Trans-selective hydrogermylation of alkynes promoted by methyliron and bis(germyl)hydridoiron complexes as a catalyst precursor

Article abstract of DOI:10.1039/c1cc12530c

Catalytic trans-selective hydrogermylation of terminal and internal alkynes was attained by a methyliron complex, CpFe(CO)₂(Me), and a bis(germyl)hydridoiron(iv) complex as a catalyst precursor. The structures of (Z)-triphenyl-(2-phenylethenyl)germane and the bis(germyl)hydridoiron(iv) complexes CpFe(CO)(H)(GeR₃)₂ (R = Et, Ph) were confirmed by single crystal X-ray diffraction studies.

Full text of DOI:10.1039/c1cc12530c

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COMMUNICATION
Trans-selective hydrogermylation of alkynes promoted by methyliron and
bis(germyl)hydridoiron complexes as a catalyst precursorw
Masumi Itazaki, Masahiro Kamitani and Hiroshi Nakazawa*
Received 30th April 2011, Accepted 24th May 2011
DOI: 10.1039/c1cc12530c
Catalytic trans-selective hydrogermylation of terminal and internal
alkynes was attained by a methyliron complex, CpFe(CO)2(Me),
and a bis(germyl)hydridoiron(IV) complex as a catalyst precursor.
The structures of (Z)-triphenyl-(2-phenylethenyl)germane and the
bis(germyl)hydridoiron(IV) complexes CpFe(CO)(H)(GeR3)2 (R =
Et, Ph) were confirmed by single crystal X-ray diffraction studies.
hydrogermylation of alkynes catalyzed by an iron complex. This
reaction shows perfect regio- and stereo-selectivity, providing the
corresponding (Z)-vinylgermanes.
The mixture of phenylacetylene (0.74 mmol, 82 mL), triethyl-
germane (0.89 mmol, 144 mL), and CpFe(CO)2(Me) (7 mol%,
0.05 mmol, 10 mg) was stirred at 80 1C in a nitrogen atmosphere.
After workup, (Z)-triethyl(2-phenylethenyl)-germane 1a was
isolated in 78% yield based on phenylacetylene (Table 1,
entry 1). The corresponding E-isomer was not observed in the
1H NMR spectrum. Table 1 presents the results of hydrogermyl-
ation of several alkynes. The reactions of phenylacetylene with
Hydrometalation of alkynes is well-established because vinyl-
metals thus obtained are useful products for the synthetic
intermediates in organic chemistry.1 Especially, organic
germanium compounds have attracted considerable attention as
alternatives to harmful organic tin compounds.2 One of the
synthetic routes to vinylgermanes is a hydrogermylation reaction
of alkynes catalyzed by a transition-metal complex.3,4 However,
regio- and stereo-selectivities of such reactions are not enough.
Trans-selective radical hydrogermylation of alkynes has been
achieved by Oshima’s group,5a and Lucarini’s group5b though
only a few examples of the hydrogermylation of alkynes were
shown in these methods. In 2005, Gevorgyan and Schwier
reported that B(C6F5)3 exhibits catalytic activity for hydro-
germylation reaction with trans-addition for simple alkynes and
cis-addition for propiolate.6 However the drawback of the catalyst
is that it is moisture sensitive and expensive. Very recently
Murakami’s group reported the trans-selective hydrogermylation
of Et3GeH with 4-octyne catalyzed by a ruthenium complex.7 We
have reported that a methyliron complex CpFe(CO)2(Me)
(Cp = Z5-cyclopentadienyl), which is relatively air-stable,
is an effective catalyst precursor for the C–CN bond cleavage
of organonitriles8a–c and for the N–CN bond cleavage of
cyanamides.9 In addition, we found a metathesis reaction between
Fe–Me and R3Ge–H bonds affording Fe–GeR3 and Me–H
bonds.10 Creation of catalytic activity of iron complexes instead
of rare and expensive metal complexes is important because iron
is an inexpensive, a ubiquitous, and an environmentally friendly
transition metal. However, the examples of homogeneous iron
catalyst for hydrometalation of unsaturated hydrocarbons are
quite limited to date.11 We here report the first trans-selective
n
Ph3GeH and Bu3GeH produced 1b and 1c in excellent yields
(entries 2 and 3). The yield for the p-F phenylacetylene is similar
to that for phenylacetylene (entry 4). The yields of hydro-
germylated products slightly decreased when phenylacetylenes
having the electron-releasing Me, OMe, NH2 groups in the para
position were used (entries 5–7). For alkyl-substituted alkynes,
1-octyne and cyclohexylacetylene reacted smoothly to give the
(Z)-vinylgermanes in excellent yields (entries 8 and 9).
Table 1 Fe-catalyzed hydrogermylation of alkynesa
Entry
R1
R2
R3
Time/h
Yieldb (%)
1
Et
Ph
Ph
Ph
Ph
Ph-F-p
Ph-Me-p
Ph-OMe-p
Ph-NH2-p
Hex
H
H
H
H
H
H
H
H
18
6
78 1a
94 1b
2c
3
nBu
nBu
nBu
nBu
nBu
nBu
nBu
Et
15
18
15
15
15
15
15
15
12
18
12
6
95 1cd
93 1d
74 1e
4
5
6
7
8
9
10
11
12
13
14
15
16c
17c
65 1f
64 1g
88 1h
cHex
Ph
Ph
Ph
Ph
Pr
Pr
COOMe
COOMe
H
Me
Me
Ph
Ph
Pr
Pr
COOMe
COOMe
499 1id
499 1j
499 1k
499 1l
499 1m
43 1n
nBu
Et
nBu
Et
nBu
Et
6
6
12
55 1o
55 1p
64 1q
Department of Chemistry, Graduate School of Science,
Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku,
Osaka 558-8585, Japan. E-mail: nakazawa@sci.osaka-cu.ac.jp;
Fax: +81-6-6605-2522; Tel: +81-6-6605-2547
w Electronic supplementary information (ESI) available. CCDC
815294–815296. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c1cc12530c
nBu
a
Reaction conditions: CpFe(CO)2(Me) (7 mol%, 0.05 mmol), alkyne
b
(0.74 mmol), hydrogermane (0.89 mmol). Isolated yield. Using
d
10 mol% catalyst. In this case the E-isomer was detected only in a
c
few percent yield.
c
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Fe–Ge bond (B - C). Then, vinyl isomerization13 (C - E) takes
place through intermediate D or D0. A similar isomerization
mechanism has been reported for hydrosilylation of alkynes.14
Oxidative addition of R1 GeH toward the Fe center gives F. The
3
subsequent reductive elimination of the vinyl and the hydride
ligands from F yields the corresponding hydrogermylated product
and an intermediate A to complete the catalytic cycle. Although
the iron complex A is a very important intermediate in the
catalytic cycle, the complex is too reactive to be isolated due to
a 16e species. A 16e germyl complex A has a possibility to react
with R1 GeH present in solution to give
a bis(germyl)-
3
hydridoiron(IV) complex, CpFe(CO)(H)(GeR1 ) , through Ge–H
Fig. 1 ORTEP drawing of a Ge1 molecule of 1b with 50% thermal
ellipsoidal plots. Hydrogen atoms except vinyl protons were omitted
for simplicity.
3 2
oxidative addition. Previously, Akita and co-workers reported
that thermal reactions of an alkyl iron complex CpFe(CO)2-
(CH2CH2Ph) with R3EH (E = Si, Sn) afford hydridoiron(IV)
complexes having two group 14 element ligands, CpFe(CO)(H)-
(ER3)2.15 Therefore, our next efforts were focused on the
isolation of a bis(germyl)hydridoiron complex, and we succeeded
in isolating bis(germyl)hydridoiron(IV) complex CpFe(CO)(H)-
(GeR3)2 (R = Et: 2, Ph: 3) in the reaction of R3GeH with
CpFe(CO)2(Me) in the 2 : 1 molar ratio at 60 1C for 12 h (eqn (1)).
Compared with hydrogermylation of terminal alkynes,
examples of that of internal alkynes are quite rare.3i Our iron-
catalyzed system was found to be very effective even for internal
alkynes. The reactions of 1-phenyl-1-propyne and diphenyl-
acetylene with Et3GeH and nBu3GeH proceeded to give the
corresponding trans-addition products in quantitative yields
(entries 10–13). 4-Octyne and dimethylacetylene-dicarboxylate
also could be converted into the desired product in moderate
yields (entries 14–17). This is, to the best of our knowledge, the
first example of trans-addition for the hydrogermylation of the
alkyne having the ester group(s) catalyzed by a transition-metal
complex. Gevorgyan and Schwier reported that Lewis acid
catalyzed hydrogermylation reactions exhibit trans-addition for
simple alkynes. However, cis-addition products are obtained by
using propiolate as an alkyne with the ester group in these
reactions.6
ð1Þ
The molecular structures of 2 and 3 were determined by single
crystal X-ray diffraction studies.z Two independent molecules
crystallized in the unsymmetric unit. The ORTEP drawing of a
Fe1 molecule is shown in Fig. 2. The Fe center has a typical four-
legged piano-stool structure possessing two germyl ligands. The
The molecular structure of 1b was determined using single
crystal X-ray diffraction (Fig. 1).z The CQC bond apparently
possesses a Z configuration. The CQC bond distance for 1b
(1.339(4), 1.335(4) A) is longer than that for a typical vinyllic
C(sp2)QC(sp2) bond (1.299 A)12 and the CQC distance found in
C2H4 (1.3142(3) A).13 However, this is quite similar to that
previously reported [(Z)-b-styrylgermatrane (1.336(4) A)].3j
A proposed catalytic cycle is shown in Scheme 1. A methyl
migration in the precursor forms acyl complex CpFe(CO)-
Fe–Ge bond distances (2.4135(6)–2.4237(7)
A for 2 and
2.4102(13)–2.4275(13) A for 3) are comparable with previously
reported analogous complexes, CpFe(CO)(H)(EEt3)(E0Et3)
{C(QO)Me}, which reacts with R1 GeH to give CpFe(CO)(H)-
3
{C(QO)Me}(GeR1 ). Reductive elimination of acetoaldehyde
3
produces CpFe(CO)(GeR1 ) (A). The generation of MeCHO
3
was confirmed by the 1H NMR spectrum. The alkyne coordinates
to the iron center of A in an Z2-fashion (A - B). This species B
undergoes an insertion of the coordinated acetylene into the
Fig. 2 ORTEP drawings of Fe1 molecules of (a) 2 and (b) 3 with
50% thermal ellipsoidal plots. Hydrogen atoms except hydrido ligand
were omitted for simplicity.
Scheme 1 Proposed catalytic cycle.
c
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(E = E0 = Si,16a and E = Ge, E0 = Si, Sn),16b CpFe(CO)(H)-
(SnPh3)2.15a To the best of our knowledge, this is the first report of
the X-ray structure of a mononuclear iron complex having two
germyl ligands.
Z = 8, Dc = 1.337 g cmꢀ3, 30 715 reflections collected, 9118 unique
reflections (Rint = 0.062), R[I 4 2s(I)] = 0.0563, wR2 = 0.0962, GOF =
1.137. CCDC 815294 for 2: C18H36OGe2Fe, Mw = 469.50, colourless,
%
prism, triclinic, P1 (No. 2), a = 10.4400(12), b = 14.1000(14), c =
16.5000(14) A, a = 63.300(5)1, b = 81.670(7)1, g = 89.180(8)1, V =
2143.4(4) A3, T = 190(2) K, Z = 4, Dc = 1.455 g cmꢀ3, 21 116 reflections
collected, 9596 unique reflections (Rint = 0.0254), R[I 4 2s(I)] = 0.0477,
wR2 = 0.1009, GOF = 1.000. CCDC 815295 for 3: C42H36OGe2Fe,
We checked the catalytic activity of this bis(germyl)hydrido
%
Mw = 757.74, yellow, prism, triclinic, P1 (No. 2), a = 12.931(2),
b = 14.400(2), c = 19.151(3) A, a = 88.061(9)1, b = 87.698(8)1,
=
ð2Þ
g = 74.396(7)1, V = 3431.0(10) A3, T = 200(2) K, Z = 4, Dc
1.467 g cmꢀ3, 31 455 reflections collected, 13 765 unique reflections (Rint
=
0.0381), R[I 4 2s(I)] = 0.0802, wR2 = 0.1833, GOF = 1.032. CCDC
815296.
complex 2. Reaction of Et3GeH with phenylacetylene in the
presence of 10 mol% of 2 at 70 1C yielded the desired
(Z)-vinylgermane 1a in 83% yield (eqn (2)). It is considered
1 (a) E. Langkopf and D. Schinzer, Chem. Rev., 1995, 95, 1375;
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2 For example, see: W. N. Aldridge, in The Organometallic and
Coordination Chemistry of Germanium, Tin and Lead, ed. M. Gielen
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that complex
2 can readily create the active species
CpFe(CO)(GeEt3) A0 by the reductive elimination of Et3GeH
in solution (eqn (3)).16b These results show that the
bis(germyl)hydridoiron(IV) complex 2 is the catalyst precursor
in our system. Stoichiometric reaction of 2 with phenylacetylene
was monitored by the 1H NMR spectra, but no signal
attributable to an intermediate was observed, though the
signals due to 1a were observed.
ð3Þ
Soc. Jpn., 1987, 60, 3468; (h) M. A. Esteruelas, M. Martın and
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L. A. Oro, Organometallics, 1999, 18, 2267; (i) H. Kinoshita,
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R. G. Kultyshev, Organometallics, 2003, 22, 199.
4 For a review of hydrogermylation, see: W. Wolfsberger, J. Prakt.
Chem., 1992, 334, 453.
5 (a) Y. Ichinose, K. Nozaki, K. Wakamatsu, K. Oshima and
K. Utimoto, Tetrahedron Lett., 1987, 28, 3709; (b) S. Bernardoni,
M. Lucarini, G. F. Pedulli, L. Valgimigli, V. Gevorgyan and
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Org. Lett., 2010, 12, 1056.
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2005, 4004; (b) M. Itazaki and H. Nakazawa, Chem. Lett., 2005,
1054; (c) H. Nakazawa, M. Itazaki, K. Kamata and K. Ueda,
Chem.–Asian J., 2007, 2, 882.
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Soc., 2009, 131, 38.
10 M. Itazaki, M. Kamitani, Y. Hashimoto and H. Nakazawa,
Phosphorus, Sulfur Silicon Relat. Elem., 2010, 185, 1054.
11 For hydrosilylation: (a) S. C. Bart, E. Lobkovsky and P. J. Chirik,
J. Am. Chem. Soc., 2004, 126, 13794; (b) A. M. Archer,
M. W. Bouwkamp, M.-P. Cortez, E. Lobkovsky and P. J. Chirik,
Organometallics, 2006, 25, 4269; For hydroboration: (c) J. Y. Wu,
B. Moreau and T. Ritter, J. Am. Chem. Soc., 2009, 131, 12915.
12 R. Lide, Handbook of Chemistry and Physics, CRC Press,
New York, 76th edn, 1995.
Next, we investigated the hydrogermylation under the similar
conditions in entry 1 in Table 1 in the presence of TEMPO
(2,2,6,6-tetramethylpiperidine-1-oxyl) as a radical scavenger
because trans-selective radical hydrogermylation of alkynes
with hydrogermane has been reported.5 The mixture of phenyl-
acetylene (1.00 mmol, 106 mL), triethylgermane (1.10 mmol,
188 L), TEMPO (0.10 mmol, 16 mg), and CpFe(CO)2(Me)
(7 mol%, 0.07 mmol, 13 mg) was stirred at 80 1C for 18 h in a
nitrogen atmosphere. After workup, (Z)-triethyl(2-phenyl-
ethenyl)germane 1a was isolated in 80% yield based on
phenylacetylene. The same results were obtained when the
reaction took place without TEMPO. These results suggest
that our reaction mechanism is not a radical process but an
iron-catalyzed process.
In summary, we found effectively trans-selective hydro-
germylation of alkynes catalyzed by an iron methyl complex.
This catalytic system is applicable not only for terminal
alkynes but also for internal alkynes and an alkyne having
the ester groups as well. The consideration of the catalytic
reaction pathway suggests that a germyliron complex is an
intermediate.
13 G. J. H. Van Nes and A. Vos, Acta Crystallogr., Sect. B: Struct.
Crystallogr. Cryst. Chem., 1979, 35, 2593.
This work was supported by a Challenging Explorating
Research (No. 21655022) from the Ministry of Education,
Culture, Sports, Science and Technology of Japan, by Innovation
Promotion Program in 2010 from NEDO, and by a Research
for Promoting Technological Seeds from JST. M.I. also
acknowledges support from the Iketani Science and Technology
Foundation (0211033-A).
14 (a) R. S. Tanke and R. H. Crabtree, J. Am. Chem. Soc., 1990,
112, 7984; (b) C. H. Jun and R. H. Crabtree, J. Organomet. Chem.,
1993, 447, 177; (c) I. Ojima, N. Clos, R. J. Donovan and
P. Ingallina, Organometallics, 1990, 9, 3127; (d) L. W. Chung,
Y.-D. Wu, B. M. Trost and Z. T. Ball, J. Am. Chem. Soc., 2003,
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Notes and references
16 (a) M. Itazaki, K. Ueda and H. Nakazawa, Angew. Chem., Int. Ed.,
2009, 48, 3313; M. Itazaki, K. Ueda and H. Nakazawa, Angew.
Chem., Int. Ed., 2009, 48, 6938; (b) M. Itazaki, M. Kamitani,
K. Ueda and H. Nakazawa, Organometallics, 2009, 28, 3601.
z Crystal data for 1b: C26H22Ge, Mw = 407.05, colourless, platelet,
monoclinic, P21/n (No. 14),
a = 17.0146(9), b = 8.7374(4),
c = 27.2505(15) A, b = 93.109(3)1, V = 4045.2(4) A3, T = 100(1) K,
c
7856 Chem. Commun., 2011, 47, 7854–7856
This journal is The Royal Society of Chemistry 2011