Alkali metal and stoichiometric effects in intermolecular hydroamination catalysed by lithium, sodium and potassium magnesiates

Article abstract of DOI:10.1039/c9dt00923j

Main group bimetallic complexes, while being increasingly used in stoichiometric deprotonation and metal-halogen exchange reactions, have not yet made a significant impact in catalytic applications. This paper explores the ability of alkali metal magnesiates to catalyse the intermolecular hydroamination of alkynes and alkenes using sytrene and diphenylacetylene as principle setting model substrates. By systematically studying the role of the alkali-metal and the formulation of the heterobimetallic precatalyst, this study establishes higher order potassium magnesiate [(PMDETA)₂K₂Mg(CH₂SiMe₃)₄] (7) as a highly effective system capable of catalysing hydroamination of styrene and diphenylacetylene with several amines while operating at room temperature. This high reactivity contrasts with the complete lack of catalytic ability of neutral Mg(CH₂SiMe₃)₂, even when harsher reaction conditions are employed (24 h, 80 °C). A pronounced alkali metal effect is also uncovered proving that the alkali metal (Li, Na, or K) is not a mere spectating counterion. Through stoichiometric reactions, and structural and spectroscopic (DOSY NMR) investigations we shed some light on the potential reaction pathway as well as the constitution of key intermediates. This work suggests that the enhanced catalytic activity of 7 can be rationalised in terms of the superior nucleophilic power of the formally dianionic magnesiate {Mg(NR₂)₄}^2- generated in situ during the hydroamination process, along with the ability of potassium to engage in π-interactions with the unsaturated organic substrate, enhancing its susceptibility towards a nucleophilic attack by the amide anion.

Full text of DOI:10.1039/c9dt00923j

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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DOI: 10.1039/C9DT00923J
ARTICLE
Alkali Metal and Stoichiometric Effects in Intermolecular
Hydroamination Catalysed by Lithium, Sodium and Potassium
Magnesiates
Received 00th January 20xx,
Accepted 00th January 20xx
Laia Davin, Alberto Hernán-Gómez, Calum McLaughlin, Alan R. Kennedy, Ross McLellan and Eva
Hevia*
DOI: 10.1039/x0xx00000x
Main group bimetallic complexes, while being increasingly used in stoichiometric deprotonation and metal-halogen
exchange reactions, have not yet made a significant impact in catalytic applications. This paper explores the ability of alkali
metal magnesiates to catalyse the intermolecular hydroamination of alkynes and alkenes using sytrene and
diphenylacetylene as principle setting model substrates. By systematically studying the role of the alkali-metal and the
formulation of the heterobimetallic precatalyst, this study establishes higher order potassium magnesiate
([(PMDETA)2K2Mg(CH2SiMe3)4 (7) as a highly effective system capable of catalysing hydroamination of styrene and
diphenylacetylene with several amines while operating at room temperature. This high reactivity contrasts with the
complete lack of catalytic ability of neutral Mg(CH2SiMe3)2, even when harsher reaction conditions are employed (24h,
80oC). A pronounced alkali metal effect is also uncovered proving that the alkali metal (Li, Na, or K) is not a mere
spectating counterion. Through stoichiometric reactions, and structural and spectroscopic (DOSY NMR) investigations we
shed some light on the potential reaction pathway as well as the constitution of key intermediates. This work suggests that
the enhanced catalytic activity of 7 can be rationalised in terms of the superior nucleophilic power of the formally
dianionic magnesiate {Mg(NR2)4}2- generated in situ during the hydroamination process, along with the ability of potassium
to engage in -interactions with the unsaturated organic substrate, enhancing its susceptibility towards a nucleophilic
attack by the amide anion.
alternatives, can also act as catalysts in intramolecular as well
as more challenging intermolecular hydroamination reactions.5
Moreover, these reactions frequently occur under relatively
Introduction
Hydroamination reactions are described as the addition of an
N-H fragment across an unsaturated C-C bond. They constitute
a powerful atom-efficient and waste-minimized methodology
for accessing amines in organic synthesis.1 Such nitrogen-
containing species standout as lead structures among
numerous pharmaceuticals, biological systems, natural
products and industrially basic and fine chemicals.2 However,
the high kinetic hurdle of these processes imposes in most
cases the use of metal catalysts.3 Over the past two decades,
transition-metal systems based on rhodium, ruthenium,
titanium or palladium have seen significant progress due to
their good tolerance of polar functional groups.4 Pioneering
work by the groups of Hill and Harder have revealed that
alkaline earth metal (particularly calcium and strontium)
complexes, cheaper and more sustainable synthetic
mild conditions. Work by Hill, Carpentier and Sarazin have also
disclosed that the catalyst activity is enhanced with increasing
ionic radius of the metal, revealing that magnesium displays
poor reactivity.5b,6
A similar effect was observed in
organolanthanide hydroamination catalysis, reported by
Marks.1b,7 Examples of alkali metal based catalysts are much
more scarce, although sec-BuLi or Na metal have been known
since the 1950’s.1a,2a,3d,8 Notably alkali-metal catalysed
hydroamination reactions impose harsh reaction conditions
(high temperatures and long reaction times).
Mechanistically, it is thought that alkaline earth metal
catalysed processes proceed through
a metal amide
intermediate, followed by substrate insertion into the M-N
bond, since these main group metals cannot operate via
oxidative addition / reductive elimination steps observed with
transition metal catalysts. These examples imply activation of
the unsaturated organic fragment takes place by forming
electrostatic interactions with the group II metal, which leads
to the polarization of the π-electron density, facilitating the
nucleophilic attack by the amide group.3
Dr L. Davin, Dr A. Hernán-Gómez, C. McLaughlin Dr A. R. Kennedy, Dr R. McLellan,
Prof. E. Hevia
WestCHEM, Department of Pure and Applied Chemistry
University of Strathclyde
Glasgow, G1 1XL (UK)
E-mail: eva.hevia@strath.ac.uk
† Supporting information for this article is given via a link at the end of the
document.
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Bimetallic catalysts are emerging as alternatives to the more Combining catalytic studies with stoichiometric, sVtrieuwcAtruticrlaelOanlninde
conventional single-metal catalytic transformations, providing spectroscopic investigations, the roles oDfOthI:e10a.1l0k3a9l/i-Cm9DeTta00l 9a2n3dJ
enhanced selectivity and reactivity in a variety of processes.9 the constitution of the magnesiate anion have been assessed,
Key to their success is the proximity of the two metals, providing new mechanistic insight into s-block cooperative
affording scenarios where they can operate synergistically. The catalysis.
vast majority of bimetallic catalysts developed involve
transition-metals; whereas the number of examples using
main-group metal systems is very limited.10 Within alkali-metal
magnesiates, unique cooperativity arises because one metal
(here an alkali metal) can act as a Lewis acid allowing the
activation of the unsaturated organic substrate facilitating the
intramolecular nucleophilic attack by the highly nucleophilic
Lewis basic tris(magnesiate) anion (Figure 1).
Results and discussion
We first assessed the intermolecular hydroamination of
diphenylacetylene with piperidine, catalysed by trisalkyl (or
lower order) alkali-metal magnesiates, that is, complexes of
the formula (AM)Mg(CH2SiMe3)3 (AM = common alkali metal =
Li, Na, K), prepared by simple co-complexation of
(AM)CH2SiMe with Mg(CH2SiMe3)2.14
Reaction using
LiMg(CH2SiMe3)3 (1) with 5 mol % loading in d8-THF afforded a
49% yield of hydroamination product after heating at 80 ⁰C for
18 hours. Furthermore, the reaction proceeded with
reasonable stereoselectivity favouring the E- over the Z-isomer
by 86:14 (Table 1 entry 1). Repeating the reaction with
NaMg(CH2SiMe3)3 (2) and KMg(CH2SiMe3)3 (3) under the same
conditions (entries 2-3) gives a markedly increased yield when
catalysed by 2 (98%) and a slightly increased yield when 3 is
used as the catalyst (59%). Furthermore, when 2 is used the
selectivity decreases E:Z (70:30). This clear alkali metal effect is
interesting since reactivity does not seem to follow a trend in
line with the radius of the alkali metal, suggesting that some
more subtle effects are likely to be in operation. Previously it
has been noted through experimental and theoretical
Figure 1 Generic depiction of
magnesiate catalyst.
a cooperative bimetallic alkali metal
Recently we have shown magnesium activated within a
sodium magnesiate platform can outperform Ca and Ba
systems for hydroamination of isocyanates (RNCO), giving
higher yields and superior substrate scope under milder
reaction conditions.11 Using homoleptic tris(alkyl) sodium
magnesiate [NaMg(CH2SiMe3)3] as a precatalyst, these studies
suggest that the reaction takes places with the initial
formation of a highly reactive tris(amido) species, which in
turn undergoes nucleophilic addition to the unsaturated
electrophile RNCO. Prior coordination of RNCO to the Lewis
acidic sodium centre, promotes a more efficient nucleophilic
substitution step. We have also reported the applications of
this mixed Na/Mg system as an efficient precatalyst for the
guanylation and hydrophosphination of amines, proceeding by
a similar reaction pathway.
Westerhausen has demonstrated that potassium calciates
[K2Ca(NPh2)4] and [K2Ca{N(H)Dipp}4] can catalyse
hydroamination reactions of diphenylbutadiyne with a variety
of anilines.13 Exemplifying the cooperative effect of the ate
compound as a catalyst, no reaction product is observed when
adding diphenylamine to 1,4-diphenylbutadiyne in presence of
Ca(NPh2)2 or KNPh2. However, when the bimetallic calciate
calculations on group
2 complexes, that the metal
polarisability, charge density as well as size can greatly
influence the outcome in main-group catalysis.15 Thus we
postulate that a similar scenario is occurring here.5b Despite
the promise of our best performing catalyst
2
in
hydroamination, it is outperformed by the strontium catalyst
[Sr{CH(SiMe3)2}2(THF)2] reported by Hill.5e In this example
quantitative yields are obtained after only two hours at 60oC,
with high E:Z selectivity (91:9). Thus, we decided to investigate
the role of catalyst loading and solvent, in an effort to optimise
our process. Firstly, repeating the intermolecular
hydroamination reaction between diphenylacetylene and
piperidine with different loadings of 2 reveals that the loading
can be reduced to 2 mol% without any reactivity drop-off.
Next, we repeated the reaction in CDCl3 and C6D6 (entries 6-7).
In the former case no reactivity is observed, whereas in the
latter only 42% of hydroamination product is obtained with an
E:Z selectivity of (90:10).
12
Related to these findings,
compound K2Ca(NPh2)4 is used as
a
catalyst the
hydroamination products are formed in NMR yields exceeding
90% (Z isomer 40%, isolated yield).
Expanding wider the catalytic applications of s-block
bimetallics, herein we systematically investigate the ability of a
range of alkali-metal magnesiates to catalyse intermolecular
Entry
1
2
3
4[c,e]
AM
Li
Na
K
Solvent
d8-THF
d8-THF
d8-THF
d8-THF
Yield (%)[a]
E:Z[b]
86:14
70:30
84:16
63:37
48
98
59
hydroamination
of
alkynes
and
alkenes
using
diphenylacetylene and styrene as model substrates.
Na
100
2 | J. Name., 2012, 00, 1-3
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5[d,e]
6[e]
Na
Na
Na
d8-THF
CDCl3
C6D6
97
0
42
66:34
-
90:10
slight decrease in reactivity after the addition oVfie1w5A-rcticrloewOnnli-n5e
suggesting a secondary role of the DOalIk: 1a0li.1-m03e9/tCa9l DTin009t2h3iJs
process.13 This is consistent with forming charge-separated ion
pair species in solutions, whereas in here reactions seem to
occur via the formation of contacted ion pair species which
facilitate metal…metal communication between potassium
and magnesium.
7[e]
Table 1 Intermolecular hydroamination of diphenylacetylene with piperidine,
catalysed by lower order alkali metal magnesiates. [a] Yields determined by 1H
NMR using ferrocene as internal standard. [b] Determined by 1H NMR
spectroscopy. [c] 10 mol% loading. [d] 2 mol% loading. [e] 15 hours.
Encouraged by the initial results using homoleptic
triorganomagnesiates (1-3) we next studied the ability of the
higher order variants [(TMEDA)2Li2Mg(CH2SiMe3)4] (5)14a
[(TMEDA)2Na2Mg(CH2SiMe3)4]
(6)16a
and
[(PMDETA)2K2Mg(CH2SiMe3)4] (7), all of which contain a
formally dianionic magnesiate fragment.16b Previous studies of
these tetraorgano complexes revealed a significantly enhanced
reactivity in deprotonative metalation17 and Mg-halogen18
exchange compared to their lower order counterparts. Again
we began by studying the intermolecular hydroamination
reaction between diphenylacetylene with piperidine, catalysed
by 5-7 (5 mol%). When 5 is used as a catalyst at room
temperature in d8-THF, no reaction occurs over a 3-hour
period (Table 2 entry 1). In contrast, when catalysed by 6, only
28% of hydroamination product is obtained in an E:Z ratio of
29:71 (entry 2). Contrastingly a remarkable increase in
reactivity and selectivity is observed when employing mixed
K/Mg precatalyst 7 affording hydroamination products 4a-b in
almost quantitative yields with a greater E/Z selectivity (93:7,
entry 3). It should be noted that no isomerization between the
E/Z products is observed over time and that NMR monitoring
of the reaction shows that both products evolve separately.
Results using 4-7 as precatalysts uncover an even more
Yield
Entry
1
Amine
AM
Li
Time (h)
E:Z[b]
(%)[a]
3
0
-
2
Na
3
28
29:71
3
K
3
24
24
2
≥99
0
93:7
4[c]
5[c]
6
MgR2
-
-
K + 18-Cr-
6 (10
0
mol%)
K
K
≥99
94:6
dramatic
triorganomagnesiates 1-3 (vide supra). Furthermore, the
enhanced nucleophilic character of 5-7 enables
alkali-metal
effect
than
when
using
7
24
80
92:8
8[c]
9[c]
K
K
24
24
0
0
-
-
hydroamination of diphenylacetylene to take place at room
temperature. These reaction conditions are remarkable as
even reactive heavier alkaline-earth metal systems require
higher temperatures for hydroamination process to occur.5e
Reflecting the importance of the K/Mg partnership in 7, when
monometallic neutral magnesium Mg(CH2SiMe3)2 is used as a
precatalyst, no hydroamination occurs, even after heating at
80 ⁰C for 24 hours (entry 4, table 2). In an attempt to glean
some information into the role of potassium in this
transformation we reacted diphenylacetylene with piperidine
using 7, in the presence of an additional 10 mol% of 18-crown-
6 (entry 5). Since 18-crown-6 has a high sequestering affinity
for potassium ions, we pondered that in the presence of the
macrocycle we might saturate the coordination of the alkali
metal, thereby preventing access of the substrate and slowing
down the reaction. Monitoring the reaction via 1H NMR
spectroscopy revealed that even after heating at 80 ⁰C for 24
hours no reaction occurs. This result therefore implies that the
level of potassium coordination effects its key role as a Lewis
acid, enabling activation of the unsaturated molecule and
facilitating the nucleophilic addition by bringing it into close
proximity to the anionic amido-magnesiate intermediate. In
contrast, guanylation reactions catalysed by 2, only show a
10
11
7
7
0.25
0.25
≥99
≥99
≥99
12
7
0.25
Table 2 Intermolecular hydroamination of diphenylacetylene and styrene
catalysed by higher order alkali metal magnesiates. [a] Yields determined by 1H
NMR using ferrocene as an internal standard. [b] Determined by 1H NMR
spectroscopy. [c] 80 ⁰C.
Satisfied with the excellent catalytic ability of 7, we next
sought to examine the scope of amines suitable for this
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entries 6-9). Hydroamination of intermolecular hydroamination reactions, we neVxitewsAertticoleuOtnlitnoe
reaction (Table
2
diphenylacetylene with six or five membered cyclic amines gain some structural and spectroscopic iDnOsiIg: h10t.1i0n3t9o/Cth9DeTn0a0t9u2r3eJ
piperidine and pyrrolidine is fast at room temperature when of potentially active catalytic intermediates. Hill proposed a
catalysed by 7, taking 3 or 2 hours respectively for full mechanism for the alkaline earth catalysed intermolecular
conversion. Installing an oxygen atom into the six-membered hydroamination,5e and suggested that they operate in a similar
ring of piperidine has a profound effect on the efficiency of manner to early transition metal complexes.20.
catalysis. Thus entry 7 shows that morpholine requires 24 Using this proposal as a starting point for our studies with
hours at room temperature to afford the hydroamination heterobimetallic complexes we propose that the reaction
product in 80% (E:Z 92:8). Notably the same reactions are proceeds as follows (Scheme 1): 1/ the alkali-metal magnesiate
reported using nickel or gold catalysts, however long deprotonates the amine, thereby initiating the reaction by
timescales and high temperatures are necessary.19 generating a sufficiently nucleophilic magnesium amide (i) and
Hydroamination reactions using less nucleophilic amines, losing SiMe4; 2/ the presence of the alkali-metal next activates
dibenzylamine and diphenylamine do not occur. 1H NMR the -system of the substrate molecule drawing it into the
monitoring of these reactions is consistent with the in situ proximity of the nucleophile and priming it for insertion (ii); 3/
formation of a {K2Mg(amido)4} intermediate, which appears substrate insertion into the magnesium-nitrogen bond giving
not
to
be
active
enough
to
facilitate
the iii; 4/ protonolysis to regenerate the active alkali-metal amido
amide addition to the C-C triple bond. 7 is also a capable magnesiate. Since our experimental results demonstrate a
catalyst for hydroamination of styrene. In these examples pronounced alkali metal effect, the catalyst selection is crucial
(entries 10-12), piperidine, pyrrolidine and morpholine all gave in activating the unsaturated substrate for onward reaction.
complete hydroamination within 15 minutes at room With these steps in mind we conducted
a series of
temperature, affording selectively the anti-Markovnikov stoichiometric reactions between 2, 3, 6 and 7 with piperidine.
product. This preferred selectivity has been previously 2 and 3 were prepared in situ by mixing the monometallic
reported for the hydroamination of styrene using other metal reagents MCH2SiMe3 (M = Na, K) and Mg(CH2SiMe3)2 in a 1:1
catalysts and it has been attributed to the formation of more ratio in hexane (Scheme 2). Subsequently, 3 equivalents of
stabilised benzylic metal intermediates.8a
piperidine were added, followed by a small amount of THF to
solubilise the white precipitate. Single crystals of
[(THF)2{NaMg(NC5H10)3}]2 (15) and [(THF)3{KMg(NC5H10)3}]2
(16) were grown in 42% and 43% yields respectively.
Alkali-metal
activation
Ph
N
N
N
Mg
K
K
Ph
N
Ph
Ph
ii
( )
Insertion
4
N
H
N
N
N
K2MgR4
K
Mg
K
Ph
Ph
N
N
N
N
N
-4 SiMe4
Initiation
Mg
K
K
i
( )
N
R= CH2SiMe3
iii
(
)
H
Protonolysis
Ph
Ph
N
H
Scheme 2
Synthesis of lower order sodium and potassium amidomagnesiates
15 and 16.
Scheme 1
Proposed catalytic cycle for intermolecular hydroamination
reactions via metal amide species catalysed by alkali-metal magnesiate catalysts.
Higher order potassium magnesiate is depicted here for illustrative purposes.
The single-crystal X-ray structures of 15 and 16 are very similar
although the poor quality of the latter precludes discussion of
geometric parameters. Compound 15 displays a dimeric
contacted ion-pair structure (Figure 2) where the metals adopt
a pseudolinear [NaMgMgNa] arrangement [Na2···Mg1···Mg2,
158.48(3)o and Mg1···Mg2···Na1, 161.77(3)o]. The distorted
tetrahedral Mg centres are linked by four piperidide bridges
(average Mg-N 2.10(3) Å), anchoring the structure, whereas
the softer distorted tetrahedral Na is affixed to the
{MgN(C5H10)4} component forming elongated ancillary bonds
[mean Na-N, 2.46(1) Å]. Two THF molecules complete the
coordination sphere of each sodium. This concept of
anchoring/ancillary bonding has been previously described for
bimetallic compounds.21 The central [MgNMgN] four-
membered planar ring is orthogonal to two adjacent
[NaNMgN] rings, and this structural motif of three fused four-
The fact that hydroaminations of styrene proceed instantly,
and much more rapidly than when diphenylacetylene is used,
indicate that steric factors may play a key role in whether the
reaction is feasible. In contrast to our findings, other reports
have shown that high temperatures and long timescales are
required for these reactions. For example, LiN(SiMe3)2 (5
mol%) in the presence of TMEDA (5 mol%) in C6D6 at 120 - 150
oC, affords the hydroamination products after 1 – 3 hours in
61-82
%
yields.8a Group
2
catalysts such as
[Sr{CH(SiMe3)2}2(THF)2] achieve the piperidine hydroamination
product in 70% yield after 10 minutes at room temperature,
using 5 mol% of the strontium precatalyst.5e Satisfied that both
the lower order (specifically 2) and higher order (specifically 7)
alkali-metal magnesiates are efficient catalysts in these
4 | J. Name., 2012, 00, 1-3
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membered rings, comprising a MMgMgM arrangement has neutral organomagnesium reagents and the dramatic alkali-
View Article Online
been previously been reported in the classical work of Weiss.22 metal effect previously described.
DOI: 10.1039/C9DT00923J
Nevertheless the isolation of 15 and 16 provides strong Following a similar procedure to the synthesis of 15 and 16,
evidence that amine deprotonation is the first step in the except that MCH2SiMe3 (M = Na, K) and Mg(CH2SiMe3)2 were
catalytic profile, and generates catalytically active species.
mixed in a 2:1 ratio and 4 equivalents of piperidine were
added in hexane/toluene, the higher order alkali-metal
magnesiates
[(TMEDA)2{Na2Mg(NC5H10)4}]
17
and
[PMDETA)2{K2Mg(NC5H10)4}] 18 were prepared as colourless
crystals in 65% and 47% yields respectively (Scheme 3). Single
crystal X-ray diffraction studies reveal that 17 and 18 display
the same core architecture, however, poor data quality
prevents a meaningful discussion of geometric parameters of
18. In 17 a central distorted tetrahedral magnesium is
connected to two peripheral alkali metal cations by 4
piperidide bridges (Figure 4). The Na and K ions in 17 and 18
are further solvated by either TMEDA or PMDETA respectively.
A search of the CCDC database revealed that other bimetallic
amides have a similar core architecture. These include a
lithium manganate24 and only two higher order lithium amido-
Figure 2 Molecular structure of 15 with 30% probability displacement ellipsoids.
All hydrogen atoms have been omitted for clarity.
Probing these complexes further we next investigated the
solution state properties of 15 and 16 in d8-THF, via Diffusion
Ordered Spectroscopy (DOSY), using the method recently
reported by Stalke.23 This gave us a reliable molecular weight
estimation of the species in solution, based upon their
diffusion coefficients against an TMS as an internal standard
(Figure 3).
magnesiates.25
Scheme 3
Synthesis
of
higher
order
sodium
and
potassium
amidomagnesiates 17 and 18.
Figure 3 Suggested aggregation states of 15 and 16 in d8-THF solution, obtained
from DOSY NMR studies.
15 has an estimated molecular weight of 694 g mol-1 which
implies a dimeric structure resembling that observed in the
solid state albeit with fewer solvating THF molecules. The
expected mass for 15 with one and two solvating THF
molecules is 672 g mol-1 (-3% error) and 743 g mol-1 (7% error),
indicating that on the DOSY timescale there is likely between
one and two solvating THF molecules. The DOSY spectrum of
16 reveals related characteristics. The experiment gives a value
of 734 g mol-1 whereas the expected molecular weights with
either 1 or 2 solvating THF molecules are 704 g mol-1 (-4%
error) and 776 g mol-1 (6% error). Notwithstanding the
ambiguity of the number of solvating THF molecules, it is clear
that the dimeric contacted ion-pair arrangement observed in
the solid state is retained in solution, enabling close metal-
metal communication, consistent with the enhanced reactivity
observed for these bimetallic compounds when compared to
Figure 4 Molecular structure of 17 with 30% probability displacement ellipsoids.
All hydrogen atoms have been omitted for clarity
Finally, in an effort to probe the insertion of the alkyne into
the Mg-N bonds of 15-18 we conducted
a series of
stoichiometric experiments. Unfortunately, we were unable to
effectively characterise such a species. Instead the only
product we observed was the final hydroamination product in
each case, highlighting the instability/high reactivity of such an
intermediate, and its propensity to react further in a product
forming step. It should also be noted that these compounds
show similar catalytic activities to those reported for the
relative alkyl precursors. Thus, 18 (using a 5 mol % loading)
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catalyses the hydroamination of styrene and piperidine to full 124.7 [CH], 107.0 [C=CH], 51.2 [N(CH2CH2)2CH2], 27.2
View Article Online
DOI: 10.1039/C9DT00923J
conversion at room temperature in similar times and with the [N(CH2CH2)2CH2], 25.4 [N(CH2CH2)2CH2].
same regioselectivity as those reported for 7 (entry 10, Table Characterisation of 1-(1,2-diphenylvinyl)pyrrolidine (8a): 1H
2)
NMR (400.13 MHz, d8-THF, 298 K)  7.34-7.21 [m, 5H, Har],
6.89-6.59 [m, 5H, Har], 5.35 [s, 1H, C=CH], 3.07-2.95 [m, 4H,
N(CH2CH2)2], 1.88-1.76 [m, 4H, N(CH2CH2)2]. 13C NMR (400.13
MHz, d8-THF, 298 K)  148.7 [C=C-N], 140.4 [Cq], 139.2 [Cq],
130.3 [CH], 129.1 [CH], 128.1 [CH], 127.9 [CH], 123.2 [CH],
Experimental
General experimental:
All reactions and manipulations were conducted under a 100.7 (C=CH), 49.1 [N(CH2CH2)2], 25.6 [N(CH2CH2)2CH2].
1
protective argon atmosphere using either standard Schlenk Characterisation of 1-(1,2-diphenylvinyl)morpholine (9a): H
techniques or an MBraun glove box fitted with a gas NMR (400.13 MHz, d8-THF, 298 K)  7.37-7.21 [m, 5H, Har],
purification and recirculation unit. NMR experiments were 6.99-6.71 [m, 5H, Har], 5.63 [s, 1H, C=CH], 3.71-3.59 [m, 4H,
conducted in J. Young’s NMR tubes oven dried and flushed N(CH2CH2)2O], 2.89-2.77 [m, 4H, N(CH2CH2)2O]. 13C NMR
with Argon prior to use. Hexane, toluene and THF were dried (400.13 MHz, d8-THF, 298 K)  151.9 [C=C-N], 139.7 [Cq], 138.2
by heating to reflux temperature over sodium benzophenone [Cq], 132.3 [Cq], 131.2 [CH], 129.2 [CH], 128.8 [CH], 128.2 [CH],
ketyl and then distilled under nitrogen prior to use. All other 124.7 [CH], 107.0 [C=CH], 50.5 [N(CH2CH2)2O], 47.7
reagents were purchased commercially from Sigma-Aldrich [N(CH2CH2)2O].
and dried via distillation from the appropriate drying agent Characterisation of 1-phenethylpiperidine (12): 1H NMR
prior to use.
(400.13 MHz, CDCl3, 298 K)  7.35-7.39 [m, 2H, Har], 7.28-7.31
NMR Spectroscopy: NMR spectra were recorded on a [m, 3H, Har], 2.90-2.94 [m, 2H, PhCH2CH2N], 2.64-2.68 [m, 2H,
Bruker AV3 or AV 400 MHz spectrometer operating at 400.13 PhCH2CH2N], 2.57 [bs, 4H, N(CH2CH2)2CH2], 1.74 [m, 4H,
MHz for 1H and 100.62 MHz for 13C. All 13C spectra were proton N(CH2CH2)2CH2], 1.53-1.60 [m, 2H, N(CH2CH2)2CH2]. 13C NMR
1
decoupled. H and 13C NMR spectra were referenced against (400.13 MHz, CDCl3, 298 K)  140.4 [Cq], 128.4 [CH], 128 [CH],
the appropriate solvent signal.
125.8 [CH], 61.1 [PhCH2CH2N], 54.5 [N(CH2CH2)2CH2], 33.4
X-ray Crystallography: Crystallographic data were collected [PhCH2CH2N], 25.8 [N(CH2CH2)2CH2], 24.19 [N(CH2CH2)2CH2].
on Oxford Diffraction instruments with Mo Kα radiation (λ = Characterisation of 1-phenethylpyrrolidine (13): 1H NMR
0.71073 Å) or Cu K radiation ( = 1.54184 Å). Structures were (400.13 MHz, CDCl3, 298 K)  7.34-7.40 [m, 2H, Har], 7.27-7.33
solved using SHELXS-9726 while refinement was carried out on [m, 3H, Har], 2.94 [m, 2H, PhCH2CH2N], 2.80 [m, 2H,
F2 against all independent reflections by the full matrix least- PhCH2CH2N], 2.63-2.71 [m, 4H, N(CH2CH2)2], 1.86-1.95 [m, 4H,
squares method using the SHELXL-97. All non-hydrogen atoms N(CH2CH2)2].13C NMR (400.13 MHz, CDCl3, 298 K)  140.4 [Cq],
were refined using anisotropic thermal parameters. Selected 128.4 [CH], 128.2 [CH], 125.8 [CH], 58.2 [PhCH2CH2N], 54.0
crystallographic details and refinement details are provided in [N(CH2CH2)2], 35.8 [PhCH2CH2N], 23.4 [N(CH2CH2)2].
table S1. CCDC 1900501-1900502 contains the supplementary Characterisation of 1-phenethylmorpholine (14): 1H NMR
crystallographic data for these structures. These data can be (400.13 MHz, CDCl3, 298 K)  7.35-7.39 [m, 2H, Har], 7.28-7.3
obtained free of charge from the Cambridge Crystallographic [m, 3H, Har], 3.79 [t, 4H, J = 4 Hz, OCH2], 2.88 [dd, 2H, J = 4 Hz, J
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
= 8 Hz, PhCH2CH2N], 2.65 [dd, 2H, J = 4 Hz, J = 8 Hz,
PhCH2CH2N], 2.54 [s, 4H, N(CH2CH2)2O]. 13C NMR (400.13 MHz,
CDCl3, 298 K)  139.6 [Cq], 128.0 [CH], 127.7 [CH], 125.4 [CH],
Synthetic procedures
General procedure for catalytic hydroamination reactions: In 66.2 [OCH2], 60.2 [PhCH2CH2N], 53.1 [N(CH2CH2)2O], 32.7
a glovebox, the NMR tube was filled with 0.6 mmol of amine, [PhCH2CH2N].
0.5 mmol of alkene or alkyne, 10 mol% of ferrocene (0.0095 g, Synthesis of [(THF)2{NaMg(NC5H10)3}]2 (15): [Na(CH2SiCH3)]
0.05 mmol) as internal standard and 0.609 g of solvent. The (0.11 g, 1 mmol), [Mg(CH2SiCH3)2] (0.22 g, 1 mmol) and hexane
initial ratio of starting materials was calculated by integration (10 mL) were added and stirred at room temperature during
of the 1H NMR spectrum relative to the ferrocene. The one hour. Piperidine (0.3 mL, 1 mmol) was added and the
precatalyst (2, 5 or 10 mol%) was introduced and the reactions white suspension was stirred during one hour more at room
times were measured from this point in regular intervals until temperature. THF (0.32 mL) was added and the suspension
full conversion by 1H NMR spectrum (some reactions were was gently heated until a clear solution was obtained and was
o
heated in a pre-heated oil bath). All the yields were calculated placed to the freezer (-33 C). After 24 hours the white solid
1
by integration of the products relative to ferrocene in the H was filtered and placed in a glovebox (0.185 g, 42%).
NMR spectrum.
1H NMR (400.13 MHz, C6D6, 298 K)  2.06-1.77 [br. m, 12H,
Characterisation of 1-(1,2-diphenylvinyl)piperidine (4a): 1H N(CH2CH2)2CH2], 1.74-1.46 [br m, 18H, N(CH2CH2)2CH2
NMR (400.13 MHz, d8-THF, 298 K)  7.36-7.16 [m, 5H, Har], N(CH2CH2)2CH2].
+
6.95-6.70 [m, 5H, Har], 5.62 [s, 1H, C=CH], 2.90-2.82 [m, 4H, 13C NMR {1H} (100.62 MHz, C6D6, 298 K)  55.1 [CH,
N(CH2CH2)2CH2], 1.64-1.49 [m, 6H, N(CH2CH2)2CH2]. 13C NMR N(CH2CH2)2CH2], 32.9 [CH, N(CH2CH2)2CH2], 28.2 [CH,
(400.13 MHz, d8-THF, 298 K)  153.0 [C=C-N], 140.4 [Cq], 139.3 N(CH2CH2)2CH2].
[Cq], 131.2 [CH], 129.2 [CH], 129.1 [CH], 128.7 [CH], 128.3 [CH], Elemental analysis: Due to the air and moisture sensitivity of
6 | J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
the sample, elemental analysis experiments were [(CH3)2 PMDETA], 43.2 [(CH3) PMDETA], 32.4 [N(CH2CH2)2CH2],
View Article Online
DOI: 10.1039/C9DT00923J
unsuccessful.
29.0 [N(CH2CH2)2CH2].
Synthesis of [(THF)3{KMg(NC5H10)3}]2 (16): To an oven dried Elemental analysis: (C38H86MgN10K2) Calculated: C: 57.02 % H:
Schlenk [K(CH2SiCH3)] (0.13 g, 1 mmol), [Mg(CH2SiCH3)2] (0.22 10.83 % N: 17.50 %. Found: C: 58.30 % H: 11.44 % N: 17.18 %.
g, 1 mmol) and hexane (10 mL) were added and stirred at
room temperature during one hour. Piperidine (0.3 mL, 1
mmol) was added and the white suspension was stirred during
Conclusions
one hour more at room temperature. THF (3 mL) was added
and the suspension was gently heated until solution that was
placed to the fridge overnight. Colourless crystals were
obtained. The crystals were isolated and placed in a glovebox
(0.245 g, 43%).
1H NMR (400.13 MHz, C6D6, 298 K)  3.47-3.19 [br. m, 12H,
N(CH2CH2)2CH2], 1.42-1.55 [br. m, 6H, N(CH2CH2)2CH2], 1.15-
1.39 [br. m, 12H, N(CH2CH2)2CH2].
13C NMR {1H} (100.62 MHz, C6D6, 298 K)  55.5 [CH,
N(CH2CH2)2CH2], 32.0 [CH, N(CH2CH2)2CH2], 28.4 [CH,
N(CH2CH2)2CH2].
Elemental analysis: Due to the air and moisture sensitivity of
the sample, elemental analysis experiments were
unsuccessful.
Synthesis of [(TMEDA)2Na2Mg(NC5H10)4] (17): To an oven
dried Schlenk [Na(CH2SiCH3)] (0.22 g, 2 mmol), [Mg(CH2SiCH3)2]
(0.20 g, 1 mmol) and hexane (10 mL) were added and stirred at
room temperature during one hour. TMEDA (0.3 mL, 2 mmol)
was added and the suspension was stirred during one hour.
Piperidine (0.4 mL, 4 mmol) was added and the white
suspension was stirred during one hour more at room
temperature. The volume of the solution was halved by
evaporation in vacuum and toluene (2 mL) was added. The
suspension was gently heated until solution that was placed in
a hot water bath. After 24 hours colourless crystals were
obtained, isolated and placed in a glovebox (0.415 g, 65%).
1H NMR (400.13 MHz, C6D6, 298 K)  3.51 [m, 16H,
N(CH2CH2)2CH2], 2.05 [m, 32H, N(CH2CH2)2CH2 + (CH3) TMEDA],
1.83 [s, 8H, (NCH2) TMEDA], 1.69 [m, 16H, N(CH2CH2)2CH2].
13C NMR {1H} (100.62 MHz, d8-THF, 298 K)  57.3 [(CH2)
TMEDA)], 55.6 [N(CH2CH2)2CH2], 46.2 [(CH3) TMEDA], 32.8
[N(CH2CH2)2CH2], 28.7 [N(CH2CH2)2CH2].
Expanding wider the catalytic potential of s-block cooperative
bimetallic reagents, this work establishes alkali-metal
magnesiates as powerful catalysts to promote intermolecular
hydroamination of diphenylacetylene and styrene. Reflecting
the relevance of the nucleophilicity of the mixed-metal
reagents, higher order (formally dianionic) magnesiates
([(donor)2(AM)2Mg(CH2SiMe3)4] AM = Li, Na, K) proved to be
efficient catalysts, capable of promoting hydroamination
processes at room temperature, while lower-order
([(AM)Mg(CH2SiMe3)3] AM = Li, Na, K) require harsher reaction
conditions (80 oC and long timescales). Interestingly, a
pronounced alkali-metal effect is witnessed in higher order
magnesiate precatalysts, with the K/Mg partnership being the
most efficient of them all, mediating the smooth
hydroamination of styrene at room temperature in 15
minutes. These findings contrast with the complete lack of
activity shown by homometallic [Mg(CH2SiMe3)2]. Initial
reactivity and structural studies suggest that the role of the
alkali metal is to act as a potent Lewis acid, coordinating the
unsaturated organic molecule to facilitate the addition of an
amide anion. X-ray crystallography combined with 1H DOSY
NMR experiments have been used to help identify important
reaction intermediates. These initial stoichiometric studies
suggest similar mechanisms to those proposed for calcium or
strontium catalysts.
Conflicts of interest
The authors confirm there are no conflicts of interest to
declare.
Elemental analysis: (C32H72MgN8Na2) Calculated: C: 60.12 % H:
11.35 % N: 17.53 %. Found: C: 59.57 % H: 9.14 % N: 16.95 %.
Synthesis of [(PMDETA)2K2Mg(NC5H10)4] (18): To an oven
dried Schlenk [K(CH2SiCH3)] (0.26 g, 2 mmol), [Mg(CH2SiCH3)2]
(0.20 g, 1 mmol) and hexane (10 mL) were added and stirred at
room temperature during one hour. PMDETA (0.4 mL, 2 mmol)
was added and the suspension was stirred during one hour.
Piperidine (0.4 mL, 4 mmol) was added and a clear yellow
solution was formed. After stirring for one hour at room
temperature the solution was placed in the fridge. After 24
hours big yellow crystals were obtained, isolated and placed in
a glovebox (0.3703 g, 47%).
1H NMR (400.13 MHz, C6D6, 298 K)  3.46 [m, 16H,
N(CH2CH2)2CH2], 2.09 [s, 30H, (CH3) PMDETA], 2.08 [m, 8H,
N(CH2CH2)2CH2], 2.02 [s, 16H, (NCH2) PMDETA], 1.78 [m, 16H,
N(CH2CH2)2CH2].
13C NMR {1H} (100.62 MHz, d8-THF, 298 K)  57.5 [CH2
PMDETA], 56.1 [N(CH2CH2)2CH2], 55.8 [CH2 PMDETA], 45.8
Acknowledgements
We thank the European Research Council (ERC-Stg
Mixmetapps to EH) for its generous sponsorship. Authors also
thank R. E. Mulvey for his insightful comments.
Notes and references
‡
Electronic Supplementary Information (ESI) available:
Providing experimental procedures, NMR spectra and
crystallographic data.
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View Article Online
Cooperative catalytic hydroamination of diphenylacetylene and styrene is
accomplished by magnesiates with efficiency dependent on the alkali DmOI:e1t0a.10l39a/Cn9dDT00923J
monoanionic
[(PMDETA)2K2Mg(CH2SiMe3)4] performing best.
or
dianionic
nature
of
the
ate
with
dianionic
NR2
R
R
R
R
N
N
N
K
Mg
K
N
N
N
5 mol%
R2NH
+
d8-THF, RT, 15 min