A photo-reagent system of benzimidazoline and Ru(bpy)3Cl 2 to promote hexenyl radical cyclization and Dowd-Beckwith ring-expansion of α-halomethyl-substituted benzocyclic 1-alkanones

Article abstract of DOI:10.1016/j.tet.2014.02.078

A combination of 2-aryl substituted 1,3-dimethyl-benzimidazolines (DMBIHs) and tris(2,2′-bipyridine)ruthenium(II) chloride, Ru(bpy) ₃Cl₂ was used to promote photoinduced electron-transfer (PET) reactions of α-halomethyl-substituted benzocyclic 1-alkanones. This photo-reagent system stimulates free radical forming, cleavage of both carbon-bromine and carbon-chlorine bonds that are not activated by carbonyl groups. The resulting free radicals undergo 5-exo hexenyl cyclization as well as sequential cyclization and ring-expansion (Dowd-Beckwith) reactions to form radicals that abstract hydrogen atoms from the radical cation of DMBIH to yield the observed products. The results of a study of the effects of substituents located on the 2-aryl ring of DMBIH suggest that steric and hydrogen-bonding interactions influence the nature of the reaction pathways followed by the radical intermediates. PET reactions using an iridium complex and DMBIH were also investigated.

Full text of DOI:10.1016/j.tet.2014.02.078

Tetrahedron 70 (2014) 2776e2783
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
A photo-reagent system of benzimidazoline and Ru(bpy)3Cl2 to
promote hexenyl radical cyclization and DowdeBeckwith ring-
expansion of
a
-halomethyl-substituted benzocyclic 1-alkanones
,
a
a
a
a
Eietsu Hasegawa a , Minami Tateyama , Tsuneaki Hoshi , Taku Ohta , Eiji Tayama ,
*
Hajime Iwamoto a, Shin-ya Takizawa b, Shigeru Murata b
a Department of Chemistry, Faculty of Science, Niigata University, Ikarashi-2 8050, Niigata 950-2181, Japan
b Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, Meguro-ku, Tokyo, 153-8902, Japan
a r t i c l e i n f o
a b s t r a c t
A combination of 2-aryl substituted 1,3-dimethyl-benzimidazolines (DMBIHs) and tris(2,20-bipyridine)
ruthenium(II) chloride, Ru(bpy)3Cl2 was used to promote photoinduced electron-transfer (PET) reactions
of a-halomethyl-substituted benzocyclic 1-alkanones. This photo-reagent system stimulates free radical
forming, cleavage of both carbonebromine and carbonechlorine bonds that are not activated by carbonyl
groups. The resulting free radicals undergo 5-exo hexenyl cyclization as well as sequential cyclization and
ring-expansion (DowdeBeckwith) reactions to form radicals that abstract hydrogen atoms from the
radical cation of DMBIH to yield the observed products. The results of a study of the effects of sub-
stituents located on the 2-aryl ring of DMBIH suggest that steric and hydrogen-bonding interactions
influence the nature of the reaction pathways followed by the radical intermediates. PET reactions using
an iridium complex and DMBIH were also investigated.
Article history:
Received 23 January 2014
Received in revised form 24 February 2014
Accepted 26 February 2014
Available online 5 March 2014
Keywords:
Photoinduced electron-transfer
Benzimidazoline
Ru complex
a
-Halomethyl benzocyclicalkanones
Ó 2014 Elsevier Ltd. All rights reserved.
Hexenyl radical cyclization
DowdeBeckwith ring-expansion
1. Introduction
Another pertinent topic in the area of synthetic applications of
PET reactions is visible (solar) light-promoted organic trans-
2-Aryl substituted 1,3-dimethyl-benzimidazolines (DMBIHs) are
versatile and effective reagents that promote reduction reactions of
various organic compounds.1e3 These reactions are initiated by
a variety of DMBIH oxidation processes including hydride, hydro-
gen atom, and electron transfer. In addition, DMBIH derivatives
serve as hydrogen storage compounds owing to the fact that they
evolve hydrogen gas upon treatment with certain Bron-
steadeLowry acids.4 The characteristic redox properties of DMBIH
have been demonstrated in an investigation carried out by Zhu and
co-workers.5 We have also shown that DMBIH derivatives act as
effective reducing reagents for carbonyl, carbonehalogen, and
other functional groups under photoinduced electron-transfer
(PET) conditions.6 These PET reactions are initiated by either di-
rect irradiation of substrates or photosensitization employing ap-
propriate photosensitizers, the latter approach being particularly
useful for the substrates that do not effectively absorb light filtered
by Pyrex glass in ordinary reaction vessels.7
formations that are catalyzed by using visible light absorbing
transition metal complexes, such as those of ruthenium and irid-
ium.8,9 In a recent study, we were the first to demonstrate that
a combination of DMBIH and tris(2,20-bipyridine) ruthenium(II)
chloride (Ru(bpy)3Cl2) could be employed to promote the PET
transformation of acetyl epoxide I to hydroxy ketone II that takes
place via selective CaeO bond cleavage (Scheme 1).6h,10
Scheme 1. Ru(bpy)3Cl2 and DMBIH photosensitized reaction of epoxy ketone.
In order to further explore the scope of photochemical reactions
that are promoted using a combination of DMBIH and visible light
absorbing metal complexes,11 a study of reactions of
a-bromo-
methyl-substituted benzocyclic 1-alkanones III was carried out
(Scheme 2). PET reactions of III with DMBIH, induced by both direct
irradiation and photosensitization using organic sensitizers such as
* Corresponding author. E-mail address: ehase@chem.sc.niigata-u.ac.jp (E. Hasegawa).
0040-4020/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2014.02.078

E. Hasegawa et al. / Tetrahedron 70 (2014) 2776e2783
2777
substituted pyrenes and anthracenes, had already been exam-
ined.6b,c,e,hej The results of the earlier effort showed that DMBIH-
promoted single electron-transfer (SET) photoreduction of III gives
rise to corresponding primary alkyl radicals IV via selective CbeBr
bond cleavage. The ensuing competitive reaction pathways fol-
expectation that the yield of formation of VI from IIIb might be
improved over that observed when an organic photosensitizer is
employed.6j
Below, we describe the results of a study of PET 5-exo hexenyl
cyclizations as well as DowdeBeckwith ring-expansion reactions
lowed by IV are governed by the nature of the carbonyl
a
-sub-
of
a-halomethyl-substituted benzocyclic 1-alkanones that are
stituent (R). While the butenyl-substituted radical IVa undergoes
preferential 5-exo hexenyl radical cyclization to form the precursor
of spirocyclic ketone V,12 the alkoxy carbonyl-substituted analog
IVb reacts by DowdeBeckwith like cyclization and ring-expansion
to yield the precursor of the ring-expanded ketone VI.13,14
sensitized by using a combination of Ru(bpy)3Cl2 or dimethyl-
2,20-bipyridine
di(phenylpyridyl)
iridium(III)
hexa-
fluorophosphate, Ir(ppy)2(dmpy)PF6,17 with DMBIH. The struc-
tures of the substrates, products, DMBIHs, and metal complexes
are provided in Fig. 1.
Scheme 2. Radical reactions of a-bromomethyl-substituted benzocyclic 1-alkanones.
The above observations suggest that the bromomethyl ketones
III are ideal substrates to probe the compatibility of DMBIH with
Ru(bpy)3Cl2-promoted PET reactions. At the outset, we believed
that a study of this reaction system would provide information
about whether a carbonebromine bond, that is, not directly acti-
vated by other functional groups can be cleaved by using the visible
light absorbing, solar energy compatible photocatalyst Ru(b-
py)3Cl.6h,15,16 Another motivation for this effort comes from the
2. Results and discussion
Reaction of the butenyl-substituted bromomethyl ketone 1a was
investigated first in order to ensure that carbonebromine bond
cleavage is taking place to generate the expected primary alkyl
radical IV (Scheme 2). Irradiation of a N2 pre-purged, MeCN or DMF
solution containing 1a, 2a, or 2b, and Ru(bpy)3Cl2 with a 500 W
XeeHg lamp through a l>390 nm cut-off glass filter did indeed give
Fig. 1. Substrates, products, DMBIHs, and metal complexes.

2778
E. Hasegawa et al. / Tetrahedron 70 (2014) 2776e2783
rise to formation of spirocyclic ketone 3 (entries 1e3 in Table 1).
While 2a and 2b are similarly effective (entries 1 and 3), the use of
the silicone-activated amine Et2NCH2SiMe3 (TMSA)18 (5.0 equiv) or
1,4-diazabicyclo[2.2.2]octane (DABCO) (5.0 equiv) in place of 2
(1.2 equiv) in these processes results in either a low yield or none of
3, respectively (entries 4 and 5). These observations are consistent
these substances form 4c more selectively (entries 2 and 3).19 While
the use of DMF in place of MeCN as solvent brings about the same
results (entry 4), reactions in DMSO and MeOH are less efficient
(entries 5 and 6) and reaction in DMSO is complicated by a low
recovered mass balance. On the other hand, the reaction is signif-
icantly decelerated in MeOH although the total yields of 4c and 5c
based on the conversion of 1c are above 70%. Similar deceleration
occurred when other protic media, such as aqueous MeCN, were
employed in previous studies of PET reactions of 1c with 2a.6j An-
other important observation is that the concentration of 2a mark-
edly affects the 4ce5c product ratio (entries 7e10). Use of a lower
concentration of 2a brings about formation of a higher yield 4c
(compare 7 and 9 with 1) while a higher concentration of this
substance leads to an increased amount of 5c (compare 8 and 10
with 1). Finally, Ir(ppy)2(dmpy)PF6 (0.02 equiv) also serves as
a photosensitizer for the PET reaction of 1c with 2a (entry 11). Al-
though the absorption profile of the Ir complex17 requires the use of
with
the
electron-donating
abilities
of
the
amines
(2>TMSA>DABCO as judged by their oxidation potentials), which
should determine the efficiencies of SET-promoted reductive
quenching of the excited state of Ru(bpy)3Cl2.19
Table 1
Ru(bpy)3Cl2 and DMBIH or amine photosensitized reactions of 1aa
shorter wavelength of light (l>360 nm), this complex promotes
a reaction that takes place in a comparable yield and selectivity to
those observed using Ru(bpy)3Cl2 (compare with entry 1).
ox
Entry
Amine
(equiv vs 1a)
E
Solvent
Conv of
1a (%)
Yield of
1=2
We next examined reactions of various halomethyl ketone
substrates 1 with 2a under Ru(bpy)3Cl2-sensitized conditions
(Table 3). The results show that reaction of indanone 1d affords 4d
as a major product along with a small amount of naphthol 6, and 5d
(entry 2). In the reaction of benzosuberone 1e, 4e is produced as the
major product although the yield is only moderate (45% based on
the conversion of 1e) (entry 3). In spite of the low yield, this result is
encouraging because direct irradiation of 1e with 2a gave only
a trace quantity of 4e.6j Notably, use of the sensitization protocol
described above leads to cleavage of carbonechlorine bond in 1f
(entry 4) and reaction of the corresponding iodide 1g produces the
expected products 4c and 5c in good yields (entry 5).
The both Ru(bpy)3Cl2 and Ir(ppy)2(dmpy)PF6-based photosen-
sitization methods were applied to promote reaction of the alkyl
ketone substrate 1h (Table 4).22 Using Ru(bpy)3Cl2 and the same
conditions employed for reaction of 1c (entry 1 in Table 2), except
for a longer irradiation time (4 h), 1h is transformed to the expected
ring-expansion product 4h albeit in low yield (recovered 1h, 23%).
Isolation of 4h could not be achieved by using chromatography and,
therefore, the yield was determined by using 1H NMR analysis of
the inseparable product mixture. Notably, use of Ir(ppy)2(dmpy)PF6
as the photosensitizer for this reaction enabled complete con-
(V vs SCE)
3b (%)
1
2a (1.2)
2a (1.2)
0.34
0.34
0.32
0.49f
0.69h
MeCN
DMF
MeCN
MeCN
MeCN
44
28
8c
22
5
2
19c
3d
4
2b (1.2)
34
10
TMSAe (5.0)
DABCOg (5.0)
5
No reaction
a
1a (0.40 mmol), Ru(bpy)3Cl2 (0.02 equiv vs 1a), solvent (4 mL),
l
>390 nm, 1 h.
b
c
Isolated yields.
Determined by 1H NMR.
Average of two independent experiments.
Et2NCH2SiMe3.
d
e
f
Ref. 6h.
g
h
1,4-Diazabicyclo[2.2.2]octane.
Ref. 20.
In contrast, reaction of bromomethyl ketone 1b, which does not
possess an alkenyl tether, under the same conditions as used for the
reaction in entry 2 of Table 1, produces debrominated ketone 5b as
the major product along with a small amount of the ring-expanded
ketone 4b (Scheme 3). The observation suggests that, in contrast to
the primary alkyl radical formed from 1a, which efficiently un-
dergoes 5-exo hexenyl radical cyclization, the one derived from 1b
abstracts a hydrogen atom preferentially.
Scheme 3. Ru(bpy)3Cl2 and DMBIH photosensitized reaction of 1b.
The above findings encouraged us to investigate metal complex
sensitized photoreactions of bromomethyl ketones 1c and 2a
(Table 2). As expected, photoreaction of 1c in MeCN sensitized by
Ru(bpy)3Cl2 and 2a proceeds smoothly to yield the ring-expanded
ketone 4c and debrominated ketone 5c (entry 1). In the absence
of 2a or Ru(bpy)3Cl2, and without irradiation, no reaction takes
place (not shown in Table 2). It should be noted that the yield and
the selectivity for formation of 4c (see the ratio 4c/5c) are high
compared with those of previously studied photochemical and free
radical reactions.21 Here again, both TMSA and DABCO are less
effective co-photocatalysts even though processes promoted by
sumption of 1h and gave 4h in a significantly improved yield. In
both reactions, the formation of debrominated ketone 5h is
negligible.
On the basis of the above observations as well as the results of
previous studies,6 it is possible to propose the plausible mecha-
nistic pathways shown in Scheme 4 for reactions of 1ae1c. The
route is initiated by reductive quenching of the excited state of
Ru(bpy)3Cl2 by DMBIH produces Ru(bpy)3Cl,6h,23,24 which then
donates an electron to 1 giving the radical anion of 1 (1ꢀꢁ, ketyl
form).25,26 Subsequent intramolecular SET to the CbeBr bond gives
a primary alkyl radical 7, as discussed in the context of previous

E. Hasegawa et al. / Tetrahedron 70 (2014) 2776e2783
2779
Table 2
Ru or Ir complex photosensitized reaction of 1c with 2aa
Entry
Sens
Amine
Solvent (mL)
Conv of
Yieldsb (%)
Ratio
(equiv vs 1c)
1c (%)
4c
5c
4c/5c
1c
2
3
4
5
6
7
8
9
Ru
Ru
Ru
Ru
Ru
Ru
Ru
Ru
Ru
Ru
Ir
2a (1.2)
MeCN (4)
MeCN (4)
MeCN (4)
DMF (4)
DMSO (4)
MeOH (4)
MeCN (4)
MeCN (4)
MeCN (20)
MeCN (2)
MeCN (2)
100
39
6
100
75
46
60
100
53
100
100
74
32e
3
75
14
6
50
57
44
61
70
23
76:24
89:11
100:0
77:23
54:46
18:82
87:13
57:43
86:14
64:36
79:21
TMSAd (5.0)
DABCOf (5.0)
2a (1.2)
4e
0
22
12e
27e
7e
2a (1.2)
2a (1.2)
2a (0.6)
2a (2.4)
2a (1.2)
2a (1.2)
2a (1.2)
43
7d
10
11
34
19
a
1c (0.40 mmol for entries 1e10, 0.20 mmol for entry 11), sensitizer (0.02 equiv vs 1c),
Isolated yields.
l>390 nm for entries 1e10 and
l>360 nm for entry 11, 1 h.
b
c
d
e
f
Average of two independent experiments.
Et2NCH2SiMe3.
Determined by 1H NMR.
1,4-Diazabicyclo[2.2.2]octane.
Table 3
Ru(bpy)3Cl2 and 2a photosensitized reaction of 1a
Entry
1
Conv of 1 (%)
Yieldsb (%)
Ratio
4
5
4/5
1c
2
3
4
5
1c (X¼Br, n¼1)
1d (X¼Br, n¼0)
1e (X¼Br, n¼2)
1f (X¼Cl, n¼1)
1g (X¼I, n¼1)
100
100
77
74
62d
35e
31
23
21
3
76:24
75:25
92:8
77:23
72:28
52e
100
8e
23
58
a
1 (0.40 mmol for entries 1e4, 0.20 mmol for entry 5), Ru(bpy)3Cl2 (0.02 equiv vs 1), MeCN (4 mL for entries 1e4, 2 mL for entry 5), l>390 nm, 1h.
Isolated yields.
Same as entry 1 in Table 2.
Included the yield of 6 (3%).
Determined by 1H NMR.
b
c
d
e
Table 4
Ru(bpy)3Cl2 and 2a photosensitized reaction of 1ha
Entry
Sensitizer
Conv of 1h (%)
Yieldsb (%)
Ratio
4h/5h
100:0
4h
28c
76
5h
1
2
Ru(bpy)3Cl2
Ir(ppy)2(dmpy)PF6
77
100
0
Trace
w100:w0
a
1h (0.40 mmol), 2a (1.2 equiv vs 1h), sensitizer (0.02 equiv vs 1h), MeCN (4 mL),
l>390 nm for entry 1 and
l>360 nm for entry 2, 4 h.
b
c
Isolated yields.
Determined by 1H NMR.

2780
E. Hasegawa et al. / Tetrahedron 70 (2014) 2776e2783
Scheme 4. Plausible reaction pathways.
studies on PET reactions of
a-bromomethyl benzocyclic 1-
when a basic species co-exist with DMBIHꢀþ 6j,11
. While the dis-
alkanones.6b,e,j,27 Alternatively, direct dissociative SET of the
CbeBr bond from Ru(bpy)3Cl to give 7 is possible, however this
route would be inconsistent with the observed lower reactivity of
1h compared to that of 1d. As previously suggested,6j solvation to
1ꢀꢁ by protic media such as MeOH could interfere with intra-
molecular SET between the ketyl part and CbeBr bond of 1 (see
entry 6 in Table 2). Radical 7a, possessing an alkenyl tether,
tonic radical anions derived from CaeO bond cleavage of epoxy
ketone radical anions contain alkoxide ions, which are sufficiently
strong bases to abstract a proton from DMBIHꢀþ 6c,31
,
CbeBr bond
-bromomethyl cycloalkanone,
such as 1ꢀꢁ, does not produce a strongly basic species (Scheme 6).
cleavage of the radical anion of
a
exclusively undergoes 5-exo cyclization.28 On the other hand,
a-
ethoxycarbonyl-substituted radical 7c undergoes competitive
hydrogen atom abstraction and DowdeBeckwith ring-expansion to
give 5c and 4c, respectively. Moreover, formation of 5b is pre-
dominant in reaction of the a
-methyl-substituted radical 7b.29 The
hydrogen atom donor in these processes is most probably
the radical cation of DMBIH (DMBIHꢀþ) that is formed by SET to the
excited state of the metal complex. DMBIH may also donate hy-
drogen atom to 7 giving 5, which would become more significant as
the concentration of DMBIH increases (not shown in Scheme 4).
Alkyl radicals 7, 8, and 9 abstract a hydrogen atom from DMBIHꢀþ to
give the respective products 5, 3, and 4 along with the stable imi-
dazolium cation (DMBIþ) (path A in Scheme 5). It is also possible
that DMBIHꢀþ releases a proton and electron sequentially to form
DMBIþ via the imidazolyl radical (DMBIꢀ) (path B in Scheme 5),5,30
Scheme 6. Bond cleavage of radical anions of epoxy ketones and
benzocyclic 1-alkanones.
a-bromomethyl
In order to explore how the structure of DMBIH influences this
reaction process, we examined the effect of 2-aryl substituents of
DMBIH (Table 5). Most of the DMBIHs except for 2f are effective in
promoting processes that produce 4c and 5c in high yields (total
yields: 80%equant). In the case of 2f, the yields of 4c and 5c, based
on the conversion of 1c, are similarly high (91%) but substrate
conversion is low (entry 8). In order to explore the origin of the
difference between the reaction promoted by the 2-o-HOC6H4-
substituted DMBIH 2f and that induced by the 2-p-HOC6H4-
substituted analog 2e (compare entry 8 with entry 7), PhOH was
added to the reaction of 1c co-sensitized by 2a (entry 2). The result
shows that external PhOH does not have a significant effect on the
yields of 4c and 5c (68% and 20%, respectively, at quantitative
conversion of 1c). This observation suggests that only an o-HO
substituent on the phenyl ring significantly deters the reaction
promoted by DMBIH. It was reported that intramolecular hydro-
gen-bonding interaction exists between o-OH and nitrogen lone
pair in 2f.4a,5,32 This specific interaction, which can also occur in
Scheme 5. Reaction pathways from DMBIꢀþ to DMBIþ
.

E. Hasegawa et al. / Tetrahedron 70 (2014) 2776e2783
2781
Table 5
Ru(bpy)3Cl2 and 2 photosensitized reaction of 1ca
Yieldsb (%)
Ratio
ox
Entry
2 (Ar)
E
Solvent
Conv of 1c (%)
1=2
(V vs SCE)
4c
5c
4c/5c
1c
2d
3e
4
5
6
7
8
9
10
2a (C6H5)
2a (C6H5)
2a (C6H5)
2b (p-MeOC6H4)
2c (p-ClC6H4)
2d (o-ClC6H4)
2e (p-HOC6H4)
2f (o-HOC6H4)
2g (1-Naph)
2g (1-Naph)
0.34
0.34
0.34
0.32
0.38
0.38
0.31
0.33
0.37
0.37
MeCN
MeCN
DMF
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
DMF
100
100
100
100
100
99
74
68
75
77
69
73
73
13
72
68
23
20
23
23
26
7
76:24
77:23
77:23
77:23
73:27
91:9
74:26
62:38
85:15
78:22
100
26
22f
8f
100
100
13
19
a
1c (0.40 mmol), 2 (1.2 equiv vs 1c), Ru(bpy)3Cl2 (0.02 equiv vs 1c), solvent (4 mL), l>390 nm, 1h.
Isolated yields.
Same as entry 1 in Table 2.
PhOH (1.2 equiv vs 1c).
Same as entry 4 in Table 2.
Determined by 1H NMR.
b
c
d
e
f
2fꢀþ, might cause hydrogen atom release from the radical cation to
be disfavored because no such stabilizing interaction is expected in
the resulting imidazolium ion.5,33 Another notable observation is
that the yield of 5c is relatively low in the reactions of 1c with 2d
and 2g (entries 6, 9, and 10). This observation might be a conse-
quence of a steric interaction interfering with hydrogen atom
transfer from 2ꢀþ to the precursor radical 7c (see Scheme 5).
NMR and 100 MHz for 13C NMR. Oxidation potentials in MeCN were
measured using cyclic voltammetry and the reported procedure.6h
Calibration of the potential values were performed using the formal
potentials of ferrocene/ferrocenium couple, Eꢂ¼0.064e0.072 V and
0.433e0.448 V versus Ag/AgNO3 and SCE, respectively. Photoreac-
tions were conducted on solutions in Pyrex test tubes (1.4 cm di-
ameter) immersed in a water bath at room temperature with
a 500 W XeeHg lamp as a light source. Column chromatography
was performed using silica gel. Preparative thin layer chromatog-
raphy (TLC) was performed on 20 cmꢃ20 cm plates coated with
silica gel. MeCN was distilled over P2O5 and subsequently with
K2CO3. Anhydrous N,N-dimethylformamide (DMF), dimethyl sulf-
oxide (DMSO), and MeOH were purchased and used without dis-
tillation. Other reagents and solvents were purchased and used
without further purification.
3. Conclusion
In the investigation described above we demonstrated that Ru
and Ir complexes, such as Ru(bpy)3Cl2 and Ir(ppy)2(dmbpy)PF6, act
as effective partners in DMBIH cooperatively sensitized PET re-
actions of a-halomethyl-substituted benzocyclic 1-alkanones. Par-
ticularly, noteworthy is the observation that not only do
carbonebromine bonds not directly activated by carbonyls cleave
in these processes but carbonechlorine bonds also undergo radical
forming cleavage reactions. The resulting radicals participate in 5-
exo hexenyl cyclization as well as DowdeBeckwith ring-expansion
reactions. It is noteworthy that both the yield and the selectivity of
the ring-expansion process are significantly improved compared
with those of previously reported PET reactions. In these processes,
DMBIHꢀþ is proposed to be the key hydrogen atom donor to radical
intermediates. While o-chloro substituent on the phenyl at the C2
position of DMBIH leads to reactions that form the pre-
rearrangement ketone 5c in decreased yields, the analog with an
4.2. Preparations of haloalkyl-cycloalkanones
Substrates 1a,6h 1b,34 1c,13f,l 1d,6j and 1e,13f,l are known and
prepared using literature procedures. Substrate 1h is known,
however structural information was not available,6c,14a and there-
fore the spectral data of 1h are reported below. Preparation of ethyl
2-chloro-1-tetralone-2-carboxylate (1f) was performed as follows.
To a suspension of NaH (ca. 60% in oil, 484 mg, 12.1 mmol) in THF
(9.0 mL) was added HMPA (2.1 mL, 12.1 mmol). Subsequently, ethyl
1-tetralone-2-carboxylate (1.31 g, 6.0 mmol) in THF (7.0 mL) was
added, and the mixture was stirred under N2 at room temperature.
After 1 h, CH2ClBr (2.0 mL, 30.1 mmol) was added, and the mixture
was stirred at 75 ꢂC for 22 h, cooled, diluted with water, and
extracted with Et2O. The extract was washed with water, saturated
aqueous Na2S2O3, saturated aqueous NaHCO3, and brine, dried over
anhydrous MgSO4, and concentrated in vacuo, giving a residue that
was subjected to column chromatography (AcOEt/n-hexane¼1:4)
to give ethyl 2-chloro-1-tetoralone-2-carboxylate 1f (862 mg,
3.2 mmol, 54%). Preparation of 1g6b was similarly performed using
CH2I2.
o-hydroxy substituent promotes
a significantly less efficient
reaction. Although steric effects and intramolecular hydrogen-
bonding of DMBIH are proposed to rationalize these observations,
further investigations of these issues are necessary. In addition, in
order to expand the generality of this photo-reagent system con-
sisting of DMBIH and visible light absorbing transition metal
complexes, applications to wider range of substrates need to be
explored.
4. Experimental section
4.1. General
4.2.1. Ethyl 2-chloro-1-tetoralone-2-carboxylate 1f. Pale yellow oil;
IR (neat) 1733, 1689 cmꢁ1
;
1H NMR (400 MHz)
d
1.19 (t, J¼7.1 Hz,
NMR spectra were recorded using CDCl3 solutions with tetra-
methylsilane (Me4Si) as an internal standard at 400 MHz for 1H
3H), 2.46e2.53 (m, 1H), 2.61e2.66 (m, 1H), 2.99 (dt, J¼17.2, 4.6 Hz,
1H), 3.15e3.24 (m, 1H), 4.05 (s, 2H), 4.15e4.22 (m, 2H), 7.05 (td,

2782
E. Hasegawa et al. / Tetrahedron 70 (2014) 2776e2783
J¼7.6, 1.2, 1H), 7.25 (d, J¼8.0 Hz, 1H), 7.32 (td, J¼7.6, 0.7 Hz, 1H) 8.04
J¼14.8, 4.0 Hz, 1H), 2.85e2.91 (m, 1H), 2.98e3.06 (m, 1H), 7.20 (d,
J¼7.2 Hz, 1H), 7.28 (t, J¼7.6 Hz, 1H), 7.40 (td, J¼7.6, 0.7, 1H), 7.72 (dd,
(dd, J¼8.0, 0.6 Hz, 1H); 13C NMR (100 MHz)
d 13.9, 25.4, 29.0, 46.2,
58.7, 61.9, 126.8, 128.0, 128.8, 131.4, 133.9, 143.1, 169.2, 192.5; HRMS
(ESI) calcd for C14H15O335ClNa [MþNa]þ 289.0602, found 289.0607,
calcd for C14H15O337ClNa [MþNa]þ 291.0572, found 291.0577.
J¼7.6, 0.6 Hz, 1H); 13C NMR (100 MHz)
d 21.7, 28.6, 32.1, 34.4, 49.0,
126.5, 128.4, 129.7, 131.8, 138.8, 142.4, 204.5; HRMS (ESI) calcd for
C
12H15O [MþH]þ 175.1117, found 175.1115.
4.2.2. Ethyl 2-iodo-1-tetoralone-2-carboxylate 1g. Pale yellow oil;
Acknowledgements
IR (neat) 1687, 1731 cmꢁ1
;
1H NMR (400 MHz)
d
1.21 (t, J¼7.2 Hz,
3H), 2.36e2.43 (m, 1H), 2.62e2.68 (m, 1H), 2.97 (dt, J¼17.8, 1.5 Hz,
1H), 3.11e3.18 (m, 1H), 3.67 (d, J¼10.4 Hz, 1H), 3.73 (d, J¼10.0 Hz,
1H), 4.14e4.26 (m, 2H), 7.23 (d, J¼7.6,1H), 7.32 (t, J¼7.6 Hz,1H), 7.50
(td, J¼7.6, 1.2 Hz, 1H), 8.04 (d, J¼7.6 Hz, 1H); 13C NMR (100 MHz)
This study was partially supported by grants from the Uchida
Energy Science Promotion Foundation and the Sasaki Environment
Technology. We thank Professor Masaki Kamata (Niigata Univer-
sity) for his useful comments. We also thank Mr. Takashi Kitazume
(Niigata University), Miss Emi Tosaka (Niigata University), and Mr.
Takayuki Seida (Niigata University) for their assistance in per-
forming related experiments.
d
7.3, 14.0, 25.3, 32.5, 57.5, 62.0, 126.9, 128.2, 128.8, 131.1, 133.9,
142.9, 168.8, 192.2; HRMS (ESI) calcd for C14H15O3INa [MþNa]þ
380.9958, found 380.9967.
4.2.3. Ethyl 2-bromo-1-cyclopentanone-2-carboxylate 1h. Colorless
oil; IR (neat) 1726, 1755 cmꢁ1 1H-MNR (400 MHz)
; d 1.28 (t,
Supplementary data
J¼7.0 Hz, 3H), 2.06e2.12 (m, 2H), 2.28e2.35 (m, 2H), 2.46e2.61 (m,
These data include 1H- and 13C NMR of compounds 1f, 1g, 1h,
and 4b. Supplementary data associated with this article can be
found in the online version, at http://dx.doi.org/10.1016/
j.tet.2014.02.078.
2H), 3.64 (d, J¼10.4 Hz, 1H), 3.78 (d, J¼10.4 Hz, 1H), 4.17e4.23 (m,
2H); 13C NMR (100 MHz)
d 14.0, 19.4, 32.4, 33.8, 38.3, 61.0, 62.1,
168.7, 211.7; HRMS (ESI) calcd for C9H13O739BrNa [MþNa]þ
270.9940, found 270.9939, calcd for C9H13O831BrNa [MþNa]þ
272.9920, found 272.9918.
References and notes
4.3. Preparations of 2-aryl-1,3-dimethylbenziimidazoline
(DMBIH) (2)
1. (a) Chikashita, H.; Itoh, K. Bull. Chem. Soc. Jpn. 1986, 59, 1747e1752; (b) Chika-
shita, H.; Ide, H.; Itoh, K. J. Org. Chem. 1986, 51, 5400e5405; (c) Chikashita, H.;
Miyazaki, M.; Itoh, K. J. Chem. Soc., Perkin Trans. 1 1987, 699e706.
2. (a) Chen, J.; Tanner, D. D. J. Org. Chem. 1988, 53, 3897e3900; (b) Tanner, D. D.;
Chen, J. J. J. Org. Chem. 1989, 54, 3842e3846; (c) Tanner, D. D.; Chen, J. J.; Chen,
L.; Luelo, C. J. Am. Chem. Soc. 1991, 113, 8074e8081; (d) Tanner, D. D.; Chen, J. J.;
Luelo, C.; Peters, P. M. J. Am. Chem. Soc. 1992, 114, 713e717; (e) Tanner, D. D.;
Chen, J. J. J. Org. Chem. 1992, 57, 662e666.
DMBIH 2a,1a,6c 2b,6g 2c,3a 2d,5 2e,6g 2f,4a,6g and 2g35 are known
compounds and were prepared by using literature procedures.6g
5. Metal complexes
3. (a) Lee, I.-S. H.; Jeoung, E. H.; Kreevoy, M. M. J. Am. Chem. Soc. 1997, 119,
2722e2728; (b) Lee, I.-S. H.; Jeoung, E. H. J. Org. Chem. 1998, 63, 7275e7279; (c)
Lee, I.-S. H.; Jeoung, E. H.; Kreevoy, M. M. J. Am. Chem. Soc. 2001, 123,
7492e7496.
4. (a) Montgrain, F.; Ramos, S. M.; Wuest, J. D. J. Org. Chem. 1988, 53, 1489e1492;
(b) Brunet, P.; Wuest, J. D. Can. J. Chem. 1996, 74, 689e696; (c) Schwarz, D. E.;
Cameron, T. M.; Hay, P. J.; Scott, B. L.; Tumas, W.; Thorn, D. L. Chem. Commun.
2005, 5919e5921.
A solution of purchased Ru(bpy)3Cl2$6H2O in MeOH was diluted
with Et2O to give a precipitate, which was isolated by filtration and
dried by heating at 120 ꢂC under vacuo for 16 h. Ir(ppy)2(dmbpy)PF6
was prepared using the literature procedure.17a
5.1. General procedure for metal complex photosensitized
reactions of 1 with 2
5. Zhu, X.-Q.; Zhang, M.-T.; Yu, A.; Wang, C.-H.; Cheng, J.-P. J. Am. Chem. Soc. 2008,
130, 2501e2516.
6. (a) Hasegawa, E.; Kato, T.; Kitazume, T.; Yanagi, K.; Hasegawa, K.; Horaguchi, T.
Tetrahedron Lett. 1996, 37, 7079e7082; (b) Hasegawa, E.; Tamura, Y.; Tosaka, E.
Chem. Commun. 1997, 1895e1896; (c) Hasegawa, E.; Yoneoka, A.; Suzuki, K.;
Kato, T.; Kitazume, T.; Yanagi, K. Tetrahedron 1999, 55, 12957e12968; (d) Ha-
segawa, E.; Chiba, N.; Nakajima, A.; Suzuki, K.; Yoneoka, A.; Iwaya, K. Synthesis
2001, 1248e1252; (e) Hasegawa, E.; Takizawa, S.; Iwaya, K.; Kurokawa, M.;
Chiba, N.; Yamamichi, K. Chem. Commun. 2002, 1966e1967; (f) Hasegawa, E.;
Chiba, N.; Takahashi, T.; Takizawa, S.; Kitayama, T.; Suzuki, T. Chem. Lett. 2004,
33, 18e19; (g) Hasegawa, E.; Seida, T.; Chiba, N.; Takahashi, T.; Ikeda, H. J. Org.
Chem. 2005, 70, 9632e9635; (h) Hasegawa, E.; Takizawa, S.; Seida, T.; Yama-
guchi, A.; Yamaguchi, N.; Chiba, N.; Takahashi, T.; Ikeda, H.; Akiyama, K. Tet-
rahedron 2006, 62, 6581e6588; (i) Hasegawa, E.; Hirose, H.; Sasaki, K.;
Takizawa, S.; Seida, T.; Chiba, N. Heterocycles 2009, 77, 1147e1161; (j) Hasegawa,
E.; Tosaka, E.; Yoneoka, A.; Tamura, Y.; Takizawa, S.; Tomura, M.; Yamashita, Y.
Res. Chem. Intermed. 2013, 39, 247e267.
7. (a) Fagnoni, M.; Dondi, D.; Ravelli, D.; Albini, A. Chem. Rev. 2007, 107,
2725e2756; (b) Hoffmann, N. J. Photochem. Photobiol. C 2008, 9, 43e60; (c)
Ravelli, D.; Dondi, D.; Fagnonia, M.; Albini, A. Chem. Soc. Rev. 2009, 38,
1999e2011; (d) Galian, R. E.; Prieto, J. P. Energy Environ. Sci. 2010, 3, 1488e1498;
(e) Ravelli, D.; Fagnoni, M.; Albini, A. Chem. Soc. Rev. 2013, 42, 97e113.
8. Representative publications that instigated the booming trend: (a) Nicewicz, D.
A.; MacMillan, D. W. C. Science 2008, 322, 77e80; (b) Ischay, M. A.; Anzovino,
M. E.; Du, J.; Yoon, T. P. J. Am. Chem. Soc. 2008, 130, 12886e12887; (c) Nar-
ayanam, J. M. R.; Tucker, J. W.; Stephenson, C. R. J. J. Am. Chem. Soc. 2009, 131,
8756e8757.
A solution of 1 (0.40 mmol), an appropriate quantity of 2, and
metal complexes (0.02 mmol) in a solvent (MeCN, DMF, DMSO,
MeOH) with or without additives was purged with N2 for 5 min
prior to irradiation. This solution was irradiated with the light,
l>390 nm for Ru(bpy)3Cl2 or l>360 nm for Ir(ppy)2(dmbpy)PF6
using a cut-off glass filter for an appropriate irradiation time. A
photolysates obtained from reactions in MeCN or MeOH were
usually concentrated directly while photolysates from reactions in
DMF or DMSO were first diluted with water and then extracted
with EtOAc to give extracts that were diluted with water, washed
with satd aqueous NaCl, dried over anhydrous MgSO4, and con-
centrated in vacuo, giving a residue that was subjected to column
chromatography and/or TLC separation using an appropriate ratio
of the mixed solvent consisting of EtOAc and n-C6H14 to give the
recovered 1 and the products. When isolation of the products by
chromatography could not be achieved, yields were determined by
1H NMR using appropriate internal references such as tri-
phenylmethane and 1,3,5-trimethoxybenzene. Photoproducts were
identified by comparisons of their 1H NMR spectra with those of
known compounds. Photoproducts 3,6h 4c,13f.l 4d,13f.l 4e,13f.l 4h,6c,13d
5b,36 5c,13f.l 5d,13l 5e,13f.l 5h,13d and 66j are known compounds.
9. Representative reviews: (a) Zeitler, K. Angew. Chem., Int. Ed. 2009, 48,
9785e9789; (b) Yoon, T. P.; Ischay, M. A.; Du, J. Nat. Chem. 2010, 2, 527e532; (c)
Narayanam, J. M. R.; Stephenson, C. R. J. Chem. Soc. Rev. 2011, 40, 102e113; (d)
Xuan, J.; Xiao, W.-J. Angew. Chem., Int. Ed. 2012, 51, 6828e6838; (e) Tucker, J. W.;
Stephenson, C. R. J. J. Org. Chem. 2012, 77, 1617e1622; (f) Prier, C. K.; Rankic, D.
A.; MacMillan, D. W. C. Chem. Rev. 2013, 113, 5322e5363.
5.1.1. 3-Methyl-1-benzosuberone 4b. Pale yellow oil; IR (neat)
10. Tanner et al. first reported the use of DMBIH with Ru(bpy)3(ClO4)2.2c Recently
successful application of Ru and Ir complexes to reductive ring-opening of aroyl
epoxides was also reported: Larraufie, M. H.; Pellet, R.; Fensterbank, L.;
1678 cmꢁ1
;
1H NMR (400 MHz)
d
1.06 (d, J¼6.4 Hz, 3H), 1.51e1.57
(m, 1H), 2.00e2.11 (m, 2H), 2.59 (dd, J¼14.8, 9.2 Hz, 1H), 2.77 (dd,

E. Hasegawa et al. / Tetrahedron 70 (2014) 2776e2783
2783
Goddard, J. P.; Lacote, E.; Malacria, M.; Ollivier, C. Angew. Chem., Int. Ed. 2011, 50,
4463e4466.
11. Recently, application of DMBIH to a artificial photosynthetic system in which
DMBIH cooperates with rutheniumerhenium complex was reported by Ishitani
and coworkers: Tamaki, Y.; Koike, K.; Morimoto, T.; Ishitani, O. J. Catal. 2013,
304, 22e28.
12. 5-exo-Hexenyl radical cyclization: (a) Griller, D.; Ingold, K. U. Acc. Chem. Res.
1980, 13, 317e323; (b) Beckwith, A. L. J.; Schiesser, C. H. Tetrahedron 1985, 41,
3925e3941.
93 V for Hantzsch ester6i and 0.94 V for Hunig’s base (Mann, C. K.; Barnes, K. K.
Electrochemical Reactions in Nonaqueous Systems; Marcel Dekker: New York, NY,
1970).
20. Hasegawa, E.; Tamuta, Y.; Suzuki, K.; Yoneoka, A.; Suzuki, T. J. Org. Chem. 1999,
64, 8780e8785.
21. Typically, direct irradiation of 1c ([1c]¼0.064 M) with 2a produces ca. 60%
yields of 4c and 5c with the ratio of 4c/5c (w60:w40) while arene sensitized
reaction ([1c]¼0.10 M) leads to formation of ca. 44% of 4c and 5c (4c/5c ¼w40:
w60).6j On the other hand, treatment of 1c with tris(trimethylsilylsilane) and
Et3B under air at room temperature produces 5c along with minor 4c (4c/
5c¼w28:w72) under the even more dilute conditions ([1c]¼0.05 M).13l
22. 1,6-Bisdimethylaminopyrene is an effective sensitizer cooperating with 2a to
promote reaction of 1h to give 4h in moderate yield (56%).6c
13. Tin or silicon-promoted DowdeBeckwith ring-expansion reactions: (a) Dowd,
P.; Choi, S. C. J. Am. Chem. Soc. 1987, 109, 3493e3494; (b) Dowd, P.; Choi, S. C. J.
Am. Chem. Soc. 1987, 109, 6548e6549; (c) Beckwith, A. L. J.; O’Shea, D. M.;
Westwood, S. W. J. Chem. Soc., Chem. Commun. 1987, 666e667; (d) Beckwith, A.
L. J.; O’Shea, D. M.; Westwood, S. W. J. Am. Chem. Soc. 1988, 110, 2565e2575; (e)
Dowd, P.; Choi, S. C. Tetrahedron 1989, 45, 77e90; (f) Bowman, W. R.; Westlake,
P. J. Tetrahedron 1992, 48, 4027e4038; (g) Dowd, P.; Zhang, W. Chem. Rev. 1993,
93, 2091e2115 and references cited therein; (h) Chatgilialoglu, C.; Timokhin, V.
I.; Ballestri, M. J. Org. Chem. 1998, 63, 1327e1329; (i) Sugi, M.; Togo, H. Tetra-
hedron 2002, 58, 3171e3175; (j) Ardura, D.; Sordo, T. L. Tetrahedron Lett. 2004,
45, 8691e8694; (k) Ardura, D.; Sordo, T. L. J. Org. Chem. 2005, 70, 9417e9423; (l)
Hasegawa, E.; Ogawa, Y.; Kakinuma, K.; Tsuchida, H.; Tosaka, T.; Takizawa, S.;
Muraoka, H.; Saikawa, T. Tetrahedron 2008, 64, 7724e7728.
14. DowdeBeckwith ring-expansion reactions under electron-transfer conditions:
(a) Shono, T.; Kise, N.; Uematsu, N.; Morimoto, S.; Okazaki, E. J. Org. Chem. 1990,
55, 5037e5041; (b) Hasegawa, E.; Kitazume, T.; Suzuki, K.; Tosaka, E. Tetrahe-
dron Lett. 1998, 39, 4059e4062; (c) Piers, E.; Gilbert, M.; Cook, K. L. Org. Lett.
2000, 2, 1407e1410; (d) Chung, S. H.; Cho, M. S.; Choi, J. Y.; Kwon, D. W.; Kim, Y.
H. Synlett 2001, 1266e1268; (e) Sugi, M.; Sakuma, D.; Togo, H. J. Org. Chem.
2003, 68, 7629e7633; (f) Takano, M.; Umino, A.; Nakada, M. Org. Lett. 2004, 6,
4897e4900; (g) Yoshizumi, T.; Miyazoe, H.; Sugimoto, Y.; Takahashi, H.; Oka-
moto, O. Synthesis 2005, 1593e1600; (h) Shimakoshi, H.; Abiru, M.; Izumi, S.;
Hisaeda, Y. Chem. Commun. 2009, 6427e6429; (i) Tahara, K.; Hisaeda, Y. Green
Chem. 2011, 13, 558e561; (j) Nishikawa, K.; Ando, T.; Maeda, K.; Morita, T.;
Yoshimi, Y. Org. Lett. 2013, 15, 636e638.
23. The quenching rate constant for the excited state of the related Ru complex by
2a was reported to be about 50 times greater than that by 1-benzyl-1,4-
dihydronicotinamide.11
24. Excited state reduction potential of a RuðbpyÞ3
2þ
salt (0.77 V vs SCE) is re-
ported: Bock, C. R.; Conner, J. A.; Gutierrez, A. R.; Meyer, T. J.; Whitten, D. G.;
Sullivan, B. P.; Nagle, J. K. J. Am. Chem. Soc. 1979, 101, 4815e4824. Excited state
reduction potential of Ir(ppy)2(dmbpy)þ (1.15 V vs SCE) can be obtained from
the ground state reduction potential (ꢁ1.46 V vs SCE), estimated from the lit-
erature data (E1/2¼ꢁ1.90 V vs ferrocene/ferrocenium)17b using the oxidation
potential of ferrocene (Eformal¼0.44 V vs SCE),6h and the absorption/emission
spectra (
l
¼470e480 nm) of this complex.17b The literature discussion, the
LSOMO (lowest singly occupied molecular orbital) of the triplet excited state of
the related iridium complex Ir(ppy)2(bpy)2þþ is delocalized in the metal and ppy
while the LSOMO of the excited RuðbpyÞ3 is localized in the metal center,17d,e
suggests that the former complex would act as a more efficient oxidant than
the latter in the initial electron-transfer step.
2þ
25. On the basis of the reduction potentials (V vs SCE) of RuðbpyÞ3 (Eformal¼ꢁ1.
33 V)24 and 1 (Ered ¼ꢁ1.82 V for 1a, ꢁ1.60 V for 1b, and ꢁ1.64 V for 1c), SET
1=2
þ
between RuðbpyÞ3 and
1 would not be fast. On the other hand, Ir(p-
py)2(dmbpy) should act as a stronger reductant, although initial SET is still
endergonic, since Ir(ppy)2(dmbpy)þ has more negative reduction potential (ꢁ1.
46 V vs SCE).24
15. Discussion of the ability of PET generated low valent transition metal com-
plexes, such as Ru(I) and Ir(II), to promote reductive cleavage of carbon-
ehalogen bonds: Nruyen, J. D.; D’Amato, E. M.; Narayanam, J. M. R.;
Stephenson, C. R. J. Nat. Chem. 2012, 4, 854e859.
26. One alternative rationalization is that proton generated from DMBIHꢀþ would
assist the endergonic electron transfer between the low valent metal com-
plexes and 1 as a proton coupled electron transfer suggested by Knowles
(Taranitino, K. T.; Liu, P.; Knowles, R. R. J. Am. Chem. Soc. 2013, 135,
10022e10025). In this case, however, the following intramolecular SET from
the protonated ketyls to the carbonebromine bonds would be less efficient
than that in the free form of ketyls (1ꢀꢁ). The other possibility is that DMBIꢀ
generated by the deprotonation of DMBIHꢀþ, although basic species cannot be
specified, acts as a reductant against 1 since DMBIꢀ is apparently stronger re-
ductant than the low valent states of those complexes,5,25which is similar to
the mechanism proposed by Scaiano (Ismaili, H.; Pitre, S. P.; Scaiano, J. C. Catal.
Sci. Technol. 2013, 3, 935e937).
27. Intramolecular electron transfers between the carbonyl radical anion (ketyl)
parts and the carbonehalogen bonds of haloalkyl-substituted ketones have
been previously reported. (a) Kimura, N.; Takamuku, S. Bull. Chem. Soc. Jpn. 1991,
64, 2433e2437; (b) Kimura, N.; Takamuku, S. J. Am. Chem. Soc. 1994, 116,
4087e4088; (c) Shaik, S.; Danovich, D.; Sastry, G. N.; Ayala, P. Y.; Schlegel, H. B. J.
Am. Chem. Soc. 1997, 119, 9237e9245.
16. Other representative publications, except for Refs. 8a,8c,15, on the carbon-
ehalogen bond cleavage promoted by Ru or Ir complexes sensitized PET re-
actions: (a) Nagib, D. A.; Scott, M. E.; MacMillan, D. W. C. J. Am. Chem. Soc. 2009,
131, 10875e10877; (b) Tucker, J. W.; Narayanam, J. M. R.; Krabbe, S. W.; Ste-
phenson, C. R. J. Org. Lett. 2010, 12, 368e371; (c) Furst, L.; Matsuura, B. S.;
Narayanam, J. M. R.; Tucker, J. W.; Stephenson, C. R. J. Org. Lett. 2010, 12,
3104e3107; (d) Andrews, R. S.; Becker, J. J.; Gagne, M. R. Angew. Chem., Int. Ed.
2010, 49, 7274e7276; (e) MacMillan. J. Am. Chem. Soc. 2010, 132, 13600e13603;
(f) Nguyen, J. D.; Tucker, J. W.; Konieczynska, M. D.; Stephenson, C. R. J. J. Am.
Chem. Soc. 2011, 133, 4160e4163; (g) Tucker, J. W.; Stephenson, C. R. J. Org. Lett.
2011, 13, 5468e5471; (h) Dai, C. H.; Narayanam, J. M. R.; Stephenson, C. R. J. Nat.
Chem. 2011, 3, 140e145; (i) Wallentin, C.-J.; Nguyen, J. D.; Finkbeiner, P.; Ste-
phenson, C. R. J. J. Am. Chem. Soc. 2012, 134, 8875e8884; (j) Freeman, D. B.;
Furst, L.; Condie, A. G.; Stephenson, C. R. J. Org. Lett. 2012, 14, 94e97; (k) Kim,
H.; Lee, C. Angew. Chem., Int. Ed. 2012, 51, 12303e12306; (l) Iqbal, N.; Jung, J.;
Park, S.; Cho, E. J. Angew. Chem., Int. Ed. 2013, 52, 1e5; (m) Cheng, Y.; Gu, X.; Li, P.
Org. Lett. 2013, 15, 2664e2667; (n) Jiang, H.; Cheng, Y.; Zhang, Y.; Yu, S. Org. Lett.
2013, 15, 4884e4887; (o) Wei, X. J.; Yang, D. T.; Wang, L.; Song, T.; Wu, L. Z.; Liu,
Q. Org. Lett. 2013, 15, 6054e6057; (p) Jiang, H.; Cheng, Y.; Wang, R.; Zheng, M.;
Zhang, Y.; Yu, S. Angew. Chem., Int. Ed. 2013, 52, 13289e13292.
17. (a) Lowry, M. S.; Hudson, W. R.; Pascal, R. A., Jr.; Bernhard, S. J. Am. Chem. Soc.
2004, 126, 14129e14135; (b) Lepeltier, M.; Lee, T. K.-M.; Lo, K. K.-W.; Toupet, L.;
Bozec, H. L.; Guerchais, V. Eur. J. Inorg. Chem. 2005, 110e117; (c) Takizawa, S.;
Aboshi, R.; Murata, S. Photochem. Photobiol. Sci. 2011, 10, 895e903 Also see the
representative publications of other Ir complexes: (d) Tinker, L. L.; McDaniel, N.
D.; Curtin, P. N.; Smith, C. K.; Ireland, M. J.; Bernhard, S. Chem.dEur. J. 2007, 13,
8726e8732; (e) Tinker, L. L.; McDaniel, N. D.; Bernhard, S. J. Mater. Chem. 2009,
19, 3328e3337.
18. (a) Hasegawa, E.; Xu, W.; Mariano, P. S.; Yoon, U. C.; Kim, J. U. J. Am. Chem. Soc.
1988, 110, 8099e8111; (b) Zhang, X.; Ye, S. R.; Hong, S.; Freccero, M.; Albini, A.;
Falvey, D. E.; Mariano, P. S. J. Am. Chem. Soc. 1994, 116, 4211e4220; (c) Su, Z.;
Mariano, P. S.; Falvey, D. E.; Yoon, U. C.; Oh, S. W. J. Am. Chem. Soc. 1998, 120,
10676e10686.
28. The rate constant for 5-exo hexenyl radical cyclizations for
a-disubstituted
primary alkyl radical is reported to be 3.6>106 sꢁ1 at 25 ꢂC.12b
29. The rate constant of DowdeBeckwith rearrangement of 7c was estimated to be
3.8ꢃ103
s
ꢁ1 (the reported 1.5ꢃ104
s
ꢁ1 is incorrect) and 1.2ꢃ105 sꢁ1 at 25 ꢂC and
80 ꢂC, respectively.13l It was also found that ratio of 4ce5c (81:19)13l was
greater than that of 4be5b (44:56) for the reaction with tris(trimethylsilyl)
silane in refluxed benzene (unpublished).
30. The last step of path B (from DMBIꢀ to DMBIþ) is a feasible owing to the ex-
tremely low oxidation potential of DMBIꢀ, which is same as the reduction po-
tential of DMBIþ (Epred¼ꢁ2.055
V
vs ferroceneeferrocenium).5 We also
measured the reduction potentials (Epred V vs Ag/AgNO3) of DMBIþBF4e and
DMBIþClOe4 to obtain ꢁ2.013 V both for BF4 and for ClO4 salts (unpublished).
This value is transferred to ꢁ2.08 V as Epred (V vs ferroceneeferrocenium) and
red
ꢁ1.61 V as E
(V vs SCE).6h
1=2
31. (a) Hasegawa, E.; Ishiyama, K.; Horaguchi, T.; Shimizu, T. J. Org. Chem. 1991, 56,
1631e1635; (b) Hasegawa, E.; Ishiyama, K.; Fujita, T.; Kato, T.; Abe, T. J. Org.
Chem. 1997, 62, 2396e2400; (c) Tanko, J. M.; Gillmore, J. G.; Friedline, R.;
Chahma, M. J. Org. Chem. 2005, 70, 4170e4173.
19. Ru(bpy)3Cl2-sensitized photoreactions of 1a and 1c with diethyl 1,4-dihydro-
2,6-dimethyl-3,5-pyridinedicarboxylate (Hantzsch ester, 1.2 equiv) and N,N-
diisopropylethylamine (Hunig’s base, 5.0 equiv), which are frequently
employed for transition metal-sensitized photoreactions,9 were examined. In
the case of Hantzsch ester, no reaction of 1a and 1c proceeded. Although 3 and
4c were detected by 1H NMR, most of 1a and 1c were recovered in the reactions
32. Beauchamp, A.; Montgrain, F.; Wuest, J. D. Acta Crystallogr. 1987, C43,
1557e1560.
33. In contrast, both 2e and 2f are similarly effective in promoting transformations
of epoxy ketones to hydroxy ketones.6f,h
34. Tsuchida, H.; Tamura, M.; Hasegawa, E. J. Org. Chem. 2009, 74, 2467e2475.
35. Bachand, B.; Ramos, S. M.; Wuest, J. D. J. Org. Chem. 1987, 52, 5443e5446.
36. Hasegawa, E.; Yamaguchi, N.; Muraoka, H.; Tsuchida, H. Org. Lett. 2007, 9,
2811e2814.
with Hunig’s base. Less efficiency of these amines than the amines reported in
ox
the text would be due to their low electron donating abilities: E
(vs SCE)¼0.
1=2