Antonino Fava

ISOMERIZATION OF ORGANIC THIOCYANATES

Abstract The mechanisms of isomerization of alkyl thiocyanates are reviewed. There appear to be two pathways generally available for the change R—SCN → R—NCS. These are: (a) Ionization and (b) Nucleophilic attack by the N-atom of the thiocyanate ion. A third pathway, an intramolecular non-ionic cyclic mechanism, is available to allylic thiocyanates, and possibly to other rearranging substrates. Tracer and stereochemical evidence is reported which indicates that the ionization mechanism is not the simple dissociation: R—SCN (R+ + SCN−) R—NCS. The results rather suggest an internal ion-pair mechanism. The isomerization via thiocyanate ion is a direct displacement by the N-end of SCN−. Thus: SCN− + R—SCN → NCS—R + SCN−. This mode of isomerization is relatively more important with substrates such as primary ones which, while not undergoing facile ionization, are more prone to nucleophilic substitution. The comparison of the rate of isomerization by this mechanism with the rate of isotopic exchange (attack by the S-end) provides interesting observations concerning the ambident nucleophilic reactivity of thiocyanate ion. The equilibrium thiocyanate-isothiocyanate is briefly discussed. The isomerization is practically complete in the vast majority of cases. However, small but definite amounts of thiocyanate may be detected at equilibrium in polar solvents. Furthermore, the equilibrium fraction of thiocyanate may become large or predominant in the case of allylic substrates where the isomeric shift gives rise to a less stable carbon skeleton. The behaviour of the related class, the selenocyanates, with respect to isomerization is examined. In general, selenocyanates isomerize more rapidly than the corresponding thiocyanates, although the rate increase is not large. The more striking difference between the two classes is concerned with equilibrium which, in the selenium class, is much more displaced toward the normal compound than in the sulfur class. Finally, the question of the isomerism of metal thiocyanate complexes is examined in the light of the more recent advances in this field.

Chiral building blocks from 1,4:3,6-dianhydrohexitols. iii. an expeditious enantiospecific synthesis of the geissman-waiss lactone

Abstract A new synthesis is presented of the R.R enantiomer of the title lactone, which is an important intermediate in the synthesis of necine alkaloids. The approach opens a new route to the synthesis of polyhydroxylated nitrogen heterocycles in enantiopure form

Kinetic versus thermodynamic acidity of α-sulphonium protons

Abstract The kinetic acidity of structurally different, or stereochemically different, protons α to a sulphonium function may not parallel their thermodynamic acidity.

Acid catalysis in the intramolecular addition of α-lithiosulphoxides to isolated double bonds

Intramolecular addition of a α-sulphinyl carbanion to an isolated double bond is observed with mesocyclic metallated E homoallylic sulphoxides (1) and (2); kinetic data suggest that this process is acid catalysed and that free (1) is the proton donor species.