Thiocyanate solvent effects

A decisive solvent effect is also observed with other a,/ -epoxy ketones. Specifically, 3jS-hydroxy-16a,17a-epoxypregn-5-en-20-one and its acetate do not react with thiocyanic acid in ether or chloroform. However, the corresponding thiocyanatohydrins are formed by heating an acetic acid solution of the epoxide and potassium thiocyanate. As expected, the ring opening reaction is subject to steric hindrance. For example, 3j6-acetoxy-14f ,15f5-epoxy-5) -card-20(22)-enoIide is inert to thiocyanic acid in chloroform, whereas the 14a,15a-epoxide reacts readily under these conditions.Reactions of 14a,15a-epoxides in the cardenolide series yields isothiocyanatohydrins, e.g., (135), in addition to the normal thiocyanatohydrin, e.g., (134). 

The importance of solvent effects in the preparation of perfluoroalkyzinc reagents is further illustrated in the reaction of perfluoroalkyl iodides with zinc-copper couple. In DMSO, DMF, and HMPA, the main products are the fluo-roolefins The formation of the fluoroolefin is facilitated when the reaction is carried out in the presence of potassium thiocyanate [30] (equation 21)... 

Solvent effects on quantum yields have been studied to some extent. Yields for substitution fall precipitously and the nature of the reaction may change if the medium consists of a noncoordinating solvent. Thus tra s-[Cr(NCS)4(NH3)2] shows a 0 of about 0.3 in aqueous media (for thiocyanate aquation), but is photoinert in nitromethane. In a mixed solvent study, it was concluded that the photochemical behavior of this complex depended on the solvent composition of the solvation shell rather than on the stoichiometric composition.41 42... 

Taking into account the fact that the solvation of ambident anions in the activated complex may differ considerably from that of the free anion, another explanation for the solvent effect on orientation, based on the concept of hard and soft acids and bases (HSAB) [275] (see also Section 3.3.2), seems preferable [366]. In ambident anions, the less electronegative and more polarizable donor atom is usually the softer base, whereas the more electronegative atom is a hard Lewis base. Thus, in enolate ions, the oxygen atom is hard and the carbon atom is soft, in the thiocyanate ion the nitrogen atom is hard and the sulfur atom is soft, etc. The mode of reaction can be predicted from the hardness or softness of the electrophile. In protic solvents, the two nucleophilic sites in the ambident anion must interact with two electrophiles, the protic solvent and the substrate RX, of which the protic solvent is a hard and RX a soft acid. Therefore, in protic solvents it is to be expected that the softer of the two nucleophilic atoms (C versus O, N versus O, S versus N) should react with the softer acid RX. 

Summary of Cooperative Ligand and Solvent Effects ON Thiocyanate Coordination... 

In the ionization mechanism exemplified by benzhydryl thiocyanate, the reaction is strictly first order over a wide concentration range. The rate of isomerization increases with increasing solvent polarity Tracer and stereochemical evidence indicates that this involves an internal ion-pair mechanism. Isomerization is faster than isotopic exchange and so it was concluded that the former process occurs via an intimate ion pair which was shown to collapse to thiocyanate and isothiocyanate in the ratio of 5 1. In the case of optically active 4-chlorobenzhydryl thiocyanate in acetonitrile, racemization occurs at a rate comparable with isomerization. With a given solvent the structural effect acts essentially on the energy term and for a given substrate, the solvent effect acts essentially on the entropy term. 

Yang, K.S. Theil, M.H. Cuculo, J.A. Lyotropic mesophases of cellulose in the ammonia-ammonium thiocyanate solvent system. Effects of system composition on phase types. ACS Symp. Ser. 1989, 384, 156-183. 

Thermal ruthenium-sulfur bond breaking in cis-[Ru"(bipy)2(pheno-thiazine)Cl] is markedly affected by the nature of the solvent, being faster in acetonitrile or in acetone than in methanol or in ethanol, and much slower in dimethyl sulfoxide or in dimethylformamide. Such bond breaking is readily photo-induced, particularly in acetone or dichloromethane. Relative quantum yields for photodissociation of cis-[Ru(CN)2(phen)2] in the presence of thiocyanate in a range of solvents correlate well with their Gutman acceptor numbers. The photoreaction is favored by solvents of high Solvent effects on photo-... 

Complex Formation Labile Cations. Solvent effects on reactivity in the formation of complexes of metal(n) cations with unidentate ligands have been reviewed, with special reference to magnesium(n) and to the solvents methanol, acetonitrile, DMF, and DMSO. There has been controversy over the mechanism of reaction of thiocyanate with nickel(n) in DMSO, with supporters of the usual Eigen-Wilkins la mechanism and of a D mechanism. The most recent investigators of this reaction report rate constants and activation parameters and favour the la mechanism. There has been further discussion of the mechanism of the reaction between nickel(n) and bipy in DMSO an earlier suggestion that the rate-determining step is ring closure is not supported by recent observations. Rate constants for the reaction of acetate, of other carboxylates, and of pada with nickel(ii) in several non-aqueous solvents have been determined. 

The clue for these deviations possibly lies in the sample preparation. Whereas Kidd and Spinney prepared the complexes and dissolved them in dry MeCN, Tarasov and Buslaev used mixtures of NbClj/NbBrs and NbCl5/NH4SCN, and their MeCN may have contained water and amines which (i) give solvent effects (which may account for variations around 20-30 ppm), (ii) initiate hydrolysis, which may give anionic species, and (iii) may steer the coordination of the thiocyanate in such a manner that only complexes containing N-bonded SCN are formed. In dry MeCN, resonances for the neutral complexes [NbCl5 Br (MeCN)] are observed. ... 

The addition of water to the methanol decreases the rate constant while increasing the dielectric constant of the medium. Likewise the addition of an inert salt as sodium chloride slows the reaction and the product, sodium thiocyanate, also slightly slows the reaction. Both the solvent effect and the salt effects are consistent with the reaction of an ionic species (cyanide ion) with a neutral molecule (sulfur, Sg). Had the rate-determining step involved the reaction of an anion with another anion, increasing the dielectric constant would have increased the rate and the presence of more salts would have also increased the rate. 

Thiocyanates are rather stable to air, oxidation, and dilute nitric acid. Of considerable practical importance are the reactions of thiocyanate with metal cations. Silver, mercury, lead, and cuprous thiocyanates precipitate. Many metals form complexes. The deep red complex of ferric iron with thiocyanate, [Fe(SCN)g] , is an effective iadicator for either ion. Various metal thiocyanate complexes with transition metals can be extracted iato organic solvents. 

However, the energy difference between N- and S-bonded thiocyanate is very small and is influenced by an interplay of several factors steric effects, solvent and the counter-ion in ionic complexes. To illustrate the last point, in complexes [Pd[Et2N(CH2)2NH(CH2)2NH2]NCS]+, the PFg salt is N-bonded, as it is in the unsolvated BPhg salt. However, though the acetone solvate of the tetraphenylborate is N-bonded, the methanol solvate is S-bonded [126],... 

Table 15.1. Effect of Solvents on Thiocyanate Hydrogenolysis to Thiol.

In addition to benzenoid diazo components, diazotised heterocyclic amines in which the amino group is attached to a nitrogen- or sulphur-containing ring figure prominently in the preparation of disperse dyes [87,88], since these can produce marked bathochromic shifts. The most commonly used of these are the 6-substituted 2-aminobenzothiazoles, prepared by the reaction of a suitable arylamine with bromine and potassium thiocyanate (Scheme 4.31). Intermediates of this type, such as the 6-nitro derivative (4.79), are the source of red dyes, as in Cl Disperse Red 145 (4.80). It has been found that dichloroacetic acid is an effective solvent for the diazotisation of 2-amino-6-nitrobenzothiazole [89]. Subsequent coupling reactions can be carried out in the same solvent system. Monoazo disperse dyes have also been synthesised from other isomeric nitro derivatives of 2-aminobenzothiazole [90]. Various dichloronitro derivatives of this amine can be used to generate reddish blue dyes for polyester [91]. 

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