Ketenimines reactions with enolates

Because of their predictable behavior and reactivity, thioacyl isocyanates comprise the bulk of this work, and extensive studies of their [4 -I- 2] reactions with olefins, enamines, enol ethers, thioacyl isocyanates, imines, carbodiimides, isocyanates, azirines, /3-enaminoke-tones, dianils, azines, hydrazones, imidazoline-4,5-diones, aryl cyanates, disubstituted cyanamides, aldehydes, ketones, ketenes, alkyl or aryl iminodithiocarbonates, and the carbon-carbon double bond of ketenimines have been detailed. In an extensive comparative study of the [4 + 2] cycloaddition reactions of thioacyl isocyanates, the heterocu-mulenes bearing strong electron-withdrawing substituents were found to be more stable and less prone to participate in cycloaddition reactions. Representative examples are summarized in Scheme 9-IV. 

In a manifestation of the reaction shown above, quinoline rings have also been formed by the cycloaddition of /V-arylketenimines 543 with 3,4-dihydro-2//-pyran 455 under high-pressure conditions (Scheme 100) <2001H(55)1971>. The reaction is proposed to proceed via the initial formation of 544 by attack of the enol ether on the protonated ketenimine subsequent electrophilic aromatic substitution gives 545. Protonation of the enamine to give 546 is followed by elimination to produce 547. Protection of the alcohol with 455 gives 548. 

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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