Metal-free carbonyl C(sp2)-H oxidative alkynylation of aldehydes using hypervalent iodine reagents leading to ynones

Article abstract of DOI:10.1039/c5cc03362d

A new metal-free tert-butyl hydroperoxide (TBHP)-mediated carbonyl C(sp²)-H oxidative alkynylation of aldehydes with ethynyl benziodoxolones (EBX) for the synthesis of ynones is described. This method is based on a carbonyl C(sp²)-H

Full text of DOI:10.1039/c5cc03362d

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Metal-Free Carbonyl C(sp2)-H Oxidative Alkynylation of Aldehydes
View Article Online
DOI: 10.1039/C5CC03362D
With Hypervalent Iodine Reagents Leading to Ynones
Xuan-Hui, Ouyang,a Ren-Jie Song,*a Cheng-Yong Wang,a Yuan Yang,a and Jin-Heng Li*a,b
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
First published on the web Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
5
of the most powerful methods for the C-C bond formation.7 In
this content, the oxidative couplings of the aldehyde C(sp2)-H
bonds offer particularly effective and highly atom-
A new metal-free tert-butyl hydroperoxide (TBHP)-mediated
carbonyl C(sp2)-H oxidative alkynylation of aldehydes with
ethynyl benziodoxolones (EBX) for the synthesis of ynones is
a
10 described. This method is based on a carbonyl C(sp2)-H oxidative
radical coupling process according to mechanistic studies, and
provides a general route to the assembly of diverse ynones with
broad substrate scope and excellent functional-group
compatibility.
55 economical tool to incorporate an acyl group into the product
through the construction of the C-C bonds.8 Herein, we report
the first example of a TBHP-mediated carbonyl C(sp2)-H
oxidative radical alkynylation of aldehydes with enynyl
benziodoxolones9 for ynone synthesis under metal-free
60 conditions that exhibits broad substrate scope and excellent
functional group tolerance (Scheme 1b); this method proceeds
through a carbonyl C(sp2)-H oxidative radical coupling, and
provides a highly efficient practical route to constructing the
C(sp2)-C(sp) bonds.
15 Alkynes, including ynones, are arguably important construct
motifs in bioactive molecules and materials, as well as are
versatile intermediates for the assembly of structurally diverse
molecules in synthesis.1,2 Consequently, considerable efforts
have been dedicated to the development of efficient methods
20 for such compound synthesis.2-5 Despite impressive progress
in the field, efficiently practical synthesis of ynones still
presents a more stringent challenge. The most frequent classic
method for ynone synthesis is the oxidation of the
corresponding propargyl alcohols.3 However, many propargyl
25 alcohols are unavailable, and among the oxidant processes the
competing side reactions (e.g. site- or over-oxidation) restrict
the substrate scopes and the selectivity control. The
Sonogashira cross-couplings2 of terminal alkynes with acyl
chlorides4 or aryl halides/CO5 represent a general and
30 convenient alternative to achieve this goal, but such
successful C-alkynylation approaches face limitations
associated with the use of pre-functionalized organic
electrophiles (e.g., organic halides) and the requirement of the
expensive transition-metal/ligand catalytic system (e.g.,
35 palladium/phosphine). Recent successes in the C-alkynylation
of aldehydes with alkynes provided an appealing route to
these ynone molecules.6 Huang and co-workers6a have
reported a new gold/amine synergistic catalysis for the
synthesis of 1-(triisopropylsilyl)-3-alkyl-1-yn-3-ones from
a) Previous work:
Huang et al. (ref 6a)
R2
L =
R1
O
R1
I
O
O
O
N
H
O
AuCl3/L
Et2O
O
+
H
O2
R1
40 o
C
R2
R2
EBX (R2 = TIPS)
N
N
Li et al. (ref 6c)
R2
I
O
O
O
[IrCp*Cl2]2, CsOAc, air
MeOH or dioxane, rt
H
O
+
R1
R1
or [RhCp*Cl2]2, CsOAc
R2
DCM, N2, 50 o
C
DG
DG
R2-EBX
DG = directing group
b) This work:
Metal-free
C-H oxidative radical
coupling strategy
R2
I
O
O
O
O
TBHP
+
R1
PhCl
120 oC, 3 h
R1
H
No directing group
R2
1
Broad substrate scope
3
2 (R2-EBX)
65
Scheme 1. Reactions of Aldehydes with Ethynyl Benziodoxolones.
Our initial investigation focused on the alkynylation
between 1H-indole-3-carbaldehyde (1a) and TIPS-EBX 2a
(Table 1). The results demonstrated that treatment of aldehyde
70 1a with 1.5 equiv TIPS-EBX 2a and 2 equiv TBHP (5M in
o
decane) in PhCl at 120 C for 3 h afforded the desired ynone
3aa in 81% yield (entry 1). The reaction temperatures were
found to affect the alkynylation, as a higher or lower reaction
temperature had a negative effect on the reaction (entries 1-3).
75 Notably, increasing amount of TBHP to 3 equiv gave the
identical results to those of 2 equiv TBHP (entries 1 and 4).
However, no expected alkynylation reaction occurred without
TBHP oxidant (entry 5). Three other peroxide oxidants,
namely, 70% aqueous TBHP, di-tert-butyl peroxide (DTBP)
80 and dicumylperoxide (DCP), showed activity for the
alkynylation, but less effective than TBHP in decane (entry 1
versus entries 6-8). DMF as medium proved ineffective for
the alkynylation (entry 9), and other solvents, including
CH2ClCH2Cl, n-BuOAc and MeCN, were found to be less
85 effective than PhCl (entry 1 versus entries 10-12).
40 aliphatic
benziodoxolones (TIPS-EBX) by
aldehydes
and
(triisopropylsilyl)ethynyl
sequence of -
a
vinylidenation6b and in situ aerobic cleavage of the C-C bond.
Very recently, Li and co-workers6c illuatrated an ortho-
chelation-assistant strategy to access 1-aryl-2-yn-1-ones by
45 iridum- and rhodium-catalyzed C-H activation and formyl
alkynylation of benzaldehydes with enynyl benziodoxolones.
However, these approaches also suffer from the requirement
of noble transition metals (Au, Ir or Rh) and the narrow
substrate scope (aliphatic aldehydes in the Huang method6a
50 and ortho-substituted benzaldehydes in the Li method6b).
The C-H oxidative coupling reactions have emerged as one
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 1

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Page 2 of 4
Table 1 Screening of the optimal reaction conditionsa
of Ar-EBX 2c-i with several aryl groups, such as Ph, 4-
View Article Online
MeC6H4, 4-ClC6H4,
DOI: 10.1039/C5CC03362D
O
TIPS
CHO
TIPS
I
O
TIPS
O
I
O
TBHP (2 equiv)
O
Conditionsa
+
O
PhCl
120 oC, 3 h
+
TIPS
O
R1
H
N
R1
N
H
3
1
H
2a
1a
2a
3aa
O
O
O
TIPS
TIPS
TIPS
R
Entry
1
Variation from the Standard Conditions
none
[O] (equiv)
N
N
N
81
56
61
78
0
H
H
2
at 80 oC
R
CO2Me
R = Me, 3ea, 62%
R = Br, 3fa, 56%
R = CN, 3ga, 54%
R = Me, 3ba, 58%
R = Bn, 3ca, 75%
R = Ac, 3da, trace
3b
4
at 140 oC
TBHP (3 equiv)
3ha, 59%
TIPS
TIPS
TIPS
O
TIPS
O
5
without TBHP
6
7
8
9
10
11
12
70% aqueous TBHP
DTBP instead of TBHP
DCP instead of TBHP
DMF instead of PhCl
CH2ClCH2Cl instead of PhCl
n-BuOAc instead of PhCl for 12 h
MeCN instead of PhCl for 12 h
40
29
15
trace
30
18
68
O
O
N
H
3la, 50%
O
N
N
H
O
3ja, 61%
3ka, 45%
3ia, 64%
O
TIPS
O
TIPS
TIPS
R
R = OMe, 3pa, 70%
R = Cl, 3qa, 59%
R = CO2Me, 3ra, 50%
S
TIPS
O
O
3oa, 68%
3ma, 54%
3na, 77%
a
O
O
O
Reaction conditions: 1a (0.2 mmol), 2a (1.5 equiv), TBHP (5 M in
5 decane; 2 equiv) and PhCl (3 mL) at 120 oC under argon atmosphere for 3
MeO
O
O
h. b Some substrate 2a was decomposed.
TIPS
TIPS
TIPS
TIPS
Br
3ua, 47%
3ta, 68%
3sa, 67%
The scope of this metal-free C-H oxidative radical
alkynylation protocol with respect to aldehydes 1 (Scheme 2)
and ethynyl benziodoxolones 2 (Scheme 3) was probed under
10 the optimal reaction conditions. The optimal conditions were
found applicable to a wide range of aldehydes, including aryl
(1b-v) and alkyl aldehydes (1w-x) (Scheme 2). In the
presence of TIPS-EBX 2a and TBHP, 1H-indole-3-
carbaldehydes 1b or 1c, having a methyl group or a benzyl
15 group on the nitrogen atom, gave the expected ynones 3ba
and 3ca in good yields. However, 1H-indole-3-carbaldehyde
1d with a N-Ac group was inert and led to no formation of
3da. Moderate yields were obtained with 1H-indole-3-
carbaldehydes 1e-i possessing varying electronic properties
O
O
O
Ph
TIPS
TIPS
3va, 58%
3wa, 48%
3xa, 56%
O
Ph
TIPS
3ya, 52%
3zc, 50%
a
45 Scheme 2. Variation of the Aldehydes (1). Reaction conditions: 1 (0.2
mmol), 2a (1.5 equiv), TBHP (5 M in decane; 2 equiv) and PhCl (3 mL)
at 120 oC under argon atmosphere for 3 h.
4-BrC6H4, 4-CNC6H4, 2-PhC6H4 and naphthalen-1-yl, at the
terminal alkyne (3ac-ai). High yield of 3ac was observed in
50 the alkynylation of Ph-EBX 2c with aldehyde 1a and TBHP.
Ar-EBX 2d-h having aryl groups of varying electronic and
steric properties could be converted to 3ad-ah with 48-75%
yields. Using naphthalen-1-yl-EBX 2i to couple with aldehyde
1a and TBHP led to 3ai in moderate yield. We were pleased
55 to find that aliphatic EBX 2j was also viable to furnish ynone
3aj in moderate yield.
20 (3ea-ia). It was noted that indoles 1j-k containing
a
carbaldehyde group at the 2 or 5 position were also viable to
furnish ynone 3ja and 3ka. Gratifyingly, other types of
heteroaromatic ynone 3la-na were obtained through the C-H
oxidative alkynylation of aldehydes bearing furan (1l−m) and
25 thiophene (1n) rings. The optimal conditions were also
compatible with various benzaldehydes with electron-
donating or electron-withdrawing substituents on the aromatic
ring (3oa-3va). Importantly, halogen groups, Cl and Br, were
perfectly tolerated, thus providing opportunities for further
30 additional modifications of the product (3qa and 3ua).
Efficient reactivity was still observed with aliphatic aldehydes
1w-z (3wa-ya and 3zc). For example, treatment of 3-
phenylpropanal (1x) with TIPS-EBX 2a and TBHP afforded
ynone 3xa in 56% yield. We were pleased to find that tertiary
35 aliphatic aldehyde 1z was also viable to furnish alkyne 3zc in
moderate yield via decarbonylation.We next set out to
investigate the feasibility of ethynyl benziodoxolones (R2-
EBX) 2 for the alkynylation with 1H-indole-3-carbaldehyde
(1a) and TBHP (Scheme 3). Gratifyingly, TMS-EBX 2b was a
40 suitable coupling partner and delivered 3ab in 65% yield. The
optimal conditions were found to be consistent with an array
R2
O
CHO
R2
I
O
Consitionsa
O
+
N
H
N
H
1a
2
3
O
O
R2
R2 = 4-MeC H ,
TMS
Ph
O
, 73%
3ad
6
4
R
2 = 4-ClC H ,
R2 = 4-BrC H ,
, 75%
3ae
4
6
, 69%
3af
4
6
R2 = 4-CNC H ,
, 48%[b]
3ag
4
N
N
N
H
6
H
H
, 65%[b]
3ac
, 75%
3ab
O
O
O
Ph
N
N
N
H
H
H
, 45%[b]
3aj
, 50%
3ah
, 56%
3ai
a
Scheme 3 Variation of the Ethynyl Benziodoxolones (2). For reaction
conditions, see Table 1 and Scheme 2. b In MeCN (3 mL) for 12 h.
60
Control experiments in Scheme 4 disclosed that the
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The utilizations of ynones are summarized in Scheme 6.
reaction of aldehyde 1a with TIPS-EBX 2a and TBHP was
View Article Online
30 Ynone 3ac underwent the electrophilicDcOyI:c1li0z.1a0ti3o9n/Cs5mCCo0o3t3h6ly2D,
providing benzo[cd]indol-3(1H)-one 5 in 50% yield.11a TMS-
containing ynone 3ab readily converted into acetal 6 in the
presence of MeOH and K2CO3.4b Terminal alkyne 7 was
synthesized from ynone 3aa reacted with TBAF and acetic
35 acid in excellent yield.6a Having terminal alkyne 7 in hand, 4-
(1H-indol-3-yl)pyrimidin-2-amine 85d and ethyl 6-(1H-indol-
3-yl)-2-methylnicotinate 911b were prepared in high yields.
completely suppressed when a stoichiometric amount of
radical inhibitors, including TEMPO, hydroquinone and BHT,
were added. In addition, product 4a was formed from
aldehyde 1a reacted with TEMPO. These results suggest that
the C-H oxidative alkynylation proceeds via a radical process.
5
R2
O
I
O
R
2 = TMS (3ab)
R2 = Ph (3ac)
O
Ph
OMe
OMe
K2CO3 (2 equiv)
ICl (3 equiv)
MeOH, rt, 4 h
MeCN, rt, 6 h
N
N
H
H
N
3
6, 62%
H
HOAc (2 equiv)
n-Bu4NF (10 equiv)
THF, from 0 oC to rt, 15 h
5, 50%
H2N
N
R
2 = TIPS (3aa)
CO2Et
N
NH
Scheme 4 Control Experiments.
O
O
HCl
NH2
(2.5 equiv)
O
N
H2N
COOEt
(2 equiv)
The mechanisms for the C-H oxiative alkynylation reaction
K2CO3 (2.5 equiv)
MeCN/tBuOH (1:1)
100 oC, 15 h
NH4OAc (2 equiv)
EtOH, reflux, 15 h
10 was proposed on the basis of the present results and previous
reports (Scheme 5).6-10 Initially, carbonyl radical A is formed
from aldehyde 1 by reacting with TBHP under heating
conditions.6,7 Addition of the carbonyl radical A to a C-C
triple bond in EBX 2 affords intermediate B,9s-v followed by
15 -elimination of intermediate B leads to the desired ynone 3
and benziodoxolonyl radical C.10 Finally, benziodoxolonyl
radical C is transformed into 2-iodobenzoic acid (10) via H-
abstraction of aldehyde 1 (Pathway I).
N
H
N
N
H
H
8, 66%
(96%)
9, 81%
7
Scheme 6 Utilizations of 3.
40
In summary, we have developed a novel carbonyl C(sp2)-
oxidative alkynylation of aldehydes with ethynyl
benziodoxolones using TBHP oxidant under metal-free
conditions, which provides a simple and efficient tool to
access a variety of highly functionalized ynones. This method
H
45 features an incorporation of an acyl group into the product by
the C-H oxidative radical coupling. Moreover, the unitizations
of ynones were performed that show the ynone chemistry with
widely potential applications in synthesis.
Pathway II was also proposed. Initially, the ligand
20 exchange on hypervalent iodine center to yield alkynyl(tert-
butylperoxy)iodoarene E, which is followed by the homolytic
cleavage of I-O bond affording tert-butylperoxy radical F.9w
Addition of aldehyde 1 to the carbon-carbon double bond of
radical F affording radical intermediate G. Finally, beta-
25 hydrogen elimination and reductive elimination of
intermediate G generates the desired alkyne 3.
This research was supported by the NSFC (Nos. 21402046,
50 21472039 and 21172060) and Hunan Provincial Natural
Science Foundation of China (No. 13JJ2018).
Notes and references
a State Key Laboratory of Chemo/Biosensing and Chemometrics, College
of Chemistry and Chemical Engineering, Hunan University, Changsha
55 410082, China. Fax: 0086731 8871 3642; Tel: 0086731 8882 2286; E-
mail: jhli@hnu.edu.cn and E-mail: srj0731@hnu.edu.cn
b State Key Laboratory of Applied Organic Chemistry, Lanzhou
University, Lanzhou 730000, China
† Electronic Supplementary Information (ESI) available: [details of any
60 supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
‡ Footnotes should appear here. These might include comments relevant
to but not central to the matter under discussion, limited experimental and
spectral data, and crystallographic data.
65 1
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2
75
80
Scheme 5 Possible Reaction Mechanisms.
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