Aza-2-azoniaallene Salts

Monocations derived by replacing carbon atoms in allenes with nitrogen atoms are of recent vintage. They were synthesized by Jochims and coworkers at the University of Konstanz in Germany in the 1980s. The l-aza-2-azoniaallenes, R2C=N+=NR, treated in this current chapter, react as 1,3-dipols in stepwise [3-1-2] cycloaddition reactions. 2-Azoniaallene salts, R2C=N+ R2, l-oxa-3-azonia-butatriene salts, R2C=N+=C=0, and l-thia-3-azoniabutatriene salts, R2C=N+=C=S, are treated in Chapter 5. 

The l-aza-2-azoniaallene salts are usually generated in situ and reacted at low temperatures with numerous dipolarophiles. The initially formed cycloadducts often undergo a 1,2-shift to form isomeric heterocycles. 

To carbon multiple bonds [3-1-2] Cycloadducts are obtained from 1 -aza-2-azoniaallene salts and alkynes and olefins. For example, from Mc2C=N+=NR (R=2,4,6-trichlorophenyl-) and diethylacetylene an 88 % yield of the [3-1-2] cycloadduct 36 is obtained at 0 °C. 

In the reaction of the l-aza-2-azoniaallene salts with ( )-3-hexene a single stereoisomer is obtained. However, in the reaction with allylcyanide, exclusive reaction of the nitrile group is observed. 

The [3-1-2] cycloaddition reaction of l-aza-2-azoniaallene salts with alkynes to give pyrazolium salts is a general reaction. The initially formed 3//-pyrazolium 

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Table 2.4 [3- -2] cycloaddition reactions of l-(chloroalkyl)-1 -aza-2-azoniaallene salts with olefins... 

In a recent theoretical study of the reaction of 1 -aza-2-azoniaallene salts, with isocyanates it was concluded that these reactions proceed in asynchronous but concerted pathways. The presence of chloro groups in the l-aza-2-azoniaallene salts, as well as methyl groups in the isocyanates, favor the cycloaddition reactions . 

In the reaction of 2,3,4-tetra-O-acetyl- -D-glucopyranosyl isothiocyanates with 1-aza-2-azoniaallene salts the cycloaddition also proceeds across the C=S bond. Also, glycosyl... 

The [3+2] cycloaddition reactions of isocyanates with 1 -aza-2-azoniaallene salts proceed by an asynchronous concerted mechanism . 

In a similar manner the l-aza-2-azoniaallene salts derived from coumarin and camphor react with diisopropylcarbodiimide to give the [2-1-3] cycloadducts. °i Also, l,3-diaza-2-azoniaallene salts 304 undergo the [2-1-3] cycloaddition reaction with diisopropylcarbodiimide or DCC to give 1,3,4,5-tetrasubstituted 4,5-dihydroten azolium salts. 305.11 ... 

Jochims [134] and Hassan s [135] groups reported the efficient use of l-aza-2-azoniaallene salts 119 and 0-acetylated j6-D-glucopyranosyl isothiocyanate 89 for the preparation of glucopyranosylimino-l,3,4-thiadiazoles 121 and 122. These salts, prepared from chloroalkylazo derivatives 118 and antimony (V) chloride, can undergo 1,3-dipolar cycloadditions with multiple bonds-containing compounds, such as alkynes, alkenes, isocyanates, isothio-... 

The 1,3-dipolar cycloaddition of 2-diazopropane with diarylideneacetones is temperature dependent, leading to mono-, di-, and tri-substitution products. The 1,3-dipolar cycloaddition of diazomethane and diazoacetate with 5-sulfur-2(5//)furanones provides a synthesis of furoisoxazolines. The 1,3-dipolar cycloaddition of 1-aza-2-azoniaallene cations with isothiocyanates produces 1,3,4-thiadiazolium salts and 1,2,4-thazolium salts. 

The reaction of l-aza-2-azoniaallene salts derived from coumarins with alkynes does not afford cycloadducts but rather leads to intramolecular cyclization. In contrast, a [3+2] cycloadduct is obtained in 92 % yield (mixture of diastereoisomes, ratio 2 1) from the same salt in the reaction with norbornene. From the camphor salt 41 (R = 2,4,6-trichlorophenyl) and phenylacetylene a 53 % yield of the cycloadduct 42 is obtained . 

In contrast to the isocyanate reaction with l-aza-2-azoniaallene salts, isothiocyanates react across their C=S bond in the [3+2] cycloaddition reaction to give 1,3,4-thiadiazolium salts. The initially formed [3+2] cycloadducts 53 undergo a 1,2-shift to give the isomeric adducts 54 °. 

Carbodiimides also undergo [3-1-2] cycloaddition reactions with l-aza-2-azoniaallene salts to give 4,5-dihydro-5-imino-li/-l,2,4-triazolium salts 56. The reaction proceeds stepwise, but the linear intermediate cannot be detected. 

In the [3-1-2] cycloaddition reactions of isothiocyanates with l-aza-2-azoniaallene salts the reaction also proceeds across the C>=S bond of the isothiocyanate to give 1,3,4-thiadiazolium salts in high yields 2. ... 

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