Hydroxy nitrene

Esters of 2-(2-azidophenyl)ethyl alcohol are photolyzed under a high-pressure mercury lamp to a reactive nitrene intermediate which, following insertion into the alkyl side-chain, undergoes elimination to give the free carboxylic acid (up to 32%) and producing indole. The photochemical release was somewhat improved (65-80%) when 5-azido-4-(hydroxy-methyl)-l-methoxy naphthalene was used (see Scheme 27). 

Disubstituted adamantanes are most frequently obtained from internal cyclization reactions of a bridgehead substituent. Thus, photolysis of the azido-formate 79 gives 80 by nitrene insertion 279). 80 may be readily hydrolyzed to 2-amino-1-adamantanol (81) 75 Subsequent oxidation gives l-hydroxy-2-ada-mantanone (82) from which other derivatives may be prepared 280>2S1). An analogous carbene insertion is initiated by the thermal decomposition of the... 

Treatment of 2-azido-3-hydroxy-l,4-diones 68 with mesyl chloride in the presence of base affords 5-substituted 3-acylisoxazoles 71, probably through vinyl azide 69 and nitrene 70 intermediates (Scheme 42) <2002EJ03055>. In a similar way, thermolysis of 3-azido-2-halopropenones gave 4-haloisoxazoles in high yields <2002S605, CHEC-III (4.03.9.1.3)426>. 

Ethoxycarbonyl- E19b, 1167 (Nitren-N-Verknupfung) Pyridazine 6-Hydroxy-3-(4-hydroxy-phenyl)-4,5-dihydro- E9a, 570 (4-Oxo — 4-Ar — butanoic Acid + N2H4)... 

When stericaUy demanding peptide fragments are coupled in the presence of HOSu, a side reaction that leads to a P-alanine derivative becomes donninant (Scheme In this side reaction, HOSu is activated by DCC and attacked by a second equivalent of HOSu. The nitrene is probably not a real intermediate as it is rearranged conconoitantly to its formation to an isocyanate moiety, which is trapped by a third equivalent of HOSu.t By replacing the HOSu additive with A-hydroxy-5-norbornene-2,3-dicarboximide (18)t or other dinucleo-phUes, this side reaction is efficiently bypassed. 

Heating of aryl azides in acetic anhydride resulted in insertion of the nitrene into the anhydride molecule. The phenylhydroxylamine derivative (38) formed readily rearranged to A, 0-diacetyl-o-hydroxy-aniline (39) . ... 

Time-resolved IR studies of the photolysis of 2-(methoxycarbonyl)phenyl azide in solution at room temperature showed that the didehydroazepine (47) was the sole intermediate, at least on the ps time-scale. This contrasts with photolysis of the same compound in matrices at 10 K, where the nitrene, iminoketene (48) and azetinone (49) were observed as well as (47). Matrix photolysis of 2-hydroxy-phenyl azide gives at least three major products, all of which are photo-interconvertible. Two of these are identified as the EtZ mixture of iminodi-enones (50), while the third is the ring-opened compound (51), existing as a mixture of conformers. 2-Aminophenyl azide behaves in a similar manner. Rapid H-transfer from the ortho hydroxy or amino group to the nitrene centre in each case appears to suppress ring expansion completely. 

A final example of an allylic C-H animation process involves a mechanism that does not fall into the classification of either a Cu-bound nitrene or N-centered radical-type process. In this case, A-Boc-hydroxylamine serves as the nitrogen source and is converted to the acylnitroso species via a disproportionation mechanism facilitated by P(OEt)3 and CuBr [50]. Such compounds will react with olefin substrates through a thermal ene-like rearrangement to give A-Boc-A-hydroxy allylic amines. The Cu catalyst is not believed to play a specific role in the actual C-H oxidation event. 

The mechanistic dichotomy for conversion of ACC to ethylene seems clear from the large body of work presented above. Formation of N-heteroatom derivatives leads to the nitrene or nitrenium ion and results in a concerted mechanism, while electron transfer/free radical oxidants lead to a radical cation and result in a non concerted mechanism. Despite the significant evidence in favor of the radical pathway, reference to N-hydroxylation and nitrenium ion formation as a key step in ethylene biosynthesis has persisted, particularly in the plant physiology literature (2, 43-46). The sequence similarity of the EFE and several hydroxylase enzymes (vide supra) has only added fuel to this fire. However, consideration of the mechanisms for known hydroxylation processes makes the intermediacy of N-hydroxy-ACC very unlikely. 

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