Diazirine, decomposition

In the last decade, experimental structural methods have been applied in specialized facets of diazirine chemistry, particularly with regard to supramolecular diazirine species and a detailed examination of diazirine decomposition. 

Hemicarcerands have been used to encapsulate the aryldiazirines 17, and 18a and 18b and thereby to investigate inner-phase diazirine decomposition <2002AGE96>. Detailed H NMR studies allowed the preferred inner-phase orientation of the guests to be deduced. 

R. Warmuth, J.-L. Kerdelhue, S. Sanchez Carrera, K. J. Langenwalter, N. Brown, Rate acceleration through dispersion interactions insight into the effect of a hemicarcerand on the transition state of inner phase diazirine decompositions, Angew. Chem. Int. Ed., 2002, 41, 96-99. 

Prepared by the latter route, decomposition is - 10% in 24 h. This reaction involves direct Cl transfer to NH2CI via the intermediate NH CE NH2CI hydrolysis playing Htde or no role. The presence of NH" 4 improves stabiUty by reacting with HOCl which tends to increase decomposition (39). Trichloramine also increases decomposition, whereas NH2CI has Httle effect. Dichloramine is useful for preparation of diazirine (40). 

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). 

First order decomposition was established for dimethyldiazirine (215) and ethylmethyl-diazirine (216). The activation energy is 139 kJ moF for (215) the half life at 100 °C is 97 h. On decomposition of (216) the products formed and their respective yields are as indicated. The products correspond qualitatively and quantitatively with the results of thermal decomposition of 2-diazobutane formed in situ in aprotic solvents. Analogous comparisons of decomposition products of diethyldiazirine, isopropylmethyldiazirine, n-butyl- and t-butyl-diazirine agree equally well 66TL1733). 

Diazirines are in most cases more easily available than linear diazo compounds. Moreover, their decomposition via true carbenes is free of side reactions, whereas linear diazo compounds in presence of H-donors may react by a cationic pathway. Only where reactions of linear diazo compounds are optimized for carbene formation do they give the same products as do decomposing diazirines. 

Photolysis of dlazirines to nitrogen and carbenes is a general reaction and plays a greater role in carbene chemistry than photolysis of linear diazo compounds. Whereas the latter are often obtained only under the conditions of their thermal decomposition from suitable precursors, diazirines are obtainable in a pure state in most cases. Photolysis has the further advantage to permit nitrogen extrusion at atmospheric pressure, even with low-boiling materials. 

Correlation diagrams include the product orbitals while perturbation approaches require knowledge of the empty orbitals of the reactant. However, the occupied molecular orbitals of diazirine (II), compared with those of cyclopropene (I), do seem to give some indication of a preferred thermal decomposition of (II) compared with the rearrangement of (I). Moreover these molecular orbitals are a typical illustration of the localization obtained in the presence of an electronegativity perturbation. 

It is suggested that the real carbene, generated by thermolysis of the diazo or diazirine precursors or photolysis of 40, gives mostly 1,3-insertion, whereas photolysis of either the diazoalkane or diazirine yields much 1,2-Me migration directly from precursor excited states.15 1 An analogous intervention of 1,2-Me migration via RIES was also observed in the photolytic decomposition of f-butylchlorodiazirine (24) to f-butylchlorocarbene (18) cf. Eq. 14.27... 

Photolytic decomposition of diazirine 9a in methylcyclohexane led to substantial C-H insertion of PI1CH2CCI into the solvent, although azine was a minor product. At 25°C, there were 74% of 1,2-H shift products and 14% of C-H insertion. Insertion increased to 44% at —75°C. Here too, a curved Arrhenius correlation reflected the competition of two classical reactions, not the incursion of QMT.71... 

Diazirine and several of its 3-substituted homologues, formally cyclic azo compounds, are explosive on heating or impact [1]. The shock-sensitivity of all diazirine compounds and the inadvisability of their handling in the undiluted state have again been stressed [2], In a description of the synthesis of 27 3-(4-substituted)halodiazirines, the need is stressed to handle the compounds at below 30°C to prevent thermal decomposition, or, for the pure compounds, explosion [3],... 

Photolytic decomposition of diazirine (25) in the presence of ethyl acetate led to the formation of the ester derived carbonyl ylide (26). Trapping of the ylide with diethyl fumarate led to the formation of the dihydrofuran 28, generated by dehydrochlorination of the initial cycloadduct 27. It occurred with a rate constant... 

As illustrated in Section 4.1.1, the addition of nonstabihzed carbenes to the oxygen atom of a carbonyl derivative can lead to the production of carbonyl yhdes. However, these methods are not always practical for preparative scale since many side reactions can accompany the decomposition of alkyl diazo and diazirine derivatives. Landgrebe and co-worker (8) extensively studied the thermal decomposition of organomercurials in the presence of carbonyl compounds for the preparative generation of carbonyl yhdes (Scheme 4.6). 

Diazo compounds are usually easy to synthesize and handle if the N2 group is next to a carbonyl or an aryl group. In other cases, especially when electronreleasing substituents such as halogen or oxygen atoms are adjacent to the diazo moiety, diazirines prove to be more practical. Sometimes, diazirines undergo photorearrangement to diazo compounds prior to decomposition (or vice versa, as in the case of phenyldiazomethane" ), but this does not impede the formation of the desired carbene. 

A disadvantage of diazo compounds is that they are quite polar, which endows them with a propensity to form dimers in the gas phase, even at concentrations as small as 1 5000. This feature becomes evident after their decomposition, which sometimes leads predominantly to carbene dimers (e.g., pentafulvalene in the case of diazocyclopentadiene" ), or adducts of the target carbene with its diazo precursor, as in the case of methylene, where the main products are CH2=NH and HCN." The problem disappears at very high guest/host ratios, but it is often impractical to achieve these. Fortunately, diazirines are less prone to dimerization in the gas phase. 

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