Catalytic hydroacetylenation of carbodiimides with homoleptic alkaline earth hexamethyldisilazides

Article abstract of DOI:10.1039/c3dt53542h

The homoleptic alkaline earth hexamethyldisilazides, [M{N(SiMe₃)₂}₂(THF)₂] (M = Mg 1a; Ca 1b; Sr 1c), have been shown to act as efficient precatalysts for the hydroacetylenation of organic carbodiimides with alk

Full text of DOI:10.1039/c3dt53542h

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Catalytic hydroacetylenation of carbodiimides
with homoleptic alkaline earth
hexamethyldisilazides†
Cite this: Dalton Trans., 2014, 43,
14249
Merle Arrowsmith,a Mark R. Crimmin,b Michael S. Hill,*a Sarah L. Lomas,a
Myra Sae Heng,b Peter B. Hitchcockc and Gabriele Kociok-Köhna
The homoleptic alkaline earth hexamethyldisilazides, [M{N(SiMe3)2}2(THF)2] (M = Mg 1a; Ca 1b; Sr 1c),
have been shown to act as efficient precatalysts for the hydroacetylenation of organic carbodiimides with
alkyl- and arylacetylenes. Catalytic activity was observed to increase with the size of the group 2 metal
centre employed and to be strongly influenced by the steric properties of the carbodiimide substrate. The
intermediate dimeric calcium and strontium bis(amidinate) complexes, [{PhCuCC(NiPr)2}2M]2 (M = Ca 2b,
Sr 2c), have been isolated and crystallographically characterised. Kinetic studies using the strontium pre-
cursor, 1c, provided a reaction rate law independent of [acetylene] but proportional to [carbodiimide]2
and inversely proportional to the concentration of the amidine product in solution.
Received 17th December 2013,
Accepted 8th January 2014
DOI: 10.1039/c3dt53542h
www.rsc.org/dalton
trivalent rare earth amide or alkyl precatalysts as well as homo-
leptic divalent lanthanide precursors.4 The resulting ethynyl-
Introduction
The atom-efficient catalytic formation of carbon–carbon bonds amidines, which may not be obtained through hydrolysis of
is one of the most actively researched areas of modern small metal amidinate precursors due to their sensitivity to hydro-
molecule synthesis. While transition metal-based processes lysis,4e have found use as valuable building blocks in the syn-
continue to dominate the field, recent advances have also seen thesis of more complex organic heterocycles, such as
the emergence of a number of non-redox active d0 metal com- isoxazoles, pyrazoles, pyrroloquinolines and aminoquino-
plexes whose catalytic activity is based on sequences of σ-bond lines.5 In 2006 Richeson and co-workers reported that the com-
metathesis and polarised insertion of CvX (X = e.g. CR2, O, mercially available lithium precursor, [Li{N(SiMe3)2}], was also
NR) bonds into a reactive M–C bond.1 Among d0 organometal- an efficient catalyst for this transformation.6 In all cases cata-
lic species, acetylide complexes occupy a privileged position lysis was shown to proceed via the insertion of a carbodiimide
owing to their ease of preparation by protonolysis of metal molecule into the metal–acetylide bond of the catalyst to form
amide or alkyl precursors with a terminal alkyne. The stoichio- an intermediate ethynylamidinate complex, followed by proto-
metric insertion of polarised multiple bonds (e.g. CO2, CS2, nolysis with acetylene to release the ethynylamidine and regen-
R2CvO, RNvCvO, RNvCvS, RCuN, RNuC, RNvCvNR) erate the active metal acetylide species as shown in Scheme 1.
into the M–C bond of rare earth and actinide acetylides is par-
In recent years, we and others have developed a rich cata-
ticularly well-documented2 and ethynylamidinate complexes, lytic chemistry of the relatively inexpensive and earth-abundant
synthesised by insertion of carbodiimides into rare earth acety-
lides, have found numerous applications in catalysis.3 Several
groups have also reported the catalytic hydroacetylenation of
carbodiimides using amidocyclopentadienyl-, ethylenebis-
(indenyl)-, tethered diamido- and tris(pyrazolyl)-stabilized
aDepartment of Chemistry, University of Bath, Claverton Down, Bath, BA2 7AY, UK.
E-mail: msh27@bath.ac.uk
bDepartment of Chemistry, Imperial College London, Exhibition Road,
South Kensington, London, SW7 2AZ, UK
cThe Chemistry Laboratory, University of Sussex, Falmer, Brighton, BN1 9QJ, UK
†Electronic supplementary information (ESI) available: Full synthetic details
and descriptions of the kinetic analysis. CCDC 977402–977405. For ESI and crys-
tallographic data in CIF or other electronic format see DOI: 10.1039/c3dt53542h
Scheme 1
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Scheme 2
heavier alkaline earth metals (Mg, Ca, Sr, Ba) encompassing (Scheme 3). Colourless single crystals of both compounds were
an ever-increasing variety of heterofunctionalisation, dehydro- obtained from saturated toluene solutions stored at −30 °C for
coupling and polymerisation reactions.7 In 2008 we reported 24 hours. Details of the X-ray crystallographic analyses are
the facile stoichiometric insertion of carbodiimides into the listed in Table 1 while a selection of bond lengths and angles
calcium–carbon bonds of heteroleptic β-diketiminato calcium are provided in Table 2 for comparative purposes. In contrast
acetylide complexes (Scheme 2A).8 The addition of phenyl- to Coles’ monomeric (THF)-solvated magnesium bis(amidi-
acetylene to N,N′-di-isopropylcarbodiimide was shown to be nate) complex, [{PhCuCC(NiPr)2}2Mg(THF)2] (2a),9 both the
catalysed by calcium complex I at 80 °C in toluene to provide calcium and strontium analogues, 2b and 2c, depicted in
the corresponding N,N′-di-isopropyl-2-phenylethynylamidine in Fig. 1, crystallise as homoleptic dimers, in which each group 2
quantitative yield (Scheme 2B). In this case, however, NMR centre is coordinated by one terminal bidentate ethynylamidi-
spectroscopic analysis showed that the supporting β-diket- nate ligand and two bridging amidinates. In the case of the
iminate ligand was protonated during the reaction as a result larger strontium dication the dimer is centrosymmetric, with
of the acidity of the acetylene substrate. In related work Coles the bridging amidinates coordinating to both strontium
has recently demonstrated that the heteroleptic magnesium centres in a (η2, μ2) fashion. In contrast, for the smaller
amidinate complex, [{MesC(NCy)2}Mg{N(SiMe3)2}(THF)] (Mes = calcium dication one of the amidinate ligands bridges in a
2,4,6-trimethylphenyl, Cy = cyclohexyl), as well as a number of symmetric (η2, μ2) fashion to both metal centres, while the
commercially available magnesium reagents, including other bridges asymmetrically, coordinating to the six-coordi-
MgBu2, [Mg{N(SiMe3)2}2]2, MeMgBr and PhMgBr, may be used nate Ca(1) centre through both N(5) and N(6), and to the five-
to effect the hydroacetylenation of di-isopropyl- and dicyclo- coordinate Ca(2) centre via N(6) only. Several trends can be dis-
hexylcarbodiimides with a variety of aryl-, alkyl- and trialkyl- cerned over the series of homoleptic complexes 2a–2c. The
silylacetylenes.9 Herein we report that the series of readily metal–nitrogen bond lengths to the terminal ethynylamidinate
available homoleptic heavier group 2 amide complexes, [M{N- ligands increase gradually with the size of the metal centre
(SiMe3)2}2(THF)2] (M = Mg 1a; Ca 1b; Sr 1c) are also efficient from magnesium in 2a (ca. 2.17 Å) to strontium in 2c
precatalysts for the hydroacetylenation of carbodiimides and (ca. 2.53 Å). The N–C–N angles of both the terminal and brid-
provide a study of the reaction mechanism using kinetic ging ligands increase by ca. 2° from 2a to 2c in order to accom-
analysis.
modate the larger metal centres while the N–M–N bite angles
decrease by ca. 5.4° from 2a to 2b and ca. 3.5° from 2b to 2c,
in line with the more significant cationic radius increase
between magnesium and calcium. The Ca–N bond lengths to
the terminal amidinate ligands in complex 2b [2.367(3)–2.382(2)
Å] are significantly shorter than those observed in our pre-
viously reported heteroleptic N,N′-di-isopropyl-2-p-tolylethynyl-
amidinate complex [2.3918(14), 2.4294(14) Å], which may be
ascribed to the high steric demands of the β-diketiminate co-
ligand in the latter species.8 Due to the coordination to both
Stoichiometric studies
Reactions between [M{N(SiMe3)2}2(THF)2] (M = Ca, Sr), two
equivalents of N,N′-di-isopropylcarbodiimide and two equi-
valents of phenylacetylene in toluene yielded the corresponding
alkaline earth bis(N,N′-di-isopropyl-2-phenylethynylamidinate)
complexes, 2b and 2c, in essentially quantitative yields
Scheme 3
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Table 1 Details of X-ray crystallographic analyses for compounds 2b, 2c, 3 and 4
2b
2c
3
4
Formula
Mw
T (K)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C
60H76Ca2N8
C60H76N8Sr2
1084.53
173(2)
Monoclinic
P21/c
12.9424(2)
19.6115(4)
11.7596(3)
90
98.832(1)
90
2949.43(11)
2
1.22
1.851
3.50 to 26.05
39 387
C45H57BaKN6
858.41
150(2)
Monoclinic
C2/c
26.4080(3)
16.4240(2)
21.1240(3)
90.00
97.322(1)
90.00
9087.3(2)
8
1.255
1.001
C33H40N2·C6H6
542.78
150(2)
Monoclinic
C2/c
22.5370(2)
12.92390(10)
25.3579(3)
90
115.4780(4)
90
6667.62(11)
8
1.081
0.062
989.45
173(2)
Triclinic
ˉ
P1
10.8208(3)
16.1524(4)
17.4648(3)
96.295(2)
94.289(1)
93.976(1)
3016.36(12)
2
1.09
0.23
3.53 to 26.05
42 730
11 828
γ (°)
V (Å3)
Z
ρ (mg m−3
)
μ (mm−1
θ range (°)
)
3.62 to 27.48
92 722
10 399
3.560 to 25.096
48 040
48 040
Reflections measured
Independent reflections
Rint
5802
0.0612
0.0417
0.0984
—
Final R1 values (I > 2σ(I))
Final wR(F2) values (I > 2σ(I))
Final R1 values (all data)
Final wR(F2) values (all data)
0.0716
0.1857
0.1012
0.2077
0.0348
0.0749
0.0496
0.0807
0.0384
0.0675
0.0799
0.0788
0.0645
0.1364
0.0826
0.1473
Table 2 Selected bond lengths (Å) and angles (°) for compounds 2b, 2c, 3 and 4
2ba
2cb
3c
4
[M–N]t
2.382(2), 2.370(3),
2.371(3), 2.367(3)
2.361(3), 2.733(3)
2.527(2), 2.551(2)
—
—
—
[M–N]a.b.
—
2.760(2),d 2.808(2),d
2.759(2),d 2.820(2)d
2.789(2),e 2.897(2)e
3.178(2),d 2.914(2),d
2.891(2),d 2.955(2)d
2.936(2),e 3.036(2)e
—
[M–N]s.b.
[M′–N]a.b.
2.650(3), 2.464(2)
3.448(3), 2.404(3)
2.751(2), 2.668(2)
—
—
—
[M′–N]s.b.
[N–C]t
2.445(3), 2.602(2)
1.333(4), 1.327(4),
1.332(4), 1.320(5)
1.428(5), 1.242(4)
2.723(2), 2.732(2)
1.335(3), 1.321(3)
—
1.325(3), 1.326(3)
[N–C]a.b.
—
1.332(4),d 1.331(4)d
1.334(4),d 1.332(4)
1.336(3),e 1.332(3)e
—
—
[N–C]s.b.
[N–M–N]t
1.324(4), 1.331(4)
57.31(8), 57.27(10)
51.84(12)
52.04(8)
40.54(9)
52.83(8)
117.9(3)
114.9(3)
115.9(3)
1.338(3), 1.334(3)
53.56(6)
—
49.75(6)
—
49.42(6)
118.9(2)
—
—
—
—
—
—
—
119.3(2)
—
—
[N–M–N]a.b.
[N–M–N]s.b.
[N–M′–N]a.b.
[N–M′–N]s.b.
[N–C–N]t
[N–C–N]a.b.
[N–C–N]s.b.
48.53(7),d 48.46(7)d
47.38(6)e
43.86(6),d 46.11(7)d
44.99(6)e
—
118.6(2),d 118.4(2)d
117.9(2)e
117.2(2)
a M/M′ = Ca. b M/M′ = Sr. c M = Ba, M′ = K. d N(1) to N(4). e N(5)/N(6); t = terminal; a.b. = asymmetric bridging; s.b. = symmetric bridging.
calcium centres the Ca–N bonds of the symmetrically bridging reported by Cameron and co-workers.10 Conversely the stron-
ligand in 2b are significantly elongated [2.445(3)–2.650(3) Å] tium dimer, 2c, exhibits two (η2, μ2) bridging ligands, in a
while the corresponding N–Ca–N bite angles [ca. 52.5°] are similar fashion to the analogous strontium bis(N,N′,N″,N″-
considerably smaller than those of the terminal ligands tetra-isopropylguanidinate) dimer reported by the same
[ca. 57.3°]. For the asymmetrically bridging ethynylamidinate authors; in this latter case, however, the Sr–N distances are
ligand, the Ca(1)–N(5) [2.361(3) Å] and Ca(2)–N(6) bond somewhat longer and the N–Sr–N angles slightly more acute
lengths [2.404(3) Å] are within the range of those observed for than in 2c due to the higher steric demands of the di-isopropyl-
terminal calcium amidinate ligands, whereas the Ca(1)–N(6) amido ligand backbone substituents.
bond is lengthened to 2.733(3) Å. A similar symmetric/asym-
NMR studies of complexes 2b and 2c showed that the com-
metric bridging motif has been observed for the related stron- pounds retain their solid-state nuclearity in solution at room
tium bis(N,N′-di-isopropyl-N″,N″-dimethylguanidinate) dimer temperature. 1H and 13C NMR spectra for both compounds
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Fig. 1 ORTEP representations of the calcium and strontium bis(ethynylamidinate), complexes 2b (left) and 2c (right). Thermal ellipsoids drawn at
30% probability. Hydrogen atoms removed for clarity.
displayed two distinct sets of resonances for the terminal and the analysis and selected bond length and angle data are pro-
bridging ethynylamidinate ligands at room temperature. vided in Tables 1 and 2, respectively. Compound 3 crystallises
Whereas the isopropyl 1H NMR resonances of 2b were well- as a 1-dimensional polymer propagated by η2–μ2-interactions
defined, those of 2c remained broad until cooled to 238 K, between alternate barium and potassium centres and the ami-
indicating some fluxional behaviour presumably caused by the dinate ligands. While the bimetallic assembly of 3 is unprece-
greater degree of conformational freedom around the larger dented for an amidinate-based system, the Ba–N bond
strontium centres. The 13C resonances of the metallacyclic distances [2.759(2)–2.897(2) Å] observed within the structure
carbon [2b δ 160.4, 158.5 ppm; 2c δ 158.8, 158.6 ppm] are very for the Ba–N–K bridging amidinate linkages are significantly
similar to that reported for the analogous monomeric mag- elongated in comparison to the terminal Ba–N bonds observed
nesium complex.9
in the only previously reported barium amidinate, a 6-coordi-
Attempts to isolate the analogous homoleptic barium N,N′- nate monometallic species containing chelating N,N′-bis(2,6-
di-isopropyl-2-phenylethynylamidinate by the same route di-isopropylphenyl)formamidinate ligands [Ba–N 2.685(3)–
invariably yielded the polymeric mixed barium–potassium 2.769(3) Å].11 The structure of compound 3 suggests that
complex, [{PhCuCC(NiPr)2}3BaK]n, compound 3, which was the barium bis(hexamethyldisilazide) starting material
identified by a single crystal X-ray diffraction analysis. The was heavily contaminated with the [BaK{N(SiMe3)2}3] ‘ate’
results of this experiment are shown in Fig. 2, while details of complex and closer analysis of the supposed barium
Fig. 2 ORTEP representation of polymeric complex 3. Thermal ellipsoids drawn at 30% probability. Hydrogen atoms and isopropyl methyl groups
omitted for clarity. Symmetry transformations used to generate equivalent atoms: #1 −x + 1/2, y − 1/2, −z + 1/2 #2 −x + 1/2, y + 1/2, −z + 1/2.
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bis(hexamethyldisilazide) starting material revealed a ca. 1 : 10 similarly improved catalytic activity upon addition of donor
ratio of the desired [Ba{N(SiMe3)2}2(THF)2] and [BaK{N- THF over a range of rare earth precatalysts.4 In contrast, and as
(SiMe3)2}3], the mixture being virtually undistinguishable by already observed by Coles,9 the magnesium-catalysed reaction
NMR spectroscopy from the solvent-free complex, [Ba{N- was significantly impeded by the addition of THF. It is there-
(SiMe3)2}2]2. Repeated attempts with different batches of the fore likely that the increased reactivity of 1b and 1c in the pres-
barium precursor, obtained by salt metathesis from barium ence of small amount of THF is the result of the breakup of
iodide and potassium hexamethyldisilazide in THF, yielded the bis(ethynylamidinate) dimers, 2b and 2c, to form catalyti-
the same result, demonstrating the unreliability of this route cally-active monomeric THF-solvated calcium and strontium
for the synthesis of pure [Ba{N(SiMe3)2}2(THF)2].
bis(amidinates) similar to 2a.
The substrate scope of the reaction was examined using pre-
catalyst 1c in a 9 : 1 mixture of C6D6 and d8-THF (Scheme 5).
As expected, the increase in steric demands of the carbodi-
imide N-substituents caused a drastic drop in reactivity in the
Catalytic studies
order of Pr ≈ Cy > Bu ≫ 2,6-iPr2C6H3 (Table 4, entries 1–4) as
previously observed by Schwamm and Coles.9 It is notable that
this is the first report of catalytic hydroacetylenation for highly
hindered carbodiimides such as the tert-butyl and the 2,6-di-
isopropylphenyl derivatives. Colourless single crystals of N,N′-
bis(2,6-di-isopropylphenyl)-2-phenylethynylamidine, compound 4,
were obtained from the NMR scale reaction in C6D6–THF 9 : 1
at 4 °C. In contrast to other isolated 2-ethynylamidine com-
pounds, which have been shown to be prone to hydrolysis, the
latter compound was air- and moisture-stable, presumably a
consequence of the steric shielding provided by the large
N-aryl groups. Compound 4, as deduced by a single crystal
X-ray diffraction analysis (Fig. 3; details of X-ray analysis and
selected bond length and angle data are presented in Tables 1
and 2), crystallises as the cis isomer with respect to the CvN
bond and forms hydrogen-bonded dimers [N(1′)–H(1) 1.95(6),
N(2)–H(2′) 1.99(7) Å]. Both the C(1)–N(1) and C(1)–N(2) bond
lengths [1.325(3) and 1.326(3) Å] are intermediate between
formal C–N single and CvN double bonds due to the amino
proton being equally disordered over both nitrogen atoms.
The catalytic reactions were also found to be sensitive to
the nature of the acetylene substituent, with aryl functional-
ities requiring lower reaction temperatures (entries 1 and 8)
i
t
Complexes 1a–1c (5 mol%) were screened for their efficacy in
the catalytic hydroacetylenation of N,N′-di-isopropylcarbodi-
imide with phenylacetylene at room temperature (Scheme 4).
The magnesium complex, 1a (Table 3, entry 1), displayed
higher activity than the heteroleptic magnesium amidinate
complex, [{MesC(NCy)2}Mg{N(SiMe3)2}(THF)], which only pro-
vided 44% conversion after 24 h at room temperature, with a
catalyst loading of 10 mol%.9 NMR yields after 24 hours
showed increased conversion with increasing size of the alka-
line earth metal dication used (entries 1–3). The barium ‘ate’
complex, [BaK{N(SiMe3)2}3], displayed even higher activity than
the strontium precursor, 1c (entry 4). In subsequent catalytic
and mechanistic studies, however, the latter precatalyst was
used to provide a more well-defined system. A similar trend of
increasing activity with increasing cation size was observed
by Zhou and co-workers when comparing the homoleptic
rare earth(II) hexamethyldisilazide precatalysts, [M{N(SiMe3)2}2-
(THF)3] (M = Sm, Eu, Yb), for the same reaction.4c
In all cases a marked product inhibition effect was
observed, as well as an increase in reactivity when performing
the reaction in a 9 : 1 mixture of C6D6 and d8-THF using the
calcium and strontium precatalysts, 1b and 1c. Performing the
reaction in d8-THF alone did not provide any significant
improvement in reactivity. Zhou and co-workers have reported
Scheme 4
Scheme 5
Table 3 Catalyst and solvent screening for the catalytic hydroacetylenation of carbodiimides
Entry
R1
R2
Catalyst (mol%)
Solvent
Time (h)
Temp. (°C)
NMR yielda (%)
1
2
3
4
5
6
7
8
iPr
iPr
iPr
iPr
iPr
iPr
iPr
iPr
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
1a (5.0)
1b (5.0)
1c (5.0)
[BaK{N(SiMe3)2}3] (5.0)
1c (2.5)
1c (2.5)
1c (2.5)
1c (2.5)
C6D6
C6D6
C6D6
C6D6
C6D6
C6D6–d8-THF 9 : 1
d8-THF
24
24
24
24
0.5
0.5
0.5
18
25
25
25
25
25
25
25
60
61
66
79
89
6
28
25
93
C6D6–d8-THF 9 : 1
a Measured by integration against bis(trimethylsilyl)amine released from catalyst activation.
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Table 4 Scope of carbodiimide hydroacetylenation in C6D6–d8-THF 9 : 1
Entry
R1
R2
Cat. (mol%)
Time (h)
Temp. (°C)
NMR yielda,b (%)
1
2
3
4
5
6
7
8
9
iPr
Ph
Ph
Ph
1c (2.5)
1c (2.5)
1c (5.0)
1c (10.0)
1c (5.0)
1c (5.0)
1c (5.0)
1c (2.5)
1a (5.0)
1a (5.0)
18
18
18
48
18
24
48
18
48
48
60
60
100
100
80
80
100
60
93 (98)
94 (99)
84 (94)
71 (91)
72 (82)
73 (83)
79 (89)
78 (83)
77 (87)
74 (84)
Cy
tBu
2,6-iPr2C6H3
Ph
iPr
Ph
Ph
Ph
Ph
Ph
nBu
Cy
tBu
4-MeC6H4
CH2NMe2
CH2OMe
80
80
10
a Measured by integration against bis(trimethylsilyl)amine released from catalyst activation. b Yield of free amidine (in parentheses: NMR yield
after hydrolysis with a few drops of D2O).
using the calcium and strontium precatalysts, 1b and 1c. In
contrast the magnesium precursor 1a yielded the corres-
ponding propargylamidines cleanly at reaction temperatures
up to 80 °C, above which negligible amounts of the butatriene
product were observed (entries 9 and 10). The presence of the
potentially coordinating methoxy and dimethylamino moieties
also suppressed the reaction rate compared to the analogous
reaction with 1-hexyne.
Mechanistic studies
Kinetic analysis of a NMR scale reaction ([PhCuCH] =
[iPrNvCvNiPr] = 0.04 M, [1b] = 8 mM) performed at 60 °C
Fig. 3 ORTEP representation of the hydroacetylenation product of N,
N’-bis(2,6-di-isopropylphenyl)carbodiimide with phenylacetylene, com-
revealed pseudo-second-order kinetics over a period of three
pound 4. Thermal ellipsoids drawn at 30% probability. Hydrogen atoms
half-lives, with an apparent rate constant of kH = 0.15 min
removed for clarity except for the amino proton, H(1) and H(2)’. Hydro-
gen bonds drawn as dashed bonds.
M−1. An identical experiment but using 1-deuterophenylacetyl-
ene, PhCuCD, yielded an apparent rate constant of kD
=
0.19 min M−1, providing a negligible kinetic isotope effect of
than alkyl functionalities (entries 5–7 and 9–10) to reach com- 1.23 (Fig. 4, left). Protonolysis of the amidinate complex by the
pletion. A significant steric effect of the acetylene functional acetylene is therefore unlikely to be rate-determining in the
group was only observed for the largest substrate, tert-butyl- catalytic cycle. A further kinetic experiment performed with
acetylene, which required forcing conditions and long reaction 10 molar equivalents of phenylacetylene per carbodiimide
times to achieve good conversions (entry 7). In all catalytic yielded a pseudo second-order reaction in [iPrNvCvNiPr] over
reactions the intermediate strontium bis(ethynylamidinate) three half-lives, confirming that phenylacetylene is not
complex was observed by 1H NMR spectroscopy throughout involved in the rate-determining step (Fig. 4, right). The latter
the course of the reaction and remained visible in solution process was, therefore, deduced to be the insertion of the
after consumption of all substrates. At this point addition of a carbodiimide into the strontium acetylide bond. Irrespective of
few drops of D2O to the reaction mixture enabled the liber- this apparent independence of reaction rate upon [PhCuCH]t
ation of two further equivalents of the amidine product per the apparent rate constant was observed to experience a
strontium centre (see yields in parentheses in Table 4). The 10-fold increase under these conditions, from 0.15 to 1.90 min
NMR resonances of the intermediate strontium bis(amidinate) M−1, suggesting a dependence of reaction rate on the initial
complex also suggested a monomeric species analogous to the concentration of acetylene, [PhCuCH]0. Conversely perform-
magnesium complex, 2a, rather than the dimeric structure of ing the reaction in the presence of the amidine product led to
2c. This is likely the result of the large excess of THF solvent, a drastic rate decrease. We suggest that this observation is
which stabilises the monomeric species during catalysis. With most likely due to competitive protonolysis of the strontium
donor-functionalised propargyl substrates, such as methyl- amidinate intermediate by the amidine product itself rather
propargyl ether and 3-(N,N-dimethylamino)-1-propyne, which than by the acetylene substrate. A plot of the rate constants
are known to undergo dimerisation in the presence of group 2 versus five different inverse initial amidine concentrations pro-
bis(amido) precatalysts,12 competitive acetylene dimerisation vided a linear correlation (Fig. 5, left, see Fig. S2 in ESI† for
and carbodiimide hydroacetylenation were observed when corresponding kinetic analyses). Furthermore a series of
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Fig. 4 Left: 2nd-order kinetic analysis of the NMR scale reaction of phenylacetylene or 1-deuterophenylacetylene (0.04 M) and di-isopropylcarbo-
diimide (0.04 M) in 0.5 mL of C6D6–d8-THF 9 : 1 with 2.5 mol% of 1c at 60 °C. Right: 2nd-order kinetic analysis of the reaction of 10 molar equivalents
of phenylacetylene (0.4 M) and di-isopropylcarbodiimide (0.04 M) in 0.5 mL of C6D6–d8-THF 9 : 1 with 2.5 mol% of 1c at 60 °C.
Fig. 5 Left: Linear correlation between the apparent rate constant k (min M−1) and 1/[amidine]t=0 (mol−1) for kinetic runs performed in 0.5 mL of
C6D6–d8-THF 9 : 1 with 2.5 mol% of 1c at 60 °C. Right: Linear correlation between the apparent rate constant k (min M−1) and catalyst loading (%)
using a 0.04 M mixture of phenylacetylene and di-isopropylcarbodiimide in 0.5 mL of C6D6–d8-THF 9 : 1 at 60 °C.
experiments conducted at increasing catalyst loading while step and that the reactions are self-inhibiting. We are currently
maintaining the substrate concentration at 0.04 M yielded a assessing the use of these amidines as synthons for further
first-order plot in [1c] (Fig. 5, right, see Fig. S2 in ESI† for group 2 catalysed functionalisation reactions and will report
corresponding kinetic analyses), supporting the participation our findings in due course.
of a monomeric THF-supported strontium bis(amidinate)
complex, which is present throughout the course of the cata-
lytic reaction (vide supra).
Acknowledgements
The rate of reaction can thus be expressed as:
We thank the EPSRC (UK) for funding.
2
d½CDIꢀ
dt
½CDIꢀ
¼ ꢁk½RCCHꢀ0½1cꢀ
ðCDI ¼ carbodiimideÞ
½amidineꢀ
References
1 For a selection of recent publications in the field, see:
(a) B. T. Guan, B. Wang, M. Nishiura and Z. Hou, Angew.
Chem., Int. Ed., 2013, 52, 4418; (b) J. Oyamada and Z. Hou,
Angew. Chem., Int. Ed., 2012, 51, 12828; (c) B. T. Guan and
Z. Hou, J. Am. Chem. Soc., 2011, 133, 18086; (d) R. Kubiak,
I. Prochnow and S. Doye, Angew. Chem., Int. Ed., 2009, 48,
1153; (e) A. D. Sadow and T. D. Tilley, J. Am. Chem. Soc.,
2003, 125, 7971. For a selection of recent reviews on the
subject, see: (f) S. Kobayashi and Y. Yamashita, Acc. Chem.
Conclusion
Simple THF-solvated magnesium, calcium and strontium bis-
(bis(trimethylsilylamide)) complexes have been assessed as
precatalysts for the hydroacetylenation of carbodiimides. The
strontium precursor proved to be the most efficient precatalyst,
providing good yields of the desired amidines even for the
most hindered carbodiimide substrates. Kinetic analyses
revealed that carbodiimide insertion provides the rate-limiting
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Paper
Dalton Transactions
Res., 2011, 44, 58; (g) K. C. Hultzsch and A. L. Reznichenko,
Organometallics, 2007, 26, 3755; (e) W.-X. Zhang,
M. Nishiura and Z. Hou, J. Am. Chem. Soc., 2005, 127,
16788.
5 (a) G. Himbert, W. Schwickerath and G. Maas, Liebigs Ann.
Chem., 1985, 7, 1389; (b) G. Himbert and W. Schwickerath,
Liebigs Ann. Chem., 1984, 1, 85; (c) H. Fujita, R. Endo,
A. Aoyama and T. Ichii, Bull. Chem. Soc. Jpn., 1972, 45,
1846.
Catalytic σ-Bond Metathesis, in Structure and Bonding, ed.
P. W. Roesky, Springer, 2010, vol. 137, p. 1;
(h) U. Rosenthal, V. V. Burlakov, M. A. Bach and
T. Beweries, Chem. Soc. Rev., 2007, 36, 719; (i) U. Rosenthal,
P. Arndt, W. Baumann, V. V. Burlakov and A. Spannenberg,
J. Organomet. Chem., 2003, 670, 84; ( j) M. Shibasaki and
N. Yoshikawa, Chem. Rev., 2002, 102, 2187; (k) J. Inanaga,
H. Furuno and T. Hayano, Chem. Rev., 2002, 102, 2211.
2 (a) I. J. Casely, J. W. Ziller and W. J. Evans, Organometallics,
2011, 30, 4873; (b) E. M. Matson, P. E. Fanwick and
S. C. Bart, Organometallics, 2011, 30, 5753; (c) W. Yi,
J. Zhang, M. Li, Z. Chen and X. Zhou, Inorg. Chem., 2011,
50, 11813; (d) M. L. Cole, G. B. Deacon, C. M. Forsyth,
P. C. Junk, D. Polo Ceron and J. Wang, Dalton Trans., 2010,
39, 6732; (e) W. J. Evans, J. R. Walensky and J. W. Ziller,
Organometallics, 2010, 29, 945; (f) W. J. Evans,
J. R. Walensky, J. W. Ziller and A. L. Rheingold, Organo-
metallics, 2009, 28, 3350; (g) D. Baudry, A. Dormond and
A. Hafid, J. Organomet. Chem., 1995, 494, C22.
6 T.-G. Ong, J. S. O’Brien, I. Korobkov and D. S. Richeson,
Organometallics, 2006, 25, 4728.
7 (a) M. Arrowsmith and M. S. Hill, Alkaline Earth Chemistry:
Applications in Catalysis, in Comprehensive Inorganic Chem-
istry II, ed. T. Chivers, Elsevier, 2013, vol. 1, p. 1189;
(b) A. G. M. Barrett, M. R. Crimmin, M. S. Hill and
P. A. Procopiou, Proc. R. Soc. London, Ser. A, 2010, 466, 927;
(c) M. R. Crimmin and M. S. Hill, Homogeneous Catalysis
with Organometallic Complexes of Group 2, in Topics in
Organometallic Chemistry, ed. S. Harder, 2013, vol. 45,
p. 191; (d) S. Harder, Chem. Rev., 2010, 110, 3852.
8 A. G. M. Barrett, M. R. Crimmin, M. S. Hill,
P. B. Hitchcock, S. L. Lomas, M. F. Mahon, P. A. Procopiou
and K. Suntharalingam, Organometallics, 2008, 27, 6300.
9 R. J. Schwamm and M. P. Coles, Organometallics, 2013, 32,
5277.
3 (a) F. T. Edelmann, Chem. Soc. Rev., 2012, 41, 7657;
(b) F. T. Edelmann, Homogeneous Catalysis Using Lantha-
nide Amidinates and Guanidinates, in Structure and
Bonding, ed. P. W. Roesky, Springer, 2010, vol. 137, p. 109.
4 (a) W. Yi, J. Zhang, M. Li, Z. Chen and X. Zhou, Inorg. 10 T. M. Cameron, C.-Y. Xu, A. G. Dipasquale and
Chem., 2011, 50, 11813; (b) Y. Wu, S. Wang, L. Zhang, A. L. Rheingold, Organometallics, 2008, 27, 1596.
G. Yang, X. Zhu, Z. Zhou, H. Zhu and S. Wu, Eur. J. Org. 11 M. L. Cole and P. C. Junk, New J. Chem., 2005, 29, 135.
Chem., 2010, 326; (c) Z. Du, W. Li, X. Zhu, F. Xu and 12 M. Arrowsmith, M. R. Crimmin, M. S. Hill, S. L. Lomas,
Q. Shen, J. Org. Chem., 2008, 73, 8966; (d) S. Zhou, S. Wang,
G. Yang, Q. Li, L. Zhang, Z. Yao, Z. Zhou and H.-B. Song,
D. J. MacDougall and M. F. Mahon, Organometallics, 2013,
32, 4961.
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