Isobutene photolysis

In addition, 18-19% of isobutene and chloroacetylene formed via fragmentation. Photolysis of the diazirine in up to 9 M trimethylethylene in pentane led to a sharp decrease in 27 and 28 (to 32% and 8.5%), along with 40% of cyclopropanes formed via the capture of 19. However, the yield of isobutene and chloroacetylene was unchanged, indicating that these products did not stem from the carbene, but arose directly by fragmentation of its excited diazirine precursor.45... 

The suppression of polymerisation by 02 and benzoquinone was due to the fact that these species captured a significant fraction of the secondary electrons produced and not to their ability to scavenge free radicals. The negative ions thus produced underwent charge neutralisation with (CH3)3C+ thus reducing the yield of polymerisation initiators. In confirmation of the fact that ionic rather than free radical processes were important was the observation that the production of free radicals within the solution by the photolysis of diacetyl failed to initiate polymerisation. Spectroscopic evidence for the existence of (CH3)3C+ in the pulse radiolysis of isobutene was obtained subsequently31. 

Figure 1. Plots of In CJC0 against t for DMSO and isobutene obtained from a DMSO/isobutene/N02 photolysis system in 760 Torr synthetic air at 298 K.

Photochemically generated trimethylsilylphenylsilylene also adds to the carbon-carbon double bonds of many types of olefins (54). Thus, the photolysis of a hexane solution of tris(trimethylsilyl)phenylsilane (20) in the presence of isobutene by irradiation with a low-pressure mercury lamp produces, after subsequent treatment of the photolysis mixture with methanol, fert-butylphenyI(trimethylsilyl)methoxysilane in 52% yield, as the sole insertion product. Direct evidence for the formation of 1-trimeth-ylsilyl-l-phenyl-2,2-dimethyl-l-silacyclopropane in this photolysis can be obtained by NMR spectroscopic analysis of the reaction mixture. 

Unlike the 1-disilanyl-substituted naphthalenes, photolysis of 2-disilan-ylnaphthalenes (46) gives intermediate C, which readily adds to the substrate present (95). Thus, irradiation of 46a in the presence of isobutene in benzene affords an adduct, 2-isobutyldimethylsilyl-3-trimethylsilyl-naphthalene in 37% yield, as the sole volatile product. Interestingly, pho-... 

Diem et al. investigated the photolysis of adamantyl iodide as a po ible source of carbocationic initiation for the polymerisation of isobutene. Althou no polymer was obtained with this simple tystem (or with f-butyl iodide as photolyte), addition of iodine scavengers such as zinc, zinc fodide or both together gave some polymerisation, indicating that the carbocations produced in the photolysis of alkyl iodides possess a modest initiating power if generated in the presence of isobutene. 

More direct evidence for the formation of difluorocarbene as an intermediate comes from the photolysis of difluorodiazirine in the presence of chlorine, iodine, dinitrogen tetroxide, and nitryl chloride when the corresponding difluoromethane derivatives were formed, and in the presence of isobutene, buta-l,3-diene, and cis- and frans-but-2-ene when the corresponding 1,1-difluorocyclopropane derivatives were formed. Even with considerable excesses of olefins, appreciable tetra-... 

From a succession of investigations of the photolysis of methane has emerged a fairly complete understanding of its mechanism. The earliest studies were inconclusive owing to incomplete product analysis. More recent studies ° have indicated that the major products are hydrogen, ethane, propane and ethylene with smaller quantities of acetylene, -butane, isobutane, propene, isobutene and allene. 

The photolysis of neopentane has been studied by Lias and Ausloos at 1236 and 1470 A. The two main processes which occur are molecular methane elimination from, and fragmentation of, the excited neopentane. The products of photolysis are methane, isobutene, hydrogen, ethane, and propene, with smaller amounts of isobutane, ethylene, propane, acetylene and 2,2-dimethylbutane. The suggested reactions are... 

Through scavenging of D atoms by HjS in the photolysis of neo-C5Di2 it was found that the hydrogen formed is approximately equal to the excess isobutene. 

The photolysis of isobutene was first examined by Kieffer and Howe and has been examined in more detail recently by BorrelP . Fewer molecular products are formed compared with the photolysis of the other butenes. The main decomposition mode is the rupture of the C-H bond J3 to the double bond, viz. 

Dissociation of acyinitrenes occurs, presumably from the triplet state, when carbonylnitrenes are produced in very unreactive media, or in a vacuum. Ethoxycarbonylnitrene, produced by flash photolysis in vacuo, dissociates to "NCO and EtO, the former being identified by its detailed and intense u.v. absorption spectrum. The NCO radical is not produced, however, when cyclohexene vapour is present. In this case, the normal double bond adduct, 7-ethoxycarbonyl-7-azabi-cyclo[4.1.0]heptane is formed, as identified by its i.r. spectrum . Pivaloylnitrene, i-BuCO—N, when produced in unreactive solvents, such as dichloromethane or neopentane, apparendy dissociates in a similar manner. A polymer suggested as being the product from <-Bu and NCO is formed, and isobutene can be obtained in 22% yield (based on nitrene formed) when a stream of nitrogen is passed through the reaction mixture . ... 

Solely the photolysis of 1 with the heterocyclic, five-membered ring compounds fliran, thiophene and selenophene furnished other products instead of the respective siliranes (see below). The reactions of tetra-t-butyldisilene 3 with alkenes gave rise to a more complex palette of products. In the cases of cyclopentene and cyclohexene, no reactions with the alkene were observed. Instead, compounds 24, 25, 27, and 28 were formed these results can be rationalized in terms of a dimerization of the disilene with / elimination of isobutene 26 and subsequent reactions of the disilene and the silylene with 26 [7, 8],... 

Comparison of reactions of halocarbenes, which were produced from either alkylhalodiazirines by nitrogen extrusion, or by the usual method of eliminating hydrogen halide from dihaloalkanes by means of strong bases, bears special interest. Thus phenylchlorocarbene 177 was produced both from benzylidene chloride by potassium-t-butoxide, and by photolysis of phenylchlorodiazirine. ° Competition experiments with mixtures of olefins showed rate differences, especially remarkable for the isobutene-tetramethylethylene pair. The rate proportion was 0.20 when the diazirine... 

The major products, in order of decreasing amounts, of the photolysis k = 147 nm) of 1,1-dimethylcyclopropane have been shown to be isobutene, ethylene, hydrogen, buta-1,3-diene, 2-methylbuta-1,3-diene, propylene, allene,... 

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