Facile C-C bond cleavage of β-diketones by tin(IV) porphyrin complex

Article abstract of DOI:10.1016/j.tetlet.2012.09.075

Reactions of trans-dihydroxo(meso-tetraphenylporphyrinato)tin(IV) [Sn(TPP)(OH)₂] with β-diketones such as acetylacetone (1), dibenzoylmethane (2), 1-phenylbutane-1,3-dione (3), or 4,4-dimethyl-1- phenylpentane-1,3-dione (4) were studied. All re

Full text of DOI:10.1016/j.tetlet.2012.09.075

Tetrahedron Letters 53 (2012) 6456–6459
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Facile C–C bond cleavage of b-diketones by tin(IV) porphyrin complex
⇑
Atul Pratap Singh, Bu Bae Park, Hee-Joon Kim
Department of Applied Chemistry, Kumoh National Institute of Technology, 1 Yangho-dong, Gumi 730-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 June 2012
Revised 14 September 2012
Accepted 19 September 2012
Available online 26 September 2012
Reactions of trans-dihydroxo(meso-tetraphenylporphyrinato)tin(IV) [Sn(TPP)(OH)2] with b-diketones
such as acetylacetone (1), dibenzoylmethane (2), 1-phenylbutane-1,3-dione (3), or 4,4-dimethyl-1-phen-
ylpentane-1,3-dione (4) were studied. All reactions afforded dicarboxylato tin(IV) porphyrin complexes
[Sn(TPP)(O2CR)2] (R = Me, Ph, t-Bu) and ketonic compounds due to the C–C bond cleavage of b-diketones.
The products were characterized by 1H NMR spectroscopy and ESI mass spectrometry, and further proved
by X-ray crystallographic analysis of Sn(TPP)(O2CCH3)2. Possible reaction mechanisms involving four-
centered intermediate species are discussed, in which the C–C and O–H bonds are cleaved along with
the simultaneous formation of (Sn)O–C and H–C bonds.
Keywords:
C–C bond cleavage
1,3-Diketones
Tin(IV) porphyrin
Hydroxo ligand
Ó 2012 Elsevier Ltd. All rights reserved.
The study on the C–C bond cleavage is one of the prime areas of
research in chemistry for a long time due to its significant use in
the degradation of environmentally hazardous and xenobiotic
compounds as well as in the synthesis of biologically active organic
compounds. Acetylacetone (2,4-pentanedione), one of widely used
industrial chemicals, shows central neurotoxicity and toxic side ef-
fects on the immune system of mammals.1 Acinatobacter johnsonii
acetylacetone dioxygenase (Dke1) is a representative example to
catalyze the oxidative degradation of acetylacetone and related
b-carbonyl compounds capable of undergoing enolization to a
cis-b-keto-enol structure.2 Two enzymes are also known to be capa-
ble of degrading acetylacetone via hydrolytically cleaving the cen-
tral C–C bond of the b-diketone moiety.3 The C–C bond cleavage
phenomenon of b-diketones in aqueous alkali solution is also evi-
dent to give carboxylic acids and ketones, and reveals the involve-
ment of ketonic form of b-diketones rather than enolic form.4 The
facilitation of the reaction proceeds through the nucleophilic at-
tack of hydroxide anion on the ketonic carbon to induce C–C bond
Tin(IV) porphyrin complexes have been readily synthesized by
the acidolysis of the hydroxo ligands coordinated to tin(IV)
porphyrins with a variety of carboxylic acids or alcohols because
hydroxo-tin(IV) porphyrins preferentially recognize carboxylic
acids or alcohols.9 These previous results prompted us to explore
the use of the hydroxo ligands in tin(IV) porphyrin complexes as
a base in organic synthesis. We here report facile C–C bond cleav-
age of b-diketones generating carboxylates and ketones by trans-
dihydroxo(meso-tetraphenylporphyrinato)tin(IV), [Sn(TPP)(OH)2,
SnP1]. So far, tin(IV) porphyrin complexes with weakly coordinat-
ing anions such as perchlorate,10 trifluoromethanesulfonate,11 and
tetrafluoroborate12 have been used only as mild Lewis acids in or-
ganic transformation.
The reaction of SnP1 with acetylacetone (1) was taken to study
the C–C bond cleavage of b-diketones. A preliminary reaction was
performed with SnP1 and 10 times excess of 1 in CHCl3 at room
temperature for 3 days.13 The reaction proceeded more efficiently
when the reaction was carried out in neat 1 at higher temperature.
Typically, the reaction using a mixture of SnP1 and 10 times excess
of 1 without any solvents was accomplished within 3–4 h at 90–
100 °C.
cleavage adjacent to ketonic group. The cleavage of b-diketones
5
was expected in the formation of Y2(l2-O2CCH3)2(acac)4(H2O)2
and Cu(py)4(O2CCF3)2,6 where the carboxylato ligands may come
from the degradation of the corresponding b-diketones. It was re-
cently reported that the C–C bond cleavage of 1,3-diketones affords
the 1,2-diketones by use of FeCl3 as the catalyst and tert-butyl ni-
trite as the oxidant without the use of solvent.7
The resulting tin porphyrin complex was characterized by 1H
NMR spectroscopy. As shown in Figure S1, this compound displays
very similar 1H NMR spectroscopic properties to acetato-tin(IV)
porphyrin Sn(TPP)(O2CCH3)2 (SnP2) reported earlier.14 We
evidently proved the formation of acetato-tin(IV) porphyrin SnP2
by X-ray crystallography. X-ray crystal structure of
[Sn(TPP)(O2CCH3)2]ꢀCHCl3 shown in Figure 1 is similar as reported
earlier by Liu et al. where two molecules of CH3COOH are addition-
ally present in a unit cell.15
On the other hand, tin(IV) porphyrins readily form stable six-
coordinate complexes with the two trans axial ligands of oxyanions
due to the oxophilic nature of the high-valent tin(IV) center.8
The production of SnP2 from the reaction implies that the deg-
radation of 1 can be engaged in generating acetate species during
⇑
Corresponding author. Tel.: +82 54 478 7822; fax: +82 54 478 7710.
E-mail address: hjk@kumoh.ac.kr (H.-J. Kim).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.09.075

A. P. Singh et al. / Tetrahedron Letters 53 (2012) 6456–6459
6457
But even after best efforts, we could not observe the 1H NMR
signals and ESI-MS cluster bands related to acetone, most probably
due to its low boiling point and high vapor pressure. To prove the
mechanism and validate the generation of acetone as by-product
from the reaction, we opted dibenzoylmethane (2) as a reactant
in place of 1. It was speculated that if it would follow the proposed
route then the generation of acetophenone would be more feasible.
The higher boiling point and lower vapor pressure of acetophenone
are vital to prove the generation of this ketonic compound by
means of 1H NMR and ESI mass spectral studies. Indeed, the 1H
NMR spectrum of the crude reaction mixture exhibits the presence
of a singlet at 2.50 ppm for CH3 group of acetophenone while sig-
nals related to phenyl groups merge with the phenyl signals of TPP
ligand in Sn(TPP)(O2CPh)2 (SnP3) and residue 2 (Fig. S2). Further-
more, chemical shifts of the signals of SnP3 are identical to those
reported earlier.16 This crude material was further subjected to
ESI mass spectral study, in which several diagnostic peaks appear
at m/z 105.1, 121.2, and 123.1 corresponding to [PhCO]+ (C7H5O re-
quires 105.03), [PhC(O)CH3+H]+ (C8H9O requires 121.07), and
[PhCOOH+H]+ (C7H7O2 requires 123.04) fragments, respectively
(Fig. S4). These results provide sufficient base to anticipate that
the hydroxo ligands are playing a crucial role in the bond cleavage
via attacking on the electron-deficient carbonyl carbons and pro-
moting the C–C bond cleavage adjacent to the carbonyl groups.
We also examined the reactivity of SnP1 toward unsymmetrical
b-diketones such as 1-phenylbutane-1,3-dione (R1 = Ph, R2 = Me;
3) or 4,4-dimethyl-1-phenylpentane-1,3-dione (R1 = Ph, R2 = tBu;
4). Reactions with the two unsymmetrical b-diketones proceeded
slowly compared to those with the symmetrical b-diketones under
similar reaction condition. The reactions for the unsymmetrical b-
diketones were almost completed after heating up to 200–250 °C.
Based on our proposed mechanism, the reaction of SnP1 with
unsymmetrical b-diketones can produce a mixture of three por-
phyrin complexes as shown in Scheme 3. As shown in Figure S5,
the 1H NMR spectrum for a reaction mixture obtained from a reac-
tion of SnP1 with 3 indeed exhibits three sets of signals attributed
to the ligated acetate and benzoate groups of expected porphyrinic
products.
Figure 1. X-ray crystal structure of [Sn(TPP)(O2CCH3)2]ꢀCHCl3 with the atom
numbering scheme. Displacement ellipsoids are drawn at the 50% probability level.
H atoms and solvate CHCl3 molecule are omitted for clarity.
the course of reaction, as shown in Scheme 1. The enzymatic cleav-
age reaction of acetylacetone by Dke1 affords equimolar amounts
of acetate and methylglyoxal.2 The alkaline hydrolysis of
b-diketones in aqueous alkali solutions also gives carboxylic acids
and ketones as products.4 Our further study was thus directed to
uncover reaction pathways for the degradation of acetylacetone
mediated by SnP1.
The incorporation of molecular O2 was examined firstly to find
out possible reaction routes. The cleavage of acetylacetone cata-
lyzed by enzyme Dke1 produces methylglyoxal as well as acetic
acid due to the incorporation of molecular O2.2 We thus conducted
two different reaction conditions for the cleavage of acetylacetone
mediated by SnP1: in the presence and absence of molecular O2.
We could not find methylglyoxal compound in the products ob-
tained from either reactions, judged by 1H NMR spectroscopy
and ESI mass spectrometry. These results provide a substantial
ground that the C–C bond cleavage of 1 mediated by SnP1 does
not follow the oxidative reaction pathway involving the incorpora-
tion of molecular O2.
A mixture of porphyrin products after preparative TLC was fur-
ther characterized by ESI mass spectrometry to identify each por-
phyrin product. Several diagnostic peaks for SnP3 appear in the
ESI mass spectrum (Fig. S6). We suspect that other porphyrinic
products may be less stable under the condition of ESI mass mea-
surement. The reaction with 4 also showed the similar reaction
pattern to produce Sn(TPP)(O2CtBu) (SnP4), judged by 1H NMR
(Fig. S7) and ESI mass (Fig. S9) spectral studies.
The basic nature of the hydroxo ligand bonded to tin(IV) center
was found in the formation of (b-diketonato)organotin complexes.
For example, the reaction of R2Sn(OH)(OSO2R0) (R = n-Pr,
n-Bu; R0 = Me, Et, n-Pr) compounds with b-diketones produced
To further seek out plausible mechanisms for the cleavage, we
have herein proposed a reaction pathway via the nucleophilic at-
tack by the hydroxo ligands in SnP1. As presented in Scheme 2,
four centered intermediate species provides a platform for the C–
C and O–H bond cleavage along with the simultaneous O–C and
H–C bond formation.
Scheme 1. Conversion of Sn(TPP)(OH)2 to Sn(TPP)(O2CCH3)2 by the reaction with acetylacetone.

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A. P. Singh et al. / Tetrahedron Letters 53 (2012) 6456–6459
Scheme 2. A possible reaction pathway for the C–C bond cleavage of b-diketones mediated by Sn(TPP)(OH)2.
Scheme 3. Possible porphyrin products based on the proposed mechanism for the reaction of Sn(TPP)(OH)2 with unsymmetrical b-diketones.
(b-diketonato)tin complexes, R2Sn(b-diketonato)(OSO2R0) where
the b-diketonates act as O,O-chelating ligands.17 This report
obviously shows the basicity of the hydroxo ligand coordinated
to tin(IV) center to be able to accept protons, which is very similar
to the reactivity found in the acidolysis of the hydroxo-tin(IV)
porphyrin complexes. By comparison with the results of both
studies, we thus believe that the enhanced nucleophilicity of
hydroxo ligands in SnP1 capable of the C–C bond cleavage of
b-diketones is attributed to the electron-donating influence of por-
phyrin ligand to stabilize high-valent tin(IV) center besides the
intrinsic basic nature of hydroxo ligands.
In conclusion, we have demonstrated the facile C–C bond
cleavage of b-diketones such as acetylacetone, dibenzomethane,
1-phenylbutane-1,3-dione, and 4,4-dimethyl-1-phenylpentane-
1,3-dione by tin(IV) porphyrin complex, Sn(TPP)(OH)2. The investi-
gated reaction pathway implies that the cleavage phenomenon
apparently proceeds via simultaneous C–C and O–H bond cleavage
through four-centered intermediate species. The C–C bond cleav-
age of b-diketones is not unprecedented, but the use of metallopor-
phyrin in the C–C bond cleavage of b-diketones is a new finding. To
the best of our knowledge, our result is the first example showing
that metalloporphyrin complexes cleave the C–C bond of b-
diketones. It has been studied in recent years that metalloporphy-
rin complexes of trivalent group 9 (Rh(III)18 and Ir(III)19) undertake
Acknowledgment
This work was supported by the Kumoh National Institute of
Technology.
Supplementary data
Crystallographic data for the structural analysis have been
deposited at the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
with
the
deposition
numbers
CCDC
882215
for
[Sn(TPP)(O2CCH3)2]ꢀCHCl3. Copies of this information can be ob-
tained free of charge via, e-mail: deposit@ccdc.cam.ac.uk or
http://www.ccdc.cam.ac.uk; fax: +44 1223 336 033.
Supplementary data (experimental details) associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.tetlet.2012.09.075.
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