Reactions of N-Monosubstituted Amidines with AlMe₃ and AlMeCl₂: Formation of Fused Triazaalane Heterocycles

Abstract

Reactions of N-monosubstituted amidines of the formula HN═C(R)–NH(R′) (R = Ph, 4- tert -butylphenyl, Me; R′ = 2,6-diisopropylphenyl, Ph) with AlMe 3 and AlMeCl 2 are reported. All the N -(Dipp)amidines (Dipp = 2,6-diisopropylphenyl) react with AlMe 3 in a 1:1 ratio to yield tetrameric aluminum amidinates ( 1, 2, and 3 ) in good yields. In these compounds, the amidinate ligand chelates to Al while bridging to another Al. However, when N -phenylamidines are employed, tetracyclic ( 4 – 9 ) and pentacyclic ( 10 ) Al–N–C heterocycles are formed. In the case of N -phenylbenzamidine, formation of a hexacyclic Al–N–C heterocycle ( 11 ) is observed when a slight excess of AlMe 3 (1:1.2 equiv) is used. In these compounds, the amidinate ligand coordinates to Al atoms in a bridging fashion. Further, N -(Dipp)acetamidine and N -(Dipp)benzamidine are also treated with AlMeCl 2 . Whereas N -(Dipp)acetamidine affords an ionic 15-membered aluminum amidinate chain ( 12 ), N -(Dipp)benzamidine gives a bicyclic heterocycle ( 13 ) and the AlCl 3 adduct of N -(Dipp)benzamidine ( 14 ). However, from a reaction between N -phenylbenzamidine and AlMeCl 2, only the AlCl 3 adduct, 15, was isolated. Compounds 3, 4, 6, 8, and 10 – 15 have been structurally characterized.