Thermolysis of polyazapentadienes. Part 12. 1 The 1,1-dialkyl- 5-aryl-1,3,5-triazapentadiene problem

Flash vacuum pyrolysis (FVP) of the 3-azavinamidine 11 at 850 °C gives 1,4-dimethylimidazole 16 (18%) aniline 18 ( ca . 24%) and 2-methylquinazoline 17 (7%) as the major products. These results are compared with the thermal behaviour of related vinamidines and 2-azavinamidines


Introduction
Under flash vacuum pyrolysis (FVP) conditions a number of competitive routes are available for the thermal cleavage of 1,5-diazapentadienes (vinamidines 2 ) and their aza-analogues.First, the 6π 'electrocyclisation with elimination' route to quinolines 2 and related compounds (Scheme 1), pioneered by Jutz and co-workers in condensed-phase, 3 is observed exclusively in the gas-phase only for 1,5-diarylvinamidines 1. 4  Unexpectedly, such quinoline formation is usually only a minor pathway for the 1,1-dialkyl-5-phenyl vinamidines 3 which instead undergo elimination of aniline (initiated by hydrogen transfer from the N-alkyl group) with concomitant cyclisation to give pyrroles 4. 5 The relative amount of the pyrrole is reduced relative to the quinoline when an alkyl group is present as a Csubstituent in the vinamidine chain, and this has been interpreted as circumstantial evidence for a dipolar rather than a radical mechanism for the hydrogen transfer as shown in Scheme 2.

Scheme 4
In view of the diversity of this chemistry, we were intrigued to discover the thermal properties of the 1,3,5-triazapentadiene system 9 (Scheme 5) which has the necessary structural features for (a) 6π electrocyclisation with elimination to give a quinazoline (c.f.Scheme 1) (b) elimination of aniline and cyclisation to give an imidazole (c.f.Scheme 2) and (c) 4π electrocyclisation with elimination of HCN to give an amidine (c.f.Scheme 4).Here we describe the results of the pyrolysis of such a precursor.

Results and Discussion
To allow direct comparison with our earlier work, 5 we required a 1,3,5-triazapentadiene system without C-substitution or with only C-alkyl substitution.Although compounds of these types are not reported in the literature, we were able to adapt the route to well-known 4-aryl compounds 10 8 to give the 4-methyl derivative 11 (Scheme 6).Thus treatment of N-phenylacetamidine 13 with N,N-dimethylformamide diethyl acetal 14 provided the pentadiene 11 as a yellow oil in 42% yield.Our attempts to prepare the parent compound 12 by a similar route were not successful (see Experimental section).The triazapentadiene 11 was characterised by its spectra and in particular by comparison of its NMR spectra with those of the corresponding 1,5-diazapentadiene 15. 5 Although these spectra show close similarities, the diazapentadiene 15 is present in solution as two isomers (E and Z about the C=N unit) whereas the triazapentadiene 11 adopts just one configuration.On the other hand, the N-methyl groups of the two isomers of 15 appear as six-proton singlets at room temperature whereas those of 11 are separated into two three-proton singlets due to restricted rotation about the C-N unit.Variable temperature NMR studies of the two isomers of 15 at 200 MHz showed coalescence temperatures of 221 K and 244 ([ 2 H 6 ]acetone) corresponding to free energies of activation of 44.5 and 48.1 kJ mol -1 respectively, whereas the coalescence temperature of 11 was 346 K in [ 2 H 6 ]DMSO solution (E a 73.5 kJ mol -1 ).Electron donation from the dimethylamino group is therefore promoted by the presence of the central nitrogen atom in 11 in accord with the relative stability of the resonance structure 11a compared with that of 15a.The electron impact mass spectrum of 11 shows a molecular ion of moderate intensity; major breakdown peaks are derived from α-cleavage following ionisation at the imino-nitrogen atom (Scheme 7).In addition a small peak is observed at m/z 148 (6%) due to loss of MeCN from the molecular ion.This electron impact induced behaviour is reminiscent of the thermal loss of HCN in 1,2,5-triazapentadiene chemistry (Scheme 4).Under our standard FVP conditions, a furnace temperature of 800 °C was required for complete transformation of 11 to products.These conditions are comparable to those required for quinoline formation or pyrrole formation from vinamidines (Schemes 1 and 2), [though only 650 °C is required for HCN elimination from 1,2,5-triazapentadienes (Scheme 4)].Four products were identified in the complex low-yielding pyrolysate (Scheme 8) viz.1,4-dimethylimidazole 16 (18%) (identical with an authentic sample), 2-methylquinazoline 17 (7%) (spectra compatible with those in the literature -see Experimental section), aniline 18 (24%) and a trace of Nmethylaniline (4%).None of the amidine 8 could be detected by G.C. comparison with an authentic sample, 9 and so the imine 11 does not decompose by a route analogous to Scheme 4. This latter result is also surprising because the analogous azaenaminone 19 (R = Me) cleaves by loss of MeCN in just this way, though at a higher temperature than required for the azoalkenes. 1 Instead, the 1,3,5-triazapentadiene system undergoes pyrolysis preferentially by the hydrogen transfer-cyclisation-elimination sequence to give the imidazole 16, together with aniline 18 as the co-product (c.f.Scheme 2).Electrocyclisation to the quinazoline 17 (c.f.Scheme 1) forms a minor component of the reaction profile.The formation of the small amount of N-methylaniline may be due to adventitious coupling of anilino and methyl radicals.These results are placed in the context of other vinamidine and azavinamidine pyrolyses in Table 1.In these series, the 4π electrocyclisation with elimination route (Scheme 4) is apparently unique to the 2-azavinamidine and other azoalkene units (despite the behaviour of the corresponding azaenaminones. 1 ) The 6π electrocyclisation with elimination route (Scheme 1) and azole formation by elimination of aniline (Scheme 2) are in competition in both the vinamidine and 3-azavinamidine series, though the unusual azole formation is either relatively favoured, or the electrocyclisation is disfavoured, in the 3-azavinamidine case.These results are consistent with the observation that an electron donating 3-methyl substituent causes a relative increase in the electrocyclisation pathway in the vinamidine series. 5Since an electron withdrawing group at the corresponding position to the additional nitrogen atom apparently causes a small acceleration in hexatriene electrocyclisation processes, 10 we conclude that the additional nitrogen atom in the vinamidine system is able to accelerate the azole formation further, probably by providing additional stability to the dipolar intermediate.

N-Phenylacetamidinium picrate (13).
Following the method of Oxley et al, 11 powdered aluminium chloride (13.3 g, 0.1 mol) was added, in small quantities over 45 min, to a mixture of acetonitrile (4.1 g, 0.1 mol) and aniline (9.3 g, 0.1 mol) so that the temperature did not rise above 90-100 °C.After the addition was complete, the semi-solid mixture was kept at 100 °C for 1 h.The reaction product, which consisted of a complex of the amidine with the catalyst, was carefully decomposed by the dropwise addition of water until the vigorous reaction ceased.The amidine base was liberated by the addition of a solution of sodium hydroxide (15 g, 0.38 mol) in water (100 ml) and the mixture was extracted three times with chloroform.The organic extracts were dried (Na 2 SO 4 ) and the solvent was removed to give a dark brown liquid (8.84 g) which was immediately converted into the picrate by the addition of an excess of picric acid in ethanol.A clean yellow solid was obtained (5.50g, 15%).5-Phenyl-1,1,4-trimethyl-1,3,5-triazapentadiene (11).N-Phenylacetamidinium picrate (1.81 g, 5 mmol) was suspended in dichloromethane (20 ml) and a solution of sodium hydroxide (1.0 g, 25 mmol) in water (30 ml) was added.The mixture was shaken vigorously and the organic layer was separated off.The aqueous layer was extracted twice more with dichloromethane, the organic extracts were dried (Na 2 SO 4 ) and the solvent was removed in vacuo (with a cold waterbath).The yellow-brown amidine 13 (0.33 g, 49% recovery) was then used immediately.A solution of N,N-dimethylformamide diethylacetal 14 (0.44 g, 3 mmol) in dioxan (1 ml) was added to the amidine 13 (0.33 g, 2.5 mmol) and the mixture was heated at 80 °C for 2 h.The dioxan and the excess of acetal were removed by distillation before the product was purified by bulb to bulb distillation to give the pentadiene 11 as a yellow oil (0.40 g, 42%), bp 106-108 °C (0. ; although no authentic sample was available this compound showed the correct molecular ion and similar EI fragmentation to that reported. 12n addition the literature value 13 for the methyl peak in the Scheme 1

Table 1 .
Ratios of major pyrolysis routes found for the vinamidines and azavinamidines 7, 11 and 15 13 and13C NMR spectra were recorded at 80 (or 200) and 20 MHz respectively for solutions in [ 2 H]chloroform.Mass spectra were obtained under electron impact (EI) ionisation conditions.
4) and the chloroform was removed in vacuo.The dark coloured liquid which remained was purified by bulb to bulb distillation to give the imidazole as a colourless liquid ISSN 1424-6376 Page 102 © ARKAT USA, Inc(