Further demonstration of the higher reactivity of the vinyl hydrogen rather
than the allyl hydrogen towards singlet oxygen
Hajime Mori, Keiichi Ikoma and Shigeo Katsumura*
School of Science, Kwansei Gakuin University, Uegahara 1-1-155, Nishinomiya 662, Japan
In a new linear twisted 1,3-diene, an anomalously high
reactivity of the vinyl hydrogen rather than the allyl
hydrogen towards singlet oxygen is observed.
1b–e are significantly twisted around the central single bond of
the 1,3-diene. The most stable conformations of 1b–e obtained
by molecular mechanics calculations and molecular obrital
calculations also support the significantly twisted struc-
tures.1§
Previously, we reported the photosensitized oxygenation of
significantly twisted cis-b-ionol derivatives, and found the
vinyl hydrogen to be anomalously more reactive towards singlet
oxygen than the allyl hydrogen in some significantly twisted
compounds of cis-b-ionol derivatives. We proposed that this
unusually high reactivity of the vinyl hydrogen in a significantly
twisted 1,3-diene should be attributed to a large interaction
between the s* orbital of the vinyl hydrogen and the p orbital of
another double bond.1 Since only a few examples of sig-
nificantly twisted 1,3-dienes are found in the literature,2
although conjugated dienes are some of the most common
systems in organic reactions as represented by pericyclic
reactions, further investigation on the design, synthesis and
photosensitized oxygenation of significantly twisted 1,3-dienes
has been required. Here we report the photosensitized oxy-
genation of a newly designed linear twisted 1,3-diene, and
describe a further demonstration of the unusually high reactivity
of the vinyl hydrogen towards singlet oxygen.
R
X
OR′
Y
R
OR′
2a,b,e X = Br
2c,d X = OTf
3 R′ = Y = ZnCl
4 R′ = H, Y = Bu3Sn
5 R′ = Et3Si, Y = ZnCl
6a–d R′ = H
6e R′ = Et3Si
Ha
R′
Ha
Hc
R
2
2
R′
5
6
5
6
a R = H
b R = Me
OR′
OSiEt3
c R = Pri
d R = But
e R = Me2C(OH)
Hb
7a–e R′ = H
1a–e R′ = Et3Si
Hb
1b R′ = H
c R′ = Me
The substrates 1a–e were chosen for the photosensitized
oxygenation because the substituent R on the olefin can be
changed from hydrogen to methyl, isopropyl and tert-butyl, and
when R is a bulky substituent the 1,3-diene structure would be
twisted. Syntheses of these cis-1,3-diene derivatives were
accomplished by palladium catalysed coupling of the appro-
priate vinyl halides or enol trifluoromethanesulfonates (tri-
flates) with zinc or stannyl acetylides followed by partial
hydrogenation and then protection of the tertiary hydroxy
group. Thus, compounds 6a,b were prepared from commer-
cially available vinyl bromides 2a,b by reaction with zinc
The photosensitized oxygenation of 1a–e was carried out in
CH2Cl2 (1.0–2.0 mmol scale) by irradiation using a halogen
lamp under an oxygen atmosphere in the presence of a catalytic
amount of tetraphenylporphine and 1 equiv. of triphenylphos-
phine at 0 °C. The products were isolated by column
chromatography and all of them were characterized by 1H, 13
C
NMR and IR spectroscopy, and by elemental analysis or high
resolution mass spectra for some compounds. The ene reaction
of Ha and Hb gave allene 8 and isopropenyl compound 9,
respectively. In the case of 1b,c, the ene reaction of Hc gave
compounds 10b,c, which might produce allene 11 by further
oxygenation. The results of the photosensitized oxygenation are
shown in Table 1. For compounds 1d,e, allenes 8d,e (mp
76–77 °C) were produced in 75 and 74% yield along with
isopropenyl compounds 9d,e in 8 and 9% yield, respectively. In
these cases, the abstracted vinyl hydrogen Ha and allyl
hydrogen Hb are located at the opposite sides of the C5–C6
double bond, and the vinyl hydrogens Ha were much more
reactive than the allyl hydrogens Hb toward singlet oxygen.7¶
acetylide
3
in the presence of tetrakis(triphenyl-
phosphine)palladium (THF, 60 °C; 6a, 80%; 6b, 45% yield),
respectively.3 Compounds 6c,d which have bulky substituents
were synthesized by coupling of stannyl acetylide 4 with enol
triflates 2c,d,4 which were prepared from the corresponding
ketones5 (LDA, N-phenyltrifluoromethanesulfonimide, THF),
in the presence of the same palladium reagent (THF, room
temp.; 6c, 45%; 6d, 52% yield).6 Another enyne compound 6e
was also synthesized by coupling of vinyl bromide 2e, prepared
from methyl 3,3-dimethylacrylate (Br2–CH2Cl2, DBU–THF,
MeMgI–THF; 46% yield), with 5 under the same conditions as
those of 2a,b (6e, 84% yield). Partial reduction of the acetylene
groups of 6a–e to the corresponding cis-olefins by catalytic
hydrogenation in hexane reflected the reactivity of their
acetylene groups. Thus, 7a,b were obtained with Lindlar
catalyst in the presence of a catalytic amount of quinoline (7a,
47%; 7b, 49% yield). In the absence of quinoline with Lindlar
catalyst, 7c was obtained (47% yield). Hydrogenation of 6d
with palladium on barium sulfate gave 7d (60% yield) and of 6e
with palladium on charoal gave 1e (MeOH, 90% yield). The cis-
alcohols obtained were transformed into the corresponding
triethylsilyl ethers 1a–d in excellent yield (Et3SiCl, DMAP,
Et3N, DMF).† The maximum absorption values in the elec-
tronic spectra of compounds 1b–e were less than 210 nm,
whereas that of 1a was 238 nm. These data clearly show that
Table 1 Ene reaction of acyclic twisted 1,3-dienes with singlet oxygen
Yield (%)
Substrate
8a
9a
10a,b
11a,b
1a
1b
1c
1d
1e
29b
32b
38b
75b
74c
45b
12b
27b
8b
—
42
8
—
—
—
—
17
—
—
9c
a
b
Isolated yield.
The yields are over three steps; silylation of the
corresponding alcohol followed by photosensitized oxygenation and then
c
desilylation. The yields are over two steps; photosensitized oxygenation
and desilylation.
Chem. Commun., 1997
2243
Mori, Hajime
