PREPARATION AND REACTIONS OF SOME ORGANOSILICON CYANATES

Reactions of the iodides TsiSiRR'I (Tsi=(Me3Si)3C; RR'=Me2, Me(OMe), (OMe)2, Me(NCO)) with AgOCN in CH2Cl2 give the corresponding normal cyanates TsiSiRR'(OCN), which have been isolated but shown to isomerize when heated to give the corresponding isocyanates TsiSiRR'(NCO). Similar treatment of TsiSiPh2I gives the rearranged cyanate (Me3Si)2C(SiPh2Me)(SiMe2OCN), which also isomerizes to the isocyanate (Me3Si)2C(SiPh2Me)(SiMe2NCO) when heated. Reaction of the less sterically-hindered iodide TsiSiMeHI with AgOCN in CH2Cl2 initially gives a mixture of the cyanate and isocyanate in solution but after more prolonged reaction only the isocyanate is present. The thermal isomerization of TsiSiMe2OCN (1) in Ph2O (at 155-200-degrees-C) is of second order with respect to 1. The isomerization in CCl4 is strongly catalysed by ICl, and the reaction is of second order with respect to both 1 and ICl. Isomerization occurs rapidly in MeOH containing NaOMe. Substitution to give the corresponding product TsiSiMe2X occurs rapidly and very predominantly when 1 is treated with salts MX, viz. NaN3, CsF and KOCN in MeOH, but with LiCl under similar conditions some of the isocyanate is formed along with the chloride TsiSiMe2Cl and the related methoxide and hydroxide (the latter from traces of water). Reaction of 1 with KSCN in MeCN gives exclusively the substitution product TsiSiMe2NCS, but CsF and NaN3, which are only very slightly soluble, promote very predominant isomerization. Possible mechanisms of the isomerizations are discussed. Cyanate is a markedly better leaving group from silicon than iodide, and comparable with triflate.