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').

Zwitterionic 1,3‐Shift Intermediates in the Interconversions of (Thio)carbamoyl Isocyanates and Carbamoyl Iso(thio)cyanates, R2N–CX–NCO ⇌ R2N–CO–NCX, and in Carbamoylketenes R2N–CO–CH=C=O

“…On introducing the two electron‐donating methyl groups on to the migrating amino substituent, the activation energies for the rearrangements (Table 2, R = Me) are predicted to be about 10 kcal/mol lower than the facile NH 2 migrations, in line with previously reported data 5,6,8. More importantly, intermediates are now located for all reactions by all methods.…”

“…Recently, the rearrangements of acyl and thioacyl cyanates and thiocyanates 1 to the corresponding isocyanates and isothiocyanates 2 were investigated computationally 5. These reactions (Scheme ) have calculated activation barriers in the range 20–40 kcal/mol, that is, they may take place at ordinary or moderately elevated temperatures.…”

Nitrogen- and Sulfur-Containing Energetic Compounds. 64 Years of Fascinating Chemistry

“…[71] The silver salt of N-methoxydithiocarbamic acid had been prepared previously by Traube, who recorded a sharp, pungent odour of the products of thermal decomposition, reminiscent of isothiocyanates. [72] We have investigated the interconversions of acyl fulminates, cyanates, isocyanates and their thio derivatives, as well as imidoyl iso(thio)cyanates and (thio)acyl carbodiimides (Scheme 13) [73] and reviewed the preparations and rearrangements of heteroatom-substituted cyanates, isocyanates, and nitrile oxides and their thio derivatives. [74] During my work on cyanates in 1966, a surprising observation was the formation of a peak at m/z 91 (C 6 H 5 N þ ) in the mass spectrum of phenyl cyanate, PhOCN, and this became the base peak in the mass spectrum of phenyl isocyanate, PhNCO.…”

Reactivity and diverse synthetic applications of acyl isothiocyanates

“…A thermal 1,3-shift of substituent R in acyl isothiocyanates 1 is reported to proceed via the transition state TS (Figure 4). 15,16 Goerdeler and coworkers have reported the isomerization of thioacyl isocyanates 1a to acyl isothiocyanates 1, in solution at temperatures around 100 C. 17,18 Although the acyl isothiocyanates 1 are more stable an equilibrium between the two may be achieved from either side under favorable conditions.…”