A benzene-bridged divanadium complex-early transition metal catalyst for alkene alkylarylation with PhI(O2CR)2viadecarboxylation

Article abstract of DOI:10.1039/d0dt04295a

The synthesis, structure and catalytic activity of a benzene-bridged divanadium complex were comprehensively studied. The reduction of (Nacnac)VCl₂(1) (Nacnac = (2,6-ⁱPr₂C₆H₃NCMe)₂HC) supported by β-diketiminate with potassium graphite (KC₈) by employing benzene as the solvent allows access to the benzene-bridged inverted-sandwich divanadium complex (μ-η⁶:η⁶-C₆H₆)[V(Nacnac)]₂(2a), which can catalyze alkene alkylarylation with hypervalent iodine(iii) reagents (HIRs)viadecarboxylation to generate regioselectively diverse indolinones. Furthermore, the mild nature of this reaction was amenable to a wide range of functionalities on alkenes and HIRs. Mechanistic studies revealed a relay sequence of decarboxylative radical alkylation/radical arylation/oxidative re-aromatization.

Full text of DOI:10.1039/d0dt04295a

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A benzene-bridged divanadium complex-early
transition metal catalyst for alkene alkylarylation
with PhI(O2CR)2 via decarboxylation†
Cite this: Dalton Trans., 2021, 50,
3201
Lei Zhang, *a Hongfei Zhou,a Shaokun Baia and Shaodan Li*b
The synthesis, structure and catalytic activity of a benzene-bridged divanadium complex were compre-
hensively studied. The reduction of (Nacnac)VCl2 (1) (Nacnac = (2,6-iPr2C6H3NCMe)2HC) supported by
β-diketiminate with potassium graphite (KC8) by employing benzene as the solvent allows access to the
benzene-bridged inverted-sandwich divanadium complex (μ-η6:η6-C6H6)[V(Nacnac)]2 (2a), which can
catalyze alkene alkylarylation with hypervalent iodine(III) reagents (HIRs) via decarboxylation to generate
regioselectively diverse indolinones. Furthermore, the mild nature of this reaction was amenable to a wide
range of functionalities on alkenes and HIRs. Mechanistic studies revealed a relay sequence of decarboxy-
lative radical alkylation/radical arylation/oxidative re-aromatization.
Received 18th December 2020,
Accepted 1st February 2021
DOI: 10.1039/d0dt04295a
rsc.li/dalton
bridged inverted-sandwich divanadium complex towards oxi-
dative substrates including N2O and azobenzene.12 Inspired by
Introduction
Over the past decades, the strategy of utilizing alkene difunc- the rich electron-transfer characteristics of organovanadium
tionalization has evoked surging interest to assemble complex complexes and their extensive utilization in polymerization
and value-added molecules from simple olefin feedstock and oxidation reactions,13 we sought to explore organovana-
efficiently.1 Historically, hypervalent iodine(III) reagents dium-catalyzed alkene functionalization with HIRs. Most
(HIRs)2 benefiting from their intrinsic electrophilic nature are recently, Chen and Mori developed an intermolecular 1,2-oxi-
profoundly used for alkene difunctionalization based on the dative trifluoromethylation of alkenes by using a vanadyl cata-
formation of classical tropylium ions and ensuing nucleophilic lyst and Togni reagents (Scheme 1a).14a Herein we report an
substitution. Mechanistically, the transformation pathway was intramolecular alkylarylation of alkenes by merging a benzene-
heavily dominated by the crucial three-membered iodonium bridged inverted-sandwich divanadium complex with HIRs via
intermediate. Hence, the strategic employment of HIRs for ver- decarboxylation (Scheme 1b), which proceeds regioselectively
satile alkene difunctionalization needs to be explored urgently. in a 5-exo cyclization fashion and provides easy access to
Along this line, the combination of HIRs and transition- diverse indolinones bearing a quaternary carbon center.15
metal catalysts has recently evolved as a prominent approach
to accomplish the difunctionalization of olefins.3–11 Among
them, a well-established single- and multi-electron transfer
pathway occurs between HIRs and the catalysts derived from
mid-late transition metals, such as Re,3 Pd,4 Ir,5 Cu,6 Fe,7 Co8
and so on.9–11 However, the early-transition metal vanadium
with its inexpensive and Earth-abundant nature remains
largely retarded towards alkene difunctionalization. In 2008,
Tsai et al. reported the chemical reductive reactivity of toluene-
aHebei Key Laboratory of Organic Functional Molecules, College of Chemistry and
Materials Science, Hebei Normal University, Shijiazhuang 050024, China.
E-mail: zhanglsd@hebtu.edu.cn
bCollege of Resources and Environment Science, Hebei Normal University,
Shijiazhuang 050024, China. E-mail: lishaodan@hebtu.edu.cn
†Electronic supplementary information (ESI) available. CCDC 2025708. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
d0dt04295a
Scheme 1 Organovanadium catalyzed difunctionalization of alkenes
with hypervalent iodine(III) reagents (HIRs).
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(Me3C)2-4-Me–C6H2O]U}2(μ-η6:η6-C6H6) (range 2.548(11)–2.611
(11) Å),19b {(C5Me5)[(Me3Si)2CH]U}2(μ-η6:η6-C6H6) (range 2.5323
(19)–2.6398(19) Å),19b [U(BIPMTMSH)(I)]2(μ-η6:η6-C6H5CH3)
(range 2.553(7)–2.616(7) Å),19c [U(BIPMTMS)]6(μ-I)3(μ-η6:η6-
Results and discussion
Synthesis of (μ-η6:η6-C6H6)[V(Nacnac)]2 (2a)
The
reduction
of
(Nacnac)VCl2
(1)16
(Nacnac
=
(2,6-iPr2C6H3NCMe)2HC) with an excess of KC8 17 by employing C7H8)3 (range 2.576(9)–2.863(7) Å)19d and {U[N(R)Ar]2}2(μ-η6:η6-
benzene as the solvent gives the benzene-bridged inverted- C6H5CH3) (R = adamantyl; Ar = 3,5-dimethylphenyl) (range
sandwich divanadium complex (μ-η6:η6-C6H6)[V(Nacnac)]2 (2a) 2.503(9)–2.660(8) Å).19e,f Moreover, any cyclic fragment in 2a is
in 71% yield (Scheme 2).18 Compared with the fully character- planar.
ized complex (μ-η6:η6-C7H8)[V(Nacnac)]2 (2b),12 the novel hom-
ologue (μ-η6:η6-C6H6)[V(Nacnac)]2 (2a) can be considered as a
Catalytic alkene alkylarylation with HIRs via decarboxylation
dinuclear V(I)–V(I) complex bridged by a benzene molecule,
and is also air and moisture sensitive. Moreover, complex 2a as
a brown solid is soluble in and easily recrystallized from an
n-hexane solution, and it was characterized by single-crystal
X-ray diffraction and IR spectroscopy in the solid state. The
highly symmetric molecular structure of 2a deciphered by
X-ray diffraction analysis is shown in Fig. 1. The average C–C
distance of the sandwiched benzene molecule in 2a is 1.419(4)
Å, which is close to the corresponding C–C distances of the
bridging toluene molecule in 2b. The V–Carene bond lengths
range from 2.236(2) to 2.255(2) Å, and the average V–C distance
is 2.245(2) Å for 2a; whereas the V–Carene bond lengths range
from 2.170(5) to 2.419(5) Å, and the average V–C distance is
2.262(5) Å for 2b.12 This slight difference in V–Carene bond
lengths may lead to different reactivities between two com-
plexes 2a and 2b. Moreover, the V–Carene bond length in the
range of 2.236(2)–2.255(2) Å is shorter than the U–Carene bond
lengths in uranium complexes [(C5Me5)2U]2(μ-η6:η6-C6H6)
Initially, we commenced the reaction of N-methyl-N-phenyl-
methacrylamide 3a and PhI(OAc)2 4a in the presence of the
toluene-bridged
divanadium
complex
(μ-η6:η6-C7H8)[V
(Nacnac)]2 (2b) as the catalyst. To our delight, 5aa was obtained
in 58% NMR yield by using toluene as the solvent (entry 1,
Table S2 in the ESI†).18 Surprisingly, when toluene was
replaced with benzene as the solvent, the NMR yield of 5aa
was dramatically increased to 82% at 80 °C for 10 h (entry 2,
Table S2 in the ESI†).18 Inspired by the above results and the
arene exchange reaction between complex 2b and C6D6,12 we
synthesized the benzene-bridged divanadium complex (μ-η6:η6-
C6H6)[V(Nacnac)]2 (2a) by the recrystallization of 2b in benzene
as a potentially effective catalyst. As expected, the corres-
ponding indolinone 5aa was obtained in 87% 1H NMR yield
with refluxed benzene as the solvent for 10 h (entry 1, Table 1).
Nevertheless, a reduced yield by 20% was observed in the
toluene solution (entry 2, Table 1), due to the Csp3–H bond in
toluene being more susceptible to the radicals than to the aro-
matic Csp2–H bonds.15a Screening of the solvents and reaction
(range
2.509(14)–2.733(14)
Å),19a
{[(Me3Si)2N](C5Me5)
U}2(μ-η6:η6-C6H6) (range 2.559(3)–2.631(3) Å),19a {(C5Me5)[2,6-
Table 1 Screening of the reaction parametersa
Entry
Variations in the standard conditions
Yield of 5aab (%)
Scheme 2 Synthesis of (μ-η6:η6-C6H6)[V(Nacnac)]2 2a.
1
2
3
4
5
6
7
8
None
87 (80)c
67
39
61
27
68
63
74
69
71
0
22
43
75
85
87
81
84
C7H8 instead of C6H6
CH3CN instead of C6H6
C6H5Cl instead of C6H6
THF instead of C6H6
1,4-Dioxane instead of C6H6
DME instead of C6H6
MTBE instead of C6H6
0.1 M instead of 0.2 M
0.4 M instead of 0.2 M
Without 2a
50 °C instead of 80 °C
1.0 instead of 2.0 eq. of 4a
1.3% instead of 2.5 mol% of 2a
100 °C instead of 80 °C
3.0 instead of 2.0 eq. of 4a
5% instead of 2.5 mol% of 2a
12 h instead of 10 h
9
10
11
12
13
14
15
16
17
18
a Reaction conditions: 3a (0.2 mmol), 4a (0.4 mmol), 2a (2.5 mol%),
C6H6 (1.0 mL), 80 °C, 10 h. b 1H NMR yields with 1,3,5-trimethoxyben-
zene as an internal standard. c Isolated yield on a 0.5 mmol scale.
Fig. 1 Molecular structure of 2a (thermal ellipsoids drawn at the 35%
probability level).
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concentrations led to inferior results (entries 3–10, Table 1). strongly electron-deficient nitro group, as well as halogen func-
Among these, a dramatically decreased yield (27%) was also tionalities (F, Cl, Br and I), was well tolerated in this transform-
observed in THF solution (entry 5, Table 1), again, due to the ation. Furthermore, the reaction occurred successfully with
Csp3–H bond in THF being more susceptible to the radicals ortho- and meta-substituted N-aryl-N-methylmethacrylamides
than to the aromatic Csp2–H bonds.15b Of note, no reaction to afford regioisomeric indolinones (5ka, 5la and 5la′) in good
occurred in the absence of any catalyst (entry 11, Table 1). A yields. Specifically, the meta-methyl substituent as an electron-
lower reaction temperature, reduced amount of HIRs, and less donating group does not make obvious interference on the
catalyst loading decreased the reaction outcome dramatically regioselectivity of the intramolecular cyclization process with a
(Table 1, entries 12–14). Additionally, no better reaction 1.5 : 1 regioisomeric ratio of indolinones 5la and 5la′, thereby
outcome was achieved with an increased temperature of again implying
100 °C, 3 equiv. of HIRs, double catalyst loading and extended N-phenylmethacrylamides bearing either aliphatic or aromatic
reaction time (Table 1, entries 15–18). substituents on the N-atom performed well in this transform-
a radical arylation pathway. Moreover,
Having identified the optimal reaction conditions for 5-exo ation, providing the corresponding indolinones in good to
cyclization, we firstly tested the scope of alkenes 3 with PhI excellent yields (5ma–5oa). Of note, N-benzyl-N-phenylmetha-
(OAc)2 4a as the reaction partner (Table 2). Variations in the crylamide was also highly reactive towards this reaction to
aryl moiety showed that both electron-rich and electron- deliver solely the five-membered indolinone 5pa, due to the
deficient substituents at different positions of the phenyl ring selective 5-exo cyclization on the aniline moiety rather than 6-
were not only amenable to the reaction conditions, but also exo cyclization on the benzylamine moiety. Remarkably, the
had little influence on the reaction outcome (5aa–ja), indicat- rigid 1-methacryloyl-1,2,3,4-tetrahydroquinoline was well com-
ing a likely radical arylation process. In particular, the more patible with the current reaction conditions for furnishing the
tricyclic indolinone product 5qa in 77% isolated yield.
Notably, N-phenylmethacrylamide containing an electron-
deficient acetyl substituent underwent the desired transform-
ation to produce the N-acetyl substituted indolinone 5ra in
Table 2 Reaction scope for 5-exo cyclizationa,b
synthetically useful yield. Unfortunately, the free N–H sub-
strate showed no reactivity towards this transformation. In
addition, switching R3 from the methyl group to the phenyl
group also led to the desired indolinone 5sa in good yield.
Subsequently, an array of HIRs 4 was further investigated
with N-methyl-N-phenylmethacrylamide 3a as a template sub-
strate for the catalytic protocol. Gratifyingly, HIRs derived from
diverse aliphatic carboxylic acid bearing primary, secondary
and tertiary alkyl groups were all well applicable to the alkene
alkylarylation protocol, delivering the expected indolinones via
decarboxylation. Specifically, primary alkyl chains such as
linear ethyl, 1-nonyl and 1-pentadecyl, as well as 4-chloroben-
zyl, can be smoothly introduced into the indolinone skeletons
5ab–5ae. Additionally, secondary aliphatic carboxylic acid
derived HIRs containing branched alkyls (e.g. isopropyl and
cyclohexyl) were well-tolerated to yield indolinones 5af and
5ag, respectively. Notably, the more sterically hindered pivalic
and 1-adamantanoic acid-derived HIRs were also capable of
undergoing 5-exo cyclization to deliver indolinones 5ah and
5ai in good yield, respectively.
The proposed mechanism
To gain better insights into the nature of this alkylarylation
reaction, a control experiment was initially carried out with
2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO). As a result, no
indolinone 5aa was observed upon the addition of TEMPO as a
radical scavenger to the reaction between N-acrylamide 3a and
HIR 4a.18 In contrast, 1-methoxy-2,2,6,6-tetramethylpiperidine
6 20 as the adduct formed by TEMPO and a methyl radical was
a Reaction conditions: 3 (0.5 mmol), 4 (1.0 mmol), 2a (2.5 mol%), C6H6
detected by 1H NMR (Scheme 3a), which suggested that the
(2.5 mL), 80 °C, 10 h. b Isolated yields of product 5 are shown.
c Determined by 1H NMR in crude reaction mixtures. d 4 (1.5 mmol).
initial alkylation might occur through a radical pathway. Next,
the competition experiment between the electro-donating
e 24 h.
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F while completing the redox catalytic turnover with the regen-
eration of V(I) complex-2a. Finally, F underwent deprotonation
to furnish the re-aromatized product 5aa under the promotion
of the acetate anion.
Conclusions
In conclusion, a novel benzene-bridged inverted-sandwich
divanadium complex was firstly synthesized and used as a
catalyst for the decarboxylative alkylarylation of alkenes with
HIRs derived from aliphatic acids. This transformation allows
for rapid access to an array of quaternary carbon-centered
indolinones via 5-exo radical cyclization. Mechanistic studies
revealed a relay sequence of intermolecular alkyl radical
addition/intramolecular radical cyclization/oxidative re-aroma-
tization. Further explorations on vanadium-catalysis in olefin
functionalization are ongoing and will be detailed in due
course.
Scheme 3 Mechanistic experiments. a Determined by 1H NMR.
methoxy substituted 3c and electro-withdrawing cyano substi-
tuted 3i was performed with an insufficient amount of HIR 4a,
and showed that the corresponding indolinones 5ca and 5ia
were formed in 17% and 21% NMR yields, respectively
(Scheme 3b). Thus, it seems that the intramolecular cyclization
process was insensitive to the substituent electronics and
might proceed in a radical manner rather than in a cationic
arylation manner.
Conflicts of interest
Based on the aforementioned experimental results and pre-
vious literature reports,3,5b,7b,14b,21 a plausible reaction mecha-
nism for this 5-exo cyclization is shown in Scheme 4. The first
heterolysis of one I–O bond in HIR 4a produced cationic
species A and an acetate anion. The subsequent reduction of
the generated species A by V(I) complex-2a through single elec-
tron transfer (SET) allowed access to the radical iodine inter-
mediate B and the V(II) species. Then, the homolysis of the
remaining I–O bond in B formed PhI and an acetoxy radical,
which immediately transformed to methyl radical C with the
There are no conflicts of interest to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grant No. 21801059), the Natural
Science Foundation of Hebei Province (Grant No.
B2020205005), the Education Department Foundation of
Hebei Province (Grant No. QN2018110) and the Science
Foundation of Hebei Normal University (Grant No. L2018B06).
release of CO2 via decarboxylation.3,4l,5b,7b Afterwards,
a
β-situated radical addition between methyl radical C and
alkene 3a led to the formation of an α-situated radical inter-
mediate D, which spontaneously cyclized to localized radical
species E via 5-exo radical cyclization. The resultant E was
further oxidized by the V(II) species to generate cationic species
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Scheme 4 Tentative reaction mechanism.
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