Generation by 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. 

The mechanisms proposed by both of these groups to explain the details of the chemistry of esters that give rise to short-lived nitrenium ions are not the only mechanisms that could fit these data. Further developments in this area will require application of picosecond spectroscopic methods to ion pairs generated by laser flash photolysis. 

McClelland and co-workers verified the absolute magnitude of the k Q values for 76h, 76n, and 76o from measurements of the effect of d-G on the rate constants for disappearance of these ions that had been generated by laser flash photolysis. " They provided additional data, included in Table 3, for 75g and 75p-75v. Their results confirm that k -c reaches an apparent diffusion-controlled limit of ca. 2.0 x 10 M s for the... 

For kinetic purposes, acetylperoxyl radicals were generated by laser flash photolysis of an acetylcobalt precursor in the presence of 02, Eq. (26). With use of ABTS - as a kinetic probe, the rate constant was determined for the cross-coupling reaction of Eq. (27), 27 = 1.49 x 108 M 1 s-1. 

In another set of competition experiments, N02 was again generated by laser flash photolysis and allowed to react with a mixture of Craq002+ and AcrH2. At the chosen set of concentrations, the great majority of N02 reacted with Craq002+ in submillisecond... 

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). 

Scheme 1.5 3-Pyridyl ketene generated by laser flash photolysis in acetonitrile, and trapping by n-butylamine investigated by time-resolved IR measurements [14].

A very interesting paper80 reported studies of the reactions of several substituted benzhydryl carbenium ions, generated by laser flash photolysis, with halide ions in several solvents. This work provided the nucleophilicity N of chloride and bromide ions in several solvents. These data, along with the ionization rate constants and the solvolysis rate constants for the reactions of substituted benzyhdryl halides, was used to construct quantitative energy surfaces for the. S N 1 reactions of substituted benzhydryl halides in several solvents. 

Homo- and cross-cycloaddition of 1,1-diphenylsilene and 1,1-diphenylgermene generated by laser flash photolysis from a 1 1 mixture of the corresponding 1,1-diphenylmetallacyclobutanes gives three 1,3-dimetallacyclobutane products (Scheme 30) <19990M5643>. 

When the carbocations are generated by Laser flash photolysis, the ion pair collapse with the nucleophilic counterion Cl- is so fast [136] that the decay cannot be followed with the instrumentation used for these experiments, i.e., only those carbocations which manage to escape from the [Aryl2CH + Cl ] ion pair can be observed. Consequently, all rate constants determined for the Laser photolytically produced carbocations refer to the reactions of the nonpaired entities. 

Radical cations generated by laser-flash photolysis (LFP), pH = 4.5. Conditions under which water addition to the radical cation competes with side-chain deprotonation [134]. 

Radiation has to be continuous to maintain an adequate standing concentration of the stannyl radicals. If the t-butoxyl radicals are generated by laser flash photolysis and the formation and decay of the R3Sn radicals is monitored by optical spectroscopy (e.g. BusSn, vmax 400 nm), the rate constant at 295 K for the self-reaction of the BusSn radicals is found to be ca. 1.4 x 109 M 1 s, and that for the reaction of BulO radicals with the stannane is 1.9 x 108 M 1 s 1... 

Adamantylidene, generated by laser flash photolysis of ada-mantyldiazirine (3) in a variety of solvents, reacts much more slowly with typical scavengers than does homocubanylidene. The equilibrium constant for the interconversion of homocub-1(9)-ene... 

Johnston reported a rate constant of 1.1 x 10 M" s for quenching of the singlet-excited dibenzosuberenyl cation by azide in 4% aqueous TFE [19]. The cation was generated by laser flash photolysis of 5-dibenzosuberenol, with the rate constant obtained from a plot of the observed rate constant for decay of the transient cation at 525 nm as a function of added azide concentration. The water co-solvent was necessary for solubility of the sodium azide quencher. 

Phenylchlorocarbene, generated by laser flash photolysis of phenylchlorodiazirine, reacts readily with y,y-dimethylallyl methyl sulphide to yield the s-ylide 228. This then undergoes a [2,3] sigmatropic rearrangement to 229, which hydrolyses to the ketone, 23034 7. 

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