Chemo- And regioselective nucleophilic hydrofunctionalization of unactivated aliphatic alkenes under transition-metal-free catalysts

Article abstract of DOI:10.1039/d1gc00474c

Transition-metal-free-catalyzed nucleophilic hydrofunctionalization of both terminal and internal unactivated aliphatic alkenes has been described for the first time. Most topical classes of carbon, nitrogen and oxygen nucleophiles are well-compatible. The highly chemoselective unprotected dinucleophiles are also presented in the atom-economical approach. More than 80 structurally complex β-hetero-substituted aliphatic amides were rapidly synthesized in good yield with exclusive Markovnikov selectivity, which are difficult to be achieved efficiently by the traditional Michael addition of conjugated amides due to their poor intrinsic electrophilicity.

Full text of DOI:10.1039/d1gc00474c

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Chemo- and regioselective nucleophilic
hydrofunctionalization of unactivated aliphatic
alkenes under transition-metal-free catalysts†
Cite this: Green Chem., 2021, 23,
3250
Received 6th February 2021,
Accepted 6th April 2021
Ying Zhan,‡a Yao Zhao,‡a Qiang Du,a Jiacheng Rui,a Rizhi Chen,b Xintao Zhenga and
a
DOI: 10.1039/d1gc00474c
Xiaojin Wu
*
rsc.li/greenchem
Transition-metal-free-catalyzed nucleophilic hydrofunctionaliza- hydrofunctionalization of unactivated aliphatic alkenes have
tion of both terminal and internal unactivated aliphatic alkenes has been achieved, the requirement of precious, eco-unfriendly
been described for the first time. Most topical classes of carbon, transition-metal catalysts, poor compatibility of diverse nucleo-
nitrogen and oxygen nucleophiles are well-compatible. The highly philes in each method, and less success in divergent regio-
chemoselective unprotected dinucleophiles are also presented in selectivity have severely hampered the potential industrial
the atom-economical approach. More than 80 structurally application. Thereby, the development of a more general and
complex β-hetero-substituted aliphatic amides were rapidly syn- practical transition-metal-free-catalyzed nucleophilic hydro-
thesized in good yield with exclusive Markovnikov selectivity, functionalization of unactivated aliphatic alkenes with diverse
which are difficult to be achieved efficiently by the traditional nucleophiles remains challenging but highly desirable from
Michael addition of conjugated amides due to their poor intrinsic both academic and industrial viewpoints.
electrophilicity.
Base-catalyzed intermolecular nucleophilic hydrofunctiona-
lization of olefins is in principle an ideal atom-economical
methodology for both academia and industry;6 however, only a
few examples concerning the unactivated aliphatic alkenes
have been exploited,7 which are mostly restricted to the hydro-
Introduction
n
amination and required strong bases such as BuLi. The chal-
Nucleophilic hydrofunctionalization of alkenes represents one
of the most atom-economical and fundamental strategies that
has been widespread application in organic chemistry.1
Despite numerous transformations being devoted to this strat-
egy,2 the nucleometalation approach has been mainly
exploited to realize the intermolecular nucleophilic addition of
unactivated aliphatic alkenes.3 Hegedus et al. initially applied
nucleopalladation to realize the alkylation of unactivated
olefins with stabilized carbanions as active nucleophiles.4
Recently, by introducing a removable directing group into
unactivated aliphatic alkenes, both the hydroamination and
hydrocarbonation of amide-tethered unactivated aliphatic
alkenes have been well-developed in anti-Markovnikov selecti-
vity; however, no relative Markovnikov selectivity was realized
(Fig. 1).5 Although significant progresses in the nucleophilic
lenges in developing a general base-catalyzed nucleophilic
addition to unactivated alkenes may be attributed to unfavor-
able pairing between the electrophilicity of alkenes and
nucleophilicity of numerous nucleophiles and the reversibility
between addition and elimination under basic conditions.6,7
In our continuous effort on the regioselective functionalization
of unactivated alkenes,8 herein, we report
a
general
Markovnikov selective hydrofunctionalization of unactivated
aInstitute of Advanced Synthesis, School of Chemistry and Molecular Engineering,
Nanjing Tech University, Nanjing 211816, China.
E-mail: iamxiaojinwu@njtech.edu.cn
bState Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech
University, Nanjing 211816, China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1gc00474c
Fig. 1 Development of regiodivergent nucleophilic hydrofunctionaliza-
‡These authors contributed equally to this work.
tion of unactivated aliphatic alkenes (TM = transition metal).
3250 | Green Chem., 2021, 23, 3250–3255
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alkenes with a broad nucleophile scope under minimal solvent Cs2CO3 (Table 1, entries 6–8). Notably, the outcome of the
conditions9 (Fig. 1). Most topical classes of carbon, nitrogen nucleophilic addition was also dependent on the reaction
and oxygen nucleophiles could efficiently add to numerous temperature; temperature that was lower than 60 °C would sig-
unactivated terminal and internal aliphatic alkenes with com- nificantly reduce the reactivity (Table 1, entries 9–11). For
plete Markovnikov selectivity. Notably, the chemo- and regio- other reaction condition optimizations, see the ESI.†
selective additions of diverse unprotected dinucleophiles to
Under the optimal reaction conditions, we first investigated
unactivated aliphatic alkenes were also achieved. Several fea- the nucleophile scope in the intermolecular hydrofunctionali-
tures of this concise approach can be highlighted as follows: zation of unactivated alkene (2a). As expected, numerous
(a) transition-metal-free reaction: the simple approach avoids primary and secondary alcohols afforded the corresponding
the fundamental restriction of using toxic and expensive metal alkyoxylated products in good to excellent yields with complete
catalysts in previous strategies. (b) Catalytic amount of in- Markovnikov selectivity (Table 2, 3a–i). Sterically hindered
organic bases, which are commercially available, inexpensive alcohol (3j) also provided the product, albeit in a decreased
and easily quenchable to eco-friendly sources. (c) Minimal yield. Alcohols featuring olefin and alkyne functional groups
solvent conditions: most of the reactions are under neat con- too were well compatible in this protocol (Table 2, 3k–l). Next,
dition and avoid the usage of additional organic solvents, in line with the alcoholic nucleophile studies, a range of
which would lead to more environmental issues. (d) diverse functionalized primary and secondary amines were
Operational simplicity and atom-economical process, which well-tolerated, including olefin, alkyne, Bn, thiophene, Ts, pyr-
could afford more than 80 structurally complex amides under rolidine, morpholine, pyrrolidine-2,5-dione and imidazole
concise and eco-friendly reaction system.
group (Table 2, 3m–y). Moreover, an array of carbonyl deriva-
Results and discussion
Table 2 Nucleophile scope in hydrofunctionalizationa
Initially, methanol (1a) and 3-butenamide (2a) were chosen as
the model substrates. Through extensive screening of reaction
conditions, a catalytic amount of Cs2CO3 was found to
promote the reaction at 100 °C, affording 3-methoxy-N-(quino-
lin-8-yl) butanamide (3a) in 99% yield with exclusive
Markovnikov selectivity (Table 1, entry 1). Remarkably, no
additional solvent was needed for the nucleophilic hydro-
methoxylation. A decrease in the basicity of inorganic bases
led to a lower yield (Table 1, entries 2–4). The organic base
such as Et3N was catalytically inactive (Table 1, entry 5).
Besides, the amount of Cs2CO3 could be reduced to 5 mol%,
while diminished reactivity was detected using 1 mol% of
Table 1 Variation
hydroalkyoxylationa
of
parameters
in
model
nucleophilic
Entry
Deviation from standard conditions
Yieldb (%)
1
2
3
4
5
6
7
8
None
99
93
72
0
K2CO3 instead of Cs2CO3
Na2CO3 instead of Cs2CO3
Li2CO3 instead of Cs2CO3
Et3N instead of Cs2CO3
5 mol% Cs2CO3
3 mol% Cs2CO3
1 mol% Cs2CO3
80 °C instead of 100 °C
70 °C instead of 100 °C
60 °C instead of 100 °C
0
99
92
84
93
85
76
a Reactions were run on a 0.1 mmol scale. Percentages represent iso-
lated yields. b 80 °C. c 60 °C. d 2 equiv. NuH and 0.1 mL THF. e 2 equiv.
NuH and 0.1 mL MeCN. f 20 mol% CsOAc. g 2 equiv. NuH, 50 mol%
CsOAc, 0.1 mL MeCN. h 2 equiv. NuH, 20 mol% CsOAc, 0.1 mL MeCN.
9
10
11
a Reactions were run on a 0.1 mmol scale. b Determined by GC i 20 mol% Cs2CO3. j 50 mol% Cs2CO3. k 2 equiv. NuH, 50 mol% KOH,
analysis.
0.1 mL THF. l 0.1 mL NuH and 50 mol% KOH.
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Table 4 Alkene scope in nucleophilic hydrofunctionalizationa
tives was explored due to the easy formation of nucleophilic
carbanion under the base catalyst. Diverse carbon nucleo-
philes including dimethyl malonate, cyclic or aliphatic
ketones, indolin-2-one and nitrile were generally well-tolerated
as carbanion precursors (Table 2, 3z–3ah). Collectively, this
atom-economical and practical protocol has significantly
extended the nucleophile scope in the base-catalyzed branched
selective addition of unactivated alkenes.
To further demonstrate the generality of this reaction, we
next evaluated the scope of unprotected dinucleophiles in the
chemo- and regioselective hydrofunctionalization reactions of
unactivated alkene (2a). Numerous diols and diamines were
tested; the corresponding mono-adducts were afforded in good
yields with exclusive Markovnikov selectivity (Table 3, 5a–f).
Secondary free nucleophilic group (OH or NH) could be well-
tolerated in the standard basic condition. Furthermore, reac-
tion involving an array of ambident N/O dinucleophiles includ-
ing amino alcohols, amino diol, amino phenol and piperidinol
proceeded well too (Table 3, 5g–m). Only the relevant amino-
adducts were afforded in complete chemo- and regioselectivity.
The length of an aliphatic chain and the steric difference in
N/O dinucleophiles have no obvious effect on the reactivity
and selectivity. In summary, the addition of inherently more
nucleophilic amino group of unprotected dinucleophiles has
been achieved in our base-catalyzed protocol. The reason
a Reactions were run on a 0.1 mmol scale. Percentages represent iso-
being exclusive chemoselectivity was favored by the innate lated yields. b 10 mol% CsOAc. c 2 equiv. NuH, 50 mol% KOH, 0.1 mL
THF.
nucleophilic ability of dinucleophiles.
Encouraged by the aforementioned extensive nucleophile
scopes of this concise protocol, we next attempted to assess
cation of amide groups has no effect on the reactivity of
N-nucleophilic hydrofunctionalization (Table 4, 7i–7k).
Gratifyingly, while the amine was changed to the dinucleophile
such as aminoethanol, dense functionalized N-adducts were
produced in the reactions of both terminal and internal unac-
tivated alkenes (Table 4, 7l–7v). Encouraged by the results, less
reactive 1,2-diphenylethanone was chosen as the model carbon
nucleophile, and various sterically different terminal and
internal alkenes also performed well in our atom-economic
approach (Table 4, 7w–7af).
To showcase further synthetic practicality of this strategy, a
multigram scale reaction was conducted to afford the
branched product (3a) in 99% yield, while the catalyst loading
was reduced to 2 mol%, 84% yield of the adduct could also be
obtained (Scheme 1, eqn (1)). Furthermore, high yield of
β-aminobutyric acid (8a) could be synthesized under this step-
the potential tolerance of unactivated alkene. By using allyl
amine as a representative nucleophile, an array of internal
unactivated alkenes were examined, and moderate to good
yield of the exclusive branched adduct was afforded in consist-
ence with the relative steric hindrance (Table 4, 7a–7f).
Sterically hindered α-alkyl substituted terminal alkenes too
could be compatible well (Table 4, 7g–7h). Notably, the modifi-
Table 3 Chemo- and regioselective addition of unprotected
dinucleophilesa
a Reactions were run on a 0.1 mmol scale. Percentages represent iso-
lated yields. b 10 mol% CsOAc. c 2 equiv. NuH, 10 mol% CsOAc, neat
condition. d 2 equiv. NuH, 10 mol% CsOH·H2O, 0.1 mL THF. e 2 equiv.
NuH, 20 mol% CsOAc, 0.1 mL MeCN. f 2 equiv. NuH and 0.1 mL THF.
Scheme 1 Synthetic practicality and drug synthesis.
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economic protocol too (Scheme 1, eqn (2)), which is an impor- intrinsic poor electrophilicity of conjugated amides. Collectively,
tant plant resistance inducer.10
although preliminary studies partially supported the elementary
In order to gain possible mechanistic insight, the deuter- steps of our proposed pathway, we still cannot exclude the
ium-labelling experiment was conducted first. The reaction above-mentioned reaction pathway. We reasoned that the
using MeOD produced the desired adduct (3a) with the complex allyl amides might be more reactive in our in situ
D-incorporation ratio of 92% and 87% at the α- and occurring tandem equilibrium than in two separated elemen-
γ-positions, respectively (Scheme 2, eqn (1)). Based on the pre- tary steps. Remarkably, despite well exploiting the Michael reac-
liminary result and literature studies,11 we proposed that a tion,12 the general application of the Michael addition to conju-
possible mechanism might involve the base-catalyzed tandem gated amides is still challenging and mainly limited to acryl-
allylic isomerization and nucleophilic addition of the resulting amide,13 which is consistent with the results of our control
conjugative amides (Scheme 2, eqn (2)). Subsequently, a series experiments (Scheme 2, eqn (4)). Thereby, our approach has
of control experiments were conducted to explore the possi- provided a powerful complementary and environmental-friendly
bility of our hypothesis. Initially, a variety of allylic amides approach for the synthesis of structurally diverse β-hetero-sub-
were tested for the proposed conjugative isomerization of stituted aliphatic amides, which are difficult to be achieved by
unactivated double bonds under the base catalyst (Scheme 2, the traditional Michael addition of conjugated amides owing to
eqn (3)); however, only allyl amide (2a) was converted to the their intrinsic poor electrophilicity.
conjugated amide in 75% yield, and neither steric hindered
terminal nor internal alkenes could isomerize efficiently,
despite the fact that they were well-tolerated substrates in our Conclusions
approach. Furthermore, the aza-Michael addition of the corres-
In conclusion, a general and efficient transition-metal-free-
ponding conjugated amides was tested under either catalytic
catalyzed nucleophilic hydrofunctionalization of unactivated
or stoichiometric amount of base, where most of the reactions
aliphatic alkenes under the minimal solvent conditions was
exhibited quite poor reactivity (Scheme 2, eqn (4)) due to the
developed. Most topical classes of carbon, nitrogen and
oxygen nucleophiles proceeded efficiently with both terminal
and internal unactivated aliphatic alkenes. The highly chemo-
selective unprotected dinucleophiles could also be presented
in the concise approach. More than 80 structurally complex
β-hetero-substituted aliphatic amides were rapidly synthesized
in good yield with exclusive Markovnikov selectivity, which are
difficult to be achieved efficiently by the traditional Michael
addition of conjugated amides due to their intrinsic poor elec-
trophilicity. The gram scale reaction and drug molecule syn-
thesis have further demonstrated the potential synthetic utility
of this scalable strategy. In contrast to previous transition-
metal-catalyzed nucleophilic additions, this base-catalyzed
protocol has made a comprehensive contribution to hydro-
functionalization of unactivated alkenes.
Author contributions
Xiaojin Wu: conceptualization, funding acquisition, investi-
gation, project administration, resources, supervision, writing
– original draft, writing – review & editing; Ying Zhan: data
curation, formal analysis, investigation, methodology, soft-
ware; Yao Zhao: data curation, formal analysis, investigation,
methodology, software; Rizhi Chen: investigation, method-
ology, validation; Qian Du: investigation, methodology, vali-
dation; Jiacheng Rui: investigation, methodology, validation;
Xintao Zheng: investigation, methodology, validation; all
authors approve the current version of the manuscript.
Conflicts of interest
Scheme 2 Study on the possible reaction pathway.
The authors declare no competing financial interest.
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J. A. Gurak Jr., Z. Liu and K. M. Engle, J. Am. Chem. Soc.,
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Acknowledgements
We gratefully acknowledge funding from the National Natural
Science Foundation of China (21602104), Natural Science
Foundation of Jiangsu Province, China (BK 20160986), the
Starting Funding of Research (3983500176) from Nanjing Tech
University.
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