Ketenimines dimerization reactions

The synthesis of A -r-butylketenimines can be accomplished readily at -80 °C. On warming to - 40 °C the ketenimines dimerized to 2-iminoazetidines (99) it is suggested that the reaction is initiated by protonation of the ketenimine. 

Acyl sulfenes, like all sulfenes, prefer to participate as 2ir components of [2 -I- 2] or [4 + 2] cycloadditions (Chapter 5). Nonetheless, a range of [4 -I- 2] cycloaddition reactions of acyl sulfenes have been described - (Scheme 8-XII), including their 47t participation in dimerization reactions and reactions with imines, carbodiimides, ketenimines, 1-azirines, vinyl ethers,and ketenes. 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. 

In the reaction of perfluoroisobutylene with A-phenylbis(trifluoromethyl)ketenimine, a mixture of the unsymmetrical cyclodimer of the ketenimine and the expected [2+2] cycloadduct are obtained. Unsymmetrical ketenimine dimers 1 are also obtained in a slow thermal reaction or in an AICI3-catalyzed reaction at -20 °C (50 % yield) . 

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... 

Ketenes, which are even more reactive than isocyanates, afford ketenimines at or below room temperature [62CRV247 84JOC2688 89JCS(P1)2140]. At elevated temperatures, dimerization or polymerization occurs (21HCA887). Although N-aryl- and A-vinyliminophosphoranes react smoothly with ketenes, strong acceptor substituents on the nitrogen hamper the reaction thus V-acyliminophosphoranes do not react with ketenes. Vinylketenimines such as 2-aza-l,3-dienes prepared in this way from... 

The metal-free eyclobutane-1,2-dioxime can be generated by oxidative displacement. It is interesting to note that, unlike ketene dimerization, head-to-head dimerization takes place here. The chromium ketenimine complex 20 is prepared by reaction of the Fischer-type chromium carbene complex with alkyl isocyanides.60 A cyclobutane-1,2,3,4-tetraimine 24 has been reported from the reaction of the ketenimine phosphonium ylide 22.61 Bisimine 23 has been proposed as the intermediate in this transformation. 

The addition of primary amines to fluoroolefins under anhydrous conditions yields imines The hexafluoropropene dimer, perfluoro-2-methyl-2-pcntcne, and ten butylamine react to yield a mixture of two compounds tn a 9 4 ratio 4] (equation 3) rather than just the major ketenimine-imine, as previously reported [5] It is claimed that this result is possible by means of isomerization to the terminally unsaturated difluoromethylene isomer prior to nucleophilic attack Secondary amines add to fluoroolefins under anhydrous conditions to give fluonnated tertiary amines in good yields II the fluoroolefin is added to the amine without cooling the reaction mixture, or if an excess of the secondary amine is used, there is a tendency toward dehvdrofluonnation of the tertiary amine The products... 

Ketenimines, formed as intermediates in the reactions of aromatic amines with a dimer of hexafluoropropene (34), cyclize in an interesting process that yields perfluoroalkyl derivatives of various heterocyclic systems (Eq. 33).119... 

The synthesis starts from 2,4,4-trimethyl-4,5-dihydrooxazole, a compound with marked C—H acidity in the exocyclic methyl group. Triple lithiation followed by derivatiza-tion with chlorotrimethylsilane affords a triply silylated, open-chain ketenimine which is reacted with [AuCl(PPh3)] in the presence of caesium fluoride to give a dimeric, octanu-clear gold(I) cluster. The reaction proceeds beyond the stage of a triauriomethyl oxazolinyl derivative which, in fact, can only be observed as a side product. 

Fluoroalkylquinolines can also be obtained from iV-arylfluoroalkylketenimines generated by treating hexafluoropropene dimer with primary arylamines indeed, the intermediate ketenimines can be isolated only if the aryl group possesses no available o-hydrogens (see Scheme 20). The extension of this reaction to heterocyclic synthesis is discussed in detail elsewhere (see p. 464). 

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). 

Dimerization.—The lithio-salts of thiazoles[e.g. (58)], though stable at low temperatures, rearrange and dimerize to (60) at room temperature, by way of ketenimines (59), in 90% yield. The dimers (60) are reconverted into the starting material (57) above 150 °C. The reaction is general to heteroaryl systems of type (61), and has also been performed using 2-methyl-5-phenyl-l,3,4-thiadiazole. ... 

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