Mechanistic studies on the CAN-mediated intramolecular cyclization of δ-aryl-β-dicarbonyl compounds

Article abstract of DOI:10.3762/bjoc.9.167

The synthesis of 2-tetralones through the cyclization of δ-aryl-β-dicarbonyl substrates by using CAN is described. Appropriately functionalized aromatic substrates undergo intramolecular cyclizations generating 2-tetralone derivatives in moderate to good yields. DFT computational studies indicate that successful formation of 2-tetralones from δ-aryl-β-dicarbonyl radicals is dependent on the stability of the subsequent cyclohexadienyl radical intermediates. Furthermore, DFT computational studies were used to rationalize the observed site selectivity in the 2-tetralone products.

Full text of DOI:10.3762/bjoc.9.167

Mechanistic studies on the
CAN-mediated intramolecular cyclization
of δ-aryl-β-dicarbonyl compounds
Brian M. Casey, Dhandapani V. Sadasivam and Robert A. Flowers II*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 1472–1479.
Department of Chemistry, Lehigh University, Bethlehem, PA 18015,
USA
doi:10.3762/bjoc.9.167
Received: 25 April 2013
Accepted: 13 June 2013
Published: 23 July 2013
Email:
Robert A. Flowers II* - rof2@lehigh.edu
* Corresponding author
This article is part of the Thematic Series "Organic free radical chemistry".
Guest Editor: C. Stephenson
Keywords:
CAN oxidation; β-dicarbonyls; free radical; radical arylations;
tetralones
© 2013 Casey et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
The synthesis of 2-tetralones through the cyclization of δ-aryl-β-dicarbonyl substrates by using CAN is described. Appropriately
functionalized aromatic substrates undergo intramolecular cyclizations generating 2-tetralone derivatives in moderate to good
yields. DFT computational studies indicate that successful formation of 2-tetralones from δ-aryl-β-dicarbonyl radicals is dependent
on the stability of the subsequent cyclohexadienyl radical intermediates. Furthermore, DFT computational studies were used to
rationalize the observed site selectivity in the 2-tetralone products.
Introduction
2-Tetralones are important intermediates or components of used in organic synthesis [12-14]. In a previous study, we
several natural products and biologically relevant molecules showed that when 6-phenyl-2,4-hexanedione (1a) is oxidized by
[1-7]. They are typically synthesized through transition-metal- CAN in MeCN in the absence of a radicophile, 3-phenylpropi-
mediated processes or preformed tetralin or naphthyl precur- onic acid (2a*) is obtained exclusively over the cyclized
sors [8,9]. Radical approaches can also be used to synthesize 2-tetralone product 2a (Scheme 1) [15]. Using this method, a
substituted 2-tetralones, and single-electron oxidations variety of 1,3-dicarbonyl compounds can be mildly converted to
employing δ-aryl-β-dicarbonyl compounds have been carried carboxylic acids in moderate to excellent yields. In follow up
out on arenes containing pendant β-ketoesters [10,11]. When studies, we found that 2-tetralone 2a could be obtained as the
Mn(III)-based oxidants are employed, secondary oxidations of major product when 1a was oxidized by CAN in MeOH [16]. In
the 2-tetralone products can occur [10,11].
addition, when 2.2 equivalents of CAN were employed, no
products of secondary oxidations were obtained. Based on this
Cerium(IV) ammonium nitrate (CAN) is a versatile, inexpen- observation, the single-electron oxidations of a variety of
sive and nontoxic single-electron oxidizing reagent commonly δ-aryl-β-dicarbonyl substrates with CAN in MeOH were
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Table 1: CAN-mediated oxidation of δ-aryl-β-dicarbonyl compounds in
MeOHa.
Entry
Substrate
Productb
Yield
(%)c
1
73
85
59
1a
1b
1c
2a
2b
Scheme 1: Oxidative conversion of 1,3-dicarbonyl compounds to
carboxylic acids with CAN.
2
3
performed to determine the scope of the reaction. DFT calcula-
tions were used to rationalize the observed impact of substitu-
tion on the δ-aryl ring on cyclization. The results of the syn-
thetic and computational studies are presented herein.
Results and Discussion
In an initial experiment, compound 1a was treated with
2.2 equivalents of CAN in MeOH producing 2-tetralone 2a in a
73% yield. To examine the breadth of this method, a series of
δ-aryl-β-dicarbonyl substrates was prepared by a previously
reported procedure [17]. As shown in Table 1, the intramolec-
ular cyclization of δ-aryl-β-diketones with unsubstituted aryl
rings (Table 1, entries 1 and 3) afforded 2-tetralone products in
2c
aReaction conditions: 1 equiv δ-aryl-β-dicarbonyl, 2.2 equiv CAN,
MeOH, rt, N2, 4 h. bProducts exist predominantly in the enol form by
1H NMR. cIsolated yield.
moderate to good yields. Additionally, cyclization of the oxidized. It is important to note that products 2g and 2j were
β-ketoester 1b proceeded efficiently, generating 2b in an 85% isolated as single isomers. Moreover, the isomers formed from
yield.
these reactions result from cyclization occurring at the more
hindered carbon atom of the δ-aryl ring. This observed site
Previous work by Rickards and co-workers on a related system selectivity is consistent with previous research by both
reported strong electronic effects when electron-donating MacMillan and Nicolaou on the α-arylation of aldehydes
substituents were incorporated onto the δ-aryl ring of the through organo-SOMO activation [18-21]. The preferential for-
starting material [10,11]. To probe the impact of electron mation of these isomers will be discussed below.
density of the δ-aryl ring on intramolecular cyclization, several
substrates with either electron-donating or electron-with- While the nucleophilicity of alkyl radicals is well-documented
drawing groups were synthesized and subjected to our reaction [22-24], the radicals generated from β-dicarbonyl compounds
conditions. The results of these experiments are summarized in have been shown to display more electrophilic character [25-
Table 2. As shown in Table 2, entry 1, dimethoxylated sub- 27]. As a consequence, these radicals should favor coupling
strate 1d oxidatively cyclized to the 2-tetralone derivative in a with more nucleophilic, electron-rich carbon centers. The obser-
76% yield. However, when only one methoxy group was incor- vation that intramolecular cyclization did not occur in either of
porated onto the δ-aryl ring, the expected 2-tetralone derivative the electron-deficient substrates (compounds 1h and 1i) is
was obtained only when the methoxy group was at the meta consistent with electrophilic radical intermediates.
position (Table 2, entry 4). For substrates 1e and 1f with the
methoxy group at either the ortho or para position, respectively, To obtain a better understanding of the impact of arene substitu-
methyl esters 2e and 2f (Table 2, entries 2 and 3) were the tion on the intramolecular cyclization, DFT calculations were
major products. Additionally, intramolecular cyclizations with performed at UB3LYP/6-31G(d) level by using Gaussian 03/09
electron-deficient δ-aryl rings (Table 2, entries 5 and 6) did not [28]. Previous research from our group has shown that the oxi-
occur, and oxidation of 1h and 1i instead favored the formation dation of the enol tautomer of a diketone initially forms a
of methyl esters 2h and 2i. Finally, the tricyclic product 2j was radical cation, which is deprotonated readily by MeOH to form
generated in an isolated yield of 61% when substrate 1j was the radical under the reaction conditions [15,16].
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Beilstein J. Org. Chem. 2013, 9, 1472–1479.
Table 2: Impact of ring substituents on the CAN-mediated oxidation of δ-aryl-β-dicarbonyl compounds in MeOHa.
Entry
Substrate
Productb
Yield (%)c
1
76
1d
2d
2e
2
3
–d
–d
1e
1f
2f
4
83
1g
2g
2h
5
6
–d
–d
1h
1i
2i
7
61
1j
2j
aReaction conditions: 1 equiv δ-aryl-β-dicarbonyl, 2.2 equiv CAN, MeOH, rt, N2, 4 h. bProducts exist predominantly in the enol form by 1H NMR.
cIsolated yield. dGC data indicated that the major products (50–80% conversion) were the methyl esters. Attempts were not made to isolate the
methyl esters.
To probe the origin of the effect of substitution on cyclization, All structures were fully optimized and identified as minima on
the key step of the reaction, namely the cyclization of the potential energy surfaces with frequency calculations. Tran-
β-dicarbonyl radical onto the aromatic ring, was investigated. sition structures were identified with one imaginary frequency.
Energy barriers for the transition structures of the ortho cycliza- Intrinsic reaction coordinate (IRC) calculations were performed
tion leading to their corresponding product cyclohexadienyl to connect the transition structures to their respective minima on
radicals were determined.
either side of the first-order saddle point. In some cases, the
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Beilstein J. Org. Chem. 2013, 9, 1472–1479.
lowest energy structures obtained from IRCs were further opti-
mized to obtain the minima.
Table 3: Energies (R. E. kcal/mol) of calculated structures. Energies
are relative to the open form of the radical.
R. E.a
R. E. + ZPVEb low frequencyc
For the aryl diketone radical, two conformers for the ketones
were considered, one with the carbonyl groups syn to each other
and the other with the carbonyl groups in the anti orientation.
For every substrate, two transition structures were identified on
the energy surface along the reaction coordinate. The first one is
a rotational transition structure for the rotation around the C–C
bond beta to the dicarbonyl to go from the minimum to a geom-
etry from which an addition to the aromatic ring is viable. The
second transition structure corresponds to the addition of the
radical intermediate to the aromatic ring.
1a'
0.0
2.3
0.2
15.0
7.7
0.0
2.4
0.4
11.0
4.2
0.0
2.3
0.3
13.1
7.3
0.0
2.4
0.2
15.8
9.1
0.0
2.3
0.2
16.2
8.8
0.0
2.4
0.2
11.8
2.4
0.0
2.2
0.3
13.4
4.9
0.0
21.4
TS1a'
2a'
2.5
46.7i
19.7
0.6
TS2a'
3a'
15.8
8.8
477.2i
35.4
1g'
0.0
13.30
41.8i
16.6
TS1g'
2g'
2.5
0.7
TS2g'
3g'
12.0
5.7
446.3i
37.5
1g''
0.0
13.3
Recently, Houk, MacMillan and co-workers showed that for the
organo-SOMO-catalyzed oxidative α-arylation of aldehydes,
the preference for the attack of the intermediate enamine radical
cation on the substituted aromatic ring leading to ortho/para
cyclization depends on the greater stabilization of the intermedi-
ate cyclohexadienyl radical [19]. The oxidative cyclization of
δ-aryl-β-dicarbonyl substrates should proceed through a similar
intermediate. As a result, the same rationale can be applied here.
While MacMillan and Houk performed detailed computational
studies to explain the site-selective radical cyclizations with
m-methoxylated rings, the selectivity of naphthyl substrates was
not investigated.
TS1g"
2g"
2.4
45.0i
13.5
0.6
TS2g"
3g"
13.9
8.5
437.5i
34.2
1f'
0.0
21.3
TS1f'
2f'
2.4
42.3i
15.9
0.5
TS2f'
3f'
16.4
10.0
0.0
473.0i
36.1
1h'
15.1
TS1h'
2h'
2.4
40.3i
19.1
0.4
TS2h'
3h'
16.9
10.2
0.0
489.8i
36.2
1j'
17.45
42.1i
16.5
The energy values for the cyclization of all substituents are
given in Table 3, and their corresponding energy diagrams are
given in Figure 1. For unsubstituted 1a’, the initial rotational
transition structure TS1a’ has a barrier of 2.5 kcal/mol, and the
corresponding radical intermediate 2a’ is 0.6 kcal/mol above
1a’. For the radical addition to the aromatic ring, the energy
barrier is 15.8 kcal/mol, and the product cyclohexadienyl
radical 3a’ is 8.8 kcal/mol higher in energy. The syn isomer of
1a’ is 4.5 kcal/mol higher in energy compared to the anti isomer
(see Table S2 in Supporting Information File 1). Only the ener-
gies of the anti isomers are discussed here.
TS1j'
2j'
2.5
0.5
TS2j'
3j'
12.6
4.0
445.9i
34.9
1j"
0.0
17.5
TS1j"
2j"
2.3
41.3i
11.5
0.6
TS2j"
3j"
14.1
6.2
463.8i
35.7
aUB3LYP/6-31G(d) geometry optimized. bFrom (a) with unscaled zero-
point vibrational energy (ZPVE) corrections. cLow or imaginary
frequencies (cm−1).
In the case of the electron-rich m-methoxy substituent 1g’, the
energy barrier for the cyclization is 12.0 kcal/mol, which is
approximately 4 kcal/mol lower than TS2a’. Due to the electro- m-methoxy substituent. The corresponding cyclohexadienyl
philic character of the diketo radical intermediate, electron- radical 3h’ is 10.2 kcal/mol higher in energy compared to the
rich systems are expected to favor cyclization. Similarly, parent β-dicarbonyl radical 1h’. The rotational transition struc-
the cyclized cyclohexadienyl radical 3g’ is approximately ture has an energy barrier of 2.4 kcal/mol, which is similar to
3.0 kcal/mol more stable compared to 3a’. The stability of the other radical intermediates. The higher barrier for cyclization is
cyclized radical is reflected in the activation barrier, and rota- consistent with synthetic data showing that no cyclized product
tional transition structure TS1g’ has a barrier of 2.5 kcal/mol. is obtained in this case. In the case of the naphthyl system (1j’),
For the electron-withdrawing chloro-substituted arene 1h’, the the energy barrier for the cyclization is 12.6 kcal/mol. The
energy barrier for the cyclization on the aromatic ring is cyclized product radical 3j’ is only 4.0 kcal/mol higher in
16.9 kcal/mol, nearly 5 kcal/mol higher compared to the energy when compared to the parent aryl diketone radical, and
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Beilstein J. Org. Chem. 2013, 9, 1472–1479.
Figure 1: Energy diagram for the unsubstituted arene with the carbonyl groups anti to each other. For TS1a’ the dihedral angle between atoms
abcd = 6.4°. *The energy values for 1a’, 1g’, 1h’ and 1j’ are shown. 1g”, 1f’ and 1j” were not included for clarity.
the rotational transition structure has a comparable barrier of
2.5 kcal/mol.
While it is evident why electron-rich systems should more
readily cyclize, the observed site selectivity in products 2g and
2j warranted further investigation. For these substrates, there
are two possible sites of ortho cyclization (Figure 2). For the
m-methoxy substrate 1g, the energy barrier for the cyclization
(TS2g’’), which would lead to product 2g”, is 1.9 kcal/mol
higher relative to the other ortho cyclization (TS2g’). This
finding is consistent with the synthetic observation that 3g’ is
the only product observed. A similar trend was observed for the
2-naphthyl substrate 1j. The barrier for TS2j”, which would
Figure 2: Possible products from the ortho cyclization of 1g and 1j.
form anthracene-derived product 2j”, is 1.5 kcal/mol higher
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Beilstein J. Org. Chem. 2013, 9, 1472–1479.
than TS2j’ and provides a rationale for the selective formation hexadienyl radical intermediate falls between the calculated
of the phenanthrene-derived product (2j).
values for the m-chloro and m-methoxy systems. For the
2-naphthyl system, delocalization of the cyclized radical inter-
Finally, in order to better understand how the position of the mediate results in a more stable product and hence a lower
methoxy group on the δ-aryl ring affects the formation of the barrier providing a pathway to formation of phenanthrene-
β-tetralone product, the energies of the intermediates for the derived 2j.
p-methoxy substrate 1f were calculated. The energy barrier for
the cyclization (TS2f’) is 16.4 kcal/mol, 4.4 kcal/mol higher In a previous study by our research group, the rates of oxi-
than the corresponding TS2g’. Furthermore, the product cyclo- dation of several β-diketones and their related silyl enol ethers
hexadienyl radical 3f’ is 10.0 kcal/mol less stable than the by CAN and the more lipophilic ceric tetra-n-butylammonium
parent 1f’, a value very close to the 10.2 kcal/mol for the elec- nitrate (CTAN) were measured in MeOH, MeCN and CH2Cl2
tron-poor m-chloro substrate 3h’. Taken together, these data by using stopped-flow spectrophotometry [16]. In these experi-
support the experimental observations that oxidative cycliza- ments, initial oxidation of substrates generated radical cation
tion occurs only for the methoxy substrate substituted at the intermediates. The rates of formation and subsequent decay of
meta position.
these radical cations were measured in all three solvents. The
results from these studies [15,16] provide two important
Overall the DFT calculations show that the origin of the reactiv- insights into the mechanism of the oxidation of δ-aryl-β-dicar-
ity as well as the selectivity in these reactions depends on the bonyl compounds in MeOH. First, MeOH is intimately involved
stability of the product cyclohexadienyl radical, which is in the decay of the initial radical cation through solvent-assisted
reflected in the activation barriers (TS2’s) for the cyclization, deprotonation. Second, intramolecular cyclization of 1a’ occurs
consistent with previous studies of MacMillan and Houk [19]. after the rate-limiting step of the reaction.
The m-methoxy substituent on the aromatic ring provides the
lowest barrier for cyclization, and the corresponding cyclized Based on the synthetic and computational data presented herein
cyclohexadienyl radical is more stable. Conversely, the elec- and findings from previous studies, the mechanism in Scheme 2
tron-withdrawing m-chloro substituent has the highest barrier is proposed for the oxidation of δ-aryl-β-dicarbonyl compounds
among the systems studied and leads to the least stable cyclo- in MeOH with CAN. Initial oxidation of the enol tautomer (4’)
hexadienyl radical. For the unsubstituted arene substrate, the by CAN produces protonated radical 5. Intermediate 5 is depro-
energy barrier to cyclization as well as the energy of the cyclo- tonated by MeOH to radical species 6. When the radical
Scheme 2: Proposed mechanism for the conversion of δ-aryl-β-dicarbonyl compounds to β-tetralones (path A) and methyl esters (path B).
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Beilstein J. Org. Chem. 2013, 9, 1472–1479.
contains a δ-aryl group with an appropriate substitution at the was added dropwise forming a clear yellow solution, which was
meta position, path A is followed. Intramolecular cyclization of stirred for an additional 10 min. The appropriate organohalide
7 occurs through radical addition to the aromatic ring forming (11 mmol) was dissolved in 2 mL of THF and rapidly injected
intermediate 8. As demonstrated by the oxidation of substrate into the reaction at 0 °C. The reaction mixture was warmed
1j, intramolecular cyclization of this radical occurs at the more gradually to room temperature over 30 min. The reaction was
electron-rich carbon atom of the δ-aryl rings. A second equiva- slowly quenched with an HCl solution (2 mL of concentrated
lent of CAN oxidizes 8 to cation 9. Rearomatization through HCl in 5 mL H2O). The organic layer was separated, and the
deprotonation of intermediate 9 yields the 2-tetralone deriva- aqueous layer was washed three times with ether. The organic
tive 10. Conversely, when the δ-aryl ring has electron-with- layers were combined, washed with brine, dried with MgSO4,
drawing substituents (Table 2, entries 5 and 6), the reaction filtered, and concentrated. The crude product was purified by
follows path B [16,29-31].
automated flash chromatography.
Conclusion
General procedure for the oxidation of δ-aryl-β-dicarbonyl
A protocol for the conversion of δ-aryl-β-tetralones using CAN compounds with CAN in MeOH. CAN (1.1 mmol) was
has been developed. The Ce(IV)-mediated synthesis of dissolved in 4 mL MeOH. This CAN solution was then added
2-tetralones has short reaction times and affords the desired dropwise in 1 min to the δ-aryl-β-dicarbonyl compound (0.5
products in moderate to very good yields under mild conditions. mmol), which was dissolved in 15 mL of MeOH. The reaction
While 2-tetralones were not generated for all substrates, cycliza- was stirred for 30 min. The solvent was then removed
tion does occur for the unsubstituted arene 1a suggesting that by rotary evaporation. Ice-cold H2O (15 mL) was poured
the electrophilicity of the radical provides some driving force into the reaction and extracted three times with ether. The
for the cyclization. The DFT computational studies indicated organic layers were combined, dried with MgSO4, filtered, and
that the formation of 2-tetralones from the cyclization of δ-aryl- concentrated. The crude product was purified by automated
β-dicarbonyl radicals is dependent on the stability of the prod- flash chromatography.
uct cyclohexadienyl radicals.
Supporting Information
Experimental
General methods and materials. Methanol (MeOH) was
Supporting Information File 1
degassed with argon and dried with activated 3 Å molecular
sieves prior to use. THF was purified with a Pure Solv solvent
purification system from Innovative Technology Inc. CAN was
purchased commercially and used without further purification.
The glassware was flame dried prior to use. Unless otherwise
stated, reactions were performed under an inert atmosphere of
nitrogen. Products were separated by using prepacked silica gel
columns with a gradient elution of ether/hexanes in an auto-
mated CombiFlash® Rf system from Teledyne Isco, Inc. All
new compounds were characterized by 1H NMR, 13C NMR,
GC–MS, IR, and LC–HRMS. Known compounds were charac-
terized by 1H NMR, 13C NMR and GC–MS. 1H NMR and
13C NMR spectra were recorded on a Bruker 500 MHz spec-
trometer. Mass spectra were obtained by using a HP 5890 series
GC–MS instrument. A Satellite FTIR from Thermo-Mattson
was used to obtain IR spectra. LC–HRMS data were recorded at
the Mass Spectrometry Facility at Notre Dame University.
Characterization data for all compounds, copies of 1H and
13C NMR spectra of final products, computational details,
absolute energies, and Cartesian coordinates of all
optimized structures.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-167-S1.pdf]
Acknowledgements
R.A.F. is grateful to the National Institutes of Health
(1R15GM075960-03) for support of this work.
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