Diazirine ring

According to measurements on steroidal diazirines (49) there is also a high field shift of equatorial methyl groups, although their protons are more distant by one position from the diazirine ring. Reported values lie between 0.1 and 0.2 p.p.m., about 0.8 p.p.m. higher than in alkanes (65JA2665). 

It was not their reactivity but their chemical inertness that was the true surprise when diazirines were discovered in 1960. Thus they are in marked contrast to the known linear diazo compounds which are characterized by the multiplicity of their reactions. For example, cycloadditions were never observed with the diazirines. Especially surprising is the inertness of diazirines towards electrophiles. Strong oxidants used in their synthesis like dichromate, bromine, chlorine or hypochlorite are without action on diazirines. Diazirine formation may even proceed by oxidative dealkylation of a diaziridine nitrogen in (186) without destruction of the diazirine ring (75ZOR2221). The diazirine ring is inert towards ozone simple diazirines are decomposed only by more than 80% sulfuric acid (B-67MI50800). 

A direct attack on the diazirine ring was observed only by use of almost concentrated sulfuric acid or strong Lewis acids, e.g. aluminum chloride in the case of (208). Some hydrazine was found together with cyclohexanone and cyclohexanol after decomposition of the spirodiazirine (189) with 80% sulfuric acid (B-67MI50800). 

Increased sensitivity towards acid is observed when protonation occurs on a functional group outside the diazirine ring, giving rise to electron dilution at the carbon atom adjacent to the diazirine carbon. The products isolated are in accord with the proposal (79AHC(24)63) that cation formation at this carbon atom leads to nitrogen extrusion, probably with formation of a vinyl cation. Thus protonated hydroxydiazirine (209) yields acetone, and methylvinyldiazirine (199) on treatment with acids yields butanone (67CB2093). 

The stabilization reactions of alkylcarbenes were used preparatively in some cases. The diazirine derived from adamantanone gave the dehydroadamantane (2l7) thermally in 96% yield 73ZOR430). Alkene formation was reported for a steroid with its C-3 atom part of a diazirine ring. At 140 °C a A-2-unsaturated steroid was formed 65JA2665). 

Diazirine ring formation from oxime 0-tosylates or 0-sulfonates ° and NH3 or primary amines has been described. Recently, it was found that this reaction was catalyzed... 

Fig. 6.22 The three-membered C—N—N ring in a diazirine molecule (a) Electron density through the plane of the diazirine ring. Contours are at 5 x lO e pm J. (b) Differential electron density map through the plane of the diazirine ring showing increases (solid lines) and decreases (broken lines) of electron density upon bond formation. Contours are at 4 X 10 " e pm"3. Note the build-up of electron density outside the C—N—N triangle.

C—N bond order of the diazirine. One of the two C—N bonds is weakened (or cleaved) in the transition state, probably in a similar way to that indicated in the mechanism of reaction (5-19). In contrast to the photolysis, however, in the thermal reaction the second C-N bond becomes stabilized after dissociation of the first. That stabilization is due to conjugation of the diazo group with the carbonyl group in addition to the release of the strain energy of the diazirine ring, which was calculated by Polta and Thiel (1986) to be ca. 83 kJ mol Therefore, the yield of 3-diazoindolin-2-one (5.71a) is claimed to be practically quantitative, i.e., dediazoniation is negligible. 

Pyridine s. under N-Bromosuccinimide Silver nitrate sodium hydroxide Diazirine ring from diaziridine ring... 

---