Olefins ketenimines

Although most of the photocycloaddition reactions have involved olefins, there are several examples of additions to other types of un-saturated systems (e.g., allenes, acetylenes, ketenimines, etc.) which extend their synthetic utility. It is impossible, at this time, to define the limitations of the reaction as applied to other systems however, this will unquestionably be an active and fruitful area of research in the future. 

Because of their predictable behavior and reactivity, thioacyl isocyanates comprise the bulk of this work, and extensive studies of their [4 -I- 2] reactions with olefins, enamines, enol ethers, thioacyl isocyanates, imines, carbodiimides, isocyanates, azirines, /3-enaminoke-tones, dianils, azines, hydrazones, imidazoline-4,5-diones, aryl cyanates, disubstituted cyanamides, aldehydes, ketones, ketenes, alkyl or aryl iminodithiocarbonates, and the carbon-carbon double bond of ketenimines have been detailed. In an extensive comparative study of the [4 + 2] cycloaddition reactions of thioacyl isocyanates, the heterocu-mulenes bearing strong electron-withdrawing substituents were found to be more stable and less prone to participate in cycloaddition reactions. Representative examples are summarized in Scheme 9-IV. 

Reactions of fluoroalkylketenimines which have been reported following these newly available synthetic approaches include hydrolysis and related additions of amines and alcohols, all of which occur at the olefinic bond. In contrast, the apparently similar addition of secondary phosphines occurs at the carbon-nitrogen double bond. These ketenimines appear to enter into cycloaddition reactions very readily with acetylenes they give quinolines, with nitrones they give oxindoles or oxadiazolidines (see p. 107), i > i and with isocyanides they yield iminoindolenines. These reactions are summarized in Scheme 54. 

A number of reports concern the attack of nucleophiles on perfluoro-olefins to give cumulenes or heterocumulenes, which in turn imdergo nucleophilic attack to give heterocyclic compounds with perfluoroalkyi substituents. o-Phenylenedi-amine reacts with hexafluoropropene to give 2-(o j8j8>tetrafluoroethyl)benzimidazole (301) ketenimine intermediates have also been utilized in the formation of quinolines (302) (see p. lll) 89.59o j,y intramolecular cyclization, and (303) (see p. 112) by intermolecular cyclization, naphthyridines (304), pyridopyrimidines (305), and isoquinolines (306). The pyran derivative (307) is formed via isomerization of an allene intermediate (see p. 100). 3-Fluoro-2-isopropyl-l,2,4-thiadiazolin-... 

The substituent R determines the reactivity of the isocyanate. Aromatic isocyanates react faster than aliphatic isocyanates, and carbonyl and sulfonyl isocyanates are considerably more reactive than the former. Isocyanate groups attached to oxygen or nitrogen are not stable in their monomeric forms. In cycloaddition reactions, isocyanates react preferentially across their C=N bonds, but additions across the C=0 bonds are also encountered. In this respect, isocyanates resemble ketenes (see Chapter 4, Section 4.1.). Suitable substrates for cycloaddition reactions are carbon multiple bonds (acetylenes, olefins, ketenes, etc.), C=N bond-containing compounds (imines, amidines, ketenimines, azines, carbodiimldes, etc.), C=0 bonds and C=S bond-containing substrates and phosphorus multiple-bond-containing substrates. Cycloaddition reactions of isocyanates across multiple metal bonds are also known. 

Keteniminium salts undergo [2-1-2] cycloaddition reactions with unreactive olefins, such as ethylene, cyclopentene, cyclohexene and styrol to give cyclobutane ammonium salts 23, which are readily hydrolyzed to give cyclobutanones 24. Likewise, reaction with acetylene derivatives affords cyclobutenylidene ammonium salts 25, which are also readily hydrolyzed to give the cyclobutenones 26. Some of the [2-1-2] cycloadducts obtained from keteniminium salts and olefines are shown in Table 4.17. The keteniminium salts are easily synthesized from suitable dimethylamides and phosgene, or trifluoromethanesulfonic acid anhydride. The reaction of the amide with phosgene generates a chloro compound 21, which is in equilibrium with the ketenimine salt 22. 

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