Silylamines basicity

It has been known for some time that the basicities of a heteroatom decrease upon a-silyl substitution [12], For example, alkyl silyl ethers (R3Si-0-R ) are less basic than dialkly ethers. Silylamines are weak bases compared to alkylam-ines. This electron-withdrawing effect of silyl groups has been explained in terms of the interaction between low lying vacant orbitals such as 3d orbitals of silicon or a orbitals with the nonbonding p orbitals (lone pairs) of the heteroatom (Fig. 4). This interaction decreases the HOMO level which in turn lowers the basicity of the heteroatom. Such effect may also cause the increase of the oxidation potentials, but little study has been reported on the electrochemical properties of this type of compounds. 

A series of silanol and silylamine chemical shifts were obtained in various solvents. (83) The silanols are found to be highly dependent (>5ppm shifts) upon solvent basicity with the more basic solvents causing low frequency shifts. This shielding effect is found to give an excellent linear correlation with Gutmann s donor number (DM) (130) which is a measure of the electron pair donor ability of the solvent. Figure 23 shows the correlation for five of the compounds examined. It... 

It was observed that ammonolysis of B(C2H,Si(R)H2)3 (Scheme 2, route A) requires basic catalysts such as n-butyl lithium. The reaction is performed in analogy to the potassium hydride-catalyzed cross-linking of cyclic silazanes described by Seyferth et al. [8]. Most probably, n-BuLi initially deprotonates the weak nucleophile ammonia with the formation of lithiiun amide and evaporation of n-butane. The stronger nucleophilic amide then replaces a silicon-bonded hydride, which subsequently deprotonates ammonia, leading to the evolution of molecular hydrogen. The silylamines that arise are not stable under the reaction conditions applied (refluxing solvent), and by fast condensation of ammonia the polymeric precursors form [6]. 

It is clearly established that silyl groups reduce the basicity of heteroatoms to which they are directly bonded. The first indication that silylamines are weak bases appears to... 

The high acid and moisture sensitivity of silylamines has been a major obstacle to their use in amino group protection. Primary amines have been protected as their rerr-butyidiphenylsilyl derivatives whose steric bulk militates against further reaction to form an Af,N-disilylamine. rerr-Butyldiphenylsilylamines have remarkable stability towards strongly basic conditions (e.g., 20% potassium hydroxide in refluxing methanol), but they cannot even withstand brief exposure to 80% acetic acid at room temperature. If silylamines are used at all in synthesis, they are usually prepared under anhydrous conditions, and used for one or two steps before their flimsy services are dispensed with. 

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