Diazirines, photolysis

Table 4 Primary and Secondary Products of Diazirine Photolysis...

Rate differences observed between the same bromophenylcarbene (241) when prepared by two different routes, diazirine photolysis and the reaction of benzylidene dibromide with potassium r-butoxide, vanish when a crown ether is added to the basic solution in the latter experiment. In this case the complexing potassium bromide is taken over by the crown ether, and selectivity towards alkenes reaches the values of the photolytic runs (74JA5632). 

The quantum yield of diazo compound formation from diazirines 43 and 44 was only 0.10-0.13, however, so that RIES was not considered an important product source during the diazirine photolysis. In the case of 44, for example, the substantial RIES contribution to chloroisobutene formation stems from excited 44, not from the excited state of its diazo isomer.55 The situation is analogous to the RIES formation of a-chlorostyrene during the photolysis of benzylchlorodiazirine.25... 

Although the carbenes are generated by diazirine photolysis, RIES is an unlikely complication because the alkylacetoxycarbenes (e.g., 76) can be almost completely scavenged by added alkenes, with the suppression of rearrangement products.81... 

Synthetically useful precursors of oxygen-substituted carbenes (see Houben-Weyl Vol. E19b. pp 1628-1682) are diazirines (photolysis or thermolysis), a-halogen ethers (base treatment), a-dihalogen ethers (treatment with alkyllithium compounds), and stable carbene complexes of the Fischer type (thermolysis). Only the mechanism-based stereoselectivity and the simple diastcreoselectivity of their reactions with alkenes have been studied to date. 

RRKM calculations on triplet species in diazirine photolysis,647 BEBO calculations on the activation energies for hydrogen-transfer reactions,648 stereochemistry as a probe for photochemical reaction mechanisms,649 photochemistry with polarized light,550 dimer formation,651 the formation of molecular complexes,552 hydrogen bonding in electronically excited states,553 and the interactions between excited-state aromatic molecules and 02654 have been the subjects of recent theoretical treatments. 

Phenylbromocarbene, generated by diazirine photolysis, is apparently more selective than the corresponding carbenoid from benzal bromide and potassium -butoxide. The electrophilic behavior of the carbenoid may be obscured by steric factors in the reaction with highly substituted olefins, as discussed above. In principle, however, complexation of a singlet carbene may increase or decrease its electrophilicity, depending on whether the com-plexing agent supplies electrons to the vacant p orbital (Lewis base) or withdraws electrons from the occupied sp orbital (Lewis acid). More data on this problem is clearly warranted. 

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. 

Methylene from diazirine has higher energy of vibration than the product from photolysis of ketene, but it is more discriminating in insertion reactions into primary and secondary C—H bonds. 

The primary and secondary products of photolysis of common diazirines are collected in Table 4. According to the table secondary reactions include not only isomerization of alkenes and hydrogen elimination to alkynes, but also a retro-Diels-Alder reaction of vibrationally excited cyclohexene, as well as obvious radical reactions in the case of excited propene. 

The photolysis of chlorodiazirine was investigated in several cases. From chloromethyl-diazirine (232) vinyl chloride was formed as the stable primary product of stabilization of chloromethylcarbene, with acetylene and hydrogen chloride as secondary products. Some 1,1-dichloroethane was assumed to have been formed through a linear diazo compound by reaction with HCl. Added HBr yielded 1-bromo-l-chloroethane (76MI5Q800). 

ESR spectra, 7, 19 Diaziridinyl radicals ESR, 7, 202 reactions, 7, 19 Diazirine, alkyl-photolysis, 7, 225-226 synthesis... 

Diazirine, fluoromethoxy-nitrogen extrusion, 7, 224 Diazirine, methylvinyl-rearrangement, 7, 221 Diazirines addition reactions to Grignard compounds, 7, 2 0 as carbene precursors, 7, 236 IR spectra, 7, 203 microwave spectrum, 7, 199 molecular spectra, 7, 202-204 nitrogen extrusion, 7, 223 NMR, 7, 202 photoconversion to diazoalkanes, 7, 234 photoisomerization, 7, 221 photolysis, 7, 225-227 quantum chemical investigations, 7, 197 reactions... 

Preliminary investigations on the formation of carbenes from diazirines have already been made available. Frey and Stevens recently reported the photolysis of cyclic diazomethane. Cyclic diazomethane was irradiated in the gaseous phase with light of wavelength... 

The methylethylcarbene which is formed thermally from methyl-ethyldiazirine at 160°C gives the same products as that from butanone p-toluenesulfonylhydrazone and bases in aprotic solvents." However, photolysis of the same diazirine gives a different mixture of C4H8 hydrocarbons. Considerable amounts of 1-butene are formed, the trans-butene content is reduced by half, and the amount of methyl cyclopropane increased fivefold. ... 

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. 

Figure 4.1. TRIR difference spectra averaged over the timescales indicated following 355 nm laser photolysis of diazirine 8 (15.7mM) in C02-satnrated dichloromethane. Reprinted with permission from B. M. Showalter and J. P. Toscano, J. Phys. Org. Chem. 2004, 14, 743. Copyright 2004, John Wiley Sons Limited.

Photolysis of diazirine 75 in N2 at 9 K produced carbene 76, whose IR and UV/Vis spectra fit predictions by DFT calculations. The IR spectra were most consistent with a carbene conformation with Cl aligned 90° to the adjacent C-CH bond. The carbene was found, by IR spectroscopy, to rearrange to chloroadamantene 77 slowly at 9 K in the dark. The rate of rearrangement was somewhat faster in Ar matrices at 9 K or at higher temperatures in N2. 

Some diazirines, particularly the 3-trifluoromethyl-3-aryldiazirines, can rearrange upon photolysis to a linear diazo derivative, similar in structure to the photosensitive end of the crosslinker PNP-DTP (Chapter 5, Section 3.12). These isomerized products themselves can be photolyzed to the reactive carbene. 

The photolysis of cyclic diazo ketones in hydroxylic solvents leads to ring contracted carboxylic acid derivatives via this ketocarbene -> ketene rearrangement. Examples of such reactions are given in (2.24)239) and (2.25) 240). In this last example a photoequilibrium between the diazo ketone and its valence isomer, a diazirine, has been observed, both products then eliminating nitrogen to afford the cyclobutane carboxylic acid. 

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