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Notizen: Photochemical Generation and Reactions of Benzonitrile-benzylideThe low temperature irradiation of 2,3-diphenyl-2H-azirine (1) in DMBP-glass at -196° has been reinvestigated. It was possible to convert 1 nearly quantitatively into the dipolar species benzonitrile-benzylide (3, Φ3 = 0,78), which exhibits UV.-absorptions at 344 (∊ = 48000) and 244 nm (∊ = 28500) (Fig. 1, Tab. 1). Irradiation of 3 with 345 nm light at -196° resulted in almost complete reconversion to the azirine 1 (Φ = 0,15; Fig. 2). When the solution of 3 in the DMBP-glass was warmed up to about -160° a quantitative dimerization to 1,3,4, 6-tetraphenyl-2,5-diaza-1,3,5-hexatriene (8) occurred. This proves that 8 is not only formed by the indirect route 3 + 1 → 7 \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\longrightarrow }\limits^{hv} $\end{document} 11 → 8 known before (Scheme 1), but also by dimerization of 3 either by direct head to head coupling or via the intermediate e (p. 2675), followed by a fast thermal hydrogen transfer reaction.The occurrence of the dipolar intermediate 11 in the photochemical conversion of the bicyclic compound 7 to 8 could also be demonstrated by low temperature experiments: On irradiation at -196° 7 gave the cherry red dipolar intermediate 11 (λmax = 520 nm), which at -120° isomerizes to 8. It should be noted, that neither 7 nor 11 are formed by dimerization reactions of 3.Experiments carried out at room temperature demonstrate, that both processes for the formation of 8 may compete: Irradiation of a solution of 1 (DMBP, c = 8 × 10-4 to 5 × 10-3M) with 350 nm light of high intensity (which does not excite the bicyclic compound 7) leads to a relative high photostationary concentration of the dipolar species 3. Under these conditions the formation of 8 is due to dimerization of 3 (Φ8 = 0,19). With low light intensity only a very low stationary concentration of 3 can be obtained. Therefore the reaction of 3 with 1, leading to the bicyclic intermediate 7, becomes now predominant (Φ-1 = 1,55, which corresponds with the expected value of 2 × 0,8). Irradiation of 1 at -130° with 350 nm light of high intensity gives 8 with a quantum yield of 0,44. This is in agreement with the theoretical value Φ8 = 0,4 for an exclusive formation of 8 by dimerization of 3. The lower quantum yield for the formation of 8 at room temperature makes probable that under these conditions 3 not only dimerizes to 8, but also to another, so far unidentified dimer, e.g. 2,3,5,6-Tetraphenyl-2,5-dihydropyrazine.By flash photolysis of a solution of 1 (cyclohexane, c = 10-4M, 25°) the disappearance of 3 could directly be measured by UV.-spectroscopy: At relative high concentrations (c ≥ 10-7M) 3 disappeared according to a second order reaction with the rate constant k = 5 × 107M-1S-1. At lower concentrations (c ≤ 10-7M) the rate of disappearance of 3 follows first order kinetics. The rate constant of this pseudo first order reaction (3 + 1 → 7) has been determined to be 1 → 104M-1S-1.Using Padwa's table of relative rates for the cycloaddition of the dipolar species 3 to various dipolarophiles, including the azirine 1, an absolute rate constant of k ≈ 8 × 108M-1S-1 for the addition of 3 to the most active dipolarophile fumaronitrile could be estimated. In cyclohexane at room temperature, the diffusion controlled rate constant equals 6,6 × 109M-1S-1.In Table 1 the UV.-maxima of several nitrile-ylides, among them a purely aliphatic one, are given.

Notizen: Synthesis and reactions of the valence polaromeric compound (4,4-dimethyl-2-thiazoline-5-dimethyliminium)-2-thiolate ⇌ 1-dimethylthiocarbamoyl-1-methyl-ethyl isothiocyanate from 3-dimethylamino-2,2-dimethyl-2H-azirine and carbon disulfide.3-Dimethylamino-2,2-dimethyl-2H-azirine (1) reacts with carbon disulfide to give crystals which have the dipolar structure 3a [(4,4-dimethyl-2-thiazoline-5-dimethyliminium)-2-thiolate, Scheme 1]. In solution, the non-dipolar (charge-free) isomeric form 3b (1-Dimethyl-thiocarbamoyl-1-methyl-ethyl isothiocyanate) is almost exclusively populated. Reaction products are derived from both forms: Derivatives of 3a are the hydrolysis product 6, the sodium borohydride reduction product 7 and the methylation products 9 and 10, respectively (Scheme 2). The isothiocyanate form 3b is responsible for the various reaction products with amines (Scheme 3). One of the reaction products with ammonia, namely 20, is also obtained by the reaction of 1 with thiocyanic acid.Thermolysis of the azirine/carbon disulfide adduct 3 leads to 2-dimethylamino-4,4-dimethyl-2-thiazoline-5-thione (17) in high yield. A possible mechanism is outlined in Scheme 4. The same compound is also formed by rearrangement of 3 under the catalytic influence of dimethylamine. Its structure has been established by X-ray crystallography (section 4). Again a rearrangement is involved in the reductive (NaBH4) conversion of 17 to 7, the direct reduction product of the dipolar species 3a (Scheme 5).The isothiocyanate form 3b is able to react with a second molecule of 3-dimethylamino-2,2-dimethyl-2H-azirine (1) to yield compound 25, which in the crystalline or dissolved state appears to be almost entirely populated by the carbodiimide form with structure 25b (Scheme 7), though all reaction products of 25 (reduction with sodium borohydride, addition of water or hydrogen sulfide, Schemes 7 and 8) are derived from the dipolar form 25a, not detectable as such; here again therefore there is a dynamic equilibrium 25a ⇌ 25b.The two forms of adduct 3, namely 3a and 3b, are obviously very easily interconverted at room temperature and therefore can be considered as valence polaromeric forms (section 5). A classification of the dipolar (zwitterionic) form is given, which allows a comparison of various dipolar species and gives as indication of charge stabilization by delocalization.The versatile reactivity of the 3-dimethylamino-2,2-dimethyl-2H-azirine/carbon disulfide adduct is demonstrated by the fact that with simple reagents approximately 25 derivatives have been obtained, most of them being new heterocyclic compounds.

Notizen: Synthesis and Reactions of 8-membered Heterocycles from 3-Dimethylamino-2,2-dimethyl-2H-azirine and Saccharin or Phthalimide3-Dimethylamino-2,2-dimethyl-2H-azirine (1) reacts at 0-20° with the NH-acidic compounds saccharin (2) and phthalimide (8) to give the 8-membered heterocycles 3-dimethylamino-4,4-dimethyl-5,6-dihydro-4 H-1,2,5-benzothiadiazocin-6-one-1,1-dioxide (3a) and 4-dimethylamino-3,3-dimethyl-1,2,3,6-tetrahydro-2,5-benzodiazocin-1,6-dione (9), respectively. The structure of 3a has been established by X-ray (chap. 2). A possible mechanism for the formation of 3a and 9 is given in Schemes 1 and 4.Reduction of 3a with sodium borohydride yields the 2-sulfamoylbenzamide derivative 4 (Scheme 2); in methanolic solution 3a undergoes a rearrangement to give the methyl 2-sulfamoyl-benzoate 5. The mechanism for this reaction as suggested in Scheme 2 involves a ring contraction/ring opening sequence. Again a ring contraction is postulated to explain the formation of the 4H-imidazole derivative 7 during thermolysis of 3a at 180° (Scheme 3).The 2,5-benzodiazocine derivative 9 rearranges in alcoholic solvents to 2-(5′-dimethylamino-4′,4′-dimethyl-4′H-imidazol-2′-yl) benzoates (10, 11), in water to the corresponding benzoic acid 12, and in alcoholic solutions containing dimethylamine or pyrrolidine to the benzamides 13 and 14, respectively (Scheme 5). The reaction with amines takes place only in very polar solvents like alcohols or formamide, but not in acetonitrile. Possible mechanisms of these rearrangements are given in Scheme 5.Sodium borohydride reduction of 9 in 2-propanol yields 2-(5′-dimethylamino-4′,4′-dimethyl-4′H-imidazol-2′-yl)benzyl alcohol (15, Scheme 6) which is easily converted to the O-acetate 16. Hydrolysis of 15 with 3N HCl at 50° leads to an imidazolinone derivative 17a or 17b, whereas hydrolysis with 1N NaOH yields a mixture of phthalide (18) and 2-hydroxymethyl-benzoic acid (19, Scheme 6). The zwitterionic compound 20 (Scheme 7) results from the hydrolysis of the phthalimide-adduct 9 or the esters 11 and 12. Interestingly, compound 9 is thermally converted to the amide 13 and N-(1′-carbamoyl-1′-methylethyl)phthalimide (21, Scheme 7) whose structure has been established by an independent synthesis starting with phthalic anhydride and 2-amino-isobutyric acid. However, the reaction mechanism is not clear at this stage.

Notizen: Structure of a Stable Dipolar Compound from 2,2-Dimethyl-3-dimethylamino-2H-azirine and Benzoylisothiocyanate.Benzoylisothiocyanate and 2,2-dimethyl-3-dimethylamino-2H-azirine (1) react to given the dipolar compound 4,4-dimethyl-2-thiazolin-5-dimethylimminium-2-benzcarboxamidate (2), whose structure has been proved by X-ray analysis. Compound 2, upon addition of water, yields the thiourea derivative 3, whereas by acid catalyzed hydrolysis the thiazolinone derivative 4 is formed. The dipolar structure 2 is also existent in organic solvents like dimethylsulfoxide or chloroform.

Notizen: The Crystal Structure of a Bridged 1,2,3-Oxadiazolidin-5-one Derivative3-(2-Allylphenyl)-4-phenylsydnone (1) undergoes in solution an intramolecular, 1,3-dipolar cycloaddition reaction to give 2-oxo-1-phenyl-1,5-methano-1,2,4,5,6,11-hexahydro [1,2,3] oxadiazolo [3,2-a] cinnoline (2). The unique 1,2,3-oxadiazolidin-5-one structure of this molecule has been proved by X-ray analysis. The crystal structure has been solved by direct methods and refined by full-matrix least squares calculations to R = 0,046. The crystal system is orthorhombic, space group Pbca, with unit cell dimensions a = 10,546, b = 15,482, c = 16,531 Å.

Notizen: Addition Reactions of 3-Dimethylamino-2, 2-dimethyl- 2 H-azirine and Isothiocyanates.The title azirine readily reacts with two molecules of benzyl- or methylisothiocyanate to form the zwitterionic 1:2 addition compounds 4 and 13, respectively (Scheme 2). The presumed 1:1 addition products, which are intermediates in the formation of 4 and 13, cannot be detected. The structure of 4 and 13 follows from their spectroscopic and chemical properties. With water they give the thiourea derivates 5 and 14, respectively; treatment with aqueous acid leads to the Δ2-1, 3-thiazolin-5-on-derivates 7 and 15, respectively. With sodium borohydride compounds 8 and 16, respectively, are obtained (Scheme 2).The zwitterionic compounds 4 and 13 are able to react further with one molecule of the isothiocyanates to give, in high-yield, triazines 9 and 18, respectively (Scheme 3). The structure of these compounds was again derived from their spectroscopic data. The mechanism for the formation of 9 and 18 is given in Scheme 3. Acid catalysed hydrolysis of 9 and 18 lead to the trithiocyanuric acid derivates 12 and 20, and to the spiro compounds 11 and 19, respectively (Sceme 6).Reaction of 4 with one molecule of phenylisocyanate gives triazine 10 (Scheme 5).According to the X-ray analysis of the methyl compound 18, there are strong steric interactions in this molecule which are due to the side chain. This is demonstrated by the small distances between C(2) … C(13), N(7) … C(11), and C(8) … C(11) (Table 4). These steric interactions, in addition, cause widening of the bond angles N(1)—C(2)—N(7) and C(9)—N(10)—C(11) (Fig.2). Furthermore, the triazine ring is no longer planar. This deformation of the ring diminishes repulsion between the methyl groups C(13) and C(15).

Notizen: Structure of the adduct from 3-dimethylamino-2,2-dimethyl-2H-azirine and 3-methyl-2,4-diphenyl-1,3-oxazolium-5-olate3-Dimethylamino-2,2-dimethyl-2H-azirine (1) reacts with 3-methyl-2,4-diphenyl-1,3-oxazolium-5-olate (5a) to give a 1:1 adduct (7) in a 88% yield. Its crystal structure has been determined by X-ray analysis (direct methods) and refined with 1056 structure amplitudes to R = 0,032. The crystal system is monoclinic, space group P21/c, with unit cell dimensions a = 10.663, b = 9,828, c = 18,592 Å, and β = 90,63°. It is obvious that 4-dimethylamino-5,5-dimethyl-2-[α-(N-methyl-benzamido)benzyliden]-Δ3-1,3-oxazoline (7) arises from an addition of 1 to the valence-polaromeric ketene form 5b of the mesoionic oxazolone 5a (Scheme 3).

Notizen: Reaction Products from 3-Dimethylamino-2,2-dimethyl-2H-azirine and Phthalohydrazide or Maleohydrazide3-Dimethylamino-2, 2-dimethyl-2H-azirine (1) reacts in dimethylformamide at room temperature with the six-membered cyclic hydrazides 2, 3-dihydrophthalazin-1, 4-dione (2) and 1, 2-dihydropyridazin-3, 6-dione (15) to give the zwitterionic compounds 3 and 16, respectively (Scheme 1 and 7). The mechanism of these reactions is outlined in Scheme 1 for compound 3 (cf. also Scheme 8). The first steps are thought to be similar to the known reactions of 1 with the NH-acidic compounds saccharin and phthalimide (cf. [1]). Instead of ring expansion to the nine-membered heterocycle i (X=CONH, Scheme 8), a proton transfer followed by the loss of water gives 3 (Scheme 1).The structure of the zwitterionic compounds 3 and 16 is deduced from spectral data and the reactions of these compounds (see Schemes 2, 3, 4, 6 and 7). Methylation of 3 yields the iodide 4, which is hydrolysed easily to the 2-imidazolin-5-one derivative 5 (Scheme 2). Hydrolysis of 3 under basic conditions leads to the amide 6, which undergoes cyclization to 7 at 220-230° (Scheme 3). The analogous cyclization has been realized under acidic conditions in the case of 17 (Scheme 7).Catalytic reduction of 3 yields the tertiary amine 14 (Scheme 6), whereas the reduction with sodium borohydride leads to a mixture of 14 and the 2-imidazoline derivative 13. The alcohol 11, corresponding to the amine 14, is obtained by sodium borohydride reduction of the 2-imidazolin-5-one 7 or of the amide 6 (Scheme 3). This remarkably easy reaction of 7 shows the unusual electrophilicity of the lactamcarbonyl group in this compound. The reduction of 6 to 11 is understandable only by neighbouring group participation of N (2′) in the dihydrophthalazine residue.

Notizen: Thermal (E), (Z)-Isomerizations of Substituted PropenylbenzenesThe thermal isomerizations of (E)- and (Z)-3,5-dimethyl-2-(1′-propenyl)phenol ((E)- and (Z)-3), (E)- and (Z)-N-methyl-2-(1′-propenyl)anilin ((E)- and (Z)-4), (E)- and (Z)-3,5-dimethyl-2-(1′-propenyl)anilin ((E)- and (Z)-5, (E)- and (Z)-2-(1′-propenyl)mesitylene ((E)- and (Z-6), (E)- and (Z)-2-(1′-propenyl)mesitylene ((E)- and (Z)-7), (E)- and (Z)-2-(1′-propenyl)toluene ((E)- and (Z)-8), (E)- and (Z)-4-(1′-propenyl)toulene ((E)- and (Z)-9) as well as of (E)- and (Z)-2-(2′-butenyl)-mesitylene ((E)- and (Z)-10) in decane solution were studied (Scheme 2). Whereas the isomerization of the 2-propenylphenols (E)- and (Z)-3 occurs already between 130 and 150° (cf. Table 1), the isomerization of the 2-propenylanilins 4 and 5 takes place only at temperatures between 220 and 250° (cf. Tables 2 and 3). The activation values and the experiments using N-deuterated 4 (cf. Scheme 4) show that 2-propenylphenols and -anilins isomerize via sigmatropic [1,5]-hydrogen-shifts. For the isomerization of the methyl-substituted propenylbenzenes temperatures 〉 360° are required (cf. Tables 4 and 5). The activation values of the isomerization of (E)- and (Z)-6 and (E)- and (Z)-9 are in accord with those of other (E), (Z)-isomerizations which occur via vibrationally excited singlet biradicals (cf. Table 7). Nevertheless, thermal isomerization of 2′-d-(Z)-8 (cf. Scheme 6) demonstrates that during the reaction deuterium is partially transfered into the ortho-methyl group, i.e. 1,5-hydrogen-shifts must have participated in isomerization of (E)- and (Z)-8 (cf. Scheme 8). Under the equilibrium conditions 2,4,6-trimethylindan (17) is formed slowly at 368° from (E)- and (Z)-6, very probably via a radical 1,4-hydrogen-shift (cf. Scheme 9). In a similar way 2-ethyl-4,6-dimethylindan (19; cf. Table 6) arises from (E)- and (Z)-7. Thermolysis of (E)- and (Z)-10 in decane solution at 367° results in almost no (E),(Z)-isomerization. At prolonged heating 19 and 2,5,7-trimethyl-1,2,3,4-tetrahydronaphthalene (20) are formed; these two products arise very likely from an intermolecular radical process (cf. Scheme 10).

Notizen: Photochemical Synthesis of 4-Phenyl-3-oxazolin-5-ones and their Thermal Dimerization.Irradiation of 3-phenyl-2H-azirines affords reactive benzonitrilemethylide intermediates, which can be trapped by carbon dioxide to yield 4-phenyl-3-oxazolin-5-ones (Scheme 1). The first section of the paper deals with the experimental description of this reaction, which already has been preliminarily communicated. Upon irradiation the 3-oxazolin-5-ones undergo photoextrusion of carbon dioxide to reform the corresponding benzonitrile-methylides, which can be trapped by dimethyl acetylenedicarboxylate.In the second section of the paper, reactions of 2-methyl-4-phenyl-3-oxazolin-5-one (3c) are described. Upon heating to 130°, this compound is partially converted to 2-methyl-4-phenyl-2-oxazolin-5-one (azlactone 4e). Prolonged heating of 3c affords the dimer 7 (Scheme 3) as well as the imidazole derivative 9 (Scheme 4). Compound 7 is related to the ‘Rügheimer compound’ C18H14N2O4, formed from hippuric acid methylester. The structure of 7 was determined by X-ray crystallography and this supports the formula assigned earlier to the ‘Rügheimer compound’ (12) and related pyrrolidin-2,4-diones. The possible mechanism of the thermal formation of 7, which is also base catalysed, is represented in Scheme 3, the one for the formation of the imidazol 9 in Scheme 4.Under the influence of oxygen the 2,4-diphenyl-3-oxazolin-5-one (3b) undergoes a dehydrodimerization to yield compound 16 (Scheme 7). Section three contains the structure elucidation of compound 16 and a mechanistic proposal for the formation of the pyrazine 19 upon thermolysis of 2,4-diphenyl-2-oxazolin-5-on (4b, Scheme 7).