Carbon–carbon bond formation via benzoyl umpolung attained by photoinduced electron-transfer with benzimidazolines

Article abstract of DOI:10.1016/j.tetlet.2013.10.028

A photoreaction between benzoyl compounds, such as benzoylformate derivatives, and 2-(p-anisyl)-1,3-dimethylbenzimidazoline in the presence of allyl bromide was found to give various allylated alcohols. In the reaction of benzoylformates, α-hydroxy ester enolates, for which the negative charge occurs on the carbonyl carbon of benzoyl (umpolung reactivity), are proposed to be generated as intermediates by electron-transfer from benzimidazolines to the photoexcited benzoylformates; these species react with allyl bromide to produce α-allyl-α-hydroxy esters.

Full text of DOI:10.1016/j.tetlet.2013.10.028

Tetrahedron Letters 54 (2013) 6874–6877
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Carbon–carbon bond formation via benzoyl umpolung attained
by photoinduced electron-transfer with benzimidazolines
b
a
b
Tomohito Igarashi a, , Eiji Tayama , Hajime Iwamoto , Eietsu Hasegawa
⇑
a Fundamental Sciences, Graduate School of Science and Technology, Niigata University, 8050, Ikarashi 2-no-cho, Nishi-ku, Niigata 950-2181, Japan
b Department of Chemistry, Faculty of Science, Niigata University, 8050, Ikarashi 2-no-cho, Nishi-ku, Niigata 950-2181, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A photoreaction between benzoyl compounds, such as benzoylformate derivatives, and 2-(p-anisyl)-1,3-
Received 2 September 2013
Revised 2 October 2013
Accepted 7 October 2013
Available online 12 October 2013
dimethylbenzimidazoline in the presence of allyl bromide was found to give various allylated alcohols. In
the reaction of benzoylformates, a-hydroxy ester enolates, for which the negative charge occurs on the
carbonyl carbon of benzoyl (umpolung reactivity), are proposed to be generated as intermediates by elec-
tron-transfer from benzimidazolines to the photoexcited benzoylformates; these species react with allyl
bromide to produce a-allyl-a-hydroxy esters.
Keywords:
Ó 2013 Elsevier Ltd. All rights reserved.
2-Aryl-1,3-dimethylbenzimidazolines
Benzoylformates
Photoinduced electron-transfer
Umpolung reactivity
Carbon–carbon bond formation
Organic hydride donors, such as 1-benzyl-1,4-dihydronicotina-
mide,1,2 Hantzsch ester,3 9,10-dihydroacridine,2,4 and 1-acetyl-2,3-
dimethylimidazoline,5 are attractive research targets for chemists
and biochemists. 2-Aryl-1,3-dimethylbenzimidazolines (DMBI-H),
which also belong to this group of reagents, are known to reduce
olefins,6 ketones,7 organic halides,6–8 sulfones,7,8 cyclopropanes,8
and epoxy compounds.7,9 DMBI-H also acts as a hydrogen-storing
material, releasing hydrogen gas upon contact with Brønsted
acids.10
Previously, we reported that DMBI-H acts as an effective re-
agent to promote a photoinduced reduction of ketone A to produce
alcohol B (Scheme 1, Eq. 1).7g The reaction is proposed to generate
carbanion C, in which the carbonyl carbon becomes nucleophilic
(umpolung reactivity) and, finally, C captures a proton from water
to give B.
disclose the allylation of various benzoyl substrates by photoin-
duced electron-transfer with DMBI-H.
We conducted a photoreaction (k >280 nm, 500 W Hg–Xe lamp)
using benzophenone 1a and 2-(p-anisyl)-1,3-dimethylbenzimidaz-
oline 2a, capturing the resultant species in situ with allyl bromide
in acetonitrile at room temperature for 1 h. Then, a-allyl adduct 3a
was obtained in 54% yield (Table 1, entry 1). While the reaction of
benzaldehyde 1b gave 3b, albeit in low yield (entry 2), acetophe-
none 1c was inactive (entry 3). Notably, the reaction of methyl
benzoylformate 1d gave allylated mandelate 3d in good yield
Carbon–carbon (CꢀC) bond formations promoted by inversion
operators are well-established in organic synthesis.11 However, it
is difficult to construct a tertiary alcohol via umpolung reactivity;
one of the few examples is the SmI2 promoted CꢀC bond formation
of a
-keto esters/amides reported by Fukuzawa et al.12 We thought
that CꢀC bond formation would be possible when the appropriate
carbon electrophiles are captured by C in lieu of the protons sup-
plied by water (Scheme 1, Eq. 2). Formation of a quaternary carbon
via umpolung reactivity is unprecedented within the scope of pho-
toinduced electron-transfer reactions of DMBI-H. In this Letter, we
⇑
Corresponding author.
E-mail address: f11j001h@mail.cc.niigata-u.ac.jp (T. Igarashi).
Scheme 1. The photo-reduction of
reactivity.
a benzophenone derivative via umpolung
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.10.028

T. Igarashi et al. / Tetrahedron Letters 54 (2013) 6874–6877
6875
Table 1
Table 2
A
photo-allylation reaction of benzoyl compounds
1
and DMBI-H 2a with allyl
Reaction conditions for the photo-allylation reaction of 1d and 2 or 4 with allyl
bromidea
halidesa
Entry
X
Solvent
2 or 4
Yieldb (%)
1
2c
3
4
5
6
7
8
9
10
11
12
13
14
15
Br
Br
Cl
I
MeCN
MeCN
MeCN
MeCN
CH2Cl2
THF
toluene
EtOAc
DMF
DMSO
i-PrOH
MeCN
MeCN
MeCN
MeCN
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2b
2c
2d
4
76
0
Entry
1
Benzoyl compound
Product
Yieldb (%)
14
31
63
42
48
52
40
0
Trace
59
0
38
0
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
54
2
3
20
a
Typical procedure: a solution of 1d (0.2 mmol), 2 or 4 (0.22–0.24 mmol), and
allyl halides (0.4 mmol) in solvent (4 mL) was irradiated at room temperature under
an argon atmosphere for 1 h.
0
b
Isolated yield.
Without light, 14 h.
c
4
86
a
Typical procedure: a solution of 1 (0.2 mmol), 2a (0.36 mmol), and allyl bro-
mide (0.4 mmol) in acetonitrile (4 mL) was irradiated at room temperature under
an argon atmosphere for 1 h.
b
Isolated yield.
Figure 1. A series of DMBI-H 2a–d and Hantzsch ester 4.
(entry 4). Mandelate derivatives are useful in various fields of
chemistry, such as organic synthesis,13 biochemistry,14 and inor-
ganic chemistry,15 and
a-substituted mandelate derivatives are
important units in several biologically active natural products
and versatile synthetic intermediates.16 Thus, we decided to exam-
ine the photoreaction of benzoylformates in detail.17
First, we conducted a photoreaction between 1d and 2a in the
presence of allyl halides to form the desired products (Table 2, en-
tries 1–4). When the quantity of DMBI-H was less than 1.8 equiv
shown in Table 1, 3d was still obtained in good yield (entry 1).
Without light, the reaction was not able to proceed (entry 2). Addi-
tionally, the use of allyl chloride greatly decreased the yield of 3d
(entry 3) and the reaction of 1d with allyl iodide produced 3d in
31% yield (entry 4).
Next, different solvents were explored to discover those suit-
able for the photo-allylation reaction (Table 2, entries 5–11). While
irradiation in dichloromethane, THF, toluene, ethyl acetate, and
DMF produced 3d in moderate yields (40–63% yields) (entries 5–
9), no reaction proceeded in DMSO (entry 10). The origin of this
unexpected effect caused by DMSO is unclear. Isopropanol was also
ineffective, possibly due to interfering proton transfer in the radical
ion pairs by this protic solvent (entry 11). Thus, acetonitrile was
found to be optimal among the solvents studied.
We also examined the steric and electronic effects of the 2-sub-
stituent of DMBI-H by using a series of DMBI-H derivatives 2a–d
and the applicability of Hantzsch ester 4, the most recognized
organic hydride donor in organic synthesis,3 (Fig. 1) in the photo-
Scheme 2. A photoreaction of 1d and 2a with a proton donor.
allylation reaction of 1d and allyl bromide (Table 2, entries
12–15). While 2-phenyl substituted 2b was moderately effective,
giving 3d in 59% yield (entry 12), 2-p-nitrophenyl substituted 2c
did not display any of the desired reactivity (entry 13). Using less
sterically hindered 2d,9 the yield of 3d was unexpectedly de-
creased to 38% yield (entry 14). It is considered that the generated
intermediate, DMBI radical or DMBI radical cation from 2d, cannot
be stabilized in the absence of 2-aryl substituent. In addition, 2d
was discovered to be rather unstable in air. Notably, 4 did not pro-
duce 3d at all (entry 15).

6876
T. Igarashi et al. / Tetrahedron Letters 54 (2013) 6874–6877
Scheme 3. A proposed mechanism for a photo-allylation reaction.
In order to explore the nature of the intermediate for the pho-
toreaction of 1d and 2a, allyl bromide was replaced by proton do-
nors (Scheme 2). While the reaction of 1d with water gave alcohol
3e, an addition of acetic acid produced 1,2-diol 3f. These results
were similar to those of the reactions reported in the literature,7g
that alcohol B was a major product from A in the presence of water
(Eq. 1 in Scheme 1) while benzopinacol was exclusively obtained
upon using acetic acid.
On the basis of the observations described above, we propose a
plausible mechanism for the reaction of 1d and DMBI-H with allyl
bromide (Scheme 3). The reaction pathway begins with a photoin-
duced electron-transfer in which initially formed radical anion D
Scheme 4. The photo-addition reaction of 1d with various electrophiles.
Table 3
abstracts a proton from DMBI-H radical cation (DMBI-HÅ+) to give
the radical E.18 The subsequent electron transfer between E and
DMBI radical (DMBIÅ) generates carbanion F,19 in which the car-
bonyl carbon becomes nucleophilic (umpolung reactivity). Finally,
F reacts with allyl bromide to produce 3d while F is protonated by
water to give 3e in the absence of allyl bromide. If acetic acid ex-
ists, D is protonated by acetic acid instead of DMBI-HÅ+ to give E
which dimerizes to produce 3f.
Photo-allylation reactions with electron withdrawing or conjugative benzoyl com-
pounds 1a
Entry
1
Benzoyl compound
Product
Yieldb (%)
94
We then applied the optimized conditions (1:2a:allyl bro-
mide = 1:1.8:2.0) to a variety of benzoylformates and other types
of benzoyl compounds (Table 3). When the reactions with n-butyl
ester 1g, cyclohexyl ester 1h, and t-butyl ester 1i, were performed
2
3
4
5
6
85
88
48
56
0
under the same conditions, the respective
and 3i, were obtained in good to excellent yields (entries 1–3).
Next, the reactions of -diketones, such as benzil 1j and 1-phe-
a-allyl adducts, 3g, 3h,
a
nyl-1,2-dioxohexane 1k, resulted in the formation of 3j and 3k in
48% and 56% yields, respectively (entries 4, 5). The reaction of 1k
proceeded regioselectively, with the n-butyl carbonyl remaining
intact in 3k. N,N-Diethylbenzoylformamide 1l was completely
unreactive (entry 6). Thus, either properly electron withdrawing
or conjugated substituents adjacent to benzoyl are necessary to
promote the reactions.
To further explore the nucleophilic reactivity of the proposed
anionic intermediate F (Scheme 3), we examined the reaction of
1d with various electrophiles (Scheme 4).20 The reaction of 1d with
propargyl bromide under the optimized reaction conditions gave
51% yield of 3m as a mixture of
a- and c-adduct (1.3:1) (Eq. 1).
When 1-bromo-4-phenyl-2-butyne (simply adding a terminal sub-
stituent to propargyl bromide) was used as the electrophile, a 34%
yield of 3n as a mixture of
a- and c-adduct (3.5:1) was obtained.
Notably, -adducts were preferentially formed in the above reac-
a
tions, particularly when the SN20 pathway was hindered by the ter-
minal substituents. Next, we chose several benzyl bromide
derivatives as electrophiles (Eq. 2). Reaction of 1d with benzyl bro-
mide gave benzyl adduct 3o in 71% yield. Addition of p-methylben-
zyl bromide produced 3p in 71% yield. Further, the reaction of
p-chlorobenzyl bromide resulted in the formation of 3q in 52%
a
Typical procedure: a solution of 1 (0.2 mmol), 2a (0.36 mmol), and allyl bro-
mide (0.4 mmol) in acetonitrile (4 mL) was irradiated at room temperature under
an argon atmosphere for 1 h.
b
Isolated yield.

T. Igarashi et al. / Tetrahedron Letters 54 (2013) 6874–6877
6877
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.10.
028.
References and notes
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18. The Gibbs free energy change associated with a full electron transfer from
DMBI-H to benzoylformates can be calculated using the equation
ox
ox
red
D
GET = 23.06(ED ꢀ EAred) ꢀ ET ꢀ C, in which ED and EA
are the oxidation
and reduction potentials of the donor (2a, 0.28 V vs SCE, Ref. 7h) and acceptor
(1d, ꢀ1.23 V vs SCE, Ref. 17b), ET is the triplet energy of the acceptor
(benzoylformates, 66 kcal/mol, Ref. 17b), and C is the Coulombic interaction
term (in MeCN, 0.06 eV, Ref. 7h). As
GET = ꢀ32.6 kcal/mol) suggest the electron-transfer from 2a to the excited
triplet state of 1d is feasible.
a
result,
a
negative value
(D
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