Ketenimine coupling

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... 

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... 

It was reported that o-iodoanilines were coupled with isocyanates 31, carbodiimides 32, or ketenimines 33 in the presence of 300 psi of carbon monoxide to afford 2,4-(l//,3//)-quinazolinediones 34, 2-amino-4(3/f) quinazolinones 35, and 2-alkyl-4(3/f)-quinazolinones 36 in moderate to excellent yields, respectively. The nature of the substrates, including the electrophilicity of the carbon center of the carbodiimide and the stability of the ketenimine, influenced the yields of the reaction. 

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). 

Valero C, Grehl M, Wingbermuehle D et al (1994) Evidence of ketenimine formation during the multiple CC coupling of isocyanides by stabilized Group 4 metallacyclobutanes. Organometallics 13 415 17... 

After the intermolecular palladium-catalyzed arylation of enolates had been discovered by the research groups of Buchwald, Hartwig, and Miura [63], it was an obvious idea to explore a stereoselective variant that was based on the reliable Evans auxiliaries, while catalytic, enantioselective protocols (cf. Section 5.2) were developed at the same time. However, only relatively moderate diastereoselectivity in the formation of arylation products 133 was achieved when the silyl ketenimine (S)-132 derived from Af-propionyl oxazolidinone was coupled with aryl bromides under palladium catalysis, as disclosed by Hartwig and Liu [64]. The additive zinc(ll) f-butoxide permitted to lower the temperature to 25 °C so that the diastereoselectivity could be enhanced (Scheme 4.26). The authors noticed that Ley s chiral dioxanone 134 proves itself as an auxiliary that induced excellent diastereoselectivity and led to the formation of arylated compounds 135 in very high diastereomeric ratios. The chiral auxiliary group was readily removed by treatment with chlorotrimethylsUane and methanol, so... 

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