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

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Abstract
The preparation of the adducts Et2XAI,NH2But 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,NH2But has been restudied. Thermal decomposition of the adducts leads to dimers containing (AIN)2 rings. Two crystalline stereoisomers of (EtBrAlNHBut)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 (EtXAINHBut)2, X = Et, Cl, and I. Mass spectral studies are reported which show that exchange takes place in solution between the dimers (EtBrAlNHBut)2, and (EtIAlNHBut)2, and between (EtBrAlNHBut)2 and (EtBrAlNDBut)2, which points to a complex mechanism for the isomerization process.

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