REACTIONS OF THE HIGHLY STERICALLY HINDERED COMPOUNDS (ME3SI)3CSIME2X(X = CL, BR, I, OR ONO2) WITH NAN3, CSF, KSCN, AND KOCN IN MEOH

The reaction of the nitrate TsiSiMe2ONO2, 9, (Tsi = (Me3Si)3C) with 0. 25 M solutions of salts MY, viz. NaN3, CsF, and KSCN, in MeOH at 45-de grees-C and 60-degrees-C give exclusively the corresponding substituti on products TsiSiMe2Y, but in the case of the halides TsiSiMe2X, with X = I (1), Br (2), or Cl (3), some side-products are formed, mainly th e fragmentation product (Me3Si)2CHSiMe2-Ome; the proportion of these s ide-products increases in the sequence (X =) I < Br < Cl, in line with the decreasing reactivity of the substrate towards the salts. With KO CN, the least reactive of the salts used, substantial amounts of the s ide-products are formed even from the reactions of the nitrate. The ro ugh relative reactivities of 3, 2, 1, and 9 at 60-degrees-C are: 0.1/1 .0/4.5/83 towards NaN3; 0.5/1.0/2.6/115 towards CsF; and 0.13/1.0/4.2/ 120 towards KSCN. (The rather small spread of rates for the halides is noteworthy.) Examples of rough relative rates of reaction with NaN3, CsF, and KSCN at 60-degrees-C are 0.5/1.0/0.3 for the nitrate and 1.3/ 1.0/0.45 for the iodide. Sodium methoxide in MeOH, which is ineffectiv e in bringing about substitution of the TsiSiMe2X compounds, involving attack at the SiMe2X centre, is roughly comparable with NaN3 and CsF in bringing about fragmentation, involving attack at an SiMe3 centre. Features of the substitution reactions are consistent with a rate-dete rmining transition state close to the very crowded five-coordinate sil icon species [TsiSiMe2XY]-. Reaction of the compounds 1-3 and 9 with 0 .25 M KSCN in MeCN are some 15-30 times as fast as those in MeOH, and give solely the isothiocyanate TsiSiMe2NCS.