Palladium-catalyzed arylation of butadiynes

Article abstract of DOI:10.1016/S0040-4039(00)01474-X

Diarylbutadiynes 1 undergo a palladium-catalyzed coupling reaction with aryl halides 2 to give tetraarylbutatrienes 3, which may further react to benzofulvenes 6 depending on the reaction temperature and the type of aryl groups. (C) 2000 Elsevier Science

Full text of DOI:10.1016/S0040-4039(00)01474-X

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 41 (2000) 8259–8262
Palladium-catalyzed arylation of butadiynes
Gerald Dyker,a,* Stefan Borowski,a Gerald Henkel,a Andreas Kellner,b Ina Dixc and
Peter G. Jonesc
aInstitut fu¨r Synthesechemie, Fachbereich 6, Gerhard-Mercator-Universita¨t Duisburg, Lotharstraße 1,
D-47048 Duisburg, Germany
bInstitut fu¨r Organische Chemie der Technischen Universita¨t Braunschweig, Hagenring 30,
D-38106 Braunschweig, Germany
cInstitut fu¨r Anorganische Chemie und Analytische Chemie der Technischen Universita¨t Braunschweig, Hagenring 30,
D-38106 Braunschweig, Germany
Received 23 August 2000; accepted 4 September 2000
Abstract
Diarylbutadiynes 1 undergo a palladium-catalyzed coupling reaction with aryl halides 2 to give
tetraarylbutatrienes 3, which may further react to benzofulvenes 6 depending on the reaction temperature
and the type of aryl groups. © 2000 Elsevier Science Ltd. All rights reserved.
Keywords: arylation; butadiynes; butatrienes; domino processes; palladium catalysis.
The carbopalladation of disubstituted alkynes is known as the initial key step of a variety of
palladium-catalyzed domino processes leading to annulated ring systems.1 In a preceding report
we presented the synthesis of 9,10-diarylphenanthrenes from diarylalkynes and iodoarenes: the
stereochemical lability of intermediary vinyl palladium species was responsible for the formation
of several isomeric products.1c We became interested in the analogous arylation of diaryl-
butadiynes 1, since for this type of unsaturated substrates the regioselectivity of the carbopalla-
dation step and the kinetic stability of the resulting vinyl palladium complexes were a priori
unclear; therefore the outcome of this type of reaction was difficult to predict.
Under moderate reaction conditions (80°C, Table 1, entry 2) a 53% yield of tetra-
phenylbutatriene (3a, A1=A2=Ph) was obtained from diphenylbutadiyne (1a) and an excess of
iodobenzene (2a). The formation of this rather surprising main product clearly indicates the
regioselectivity of the first carbopalladation step and is in accord with the mechanistic pathway
in Scheme 1: a vinyl palladium species 4 is formed with two geminal aryl groups. The lifetime
of this species is obviously not sufficient for a cyclometallation at the neighboring aryl group, a
pathway which would lead to the formation of a phenanthrene moiety.1c Presumably a
* Corresponding author. Tel: +208-379-2831; fax: +208-379-4192; e-mail: dyker@uni-duisburg.de
0040-4039/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01474-X

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Table 1
Pd-catalyzed coupling reactions of diarylbutadiynes 1 with aryl halides 2a
Entry
1: Ar1
2: Ar2; X
Base
T (°C)
t (d)
Yieldb(%):
3
6
1
2
3
4
5
6
7
8
9
1a: phenyl
1a: phenyl
1a: phenyl
1a: phenyl
1b: 1-naphthyl
1c: 9-anthryl
1a: phenyl
1a: phenyl
1a: phenyl
2a: phenyl; I
2a: phenyl; I
2a: phenyl; I
2a: phenyl; I
2b: 1-naphthyl; I
2c: 9-anthryl; I
2b: 1-naphthyl; Br
2d: mesityle; Br
2c: 9-anthryl; Br
Et3N
Et3N
NaOAc
NaOAc
Et3N
Et3N
Et3N
Et3N
Et3N
60
80
100
120
120
95
80
110
110
2.5
16
53
37
–
–
2.5
2.5
2.5
2.5
2.5
3
trace
41
52
–
–
–
64c
–
15d
15d
17d
15
14
–
–
a Reaction conditions: 1 mmol of 1, 5 mmol of 2, 5 mol-% of Pd(OAc)2, 10 mol-% of PPh3, 5 mmol of NaOAc or
4 ml of Et3N, 10 ml of dry DMF.
b Yields of the isolated products are given.
c Twice the amount of catalyst and ligand applied.
d Mixture of stereoisomers.
e 2,4,6-Trimethylphenyl.
Scheme 1. Mechanistic pathway for the formation of butatriene 3 from butadiyne
rearrangement to 5 takes place instead (such a rearrangement has some precedent in the double
silylation of bis(trimethylsilyl)butadiyne to tetrakis(trimethylsilyl)butatriene).2 The domino pro-
cess is terminated by a coupling reaction of Pd-complex 5 with a second equivalent of the aryl
halide 2. Irrespective of whether the latter reaction step proceeds via a PdIV-intermediate3 (by
direct oxidative addition of 2 to the PdII-intermediate 5) or involves a ligand exchange reaction4
between 5 and an aryl palladium halide, the formation of PdX2 has to be taken into account.
Therefore, an ‘in situ’ reduction to the active Pd0-catalyst is necessary in each catalytic circle.
Also, the Pd-catalyzed Ullmann coupling reaction5 (aryl halide is transformed to biaryl), which
is observed as a competing reaction, affords such a reductive step and is known to be feasible
in the presence of DMF as solvent.

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At elevated temperatures (100–120°C) butatriene 3a is replaced by the benzofulvene 6a6 as the
final product of the domino coupling process (52% yield, entries 3 and 4 of Table 1),
unambiguously identified by a X-ray structure analysis (Fig. 1).7 With preformed 3a as starting
material the yield of 6a increased to 97% (Scheme 2). This reaction is easily explained by a
carbopalladation of the central double bond of 3a to give intermediate 7a, followed by
cyclopalladation to 8a and the final reductive elimination.
Figure 1. X-ray structure analysis of selected coupling products
Scheme 2. Mechanistic pathway for the formation of benzofulvene 6a from butatriene 3a
In order to test scope and limitations of the domino coupling process, we varied the aryl
groups of starting materials 1 and 2. The 1-naphthyl substituted coupling components 1b and 2b

8262
led to the isolation of the tetranaphthylbutatriene 3b in acceptable yield (Table 1, entry 5). In
contrast, the 9-anthryl substituted starting materials 1c and 2c gave no coupling products (Table
1, entry 6).
Entries 7–9 result in butatrienes 3 with mixed substituents8 and are of importance for studying
the stereoselectivity of the coupling process: according to NMR and MS data mixture of the cis
and the trans isomers are obtained in all cases. Since the initial carbopalladation step should
proceed in a syn-manner, the cis–trans isomerization presumably takes place at the stage of the
vinyl palladium intermediates 4 and 5; the stereochemical lability of similar palladium species
has been demonstrated previously.1c
Details of the stereoselectivity of this process are to be investigated; however, in the case of
butatriene 3a/d with two mesityl substituents a fraction of a pure stereoisomer was obtained by
crystallization, identified as the trans isomer by an X-ray structure analysis (Fig. 1).7,9 Bond
lengths of the central butatriene unit are virtually identical with those of the tetraphenyl-substi-
tuted derivative 3a.10,11 The two sterically demanding mesityl substituents in structure 3a/d are
of course more twisted with respect to the butatriene unit than the neighboring phenyl groups.
For the two distinct positions in the elemental cell torsion angles of 68 and 70° are found for
the mesityl groups, and 24 and 1° for the phenyl groups, respectively.
Acknowledgements
Financial support of the Fonds der Chemischen Industrie is gratefully acknowledged. We are
indebted to Degussa AG for a generous donation of palladium acetate.
References
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