Methylene, from photolysis

Methylene from diazirine has higher energy of vibration than the product from photolysis of ketene, but it is more discriminating in insertion reactions into primary and secondary C—H bonds. 

Fig. 12. Internal energy spectrum of the CH2 fragment from photolysis of the CH3 radical at 216.3 nm. The combs above the figure indicate the expected TOFs of H atoms formed, in association with triplet methylene CH2(X3Bi) or singlet CH2( i1Ai) respectively as a function of V2, the vibrational quantum number for their respective bending mode. (From Wilson et al,113)...

From photolysis of methylene blue by ruby-laser giant pulses, Danzinger et al. found that a 0.5 Joule, 30 nsec laser pulse causes almost total conversion of the original molecules into transients, but that the photochemical change is completely reversible. The lifetimes of three transients have been measured as 2, 30 and 140 jusec resp. at a 5.5 x 10 M dye solution. 

It will now be shown that the discrepancy can be resolved by considering the possibility of a more energetic methylene in the experiments of Bell and Kistiakowsky, who used diazomethane photolysis to generate methylene. Frey41 has shown that methylene from diazomethane may be more energetic than methylene from ketene by 10-15 kcal. 

In some cases the nature of the products resulting from photolysis of dienes and trienes depends on whether the reaction occurs from the excited singlet state or the triplet state. The triplet states can be populated exclusively by suitable sensitizers. For example, 3-methylene-1, 5-hexadiene gives a cyclobutene derivative (A) from the singlet state and tricyclo ring compound (B) from triplet state ... 

Assuming that reaction (4) proceeds with unit collisional deactivation efficiency, then the value of is computed as 3 X 10 sec. . This value is close to the value obtained when diazomethane is used as the methylene precursor and implies that the total energy of the methylene produced by the photolysis of diazomethane and diazirine is the same. These results were rationalized on the basis that methylene from diazirine contained less translational energy and was more vibrationally excited than methylene from diazirine. 

There is much evidence that the photolysis of ketene, diazomethane, and diazirine produces methylene predominantly in the singlet state. It is, however, possible, by using mercury photosensitization or by photosensitization using benzophenone to produce triplet methylene. Since the triplet is the ground state for methylene, it is also possible to produce triplet methylene from singlet methylene by carrying out reactions in the presence of a large excess of inert gas. Recently, much evidence has accumulated to indicate that even in the normal photolysis of all three of the methylene precursors, some of the methylene is produced in the triplet state. ... 

The photolysis of diazirine °° at 3130 A yields ethylene and nitrogen methylene is probably formed in the primary process. The long wavelength absorption ( max = 3200 A) had been identified as the allowed (a, n ) transition and semi-empirical Huckel calculations indicate that the methylene produced must be (Ai) or (Bi). There is evidence that methylene produced from the photolysis of diazirine is more selective than methylene from diazomethane, owing to decreased excess translational energy. At low pressures 5-30 torr, diazomethane was identified as an intermediate by its absorption spectrum and the question arises, is methylene formed directly, or does it arise from decomposition of diazomethane The quantum yield of disappearance of diazirine is 2.0+0.5 and the quantum yield of diazomethane formation is about 0.2. The intermediate diazomethane is... 

On the basis of eqn. (36), the yields of triplet methylene from ketene are found to be roughly 15% at 2800 A, 15% at 3130 A, 30% at 3340 A and 40% at 3660 A . Equation (36) neglects the formation of singlet-type products from triplet methylene via radical recombination (35). If the reduction in yield of rra/ti-2-pentene and 2-methyl-2-butene by added oxygen is also considered to be the result of triplet CH2 reactions, a recalculation of triplet methylene fractions yields 20% triplet at 2800 A, 55% at 3340 A and 60% at 3660 A for ketene photolysis . Assuming that 10% added oxygen suppresses all triplet products and measuring product yields relative to an internal standard, Eder and Carr " arrived at 29 3% triplet CH2 at 3130 A, and 87 2% at 3660 A. For the diazomethane-troni-2-butene system, the fraction of triplet methylene was found to be 12% at 3550-4000 A . 

Early work on the gas-phase interaction of methylene with carbon-hydrogen bonds showed selective reactivity in the order, tertiary C-H > secondary C-H > primary C-H. Methylene from diazomethane was found to be less selective than methylene from ketone . The differences in the relative reactivity of methylene from the two sources have been attributed to excess translational energy of methylene from CH2N2 . Recently, however, it has been shown that the photolysis of ketene produces more triplet CH2 than the photolysis of diazomethane cf. preceeding section). Part of the reported selectivity must be attributed to triplet-methylene effects. 

Dewar benzene was prepared in 1963 by lead tetraacetate decarboxylation of the diacid from the photolysis of 1,2-dihydrophthalate esters. Benzvalene was isolated from photolysis of benzene in 1967 " and prepared in quantity from lithium cyclopentadienide, methylithium, and methylene chloride. Prismane was prepared from benzvalene in 1972. Bicyclopropenyl itself was prepared in 1989 although alkyl derivatives were characterized earlier and shown to undergo 3,3-sigmatropic shifts, vide infra. Substituted derivations of all the (CH)6 hydrocarbons, particularly the permethyl materials. 

Kinetics of the reaction of [ H2]methylene from the photolysis of [ Hjlketen with perfluoroalkenes in the gas phase have been examined and correlated with... 

The reactions of Scheme 1, involving the addition of methylene from the photolysis of keten, have been investigated in an interesting study of intramolecular energy relaxation. The intermediate hexafluorobicyclopropyl, which is chemically activated by some 465 kJ mol, decays to the extent of some 3.5% by a non-random process involving decomposition of the newly... 

Methylene (CHj) was generated from diazomethane in some of the earhest matrix studies of organic reactions, and its dimerization to ethene was observed, although its IR spectrum proved somewhat elusive.Various halomethylenes were also observed among the products from the vacuum-UV irradiation of halomethanes in matrices. Examples are CHF from photolysis of methyl fluoride and CClj from dichloromethane. ... 

Electronic excitation from atom-transfer reactions appears to be relatively uncommon, with most such reactions producing chemiluminescence from vibrationaHy excited ground states (188—191). Examples include reactions of oxygen atoms with carbon disulfide (190), acetylene (191), or methylene (190), all of which produce emission from vibrationaHy excited carbon monoxide. When such reactions are carried out at very low pressure (13 mPa (lO " torr)), energy transfer is diminished, as with molecular beam experiments, so that the distribution of vibrational and rotational energies in the products can be discerned (189). Laser emission at 5 p.m has been obtained from the reaction of methylene and oxygen initiated by flash photolysis of a mixture of SO2, 2 2 6 (1 )-... 

As might be expected, a complex mixture of lactones corresponding to abstraction of a hydrogen atom from various sites along the methylene chain was obtained from the photolysis. The mixture of lactones was converted by dehydration, ozonolysis, and hydrolysis to a mixture of ketones. It was found that no functionalization occurs with ester side chains of less than nine carbon atoms. This is probably due to the inability of the carbonyl to approach any methylene closely enough to abstract a hydrogen. The data for side chains of nine carbons or greater is presented in Table 3.11. 

Flash photolysis studies<22) have indicated singlet methylene to be produced from the diazomethane-excited singlet upon loss of nitrogen followed by collisional deactivation to the triplet, the ground state multiplicity for this molecule. 

The yield of trans product (18) is decreased by the presence of a radical scavenger such as 1,1-diphenylethylene and increased by dilution of the reactants with methylene chloride or butane, indicating this product to result from the triplet carbene. A heavy-atom effect on the carbene intermediate was observed by photolysis of a-methylmercuridiazoacetonitrile. With c/s-2-butene as the trapping agent either direct photolysis or triplet benzophenone-sensitized decomposition results in formation of cyclopropanes (19) and (20) in a 1 1 ratio ... 

Methylene cyclopropene (5), the simplest triafulvene, is predicted to be of very low stability. From different MO calculations5 it has been estimated to possess only minor resonance stabilization ranging to 1 j3. Its high index of free valency4 at the exocyclic carbon atom causes an extreme tendency to polymerize, a process favored additionally by release of strain. Thus it is not surprising that only one attempt to prepare this elusive C4H4-hydrocarbon can be found in the literature. Photolysis and flash vacuum pyrolysis of cis-l-methylene-cyclopropene-2,3-dicarboxylic anhydride (58), however, did not yield methylene cyclopropene, but only vinyl acetylene as its (formal) product of isomerization in addition to small amounts of acetylene and methyl acetylene65 ... 

While carbene lp is completely stable towards irradiation with visible or UV light, carbenes lq and lr bearing methyl groups in the peri-position adjacent to the carbene center slowly rearrange on irradiation with k > 420 nm." During this photolysis the color of the matrix changes from yellow (carbene) to purple. By comparison of the experimental with the DFT-calculated IR spectrum the purple compound was identified as 5-methylene-5//-anthracenc-1 -one (23) with an extended rr-system that is responsible for the intense color. 

Preparative Photolysis. The preparative photolysis of an aqueous solution (pH=8.5) of AETSAPPE (2.5 M) was conducted in a 1-inch diameter quartz test tube in a Rayonet Reactor (Southern New England Radiation Co.) fitted with 254 nm lamps. Within two hours the solution gelled and the reaction was terminated. Upon acidification the solution cleared, and the product could be re-precipitated by addition of base. This indicates loss of the thiosulfate functionality. The product was dissolved in dilute HC1, precipitated with acetone, and filtered. This process was repeated three times, and the final precipitate was washed with water. The product (20 to 30 mg) was dried in vacuo for 24 hours and stored in a dessicator until use. Comparison of the13 C NMR spectrum of the product with the starting AETSAPPE 13C NMR spectrum clearly shows that the thiosulfate methylene peak shifted upfield, from 39 ppm to 35 ppm. The complete 13 C NMR and IR analysis of the product were consistent with the disulfide product. Further, elemental analysis of the product confirmed that the product was the desired disulfide product 2-amino (2-hydroxy 3-(phenyl ether) propyl) ethyl disulfide (AHPEPED) Expected C 58.39, H 7.08, N 6.20, S 14.18 actual C 58.26, H 7.22, N 6.06, S 14.28. 

Sensitizers have also been used to induce inters5retem crossing to the triplet The insertion reaction of methylene into C—H bonds in solution is widely acknowledged to be a concerted process resulting from the singlet state. Photolysis of... 

Sensitization, which can populate the triplet manifold, was used in a number of instances. Sensitization with benzophenone was used in the photolysis of diazomethane to generate triplet methylene. The triplet methylene thus produced, however, failed to abstract much hydrogen from alkanes (cyclohexene), but... 

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