Separation and identification of some stereoisomers of cyclodi-µ-t-butylamido-di(ethylbromoaluminium). Spectral studies of their interconversion in solution and the similar behaviour of some related compounds

Abstract

The preparation of the adducts Et₂XAI,NH₂Bu^t X = Cl, Br, and I, is described and the n.m.r. spectra are analysed to show that there is no restriction of rotation about Al–C or N–C bonds at room temperature. Et₃Al,NH₂Bu^t has been restudied. Thermal decomposition of the adducts leads to dimers containing (AIN)₂ rings. Two crystalline stereoisomers of (EtBrAlNHBu^t)₂ have been isolated by fractional sublimation and are shown to isomerize in solution at room temperature, at different rates, to give an equilibrium mixture containing three principal isomers. A preliminary X-ray study indicates that the predominant and most stable isomer is 1α,3α-di-t-butyl-2α,4α-dibromo-1β,3β-dihydro-2β,4β-diethyl-cyclodialuminatazoniane. ¹H N.m.r. spectra (250 MHz) are used to show that the second crystalline isomer is 1α,3β-di-t-butyl-2α,4β-dibromo-1β,3α-dihydro-2β,4α-diethylcyclodialuminatazoniane and a third isomer, evident in solution, is probably 1α,3α-di-t-butyl-2α,4β-dibromo-1β,3β-dihydro-2β,4α-diethyl-cyclodialuminatazoniane. It is proposed that isomer stability is principally a function of steric interactions. N.m.r. studies indicate that mixtures of isomers are also present in solutions of (EtXAINHBu^t)₂, X = Et, Cl, and I. Mass spectral studies are reported which show that exchange takes place in solution between the dimers (EtBrAlNHBu^t)₂, and (EtIAlNHBu^t)₂, and between (EtBrAlNHBu^t)₂ and (EtBrAlNDBu^t)₂, which points to a complex mechanism for the isomerization process.