Synthesis and reactivity of boryl substituted silaimines

Reactions of two N-heterocyclic silylenes with a boron azide have given the first N-boryl substituted silaimines, (HCNR)₂Si[double bond, length as m-dash]N{B(DipNCH)₂} (R = Dip or Bu^t, Dip = 2,6-diisopropylphenyl) which are either monomeric or dimeric in the solid state. The compounds shows divergent reactivity with CO₂, in that the bulkier silaimine undergoes a [2+2] cycloaddition with CO₂ to form (HCNDip)₂Si{OC([double bond, length as m-dash]O)N[B(DipNCH)₂]} under mild conditions, while the smaller silaimine undergoes a bond metathesis reaction with CO₂, affording the boron isocyanate, (HCNDip)₂B(NCO), and a known oxo-bridged silicon(iv) system. Both silaimines add H₂O across their Si[double bond, length as m-dash]N bonds to give the boryl-aminosilanols, (HCNR)₂Si(OH)-(H)N{B(DipNCH)₂}. The larger silaimine does not react with the bulky boron azide (HCNDip)₂BN₃, whereas the smaller system undergoes a [3+2] cycloaddition reaction with the azide to give spirocyclic borasilatetrazoline, (HCNBu^t)₂Si{[(HCNDip)₂B]NN}₂. Overall, the reactivity of the prepared N-boryl substituted silaimines is similar to that reported for other silaimines. However, the steric differences between the two Si[double bond, length as m-dash]N bonded compounds plays a definitive role in the outcomes of their reactions with the small molecule substrates studied.