Palladium-catalyzed highly regio-, stereo- and chemoselective carbogermanylation of allenes: A novel method for the synthesis of 2-arylallylgermane derivatives

Article abstract of DOI:10.1039/b305370a

Palladium complexes effectively catalyze the three-component assembly of allenes, aryl iodides and stannylgermane in a highly regio-, stereo- and chemoselective fashion to yield (Z)-allylgermanes in good to excellent yields.

Full text of DOI:10.1039/b305370a

Palladium-catalyzed highly regio-, stereo- and chemoselective
carbogermanylation of allenes: a novel method for the synthesis of
2-arylallylgermane derivatives†
Masilamani Jeganmohan, Muthian Shanmugasundaram and Chien-Hong Cheng*
Department of Chemistry, Tsing Hua University, Hsinchu, Taiwan 300. E-mail: chcheng@mx.nthu.edu.tw;
Fax: 886-3-5724698; Tel: 886-3-5721454
Received (in Cambridge, UK) 15th May 2003, Accepted 30th May 2003
First published as an Advance Article on the web 16th June 2003
Palladium complexes effectively catalyze the three-compo-
nent assembly of allenes, aryl iodides and stannylgermane in
a highly regio-, stereo- and chemoselective fashion to yield
(Z)-allylgermanes in good to excellent yields.
reaction of 1a with 2a and 3 were examined. Phosphine–
palladium complexes such as PdCl2(PPh3)2 and Pd(PPh3)4 were
totally ineffective for the three-component assembling reaction.
Pd(OAc)2 and PdCl2(PhCN)2 were less effective, giving 4a in
53 and 42% yields, respectively. A brief examination of the
effect of solvent on the yield of 4a using Pd(dba)2 as the catalyst
revealed that toluene was the solvent of choice. Other solvents
such as THF, CH3CN, DMF and EA (ethyl acetate) gave 4a in
84, 82, 69 and 51% yields, respectively. The above optimization
studies indicate that the selection of phosphine-free palladium
complexes is crucial for the success of the three-component
reaction.
The results for the three-component assembly of allenes, aryl
iodides and stannylgermane catalyzed by the Pd(dba)2–toluene
system are compiled in Table 1. Several aryl iodides 1b–g
effectively undergo three-component assembling with 2a and 3
to afford the corresponding allylgermanes 4b–g in good to
excellent yields with very high Z selectivity (entries 2–7).
Electron-withdrawing substituents and electron-donating
groups on the aromatic ring show little effect on the yields of the
catalytic reaction (entries 1–4). The catalytic reaction is highly
The transition-metal-catalyzed addition of an electrophile and a
nucleophile to an unsaturated species is an efficient route to
construct two carbon–carbon bonds for organic synthesis.1
However, the control of both regio- and stereoselectivity in the
multi-component coupling reactions remains a great challenge
to organic chemists. Recent disclosures from our laboratories
revealed that allenes are very useful unsaturated substrates for
palladium-catalyzed three-component assembling reactions.2
Other groups have also studied this type of three-component
reaction of allenes3–5 but most of these reactions gave poor
regio- and stereoselectivity of the products.3,4 While several
types of nucleophiles2–5 including amines, malonates, and
organometallic compounds (B–B, Sn–Sn and Si–Sn) have been
used in the three-component assembling reactions of allenes, to
the best of our knowledge, no example using stannylgermanes
as nucleophiles has been reported.
Allylgermanes are versatile organometallic reagents in
organic synthesis.6 For example, these reagents are widely used
in allylation reactions with imines7 and in [3 + 2] cycloaddition
reactions.8 Although considerable attention has been paid to the
synthesis of allylmetal reagents such as allylboranes, allylsi-
lanes and allylstannanes, very few studies on the synthesis of
allylgermanes have been reported.9 In view of the high synthetic
utility and lower toxicity of allylgermanes than the correspond-
ing organotin compounds, the development of a novel method
for the synthesis is highly demanded. Our continuing interest in
metal-mediated allene chemistry10 prompted us to explore the
possibility of using stannylgermanes as nucleophiles in the
three-component assembling reactions. In this communication,
we wish to report that phosphine-free palladium complexes
effectively catalyzed the three-component assembly of allenes,
aryl iodides and stannylgermane leading to (Z)-allylgermanes in
good to excellent yields in a highly regio-, stereo- and
chemoselective fashion.
Scheme 1
Table 1 Results of palladium-catalyzed three-component assembly of aryl
iodides 1, allenes 2 and stannylgermane 3a
Treatment of 4-iodoanisole 1a with cyclohexylallene 2a and
trimethyl(tributylstannyl)germane 3 in the presence of Pd(dba)2
(5.0 mol%) in toluene at 80 °C for 8 h led to three-component
assembling product 4a in 93% yield (Scheme 1; Table 1, entry
1). No isomer other than 4a was detected under these conditions
Aryl
iodide
Entry
Allene
Product
Z : Eb
Yield (%)c
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
1g
1h
1a
1a
1a
2a
2a
2a
2a
2a
2a
2a
2a
2b
2c
2d
4a
4b
4c
4d
4e
4f
4g
4h
4i
99 : 1
99 : 1
99 : 1
98 : 2
98 : 2
98 : 2
94 : 6
98 : 2
98 : 2
94 : 6
99 : 1
88 (93)
82
89
90
88
80
81
83
88
1
as evidenced by the H NMR spectrum of the crude reaction
mixture. The catalytic reaction is completely regioselective, the
aryl and the germyl group adding to the middle and terminal
carbon of the allene moiety, and highly stereoselective, giving
4a with Z-stereoselectivity greater than 99%. Control experi-
ments verified that in the absence of a palladium catalyst, no
reaction occurred.
10
11
4j
4k
84
82
To understand the nature of the catalytic reaction, the
activities of various palladium catalysts in toluene for the
a All reactions were carried out using aryl iodide 1 (1.00 mmol), allene 2
(1.50 mmol), stannylgermane 3 (1.00 mmol), Pd(dba)2 (0.05 mmol) and
toluene at 80 °C for 8 h. b Determined by 1H NMR spectroscopy. c Isolated
yields; yields in parentheses were measured from the crude products by the
1H NMR integration method using mesitylene as an internal standard.
† Electronic supplementary information (ESI) available: synthesis and
characterization of compounds 4 and 6. See http://www.rsc.org/suppdata/
cc/b3/b305370a/
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CHEM. COMMUN., 2003, 1746–1747
This journal is © The Royal Society of Chemistry 2003

chemoselective as evidenced by the selective reaction of
4-bromoiodobenzene 1h with 2a and 3 to afford 2-(4-bromo-
phenyl)allylgermane 4h in 83% yield (entry 8). The present
arylgermanylation of allenes tolerates a variety of functional
groups such as methoxy, nitro, acetyl, sulfur and bromo on the
aromatic ring of the aryl iodide. In addition to 2a, cyclopenty-
lallene 2b, n-butylallene 2c and t-butylallene 2d also undergo
three-component reaction with 1a and 3 to furnish the
corresponding products 4i–k in 88, 84 and 82% yields,
respectively, with high Z selectivity. In all cases, no trace of
allylstannanes was observed. Moreover, only a single or
predominantly (Z)-stereoisomer of allylgermane products was
obtained with a Z : E ratio falling in the narrow range 94 : 6 to
99 : 1. From the above data, it seems that the stereoselectivity of
the reaction is little affected by the substituent present on the
allenes and aryl iodides.
Fig. 1 Proposed cyclic SE2 intermediate.
mediate 9. It is likely that a cyclic SE2 pathway is involved in
the transmetalation step (Fig. 1). Such an SE2 type mechanism
has been proposed for the palladium-catalyzed Stille coupling
of arylstannanes with aryl halides.11 While the exact reason for
the high chemoselective transmetalation is not clear, the great
affinity of the –SnR3 moiety for halide is probably the driving
force for the chemoselectivity. The accompanying formation of
Bu3SnI was evidenced by the observation of the corresponding
signals in the 1H NMR spectrum of the crude reaction mixtures
of 1 with 2 and 3.2a
It is to be noted that palladium-catalyzed addition of
stannylgermane to allene is known in the literature.12 However,
under our standard conditions, no such addition product was
observed. These results strongly indicate that the oxidative
addition of Ar–I to Pd(0) is faster than that of the Ge–Sn
bond.
The present three-component assembling reaction can be
applied successfully to a partially intermolecular coupling
reaction. Treatment of allene 5 with 3 in the Pd(dba)2–toluene
system afforded chromene derivative 6 in 78% yield (Scheme
2).
In summary, we have developed a new phosphine-free
palladium-catalyzed three-component assembly of allenes, aryl
iodides and stannylgermane. This method allows an efficient
synthesis of various allylgermane derivatives in good to
excellent yields. The catalytic reaction is highly regio-, stereo-
and chemoselective. Further work is in progress to extend the
scope of this reaction to partially intermolecular coupling
reactions and to study the application of these allylgermane
products.
Scheme 2
Based on the known palladium chemistry, a mechanism
involving face-selective coordination of allene to the palladium
center is proposed to account for the observed regio- and
stereochemistry of products (Scheme 3). The catalytic reaction
is likely initiated by the oxidative addition of aryl iodide to
Pd(0) to give aryl palladium(II) intermediate 7. The terminal
double bond of allene is then coordinated favorably to the
palladium center of 7 at the face opposite to the substituent R1
to avoid steric congestion. Insertion of the coordinated double
bond of the allene into the Pd–R bond affords p-allyl palladium
complex 8 with the R1 group anti to the R group of the p-allyl
moiety. Transmetalation of 8 with stannylgermane 3 furnishes
intermediate 9. Subsequent reductive elimination of 9 affords
the final product. The anti stereochemistry of p-allyl complex 9
accounts for the high Z stereoselectivity of products.
We thank the National Science Council of the Republic of
China (NSC-91-2113-M-007-053) for the support of this
research.
Notes and references
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allylstannanes can be attributed to the highly chemoselective
transmetalation of 8 with stannylgermane 3 to form inter-
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Scheme 3
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