A simple Lewis acid induced reaction of phenols with electrophiles: Synthesis of functionalized 4H-chromenes and ortho-benzylphenols

Article abstract of DOI:10.1080/00397911.2019.1689268

Lewis acid ZnCl₂ promoted cyclization protocol to 4H-chromenes is accomplished, using readily available phenols and acetophenones as starting materials. Interestingly, the process is feasible under the solvent free environment. Synthesis of a variety of 4H-chromenes have been accomplished using this strategy. In addition, this concept is extended to the synthesis of ortho-benzylphenols by treating phenols either with styrenes or secondary benzylic alcohols.

Full text of DOI:10.1080/00397911.2019.1689268

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: 0039-7911 (Print) 1532-2432 (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
A simple Lewis acid induced reaction of phenols
with electrophiles: Synthesis of functionalized 4H-
chromenes and ortho-benzylphenols
Chinnabattigalla Sreenivasulu, Ditto Abraham Thadathil, Sumit Pal &
Satyanarayana Gedu
To cite this article: Chinnabattigalla Sreenivasulu, Ditto Abraham Thadathil, Sumit Pal &
Satyanarayana Gedu (2019): A simple Lewis acid induced reaction of phenols with electrophiles:
Synthesis of functionalized 4H-chromenes and ortho-benzylphenols, Synthetic Communications,
DOI: 10.1080/00397911.2019.1689268
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SYNTHETIC COMMUNICATIONSV
https://doi.org/10.1080/00397911.2019.1689268
A simple Lewis acid induced reaction of phenols with
electrophiles: Synthesis of functionalized 4H-chromenes
and ortho-benzylphenols
Chinnabattigalla Sreenivasulu, Ditto Abraham Thadathil, Sumit Pal, and
Satyanarayana Gedu
Department of Chemistry, Indian Institute of Technology Hyderabad, Kandi, India
ABSTRACT
ARTICLE HISTORY
Received 1 September 2019
Lewis acid ZnCl2 promoted cyclization protocol to 4H-chromenes is
accomplished, using readily available phenols and acetophenones as
starting materials. Interestingly, the process is feasible under the
solvent free environment. Synthesis of a variety of 4H-chromenes
have been accomplished using this strategy. In addition, this concept
is extended to the synthesis of ortho-benzylphenols by treating phe-
nols either with styrenes or secondary benzylic alcohols.
KEYWORDS
4H-chromenes; phenol;
ketones; chalcones;
Lewis acid
GRAPHICAL ABSTRACT
4H-Chromene is ubiquitous fused bi-cyclic core found in natural[1] as well as pharma-
ceutical products,[2] which exhibit interesting biological properties.[3] As a result, drawn
considerable attention from synthetic community toward their synthesis. Consequently,
new methods have been developed for the preparation of 4H-chromenes.[4] An interest-
ing approach has been developed by Liu and Xu, using a combination of two Lewis
acid catalysts (AuCl3/3AgOTf), from phenols and ketones.[5a] It is worth mentioning
that catalytic quantities of ZnCl2 was unsuccessful to drive the reaction. In this regard,
Wu et al. established the preparation of chromenes by using acetophenones and poly-
phenols with para-toluenesulfonic acid (p-TSA).[5b] Recently, Hui-Jing Li and coworkers
demonstrated a mild method to produce 4 H-chromenes using Brønsted acid
(H2SO4).[5c] Very recently, the research groups of Anil Kumar[5d] and Yan-Chao Wu[5e]
have demonstrated the synthesis of 4 H-chromenes using acid catalysts.
CONTACT Satyanarayana Gedu
gvsatya@iith.ac.in
Department of Chemistry, Indian Institute of Technology
Hyderabad, Sangareddy, Kandi, Telangana 502 285, India.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lsyc.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2019 Taylor & Francis Group, LLC

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C. SREENIVASULU ET AL.
It is noteworthy to mention that the synthesis of 2 H-chromenes[6] has been well
explored, while, 4 H-chromene’s synthesis is scarcely established. Though some interest-
ing methods have been presented, for the preparation of 4 H-chromene,[7–11] still there
is enough room to investigate efficient methods by merely making use of benchtop
chemicals, in particular, using simple and inexpensive agents/catalysts as promoters to
drive reactions. Based on our interests on developing one-pot strategies, specifically, by
employing Brønsted or Lewis acid catalysts[12] in recent times, accomplished the synthe-
sis of benzofurans mediated by simple Lewis acid (ZnCl2).[12a] Encouraged by these
interesting results, very recently, we have disclosed the formation of 2 H-/4H-chromenes
via cyclization of phenols and terminal-acetylenes, facilitated by ZnCl2.[12b] Herein, we
report ZnCl2 mediated synthesis of 4 H-chromenes through condensation reaction of
phenols with acetophenones. Further, ZnCl2 mediated reaction was applied to ortho-
benzylphenols by performing the reaction between phenols and styrenes or secondary
benzylic alcohols.
Results and discussion
For the screening study, phenol 1a and methyl phenyl ketone 2a were chosen as starting
materials and the observations are shown in Table 1. Firstly, phenol 1a (1 equiv.) was
Table 1. Screening conditions to give 4 H-chromene 3aa.a–g
Entry
Solvent (1 mL)
Lewis acid (equiv.)
Temp (ꢀC)
Time (h)
Yield 3aab
c
1
2
3
4
5
6
7
8
–
ZnCl2 (0.2)
ZnCl2 (0.5)
ZnCl2 (1)
ZnCl2 (2)
ZnCl2 (1)
100
100
100
100
100
100
100
100
100
100
100
50
24
48
24
24
48
48
24
24
24
24
24
24
24
24
24
24
44%
51%
91%
88%
42%d
c
–
c
–
c
–
c
–
c
e
f
–
–
–
Toluene
DCE
ZnCl2 (1)
ZnCl2 (1)
ZnCl2 (1)
ZnI2 (1)
ZnI2 (1)
83%
32%
f
9
DMF
–
c
10
11
12
13
14
15
16
–
74%
65%
40%
45%
38%
52%
Toluene
,,
,,
,,
,,
,,
FeCl3 (0.2)
FeCl3 (0.2)
TiCl4 (0.5)
SnCl2ꢁ2H2O (1.0)
AlCl3 (0.2)
100
80
100
80
g
–
aReactions were performed with phenol 1a (0.5 mmol, 1 equiv.), acetophenone 2a (1.5 mmol, 3.0 equiv.).
bIsolated yields of 4 H-chromene 3aa.
cReactions were performed without solvent.
dReaction was performed with acetophenone 2a (1.0 mmol, 2.0 equiv.).
eReaction was conducted without ZnCl2.
fNo progress of the reaction.
gStarting materials decomposed.
The bold values represents the best optimized conditions.

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reacted with acetophenone 2a (3 equiv.) using the Lewis acid ZnCl2 (0.2 equiv.), under
neat conditions at 100 ꢀC for 24 h. The expected 4 H-chromene 3aa was formed in 44%
yield (entry 1, Table 1). With the increasing amount of ZnCl2 (0.5 equiv.) at 100 ꢀC, for
longer reaction time (48 h), showed a slight improvement in yield (entry 2, Table 1).
Gratifyingly, the reaction by using 1 equiv. (0.5 mmol) of ZnCl2 for 24 h at 100 ꢀC,
afforded 3aa in 91% yield (entry 3, Table 1). Although, the reaction demands stoichio-
metric amounts of ZnCl2 (1 equiv.), still viable and equitable, as ZnCl2 is cost-effective
[$41.73/500 g, reagent grade (ꢂ98%)]. With further raising the amount of ZnCl2
(1 mmol, 2 equiv.), not much improvement was noted (entry 4, Table 1). While treating
the phenol 1a (1 equiv.) with 2 equiv. of acetophenone 2a and with 1 equiv. (0.5 mmol)
of ZnCl2, for 48 h, gave 3aa only in 42% yield (entry 5, Table 1). The transformation
without the Lewis acid ZnCl2, showed no progress, thus, reveals importance of ZnCl2 to
trigger the reaction (entry 6, Table 1). When toluene was employed as solvent, 83% of
3aa was formed (entry 7, Table 1). The reaction was inferior in DCE solvent (entry 8,
Table 1). Whereas the reaction showed no progress in DMF (entry 9, Table 1). Using
ZnI2 as the catalyst, under neat conditions as well as in toluene, afforded 3aa, in 74%
and 65% yields, respectively (entries 10 and 11, Table 1). In contrast, the reaction with
various catalysts, such as FeCl3, TiCl4, SnCl2ꢁH2O and AlCl3, were proved to be inferior
(entries 12–16, Table 1).
With the above standard conditions (entry 3, Table 1), for generating 4 H-chro-
mene 3aa, and to examine the feasibility of the process, different phenols 1 and ace-
tophenones 2 have been subjected to the reaction. Gratifyingly, the process was
amenable and delivered 4 H-chromenes 3aa–he, as depicted in Table 2. For example,
apart from simple phenol, the reaction was also smooth between meta-/para-cresols/
para-tertiarybutylphenol 1b/1c/1d and various acetophenonoes. In addition, the reac-
tion with 3,4-dimethylphenol 1e was amenable with acetophenones 2a and 2b, and
afforded the corresponding 4 H-chromenes 3ea and 3eb. Further, the reaction was
feasible with electron rich OMe bearing phenol 1f with para-chloroacetophenone 2d.
Notably, the method was found to be smooth with a- as well as b-naphthols (1g and
1h) with acetophenones having different functional groups. Overall, this one-pot pro-
cess was quite successful with acetophenones possessing Me, OMe, Cl and Br func-
tional moieties. Notably, these halo-substituted products 3bd–he are of particular
interest, as they can be utilized as precursors for transition-metal catalyzed cou-
pling reactions.
In contrast, treatment of para-cresol 1c using electron deficient meta-nitroacetophe-
none 2f, under standard conditions, furnished the chalcone 4f, as an exclusive product
via self-aldol condensation reaction instead of yielding the expected 4 H-chromene
(Scheme 1a). This can be justified on the basis that aldol condensation is feasible either
under sufficient basic or under acidic conditions. Moreover, the strong electron with-
drawing nitro groups that are flanked on either aromatic rings of chalcone 4f,
would make it less reactive for further reacting with para-cresol 1c to give the final 4 H-
chromene product. To further confirm the formation of chalcones[13] under these acidic
conditions, the reaction was performed only with acetophenones 2b and 2d in absence
of phenol, under established conditions. As anticipated, the chalcones 4b (76%) and 4d
(72%) were isolated, as mentioned in Scheme 1b.

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C. SREENIVASULU ET AL.
Table 2. Scope of 4 H-chromenes 3 with phenols 1 and acetophenones 2.a,b
Reaction conditions: aPhenols 1 (0.5 mmol, 1 equiv.), ketones 2 (1.5 mmol, 3.0 equiv.) and ZnCl2 (0.5 mmol, 1 equiv.) at
100 ꢀC for 24 h, under neat conditions. bIsolated yields of products 3.
It was evident from the above Scheme 1 that the chalcones 4 would be the probable
intermediates in the formation of 4 H-chromenes 3. To further ascertain this hypothesis,
the chalcone 4d was treated the phenol 1g in toluene solvent (2 mL), under standard
reaction conditions (Scheme 2). To our delight, as expected, the 4 H-chromene 3gd was
isolated in 72% yield. From this evidence, it can be confirmed that the reaction might
be proceeding via the chalcone intermediate.
Moreover, when the reaction was carried out with para-fluorophenol 1i with para-
chloroacetophenone 2d, furnished an inseparable mixture of 4 H-chromene 3id and tri-
arylbenzene 5d (Scheme 3a). This can be attributed to the less nucleophilic nature of

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Scheme 1. Formation of chalcones 4 from acetophenones 2.
Scheme 2. Synthesis of 4 H-chromene 3gd from 4d and 1g.
para-fluorophenol 1i due to deactivating fluoro functionality on its aromatic ring. The
formation of triarylbenezene 5d[14] is further established by performing the reaction
separately with each of the acetophenones 2a-d without the external phenol
(Scheme 3b).
To further extend the concept of reaction of phenols with electrophiles, it was pre-
dicted that alkenes would serve as electrophiles to combine with phenols in the
presence of ZnCl2. Thus, phenols 1a–l were reacted with different styrenes 6a–b with
ZnCl2 under neat conditions for 24 h at 80 ꢀC. Delightfully, the protocol was found to
be amenable and furnished mainly ortho-benzylphenols[15] 7aa–lb, as detailed in
Table 3.
Moreover, in the similar context, it was presumed that benzylic alcohols could act as
electrophiles to react with phenols. Consequently, after several attempts, it was found
that the reaction was feasible between phenols 1 and secondary benzylic alcohols 8 in
solvent toluene at 120 ꢀC for 12 h and afforded ortho-benzylated phenols 9da, 9bd and
9jb (Table 4). Notably, ortho-bis-benzylation was also found amenable by emptying
more amount (3 equiv.) of secondary benzylic alcohols[16] 8a and 8b. The chemical
structure of 9la was further confirmed with the help of single crystal x-ray analysis
(Figure 1).

6
C. SREENIVASULU ET AL.
Scheme 3. Synthesis of 1,3,5-triaryl benzenes 5 from acetophenones 2.
Table 3. Synthesis of ortho-benzylphenols 7aa–lb from phenols 1a–l and styrenes 6a–b.a,b
a
Reaction conditions: Phenols 1 (0.5 mmol, 1 equiv.), styrene 6 (0.75 mmol, 1.5 equiv.) and ZnCl2 (0.5 mmol, 1 equiv.) at
80 ꢀC for 12 to 24 h, under neat conditions. bYields are for isolated products 7.

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Table 4. Synthesis of ortho-benzylphenols 9 from phenols 1 and alcohols 8.a,b,c
Reaction conditions: aPhenols 1 (0.5 mmol, 1 equiv.), alcohols 8 (0.75 mmol, 1.5 equiv.), ZnCl2 (0.5 mmol, 1 equiv.) and
c
toluene (2 mL) 120 ꢀC for 12 h. bIsolated yields of products 9. The reaction with 1.5 mmol (3 equiv.) of alcohols 8.
Figure 1. X-ray structure of compound 9la (CCDC 1945253).
Conclusion
We have demonstrated a practical protocol to access 4 H-chromenes using mild Lewis
acid induced reaction of phenols and acetophenones. Particularly, the reaction is feasible
under solvent free environment. In addition, it was demonstrated the synthesis of
ortho-benzylphenols from phenols and styrenes or secondary benzylic alcohols.
Experimental section
General
1
IR spectra were recorded on a Bruker Tensor 37 (FTIR) spectrophotometer. H NMR
spectra were recorded on Bruker Avance 400 (400 MHz) spectrometer at 295 K in
CDCl3; chemical shifts (d ppm) and coupling constants (Hz) are reported in standard

8
C. SREENIVASULU ET AL.
fashion with reference to either internal standard tetramethylsilane (TMS)
(dH ¼ 0.00 ppm) or CDCl3 (dH ¼ 7.26 ppm). 13C NMR spectra were recorded on Bruker
Avance 400 (100 MHz) spectrometer at RT in CDCl3; chemical shifts (d ppm) are
reported relative to CDCl3 [dC ¼ 77.00 ppm (central line of the triplet)]. In 13C NMR
data, the nature of carbons (C, CH, CH2 and CH3) was determined by recording the
DEPT-135 spectra, and is given in parentheses and noted as s ¼ singlet (for C),
1
d ¼ doublet (for CH), t ¼ triplet (for CH2) and q ¼ quartet (for CH3). In H-NMR data,
the following abbreviations were used throughout: s ¼ singlet, d ¼ doublet, t ¼ triplet,
q ¼ quartet, qui ¼ quintet, sept ¼ septet, dd ¼ doublet of doublet, m ¼ multiplet and br.
1
s ¼ broad singlet. The assignment of signals was confirmed by H, 13C CPD, and DEPT
spectra. High-resolution mass spectra (HR-MS) were recorded on an Agilent 6538 UHD
Q-TOF using electrospray ionization (ESI) mode. Melting points are recorded using
Tempo and Mettler FP1 melting point apparatus in capillary tubes and are uncorrected.
All small-scale dry reactions were carried out using Schlenk tubes under an inert atmos-
phere. Reactions were monitored by TLC on silica-gel using a combination of hexane
and ethyl acetate as mobile phase. Reactions were generally carried out under argon or
a nitrogen atmosphere. Solvents were distilled prior to use; petroleum ether with a boil-
ing range of 60 to 80 ꢀC was used. Dichloroethane (DCE) was dried over CaH2. Silica-
gel (60–120 mesh) was used for column chromatography (approximately 20 g per one
gram of crude material). The reactants phenols, Acetophenones and styrenes were
bought from local suppliers (Avra, Finar, SD Fine-Chem) and they have been used as
such. The reagent ZnCl2 was bought from Sigma-Aldrich and other Lewis acids (FeCl3,
TiCl4, SnCl2ꢁH2O and AlCl3) are purchased from the local suppliers. These sort of
experimental procedure have already been reported.[12a,b]
GP-1 (General procedure for preparation of chromenes 3)
In an oven dried Schlenk tube, were added phenol 1 (47–75 mg, 0.5 mmol) and ketone
2 (202–225 mg, 1.5 mmol) followed by ZnCl2 (68 mg, 0.5 mmol) under nitrogen atmos-
phere. The resultant reaction mixture was stirred at 100 ꢀC for 24 h. The reaction pro-
gress was monitored by using TLC until the reaction is completed. The reaction
mixture was cooled to room temperature and quenched with aqueous NH4Cl solution
and extracted with ethyl acetate (3 ꢃ 15 mL). The organic layers were washed with satu-
rated NaCl solution, dried (Na2SO4) and filtered. Evaporation of the solvent under
reduced pressure and purification of the crude material by silica-gel column chromatog-
raphy (petroleum ether/ethyl acetate) furnished the 4 H-chromene 3 (64–95%) as a vis-
cous liquid.
GP-2 (General procedure for preparation of ortho-benzylphenol 7 from styrene 6):
In an oven dried Schlenk tube, were added phenols 1 (54–75 mg, 0.5 mmol) and styr-
enes 6 (78–123 mg, 0.75 mmol) followed by ZnCl2 (68 mg, 0.5 mmol) under nitrogen
atmosphere. The resultant reaction mixture was stirred at 80 ꢀC for 24 h. The reaction
progress was monitored by using TLC until the reaction is completed. The reaction
mixture was cooled to room temperature and quenched with aqueous NH4Cl solution

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9
and extracted with ethyl acetate (3 ꢃ 15 mL). The organic layers were washed with satu-
rated NaCl solution, dried (Na2SO4) and filtered. Evaporation of the solvent under
reduced pressure and purification of the crude material by silica-gel column chromatog-
raphy (petroleum ether/ethyl acetate) furnished the ortho-benzylphenol 7 (58–76%) as a
pale yellow liquid.
Supporting Information: Full experimental detail, characterization data for unknown
compounds, 1H and 13C NMR spectra. This material can be found in the
“Supplementary Content” section of this article’s webpage.
Acknowledgment
We are very much thankful to the Council of Scientific and Industrial Research [(CSIR), No.
02(0262)/16/EMR-II], New Delhi, for financial support and C.B.S. thank UGC for the award of a
research fellowship.
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