Reactivity of Diazirines

1 Reactions with conservation of the three-membered ring 

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). 

The chemical inertness of the three-membered ring permitted many conversions of functional groups in diazirines. Esterifications, cleavage of esters and acetals, synthesis of acid chlorides, oxidation of hydroxy groups to carboxyl groups as well as Hofmann alkenation all left the three-membered ring intact (79AHC(24)63). 

Reductions of the N —N double bond yield diaziridines and were carried out for proof of structure, using for example sodium amalgam or catalytic hydrogenation. They are unimportant beyond that, because most diazirine syntheses start with diaziridines. 

Diazirines (3) smoothly add Grignard compounds to the N—N double bond, giving 1-alkyldiaziridines. Reported yields are between 60 and 95% without optimization (B-67MI50800). The reaction is easily carried out on a preparative scale without isolation of the hazardous diazirines and may serve as an easy access to alkylhydrazines. The reaction was also used routinely to detect diazirines in mixtures. The diaziridines formed are easily detected by their reaction with iodide. Phenyllithium or ethylzinc iodide also add to (3) with diaziridine formation. 

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As in prior editions, the reactivity of diazirines is categorized as proceeding with or without ring cleavage discussion here follows this respective order. 

The reactivity of cage-annulated carbene (53) was found to depend markedly on the method of its formation." Pyrolysis of the corresponding tosylhydrazone sodium salt gave products of intramolecular CH insertion or H-abstraction. Photolysis of a diazirine precursor gave only azine products by reaction of the carbene with the precursor or diazo compound. Treatment of the m-dibromoalkane with BuLi gave products due to intermolecular insertion of the carbene into CH bonds of the solvent. 

The reactivity of palladium and copper cluster models toward diazirine has been compared using the LCGTO-MCP-LSD method <1996SUS11> such calculations were performed to give an insight into the differential bond scission experimentally observed in the thermal decomposition of diazirine on palladium and copper surfaces. Stronger chemisorption was evident with palladium and furthermore, partial diazirine lowest-unoccupied molecular orbital (LUMO) occupation only occurred for the copper cluster model systems. The calculated N-N bond order was significantly decreased in the copper complexes of excited state diazirines, whereas palladium complexes remained unperturbed. 

The results are in accord with the proposed supramolecular structures of diazirine 45 and its hosts (vide supra). No azine, di(3-nortricyclanylidene)hydrazine (53), was formed upon photolysis of the dimeric 45 (oc-CyD)2 complex because of a complete encapsulation of the reactive guest. Bimolecular reaction of the guests within the (45 /J-CyD)2 dimer should, of course, produce azine 53 by virtue of their geminated assemblage. Yet, the affinity of carbene 46 for the O H bonds present in both CyDs predominated, giving D-substituted CyDs (55a, 55b, iso-55a, and /.w-55b), via in-nermolecular reactions,21 that were detected using FAB MS analysis (Table 5).133... 

To determine whether room-temperature supramolecular constraint of carbene 64 could alter its reactivity by yielding ring-expanded allene 65, triplet carbene iso-64, or benzocyclopropene 68, diazirine 63 was prepared,166 and then included within CyDs and FAUs prior to irradiation.167 The photolysis of diazirine 63 is expected to form relatively long-lived carbene 64, which usually decomposes via intermol-ecular reactions (Scheme 14). 

Rearrangement in the excited state (RIES) of diazirine 45 might be the actual route to enyne 47 because MeOH, which is relatively reactive and in excess, is expected to trap carbene 46 completely... 

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