Light-enabled, AlCl3-catalyzed regioselective intramolecular nucleophilic addition of non-nucleophilic alkyls to alkynes

Article abstract of DOI:10.1039/d0cc04636a

Light-enabled, AlCl3-catalyzed regioselective intramolecular nucleophilic cyclization of alkynes using non-nucleophilic alkyls as the nucleophile is reported. Upon photoexcitation, o-alkylphenyl alkynyl ketones can be transferred into (E)-photoenols. Thus, a nucleophilic methylene is formed from the non-nucleophilic alkyl. An AlCl3 catalyst can stabilize the (E)-photoenol intermediate and facilitate further intramolecular nucleophilic cyclization. DFT calculations indicated that the AlCl3-catalyzed cyclization is the regioselectivity determining step.

Full text of DOI:10.1039/d0cc04636a

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DOI: 10.1039/D0CC04636A
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Light-enabled, AlCl3-catalyzed regioselective intramolecular
nucleophilic addition of non-nucleophilic alkyl to alkyne
Yanbin Zhang,a Ruiwen Jin,a Guangxing Panb and Hao Guo*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
(A)
Classic Lewis acid-catalyzed nucleophilic addition of alkyne
Light-enabled, AlCl3-catalyzed regioselective intramolecular
nucleophilic cyclization of alkyne using non-nucleophilic alkyl as the
nucleophile was reported. Upon photoexcitation, o-alkylphenyl
alkynyl ketones can be transferred into (E)-photoenols. Thus, a
nucleophilic methylene is formed from the non-nucleophilic alkyl.
AlCl3 catalyst can stabilize (E)-photoenol intermediate and facilitate
further intramolecular nucleophilic cyclization. DFT calculations
indicated that the AlCl3-catalyzed cyclization is the regioselectivity
determining step.
LA
LA
E
Nu
E
R'
R'
R
R'
R
R
nucleophilic attack
electrophiles quenching
Nu
Nu
normal alkyl has NO nucleophilicity
(B)
Generation of enol from non-nucleophilic alkyl
O
R
OH
R
H
OH
Rotation & ISC
R
1,5-H transfer
R'
R'
R'
nucleophilic C(sp2)
non-nucleophilic C(sp3)
Nucleophilic addition of alkyne is one of the well-studied
reactions,1 as it provides a convenient way to synthesize
functionalized alkenes. The general activation mode is
summerized in Scheme 1A. The alkyne moiety normally requires
the coordination with Lewis acid or π acid to increase its
electrophicility.2 Upon activation, a suitable nucleophile can
attack the alkyne, forming an alkene intermediates. This is
normally the rate-determining step (RDS) of the whole
reaction.3 Finally, quenching by electrophiles gives the
corresponding multi-substituted alkene products. Nucleophile
plays an essential role in this reaction process. Nucleophiles can
be generally divided into three types: (1) σ type, like NaBH4,
lithium reagents, Grignard reagents, etc.; (2) π type, like enol
ethers, alkenes, aromatic rings, etc.; (3) lone pair electron type,
like ROH, RNH2, RSH, etc.4 Alkyls are not considered as a
nucleophile compared with those typical examples due to its
lack of any nucleophilic atoms. To the best of our knowledge,
alkyls have not been reported as a nucleophile in the addition
to alkynes.
Organic photoreactions normally proceed via excited state
of reactants,5 which shows different reactivity toward thermal
reactions at ground state. One example is the photochemistry
of o-alkylaromatic ketones (Scheme 1B).6 At the ground state,
the pKa of the benzylic proton of the substrate is about 27
(referenced by phenyl(o-tolyl)methanone), indicating the
benzylic H is non-acidic.7 Thus, the benzyl group can be
rationalized to be a rather weak nucleophile. At the
(C) This work: AlCl3-catalyzed nucleophilic cyclization of non-nucleophilic alkyl
O
O
[Al] &
5
R
R
R'
R'
1
2
OH
R'
non-nucleophilic alkyl
5-exo-dig selective
[Al]
photoenolization
nucleophilic addition
R
nucleophilic methylene
Scheme 1 Nucleophilic addition of alkynes.
photoexcited state of o-alkylaromatic ketones, 1,5-H transfer
occur via the triplet hyperface. Further rotation and
intersystem-crossing (ISC) results in the (E)-photoenols.8 Thus,
the non-nucleophilic benzylic C(sp3) is transformed into a
nucleophilic C(sp2), which has been proved useful in many
classic enol type reactions.9 Our previous study also showed
that the similar strategy could achieve 6π-photocyclization of
non-6π substrates.10 With continued interest in photo-induced
cyclization reactions, we questioned whether this photoenol
can be used in more challenging nucleophilic addition to
alkynes. Specifically, if o-alkylphenyl alkynyl ketones are
employed as potential substrates, the in situ generated
photoenols may attack the activated alkyne (Scheme 1C).
Herein, we wish to report our recent results in AlCl3-catalyzed
regio- and stereoselective intramolecular nucleophilic
cyclization of o-alkylaromatic alkynyl ketones.
The initial attempt was carried out using o-tolyl 4-
methoxyphenylacetylenyl ketone (1a) as the model substrate. A
series of Lewis acids were tested and AlCl3 gave 12% yield of 5-
exo-dig product (2a) (Table 1, entries 1-4). Notably, 6-endo-dig
product (3a) was also formed. The regioselectivity of 2a and 3a
was 12:1 for AlCl3 (Table 1, entry 4). Considering the high
a. Department of Chemistry, Fudan University, 2005 Songhu Road, Shanghai
200438, PR China.
Email: Hao_Guo@fudan.edu.cn
b. Academy for Engineering and Technology, Fudan University, Shanghai 200433, PR
China.
Electronic Supplementary Information (ESI) available: Experimental procedures and
NMR copies. See DOI: 10.1039/x0xx00000x

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regioselectivity, AlCl3 was chosen as the optimal catalyst. Cl (2g) and 4-Br (2h) aryl also gave good yields. Strong electron-
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Solvent screening revealed that 1,4-dioxane gave the highest withdrawing groups (EWGs), like methoDxyOcIa: 1r0b.o10n3y9l/D(20iC),Cf0o4r6m36yAl
regioselectivity and a promising yield (Table 1, entries 4-9). To (2j) and trifluormethyl (2k), at the aryl group slightly decreased
further accelerate the 5-exo-dig cyclization, higher the yields. To further expand the scope at R1, function groups
temperatures were tested (Table 1, entries 10-12). The reaction other than aryl were also tried. TMS-bearing substrate 2l gave a
o
at 60 C showed higher yield than that of room temperature moderate yield, probably due to the reactive nature of TMS
o
(Table 1, entry 10). Further raising the temperature to 80 C under Lewis acidic conditions. Alkyl groups, like n-butyl (2m),
resulted in 88% yield and >20:1 regioselectivity (Table 1, entry cyclopropyl (2n) and cyclohexyl (2o), were also tried and gave
11). At a higher reaction temperature of 100 oC, the yield slightly good yields. Next, modifications at R2 were explored. Strong
decreased (Table 1, entry 12). Decreasing the catalyst loading to EDG (2l and 2m), weak EDG (2n) and halogen (2o and 2p) at R2
5 mol% gave 92% of 2a with excellent regioselectivity (Table 1, did not affect the reaction. Substrates with modifications on
entry 13). Notably, only (E)-product was formed in the above both R1 & R2 were also synthesized and tried. Di-EDG (2q),
reactions, showing the high stereoselectivity of the conversion.
Purple LED
Control experiments without AlCl3 showed that 2a can be
O
O
5 mol%
3, 80 oC, 1,4-dioxane
AlCl
formed by simple heating, but with a lower efficiency and
poorer regioselectivity (Table 1, entry 14). Further mechanistic
studies indicated that AlCl3 could stabilize the (E)-photoenol
intermediate and facilitate the nucleophilic addition to form 2a,
and unexpected reaction pathway could be inhibited (vide
infra). Light was proved to be essential for the formation of 2a
(Table 1, entry 15). Thus, conditions of entry 13 (1 and 5 mol%
of AlCl3 in 1,4-dioxane (0.02 M) irradiated by purple LED under
argon atmosphere at 80 oC for 72 h) was chosen as the
optimized conditions for further studies.
With optimized conditions in hand, the substrate scope of
this reaction was next examined (Scheme 2). The substrate
scope on R1 group was first studied. Aryl groups at R1 were tried.
Strong electron-donating groups (EDGs) substituted aryl, such
as methoxy (2a) and acetoxy (2b), showed good reactivity. Aryl
groups with weak EDGs, such as tert-butyl (2c) and methyl (2d),
were also tolerated in this reaction. Substrates with 4-F (2f), 4-
R2
R2
Ar, 72 h
5
R1
R1
R3
R3
1
2
5-exo:6-endo > 20:1
Scope of R1 (R2 = H, R3 = H)
O
E
O
E
H
H
R1
NOE
R
R1
R = MeO 2a
Cl
Br
COOMe
CHO
2g
2h
, 90%
, 78%
, 75%
78%
TMS
=
n-Bu
2l
,
65%
2m
, 80%
AcO
t-Bu
Me
H
2b
, 84%
2c
, 87%
2d
, 87%
2e
, 86%
2i
2j
,
,
2n
2o
,
81%
80%
80%
CF3
2k
, 76%
,
F
2f
,
80%
Scope of R2 (R1 = phenyl, R3 = H)
O
7
4
R2
=
2p
2q
5-Br
5-F
2s
, 88%
, 88%
, 89%
6
5
4-MeO
4-AcO
6-Me
E
2t
R2
,
88%
2r
,
85%
Ph
Scope of R1 & R2 (R3 = H)
O
O
O
Table 1 Screening of the conditionsa
E
E
E
OH
Purple LED
LA, T oC, Solvent (0.02 M)
Ar, 72 h
O
O
OMe
OMe
OMe
E
+
5
6
Ar
OMe
COOMe
CHO
Ar
Ar
Ar =
OMe
2u, 93%
2v, 87%
2w
, 88%
1a
2a
3a
6-endo-dig
5-exo-dig
T (oC)
rt
O
O
O
Entry
1
2
LA (mol%)
La(OTf)3 (10)
Zn(OTf)2 (10)
Sc(OTf)3 (10)
AlCl3 (10)
AlCl3 (10)
AlCl3 (10)
AlCl3 (10)
AlCl3 (10)
AlCl3 (10)
AlCl3 (10)
AlCl3 (10)
AlCl3 (10)
AlCl3 (5)
Solvent
THF
THF
THF
THF
EA
CH3CN
DCM
Toluene
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
2a (%)
15 (6)b
12 (6)b
6 (16)b
12 (31)b
23 (26)b
24 (43)b
17 (36)b
47 (6)b
45 (40)b
64
2a:3a
2:1
F
F
E
E
E
Cl
rt
rt
rt
rt
rt
rt
rt
rt
60
80
100
80
80
80
3:1
6:1
OMe
3
4
5
6
CF3
OMe
OMe
2x
2y
, 91%
, 81%
2z
, 91%
12:1
3:1
Scope of R3 (R1 = Ar, R2 = H)
O
O
O
5:1
6:1
9:1
7
8
9
Ph
Ph
Ph
Ph
>20:1
>20:1
>20:1
>20:1
>20:1
5:1
2aa
E:Z = 1.2:1
2ab
, 90%
E:Z = 1.3:1
, 94%
2ac
, 90%
E:Z = 1.0:1
10
11
12
13
14
15d
O
O
88
82 (7)b
92 (90)c
65
--
Br
Br
AlCl3 (5)
n.r. (85)b
--
2ad
, 93%
E:Z = 1.1:1
2ae
, 93%
E:Z = 1.1:1
a A solution of 1a (0.2 mmol) and LA in anhydrous solvent (10 mL) was irradiated
by purple LED under argon atmosphere for 72 h. Yield, recovery, regio- and
stereoselectivity was determined by 1H NMR analysis (400 MHz) of the crude
reaction mixture, using CH2Br2 (0.2 mmol) as the internal standard; b Recovery of
1a; c Isolated yield of 2a; d Dark reaction. n.r. = no reaction.
Scheme 2 Substrate scope.
Conditions: 1 and 5 mol% of AlCl3 in 1,4-dioxane (0.02 M) irradiated by purple LED
under argon atmosphere at 80 oC for 72 h. Isolated yield was reported. The E:Z
ratio was calculated by isolated yield.
2 | J. Name., 2012, 00, 1-3
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EDG/EWG (2r-t), EWG/EDG (2u) and tri-substituted (2v) than 6-endo-dig transition state (TS-6). Both energy barrier and
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DOI: 10.1039/D0CC04636A
reactants worked well. Notably, neither (Z)- nor 6-endo-dig the conformation indicated TS-5 is the favored transition state
products were detected for the substrates tested above, which than TS-6. Thus the regioselectivity originates from this
highlighted the high regio- and stereoselectivity when R3 = H.11 cyclization process, leading to the 5-exo-dig product 2 as the
Finally, the scope of R3 was studied. Ethyl (2aa & 2ad), isopropyl predominant product.
(2ab & 2ae) and benzyl (2ac) were installed at ortho position
Deuterium labeling and KIE experiments were also carried
forming the corresponding products in excellent yields. out to understand the H atom transfer process in this reaction.
However, with the increasing of the steric hindrance, the E:Z 1a-d3 was synthesized and subjected to the standard
ratio decreased to ca. 1:1.12
conditions. 62% of 2a-d was formed with 26% of 1a-d3
After the substrate scope investigation, mechanistic studies recovered. 1H NMR analysis indicated that the deuterium in 2a-
were next carried out. UV-Vis spectrum indicated that AlCl3 d located at both benzylic position and alkenyl position (Scheme
could enhance the absorption of 1a, but led to no appreciable 4). The D at alkene strongly suggested that an intramolecular
wavelength change (for the spectrum, see Figure S1). Thus, protolysis occurred in the reaction process. The deuterium
upon the light irradiation, AlCl3-1a complex is more likely to content difference between benzylic position and alkenyl
absorb light to be excited than the un-coordinated 1a. Next, DFT position might be due to the intermolecular protolysis process,
calculations of the intermediates and transition states energies which could bring H to the alkene position. 26% of 1a-d3 was
were computed, and the results were shown in Scheme 3. First, recovered with 82% deuterated incorporation. The deuterium
the AlCl3-coordinated (E)-photoenol (5) showed lower energy (- content loss (both in recovered 1a-d3 and benzylic position of
2.6 kcal/mol) than the uncoordinated (E)-photoenol. This result 2a-d) could be reasoned in H-D exchange in 1,4-biradical
indicated that the AlCl3 not only serves as a Lewis acid to intermediate and (Z)-photoenol (vide infra) with solvent or
activate the alkyne, but also stabilizes the (E)-photoenol moisture in the solvent. (for detailed discussion, see Figure S3
intermediate. This calculated energetic result also met the in ESI)8b, 13 This could also be an evidence for occurrence of
experimental observation without AlCl3 catalyst (Table 1, entry Norrish Type II process of 1a.10 Kinetic isotopic experiment
14). As the unstabilized (E)-photoenol might undergo showed kH/kD = 1.3, indicating that H abstraction process is not
unexpected decomposition leading to lower yield, AlCl3 could the rate-determining step.
prevent (E)-photoenol from decomposition and facilitate the
With the information above and literature precedent, a
following cyclization to give higher yield. Next, the transition possible reaction pathway was proposed in Scheme 5.
state energy surfaces were calculated to explain the Coordination of AlCl3 to the alkyne moiety of 1 gives complex
regioselectivity between 5-exo-dig product and 6-endo-dig 3.2a, b Then, the photoexcitation and ISC of 3 results in 3* (T1).
product. Compared with 6-endo-dig pathway, 5-exo-dig 1,5-Hydrogen transfer affords 1,4-biradical 4 as the key
pathway has a much lower activation energy (ΔΔG = -7.7 intermediate. 4 undergoes rotation and ISC affording AlCl3-
kcal/mol). The conformation of AlCl3-coordinated (E)-photoenol coordianted (E)-photoenol 5.8b With the activation of AlCl3,
(5) is also more similar with 5-exo-dig transition state (TS-5)
G (kcal/mol)
TS-6
6.3
(E)-photoenol
0.0
TS-5
-1.4
5
-2.6
5
( )
(E)-photoenol-AlCl3
Int-5
-49.4
Int-6
-57.1
5-exo-dig
6-endo-dig
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J. Name., 2013, 00, 1-3 | 3
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Scheme 3 DFT calculations of the energy surface.
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DOI: 10.1039/D0CC04636A
O
1
(a) S. M. Abu Sohel and R. Liu, Chem. Soc. Rev., 2009, 38, 2269-
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2015, 115, 9028-9072; (d) G. Fang and X. Bi, Chem. Soc. Rev.,
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Soc. Rev., 2016, 45, 4471-4503; (f) L. Chen, K. Chen and S. Zhu,
Chem, 2018, 4, 1208-1262.
O
C
O
H/D
50% D
Standard Conditions
+
kH/kD = 1.3
Ar
Ar
Ar
H/D
(H/D)
3
D
C
3
D/H
84% D
2a
>99% D
82% D
26% of -d3
1a
1a
-d3
Ar = 4-Methoxyphenyl
62% of -d
Scheme 4 Deuterium labelling and KIE experiments
2
3
(a) T. Schwier and V. Gevorgyan, Org. Lett., 2005, 7, 5191-
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O
O
R
R
2
1
[Al]
R
Protolysis
Coordination
H
O
[Al]
[Al]
5
O
R
transfer
6
1,5-H
3
4
5
R
Nucleophilic
addition
Excitation
ISC
[Al]
R
OH
OH
[Al]
[Al]
*
(Z)-photoenol
ISC without
R
O
T1
H
5
Rotation
Rotation
3
*
6
1,5-H transfer
R
ISC
[Al]
OH
4
Scheme 5 Proposed mechanism.
7
8
F. G. Bordwell, D. Algrim and N. R. Vanier, J. Org. Chem., 1977,
42, 1817-1819.
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613.
intramolecular nucleophilic addition of 5 yields 5-exo-dig
intermediate 6 with high regioselectivity. Subsequent inter- or
intramolecular protolysis of 6 finally yields 2. On the other hand,
the direct ISC of 1,4-biradical 4 forms (Z)-photoenol. The (Z)-
photoenol is conformationally-favored for another 1,5-H shift
to return to the starting complex 3.8b The H-D exchange of the
hydroxyl in 1,4-biradical 4 and (Z)-photoenol accounts for the
deuterium loss in deuterium labelling experiments.
In conclusion, we successfully achieved a regioselective
intramolecular nucleophilic addition of a non-nucleophilic
carbon to alkyne. With the irradiation of purple LED, the non-
nucleophilic methyl in o-alkylphenyl alkynyl ketones were
transferred into a nucleophilic (E)-photoenol. Under the
catalysis of AlCl3, intramolecular nucleophilic cyclization
occurred with high regioselectivity. Mechanistic studies
indicated that AlCl3 showed triple functions: enhancing
absorption, stabilizing (Z)-photoenol and increasing selectivity.
Further photophysics studies and application of the strategy are
ongoing in our group.
9
10 Y. Zhang, R. Jin, W. Kang and H. Guo, Org. Lett., 2020, 22,
5502-5505.
11 The stereochemistry of 2a was determined by 2D NOESY
analysis. 2a, 2d, 2e, 2f, 2g, 2h, 2j, 2k, 2l, 2m, 2o, 2p, 2r, 2s and
2t are known compounds. The stereochemistry of those
compounds was determined by 1H NMR comparison with
literature reports. For details, see ESI.
12 The stereochemistry of (E)-2ad, (Z)-2ad, (E)-2ae and (Z)-2ae
was determined by 2D NOESY analysis. (E)-2ab, (Z)-2ab and
(E)-2ac are known compounds. The stereochemistry of those
compounds was determined by 1H NMR comparison with
literature reports. For details, see ESI.
13 S. K. Sarkar, G. K. Weragoda, R. A. A. U. Ranaweera and A. D.
Gudmundsdottir, J. Phys. Chem. B, 2014, 119, 2668-2676.
We greatly acknowledge the financial support from
Shanghai Science and Technology Committee (18DZ1201605).
Conflicts of interest
There are no conflicts to declare.
Notes and references
4 | J. Name., 2012, 00, 1-3
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[Al]:
DOI: 10.1039/D0CC04636A
Enhancing Absorption
Stablizing intermediate
Facilitating Nu addition
O
O
5
R
R
R'
R'
non-nucleophilic alkyl
5-exo-dig selective
OH
[Al]
Ar
R'
nucleophilic methylene
Light enabled AlCl3 catalyzed regioselective intramolecular nucleophilic
addition of non-nucleophilic alkyl to alkyne