On the Stereochemistry of the NH Bond Activation of Ammonia or Amines by Disilenes: Predictability.

The stereochemistry of the addition of NH3 to the stereoisomers of 1,2-di-tert-butyl-1,2-bis(2,4,6-triisopropylphenyl)disilene (Z-5 or E-5) is 100 % stereospecific giving two isomeric disilylamines 6 and 7, respectively, derived from syn-addition to the stereoisomeric disilenes. Variable time normalization analysis studies of the reaction of tetramesityldisilene (3) with isopropylamine (i PrNH2 ) revealed that the order in both amine and disilene is 1. The kinetic isotope effect for the addition of i PrNH2 /i PrND2 to tetramesityldisilene was determined to be 3.04±0.06 at 298 K, a primary KIE, indicating that the proton is transferred in the rate determining step. Competition studies between the addition of PrNH2 and i PrNH2 to tetramesityldisilene resulted in the exclusive formation of the PrNH2 adduct consistent with a nucleophilic addition. Computational studies of the mechanism of the addition of ammonia to E-5 revealed the lowest energy pathway involves the formation of the donor adduct derived from syn-addition, followed by intramolecular syn-transfer of the proton. The formation of the donor adduct is the rate determining step. The results of this study, together with previous studies on the addition of ammonia and amines to disilenes, allow for a refinement of our understanding of the mechanism of this important fundamental reaction in disilene chemistry, and allow us to understand our ability to reliably predict the stereochemical outcomes of future NH σ-bond activation reactions.