entry 6). Common solvents
such as toluene, hexanes and THF (entries 6–8) can all be
used at room temperature, with toluene being preferred. In
order to compare in situ prepared precatalysts with isolated
complexes, salt metathesis has been used to prepare the
desired complex 4 as yellow crystalline needles (77% yield,
Scheme 3). X-ray crystallographic analysis of this complex
We identified the minimum temperature required to
achieve at least three turnovers within 20 h for several TaV
precatalysts. The commercially available Ta(NMe2)5 requires
a high reaction temperature of 1308C to realize three catalyst
turnovers within 20 h.[5a] Our reported N,O-chelating amidate
precatalyst[5c] can realize this transformation at 1108C.
Previous investigations have shown that sufficient steric
bulk is critical for promoting efficient reactivity in this class
of precatalysts, while N,O-chelating ligands promote
enhanced substrate scope.[5c,h] Thus our attention turned to
other readily available N,O-chelating motifs that could be
easily modified to support enhanced steric bulk. Phosphor-
amidates are attractive due to their increased electron-
withdrawing properties,[8] tunable steric bulk at both phos-
phorus and nitrogen, and their facile modular syntheses.[9]
The mixed phosphoramidate–Ta(NMe2)4 complex pre-
pared in situ by protonolysis of Ta(NMe2)5 with the proligand
can catalyze the desired hydroaminoalkylation reaction at
908C. This result is comparable to the recently reported
Cl2TaMe3 precatalyst which also requires 908C for observable
catalytic turnover within 20 h.[5j] Gratifyingly, the combina-
tion of the steric bulk of the phosphoramidate ligand with the
generation of a highly electrophilic metal center through the
incorporation of both electron-withdrawing chloride and
phosphoramidate ligands resulted in seven turnovers within
20 h at only room temperature.
Scheme 3. Salt metathesis to give crystalline precatalyst 4.
reveals a distorted trigonal bipyramidal geometry with the k2-
N,O-chelated phosphoramidate ligand trans to the axial
chloride ligand.[10] Comparison of the metric parameters for
the phosphoramidate ligand in 4 with the starting L2-H (see
Supporting Information) proligand shows elongation of the
=
À
P O double bond, accompanied by a slight shortening of P O
À
and P N s bonds, as well as a significant reduction of the
OPN angle from 113.018 to 98.678.[10] These observations are
all consistent with an N,O-chelating bonding mode. Solution-
phase characterization data gives 1H, 13C, and 31P NMR
spectra consistent with one species in solution, suggesting
either static k2 chelation, as observed in the solid state, or
a highly fluxional complex (k2–k1) on the NMR timescale.
Most importantly, the use of this isolated precatalyst gives the
highest yielding room-temperature transformation (Table 1,
entry 9).
Reaction optimization is shown in Table 1. A closer look
at the steric bulk provided by the ligand, through 2,6-
substitution of the aniline moiety, showed that 2,6-dimethyl-
aniline enabled improved conversion (52%, entry 2). How-
ever, even bulkier groups at the 2,6-positions (entry 3) or on
the alkoxy substituents of phosphorus (entry 4) were detri-
mental to catalyst performance. Notably, the use of an N-alkyl
With the straightforward synthesis of precatalyst 4 estab-
lished, investigations of substrate scope showed that a variety
of secondary amines are reactive with this catalyst system
(Table 2). Electron-rich N-methyl anilines are preferred
substrates, such that 4-methoxy- or 4-dimethylamino-N-
methylanilines furnish the expected branched product 3a
and 3b in high yields (entries 1 and 2). Consistent with this
observation is the fact that N-methylaniline gives only 19%
yield under the same reaction conditions, yet the addition of
electron-donating alkyl substituents improves reactivity such
that N-methyl-p-toluidine gives 3c in 46% yield (entry 3),
and the introduction of two methyl groups in the 3,4-positions
of the phenyl ring enable the isolation of 3d in 62% yield
(entry 4).[11] Halogen substituents are tolerated as shown with
product 3e (entry 5) to give amine building blocks ready for
catalyzed coupling protocols.[5j] Even chelating catechol
derivatives[12] can be used (entry 6) to give the expected
compound 3 f in good yield (80%), with extended reaction
times. Using the same reaction conditions N,N’-dimethyl-1,4-
phenylenediamine was monoalkylated to give 3g (entry 7)
and unreacted starting material.[13] Most importantly, in
addition to arylamines, this precatalyst promotes room-
temperature reactivity with known challenging dialkylamine
Table 1: Optimization of reaction conditions.[a]
Entry
L
x mol%
Solvent
Conv. [%]
1[b]
2[b]
3[b]
4[b]
5[b]
6[b]
7[b]
8[c]
9[c]
L1
L2
L3
L4
L5
L2
L2
L2
L2
5
5
5
5
5
10
10
10
10
[D8]toluene
[D8]toluene
[D8]toluene
[D8]toluene
[D8]toluene
[D8]toluene
hexanes
37
52
28
24
0
76
62
26
86
[D8]THF
[D8]toluene
[a] Reaction conditions: 1a (0.5 mmol), 2a (0.75 mmol), solvent
(0.6 mL), [Ta] (x mol%), 20 h. Conversion determined by 1H NMR
spectroscopy. [b] In situ generated complex. [c] Isolated complex.
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Table 2: Amine substrate scope.[a]
Table 3: Alkene substrate scope.[a]
Entry Amine
1
Product
Yield [%]
84
Entry Amine
1
Product
Yield [%]
88
2
87
2
3
59
78
3
4
46
62 (44 h)
4
5
6
70
86
56
5
6
7
41
80 (64 h)
52
88 [68:1][b] (508C)
87 (168 h)
7
84 (508C)
62 (168 h)
8
9
87 [14:1][b] (508C)
93 [26:1][b] (168 h)
8
15
58 [10:1][b] (508C)
93 [10:1][b] (168 h)
[a] Reactions conditions: 1a–i (1 mmol), 2a (1.5 mmol), toluene (1.2 g),
4 (0.1 mmol). Yield of isolated products.
9
91 [1.6:1][b] (508C)
78 [1.8:1][b] (168 h)
10
11
substrates.[5b] Sterically comparable N-methylcyclohexyl-
amine can be used in the hydroaminoalkylation of 1-octene
to give 3h in 62% yield. Here, prolonged reaction times are
required at room temperature, but notably the reaction
duration can be considerably shortened and the product yield
can be significantly enhanced by warming to 508C over 20 h
(entry 8).[14] This precatalyst is very sensitive to steric bulk
such that tert-butylmethylamine cannot be used for hydro-
aminoalkylation; however, dibutylamine can be selectively
monoalkylated, albeit in low yield (3i, 15% yield, entry 9).
Various alkenes can successfully undergo room-temper-
ature hydroaminoalkylation (Table 3). In addition to unac-
tivated 1-octene, alkenes with protected alcohols at various
positions (distal, 3j and allyl, 3k) are well tolerated (Table 3,
entries 1 and 2), opening routes to further synthetic manip-
ulation.[5h] Phenyl-substituted alkenes are well tolerated with
3l isolated in 78% yield while 3m, which is prepared using
isomerizable allylbenzene, is reached in 70% yield (entries 3
and 4). When using bicyclic alkene substrates such as
norbornene and norbornadiene, only the exo-diastereomer
is prepared in both cases (3n, 3o, entries 5 and 6).[15] In the
case of norbornadiene the mono-substituted product is the
major product isolated, although the overalkylated product
can be detected.
66 [1:7][c]
[a] Reaction conditions: 1a (1 mmol), 2b–l (1.5 mmol), toluene (1.2 g),
4 (0.1 mmol). Yields of isolated products. [b] Major isomer presented,
[branched/linear] ratio determined by GC of the crude material. Yields
refer to combined regioisomers. [c] Major isomer presented, [branched/
linear] ratio determined by GC of the inseparable purified isomers. Yield
refers to combined regioisomers.
Room-temperature catalysis permits the exploration of
temperature-sensitive substrates. To this end, styrenes were
reacted with p-methoxy-N-methylaniline. In contrast to
reports using Group 4 catalysts,[4e,g] in this case, electron-rich
and electron-deficient styrenes are very well tolerated,
although prolonged reaction times are required (entries 7–
10, 3p–3s). Here again, performing the reaction at 508C
shortens the required reaction time. When p-methoxystyrene
is used at 508C, a trace amount of the linear product can be
detected by GC-MS analysis (3p, entry 7). While such linear
products have been disclosed for Group 4 catalyzed hydro-
aminoalkylation,[4] this is the first example of Group 5
complexes giving both regioisomers with styrene substrates.
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Interestingly, the p-substituent plays an important role in the
branched:linear ratio. Indeed, the amount of linear adduct in
3p–s increases when a stronger electron-withdrawing group is
used, reaching 1.8:1 when a trifluoromethyl group is used
(entry 10). It is noteworthy that in the case of the more
electron-rich styrenes (entries 7 and 8), room-temperature
reactions promote a more selective transformation with only
the branched isomer being detected in the case of
p-methoxystyrene.
As the observed amount of linear adduct in 3p–s increases
when a stronger electron-withdrawing group is used for
styrene derivatives, the alkene is hypothesized to evolve
transiently charged species. Here, as the substitution of the
distal para-position with increasingly electron-withdrawing
moieties is explored, the generation of such transiently
charged species could be destabilized. Thus, modified sub-
strate electronic effects may enable a competitive reaction
pathway and varying amounts of linear products can be
accessed (Table 3, entries 7–10).
In order to further test such substrate electronic effects,
we explored the b-silicon effect.[16] A transient positive charge
would then be preferentially stabilized at the terminal
position, and in this case would provide access to linear
amines. Indeed, when vinyltrimethylsilane was used
(entry 11), linear product 3t was found to be the main
regioisomer with a branched:linear ratio of 1:7, in 66%
overall yield. This selectivity is in sharp contrast to previous
reports for Group 5 catalyzed hydroaminoalkylation includ-
ing vinyltrimethylsilane, where the branched product is
obtained virtually exclusively.[5a,g,17]
In summary, the first room-temperature catalytic hydro-
aminoalkylation reactions have been realized through the
development of a phosphoramidate–ClTaMe3 precatalyst (4).
This easily prepared complex enables the atom-economic
synthesis of amines directly from simple amines and alkenes,
including challenging substrates such as styrenes and dialkyl
amines, and also tolerates the incorporation of functional
groups suitable for further synthetic elaboration on both the
amine and alkene substrates. Here we disclose the first
Group 5 catalytic examples in which significant substrate
electronic effects dramatically impact regioselectivity, includ-
ing the first example of linear regioselectivity using a Ta
hydroaminoalkylation precatalyst. Most importantly, this
catalyst is amenable to application using solvent-free reaction
conditions resulting in improved synthetic efficiency for
a variety of substituted amine products. These preliminary
results set a new benchmark for advancing sustainable
approaches for amine synthesis. On-going work includes
kinetic, mechanistic, and catalyst investigations to inform
further development efforts. We are working to exploit this
new, flexible ligand motif to access robust and efficient
catalysts that can be used under solvent-free conditions to
realize regio- and enantioselective transformations with
a broad substrate scope.
Received: May 14, 2013
Published online: && &&, &&&&
À
Keywords: amines · C H activation · hydroaminoalkylation ·
.
room-temperature synthesis · tantalum
In addition to targeting mild reaction conditions for
improving energy efficiency in catalytic transformations, the
principles of green chemistry[18] promote the elimination of
the use of solvent. Indeed, transition metal catalyzed
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C H functionalization reactions have recently evolved to
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Scheme 4. Solvent-free hydroaminoalkylation.
stirred for 20 h at room temperature to provide results that
could be directly compared to those listed in Tables 2 and 3. In
the case of compounds 3a and 3l the yields using the solvent-
free conditions were superior to the more traditional catalytic
reactions (Table 2, entry 1 and Table 3, entry 3, respectively),
while reactivity with norbornadiene afforded product 3o in
comparable yield (Table 3, entry 6). These representative
examples show that phosphoramidate complex 4 is suitable
for use in solvent-free conditions.
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6
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