Anchimeric assistance in the reactions of the crowded organosilicon iodide (Me3Si)2(Ph2MeSi)CSiMe2I with electrophiles

The relative reactivities of the iodides (Me 3 Si) 2 (Ph 2 MeSi)CSiMe 2 I, 1, (Me 3 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 that for 1 and 2 the rate-determining step involves generation of a Ph-bridged cation, and so anchimeric assistance by a Ph group to the departure of I-. Thus the fall in reactivity in the sequence 1 > 2 > 5 reflects the presence of two γ-Ph groups in 1 and one in 2 able to provide such assistance. Towards AgO 3 SC 6 H 4 Me-p in CH 2 Cl 2 1 is > 50 times as reactive as 5, but in the reactions with AgNO 3 in MeOH the effects 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 CCl 4 the corresponding ratio is ca 150:70:1. In the reactions of 1 there is little if any formation of rearranged products arising from migration of a Ph group to the α-Si, the lower steric hindrance at the α-site in the 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 CH 2 Cl 2 with AgO 3 SCF 3 , towards which (Me 3 Si) 3 CSiMe 2 Cl is inert. The reactivity ratio for reaction of (Me 3 Si) 3 CSiPh 2 I, (Me 3 Si) 3 CSiPhMeI, and 5 with ICl in CCl 4 is ca 6:1.5:1, indicating that α-Ph groups can give some help to formation of a Me-bridged cation.