Ketenimine adducts

Hydrostannation of highly polarized a,)8-unsaturated nitriles leads to the formation of N-stannylketenimines 31, 33) in what is considered to be a polar 1,4-addition. Thus, polar mechanisms are ascribed to reactions leading to either 1-cyanoalkyl or ketenimine adducts. 

The oxime 299 is silylated in the presence of catalytic amounts of TMSOTf 20 to 300, which affords, via the Beckmann fragmentation intermediate 301 and alkylation with allyltrimethylsilane 82, 66% of the seco nitrile 302 [101, 102] (Scheme 4.39). Tris(trimethylsilyl) ketenimine 303 reacts with aldehydes such as benzaldehyde in the presence of Bp3-OEt2, via the aldol adduct 304, to give the unsaturated nitriles 305, in 99% yield, and HMDSO 7 [103]. 

Experimental observations support these views. Photolysis of 1-naphthylazide in the presence of diethylamine and tetramethylethylenediamine (TMEDA) yields azirine, but no ketenimine-derived adducts at ambient temperature. " In the presence of diethylamine but in the absence of TMEDA, good yields of 1-amino-naphthalene and 1,1 -azo-naphthalene, products attributable to the triplet nitrene are observed. Good yields of 46 are also achieved when the photolysis of 1-naphthylazide and diethylamine is performed at —60 °C in the absence of TMEDA. Presumably, lowering the temperature extends the hfetime of azirine 43 by reducing its rate of reversion to singlet 1-naphthylnitrene more than it retards the rate of its reaction with diethylamine. 

Fig. 7. Dependency of the reciprocal of the quantum yield for adduct formation on ketenimine concentration in the fluorenone-dimethyl-iV-(cyclohexyl)ketenimine reaction. Slope = 0.006M intercept = 1.08. (From Singer and Davis220 with permission of the American Chemical Society.)...

More recent experiments219 with added di-f-butylnitroxide, a quencher of both singlets and triplets, and detailed kinetic analysis of the results led to the conclusion that the complex proceeds to adduct with about 60% efficiency but that complex formation still could not account for the sharp rise in Figure 7. Deactivation of the complex by ketenimine would explain this curvature, but at present this idea is only speculative.220... 

Both carbodiimides and ketenimines are reported to add to N-sulfinylsulf amides to give 3-imino- and 3-alkylidene-l,2,4-thiadiazetidine 1-oxides, (Scheme 135) (73BCJ2156, 75BCJ3259). The adducts were identified from both IR and NMR spectra and from hydrolysis products. 

The nucleophilic C=C bond of ketenimines reacts with isocyanates to yield a single [2 + 2] adduct (equation 72). 

The cycloaddition reactions of ketenimines to thiocarbonyl groups to give both 1,2-(iminothietanes) and 1,4-adducts have been reviewed. The frontier molecular orbitals of the reactants are important for the addition that is classed as a ( 2 + 2a) process. Steric factors play a role and the rate of 1,2-addition to give 2-iminothietanes depends more on the substituents at the terminal carbon atom of the ketenimine than 1,4-addition to give a six-membered ring. The rate of 1,2-... 

Staudinger and Pfenninger long ago suggested that addition of sulfur dioxide to diphenylketene gives an intermediate, 3,3-diphenyl-l,2-oxathietane-4-one 2-oxide, that decomposes to diphenylsulfene and carbon dioxide, the former yielding tetra-phenylethylene. Similar intermediates were proposed for the addition of sulfur dioxide to pentamethyleneketene, ketenimines, " ethylene, 1,1-difluoro-ethylenes, and an aluminum trichloride derivative of the 1,2,3,4-tetramethyl-cyclobutene cation. An intermediate adduct 494 actually has been isolated at... 

Reactions of triphenyl-2,2-bis(trifluoromethyI)vinylidenephosphorane, synthesized from a cyclic ylide-ketone adduct. 7 Cycloaddition reactions of triphenylphosphoranylideneketenc. 8 Synthesis of triphenylphosphoranylidene-ketenimines. 9... 

The formation of 1,3-oxathiolane with chloral and 1,3-dithio-lanes with cycloaliphatic thioketones occurs regioselectively to yield the sterically less hindered products. On the other hand, aromatic thioketones, e.g., thiobenzophenone or 9/ fluorene-9-thione, intercept 8 to give comparable amounts of both regio-isomeric adducts. Usually, stereoisomeric dipolarophiles such as fumaronitrile and maleonitrile as well as dimethyl fumarate and maleate form 18 in a stereoselective manner. However, in the case of extremely electron-poor dipolarophiles, e.g., dimethyl 1,2-dicyanofumarate or ( )-l,2-bis(trifluoromethyl)ethylene-l,2-dicarbonitrile, non-stereospecific formations of the corresponding tetrahydrothiophenes are described. i- s This result is interpreted in terms of a stepwise reaction mechanism with a zwitterion as the key intermediate. Alternatively, this intermediate can cyclize to form seven-membered ketenimines of type 20. With R = Cp3, this product can be isolated in a crystalline form, whereas in the case of R = CN, stable lactam 21 is obtained only after addition of water (eq 10). 

The use of ketene equivalents in 4- -2-cycloaddition reactions for organic synthesis has been reviewed. " a-Carbonyl ketenes behave as dienes in Diels-Alder reactions with 4-aryl-2-methyl-2,3-dihydro-l,5-benzothiadiazepines to yield regiospecific cycloadducts. The reactions of diphenylketene with cyclic ( -cw)-l,3-dienes such as cyclopentadiene and cyclohexadiene initially yield the Diels-Alder adducts, which are converted into the Staudinger cyclobutanones by [3,3]-sigmatropic rearrangementsl-Benzyl-l,3-diazabuta-l,3-dienes react with ketenes to produce the 4 - - 2-cycloadducts, substituted l-benzyl-4-(benzylidenimino)-4-phenylazetidin-2-ones, which rearrange into the more stable 5,6-dihydro-3//-pyrimidin-4-ones. The formal 4 - - 2-cycloaddition reaction of ketenimines has been used to synthesize benzoimidazo[l,2-Z ]isoquinolines. ... 

Four-membered Rings.—Open-chain sulphinate esters are readily prepared but their cyclic analogues are less easily come by. [2 + 2]Cycloadditions involving SO2 are limited to a few specialized examples, and it has been suggested that it is this mode of reaction that leads to the initially formed adduct (274) between SO2 and ketenimines. These compounds are unstable and rearrange to thiazetidine 5-oxides (275) in 90% yield. A versatile approach has been... 

In the [3+2] cycloaddition reaction with ketenimines two isomeric adducts 12 and 13 are formed by reaction across either the C=C or the C=N bond of the ketenimines . 

From Af-phenyl substituted ketenimines 15 and ynamines the linear 1 1 adducts 16 are formed, which undergo an intramolecular cyclization reaction to give 4-aminoquinoline derivatives 17. 

Also, the highly strained seven-membered ring ketenimine 29 adds the vinyl ether at room temperature to give the bicyclic adduct 30 in 81 % yield (two diastereoisomers, ratio 57 43)2. ... 

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