Ketenimines, acyl

The (aziridin-l-ylimino)phosphoranes (57) react with ketens to give nitrile derivatives, presumably from an intermediate ketenimine by breaking the N—N bond and migration of the aziridinyl group.60 They also react with acyl halides and... 

Ketenes are oxo compounds with cumulated carbonyl and carbon—carbon double bonds of the general structure R R C—C—O, where R1 and R2 may be any combination of hydrogen, alkyl, aryl, acyl, halogen, and many other functional groups. Ketenes with R1 = H,R2 H are sometimes called aldoketenes, those with R R- H, ketoketenes. The S— and N— analogues of ketenes are called thioketenes (R1R2C—C—S) and ketenimines (R1R C—C—NR), respectively. 

C-H bond dissociation energies, 1, 298 C-N triple bond additions, 10, 427 C02, ketene, ketenimine complexes, 5, 84 CO and CNR reductive coupling, 5, 66 cyclopentadienyl alkyl and aryls, 5, 66 cyclopentadienyl carbonyl complexes, 5, 64 cyclopentadienyl hydrides, 5, 69 cyclopentadienyl isocyanide complexes, 5, 65 rf-acyl and rj2-iminoacyl complexes, 5, 82 fj3-complexes, 5, 87... 

In addition to these effects, electronic effects on the azacycloheptatetraene intermediates may hold the key to an understanding of the selectivities. Scheme 44 shows all the possible intermediates, which for simplicity are supposed to be in equilibrium. Ortho substituents (R = alkyl or aryl) would be expected to stabilize the ketenimines 222 more than 225, thus leading to the observed dominant regiochemistry. On the other hand, when R = o-acyl, 222 would be destabilized electronically, presumably more so than 225. Electro-negatively substituted ketenimines appear to be generally unstable.29,271,272 If thermodynamic equilibrium prevails, 225 would then dominate, but 222 would be more reactive toward nucleophiles. The products derived from both were obtained.257... 

Acyl sulfenes, like all sulfenes, prefer to participate as 2it components of [2 + 2] or [4 + 2] cycloadditions (Chapter 5). Nonetheless, a range of [4 + 2] cycloaddition reactions of acyl sulfenes have been described46,47 (Scheme 8-XII), including their 4n- participation in dimerization reactions46-48 and reactions with imines,49 carbodiimides,50 ketenimines,51 1-azirines,52 vinyl ethers,53 and ketenes.47 The reactions often provide mixtures of [4 + 2] and [2 + 2] cycloadducts, and the observed course of the reaction usually depends on the reaction conditions. Consequently, many of the observed [4 + 2] cycloadditions of acyl sulfenes proceed by a stepwise, polar addition-cyclization reaction. 

Acyl isothiocyanates have been shown to participate in [4 + 2] cycloadditions with enamines,98 imines," ketenimines,100 and hydrazones.101 Similar reports have detailed the [4 + 2] cycloadditions of thioacyl isothiocyanates with the carbon-carbon double bond of ketenes, ketenimines, and enamines as well as the carbon-nitrogen double bond of imines, carbodii-mides, and isocyanates.102... 

In contrast, C-3 deprotonation of quatemized isoxazolium salts causes fission of the 0-N bond with formation of acyl ketenimines ... 

The photochemically induced isomerization of isoxazole (279) to oxazole (281) is known to proceed via the intermediate keto-azirine (280). An M.O. study of the isomerization reactions of the azirine that are involved has been reported. Irradiation of (280) at >300 nm gives n tt excitation of the CO chromophore to 5i, intersystem crossing to Ti, and then C—N bond rupture, to form (279). Irradiation at 254 nm, however, gives n - n excitation of the C=N chromophore to S2, intersystem crossing to T, and then C—C bond rupture, to form (281). Such an isomerization reaction resulted in the photochemical conversion of isoxazolophane (282) into (283) (40%). Irradiation of (283) at 254 nm produced a quantitative yield of the expected oxazolophane. A new rearrangement was found, however, on irradiation of (283) at >300 nm, when the acyl-ketenimine (284) was formed, which could be converted into (285) by acid hydrolysis. 

Af-Acyl ketenimines react with carbodiimides via a [4-1-2] cycloaddition reaction to give the expected [4-1-2] cycloadducts... 

In contrast, the A -silylenamine (186) reacts in conventional fashion with acyl halides R COCl in the presence of KF to give (187), and in the same vein tris(trimethylsilyl)ketenimine when treated sequentially with an aldehyde and alkali gives (Z)-alk-2-enenitriles in good jdeld and high isomeric purity. A-Silylimines derived from non-enolizable aryl aldehydes and ketones give secondary or tertiary carbinamines respectively on reaction with organolithium compounds. ... 

Theoretical studies [59] indicate that the lowest unoccupied molecular orbital (LUMO) of such molecules is localized primarily on the acyl carbon atom, similarly to the situation in Fischer complexes. An example of such a compound is shown in Fig. 23.9, where the shortness of the Th-O distance (2.37(2) A) relative to Th-Cg (2.44(2) A) is unprecedented for a dihapto-acyl. A second example of an actinide dihapto-acyl is shown in Fig. 23.10. It should be noted that the orientation of the C-O vector is in the opposite direction from that in Fig. 23.9. The relative magnitudes of the Th-O and Th-C distances appear to reflect both the orientation of the C-O vector and conjugation with the arene n system. The intricate chemistry exhibited by actinide dihapto-acyls is summarized in Fig. 23.11. Important reactions include C-C coupling to form monomeric (10) or dimeric enediolates [57,58,60,61], isomerization to yield enolates (//) [60,62], catalytic hydrogenation to yield alkoxides (/2) [63], CO tetramerization to form dionediolates (13) [62, 64], coupling with ketenes 14), coupling with CO and phosphines 15) [62, 64], and addition to isocyanides to yield ketenimines [62, 64] 16). 

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