Ketenes photodecomposition

Carbonylcarbene reacts with oxetane in a complex manner, giving cyclopropane and propene by a deoxygenation process and ketene plus ethylene as apparent insertion-fragmentation products (equation 66). The reaction is carried out by irradiation of a solution of carbon suboxide in oxetane under these conditions, photodecomposition of intermediates is possible. The initial attack is believed to be at the ring oxygen atom to give a zwitterionic species (78JA6425). 

The light-induced reaction of o-quinone diazides finds occasional use in the synthesis of a large number of compounds which are used in photography.273,277 Photolysis of diazoindauoues (CCVIII and CCX) provides a new synthetic route to cyclobutene carboxylic acid derivatives CCIX and CCXI, respectively.117 A good demonstration that ketenes are actually involved in the photodecomposition of o-quinone... 

Photodecomposition. The facts that the absorption spectrum is diffuse and that there has been no emission observed from ketene suggest that the excited singlet state of ketene is extremely short lived. An upper-limit lifetime of t 4 x 10 s has been deduced by Zabransky and Carr (257). The study of ketene photochemistry has had to rely on the measurement of CH2(x3bi) and CH2(al-A ) yields - a notoriously difficult task. 

A theoretical calculation shows that 80 kcal/mol is required to form oxirene from ketene (73). The fact that hot ground-state ketene has no low-energy decomposition pathway available may make the explanation of the pressure dependence of photodecomposition yields plausible. The thresholds for 3g and A production have been measured as 75.7 1.0 kcal/mol and 84.0 0.6 kcal/ mol, respectively (225). These high barriers should therefore make the decomposition relatively slow for a molecule this size at low excitation energies, and thus subject to pressure quenching at moderately high pressures (10-100 torr). 

Basch investigated the photodecomposition of keten and carried out both SCF and MCSF calculations with an augmented DZ basis set. It was concluded that the first excited triplet state of keten can form CH2( Ba) and CHaCMi) relatively rapidly and the first excited singlet state of keten can give CH2( R2) easily in a near-least-motion path. However, the formation of CHj(. 4i) from the first excited singlet state of keten by a near-least-motion path seemed to be very improbable. 

Photolysis of the cyclic vinyl ketone 5,5-dimethyl-3-azido-2-cyclo-hexene-l-one (61) in aqueous tetrahydrofuran resulted in ring enlargement to the azepine (62). Irradiation of the same azide in benzene did not however lead to photodecomposition. A reaction path involving a cyclic ketene-imine was put forward. 

Photodecomposition of the diazo group containing acetals (472 Scheme 87) furnishes lactam acetals (473) similar compounds are accessible from ketenes and azidoformates. From 1,3-dioxolanes and azidoformate the aminodioxolanes (474) were prepared. " ... 

Carboxylate esters readily undergo photodecomposition with loss of carbon dioxide. Not surprisingly, lactones and related oxygen heterocycles undergo related transformations. A wide variety of lactones behave in this fashion" for example, the cyclic dilactone (505) is converted on irradiation to the [2.2]paracyclophane (506). Of particular interest is the use of the jS-lactone (507) as a precursor of matrix-isolated cyclobutadiene (508)." Phthaloyl peroxide (509) has similarly been employed as a precursor of benzyne (510). A careful examination in an argon matrix at 8 K has revealed that the / -lactone (511) and the keto ketene (512) are intermediates... 

A new statistical quantum mechanical approach - the statistical adiabatic product distribution method has been applied to the photodissociation of ketene in parallel theoretical and experimental studies. The main focus of the work, however, was H-atom production via C-H cleavage rather than the elimination of CO. The photodecomposition of formohydroxamic acid (HCONHOH) has been investigated in matrix-isolation FTIR and DFT stud-ies. " Irradiation of the acid in Ar or Xe matrices with the full output of a xenon arc lamp generated H-bonded HNCO- H2O and NH20H- CO complexes. In the latter case, the IR spectra also suggest the existence of a structure with the NH2 group interacting with the carbon atom. 

A few attempts to test for the involvement of a benzooxirene are outlined in Figs. 3.9 and 3.10. Neither the phenol derivative 15 [56] nor the naphthols 16/17 [57] showed evidence of oxygen migration in reactions that may have generated carbenes (Fig. 3.9). Photodecomposition of the labeled diazo ketone 18 gave only ketene 19, indicating that a benzooxirene did not participate (Fig. 3.10). 

When cyclobutanone was photolyzed in Ar, CO, and Nj matrices with UV or vacuum-UV light, photodecomposition occurred by at least two and possibly three primary processes. Photolysis with the 302 nm Hg line produced only CO and cyclopropane, while photolysis with the full Hg arc, with major Hnes at 302, 297, 289, and 280 nm, gave ketene and propene as IR detectable products, as well. It was concluded that the processes operating were (1) c-CjH CO — c-CjH + CO, (2) c-CjH CO —> CjH + CH2=C=0, and possibly (3) c-CjH CO —> CH3CH=CH2 + CO. Additional secondary photolysis products were detected when the matrices were irradiated in the vacuum-UV. 

Although uncertainty exists in the rate coefficients of OH with ketene and its derivatives, it is very likely that these reactions will be important atmospheric loss mechanisms. Note that the lifetime will only be about 8 h for ketene itself, using the rate coefficient recommended above and assuming daytime [OH] = 2.5 x 10 molecule cm . Reaction with NO3 is also very rapid, and will likely contribute in polluted nighttime atmospheres for example, the lifetime of ketene against NO3 reaction will be less than 1 h for [NO3] = 3 x 10 molecule cm . The lifetime for photodecomposition of ketene in the lower troposphere with an overhead sun is about 1 h, and this will clearly be an important loss process for ketene. See section IX-E-6 for further details. 

The observation of ketene and acetylene formation in photolyses at 248 nm provides strong evidence for the occurrence of process (I) the observed HCO formation confirms the occurrence of process (II) in photolyses at this wavelength. CO formation could arise from processes (II) through (V) or as a secondary product from photolysis of primary product ketene. Although process (I) was considered as a possible source of acetylene, the authors suggest that it is likely formed from the photodecomposition of the initial product, Z-3,4-diformyl-cyclobutene, formed in (VI). This cyclobutene product was suggested as a primary product in the studies of Klotz et al. (1995, 1999). 

The biradicals shown may be involved as precursors to the formation of alkenes and ketenes in the photodecomposition processes (I) and (II). Those formed from singlet and triplet excited states are expected to form singlet and triplet biradicals, respectively. The biradical precursor to process (II) appears to be favored over that suggested for process (I), as one might expect from the difference in bond dissociation energies involved. The extent of stereospecificity of the products formed was rationalized by Carless et al. (1972) in terms of the relative dissociative lifetimes of the biradicals of different spin states involved compared to rotation about a C—C bond. 

Figure IX-E-15. Quantum yield of singlet methylene formation as a function of wavelength in the photodecomposition of ketene at low pressures.

The quantum yields of photodecomposition of ketene versus wavelength were estimated using the data given in figures IX-E-16 and IX-E-17. These are given in figure IX-E-21... 

Figure IX-E-21. Quantum yields of photodecomposition of ketene versus wavelength (solid hlack line) cross sections of ketene versus wavelength, gray curve wavelengths of onset of CH2 and CH2 are indicated by the long-dashed line and short-dashed line, respectively.

We have calculated the photolysis frequencies for ketene using the quantum yields for photodecomposition at atmospheric pressure and the absorption cross sections for ketene (see table IX-E-3) together with the appropriate actinic flux data for a cloudless day with an overhead ozone column of 350 DU see figure IX-E-22. These data give a photochemical lifetime of about 1 h with an overhead Sun. 

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