Butenes, addition from photolysis

The photolysis of a-diazosulfones dissolved in alkenes provides sulfonyl-substituted cyclopropanes in high yields. This is exemplified by the preparation of l-(p-methoxyphenylsulfonyl)-2,2,3,3-tetra-methylcyclopropane in 75% yield from -methoxybenzenesulfonyl-diazomethane and 2,3-dimethyl-2-butene. A similar addition to [Pg.101]

Enhancement of the total butene yield is observed when various additives whose ionization potential falls below about 9.4 e.v. are present during ethylene radiolysis (35). This is consistent with the above interpretation (Figure 2). In the vacuum ultraviolet photolysis of cyclobutane the yield of butenes varies with the ionization potential of the additives in the same way as observed here (12). The maximum enhancement corresponds closely to the yield of C4H8+, as expected from our mechanism. 

The addition of carbenes is also a stereospecific reaction, each geometrical isomer forms the cis addition product. Thus carbene generated from the photolysis of diazomethane adds in a cis manner to both cis and trans butene-2. 

An added complication in the interpretation of long-wavelength ketene photolysis is demonstrated by Cundall s discovery of ketene sensitized cis-trans isomerization of the 2-butenes.33 As the pressure of olefin increases, the rate of ketene decomposition decreases and the rate of olefin isomerization increases. At high olefin concentrations part of the apparent nonstereospecificity of cyclopropane formation can thus result from stereospecific singlet addition to already isomerized olefin. 

Likewise, 03 reacts with hydrocarbons to produce unknown numbers of H02 and R02 (or RC002) [see below]. From the computer analysis of simulated smog formation involving the hypothetical illumination of N0-N02-H20-butene-aldehydes-C0-CH4 mixtures in air, Calvert and McQuigg (184) estimate that H02 and R02 radicals, formed mainly by the addition of OH to butene, account for 10% of NO to N02 conversion. The H02 and R02 radicals formed from the photolysis of aldehydes and OH reactions with aldehydes are responsible for 25% of the conversion. Carbon monoxide is only 5% effective for the NO to N02 conversion. The effect of paraffins on the NO to N02 conversion rate is very small. 

The photolysis and pyrolysis of difluorodiazirine has been shown to involve difluoromethylene and stereospecific addition to 2-butene indicates that it is probably in a singlet state. In argon or nitrogen matrices, Cp2 has been detected by spectroscopic measurements . Products arising from the thermolysis or photolysis of fluoromethoxydiazirine, cyanofluorodiazirine, difluoroaminodiazirine and chlorofluorodiazirine have been reported the main feature of these reactions is that the intermediate carbene in all cases can be trapped by olefin reagents, whereas fluorocarbenes from other sources are quite unreactive. 

Although the primary C-H bond rupture has a low quantum yield in the photolysis of 1-aikene, Niedzielski et al. have confirmed the possibility that hydrogen atoms formed in the 8.4 and 10.0 eV photolysis of 1-butene may be hot (74). In their experiments, the addition of a hydrogen atom to the double bond gives rise to a vibrationally hot butyl radical. This hot butyl radical may eventually decompose, leading to the formation of propene. From kinetic treatment and using a plot of the rate constant for dissociation versus excess... 

There is eertainly strong experimental evidenee for the existenee of radieal-solvent eom-plexes. For instanee, Russelk and eo-workers eolleeted experimental evidenee for radi-eal-eomplex formation in studies of the photoehlorination of 2,3 -dimethylbutane in various solvents. In this work, different produets were obtained in aliphatie and aromatie solvents, and this was attributed to formation of a jr-eomplex between the Cl atom and the aromatie solvent. Complex formation was eonfirmed by flash photolysis. Complex formation was also proposed to explain experimental results for the addition of triehloromethane radieal to 3-phenylpropene and to 4-phenyl-1-butene and for hydrogen abstraetion of the t-butoxy radieal from 2,3-dimethylbutane. Furthermore, eomplexes between nitroxide radieals and a large number of aromatie solvents have been deteeted. Evidenee for eomplexes between polymer radieals and solvent moleeules was eolleeted by Flatada et al., in an analysis of initiator fragments from the polymerization of MMA-d with AIBN and BPO initiators. They diseovered that the ratio of disproportionation to eombination depended on the solvent, and interpreted this as evidenee for the formation of a polymer radieal-solvent eomplex that suppresses the disproportionation reaetion. 

Some reaction systems, which have been described in the patent literature for the production of meat aromas, regard thiamine as precursor. 3-Methyl-2-butene-l-thiol is one of the roast odorants of coffee (cf. 21.1.3.3.7) and can cause on off-flavor in beer (cf. Table 5.5). In general, only very small amounts are formed which are still aroma active on account of the very low odor threshold (Table 5.21). The formation of the thiol is explained by the fact that the 3-methyl-2-butene radical is formed from terpenes by photolysis (beer) or under the drastic conditions of the roasting process (coffee). This radical then meets a SH -radical formed from cysteine under these conditions. In the case of beer, humulons (cf. 20.1.2.3.2) are under discussion as the source of the alkyl radical. In coffee 3-methyl-2-butene-l-ol (prenyl alcohol) is also a possible precursor, which yields the thiol after water elimination and hydrogen sulfide addition. 

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