Labile intermediates

It has been tentatively suggested that one mechanism underlies the Willgerodt reaction and the Kindler modification of it. A labile intermediate is first formed which has a carbon—carbon bond in the side chain. The scheme is indicated below it postulates a series of steps involving the addition of ammonia or amine (R = H or alkyl), elimination of water, re addition and eUmination of ammonia or amine until the unsaturation appears at the end of the chain then an irreversible oxidation between sulphur and the nitrogen compound may occur to produce a thioamide. 

Little is known about the existence of alkyl hydrotetraoxides, R—OOOOH. There is some kinetic evidence supporting methyl hydrotetraoxide [23594-84-5] as a very labile intermediate in the reaction of methylperoxy radical, , and hydroperoxy radical, OOH (63). 

Dehydrochlorination and Chlorination. The simultaneous chlorination and dehydrochlorination of 1,1,2,2-tetrachloroethane proceeds via formation of labile intermediate, CI2CCHCI2 (123). Chlorination of tetrachloroethane to hexachloroethane is accelerated by 315—354 nm light (124). 

Dihydropyridazines are known as labile intermediates, and lose nitrogen at -78 °C with a half-life of 30 seconds or less. On the other hand, the corresponding 2-oxides are stable compounds which lose N2O only at 300 °C or above (77JA8505). 

Finally, a quinonoid 6,7,8-trihydropterin structure (49) absorbing at 303 nm plays an important role as a labile intermediate in the tetrahydrobiopterin-dependent enzymatic hydroxylation of phenylalanine <67JBC(242)3934). 

Thus, glycoproteins such as HRP, GO, or most antibody molecules can be activated for conjugation by brief treatment with periodate. Crosslinking with an amine-containing protein takes place under alkaline pH conditions through the formation of Schiff base intermediates. These relatively labile intermediates can be stabilized by reduction to a secondary amine linkage with sodium cyanoborohydride (Figure 20.8). 

Finally, it is known that BENA are thermally unstable (464). a-Nitrosoalkenes generated from BENAs are very labile intermediates having multiple reactivities (503). Hence, special efforts should be made to choose the reaction conditions under which the transformations of BENA follow the desired pathway. 

Perhaps the most controversial suggestion is that spontaneous isomerizations may proceed through a dissociative mechanism, invoking two three-coordinated intermediates (probably T-shaped). The labile intermediates may be cis-like (A) or trans-like (B) in structure. Applied to the monoorganoplatinum(II) complexes, PtL2XR, the scheme is ... 

Where we have reason to suspect the involvement of a particular species as a labile intermediate in the course of a reaction, it may be possible to confirm our suspicions by introducing into the reaction mixture, with malice aforethought, a reactive species which we should expect our postulated intermediate to react with particularly readily. It may then be possible to divert the labile intermediate from the main reaction pathway—to trap it—and to isolate a stable species into which it has been unequivocally incorporated. Thus in the hydrolysis of trichloromethane with strong bases cf. p. 46), the highly electron-deficient dichlorocarbene, CClj, which has been suggested as a labile intermediate (p. 267), was trapped by introducing into the reaction mixture the electron-rich species cis but-2-ene (11), and then isolating the resultant stable cyclopropane derivative (12), whose formation can hardly be accounted for in any other way ... 

The structure of the labile intermediate compound "Y" is uncertain. On the basis of its physical properties and chemical behaviour (which cannot be detailed here), possible formulations that were originally (485) considered included a six-membered bridged carbonyl-structure, as well as seven- or five-membered rings. The Italian group of workers (536) favoured the seven-membered ring-structure (CXVI) for the intermediate "Y", and 5-phenylguanamine structures (CXXI) for the final stable products. 

A recently published new synthesis of dinitrosomethanides salts (DNM = rfinitroso-methanide) starts from formamidinium nitrate (8), which represents an easily accessible source for an NC(H)N unit . Treating a methanolic solution of 8 and hydroxylammonium nitrate (two equivalents) with a methanolic solution of KOBu-t (two equivalents) results in the formation of the labile intermediate Af,Af -dihydroxyformamidinium nitrate (9) (Scheme 8). The reaction of 9 with MOBu-t (two equivalents, M = alkali metal) in the presence of oxygen yields the deep blue alkali DNM salt (7), which can easily be purified by recrystaUization from methanol (yield 60-70%) . 

Hydrolytic Decomposition of Phosphoprotein. The kinetic analysis of phosphate liberation revealed that phosphate is not steadily liberated but that an initial lag period occurs which is followed by a transient burst of phosphate liberation123, 177> 192 This burst has been interpreted as resulting from the accumulation of an acid-labile intermediate arising from an acid-stable precursor. The burst is fol-... 

The exchange reaction may be interpreted by assuming the intervention of a labile intermediate of the type R22A10CR NPh A1R13, which disproportionates very easily. By assuming the labile intermediate complex, the observed decrease in the catalytic activity of... 

Again in the carbohydrate field, Raphael and Roxburgh14 4 described the preparation of a labile intermediate assumed to possess a monomeric epoxy ether structure but too reactive to allow its isolation. Unsuccessful attempts by Huffman and Tarbcll 48 to prepare an epoxide from 2-benzhydrylidenetotrahydrctfuran constitute additional evidence of the instability of bicyolio epoxy ethers. 

Asymmetric aryl isocyanate dimers, in which the C=0 group of one molecule reacts with the C=N group of another, have been postulated as labile intermediates in the formation of carbodiimides (17) upon heating isocyanates. 

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