Allene, from photolysis

By photolyzing a solution of the allyl aryl sulfoxide at slightly elevated temperature at appropriate wavelengths, allyloxy radicals were produced from photolysis of the steady state concentration of the sulfenate. Another study examined the regioselectivity of addition of the alkyl and sulfenyl radicals across olefins and allenes [155], Control of these elements allowed potentially useful synthetic transformations to be designed [156], particularly as the sulfenate may be viewed as an O-H abstraction synthon . 

Because of the exceptional C-F bond strength, the successful preparation of a-halocyclopropyl c-complexes is realized by substitution of 1-bromo-l-fluoro-trans-2,3-dimethylcyclopropane 179 with Fp [90], Silica gel column chromatography of the thus obtained cr-complex 180 results in ring opening to the alcohol 181 as a single stereoisomer. The allene complex 182 is produced by treatment with BF3OEt2, indicating that 181 is derived from 182 and water. The 7i-allyl complex 183 is formed by photolysis via a disrotatory process. 

According to results from laser flash photolysis, the p-(methoxyphenyl) sulfanyl radical adds exclusively to the central atom in of 2,4-dimethylpenta-2,3-diene (If) with a rate constant of 1.1 x 10s M-1 s-1 (23 1 °C) (Scheme 11.6) [45], A correlation between the measured rate constants for addition of para-substituted arylsulfanyl radicals to allene If was feasible using Brown and Okomoto s o+ constant [46], The p+ value of 1.83, which was obtained from this analysis, was interpreted in terms of a polar transition state for C-S bond formation with the sulfanyl radical being the electrophilic part [45]. This observation is in agreement with an increase in relative rate constant for phenylsulfanyl radical addition to 1-substituted allene in the series of methoxyallene lg, via dimethylallene Id, to phenylsulfanylallene lh, to ester-substituted 1,2-diene li (Table 11.2). 

It was found that the azirine-nitrile ylide isomerization was a completely reversible process. The unlabeled nitrile ylide showed a prominent band at 1926 cm that underwent a 66-cm shift with N substitution. This shift was interpreted as being consistent with an allene-like skeleton (8) rather than the alternative pwpargyl-like stmcture (9). This conclusion was supported by the spectra from the C- and H-labeled variants. Warming the nitrile ylide in a xenon matrix from 12 to 82 K provided no new absorptions suggesting that the allene-like structure may also be adopted in solution. Some absorption spectra for benzonitriho benzylide (DPNY) and some substituted benzonitrilio methylides obtained via pulsed-laser photolysis of azirines are given in Table 7.1 (5). 

Because only a small number of stable selenoketones exist, studies of their decomposition are scarce. Irradiation of dwerf-alkylselones with UV light in hydrogen-donating solvents affords diselenides (80CJC6 87MI1), and the selone 37 behaves likewise. A second path of photolysis involves extrusion of molecular nitrogen from 37 to yield eventually tetramethyl-allene and elemental selenium, probably via the elusive... 

Carbene 132 is implicated in the photolysis of 1 since the observed289 photodimerization to 9,10-dihydrophenanthrene and -anthracene is best explained by head-to-head and head-to-tail coupling of this species. Moreover, the fact that allene 134 is isolated289,290 as the major product from irradiation of diesters 31 (equation 35) is fully consistent with a photo-Wolff rearrangement of the carbene. The minor product here involves cyclization... 

Silacyclopropenes are commonly formed from the addition of a silylene to an alkyne, or in some cases as the result of photolysis of an alkynyldisilane (see Section III.C). Substituted silacyclopropenes have been shown to undergo both 1,2- or 1,3-shifts when photolyzed, yielding silyl-substituted allenes or alkynes, respectively2. More complex behavior was observed with methylenesilacyclopropenes such as 2323 which ring-opened to a diene, as shown in Scheme 4. 

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 effect of nitric oxide or oxygen on the photolysis of cis- or trflnj-butene-2 was quite striking The yields of ethane, propene, -butane, butene-1, isobutane and Cj to Cg compounds were reduced sharply to levels well below those from corresponding runs with nitrogen. In contrast, allene, methane, ethylene, acetylene, butene-2 and butadiene were affected only to the same extent as the runs with nitrogen. It is concluded that the products in the latter group are primary while those of the former group are secondary and arise from free radicals produced in primary steps. 

A brief study by Jackson et at 1470 A gave results very different from those reported above. The photolysis was conducted at pressures of 200 n and 10 torr of benzene. No hydrogen, allene, cyclohexadienes, biphenyl or dihydrobiphenyls was observed, while acetylene, ethylene, methylacetylene and vinyl acetylene were found. These authors conclude that neither the atomic nor molecular elimination of hydrogen occurs, while Hentz and Rzad conclude that both maybe operative. Thus the situation is confused at present, but both studies agree that polymer formation is extensive at 1470 A. 

The most important products of decomposition are ethylene, allene, hydrogen, butenes and ethane. Ethylene is found to arise almost entirely from the primary decomposition. Photolysis of cyclo-C3H6-cyclo-C3D6 mixtures gives about 20% HD in the hydrogen, and indicates a significant contribution of an atomic hydrogen process. The formation of propene in a primary step may represent a small fraction of the total primary processes. 

Wavelengths longer than 220 nm. The absence (2) of fluorescenci or phosphorescence in carbon suboxide excited in the ultraviolet would suggest that it is efficiently photodecomposed in this region of the spectrum. From its structural similarity to ketene, it is expected that this will involve production of C2O in step 1. The work of Bayes has in fact shown that this is the most probable process in the absorption region with a maximum at 265 nm. Bayes (11) found that photolysis of 02/02 mixtures led to production of allene rather than the acetylene, which is known (12) to be formed by insertion of carbon atoms into C2H. ... 

Allenes can also be synthesized from diazocyclopropanes by chemical or photochemical [l,2,l]-elimination of nitrogen. In the thermolysis of 28 to give 30 the carbene-intermediate 29 could be trapped 18), and in the low-temperature photolysis of 31 the triplet carbene 32 could be detected by EPR-spectroscopy 17). 32 is longlived in a polycrystalline matrix and rearranges to 33 (28 %) at a temperature of —154 °C 19>. Numerous applications are included in Ref. 20). Especially noteworthy are the syntheses of stable cyclobutadienes by Masamune (90%)21) and Regitz (67%)22). 

The higher members of the [l,n,l]-eliminations are also of preparative importance. Thus, the Doering allene synthesis 14) leads to bicyclobutanes (e.g. 66, 28 %) 45), if bulky substitution as in 65 favours the [l,3,l]-elimination. The related cyclopropene syntheses from 6746) and 6946, respectively (40 and 6% 68 in derivatized form lithiation and carboxylation), are to be classified as [l,3,(2)l]-eliminations of bromine (reductive) as well as of hydrogen chloride. The thermolysis or the photolysis of diazo... 

The involvement of trimethylenemethane diradicals in deazetization of diazoalkane-allene adducts or trimethylene diradicals in the deazetization of the adducts of acyclic alkenes often leads to mixture of regioisomers and stereoisomers and from the standpoint of cyclopropane syntheses, this is undesirable. Far fewer problems of this type attend deazetization of the adducts of cyclic or polycyclic alkenes and, furthermore, even a modest amount of strain in the system activates the alkene to diazoalkane addition so that there is no need for activating substituents on the double bond. Cyclopropene is highly reactive towards diazoalkanes (see also Section 1.1.5.1.5.3.1.) and cycloaddition reactions of this type provide a ready entry into the bi-cyclo[1.1.0]butane series. The addition of diphenyldiazomethane to cyclopropene gave 4,4-diphenyl-2,3-diazabicyclo[3.1.0]hex-2-ene (1), which on photolysis gave a mixture of 2,2-diphenylbicyclo[1.1.0]butane (2) and 1,1-diphenylbuta-l,3-diene (3). ... 

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