PhSeBr mediated hydroxylative oxidative dearomatization of naphthols-an open air facile one-pot synthesis of ketols

Article abstract of DOI:10.1039/c6ra00036c

A new methodology for oxidative-dearomatization of planar phenols is described. An economic, viable one-pot metal free protocol for direct conversion of naphthols to α-ketols is reported. Naphthols were found to undergo facile unprecedented oxidative dearomatization with regioselective hydroxylation with phenyl selenyl bromide in open air conditions. Quaternary stereocenters were developed along with formation of sterically demanding α- and γ-ketols with high yields. Functional group tolerance like esters is revealed. A thorough study of the stereoelectronic demands of the unusual oxy-selenium reactive intermediate involved in dearomatization of 1- and 2-naphthols is carried out. 4-Hydroxy cyclohexadieneone and cyclohexadieneone aryl ethers were generated from dialkyl-phenols under similar reaction conditions providing direct evidence of the mechanical postulate. The first instance of the phenoxy-selenium interaction leading to facile dearomatization of arenes is highlighted in this manuscript.

Full text of DOI:10.1039/c6ra00036c

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PhSeBr mediated hydroxylative oxidative
dearomatization of naphthols – an open air facile
one-pot synthesis of ketols†
Cite this: RSC Adv., 2016, 6, 26886
Received 1st January 2016
Accepted 5th March 2016
a
a
a
b
Debayan Sarkar,* Manoj Kumar Ghosh, Nilendri Rout and Santanab Giri
DOI: 10.1039/c6ra00036c
www.rsc.org/advances
A
new methodology for oxidative-dearomatization of planar observed to make signicant progress in recent years. The
phenols is described. An economic, viable one-pot metal free hypervalent iodine compounds have been a keen point of
protocol for direct conversion of naphthols to a-ketols is reported. sufficient attention due to their chemoselective oxidising
Naphthols were found to undergo facile unprecedented oxidative properties and also being environmentally acceptable. There
dearomatization with regioselective hydroxylation with phenyl have been limited eye-catching reports on this reaction,
selenyl bromide in open air conditions. Quaternary stereocenters Krohn reported a zirconium catalysed TBHP mediated
6
were developed along with formation of sterically demanding a- and oxidation of naphthols to such ketols. Quideau and
g-ketols with high yields. Functional group tolerance like esters is coworkers came up with an asymmetric version of this
revealed. A thorough study of the stereoelectronic demands of the transformation employing in situ generated iodanes from
7
unusual oxy-selenium reactive intermediate involved in dearomati- chiral iodoarenes and m-CPBA as a co-catalyst. Several
zation of 1- and 2-naphthols is carried out. 4-Hydroxy cyclo- asymmetric approaches have been developed using chiral
8
hexadieneone and cyclohexadieneone aryl ethers were generated iodine complexes, interestingly metal carbonyls have also
9
from dialkyl-phenols under similar reaction conditions providing been extensively used in this transformation. Fewer
direct evidence of the mechanical postulate. The first instance of the attempts on organocatalytic transformations have also been
1
0
phenoxy–selenium interaction leading to facile dearomatization of fairly accomplished. A closer study on the natural product
arenes is highlighted in this manuscript.
reports, upholds an impression that only phenyl iodo-
benzene diacetate (PIDA) has been effectively utilized as an
effective dearomatization reagent in wide scale natural
Controlled generation of well-functionalized three-dimensional
structures from simple planar aromatics allows a fast access to
solve molecular intricacy at higher levels. Dearomatization of
abundant arenes is thus considered to be the shortest and most
11
product syntheses. Some catalytic processes are also
12
attempted with moderate results and fewer substrate scope.
Hypervalent iodine complexes are synthesised under
rigorous conditions in presence of drastic oxidising agents
like sulphuric acid and potassium bromate. Again, the
utmost signicance of these complexes in dearomatisation
reaction and drawbacks like not being generalizable, scal-
able, non-economic and creating metal-free environment
associated with all such processes has put us an urge for
exploring newer simplied pathways of this reaction.
Undoubtedly, the chemist community has no generalised &
judicious alternative to dearomatisation till now.
1
powerful approach towards this goal.
In the recent years, there has been continued interest in
accomplishing oxidative and alkylative dearomatization reac-
2
tions via metal–arene complexation, hypervalent iodine medi-
3
ated oxidation and reductive alkylation under Birch
4
conditions where a stoichiometric amount of metal reagent or
oxidant, in general is required. Transition-metal catalyzed
5
dearomatization of aromatic compounds has also been
Parallely, during one of our endeavours towards natural
products like lacinilene C(A), lacinilene D(B), and cyclosordurine
D(C) encompassing a a-ketol framework, it was surprisingly felt
a
Organic Synthesis and Molecular Engineering Laboratory, Department of Chemistry,
6
National Institute of Technology, Rourkela, Odisha, India-769008
b
that scanty direct protocols are available for transformation of
Theoretical Chemistry Laboratory, Department of Chemistry, National Institute of
13
alkyl-naphthones to a-ketols which certainly act as important
precursors in modern organic synthesis (Fig. 1).
Technology, Rourkela, Odisha, India-769008. E-mail: sarkard@nitrkl.ac.in; giris@
nitrkl.ac.in
†
Electronic supplementary information (ESI) available: Detailed experimental
An oxidative dearomatization protocol employing diphe-
nylseleninic anhydride acted as a key step in the total synthesis
1
13
procedures, spectral data for all compounds, including copies of H, C, NMR
spectra. See DOI: 10.1039/c6ra00036c
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(
b) Asymmetric induction by in situ generated chiral ben-
7
ziodoxolone reagent (previous work).
Fig. 1 Biologically active a-ketol natural products.
14
of lacinilene methyl ether (LCME). Facile generation of a-
ketols from planar alkyl napthols is a challenging task. Also
scanty protocols have been reported in literature for the direct
one-pot benzylic C–H activation of the a-H of a naphthoquinone
to corresponding hydroxy, delivering such ketols. Cyclic dike-
tone like 1,2-napthoquinones act as a versatile starting
templates for numerous organic transformations, which makes
this attempt an attractive one for the chemist community
(
c) Sir D. H. R. Barton's report – oxidation of phenols to o-
15
hydroxy-dienones with benzeneseleninic anhydride.
(Fig. 2).
An extensive literature survey fetched us with an impor-
1
5
tant report from Barton et al. where benzeneseleninic
anhydride has been successfully employed in transforming
1
4
phenol to ortho-hydroxy dieneones. Meyer's et al. has also
referred to this protocol in the synthesis of lacinilene methyl
ether. Sir D. H. R. Barton's report of benzene seleninic
anhydride being used as a useful protocol for regioselective
ortho-hydroxylative dearomatization of phenols, though the
yields were not satisfactory in most of the substrates. The
To our information, no other elegant protocol is reported in
report also conveys an important information on the mech- literature for such straightforward regioselective hydroxylation
anistic articulation where the author has pointed out that the of naphthols encompassing a dearomatization reaction.
oxidative dearomatization using benzeneseleninic anhydride
may proceed in a variety of ways which seem indistinct. One
of the plausible mechanisms involve a phenoxy–seleno
reactive intermediate (A). In all cases diphenyl diselenide was
(d) This work:
1
5
found to be a major byproduct. Selenium exhibiting both
electrophilic and nucleophilic tendencies and associated
inconsistencies in the mechanisms in oxidative dearomati-
zation with selenium reagents prescribed till now acted as an
addendum to study its application in these reactions. This
manuscript was a lucrative source which led us to look into
the selenium mediated dearomatization reaction with a new
ideology.
Herein, we report a viable, economic and high yielding
protocol of oxidation of alkyl-naphthols to benzylic a-ketols
employing phenyl selenium bromide. 1,2-Naphthoquinones are
(
a) Zr catalysed hydroxylative dearomatisation of naphthols
6
(
previous work).
Scheme 1
Fig. 2 Scanty Protocols.
Scheme 2
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synthesized from halogenated naphthols (Scheme 2). The Table 3 Optimization of reaction conditions
reaction is an open air transformation.
Sl no.
Solvent
Base
Yield (%)
In an initial attempt, the envisioned oxidative dearomatiza-
tion of 1-methylnapthalene-2-ol [S ] was investigated with
3
1
2
3
4
5
6
7
8
9
THF/MeOH
MeOH
THF/dioxane
THF/MeOH
DCM
DMF
THF
DMSO
Py
60
55
20
60
65
30
55
60
50
75
95
80
phenyl selenyl bromide and potassium carbonate as a base and
THF as a solvent in open air atmosphere, which delivered 1-
hydroxy-1-methylnaphthalen-2(1H)-one (1(c)) in fair yields only
aer execution of an aqueous workup (Scheme 1).
The reaction required 2 equivalents of PhSeBr for comple-
tion, however use of 1 equivalent did not pose any retarding
effect on the reaction rate but only retaining of the substrate
which claimed for a unimolecular rate of reaction w.r.t to
PhSeBr, however the addition of two equivalents resulted into
complete consumption of the substrate (Table 1).
NEt3
MeONa
KOH
K
2
CO
MeONa
Na CO
K2CO3
CO
Cs CO
CO
3
2
3
Et
2
O
K
2
3
1
1
1
1
0
1
2
3
THF
THF
THF
2
3
K
2
3
t
KOBut
Et
2
O
KOBu
72
In our case, though it was apprehended that the transformation 14
THF
DBU
40
happened to be aer addition of water, but to our surprise no 15
reaction occurred in THF : H O (10 : 1). A thorough study of
2
equivalents of water used in combination with THF as solvent is
given below, (Table 2) presence of one equivalent of water
completed the reaction whereas increase in amounts retarded the
reaction or no reaction at all.
H
H
2
O
O
K
2
CO
3
No reaction
No reaction
No reaction
No reaction
No reaction
16
17
18
19
2
Cs CO
2
3
THF : H
THF : H
MeOH : H
2
O (1 : 1)
K
2
CO
3
3
2
O (10 : 1)
Py
K
2
2
O
CO
This observation pointed towards a reactive intermediate
which was only stable in minimal amounts of water. Combi- reused and thus use of two equivalents initially can be
nation of different bases and solvents suggested K
THF to be the best combination (Table 3).
2
CO
3
and partially compensated.
Interestingly, 1-halo-2-naphthols (Br, Cl, I) (Table 4-S , S )
1
0
11
Exposure of a wide range of 2-alkyl-naphthols to similar delivered napthoquinones 1(j) in appreciable yields [Table 4].
reaction conditions delivered a-ketols in appreciable yields
Dearomatization of 1-naphthol derivatives have been rare
7
in every case with concomitant formation of PhSe–SePh as encounters to the best of our knowledge. Suitably substituted
a by-product which was again found to be a similar obser- a-naphthols also delivered the g-ketols in moderate to good
1
5
vation as reported by Barton et al. Functional group toler- yields respectively (S , S ). Thus the reported protocol unam-
1
3
14
8 9
ance with esters is depicted (S , S , Table 4). More electron biguously presents a successful strategy towards open air
donating alkyl groups delivered better yields providing dearomatization of substituted planar 1-naphthol rings into g-
a sensible evidence of more stable oxy-seleno reactive inter- ketols making them viable for further signicant trans-
mediate being involved in this reaction. The diphenyl-di- formations at ease.
15
selinide can be easily converted to PhSeBr employing
Barton et al. reported an involvement of a reactive inter-
a simple bromination in THF and thus the reagent can be mediate (A) during the benzeneseleninic anhydride oxidations
for effective ortho-hydroxylation in the phenolate anions. In our
case, no chances of seleno-oxidation was prevalent in the
employed reaction conditions which might deliver the ortho-
Table 1 Equivalents of PhSeBr required for substrate completion
hydroxylated products. Thus, the intermediate as proposed by
PhSeBr (no. of
Substrate
Barton et al. seems implausible in our case, rather we do
propose an oxy-seleno naphthol reactive intermediate (B) being
operative with the alkyl napthaloate anion and which exhibits
a persistence deep red coloration during the reaction condition
and decolourises slowly or immediately aer addition of water,
breaking the oxy-selenium bond. As an additional evidence, we
Sl no.
equivalents used)
consumed (in%)
1
2
3
4
5
1
50
60
75
80
100
1.25
1.5
1.75
2
Table 2 Amount of water required for substrate completion
Amount of alkyl
naphthol (mmol)
Amount of
THF solvent (ml)
Water added
(mmol)
Time required for
completion
Percentage
yield
Sl no.
1
2
3
4
5
6
0.12
0.12
0.12
0.12
0.12
0.12
3
3
3
3
3
3
0.2
0.4
0.6
0.8
1.0
1.2
8 h
8 h
10 h
18 h
No reaction
No reaction
85%
75%
65%
60%
—
—
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Table 4 Dearomatization on substituted naphthols
Table 4 (Contd.)
Yields Entry
Substrates
Entry
Substrates
Products
Products
Yields
80%
(i)
97%
(viii)
(ii)
87%
(ix)
60%
(iii)
91%
(x)
50%
(
(
(
iv)
98%
80%
67%
(
xi)
77%
v)
(xii)
65%
vi)
have taken a UV-data of the colored intermediate which exhibits
a hypsochromic shi as expected due to losing of partial
aromaticity, attesting our postulate (Fig. 3).
Though C–Se bond formation is a favorable situation with
comparable electronegativity of (2.5 and 2.55) for C & Se
respectively, but nucleophilic substitution at quaternary
benzylic position in intermediate (C) delivering the ketol seems
insensible.
Exposing the reaction mixtures to H, C and Se NMR
clearly demonstrated the presence of the oxy-seleno interme-
diate. Running crude NMR experiments with the reaction
mixtures clearly depicted a C]O at 206 ppm, C–O at 145 ppm
(vii)
73%
1
13
77
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Fig. 3 Ultra-violet spectra of the reaction mixture [1-methylnaph-
thalen-2-ol, PhSeBr & K CO (as base)].
2 3
1
3
and C–OH at 68 ppm in the C NMR spectrum which
concluded for the presence of both ketol & naphthols in the
7
7
reaction mixture. The clear notations of 1116 ppm in Se NMR
also conrmed the presence of the designated intermediate, the
standards being d ¼ 425 and 870 for PhSe–SePh and PhSeBr
25
respectively. (Fig. 4) it is clearly noticeable that the newly
generated olenic and aromatic signals of the reactive inter-
mediate B [Fig. 4(d)] start disappearing with emergence of new
olenic signals of the ketol on addition of catalytic amounts of
water [Fig. 4(c)].
We conclude that a nucleophilic attack at the aromatic
carbon by a water molecule with the concomitant migration of
the double bond in the reactive intermediate (B) delivered the
ketols as the major product (Scheme 3).
The leaving phenyl selenide anion reacts with another
molecule of the oxy-seleno-napthoxy intermediate to deliver
diphenyldiselenide thus regenerating the naphthol. Speci-
cally, one equivalent of PhSeBr is required for complete gener-
ation of the phenoxy–seleno intermediate.
1
Fig. 4 (a) H NMR of reaction mixture (1-methylnaphthalen-2-ol,
PhSeBr) with K CO after addition of water (1 equiv.) (b) corresponding
C NMR (c) H NMR of reaction mixture after addition of catalytic
IR spectra of the crude reaction mixture shows a notable
decrease in intensity of the naphthol O–H stretching frequen-
2
3
13
1
ꢀ
1
cies at 3000 cm which clearly puts forward an additional amount of water & methanol, olefinic peaks of the ketol start
1
evidence towards the formation of the emphasized oxy-seleno appearing (d) H NMR of reaction mixture before addition of water (e)
13
77
corresponding C NMR (f) Se NMR of the reaction mixture before
reactive intermediate. The reaction was repeated with 1-ethyl-
-naphthol, and in both cases ended up with the ketols. Bar-
1
addition of water (g) H NMR of 1-methyl-2-naphthol (h) corre-
2
13
sponding C NMR.
15
ton et al. reported equivalent chances of benzeneseleninic
anhydride acting as electrophilic oxidant rather than only
formation of the reactive intermediate (A), which conrms the
above prediction and provides a signicant evidence of the
possible involvement of the intermediate (B) in the dearomati-
zation reaction with similar selenium reagents. Replacement of
PhSeBr with PhSeCl exhibited notable reduced yields and
reaction rates.
ethyl-2-bromo-3-(2-hydroxynaphthalen-1-yl)acrylate [as per NOE
studies] further proving the oxidative dearomatization mechanism
being involved in the reaction. Thus, this strategy also puts forward
an easy approach for efficient vinylic bromination which can be
utilised in broader directions (Scheme 4).
Further affirmation of the mechanistic postulate turned up
when 2,6-dialkylphenol were found to deliver cyclohexadieneone
aryl and bi-aryl ether under same reaction conditions. The
formation of this ethers provided a clear evidence in support of
the prevalence of the phenoxy–seleno intermediate (F).
Notable changes were observed, when 2,4,6-trimethyl-phenol
Reaction of unsubstituted naphthols delivered bromo-
naphthols as a prime product which is expected to be
a kinetic outcome where bromide ion acts a nucleophile.
Attempting a more insight to the reaction, (E)-ethyl 3-(2-
hydroxynaphthalen-1-yl)acrylate (S ) was treated in the same
1
5
reaction conditions. Direct exposure to column chromatog-
raphy without any aqueous work delivered stereoselective (E)-
1
3
was exposed to similar experimental conditions, C NMR of
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Scheme 3
1
Fig. 5 In situ NMR studies with 2,4,6-trimethyl phenol (S17) (a) H NMR
1
3
of reaction mixture before addition of water (b) corresponding
C
1
13
NMR (c) H NMR of 2,4,6-trimethyl phenol (d) corresponding C NMR
[full spectra are available in the ESI†].
Scheme 4
crude reaction mixture cleanly indicate the presence of two
aromatic phenoxy C-13 signal at 149.88 and 147.20 pointing out
towards the phenol and phenoxy–selenium intermediate [G]
respectively. The growing signal at 147.20 ppm exhibited
a notable increase with time and completely vanished on
addition of water delivering the 4-hydroxy-2,4,6-trimethyl
cyclohexenedieneone as the sole product (Fig. 5).
In this case the biaryl ether formation is restricted due to an
existent steric crowding at the para position. Diphenyl dis-
elenide was the prime by-product in all observations
(Scheme 5).
In order to see which intermediate was exhibited enhanced
susceptibility towards nucleophilic addition of H
2
O to form the
desired product, we have optimized both the intermediates “B”
26
and “C” using B3LYP/6-311+G(d,p) level of theory using G09w
27
program (Fig. 5) depicts the optimized geometries of reactive
intermediate “B” and (Fig. 6) depicts the optimization of reac-
tive intermediate C. Frequency calculation is also performed in
the same level and basis set to make sure that these structures
are on the minima of the potential energy surface. We have
found that both the intermediates are in ground state due to the
Scheme 5
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absence of any negative frequency in the vibration level. Further
energy calculation have enabled us to conclude that reactive
ꢀ1
intermediate B is 3.2 kcal mol less stable than C. We have also
calculated the hardness which is a conceptual density func-
28
tional theory (CDFT) based global reactivity descriptor of
reactive intermediate B and C from the frontier molecular
orbitals to conrm the stability of both the intermediate. The
hardness value also reveals that reactive intermediate B is less
stable than C. In other sense we can say that intermediate B is
more reactive than intermediate C.
To investigate the reactive sites of the intermediates in
detail we have calculated another CDFT based reactivity
2
9
descriptor called dual descriptor. From this descriptor we
can easily identify the electrophilic and nucleophilic regions
inside the molecule at the same time. Fig. 5 tells us about the
reactive sites of reactive intermediate B. In this gure red
colored region indicates the electrophilic regions whereas blue
corresponds nucleophilic regions. If we look at the dual
descriptor of reactive intermediate B we have found that the
quarternary C-center (attached to Me) belongs to electrophilic
2
region which suggest that nucleophilic attack of H O is
possible in this center to form the desired product. But in the
case of isomer C, we could not nd any nucleophilic/
electrophilic region on the same carbon center. This obser-
vation clearly demonstrates that intermediate B is more
susceptible for nucleophilic addition of H O as compared to
2
Fig. 6 Optimized geometry of intermediate B with electrophilic and intermediate C. These ndings nicely correlate with the
nucleophilic regions (red: electrophilic; blue: nucleophilic).
experimental ndings (Fig. 7).
Conclusions
There have been very limited reports in the literature available
for these transformations. Either transition metal or hyper-
valent iodine along with peroxyacids have been used in wider
scales. In case of benzeneseleninic anhydride oxidations, the
yields are unsatisfactory along with indistinct mechanisms
being postulated. In this report, though two equivalents of
PhSeBr is consumed for completion of the reaction, the overall
importance of this reaction and the distinct presence of an
phenoxy–seleno intermediate in the reaction medium encom-
passing an oxidative dearomatization of naphthol core creates
an uniqueness of this unprecedented synthetic methodology. In
summary, an easy, viable, economic metal-free protocol for one-
pot synthesis of a-ketols from alkyl substituted naphthols is
achieved. Halogenated 2-naphthols delivered naphthoquinones
whereas the alkyl-1-naphthols delivered g-ketols in good yields.
The understanding of selenium mediated dearomatizations has
been put up to an advanced step which had remained dormant
for the last two decades. A fair functional group tolerance is also
exhibited. No inert atmosphere is required for this reaction, as
it can be carried out in open sample vials. Thus the strategy
outweighs the present available strategies with respect to its
easy availability, reaction conditions and economy. The meth-
odology also provides a facile access to quaternary benzylic
stereocentres with the formation of ketols which can be facile
entry points to solve three dimensional molecular complexities.
Fig. 7 Optimized geometry of intermediate C with electrophilic and
nucleophilic regions (red: electrophilic; blue: nucleophilic).
The NMR studies of crude reaction mixtures unambiguously
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prove the presence of a phenoxy–seleno intermediate (B). The
formation of cyclohexadienyl aryl ethers and hydroxyl-
cyclohexadieneone from 2,6-dialkyl phenol and 2,4,6-trialkyl
phenol further attest to this manifestation. Theoretical inter-
pretation also correlate well with the experimental ndings. The
developed protocol can be one of the most facile accessible
entry points to three dimensional molecular complexities from
abundant and cheap planar naphthols for developing sterically
demanding benzylic alcohols. The asymmetric variant of this
methodology would be reported soon.
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Acknowledgements
10, 2381–2384; (e) R. B. Bedford, C. P. Butts, M. F. Haddow,
We sincerely acknowledge Department of Science and Tech-
nology (DST), Govt. of India (Grant no. SB/FT/CS-076/2012) and
DST-INSPIRE Faculty Award (Grant No. 04/2013/000751). MKG
thanks to NIT Rourkela for research fellowship. NR thanks to
DST fellowship. SG thanks Department of Science and Tech-
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