Silanediol peptidomimetics have been prepared, designed to inhibit the serine protease enzyme Factor XIa (FXIa) for treatment of thrombosis without complete interruption of normal hemostasis. These Arg-[Si]-Ala analogues of the FXIa substrate (FIX) are the first silanediol dipeptide analogues to carry a basic guanidine group. Control of stereochemistry was accomplished using catalytic asymmetric hydrosilylation and addition of a silyllithium intermediate to the Davis-Ellman sulfinimine.