The allyl leaving group approach to tricoordinate silyl, germyl, and stannyl cations

A group 14 atom bonded to three mesityl groups (2,4,6-trimethylphenyl) and to one allyl group serves as a novel precursor to tricoordinate group 14 ca tions, the analogues of the carbocation. The double bond of the allyl group provides an accessible reaction site that is located beyond the ortho meth yl groups. Reaction of various electrophiles with the double bond releases the allyl group and leads to formation of the group 14 cations. The mesityl groups then are of sufficient steric bulk to protect the tricoordinate met al center from attack by nucleophiles. This approach is used herein with si licon, germanium, and tin as the central atom. The Si-29 chemical shift (de lta 225) indicates full cationic character for the silicon system. The Sn-1 19 chemical shift (delta 806) indicates less than full cationic character f or the tin system. The positive charge for the germanium system has been as sessed by examination of the aromatic C-13 chemical shifts. These results p rovide the highest current cationic character for silylium and stannylium i ons.