Harnessing hypervalent iodonium ylides as carbene precursors: C-H activation of: N -methoxybenzamides with a Rh(iii)-catalyst

Article abstract of DOI:10.1039/d0cc06038k

Hypervalent iodonium ylides expeditiously generate carbenes which undergo domino intermolecular C-H activation followed by intramolecular condensation in the presence of N-methoxybenzamide as a starting material and a Rh(iii)-catalyst to afford dihydrophenanthridines. KIE studies and DFT calculations were performed to substantiate the mechanistic pathway. To extend the synthetic utilisation, fluorescent pyranoisocoumarins were achieved by using Rh(iii)-catalyzed peri-C-H/O-H activation/annulation reactions.

Full text of DOI:10.1039/d0cc06038k

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DOI: 10.1039/D0CC06038K
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Harnessing hypervalent iodonium ylide as carbene precursors; C-H
activation of N-methoxybenzamides by Rh(III)-catalyst
Sivakalai Mayakrishnan,a Masilamani Tamizmani b and Naryanan Uma Maheswari *a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
An expeditious generation of carbene from hypervalent iodonium functionalization has been enormously implied in organic
ylide incorporating N-methoxybenzamide as a starting material synthesis and revolutionized synthetic strategies.11
upon Rh(III)-catalyst undergoes domino intermolecular C−H
activation followed by intramolecular condensation to afford
dihydrophenanthridines. KIE studies and DFT calculations were
performed to substantiate the mechanistic pathway. To extend the
synthetic utilisation, fluorescent pyranoisocoumarins were achived
by using a Rh(III)-catalyzed peri-C-H/O-H activation/annulation
reactions.
Hypervalent iodine-(III) ylides (HVI) have experienced
a
tremendous growth for its versatile synthetic applications.1
Iodonium ylides exhibit their characteristic thermal stability and
solubility in common organic solvents were being sought as
precursors2 for carbenes or metallocarbenoid intermediates for
transformations in which they act as electrophiles or
nucleophiles.3 Among the deployed carbene precursors,4 diazo
surrogates are widely used.5 HVI compounds have gained
considerable attraction due to their reactivity is reminiscent of
toxic and costly transition metals. Hence, these aryl iodides are
environmentally benign and widely utilized.6 Previously,
sulfoxonium and sulfonium ylides7 act as good carbene
counterparts. The aryl iodonium ylides are well known
surrogates utilized both in transition metal-catalyzed and a
catalyst free reactions with alkenes, ketenes, alkynes, nitriles,
isocyanates, thiols, and carbodiimides.8 Owing to their
affordability and ease of availability with special emphasis
about its safety aspects compared to diazo surrogates, HVI has
been deployed as the most preferred carbene counterpart and
it provides access for a variety of polycyclic heteroarenes,
carbocyclic and heterocyclic scaffolds.
Scheme 1. C-H bond functionalization of different carbene precursor
Developing a protocol to validate a methodology for the
selective functionalization of different C-H bonds in a given
substrate has been a long-standing effort. In this context
especially, the Rh(III)-catalysed transformation reactions were
exploited predominantly with researcher’s attention at large.12
Meanwhile, Rh(III)-catalysed construction of nitrogen-
containing heterocycles mediated by C−H activation explored
extensively.13 Previously, Glorius and co-workers have
developed a C–H activation of N-methoxybenzamide and
accomplished the synthesis of isoquinolones with α-
MsO/TsO/Cl ketones as oxidized alkyne equivalents [Scheme 1,
a].14a Li and co-workers utilized the sulfoxonium ylide as a
carbene precursor for chemo divergent cyclization of N-
methoxybenzamide that leads to N-methoxyisoquinolones
[Scheme 1, b].14b Ramana and co-workers have developed the
Ir(III)-catalyzed carbenoid functionalization of benzamides with
diazo compounds towards the synthesis of N-
Methoxyisoquinolinediones and N-Methoxyisoquinolinones
[Scheme 1, c].14c However, to the best of our knowledge, Rh(III)-
catalyzed oxidative C−H functionalization/intramolecular
cyclisation with hypervalent iodonium ylide has not been
reported in the literature ever before. To extrapolate our work
in this context,15 we have accomplished an efficient Rh(III)-
catalyzed synthesis of 3,4-dihydrophenanthridins via cascade
reaction of hypervalent iodonium ylides with N-
methoxybenzamides under the mild reaction conditions
[Scheme 1, d]. To investigate the present Rh(III)-catalyzed C-H
The transition-metal-catalysed activation of C-H/C-C bonds,9
with insertion of π-systems has become well-developed for the
synthesis of carbocycles and heterocyclic compounds.10 The
discovery
of
transition-metal-catalysed
C-H
bond
a.
Organic & Bioorganic Chemistry Laboratory, CSIR-Central Leather Research
Institute, Chennai 600020, India. Email: umamaheswari@clri.res.in
b. State Key Laboratory of Organometallic Chemistry, Shanghai institute of Organic
Chemistry.
† Electronic Supplementary Information (ESI) available: [Experimental section and
characterisation]. See DOI: 10.1039/x0xx00000x
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activation/annulation reaction, N-methoxybenzamide 1a and hypervalent iodonium ylides (2h-k) had significantVeiefwfeAcrtticolenOntlhinee
5,5-dimethyl-2-(phenyl-l3-iodanylidene)cyclohexane-1,3-dione outcome of reaction producing 3ad-3ag wDOitIh: 1m0.1o0d3e9/rDa0teCCy0ie6l0d3s8K
.
2a were used as a trail substrates under different reaction Table 2. Substrate scope of 2b-k with 1aa
conditions (for details see supporting information (ESI)).
Table 1. Substrate scope of 1a-w with 2aa
Reaction conditions: 1a (0.6 mmol), 2b-k (0.6 mmol, 1.0 equiv), [RhCp*Cl2]2 (3.0
mol%), CsOAc (1.0 equiv.) and AgSbF6 (12 mol%) in Acetone (5.0 mL) at 85 oC for
12 h. a Isolated yields.
The competitive experiment was performed between N,4-
dimethoxybenzamide having an electron-donating 1g and 4-
fluoro-N-methoxybenzamide an electron withdrawing 1h with
2a enabled preferential formation of 3g (3g:3h = 1.0 :0.33,
Scheme 2, a). We envisaged the reaction efficiency of 1a with
2l under the chosen reaction conditions (Scheme 2, b). The
reaction led to the formation of isochromenone derivative 3ah’
in 75% of yield instead of above-mentioned product
isoquinolinone derivative 3ah. When the reaction was carried
out using N-methyl benzamide 1x or N-phenyl benzamide 1y
instead of N-methoxybenzamide 1a with 2a under the
optimized reaction condition, resulted in no formation of the
product 3ai and 3aj (See ESI, Scheme 9). Similarly, the control
reaction between 1a with 1,3-diketone 2m instead of 2a did not
provide the desired product 3a (See ESI, Scheme 10). Also, the
addition of (diacetoxyiodo)benzene (PIDA) to the reaction
between 1a with 1,3-diketone 2m under optimized reaction
condition resulted in traces of 3a only (See ESI, Scheme 11).
Meanwhile, the five membered HVI 2m was tested with 1a with
the same reaction condition has led to no formation of desired
Reaction conditions: 1a (0.6 mmol), 2b-k (0.6 mmol, 1.0 equiv), [RhCp*Cl2]2 (3.0
mol%), CsOAc (1.0 equiv.) and AgSbF6 (12 mol%) in Acetone (5.0 mL) at 85 oC for
12 h. a Isolated yields.
With the optimized reaction conditions in hand, the generality
of this procedure was first investigated by testing the catalytic
reaction with various electronically substituted N-
methoxybenzamides 1b-1w with 2a as shown in (Table 1). The
para alkyl substituted N-methoxy benzamide 1b-f underwent
cyclization with 2a, afforded the titled dihydrophenanthridines
(
3b-3f) in excellent yield. Next, we treated electron donating
group (methoxy) substitution at 4th position with 2a which
provided the corresponding dihydrophenanthridine 3g in 92%
of yield. It was found that halogen substituted benzamides F, Cl,
Br and I at the p-position could be coupled smoothly with 2a in
3h, 3i, 3j and 3k afforded in excellent yield. Substitution of the
strong electron withdrawing groups at the p-position of phenyl
ring CF3 and NO2 of the benzamide provided 3l and 3m in 79%
and 38% of the yield. In addition, the reaction was extended to
biphenyl and naphthyl fused N-methoxybenzamide which also
coupled with 2a gives 84% and 82% of 3n and 3o of yields. The
cyclization of o-substituted N-methoxybenzamides 2p-r with 2a
provided 3p-r in 84%, 88% and 99% yields, respectively. The
structure of compound 3s unambiguously confirmed by using
XRD analysis (See ESI table 3). Subsequently, the reaction was
tested with m-methyl, methoxy and chlorine substituted N-
methoxybenzamides treated with 2a afforded 3s-u in 85%, 86%
and 80% of yield, respectively. Further, 3,4-dimethoxy
benzamide 1u, N-methoxybenzo[d][1,3]dioxole-5-carboxamide
1v with 2a afforded 3v and 3w in 99% and 76% of yields.
product 3ak (See ESI, Scheme 12
)
Scheme 2. Competitive experiments and reaction with 2l
The scope of annulation reaction was examined with various
substituted hypervalent iodonium ylides 2b−k (Table 2). The
To gain a preliminary mechanistic insight, the deuterium
labelling experiment was performed.
A 49% deuterium
reaction was compatible with R1, R2 = H substituted ylide 2b
with 1a provided annulated 3x in 48% yield. In the reaction, 3-
arylated hypervalent iodonium ylides 2c were treated with 1a
provided good yield 75% of 3y. A hypervalent iodonium ylide
bearing an aryl at 4th substitution such as 4-Me, 4-OMe, 3,4-
dimethoxy and 3,4,5-trimethoxy on the aromatic group reacted
incorporation at ortho-C-H bonds of 1b was observed in the
presence of D2O (2.5 equiv.), which indicates that the C-H bond
cleavage step is reversible (Scheme 3, a). Further, in the
reaction of 1a-d5 with 2a under the same reaction condition, the
expected cyclized products 3a-d4 were observed in the yield of
72% (Scheme 3, b). The competitive parallel kinetic isotopic
experiments (KIE) were performed using substrates 1a and 1a-
d5 with 2a that provided kH/kD = 1.28. These results indicate that
smoothly to give 3z, 3aa, 3ab and 3ac in 65%, 70%, 72% and 55%
of yields, respectively. While 3-aryl halide substituents with
2 | J. Name., 2012, 00, 1-3
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the C-H bond activation process is not the rate-determining step intermediate Int4 readily provides Rh-carbene intermediate
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of this reaction (Scheme 3, c). We carried out a scale-up Int5 that located at -139.6 kcal/mol. The DtrOaIn: 1s0it.i1o0n39s/tDa0tCeCT0S6203f8oKr
synthesis of 3a using 1a (1.05 g, 7.0 mmol) and 2a (2.45 g, 7.0 the migratory insertion step is located at -132.8 kcal/mol with a
mmol). The desired product 3a (1.5 g) was isolated in 80% of barrier of 6.8 kcal/mol, which forming six-membered Rh(III)
yield. Also, N-deprotection reaction was performed as a complex Int6 (-164.2 kcal/mol). Subsequently, the intermediate
representative for 3a (Scheme 3, d). The transformation of 3a Int7 is formed by the protonation of C−Rh bond of Int6 with
was accomplished using NaH in DMSO at room temperature for acetic acid, and regenerates the active catalyst. Further, the
30 minutes. The corresponding free N-H product 4a was enamine type reaction of Int6 takes place to provide the
obtained in excellent yield (95%).
product 3a. Our DFT studies revealed that the C-H activation
Scheme 3. Primary mechanistic investigation and gram scale synthesis
process is not the rate-determining step, which was in good
agreement with our small experimental KIE results (kH/kD
1.28).17
=
Figure 1. Gibbs free energy profile (kcal/mol) obtained at the SMD/M06/6-
311++G(d,p), SDD(Rh) level of theory in acetone solvent.
To understand the mechanism (Scheme 4), DFT calculations Although numerous approaches in formulating isoquinoline
were performed with the SDD and M06 hybrid functional to get derivatives have been developed,18 owing to their importance,
a Gibbs free energy profile diagram at 273.15 K. The the development of novel and expeditious methods for the
intermediate Int1 undergoes C-H bond activation via transition divergent construction of polycyclic isoquinolines from readily
state TS-1, in which the acetate ligand deprotonates an ortho C- available starting materials are still highly desirable. When 4a
H to afford the intermediate Int2 at relative energy of 6.8 was treated with 5a in the presence of [RhCp*Cl2]2 (5.0 mol%),
kcal/mol (Figure 1).16
AgSbF6 (20 mol%) and Cu(OAc)2.H2O (1.0 equiv) in 1,2-DCE
Scheme 4. Proposed catalytic pathway for the reaction of 1a with 2a using Rh(III)- heated at 110 oC under nitrogen for 12 hours (See, the detailed
catalyst
optimization study ESI), the annulated product 6a was isolated
as yellow solid with 85% yield. The substrate scope (Scheme 5
)
of the annulation reaction was examined with differently
substituted alkyne 5b-e smoothly coupled with 4a to give
corresponding 6b-e in 65-78% of yield, respectively.
Scheme 5. Rh(III)-catalyzed synthesis of pyranoisocoumarins by using 4a with
alkynes
These concerted metalation-deprotonation (CMD) step is not as
much of energy demanding process (11.3 kcal/mol), which
suggests that the C-H activation is not the rate-determining
step. Consecutively, the five-membered rhodacycle
intermediate Int2 loses -AcOH, which providing a vacant binding
site on Rh(III) for the iodosocarbene coordination. The resulting
intermediate Int3 is located at -4.5 kcal/mol. The coordination
of the iodosocarbene 2a to Int3 forms intermediate Int4 with
exothermicity (-10.7 kcal/mol). The subsequent de-iodination of
Additionally, the structure of compound 6c unambiguously
confirmed by X-ray diffraction analysis (for details see ESI table
4). The unsymmetrical alkyne (5f) underwent smoothly with 6f
(70%) yield and exhibits an excellent regioselectivity. Likewise,
symmetrical aliphatic alkyne was annulated thus resulted the
product 6g with 65% of yield. We surprisingly found that
pyranoisocoumarins could exhibit bright fluorescence, which
can be utilized to measure photoluminescence properties,
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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excited states for 6a-e in solution and solid state. Our studies
can further be applied in developing new organic
fluorophores,15 by investigating their photophysical properties
and potential applications of organic fluorescent molecules in
smart materials, optoelectronic devices, fluorescent sensors,
and biomedical imaging.19 We have measured the absorption
and emission spectra in DCM solution and thin-film are listed in
(Figure 2). The fluorescence of compound 6a–6e shows
emission maxima at 456–472 nm in DCM solution and 492-529
nm in thin-film (for details of PL study see ESI).
View Article Online
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line), and thin film (dotted line); (b) Normalized fluorescence 6a, 6b, 6c, and 6e in
DCM solution (solid line), and thin film (dotted line); Con 5.0 µM; Excitation
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In summary, the first example of Rh(III)-catalysed C-H bond
activation of N-methoxybenzamide with HVI deployed as a
carbene precursor has been reported. The cyclisation reaction
is compatible with various electronically substituted functional
groups, substituted benzamides and HVI’s. The detailed
mechanistic investigation was demonstrated by deuterium
labelling studies, competitive experiments and DFT calculations.
Furthermore, DFT calculations revealed that the C−H activation
process is the rate-limiting step, which is in good agreement
with experimental KIE results. Besides, the N-deprotected
dihydrophenanthridines were subjected to peri C-H/O-H bond
activation/annulation with alkyne upon Rh(III)-catalyst and the
resulted pyranoisocoumarins and their photophysical
properties were investigated.
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N.U.M. thanks the Director Dr. K. J. Seeram CSIR-CLRI and S.
M. acknowledges the CSIR, India, for providing a Senior
Research Fellowship (Grant No. 31/6(422)/2017-EMR-I).
Conflicts of interest
There are no conflicts to declare
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4 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/D0CC06038K
Harnessing hypervalent iodonium ylide as carbene precursors; C-H
activation of N-methoxybenzamides by Rh(III)-catalyst
Sivakalai Mayakrishnan,a Masilamani Tamizmani,b Narayanan Uma Maheswari *a
a Organic & Bio-organic Chemistry Laboratory, CSIR-Central Leather Research Institute, Adyar, Chennai-600020,
India, Corresponding author E-mail: umamaheswari@clri.res.in
b State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy
of Sciences, 345 Lingling Road, Shanghai 200032, China
The first ever attempt of Hypervalent Iodonium ylides with readily available N-
methoxybenzamides by using Rh(III)-catalyst has been well exploited