Alkylation of Allyl/Alkenyl Sulfones by Deoxygenation of Alkoxyl Radicals

Article abstract of DOI:10.1002/chem.201806138

A challenging deoxygenation of alkoxyl radicals from readily accessible alcohol derivatives was developed, affording facile synthesis of functionalized alkenes with good functional group tolerance under mild reaction conditions. Because alkoxyl radicals can easily undergo β-fragmentations or hydrogen abstractions, this new strategy for deoxygenation of alkoxyl radicals is highly valuable. Moreover, mechanistic studies revealed that the electron-neutral phosphine acts as the deoxygenation reagent.

Full text of DOI:10.1002/chem.201806138

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Accepted Article
Title: Alkylation of Allyl/alkenyl Sulfones by Deoxygenation of Alkoxyl
Radicals
Authors: Jia-Bin Han, Ao Guo, and Xiangying Tang
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Alkylation of Allyl/alkenyl Sulfones by Deoxygenation of Alkoxyl
Radicals
Jia-Bin Han, Ao Guo and Tang Xiang-Ying*
Abstract: A challenging deoxygenation of alkoxyl radicals from
readily accessible alcohol derivatives was developed, affording facile
synthesis of functionalized alkenes with good functional group
tolerance under mild reaction conditions. Since alkoxyl radicals can
easily undergo β-fragmentations or hydrogen abstractions, this
new strategy for deoxygenation of alkoxyl radicals is highly valuable.
Moreover, mechanistic studies revealed that the electron-neutral
phosphine acts as the deoxygenation reagent.
Alcohols, as one of the most abundant feedstock chemicals, are
ubiquitous in drugs and natural products, thus convertion of
alcohols or their derivatives to other functionalized moleculars is
of high value. Especially, the radical transformation of alcohols
has gained much attention in recent years. To date, great
achivements have been made in using alcohols[1] or their
derivatives, such as xanthate[2], oxalate[3], benzoate[4],
phosphate[5], and ethers[6] as alkylation agents. MacMillan and co-
Scheme 1. Alcohols or their derivatives as alkylation agents. Nphth
=
workers reported an elegant Minisci reaction under photoredox
phthalimido, HE = Hantzsch ester.
catalysis with simple alcohols as alkylation agents (Scheme 1,
Ia).[1d] Suga and Ukaji disclosed a low-valent titanium-mediated
The alkoxyl radical is a versatile reactive intermediate in many
organic transformations and biologic processes.[7] It has been well
established that alkoxyl radical can easily undergo β-
fragmentations,[8] hydrogen abstractions,[9] or addition to
unsaturated bonds.[10] Whereas, reactions involving alkoxyl
radicals were always hampered by the instability of alkoxyl radical
precursors or harsh reaction conditions.[9a, b, 10a, 11] Thanks to the
recent development of visible-light catalysis,[12] the generation of
alkoxyl radicals has been realized in mild conditions with excellent
functional group tolerance.[9c-g, 10b] Chen and Meggers disclosed
the generation of alkoxyl radicals under photoredox catalysis and
the subsequent δ-selective functionalization after 1,5-hydrogen
atom transfer (HAT). [9c, d] Zuo and co-workers reported a similar
1,5-HAT strategy in which alkoxyl radicals were generated from
simple alcohols.[9g] Recently, Chen reported a donor-acceptor
complex enabled alkyl radical generation from β-fragmentation of
the alkoxyl radical (Scheme 1, II).[8c] Very recently, Dagousset
reported the synthesis of ethers via alkoxyl radicals generated
from N-alkoxypyridinium salts under photoredox catalysis.[10b]
Although great achievements involving alkoxyl radicals have been
made recently, reactions involving alkoxy radicals against
hydrogen abstraction or β-fragmentation are rarely reported.[13] To
radical conjugate addition of electro-deficient alkenes with benzyl
alcohols as benzyl radical sources (Scheme 1, Ib).[1f] In Addition,
Overman and coworkers developed an efficient radical alkylation
using oxalates as alkylation agents under photoredox catalysis
(Scheme 1, Ic).[3d] Recently, Zhu and Xie reported an efficient
method for trifluoromethylthiolation and difluoromethylthiolation of
tertiary alcohols through selective cleavage of tertiary C(sp3)–O
ether bonds in methoxymethyl protected tertiary alcohols
(Scheme 1, Id).[6] However, to the best of our knowledge, a
general alkylation protocol via alkoxyl radical generated from
alcohols or their derivatives remains a challenge.[5]
[*]
J.-B. Han, A. Guo, and Prof. X.-Y. Tang
Hubei Key Laboratory of Bioinorganic Chemistry and Materia
Medica
School of Chemistry and Chemical Engineering
Huazhong University of Science and Technology
1037 Luoyu Road, Wuhan 430074 (China)
E-mail: xytang@hust.edu.cn.
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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this regard, the direct deoxygenative alkylation using alcohol
hypervalent iodine reagents or Mn(OAc)3.[19] Unfortunately, only
trace amount of desired products could be obtained. We then
chose N-alkoxyphthalimides as alkoxyl radical precursors.[8b,9a-
c,13] In the presence of 2 mol% of Ir-I, Hantzsch ester, and P(OEt)3,
the reaction in 1,4-dioxane under the irradiation of 3 W blue LED
proceeded smoothly to afford the desired product 3a in 44% yield
together with 32% yield of 3a' resulting from H abstraction (Table
1, entry 1). Other photocatalysts were observed to be less
effective than Ir-I (Table 1, entries 2-7). It is worthnoting that the
choice of phosphines was crucial to the product distritution.
P(OMe)3 was found to be more efficient than P(OEt)3 and P(OiPr)3
(Table 1, entries 1 and 8-9). To our surprise, the reactions using
PPh3 or P(OPh)3 produced much diminished yields, and this result
was quite different from some recent related works[13,17] (Table 1,
entries 10 and 13). When P(OMe)Ph2 or Ph(OH)(OEt)2 was
employed, 3a' was given in high yield but no 3a was not obtained
(Table 1, entries 11 and 12). Further screening of reaction
conditions revealed that 1 mol% of Ir-1 was enough for this
reaction, and MTBE was proved to be better solvent than 1,4-
dioxane (Table 1, entry 14). To further improve the yield of 3a, 8
equiv of P(OMe)3 was required to suppress the H abstraction
process, giving 3a in to 83% yield with only 8% yield of 3a'
obtained. In addition, low concentration of reactants gave good
yield of 3a (Table 1, entry 15). Control experiments demonstrated
Hantzsch ester, photocatalyst, and blue LED were all necessary
in this reaction (Table 1, entry 16. For details, see SI).
substrates is highly desirable.
We speculated that if a rapid deoxygenation took place
before the intrinsic β-fragmentation or hydrogen abstraction, an
innovative alkylation method by deoxygenation of alkoxyl radical
would be rationally realized. Because most alkoxyl radical
precursors could be readily prepared from cheap and abundant
[9c,d, 10a,b]
alcohols,
this method would be highly practical and
valuable. Very recently, Schmidt disclosed an anti-markovnikov
alkene hydroamination via phthalimidyl radicals, which was
accessed from phosphite promoted radical deoxygenation of N-
hydroxyphthalimide.[14] Doyle disclosed a novel deoxygenative
hydrogenation of benzylic alcohols by phosphine radical cation
(Scheme 1, III).[15] Encouraged by these reports and other
studies[13,17], we envisioned that phosphine would complete such
process due to strong P-O bond strength (148 kcal/mol for
P(O)(OEt)3).[18] Herin, we wish to report a novel deoxygenative
alkylation of allyl/alkenyl sulfones from readily accessable N-
alkoxyphthalimides under visible light (Scheme 1, IV).
Table 1. Screening of reaction conditions.
Yieldb
With the optimized reaction conditions in hand, we sought to
investigate the scope of alcohol derived N-alkoxyphthalimides. As
entrya
photocatalyst
additive
1a/2a/PR3
(3a/3a', %)
44/32
18/54
24/51
22/60
0/6
1
2
Ir-I
Ir-II
Ir-III
Ir-IV
Ru-I
Ru-I
Eosin Y
Ir-I
P(OEt)3
P(OEt)3
1/2/2
1/2/2
1/2/2
1/2/2
1/2/2
1/2/2
1/2/2
1/2/2
1/2/2
1/2/2
1/2/2
1/2/2
1/2/2
1/2/2
1/3/8
1/2/2
demonstrated in Scheme 2,
a
broad array of N-
3
P(OEt)3
alkoxyphthalimides could serve as alkylation agents in this
deoxygenative alkylation reaction. Deoxygenation of alkoxyl
radicals generated from 1-arylethan-1-ol derivatives occurred
smoothly to afford the 1,1-disubstituted alkenes in moderate to
good yields (3b-3h, 50%-90%). N-alkoxyphthalimides with
electron-donating groups, such as Me, OMe on the phenyl ring
afforded better yields than N-alkoxyphthalimides with electron-
withdrawing groups (3b and 3c vs 3d-3h). It is worth noting that
the halide substituents on the phenyl ring were compatible under
the reaction conditions (3e-3g). Ethyl group adjacent to the
alkoxyl radical did not impede the deoxynation process, giving
desired product 3i in 77% yield. 1,1-Disubstituted alkenes with an
indane, chromanone and furan skeleton could also be obtained in
moderate and good yields (3j-3l). Benzyl alcohol derived N-
alkoxyphthalimides bearing alkyl, alkynyl, phenyl, halide,
trifluoromethoxyl, cyanide, and ester groups were also suitable
substrates for this reaction, providing the 1,1-disubstituted
alkenes in 34-79% yields (3m-3u). In addition, 3m could be
4
P(OEt)3
5
P(OEt)3
6
P(OEt)3
2/3
7
P(OEt)3
5/6
8
P(OMe)3
P(OiPr)3
P(OPh)3
P(OMe)Ph2
P(OH)(OEt)2
PPh3
50/25
35/47
14/45
0/95
9
Ir-I
10
11
12
13
14c,d
15c,e
16
Ir-I
Ir-I
Ir-I
0/82
Ir-I
13/27
56/22
83 (80)/8
4/7
Ir-I
P(OMe)3
P(OMe)3
P(OEt)3
Ir-I
—
[a] Reaction condition: 1a (0.2 mmol), 2a, PC (2 mol%), Hantzsch ester (0.3
mmol), and 1,4-dioxane (2.0 mL) stirred at RT under irridation of 3W blue LED
for 12 h. [b] Yields were determined by GC analysis with n-dodecane as an
internal standard. [c] PC (1 mol%). [d] MTBE (2 mL). [e] MTBE (4 mL). Ir-I:
Ir[(dtb-bpy)(ppy)2](PF6); Ir-II: Ir[bpy((dF(CF3)ppy)2](PF6); Ir-III: Ir[(dtb-
bpy)((dF(CF3)ppy)2](PF6); Ir-IV: Ir(ppy)3; Ru-I: Ru(bpy)3Cl2; Ru-II:
Ru(bpy)3(PF6)2.
To test our hypothesis, we first investigated the
deoxygenative alkylation of ally sulfone with alcohols as alkoxyl
radical precursors in the presence of oxidants such as
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obtained in 78% and 50% yield if performing the reaction on 2 and
5 mmol scale. It was found that the position of the substituents on
the phenyl rings had small influence on the reaction. N-
aryloxylphthalimide with m-Me on the phenyl ring gave the
product in slightly lower yield (3o vs 3n and 3p). Notably, the
efficiency of this reaction was not hindered by ortho substituents
on the phenyl ring (3n, 3t, 3w, and 3z). Moreover, 2,3,4,5,6-
pentafluorophenyl group was not active in this reaction, provided
3z in 34% yield probably due to low nucleophilicity of the radical
intermediate. Similar result was also observed in the case of 3y.
Piperonyl and difluoromethylated piperonyl alcohol derived N-
alkoxyphthalimides went through the reaction smoothly to give
3aa and 3ab in good yields. Notably, indole- and pyridine-
containing heterocycle substrates were both successful alkylation
agents, delivering allylation adducts 3ac and 3ad in 70% and 72%
yield, respectively.
Scheme 3. Scope with respect to allyl/alkenyl sulfones. Reaction condition: 1
(0.2 mmol), 2 (0.6 mmol), Ir-I (1 mol%), HE (0.3 mmol), and P(OMe)3 (1.6
mmol) stirred in MTBE (4 mL) under the irridation of 3W blue LED for 12 h.
Scheme 4. Deoxygenation vs β-fragmentation with respect to N-
alkoxyphthalimides. [a] Reaction condition: 1 (0.2 mmol), 2 (0.6 mmol), Ir-I (1
mol%), Hantzsch ester (0.3 mmol), P(OMe)3 (1.6 mmol) and MTBE (4.0 mL)
stirred at RT under irridation of 3W blue LED for 12 h. [b] 1H NMR yield with
CH2Br2 as an internal standard.
Scheme 2. Scope with respect to N-alkoxyphthalimides. Reaction condition: 1
(0.2 mmol), 2a, Ir-I (1 mol%), Hantzsch ester (0.3 mmol), P(OMe)3 (1.6 mmol)
and MTBE (4.0 mL) stirred at RT under irridation of 3W blue LED for 12 h. b
1,4-dioxne in stead of MTBE. c 2.0 mmol scale. d 5.0 mmol scale.
After demonstrating that benzoxyl radicals could be smoothly
deoxygenated in this reaction, we wondered if this deoxygenation
strategy could be extended to other alkoxyl radicals. It was found
that 2-phenyl-ethanoxyl radical was deoxygenated in a much low
efficiency, providing 5a in only 22% yield, together with the
generation of small amount of 3m (8%). The substituents have
dramatic influence on the deoxygenation process. β-
Fragmentation of 2-aryl-ethanoxyl radical with para-Br substituent
was easily suppressed, whereas β-fragmentation of 2-aryl-
ethanoxyl radical with para-OMe substituent occurred more
rapidly (5b, 3s and 5c, 5c'). This could be attributed to the stability
of the benzyl radical generated from β-fragmentation (Scheme
4a), which can also explain the product distribution in the
reactions (Scheme 4b-4c).[20] While in the case of isopropanol
derived N-alkoxyphthalimides, β-fragmentation was more favored
We next sought to evaluate the generality of this deoxygenative
alkylation reaction with respect to allyl/alkenyl sulfones. As shown
in Scheme 3, a wide range of allyl/alkenyl sulfones were
sucessfully alkylated under the typical reaction conditions. Allyl
sulfones bearing a methyl-, benzyl- and naphthalene benzyl-
carboxylic esters are suitable for this reaction, giving the desired
products in moderate yields (4a-4c). Allyl sulfones bearing
aromatic groups were also suitable reaction partners, providing
phenyl substituted alkenes in 48-65% yields, in which electron-
deficient allyl sulfones provided higher yields (4d-4h). Vinyl
sulfones were also suitable radical acceptors, giving aryl
substituted styrenes in acceptable yields (4i-4l).
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than deoxygenation (Scheme 4d). It is interesting that alkoxyl
radical generated from 1al could also undergo β-fragmentation to
afford ring opening product 5h' (Scheme 4f).[21]
alkoxyl radical and phthalimide.[8c,9c] This alkoxyl radical can
easily undergo β-fragmentation or hydrogen abstraction, while
in the presence of P(OMe)3, alkoxyl radical was added to
P(OMe)3 to form a phosphoranyl radical which could easily
undergo β-scission to give an alkyl radical.[16] This alkyl radical
then reacts with allyl/alkenyl sulfones to furnish the product and
benzenesulfonyl
radical
by
an
addition-elimination
sequence.[23] The benzenesulfonyl radical abstract a hydrogen
radical from Hanzstch ester radical, through the persistent
radical effect, to form PhSO2H and pyridine species to furnish
the catalytic cycle.[9d, 24]
Scheme 5. Scope with respect to N-alkoxyphthalimides. Reaction
condition: 1 (0.2 mmol), 2 (0.6 mmol), Ir-I (1 mol%), Hantzsch ester (0.3
mmol), P(OMe)3 (0.8 mmol) and MTBE (4.0 mL) stirred at RT under
irridation of 3W blue LED for 12 h.
It was found that aliphatic alcohols derived N-
alkoxyphthalimides could be successfully employed in our
reaction, affording the corresponding olefines (Scheme 5).
Unlike the reaction of 2-phenyl-ethanoxyl radical, 3-phenyl-
propanoxyl radical was deoxygenated by P(OMe)3, providing
6a in 33% yield with trace product from β-fragmentation
observed. Geraniol derived N-alkoxyphthalimide could also be
used for alkylation, giving the product 6b as an E/Z isomer
mixture in 30% yield. 4-Phenylbutan-2-ol derived N-
alkoxyphthalimide provided 36% yield of the desired product
6c. Moreover, menthol derived N-alkoxyphthalimide is also a
successful alkylation agent in our reaction and product 6d was
obtained in 38% yield. α-Carbonyl radical was also generated
by this deoxynative strategy, affording 6e in 38% yield. t-Butyl
radical was obtained in the presence of P(OMe)3, and desired
product 6f was produced in 55% yield with a little methylated
product detected by 1H NMR.
Scheme 6. Proposed mechanism.
In summary, we have developed a general strategy for
alkylation reactions via deoxygenation of alkoxyl radical
generated from N-alkoxyphthalimide. A wide range of N-
alkoxyphthalimides derived from benzyl alcohols serves well
as alkylation agents to react with allyl/alkenyl sulfones.
Moreover, N-alkoxyphthalimides from alkyl alcohols can also
be functionalized by our protocol. The synthetic usefullness of
this deoxygenation strategy has been demonstrated through
the functionalization of natural occurring products, such as
geraniol and menthol. Further investigations to understand the
mechanism more deeply and expansion of the substrate scope
is ongoing in our lab.
A Stern−Volmer quenching experiment was performed to
exploit whether P(OMe)3 or Hantzsch ester is the real
quencher of the photoexited state of [Ir(dtb-bpy)(ppy)2](PF6)
(For details, see SI). As expected, hantzsch ester successfully
quenched *[Ir(dtb-bpy)(ppy)2](PF6), while P(OMe)3 was not
able to decrease the intensity of *[Ir(dtb-bpy)(ppy)2](PF6).
Based on Stern−Volmer quenching experiment and recent
studies,[8c,9c-d] a plausible mechanism was proposed as shown
in Scheme 6. IrIII is activated upon being exposed to blue LED
to give IrIII*, and then reduced by Hantzsch ester to afford IrII.[9c,
22] The resulting IrII species can reduce N-alkoxyphthalimide by
single electron transfer to give N-alkoxyphthalimide anion,
which is further protonated by Hantzsch ester radical cation to
facilitate a homolytic N-O bond cleavage with formation of an
Acknowledgements
We are grateful for the financial support from National Natural
Science Foundation of China (21871100), Huazhong University
of Science and Technology (HUST), Opening fund of Hubei Key
Laboratory of Bioinorganic Chemistry & Materia Medica (No.
BCMM201805) and the Fundamental Research Funds for the
Central Universities (2017KFYXJJ166). We thank Analytical &
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10.1002/chem.201806138
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COMMUNICATION
Testing Center of HUST, Analytical and Testing Centre of the
School of Chemistry and Chemical Engineering (HUST) for
access to their facilities.
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Conflict of interest
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The authors declare no conflict of interest.
Keywords: alkoxyl radical • deoxygenation • photoredox
catalysis • alkenes • radical alkylation
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10.1002/chem.201806138
Chemistry - A European Journal
COMMUNICATION
COMMUNICATION
Han Jia-Bin, Guo-Ao, Tang Xiang-Ying*
Page No. – Page No.
Alkylation of Allyl/alkenyl Sulfones by
Deoxygenation of Alkoxyl Radicals
A challenging deoxygenation of alkoxyl radicals from readily accessible alcohol
derivatives is described, affording facile synthesis of synthetic valuable alkenes with
good functional group tolerance under mild reaction conditions. Since alkoxyl
radicals can easily undergo β-fragmentations or hydrogen abstractions, this new
strategy for deoxygenation of alkoxyl radicals is of high significance.
This article is protected by copyright. All rights reserved.