Metal-free CH bond activation of branched aldehydes with a hypervalent iodine(III) catalyst under visible-light photolysis: Successful trapping with electron-deficient olefins

Article abstract of DOI:10.1002/anie.201406513

Direct acyl radical formation of linear aldehydes (RCH₂-CHO) and subsequent hydroacylation with electron-deficient olefins can be effected with various types of metal and nonmetal catalysts/reagents. In marked contrast however no successful reports on the use of branched aldehydes have been made thus far because of their strong tendency of generating alkyl radicals through the facile decarbonylation of acyl radicals. Here use of a hypervalent iodine(III) catalyst under visible light photolysis allows a mild way of generating acyl radicals from various branched aldehydes thereby giving the corresponding hydroacylated products almost exclusively. Another characteristic feature of this approach is the catalytic use of hypervalent iodine(III) reagent which is a rare example on the generation of radicals in hypervalent iodine chemistry.

Full text of DOI:10.1002/anie.201406513

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Angewandte
Communications
DOI: 10.1002/anie.201406513
Photochemistry
À
Metal-Free C H Bond Activation of Branched Aldehydes with
a Hypervalent Iodine(III) Catalyst under Visible-Light Photolysis:
Successful Trapping with Electron-Deficient Olefins
Shin A. Moteki, Asuka Usui, Sermadurai Selvakumar, Tiexin Zhang, and Keiji Maruoka*
Abstract: Direct acyl radical formation of linear aldehydes
(RCH2-CHO) and subsequent hydroacylation with electron-
deficient olefins can be effected with various types of metal and
nonmetal catalysts/reagents. In marked contrast, however, no
successful reports on the use of branched aldehydes have been
made thus far because of their strong tendency of generating
alkyl radicals through the facile decarbonylation of acyl
radicals. Here, use of a hypervalent iodine(III) catalyst under
visible light photolysis allows a mild way of generating acyl
radicals from various branched aldehydes, thereby giving the
corresponding hydroacylated products almost exclusively.
Another characteristic feature of this approach is the catalytic
use of hypervalent iodine(III) reagent, which is a rare example
on the generation of radicals in hypervalent iodine chemistry.
radicals, which subsequently form the alkylated products 3.[4j]
Therefore, as shown in Scheme 2, only limited reports on
hydroacylation by branched aldehydes have been reported so
far. Fagnoni and co-workers conducted the hydroacylation of
electrophilic olefins through photocatalyzed activation of
various aldehydes.[7] The catalytic use of tetrabutylammonium
decatungstate (TBADT), which has a maximum light absorb-
ance around l = 323 nm, effectively generates acyl radicals
under UV irradiation. However, for branched aldehydes, use
of such high-energy light causes a significant decarbonylation
of acyl radicals at ambient temperature. Decarbonylation of
acyl radicals can be minimized by lowering the reaction
temperature (À20–À508C), and hydroacylation products
were obtained in up to approximately 45% yield after
24 hours. To overcome this drawback, Ryu, Fagnoni, and co-
workers efficiently generated acyl radicals through carbon-
À
C
onstruction of C C bonds through the use of carbon-
centered radicals is one of the growing fields in synthetic
organic chemistry.[1] Various efficient and practical
À
approaches have been developed, and the generation of C
C bonds through addition of acyl radicals to olefins is very
challenging.[2,3] A most widely used strategy in the generation
of acyl radicals is to employ modified acyl derivatives such as
acyl selenides, tellurides, and thioesters (Scheme 1, path a).[4]
Alternatively, acyl radicals can be generated in situ by
carbonylation of aryl, alkyl, and vinyl radicals with carbon
monoxide under high pressure (Scheme 1, path b).[5] How-
ever, the most attractive approach, being free of harmful
precursors and/or extra synthetic steps, is the direct gener-
ation of acyl radicals from the corresponding aldehydes
1 (Scheme 1, c). To this end, several approaches have been
developed, and include acetyl peroxide/UV light, photogen-
eration of alkoxy radicals, use of various photoinitiators, and
aerobic auto-oxidation.[6–8] Unfortunately, the efficiency of
these approaches are rather limited to the use of linear
aldehydes. Because of their higher stability, acyl radicals
generated from linear aldehydes can be efficiently trapped by
electrophiles (E) to form the ketone derivatives 2. In marked
contrast, acyl radicals generated from branched aldehydes
tend to undergo facile decarbonylation to produce alkyl
Scheme 1. Generation of acyl radicals.
[*] Dr. S. A. Moteki, A. Usui, Dr. S. Selvakumar, Dr. T. Zhang,
Prof. Dr. K. Maruoka
Laboratory of Synthetic Organic Chemistry and Special Laboratory of
Organocatalytic Chemistry, Department of Chemistry, Graduate
School of Science, Kyoto University
Sakyo, Kyoto 606-8502 (Japan)
E-mail: maruoka@kuchem.kyoto-u.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201406513.
Scheme 2. Acyl radical formation from branched aldehydes.
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ylation of alkyl radicals with CO under high pressure (up to
80 atm).[5e] Although this approach is effective toward gen-
eration of branched acyl radicals, the applicability is limited to
unsubstituted cyclic alkanes because of an issue with site
specificity in generating carbon radicals from substituted
cyclic or acyclic alkanes. More recently, Caddick and co-
workers have taken the aldehyde auto-oxidation approach to
generate acyl radicals.[8] However, this approach was effective
only for linear aldehydes, and issues such as low reactivity,
major decarbonylation of acyl radicals, as well as prolonged
reaction times, with respect to branched aldehydes, need to be
addressed.
ation of acyl radicals from branched aldehydes and its
subsequent trapping with electrophiles have not been pre-
viously developed to a synthetically useful level.[11] To our
delight, catalytic use of DIB (4a) under the irradiation by
visible light gave the hydroacylation product 7a in 32% yield
with high selectivity for ketone formation (7a/8a = 8.0:1;
entry 1). Only trace amounts of reaction products were
detected in the absence of 4a (entry 2) or visible light
(entry 3) under the given conditions. These results strongly
indicate that the reaction undergoes a radical pathway, thus
generating acyl radicals in the reaction mixture. In addition,
the reaction can be conveniently conducted using pyrex
glassware, so use of an expensive quartz reactor is not
necessary. We also investigated other iodine(III) catalysts
(4b–e; entries 4–7), and found that 4b and 4e gave satisfac-
tory results (entries 4 and 7), whereas 4c and 4d exhibited
lower reactivity and selectivity (entries 5 and 6). In general,
we prefer the use of 4e because of the high solubility in
CH3CN. Use of a light source containing a short-wavelength
UV light (100 W high-pressure mercury lamp or l = 365 nm
black light) accelerated the overall reaction rate, however, it
increased the amount of the alkylated product 8a (entries 8
and 9). Among the various solvents screened, CH3CN
provided the best result. The concentration of the reaction
mixture is also important for obtaining the high yield as well
as selectivity, and indeed use of a more concentrated CH3CN
(1.6m) solution significantly enhanced both the selectivity
(> 24:1) and the yield (89%) in favor of 7a (entry 10).
Lowering the catalyst loading decelerates the reaction
progress, however, high yield was obtained with a prolonged
reaction time (entry 11).
We believe the key to minimizing the decarbonylation of
branched acyl radicals lies in an appropriate choice of radical
À
catalyst, which can effectively activate the C(O) H bond
under mild reaction conditions. Accordingly, we are inter-
ested in the possibility of using hypervalent iodine(III)
catalysts,[9] which easily undergo photodecomposition by
weak UV light or visible light to furnish iodanyl and carboxyl
radicals, for the smooth generation of acyl radicals.[10]
To probe the potency on the effective use of hypervalent
iodine(III) reagents with visible light, the hydroacylation
reaction between 2-ethylbutyraldehyde (5a) and dimethyl
maleate (6a) was investigated (Table 1). The in situ gener-
Table 1: Hydroacylation of 2-ethylbutylaldehyde with dimethyl maleate by
hypervalent iodine(III) catalysts (4a–e).[a]
With the optimum reaction conditions in hand, we
examined the applicability of other branched aldehydes as
well as electrophilic olefins (Table 2). First, various branched
aldehydes were screened against dimethyl maleate. None of
the alkylated product 8b was obtained in the reaction with
cyclohexanecarbaldehyde (5b), thus giving the hydroacyla-
tion product 7b almost exclusively in 98% yield. In contrast,
the same reaction using an aerobic protocol gave a mixture of
7b and 8b in 56 and 21% yield, respectively, after ten days.[8c]
Introduction of a heteroatom into the cyclohexane ring can be
tolerated (7c), and does not affect stereocenters on hetero-
cyclic rings (7d). The alteration of ring geometry has no effect
on the yield (7e). It is noteworthy that the stereocenter in
endo-5e was maintained during the reaction, thus giving
endo-7e in nearly quantitative yield. 2-Ethylhexanal was
previously reported to give a significant amount of the
alkylated product 8 f.[8c] However, the extremely high prefer-
ence for the hydroacylation product 7 f was observed when
using our mild approach. No benzylic oxidation of the
substrate 5g was observed under the given reaction con-
ditions (7g). Having confirmed the tolerance to various
branched aldehydes, we screened substituted alkenes having
distinct geometries as well as different electrophilicities. The
high selectivity toward hydroacylation appears to be inde-
pendent of the alkene geometry, as dimethyl fumarate (6h;
R3 = R6 = H; R4 = R5 = CO2Me) resulted in high selectivity
for hydroacylation (7h). No alkylation was observed with the
trisubstituted alkene 6i (R3 = H; R4 = Me; R5 = R6 = CO2Et),
Entry
Catalyst
Yield [%][b]
7a/8a[c]
1
2
4a
none
4a
4b
4c
32
8.0:1
n.d.
n.d.
trace
trace
39
24
11
3[d]
4
8.1:1
7.9:1
7.6:1
8.1:1
3.2:1
5.3:1
>24:1
>24:1
5
6
7
8
9
10
11
4d
4e
41
4e[e]
4e[f]
4e
>98
97
89 (82)[h]
83
4e[i]
[a] Unless otherwise specified, reaction of 5a (0.75 mmol) and 6a
(0.5 mmol) was conducted in the presence of the catalyst 4 (10 mol%) in
CH3CN (0.5m) with irradiation by visible light under the given
conditions. [b] Yield was obtained for crude reaction mixture of 7a and
8a by 1H NMR spectroscopy using 1,1,2,2-tetrachloroethane as internal
standard. [c] The ratio of the hydroacylation versus alkylation product
was obtained by 1H NMR spectroscopy using 1,1,2,2-tetrachloroethane
as an internal standard. [d] Reaction in the dark. [e] Use of 100 W
mercury lamp in quartz reactor. [f] Use of l=365 nm black light.
[g] Reaction in 1.6m CH3CN. [h] Yield of isolated product. [i] Reaction
with 5 mol% 4e for 20 h.
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Table 2: Selective hydroacylation of branched aldehydes with electro-
philic olefins by the hypervalent iodine(III) catalyst 4e.[a,b]
decomposition of DIB (4a) or PIFA (4d) is known to give the
iodanyl radical 12 (Ar= Me or CF3) as well as the acyloxy
radical 13 (Ar= Me or CF3).[10] At this stage, we believe that
12, rather than 13, is mainly responsible for generating the
acyl radical 11 for the following reasons: 1) In the kinetic
isotope experiment using [D2]-5b, the kH/kD value was found
to be 4.5, thus implying the acyl radical formation to be the
rate-determining step (Scheme 4a).[14] 2) When the sterically
hindered hypervalent iodine(III) catalyst 4 f was employed,
reduction in the reaction progress was observed, thus
suggesting that 12 plays an important role in the rate-
determining step of the reaction (Scheme 5). Moreover, the
rate of reaction was closely correlated with the amount of
I(III) catalyst employed (Table 1, entry 10 versus 11). In
contrast, 13 may also abstract a hydrogen from the aldehyde
to furnish 11 in addition to the acid 14, which is not utilized for
the donation of the hydrogen to the carbon radical 15.[15]
Indeed, the deuterium incorporation into 7 was observed only
in trace amounts (< 5%) when p-tBuC6H4COOD was added
to the reaction mixture (Scheme 4b). The ability of 15 to
abstract hydrogen from the aldehyde 5 is rather low, and thus
often requires combined use of auxiliary reducing reagents
such as thiols to maintain active chain propagation.[2,16]
However, in our catalytic system, we believe that the iodine-
(III) intermediate 16 can serve as a hydrogen source for 15 to
produce final product 7 with regeneration 12 for further use in
this catalytic cycle (SCheme 3). In the present selective
hydroacylation reaction, the hypervalent iodine(III) catalyst
[a] Unless otherwise specified, reactions were conducted in the presence
of 4e (10 mol%), aldehyde 5b–n (2.4 mmol) and olefin 6b–n (1.6 mmol)
in CH3CN (1.6m) under the given reaction conditions to furnish 7b–
n almost exclusively. [b] Yields of isolated 7b–n. The alkylated products
8b–n were not detected by 1H NMR analysis of the crude reaction
mixture. [c] Use of 4e (30 mol%). [d] Use of 0.8m CH3CN/H2O (1:1.5)
for 18 h.
thus giving the hydroacylation product 7i in nearly quantita-
tive yield. Excellent yield of 7j was obtained, by using diethyl
ethoxymethylenemalonate (6j; R3 = H; R4 = OEt; R5 = R6 =
CO2Et), without any of the alkylated product 8j. The
sterically hindered diethyl isopropylidenemalonate (6k;
R3 = R4 = Me; R5 = R6 = CO2Et) was previously reported to
give merely a trace amount of the hydroacylation product
7k.[8c] To our delight, we obtained the desired 7k in moderate
yield. Here, no alkylated product (8k) was observed with this
bulky substrate (6k) because of the possible steric interac-
tions between the cyclohexyl radical and isopropylidenemal-
onate. The substrate scope of electrophilic olefins can be
extended to those with single electron-withdrawing groups,
and give the desired products 7l–n in good to high yields with
excellent selectivities. These results further indicate the
superiority of this approach.
Scheme 3. Proposed mechanism of hydroacylation reaction catalyzed
by 4.
With these results in hand, we propose a reaction
mechanism as depicted in Scheme 3. When the reaction of
5a and 6a was conducted in the presence of a radical trap,
2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), the TEMPO
adduct from the acyl radical 11 [R1 = CH(CH2CH3)2] was
isolated with only a trace amount of 7a.[14] The photochemical
Scheme 4. Mechanic studies of hydroacylation reaction with the cata-
lyst 4e.
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10.8 Hz), 2.33 (1H, dd, J = 17.0, 4.0 Hz), 1.61–1.54 (1H, m), 1.40–1.19
(4H, m), 0.91 (3H, t, J = 7.4 Hz), 0.88 ppm (3H, t, J = 7.6 Hz);
13C NMR (125 MHz, CDCl3): d = 175.3, 173.1, 51.8, 51.7, 43.2, 42.9,
32.1, 23.6, 23.3, 11.8, 11.6 ppm.
Received: June 24, 2014
Published online: August 25, 2014
Keywords: aldehydes · hypervalent compounds · iodine ·
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photochemistry · radicals
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Scheme 5. Hydroacylation with 4e and the bulky catalyst 4 f. Unless
otherwise specified, reaction of 5b (0.75 mmol) and 6a (0.5 mmol)
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1
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4 does not require a high-energy input to undergo homolysis,
and this is the key factor for the origin of superb selectivity
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In summary, we have succeeded the mild generation of
acyl radicals from branched aldehydes by using hypervalent
iodine(III) catalysts under visible-light photolysis, and sub-
sequent trapping of such acyl radicals with some electrophilic
olefins. Further investigations on more detailed mechanism
and synthetic application toward other systems are currently
underway in our laboratory.
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Experimental Section
Representative procedure (Table 1, entry 10): In a reaction tube
containing dimethyl maleate (1.6 mmol), hypervalent iodine(III)
catalyst 4e (0.16 mmol), and 2-ethylbutanal (2.4 mmol) were mixed
in dry acetonitrile (1.6m) under argon atmosphere. The mixture was
irradiated by visible light (l > 400 nm) at room temperature with
stirring for 12 h. Upon completion, acetonitrile was removed under
reduced pressure. The crude reaction mixture was purified by flash
column chromatography (eluting with n-hexane/ethyl acetate = 20:1
to 4:1) to afford a colorless oil (7a: 308 mg, 1.26 mmol, 79% yield and
7b: 11.0 mg, 0.052 mmol, 3% yield); 7a: 1H NMR (500 MHz,
CDCl3): d = 4.11 (1H, t, J = 7.1 Hz), 3.74 (3H, s), 3.68 (3H, s), 2.91
(1H, dd, J = 17.4, 7.2 Hz), 2.84 (1H, dd, J = 17.3, 7.1 Hz), 2.75–2.67
(1H, m), 1.76–1.62 (2H, m), 1.56–1.39 (2H, m), 0.89 (3H, t, J =
7.4 Hz), 0.81 ppm (3H, t, J = 7.4 Hz); 13C NMR (125 MHz, CDCl3):
d = 206.5, 171.7, 168.8, 54.3, 54.1, 52.6, 52.0, 31.9, 24.2, 22.9, 11.6,
11.3 ppm; 8a: 1H NMR (500 MHz, CDCl3): d = 3.69 (3H, s), 3.67
(3H, s), 3.01 (1H, dt, J = 10.8, 4.1 Hz), 2.74 (1H, dd, J = 17.0,
[8] a) R. J. Fitzmaurice, J. M. Ahern, S. Caddick, Org. Biomol.
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[11] In a previous report of photocatalysis using TBADT at room
temperature, this reaction gave mostly the alkylated product
(7a/8a = 1:2.3, 7a = 15% yield) after 24 h. The enhanced
selectivity was obtained when the reaction was conducted at
À208C (7a/8a = 8:1, 7a = 40% yield). See Ref. [7d].
À
[12] In a previous report of the aerobic C H activation at 608C, this
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Ref. [8c].
[13] See the Supporting Information for characterization of the
TEMPO adduct of the acyl radical 11 [R1 = CH(CH2CH3)2)].
[14] Full experimental details are described in the Supporting
Information.
[15] Under the given conditions, benzoyl peroxide (BPO) did not
catalyze the reaction.
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