Azoniaallene salts

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... 

Furthermore, a cycloreversion has been observed in the case of the thermally labile azoniaallene salts (61) (Z = O, S). When Z = S, this proceeds within an hour at room temperature to yield the aryl isothiocyanate and the charged cumulene (66) which is isolated as the acyliminium salt (67) <9ICB2537>. When Z = O and R = Ph, cycloreversion occurs at 83°C (refluxing 1,2-dichloroethane) to give the aryl isocyanate and (66), which is further cyclized to the quinazolinium salt (68) (Scheme 4) <89T5825>. 

A new approach to spirolactams involving the thermal reaction of A-unsaturated alkyl /1-carboxamidoenamines in which the enamine is the hetero-ene component has been reported.326 A-Alkylnitrilium salts have been shown to undergo ene reactions with electron-rich alkenes to afford either 2-azoniaallene salts (where the nitrilium salt acts... 

Passing gaseous H2S through a solution of l,3-dichloro-2-azoniaallene salts 150 in dichloroethane results in 3,5-diaryl-1,2,4-dithiazolium hexachloroantimonates 151 (Equation 23) <1995JPR274>. 

Several azoniaallene salts, generated in situ, undergo a [2+3] cycloaddition reaction with carbodiimides. For example, generation of 300 from the chloroalkylazo compound 299 in... 

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 1,5-dichloro-substituted 2-azoniaallene salt 376 reacts as a trivalent electrophile with benzohydrazine 377, to furnish the bicyclic oxadiazinium salt 5-methylthio-2,7-diphenyl[l,2,4]triazolo[l,5-f][l,3,5]oxadiazinium hexachloro-antimonate S3, which represents a new ring system (Equation 54). Very little is known about these compounds however, the structure of the oxadiazinium salt is confirmed by X-ray analysis (see Section 9.09.3.2) <1995JPR274>. 

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-... 

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... 

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 . 

Also, RCH=N+=NR SbCle reacts with acetylenes and olefins to give the [3+2] cycloadductsSimilar cycloaddition reactions with alkynes and alkenes are also observed with l-(chloroalkyl)-l-aza-2-azoniaallene salts to give 43 examples are shown in Table 2.4 2. 

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

Azomethines undergo nucleophilic addition to l-aza-2-azoniaallene salts to give linear iminium salts. 

Heterocumulenes, such as isocyanates and carbodiimides also react with l-aza-2-azoniaallenes across the C=N bond to give [3+2] cycloadducts. For example, aliphatic and aromatic isocyanates add at -20 °C to l-aza-2-azoniaallene salts 50 to give 4,5-dihydro-5-oxo-3/f-l,2,4-triazolium salts 51 or the rearranged triazolium salts 52. ... 

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 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 °. 

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... 

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. 

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

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. ... 

Azoniaallene salts, such as the in situ generated 178, react with carbodiimides to give 4,5-dihydro-5-imino-l//-l,2,4-triazolium salts 179, in 65-100% yield (for more details, see Chapter 2, Section 2.5) . ... 

---