ANCHIMERIC ASSISTANCE IN THE REACTIONS OF THE CROWDED ORGANOSILICON IODIDE (ME(3)SI)(2)(PH(2)MESI)CSIME(2)I WITH ELECTROPHILES

The relative reactivities of the iodides (Me(3)Si)(2)(Ph(2)MeSi)CSiMe( 2)I, 1, (Me(2)Si)(2)(PhMe(2)Si)CSiMe(2)I, 2, (Me(3)Si)(3)CSiPhMeI 3, ( Me(3)Si)(3)CSiPh(2)I, 4, and (Me(3)Si)(3)CSiMe(2)I, 5, towards silver salts or ICl have been studied and the results support the proposal th at for 1 and 2 the rate-determining step involves generation of a Ph-b ridged cation, and so anchimeric assistance by a Ph group to the depar ture of I-. Thus the fall in reactivity in the sequence 1 > 2 > 5 refl ects the presence of two gamma-Ph groups in 1 and one in 2 able to pro vide such assistance. Towards AgO(3)SC(6)H(4)Me-p in CH2Cl2 1 is > 50 times as reactive as 5, but in the reactions with AgNO3 in MeOH the ef fects of the assistance are smaller, and the relative reactivities of 1, 2 and 5 are ca 7:2:1. In the reaction with ICl in CCl4 the correspo nding ratio is ca 150:70:1. In the reactions of 1 there is little if a ny formation of rearranged products arising from migration of a Ph gro up to the alpha-Si, the lower steric hindrance at the alpha-site in th e Ph-bridged cation favouring attack there. Because of the anchimeric assistance, even the chloride (Me(3)Si)(2)(Ph(2)MeSi)CSiMe(2)Cl reacts in CH2Cl2 with AgO3SCF3, towards which (Me(3)Si)(3)CSiMe(2)Cl is iner t. The reactivity ratio for reaction of (Me(3)Si)(3)CSiPh(2)I, (Me(3)S i)(3)CSiPhMeI, and 5 with ICl in CCl4 is ca 6:1.5:1, indicating that a lpha-Ph groups can give some help to formation of a Me-bridged cation.