Diazirine, matrix photolysis

Fluorodiazirines may photoisomerize to diazo compounds, as illustrated with diazirine 278. Photolysis of 278 at 334 nm in a nitrogen matrix at 8K primarily... 

During photolysis of [29] in an argon matrix doped with 4% CO, bands belonging to the ketene [31] were observed along with those of the carbene [30], Upon further warming to 40-45 K the carbene bands disappeared and were replaced with bands of the ketene [31], indicating a direct interaction of carbene (CF3)2C with CO. Photolysis of diazirine [29] in a matrix doped with 17% CO resulted in the appearance of only ketene [31] bands the carbene bands were not detected in this experiment. 

Generation of 78 by thermolysis or photolysis of a diazoalkane or diazirine precursor, however, affords the singlet carbene, whose 1,2-H shift to ethene is opposed by a barrier of only 0.678 to 1.298 kcal/mol. Consequently, even in cryogenic matrices, singlet 78 rearranges more rapidly than it intersystem crosses to the triplet, which has therefore not been detected by UV or ESR in either an Ar matrix at 8 K or a Xe matrix at 15 K." The lifetime of singlet 78 at ambient temperature has been estimated at <0.5 ns.89,98b (Note the enormous spectator substituent effect of Cl the lifetime of MeCCl is 740 ns,60 at least 1500 times longer than that of MeCH.)... 

The ESR spectrum of methylene in a xenon matrix (4 °K) was studied by Wasserman Photolysis of diazomethane or diazirin gave anESR-signed which persisted up to 20 °K but disapj)eared above 77 °K. 

Most carbenes are relatively easily generated by photolysis of nitrogeneous precursors such as diazo compoundsor diazirines. The reaction is clean, as nitrogen is the only byproduct and it is also very efficient as nitrogen evolution is a highly exothermic reaction. Therefore carbenes can be easily generated even under very inert conditions, such as in a noble gas matrix at very low temperatures. 

The photolysis of dimethylsilyldiazomethane 251 in an argon matrix at 10 K (Scheme 45) was shown by Sander and coworkers to lead to the diazirine 252 and, via the carbene 253 and a 1,2-H shift, to dimethylsilene 254, used to study the thermal reaction of the silene with formaldehyde132. 

In addition, reference to calculated data will be found at various points throughout Section 7.10.2 of this chapter. Photolysis of the diazirine 35 in an N2 matrix at 10 K generates 2-benzothienylchlorocarbene. Infrared spectroscopy, supported by B3LYP/6-31G calculations, indicates the presence in the matrix of both anti and syn carbenes,... 

Recently, Moore and Pimentel have photolyzed diazirine isolated in a solid nitrogen matrix. They found that the photolysis did produce diazomethane. In order to decide whether the diazomethane arose by a photoisomerization reaction or by the reaction of methylene with the nitrogen of the matrix, experiments were carried out in a nitrogen matrix enriched in N. On the basis of these experiments they conclude that the photolysis of the diazirine produces methylene and nitrogen, and that the diazomethane arises from the reaction of the methylene with the matrix and not by photoisomerization. Photolysis of diazomethane does not... 

Fluorodiazirines have also been used extensively as carbene photoprecursors for cryogenic matrix isolation " and laser flash photolysis studies. Photolysis of fluorophenoxydiazirine has made observation of the corresponding carbene directly observable in yet another fashion encapsulated in a hemicarcer-and. Diazirines have been used for studies in solution of carbene philicities as well as for synthesis of cyclopropanes and cyclopropenes.Of course, diazo compounds can also serve as carbene photoprecursors, but for carbenes such as difluorocarbene, the diazo precursor is either nonexistent or extremely unstable. 

The only route to carbonyl O oxides 1 which is suitable for matrix isolation is the oxidation of free carbenes 2 with molecular oxygen 02 [5-7]. The carbenes are generated by photolysis of the corresponding diazo compounds or diazirines. Prerequisite for this method is that the carbenes are stable under the conditions of matrix isolation (solid rare gas at 10 K). This excludes simple alkyl substituents, because these carbenes rapidly undergo [1,2]-H shifts to yield alkenes in essentially quantitative yields. During the last years we were able to study the spectroscopy and chemistry of a number of carbonyl oxides (R and R were H, Ph, HC=C, CF3, cyclic systems etc.) [8-12] by IR and UV/vis spectroscopy. 

Photolysis of biphenylchlorodiazirine (36) in an Ar matrix at 10 K yielded the corresponding carbene in the presence of acetonitrile, a nitrile yUde is obtained whose IR spectrum is in good agreement with 2-azonia-l-allenide (37). Subjecting this diazirine to LFP in the presence of acetonitrile showed an equilibrium between the carbene and the less reactive nitrile ylide. ... 

The parent 3H-diazirine reveals a weak absorption = 200) in the range of 280 to 350 nm. Photolysis of 3H-diazirine in a xenon matrix at 4 K (Scheme 7) gives triplet ground-state methylene at 77 K, subsequent dimerization and irradiation afford ethenyl radicals. ... 

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