Institut für Reine und Angewandte Chemie and Center of Interface Science, Carl von Ossietzky Universität Oldenburg, PO Box 2503, 26111 Oldenburg, Germany.
J Org Chem. 2013 Mar 1;78(5):1802-10. doi: 10.1021/jo3013786. Epub 2012 Sep 20.
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').
两种类型的重排反应已在 B3LYP/6-311+G(d,p) 水平上进行了计算研究。酰基硫氰酸酯 RCO-SCN 重排成相应的异硫氰酸酯 RCO-NCS 的活化能垒为 30-31 kcal/mol，这与硫氰酸盐在某些情况下是可分离的尽管非常敏感的化合物的观察结果一致。烷氧基羰基-、（烷基硫代）羰基-和氨甲酰基硫氰酸盐是可分离的，并且向异硫氰酸盐重排的计算出的能垒较高（约 40 kcal/mol），而所有硫代酰基硫氰酸酯衍生物都是相当不稳定的化合物，其向异硫氰酸盐重排的能垒在 20-30 kcal/mol 范围内。酰基、烷氧基羰基-和氨甲酰基氰酸酯 R-CO-OCN 预计在某些情况下是可分离的化合物，其向异氰酸酯 RCO-NCO 重排的能垒高达约 40 kcal/mol。所有此类重排反应都涉及近乎线性（硫）氰酸根阴离子的 HOMO 和酰基阳离子的 LUMO，特别是酰基 C═X π*轨道。第二种类型的重排涉及与（硫）酰基基团相连的基团 R 的 1,3-迁移，即酰基异硫氰酸酯-硫代酰基异氰酸酯和亚氨酰基异硫氰酸酯-硫代酰基碳二亚胺重排。这些反应涉及到四元环状、两性离子过渡态，由迁移 R 基团与相邻的异（硫）氰酸酯官能团之间的孤对-LUMO 相互作用来促进。两种重排的组合导致了一般的反应方案 RCX-YCN ⇌ RCX-NCY ⇌ RCY-NCX ⇌ RCY-XCN（X 和 Y = O、S、NR'）。
