Alkenes 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. 

Ab initio and RRKM calculations indicate that the reactions of C, CH, and (H2C ) with acetylene occur with no barrier." Laser flash photolysis of the cyclopropanes (69) and (70) was used to generate the corresponding dihalocarbenes. The absolute rate constant for the formation of a pyridine ylide from Br2C was (4-11) x 10 lmoP s. The rates of additions of these carbenes to alkenes were measured by competition with pyridine ylide formation and the reactivity of BrClC was found to resemble that of Br2C rather than CI2C . 

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 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... 

Electron impact mass spectrometry of the cyclobutanedione (24) gives rise to dimethylcarbene radical cation.35 Appearance energy measurements and ah initio calculations indicated that the radical cation lies 84 kJ mol 1 above the propene radical cation and is separated from it by a barrier of 35 kJ mol-1. Diarylcarbene radical cations have been generated by double flash photolysis of diaryldiazomethanes in the presence of a quinolinium salt (by photo-induced electron transfer followed by photo-initiated loss of N2).36 Absolute rate constants for reactions with alkenes showed the radicals to be highly electrophilic. In contrast to many other cation radicals, they also showed significant radicophihc properties. 

Recent studies by Schuster and collaborators67,54, based on nanosecond laser flash techniques, revealed important conclusions, including (a). The enone excited state responsible for the photocycloaddition is the jt-Tt which possesses different polarization than the n-7T state, considered in rationalizing the effect of the oriented jr-complex. (b) Direct measurement of the reactivity scale of alkenes measured by nanosecond flash photolysis provided different results from those obtained with no consideration of the diradical fragmentation to starting materials. 

Recent laser flash photolysis (LFP) studies have provided absolute rates of addition of perfluoro-n-alkyl radicals to a variety of alkenes in solution [ 114,115]. In these studies, C2Fj, C3F7, and n-C7F,5 were generated instantaneously by photolysis of the respective diacyl peroxides. The initially-formed perfluoroacyloxyl radicals decarboxylated rapidly to yield the perfluoroalkyl radicals, after which the additions of these radicals to styrene, a-methylstyrene, etc. were monitored directly via observation of the growth of UV absorption due to the transient benzylic radicals. 

Insight into the nature of the intermediate in the cyclopropanation of electron-rich and electron-poor alkenes was given by laser flash photolysis (LFP) studies of phenyl-bromodiazirine in the presence of various amounts of tetrabutylammonium bromide.23 Electron-rich alkenes react exclusively with the carbene (11) leading to (12). Electron-poor alkenes yield cyclopropanes (13) only slowly with the carbene (11) and more rapidly with the carbanion (14) arising from the addition of the bromide to the carbene. 

Since the lifetime of fluorenone radical anions lies in the ps-time region, conventional flash photolysis has been used to measure the transient absorption spectra [32]. The effect of solvent and added salts on the reactions of triplet fluorenone and electron rich alkenes is shown in Figs. 1 and 2, respectively. 

Two-laser two-photon results revealed photoisomerization of the cation E,E-11 to its stereoisomer Z,E-11, which undergoes thermal reversion with a lifetime of 3.5 ps at room temperature. Absolute rate constants for reaction of styrene, 4-methylstyrene, 4-methoxystyrene and /i-methyl-4-methoxystyrene radical cation with a series of alkanes, dienes and enol ethers are measured by Laser flash photolysis [208]. The addition reactions are sensitive to steric and electronic effects on both the radical cation and the alkene or diene. Reactivity of radical cations follows the general trend of 4-H > 4-CH3 > 4-CH3O > 4-CH30-jff-CH3, while the effect of alkyl substitution on the relative reactivity of alkenes toward styrene radical cations may be summarized as 1,2-dialkyl < 2-alkyl < trialkyl < 2,2-dialkyl < tetraalkyl. 

In many synthetically useful radical chain reactions, hydrogen donors are used to trap adduct radicals. Absolute rate constants for the reaction of the resulting hydrogen donor radicals with alkenes have been measured by laser flash photolysis techniques and time-resolved optical absorption spectroscopy for detection of reactant and adduct radicals Addition rates to acrylonitrile and 1,3-pentadienes differ by no more than one order of magnitude, the difference being most sizable for the most nucleophilic radical (Table 8). The reaction is much slower, however, if substituents are present at the terminal diene carbon atoms. This is a general phenomenon known from addition reactions to alkenes, with rate reductions of ca lOO observed at ambient temperature for the introduction of methyl groups at the attacked alkene carbon atom . This steric retardation of the addition process either completely inhibits the chain reaction or leads to the formation of rmwanted products. 

The spectra of alkene selenides persists for several minutes after flashing. A spectrum observed in flashed COSe and ethylene mixture is identical to that produced in flashed CSe2 and ethylene mixture. In Ar or N2, flash photolysis of COSe produces Se2 but no CSe. An added alkene inhibits Se2 formation in experiments either with COSe or CSe2. In flashed CSe2, the rate of decay of Se (4 P) is identical to the rate of appearance of the far ultraviolet bands, within an experimental scatter of + 20% in the pseudo first-order rate constant. Therefore the absorption in the far... 

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