Rearrangements of Acyl, Thioacyl, and Imidoyl (Thio)cyanates to Iso(thio)cyanates, Acyl Iso(thio)cyanates to (Thio)acyl Isocyanates, and Imidoyl Iso(thio)cyanates to (Thio)acyl Carbodiimides, RCX-YCN ⇌ RCX-NCY ⇌ RCY-NCX ⇌ RCY-XCN (X and Y = O, S, NR′)

Abstract

Two types of rearrangements have been investigated computationally at the B3LYP/6-311+G­(d,p) level. The activation barriers for rearrangement of acyl thiocyanates RCO–SCN to the corresponding isothiocyanates RCO–NCS are 30–31 kcal/mol in agreement with the observation that the thiocyanates are in some cases isolable albeit very sensitive compounds. Alkoxycarbonyl-, (alkylthio)­carbonyl- and carbamoyl thiocyanates are isolable and have higher calculated barriers (ca. 40 kcal/mol) toward rearrangement to isothiocyanates, whereas all thioacyl thiocyanate derivatives are rather unstable compounds with barriers in the range 20–30 kcal/mol for rearrangement to the isothiocyanates. Acyl-, alkoxycarbonyl-, and carbamoyl cyanates R–CO–OCN are predicted to be in some cases isolable compounds with barriers up to ca. 40 kcal/mol for rearrangement to the isocyanates RCO–NCO. All of the rearrangements of this type involve the HOMO of a nearly linear (thio)­cyanate anion and the LUMO of the acyl cation, in particular the acyl CX π* orbital. The second type of rearrangement involves 1,3-shifts of the groups R attached to the (thio)­acyl groups, that is, acyl isothiocyanate–thioacyl isocyanate and imidoyl isothiocyanate–thioacyl carbodiimide rearrangements. These reactions involve four-membered cyclic, zwitterionic transition states facilitated by lone pair–LUMO interactions between the migrating R group and the neighboring iso­(thio)­cyanate function. Combination of the two rearrangements leads to the general reaction scheme RCX–YCN ⇌ RCX–NCY ⇌ RCY–NCX ⇌ RCY–XCN (X and Y = O, S, NR′)