chapter 4 cycloaddition reactions of 0-thioquinones .chapter 4 cycloaddition reactions of...
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CYCLOADDITION REACTIONS OF
0-THIOQUINONES WITH ALKENES AND ALKYNES
Only scant information has been available on the cycloaddition chemistry of
o-benzoquinones with alkenes and alkynes.'-3 Recent investigations in our
laboratory have clearly established the reactivity of o-benzoquinones with alkenes
and a ~ k ~ n e s . ~
In vlew of the facility with which o-thioquinones underwent cycloaddition
to various conjugated n-systems (see Chapters 2 and 3), it was of interest to
investigate their reactivity towards alkenes and alkynes. It may be mentioned that
there has been only one isolated report on the cycloaddition reactions of
o-thioquinones with aryl alkenes and alkynes.' CMethoxystyrene has been shown
to react with 1-thionaphthoquinone leading to the formation of 1 in 62% yield
Ar = p-anisyl
It 1s reported in the same paper that I-thionaphthoquinone on treatment with
ethoxyethyne underwent two consecutive [4+2] cycloadditions to afford the
bisadduct 2 (Scheme 2).
4.2 THE PRESENT WORK
It was clear Gom the literature survey that the reactivity profile of
0-thioquinones with alkenes and alkynes has received only limited attention. The
results of our studies on the cycloaddition of o-thioquinones with alkenes and
some acetylenic compounds are presented in this chapter.
The o-thioquinones and, alkenes and a w e s selected for our study are
given respectively in Figures 1 and 2.
4.3 RESULTS AND DISCUSSION
COOMe COPh R
4.3.1 Cycloadditions with Alkenes
Initially we investigated the [4+2] cycloaddition reactions of various
substituted o-thioquinones with acenaphthylene. 4-Isopropyl-2-thiobenzoquinone,
generated in srtu from the thiophthalimide 3, on treatment with 4 in chloroform at
70 OC afforded the 1,4-benzoxathiin adduct 5 in 96% yield (Scheme 3).
COOMe COPh R
I I I I
/ \ + -
Me2HC SNPhth \ ' Me2HC \ 4 H \ /
I R = n-propyl
3 4 5
i. Pyridine, CHCI,, Sealed tube, 70 OC, 20 h, 96%
The structure of the product was established on the basis of spectroscopic
data. The IR spectrum of 5 showed ether absorption at 1222 cm-'. In the 'H NMR
spectrum, the (=3)2CH protons resonated as a doublet at 6 1.09 (J = 6.8 Hz). The
( C H 3 ) * B proton was observed as a multiplet centered at 6 2.70. The proton on
C-3 resonated as a doublet at 6 5.22 (J = 7.2 Hz). The signal at 6 6.22
(d, J = 7.2 Hz) can be assigned to C-2 proton. The aromatic protons were visible as
multiplets centered at 6 7.22. In the I3c NMR spectrum, the signals at 649.66 and 84.66 were assigned to the C-3 and C-2 carbons respectively.
Simllar reactions were observed with other o-thiobenzoquinones also and
the results are summarized in Table 1.
Table 1: Cycloaddition reactions of substituted o-thiobenzoquinones with acenaphthylene
Thioquinone precursor product
Me3c177;1phth \ CMe3 12
\ Me2HCaoH SNPhth 14
Reaction conditions: Pyridine, CHCI,, Sealed tube, 70 OC, 20 h
1-Thlonaphthoquinone also reacted with 4 in a similar fashion as illustrated
in Scheme 4
--.-. \ /
4 H \ / /
16 4 17
i. Pyridine, CHCI,, Sealed tube, 70 OC, 20 h, 92%
The structure of the product was ascertained fiom its spectroscopic data. In
the 'H NMR spectrum, C-3 proton resonated as a doublet at S 5.34 (J = 6.8 H z ) ,
while the C-2 proton appeared as a doublet at 86.39 (J= 6.8 H z ) . In the 13c NMR spectrum of 17, the characteristic signals at S 49.06 and 84.66 can be assigned to
the C-3 and C-2 carbons respectively.
Subsequent to the above studies, we have carried out the [4+2]
cycloaddition reactions of o-thioquinones with trans-stilbene, 18. 4-Isopropyl-2-
thiobenzoqu~none on treatment with 18 yielded product 19 in 95% yield
'. Me2HC SNPhth Ph
3 18 19
i. Pyridine, CHC13, Sealed tube, 70 OC, 24 h, 95%
The IK spectrum of 19 showed ether absorption at 1239 cm-'. The 'H NMR
spectrum displayed the signals due to methyl protons, (CJ&CH at 6 1.22 (d, J =
6.9 Hz) and the methine proton of the isopropyl group, (CH3l2m was discernible
at 62.81-2.85 as a multiplet. The proton on C-3 resonated as a doublet at 6 4.37
(J = 8.9 H z ) , while the C-2 proton appeared as a doublet at 6 5.00 (J = 8.9 H z ) .
The aromatic protons were visible as multiplets centered at 6 6.99. In the I3c NMR
spectrum, the C-2 and C-3 carbons were observed at 6 82.07 and 49.47
The reaction of other o-thiobenzoquinones with trans-stilbene also followed
the same pathway and the results are summarized in Table 2.
Table 2: Cycloaddition reactions of substituted o-thiobenzoquinones with trans-stilbene
Thioquinone precursor product
Me3C SNPhth Me3C
M e 0 SNPhth M e 0 Ph
Me SNPhth Me
SNPhth Ph CMe3 CMe3 12 23 (87%)
Reaction conditions: Pyridine, CHCh, Sealed tube, 70 OC, 24 h
In an analogous manner, reaction of I-thionaphthoquinone with trans-
stilbene afforded 1,4-oxathiin adduct 25 in 83% yield (Scheme 6).
16 18 25
i. Pyridine, CHC13, Sealed tube, 70 OC, 24 h, 83%
As usual the structure of the product was assigned on the basis of
spectroscopic data. In the 'H NMR spectrum, the proton on C-3 resonated as a
doublet at 6 4.34 (J = 8.7 Hz), while the C-2 proton appeared as a doublet at 65.08
(J = 8.7 Hz). In the 13c NMR spectrum, the C-3 and C-2 carbons were visible at 648.96 and 82.08 respectively.
When indene was used in place of trans-stilbene, with 4-isopropyl-2-
thiobenzoqumone, the product 27 was obtained regioselectively in 92% yield. The
reaction is illustrated in Scheme 7.
\ Me2HC SNPhth Me2HC S 4
3 26 27
i. Pyridine, CHC13, Sealed tube, 70 OC, 16 h, 92%
The structure of the adduct was assigned on the basis of spectroscopic data.
In the IR spectrum of 27, the characteristic stretching frequency of C-0 bond was
observed at 1243 cm-I. The 'H NMR spectrum showed a doublet at 6 1.18 (J = 6.9
Hz) correspondmg to (B3)*CH protons. The (CH3)2m proton resonated as a
multiplet in the region 62.76-2.80. The multiplets centered at 63.15 and 3.32 were
assigned to the ~nethylenic protons of the indene ring. The C-3 proton was
observed as a multiplet centered at S 3.97, which is diagnostic for the observed
regioisomer. The proton on C-2 appeared as a doublet at 65.28 (J = 5.3 Hz). The
aromatic protons were visible as multiplets in the region 6 6.84-7.49. In the 13c NMR spectrum, C-3 carbon resonated at 6 42.83 and the signal at 6 81.54 can be
attributed to C-2 carbon.
In follow-up studies, we examined the generality of the reaction with other
substituted thloquinones and the results are summarized in Table 3.
Table 3: Cycloaddition reactions of substituted o-thiobenzoquinones with indene
M e 0 'SNPhth
Reaction conditions: Pyridine, CHCls, Sealed tube, 70 OC, 16 h
In a similar fashion, 1-thionaphthoquinone on heating with indene afforded
the l,4-oxathiin adduct 33 in 92% yield (Scheme 8).
16 26 33 i. Pyridine, CHCI3, Sealed tube, 70 OC, 16 h, 92%
The IR spectrum of 33 showed ether absorption at 1229 cm-' and thioether
absorption at 751 cm-'. The 'H NMR spectrum showed the C-3 proton as a
multiplet centered at 64.07 and the proton on C-2 as a doublet at 6 5.47 (J = 5.0
Hz). In the 'k NMR spec- signals at 642.29 and 81.24 were assigned to the C-3 and C-2 carbons respectively.
2-Hydroxy-5-isopropylbenzothiophthalimide, 3 on treatment with
1-phenylcyclohexene under sealed tube conditions in presence of pyridine afforded
1,4-benzoxathiin adduct 35 in 17% yield (Scheme 9).
,fJOH+ ph\O i- Me2HC SNPhth Me2HC
3 34 35 i. Pyridine, CHCI3, Sealed tube, 70 OC, 15 h, 17%
The structure of the adduct 35 was ascertained from its spect~oscopic data.
The IR spectrunl showed ether absorption at 1232 cm-'. In the 'H NMR spectrum,
proton on C-3 appeared as a multiplet centered at 63.36, which is diagnostic for a
product resulting from th~s mode of addition. In the l3c NMR spectrum, C-2 and
C-3 carbons were discernible at 6 77.82 and 42.96 respectively. All other signals
were in accordance with the proposed structure.
Similarly, 4,6-di-tert-butyl-2-hydroxybenzothiophth on reaction
with cyclohexene in presence of pyridine afforded 1,4-benzoxathiin adduct 37 in
33% yield (Scheme lo).
12 36 37
i. Pyridine, CHC13, Sealed tube, 70 OC, 15 h, 33%
The IR spectrum of 37 showed ether absorption at 1303 cm-'. In the 'H
NMR spectrum, the protons on C-2 and C-3 were observed as two separate
multiplets centered at 6 4.37 and 3.11 respectively. In the I3c NMR spectrum, C-2
and C-3 carbo