Reactions with boron—sulfur bonds

Thioketals are readily formed by acid-catalyzed reaction with ethane-dithiol. Selective thioketal formation is achieved at C-3 in the presence of a 6-ketone by carrying out the boron trifluoride catalyzed reaction in diluted medium. Selective protection of the 3-carbonyl group as a thioketal has been effected in high yield with A" -3,17-diketones, A" -3,20-diketones and A" -3,l 1,17-triones in acetic acid at room temperature in the presence of p-toluenesulfonic acid. In the case of thioketals the double bond remains in the 4,5-position. This result is attributed to the greater nucleophilicity of sulfur as compared to oxygen, which promotes closure of intermediate (66) to the protonated cyclic mercaptal (67) rather than elimination to the 3,5-diene [cf. ketal (70) via intermediates (68) and (69)]." " ... 

The formation of aryl chlorides from arenediazonium ions on reaction with copperfi) chloride is known as the Sandmeyer reaction and also involves the intermediacy of aryl radicals [192]. Copper salts are similarly involved in the reactions of diazonium ions with cyanide, nitrite, and sulfur dioxide. The uses of arenediazonium salts in synthetic reactions forming carbon-carbon, carbon-sulfur, and carbon-boron bonds have been summarized recently [193]. 

Compounds 66 and 67 typically reacted through the boron-boron multiple bond to afford n-complexes with coinage or alkali metals (68 and 69, Figure 5.10). Additionally, complexes 62, 66, and 67 were shown to undergo simultaneous B-B bond cleavage/cycloaddition reactions by oxidation with elemental sulfur or selenium (70 and 71). " Diboron complexes 66 and 67 also reacted with carbon monoxide and isonitriles, respectively, to afford NHC-stabilized bis(boralactone) 72 and bis(boraketenimine) 73. ... 

This reaction typifies the two possibilities of reaction routes for M-catalyzed addition of an S-X (or Se-X) bond to alkyne (a) oxidative addition of the S-X bond to M(0) to form 94, (b) insertion of alkyne into either the M-S or M-X bond to provide 95 or 96 (c) C-X or C-S bond-forming reductive elimination to give 97 (Scheme 7-21). Comparable reaction sequences are also discussed when the Chalk-Harrod mechanism is compared with the modified Chalk-Harrod mechanism in hydrosily-lations [1,3]. The palladium-catalyzed thioboratiori, that is, addition of an S-B bond to an alkyne was reported by Miyaura and Suzuki et al. to furnish the cis-adducts 98 with the sulfur bound to the internal carbon and the boron center to the terminal carbon (Eq. 7.61) [62]. 

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