Thiocyanates thioketones

Allylic selenocyanates and thiocyanates are prepared from the reaction of allylsilanes with KSeCN 2CuBr2 and KSCN 2CuBr2, respectively. Both cyanoselenation and cyan-othianation take place at the a-position of allylsilanes (equation 106). Presumably, the electrophile XCN generated in situ is first added to the /-position, followed by a [l,3]-migration196. Fluoride induced reactions of allylsilanes with thioketones yield the corresponding allyl sulfides197. Displacement via an Se route is noted. 

In addition to pyrolysis or photolysis, S,C cleavage may be induced by nucleophiles (or bases) in substrates with an electrophilic (or acidic) center and a leaving group next to the sulfur1. This is exemplified in the treatment of a bridged thiocyanate with potassium terf-butoxide leading to an a-oxo thioketone which spontaneously dimerizes (equation 58)324. 

Aqueous sodium sulfide reduced the aryl thiocyanate 85, prepared from ethidium bromide, affording the air-sensitive dithiol 86 in 63% yield381 (equation 92). Also, Nicolaou s group has reported on the cleavage of dithiatopazine 87, a stable 1,2-dithietane system with loss of one sulfur atom and formation of a rearranged thioketone 88382 (equation 93). 

Phenols react with phthalimidesulphenylchloride (PhthNSCl) to yield N-thiophthalimides. These when treated with tertiary amines afford unstable ortho-thioquinones. Base-catalysed CN-elimination from camphoryl thiocyanate yields the a n -l,3-dithietane as the sole stereoisomer via thioketone (Scheme 9). 

From -Chlorovinylmethine-immonium and Bunte Salts Types C and E). j8-Chloro-vinylmethine-immonium salts (9) react with alkali-metal thiocyanates in the presence of aromatic amines to give isothiazoles (10). The reaction is considered to proceed by a nucleophilic displacement to give eventually the jS-thiocyanovinyl aldehyde anils ArC(SCN)=CHCH=NR, which cyclize in situ. The reaction of (9) with sodium thiosulphate gives Bunte salts, which react with amines (R R NH) to give the stable j8-aminovinyl thioketones (11), and hence, by oxidative cycliza-tion, the isothiazolium salts (10 R = H) (Scheme 1). 

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