Tricoordinate silyl cations (silylium ions)

Being silicon analogues of carbenium ions, which are ubiquitous in organic chemistry, silylium ions play a pivotal role in organosilicon chemistry. However, unlike widespread carbenium ions, silylium ions are much less accessible and much more difficult to stabilize due to the lower electronegativity of the silicon atom, its greater polarizability and notably larger size (compared to those of carbon atom), which results in extraordinarily high electrophilicity and Lewis acidity of silylium ions. These peculiar characteristics of silylium ions require very special, sophisticated synthetic approaches for their generation, isolation and structural elucidation, on the one hand, and open the door for their use as exceptionally potent Lewis acids in a number of catalytic transformations, on the other hand. In this review, we present and discuss the latest advances in the field of stable silylium ion derivatives, particularly emphasizing the state-of-the-art procedures for their synthesis, structural studies both in solution and in the solid state, most remarkable recent representatives of the isolable silylium ions, and finally, discuss the use of silylium ions in synthetic organic chemistry.

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