Organomagnesium-catalyzed isomerization of terminal alkynes to allenes and internal alkynes

Article abstract of DOI:10.1002/chem.201500179

Organomagnesium complexes 2 were synthesized from N,N-dialkylamineimine ligands 1 and dibenzylmagnesium by benzylation of the imine moiety. 3-Aryl-1-propynes reacted with 2 to form the corresponding tetraalkynyl complexes, which acted as catalysts for the transformation of these terminal alkynes into allenes and further to internal alkynes under mild conditions. To the best of our knowledge, this example is the first of an organomagnesium-catalyzed isomerization of alkynes. Notably, the reactions proceeded through temporally separated autotandem catalysis, thus allowing the isolation of the allene or internal alkyne species in good yields. Mechanistic experiments suggested that the catalytically active tetraalkynyl complexes consist of a tautomeric mixture of alkynyl-, allenyl-, and propargylmagnesium species.

Full text of DOI:10.1002/chem.201500179

DOI: 10.1002/chem.201500179
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Magnesium Catalysis |Hot Paper|
Organomagnesium-Catalyzed Isomerization of Terminal Alkynes
to Allenes and Internal Alkynes
Raphaꢀl Rochat, Koji Yamamoto, Michael J. Lopez, Haruki Nagae, Hayato Tsurugi,* and
Kazushi Mashima*[a]
Abstract: Organomagnesium complexes 2 were synthesized
from N,N-dialkylamineimine ligands 1 and dibenzylmagnesi-
um by benzylation of the imine moiety. 3-Aryl-1-propynes
reacted with 2 to form the corresponding tetraalkynyl com-
plexes, which acted as catalysts for the transformation of
these terminal alkynes into allenes and further to internal al-
kynes under mild conditions. To the best of our knowledge,
this example is the first of an organomagnesium-catalyzed
isomerization of alkynes. Notably, the reactions proceeded
through temporally separated autotandem catalysis, thus al-
lowing the isolation of the allene or internal alkyne species
in good yields. Mechanistic experiments suggested that the
catalytically active tetraalkynyl complexes consist of a tauto-
meric mixture of alkynyl-, allenyl-, and propargylmagnesium
species.
Introduction
Allenes are interesting building blocks for organic synthesis.
Various methods for their preparation have been developed,
including alkaline-metal-base-mediated or -catalyzed isomeriza-
tions of terminal alkynes to allenes. However, these reactions
may afford mixtures of allenes and internal alkynes, depending
on the substrates and conditions.[6] Therefore, the quest for se-
lective allene-formation reactions by using electropositive or-
ganometallics is an important and still ongoing task.
Organomagnesium compounds have become a highly impor-
tant class of reagent in organic chemistry since the discovery
of the Grignard reaction more than 100 years ago. These com-
pounds are powerful and inexpensive reagents that can be
readily prepared from metallic magnesium and organic halides,
and many applications have been developed.[1] Along with the
original synthetic protocol of the Grignard reagent, CꢀH bond
activation of functionalized aromatic compounds were recently
developed for the synthesis of organomagnesium reagents
with functionalized groups (for example, Knochel–Hauser
bases).[2] The application of such reagents to the formation of
new CꢀC and CꢀE (E=B, O, Si, P, S, Sn, etc.) bonds in addition
or substitution reactions has been continuously and intensively
studied. However, organomagnesium compounds are used as
stoichiometric reagents in the vast majority of these reactions.
Besides Lewis acid catalysis and magnesium-initiated polymeri-
zation reactions, there are only few reports of homogeneous
magnesium catalysis, namely hydroamination,[3] hydrobora-
tion,[4] and various coupling reactions (that is, cross-dehydro-
coupling of silanes with amines, dehydrocoupling of aminobor-
anes, coupling of alkynes with carbodiimides, and dehydrocou-
pling of silanes with 2,2,6,6-tetramethyl-1-piperidinyloxy).[5]
Herein, we report that alkylmagnesium complexes bearing
monoanionic N,N-bidentate ligands can catalyze the isomeriza-
tion of terminal alkynes to allenes and further to internal al-
kynes, and CꢀH activation is a key event in both reactions. To
the best of our knowledge, this example is the first homogene-
ous organomagnesium-catalyzed alkyne isomerization. The re-
actions from terminal alkynes to allenes and further to internal
alkynes notably proceed through a temporally separated auto-
tandem catalysis, so that the macroscopic temporal separation
allows for the isolation of the allene or the internal alkyne, ac-
cording to demand. Furthermore, methyl C(sp3)ꢀH bond acti-
vation of 1-phenyl-1-propyne by the alkylmagnesium moiety
and subsequent tautomerization produces a dimeric alkynyl-
magnesium complex.
Results and Discussion
[a] Dr. R. Rochat, K. Yamamoto, M. J. Lopez, H. Nagae, Dr. H. Tsurugi,
Prof. Dr. K. Mashima
Preparation and characterization of alkylmagnesium com-
plexes 2 with aminoamido ligands
Department of Chemistry, Graduate School of Engineering Science
Osaka University and CREST, JST
Toyonaka, Osaka 560-8531 (Japan)
E-mail: mashima@chem.es.osaka-u.ac.jp
tsurugi@chem.es.osaka-u.ac.jp
The treatment of [Mg(CH2Ph)2(thf)2] with one equivalent of N1-
benzylidene-N2,N2-diisopropylethane-1,2-diamine (1a) in tolu-
ene at 608C for 18 hours gave monobenzyl complex 2a in
87% yield as a product of the intramolecular benzylation of
the C=N moiety of the ligand [Eq. (1)]:
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500179.
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cores.[8] Notable differences between the two complexes are
that the MgꢀN1 bond distance in anti-2a is elongated relative
to anti-2b (2.232(3) versus 2.179(4) ꢂ, respectively), and the
angle N1-Mg-C(benzyl) in anti-2a is larger than that in anti-2b
(120.42(11) versus 113.52(16)8, respectively) which is due to the
steric hindrance of the isopropyl group on the amine nitrogen
atom of the ligand. These findings are consistent with the fact
that anti-2a is thermodynamically more stable than syn-2a.
In sharp contrast to the reaction in Equation (1), the reaction
of one equivalent of N1-benzylidene-N2,N2-dimethylethane-1,2-
diamine (1b) with [Mg(CH2Ph)2(thf)2] (ꢀ788C!room tempera-
ture, 3 h) resulted in the formation of a mixture of syn- and
anti-conformers of 2b in 90% yield (Eq. (2), based on the
1H NMR resonances). The selective formation of only one
isomer at room temperature was not observed; however, pro-
longed heating of the syn/anti mixture of 2b in a mixture of
toluene and pyridine at 808C for 18 hours led to the selective
formation of anti-2b, thus indicating that anti-2b is thermody-
1
The H NMR spectrum of 2a showed nonequivalent methyl-
ene protons for the benzyl moiety attached to the magnesium
center. The chemical-shift value of the magnesium-bound
benzyl CH2Ph carbon atom was in good accordance with typi-
cal benzylmagnesium complexes.[7] The overall molecular struc-
ture of 2a was determined by a single X-ray diffraction study
(Figure 1), which revealed the centrosymmetric dimer unit
{Mg2(m-N)2}. Both magnesium atoms adopted a distorted-tetra-
hedral geometry and were bridged by the amido nitrogen
atom of the ligand. The two benzyl groups were arranged anti
with respect to the {Mg2(m-N)2} core. The bond lengths of Mgꢀ namically more stable than syn-2b. It is assumed that the iso-
N2 and MgꢀN2* (2.13–2.14 ꢂ) are consistent with the reported
lengths of the bridging MgꢀN(amido) bonds in the {Mg2N2}
merization proceeds through the pyridine-induced dissociation
of 2b into monomeric species prior to the facial rearrange-
ment at an elevated temperature. Thus, we isolated anti-2b in
87% yield under these reaction conditions. The molecular
structure of anti-2b was determined by a single-crystal X-ray
diffraction study (Figure 1), which confirmed that the structure
of anti-2b is essentially the same as that of complex 2a.
Magnesium-catalyzed isomerization of terminal alkynes to
allenes and to internal alkynes
We first observed that complex 2b catalytically (2 mol%) trans-
formed 3-phenyl-1-propyne (3a) into phenylallene (4a) almost
completely within 18 hours at 608C. Moreover, the allene was
further converted into 1-phenyl-1-propyne (5a) when the reac-
tion mixture was maintained at 608C. Because both isomeriza-
tion reactions occurred subsequently on a macroscopic level
and without any need to the change the catalytic conditions
or add reagents, this reaction can be categorized as “temporal-
ly separated autotandem catalysis” [Eq. (3)].[9] Such systems are
advantageous because the reaction can be terminated after
the first or second cycle, thus yielding in our case the allene or
internal alkyne, respectively.
Figure 1. ORTEP representations of monobenzyl complexes anti-2a (top)
and anti-2b (bottom). Ellipsoids are set at 30% probability. All the hydrogen
atoms and solvents have been omitted for clarity. Selected bond lengths [ꢂ]
and angles [8] for anti-2a: Mg1–N1 2.232(3), Mg1–N2 2.133(2), Mg1–N2*
2.141(3), Mg1–Mg1* 3.0331(18), Mg1–C23 2.204(3); Mg1-N2-Mg1* 90.41(9),
N2-Mg1-N2* 89.59(10), N1-Mg1-C23 120.42(11), N2-Mg1-C23 129.11(11). For
anti-2b: Mg1–N1 2.179(4), Mg1–N2 2.132(4), Mg1–N2* 2.144(4), Mg1–Mg1*
2.955(3), Mg1–C19 2.173(5); Mg1-N2-Mg1* 87.46(14), N2-Mg1-N2* 92.54(14),
N1-Mg1-C19 113.52(16), N2-Mg1-C19 130.99(16).
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Table 1. Screening of conditions for the catalytic isomerization of 3-
phenyl-1-propyne (3a) to phenylallene (4a) and further to 1-phenyl-1-
proypne (5a), and the formation of the cycloaddition byproduct 6a.
Figure 2 shows the reaction profile of 3a in the presence of
1
2b (4 mol%) monitored by H NMR spectroscopic analysis at
808C in [D8]toluene over 34 hours. Initially, 2b reacted with
four equivalents of 3a to generate the catalytically active spe-
Cat. precursor
T 4a
[8C]
5a
6a
t1
Yield
[%][b]
92
>95
78
t2
Yield Yield
[%][b] [%][b]
1[c] 2b
40 2.3 d
60 9.5 h
28 d
5 d
30 h
4 h
>95
>95
86
85
85
2
5
5
7
7
7
10[e]
<2
6[e]
0
2[c] 2b
3[c] 2b
80
4 h
4[c,d] 2b
100 0.5 h
120 0.35 h
80 7.7 d
5 d
80 20 h
60 18 h
80
87
5[c,d] 2b
2 h
6[c,d] 2a
27 term.[g]
8 term.[g]
51 5.5 d
0 term.[g]
7[d] [MgBn2(thf)2][f]
80
<1
6
8
9
[MgBn2(TMEDA)][f]
None
0
[a] Reaction mixtures: substrate=1.00 mmol, benzene=0.5 mL, catalyst
precursor=0.04 mmol. [b] Yields and conversions were determined by
1H NMR spectroscopic analysis with phenanthrene as an internal standard
and were based on the amount of substrate 3a available for isomeriza-
tion (100%=0.84 mmol for 4a and 5a, 100%=0.42 mmol for 6a).
[c] The catalyst was generated in situ from complex 2 and 4 equivalents
of 3a (relative to 2). [d] In [D8]toluene. [e] Other unknown byproducts
were also formed. [f] 5.0 mol%. [g] Terminated after time t1 owing to
poor reactivity.
Figure 2. Reaction profile for the temporally separated autotandem-cata-
lyzed isomerization of 3-phenyl-1-propyne (3a, green triangles) to phenylal-
lene (4a, blue diamonds), and further to 1-phenyl-1-propyne (5a, red circles)
over 34 h. Small amounts of 1,2-dimethylene-3,4-diphenylcyclobutane (6a,
yellow crosses) were also formed. Reaction conditions: 2b=0.02 mmol, sub-
strate 3a=0.50 mmol, 808C, [D8]toluene=0.5 mL.
cies (see below) in a very fast reaction (<2 min). In the first
catalytic cycle, terminal alkyne 3a was transformed into phe-
nylallene (4a). After reaching the maximum concentration of
the bidentate secondary-amine-based ligand held an impor-
tant role in the deprotonation of 3a and the tautomerization
of the alkynylmagnesium species by further polarizing the
MgꢀC bond.
1
4a, we observed a shift of one set of H NMR signals assigned
to a Mg complex (see below), thus indicating a change of the
catalyst when switching to the second isomerization cycle (see
the Supporting Information for details). In the second stage,
4a was isomerized further to 1-phenyl-1-propyne (5a). When
4a was present at sufficiently high concentrations, a [2+2] cy-
cloaddition reaction occurred as a side reaction to produce
mainly 1,2-dimethylene-3,4-diphenylcyclobutane (6a)[10] in
small amounts (<5%). Notably, 6a was only formed in traces
when 4a was heated at 808C for 20 hours in the absence of
a catalyst.
Having determined the best precatalyst (that is, 2b), we
next examined the substrate scope (the results are summarized
in Table 2). Aromatic substrates were converted well, whereas
the isomerization of nonbenzylic alkynes, such as 1-hexyne, 4-
phenyl-1-butyne, or terminal alkynes with a propargylic hetero-
atom, such as 3-methoxypropyne and 3-(dimethylamino)pro-
pyne, did not, or only very poorly, proceed, thus suggesting
that this magnesium-catalyzed reaction was different from
simple base-promoted alkyne isomerizations. In the case of 3a,
the thermodynamic stability of the three isomers is in the
order PhCCMe (5a)>PhCH=C=CH2 (4a)>PhCH2CCH (3a). De-
pending on the substituent on the C3 fragment however, the
series can differ and exclude a catalytic reaction of a terminal
to an internal alkyne through the allene.[11] Electron-poor ben-
zylic terminal alkynes were converted into the corresponding
allenes much faster than electron-rich alkynes. The rates of iso-
merization of the allenes to the internal alkynes were in about
the same range for all substrates. This finding is consistent
with the fact that establishing conjugation of the aromatic ring
with the allene system in the first isomerization is the step that
is most dependent on the electronic properties of the aromatic
group.
We then investigated the influence of the temperature on
the efficiency and product distribution by using 3a in the pres-
ence of 2a and 2b as precatalysts (Table 1). We found that
higher temperatures are beneficial because they decrease the
reaction time while not significantly increasing the formation
of the cycloaddition byproducts (less than 10% in all cases).
When 2a was used as a precatalyst, the catalytic activity was
lower (27% after 7.7 days) than in the case of 2b owing to
steric hindrance around the amine moiety. It is noteworthy
that [MgBn2(thf)2] and [MgBn2(TMEDA)] (TMEDA=tetramethy-
lethylenediamine) acted as catalyst precursors for this system
as well, but these complexes were much less efficient than the
systems of 2a and 2b (Table 1, entry 7), thus indicating that
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also possible, as shown by the reaction of two equivalents of
1-phenyl-1-butyne with 2b for 20 hours at 1008C, thus produc-
ing 1-phenyl-1,2-butadiene after quenching with water. Com-
plex 7 was analyzed by means of X-ray crystallography studies
Table 2. Substrate screening.
1
and NMR spectroscopic analysis (Figure 3). The H NMR spec-
trum of 7 showed broad signals when recorded in benzene,
but displayed sharp resonances in pyridine, thus indicating
syn/anti fluxionality in solution with benzene. Figure 2 shows
the molecular structure of 7, which has features that are con-
Substrate 3
4
5
6
t1
Yield
[%][b]
t2
Yield
[%][b]
Yield
[%][b]
1
2
3
3b 9 h
72
25.5 h 67
37 h 61
21.5 h 57
6
7
6
3c 10 h 77
3d 3 h 75
4
3e 12.5 h 47
11 h 64
2
5
6
3 f 5.5 h 83
3g 5.5 h 89
27 h 66
34 h 82
6
5
7
8
3h <4 h >71
25 h 62
2[c]
3i 1 h
52
21 h[d] 11[d]
>8[c]
9
3j 29 h 54
3k 40 h 58
28 h 50
30 h 63
8
8
10
11
3l 6.3 d[e] 59
–
–
12[c]
[a] Reaction mixtures: substrate=0.5 mmol, [D8]toluene=0.4 mL, 2b=
0.02 mmol, 808C. The catalyst was generated in situ from complex 2b
and 4 equivalents of the alkyne (relative to 2). [b] Yields were determined
by 1H NMR spectroscopic analysis with phenanthrene as an internal stan-
dard and were based on the amount of substrate available for isomeriza-
tion (100%=0.42 mmol for 4 and 5, 100%=0.21 mmol for 6). [c] Other
unknown byproducts were also formed. [d] Terminated owing to poor re-
activity and side reactions. [e] Conditions: 4.3 days at 608C and 2 days at
808C in C6D6.
Mechanistic studies
The reaction of 2b with only two equivalents of 3a afforded
bis-3-phenyl-1-propyne-1-yl dinuclear magnesium complex 7
after 20 minutes at 608C. Notably, complex 7 was also ob-
tained by treating 2b with two equivalents or excess of 5a for
2 hours at 808C, although simple dibenzylmagnesium
([Mg(CH2Ph)2(thf)2]) did not react with 5a, even upon heating
at 1008C for 12 hours, thus suggesting that the monoanionic
aminoamido ligand increased the basicity of the MgꢀC frag-
ment enough to activate the terminal C(sp3)ꢀH bond of inter-
nal alkynes. The activation of an internal C(sp3)ꢀH bond was
Figure 3. ORTEP representations of intermediate complex 7 (top), dialkynyl
complex 12 (middle), and tetraalkynyl complex 13 (bottom). Ellipsoids are
set at 30% probability. All the solvents and hydrogen atoms have been
omitted for clarity, except for the hydrogen atoms on the nitrogen atoms.
Selected bond lengths [ꢂ] and angles [8] for 7: Mg1–N1 2.163(3), Mg1–N2
2.103(3), Mg1–N2* 2.132(3), Mg1–C19 2.095(3), C19–C20 1.211(5), C20–C21
1.481(5); Mg1-C19-C20 174.9(3), C19-C20-C21 178.5(4). For 12: Mg1–N1
2.153(5), Mg1–N2 2.094(6), Mg1–N2* 2.134(6), Mg1–C19 2.104(7); Mg1-C19-
C20 174.2(5). For 13: Mg1–N1 2.208(3), Mg1–N2 2.324(4), Mg1–C19 2.127(4),
Mg1–C27 2.401(3), Mg1–C27* 2.181(4), C19–C20 1.217(5), C27–C28 1.218(5);
Mg1-C19-C20 171.1(3), Mg1-C27-Mg1* 90.89(13), Mg1-C27-C28 106.5(3),
Mg1*-C27-C28 153.9(3), C27-Mg1-C27* 89.11(13), N2-Mg1-C27* 169.44(12).
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sistent with its parent complex 2b, except that the two benzyl
moieties on the magnesium atom are substituted with two al-
kynyl groups. Interestingly, despite the activation of the termi-
nal position of the alkyne and that the benzylic CH2 group re-
mains intact, quenching 7 with an excess of TMSCl (10 equiv)
yielded almost solely bis-silylated product 10a, whereas
quenching with two equivalents of TMSCl afforded a mixture
of mono- and bis-silylated alkyne and allene products 8a–11 a
(Scheme 1). Accordingly, we concluded that 7 is a tautomeric
mixture of alkynyl-, propargyl-, and allenylmagnesium species
in solution, among which the bis-3-phenyl-1-propyne-1-ylmag-
nesium complex was isolated as the thermodynamically most
stable tautomer in the solid state (see the Supporting Informa-
tion).
Scheme 2. Formation of di- (12) and tetraalkynylmagnesium (13) complexes
through the deprotonation of phenylacetylene by the MgꢀC and MgꢀN
bonds.
adopted a five-coordinated distorted-trigonal planar geometry.
It is noteworthy that the bond length of MgꢀN2 in 13 was sig-
nificantly elongated relative to the analogous bond in 2b
owing to the change from monoanionic to neutral coordina-
tion to the metal center,[12] thus indicating that the amidomag-
nesium moiety in 2b acts as a proton acceptor to induce de-
protonation of phenylacetylene. The formation of the amine
1
group in complex 13 was also confirmed by H NMR spectro-
scopic analysis in which one broad resonance assignable to
the amine protons was observed, and by a weak band at n˜ =
3276 cmꢀ1 in the IR spectrum. The 13C NMR spectrum of 13 in
toluene showed only one set of broad signals for the alkylnyl-
magnesium moiety, although terminal and bridging alkynyl
groups were present in the solid state, thus suggesting that
such five-coordinated tetraalkynyl complexes readily dissociate
into the corresponding mononuclear complexes in solution
owing to the weak bridging nature of the alkynyl group rela-
tive to the amido bridge. This finding was further supported
by the 13C NMR spectrum of 13 in pyridine, in which there was
one set of sharp signals for the alkynyl groups at d=112.8 and
130.4 ppm, thus indicating that the mononuclear species was
stabilized by the coordination of pyridine. Previously, some-
what similar four-coordinated alkyne-bridged dinuclear com-
plexes of Group 2 metals were reported to keep their dimeric
nature in non-coordinating solvents.[13]
Scheme 1. Preparation of dialkynyl complex 7 and its reactivity with TMSCl
(R=CH(Ph)CH2Ph). TMS=trimethylsilyl.
We then examined the reaction of 2b with a nonisomerizable
terminal alkyne, phenylacetylene, as a model reaction to
detect that magnesium species in the catalytic reaction. The
acidic C(sp)ꢀH hydrogen atom of terminal alkynes is readily de-
protonated by MgꢀC bonds. Complex 2b was treated with
two equivalents of phenylacetylene to give dialkynylmagnesi-
um complex 12 after 20 minutes at 608C in 61% yield
(Scheme 2), whereas the reaction of 2b with excess amounts
of phenylacetylene afforded the dinuclear tetraalkynylmagnesi-
um complex 13 in 91% yield, in which the two magnesium
centers were bridged by two alkynyl groups and each magne-
sium atom accommodated one terminal alkynyl moiety.
Because the stoichiometric isomerization of 5a to 3a by
using 2b seemed to proceed through propargyl and allenyl
tautomers of 7, we proposed a plausible pathway for the cata-
lytic reaction outlined in Scheme 3. In the presence of excess
amounts of terminal alkyne, 2b was first converted into 7, fol-
lowed by the activation of another alkyne through the basic
magnesium–amido bond to form a tautomeric mixture of di-
alkynyl complexes A–C, which were in equilibrium through
1,3-hydride shifts, and acted as key intermediates in the cata-
lytic cycle. The experimental evidence for isomerization in the
presence of THF and pyridine supports the proposed mononu-
clear species. Furthermore, we assumed that the dialkynyl spe-
The molecular structures of complexes 12 and 13 were clari-
fied by means of X-ray diffraction studies (ORTEP drawings are
shown in Figure 3). Complex 12 possessed a {Mg2(m-N)2} dimer
core in which two benzyl groups were replaced with two phe-
nylacetylenyl moieties relative to 2b. The overall structure of
12 is essentially the same as that of anti-2b. In contrast, the
molecular structure of 13 is centrosymmetric with a four-mem-
bered {Mg2(m-C)2} core. The two magnesium centers are
bridged by sp-hybridized phenylacetylenyl groups through
three-center two-electron bonds, and each magnesium center
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labeled substrates were not expected to be informative in our
case because both inter- and intramolecular scrambling on the
two alkynyl/allenyl/propargyl fragments of the catalytically
active species may occur.
Conclusion
The first example of an organomagnesium-catalyzed isomeriza-
tion of 3-aryl-1-propynes to arylallenes and further to 1-aryl-1-
propynes is described. Notably, the reactions proceeded
through temporally separated autotandem catalysis, thus al-
lowing the isolation of the allene or the internal alkyne species,
as desired. In the course of our mechanistic experiments, we
observed the stoichiometric magnesium-mediated isomeriza-
tion of internal alkynes to terminal alkynes. The mechanisms
for both type of transformation are proposed to involve alkyn-
yl-, allenyl-, and propargylmagnesium tautomers as key struc-
tures.
Although stoichiometric magnesium chemistry has a rich
past and much progress has been made in the last decades,
only few types of organomagnesium-catalyzed reaction have
been reported to date. In this context, our work on the magne-
sium-catalyzed isomerization of the carbon linkage, a funda-
mental reaction, is an attractive demonstration of the large po-
tential of magnesium chemistry, which is inexpensive and non-
toxic.
Scheme 3. Proposed catalytic cycles for the temporally separated autotan-
dem isomerization of phenylpropynes/phenylallene with catalyst precursor
2b. R=CH(Ph)CH2Ph, R’=alkynyl, allenyl, propargyl.
Experimental Section
cies is involved in the catalytic cycle based on the similar cata-
lytic activities of complexes 13 and 2b. In the initial period,
phenylallene 4a could be released from allenylmagnesium B
by s-bond metathesis with 3a to regenerate alkynylmagnesi-
um A. After full consumption of 3a, propargylmagnesium C
acted as a key intermediate to produce 1-phenyl-1-propyne
(5a) through C(sp2)ꢀH bond cleavage of 4a. In the presence of
one equivalent of phenylacetylene, the isomerization stopped
at the allene stage. The formation of 5a was suppressed be-
cause phenylacetylene was acidic enough to protolytically re-
lease 4a from B before allowing further tautomerization to C
and the formation 5a. In the absence of phenylacetylene, 3a
acted as an acid to release 4a from B, and therefore generated
temporal separation of the two isomerization steps. Further-
more, the reaction of 7 with 4a (2 equiv) produced 5a as the
alkyne-isomerized product, which was clear proof of the in-
volvement of the C(sp2)ꢀH bond cleavage step of 4a.
General procedures
All manipulations involving air- and moisture-sensitive organome-
tallic compounds were performed under argon by using standard
Schlenk techniques or glove-box techniques. Toluene, hexane, pen-
tane, THF, and diethyl ether were dried and deoxygenated by distil-
lation over sodium benzophenone ketyl under argon or by using
Grubbs columns (Glass Counter Solvent Dispensing System; Nikko
Hansen & Co., Ltd., Osaka, Japan) and stored over sodium. Magne-
sium (turnings), N,N-dimethylethane-1,2-diamine, N,N-diisopropyl-
ethane-1,2-diamine, benzaldehyde, and CDCl3 were purchased
and used as received. The following complexes and substrates
were prepared according to reported procedures, and
their analytical data were in accordance with the reported
values:
[{Mg(CH2Ph)2(OEt2)}2]
and
[Mg(CH2Ph)2(thf)2],[16]
[Mg(CH2Ph)2(TMEDA)],[7b] 3-phenyl-1-butyne,[17] 3-(4-methoxyphen-
yl)-1-propyne,[17a] 3-(4-dimethylaminophenyl)-1-propyne,[17] 3-(4-
chlorophenyl)-1-propyne,[18] 3-(4-fluorophenyl)-1-propyne,[18b,19] 3-
(2-bromophenyl)-1-propyne,[18b,20] 3-(4-tolyl)-1-propyne,[18] 3-(1,1’-
biphen-4-yl)-1-propyne,[18] 3-(2-napthtyl)-1-propyne,[18] 3-(2-tolyl)-1-
propyne,[18] 3-(3,5-bistrifluoromethylphenyl)-1-propyne.[18] Terminal
alkynes were distilled, degassed by three freeze–pump–thaw
cycles, dried over molecular sieves (4 ꢂ; activated for 18 h at 3208C
under vacuum), and stored under argon. [D6]Benzene, [D5]pyridine,
[D8]THF, [D8]toluene, PhCH2Cl, 3-phenyl-1-propyne, 4-phenyl-1-
butyne, phenylacetylene, and pyridine were distilled over CaH2, de-
gassed, and stored under argon. 1H and 13C NMR (400 and
100 MHz, respectively) spectra were measured on a Bruker Avance
III-400 spectrometer. The NMR spectra were referenced to the re-
sidual solvent signals at dH =7.16 and dC =128.0 ppm for C6D6,
dH =8.74 and dC =150.4 ppm for [D5]pyridine, dH =3.58 and dC =
The exact nature of the rearrangements from alkynyl- to al-
lenyl- and to propargylmagnesium species remains unclear.
This question was also left unanswered in a recent report in
which an alkyne/allene isomerization at heavier Group 2 metal
species occurred as the key step in the hydroxyalkylation of al-
kynes.[14] However, the base-mediated process was proposed
to occur through intramolecular 1,3-hydride shifts.[15] As indi-
cated by the screening results with [MgBn2(TMEDA)] (Table 1),
the secondary amine moiety on our ligand seems to be impor-
tant and supposedly assists the 1,3-hydride shifts in the tauto-
merization processes. Crossover experiments with deuterium-
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67.2 ppm for [D8]THF, dH =2.08 and dC =20.43 ppm for [D8]toluene,
and dH =7.26 and dC =77.2 ppm for CHCl3. Yields determined from
1H NMR spectroscopy used an internal standard, a pulse of 308,
1 scan, an acquisition time of 4 s, and a relaxation delay of 10 s.
The IR spectra were recorded on a JASCO FT/IR-4200 spectrometer
by using air-tight cells. All the melting points were measured in
sealed tubes in an argon atmosphere. Low- and high-resolution
mass spectra were recorded by JEOL JMS-700. Elemental analyses
were performed on a PerkinElmer 2400 microanalyzer at the Facul-
ty of Engineering Science, Osaka University. CCDC 1026445 (anti-
2a), 1026446 (anti-2b), 1026447 (7), 1026448 (12), 1026449 (13),
and 1026450 ([{Mg(CH2Ph)2(Et2O)}2]) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif. See the Supporting Infor-
mation for further details of the X-ray crystallography.
Preparation of complex 2b (as a mixture of anti-2b and syn-2b):
A solution of 1b (522 mg, 2.96 mmol, 2.0 equiv) in toluene (5 mL)
was added to
a solution of [{Mg(CH2Ph)2(Et2O)}2] (832 mg,
1.48 mmol, 1.0 equiv) or [Mg(CH2Ph)2(thf)2] (1.038 g, 2.96 mmol,
2.0 equiv) in toluene (10 mL) at ꢀ788C. The mixture was allowed
to warm to room temperature and stirred for 3 h. After all the vola-
tiles were removed under reduced pressure, the resulting residue
was washed with hexane (3ꢃ5 mL) to give 2b as a white powder
(1.02 g, 90%). Characterization by NMR spectroscopic analysis
showed this compound to be
[D5]pyridine. M.p.>2108C (decomp);
a
mononuclear species in
1H NMR
(400 MHz,
[D5]pyridine, 308C): d=7.56–7.53 (m, 1H; Ph), 7.48–7.43 (m, 9H;
Ph), 7.30–7.20 (m, 5H; Ph), 4.10 (dd, J=9.5, 4.8 Hz, 1H;
NCH(CH2Ph)Ph), 3.51 (dd, J=12.8, 9.5 Hz, 1H; NCH(CH2Ph)Ph), 3.27
(ddd, J=11.1, 6.1, 4.5 Hz, 1H; NCH2CH2N), 3.04 (s, 2H MgCH2Ph),
2.98 (m, 1H; NCH(CH2Ph)Ph), 2.91 (ddd, J=11.1, 7.0, 4.8 Hz, 1H;
NCH2CH2N), 2.47 (m, 2H; NCH2CH2N), 1.90 ppm (s, 6H; Me);
13C NMR (100 MHz, [D5]pyridine, 308C): d=143.6, 141.3, 130.5,
129.9, 128.0, 127.9, 127.9, 127.5, 125.2, 125.1, 67.1, 62.0, 51.5
(MgCH2Ph), 44.7, 44.5, 39.4 ppm; elemental analysis calcd (%) for
C50H60Mg2N4: C 78.43, H 7.90, N 7.32; found: C 78.24, H 8.18, N 7.31.
Preparation of N1-benzylidene-N2,N2-diisopropylethane-1,2-dia-
mine (1a): Compound 1a was prepared following the same proce-
dure as for 1b, thus giving a colorless oil in 76% yield. 1H NMR
(400 MHz, CDCl3, 308C): d=8.24 (brs, 1H; N=CH), 7.75–7.68 (m,
2H; Ph), 7.42–7.36 (m, 3H; Ph), 3.62 (td, J=7.2 Hz, J=1.3 Hz, 2H;
NCH2CH2NiPr2), 3.03 (sept, J=6.5 Hz, 2H; CH(CH3)2), 2.75 (t, J=
7.2 Hz, 2H; NCH2CH2NiPr2), 1.02 ppm (d, J=6.5 Hz, 12H; CH(CH3)2);
13C NMR (100 MHz, CDCl3, 308C): d=161.5, 136.6, 130.5, 128.7,
128.1, 63.6, 49.2, 46.3, 21.0 ppm; MS (EI): m/z: 232.3, 217.2, 189.2,
175.2, 114.2, 72.1, 58.1; HRMS (EI): m/z calcd for C15H24N2: 232.1939;
found: 232.1949.
Preparation of complex anti-2b: Pyridine (63.0 mL, 7.84ꢃ
10ꢀ1 mmol, 1.0 equiv) was added to a solution of 2b (300 mg,
7.84ꢃ10ꢀ1 mmol, 1.0 equiv) in toluene (10 mL) at room tempera-
ture. The mixture was warmed to 808C and stirred for 18 h. After
all the volatiles were removed under reduced pressure, the result-
ing residue was washed with hexane (3ꢃ5 mL) to give 2b-anti as
a white powder (87%). 1H NMR (400 MHz, C6D6, 308C): d=7.26–
7.30 (m, 4H; Ar), 7.16–7.22 (m, 5H; Ar), 6.98–7.02 (m, 4H; Ar), 4.26
(dd, J=12.5, 3.3 Hz, 1H; NCHPh(CH2Ph)), 3.61 (dd, J=15.0, 12.5 Hz,
1H; NCHPh(CH2Ph)), 2.92 (m, 1H; NCH2CH2N), 2.68 (dd, J=15.0,
3.3 Hz, 1H; NCHPh(CH2Ph)), 2.4–2.6 (m, 2H; NCH2CH2N), 1.93 (d, J=
9.0 Hz, 1H; CH2Ph), 1.75 (d, J=9.0 Hz, 1H; CH2Ph), 1.66 (s, 3H; Me),
1.60–1.65 (m, 1H; NCH2CH2N), 1.00 ppm (s, 3H; Me); 13C NMR
(100 MHz, C6D6, 308C): d=156.2, 142.1, 139.7, 129.8, 128.7, 128.6,
126.9, 125.7, 124.4, 117.6, 62.5, 60.5, 47.0, 41.2, 40.1, 39.9,
24.2 ppm.
Preparation of N1-benzylidene-N2,N2-dimethylethane-1,2-diamine
(1b): Benzaldehyde (4.7 mL, 46 mmol, 1.0 equiv) was added to
a suspension of N,N-dimethylethane-1,2-diamine (5.0 mL, 46 mmol,
1.0 equiv) and MgSO4 (ca. 5 g) in diethyl ether (20 mL). The reac-
tion mixture was stirred overnight at room temperature, and the
MgSO4 was removed by filtration. All the volatiles were removed
under reduce pressure to give 1b as a colorless oil (8.87 g, 85%).
1H NMR (400 MHz, CDCl3, 308C): d=8.30 (s, 1H; N=CH), 7.76–7.66
(m, 2H; Ph), 7.38 (m, 3H; Ph), 3.74 (t, J=7.0 Hz, 2H;
NCH2CH2N(CH3)2), 2.64 (t, J=7.0 Hz, 2H; NCH2CH2N(CH3)2),
2.31 ppm (s, 6H; NCH2CH2N(CH3)2); 13C NMR (100 MHz, CDCl3,
308C): d=161.7, 136.2, 130.5, 128.5, 128.1, 60.1, 59.9, 45.9 ppm;
MS (EI): m/z: 176.2, 132.2, 117.1, 91.1, 58.1; HRMS (EI): m/z calcd. for
C11H16N2: 176.1313; found: 176.1310.
Preparation of dialkynyl complex 7: From 3a: 3-phenyl-1-propyne
(12.4 mL, 1.00ꢃ10ꢀ1 mmol, 2.0 equiv) was added to a solution of
2b (38.3 mg, 5.00ꢃ10ꢀ2 mmol, 1.0 equiv) in toluene (0.5 mL). The
mixture was heated to 608C for 20 min. From 5a: 1-phenyl-1-pro-
pyne (13.8 mL, 1.10ꢃ10ꢀ1 mmol, 2.2 equiv) was added to a solution
of 2b (38.3 mg, 5.00ꢃ10ꢀ2 mmol, 1.0 equiv) in toluene (0.5 mL).
The mixture was heated to 808C for 2 h. In both cases, the volatiles
were removed in vacuo and the remaining oil was washed several
times with pentane to give a white-yellow powder. Owing to tau-
tomerization, the NMR spectra could not be unambiguously as-
signed, even at low temperature and in pyridine. 2D NMR spectra
suggested two sets of overlapping signals. M.p. 588C (decomp);
1H NMR (400 MHz, [D5]pyridine, 308C): d=7.77–7.17 (m, Ar), 4.04
(m, 1.3H), 3.97 (m, 1.7H), 3.53 (t, J=11.9 Hz, 1H), 3.26 (m, 1H), 2.99
(m, 3.4H), 2.86 (m, 1H), 2.58 (m, 1H), 2.50 (m, 1.5H), 2.37 (m, 1H),
2.26 (m, 1.5H), 1.97 (s, 7.8H), 1.85 ppm (brs, 1.4H); 13C NMR
(100 MHz, [D5]pyridine, 308C): d=144.9, 143.9, 141.3, 139.5, 131.6,
130.3, 129.9, 129.5, 128.5, 128.4, 128.3, 128.2, 128.1, 128.0, 127.8,
127.6, 127.4, 127.0, 126.2, 125.8, 125.2, 125.1, 124.8, 66.4, 65.3, 65.2,
61.0, 59.0, 51.6, 45.5, 45.4, 45.1, 44.4, 44.4, 38.3, 28.0 ppm (brs); IR
(KBr): n˜ =3059 (w), 3025 (m), 2878 (m), 2842 (m), 2070 (w), 1944
(w), 1869 (w), 1602 (m), 1494 (s), 1453 (s), 1333 (w), 1282 (w), 1181
(w), 1087 (m), 1029 (m), 981 (s), 943 (s), 787 (w), 731 (m), 699 (s),
608 (w), 551 cmꢀ1 (w); a satisfactory elemental analysis could not
be obtained owing to the high reactivity of the Mg–alkynyl
Preparation of complex anti-2a: A solution of 1a (414 mg,
1.78 mmol, 2.0 equiv) in toluene (5 mL) was added to a solution of
[{Mg(CH2Ph)2(Et2O)}2] (500 mg, 8.90ꢃ10ꢀ1 mmol, 1.0 equiv) or
[Mg(CH2Ph)2(thf)2] (624 mg, 1.78 mmol, 2.0 equiv) in toluene
(10 mL) at room temperature. The mixture was stirred at 608C for
18 h. After all the volatiles were removed under reduced pressure,
the resulting residue was washed with hexane (3ꢃ5 mL) to give
anti-2a as a white powder (678 mg, 87%). M.p.>1308C (decomp);
1H NMR (400 MHz, C6D6, 308C): d=7.43–7.40 (m, 2H; Ph), 7.38–7.32
(m, 4H; Ph), 7.11–7.07 (m, 2H; Ph), 7.01–6.95 (m, 4H; Ph), 6.96–
6.95 (m, 2H; Ph), 6.90–6.89 (m, 1H; Ph), 4.59 (dd, J=12.5, 3.2 Hz,
1H; NCH(CH2Ph)Ph), 3.70 (dd, J=16.5, 12.5 Hz, 1H; NCH(CH2Ph)Ph),
3.38 (m, 1H; NCH2CH2N), 2.95 (dd, J=16.5, 3.2 Hz, 1H;
NCH(CH2Ph)Ph), 2.94 (m, 1H; NCH2CH2N), 2.63 (m, 2H; NCH2CH2N),
2.53 (br, 2H; CHMe2), 2.14 (d, J=9.4 Hz, MgCH2Ph), 1.94 (d, J=
9.4 Hz, MgCH2Ph), 0.70 ppm (brd, J=6.7 Hz, 12H; Me); 13C NMR
(100 MHz, C6D6, 308C): d=156.1, 142.5, 139.7, 129.8, 128.64, 128.59,
128.50, 127.0, 125.9, 125.2, 118.1, 62.5, 41.3, 27.9 (MgCH2Ph),
20.2 ppm; a satisfactory elemental analysis could not be obtained
owing to the high reactivity of the Mg–alkyl moiety: calcd (%) for
C58H76Mg2N4: C 79.35, H 8.73, N 6.38; found: C 78.36, H 8.96, N 6.57.
Chem. Eur. J. 2015, 21, 1 – 10
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moiety: calcd (%) for C54H60Mg2N4: C 79.71, H 7.43, N 6.89; found:
C 78.71, H 7.54, N 6.94.
quenched with a few drops of water and filtered over a short plug
of silica with toluene after precipitation of magnesium salts. The
crude product was concentrated and purified by chromatography
with hexane as the eluent.
Preparation of dialkynyl complex 12: Phenylacetylene (11.0 mL,
1.00ꢃ10ꢀ1 mmol, 2.0 equiv) ) was added to a solution of 2b
(38.3 mg, 5.00ꢃ10ꢀ2 mmol, 1.0 equiv) in C6D6 (0.5 mL). The mixture
was heated to 608C for 20 min before it was filtered and the vola-
tiles removed under reduced pressure. The solid was washed with
pentane (3ꢃ2 mL) to afford dialkyl complex 12 (24.0 mg, 61%).
Acknowledgements
1
K.Y. and H.N. express their special thanks for the financial sup-
port provided by the JSPS Research Fellowships for Young Sci-
entists. H.T. acknowledges financial support by a Grant-in-Aid
for Young Scientists (A) from the Ministry of Education, Culture,
Sports, Science, and Technology, Japan. This work was support-
ed by the Core Research for Evolutional Science and Technolo-
gy (CREST) Program of the Japan Science and Technology
Agency.
M.p. 1808C (decomp); H NMR (400 MHz, C6D6, 308C): d=7.89–7.76
(m, 4H; Ph), 7.35–6.95 (m, 11H; Ph), 4.48 (m, 1H; NCHPh(CH2Ph)),
3.57 (m, 1H; NCHPh(CH2Ph)), 3.31 (m, 1H; NCHPh(CH2Ph)), 2.68 (m,
1H; NCH2CH2N), 2.54 (m, 1H; NCH2CH2N), 1.92 (m, 2H; NCH2CH2N),
1.72 (s, 3H; NMe2), 1.55 pm (s, 3H; NMe2); 13C NMR (100 MHz, C6D6,
308C): d=142.6, 139.6, 131.8, 129.7, 129.0, 128.2, 128.1, 127.7,
127.5, 126.8, 125.8, 125.7, 120.9 (Mg-CCPh), 110.9 (Mg-CCPh), 64.3
(CH), 59.6 (CH2), 46.1 (CH3), 43.4 (CH3), 42.5 (CH2), 41.9 ppm (CH2);
IR (KBr): n˜ =3059 (m), 3025 (m), 2961 (m), 2890 (m), 2846 (m), 2059
(w), 1945 (w), 1869 (w), 1793 (w), 1594 (m), 1495 (m), 1484 (s),
1454 (s), 1357 (w), 1281 (w), 1262 (w), 1199 (m), 1084 (m), 1069
(m), 1023 (m), 938 (w), 852 (w), 779 (m), 758 (s), 695 (s), 597 (w),
547 cmꢀ1 (m); a satisfactory elemental analysis could not be ob-
tained owing to the high reactivity of the Mg–alkynyl moiety:
calcd (%) for C52H56Mg2N4: C 79.50, H 7.18, N 7.13; found: C 78.22,
H 7.03, N 7.01.
Keywords: alkynes · allenes · CꢀH activation · homogeneous
catalysis · magnesium
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Preparation of tetraalkynyl complex 13: Phenylacetylene
(0.275 mL, 2.50 mmol, 25 equiv) was added to a solution of 2b
(76.6 mg, 1.00ꢃ10ꢀ1 mmol, 1.0 equiv) in toluene (3 mL) at room
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7.99 (d, 4H; MgCCPh), 7.38 (d, 2H; NHCHPhCH2Ph), 7.18 (t, 4H;
MgCCPh), 6.95–7.16 (m, 10H; CHArom), 4.55 (m, 1H;
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NMe2), 2.12 (m, 1H; NCH2CH2N), 2.01–2.02 (m, 2H; NCH2CH2N),
1.64 ppm (m, 1H; NCH2CH2N); 13C NMR (100 MHz, C6D6, 308C): d=
141.3, 139.1, 132.4, 130.3, 128.7, 128.5, 128.3, 128.2, 128.0, 127.6,
126.4, 125.8, 114.6, 65.6, 59.1, 46.2, 44.0, 43.2 ppm; 1H NMR
(400 MHz, [D5]pyridine, 308C): d=7.83 (d, J=7.8 Hz, 4H; Ph), 7.45
(d, J=7.8 Hz, 2H; Ph), 7.33 (t, J=7.8 Hz, 6H; Ph), 7.26–7.17 (m, 8H;
Ph), 3.96 (m, 1H; NHCHPh(CH2Ph)), 2.98 (m, 2H; NHCHPh(CH2Ph)),
2.50 (m, 2H; NCH2CH2N), 2.25 (m, 2H; NCH2CH2N), 2.15 (br s, 1H;
NHCHPh(CH2Ph)), 1.96 ppm (s, 6H; NMe2); 13C NMR (100 MHz,
[D5]pyridine, 308C): d=145.0, 140.7, 139.5, 131.4, 130.4, 129.5,
128.4, 128.3, 127.6, 127.0, 126.2, 124.7, 112.8, 65.3, 59.1, 45.5, 45.4,
45.0 ppm; IR (KBr): n˜ =3276 (w, NꢀH), 3059 (m), 3025 (m), 2921
(m), 2843 (m), 2063 (w), 1951 (w), 1888 (w), 1810 (w), 1749 (w),
1595 (m), 1484 (s), 1454 (s), 1283 (w), 1246 (w), 1199 (m), 1097 (m),
1069 (m), 1027 (m), 958 (m), 914 (m), 855 (m), 778 (s), 758 (s), 696
(s), 540 (m), 511 cmꢀ1 (m); a satisfactory elemental analysis could
not be obtained owing to the high reactivity of the Mg–alkynyl
moiety: calcd (%) for C68H68Mg2N4: C 82.51, H 6.92, N 5.66; found:
C 79.62, H 6.96, N 5.84.
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Received: January 15, 2015
Published online on && &&, 0000
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FULL PAPER
Selective and important: Magnesium
complexes bearing aminoamido ligands
are precursors for the isomerization of
terminal alkynes to allenes and further
to internal alkynes through temporally
separated autotandem catalysis (see pic-
ture). The catalytically active tetraalkynyl
complexes consist of a tautomeric mix-
ture of alkynyl-, allenyl-, and propargyl-
magnesium species.
&
Magnesium Catalysis
R. Rochat, K. Yamamoto, M. J. Lopez,
H. Nagae, H. Tsurugi,* K. Mashima*
&& – &&
Organomagnesium-Catalyzed
Isomerization of Terminal Alkynes to
Allenes and Internal Alkynes
In a first step, organomagnesium…
…catalysts completely transform terminal alkynes into
allenes before these are further converted to the final
products, internal alkynes, in a second step, allowing
for isolation of the intermediate allenes. This is illus-
trated by three water tanks, symbolizing the substrate,
the intermediate product, and the final product, which
are connected by tubes that are regulated by linked
magnesium stopcocks, allowing only one transforma-
tion at a time. For more details, see the Full Paper by
H. Tsurugi, K. Mashima, and co-workers on page && ff.
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Chem. Eur. J. 2015, 21, 1 – 10
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ꢁ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!