Preferential carbene insertion into Ge-H vs. other heavier group 14 hydrides via samarium carbenoids

Article abstract of DOI:10.1039/c1cc11256b

The relative reactivities of Zn, Al, and Sm carbenoids in the chemoselective carbene insertion reaction of heavier group 14 hydrides were studied. By variation of the reaction protocols using Sm carbenoids, insertion reaction can favour the Ge-H bonds to give Ge-alkylated derivatives in good to high yield.

Full text of DOI:10.1039/c1cc11256b

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Preferential carbene insertion into Ge–H vs. other heavier group 14
hydrides via samarium carbenoidsw
Hitoshi Kondo, Yoshinori Yamanoi* and Hiroshi Nishihara*
Received 3rd March 2011, Accepted 21st April 2011
DOI: 10.1039/c1cc11256b
The relative reactivities of Zn, Al, and Sm carbenoids in the
chemoselective carbene insertion reaction of heavier group 14
hydrides were studied. By variation of the reaction protocols
using Sm carbenoids, insertion reaction can favour the Ge–H
bonds to give Ge-alkylated derivatives in good to high yield.
Metal carbenoid insertion into heteroatom–hydrogen bonds is
a well-established process that surprisingly has not yet found
widespread application in organic synthesis.1 Although
excellent yields can be obtained from carbenoid insertion into
Si–H bonds,2 chemoselective carbene insertions into group 14
hydrides have not been reported. This lack of attention is
surprising because many protocols and reagents have been
developed for catalytic enantioselective metal carbene insertion
into Si–H bonds.3 Samarium carbenoids are generated from
Sm or SmI2 and gem-dihaloalkanes under mild conditions.4
Among the various metal carbenoids, samarium carbenoids
exhibit unique reactive properties. Their notable uses include
the chemoselective Simmons–Smith cyclopropanation,5 methylene
insertions into B–H or C–H bonds,6,7 and several rearrange-
ment reactions.8 However, the reactivity of samarium
carbenoids toward heavier group 14 hydrides has not been
reported. Here, we describe chemoselective carbene insertion
into heavier group 14 hydrides. We found that the samarium
carbenoids undergo extremely preferential carbene insertion
into hydrogermanes over hydrosilanes and hydrostannanes.
Initially, we compared the reactivities of Zn, Al, and Sm
carbenoids toward methylene insertion into triphenylsilane,
triphenylgermane, and triphenylstannane as model reactions.9
As shown in Scheme 1, an equimolar solution of the heavier
group 14 hydrides was treated with 4 equiv. of Zn, Al, and Sm
carbenoids prepared in situ. The reaction was monitored by
GC-MS and quenched with H2O once one of the starting
materials had disappeared. The Zn and Al carbenoids led to
poor selectivity in the methylene insertion reaction.10–12
In contrast, the samarium carbenoid was found to attack
Scheme 1 Intermolecularly competitive methylene insertion reaction
of heavier group 14 hydrides using metal carbenoids. aDecomposition
of triphenylstannane was observed by GC-MS. bAlmost recovery of
triphenylsilane was observed by GC-MS.
triphenylgermane exclusively. Methylene insertion using the
samarium carbenoid was limited to triphenylgermane with
almost complete selectivity. Triphenylsilane did not react with
the samarium carbenoid and was recovered quantitatively.
Triphenylstannane gave a complex mixture during the trans-
formation in the presence of the Sm carbenoid. A samarium
carbenoid prepared from Sm and CH2BrI showed a slightly
better selectivity compared with that prepared from Sm and
CH2I2, probably because the lifetime of the former samarium
carbenoid was longer than that of the latter. Deuterium labeling
of Sm using CD2I2 led to the selective incorporation of the CD2
group into triphenylgermane (499%D) (eqn (1)), confirming
that the methylene unit inserted directly into the Ge–H bond.
ð1Þ
Department of Chemistry, School of Science, The University of
Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
E-mail: yamanoi@chem.s.u-tokyo.ac.jp,
nisihara@chem.s.u-tokyo.ac.jp; Fax: +81-3-5841-8063;
Tel: +81-3-5841-4346
w Electronic supplementary information (ESI) available: Compound
characterization data and copies of the 1H and 13C NMR spectra. See
DOI: 10.1039/c1cc11256b
The scope and limitations of the carbene insertion
methodology were examined by conducting reactions of the
hydrogermanes with the samarium carbenoids.z The results
c
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Chem. Commun., 2011, 47, 6671–6673 6671

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Table 1 Alkylidene insertions into the Ge–H bond via the
samarium carbenoid
Fig. 1 (a) Selective carbene insertion into the Ge–H bond (b) 1H NMR
(400 MHz, CDCl3) showing the hydride region of the starting material
and 13.
a
Reaction conditions: substrate (1.0 mmol), Sm (3.0 mmol), R0CHI2
b
(3.0 mmol), THF (3.0 mL), ꢀ78 1C to rt. GC yield. (These com-
ordering arises from electronegativity differences among the
group 14 elements (Allen values. H: 2.300, Si: 1.916, Ge: 1.994,
and Sn: 1.824).18 The non-monotonic trend in electronegativity
among the group 14 elements (Si–Ge–Sn) is due to incomplete
screening of the 4s electrons by the 3d electron in the first
transition series, which can influence the reactivity of the
element. In previous work, samarium carbenoids were inserted
into B–H and P–H bonds, which exhibited only weak hydride
properties (electronegativity of B is 2.051, and P is 2.253).6,18
These results suggested that weak polarization is essential for
the transformation, although the mechanistic details of this
reaction remain unclear.
c
pounds were volatiles.) Reaction conditions: substrate (1.0 mmol), Sm
(3.0 mmol), R0CHI2 (3.0 mmol), THP (3.0 mL), ꢀ40 1C to rt.
d
Reaction conditions: substrate (1.0 mmol), Sm (5.0 mmol), R0CHI2
e
(5.0 mmol), THF (5.0 mL), ꢀ78 1C to rt. Reaction conditions:
substrate (1.0 mmol), Sm (6.0 mmol), R0CHI2 (6.0 mmol), THF
f
(6.0 mL), ꢀ78 1C to rt. Reaction conditions: substrate (1.0 mmol),
Sm (6.0 mmol), R0CHI2 (6.0 mmol), THP (6.0 mL), ꢀ40 1C to rt.
are shown in Table 1. Tri-n-butylgermane led to an alkylated
product (5) in good yield. 1,1-Diiodoethane was also found to
be effective and gave the ethylated product, 6, in good yield.
The reaction efficiency was not hampered by the presence of a
proximal phenyl moiety (7). The highly sterically hindered
2,2-dimethyl-1-propylene group was also smoothly inserted
into the Ge–H bond (8). It is worth noting that the presence of
amino (–NH2) and cyano (–CN) groups on the aromatic ring
of hydrogermane was tolerated under the transformation,
whereas these functional groups were highly sensitive and
coordinative toward organometallic reagents (9 and 10).13
The reaction of the Sm carbenoid with the secondary or
primary germanes gave, respectively, doubly and triply methylene-
inserted compounds (11 and 12). Neither monoalkylated
(for 11 and 12) nor doubly alkylated (for 12) products were
observed by GC-MS.14,15 In all cases, reaction of the corres-
ponding hydrosilanes with the samarium carbenoid gave
carbene-inserted products in very low yield (o5% GC yield)
under the same reaction conditions.
In summary, samarium carbenoids were shown to
prefer insertion into hydrogermanes over hydrosilanes or
hydrostannanes, as demonstrated in competition experiments.
The synthesis is suitable for the selective alkylation of a wide
range of hydrogermanes. Further investigations are in
progress.
The present work was financially supported by Grant-in-
Aids for Young Scientists (B) (No. 21750036), Scientific
Research on Innovative Areas ‘‘Coordination Programming’’
(area 2107, No. 21108002), and the Global COE Program for
‘‘Chemistry Innovation through the Cooperation of Science
and Engineering’’ from the Ministry of Education, Culture,
Sports, Science, and Technology, Japan.
Notes and references
z A typical experimental procedure for methylene insertion reaction to
the Ge–H bond using a samarium carbenoid. To a mixture of Sm
powder (ca. 50 mesh, 227 mg, 1.51 mmol) and triphenylgermane
(154 mg, 0.504 mmol) in THF (1.5 mL) was added diiodomethane
(120 mL, 1.50 mmol) at ꢀ78 1C. After the reaction mixture was
warmed to room temperature, the reaction mixture was quenched
with water, extracted with CH2Cl2 three times, and dried over Na2SO4.
The solvent was evaporated under a reduced pressure, and fraction-
ated column chromatography was carried out on silica gel to afford the
desired methylated product 2 as a colorless solid (139 mg, 85%).19
1H NMR (400 MHz, CDCl3) d 7.50–7.45 (m, 6H), 7.38–7.32 (m, 9H),
0.90 (s, 3H); 13C NMR (100 MHz, CDCl3) d 138.0 (Cq), 134.5 (CH),
128.8 (CH), 128.2 (CH), ꢀ4.2 (CH3); EI-MS m/z 320 (M+).
To demonstrate the synthetic utility of this methodology, we
selectively inserted a carbene into a compound containing
both Si–H and Ge–H moieties in the same molecule
(Fig. 1(a)).16 As expected, the Sm carbenoid exhibited
complete Ge–H selectivity, and the Si–H functionality remained
unaffected during the transformation (13).17 The corresponding
reaction product was fully identified by the presence of hydride
1
peaks in the H NMR spectrum (Fig. 1(b)).
The relative reactivity of the samarium carbenoid with
group 14 hydrides was R3Sn–H o R3Si–H { R3Ge–H. This
c
6672 Chem. Commun., 2011, 47, 6671–6673
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c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 6671–6673 6673