Pseudohalogen

The characteristic absorption at 1746 cm of 1-acetylpyrazole (89) has been related to the pseudohalogen behaviour of the 1-pyrazolyl substituent (o-p = 0.23) by the equation... 

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... 

A useful theme for cohering much of the chemistry of compounds containing the CN group is the concept of pseudohalogens, a term introduced in 1925 for certain strongly bound, univalent radicals such as CN, OCN, SCN, SeCN, (and N3, etc.). These groups can form anions X , hydracids HX, and sometimes neutral species X2. 

The pseudohalogen concept (p. 319) might lead one to expect the existence of a cyanate analogue of cyanogen but there is little evidence for NCO-OCN, consistent with the known reluctance of oxygen to catenate. By contrast. 

Pseudohalides of Se in which the role of halogen is played by cyanide, thiocyanate or selenocyanate are known and, in the case of Se are much more stable with respect to disproportionation than are the halides themselves. Examples are Se(CN)2, Se2(CN)2, Se(SeCN)2, Se(SCN)2, Se2(SCN)2. The selenocyanate ion SeCN is ambidentate like the thiocyanate ion, etc., p. 325), being capable of ligating to metal centres via either N or Se, as in the osmium(IV) complexes [OsCl5(NCSe)], [OsCl5(SeCN)], and trans-[OsCU(NCSe)(SeCN)]2-.920) Tellurium and polonium pseudohalogen analogues include Te(CN)2 and Po(CN)4 but have been much... 

Dimethyltin dichloride has a similar chain structure (375). In diethyltin diiodide (374), dimethyltin diisothiocyanate (376, 377), and di-chloro bis(chloromethyl)stannane (378), however, the distorted, trans-RgSnX geometry of each tin atom is completed by two bridging bonds involving the halogen or pseudohalogen atoms on the same, neighboring molecule. 

Tbe most outstanding feature of RjTl compounds with respect to chemical reactivity is the great ease with which one of the substituent groups may be displaced and R2TIX compounds formed. Cleavage has been found to occur readily not only with a wide variety of proton acids (82) [Eq. (1)], but with halocarbons (82) [Eq. (2)], pseudohalogens (106) [Eq. (3)], and... 

Chloro- and A TV-dichloro-phosphoramidate esters (20) and (21) are readily prepared from the parent phosphoramidate by direct chlorination in mildly acidic solution but when R = Ph, the use of t-butyl hypochlorite is preferable, to avoid chlorination of the aromatic nucleus. These compounds behave as pseudohalogens, (21) reacting with olefinic compounds such as styrene to give (22), which is also formed by chlorination of the N-phosphorylaziridine (23). ... 

The imide nitrogen atom was also most reactive to a variety of electrophilic species (hydrogen halides, pseudohalogens, and alkyl halides) in the parent Rimidophosphazenes, R(C—NH)-N=PPh3. With t-butyl hypochlorite the /V-chloro-derivatives, R(C=NCl)-N=PPh3, were obtained. R/ -Vinyl-phenylphosphazenes have been prepared by condensation of aldehydes with active methylene compounds ... 

This, and the cyanato and selenocyanato (pseudohalogen) analogues are explosive in the solid state, but may be handled with comparative safety. They are also sensitive to laser light. 

Using the tri-iodide/iodide redox couple and the sensitizers (22) and (56), several groups have reported up to 8-10% solar cell efficiency where the potential mismatch between the sensitizer and the redox couple is around 0.5 V vs. SCE. If one develops a suitable redox couple that decreases the potential difference between the sensitizer and the redox couple, then the cell efficiency could increase by 30%, i.e., from the present value of 10% up to 13%. Towards this goal, Oskam et al. have employed pseudohalogens in place of the triiodide/iodide redox couples, where the equilibrium potential is 0.43 V more positive than that of the iodide/iodide redox couple.17 Yamada and co-workers have used cobalt tris-phenanthroline complexes as electron relays (based on the CoII/m couple) in dye-sensitized solar cells.95... 

---