Palladium-catalyzed synthesis of α-diketones from acylchromates, iodoarenes, and carbon monoxide

Article abstract of DOI:10.1246/cl.2000.168

A three-component coupling of tetramethylammonium pentacarbonyl(acyl)chromates, aryl iodides, and CO takes place under CO atmosphere in the presence of a catalytic amount of Pd(PPh₃)₄ to afford the corresponding unsymmetrical α-diketones in good yield. By performing the reaction under argon atmosphere, unsymmetrical ketones are prepared instead of α-diketones.

Full text of DOI:10.1246/cl.2000.168

168
Chemistry Letters 2000
Palladium-Catalyzed Synthesis of α-Diketones from Acylchromates, Iodoarenes,
and Carbon Monoxide
Hidehiro Sakurai, Katsuhiko Tanabe, and Koichi Narasaka*
Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033
(Received October 26, 1999, CL-990914)
A three-component coupling of tetramethylammonium
pentacarbonyl(acyl)chromates, aryl iodides, and CO takes
place under CO atmosphere in the presence of a catalytic
amount of Pd(PPh3)4 to afford the corresponding unsymmetri-
cal α-diketones in good yield. By performing the reaction
under argon atmosphere, unsymmetrical ketones are prepared
instead of α-diketones.
Palladium-catalyzed ketone synthesis has attracted much
attention, and a variety of ketones have been synthesized start-
ing from organometallic compounds and acyl halides or alkyl
halides with CO under mild reaction conditions.1 In contrast to
the synthesis of ketones, α-diketones are hard to be prepared
by the palladium-catalyzed process because good acyl donors,
usually acylmetals, are necessary instead of alkylmetals in
order to avoid difficult double insertion of CO (Eq 1).2
Although acyltins are one of the representative acylmetal
compounds, they are unstable and give α-diketones in low
yield in the Pd-catalyzed coupling with acyl halides.3 α-
Alkoxyvinyltins are used as acylmetal equivalents and react
with alkenyl halides under the atmosphilic CO. This process is
suitable for the preparation of methyl α-diketones
(MeCOCOR) but cannot be applied to the synthesis of aromat-
ic α-diketones.4 Recently the reaction of acylzirconium and
acyl halides has been reported, though giving α-diketones in
unsatisfactory yield.5
In the previous paper, we demonstrated that acylchromates
1 can be employed as good acyl donors for the Pd-catalyzed
acylation of allylic halides.6 Based on this finding, the prepa-
ration of unsymmetrical α-diketones from iodoarenes and car-
bon monoxide was examined by using the acylchromates 1.
Tetramethylammonium pentacarbonyl(4-methoxyben-
zoyl)chromate 1a and iodobenzene were treated with a catalyt-
ic amount of Pd(PPh3)4 under 1 atm CO (Eq. 2, Table 1).
When CH2Cl2 or THF was used as a solvent, monoketone 3a
was obtained along with the desired α-diketone 2a (entries 3
and 4). The formation of the monoketone 3a was suppressed
in toluene and the α-diketone 2a was obtained as a sole prod-
uct (entry 5). Excess amounts of PhI were needed in order to
obtain the α-diketone 2a in good yield at room temperature
(entries 6-8), whereas 2a can be synthesized in 74% yield (1
mol% Pd) or 84% yield (2 mol% Pd) by employing only 1.5
molar amounts of PhI when the reaction was performed at 50
°C (entries 9 and 10).
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
169
The reactions of various aroylchromates 1 and iodoarenes
were carried out under the conditions listed in entry 10 of
Table 1, and the results are shown in Table 2. When an elec-
tron donating substiuent was introduced on aryl group of the
chromates (Ar1), the corresponding α-diketone 2 was produced
in good yield, while introduction of electron withdrawing
group such as CF3 diminished the yield of 2 and the monoke-
tone 3 was also obtained (entries 1-3). Substituent effect on
the iodoarene (Ar2) showed a similar inclination (entries 4-7).
That is, the α-diketones 2 were generally obtained selectively,
whereas the reaction with 4-iodoacetophenone gave both of 2
and 3.
Iodoalkenes are also available for this reaction as well as
iodoarenes. For example, Z-isomer (Z : E = >98 : <2) of 1-
iodo-2-phenylethene8 and the chromate 1a reacted under the
above conditions to give Z-diketone 5 (Z : E = >98 : <2) stere-
ospecifically in 68% yield (Eq. 4).
This work was partially supported by a Grant-in-Aid for
Scientific Research on Priority Area No.11119211 from the
Ministry of Education, Science, Sports and Culture of Japanese
Government.
References and Notes
When the reaction of the chromates 1 and iodoarenes was
conducted under argon atmosphere in place of CO, the unsym-
metrical ketones 3 were obtained without forming the α-dike-
tone 2 (Table 3).9
1
Reviews: J. K. Stille, Angew. Chem., Int. Ed. Engl., 25, 508
(1986); R. W. Bates, In "Comprehensive Organometallic
Chemistry II," ed by L. S. Hegedus, Elsevier, Oxford (1995),
Vol. 12, p. 349.
2
Accounts: A. Yamamoto, Bull. Chem. Soc. Jpn., 68, 433
(1995); A. Yamamoto, F. Ozawa, K. Osakada, L. Huang, T-i.
Son, N. Kawasaki, and M-K. Doh, Pure Appl. Chem., 63, 687
(1991).
3
4
5
6
7
J-B. Verlhac, E. Chanson, B. Jousseaume, and J-P. Quintard,
Tetrahedron Lett., 26, 6075 (1985).
H. B. Kwon, B. H. McKee, and J. K. Stille, J. Org. Chem.,
55, 3114 (1990).
Y. Hanzawa, N, Tabuchi, and T. Taguchi, Tetrahedron Lett.,
39, 6249 (1998).
H. Sakurai, K. Tanabe, and K. Narasaka, Chem. Lett., 1999,
309.
General procedure. A mixture of the chromium complex 1
(0.67 mmol), iodoarene (1 mmol), and Pd(PPh3)4 ( 0.013
mmol) in toluene (15 mL) are stirred at 50 °C for 10 h under
CO atmosphere. After the reaction mixture is diluted with
Et2O, the solution is passed through Celite pad and the sol-
vent is evaporated. The products 2 and 3 are separated by sil-
ica gel chromatography.
A possible mechanism is shown in Scheme 1. The reaction
of acylchromate with arylpalladium iodide seems to be very
facile to form trans-bimetallic complex A. If aryl (Ar2) migra-
tion to CO proceeds fast from the complex B, which is generat-
ed by the ligand exchange from phosphine to CO, cis-diacyl-
like complex C might be generated. From the complex C,
reductive elimination proceeds smoothly to give the α-dike-
tone 2.10 Both substituent on the aryl groups of acylchromate
(Ar1) and iodoarene (Ar2) might influence the migration step.
The substituent effects on Ar2 can be explained by decreasing
the rate of Ar2 group migration to CO, when electron with-
drawing group is introduced.12 Although substituent effects on
Ar1 are still speculative, migration might be accelerated by the
trans σ-donor ligand by some kind of trans effect.13
Acylchromate unit plays a role as a σ-donor ligand and the
introduction of electron withdrawing group such as CF3 dimin-
ishes the σ-donor character.
8
9
G. Stork and K. Zhao, Tetrahedron Lett., 30, 2173 (1989).
A similar aryl ketone synthesis from acyliron complex was
reported. See, T. Koga, S. Makinouchi, and N. Okukado,
Chem. Lett., 1988, 1141.
10 Although details of the pathway from the complex C to 2 is
unknown, cis-diacylpalladium complex and Cr(CO)6 will be
produced in the presence of CO. The related reaction mecha-
nism is discussed in ref. 6 and 11.
11 M. Yamane, Y. Ishibashi, H. Sakurai, and K. Narasaka,
Chem. Lett., 2000, 174.
12 N. Sugita, J. V. Minkiewicz, and R. F. Heck, Inorg. Chem.,
17, 2809 (1978).
13 G. K. Anderson and R. J. Cross, Acc. Chem. Res., 17, 67
(1984).