B(C6F5)3-Catalyzed Sequential Additions of Terminal Alkynes to para-Substituted Phenols: Selective Construction of Congested Phenol-Substituted Quaternary Carbons

Article abstract of DOI:10.1021/acs.orglett.1c01863

We have developed a borane-catalyzed sequential addition of terminal alkynes to para-substituted phenols, which affords a wide range of ortho-propargylic alkylated phenols bearing congested quaternary carbons. Control experiments combined with DFT calcula

Full text of DOI:10.1021/acs.orglett.1c01863

pubs.acs.org/OrgLett
Letter
B(C6F5)3‑Catalyzed Sequential Additions of Terminal Alkynes to
para-Substituted Phenols: Selective Construction of Congested
Phenol-Substituted Quaternary Carbons
Hui Chen, Mo Yang, Guoqiang Wang,* Liuzhou Gao, Zhigang Ni, Jingxiang Zou, and Shuhua Li*
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ABSTRACT: We have developed a borane-catalyzed sequential
addition of terminal alkynes to para-substituted phenols, which
affords a wide range of ortho-propargylic alkylated phenols bearing
congested quaternary carbons. Control experiments combined with
DFT calculations suggest that the reaction undergoes a sequential
phenol alkenylation/hydroalkynylation process. Further extension
of this strategy to the construction of triaryl-substituted quaternary
carbons demonstrates the broad utility of this method.
Scheme 1. Propargylic Alkylation of Phenols with Alkynes
he phenol motif is particularly prevalent in bioactive
T
molecules, pharmaceuticals, and functional materials.1
Direct functionalization of phenols or phenol ethers is highly
attractive because of their broad availability from industrialized
Hock process2 or depolymerization of lignin.3,4 Recently,
transition-metal catalysis offers an efficient approach for the
regio- and chemo-selective modification of phenols through
directed C−H bond functionalization strategy.5 Alternatively,
one can utilize the classical electrophilic aromatic substitution
strategy (EArS) to prepare substituted phenols.6 In this context,
acid-catalyzed Friedel−Crafts reaction,7,8 is one of the most
classical strategies to afford alkylated phenols. However, the
alkylation of phenolics with alkynes has been less explored.
Although the reaction of phenols with propargylic alcohols or
their congeners can give propargylic alkylated phenols, an
additional synthetic operation is required to obtain these
propargylic reactants (Scheme 1a).9 Given that the alkyne is
one of the most accessible chemicals in petrochemicals,10 the
development of a new method for direct alkylation of phenol
with alkyne is highly desired. However, direct alkylation of
phenol with alkyne may face the following challenges: (1)
phenol may undergo C- or O-alkenylation reactions with
alkyne;11,12 (2) the ortho-alkenylated product can further
undergo cascade cyclization reaction;13 (3) a mixture of mono-
and multisubstituted products may be formed.14
alkynes..17 Here, we report an OH-assisted propargylation of
phenols via sequential additions of aryl terminal alkynes to
phenols using B(C6F5)3 as catalyst (Scheme 1b). The reaction
features step- and atom-economy and enables one to directly
Inspired by our research on borane-catalyzed hydroarylation
of 1,3-diene with phenols,15 we envisioned that the association
of the OH group of phenol with B(C6F5)3 would probably lead
to the reaction of phenol and alkyne affording propargylic
alkylated phenols.16 In addition to the potential competing
pathways discussed above, another challenge is to suppress the
carboboration reaction between B(C6F5)3 and terminal
Received: June 7, 2021
Published: July 7, 2021
https://doi.org/10.1021/acs.orglett.1c01863
© 2021 American Chemical Society
Org. Lett. 2021, 23, 5533−5538
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introduce the sterically congested quaternary carbons to para-
substituted phenols with the reactive OH group untouched.
Control experiments and density functional theory (DFT)
calculations show that the reaction proceeds through a phenol
alkenylation/hydroalkynylation sequence with the assistance of
the phenolic OH group. Moreover, the prominent kinetic
differentiation between the alkenylation and hydroalkynylation
processes enables us to construct unique triaryl-substituted
quaternary carbons.
We began investigating the reaction of 4-methoxyphenol 1a
and phenylacetylene 2a in CH2Cl2 using 10 mol % of B(C6F5)3
as the catalyst (see Tables S1 for optimization details).
Treatment of 1a with 2a at room temperature could only
afford 4aa in poor yields along with 3aa as a major product. It
is probably due to the deterioration of B(C6F5)3 via the
carboboration reaction17 with terminal alkyne or other
unknown processes (see Figures S1−S2 for NMR analysis).
We were delighted to find that lowering the reaction
temperature to −20 °C could increase the yield of 4aa to
85% isolated yield. Other Lewis acids, including BCl3, BBr3,
(C6F5)CH2CH2B(C6F5)2, FeCl3, and ZnCl2, afford little or no
desired products 4aa or related alkenylation product 3aa
(Table S1 and S2), probably due to the generation of other
unidentifiable byproducts. Although the reaction was specu-
lated to involve the protonation of alkyne with B(C6F5)3-
phenol adduct, no desired product was detected in the
presence of Brønsted acids.
lation mechanism,18 and the existence of the OH group at the
ortho-position of phenol is important for the hydroalkynylation
step. Furthermore, deuterium labeling experiments confirmed
the involvement of the protonation of alkyne in the
alkenylation step and the deprotonation of terminal alkyne in
the hydroalkynyaltion step, respectively (see Scheme 2c and
Scheme S1 in the SI).
To elucidate the mechanistic details of this reaction, DFT
calculations at the M06-2X/cc-pVTZ//M06-2X/6-311G(d,p)
level19 were performed on the model reaction of 4-
methoxyphenol 1a and phenylacetylene 2a with B(C6F5)3 as
the catalyst (see SI for computational details, and other
kinetically less favorable pathways). As shown in Figure 1,
proton transfer from the OH group of INT1 to alkyne 2a
forms a tight ion-pair INT2 with a barrier of 16.6 kcal mol−1
(via TS1). INT2 consists of a highly electrophilic vinyl cation
and a borate-phenol anion as the counteranion, which could
readily undergo electrophilic addition reaction to afford the
Wheland intermediate INT3 (via TS2, ΔG‡ = 13.9 kcal
mol−1). Subsequent rearomatization of INT3 followed by the
dissociation of B(C6F5)3 affords the alkenylated phenol 3aa
and regenerates the catalyst.
In competing with catalyst regeneration, 3aa-B(C6F5)3
complex INT5 could also undergo intramolecular protonation
reaction (via TS4, ΔG‡ = 12.7 kcal mol−1) to generate a
borane-stabilized tertiary carbenium ion INT6 (Figure 1,
right). It then undergoes electrophilic addition reaction with
another molecule 2a to form a zwitterionic intermediate INT7
(via TS5). This step has a barrier of 21.8 kcal mol−1, and the
formation of INT7 is endergonic by 10.7 kcal mol−1 relative to
2a and INT6. Then, INT7 is deprotonated to provide the
neutral complex INT8 with a barrier of 12.2 kcal mol−1.
Finally, Lewis acid−base dissociation of INT8 regenerates
B(C6F5)3 and releases the ortho-propargylation product 4aa,
which is the major product obtained experimentally. Along the
whole reaction process, the rate-determining step is the
hydroalkynylation step (3aa → 4aa) with a barrier of 21.8
kcal mol−1 (via TS5). The alkenylation step is kinetically
favored over the hydroalkynylation step by 5.2 kcal mol−1, but
the former is thermodynamically less favored than the latter.
These computational results can account for the experimental
observation that the ortho-propargyl phenols are accessible
through the reaction of phenol with a terminal alkyne under
mild conditions. Besides, the proton-initiated mechanism20 or
the direct addition of phenol toward B(C6F5)3/alkyne adduct17
could also be excluded because of the involvement of high-
energy transition states (see Figures S8−S9 in the SI for
details).
To shed light on the mechanism, we subjected 2-vinylphenol
3aa to the standard reaction conditions in the presence of 4-
chlorophenylacetylene 2b, the corresponding phenol 4aab
could be obtained in 67% yield (Scheme 2a). However, the
reaction of meta-alkenyl phenol 5 with phenylacetylene 2a did
not give the related hydroalkynylation product with 76%
recovery of the starting material 5 (Scheme 2b). These results
indicate the propargylation reaction of phenol may proceed
through a sequential phenol-alkenylation/alkene-hydroalkyny-
Scheme 2. Control Experiments
Then, the scope of phenols was examined with phenyl-
acetylene 2a (Scheme 3). Para-substituted phenols bearing
electron-donating substituents, including alkoxyl, phenoxyl,
and alkyl, could undergo ortho-propargylation reactions to give
the related phenols in good to excellent yields (4ba−4ga,
61%−88%). Para-halogenated phenols are also applicable to
the method, affording the corresponding ortho-propargylation
products in moderate to excellent yields (4ha−4ka) in the
presence of 15 mol % of B(C6F5)3 at −40 °C. It should be
noted that the compatibility of synthetically valuable C−Br and
C−I bonds provide opportunities for further synthetic
manipulations. Trisubstituted phenol 1l could also be used
as the coupling partners to afford the desired product 4la in
moderate yield. However, the current strategy is not applicable
to phenols containing groups with a strong coordinating ability
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Figure 1. Catalytic cycle of the B(C6F5)3 catalyzed propargylation of 4-methoxyphenol 1a with 2a via a sequential addition mechanism. Computed
Gibbs free reaction energies and barriers (labeled with an asterisk) in kcal mol−1 are colored red.
a
not suitable for ortho- or meta- substituted phenols. Pleasingly,
when we subjected 2-vinylphenols 3m and 3n to the standard
conditions, the corresponding phenols 4ma and 4na could be
obtained in 79% and 55% yield respectively (Scheme 3,
bottom). This strategy provides a complementary way to
synthesize the products that cannot be achieved via the direct
reaction of phenol and terminal alkynes.
Scheme 3. Scope of Phenols
Subsequently, the substituent effect of alkynes was explored
(Scheme 4). The position of substituents on arylacetylene has
a significant influence on this reaction. For example, aryl
acetylenes bearing neutral, electron-rich, and weak electron-
withdrawing substituents at the para-position reacted effec-
tively to produce the desired product in moderate to good
yields (4ab−4ah). Meta-substituted arylacetylenes could also
smoothly react with 1a to afford ortho-propargylic alkylated
phenols (4ai−4al), although 20 mol % of B(C6F5)3 is required
for m-F or m-Cl substituted arylacetylene. However, the
reaction of ortho-substituted arylacetylene 2m with 1a stopped
at the initial alkenylation step, affording 2-vinylphenol 3am in
excellent yield. It is perhaps because the substituents at ortho-
position increase the steric crowding at the reaction center.
Gratifyingly, the trimethylsilyl (TMS) group which is known to
be labile under acidic conditions, is also compatible with our
method. The corresponding arylacetylene (2n) could be
converted to the ortho-propargylation product with good
efficiency (4an, 76% yield). Finally, the reaction of ortho-
alkenyl phenol 3aa and ortho-methyl phenylacetylene 2m was
conducted to explore the influence of the steric effect (Scheme
4, bottom). Alkene hydroalkynylated product 4aam could be
obtained in 74% yield, which suggests that the introduction of
methyl on the ortho-position of alkyne did not affect the
hydroalkynylation step.
a
b
c
Isolated yields. 15 mol % B(C6F5)3. −40 °C with 15 mol %
d
B(C6F5)3. a complicated mixture.
(e.g., carbonyl, nitro, nitrile, and amide, see Scheme S2 for
unsuccessful substrates). In addition, the reaction of phenol
1m could not produce the related C−H functionalized product
4ma, only resulting in a messy mixture. The procedure is also
The practicality of this method could be demonstrated by
performing a gram-scale synthesis of 4aa (Scheme 4). With 7.5
mol % B(C6F5)3, the desired product 4aa could be obtained in
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a
Scheme 4. Scope of Terminal Alkynes
Scheme 5. Scope Extension: B(C6F5)3-Catalyzed One-Pot
Construction of Triaryl Substituted Quaternary Carbons
a
b
c
Isolated yields. 15 mol % B(C6F5)3. 20 mol % B(C6F5)3.
involving borane-assisted sequential inter- and intramolecular
protonation reaction is responsible for the formation of the
desired products. Further extension of this strategy to the
construction of 1,1,1-triaryl ethane also supports the proposed
mechanism.
good yield (952 mg, 72% yield). Both the phenol and alkyne
fragments in 4aa are amendable to afford heterocycles or other
phenol analogues, potentially relevant to medicinal applica-
tions (see the Supporting Information for these transformation
details and related discussion).
ASSOCIATED CONTENT
* Supporting Information
Furthermore, the current strategy is also applicable to the
construction of triaryl-substituted quaternary carbons, which
are significant motifs in biologically active compounds.21
Under slightly modified reaction conditions, 1,1,1-triaryl
ethanes could be obtained in good yields by the direct
addition of electron-rich heterocycles into a 2:1 reaction
mixture of 1a and 2a. As shown in Scheme 5, the reaction
proceeds well for both N-protected and unprotected indoles to
afford the corresponding products in good efficiency (9aa−
9ag). Alternatively, tricyclic indole derivative lilolidine 7h
formed 1,1,1-triaryl ethane 9ah in 77% yield. More challenging
furan-derived substrates formed products 10aa−10ac in 51−
65% yields. Attempts to extend this process to other
heterocycles such as benzofuran, pyrrole, and thiophene were
unsuccessful, probably due to the low nucleophilicity of these
substrates. This procedure not only provides a facile protocol
to congested triaryl quaternary carbons but also supports the
intermediacy of tertiary carbocation via the intermolecular
protonation of the in situ formed 2-vinylphenol.
In summary, we have developed a metal-free, step- and
atom-economic method for the propargylation of para-
substituted phenols with arylacetylenes using B(C6F5)3 as the
catalyst. The new reaction involves a phenol alkenylation/
hydroalkynylation sequence, eventually affording ortho-prop-
argyl phenols rather than alkenylated phenols that were
observed in metal Lewis acid catalysis. The propargylation
products containing a phenolic group and an internal alkyne,
could undergo a wide range of transformations to afford
heterocycles and other phenol analogues. Control experiments
combined with DFT calculations show that the pathway
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c01863.
Calculated full free energy profiles, optimized geo-
metries, experimental procedures, compound character-
ization, and spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Shuhua Li − Key Laboratory of Mesoscopic Chemistry of
Ministry of Education, Institute of Theoretical and
Computational Chemistry, School of Chemistry and Chemical
Engineering, Nanjing University, Nanjing 210093, P. R.
China; orcid.org/0000-0001-6756-057X;
Email: shuhua@nju.edu.cn
Guoqiang Wang − Key Laboratory of Mesoscopic Chemistry
of Ministry of Education, Institute of Theoretical and
Computational Chemistry, School of Chemistry and Chemical
Engineering, Nanjing University, Nanjing 210093, P. R.
China; orcid.org/0000-0001-9666-1919;
Email: wangguoqiang710@nju.edu.cn
Authors
Hui Chen − Key Laboratory of Mesoscopic Chemistry of
Ministry of Education, Institute of Theoretical and
Computational Chemistry, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210093,
P. R. China
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Mo Yang − Key Laboratory of Mesoscopic Chemistry of
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Computational Chemistry, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210093,
P. R. China
Liuzhou Gao − Key Laboratory of Mesoscopic Chemistry of
Ministry of Education, Institute of Theoretical and
Computational Chemistry, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210093,
P. R. China
Zhigang Ni − Key Laboratory of Mesoscopic Chemistry of
Ministry of Education, Institute of Theoretical and
Computational Chemistry, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210093,
P. R. China; orcid.org/0000-0002-0109-3275
Jingxiang Zou − Key Laboratory of Mesoscopic Chemistry of
Ministry of Education, Institute of Theoretical and
Computational Chemistry, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210093,
P. R. China
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Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (22073043, 21833002, 21673110, and
21903043), and the Fundamental Research Funds for the
Central Universities (020514380243).
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