The mechanism of the isomerization of organosilicon cyanates to isocyanates

Isomerization of the cyanate (Me(3)Si)(3)CSi(CD3)(2)OCN in the molten state at 150 degrees C gives the unrearranged isocyanate (Me(3)Si)(3)CSi(CD3)(2)NCO and the rearranged isocyanate (Me(3)Si)(2)[(CD3)(2)MeSi]CSiMe(2)NCO in a ratio of ca. 75/25, and isomerization in solution in Ph(2)O at 220 degrees C gives the same products in a ratio of ca. 85/15. The occurrence of a significant amount of rearrangement is consistent with a mechanism involving initial ionization of the cyanate. In contrast, no rearrangement accompanies the isomerization in CCl4 catalyzed by ICI, indicating that ionization is not involved; this conclusion is supported by the observation that the isomerization of (Me(3)Si)(2)(PhMe(2)Si)CSiMe(2)OCN, in which the Ph group would provide anchimeric assistance to ionization, proceeds at a similar rate to that of (Me(3)Si)(3)CSiMe(2)OCN.