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  • CHAPTER 4

    CYCLOADDITION REACTIONS OF

    0-THIOQUINONES WITH ALKENES AND ALKYNES

    4.1 INTRODUCTION

    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

    (Scheme 1).

    Ar = p-anisyl

    Scheme 1

  • 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).

    L

    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.

    Figure 1

  • Alkenes

    Figure 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

    I

    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%

    Scheme 3

    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

  • Chapter 4

    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

    Me3(: SNPhth

    Me0 SNPhth

    Me SNPhth

    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

  • SNPhth

    + \

    --.-. \ /

    4 H \ / /

    16 4 17

    i. Pyridine, CHCI,, Sealed tube, 70 OC, 20 h, 92%

    Scheme 4

    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

    (Scheme 5)

    I

    '. Me2HC SNPhth Ph

    3 18 19

    i. Pyridine, CHC13, Sealed tube, 70 OC, 24 h, 95%

    Scheme 5

    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

    respectively.

    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

    Me3C

    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).

  • Chapter 4

    SNPhth

    I

    Ph

    16 18 25

    i. Pyridine, CHC13, Sealed tube, 70 OC, 24 h, 83%

    Scheme 6

    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%

    Scheme 7

    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

  • Chapter 4

    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

    Thioquinone precursor

    Me3C: SNPhth

    M e 0 'SNPhth

    product

    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).

  • SNPhth

    __C

    \ 4

    16 26 33 i. Pyridine, CHCI3, Sealed tube, 70 OC, 16 h, 92%

    Scheme 8

    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%

    Scheme 9

    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).

  • MejC

    SNPhth

    12 36 37

    i. Pyridine, CHC13, Sealed tube, 70 OC, 15 h, 33%

    Scheme 10

    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

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