Chloroboration and allied reactions of unsaturated compounds. Part VIII. Insertion of isocyanates and isothiocyanates into Al–Et and Al–Br bonds

Abstract

The reactions of triethylaluminium, ethylaluminium chlorides (Et–Al cleavage), and aluminium tribromide with isocyanates and isothiocyanates (mainly PhNCO and PhNCS) have been examined. Under mild conditions, solid 1 : 1-insertion (into Et–Al or Br–Al bonds) compounds [e.g. Br₂AlN(Ph)C(:O)Br from Al₂Br₆ and 2PhNCO] were isolated in every instance. With Al₂Br₆ and either PhNCO or MeNCO metastable co-ordination compounds (formulated as RNC[graphic omitted]–l[graphic omitted]Br₃, and characterised by an abnormally-high NCO asymmetric stretching frequency at ca. 2400 cm.^–1) were identified as the first-formed products. The insertion compounds were dimeric in solution and are assigned an eight-membered ring structure. Exceptions are Br₂AlN(Ph)C(:Q)Br (Q = O or S), which are monomeric, and are believed to be chelates, with tetra-co-ordinate aluminium.