1.3- Diaza-2-azoniaallene salts

The 1,3-dipolar character of triazaallenium salts, from now on referred to as l,3-diaza-2-azoniaallene salts, is evidenced by the many [3+2] cycloaddition reactions these types of compounds can participate in. The l,3-diaza-2-azoniaallene salts are generated in situ and trapped with suitable dipolarophiles. For example, l,3-diaza-2-azoniaallene salts undergo stereospecific [3+2] cycloaddition reactions with alkynes and olefins. However, they fail to react with isocyanates, isothiocyanates and azo compounds. Also, [3+2] cycloadditions to carbodiimides and cyanamides are observed. In contrast, nitriles fail to react. 1,3-Diaza-2-azoniaallene salts are obtained in the oxidation of 1,3-disubstituted triazenes with t-butyl hypochlorite. The resultant Al-chlorotriazenes react with antimony pentachloride to form the salts as reactive intermediates. Above —25°C, l,3-diaza-2-azoniaaUene salts disproportionate into diazonium salts and azo compounds. 

Table 7.1 Cycloaddition reactions of 1,3-diaza-2-azoniaallene salts with alkynes...

Table 7.2 13+2] Cycloaddition reactions of 1,3-diaza-2-azoniaallene salts with olefins ArN = N =NR + R R C=CR2R ------------------ ...

The reaction of buta-2,3-dienoates (44) with electron-deficient imines (45) in the presence of triphenylphosphene yields 3 - - 2-cycloadducts (46) in excellent yields and high chemoselectivity (Scheme 16)." The l,3-diaza-2-azoniaallene salts (47) react with alkynes, iV,iV -dialkylcarbodiimides, and Al,Al-dialkylcyanamides to produce... 

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 ... 

The [3+2] cycloaddition reaction of the l,3-diaza-2-azoniaallene salts 28 with l,4-bis(4-bromophenyl)-l,4-di-t-butylbutatriene affords the cycloadduct 29, resulting from addition across the terminal double bond . 

The reactions of l,3-diaza-2-azoniaallene salts 1, generated in situ, with alkynes at low temperatures, afford the [3+2] cycloadducts 2 as shown in Table 7.1 ... 

In the reaction of l,3-diaza-2-azoniaallene salts, R N=N+=NR, with dienes, such as butadiene and 2,3-dimethylbutadiene, mono [3+2] cycloadducts are obtained. From cyclooctatetraene a bis-cycloadduct was isolated. Also, allenes and higher cumulenes undergo [3+2] cycloaddition reactions with the same salts. With butatrienes, reaction proceeds across the terminal double bonds, while pentatetraenes react across the center double bonds. 

Likewise, l,3-diaza-2-azoniaallene salts (Ar = 2,4,6-Cl3Ph) react with carbodiimides at -60 °C to give the [3+2] cycloadducts 5. ... 

Following a different synthetic procedure for the synthesis of the triazolium salts, it was possible to introduce exclusively aryl substituents on the heterocycle. Hence, Afl,Af3-diaryl triazolium salt 3 was prepared directly via 1,3-dipolar cycloaddition between l,3-diaza-2-azoniaallene salts and alkynes (Scheme 3.1). The presence of aryl substituents exclusively on the heterocycle suppresses any migration of the substituents and therefore enhances the stability of the free triazolylidenes substantially. 

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