Pd(O)-catalyzed reaction of alkynes with trifurylgermane and CO providing acylgermanes: The example of hydrometalcarbonylation of alkynes

Article abstract of DOI:10.1021/ja0257016

A direct synthesis of acylmetals from alkynes with group 14 metal hydride and CO is reported. The Pd(0)-catalyzed reaction of alkynes with tri(2-furyl)germane under ambient pressure of CO provides α,β-unsaturated acylgermanes in good yields. Conversion of acylgermanes to the corresponding amides is achieved. Copyright

Full text of DOI:10.1021/ja0257016

Published on Web 03/27/2002
Pd(0)-Catalyzed Reaction of Alkynes with Trifurylgermane and CO Providing
Acylgermanes: The Example of Hydrometalcarbonylation of Alkynes
Hidenori Kinoshita, Hiroshi Shinokubo, and Koichiro Oshima*
Department of Material Chemistry, Graduate School of Engineering, Kyoto UniVersity, Kyoto 606-8501, Japan
Received January 24, 2002
Scheme 1
Rhodium complexes catalyze the carbonylation reaction of
alkynes with triorganosilanes and carbon monoxide. This reaction
has been extensively investigated and is now regarded as a
“silylformylation” reaction.1,2 The process represents metalformyl-
ation reactions to provide â-metalated R,â-unsaturated aldehydes
(Scheme 1, Path A).3 On the other hand, hydrometalcarbonylation
is very rare (Scheme 1, Path B). Murai et al. have reported an
iridium complex-catalyzed reaction of alkenes to furnish acylsilanes
indirectly.4 To our best knowledge, there are no precedent reports
Scheme 2
on direct synthesis of acylmetals from alkynes with group 14 metal
hydrides. Herein we wish to report that the Pd(0)-catalyzed reaction
of alkynes with germanium hydride under CO provided R,â-
unsaturated acylgermanes in good yields.5
To a solution of [PdCl(η3-C3H5)]2 and phosphite 4 in toluene
was added tri(2-furyl)germane6 under 1.0 atm of CO. After 5 min,
1-octyne was introduced dropwise at room temperature. The
reaction mixture was stirred for 5.5 h, and the solution turned yellow
during this period. Concentration and purification of the reaction
mixture provided 2-nonenoylgermane 1a in 58% yield with a small
amount of the regioisomer 2a (1%) (Scheme 2). Dienylgermane
3a via double insertion of 1-octyne was also obtained in 5% yield.6d
After extensive optimization of reaction conditions, we found
that quinoline can suppress the formation of dineylgermane 3a and
improve the yield of acylgermane 1a up to 78%. The use of
phosphine ligands such as Ph3P or trifurylphosphine resulted in
predominant formation of 3a. Toluene proved to be the best among
reaction solvents we examined.7 Interestingly, the reaction in water
as the solvent instead of toluene provides 1a in moderate yield
(54%).
Scheme 3
Having optimized reaction conditions, we examined various
alkynes to prepare R,â-unsaturated acylgermanes. The results are
summarized in Table 1. Several characteristics of this process are
noteworthy. A variety of alkynes afforded the desired products in
good yields. A free hydroxy group, an amino group, and an olefinic
moiety are compatible (entries 5-8). A conjugated enyne, isopro-
penylacetylene, provided dienoylgermane 1d in moderate yield
(entry 4). None of cyclized products were obtained in the reaction
with 1,6-enyne (entry 8). Unfortunately, the use of internal alkynes
such as 6-dodecyne yielded none of the cylgermanes under the same
conditions.
We propose the plausible reaction mechanism as depicted in
Scheme 3.8 (1) Oxidative addition of tri(2-furyl)germane to Pd(0)
providing hydridopalladium 5, (2) regioselective hydropalladation
across an alkyne, and (3) CO insertion into vinylpalladium species
6 followed by reductive elimination produce acylgermane 1, and
regenerate the Pd(0) catalyst. Insertion of a second alkyne into 6 is
a minor pathway furnishing dienylgermane 3 as a byproduct.
Quinoline coordinated to palladium likely retards the second alkyne
insertion but not the CO insertion. In general, analogous alkenyl-
stannylpalladiums do not allow insertion of CO, because of their
rapid reductive elimination to furnish alkenylstannanes. In fact, the
use of triphenylstannane instead of tri(2-furyl)germane provided
none of the carbonylation products.9 We speculate that a relatively
long lifetime of alkenylgermylpalladium 6 enables insertion of CO.
We have also commenced a preliminary study of synthetic
application of acylgermanes.10 We found that acylgermanes reacted
with secondary amines to provide the corresponding amides in good
yields in the presence of a catalytic amount of 4-(N,N-dimethyl-
amino)pyridine (Scheme 4).11 Tri(2-furyl)germane can be recovered
in good yield. Furthermore, carbonylation and amide formation can
be conducted in a one-pot operation. This two-step procedure
achieves highly regioselective hydrocarbamoylation of alkynes.12
* Corresponding author. E-mail: oshima@fm1.kuic.kyoto-u.ac.jp.
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10.1021/ja0257016 CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Conversion of Alkynes into R,â-Unsaturated
Acknowledgment. We thank Professor Tamejiro Hiyama and
Mr. Fumitoshi Shibahara (Kyoto University) for help with high
pressure reactions. This work was supported by a Grant-in-Aid for
Scientific Research on Priority Areas (No. 412: Exploitation of
Multi-Element Cyclic Molecules) from the Ministry of Education,
Culture, Sports, Science, and Technology, Japan.
Acylgermanesa
Supporting Information Available: General procedures and
spectral data for compounds. This material is available free of charge
via the Internet at http://pubs.acs.org. See any current masthead page
for ordering information and Web access instructions.
References
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(3) Germylformylation of alkynes with a rhodium complex has been reported,
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N.; Ikeda, S.; Ohe, K.; Murai, S. J. Am. Chem. Soc. 1992, 114, 9710.
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Hiyama, T.; Kusumoto, T. In ComprehensiVe Organic Synthesis: Trost,
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a Reaction conditions: alkyne (1.0 mmol), tri(2-furyl)germane (1.0
mmol), [PdCl(η3-C3H5)]2 (0.025 mmol), phosphite 4 (0.1 mmol), quinoline
(0.1 mmol), toluene (5 mL), CO (1.0 atm), room temperature.
Scheme 4
(7) The reaction in THF or CH2Cl2 provided 1a in 59% or 65% yield,
respectively.
(8) The alternative mechanism that involves insertion of CO into HPdGe 5
prior to hydropalladation cannot be ruled out. However, preliminary
calculations at the B3LYP/LANL2DZ level indicate that hydropalladation
(Ea ) 15. 2 kcal/mol) is more feasible than CO insertion (Ea ) 57.2 kcal/
mol). See Supporting Information.
(9) No reactions proceeded with Ph3GeH, Ph3SiH, or tri(2-furyl)silane under
otherwise the same reaction conditions.
(10) For recent examples of acylgermanes in organic synthesis, see: (a) Jiaang,
W. T.; Lin, H. C.; Tang, K.-H.; Chang, L.-B.; Tsai, Y.-M. J. Org. Chem.
1999, 64, 618. (b) Iserloh, U.; Curran, D. P. J. Org. Chem. 1998, 63,
4711. (c) Diederichsen, U.; Curran, D. P. J. Organomet. Chem. 1997,
531, 9. (d) Curran, D. P.; Diederichsen, U.; Palovich, M. J. Am. Chem.
Soc. 1997, 119, 4797.
(11) The reaction of 1b with diethylamine provided the corresponding amide
in 88% yield. No reaction proceeded with methylaniline.
In conclusion, we have found the first example of hydrometal-
carbonylation of alkynes to furnish an acylmetal. The palladium-
phosphite complex efficiently catalyzes the reaction of alkynes with
tri(2-furyl)germane providing R,â-unsaturated acylgermanes under
ambient pressure of CO.13 We have also accomplished conversion
of acylgermanes to the corresponding amides in good yields. Further
synthetic application of acylgermanes obtained with this protocol
is currently under investigation.
(12) Direct hydrocarbamoylation of alkynes with palladium catalysts provides
the regioisomeric R,â-unsaturated amides (2-alkylacrylamides), see: (a)
Kiss, G. Chem. ReV. 2001, 101, 3435. (b) Beller, M.; Cornils, M.;
Frohning, C. D.; Kohlpaintner, C. W. J. Mol. Catal. A 1995, 104, 17. (c)
Torii, S.; Okumoto, H.; Sadakane, M.; Xu, L. H. Chem. Lett. 1991, 1673.
(13) In contrast, tri(2-furyl)germane was not effective for germylformylation
with a rhodium catalyst. None of the aldehydes were obtained in the
reaction of 1-octyne with tri(2-furyl)germane in the presence of Rh4(CO)12
under CO atmosphere.
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