Esters, nitrene derivatives

A nitrene (derived from a nitro group and triethyl phosphite) reacts with an adjacent ethoxycarbonylvinyl group to form a quinoline in moderate yield. Replacing the ester by a keto carbonyl group results in much lower yields. 

Preliminary efforts to examine the mechanism of C-H amination proved inconclusive with respect to the intermediacy of carbamoyl iminoiodinane 12. Control experiments in which carbamate 11 and PhI(OAc)2 were heated in CD2CI2 at 40°C with and without MgO gave no indication of a reaction between substrate and oxidant by NMR. In Hne with these observations, synthesis of a carbamate-derived iodinane has remained elusive. The inability to prepare iminoiodinane reagents from carbamate esters precluded their evaluation in catalytic nitrene transfer chemistry. By employing the PhI(OAc)2/MgO conditions, however, 1° carbamates can now serve as effective N-atom sources. The synthetic scope of metal-catalyzed C-H amination processes is thus expanded considerably as a result of this invention. Details of the reaction mechanism for this rhodium-mediated intramolecular oxidation are presented in Section 17.8. 

The para-nitro ester 71d generated only 4-nitroaniline (70%) and 4,4 -dinitroazoxybenzene (10%) when it underwent decomposition (Scheme 29). These products could have been derived from either a triplet nitrene or a triplet nitrenium ion precursor. Homolysis of the N—O bond to generate radical intermediates was ruled out because of the nearly quantitative yield of pivalic acid derived from 71d. The pivaloxy radical would have undergone rapid decarboxylation to generate CO2 and the rert-butyl radical under these conditions. Since no rearrangement product was observed, it was tentatively concluded that this ester underwent direct decomposition to 4-nitrophenyl-nitrene without the intermediacy of a nitrenium ion. ... 

The comparison of thiophene with thioethers on the one hand and with enol thioethers on the other, in regard to its behaviour towards conventional electrophiles, has been made in Section 3.02.2.3. Attack on carbon is the predominant mode of reaction (Section 3.14.2.4) reaction at sulfur is relatively rare (Section 3.14.2.5). Carbenes are known to act as electrophiles attack at both carbon and sulfur of thiophene has been reported. The carbene generated from diazomalonic ester by rhodium(II) catalysis attacks the sulfur atom of thiophene, resulting in an ylide. It has also been shown that the carbenoid species derived by thermolysis of such an ylide functions as an electrophile, attacking the a-carbon of a second molecule of thiophene (Section 3.14.2.9). Singlet nitrene is electrophilic. However, in contrast to carbenes, it invariably attacks only the carbon atom (Section 3.14.2.9). 

Anodic oxidation of N, N.-dimethylaniline in methanol yields the -methoxylated derivative (72), which reacts with substituted alkenes in the presence of Lewis acid to give a wide range of tetrahydroquinolines (e.g. 73).94- The adduct (75) from N-methylaniline and allene-1,3-dicarboxylic acid dimethyl ester (7 ) can be converted into the 4-quinolone (76) on treatment with polyphosphoric acid. 95 The use of nitrobenzenes as quinoline precursors is exemplified by the reaction sequence depicted in Scheme 17. The transition-metal-catalysed transformation is thought to involve the reduction of the nitrobenzene to the corresponding aniline via a nitrene intermediate.96... 

Appropriate oxidations of ortho-aminoaiyl ketones or esters produce 2,1-benzisoxazoles these ring closures may involve the intermediate formation of a nitrene, or a conventional sequence like that shown for the formation of the 3-styryl derivative below. 

Conversion of acid derivatives into amines with the loss of the carbonyl group can be done in various ways. In chapter 36 we recommended the Curtius and the Hofmann. The Hofmann degradation is the easier if we start with an ester, converting into the amide with ammonia and then treating with bromine in basic solution. The N-bromo amide undergoes a-elimination to a nitrene that rearranges to an isocyanate. 

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