High-yield synthesis of 1,3-dimesityl-propane-1,3-dione: Isolation of its aluminum complex as a stable intermediate

Article abstract of DOI:10.1080/00397910802138702

1,3-Dimesityl-propane-1,3-dione was synthesized in high yield, via a stable intermediate (its aluminum complex) from malonyl dichloride and mesitylene by Friedel-Crafts reaction using anhydrous aluminum chloride as catalyst. The intermediate aluminum complex was isolated and characterized by X-ray diffraction analysis, which decomposed upon reflux in concentrate hydrochloric acid to give the title compound. Copyright Taylor & Francis Group, LLC.

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High-Yield Synthesis of 1,3-
Dimesityl-propane-1,3-
dione: Isolation of Its
Aluminum Complex as a Stable
Intermediate
Cui Zhang a b , Peiju Yang a , Yue Yang a b , Xiaojuan
Huang a , Xiao-Juan Yang a & Biao Wu a
a State Key Laboratory for Oxo Synthesis and
Selective Oxidation, Lanzhou Institute of Chemical
Physics, Chinese Academy of Sciences , Lanzhou,
China
b Graduate University of Chinese Academy of
Sciences , Beijing, China
Published online: 11 Jul 2008.
To cite this article: Cui Zhang , Peiju Yang , Yue Yang , Xiaojuan Huang , Xiao-
Juan Yang & Biao Wu (2008) High-Yield Synthesis of 1,3-Dimesityl-propane-1,3-
dione: Isolation of Its Aluminum Complex as a Stable Intermediate, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 38:14, 2349-2356, DOI: 10.1080/00397910802138702
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Synthetic Communications1, 38: 2349–2356, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910802138702
High-Yield Synthesis of 1,3-Dimesityl-propane-1,
3-dione: Isolation of Its Aluminum Complex as a
Stable Intermediate
Cui Zhang,1,2 Peiju Yang,1 Yue Yang,1,2 Xiaojuan Huang,1
Xiao-Juan Yang,1 and Biao Wu1
1State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou
Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, China
2Graduate University of Chinese Academy of Sciences, Beijing, China
Abstract: 1,3-Dimesityl-propane-1,3-dione was synthesized in high yield, via a
stable intermediate (its aluminum complex) from malonyl dichloride and mesityl-
ene by Friedel–Crafts reaction using anhydrous aluminum chloride as catalyst.
The intermediate aluminum complex was isolated and characterized by X-ray
diffraction analysis, which decomposed upon reflux in concentrate hydrochloric
acid to give the title compound.
Keywords: Aluminum complex; 1,3-dimesityl-propane-1,3-dione; Friedel–Crafts
reaction; Synthesis
INTRODUCTION
b-Diketone compounds are important intermediates in organic syn-
thesis.[1,2] According to the literature, there are a number of commonly
utilized synthetic approaches to b-diketones.[3] For example, benzoylace-
tone can be obtained by classical Claisen condensation,[4] whereas
1,3-diphenyl-propane-1,3-dione is synthesized from acetophenone and
benzaldehyde by consequent aldol condensation,[5] bromination, hydrox-
ylation, and dehydration.[6] Moreover, some b-diketones (e.g., dimesityl
Received October 12, 2007
Address correspondence to Biao Wu, State Key Laboratory for Oxo Synthesis
and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Acad-
emy of Sciences, Lanzhou, China. E-mail: wubiao@lzb.ac.cn
2349

2350
C. Zhang et al.
Scheme 1. Synthesis of 1,3-dimesityl-propane-1,3-dione (HL).
diketones) and some monoketones can be prepared by Friedel–Crafts
reaction.[7–10]
We have recently employed the Friedel–Crafts reaction for the syn-
thesis of a sterically demanding b-diketone, 1,3-dimesityl-propane-1,
3-dione (HL). During the reaction process, an unexpected product,
tris(1,3-dimesityl-propane-1,3-dionato)aluminum(III) complex [AlL3]
was obtained. This intermediate is considerably stable even in diluted
hydrochloric acid. Upon reflux in concentrate HCl, the Al(III) com-
plex decomposed to give the desired b-diketone, HL, in good yield
(Scheme 1). Herein we report the synthetic procedure for HL as well as
the crystal structure of the Al(III) complex, [AlL3].
RESULTS AND DISCUSSION
Initially, following the method used for 1,3-diphenyl-propane-1,3-
dione,[5,6] we attempted to synthesize the title compound by the conden-
sation of 2,4,6-trimethylbenzaldehyde with 2,4,6-trimethylacetophenone.
However, the two reactants did not undergo condensation, although both
the reactions of 2,4,6-trimethylbenzaldehyde with acetophenone and
2,4,6-trimethylacetophenone with benzaldehyde could occur easily. We
attributed the difficulty of condensation of the two trimethyl-substituted
starting materials to the high steric hindrance of the six methyl groups.
As an alternative approach, we chose Friedel–Crafts reaction for the
desired diketone.
The synthesis began from the preparation of malonyl dichloride
by reaction of thionyl chloride and malonic acid.[11] It was then reacted
with mesitylene in the presence of anhydrous aluminum chloride, fol-
lowed by treatment with HCl=ice (Scheme 1). An off-white solid was
obtained in very high yield, which was characterized by electrospray
ionization mass spectrometry (ESI-MS) spectrometry. Surprisingly, the
product showed m=z values at 641.6 and 949.8, approximately

1,3-Dimesityl-propane-1,3-dione
2351
Figure 1. ORTEP view (thermal ellipsoids are drawn at the 30% probability level)
of the molecular structure of [AlL3]. Hydrogen atoms are omitted for clarity.
two- and three-fold of the molecular weight of dimesityl-propane-1,3-
dione (308), respectively, plus an aluminum atom. It was therefore sup-
posed that the diketone ligand may have been coordinated to Al3þ
.
The ESI-MS spectrum agrees well with the formation of [AlL2]þ
(641.3) and [Al(HL)L2]þ (949.5), where L denotes the deprotonated eno-
lic form of the diketone HL.
The presence of the Al(III) complex [AlL3] was further confirmed by
X-ray structural studies. Single crystals of [AlL3] were grown from
CH2Cl2=EtOH. As expected, the complex shows a 1 : 3 metal-to-ligand
ratio where the Al3þ ion is coordinated by six oxygen atoms from three
deprotonated enol ligands L, resulting in a neutral complex [AlL3]
(Fig. 1). The Al–O distances are similar to each other, and angles involv-
ing Al as the central atom are close to 90ꢀ or 180ꢀ, indicating that the
aluminum atom resides in a nearly ideal octahedral geometry (Table 1).
Similar structures have been reported for Al(III) complexes with closely
related diketones, which were prepared by the reaction of AlCl3 with
the corresponding diketone ligands buffered with NaOAc.[12,13] The mean
˚
Al–O distance (1.881 A) in the complex reported here is readily compared

2352
C. Zhang et al.
˚
Table 1. Selected bond lengths (A) and angles (deg) for [AlL3]
Parameter
Value
Al–O(1)
Al–O(2)
Al–O(3)
Al–O(4)
Al–O(5)
Al–O(6)
1.880(2)
1.894(2)
1.876(2)
1.882(2)
1.877(2)
1.875(2)
O(6)–Al–O(3)
O(6)–Al–O(5)
O(3)–Al–O(5)
O(6)–Al–O(1)
O(3)–Al–O(1)
O(5)–Al–O(1)
O(6)–Al–O(4)
O(3)–Al–O(4)
O(5)–Al–O(4)
O(1)–Al–O(4)
O(6)–Al–O(2)
O(3)–Al–O(2)
O(5)–Al–O(2)
O(1)–Al–O(2)
O(4)–Al–O(2)
86.05(10)
91.95(10)
89.61(10)
87.74(10)
90.57(10)
179.63(11)
175.96(11)
90.78(10)
90.54(10)
89.78(10)
91.97(10)
176.62(11)
87.72(10)
92.09(10)
91.31(9)
to that in the tris(1,3-diphenylpropane-1,3-dionato)aluminum complex
[12]
˚
(1.874 A)
or in the dimethyl substituted analog, tris(1,3-dimethylpro-
[13]
˚
pane-1,3-dionato)aluminum complex (1.892 A).
The mean O–Al–O
bite angle of [AlL3] (91.61ꢀ), however, is slightly wider than in the other
two complexes (90.30ꢀ), possibly because of the larger steric hindrance of
the mesityl groups in the ligand L. The aryl rings are twisted from the
central C3O2Al chelate rings to different extents, with dihedral angles
in the range 56.0–78.1ꢀ (mean value 70.0ꢀ).
According to literature reports,[8–10] diketones can be obtained by
acidification of the intermediate with HCl=ice. We carried out the reac-
tion; however, it turned out that under such mild conditions the Al(III)
complex remained intact, which may be attributed to the stability of
the complex gained from the six-membered ring chelation together with
the ligand steric protection. To promote the dissociation of the Al(III)
complex and formation of the product, concentrated HCl was used,
and the mixture was heated to reflux for 24 h under stirring. The desired
diketone compound HL was then isolated. The yield is higher than the
compounds prepared using similar methods,[8–10] and the product was
1
characterized by H NMR and IR spectroscopy.

1,3-Dimesityl-propane-1,3-dione
2353
Scheme 2. Keto–enol tautomerism of 1,3-dimesityl-propane-1,3-dione.
b-Diketones are capable of keto–enol tautomerism, and the tauto-
mers exist in equilibrium with each other in solution (Scheme 2). In most
organic solvents, b-diketones are predominately ( >90%) enolized.[14]
The 1H NMR spectrum of HL in CDCl3 showed the characteristic enolic
proton absorptions at around 16.0 ppm, and the ꢁCH2 protons of the
keto form are not visible, which is an indication that the enol tautomer
is the most dominant form in CDCl3 solvent.
The infrared (IR) spectra of b-diketones have been previously
reported,[15,16] which generally exhibit three very strong bands in the
1200–1650 cmꢁ1 region. For our compound, the band at about 1615 cmꢁ1
is assigned to the chelated carbonyl group, and the lower frequency band
situated around 1435 cmꢁ1 is due to the C C double bond. The lowest fre-
=
quency band at around 1272 cmꢁ1 can be assigned to the CꢁO vibration.
In conclusion, we have synthesized a sterically demanding b-
diketone, 1,3-dimesityl-propane-1,3-dione (HL). The intermediate Al(III)
complex [AlL3] has been isolated and structurally characterized, which
bears considerably tolerance for diluted HCl, and decomposes upon
treatment with concentrate HCl by heating to yield the title compound.
EXPERIMENTAL
Malonic acid and anhydrous aluminum chloride were commercially
available and used without further purification. Thionyl chloride, mes-
itylene, and carbon disulfide were distilled before use. IR spectra were
1
obtained on an HP589011 GC=Nexus 870 instrument. The H NMR
spectra were obtained from a Mercury Plus 400 spectrometer using
CDCl3 as solvent and tetramethylsilane (TMS) as internal standard.
The ESI-MS spectra were recorded on a Waters ZQ4000 spectrometer.
Malonyl Dichloride[11]
In a 250-ml flask fitted with a ground-glass joint to a reflux condenser
capped with a calcium chloride drying tube, 10.4 g (0.1 mol) of finely
powered, dry malonic acid and 24 ml (ꢂ0.33 mol) of thionyl chloride

2354
C. Zhang et al.
were placed. The flask is heated at 50 ꢀC for 3 d and then at 60 ꢀC for 6 h.
The mixture was cooled and distilled at reduced pressure. After a small
forerun of thionyl chloride, the product of malonyl dichloride was col-
lected at 48–50 ꢀC=12 mmHg.
Tris(1,3-dimesityl-propane-1,3-dionato)aluminum(III) [AlL3]
Malonyl dichloride (2.82 g, 0.02 mol) was added dropwise to a mixture of
mesitylene (12 ml, 0.08 mol) and anhydrous aluminum chloride (12.0 g,
0.09 mol) in 60 ml of carbon disulfide cooled with an ice bath. After
the addition was complete, the reaction mixture was stirred at 50ꢀC for
3 h and then poured into 10 ml of concentrate HCl and 40 g of ice with
vigorous stirring. The carbon disulfide solution was separated from the
aqueous layer and washed several times with water. The solvent was
removed, and the residue was washed several times with ether to give
an off-white solid. Yield 5.94 g (94%). Mp: >300 ꢀC. ESI-MS: 949.8
[M þ H]þ, 641.6. [M ꢁ L]þ 1H NMR(CDCl3, 400 MHz, d=ppm): 2.21
(s, 18H, CH3), 5.71 (s, 1H, enol CH), 6.72 (s, 4H, ArH). IR (KBr,
n=cmꢁ1), 2918, 2857, 1612, 1575, 1514, 1420, 1390, 1165, 1063, 850.
1,3-Dimesityl-propane-1,3-dione (HL)
The solid of [AlL3] (5.94 g, 6.25 mmol) was dissolved in chloroform and
poured into concentrate HCl (10 ml), and the mixture was refluxed for
24 h to yield the title diketone, which was recrystallized from ethanol as
colorless shining flakes. Yield: 4.54 g (78.5%). Mp: 103–104 ꢀC. ESI-
1
MS: 309.5 [M þ H]þ. H NMR (CDCl3, 400 MHz, d=ppm): 2.28–2.32
(m, 18H, CH3), 5.74 (s, 1H, enol CH), 6.88 (s, 4H, ArH), 16.01 (br, enol
=
OH). IR (KBr, n=cmꢁ1), 3430 (OH), 3106 (C CꢁH), 2951 (CH2), 2918
=
=
(CH3), 1615 (C O), 1435 (C C), 1377 (CꢁH), 1272 (CꢁO).
X-RAY CRYSTAL STRUCTURE DETERMINATION
Crystals suitable for X-ray structure determination were obtained by
slow evaporation of its CH2Cl2=EtOH solution. Data collections were
performed at room temperature on a Bruker Smart Apex II X-ray
diffractometer with graphite-monochromated Mo Ka radiation
˚
(k ¼ 0.71073 A) using the x-scan technique. An empirical absorption cor-
rection using SADABS[17] was applied. The structure was solved by direct
methods using the SHELXL-97 program.[18] All nonhydrogen atoms
were located in successive difference Fourier syntheses and refined by full
matrix least-squares methods with anisotropic thermal parameters on F 2.

1,3-Dimesityl-propane-1,3-dione
2355
Table 2. Crystallographic data for [AlL3]
Parameter
Value
Empirical formula
FW
C63H69AlO6
949.16
Crystal system
Space group
Shape=color
Monoclinic
P2(1)=c
Needle, colorless
13.4110(15)
15.7336(18)
28.784(3)
112.431(4)
5614.0(11)
4
293(2)
1.123
0.085
2032
0.30 ꢃ 0.11 ꢃ 0.08
1.64–28.30
33850
13120
13120=0=632
0.958
˚
a (A)
˚
b (A)
˚
c (A)
b (deg)
3
˚
V (A )
Z
T (K)
Dc (g cm ꢁ3
)
l (mmꢁ1
F(000)
)
Crystal size (mm3)
h range (deg)
Reflections collected
Independent reflections
Data=restraints=parameters
GOF on F2
R1, wR2 [I > 2r(I)]
R1, wR2 (all data)
0.0694, 0.1723
0.1982, 0.2295
Hydrogen atoms were added in theoretical riding mode on the concerned
atoms. Further details of the structural analysis are given in Table 2. It is
noted that the final R values for all data are quite high because of the lack
of sufficient strong diffractions. Nevertheless, the structure can be clearly
determined from the dataset.
Crystallographic data for the structure reported in this article have
been deposited with the Cambridge Crystallographic Data Center as sup-
plementary publications (CCDC No. 670333). Copies of this material can
be obtained free of charge on application to the director, CCDC, 12
Union Road, Cambridge CB2 1EZ, UK (fax: þþ44 12 23 336 033; email:
deposit@ccdc.cam.ac.uk).
ACKNOWLEDGMENT
We thank the Bairen Jihua program of the Chinese Academy of Sciences
for funding.

2356
C. Zhang et al.
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