Cyclopentane photolysis

In addition, ring-opening of the CsHjo ions was found in cyclo-CjHjo-NO mixtures which lead to pentene-2, presumably through the charge-transfer reaction with NO. Pentene-2 was not a product at 1236 or 1470 A, for similar mixtures. The presence of CsHs ions was also observed in the cyclopentane photolysis at 1048-1067 A, and has been attributed to the decomposition of hot cyclopentane ions, viz. 

Photolysis of alkoxysilyl-diazoacetic esters 1 generates l-oxa-2-sila-cyclopentanes 2 by carbenic intramolecular C/H insertion. Four- and six-membered rings are not observed. 

That propane is indeed formed by H2 reaction is known by observing the distribution of yields of various isotopic compositions of propane from the radiolysis of an equimolar mixture of cyclopentane and deuterated cyclopentane. Further evidence is provided by the facts that (1) propane is not formed by photolysis below the ionization threshold, and (2) an electric field has no effect on the yield. 

The photodecomposition of -alkanes at excitation energies slightly above the absorption onset involves both C-H and C-C bond decompositions [18]. The dominant process is the C-H scission, (H2) 0.8-0.9, and the contribution of C-C decomposition is small. In the photolysis of cyclohexane, cycloheptane, cyclooctane, and cyclodecane, however, only hydrogen evolution was observed [[Pg.375]

Figure 8 Connection between primary C-H and C-C bond ruptures during radiolysis and photolysis. Alkanes (1) propane, (2) w-butane, (3) -pentane, (4) -hexane, (5) w-heptane, (6) n-octane, (7) w-decane, (8) isobutane, (9) neopentane, (10) 3-methylpentane, (11) 2,2-dimethylbutane, (12) isooctane, (13) cyclopentane, (14) cyclohexane, (15) cycloheptane, (16) cyclooctane, (17) cyclodecane, (18) methylcyclopentane, (19) methylcyclohexane, (20) ethylcyclohexane, (21) 1,1-dimethylcyclohexane, (22) cis-l,2-dimethylcyclohexane, (23) fraw5-l,2-dimethylcyclohexane, (24) cis-1,3-dimethylcyclohexane, (25) trarw-l,3-dimethylcyclohexane, (26) cw-l,4-dimethylcyclohexane, (27) trawi-l,4-dimethylcyclohexane. (From Refs. 18, 29, 91, 92, 99, 100, 108, 110, 111, 113, 114, and 160.)...

The reaction of CH2 from CH2N2 photolysis with cyclopentane at — 75°C. and 15°C. in the liquid phase gave methylcyclopentane with less than 0.1% cyclohexane, showing that ring enlargement by insertion of CH2 into C—C bonds did not occur.24... 

Photolysis of cyclohexanone gives rise to carbon monoxide, ethylene, propylene (1), cyclopentane, 1-pentene (3), and 5-hexenal (29). Cyclo-hexenyl cyclohexanone, water, and a polymer have also been reported as products, especially when the photolysis is conducted in the temperature range from 100°-300° in the presence of short wavelength radiation (3). At 3130 A. and over the temperature range of 100°-300° the ketone that is decomposed is almost fully accounted for in the products (5) and the stoichiometry of the products fits the eq. 15-18 (3,27) ... 

The photolysis of cyclobutanone in the presence of 20- to 50-fold excess of ethylene has been reported (17). In this instance various five-carbon olefins but not cyclopentane are said to be observed among the products. The hydrocarbons are believed to be formed by a reaction between a tri-... 

In a more recent study, the enamide photocyclization with very similar photosubstrates was examined in the presence of chiral amino alcohols and chiral amines as asymmetric inductors [47]. The achieved enantioselectivities are in the same range as the ones reported by Ninomiya and Naito, but in this approach the asymmetric induction was more effective for the cis products. In cyclopentane at — 40°C, 0.1 equivalents of the most effective inductor, (— )-ephedrine (entity, gave the cis cyclization products with up to 37% ee and the trans products with only 2% ee. The role of the chiral inductor as a Br0nsted acid was supported by flash photolysis experiments. The presence of the chiral amino alcohol led to an increase in the rate of disappearance of a transient that was assigned to the primary cyclization intermediate of type 29, i.e., the chiral inductor accelerates the protonation/deprotonation sequence that reestablishes the aromatic ring. 

The reactivity of a number of alkane complexes has been examined and this field has been reviewed through 1996 by Hall and Perutz. Flash photolysis of Cr(CO)6 in cyclohexane showed that solvation occurs within the first picosecond after photolysis, a fact that appears to rule out spin crossing as an important component in the dissociation of CO from Cr(CO)6. The stability of CpRe(CO)2(alkane) is particularly striking. Comparison of the rate constants for heptane solvated metal complexes with CO, Table 1, reveals that the rate constant for CpRe(CO)2(heptane) is five orders of magnitude slower than that of CpV(CO)3 (heptane). In fact, the stability of the CpRe(CO)2(alkane) complexes is so high that it has been possible to carry out low-temperature NMR on the cyclopentane complex generated by continuous photolysis of... 

When S( D) atoms react with C2H6, CgHg, iso-CgHio, cyclopropane, cyclobutane, or cyclopentane, only one type of product—the corresponding mercaptan—is detected in photolysis experiments with the mercury arc. Specific search in careful studies has failed to reveal the presence of any additional products in these systems. This fact can be taken as compelling evidence that in the mechanism of mercaptan formation, no free-radical species are involved. The only exception to this pattern is... 

A study very similar in nature to the one described above for cyclobutane has been undertaken by Ausloos et for the photolysis of cyclopentane at 1470, 1236 and 1048-1067 A. The photolysis mechanism is not as clear-cut as with cyclobutane, and it appears that more reactions contribute to the photolysis, viz. 

The photolysis of cyclopentane at 1048-1067 A was found to produce photoionization with an ionization efficiency of about 0.43. The presence of CjHjo ions was shown by the H2 transfer reaction to propene, viz. 

Pyrolysis of 4,5-dihydro-3//-pyrazole (3) at 223 °C gave an 89% yield of cyclopropane. In a similar manner, 2-(aminomethyl)cyclopentan-l-amine gave 2,3-diazabicyclo[3.3.0]oct-2-ene yield 80% bp 73.5 C/14Torr. On photolysis, this gave bicyclo[3.1.0]hexane in 68% yield. 2,2-Dimethylpropane-l, 3-diamine gave 4,4-dimethyl-4,5-dihydro-3//-pyrazole yield 65% bp 66-67°C/40Torr. Pyrolysis at 249°C gave a 97% yield of 1,1-dimethylcyclopropane. 

TABLE 8 The Distribution of Products Formed on Photolysis of Pentafluorophenyl Azide in Cyclopentane [105 ... 

Figure 13. Arrhenius treatment of the ratio of CH insertion adduct to the yield of decafluoroazobenzene and pentafluoroaniline formed on photolysis of pentafluoro-phenyl azide in cyclopentane.

In the vapour-phase photolysis of cyclopentene, photoionization occurs at an excitation wavelength of 123.6 nm with an efficiency of 0.16, and leads to the formation of cyclopentane through ion-molecule reactions.14 At 147 and... 

Davies et al. (1971b) also studied the reactions of I-C4H90, prepared from (r-C4H90)2 photolysis, with boron compounds in competition with the reaction with cyclopentane in peroxide or isooctane solutions. They monitored the radicals produced from the respective reactions by electron spin resonance. The reactions of I-C4H90 with boron compounds had rate coefficients of 2 X 10 — 3 X 10 A/ -sec at 30 C. This work was extended by Davies et al. (1972a), who used r-butylhypochlorite as the source of I-C4H90 radicals and measured the amount of alkyl chlorides produced from the alkyl radical products at 40°C. 

Competitive rate measurements from the original publication combined with the rate for cyclopentane from laser photolysis measurements (Paul et al., 1978). The analysis usually assumes that the intermediate decays solely to give scission products, a condition which is met at low readical concentrations. 

Transformation of 945 into the thiohydroxamate ester 947 followed by visible-light photolysis in the presence of tributyltin hydride allows large scale preparation of the methylene cyclopentane 948. The overall yield of 948 from 945 after chromatographic purification on silica gel is 55—65%. Desilylation of 948 and subsequent epoxidation of the exocyclic olefin with freshly prepared dimethyldioxirane solution provides the epoxyalcohol 949 in high yield. A Dess—Martin oxidation to the epoxyketone followed by rearrangement on silica gel to the enone, a Luche reduction from the convex face, and a final isopropylidene ketal hydrolysis provides 950 as a single diastereomer [262] (Scheme 205). 

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