O-Thioquinone methide

The lower reactivity of the o-thioquinone methide 81 (7.0 x 104 M-1 s 1) compared with the o-l (8.4 x 105 M-1 s 1) in acidic solution contrasts with the much higher reactivity of 81 at neutral pH (Table 1). This may represent the balance between the smaller concentration of the protonated thioquinone methide compared with protoned o-l due to the weaker basicity of o-S (pATa < —3 Table 1) compared with o-S (pKa < —1.7) and the presumably greater intrinsic reactivity of H-81 +. 58 However, the observed effects of sulfur for oxygen substitution on carbocation reactivity have in the past proven very difficult to rationize,130,160 and in the present case are probably not fully understood. 

Benzo[3]thiete is a source of o-thioquinone methides, heterodienes which react with 1,4-naphthoquinones or 1,4-epoxynaphthalenes to give the benzo[ ]thioxanthene system. With the former reagents, the initial products undergo an autooxidation and 6,ll-dihydro-12//-benzo[7]thioxanthen-6,ll-diones result. The 6,11-epoxy adducts which are formed from the epoxynaphthalenes can be dehydrated to 12//-benzo[7]thioxanthenes or converted into the dihydrobenzothioxanthene-6-ol (Scheme 131) <1995JPR379>. 

The photolysis of 4-phenyl-l,2-dithiole-3-thione (71) gives a dimer (72) (73TL1561), but benzo-l,2-dithiolane 2,2-dioxide (73a) gives an o-thioquinone methide (74) (78JOC3374). 

Benzo-l,2-dithiole-3-thione (77b) also reacts, but the monoadducts corresponding to (95) have o-thioquinone methide structures (cf. structure 104) and form diadducts rapidly. The use of dehydrobenzene (benzyne) gives 2-thioacylmethylenebenzo-l,3-dithioles (95 R3, R4 = (CH=CH)2). 

A series of stabilized and isolable o-thiobenzoquinone methides and related o-thioquinone methides, initially prepared by the photochemical addition of olefins to 1,2-benzodithiole-3-thione [Eq. (15)],33 34... 

The o-thioquinone methide (39), generated by the photoreaction of benzo-l,2-dithiole-3-thione with olefins, dimerizes to give a head-to-head dimer. Further... 

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