Sulfoxonium Ylides as Carbene Precursors: Rhodium(III)-Catalyzed Sequential C?H Functionalization, Selective Enol Oxygen-Atom Nucleophilic Addition, and Hydrolysis

Article abstract of DOI:10.1002/adsc.201900861

Herein, we report a protocol for Rh(III)-catalyzed annulation reactions between oxazolines and sulfoxonium ylides via a sequence involving C?H activation, selective enol oxygen-atom nucleophilic addition, and hydrolysis. This practical, operationally simple protocol has a wide substrate range, excellent regioselectivity, and moderate to good yields.

Full text of DOI:10.1002/adsc.201900861

UPDATES
DOI: 10.1002/adsc.201900861
Sulfoxonium Ylides as Carbene Precursors: Rhodium(III)-
Catalyzed Sequential CÀ H Functionalization, Selective Enol
Oxygen-Atom Nucleophilic Addition, and Hydrolysis
Yuanqiong Huang,a Xueli Lyu,a Hongjian Song,a,* and Qingmin Wanga, b,*
a
State Key Laboratory of Elemento-Organic Chemistry, Research Institute of Elemento-Organic Chemistry, College of Chemistry,
Nankai University, Tianjin 300071, People’s Republic of China
E-mail: songhongjian@nankai.edu.cn; wangqm@nankai.edu.cn
Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300071, People’s Republic of China
b
Manuscript received: July 12, 2019; Revised manuscript received: September 29, 2019;
Version of record online: ■■■, ■■■■
Supporting information for this article is available on the WWW under https://doi.org/10.1002/adsc.201900861
approach has been used to construct complex mole-
Abstract: Herein, we report a protocol for Rh(III)-
catalyzed annulation reactions between oxazolines
and sulfoxonium ylides via a sequence involving
cules from simple, readily available building
blocks.[1m,3] Oxazoline moieties are often present in
pharmaceutical molecules[4] and would therefore be a
CÀ H activation, selective enol oxygen-atom nucleo-
good choice as directing groups for CÀ H activation. In
philic addition, and hydrolysis. This practical,
fact, Kakiuchi and co-workers developed a protocol for
operationally simple protocol has a wide substrate
Ru3(CO)12-catalyzed oxazolinyl-assisted silylation of
range, excellent regioselectivity, and moderate to
aromatic CÀ H bonds;[5] and Ru(II)-catalyzed oxazolin-
good yields.
yl-assisted aromatic CÀ H bond alkenylation,[6] (hetero)
arylation,[6a,7] and silylation[8] reactions have been
Keywords: nucleophilic addition; oxazolines; Rho-
reported by other investigators (Scheme 1a).
dium; regioselectivity; ylides
In addition, oxazolines not only can act as directing
groups for CÀ H activation but also can be incorporated
into heterocycles. For example, Kapur and co-workers
Transition-metal-catalyzed CÀ H activation of arenes reported the use of oxazolinyl-assisted Ru(II)-catalyzed
has emerged as a powerful, atom-economical strategy CÀ H functionalization reactions involving migratory
for the construction of heterocycles.[1] In transforma- carbene insertion for the synthesis of isoquinolones
tions involving CÀ H activation, sulfoxonium ylides (Scheme 1b).[9] In addition, Cui and co-workers synthe-
have recently been used as precursors of metal sized N-(2-acetoxyalkyl) isoquinolones by reactions of
carbenes for rapid construction of complex hetero- oxazolines and alkynes via a Rh(III)-catalyzed process
cycles. Vaitla and co-workers used reactions between involving CÀ N bond formation and opening of the
anilines and Ir(II) carbene complexes generated from
sulfoxonium ylides to synthesize indoles.[2a] In addi-
tion, Rh(III)- and Ru(II)-catalyzed transformations
involving CÀ H activation of arenes and heteroarenes
and migratory insertion of sulfoxonium ylides have
witnessed great advancements in recent years.[2b–q]
Among the transition metals typically used for CÀ H
activation, Rh(III) species have achieved prominence.
Directing groups such as pyridines, pyrimidines,
pyrazoles, and carbamates are usually required to
control the regioselectivity of CÀ H activation reac-
tions, but these groups often need to be removed after
the reaction, which reduces the atom and step economy
of such processes. However, functional groups that are
present in the target molecule can be used as directing
groups for CÀ H activation, and this elegant, succinct
Scheme 1. Oxazoline-directed CÀ H activation reactions.
Adv. Synth. Catal. 2019, 361, 1–6
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Table 1. Optimization of reaction conditions.[a]
Entry
Catalyst/Additive
Base
Acid
Solvent
Yield (%)[b]
1
2
3
4
5
6
7
8
[Cp*RhCl2]2/AgNTf2
[Cp*RhCl2]2/AgNTf2
[Cp*RhCl2]2/AgNTf2
[Cp*RhCl2]2/AgNTf2
[Cp*RhCl2]2/AgNTf2
[Cp*RhCl2]2/AgSbF6
[Cp*RhCl2]2/AgNTf2
[Cp*RhCl2]2/AgNTf2
[Cp*RhCl2]2/AgNTf2
[Cp*RhCl2]2/AgNTf2
[Cp*RhCl2]2/AgNTf2
[Cp*RhCl2]2/AgNTf2
[(p-cym)RuCl2]2/AgNTf2
[Cp*IrCl2]2/AgNTf2
[Cp*RhCl2]2/AgNTf2
[Cp*RhCl2]2/AgNTf2
CsOAc
AgOAc
KOAc
PhCOOH
PhCOOH
PhCOOH
PhCOOH
PhCOOH
PhCOOH
HOAc
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
CH2Cl2
CHCl3
DCE
DCE
DCE
DCE
52
35
56
65
54
57
30
37
48
59
57
54
trace
trace
76
NaOAc
Cu(OAc)2
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
PivOH
9
1-AdCOOH
(PhO)2P(O)OH
PhCOOH
PhCOOH
PhCOOH
PhCOOH
PhCOOH
PhCOOH
10
11
12
13
14
15[c]
16[c][,][d]
66
[a] Reaction conditions, unless otherwise noted: 1a (0.2 mmol), 2a (0.4 mmol), catalyst (2.5 mol%), additive (AgX, 10 mol%), base
°
(0.1 mmol), and acid (0.4 mmol) in solvent (2 mL) under argon at 90 C for 18 h.
[b] Yields were obtained by NMR analysis of the crude product with 1,1,2,2-tetrachloroethane as an internal standard.
[c] The amount of DCE was decreased to 1 mL.
[d]
°
The reaction temperature was increased to 100 C.
oxazoline ring followed by reaction with acetate anion alkane solvents (compare entry 4 with entries 11 and
(Scheme 1c).[10] In both of these reactions, isoquino- 12). Only trace amounts of 3aa were obtained when
lones skeleton was constructed, especially in Kapur’s other transition metal catalysts (Ru(II) and Ir(III)) were
work, nucleophilic attack of the carbonyl functional used (entries 13 and 14). The yield improved to 76%
group occurs chemoselectively via the nitrogen atom when the concentration of the reactants was increased
of the oxazolinyl group to generate isoquinolone (compare entry 15 with entry 4), but increasing the
derivatives. However, no instances of the oxygen atom temperature was detrimental (compare entry 16 with
acting as a nucleophile to afford isocoumarins have entry 15). It is worth mentioning that the reaction was
been reported.[11] Therefore, in this study we set out to completely selective for monoalkylated products.
explore this possibility.
These experiments revealed that the optimal reaction
The initial reactions were performed with 2-phenyl- conditions were as follows: 2.5 mol% [Cp*RhCl2]2,
oxazoline (1a) and sulfoxonium ylide 2a as substrates, 10 mol% AgNTf2 as an additive, 0.5 equiv. of NaOAc
[Cp*RhCl2]2 (2.5 mol%) as the catalyst, AgNTf2 as a base, and 2.0 equiv. of PhCOOH as an acid at
°
(10 mol%) as an additive, CsOAc (0.1 mmol) as a 90 C under argon in DCE for 18 h.
base, and PhCOOH (0.4 mmol) as an acid under argon
With the optimized reaction conditions established,
°
in 1,2-dichloroethane (DCE) at 90 C for 18 h, and we we explored the generality of the reaction and were
were gratified to find that these conditions afforded pleased to discover that a variety of phenyl sulfoxo-
desired product 3aa in 52% yield (Table 1, entry 1). nium ylides underwent annulation reactions with
Screening of several other inorganic bases revealed oxazolines to furnish the desired products (3aa–3ao)
that the best yield was obtained with NaOAc (en- in moderate to good yields (Scheme 2). Ylides with
tries 2–5). Next, we evaluated various additives and phenyl rings bearing an electron-donating group (Me,
found that the yield of 3aa was 57% when AgSbF6 tBu, or Ph) or an electron-withdrawing group (F, Cl,
(10 mol%) was employed (entry 6). Different acids- Br, or CF3) at the para position were compatible with
HOAc, PivOH, 1-AdCOOH, and (PhO)2P(O)OH (en- the catalytic system. In addition, a gram-scale reaction
tries 7–10) – were then screened, but the results were of 1a and 2a with a catalyst loading of only 1 mol%
no better than those obtained with PhCOOH (entry 4). gave a 65% isolated yield of 3aa. Substituents at the
DCE was found to be superior to other chlorinated ortho position also proved to be tolerated: 3ai and 3aj
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Scheme 3. Scope of the reaction with respect to the oxazoline. a
Reaction conditions: 1 (0.2 mmol), 2a (0.4 mmol), [Cp*RhCl2]2
(2.5 mol%), AgNTf2 (10 mol%), NaOAc (0.1 mmol), and
Scheme 2. Scope of the reaction with respect to the sulfoxo-
nium ylide. Reaction conditions, unless otherwise noted: 1a
(0.2 mmol), 2 (0.4 mmol), [Cp*RhCl2]2 (2.5 mol%), AgNTf2
(10 mol%), NaOAc (0.1 mmol), and PhCOOH (0.4 mmol) in
a
°
PhCOOH (0.4 mmol) in DCE (1 mL) under argon at 90 C for
18 h. Isolated yields are provided.
°
DCE (1 mL) under argon at 90 C for 18 h. Isolated yields are
provided. The catalyst loading was reduced to 1 mol%, and
the reaction was performed on a 7 mmol scale.
b
were obtained in 77% and 68% yields, respectively.
Moreover, meta-Me- and meta-Cl-substituted aryl
sulfoxonium ylides 2k and 2l also reacted smoothly,
giving the corresponding products in 80% and 73%
yields, respectively. Furthermore, trisubstituted aryl
sulfoxonium ylide 2m gave expected product 3am in
70% yield, and reactions of 1-naphthyl- and 2-
naphthyl-substituted sulfoxonium ylides delivered iso-
coumarins 3an and 3ao in more than 70% yield.
Finally, furyl and thienyl sulfoxonium ylides also
Scheme 4. Deuterium-labeling studies.
To investigate the reaction mechanism, we con-
coupled with 1a to afford 3ap and 3aq, respectively, ducted
in good yields; and alkyl sulfoxonium ylides 2r–2t (Scheme 4). When the reaction of 1a was carried out
likewise proved to be viable substrates. under the standard conditions except that the sulfoxo-
some
deuterium-labeling
experiments
Next, we explored the scope of the reaction with nium ylide was omitted and MeOD was used as the
respect to the oxazoline by carrying out a number of solvent, 93.5% of the deuterium was found at the ortho
reactions with sulfoxonium ylide 2a (Scheme 3). Aryl positions of the benzene ring (eq 1); this high level of
oxazoline substrates with an electron-donating or deuterium incorporation indicates that the reaction may
-withdrawing group in the para position of the benzene have proceeded via activation of the ortho CÀ H bonds.
ring smoothly afforded the corresponding isoquinoli- To further explore this possibility, we undertook a
nones (3ba–3ja) in moderate to good yields. Moderate primary kinetic isotope effect experiment involving 1a
steric bulk was also tolerated: specifically, ortho-Me- and 2a (eq 2) and obtained a kH/kD ratio of 1.1, which
substituted aryl oxazoline 1k was found to be a viable indicates that CÀ H cleavage was unlikely to have been
substrate, although the yield of 3ka (72%) was slightly the rate-limiting step.
lower than that of 3ea (77%). Oxazolines bearing meta
On the basis of literature precedents[2d,12] and our
substituents also regioselectively provided the corre- preliminary mechanistic results, we propose the mech-
sponding products (3la and 3ma) in good yields. anism shown in Scheme 5 for the coupling of oxazo-
Moreover, the reactions of 1-(naphthalen-1-yl)-4,5- line 1a with sulfoxonium ylide 2a. The process begins
dihydrooxazole and 2-(naphthalen-1-yl)-4,5-dihy- with coordination of the N atom of 1a with the active
drooxazole furnished good yields of the expected catalyst Cp*RhX2 and subsequent formation of the key
products; particularly notable was the fact that the five-membered rhodacycle A, in which the base
reaction of 2-(naphthalen-1-yl)-4,5-dihydrooxazole fur- facilitates CÀ H activation. Then coordination of 2a
nished 3oa as a single regioisomer.
generates Rh(III) alkyl species B, and extrusion of
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Acknowledgements
We are grateful to the National Key Research and Development
Program of China (2018YFD0200100) and the National
Natural Science Foundation of China (21977056, 21732002,
21602117, 21672117) for generous financial support for our
programs.
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Scheme 5. Proposed mechanism.
DMSO affords reactive rhodium carbene species C,
which undergoes migratory insertion of the RhÀ C
bond to give six-membered rhodacycle D. Proto-
demetallation of the RhÀ C and RhÀ N bonds of D
affords E and regenerates the active catalyst. The
tautomerization of E to enol intermediate F. Finally,
nucleophilic attack on the imine of F by enol oxygen
and subsequent hydrolysis, which is promoted by acid,
affords the target product.
In summary, we have developed a protocol for Rh
(III)-catalyzed annulation reactions of oxazolines with
sulfoxonium ylides to afford a variety of isocoumarins.
The transformation proceeds via a sequence involving
Rh(III)-catalyzed CÀ H activation, enol oxygen-atom
nucleophilic addition on the imine, and hydrolysis. The
reaction showed a broad substrate scope and good
functional group tolerance and could be carried out on
a gram scale. We are currently exploring the synthetic
applications of this method, which we expect to serve
as a powerful tool for the synthesis of isocoumarins for
drug discovery.
Experimental Section
General Procedure for the Synthesis of Products
To a 25 mL of Schlenk tube were added oxazoline (0.2 mmol),
sulfoxonium ylide (0.4 mmol), [RhCp*Cl2]2 (2.5 mol%),
AgNTf2 (10.0 mol%), NaOAc (0.1 mmol), and PhCOOH
(0.4 mmol) under air. The mixture was then evacuated and
backfilled with Ar (3 times). DCE (1.0 mL) were added
subsequently. The Schlenck tube was screw capped and stirred
°
at 90 C for 18 h. After that, removal of the solvent under
reduced pressure, purification was performed by flash column
chromatography on silica gel to afford the desired product.
Adv. Synth. Catal. 2019, 361, 1–6
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UPDATES
Sulfoxonium Ylides as Carbene Precursors: Rhodium(III)-
Catalyzed Sequential CÀ H Functionalization, Selective Enol
Oxygen-Atom Nucleophilic Addition, and Hydrolysis
Adv. Synth. Catal. 2019, 361, 1–6
Y. Huang, X. Lyu, H. Song*, Q. Wang*