Diazirine, laser flash photolysis

Photolysis ofbenzylchlorodiazirine (3) in the presence of tetramethylethylene (TME) is known to produce ( )- and (Z)-/l-chlorostyrene (4) and the cyclopropane (5). Plots of [5]/[4] vs [TME] are curved, consistent with the existence of two pathways for the formation of the alkenes (4). Benzylchlorocarbene (BnClC ) was generated by laser flash photolysis of the phenanthrene (6) in the presence of TME. In this case, plots of [5]/[4] vs [TME] are linear, mling out the possibility that the second pathway to the alkenes (4) involves reaction of a carbene-alkene complex. Time-resolved IR spectroscopy revealed that diazirine (3) rearranges to the corresponding diazo compound, but this process is too inefficient to account for the curvatures. It is proposed that the second pathway to alkene formation involves the excited state of the diazirine. 

The absolute rate constants for reaction of /t-tolyl(trifluoromethyl)carbene, generated by laser flash photolysis of the corresponding diazirine, with pyridine (4 x lO lmor s ... 

The absolute rate constants for oxygen and sulfur transfer to a range of carbenes (dialkyl, cycloalkylidene, alkylchloro, diaryl, arylchloro, arylalkoxy, and dialkoxy), generated by laser flash photolysis of diazirine or oxadiazoline precursors, were determined. No evidence was seen for ylide formation and a concerted mechanism via an ylide-like transition state was proposed. 

Confirmation was provided by the observation that the species produced by the photolysis of two different carbene sources (88 and 89) in acetonitrile and by photolysis of the azirine 92 all had the same strong absorption band at 390 nm and all reacted with acrylonitrile at the same rate (fc=4.6 x 10 Af s" ). Rate constants were also measured for its reaction with a range of substituted alkenes, methanol and ferf-butanol. Laser flash photolysis work on the photolysis of 9-diazothioxan-threne in acetonitrile also produced a new band attributed the nitrile ylide 87 (47). The first alkyl-substituted example, acetonitrilio methylide (95), was produced in a similar way by the photolysis of diazomethane or diazirine in acetonitrile (20,21). This species showed a strong absorption at 280 nm and was trapped with a variety of electron-deficient olefinic and acetylenic dipolarophiles to give the expected cycloadducts (e.g., 96 and 97) in high yields. When diazomethane was used as the precursor, the reaction was carried out at —40 °C to minimize the rate of its cycloaddition to the dipolarophile. In the reactions with unsymmetrical dipolarophiles such as acrylonitrile, methyl acrylate, or methyl propiolate, the ratio of regioisomers was found to be 1 1. 

The reaction of triplet diphenylcarbene with pyridine has been well studied, and a mechanism proposed from kinetic data (90TL953). The carbenes generated from laser flash photolysis of alkylbromo- and alkylfluoro-diazirines were trapped by pyridine to form the pyridinium ylides... 

Kinetic analysis of the rearrangement of benzylfluorocarbene, generated by laser flash photolysis of the corresponding diazirine, gave a rate constant of 9.2 x 106 s 1 at 26 °C with activation entropy —17.2 eu and activation energy 3.25 0.34 kcal mol-1, very similar to the values for the chlorocarbene.80 A product analysis study of the thermolysis and photolysis of the diazirine (73) in the presence of tetramethylethylene showed tiiat die ring-expanded cyclobutene and the cyclopropanation products do not arise via a common intermediate.81 The ring expansion was proposed to occur by loss of N2 from the diradical intermediate (74). 

Condensed-phase carbenes will often react with the surrounding solvent medium, but they can also combine with other solute molecules. Indeed, these intermolecular reactions commonly occur at rates that are limited only by diffusion. Consequently, such carbenes are just short-lived intermediates. Nevertheless, arylhalocarbenes tend to have much longer lifetimes than typical alkylhalocarbenes, e.g., three orders of magnitude longer, because the latter also undergo rapid 1,2-H shift to form al-kenes.168,169 Indeed, the lifetime (t) of singlet carbene 64 (2max = 310 nm)170 in 2,2,4-trimethylpentane (isooctane) is reported to be ca. 3.6 ps, as determined via laser flash photolysis (LFP) of diazirine 63.171... 

Laser flash-photolysis investigations have been carried out for benzylchloro-carbene, phenylchlorocarbene, methoxyphenylcarbene and fluorenylidene, and some other carbenes, generated from diazirine, diazo and other precursors, in the presence of oxiranes and thiiranes. These carbenes abstract oxygen or sulfur atoms with bimolecular rate constants in the range 10 to 10 s ... 

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. 

Figure 4. Growth of ylide 21 absorbance following laser flash photolysis of dimethyl-diazirine in the presence of pyridine. Reprinted with permission from Ford, R Yuzawa, T. Platz, M.S. Matzinger, S. Fiilscher, M. J. Am. Chem. Soc., 1998,120, 4430...

The probe kinetic methodology developed by Scaiano [23] to study radicals and biradicals by laser flash photolysis (LFP) methodology is readily extended to carbenes. LFP (308, 337, 351, or 355 nm) of diazirine or diazo compounds in the presence of pyridine produces carbenes, which generally react rapidly lO M sec" ) to form ylides. [28,29] Ylides are much easier to monitor than the carbenes because they have intense UV-Vis absorption and microsecond lifetimes. Pyridine ylide methodology has enabled LFP studies of alkyl, dialkyl, alkylhalo, dihalo and carbonylcarbenes. It is now a standard tool and will be used as long as LFP studies of simple carbenes are performed. 

The formation of ylides by the reaction of 4-nitrophenylchlorocarbene (generated by laser flash photolysis, LFP, of the parent diazirine) with ethers has been observed by UV-vis spectroscopy and was shown to be reversible. LFP combined with UV-vis spectroscopy has also enabled determination of the equilibrium constant for dihalocar-banion formation on reaction ofphenylchlorocarbene with halides. These experiments established a very fast equilibrium between the electrophilic carbene and the nucleophilic carbanion, which can be adjusted by careful control of the halide concentration. 

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