Bis-Amidines as Useful Building Blocks for Heterofulvenes and -Fulvalenes

Abstract

An easily feasible synthesis for bis-amidines of type 3 was developed via an aminolysis reaction of imidoyl chlorides 1 with gaseous ammonia. Depending on the nature of the electrophilic cyclization reagent, these amidines show a different reactivity. On treatment with orthoesters, the fused imidazoles 5 were obtained which involved the participation of two different kinds of nitrogen atoms in the cyclization. In contrast to this, carboxylic acid chlorides are able to react selectively at the NH 2 -groups under formation of the fulvenoid 4H-imidazoles 6. The same regioselectivity was observed for chlorodimethylimmonium chloride thus leading, via the dimerization of cyclic orthoamides, to the 1,4,5,8-tetraazafulvalenes 8.Bis-amidines of type 2 are versatile building blocks for several heterocycles 1-3 as well as being ligands for catalytical active metals. 4-6 Their syntheses are easily feasible by aminolysis reactions of bis-imidoylchlorides 1 with various amines. In contrast to this, there exist only a few bisamidines of type 3 which possess two primary NH 2 groups. A limiting factor for this might be the inconvenient handling of dicyane as the starting material. Thus, only a small number of amidines 3 have been reported as being synthesized by amine additions to dicyane 7-9 or by aminolysis reactions of derivatives of dicyane as imino esters 10-15 or diiminosuccinonitrile. 16 However, another route to 3 starting from bis-imidoylchlorides 1 proceeds via an aminolysis reaction with ammonia. 17 According to an existing protocol 17 in the course of which ethanol has been employed as the solvent only a small amount of amidine 3a was obtained, in addition to phenyl isonitrile and tarry material. Obviously, imidoylchlorides of type 1 behave similarly to oxalyl chloride, which in the presence of nucleophiles often undergoes a C-C cleavage reaction.We report here on the aminolysis of bis-electrophiles 1 with gaseous ammonia to give the desired amidines 3 in high yields. On treatment of a suspension of bis-imidoylchlorides 1 in dry THF with a stream of dried gaseous ammonia, the products 3a-g were formed in a smooth reaction (Scheme 1). The progress of this aminolysis can be followed by fading of the intensive yellow color of the reaction mixture and was effectively monitored by TLC. Simple recrystallization gives the amidines 3a-g as white to yellowish crystals in high purity. The application of elevated pressure (5 bar, glas autoclave) lead to higher yields and a shortening of the reaction times (1 hour, starting from 0.1 mol of 1). The X-ray structural determination of 3b revealed a transoid arrangement of both aryl groups (Figure 1). The C=N fragments are located at the aromatic residues whereas the bond length for C-NH 2 corresponds to a typical C-N single bond. Contrary to the solid state structures, in solution, 3 undergoes facile E/Z-interconversion and prototropic processes called "tautomeric rotations". 18 NMR experiments performed at -60°C for the trifluoromethyl substituted derivative 3g revealed the coexiste...

Polyazaacenes – On the Way to Stable, Fluorescent and Redox‐Active Derivatives

“…Only a few literature reports exist that deal with the synthesis of systems possessing more than two linear condensed pyrazine rings including their hydro derivatives such as fluorubine, [4] 5,10-dihydropyrazino[2,3-b:2Ј,3Ј-e]-pyrazines, [5,6] decaazapentacenes [7] and octaazadihydrohexacenes. [8] Disubstituted oxalic amidines 1, [9] which were already used as building blocks for five-membered heterocyclic ring systems, [10,11] should also be applicable to six-membered rings. Moreover, we demonstrate that the dyotropic rearrangement of 1,4,5,8-tetraazafulvalenes allows easy access to tetrasubstituted pyrazino[2,3-b]pyrazines 2.…”

Cycloaminals of trichloroacetaldehyde: synthesis and their use as carbene-precursors

“…In contrast to the s-trans conformation present in the solid state of 1 (Ar = 4-MeC 6 H 4 ) in solution these derivatives exist in several prototropic isomers depending on the solvent, e.g. 1' 1 . Thus, trialkyl orthoformates react with 1 including all four nitrogen atoms to form mixtures of diastereomeric tetrahydroimidazol [4,5-d]imidazoles 2 1 .…”

“…Unfortunately, these aminal esters are unsuitable for the use as carbene precursors because the elimination of alcohol occurs only at higher temperatures under the formation of tarry-like materials. Previously, we reported a simple reaction of 1 with the Vilsmeier reagent which resulted in the formation of tetraazafulvalenes 5 1 . As keyintermediate in a complex reaction, most-likely the cyclisation product 3 was formed in a regioselective manner.…”

“…1' 1 . Thus, trialkyl orthoformates react with 1 including all four nitrogen atoms to form mixtures of diastereomeric tetrahydroimidazol [4,5-d]imidazoles 2 1 . Unfortunately, these aminal esters are unsuitable for the use as carbene precursors because the elimination of alcohol occurs only at higher temperatures under the formation of tarry-like materials.…”

“…1 Due to their four nucleophilic centres they are of interest as starting materials for several heterocycles. However, the amidines 1 undergo facile E/Zinterconversion and prototropic processes called "tautomeric rotation" 2 , therefore cyclisation reactions with electrophiles often tend to the formation of regioisomeric compounds.…”