Pseudohalogens, addition reactions

Pseudohalogen addition reactions reported recently include those of iodine azide (IN3), which readily attacks olefinic steroids (e.g. 139) to give iodo-azides... 

Iodine isocyanate was used to synthesize the first steroidal aziridine, 2, 3 -iminocholestane (95). from 5a-cholest-2-ene (91). This reaction sequence which is believed to proceed through a three-membered ring iodonium ion (92) illustrates the limitation of pseudohalogen additions for the synthesis of -aziridines. The iodonium complex forms from the least hindered side (usually alpha) and is opened tmK5-diaxially to give a -oriented nitrogen function. The 3a-iodo-2 -isocyanate (93) is converted by treatment with... 

Bis [(trifluoromethyl)thio] acetaldehyde (83a) has been prepared from an enam-ine precursor (84), although refluxing in aqueous ethanolic HCl is required to effect this reaction.The aldehyde is less stable than its enol tautomer (83b), and many reactions typical of aldehydes fail. For example, addition of aqueous silver nitrate immediately yields the silver salt of (83b), rather than giving precipitation of (elemental) silver. The (trifluoromethyl)thio substituent has pseudohalogenic character and, together with the hydroxy group, stabilizes the alkene tautomer in the manner of a push-pull alkene. The enol-aldehyde equilibrium mixture in acetonitrile shows an apparent of 2.6 when titrated with aqueous hydroxide. 

There are three important routes to the formation of the mercury-transition metal bond (a) displacement of halogen or pseudohalogen from mercury(II) salts with carbonyl metallate anions (b) reaction of a halo-phenylmercury compound with a transition metal hydride and (c) oxidative addition of a mercury halide to neutral zero valent metals.1 We report here the syntheses of three compounds containing three-centre, two-electron, mercury-ruthenium bonds utilizing trinuclear cluster anions and mercury(II) halides.2-4... 

The use of iodine-containing pseudohalogens has been demonstrated to cause a variety of side reactions. Treatment of methyl 5-0-benzoyl-2,3-dideoxy-3-D-g/ycero-pent-2-enofuranoside (83, Scheme 22) with nitryl iodide afforded none of the desired addition product, but gave a quantitative yield of furfuryl benzoate (86). The same result was obtained by treating 83 with iodine nitrate, or with iodine alone.125 The rationale for this undesired result lies in the ability of iodine to act as a Lewis acid, polarizing the glycosidic bond, generating an oxacarbenium cation, which then decomposes to the stable aromatic 86. 

Addition of pseudohalogens, such as BrNs and INS, to unsaturated compounds has been shown to occur either by a free radical (via N3) or by an electrophilic (via X+) stepwise mechanism, depending on the reaction conditions (Hassner and Boerwinkle, 1969). Electrophilic addition of BrNs to l-phenylpropyneisnon-regiospecifio (Hassner, 1909) and both products 50 and 51 are formed. The addition of IN3 is instead... 

The reactions of pentafluorothio olefins are broad, with cycloadditions being rare l-SF5-l,3-cyclohexadiene can be dehydrogenated to QH5SF5 (20). Additions of halogens and pseudohalogens take place easily nucleophilic and electrophilic additions or nucleophilic substitutions may occur (20,21). 

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