Retention of Configuration in Two Photochemical Reactions: Formation of Cyclopropanimines by Extrusion of Molecular Nitrogen from Tetraalkyl‐4‐imino‐1‐pyrazolines and [2 + 1] Cycloreversion of Cyclopropanimines to Isocyanides and Alkenes

Abstract

The 1-pyrazolin-4-ones 7 and 9 and the pyrazolidin-4-one 13 are condensed with alkanamines 8 to produce the imines 3, lO and 12 in high yields. Direct irradiation of 3 with 350-nm light at 90°C in deuterated hydrocarbon solvents affords the cyclopropanimines 4 in almost quantitative yields besides molecular nitrogen and small amounts of the imines 12 as a result of photoreduction. In [D,]acetone, the cyclopropanimine 4 b isomerizes in part of the a$-unsaturated imine 14. Direct irradiation of 3 with the unfiltered light of the high-pressure mercury lamp results in quantitative [2 + 1] cycloreversion of the primary photoproducts 4 into the alkene 6 and an isocyanide 5. At low temperature (lO°C), photolysis of 3 occurs much more slowly giving rise to photoextrusion of nitrogen (-4) and photoreduction (-12) to about the same extent. -Photolysis of the stereochemically labelled iminopyrazolines cis-and trans-10 (d.e. 99%) at 90°C produces the cyclopropanimines cis-and trans-16 (d. e. 94%) with high stereospecificity. The configuration of cis-and trans-16 is established by a comparison with the corresponding methylenecyclopropanes cis-and trans-19 and the quantitative and completely stereospecific [2 + 1 1 cycloreversion into methyl isocyanide (5a) and the 3,4-dimethyl-3-hexenes (4-and (4-17 on irradiation with the unfiltered light of the mercury arc. The necessity of thermal activation for efficient nitrogen extrusion from the '(n,n*) state of 3 and 10 is indicative of a considerable energy barrier towards the transition into a dissociative state. At low temperature, hydrogen abstraction from the solvent or other molecules becomes important for the deactivation of the '(n,x*) state, in addition to decay and fluorescence. The stereospecific formation of cis-and trans-16 is interpreted in terms of diastereomeric bis-orthogonal azatrimethylenemethane diradicals as intermediates which retain the configuration on cyclization. The minor non-stereospecific path may involve mono-orthogonal azatrimethylenemethane diradicals. Thus, mechanisms that involve the same types of diradical intermediates can rationalize the photolysis of the iminopyrazolines 3, 10 and of the methylenetriazoline 1 as well. The [2 + 1 1 cycloreversion of cis-and trans-16 into the alkenes (2) and (E)-17 and methyl isocyanide (5a) demonstrates for the first time that such photoreactions can be entirely stereospecific.The chemistry of cyclopropanones 3, has been enriched and extented by the syntheses4-') and reactions8) of cyclopropanimines. A Favorskii-type 1,3-dehydrobromination of cl-bromo ketimines is the method of choice for the synthesis of cyclopropanimines provided that, by virtue of a shielding substitution pattern, they are not too reactive towards nucleophile~~"). On the other hand, highly reactive cyclopropanimines, devoid of ring substituents, are readily available by thermal isomerization of methylena~iridines~~). Eventually, the parent compound has been generated by heterogeneous 1,2-dehydrochlorination of N-chlorocyclopropanamine and iden...

Photochemical Isomerizations of 5-Alkylidene-4,5-dihydro-3H-1,2,4(λ3)-diazaphospholes (4-Phosphapyrazolines): (5→4) Ring Contraction Generates Azomethineimine Dipoles

“…[30] In a series of detailed investigations, Quast and co-workers have studied the thermal and photochemical behavior of D 1 -pyrazolines with p bonds at the C-4 position of the ring but lacking 3-(or 5-)alkylidene substitution. While N 2 elimination was the major pathway in most cases, some photochemical side reactions were also observed, such as the cleavage of 3,3,5,5-tetramethyl-1-pyrazolin-4-one into acetone azine and carbon monoxide, [31] and the photoreduction of the azo group of 3,3,5,5-tetramethyl-4-methylene-1-pyrazoline [32] and 4-imino-3,3,5,5-tetraalkyl-1-pyrazolines [33] in hydrogen-donating solvents. Upon irradiation of 3,3,5,5-tetramethyl-1-pyrazolin-4-one azine, cleavage of the NÀN bond of the azino group is the major reaction.…”

Allenyl Azide Cycloaddition Chemistry: Exploration of the Scope and Mechanism of Cyclopentennelated Dihydropyrrole Synthesis through Azatrimethylenemethane Intermediates

“…The reaction mixture was held at 0 °C for 1.5 h and then slowly warmed to room temperature over an additional 1 h. The reaction mixture was poured into 30 mL of saturated NH 4 Cl solution, and the organic layer was extracted into Et 2 O, washed with water and then brine. Drying the organic phase over Na 2 SO 4 , followed by evaporation of the solvent in vacuo, gave a yellow oil.…”

“…To a solution of propargyl azidomesylate 19 (0.14 g, 0.65 mmol) and Pd(PPh 3 ) 4 (36 mg, 0.03 mmol, 5 mol%) in 10 mL of THF were added commercially available 3-furanyl boronic acid (0.17 g, 1.5 mmol) and sodium carbonate (95 mg, 0.9 mmol). After the reaction mixture was refluxed for 4 h, the solution was poured into water and the mixture was extracted with 15 mL of Et 2 O. Drying over Na 2 SO 4 , followed by evaporation of solvent and chromatography on silica gel using 5% Et 2 O in hexane afforded 26 as a pale yellow oil (35 mg, 28%).…”

“…To a solution of propargyl azidomesylate 19 (108 mg, 0.50 mmol) and Pd(PPh 3 ) 4 (30 mg, 0.02, 5 mol%) in 10 mL of THF was added (Z)-ethoxycarbonylethenylzinc iodide 20 (1.5 mmol), and the mixture was heated at 50 °C for 3 h. After the reaction solution was cooled to room temperature, water was added and the mixture was extracted with 15 mL of Et 2 O. The organic layer was dried over Na 2 SO 4 and solvent was evaporated in vacuo.…”

“…Extrapolating from this observation, we speculated that combination of two highly substituted carbon radicals may provide a facile means to forge a C-C bond in an extremely sterically hindered environment. In particular, an interest in the synthesis of certain structural classes of alkaloids focused our attention on di (carbon) radical generation from substrates bearing strategically placed nitrogens, and that restriction led to the work of Quast,4 who provided evidence for the intermediacy of a nitrogen version of the classic trimethylenemethane diyl, azatrimethylenemethane (ATMM) 11, upon either thermolysis or photolysis of the triazoline 10. 4b This diyl may have been implicated but unrecognized in allene/azide cycloaddition chemistry reported earlier by Shechter 5 and later it was posited as a reactive intermediate by Gilbert,6 but no deliberate effort to exploit ATMM chemistry in reaction design has appeared prior to our work.…”