Ketenimines

Ketenes and related compounds have been reviewed extensively (1 9). For the synthesis and synthetic uses of conjugated ketenes see Reference 10. Ketenes with three or more cumulated double bonds have been prepared (11,12). The best known is carbon suboxide [504-64-3] 3 2 preparative uses and has been reviewed (13—16). Thioketenes (17,18), ketenimines (19—21), and their dimers show interesting reactivity, but they have not achieved iadustrial importance to date. 

Physical Properties. Ketenes range ia their properties from colorless gases such as keteae and methylketene [6004-44-0] to deep colored hquids such as diphenylketene [525-06-4] and carbon subsulftde [627-34-9]. Table 1 lists the physical state mp, and bp for certain ketenes, thioketenes, and ketenimines. 

Table 1. Properties of Some Ketenes, Thioketenes, and Ketenimines...

StericaHy hindered or very electrophilic substituted ketenes, such as diphenylketene, di-Z rZ-butylketene [19824-34-17, and bis(trifluoromethyl)ketene, are quite stable as monomers. Ketenimines tend to polymerize. The dimerization of thioketenes results in 1,3-dithiacyclobutanones (6) (45), a type of dimer not observed with ketenes. 

Ketenimines are usually prepared from carboxyHc acid derivatives such as amides and imino chlorides via elimination and from nitriles via alkylation with alkyl haHdes under strong basic conditions (21,64). 

With malonic acid as the C3 fragment in the presence of acetic anhydride, 6-substituted 5,7-dihydroxy compounds are obtained (64JOC219,61M1184), whilst the 6-IV-lithio derivative of a uracil (251) reacted with a ketenimine to give the 7-t-butylamino compound (252) (77JOC221). 

Isoxazoles largely undergo photochemical isomerization to azirines, which sometimes undergo a further thermal or photochemical reaction. 3,4,5-Triarylisoxazole (529) formed the 2,3-diphenyl-3-benzoylazirine (530) which underwent further reaction to the oxazole (531) 72JA1199). A small amount of the corresponding benzoyl ketenimine was also obtained. 

The cycloaddition of nitrones with ketenes produced 5-isoxazolidinones as well as oxazolones, as shown in Scheme 162 (78H(9)457, 79JOC2961). In a similar fashion, nitrones also react with ketenimines to generate the 5-isoxazolidinone imines (75JHC175, 68JHC881). 

The 27T-electrons of the carbon-nitrogen double bond of 1-azirines can participate in thermal symmetry-allowed [4 + 2] cycloadditions with a variety of substrates such as cyclo-pentadienones, isobenzofurans, triazines and tetrazines 71AHC(13)45). Cycloadditions also occur with heterocumulenes such as ketenes, ketenimines, isocyanates and carbon disulfide. It is also possible for the 27r-electrons of 1-azirines to participate in ene reactions 73HCA1351). 

The reactions of 1-azirines with ketenes and ketenimines represent non-concerted additions and are formally different from the additions to 47r-systems of dienes and 1,3-dipolar compounds (73JOC3466, 71CB2786). 

A variety of 1-azirines are available (40-90%) from the thermally induced extrusion (>100 °C) of triphenylphosphine oxide from oxazaphospholines (388) (or their acyclic betaine equivalents), which are accessible through 1,3-dipolar cycloaddition of nitrile oxides (389) to alkylidenephosphoranes (390) (66AG(E)1039). Frequently, the isomeric ketenimines (391) are isolated as by-products. The presence of electron withdrawing functionality in either or both of the addition components can influence the course of the reaction. For example, addition of benzonitrile oxide to the phosphorane ester (390 = C02Et) at... 

Other potential synthetic routes to these unsaturated aziridine derivatives which involve the addition of nitrenes to allenes <75JOC224), carbenes to imines with subsequent hydrolysis <67JA362), and of carbenoid species to ketenimines <76TL1317,79TL559) have been investigated but are collectively of little or no preparative value. 

The use of ynamines, ketenimines and mercuric acetate as electrophiles in DMSO oxidations has also been reported but, as yet, appear to offer no advantages over the reagents described above. 

The addition of a primary amine to methyl perfluoromethacrylate leads to a ketenimine derivative [97] (equation 83). 

The byproducts of decomposition of certain dialkyldiazcncs can be a concern. Consider the case of AIBN decomposition (Scheme 3.13). The major byproduct is the ketenimine (lO).61 100"102 This compound is itself thermally labile and reverts to cyanoisopropyl radicals at a rate constant similar lo that for AIBN thermolysis.59,60 102 This complicates any analysis of the kinetics of initiation/2,60... 

Some of the complications associated with the use of AIBN may be avoided by use of alternative azo-initiators. Azobisfmelhyl isobutyrale) (AIBMe) has a decomposition rate only slightly less than AIBN and has been promoted for use in laboratory studies of polymerization85 because the kinetics and mechanism of its decomposition kinetics are not complicated by ketenimine formation. 

Even though AIBN has a low transfer constant, the ketenimine formed by combination of cyanoisopropyl radicals (Scheme 3.13) is anticipated to be more susceptible to induced decomposition (Scheme 3.22).1Cb... 

Cyanoisopropyl radicals (15) undergo unsymmetrical C-N coupling in preference to C-C coupling.11 1 The preferential formation of the ketenimine is a reflection of the importance of polar and steric influences.1"1 However, the ketenimine is itself thermally unstable and a source of 15, thus the predominant isolated product is often from C-C coupling. 

Preferential C-N coupling is also observed for oligomeric radicals (Scheme 5.9).117 A ketenimine (21) is the major product from the reaction of the "dimeric" MAN radical 18 with cyanoisopropyl radicals (15). Only one of the two possible ketcnimincs was observed a result which is attributed to the thermal lability of ketenimine 19. If this explanation is correct then, although C-N coupling may... 

The reaction of [p-MeC6H4C(NSiMe3)2]2Ta( = CH2)CH3 with 2,6-dimethyl-phenyl isocyanide afforded an f/ -ketenimine complex (Scheme 118). Carbon-sulfur cleavage reactions produced tantalum thioformaldehyde and tantalum sulfido complexes. ... 

DCC, and l-methyl-2-chloropyridinium iodide (Mulcaiyama s reagent). Analogously, amides can be dehydrated with P2O5, pyridine, and AI2O3 to give ketenimines ... 

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]. 

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