Flash photolysis rearrangement

Carbon tunneling in a second singlet chlorocarbene has also been proposed. It has proved impossible to observe noradamantylcarbene 73 spectroscopically, either by solution laser flash photolysis or with matrix isolation at low temperatures. It has been suggested that the carbene rearranges too rapidly, possibly via carbon tunneling, to adamantene (74). 

Laser flash photolysis (LFP) of quinone diazide 2d in Freon-113 at room temperature produces carbene Id, which could be monitored indirectly by addition of trapping reagents.25 At 2.0 xs the lifetime of Id is slightly longer than that of la (1.65 xs), otherwise the reactivities of these carbenes are very similar. The Id —> 11 rearrangement is not observed in the LFP experiments. All trapping products with a variety of reagents (O2, acetonitrile, pyridine etc.) are derived from carbene Id. 

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 iron(II) complexes of the hexadentate 2-pyridylmethyl derivatives of bipy (94) and (95) are spin cross-over compounds, whose light-induced high-spin to low-spin conversion has been monitored in solution by laser flash photolysis. Single exponential kinetics (A h l = 6.7 x 10 s , at 273 K) were observed for [Fe(94)] ", but for [Fe(95)] " kinetics were biphasic, with the spin-conversion step (ku i = 2.5 x 10 s ) followed by a slower step (k = 3.7 x 10 s ) involving rearrangement of the pyridylmethyl pendant arms. ... 

Sundberg and co-workers studied the photochemistry of phenyl azide by conventional flash photolysis in 1974. They detected the transient UV absorption of ke-tenimine 30 and measured its absolute rate constant of reaction with diethylamine to form the IH azepine, which subsequently rearranges to Doering and Odum s 3H-azepine (27). ... 

Fats, rancidity, 328 Favorskii rearrangement, 294 Fenton s reagent, 306 Ferrocene, 275 Field effects, 22, 152, 364 Flash photolysis, 304 Fluorination, 121,140,170, 315, 326 Free energy, Gibbs, 34 change and K, 35, 359 of activation, 38, 359 standard, 35... 

The facile isomerization of simple alkylcarbenes by alkyl or hydride shifts is so rapid that these species cannot be conveniently studied even by laser-flash photolysis.128- 130 a-Fluori-nation of alkylcarbenes dramatically reduces the rate of rearrangements.131-134 e.g. fluoro(methyl)carbeneis sufficiently stabilized that it can be trapped by cyclopenta-1,3-diene.135 This type of insertion reaction is quite typical of a-fluorinated carbenes. oc-Fluorinated earbenes are less prone to CH insertion, generally adding stereoselectively to alkenes.136... 

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). 

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